The cover depicts a crystal of the mineral zircon (ZrSi04), which is contained by most igneous 

rocks in small amounts (usually less than 0.1 percent). Because of its refractory nature and 

relatively high concentrations of uranium and thorium, zircon is commonly used for isotopic 

dating. 

Artwork by Arthur L. lsom 
SHORTER CONTRIBUTIONS TO 

ISOTOPE RESEARCH 

Edited by Zell E. Peterman and Diane C. Schnabel 

Topical reports on 

geochronology and 

isotope geochemistry 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1986 

For sale by the Branch of Distribution 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Shorter contributions to isotope research. 

(Geological Survey Bulletin ; 1622) 

Includes bibliographies. 

Supt. of Docs. no.: 119.3:1622-A 

1. Geological time-Addresses, essays, lectures. 2. Isotope geologyUnited States-Addresses, essays, lectures. 3. Geology-United StatesAddresses, essays, lectures. I. Peterman, Zell E. II. Schnabel, Diane 

C. Ill. Series: U.S. Geological Survey Bulletin ; 1622. 

QE75.B9 no. 1622 557.3s [551.7'01] 84-600171 [QE508] 
SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

CONTENTS 

[For convenience in cross-referencing, each chapter of this publication has a letter designation] 

A. Geochronology of Precambrian and Tertiary rocks from the Mineral Mountains, 

south-central Utah, by John N. Aleinikoff, Dennis L. Nielson, 

Carl E. Hedge, and Stanley H. Evans, Jr. 1 

B. Uranium-lead zircon ages and common lead measurements for the Archean 

gneisses of the Granite Mountains, Wyoming, by Lynn B. Fischer and 

John S. Stacey 13 

C. Fission-track ages of air-fall tuffs in Miocene sedimentary rocks of the Espanola basin, 

Santa Fe County, New Mexico, by Glen A. Izett and C. W. Naeser 25 

D. Muscovite-phenocrystic two-mica granites of northeastern Nevada 

are Late Cretaceous in age, by Donald E. Lee, JohnS. Stacey, 

and Lynn B. Fischer 31 

E. Geochronology of basement granite, Stephenson County, Illinois, by ZellE. 

Peterman, S. S. Goldich, Lynn B. Fischer, and K. Futa 41 

F. A protracted Archean history in the Watersmeet gneiss dome, northern 

Michigan, by Zell E. Peterman, R. E. Zartman, and P. K. Sims 51 

G. Age of the basement straurolite-biotite schist of northeastern South Dakota, by 

J. R. Richards, D. M. Baron, and S. S. Goldich 65 

H. Apatite fission-track age for the Bull Domingo boulder pipe, Custer County, Colorado, 

by W. N. Sharp and C. W. Naeser 77 

I. Uranium-lead systematics of a mixed zircon population-The Granite at Yale Farm, 

Berkshire massif, Connecticut, by Robert E. Zartman, 

Loretta M. Kwak, and Ralph P. Christian 81 

J. Fission-track dating of the Illinois drill-hole core, by Robert A. Zimmermann 

K. Oxygen isotopic constraints on the origin of the Precambrian granites from the 

southern Wind River Range and the Granite Mountains, 

central Wyoming, by K. K. Cheang, D. B. Wenner, and J. S. Stuckless 109 

99 

L. Origin of the lithium-rich brine,Clayton Valley, Nevada, by Joseph R. Davis, Irving 

Friedman, and J. D. Gleason 131 

M. Chemical and stable isotope compositions of anorogenic granite from 

Stephenson County, Illinois, by T. Kurtis Kyser 139 

Contents Ill 
IV Contents 

N. Negative B180 values found for eastern Nevada plutonic rocks deformed by 

stresses resulting from post-crystallization movement along 

spatially related thrust faults, by Donald E. Lee, 

Irving Friedman, and Jim D. Gleason 151 

0. Preliminary study of hydrogen, oxygen, and sulfur isotope systematics of the 

Xihuashan quartz-wolframite deposit, China, by Robert 0. Rye, 

T. P. Ding, Joseph F. Whelan, and Gary P. Landis 157 

P. The strontium isotope composition of granitoid rock from the southern 

Snake Range, Nevada, by Donald E. Lee, Ronald W. Kistler, and 

Allen C. Robinson 171 

Q. Lead-isotope evidence for a Pre-Grenville crust under the piedmont of Georgia, 

by J. S. Stuckless, D. B. Wenner, and I. T. Nkomo 181 

R. Soil-gas helium distribution at the Shadow Mountain collapse, 

Cameron uranium district, Arizona, by C. G. Bowles, 

G. M. Reimer, J. M. Been, and D. G. Murrey 201 

S. An automated helium analysis station, by Irving Friedman, Joe Jurceka, 

Willis Doering, William Long, and Don McNair 213 

T. Constraints on time-efficient data-taking strategies for single-collector, isotope-ratio 

mass spectrometers, by Kenneth R. Ludwig 219 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

GEOCHRONOLOGY OF PRECAMBRIAN 

AND TERTIARY ROCKS IN THE 

MINERAL MOUNTAINS, SOUTH-CENTRAL UTAH 

Chapter A 

By JOHN N. ALEINIKOFF, DENNIS L. NIELSON1 , CARL E. HEDGE, and 

STANLEY H. EVANS, JR.1 

CONTENTS 

Abstract ..... 

Introduction . . . 

Acknowledgments 

Geology ..... 

Analytical procedures 

FIGURES 

Page 

2 

2 

2 

2 

4 

Geochronology . . . . . . . . 

Metamorphic rocks . . . 

Hornblende granodiorite . 

Discussion 

References cited . . . . . . . 

A1. Generalized geologic map of the central part of the Mineral Mountains, Utah 

A2. Photomicrographs of zircons from metamorphic rocks . . 

A3. Uranium-lead concordia plot of zircons from metamorphic rocks 

A4. Rubidium-strontium isochron plot of metamorphic rocks . . . . . 

A5. Photomicrographs of uraniferous minerals from Tertiary granodiorite . 

A6. Uranium-lead concordia plot of zircons, sphene, and thorite from hornblende granodiorite 

A 7. Model uranium-lead concordia plot showing possible multistage history for zircons in granodiorite, 

Mineral Mountains, Utah . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

TABLES 

A 1. Chemical analyses of Precambrian rocks from the Mineral Mountains, Utah 

A2. Chemical analyses of foliated hornblende granodiorite . . . 

A3. Uranium-thorium-lead data for zircons, thorite, and sphene 

A4. Rubidium-strontium data . 

A5. Potassium-argon data . . . . . . . . . . . . . 

1

University of Utah Research Insitute, University of Utah, Salt Lake 

City, Utah. 

Geochronology of Precambrian and Tertiary Rocks, Mineral Mountains, Utah 

Page 

5 

5 

8 

9 

11 

Page 

3 

6 

6 

7 

9 

10 

11 

Page 

4 

4 

7 

7 

8 
Abstract 

Uranium-thorium-lead and rubidium-strontium 

studies of two metamorphic units from the Mineral 

Mountains, Utah, document the existence of Early 

Proterozoic rocks in south-central Utah. A paragneiss 

and an orthogneiss are about 1,720 million years old; 

both units are highly deformed and are in fault-bounded 

slices. The gneisses are intruded by foliated hornblende 

granodiorite previously thought to be also of Precambrian age. The granodiorite has been dated using zircon, 

sphene, and thorite, as about 25 million years old, which 

makes it an early phase of the intrusive complex of the 

Mineral Mountains. Potassium-argon determinations for 

hornblende and biotite from all three units show that 

mid-Tertiary to Quaternary thermal activity affected the 

rocks. Data for zircons from the hornblende granodiorite 

suggest a complex, multistage history involving inherited 

Proterozoic source material and magmatic zircon, both 

of which may have two-stage histories. 

INTRODUCTION 

The Mineral Mountains in south-central Utah are 

made up mostly of an intrusive complex, with minor associated sedimentary, extrusive, and metamorphic units 

(fig. A1). The intrusive complex is of Tertiary age, and 

is the largest body of plutonic rocks exposed in the state 

of Utah. 

Previous geologic mapping in the area not specifically related to this study includes the work of Liese 

(1957), Earll (1957), Condie (1960), Petersen (1975), 

Evans (1975), and Lipman and others (1978). More recently, the central and northern parts of the Mineral 

Mountains have been mapped at a scale of 1 :24,000 by 

Nielson and others (1978) and Sibbett and Nielson 

(1980a). These results also have been summarized in Sibbett and Nielson (1980b) and the present study is an outgrowth of this work. Studies centered on the Roosevelt 

Hot Springs geothermal system, which is on the central 

western margin of the range, have created an abundant 

and diverse geoscience data base for the Mineral Mountains area, and some of the results have been summarized 

in McKinney (1978) and Ward and others (1978). 

ACKNOWLEDGMENTS 

This work was partially supported by contract No. 

DE-AC07-80ID12079 from the Division of Geothermal 

Energy of the U.S. Department of Energy to the University of Utah Research Institute. We thank R. Hopper, G. 

Cebula and J. Groen for mineral separations, and L. 

Fischer and L. K wak for chemical processing. Identifica2 Shorter Contributions to Isotope Research 

tion of thorite by E. E. Foord is gratefully acknowledged. 

This manuscript was reviewed and therepy substantially 

improved by J. S. Stuckless and T. A. Steven. 

GEOLOGY 

A generalized map of the central part of the Mineral 

Mountains is shown in figure A 1. The reader is referred 

to the more complete studies of Nielson and others 

(1978), and Sibbett and Nielson (1980a), for details of 

the geology of the range. 

The oldest rocks exposed in the Mineral Mountains 

are gneisses that were assigned a Precambrian age on the 

basis of lithologic character and regional relationships. 

These rocks crop out along the west-central margin of 

the range. The Precambrian units vary widely in lithology 

and consist largely of granitic gneiss, banded gneiss, 

schist, and migmatite, with small included lenses of 

quartzite (Nielson and others, 1978; Sibbett and Nielson, 

1980a). Outcrops have a number of penetrative deformational features. Schistosity is well developed, and smallscale isoclinal folding has produced strong northeastplunging lineations on schistosity planes. A few widely 

scattered outcrops show gentle cylindrical refolding of the 

isoclinal folds along flat-lying east-west axes. 

In hand specimen, the banded gneiss is typically 

fine to medium grained and equigranular, and consists of 

biotite, hornblende, potassium-feldspar, plagioclase, and 

quartz in highly variable proportions. Preferentially 

oriented biotite books and stressed and elongate quartz 

grains mark the schistosity. Dark layers are predominantly 

plagioclase, biotite, and hornblende with lesser amounts 

of quartz and potassium-feldspar. Light-colored layers 

consisting mostly of potassium-feldspar and quartz are 

typically coarser grained and locally contain small 

amounts of muscovite. In both light and dark layers, biotite flakes commonly rim potassium-feldspar grains. Zircon is a common accessory mineral and is especially 

abundant as inclusions within biotite. Apatite also is an 

abundant accessory mineral, but sphene generally is uncommon. In addition to this typical mineralogy, a 2- to 

3-m-thick layer of distinctive sillimanite-cordierite-corundum-biotite schist is exposed in one small outcrop just 

south of Wild Horse Canyon. This layer demonstrates 

both a compositional variability within the banded gneiss 

and a lack of diffusion within the metamorphic system, 

suggesting that the banding within much of the gneiss 

represents original sedimentary layering. 

Granitic gneiss occurs as a pink, medium-grained, 

equigranular, and well-foliated rock that consists of 

quartz, potassium-feldspar, plagioclase, biotite, and muscovite, with accessory zircon, apatite, and sphene. In 

marked contrast to the layered paragneiss, this unit has 
38° 

22' 

30" 

GC:"i 

UT/MM 79-15 

UT/ MM 79-15 

UT/MM 79-151} 

UT/MM 79-4 

I 

I GC-16--....., 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

/UT/MM 

'• 

UT/MM 79-110,111,112 

UT/MM 113,114} 

MR 79-10 

UT/MM 79-11 

GC-15 

• 

EXPLANATION 

Quaternary alluvium and flows 

Tertiary intrusive complex of Mineral Mountain 

Tertiary foliated hornblende granodiorite 

Paleozoic(?) sedimentary rocks 

Early Proterozoic gneisses 

Contact 

Fault- Dashed where inferred 

Sample number and locality 

6 KILOMETERS 

~----~------~-----------~ 

3 MILES 

I 

UTAH 

~ Ml""'' , M'""'''"' 

Figure A 1. Generalized geologic map of the central part of the Mineral Mountains, Utah (simplified from Sibbett 

and Nielson, 1980a). 

Geochronology of Precambrian and Tertiary Rocks, Mineral Mountains, Utah 3 
Table A1. Chemical analyses of Precambrian rocks from 

the Mineral Mountains, Utah 

[J. Mason and F. H. Brown, analysts. Sample UT/MM 79-115 is orthogneiss; all other samples are paragneiss. LOI, loss on ignition; NA, 

not analyzed for] 

UT/MM- UT/MM- UT/MM- UT/MMSample No. ----79-10A 79-10B 79-110 79-111 1 

Oxide, in weight percent 

SiOz 77.84 72.52 73.61 78.01 

TiOz 0.08 0.68 0 . 35 0.14 

AI 2o3 12.34 11.52 14.73 14.43 

2Fe0 0.94 5.11 2.84 1.10 

MnO 0.01 0.04 0.04 0.03 

MgO 0.29 1.30 1.11 o. 75 

CaO 0.93 1.46 4.60 3. 22 

K2o 3.29 2.13 2. 79 3. 76 

Na 2o 4.35 3. 34 o. 79 1.31 

P2os 0.01 0.05 0.11 0.07 

LOI 0.57 o. 70 0.50 0.46 

Sum---------100. 76 99.42 101.47 103.28 

Trace element, in parts per million 

Nb 

Zr 

y 

Sr 

Rb 

Th 

Pb 

Ba 

10 20 10 

160 465 140 

35 125 15 

85 70 295 

85 105 30 

15 25 15 

10 15 5 

600 550 NA 

lnated by U-Pb zircon method 

2Total iron calculated as FeO. 

10 

155 

10 

235 

35 

25 

15 

NA 

UT/MM- UT/MM79-112 79-115 1 

75.31 73.30 

0.12 0.20 

14.21 14.10 

1.09 1.86 

0.03 0.03 

0.68 0.42 

0.91 1. 28 

3.40 3.13 

5.10 5.28 

0.06 0.06 

0.42 0.40 

101.33 100.06 

5 20 

105 160 

15 30 

125 100 

100 230 

20 45 

30 45 

690 680 

a uniform composition, at least on outcrop scale, and 

probably represents a metaigneous rock. 

Chemical analyses of six samples are listed in table 

A 1 . These samples of both paragneiss and orthogneiss 

were collected from the more quartzofeldspathic layers in 

the gneisses. 

Foliated hornblende granodiorite to quartz monzonite intrudes the Precambrian rocks just described. The 

foliation in this rock is nearly parallel to that in the Precambrian gneiss, and therefore Nielson and others (1978) 

proposed that it also was of Precambrian age. Essentially 

identical rocks later were found to be intrusive into Cambrian marbles in the northern part of the range, at which 

locality the foliation is nearly parallel to the contact with 

the marbles and is considered to be a flow structure. Correlation of the two bodies of foliated hornblende 

granodiorite is supported not only by their similar appearance in the field, but also by the similarities in chemical 

composition (table A2) and identical morphologies of the 

zircons separated from both of them. 

ANALYTICAL PROCEDURES 

The paragneiss (sample UT/MM 79-111), orthogneiss (sample UT/MM 79-115), and a flow-foliated 

4 Shorter Contributions to Isotope Research 

Table A2. Chemical analyses of Tertiary foliated 

hornblende granodiorite samples from the Mineral 

Mountains, Utah 

[J. Mason and F. H. Brown, analysts. LOI, loss on ignition; NA, not 

applicable] 

UT/MM- UT/MM- UT/HM- UT/MM- UT/MMSample No.---- 78-170 79-113 1 79-114 79-150 79-151 

Oxide, in weight percent 

Si02 63.52 59.88 67.83 72.08 63.57 

Ti02 0.60 0.63 0.43 0.26 0.71 

Aiz03 15.92 16.60 15.92 14.40 15.89 

2Fe0 4.57 5.33 2.89 1.88 5.04 

MnO 0.09 0.11 0.06 0.04 0.09 

~lgO 2.40 2.96 1.45 0.44 2.32 

CaO 4.32 5.00 2. 71 1.63 4.50 

K2o 3.33 4.30 3.42 3.74 3.82 

Na 2o 3.86 2. 75 4. 96 4.48 3.19 

Pzo5 0.19 0.27 0.14 0.07 0.26 

LOI o. 75 0.36 0.55 0.85 0.86 

Sum----------99. 55 98.19 100.36 99.87 100.25 

Nb 

Zr 

y 

Sr 

Rb 

Th 

Pb 

3u 

Ba 

Trace element, in parts per million 

15 10 15 10 10 

130 105 170 120 140 

20 15 30 10 15 

785 850 550 445 870 

70 90 120 125 85 

5 15 30 25 25 

15 15 30 25 15 

2 NA NA 3 (2 

925 980 1200 910 980 

!Hornblende granodiorite, dated by U-Th-Pb zircon method. 

2Total Fe calculated as FeO. 

3uranium determined fluorimetrically • 

UT/MM79-153 

65.69 

0.57 

14.99 

4.29 

0.08 

1.95 

3. 76 

3.52 

3. 74 

0.20 

0. 77 

99.56 

10 

150 

15 

685 

90 

10 

15 

(2 

855 

hornblende granodiorite (samples UT/MM 79-113 and 

UT/MM 80--4) were dated by the U-Th-Pb (uraniumthorium-lead) method. Zircons were separated from about 

50 kg (kilograms) of rock, using a crusher, pulverizer, 

Wilfley2 table, and heavy liquids. The heavy-mineral separates were at least 99 percent pure zircon. Size and magnetic fractions were separated and were upgraded by handpicking out impurities to ensure nearly absolute purity. 

Uranium, thorium, and lead were extracted from 

zircon, sphene, and thorite. Size and magnetic fractions 

of zircon and sphene were leached in separate solutions 

of warm 7N HN03 and warm 6.5N HCI. About H~--20 

mg (milligrams) of zircon were dissolved in ultrapure 48-

percent HF (2 mL (milliliters)) and 16N HN03 (1 mL) 

in a teflon bomb assembly at 21 0°C in an oven. Dissolution took three days or more. Samples for which thorium 

concentrations were determined were evaporated to dryness with 235U and 23<Yrh spike and redissolved in 1 N 

HN03 . Overnight equilibration in the oven at 210°C was 

followed by liquid-aliquoting portions for lead composition and concentration. Samples that were not analyzed 

for thorium were not equilibrated in IN HN03 and were 

spiked for uranium, liquid-aliquoted, and spiked for lead 

after dissolution. 

zuse of trade and company names is for descriptive purposes only 

and does not imply endorsement by the U.S. Geological Survey. 
Sphene was immersed in ultrapure 70-percent 

HC104 (0.50 mL) and 6.5N HCl (4 mL) in a teflon bomb 

assembly in an oven (21 0°C) overnight. The next day, 

48-percent HF (1 mL) and 16N HCl (2 mL) were added 

and the sample was dissolved overnight. The sample was 

spiked and aliquoted for uranium, thorium, and lead after 

complete dissolution. 

Thorite was immersed in 48~percent HF (0.50 mL) 

and 6.5N HCl (1 mL) in a screwtop teflon beaker and 

heated on a hotplate at 200°C overnight. After evaporation 

to dryness, 70-percent HC104 (0.50 mL) and 16N HN03 

(1 mL) were added. The sample dissolved after three days 

of boiling. After 3 mL of 6.5N HCl were added, the 

solution was ready for aliquoting. 

Lead was concentrated on a bromide-form ion-exchange column (modified from Krogh, 1973) and was 

analyzed using the single-Re-filament, H3P04/silica gel 

technique. Uranium and thorium were extracted on nitrateform ion-exchange columns and were analyzed using a 

triple-Re-filament assembly. All isotopes were analyzed 

on a 30.5-cm-radius, 90°-sector, single-focusing mass 

spectrometer with digital data collection. 

Uranium, thorium, and lead concentrations were determined by isotope dilution and are believed to be accurate to at least 1 percent. Lead isotopic compositions are 

believed accurate to at least 0.1 percent, except where 

the 206PbP04Pb is greater than 5000, as it is in samples 

UT /MM 79-111 and UT /MM 79-115. The errors for 

atomic ratios may be as high as 5-10 percent in these 

analyses but the atomic percent of 204Pb is so small that 

these large errors have no significant effect on the age 

calculations. Laboratory blanks, usually less than 1 ng 

(nanogram), determined during the course of the study, 

were of the composition 204PB: 206PB:207Pb: 208PB= 1:18.7:15.6:38.2 

and were subtracted from the analyses. Common lead was 

assumed to have the composition 

1:15.7:15.3:35.3 

(estimated from Stacey and Kramers, 1975) for the 

metamorphic rocks and 

1:18.24:15.57:38.37' 

as determined by analysis of a potassium-feldspar, for the 

hornblende granodiorite. 

Age calculations used decay and isotopic constants 

of Steiger and Jager ( 1977). Regression analysis followed 

the algorithm of Ludwig (1980). Analytical procedures for 

K-Ar (potassium-argon) determinations are similar to 

those reported in Evernden and Curtis (1965) and Dalrymple and Lanphere (1969). 

GEOCHRONOLOGY 

Previous radiometric dating of units of the intrusive 

complex of the Mineral Mountains have been published 

by Park (1968), Liese (1957), Armstrong (1970), Bowers 

(1978), and Lipman and others (1978), but no ages are 

available for the Precambrian rocks or for the foliated 

hornblende granodiorite. Stacey and others ( 1968) measured common lead isotope ratios in galenas and feldspars 

from mining districts throughout Utah, including the Milford district just to the west of the Mineral Mountains, 

and concluded that the lead was derived from an Early 

Proterozoic source rock. 

Metamorphic Rocks 

Two size fractions of zircon from each metamorphic 

rock were analyzed. Zircons in the paragneiss (UT/MM 

79-111) are- euhedral and some are slightly rounded (fig. 

A2A). They are blocky in morphology, with length-towidth ratios of about 2 or 3 to 1 and contain abundant 

opaque and clear inclusions. They do not appear to be 

detrital (that is, they are not rounded or frosted) and on 

the basis of their morphology and the compositions of 

the paragneiss, we conclude that the rock sampled is volcanic in origin. Zircons in the orthogneiss (UT /MM 79-

115) are also euhedral (fig. A2B). They are more prismatic than the zircons from the paragneiss, having length-towidth ratios of about 4 or 5 to 1, and contain few inclusions. Their general morphology is typical of plutonic zircons and the rock in which they occur is interpreted to 

be a metaquartz monzonite. 

Isotopic data for the gneisses are presented in table 

A3 and are plotted in figure A3. The 207PbP06Pb ages 

range from 1, 723 to 1, 708 Ma (million years) and the 

U-Pb ages range from 1,482 to 662 Ma. As shown in 

figure A3, a best-fit line through the four data points has 

concordia intercept -ages of 1 , 716 ± 31 and 7 ± 93 Ma (95-

percent confidence limits). Three of the four points are 

about 33 percent discordant, whereas the fourth sample 

plots about 67 percent down the discordia. A reasonable 

explanation for this array is suggested by the uranium concentration data. The three least discordant zircons have 

concentrations that range from 1,371 to 2,193 ppm (parts 

per million) uranium, whereas the most discordant size 

fraction contains 3,736 ppm uranium. Thus, a Cenozoic 

event that caused lead loss from the zircons would have 

had a significantly greater effect on the most uranium-rich 

(and thereby most radiation-damaged) sample. 

We interpret the data to indicate an age of about 

1, 720 Ma for both metamorphic units. Specifically, the 

orthogneiss was intruded at this time, during regional 

metamorphism of the paragneiss inasmuch as there is no 

evidence in the U-Th-Pb systematics for a younger 

metamorphic event. Plutonic rocks of similar age in Colorado (such as the Boulder Creek Granite) also are interpreted as being syntectonic in origin (Tweto, 1977). Igneous rocks in the next youngest age group in Colorado, 

about 1 ,400 Ma (Tweto, 1977) generally are not foliated 

and are considered to be post-tectonic. 

Geochronology of Precambrian and Tertiary Rocks, Mineral Mountains, Utah 5 
• 

Figure A2. Photomicrographs of zircons from metamorphic rocks of the Mineral Mountains, Utah. A, Zircons from 

paragneiss sample UT/MM 79-111 (size fraction, +150 mesh); 8, Zircons from orthogneiss sample UT/MM 79-115 

(size fraction, +150 mesh). Grains in both A and 8 are about 170 J.Lm long. 

0 .3 

0 .1 

/ 

~a 0 UT/MM 79-111(+150) ? /o·0 

UT/MM 79-111(-400) 

/__ ... UT/MM 79-11~-325) 

• UT/MM 79-115(+150) 

2 4 5 

Figure AJ. Uranium-lead concordia plot of zircons from 

metamorphic rocks from the Mineral Mountains, Utah. 

Sample UT/MM 79-11 (open circles) is a paragneiss; sample UT/MM 79- 115 (solid circles) is an orthogneiss. 

6 Shorter Contributions to Isotope Research 

Zircons in the paragneiss may be volcanic in origin 

or they may have formed during regional metamorphism 

and intrusion of the orthogneiss proto lith. There is no evidence in the zircon data for the age of a parent rock of 

the paragneiss. This lack of evidence may be explained 

by: ( 1) the absence of detrital zircons from the precursor 

sedimentary pile, or (2) the precursor volcanic(?) rocks 

were formed only shortly before burial and metamorphism 

and thus the slight age difference of the protolith and 

metamorphic rock cannot be resolved. The lower intercept 

of the discordia corresponds to a Cenozoic age and could 

have been caused by Quaternary dilatancy (Goldich and 

Mudrey, 1972) or, more likely, by intermittent igneous 

activity in this area since early Miocene time. Evans and 

Steven ( 1982) reported an age of about 9 Ma for emplacement of rhyolite domes and flows a few kilometers east 

of the Mineral Mountains. 

Rubidium-strontium (Rb-Sr) data for the orthogneiss 

and paragneiss are listed in table A4. Three paragneiss 

samples show scatter well beyond the limits of analytical 

error (see fig. A4). This scatter presumably is due to 
Table A3. Uranium-thorium-lead data for zircon, thorite, and sphene from the Mineral Mountains, Utah 

[Leaders(---) indicate no Th concentration determined and thus no 208PbP32Th age calculated; NM, nonmagnetic; VM, very magnetic] 

A~e (million years) 

Concentration (ppm) Atomic percent 206pb 207pb 207pb 208pb 

Mineral: (size fraction) u Th Pb 204pb 206pb 207pb 208pb 238u 23Su 206pb 232Th 

Sample UT/MM 79-111 (paragneiss) 

Zircon: (+150) 1370.5 308.25 0.0025 86.93 9.204 3.864 1320 1482 1723 

(-400) 1957.3 429.48 0.0012 86.17 9.056 4. 774 1281 1452 1712 

Sample UT/MM 79-115 (orthogneiss) 

Zircon: (+150) 3736.3 415.9 0.0010 84.53 8.879 6.593 669 963 1712 

(-325) 2192.6 458.2 0.0007 86.83 9.092 4.082 1234 1419 1708 

Sample UT/MM 79-113 (hornblende granodiorite) 

Zircon: (+100)VM 338.2 2.05 0.4299 64.69 9.401 25.48 25.8 26.4 82.8 

(+100) 363.0 162.5 1. 72 0.1866 75.16 6.297 18.36 25.4 25.8 62.8 26.9 

( -100+150) 375.5 189.5 1. 91 0.2046 73.41 6.536 19.85 26.5 27.4 103.7 27.5 

(-325+400) 531.0 2.69 0.1130 70.19 5.086 24.61 25.8 27.0 139.1 

(-400) 1122.9 7.21 0.1670 47.06 4.842 47.94 21.1 23.1 238.6 

Sample UT/MM 4-80 (hornblende granodiorite) 

Zircon: (+150)NM 512.0 4.74 0.0537 

Sphene---------------- 308.5 384.2 3.07 0.8148 

Concentration 

u Th Pb 

(wei~ht percent) (ppm) 

Thorite--------------- 12.1 58.7 666.30 0.0069 

Table A4. Rubidium-strontium data for rocks from the 

Mineral Mountains, Utah 

Rb Sr 

Sample No. (parts per million) 87Rb/86sr 87sr;BGsr 

Paragneiss 

UT/MM 79-110 28.8 269 0.3092 o. 7177 UT/MM 79-111 49.9 225 0.6429 o. 7282 UT/MM 79-112 89.4 108 2.404 0.7621 

Orthogneiss 

UT/MM 79-115 216 94.1 6.743 0.8717 

Granodiorite 

UT/MM 79-113 96.6 857 0.3262 0.70560 

heterogeneous initial 87Sr/86Sr. One orthogneiss sample 

also was analyzed. The relatively high 87Rb/86Sr of 6. 743 

allows calculation of an apparent age of 1, 750 Ma if an 

initial 87Sr/86Sr of 0. 702 is assumed. This isochron also 

passes through one of the paragneiss data points. This 

Rb-Sr age is in satisfactory agreement with the zircon age 

data, considering that the assumption of initial 87Sr/86Sr 

may not be quite accurate. 

Potassium-argon data of biotites from three samples 

of the banded gneiss are listed in table A5. Sample GC-16 

81.65 

42.82 

38.48 

0.86 

.... 0.82 (/) 

'-0 

00 

'>:;-

(/) 

~ 0.78 

0.74 

7.008 11.29 

13.974 42.39 

1.892 59.62 

55.7 88.9 1108 

20.7 20.7 20.7 20.1 

15.6 15.7 24.7 15.2 

Model age= 1750 million years 

R;=0.7020 

0. 70 '-----__L_ __ ...J__ _ ___l __ __L_ __ .J.__ _ __j_ __ _L_ _ ___j 

0 2 3 4 5 6 7 8 

87Rb/86Sr 

Figure A4. Rubidium-strontium isochron plot of 

metamorphic rocks from the Mineral Mountains, Utah. Errors (2cr) are 0.035 percent and 1.7 percent for 87Sr/86Sr 

and 87Rb/86Sr, respectively. Open circle is orthogeniss; 

solid circles are paragneiss. The line is a model isochron 

calculated through the orthogneiss data point assuming 

an initial 87Sr/86Sr (R;) of 0.702. 

Geochronology of Precambrian and Tertiary Rocks, Mineral Mountains, Utah 7 
Table AS. Potassium-argon data for rocks from the Mineral Mountains, Utah 

Weight K Moles/gram Percent Ar 40 Age 

Ar 4 0(x10- 11 Sample No. Mineral (grams) (percent) ) (atmospheric) (million years) Reference 

Gneiss 

GC-16 Biotite------ 0.45942 7.505 14.009 63.7 10.7±0.2 Bowers (1978). 

GC-15 --do.-------- 2.56070 6.323 13.797 27.8 12.5±0.1 Do. 

UT/MM 79-10 --do.-------- 0.77447 7.60 13.016 47 9.85±0.37 

Hornblende granodiorite 

MR 79-10 Biotite------ 0.56814 7.49 

Hornblende--- 2.50659 0.75 

UT/MM 78-170 Biotite------ 0.79213 7.40 

UT/MM 79-150 --do.-------- 0.52549 7.89 

UT/MM 79-151 Biotite------ 0.60145 7. 77 

Hornblende--- 2.53078 0.67 

UT/MM 79-153 Biotite------ 0.50100 7.40 

Hornblende--- 2.50158 0.79 

(Bowers, 1978) was collected from an outcrop in Negro 

Mag Wash. The outcrop is along a major fault zone and 

within 50 m of an active fumarole. Sample GC-15 was 

collected from an outcrop of gneiss that presently is interpreted as a xenolith (about 400 m2 in area) within some 

of the more felsic phases of the intrusive complex of the 

Mineral Mountains. A cataclasite found in the same outcrop indicates that the sample is near a fault zone. Sample 

UT /MM 79-10 was collected from an outcrop which is 

on the eastern margin of the Roosevelt Hot Springs geothermal field. This sample also was collected within about 

100 m of a principal range-front fault. The average of 

the three K-Ar dates is about 11 Ma. This age is near 

that of 9 Ma reported by Evans and Steven (1982) for 

rhyolite lavas just east of the Mineral Mountains and indicates, in light of the Rb-Sr and U-Pb data, extensive resetting of the K-Ar system. 

Hornblende Granodiorite 

Three accessory minerals (zircon, sphene, and thorite) from the hornblende granodiorite were analyzed for 

U-Th-Pb isotopes. The zircons are distinctly bright pink 

and have virtually no evidence, such as dark core material, for an inherited component. The crystals are euhedral 

and nearly equant with length-to-width ratios ranging from 

about 1 to 4 (fig. A5A). The sphene is anhedral, coarse 

grained, and bright yellow. Thorite is subhedral to euhedral, forming prisms similar in shape to zircons (fig. A5B), 

from which it can be distinguished by its olive- to 

emerald-green. Several grams of each accessory mineral 

were extracted from about 50 kg of rock. The thorite was 

8 Shorter Contributions to Isotope Research 

13.448 72 10.3±0.6 

5.284 39 40.3±1.4 

12.666 so 9.86±0.37 

18.362 27 13.4±0.5 

26.392 34 19.5±0.7 

4.503 59 38.4±1.6 

17.473 57 13.6±0.6 

3.744 33 27.2±0.9 

identified by Eugene E. Foord, U.S. Geological Survey, 

using X-ray diffraction of preheated crystals and X-ray 

fluorescence (EDS). 

Isotopic data for two samples of hornblende 

granodiorite are listed in table A2 and plotted on figure 

A6. The zircon data from sample UT /MM 79-113 do not 

form a linear pattern and so a complicated multistage history must be invoked to explain the data array. The 207Pb/ 206Pb ages range with decreasing grain size from 63 Ma 

( + 100 mesh) to 239 Ma (-400 mesh). All U-Pb and ThPb ages cluster around 25 Ma ± about 4 Ma In an attempt to determine more definitely the intrusive age of 

the granodiorite, another sample (UT/MM 4-80) from a 

more northerly locality in the Mineral Mountains was collected. Based on the data from UT /MM 79-113, we 

thought that the coarsest and least magnetic size fraction 

would yield the youngest 207PbP06Pb ages. As can be 

seen in table A2 and figure A6, the 207PbP06Pb age of 

UT/MM 4-80 ( + 150)NM is far older (1, 108 Ma) than 

any of the other size fractions. We attribute the older Pb/ 

Pb ages to inheritance of old radiogenic lead, derived 

from magmatic source regions, in young ( ~25-Ma-old) 

granodiorite zircons. 

Given an age of 25 m.y., sample UT/MM 79-113 

has a calculated initial 87 Sr/86Sr of 0. 70548. Such ratios 

are typical of andesitic volcanic rocks erupted at about 

this same time about 24 km to the east in the Marysvale 

volcanic field (C. E. Hedge, unpub. data). 

Potassium-argon ages for samples of foliated 

hornblende quartz monzonite and hornblende granodiorite 

are listed in table A5. There is considerably more scatter 

in the dates on these intrusive units than in the dates for 
Figure AS. Photomicrographs of uraniferous minerals 

from Tertiary granodiorite, Mineral Mountains, Utah. A, 

Zircons from granodiorite sample UT/MM 79-113 (size 

fraction, +100 mesh). Grains are about 200 J.Lm long. 8, 

Scanning electron microscope photograph of thorite 

from granodiorite sample UT/MM 79-113. Grain is about 

200 J.Lm long. 

the Early Proterozoic gneisses. The biotite ages vary by 

a factor of two and the hornblende ages are markedly 

different from the biotite ages from the same samples. 

In comparing the biotite ages with the U-Pb ages, clearly 

the biotites all have been reset. The hornblende ages are 

more problematical. One date of 27 Ma is concordant with 

the U-Pb ages. The other hornblende ages are somewhat 

older and excess argon is the likely cause of the anomalously old ages. Note that the ages of biotite~ from the 

granodiorite in the northern part of the range are older 

than those from the central part of the range. This geographic distribution of ages may represent a greater distance from the perturbing younger thermal center for these 

samples. 

Sphene and thorite samples from the granodiorite 

were analyzed also. As shown in table A3, the sphene 

yields a concordant age of about 21 Ma, younger than 

any of the zircons. Tho rite has a 207PbF06Pb age of about 

25 Ma and U-Pb and Th-Pb ages are about 15 Ma. Elsewhere, U-Th-Pb ages on sphene have been shown (Hanson and other, 1971) to have been reset by contact 

metamorphism. Such ages are thought to be more resistant 

to resetting than K-Ar hornblende ages, but less resistant 

than U-Pb zircon ages (Hanson and others, 1971). Thus, 

the igneous acitvity in this area appears to have prevented 

final closing of the U-Th-Pb system in the sphene until 

21 m.y. ago. The thorite 207PbF06Pb date of 25 Ma is 

interpreted as the age of intrusion. Subsequently, lead was 

lost, changing U-Pb and Th-Pb ratios to reflect a younger 

age. Because of the short time span and the uncertainty 

in the crystallization age, we cannot determine precisely 

when in the last 25 m. y. this lead was lost, although 

Quaternary lead loss due to dilatancy (Goldich and Mudrey, 1972) or loss during any of the several Miocene to 

Quaternary episodes of igneous activity known to have 

occurred in this area seems likely. 

DISCUSSION 

In the simple two-stage example where the lead 

isotopic composition of a zircon is a function only of the 

age of crystallization and the age of the inherited material, 

a linear array is formed. The location of a data point 

along this "inheritance line" (in our study between 25 and 

1, 720 Ma) is determined by the amount of each lead 

isotope contributed by the two end-member sources. For 

the intercepts to yield geologically reasonable dates, no 

lead may have been lost from either end member prior 

to or subsequent to the mixing. In the Mineral Mountains, 

the hornblende grandiorite shows isotopic evidence for 

disturbance to the U-Pb system after crystallization of the 

zircons 25 Ma ago. The fine-grained size fractions, 

(-325+400 mesh) and (-400 mesh), have moved down 

towards the present (0 Ma). The concentrations of uranium in the fine-grained zircons are considerably higher 

Geochronology of Precambrian and Tertiary Rocks, Mineral Mountains, Utah 9 
:J 

lO 

>") 

0.004 

1 0.003 0.. 

.0 

:::> 

"-1 

0.002 

HORNBLENDE GRANODIORITE 

Sphene 6 

0 Thorite 

0.016 0.020 

• 

• • 

• 

0.024 0.028 0.032 0.036 

Figure A6. Uranium-lead concordia plot of zircons, sphene, and thorite from hornblende granodiorite 

from the Mineral Mountains, Utah. Solid circles, zircon fractions from UT/MM 79-113. Solid circle in inset 

shows location of UT/MM 4-80 (size fraction, + 150 mesh NM); shaded area contains hornblende granodiorite data points from UT/MM 79-113. 

(between 531 and 1,123 ppm, respectively) than in the 

coarser grains (between 338 and 376 ppm). Therefore, 

the fine-grained zircons probably accumulated more radiation damage and were more susceptible to lead loss caused 

by either local thermal activity or dilatancy. This area 

of Utah is known for its repeated episodes of intrusion 

and volcanism during the past 25 Ma and is, in fact, an 

active geothermal area today. Therefore, the U-Th-Pb data 

from the coarser zircon grains indicate an intrusive age 

of about 25 Ma ago and suggest derivation from much 

older (Proterozoic) rock. 

A likely parent for the hornblende granodiorite, as 

indicated by the U-Pb isotopic data, is a unit of Early 

Proterozoic age. However, based on whole-rock chemistry 

and strontium isotope data, the paragneiss and orthogneiss 

analyzed in this study cannot have been the parent material. A more likely source rock would be lower crustal 

material, which should have been more mafic in composition and should have had lower 87Sr/86Sr. If only modem 

lead loss has occurred since intrusion of the granodiorite, 

then the data points for the granodiorite should fall on 

1 0 Shorter Contributions to Isotope Research 

or below an "inheritance chord" between 25 Ma and 1 , 720 

Ma. As previously discussed, the two fine-grained fractions of sample UT/MM 79-113 plot below this line, due, 

at least in part, to recent lead loss. However, sample UT/ 

MM 4--80 ( + 150) which has a 207PbF06Pb age of 1, 108 

Ma, plots slightly above the "inheritance chord". We ~­

plain this plot position by invoking a small amount of 

lead loss from the Proterozoic zircons probably about 

1,400 million years ago, caused by the thermal event responsible for the intrusion of Middle Proterozoic 

anorogenic plutonic rocks. Another possible cause for the 

observed scatter is that the lower crustal source material 

is slightly younger than the Proterozoic material dated in 

this study. 

The possible multistage history of the hornblende 

granodiorite is shown graphically in figure A 7. A point 

(X) would move some unknown distance (assumed to be 

a maximum of about 30 percent) from point A towards 

point B. About 1,400 million years later, during anatexis, 

partially resorbed zircons were incorporated into Tertiary 

magmas, to serve as nuclei for new zircons crystallizing 
;:) 

00 

M 

"' :o0.. 

~ 

0 

"' 

0.3 

0.2 

0.1 

2 

0 .012 

0.008 

0.004 

0 

3 

zo7Pb/z3su 

6'o 

,./"" 9'0 ,..,..... ,..,..... ,..,..... ,/ 

:::;...---

25 million years 

0 0.04 0.08 

4 5 

Figure A7. Model U-Pb concordia plot showing possible multistage history for zircons in granodiorite 

sample UT/MM 79-113, Mineral Mountains, Utah. Zircons in Precambrian rocks shift from point A towards 

point B to a position near point X on chord A-8. These discordant zircons were nuclei around which 

25-m.y.-old zircon grew when the hornblende granodiorite magma crystallized. Inset shows the present 

location of sample UT/MM 4-80 ( + 150)NM with respect to a dash-dot reference line between 25 and 

1750 m.y. Note that a line constructed from 25 m.y. through X would yield an erroneously young parent 

age. Another possibility is that the chord from 25 m.y. through X might accurately date the source rock 

at a slightly younger age than the Precambrian rocks dated in this study. 

in the hornblende granodiorite magma. Thus, a chord constructed from the probable age of intrusion (25 Ma) 

through the value for sample UT/MM 4-80 ( + 150)NM 

would give an erroneously young upper intercept. Note 

that modem lead loss in UT/MM 4-80 ( + 150)NM might 

have fortuitously moved the data point down towards the 

present so that it plotted on the "inheritance chord" between 25 Ma and 1,720 Ma. Thus, what in reality might 

be a complicated four-stage history would appear to be 

a simple two-stage history. 

In conclusion, the orthogneisses and paragneisses of 

the Mineral Mountains are about 1 , 720 Ma. Evidence for 

rocks of this age was found by Stacey and others (1968), 

based on common-lead data from galenas from the Milford district. Our study confirms their conclusions and 

identifies a possible source rock for the lead in the galenas 

from south-central Utah. 

The foliated hornblende granodiorite previously 

mapped as Precambrian is shown to be about 25 Ma, 

based on data from zircons. Ages of sphene and thorite, 

which occur in minor and trace amounts, respectively, 

confirm the zircon-age data. Zircons also show that the 

granodiorite probably was derived from Early Proterozoic 

material, although a likely parent material has not been 

found in the area. 

REFERENCES CITED 

Armstrong, R. L., 1970, Geochronology of Tertiary igneous 

rocks, eastern Basin and Range Province, western Utah, 

eastern Nevada and vicinity, U.S. A.: Geochimica et Cosmochimica Acta, v. 34, p. 203- 232. 

Geochronology of Precambrian and Tertiary Rocks, Mineral Mountains, Utah 11 
Bowers, D., 1978, Potassium-argon dating and petrology of the 

Mineral Mountains pluton, Utah: University of Utah M.S. 

thesis, 76 p. 

Condie, K. C., 1960, Petrogenesis of the Mineral Range pluton, 

southwestern Utah: University of Utah M.S. thesis, 92 p. 

Dalrymple, G. B., and Lanphere, M.A., 1969, Potassium-argon 

dating: San Francisco, W. H. Freeman and Co., 250 p. 

Earll, F. N., 1957, Geology of the central Mineral Range, 

Beaver County, Utah: University of Utah Ph. D. thesis, 

112 p. 

Evans, S. H., Jr., 1975, Geology of the central and northern 

Mineral Mountains, Utah: University of Utah, Dept. of 

Geology and Geophysics, Topical Report 77-7, 1 p. 

Evans, S. H., Jr., and Steven, T. A., 1982, Rhyolites in the 

Gillies Hill-Woodtick Hill area, Beaver County, Utah: 

Geological Society of America Bulletin, v. 93, no. 11, p. 

1131-1141. 

Evernden, J. F., and Curtis, G. H., 1965, The potassium-argon 

dating of Late Cenozoic rocks in east Africa and Italy: Current Anthropology, v. 6, p. 343-385. 

Goldich, S. S., and Mudrey, M. G., 1972, Dilatancy model 

for discordant U-Pb ages, in Tugarinov, A. (ed.), Contributions to recent geochemistry and analytical chemistry, (A. 

P. Vinogradov Volume), Moscow, USSR, NAUKA Publ. 

Office, p. 415-418. 

Hanson, G. N., Catanzaro, E. J., and Anderson, D. H., 1971, 

U-Pb ages for sphene in a contact metamorphic zone: Earth 

and Planetary Science Letters, v. 12, p. 231-23 7. 

Krogh, T. E., 1973, A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb 

for isotopic age determinations: Geochimica et Cosmochimica Acta, v. 37, p. 485-494. 

Liese, H. C., 1957, Geology of the northern Mineral Range, 

Millard and Beaver Counties, Utah: University of Utah 

M.S. thesis, 88 p. 

Lipman, P. W., Rowley, P. D., Mehnert, H. H., Evans, S. 

H., Nash, W. P.,and Brown, F.H., 1978, Pleistocene rhyolite of the Mineral Mountains, Utah-Geothermal and archeological significance: U.S. Geological Survey Journal of 

Research, v. 6, no. 1, p. 133-147. 

Ludwig, K. R., 1980, Calculation of uncertainties of U-Pb 

isotope data: Earth and Planetary Science Letters, v. 46, 

p. 212-220. 

12 Shorter Contributions to Isotope Research 

McKinney, D. B., 1978, Annotated bibliography of the geology 

of Roosevelt Hot Springs Known Geothermal Resource Area 

and the adjacent Mineral Mountains, March 1978: University of Utah Research Institute, Earth Science Laboratory 

Report, U.S. Department of Energy contract EG-78~---{}7-

1701. 

Nielson, D. L., Sibbett, B. S., McKinney, D. B., Hulen, J. 

B., Moore, J. N., and Sainberg, S. M., 1978, Geology 

of the Roosevelt Hot Springs KGRA, Beaver County, Utah: 

University of Utah Research Institute, Earth Science Laboratory Report 12, 121 p. 

Park, G. M., 1968, Some geochemical and geochronologic 

studies of the beryllium deposits in western Utah: University 

of Utah M.S. thesis, 104 p. 

Petersen, C. A., 1975, Geology of the Roosevelt Hot Springs 

area, Beaver County, Utah: Utah Geology, v. 2, p. 109-

116. 

Sibbett, B. S., and Nielson, 1980a, Geology of the central Mineral Mountains, Beaver County, Utah: University of Utah 

Research Institute, Earth Science Laboratory Report 33, 

42 p. 

---1980b, The Mineral Mountains intrusive complex, Utah 

[abs.]: Geological Society of America Abstracts with Programs, Rocky Mountain Section, v. 12, p. 305. 

Stacey, J. S., Zartman, R. E., and Nkomo, I. T., 1968, A 

lead isotope study of galenas and selected feldspars from 

mining districts in Utah: Economic Geology, v. 63, p. 

796-814. 

Stacey, J. S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth 

and Planetary Science Letters, v. 26, p. 207-221. 

Steiger, R. H. and Jager, E., 1977, Subcommission on geochronology; Convention on the use of decay constants in 

geo- and cosmochronology: Earth and Planetary Science 

Letters, v. 36, p. 359-362. 

Tweto, Ogden, 1977, Nomenclature of Precambrian rocks in 

Colorado: U.S. Geological Survey Bulletin 1422-D, 22 p. 

Ward, S. H., Parry, W. T., Nash, W. P., Sill, W. R., Cook, 

K. L., Smith, R. B., Chapman, D. S., Brown, F. H., Whelan, J. A., and Bowman, J. R., 1978, A summary of the 

geology, geochemistry, and geophysics of the Roosevelt Hot 

Springs thermal area, Utah: Geophysics, v. 43, p. 

1515-1542. 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

URANIUM-LEAD ZIRCON AGES AND 

COMMON LEAD MEASUREMENTS 

FOR THE ARCHEAN GNEISSES OF THE 

GRANITE MOUNTAINS, WYOMING 

Chapter B 

By LYNN B. FISCHER and JOHNS. STACEY 

CONTENTS 

Abstract ..... 

Introduction . . . 

Acknowledgments 

Geology .... . 

Samples .... . 

Analytical procedures 

FIGURES 

Page 

14 

14 

14 

14 

16 

16 

Results .......... . 

Uranium-lead zircon data . 

Lutetium-hafnium data 

Common lead feldspar data 

Summary of events in the Archean . 

References cited . . . . . . . . . . . 

B 1. Map of the geology of the Precambrian of the Granite Mountains, Wyo. 

B2. Uranium-lead concordia diagram ..... . 

B3. Interpretive uranium-lead concordia diagram 

B4. Common lead isochron plots .. 

B5. Common lead evolution diagram . . . . . . 

TABLES 

B 1. Description of samples from the Granite Mountains . . . . 

B2. Uranium-lead zircon data . . . . . ............ . 

B3. Lutetium-hafnium data for zircons from two gneiss samples 

B4. Lead-uranium-thorium data for some HF-leached feldspars from the Archean gneisses 

Page 

17 

17 

19 

19 

22 

22 

Page 

15 

17 

18 

20 

21 

Page 

16 

17 

19 

21 

Uranium-Lead Zircon Ages and Common Lead Measurements, Archean Gneisses, Granite Mountains, Wyo. 13 
Abstract 

Zircons from the metamorphic complex of the 

Granite Mountains, Wyoming, yield uranium-lead concordia ages of 2,880 Ma (106 years) for the massive augen 

gneisses and 3,230 Ma for the layered gneisses. This 

older 3,20~Ma event, predicted by Z. E. Peterman and 

R. A. Hildreth from the high initial 87Sr/86Sr value, is supported further by common lead measurements and by 

the 177Hf/176Hf chondrite model ages of 3,200 Ma. However, such measurements do not preclude a period of 

crustal residence for the precursors of the layered gneisses prior to 3,200 Ma. 

INTRODUCTION 

The Granite Mountains of Wyoming are part of an 

extensive Archean terrane, the Wyoming age province, 

which includes Montana, Wyoming, and adjacent parts 

of Idaho and Utah. The Precambrian basement of the 

Granite Mountains was uplifted during the Laramide 

orogeny, exposing Archean metamorphiC and igneous 

rocks. This paper presents U-Pb (uranium-lead) zircon age 

determinations and U-Th-Pb (uranium-thorium-lead) data 

for feldspars from these metamorphic rocks. Zircon and 

feldspar were concentrated from samples collected for an 

earlier study by Peterman and Hildreth (1978) who, 

primarily on the basis of Rb-Sr (rubidium-strontium) 

analyses, concluded that the Archean gneisses were involved in a high-grade metamorphism 2,860±60 Ma ago. 

Uranium-lead analyses of zircon from granite of Lankin 

Dome and Long Creek Mountain confirmed a younger 

event that has concordia ages of approximately 2,600 Ma 

(Ludwig and Stuckless, 1978). 

On the basis of Rb-Sr data and geologic evidence, 

Peterman and Hildreth ( 1978) observed two separate 

groups of Archean gneisses. The relatively high 87Sr/86Sr 

initial ratio of 0. 7048 ± 0. 0012 that they obtained on one 

group of gneisses suggested that the precursors had a crustal history several hundred million years prior to 

metamorphism. This present U-Pb study of the ancient 

gneisses was undertaken on Peterman's samples not only 

to confirm the regional metamorphic event, but also to 

investigate the pre-metamorphic history of these rocks. 

ACKNOWLEDGMENTS 

We are indebted to our colleagues Z. E. Peterman, 

S. S. Goldich, and J. N. Rosholt for helpful discussions 

in reviewing this manuscript. Thanks are due also to 

Peterman for supplying the mineral separates. Becky Hopper separated the zircons and helped with the chemical 

procedures. Loretta K wak analyzed zircon fractions 1 

and 3. 

14 Shorter Contributions to Isotope Research 

GEOLOGY 

The Precambrian geology of the Granite Mountains 

is complex, and, as Peterman and Hildreth (1978) noted, 

there is a lack of detailed mapping and laboratory studies. 

The generalized geologic map showing sample localities 

(fig. B1) and the abbreviated summary that follows are 

based on the work of Peterman and Hildreth (1978) to 

which the reader is referred. Peterman and Hildreth ( 1978) 

noted three principal rock groups: gneiss, granite, and 

diabase dikes. The granitic gneisses that contain the oldest 

Precambrian rocks show considerable variety in texture, 

structure, and composition. The two types with which we 

are concerned were designated layered gneiss and augen 

gneiss by Peterman and Hildreth (1978). The compositionally layered quartz-feldspathic gneiss generally is 

granodioritic in composition. The foliated, but not 

layered, biotite-quartz-plagioclase augen gneiss is tonalitic 

and is called "massive" gneiss to distinguish it from the 

layered rock. Most of the gneisses in the two categories 

likely were derived from igneous precursors. Peterman 

and Hildreth ( 1978), however, concluded that some of 

the biotite-rich gneisses and schists, as well as some of 

the amphibolitic rocks, probably were derived from sedimentary precursors. They proposed that an original sedimentary-volcanic sequence, similar to the sedimentaryvolcanic sequence in the southwestern Wind River Range 

northwest of the Granite Mountains, was metamorphosed 

at 2,860 Ma and later was intruded by granite. 

-+-

GM-77 X 

EXPLANATION 

TERTIARY 

Sedimentary rocks undivided 

MESOZOIC AND PALEOZOIC 

Sedimentary rocks undivided 

LATE ARCHEAN 

Mafic dikes of basaltic composition 

Medium- to coarse-grained granite with numerous dikes 

of aplite and pegmatite 

MIDDLE ARCHEAN-Order may not reflect age; relative 

ages unknown 

lnequigranular biotite-quartz-plagioclase augen gneiss 

Dark-gray, fine-graned, thinly banded biotite gneiss interpreted to be metagraywacke 

Fine- to medium-grained amphibolite. Contains many 

dikes and sills of granite and pegmatite 

Fine-grained, banded epidote-rich gneiss 

Dominantly medium-grained, well-banded, biotite-quartzfeldspar gneiss of tonalitic to granitic composition. 

lnterlayered with subordinate amounts of mice schist. 

amphibolite, epidote gneiss, serpentinite, and augen 

gneiss 

CONTACT- Dashed where approximately located 

FAULT-Dashed where approximately located 

STRIKE AND DIP OF FOLIATION AND BANDING 

STRIKE AND DIP OF HORIZONTAL FOLIATION 

SAMPLE LOCALITY AND NUMBER 
~ Ill 

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::r 

~ 

Ill 

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C'l 

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42° 

37' 

30" 

42° 

30' 

1 08°00' 52'30" 45' 1 07°37'30" 

............. __ __ ................ 

;¥zlh 

Mzfh 

~ 

--- / 

0 5 10 KILOMETERS 

0 5 MILES 

Figure 81 (above and facing page). Map of the geology of the Precambrian of the Granite Mountains, Wyo. Shown 

are localities of samples analyzed in this study. Map is modified from Peterman and Hildreth (1978) . 

4~ 

~ z 

::l 

0 

u 

!-

z 

0 

::E 

w 

0:: 

1.1.. 
Table 81. Description of samples from the Granite Mountains included in this study 

[Modified from Peterman and Hildreth (1978)] 

Sample Latitude, 

No. longitude 

GM-35-68 42°34'21" N., 

108°55'37" w. 

GM-38-68 42°35'04" N., 

107°54'55" w. 

GM-77-68 42°33'58" N., 

1 0 7° 52 , 18" w 0 

GM-129-77 42°35'04" N., 

107°54'55" w. 

W2-CR5 42°40'13" N., 

107°32'46" w. 

GH-98-68 42°32'38" N., 

108°12'40" w. 

GM-99-68 42°38'59" N., 

107°54'18" w. 

Description 

Layered gneisses 

Light-gray, fine- to medium-grained, xenoblastic 

inequigranular granodioritic gneiss. Leucocratic 

bands alternate with more biotite-rich bands. 

Plagioclase moderately sericitized. 

Medium-gray, fine- to medium-grained, xenoblastic 

inequigranular tonalitic gneiss. Alternating 

light-dark bands. Plagioclase only slightly 

sericitized. 

Dark-gray, fine- to medium-grained, xenoblastic, 

inequigranular granodioritic gneiss with thin 

discontinuous leucocratic layers alternating 

with thicker mafic layers. 

New sample collected by z. E. Peterman from 

outcrop of GM-38-68. 

Medium-grained biotite gneiss and schist. Drill 

core, 153-ft depth. 

Massive gneisses 

Medium-gray, medium-grained, xenoblastic, inequigranular tonalitic gneiss. Well-developed 

foliation, but is not banded. 

Medium- to coarse-grained biotite augen gneiss. 

Augens have been highly sheared locally into 

leucocratic streaks. 

The granitic rocks form a major part of the uplifted 

Precambrian block of the Granite Mountains. These granites that were intruded into the gneisses are massive to 

foliated, light-gray to reddish-gray biotite granite. The 

fine- to medium-grained diabase dikes, according to Peterman and Hildreth (1978), generally are fresh or are relatively unaltered. Potassium-argon data indicated that the 

dikes were intruded into the gneisses and granite about 

2,600 Ma, shortly after granitic plutonism. 

the dark-brown crystals have a relatively higher uranium 

content than the lighter colored grains, but this relationship is not constant. 

ANALYTICAL PROCEDURES 

Preparation of feldspar samples for isotope analysis 

consisted of washing the concentrates with 7N HN03 and 

6N HCl followed by leaching with warm 5-percent HF 

(Ludwig and Silver, 1977). Feldspars were then dissolved 

in a solution of 48-percent HF, 15N HN03 and 50-percent 

HC104 by heating in a teflon digestion container at 160°C. 

Zircon samples were split into size and magnetic fractions 

and washed with warm 7N HN03 and 6N HCl. Samples 

were then dissolved in a solution of 48-percent HF and 

15N HN03 in a teflon bomb assembly at 208°C (Krogh, 

SAMPLES 

Four samples of layered gneiss and two of the more 

massive gneiss are described briefly in table B 1 . Sample 

W2-CR5 is core from a drill hole; the others are surface 

samples collected by Peterman. The zircon concentrates 

were split into two or more fractions by sieving, and for 

easy reference the analyzed fractions are numbered from 

1 to 15. The morphology of the zircon is varied; some 

of the grains are rounded, but most are well-formed crystals ranging from stubby to long, slender prismatic forms 

that have dipyramidal terminations. Color ranges from 

clear or pale yellow to dark reddish brown. In general, 

16 Shorter Contributions to Isotope Research 

1973). Lead was extracted using a bromide-form, anionexchange resin column, and was analyzed on a mass spectrometer using the single rhenium filament silica-gel technique (Cameron and others, 1969). Uranium was extracted on a nitrate-form, ion-exchange resin column and 

was analyzed as a metal on a triple rhenium filament 

assembly. 
Isotopic ratios were measured using a 30-cm-radius 

90° -sector mass spectrometer, and corrections for Pb fractionation were determined from analysis of National Bureau of Standards sample SRM-981. Analytical precision 

of isotopic ratios within 95-percent confidence limits are 

± 0. 12 percent for lead ratios, and ± 1 percent for zircon 

U/Pb. Uranium and lead concentrations determined by 

isotopic dilution are ± 1 percent for zircon samples and 

± 3 percent for feldspar samples. 

RESULTS 

Uranium-Lead Zircon Data 

The analytical data and the apparent ages for the 

15 zircon fractions are reported in table B2. Most of the 

fractions have highly discordant apparent ages, indicating 

considerable lead loss from the zircon at some time in 

its past history, although uranium gain may have been 

a contributing factor. The layered gneiss samples give 

quite different apparent ages (table B2). The fractions 

Figure 82. Uranium-lead concordia diagram. The intercepts are 32±290 and 3,187±162 Ma (point A); -96±660 

and 2,819±30 Ma (point B); and 21 and 2,654 Ma (point 

C). Data are from table 82 for the 15 zircon fractions. 

from sample GM-77 give nearly concordant ages of about 

2,600 Ma, but the fractions from samples GM-129 and 

Table 82. Lead-uranium data for zircons from the Archean gneisses, Granite Mountains, Wyo. 

[Numbers in parentheses are mesh sizes of zircon separates. Suffix letters refer to magnetic (M), nonmagnetic (NM), abraded (A). Atomic ratios 

are corrected for laboratory blank] 

Atomic ratio Ase 2 Ma 

Concentration 206pb 206pb 207pb 207pb 206pb 207pb 207Pb· Fraction Sample No. (Earts Eer million) 

No. (fraction) u Pb 204pb 23Su 23Su 206pb 23Su 23Su ?06pb 

Layered gneisses 

GM-129-77: 

1. (50-100) 1047 584 1633 0.4631 16.011 0.2511 3453 2877 3191 

2. (50-100)A 1130 630 1470 0.4608 16.248 0.2557 2443 2891 3220 

3. (100-150) 1008 554 1658 0.4561 15.846 0.2523 2422 2868 3200 

4. (200-325)M 937 487 702 0.4058 13.580 0.2427 2196 2721 3138 

5. (-325)NM 950 528 1406 0.4629 15.572 0.2443 2452 2851 3148 

W2-CR5: 

6. (+150) 570 233 293 0.3006 10.566 0.2550 1694 2486 3216 

7. (150-200) 952 392 395 0.2917 9.958 0.1476 1650 2431 3169 

8. (-200) 660 252 368 0.2822 9.618 0.2472 1602 2399 3167 

GM-77-68: 

9. (+100) 494 348 417 0.4850 11.803 0.1765 2549 2589 2620 

10. ( 100-325) 484 350 328 0.5123 12.878 0.1823 2666 2671 2674 

11. (-325) 550 322 341 0.4179 10.458 0.1815 2251 2476 2667 

Massive gneisses 

GM-98-68: 

12. (+150) 1804 695 614 0.2732 7.491 0.1989 1557 2172 2817 

13. (-150) 1967 765 1722 0.2793 7.961 0.2067 1588 2227 2880 

GM-99-68: 

14. (+100) 1835 456 695 0.1836 5.262 0..-2079 1086 1863 2890 

15. (100-150) 1718 518 630 0.2180 6.175 0.2054 1271 2001 2870 

Uranium-Lead Zircon Ages and Common Lead Measurements, Archean Gneisses, Granite Mountains, Wyo. 17 
W2-CR5 give minimum ages greater than 3,100 Ma. The 

fractions from the massive gneiss (samples GM-98 and 

GM-99) give sharply discordant ages, but the 207PbF06Pb 

values are in a narrow range from 2,817 to 2,890 Ma. 

The U-Pb data plotted in a concordia diagram (fig. 

B2) show three groups of apparent ages. Linear regressions of the data give intercepts on the concordia curve 

of 3,190 Ma, 2,820 Ma, and 2,650 Ma (fig. B2, lines 

A, B, and C). The points for zircon fractions 9, 10, and 

11 of the layered gneiss sample GM-77 plot near the concordia curve, but fractions (12-15) from the massive 

gneiss (samples GM-98 and GM-99) with marked age 

discordance plot along line B well below concordia. Similarly, fractions 6, 7, and 8 of layered gneiss W2-CR5 

plot along line A well below the concordia curve. Their 

positions can be explained by lead losses on the order 

of 50 percent, providing their true or original age was 

3, 190 Ma. The scatter of the data points about lines A 

and C (fig. B2), however, is significantly greater than 

the analytical error, and a complicated geologic history 

is indicated. 

Figure B3 is a concordia diagram in which the U-Pb 

data are interpreted with the following assumptions: 

1. The 207Pbt206Pb ages (table B2) give only a 

minimum age for the layered gneisses (fig. B3, line A). 

2. The zircon data for the massive augen gneiss 

mark a distinct event that may be related to the major 

regional metamorphism dated by the Rb-Sr isochron age 

of 2,860±80 Ma by Peterman and Hildreth (1978). 

3. The nearly concordant ages of zircon fractions 

9-11 from layered gneiss sample GM-77 are metamorphic 

ages; that is, the U-Pb isotopic systems were reset at the 

time of emplacement of granite magma at about 2,600 

Ma. 

4. The age discordance, in large part, resulted 

from lead loss during the Laramide orogeny about 60 Ma. 

With these assumptions, line A (fig. B3) gives a 

minimum age of 3,230 Ma for the layered gneiss, line 

B gives an apparent age of 2,880 ± 90 Ma for the massive 

gneiss, and line C is the time of emplacement of granitic 

magma at 2,600 Ma. The lower intercepts for lines A, 

B, and C have been constrained at 60 Ma, the approximate time of the Laramide orogeny. Some of the supporting data for the assumptions and some implications of 

the interpretive diagram for the past geologic history are 

considered in the discussion that follows. 

Ludwig and Stuckless ( 1978) analyzed zircon from 

the granite of Long Creek Mountain, about 6 km west 

of sample GM-99 (fig. B1), and zircon from granite similar to that of Lankin Dome. Ages derived by the concordia-diagram technique were 2,640 ± 20 Ma and 2,595 ± 40 

Ma for granite types from the Long Creek Mountain and 

Lankin Dome, respectively. The lower intercepts on concordia were 50±40 Ma and 100±75 Ma, respectively. 

The lower intercepts are imprecise, but Ludwig and 

18 Shorter Contributions to Isotope Research 

Stuckless (1978) concluded that the zircon crystallized 

about 2,600 Ma and lost lead during the Laramide uplift. 

The two ages differ by more than analytical error and 

suggest that granitic magmatic activity was not a single 

episode sharply limited in time. For the purpose of this 

discussion, the U-Pb zircon ages of Ludwig and Stuckless 

(1978) and the Rb-Sr isochron age of 2,550 ± 60 Ma for 

the granite (Peterman and Hildreth, 1978) are the basis 

for line C in figure B3. We conclude that the apparent 

ages for zircon fractions 9, 10, and 11 from layered gneiss 

sample GM-77-68 are indeed metamorphic ages for the 

gneiss, which at this locality is in close proximity with 

the larger granitic masses of the Granite Mountains. 

The conclusion that the data points (9, 10, 11, fig. 

B3) for zircon from layered gneiss sample GM-77-68 

represent metamorphic alteration of the U-Pb systems also 

is supported by the scatter of the points. The broader implication, however, is that points 9, 10, 11 moved along 

a chord that, for the present, is anchored by point A (fig. 

B3). Whether or not this age discordance was produced 

solely by the 2,600-Ma event, or in part by intervening 

geologic events is uncertain. Clearly, however, some age 

discordance was produced since the 2,600-Ma event, and 

this secondary discordance is attributed to relatively recent 

lead loss for which the Laramide orogeny, about 60 Ma, 

is a logical assignment. A major age discordance, however, is apparent in comparing sample GM-77 with samples GM-129 and W2-CR5 (figs. B2 and B3), and the 

chord A-C (fig. B3) is a requirement, although its upper 

intercept may be questioned as is considered. 

The zircon fractions 1-5 of sample GM-129 and 

fractions 6-8 of sample W2-CR5 define line A in figure 

B2, but the deviation of the data points from line A is 

well outside analytical error. A possibility may be that 

0.6 

oL_~~--L-~-L~~~-L~--~~~--~~ 

0 4 8 12 16 20 24 28 

Figure 83. Interpretive uranium-lead concordia diagram. 

All lines are regressed thru a lower intercept of 60 Ma 

(point D). Upper intercepts are 3,230 Ma (point A); 2,880 

Ma (point B); and 2,600 Ma (point C). 
the original time of formation falls within the analytical 

error, suggesting a maximum age of 3,250 Ma; however, 

our interpretation (fig. B3, line A) gives a minimum age 

of 3,230 Ma for the precursor of the layered gneiss. Zircon samples along line A have complicated patterns of 

age discordance. Fractions 6--8 from sample W2-CR5 

have uranium contents ranging from 570 to 660 ppm, not 

much greater than the uranium concentration in fractions 

9-11 that are nearly concordant. A possible explanation 

is that the latter zircons were recrystallized or the surfaces 

were annealed during the 2,600-Ma event so that lead 

losses during the Laramide event were relatively small. 

Compared with the zircon fractions 1-5 of sample 

GM-129, the marked age discordance of fractions 6--8 

of sample W2-CR5 cannot be explained on the basis of 

uranium content. Fractions 1-5 range from 937 to 1047 

ppm uranium. The largest size fraction, 1 (-50 + 100 

mesh), has the highest content of 1,047 ppm uranium, 

and gives an apparent 207PbP06Pb age of 3, 191 Ma. The 

crystal comers and edges of a part of fraction 1 were 

abraded in an air-driven, diamond-studded cell, leaving 

more or less spherical-shaped grains. The abraded fraction 

(2) has a somewhat larger content of 1, 130 ppm uranium, 

compared with 1 ,04 7 ppm of uranium in the original 

euhedral crystal. The 207PbP06Pb age of the abraded zircon is 3,220 Ma compared with 3,191 Ma for the original 

zircon. Although the difference is relatively small (1 percent), it is considered to be real. Abrasion experiments 

could not be continued because of a lack of sample, but 

the data (table B2, figs. B2 and B3) indicate that 3,200 

Ma cannot be accepted as the unquestioned age of the 

precursor of the layered gneiss. 

The U-Pb data for zircon fractions 12 and 13 from 

sample GM-99 and for fractions 14 and 15 from sample 

GM-98 contrast sharply with the data from the layered 

gneiss and constitute a third group with an apparent age 

of 2,82~2,880 Ma (figs. B2 and B3). These zircon ages 

bracket the Rb-Sr isochron age of 2, 860 Ma that Peterman 

and Hildreth (1978) attributed to a major metamorphism. 

The possibility exists that the massive augen gneiss is of 

different origin from that of the layered gneiss. Support 

for this concept is found in the Rb-Sr data for sample 

GM-98, which plot below the 2,860 Ma isochron. From 

the data of Peterman and Hildreth (1978, table 3, p. 11) 

the Rb-Sr age for sample GM-98, using the isochron initial 87Sr/86Sr of 0. 7048, is 2,640 Ma, and for biotite from 

the rock the age is 2,440 Ma. These data suggest the 

possibility that the precursor of the massive augen gneiss 

was a granitic intrusion emplaced in the layered gneiss 

2,880 Ma and metamorphosed during the 2,600-Ma event. 

Lutetium-Hafnium Data 

In a hafnium (Ht) study of continental evolution, 

John Patchett (Patchett and others, 1981) analyzed the 

least discordant fraction of sample GM-129 zircon. In addition, for our study he analyzed zircon from the massive 

augen gneiss GM-99. Both sets of data are in table B3. 

Using the present-day 177Lu1176Hf, the 177Hf/176Hf correct 

back to chondrite model ages of about 3,200 Ma. These 

model Lu-Hf ages on zircon from the augen gneiss and 

layered gneiss are similar and support the concept that 

both gneiss types were formed at least 3,200 Ma, and 

that a more complicated geologic history is indicated for 

the massive augen gneiss than was suggested by the apparent U-Pb zircon age. 

Common Lead Feldspar Data 

Results of feldspar analyses for U and Pb concentrations and Pb isotope ratios are given in table B4. Initial 

lead isotope ratios are in some instances less radiogenic 

than were reported previously for the same samples by 

Nkomo and Rosholt (1972). This difference probably is 

due to our leaching the feldspar crystals with dilute HF 

before digestion, a procedure later initiated by Ludwig 

and Silver ( 1977) to remove surficial lead. 

In both plots of figure B4, all points are well above 

the model average growth curve. On the 207PbP04Pb206PbP04Pb plot (fig. B4), the measured data are scattered, but because the uranium and thorium contents are 

significant, each must contain a fraction of radiogenic lead 

Table 83. Lutetium-hafnium data for zircons from two gneiss samples from the 

Granite Mountains 

[Analyzed by John Patchett. Tch is the chondrite model age] 

Concentration 

(Earts per million) l76Lu l76Hf l76Hf 

Sample No. Lu Hf l77Hf l77Hf l77Hf ch Tch' in Ma 

1GM-129-77 17.1 12,090 0.000200 0.280714 0.280702 3,200 

GM-99-68 113.8 12,527 0.001288 0.28077 0.280690 3,200 

1Data for GM-129-77 is from Patchett and others (1981). 

Uranium-Lead Zircon Ages and Common Lead Measurements, Archean Gneisses, Granite Mountains, Wyo. 19 
39r-----~------------~--------------~-------------.---------------,--------, 

A 

38 

37 

F-5 •-------o 

35 

34 

EXPLANATION 

33 o Measured data 

• Data corrected for 2600 Ma 

radiogenic component 

B 

17 

14 15 

__ o 

F-s·-------

17 18 

Figure 84. Common lead isochron plots. A, Lead isotopic data for the feldspars of the samples analyzed 

for this study. Measured ratios are plotted together with those corrected for 2,600 Ma. 8, Corrected 

data for the layered gneisses form a secondary isochron that has a slope of 0.557 ± 0.013 indicating 

a common age of formation of 3,300 Ma. Average growth curves are from Stacey and Kramers (1975). 

Samples are identified by laboratory number. 

formed since the rocks were last metamorphosed, probably by emplacement of the granites 2,600 Ma. This event 

was sufficiently intense to almost completely reset the zircons of sample GM-77, and therefore may have redistri20 Shorter Contributions to Isotope Research 

buted lead into the feldspars at that time. Therefore, in 

figure B4 the feldspar lead data all are plotted with corrections for radiogenic growth in the last 2,600 Ma. A linear 

array is formed by the corrected layered gneiss data, the 
Table 84. lead-uranium-thorium data for some HFleached feldspars from the 

Granite Mountains, Wyo 

Archean gneisses of the 

Measured ratios Concentration 

SamJ2le No. 206pb 207pb 208pb (]2arts 12er million) 

Lab. Field 204pb 204pb 204pb Pb u Th 

Layered gneisses 

F-1 GM-35-68 13.951 15.096 34.340 37.4 0.04 0.04 

F-2 GM-38-68 14.572 15.232 34.825 10.9 0.21 0.19 

F-3 GM-77-68 14.676 15.493 34.896 14.2 0.02 0,01 

F-4 W2-CR-5 14.688 15.416 35,083 42.2 0.36 0.3 

Massive gneisses 

F-5 GM-98-68 17.702 16.302 35.374 15.16 0,30 0.17 

F-6 GM-99-68 18.167 17.259 38.152 55.3 0.12 0.36 

trend of which almost incorporates the data point for the 

massive augen gneiss (sample GM-99). The data point 

for the other massive gneiss (sample GM-98) is quite different and is not included. 

If the linear array is not fortuitous, then it could 

be regarded as a secondary isochron that would imply 

a single initial lead isotopic composition at a common 

time of formation for all the samples. The slope of the 

isochron is a function of two times, t1 and t2 . If t2 is 

2,600 Ma, the time of lead redistribution into the feldspars, then the source age of t 1 can be computed from 

the equation: 

(eAl'1-eAl'2) 

R= 137. 88(eA'1-eA'2) 

where X. 1 is the decay constant for 235U and X. is the decay 

constant for 238 U. 

Using the layered gneiss data only, the slope R is 

0.557 ± 0.013, and t1 is 3,360 ±50 Ma. Great precision 

cannot be expected from feldspar data because of their 

susceptibility to disturbance during such vast periods of 

time. This calculation is presented to show that the data 

are consistent with the proposed minimum age of ~3,200 

Ma for all the layered gneiss samples analyzed. We also 

note that the whole-rock U-Pb systems must have been 

effectively closed between that time and 2,600 Ma; otherwise linearity of the data, as seen in figure B4, would 

have been disturbed. 

As previously noted, the isochron determined by the 

layered gneiss samples nearly incorporates one of the massive gneisses (GM-99). If the GM-99 point is included 

in the array, then R=0.515±0.0046 and t1=3,190 Ma. 

The data suggest the possibility that sample GM-99 also 

may be derived from a 3,200-Ma precursor, as was indicated by the Lu-Hf data, and that GM-99 later was 

metamorphosed severely at 2,860 Ma. Feldspar data from 

the other massive gneiss (sample GM-98) are quite different and may have been affected by one of the later 

Proterozoic events in the region (Peterman and Hildreth, 

1978). The locality of GM-98, which should be noted, 

is more than 25 km west of the vicinity of all. other samples (fig. Bl). These facts may indicate that this gneiss 

is of a different origin and that 2,880 Ma is the minimum 

age of formation of GM-98. 

In figure B5, the feldspar line is extended back to 

intersect at point M, an isochron drawn through troilite 

lead 0 that has a slope corresponding to lead evolution 

between 4,570 and 3,200 Ma. Point M then corresponds 

to a growth in the mantle with a quite reasonable value 

of 8. 3 for JL. Point M is also the initial lead isotopic 

composition of the gneisses when they were formed 

~3,200 Ma, shortly after separation from the mantle. 

Subsequently, lead evolved in these crustal rocks with 

higher JL values, ranging from 10. 3 to 15.6 for the layered 

gneisses, and to 41.3 for the highly radiogenic augen 

gneiss (sample GM-99). Metamorphism associated with 

the granitic plutonism in the region equilibrated the lead 

within the rocks and preserved in their feldspars the lead 

isotopic compositions of the rocks at that time. Measurements made by Nkomo and Rosholt (1972) showed that 

the layered gneisses subsequently have lost much of their 

uranium. 

The high 208PbP04Pb and 207PbP04Pb, all of which 

are above the average growth curves in figure B4, are 

evidence that thorium and uranium were both differenif 

g 

17 

16 

15 

'1- 14 

0 

~ 

1:\1 13 

12 

11 

Feldspar isochron 

320Q-2600 Ma 

10L---L---L-~L-~L-~L-~L---~~--~~--

9 10 11 12 13 14 15 16 17 18 19 

206Pb/204pb 

EXPLANATION 

o Measured data 

• Data corrected for 2600-Ma 

radiogenic component 

Figure 85. Common lead evolution diagram. Proposed 

model for formation of feldspar lead in the layered gneisses: Lead formed in the mantle from troilite composition 

4,570 to -3,200 Ma with a J.L-8.3. Gneisses were formed 

from mantle-derived material at 3,200 Ma and their feldspar lead had the composition M. Final metamorphism at 

2,600 Ma introduced different amounts of radiogenic lead 

to each feldspar sample to form the data line shown. 

Uranium-Lead Zircon Ages and Common Lead Measurements, Archean Gneisses, Granite Mountains, Wyo. 21 
tiated into the crust preferentially with respect to lead. 

The ability to account for these high ratios in such ancient 

rocks is an attractive feature of the comparatively simple 

model outlined herein. 

Worth noting is that data with similarly high 207Pb/ 204Pb and 208PbP04Pb are characteristic of Archean environments around the world. For instance, they are found 

in feldspar lead from the Sacred Heart granite in the Minnesota River Valley (Doe and Delevaux, 1980), in ore 

leads from Que Que, Zimbabwe (Robertson, 1973) and 

in ores from Pilbara in Western Australia (Richards and 

others, 1981). 

SUMMARY OF EVENTS IN THE ARCHEAN 

2600-Ma granitic plutonism.-Previous studies by 

Ludwig and Stuckless ( 1978) and Stuckless and Nkomo 

( 1978) have determined the age of granitic plutonism at 

2,600 Ma. This event is represented in our data by the 

layered gneiss (sample GM-77) in which the zircons were 

reset along a chord anchored at 2,600 Ma. Feldspar common lead data for most of the gneiss samples also show 

the effects of the plutonism because lead in these minerals 

was redistributed during this time. 

2860- to 2880-Ma regional metamorphism.-The 

two massive gneiss samples, GM-98 and GM-99, both 

yield rather discordant U-Pb zircon data giving an age 

of about 2,880 Ma. Although possibly these data could 

indicate modem lead loss from a 2,6000- to 3,200-Ma 

chord on the concordia diagram, our preferred interpretation is that at 2,880--2,860 Ma there was a significant 

metamorphic event as proposed by Peterman and Hildreth 

(1978) in their Rb-Sr study. 

3200-Ma gneiss formation.-Uranium-lead zircon 

data give an upper intercept concordia age of 3, 170 ± 50 

Ma if lead loss is assumed to have been due to uplift 

in the Laramide. The scattering of data points suggests 

that a better estimate for the minimum age of formation 

can be obtained from the two fractions with the highest 207Pb;2°6

Pb. This upper intercept age is 3, 230 Ma for a 

lower intercept of 60 Ma. The Lu-Hf data from the 

layered gneiss sample GM-129 and the massive gneiss 

sample GM-99 yield chondrite model ages of 3,200 Ma. 

When corrected for in-place decay of uranium, the 

least radiogenic lead from feldspars in the layered gneisses 

has a Stacey and Kramers (1975) model age of 3,000 

Ma. Because of the complex history of the region, this 

age must be regarded as a minimum age estimate. The 

isotopic composition of the feldspar leads yields a secondary 207PbP06Pb isochron (fig. B5), the slope of which 

is consistent with 3,200--Ma time of origin. 

Crustal history prior to 3,200 Ma.-The data presented in this study suggest 3,200 Ma as a minimum age 

of formation of the layered gneisses and possibly of one 

22 Shorter Contributions to Isotope Research 

of the massive augen gneisses. The scatter of data on 

the U-Pb concordia diagram and the slightly older 207Pb/ 206Pb age from the abraded zircon suggest an event prior 

to 3,200 Ma. The Lu-Hf data shows that Hf fractionation 

from the mantle was limited to 100--200 Ma prior to 3,200 

Ma, even if the early mantle was more heterogeneous than 

is indicated by the work of Patchett and others ( 1981). 

However, the considerable U-Pb age discordance suggests 

possible disturbance of the Lu-Hf system. Analyses of our 

fairly discordant zircons then may yield younger ages. 

The lead evolution model, previously discussed and presented in figure B5, indicates that our feldspar data are 

consistent with separation from the mantle and formation 

of the layered gneisses about 3,200-3,300 Ma. However, 

the feldspar data, in addition to the zircon U-Pb and LuHf data, do not preclude a previous crustal history for 

these Archean gneisses. 

REFERENCES CITED 

Cameron, A. E., Smith, D. H., and Walker, R. L., 1969, Mass 

spectrometry of nanogram size samples of lead: Analytical 

Chemistry, v. 41, p. 525-526. 

Doe, B . R. , and Delevaux, M. H. , 1980, Lead isotope investigations in the Minnesota River Valley-Late-tectonic and 

post-tectonic granites: Geological Society of America Special Paper 182, p. 105-112. 

Krogh, T. E., 1973, A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb 

for isotopic age determinations: Geochimica et Cosmochimica Acta, v. 37, p. 485-494. 

Ludwig, K. R., and Silver, L. T., 1977, Lead-isotope inhomogeneity in Precambrian igneous K-feldspars: 

Geochimica et Cosmochimica Acta, v. 41, p. 1457-1471. 

Ludwig, K. R., and Stuckless, J. S., 1978, Uranium-lead 

isotope systematics and apparent ages of zircons and other 

minerals in Precambrian granitic rocks, Granite Mountains, 

Wyoming: Contributions to Mineralogy and Petrology, v. 

65, p. 243-254. 

Nkomo, I. T., and Rosholt, J. N., 1972, A lead-isotope age 

and U-Pb discordance of Precambrian gneiss from Granite 

Mountains, Wyoming, in Geological Survey research, 1972: 

U.S. Geological Survey Professional Paper 800---C, p. 

C169-C177. 

Patchett, P. J., Kuovo, 0., Hedge, C. E., and Tatsumoto, M., 

1981, Evolution of continental crust and mantle 

heterogeneity-Evidence from Hf isotopes: Contributions to 

Mineralogy and Petrology, v. 78, p. 279-297. 

Peterman, Z. E., and Hildreth, R. A., 1978, Reconnaissance 

geology and geochronology of the Precambrian of the Granite Mountains, Wyoming: U.S. Geological Survey Professional Paper 1055, 22 p. 

Richards, J. R., Fletcher, L. J., and Blockley, J. G., 1981, 

Pilbara galenas; Precise isotopic assay of the oldest Australian leads-Model ages and growth-curve implications: 

Mineralium Deposita, v. 16, p. 7-30. 
Robertson, D. K., 1973, A model discussing the early history 

of the Earth based on a study of lead isotope ratios from 

veins in some Archean cratons of Africa: Geochimica et 

Cosmochimica Acta, v. 37, p. 2099-2124. 

Stacey, J. S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth 

and Planetary Science Letters, v. 26, p. 207-221. 

Stuckless, J. S., and Nkomo, I. T., 1978, Uranium-lead isotope 

systematics in uraniferous alkali-rich granites from the Granite Mountains, Wyoming; Implications for uranium source 

rocks: Economic Geology, v. 73, p. 427-441. 

Tatsumoto, M., Knight, R. J., and Allegre, C. J., 1973, Time 

differences in the formation of meteorites as determined 

from the ratio of lead-207 to lead-206: Science, v. 180, 

p. 1279-1283. 

Uranium-Lead Zircon Ages and Common Lead Measurements, Archean Gneisses, Granite Mountains, Wyo. 23 

FIGURE 

1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

FISSION-TRACK AGES OF AIR-FAll TUFFS IN 

MIOCENE SEDIMENTARY ROCKS OF THE 

ESPANOLA BASIN, SANTA FE COUNTY, 

NEW MEXICO 

Chapter C 

By GLEN A. IZETT and C. W. NAESER 

CONTENTS 

Abstract .................................. . 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Fission-track dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Methods and results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Page 

26 

26 

26 

27 

29 

Cl. Generalized stratigraphic diagram showing the relationships among certain air-fall tuffs of the Skull 

Ridge and Pojoaque Members of the Tesuque Formation . . . . . . . . . . . . . . . . . . . . 

TABLE 

C 1. Fission-track ages of zircon microphenocrysts and glass shards separated from volcanic ash beds in 

the Tesuque Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Fission-Track Ages of Air-Fall Tuffs in Miocene Sedimentary Rocks, N.Mex. 

Page 

28 

Page 

29 

25 
Abstract 

Zircon fission-track ages of two air-fall tuffs in the 

middle part of the Miocene Skull Ridge Member of 

Galusha and Blick (1971) of the Tesuque Formation of 

the Santa Fe Group range in age from 14.6 to 12.7 million 

years. These ages place estimated limits of about 16--13 

million years for the age range of the 220-meter-thick 

member. The dated tuffs are stratigraphically near fossilmammal quarries of the American Museum of Natural 

History and provide age control for the correlation of 

these fossils with those of the Lower Snake Creek quarries in the Sheep Creek Formation in Nebraska and with 

fossil mammals in the upper one-third of the Troublesome Formation in Middle Park, Colo. 

Zircon fission-track ages of two air-fall tuffs of the 

overlying Pojoaque Member of Galusha and Blick (1971) 

of the Tesuque Formation are 11.4 and 9.4 million years 

and date the middle part of the 500-meter-thick 

Pojoaque Member. These dated tuffs are stratigraphically 

near fossil-mammal assemblages in the member that correlate with fossil-mammal assemblages in the Miocene 

Valentine Formation of the Ogallala Group of Galusha 

and Blick, 1971, in central Nebraska. A tuff stratigraphically near the Burge fossil-mammal quarry has a fissiontrack age of about 10 million years. 

INTRODUCTION 

An unusually large number of prominent, thick airfall tuffs are interlayered in the continental Miocene sedimentary rocks of the Santa Fe Group in the Espanola 

basin of north-central New Mexico. The names for the 

Miocene rocks of the Santa Fe Group used in this report 

are those advocated by Galusha and Blick (1971), although many of these names have not been, and are not 

herein, formally adopted by the U.S. Geological Survey. 

Miocene time as used in this report ranges from about 

24 to 5 m. y. (million years). 

At least 37 superposed air-fall tuff beds, as thick 

as 2 m (meters), occur in a nearly continuous stratigraphic 

succession in the Skull Ridge Member of the Tesuque 

Formation of the Santa Fe Group (Galusha and Blick, 

1971 , figs. 17 and 23), and these tuffs have the potential 

for establishing the isotopic age range of the upper Cenozoic rocks of the area. Large numbers of fossil land mammals have been collected in the last 100 years in the Espanola basin, and many of the collections are related stratigraphically to the tuff bed succession (Galusha and 

Blick, 1971). Reliable isotopic ages of the tuffs would 

aid in establishing the age of fossil-mammal collections 

from elsewhere in the western United States. Moreover, 

isotopic ages of the tuff marker beds could set limits on 

the rates of evolution of the different land-mammal 

groups. 

26 Shorter Contributions to Isotope Research 

The paleomagnetic stratigraphy of the upper part of 

the Santa Fe Group rocks was studied by McFadden 

( 1977), and a lower part of the Santa Fe currently is being 

studied by S. F. Barghoom of Columbia University. Reliable isotopic ages are needed to calibrate the paleomagnetic-reversal sequence in the Espanola basin. Isotopic ages 

also are required to date the times of deformation of the 

Miocene rocks of the basin. For the above-mentioned 

reasons, several of the thickest, purest-looking tuffs were 

collected for fission-track dating. 

FISSION-TRACK DATING 

The fission-track method of dating volcanic ashes 

and tuffs has proved to be a generally much more accurate 

method than the potassium-argon method because of the 

problem of contamination of the ashes and tuffs with 

much older detrital minerals of the enclosing sediments 

incorporated into the tuffs during deposition. These older, 

spurious minerals (potassium-bearing minerals in particular) are difficult to remove from the mineralogically similar primary-bearing minerals of the tuffs necessary for potassium-argon dating. These older potassium-bearing minerals supply a disproportionate amount of 40 Ar to the total 40 Ar extracted from the minerals .in the laboratory. Accordingly, anomalously old ages result. In the early 

1970's, Henry Faul and his coworkers at the University 

of Pennsylvania attempted to date, by the K-Ar (potassium-argon) method, certain of the upper Cenozoic tuffs 

in the Espanola basin and hoped to circumvent the contamination problem by using careful collection techniques 

and by rejecting those tuffs that appeared to be contaminated with detrital sediments. However, the results of 

their work were disappointing in that unrealistically old 

ages were determined in most instances (H. Faul, oral 

commun., 1979). On the other hand, the fission-track 

method allows a geochronologist the opportunity to date 

only glass-mantled microphenocrysts separated from ashes 

or tuffs, or to date only those microphenocrysts that 

clearly belong to a pyrogenic population of primary 

microphenocrysts. For example, zircon crystals of Precambrian age incorporated in a tuff during deposition can 

be recognized by their extremely high fossil fission-track 

densities (coupled with normal uranium concentrations) 

and their metamict character, and therefore they would 

not be included in the fission-track count. 

The samples used for the fission-track dating of the 

Santa Fe Group tuffs were collected by G. A. Izett in 

1974 in company with Ted Galusha (now deceased), at 

that time with the American Museum of Natural History. 

Nearly all the individual tuffs of the Miocene and Pliocene 

sequence were collected with the hope that many of the 
beds could be dated. Galusha invited Izett to collect and 

date the tuffs inasmuch as Galusha recognized the potential they have as stratigraphic markers and the potential 

they have as targets for isotopic age analyses. Unfortunately, many of the tuffs proved not to be suitable for 

dating because not enough pyrogenic zircons could be obtained from many of the pervasively contaminated tuffs. 

We dated two prominent marker tuffs of the Skull 

Ridge Member in its type area: the No. 2 and No. 4 

white ash beds. These two thick tuffs were dated because 

they can be traced throughout much of the area and because they are related stratigraphically to fossil-mammal 

quarries of the American Museum of Natural History. The 

No. 2 white ash bed, which is about 1.5 m thick, is about 

40 m above the base of the Skull Ridge Member, and 

the No. 4 white ash bed, which is about 1.5 m thick, 

is about 145 m above the base of the member. The Skull 

Ridge Member, which overlies the Nambe Member (fig. 

C1), is about 220 m thick according to Galusha and Blick 

(1971). 

The other two tuff beds in the overlying Pojoaque 

Member of the Tesuque Formation of Galusha and Blick 

(1971) were dated also: an unnamed ash bed not shown 

on the diagrams of Galusha and Blick (1971, fig. 23) 

and an overlying tuff called the blue-gray ash. These two 

tuffs are separated by about 60 m of orange-gray silty 

sandstone. The Pojoaque Member is about 500 m thick 

according to the diagrams presented by Galusha and Blick 

( 1971). The two tuffs were collected in the Pojoaque 

Bluffs area, which is the area from which most of the 

fossil-vertebrate collections were made and studied by 

Cope (1877), and from which many collections of fossil 

vertebrates were made by Galusha. 

METHODS AND RESULTS 

Zircon microphenocrysts were separated from about 

5-kilogram samples of the tuffs by conventional mineralseparation techniques, and the zircons were dated using 

fission-track dating methods described by Naeser (1969). 

The dosimeter used to measure the neutron flux at the 

time the samples were irradiated in the reactor was a N ationa} Bureau of Standards (NBS) standard glass No. 962 

originally irradiated by B. S. Carpenter of the National 

Bureau of Standards in the NBS reactor along with copper 

and gold foils. The decay constant used to calculate the 

fission-track ages is Af=7.03X 10- 17yr- 1. In addition, 

pure glass-shard fractions of the two gray ash beds (samples 74G326 and 74G327) were dated by the fission-track 

method, and the results are given in table C1. 

The fission-track ages of the glass shards are systematically younger than are the zircon fission-track ages 

from the same tuffs. The tendency for fission-track glass 

ages to be younger than zircon ages from the same tuffs 

results from track fading or annealing at ambient temperatures in the glass shards over time (Naeser and others, 

1980). 

On the basis of fission-track ages reported here, the 

Skull Ridge Member probably ranges from about 16 to 

13 m.y., and the age of the middle part of the Pojoaque 

Member ranges from about 12 to 9 m.y. According to 

Galusha and Blick (1971, p. 110), the fossil mammals 

of the Skull Ridge Member are Barstovian in terms of 

North American land mammal ages and most closely 

match fossil mammals of the Lower Snake Creek quarries 

excavated in the Sheep Creek Formation in Nebraska. 

Fossil mammals collected from the upper third of the 

Troublesome Formation (Miocene) in Middle Park, Colo., 

are also about the same age as those in the Lower Snake 

Creek quarries (Izett, 1968, p. 45). The fossils in the 

Troublesome are stratigraphically near the level of a tuff 

fission-track dated at about 14 m. y. (lzett and Barclay, 

1973). Fossil mammals of the Pojoaque Member most 

closely match fossil mammals of the Valentine Formation 

(Miocene) as used by Galusha and Blick (1971, p. 64) 

of the Valentine, Neb., area. In terms of North American 

land mammal ages, the rocks were assigned a Valentinian 

to Clarendonian age by Galusha and Blick (1971, p. 12 

and 64). A tuff stratigraphically just above the Burge fossil quarry in the Ash Hollow Formation of the Ogallala 

Group in the Valentine, Neb., area was dated by the fission-track zircon method at about 10 m.y. (lzett, 1975). 

This age of 10 m.y. falls within the age range of the 

Pojoaque Member as determined by our fission-track ages. 

Zircon fission-track ages of 12.7 ± 1.8 and 

10. 8 ± 1. 6 were reported by Manley and N aeser ( 1977), 

for two air-fall tuffs in Miocene rocks in the northeast 

part of the Espanola basin. These tuffs were thought by 

Manley and Naeser (1977) to be in the Tesuque Formation 

and fall within the age range of the uppermost part of 

the Skull Ridge Member and the lower part of the 

Pojoaque Member as determined in the present paper. 

Fission-track zircon ages reported here for the tuffs 

in the Pojoaque Member (samples 74G326 and 74G327 

of table C 1) previously were reported by Naeser and 

others (1980, table 5, p. 26--27, entries 5 and 8). Unfortunately, the age data and the descriptions were incorrectly 

published. In table 4 of Naeser and others (1980), entries 

5 and 8 were switched~ entry 5 should have read entry 

8, and entry 8 should have read entry 5. Entry 5 of table 

5 was incorrectly given to be in sec. 31, T. 20 N., R. 

9; it should have been listed in the SE1/4 sec. 36, T. 

20 N., R. 8 E. In addition, entry 5 of table 5 should 

have been assigned sample number 74G326 rather than 

74G327, and entry 8 should have been assigned sample 

number 74G327 rather than 74G326. 

Fission-Track Ages of Air-Fall Tuffs in Miocene Sedimentary Rocks, N.Mex. 27 
c: 

.g 

ca 

e 

0 

u.. 

Q) 

::I 

cr 

::I 

Ul 

Q) 

tiii 

..c 

E 

Q) 

~ 

Q) 

::I 

cr 

ca 

0 

"0' ''""·'''""''''"'·''''·'"''"'·'''''"'""'"·"'"·'""''·"'·''•'·'·''""····"'"""·'"''·'''·'"'"''·""'"·'.''"'""""" Blue-gray ash, 9.4±0.9 m.y. a.. 

\ 

\nnamed .,h, 11.4± 1.1 m.y. 

~-+------------------------4 

iii 

..c 

E 

Q) 

~ 

Q) 

..c 

E 

ca 

z 

~~~~~~~~~~~~~~ No.4 white ash, 13.4 m.y. 

(average of 2 deter.minations) 

\ 

Figure C1. Generalized stratigraphic diagram showing the relationships among certain airfall tuffs of the Skull Ridge and Pojoaque Members of the Tesuque Formation (Santa Fe 

Group) of Miocene age of Galusha and Blick (1971). Modified from diagrams of Galusha 

and Blick (1971, figs. 17 and 23). 

28 Shorter Contributions to Isotope Research 
Table C1. Fission-track ages of zircon microphenocrysts and glass shards separated from volcanic ash beds in 

the Tesuque Formation of Galusha and Blick (1971) in the Espanola basin, Santa Fe County, N. Mex. 

[F, neutron fluence; P5 , spontaneous track density; Pj, induced track density; cm2 , square centimeters; ± is 2 sigma; numbers in parentheses 

are the number of tracks counted] 

Sample 

No. 

p 

Material F tracks,cm2 

p, 

track~/cm 2 

x10 6 Age 

x106 Description and locality dated neutrons/cm2 x1o 15 

74G327 Blue-gray volcanic ash in the Pojoaque Zircon----- 1.10 1.35 (153) 

0.0149 (157) 

9.47 (493) 

0.253 (347) 

9.4±0.9 

Member of the Tesuque Formation of Galusha Glass------ 1.78 6.3±1.2 

and Blick (1971, P• 62); SW1/4SE1/4 

sec. 36, T. 20 N., R. 8 E., Espanola 

7-1/2 minute quadrangle, Santa Fe County, 

N. Mex. 

74G326 Volcanic ash in the Pojoaque Member of the Zircon----- 1.10 1.19 (132) 

0.0212 ( 55) 

6.88 (382) 11.4±1.1 

Tesuque Formation of Galusha and Blick Glass------ 2.09 0.331 (228) 8.0±2.4 

(1971); about 60 m below blue-gray ash 

74G327; this ash bed not shown on diagram 

by Galusha and Blick (1971, p. 62); SW1/4 

SE1/4 sec. 36, T. 20 N., R. 8 E., Espanola 

7-1/2-minute quadrangle, Santa Fe County, 

N. Mex. 

74G314 Volcanic ash in the Skull Ridge Member of Zircon----- 1.21 1.51 (251) 

2.12 (491) 

7.76 (647) 14.1±1.1 

the Tesuque Formation of Galusha and Blick --do.------ 0.784 7.81 (904) 12.7±1.4 

(1971); No. 4 white ash bed of Galusha 

and Blick (1971, p. 31); center sec. 14, 

T. 20 N., R. 9 E., Cundiyo 7-1/2-minute 

quadrangle, Santa Fe County, N. Mex. 

74G317 Volcanic ash in the Skull Ridge Member of Zircon----- 0.780 2.50 (255) 7.99 ( 40) 14.6±1.2 

the Tesuque Formation of Galusha and Blick 

(1971); No. 2 white ash bed of Galusha 

and Blick (1971, p. 31); center sec. 15, 

T. 20 N., R. 9 E., Cundiyo 7-1/2-minute 

quadrangle, Santa Fe County, N. Mex. 

REFERENCES CITED 

Cope, E. D., 1877, Report upon the extinct Vertebrata obtained 

in New Mexico by parties of the expedition of 1874, in 

Wheeler, G. M., Report upon United States geographical 

surveys west of the 1 OOth meridian: Washington D.C. , v. 

4, pt. 2, p. 1-370. 

Galusha, Ted, and Blick, J. C., 1971, Stratigraphy of the Santa 

Fe Group, New Mexico: American Museum of Natural History Bulletin, v. 144, no. 1, 127 p. 

Izett, G. A., 1968, Geology of the Hot Sulphur Springs quadrangle, Grand County, Colorado: U.S. Geological Survey 

Professional Paper 586, 79 p. 

---1975, Late Cenozoic sedimentation and deformation in 

northern Colorado and adjoining areas, in Curtis, Bruce, 

ed., Cenozoic history of the Southern Rocky Mountains: 

Geological Society of America Memoir 144, p. 179-209. 

Izett, G. A., and Barclay, C.S.V., 1973, Geologic map of the 

Kremmling 15-minute quadrangle, Grand County, Colorado: U.S. Geological Survey Geologic Quadrangle Map 

GQ--1115, scale 1:62,500. 

Manley, Kim, and Naeser, C. W., 1977, Fission-track ages for 

tephra layers in upper Cenozoic rocks, Espanola Basin, New 

Mexico: Isochron/West, no. 18, p. 13-14. 

McFadden, B. 1., 1977, Magnetic polarity stratigraphy of the 

Chamita Formation stratotype (Mio-Pliocene) of north-central New Mexico: American Journal of Science, v. 277, p. 

769-800. 

N aeser, C. W. , 1969, Etching fission tracks in zircons: Science, 

v. 165, no. 3891, p. 388. 

Naeser, C. W., Izett, G. A., and Obradovich, J. D., 1980, 

Fission-track and K-Ar ages of natural glasses: U.S. 

Geological Survey Bulletin 1489, 31 p. 

Fission-Track Ages of Air-Fall Tuffs in Miocene Sedimentary Rocks, N.Mex. 29 

1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

MUSCOVITE-PHENOCRYSTIC TWO-MICA 

GRANITES OF NORTHEASTERN NEVADA ARE 

LATE CRETACEOUS IN AGE 

Chapter D 

By DONALD E. LEE, JOHNS. STACEY, and LYNN B. FISCHER 

CONTENTS 

Abstract .. 

Introduction 

Analytical methods 

Muscovite-phenocrystic two-mica granites 

The age-dating problem 

Kern Mountain pluton . . . . 

Southern Snake Range pluton 

Toana Range pluton . . . . . 

FIGURES 

Page 

32 

32 

33 

33 

33 

33 

35 

36 

Equigranular two-mica granites 

Lexington Creek pluton 

Silver Creek pluton . . . . 

Willard Creek pluton . . . 

Calc-alkaline rocks of Jurassic age 

Osceola stock . . . 

Notch Peak pluton 

Conclusions . . 

References cited . . . . 

01. Map of northeastern Nevada and northwestern Utah, showing locations of muscovite-phenocrystic twoPage 

36 

36 

37 

37 

37 

37 

37 

38 

38 

Page 

mica granites, equigranular two-mica granites, and calc-alkalic rocks 32 

02. Scanning electron micrograph of zircon in apatite . . 34 

03. Uranium-lead zircon data for the Cretaceous samples 34 

04. Uranium-lead zircon data for the Jurassic samples . . 37 

TABLES 

Page 

D 1. Uranium-lead data for zircon fractions from two-mica granites and calc-alkalic rocks . . . . . . . 35 

02. Summary of radiometric age data for muscovite-phenocrystic two-mica granites of northeastern Nevada 36 

Muscovite-Phenocrystic Two-Mica Granites of Northeastern Nevada are Late Cretaceous 31 
Abstract 

Three representatives of a unique ("muscovitephenocrystic") type of two-mica granite are exposed 

along a north-south trend about 225 kilometers long in 

northeastern Nevada. This type of two-mica granite is 

distinguished in part by the presence of large phenocrysts of muscovite, and practically all the biotite in the 

rock is present as euhedra within this muscovite. Chemical, mineralogical, and isotopic data suggest that the 

muscovite-phenocrystic two-mica granites crystallized 

from anatectic magmas formed from upper Precambrian 

and Lower Cambrian pelitic sediments, along a trend 

that corresponds to the westward disappearance of a 

continental basement. 

Near the southern part of the trend just described 

are exposed three examples of a more common ("equigranular") type of two-mica granite, in which muscovite 

and biotite are present as discrete crystals. The equigranular-type two-mica granites also may have evolved 

through anatexis of the pelitic sediments. 

For various reasons potassium-argon and 

rubidium-strontium radiometric dating methods have 

been of limited usefulness in our efforts to investigate 

possible age differences between the two types of twomica granites. Moreover free (that is, not included in 

apatite) zircon crystals are so sparse in the muscovitephenocrystic type that to recover a workable amount of 

zircon from any one of these three plutons has heretofore been impossible. 

This paper presents radiometric ages for zircon 

recovered from the muscovite-phenocrystic two-mica 

granite of the Kern Mountains, White Pine County, Nev. 

Together with previously reported potassium-argon and 

rubidium-strontium age data, this new information 

suggests that all three of the muscovite-phenocrystic 

two-mica granites probably crystallized during Late Cretaceous time. The equigranular two-mica granites appear 

to range in age from Jurassic to Cretaceous, and possibly 

to Tertiary. 

This paper also presents radiometric age data for 

zircons from the Osceola stock of the southern Snake 

Range, Nevada, and from the Notch Peak pluton of the 

House Range, western Utah. Both of these calc-alkalic 

intrusives are Jurassic in age. The Jurassic igneous event 

was rather widespread in the region. 

INTRODUCTION 

During 1969-1976, one of us (Lee) visited more 

than 160 plutons while systematically sampling granitoid 

rocks in the Basin and Range Province of Nevada, Utah, 

California, and Arizona. Among these 160 plutons, three 

are two-mica granites that resemble each other and are 

unlike any of the other plutons observed during this study. 

To our knowledge, they also are unlike any plutonic rocks 

described in the literature. 

The unusual two-mica granites are along a north32 Shorter Contributions to Isotope Research 

south trend about 225 km (kilometers) in northeastern 

Nevada (fig. D1) and are distinguished in part by the presence of large phenocrysts of muscovite, most of which 

contain small inclusions of euhedral biotite. These three 

muscovite-phenocrystic (Lee, Kistler, Friedman, and Van 

Loenen, 1981) two-mica granites are present in the Pole 

Canyon-Can Young Canyon area of the southern Snake 

Range, in the Kern Mountains (sample 244-MW-6) and 

in the Toana Range (sample 415). Chemical analyses, accessory mineral contents, oxygen isotope data and initial 

87Sr/86Sr ratios indicate that these unusual granites crystalo415 

ELKO 

X 363-0l-66 

244-MW-60 o 

WHITE PINE 

I 

I BOX ELDER 

~-------

TOOELE 

I 

1---------

1 

I 

I 

X 293,294 

JUAB 

~----------- I 

I 

I 

I 

287-DL-63 ~ 1 X 298-0l-64 

177-MW-~Ox I 

295-0L-64 

MILLARD 

~c;,- ~ole ~anyon-Can Young Canyon 

Snake Creek- mtrus1ve area 

Wil~iams .canyon ~ 378 : 

mtrus1ve area 

50 100 KILOMETERS 

25 50 MILES 

Figure 01. Northeastern Nevada and northwestern 

Utah, showing locations of muscovite-phenocrystic twomica granites (open circles), equigranular two-mica 

granites (open triangles), and calc-alkalic rocks (X's). 

Numbers are those assigned to samples. The Pole Canyon-Can Young Canyon and the Snake Creek-Williams 

Canyon intrusive areas are outlined. 
lized from evolved (S-type) magmas, and Lee, Kistler, 

Friedman, and Van Loenen ( 1981) have suggested that 

they may have resulted from anatexis of upper Precambrian-Lower Cambrian clastic sediments near the westward 

disappearance of a continental basement delineated on the 

basis of lead isotope data by Stacey and Zartman (1978). 

Three additional two-mica granites, with muscovite 

and biotite in separate grains, are present in the southern 

part of the 225-km-long north-south trend just mentioned. 

These are the equigranular-type two-mica granites of Lee, 

Kistler, Friedman, and Van Loenen (1981), samples 287-

DL-63, 295-DL-64, and 378 (fig. Dl). The equigranulartype two-mica granites also might have evolved from 

anatexis of upper Precambrian-Lower Cambrian clastic 

sediments. Lee, Kistler, Friedman, and Van Loenen 

(1981) suggested that there may be systematic age differences between the muscovite-phenocrystic and equigranular types of two-mica granites, but data have been too 

few to check this possibility. The purpose of the present 

paper is to present new information on the ages of the 

two-mica granite types just described. We also present 

new information to show that calc-alkalic intrusives of 

Jurassic age are rather widespread in the region. 

ANALYTICAL METHODS 

Zircon was recovered from crushed materials using 

the Frantz 1 isodynamic separator and the heavy liquids 

bromoform and methylene iodide. The special procedure 

required for zircon sample 244-MW-60 is described separately later in this report. 

Clerici solution was used for final cleanup of some 

samples. After sizing, the crystals were heated gently in 

nitric acid to remove all traces of lead remaining from 

the Clerici solution. Chemical separation of lead and uranium followed the ion-exchange procedures of Krogh 

(1973) with minor modifications. Mass spectrometry was 

carried out on a 30-cm-radius, 90°-sector instrument, 

using the silica-gel single filament method of Cameron 

and others ( 1969) for lead, and triple filament for uranium. Analytical precision of feldspar lead-isotope ratios 

is ±0.12 percent (95-percent confidence limits). Uncertainties in zircon uranium-lead ratios are about ± 1 percent. 

Laboratory blank corrections averaged 0.5 nanograms for lead isotopic analyses. Common lead corrections for each zircon fraction were made using the isotopic 

composition of feldspar lead from sample 244-MW-60 

reported in this manuscript. 

1

Trade and company names are used for descriptive purposes only 

and do not imply endorsement by the U.S. Geological Survey. 

Constants used in the calculation are: 

A. 238U =0.155125; 235U =0.98485; A. 238UF35U = 137.88 

MUSCOVITE-PHENOCRYSTIC 

TWO-MICA GRANITES 

The Age Dating Problem 

Obtaining radiometric dates of emplacement for the 

muscovite-phenocrystic two-mica granites has been difficult for these reasons: 

1. All three of the plutons are exposed beneath 

regional thrust faults, and K-Ar (potassium-argon) ages 

determined may not be emplacement ages because they 

may have been reset by heat and stress related to later 

movement on these thrust faults (Lee and others, 1970, 

1980). 

2. Whole-rock Rb-Sr (rubidium-strontium) data 

for the muscovite-phenocrystic two-mica granite of the 

Kern Mountains failed to yield a well-defined isochron 

(Best and others, 1974), because of scatter much larger 

than possible experimental errors. This type of scatter is 

not unusual for muscovite granites derived by anatexis 

in an upper crustal environment (R. W. Kistler, written 

commun.) We will consider in a later section the recently 

obtained (chap. P, this volume) Rb-Sr age of 79 Ma for 

the muscovite-phenocrystic two-mica granite of the Pole 

Canyon-Can Young Canyon area. 

3. Almost all of the zircon in the muscovitephenocrystic two-mica granites of the Pole Canyon-Can 

Young Canyon (Lee and Van Loenen, 1971, p. 5, 39) 

and Kern Mountains (Lee and others, 1973, p. 267) areas 

is present as tiny acicular inclusions in large, equant, 

rather poorly formed, and sparsely distributed apatite crystals (fig. D2). The muscovite-phenocrystic two-mica granite of the Toana Range contains discrete crystals of stubby 

zircon, but these are very sparse ( <0.02 weight percent 

of the rock) and tiny ( <0.08 mm (millimeters) long). 

Thus, until now we have been discouraged in our attempts 

to recover enough zircon from any one of these unusual 

granites for uranium-thorium-lead isotopic age determinations. 

Kern Mountain Pluton 

In addition to the zircons included in apatite (fig. 

D2), a few scattered grains of free zircon are present in 

the muscovite-phenocrystic granites of the southern Snake 

Range and the Kern Mountains. Most of the free zircon 

grains are about 0.2 mm long and 0.06 mm wide. In 

order to recover a concentrate of this zircon, a 1 0-kg 

(kilogram) sample (244-MW-60) of the Kern Mountains 

Muscovite-Phenocrystic Two-Mica Granites of Northeastern Nevada are Late Cretaceous 33 
Figure D2. Scanning electron micrograph of zircon in 

apatite. The columnar zircon crystal is about 5 micrometers (0.005 mm) wide and 50 micrometers (0.05 mm) long. 

Sample 135-DL-62, from the Pole Canyon-Can Young 

Canyon area of the southern Snake Range. Scanning 

electron micrograph by A. J. Gude, 3d. 

pluton granite was collected at lat 39°35'55" N., long 

114°07'55" W. This sample was crushed to -35 mesh and 

run over a Wilfley table to obtain about 5 kg of heavy 

concentrate, which was further refined in bromoform by 

means of a separatory funnel. The bromoform heavies 

were centrifuged in methylene iodide and run through a 

Frantz isodynamic separator to obtain a final zircon concentrate of about 70 mg (milligrams). Assuming a 50-percent recovery, the results of this tedious procedure indicate that sample 244-MW-60 contains about 0.0007 

weight percent free zircon. 

Zircon data (table D1) for the Kern Mountains samples are shown on a concordia plot (fig. D3). Aplite sample 245-MW-60 was collected less than 3-m (meters) 

from two-mica granite sample 244-MW-60. Data for 

these two samples define a cord that has a lower intercept 

age of 75 ±9 m.y. (95-percent confidence limits). The 

upper intercept of 1970± 330 m.y. shows that sample 

244-MW-60 contains a considerable amount of inherited 

zircon of Early Proterozoic age. This observation is direct 

evidence that this Late Cretaceous granite was derived at 

least in part from Early Proterozoic material. Lead from 

the potassium feldspar of sample 244-MW -60 also has 

been analyzed and its isotopic composition is: 

206Pb;2°4Pb = 18.968; 207PbP04Pb = 15.7 61 ; 208PbP04Pb = 39.991 . 

These data are transitional between area I and area 

II types of lead as defined by Zartman (1974). Such a 

composition reflects derivation from the Proterozoic base34 Shorter Contributions to Isotope Research 

ment of Utah, with addition of radiogenic lead from continental sediments in this region, and is consistent with our 

proposed origin for this granite . For a discussion of lead 

isotopic characteristics on the Nevada-Utah State line, the 

reader is referred to the study of the Gold Hill mining 

district by Stacey and Zartman ( 1978). 

Whole-rock rubidium and strontium data were obtained for samples 244-MW-60 and 245-MW-60 by Zell 

Peterman and listed by Lee and others (1980, p. 22). A 

two-point isochron based on these data gives an initial 87 Sr/86Sr of 0. 719 at an age of 71.6 Ma. This age agrees 

with the zircon results just discussed. Best and others 

(1974) found similarly high 87Sr/86Sr for this granite. 

Muscovite samples 244-MW-60 and 245-MW-60 

gave K-Ar ages of 50.9 and 47.9 Ma, respectively, (Lee 

and others, 1980, p. 18), results that presumably reflect 

partial degassing of these muscovites by heating resulting 

from recent movement on an overlying thrust fault. About 

10 km northwest of these samples, where the muscovites 

were less completely degassed, the apparent K-Ar ages 

increase to as much as 69. 2 Ma (Lee and others, 1980). 

Best and others (197 4) listed K-Ar ages of 63.9 and 4 7. 1 

Ma for the two-mica granite of the Kern Mountains. 

From their studies, Best and others (1974) and Lee 

and others ( 1980) postulated minimum ages of 60 and 

70 Ma, respectively, for the main Kern Mountains intrusive event. Although the zircon data presented here confirm the general validity of these earlier estimates, the 

results of all these studies taken together testify to the 

complexity of the magmatic and metamorphic history of 

the Kern Mountains. 

0.030 

::l 

00 1:l 0.020 

:o0.. 

'-0 

~ 

0.010 

~0 

.---- ·----- ·----

SAMPLE 

• 244-MW-60 

o 245-MW-60 

OL-----~------~------~----~-------

0 0.05 0.10 0.15 0.20 0.25 

207Pbj235U 

Figure 03. Uranium-lead zircon data from Cretaceous 

samples studied in this paper. The regression line for 

three fractions from sample 244-MW-60 and one from 

the aplite phase 245-MW-60 has a lower intercept of 

75±9 Ma. The upper intercept of 1970±330 Ma shows 

inheritance of zircon from the Early Proterozoic material. 

The single data point from sample 37~DL-66 also shown 

inheritance of older lead, and its position indicates a 

Cretaceous rather than a Jurassic age. 
Table 01. Uranium-lead data for zircon fractions from two-mica granites and calc-alkalic rocks discussed 

in this paper 

Concentration 

(parts per 

million) 

Atomic ratios Age, million years 

Blank 

corrected 

Sample Number 

(mesh size) Pb U 

206Pb 207Pb 

238u 235u 

207Pb 

206Pb 

206Pb 

204Pb 

Kern Mountains--Two-mica granite phase 

244-MW-60: 

(+200)------- 51.2 

(-200 +400)-- 51.0 

(-400)------- 47.0 

2866 

3278 

3447 

0.01853 

0.01633 

0.01406 

0.1930 

0.1547 

0.1218 

0.07551 

0.06873 

0.06284 

118 

104 

90 

179 

146 

117 

1082 

890 

703 

9302 

11467 

3426 

Kern Mountains--aplite phase 

245-MW-60: 

All sizes---- 92.8 7475 0.01196 0.07949 0.04819 76 78 109 1361 

Lexington Creek stock 

378-DL-66: 

All sizes---- 37.9 1509 0.02466 0.2449 0.07201 157 222 986 2642 

Osceola stock 

177-MW-60: 

(+250)NM1--- 37.2 1319 0.02548 0.1804 0.05134 162 168 256 2389 

(+250)NM-A2- 36.5 1314 0.02592 0.1933 0.05409 165 179 375 3155 

(-250)NM----- 48.1 1825 0.02042 0.1418 0.05037 130 135 212 2388 

Notch Peak pluton--main intrusive phase 

298-DL-64: 

(+150)NM----- 47.9 1696 o. 02377 0.1620 0.04945 151 152 169 392 

(-400)------- 83.5 2777 0.02237 0.1524 0.04942 143 144 167 221 

Notch Peak pluton--xenolith 

299-DL-64: 

(+200)NM----- 64.8 2589 0.02171 0.1479 

1NM, nonmagnetic. 

2Abraded. 

Southern Snake Range Pluton 

Data for a muscovite, an aplite, and four samples 

of muscovite-phenocrystic two-mica granite from the Pole 

Canyon-Can Young Canyon pluton of the southern Snake 

Range give an initial 87 Sr/86Sr of 0. 7165 at an age of 

about 79 Ma (chap. P, this volume). As noted earlier, 

Rb-Sr data for a sample of the muscovite-phenocrystic 

two-mica granite of the Kern Mountains and a related aplite give a two-point isochron showing an initial 87Sr/86Sr 

of 0. 719 at an age of 71 ± 6 Ma. Thus, results of Rb-Sr 

studies, as well as the chemical, petrological, and 

mineralogical data presented by Lee, Kistler, Friedman, 

and Van Loenen ( 1981), serve to emphasize the great 

0.04941 138 140 167 1769 

similarities between these two unusual granites that appear 

in outcrop about 83 km apart (fig. 01). 

Potassium-argon ages for six micas recovered from 

the muscovite-phenocrystic two-mica granite of the Pole 

Canyon-Can Young Canyon area range from 30. 8 ± 2.1 

to 77.7 ± 2. 2 Ma (Lee and others, 1970). The younger 

ages result from partial degassing of the micas by thermal 

stresses resulting from recent (17-18 Ma) movement on 

the overlying Snake Range decollement. The oldest age 

(77. 7 ± 2.2 Ma) is in good agreement with the Rb-Sr age 

(79 Ma) just noted, and it was obtained for a muscovite 

recovered from the sample exposed farthest below the 

Snake Range decollement (Lee and others, 1970, table 

1' fig. 2). 

Muscovite-Phenocrystic Two-Mica Granites of Northeastern Nevada are Late Cretaceous 35 
Table 02. Summary of radiometric age data for muscovite-phenocrystic two-mica 

granites of northeastern Nevada 

Age 

Muscovite: 

K-Ar age 1-------

Rb-Sr age-------

Southern Snake 

Range pluton 

77. 7±2.2 Ma 

About 79 Ma (initial 87Sr/86 sr=0.7165). 

Kern Mountains 

pluton 

69.2±1.6 Ma 

71±6 Ma (initial 87sr/ 86sr=0.719). 

Toana Range 

pluton 

71.2±2.0 Ma. 

Not determined. 

Isotopic age 

of zircon------- Not determined-------- 75±9 Ma 2 Do. 

10nly maximum K-Ar ages are listed. The younger K-Ar ages noted in the 

text presumably result from degassing due to heat stress related to late 

movement on overlying thrust faults. 

2Age obtained for the source rock is 1970±330 Ma. 

Toana Range Pluton 

Only K-Ar radiometric ages are available presently 

for the muscovite-phenocrystic two-mica granite of the 

Toana Range. The muscovite and biotite recovered from 

sample 415 (fig. D1) gave markedly discordant K-Ar ages 

of 71.2 ± 2. 0 and 15 .4 ± 0. 5 Ma, respectively (Lee and 

Marvin, 1981). Muscovite from sample 415 is phengitic, 

similar to muscovites recovered from other two-mica granites in northeastern Nevada. However, biotite from sample 

415 is distinct chemically from other biotites recovered 

from these two-mica granites in that it is a hydrobiotite, 

with H30 + substituting for K + in the interlayer X -group. 

These characteristics have led Lee, Kistler, Friedman, and 

Van Loenen ( 1981) to suggest that the composition of 

this biotite was altered by postmagmatic processes. Such 

an alteration also would account for the low K-Ar age 

( 15 .4 ± 0. 5 Ma) obtained for this biotite. 

About 1 km northeast of sample 415, at lat 41°0'55" 

N. , long 114 o 17 '30" W. , the Cambrian part of the Prospect Mountain Quartzite is exposed in intrusive contact 

with the granite. Two samples of metamorphic muscovite 

recovered from this quartzite gave K-Ar ages of 

72.5 ± 2.0 and 75.6 ± 2.1 Ma (Lee and others, 1980), in 

good agreement with the K-Ar age (71.2 ± 2.0 Ma) determined for muscovite from sample 415, recovered from 

the granite itself. 

The map of Stewart and Carlson (1978) shows the 

two-mica granite represented by sample 415 as a TertiaryJurassic pluton intruding the Cambrian part of the Prospect Mountain Quartzite, with Cambrian carbonate thrust 

over the quartzite. In view of these field relations, the 

most obvious interpretation of these K-Ar ages might 

seem to be that the 71.2 ± 2. 0 age determined for igneous 

muscovite from sample 415 actually represents the time 

of crystallization. The metamorphic muscovite ages 

(72.5 ± 2.0 and 75.6 ± 2.1 Ma) might then represent de36 Shorter Contributions to Isotope Research 

gassing resulting from thermal contact action on the 

quartzite, and the degassing and chemical alteration of 

biotite from sample 415 may have resulted from postmagmatic stress related to late movement on the overlying 

thrust fault. However, the possibility exists that the K-Ar 

ages of the muscovites themselves may have been modified by stresses related to later movement on the thrust 

fault, as has been found in southern Snake Range (Lee 

and others, 1970) and elsewhere (Lee and others, 1980) 

in eastern White Pine County, Nev. 

From the radiometric age data for the muscovitephenocrystic two-mica granites summarized in table D2, 

clearly the Kern Mountains pluton and the Pole CanyonCan Young Canyon pluton of the southern Snake Range 

crystallized in Late Cretaceous time. The Toana Range 

pluton also may be Late Cretaceous in age, but this conclusion must be regarded as tentative pending further 

study. 

EQUIGRANULAR TWO-MICA GRANITES 

Lexington Creek Pluton 

Sample 378 (fig. D1) is from the equigranular twomica granite of the Lexington Creek area. (See also chap. 

P, this volume, fig. P1). Unfortunately insufficient zircon 

was recovered for more than one analysis, but the data 

are shown in table D 1 and plotted in figure D3. Although 

no really definitive age estimate is possible because of 

obvious inheritance, when the data are compared with the 

others in figures D3 and D4 the lower intercept with concordia seems more to indicate a Cretaceous rather than 

a Jurassic age. There is no doubt, however, that the sample does contain a high proportion of inherited zircon of 

Proterozoic age that is consistent with its derivation, at 

least in part, from a sedimentary source. 
Muscovites recovered from three samples of the 

Lexington Creek pluton gave Late Cretaceous K-Ar age 

results ranging from 77.9 to 86.3 Ma (Lee and others, 

1980). However, the top of this intrusive has been severed 

by the Snake Range decollement, and the dated muscovites may have been partially degassed as a result of 

stresses related to movement on the decollement. Thus, 

the intrusive may be somewhat older than indicated by 

these K-Ar age data. 

Silver Creek Pluton 

The equigranular two-mica granite of the Silver 

Creek area is Tertiary or older. The constituent muscovite 

and biotite from sample 287-DL-63 gave K-Ar ages of 

31.1 ± 1.7 and 25.5 ± 1.3 Ma, respectively (Lee and 

others, 1970), but again these results probably reflect degassing as a result of late movement along the overlying 

Snake Range decollement as discussed in the context of 

many additional K-Ar ages by Lee and others (1980). 

Uranium-thorium-lead isotopic data show that a metamorphic monazite, possibly related to contact action of this 

granite on Prospect Mountain Quartzite, apparently crystallized 80-100 Ma ago (Lee, Stem, and Marvin, 1981). 

Further work will be needed before the significance of 

these data is fully understood. 

Willard Creek Pluton 

The two-mica granite of Willard Creek (sample 

295-0L-64, fig. 01) has been regarded (Lee, Kistler, 

Friedman, and Van Loenen, 1981, p. 10,608) as a felsic 

phase of an intrusive referred to as the Osceola stock by 

Armstrong (1966). However, recent Rb-Sr work (chap. 

P, this volume) has shown that the granite of Willard 

Creek is not comagmatic with the main mass of the Osceola stock that is exposed about 3 km to the northeast 

of sample 295-0L-64. (See chap. P, this volume, fig. 

PI). Nonetheless, despite the lack of magmatic affinity 

between these two intrusive bodies, available radiometric 

age data indicate that both are Jurassic in age. The muscovite recovered from sample 295-0L-64 gave a K-Ar age 

of 151 ±4 m.y. (Lee and others, 1970), probably representing the time of crystallization of this equigranular twomica granite. The (Jurassic) age of the Osceola stock itself 

is discussed in the following section. 

CALC-ALKALINE ROCKS OF JURASSIC AGE 

Osceola Stock 

Sample 177-MW-60 is from the Osceola stock. The 

data in table 01 from the ( + 250)NM and (-250)NM zircon fractions are plotted in figure 04. If a chord were 

drawn between these two points, it would have a lower 

intercept with concordia at 82 Ma. Although this might 

correspond to the Late Cretaceous event known in the 

region, the upper intercept would give 348 Ma. This latter 

seems an unlikely age for a pluton in this part of the 

continent and so, to investigate further, the ( + 250)NM 

fraction was abraded in a sphere-former device adapted 

for the purpose by S. S. Goldich and J. N. Aleinikoff. 

The data point (point A in fig. 04) for the abraded fraction has a higher 207PbP06Pb than the unabraded fraction, 

and a chord through the two ( + 250)NM fractions has 

a lower intercept of 160 m.y. (fig. 04). This much more 

reasonable estimate should be regarded as a minimum age 

for this zircon because the unabraded fraction may have 

lost some lead later in its history, just as has the 

(-250)NM fraction. The 207PbP06Pb age of the (-250)NM 

fraction is 212 Ma and this higher value probably is due 

to zircons inherited from the source materials. Whole-rock 

Rb-Sr work on sample 177-MW-60 and on two other 

samples from the Osceola stock gives an initial 87Sr/86Sr 

value of 0.7075 at an age of 145 Ma (chap. P, this volume). The lower U-Pb intercept of 160 Ma is in fair 

agreement with these Rb-Sr results. 

Notch Peak Pluton 

Finally, we present zircon data for samples collected 

at lat 39°11'12" N., long 113°26'30" W., from the quartz0.035 ,---,----------r----~--~--,.--, 

'%o 

0.030 

:J 

0 

~ N 0.025 

0.020 

0.15 0.20 

207Pbj235U 

SAMPLE 

"- 177-MW-61 

• 298-DL-64 

o 299-DL-64 

0.25 

Figure 04. Uranium-lead zircon data from the Jurassic 

samples studied in this paper. The common age for samples 298-DL--64 and 299-DL--64 is 169 ± 3 Ma, with a lower 

intercept of 10±11 Ma. Sample 177-MW-61 shows both 

inheritance and modern lead loss. Abrasion of the 

( + 250)NM fraction produces a chord with lower intercept of -160 Ma, that is a minimum age. Dotted line 

shows 207PbP06Pb age of the (-250)NM zircons 

to be 212 Ma, but this age is artificially high because 

inherited lead has caused sideways displacement of the 

point. 

Muscovite-Phenocrystic Two-Mica Granites of Northeastern Nevada are Late Cretaceous 37 
monzonite at Notch Peak (Gehman, 1958) of the House 

Range in western Utah (fig. D1). Two fractions of sample 

298-DL--64, from the main intrusive, and one fraction 

of sample 299-DL--64, a xenolith, give three data points 

that are on a single cord with a common intercept age 

of 169 ± 3 Ma (fig. D4). The lower intercept of 10 ± 11 

Ma shows recent lead loss that may have taken place during the last uplift of the region. 

CONCLUSIONS 

Uranium-lead isotopic age results for zircon from 

sample 244-MW--60 from the muscovite-phenocrystic 

two-mica granite of the Kern Mountains confirm previously tentative Rb-Sr and K-Ar age data and show that 

this unusual granite crystallized during Late Cretaceous 

time. Taken together, these age data for the Kern Mountains intrusive also help to place in perspective Rb-Sr and 

K-Ar results for the similarly unusual Late Cretaceous 

muscovite-phenocrystic two-mica granite of the Pole Canyon-Can Young Canyon area, southern Snake Range. 

Another odd muscovite-phenocrystic two-mica granite, 

present in the Toana Range of Elko County, also may 

be Late Cretaceous in age. However, additional work is 

necessary to check the sparse K-Ar data presently available for this granite. Thus, two and possibly all three 

of the muscovite-phenocrystic granites of northeastern 

Nevada (fig. Dl) are Late Cretaceous in age. 

On the other hand, three examples of equigranular 

two-mica granites exposed in northeastern Nevada (fig. 

D1) appear to range in age from Jurassic to Cretaceous, 

and possibly to Tertiary. The equigranular two-mica granite (sample 295) of the Willard Creek area is shown by 

K-Ar work to be about 150 Ma old. The equigranular 

two-mica Lexington Creek intrusive (area of sample 378) 

is at least as old as Late Cretaceous, as indicated by both 

U-Pb and K-Ar work. The equigranular two-mica granite 

of the Silver Creek area, northern Snake Range (sample 

287) appears to be middle Tertiary or older, depending 

upon how much the K-Ar age-dated muscovite and biotite 

(31 ± 1.7 and 25.5 ± 1.3 Ma, respectively) were degassed 

as a result of movement on the overlying Snake Range 

decollement. 

The apparent similarity of the ages (Late Cretaceous) of the unusual muscovite-phenocrystic two-mica 

granites of northeastern Nevada is consistent with the idea 

that they all evolved in the same way-through anatexis 

of upper Precambrian-Lower Cambrian argillitic sediments 

under conditions of relatively low oxygen fugacity, along 

a line that roughly coincides with the westward disappearance of continental basement. However, we note that the 

initial Sr isotope values in granitoids of the region suggest 

38 Shorter Contributions to Isotope Research 

that the edge of the continent is farther to the west (R. 

W. Kistler, oral commun.) than is indicated by the lead 

isotope data of Stacey and Zartman ( 1978). 

Although one or more of the equigranular two-mica 

granites of the same general area (fig. D1) also may have 

evolved through anatexis, the apparent range in age (Jurassic-Tertiary?) of these granites suggests that they are 

not so closely related genetically. There also are 

mineralogical differences among these rocks, especially 

their accessory mineral contents (Lee, Kistler, Friedman, 

and Van Loenen, 1981). 

Although many of the two-mica granites in northeastern Nevada are Late Cretaceous in age, calc-alkalic 

plutons of Jurassic age also appear to be rather widespread 

in the eastern Great Basin. In addition to the Osceola 

stock and Notch Peak pluton discussed in this paper, Jurassic ages have been determined for the Snake CreekWilliams Canyon pluton of the southern Snake Range 

(Lee and others, 1968; chap. P, this volume), and for 

the southern part of the Gold Hill stock in western Utah 

(Stacey and Zartman, 1978). 

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for Ar analysis: Geochimica et Cosmochimica Acta, v. 30, 

no. 5, p. 565--600. 

Best, M. G., Armstrong, R. L., Graustein, W. C., Embree, 

G. F., and Ahlborn, R. C., 1974, Mica granite of the Kern 

Mountains pluton, eastern White Pine County, NevadaMobilized basement of the Cordilleran miogeosyncline?: 

Geological Society of America Bulletin, v. 85, no. 8, p. 

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Cameron, A. E., Smith, D. H., and Walker, R. L., 1969, Mass 

spectrometry of nanogram size samples of lead: Analytical 

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Gehman, H. M., Jr., 1958, Notch Peak intrusive, Millard 

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Krogh, T. E., 1973, A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb 

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research 1968: U.S. Geological Survey Professional Paper 

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Stacey, J. S., and Zartman, R. E., 1978, A lead and strontium 

isotopic study of igneous rocks and ores from the Gold Hill 

mining district, Utah: Utah Geology, v. 5, no. 1, p. 1-15. 

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Zartman, R. E., 1974, Lead isotopic provinces in the Cordillera 

of the Western United States and their geologic significance: 

Economic Geology, v. 69, p. 792-805. 

Muscovite-Phenocrystic Two-Mica Granites of Northeastern Nevada are Late Cretaceous 39 

1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

GEOCHRONOLOGY OF BASEMENT GRANITE, 

STEPHENSON COUNTY, ILLINOIS 

Chapter E 

By ZELLE. PETERMAN, S. S. GOLDICH, LYNN B. FISCHER, and K. FUTA 

CONTENTS 

Abstract ..... 

Introduction . . . 

Acknowledgments 

Samples .... 

Petrography 

Chemistry . 

FIGURES 

Page 

42 

42 

43 

43 

43 

44 

Geochronology . . . . . . . . . . . . . . . . 

Methods ............... . 

Rubidium-strontium and potassium-argon 

Uranium-thorium-lead 

Discussion 

Summary 

References cited 

Page 

45 

45 

46 

47 

48 

49 

50 

Page 

El. Map showing location of drill holes and mineral ages in northern Illinois and adjacent states . . 42 

E2. Plot of rubidium-strontium whole-rock isochron for samples from drill holes UPH-1 and UPH-3 . 46 

E3. Plot of rubidium-strontium mineral isochrons for four samples . 48 

E4. Uranium-lead concordia plot for zircon and sphene . . . . . . . . . . . . . . . . . . . . . . . . 49 

TABLES 

El. Modal analyses of samples dated ............ . 

E2. Chemical analyses of selected samples . . . . . . . . . . 

E3. X-ray fluorescence analyses for Rb, Sr, Y, Zr, and Ba . 

E4. Rubidium and strontium data for whole-rock samples and minerals 

E5. Uranium, thorium, and lead data for zircon and sphene ..... . 

Geochronology of Basement Granite, Stephenson County, Illinois 

Page 

43 

44 

45 

46 

47 

41 
Abstract 

Whole-rock samples of granite from drill holes 

UPH-1 and UPH-3 in Stephenson County, Ill., define a 

rubidium-strontium isochron of 1,430±20 million years 

with an initial 87SrtB6Sr of 0.7029 ± 0.0041. Zircons and 

sphene from the granite define a similar uranium-lead 

concordia intercept age of 1,454 ± 30 million years. Minor 

cataclasis and alteration of the granite occurred about 

1,200 million years as indicated by rubidium-strontium 

mineral ages for samples from drill hole UPH-3. This disturbance may be related to stresses imposed on the 

basement terrane during early phases of Keweenawan 

rifting to the west or Grenville orogenesis to the east. 

INTRODUCTION 

Core samples of granite recovered from three deep 

drill holes in northern Illinois (Hinze and others, 1981) 

have been made available for scientific purposes and provide an opportunity to study the physical, chemical, and 

01.178 

1.308 

I 

( 

I 

isotopic properties of a Precambrian granite well beneath 

the effects of near-surface alteration. The present report 

presents the results of a geochronological study of selected 

samples from two of these drill holes. 

The Precambrian basement in this area (fig. E 1) is 

covered by about 600 m (meters) of lower Paleozoic 

strata, but fission-track dating of apatite from the IDH 

(Illinois deep hole) granite suggests that the overlying section was considerably thicker at one time (Zimmerman, 

1981). The nearest exposed Precambrian is in central Wisconsin where Archean gneisses and Early and Middle 

Proterozoic supracrustal rocks, gneisses, and granites crop 

out. 

The IDH granite is one of several Middle Proterozoic an orogenic intrusions in the midcontinent region, the 

southern Rocky Mountains, and the southwestern United 

States (Van Schmus and Bickford, 1981; Silver and 

others, 1977; Emslie, 1978). The composite Wolf River 

batholith in central Wisconsin is a member of this 

anorogenic suite of intrusions. Isotopic dating of other 

\ 

I 

' 

WISCONSIN 

42" 

\ 

' ...___ ....... 

\ 

~-·:6: o\:--------o~P_!!-~3~~------ ---------- '\ UPH-2 Cb UPH-3 1.208 

'\ 1.138 

I 0 W A '-,'1 

} 

( 

1.308 0 j 1.298/ 

J 

) 

,,.-

_/..._/ 

/ 

I 

\.. \ 01.278 

1 1.358 

0 1.42F 

1.408 

01.308 

1.378 

1.158 0 01.248 

1.158 1.318 

ILLINOIS 

0 0.938 

1.218 

0.628 0 0.658 

50 100 KILOMETERS 

25 50 MILES 

INDIANA 

Figure El. Potassium-argon and rubidium-strontium biotite (Bi) and potassium feldspar (F) ages of Precambrian 

basement samples obtained from drill holes in northern Illinois and adjacent states. Ages are in millions of years 

(Ma). Potassium-argon ages are italicized; rubidium-strontium ages are not. Location of drill holes UPH-1, -2, and 

-3 are shown together with ages from this report. Other ages are from Lidiak and others (1966). 

42 Shorter Contributions to Isotope Research 
basement samples from eastern Iowa and western Illinois 

have identified similar Middle Proterozoic granitoids, and 

aeromagnetic data suggest the presence of a belt of these 

plutons extending from southeastern Wisconsin to southeastern Iowa (Van Schmus and others, 1981; Hoppe and 

others, 1983). 

ACKNOWLEDGMENTS 

We are indebted to H. H. Mehnert for the K-Ar 

(potassium-argon) determination on biotite from sample 

5251, to T. T. Ball for the petrography and modal 

analyses, and to Linda Martin for sample preparation and 

mineral separation. We also thank Prof. W. R. Van 

Schmus for the allocation of samples. 

SAMPLES 

The drill holes are in Stephenson County in northern 

Illinois (fig. E1). Coordinates for these holes are as follows: UPH-1, lat 42°30'20" N., long 89°51'07" W; UPH2, lat 42°26'34" N., long 89°52'01" W.; and UPH-3, lat 

42°26'14" N., long 89°51'27" W. 

Sample numbers (table E 1) are depths in feet; 2031 

and 2064 are from drill hole UPH-1 , and the others are 

from UPH-3. Samples 2064, 2209, 3333, 3514, and 5056 

wen; provided by W. R. Van Schmus in the allocation 

of material for the Illinois drill-hole project. These were 

whole pieces of core 4. 7 em (centimeters) in diameter 

and 5-10 em long. A larger sample of 2031 had been 

obtained earlier by Goldich from the Illinois Geological 

Survey, Sample designated 5251 represents the interval 

from 5245.0 to 5256.8 ft. 

PETROGRAPHY 

The dated samples (table El) differ in texture and 

varietal mineralogy. Samples 2209 and 2290 are finegrained, biotite-muscovite granites. Sample 3514 is a porphyritic granite that has groundmass texture and mineralogy like those of the fine-grained rocks but with subhedral 

to euhedral phenocrysts of microcline and plagioclase as 

much as 1 em long. The remaining samples-2031, 2064, 

5056, and 5251-are coarse-grained, biotite granites. Textures are xenomorphic inequigranular in the fine-grained 

rocks and hypidiomorphic inequigranular in the coarsegrained granites. 

Quartz, perthitic microcline, and albite to sodic oligoclase compose more than 80 percent of all the samples. 

Quartz is rutilated and contains abundant trails of fluid 

inclusions. In the fine-grained granites, and to a lesser 

extent in the coarse-grained rocks, microcline shows a 

micrographic texture with rounded blebs and rods of 

quartz. Poikilitic microcline in the coarse-grained granites 

contains inclusions of quartz, plagioclase, biotite, and accessory minerals. Plagioclase is moderately to severely 

sericitized to dense patches of very fine grained material, 

Table E1. Modal analyses of samples dated, Stephenson County, Ill. 

[Modes are based on I , 700 points for sample 203I , I ,000 points for sample 525I , and 600 points for 

the other samples. Leaders ( ---) indicate not present; tr, trace] 

Drill hole--------- UPH-1 UPH-3 

Sample No. (also 

depth, in feet)----- 2031 2064 2209 2290 3333 3514 5056 5251 

Quartz------------- 31 32 40 39 30 35 29 38 

Microcline--------- 22 41 30 30 32 35 36 27 

Plagioclase-------- 21 15 21 22 26 24 21 23 

Muscovite---------- 6.2 5.2 1.7 2.2 0.2 

Biotite------------ 6.0 6.7 1.2 3.2 5.5 1.0 7.0 7.7 

Hornblende--------- 1.0 .1 

Fluorite----------- .4 .9 .2 1.0 1.3 .7 .3 .5 

Monazite----------- .2 .2 tr tr 

Zircon------------- tr tr .5 .3 tr tr .2 

Sphene------------- .1 tr .6 

Apatite------------ .6 .2 tr tr 

Opaque minerals---- 15.4 .7 .2 .8 .2 1.0 .5 

Allanite----------- .1 

Chlorite----------- 1.9 1.0 1.5 .3 .7 1.3 .2 .3 

Sericite----------- 10.7 2.7 .8 .5 5.3 2.2 

Carbonate---------- 1.1 .3 .2 

1Includes dense iron-oxide staining associated with other alteration 

minerals generally on plagioclase. 

Geochronology of Basement Granite, Stephenson County, Illinois 43 
commonly stained with iron oxide, and to discrete, 

oriented blades of coarser white mica. Biotite ranges from 

fresh to almost completely altered to chlorite, and commonly is associated with granular sphene. Some biotite 

grains contain thin lenses of carbonate and a low-birefringent mineral. Hornblende is a trace mineral in two samples. Muscovite is present locally in fibrous intergrowths 

between biotite and microcline. Fluorite is a ubiquitous 

accessory mineral and is present as interstitial grains and 

as inclusions in other minerals. In the coarse-grained granites, biotite is present as clots in association with opaque 

oxides, sphene, and other accessory minerals. Samples 

2031 and 2064 are stained appreciably with iron oxide 

along fractures and grain boundaries. Thin carbonate veinlets are present in 2031 , and sphene is altered locally to 

an inter growth of leucoxene and carbonate. 

Strain in the samples is present in different degrees. 

Sample 2209 has well-developed mortar structure, 

through-going microshear zones, bent and broken plagioclase twins, and mutually sutured grain boundaries. 

Quartz is highly strained and recrystallized into domains 

that have sutured boundaries. The other samples show less 

deformation, but strained quartz and mortar structure 

around larger grains are common. 

CHEMISTRY 

Chemical analyses of samples 2031 (hole UPH-1) 

and 5251 (hole UPH-3) were made by a combination of 

atomic absorption and conventional chemical methods. 

These are the large samples from which zircon was concentrated for U-Pb determinations and other minerals for 

Rb-Sr analyses. The two analyses (table E2) are similar 

but show greater differences than might be surmised from 

the modal analyses (table E1), but the coarse-grained 

rocks are not represented adequately in one or two thin 

sections. The chemical analyses are similar to published 

analyses (Van Schmus and others, 1975; Anderson and 

Cullers, 1978) of granitic rocks from the Wolf River 

batholith, Wisconsin, about 300 km northeast of the drill 

holes in Stephenson County, Ill. Samples 2031 and 5251 

most closely resemble the Red River adamellite of Anderson and Cullers (1978) for which their average is given 

in table E2. The low K/Rb and high Ba/Sr and Rb/Sr 

are distinctive features of both the IDH granite and the 

granitic phases of the Wolf River batholith. Their potassic 

character is shown by K20/(K20+Na20) of0.71 for sample 2031, 0. 65 for sample 5251, and 0. 61 for the Red 

River adamellite. 

Using GSP-1 as a standard, Rb, Sr, Y, Zr, and 

Ba were determined by X-ray fluorescence (XRF), for all 

samples received in the allocation (table E3). Coefficients 

of variation (1 sigma) are estimated to be ± 7 percent 

for concentrations greater than 100 ppm (parts per million) 

and about ± 15 percent for those less than 100 ppm. 

44 Shorter Contributions to Isotope Research 

Table E2. Chemical analyses of selected samples, Stephenson County, Ill. 

[Leaders ( ---) indicate not determined] 

Drill hole-- UPH-1 

Sample No.-- 2031 

UPH-3 

5251 

Weight percent 

Si02-------- 71.8 

A1 2o3------- 14.1 

Ti02-------- .30 

Fe 2o3------- .39 

FeO--------- 1.08 

MnO--------- .02 

MgO--------- .56 

CaO--------- 1.37 

Na2o-------- 2.80 

K20--------- 6.82 

P205--------

H20+--------

H2o- -------

co2---------

F-----------

.06 

.29 

.06 

.30 

.20 

Total--- 3100.07 

71.32 

13.61 

.46 

.75 

1.99 

.os 

.39 

1. 97 

3.00 

5. 55 

.13 

.13 

.05 

.08 

99.48 

Parts per million 

Rb---------- 303 

Sr---------- 152 

Ba---------- 1,150 

Y----------- 36 

Zr---------- 143 

302 

132 

1,000 

71 

254 

Weight ratio 

K/Rb--------

K/Sr--------

Rb/Sr-------

Ba/Sr-------

Ba/Rb-------

187 

372 

1.99 

7.56 

3.80 

153 

349 

2.29 

7.58 

3.31 

72.5 

13.91 

.36 

22.65 

.06 

.47 

1.25 

3.24 

5.14 

99.59 

236 

129 

705 

181 

330 

1.83 

5.47 

2.99 

1RR is the average of seven analysee 

of the Red River adamellite of the Wolf 

River batholith, Wisconsin (Anderson and 

Cullers,.1978). 

2Total Fe reported as FeO. 

3rotal corrected for F. 

Except for sample 2185 (table E3), some systematic 

variations are apparent among these data. Strontium decreases from about 150 to 25 ppm as rubidium increases 

from about 300 to 650 ppm. The fine-grained granites 

such as 2209 and 2290 (table E1) are higher in Rb and 
Table E3. X-ray fluorescence analyses for Rb, Sr, Y, Zr, and Ba of samples, 

Stephenson County, Ill. 

[Suffix letters B and C and F and G refer to adjacent pieces of core prepared as separate samples] 

Sample Concentration (parts per million) Weight ratio 

No. Rb Sr y Zr 

Drill 

2004 306 154 71 302 

2025 301 167 59 305 

2031 296 149 36 144 

2064B 299 142 68 285 

2064C 345 152 43 197 

2071F 303 137 68 347 

2071G 299 137 64 236 

Drill 

2185 335 292 75 333 

2385 318 128 75 317 

2779 339 130 30 172 

3194 341 85 40 208 

3678 371 49 38 71 

4202 517 so 70 97 

4405 693 71 73 227 

4773 435 125 72 278 

5411 433 90 73 268 

Drill 

2209 621 25 118 212 

2290 591 34 126 264 

2397 537 25 104 254 

2422 524 39 87 174 

3333 423 68 68 233 

3514 438 45 101 291 

3880 392 100 62 246 

4059 365 103 95 331 

4609 490 55 82 260 

4689 423 55 73 225 

5056 417 96 71 266 

5251 308 135 71 254 

lower in Sr than the coarse-grained rocks. Barium correlates with strontium, and yttrium shows a crude correlation with rubidium. 

Anderson and Cullers (1978) concluded that granitic 

rocks of the Wolf River batholith were anatectic melts 

of a tonalitic to granodioritic source in the lower to intermediate crust. Fractional crystallization at higher levels 

produced some of the chemical variations among the different phases. Variations in Rb, Sr, and Ba in the IDH 

samples (table E3) could be the result of fractional crystallization in which plagioclase was an important precipitate 

phase. 

Ba Rb/Sr Zr/Y Ba/Sr 

hole UPH-1 

924 1.98 4.2 6.0 

680 1.80 5.2 4.1 

956 1.98 4.0 6.4 

507 2.10 4.2 3.6 

892 2.28 4.6 5.9 

568 2.22 5.1 4.1 

738 2.19 3.7 5.4 

hole UPH-2 

622 1.15 4.4 2.1 

638 2.48 4.3 s.o 

873 2.61 5.7 6.7 

362 4.01 5.2 4.3 

125 7.57 1.7 2.6 

243 10.3 1. 4 4.9 

537 9.76 3.1 7.6 

529 3.48 3.9 4.2 

480 4.81 3.7 5.3 

hole UPH-3 

95 25.1 1.8 3.8 

55 17.3 2.1 1.6 

106 21.2 2.4 4.2 

160 13.3 2.0 4.1 

226 6.22 2.0 5.0 

189 9.73 2.9 4.2 

531 3.92 4.0 5.3 

541 3.54 3.5 5.3 

313 8.91 3.2 5.7 

265 7.69 3.1 4.8 

520 4.34 3.8 5.4 

2.28 3.8 

GEOCHRONOLOGY 

Methods 

Following ultrasonic cleaning, the samples were 

crushed, and splits of 30-50 g (grams) were pulverized 

to -200 mesh in a shaker mill for the whole-rock analyses. 

Mineral separates were prepared from sized, crushed rock 

using heavy liquids and magnetic separation. 

Samples for Rb and Sr analyses were selected on 

the basis of XRF determinations to obtain a suitable range 

in Rb/Sr for the construction of an isochron. Rubidium 

Geochronology of Basement Granite, Stephenson County, Illinois 45 
Table E4. Rubidium and strontium data for whole-rock 

samples and minerals, Stephenson Co., Ill. 

[Sample numbers are depths in feet. Suffix letters refer to type of sampie: WR, whole rock; Ap, apatite; Sp, sphene; Bi, biotite; PI, Plagioclase; and Mi, microcline. Leaders ( ---) indicate not determined] 

Concentration 

Sample (parts per million) 

No. Rb Sr 87Rb/86sr 87sr I B6sr 1 

Drill hole UPH-1 

2031WR 303.0 151.7 . 5.845 0.82147±3 

2031WR 151.1 5.867 .82151±2 

2031Mi 452.5 162.3 8.193 .86667±3 

2031Mi 163.3 8.146 .86634±15 

2064WR 348.3 149.8 6.816 .83945±4 

Drill hole UPH-3 

2209WR 619.0 29.21 69.87 2.13036±40 

2209Mi 1,153 39.60 100.17 2.63314±32 

2290WR 560.8 27.75 66.25 2.06461±9 

2290Mi 1,181 47.91 82.91 2.36280±18 

3333WR 417.4 62.15 20.22 1.11723±30 

3514WR 431.7 43.24 30.66 1.33079±23 

5056WR 415.2 93.70 13.16 .97585±5 

5251WR 302.0 131.9 6.715 .84426±4 

5251WR 131.5 6.734 .84446±2 

5251Ap 12.19 49.41 .7159 .73369±3 

5251Sp 13.28 50.27 .7694 • 77508±13 

5251Pl 85.91 95.95 2.608 • 77382±2 

5151Pl 95.85 2.611 .77384±2 

5251Mi 503.8 113.0 13.22 .95922±19 

5155Mi 112.4 13.29 .95951±3 

5251Bi 1,364 22.73 244.8 4.89660±40 

5251Bi 22.94 242.9 

Standard sample GSP-1 

GSP-1 253.3 235.3 3.135 .76888±3 

1Uncertainties are ±2a on the mean x 10 5• 

and strontium were determined by conventional isotopedilution methods using enriched 87Rb and 84Sr spikes. 

Samples were decomposed in HF and H2S04 in covered 

platinum dishes following spiking, and Sr was purified 

for isotopic analysis on conventional ion-exchange columns elutriated with HCl. 

Analytical uncertainties (lcr) are Rb and Sr, ± 0.6 

percent; 87 Sr/86Sr, ± 0. 017 percent. The U.S. Geological 

Survey rock standard GSP-1 was analyzed in addition to 

the core samples and the results (table E4) are in agreement with generally accepted values. The average 87Sr/86Sr for the E and A standard in this laboratory is 

0.70803. 

The uncertainties shown for 87Sr/86Sr in table E4 

are 2cr on the mean determined from the in-run statistics. 

These uncertainties are 2-20 times lower than the precision determined from replicate analyses of granitic rocks. 

This difference reflects the difficulty of replicating splits 

at the 0. 1-1 g level of samples in which the constituent 

minerals differ by nearly an order of magnitude in 

s7Sr/s6sr. 

46 Shorter Contributions to Isotope Research 

Zircon and sphene (table E5) were analyzed following a method modified from that described by Krogh 

(1973). Analytical uncertainties (lcr) are about ±0.7 percent for parent-daughter ratios and ± 0.05 percent for 

207pb;206pb. 

Isotopic and decay constants recommended by the 

International Union of Geological Sciences Subcommission on Geochronology (Steiger and Jager, 1977) were 

used in calculating the ages. The regressions are calculated using Ludwig's ( 1982a, b) programs. 

Rubidium-Strontium and Potassium-Argon 

Rubidium-strontium data for whole-rock samples 

from drill holes UPH-1 and UPH-3 (table E4) define an 

isochron (fig. E2) for which the scatter is slightly greater 

than analytical error. The MSWD (mean square of the 

weighted deviates) for a model I solution is 3.2. The preferred age of 1,430 ± 20 Ma (million years) is determined 

from the model III regression which increases the variance 

in 87Sr/86Sr to explain the scatter. Slight variations in initial 87Sr/86Sr among the samples would be consistent with 

a model III solution . 

The two fine-grained granites, samples 2209 and 

2290, from near the sub-Cambrian unconformity have the 

highest Rb/Sr. Uranium-lead analyses of these samples indicate loss of lead (Doe and others, 1983; Stuckless and 

others, 1981), but this disturbance is not evident in the 

Rb-Sr systems. Exclusion of the two points from the regression yields a model III solution of 1 ,432 ± 40 Ma and 

an intercept of 0. 7026 ± 0. 0069. 

1.9 

c/5 

~ 

~ 1.5 CJ) 

..... 

00 

1.1 

20 60 80 

Figure E2. Rubidium-strontium whole-rock isochron for 

samples from drill holes UPH-1 and UPH-3, Stephenson 

County, Ill. The slope of the line corresponds to an age 

of 1,430±20 Ma, and the 87SrfK>Sr intercept is 

0.7029±0.0041. 
Table E5. Uranium, thorium, and lead data for zircons and sphene, Stephenson County, Ill. 

[Numbers in parentheses are mesh size of fractions. Isotopic composition of initial lead is assumed to be 204Pb: 206Pb: 207Pb: 208Pb=l:l6.27:15.41:35.93. 

Age in millions of years are given in square brackets below the atom ratio. Leaders(---) indicate not determined] 

Concentration Isotopic composition of lead Atom ratio 

(parts per million) 

Sample No. Material 

(mesh size) analyzed u Th Pb 

2031: 

(unsized) Zircon-- 768.1 131.3 

5251: 

(60-100) Zircon-- 494.8 111.8 

(60-100)ABR2 --do.--- 650.2 146.4 

(100-200) --do.--- 1,100 221.9 

(+200) Sphene-- 161.2 129.9 48.1 

1Corrected for laboratory blank. 

2Suffix ABR designates abraded sample. 

204pb 

0.078 

.118 

.090 

.161 

.082 

Although the initial 87 Sr/86Sr intercept is low0. 7029-the large uncertainty of ± 0. 0041 allows considerable latitude in estimating the Rb/Sr of the source material. At the 95-percent confidence level, the intercept is 

constrained to a possible range between 0.6988 and 

0. 7070. The lower limit is impossible, but minimum mantle values can be calculated using the coordinates of the 

centroid of the whole-rock data as estimates for the average granite. These values are 10.33 for 87Rb/86Sr and 

0. 9149 for 87 Sr/86Sr. Model ages and intercepts, respectively, are 1,431 Ma and 0.7028 in the bulk-earth model 

of McCulloch and Wasserburg ( 1978) and 1 ,434 Ma and 

0. 7024 in the tonalite-trondhjemite model of Peterman 

(1979). A bulk-earth value of 0. 7028 as the lower limit 

and 0. 7070 as the upper limit for the granite would allow 

a Rb/Sr between 0.029 and 0.25 for a source of 1.9 Ga. 

The 0.25 figure commonly is cited as an average Rb/Sr 

value for the upper crust. An assumed older age for the 

source would, of course, lower the maximum permissible 

Rb/Sr. Nonetheless, the possible range in the 87Sr/86Sr 

intercept for the IDH granite allows partial or complete 

derivation of the granite from older crustal material. 

Minerals from four samples were analyzed (table 

E4) to evaluate the internal isotopic integrity of the granite 

(fig. E3). Data for whole-rock, plagioclase (about 50 percent quartz), microcline, and biotite from sample 5251 

are colinear within analytical error (MSWD=0.99) with 

an age of 1,201 ± 15 Ma. Exclusion of the biotite from 

the regression does not change the age significantly 

(1 ,211 ± 22 Ma). Data for sphene and apatite (fig. E3) 

are not co linear with data for the major minerals. This 

difference probably represents the distribution and associa-

(atom percent)l 206pb 207pb 207pb 208pb 

206pb 207pb 208pb 238u 235u 206pb 232rh 

79.53 8.15 12.24 0.1555 1.9046 0.0889 

[931] [1,083] [ 1,401] 

76.26 8.51 15.12 .1952 2.4205 .0899 

[1,149] [1,249] [1,424] 

74.87 7.94 17.10 .1921 2.3684 .0894 

[1,133] [1,233] [1,413] 

71.12 8.49 20.22 .1606 1.9401 .0876 

[960] [1,095] [1,375] 

71.67 7.68 20.57 .2439 3.0682 .0912 0.0735 

[1,407] [1,425] [1,452] [1,433] 

tion of these minor phases. Sphene occurs mainly with 

clots of biotite and opaque oxides, and a small amount 

is related to chloritization of biotite. Thus, sphene could 

have incorporated a disproportionate amount of radiogenic 87 Sr released from the biotite during the disturbance. 

Some apatite is associated with the mafic clots, but much 

is distributed throughout the other minerals, such as 

quartz, where it could not readily accept mobile 87Sr. 

Data for whole-rock samples 2209 and 2290 and 

their constituent microclines (table E4) are colinear within 

analytical error (fig. E3, inset) and give an age of 

1,181 ±65 Ma (MSWD=0.79). This age is indistinguishable from the mineral isochron for sample 5251. The 

whole rock and microcline for sample 2031 are on an 

isochron of 1,357 ± 101 Ma. 

A potassium-argon age of biotite from sample 5251 

is 1,130± 12 Ma. The data are: K20, 7.70 percent; 40Ar radiogenic, 2.019X 10-10 moles/gram; and 40Ar 

radiogenic/40 Ar total, 0. 9884. 

Uranium-Thorium-Lead 

Four samples of zircon and one of sphene were processed and analyzed (table E5). The U-Pb ages for the 

zircon are appreciably discordant, and the ratios of the 207PbP35U ages to the 206PbP38U ages range from 1.09 

to 1.16, but this ratio of the U-Pb ages of the sphene 

is 1.013. The sphene, therefore, controls the plot for the 

five samples on a concordia diagram (fig. E4). A regression for the data gives upper and lower intercepts of 

1 ,454 ± 30 Ma and 181 ± 110 Ma, respectively. The concordia age agrees, within analytical error, with the Rb-Sr 

isochron age of 1 ,430 ± 20 Ma. 

Geochronology of Basement Granite, Stephenson County, Illinois 47 
2.2 

0.74 

2.0 60 80 100 

0.70 

0 2 4 6 8 10 12 14 

87Rb/86Sr 

Figure E3. Rubidium-strontium mineral isochrons for samples from Stephenson County, Ill. The isochrons and intercepts, respectively, are 1,201±15 Ma and 0.7290±0.0011 for plagioclase (PI), whole rock 

(WR), microcline (Mi), and biotite (Bi)-not shown-for sample 5251 (solid line); 1,357±101 Ma and 

0.7076±0.0098 for whole rock and microcline for sample 2031 (dashed line); and 1,181 ±65 Ma and 

0.948 ± 0.073 for whole rocks and microclines for samples 2209 and 2290 (inset). Data points for apatite 

(Ap) and sphene (Sp) from samples 5251 are excluded from the regression. 

A part of sample 5251 (60-100 mesh) was abraded 

in a sphere-former to see if the zircons were composed 

of cores and rims of different ages. Although the more 

resistant material has higher U and Pb contents (table E5), 

the parent-daughter ratios are not appreciably different 

than those of the unabraded fraction. 

DISCUSSION 

The whole-rock Rb-Sr isochron age of 1 ,430 ± 20 

Ma and the U-Pb concordia-intercept age of 1,454 ± 30 

Ma agree within experimental error. These ages agree 

with U-Pb zircon age of 1,461 ± 6 Ma (drill hole UPH-1) 

and 1,465 ± 8 Ma (drill hole UPH-3), and with a Rb-Sr 

whole-rock isochron age of 1 ,404 ± 35 Ma (drill hole 

UPH-3) reported by Hoppe and others ( 1983). All the 

isotopic ages are similar to those of the Wolf River 

batholith of central Wisconsin (Van Schmus and others, 

1975), which, when recalculated for decay constants and 

48 Shorter Contributions to Isotope Research 

regression methods used here, are 1,437 ± 20 Ma (Rb-Sr) 

and 1,469 ± 28 Ma (U-Pb). 

Concordancy of the Rb-Sr mineral ages for samples 

from drill hole UPH-3 suggests a specific event at about 

1,200 Ma. This event is correlated with the deformation 

and mineralogic alteration of the granite. The greater age 

of 1,357± 101 for microcline from drill hole UPH-1 

(sample 2031) may indicate that the disturbance was 

rather local in extent. Drill hole UPH-1 is about 8 km 

north of drill hole UPH-3 (fig. E1). 

McGinnis and others ( 1976) interpreted the gravity 

field of northern Illinois as representing discrete crustal 

blocks that are fault-bounded. These authors suggested 

that there was movement along these faults in the Precambrian and the Phanerozoic, and that most of the earthquake epicenters in Illinois are coincident with the inferred fault zones (McGinnis and Ervin, 1974). 

Potassium-argon and rubidium-strontium mineral 

ages of basement samples in this region (fig. E1) probably 
represent differential uplift of these domains and (or) 

localized tectonism at their margins. For example, biotite 

from a drill hole in northeastern Illinois (fig. E1) has unusually low ages of 620 Ma (Rb-Sr) and 650 Ma (K-Ar). 

The hole is on or close to the northwest-trending 

Sandwich fault (McGinnis and others, 1976), and these 

ages indicate Late Proterozoic movement. 

McGinnis and Ervin (1974) suggested that a horizontal stress field imposed on this mosaic of basement 

domains would produce differential horizontal and vertical 

movements of these blocks. Activity along the basement 

faults could be triggered by stresses originating at considerable distances outside the region. Thus, the 1 ,200-Ma 

ages recorded by samples from drill hole UPH-3 probably 

are related to early phases of Keweenawan rifting to the 

west or to Grenville orogenesis to the east. Depending 

upon the direction of the stress field, activity may be preferential along certain fault orientations, which would explain the regional pattern of generally concordant age 

pairs that differ substantially from one area to another 

(fig. E1). This long history of repeated activity in the 

basement contrasts with areas in the exposed Canadian 

shield, such as in the central and western Superior province, where biotite ages are remarkably uniform over 

broad areas. 

The ages reported here and other data indicate disturbances of the IDH granite in the Phanerozoic. Assuming no gain or loss of uranium, the zircon array (fig. 

E4) could have resulted from loss of 24--40 percent of 

accumulated radiogenic lead between Late Pennsylvanian 

and Late Cretaceous time. Whole-rock U-Pb data indicate 

0.28 

0.24 

0.20 

::J 0.16 00 

~ 

-.£) 

0.. 

·~ 0.12 ~ 

0.08 

0.04 

0.00 

~ z._ 5251 Sp 00 

~ 

r:9 5251 Z(60-1 00) 

L5251 Z(60-100) Abraded 

o 5251 Z(1 00-200) 0 

2031 z 

loss of lead from samples 2209 and 2290 at about this 

same time (Doe and others, 1983; Stuckless and others, 

1981). Fission-track ages of apatite decrease linearly from 

130 ± 14 Ma near the sub-Cambrian unconformity to 

50± 7 Ma at 5400 ft, and Zimmermann ( 1981) interpreted 

this pattern as resulting from cooling below the annealing 

temperature of apatite during the Cretaceous or early Tertiary. 

Correlation of these relatively young disturbances 

with known geologic events is not clear. The granite was 

exposed during the Late Proterozoic and Early Cambrian, 

and it is overlain unconformably by the Upper Cambrian 

Mount Simon Sandstone. Some disturbance of these sensitive isotopic systems would have been expected at this 

time. A Bubnoff plot of McGinnis and others (1976) 

shows a sharp increase in subsidence of the Illinois basin 

during the Mississippian, and this could have been accompanied by movement along basement faults. The event 

in the Late Cretaceous to early Tertiary recorded in the 

fission-track ages is not apparent in the stratigraphic record as it is presently known (Zimmermann, 1981). 

SUMMARY 

Isotopic dating of samples of basement granite penetrated in the Illinois deep drill holes establishes the time 

of intrusion, and, together with published ages for this 

region, records a long and complex tectonic history for 

the buried basement. 

1. The rubidium-strontium whole-rock age of 

1,430 ± 20 Ma and the zircon-sphene concordia age of 

1 ,454 ± 30 Ma agree within analytical error and establish 

the time of intrusion of this anorogenic granite. 

2. The rubidium-strontium mineral ages for samples from drill hole UPH-3 record an event at about 1,200 

Ma manifested in the granite by cataclasis and alteration. 

An age of about 1 ,360 Ma for microcline from drill hole 

UPH-1 suggests that this event may have been local in 

extent. 

3. The 1 ,200-Ma mineral ages for samples from 

drill hole UPH-3 and several published Rb-Sr and K-Ar 

biotite ages for basement samples in the region probably 

are related to differential vertical and horizontal movements of discrete crustal blocks resulting from stresses 

imposed by early phases of either Keweenawan rifting or 

Grenville orogenesis. Later activity of this kind also is 

indicated by published biotite ages. 

0 3 4 4. The granite was exposed by Late Cambrian 

time, but the effects of this uplift and denudati<?n on the 

isotopic systems, if any, have been obscured by apparent 

disturbances in middle to late Phanerozoic time. Figure E4. Uranium-lead concordia plot for zircon (Z) 

and sphene (Sp) (samples from Stephenson County, Ill.). 

Intercepts are 1 ,454 ± 30 Ma and 181 ± 110 Ma. Size fraction (if any) is shown in parentheses next to sample 

number. 

5. This long-term tectonic history recorded in 

basement rocks of the region probably is typical of the 

buried Precambrian basement of the central interior. It 

Geochronology of Basement Granite, Stephenson County, Illinois 49 
contrasts dramatically with some terranes in the Superior 

province of the Canadian shield where biotite ages indicate remarkable long-term stability over large areas. 

REFERENCES CITED 

Anderson, J. L., and Cullers, R. L., 1978, Geochemistry and 

evolution of the Wolf River batholith, a late Precambrian 

rapakivi massif in northern Wisconsin, U.S.A.: Precambrian 

Research, v. 7, p. 287-324. 

Doe, B. R., Stuckless, J. S., and Delevaux, M. H., 1983, The 

possible bearing of the granite of the UPH deep drill holes, 

northern Illinois, on the origin of Mississippi Valley ore 

deposits: Journal of Geophysical Research, v. 88, p. 7335-

7345 .. 

Emslie, R. F., 1978, Anorthosite massifs, rapakivi granites, and 

late Proterozoic rifting of North America: Precambrian Research, v. 7, p. 1-98. 

Hinze, W. J., Haimson, B. C., and Van Schmus, W. R., 1981, 

The Illinois deep hole project-An overview [abs.]: EOS, 

v. 62, no. 17, p. 387. 

Hoppe, W. J., Montgomery, C. W., and Van Schmus, W. R., 

1983, Age and significance of Precambrian basement samples from northern Illinois and adjacent states: Journal of 

Geophysical Research, v. 88, p. 7276-7286. 

Krogh, T. E., 1973, A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb 

for isotopic determinations: Geochimica et Cosmochimica 

Acta, v. 37, p. 485-494. 

Lidiak, E. G., Marvin, R. F., Thomas, H. H., and Bass, M. 

N., 1966, Geochronology of the midcontinent region, 

United States, pt. 4; eastern area: Journal of Geophysical 

Research, v. 71, p. 5427-5438. 

Ludwig, K. R. l982a, Programs for filing and X-Y plotting 

of isotopic and other data using an HP-9830 computer and 

HP-9862 plotter: U.S. Geological Survey Open-File Report 

OF 82-385, 23 p. 

---1982b, Programs for filing and plotting U-Pb isotope data 

for concordia diagrams, using an HP-9830 computer and 

HP-9862 plotter: U.S. Geological Survey Open-File Report 

OF 82-386, 23 p. 

McCulloch, M. T., and Wasserburg, G. J., 1978, Sm-Nd and 

Rb-Sr chronology of continental crust formation: Science, 

V. 200, p. 1003-1011. 

50 Shorter Contributions to Isotope Research 

McGinnis, L. D., and Ervin, C. P., 1974, Earthquakes and 

block tectonics in the Illinois basin: Geology, v. 2, 

p. 517-519. 

McGinnis, L. D., Heigold, P. C., Ervin, C. P., and Heidara, 

M., 1976, The gravity field and tectonics of Illinois: Illinois 

State Geological Survey Circular 494, 28 p. 

Peterman, Z. E., 1979, Strontium isotope geochemistry of late 

Archean to late Cretaceous tonalites and trondhjemites, in 

Barker, Fred, ed., Developments in petrology, dacites, trondhjemites, and related rocks: Amsterdam, Elsevier Scientific 

Publishing Co., p. 133-147. 

Silver, L. T., Bickford, M. E., Van Schmus, W. R., Anderson, 

J. L., Anderson, T. H., Medaris, L. G., Jr., 1977, The 

1. 4--1.5 b. y. transcontinental anorogenic plutonic perforation of North America [abs.]: Geological Society of America 

Abstracts with Programs, v. 9, no. 7, p. 1176-1177. 

Steiger, R. H., and Jager, E., 1977, Subcommission on geochronology; convention on the use of decay constants in 

geo- and cosmochronology: Earth and Planetary Science 

Letters, v. 36, p. 359-362. 

Stuckless, J. S., Doe, B. R., and Delevaux, M., 1981, U-Th-Pb 

isotope systematics and uranium distribution within granite 

recovered from drill-hole UPH-3, northern Illinois [abs.]: 

Geological Society of America Abstracts with Programs, v. 

13, n. 7, p. 562. 

Van Schmus, W. R., and Bickford, M. E., 1981, Proterozoic 

chronology and evolution of the midcontinent region, North 

America, in Kroner, A., ed., Precambrian plate tectonics: 

Amsterdam, Elsevier Scientific Publishing Co., p. 261-296. 

Van Schmus, W. R., Hoppe, W. J., and Montgomery, C. W., 

1981 , Age and geologic significance of Precambrian basement samples from northern Illinois and eastern Iowa [ abs.]: 

Geological Society of America Abstracts with Programs, v. 

13, no. 7, p. 572. 

Van Schmus, W. R., Medaris, L. G., Jr., and Banks, P. 0., 

1975, Geology and age of the Wolf River batholith, Wisconsin: Geological Society of America Bulletin, v. 86, p. 

907-914. 

Zimmermann, R. A., 1981 , Where have all the sediments 

gone?-Fission track dating of the Illinois deep hole core 

[abs.]: Geological Society of America Abstracts with Programs, v. 13, no. 7, p. 588. 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

A PROTRACTED ARCHEAN HISTORY IN THE 

WATERSMEET GNEISS DOME, 

NORTHERN MICHIGAN 

FIGURES 

Chapter F 

By ZELL E. PETERMAN, R. E. ZARTMAN, and P. K. SIMS 

Abstract ..... 

Introduction . . . 

Acknowledgments 

Geologic framework 

Geochronology . . . 

Samples and zircons 

CONTENTS 

Results . . . . ... 

Uranium-lead discordance patterns 

Thorium-lead anomalies . 

Geochronologic implications 

Conclusions . . 

References cited . . . . . . 

Fl. Geologic map of the Marenisco-Watersmeet area .. . 

F2. Geologic map of the northern Watersmeet dome .. . 

F3. Back-scatter photograph of zircons from sample M321 

F4. Concordia plot for zircons from Late Archean rocks . 

F5. Concordia plot for zircons from Early Archean rocks 

F6. Plot of 206Pb versus 238U for zircons from Early Archean gneisses 

F7. Plot of multiepisodic lead-loss trajectories . . 

F8. Diagrammatic summary of radiometric ages . . . . . . . . . . . . . 

TABLES 

Page 

52 

53 

53 

54 

54 

54 

57 

59 

61 

61 

63 

63 

Page 

52 

53 

56 

57 

58 

59 

60 

62 

Page 

Fl. Uranium-thorium-lead data for zircons . . . . . . . . . . 55 

F2. Microprobe analyses for light and dark phases of sample M321 56 

A Protracted Archean History in the Watersmeet Gneiss Dome, Northern Michigan 51 
Abstract 

Uranium-thorium-lead dating of zircon from biotite 

gneiss in the Watersmeet dome, northern Michigan, 

adds to the previous data base for Early Archean core 

rocks in this structural dome that was rejuvenated late 

in Early Proterozoic time. The new data, together with 

previously published data on tonalitic augen gneiss, define a chord with intercepts at 3.56 and 1.25 Ga (109 

years). Zircon from an amphibolite and gneiss unit are 

dated at 2.64 Ga, establishing the presence of a volcanic 

sequence that onlapped the older gneiss terrane during 

the Late Archean. Consideration of the geologic history 

and discordia relations for all the zircons suggests that 

the observed systematics stem from multiple episodes 

of lead loss. Zircons from Early Archean gneisses are 

composed of two compositionally distinct domains that 

are present in oscillatory zones and as chaotic mixtures 

of the two. One phase is essentially stoichiometric zircon (ZrSi04), and the other is enriched in CaO, U, and 

Th and may contain about 10 percent H20. Discordance 

patterns probably are related to differential lead loss 

from these domains. Spurious Th-Pb ages indicate the 

presence of phases or inclusions in some of the zircons 

that have preferentially lost thorium relative to 208Pb. 

The geochronologic results emphasize the existence of three major rock-forming and tectonic events 

in this part of the Great Lakes tectonic zone: (1) formation of Early Archean gneisses-3.5 to 3.6 Ga; (2) accretion of the Late Archean volcanic-plutonic belt-2.6 to 

2.8 Ga; and (3) reactivation of the Early Archean basement following blanketing by Early Proterozoic supracrustal rocks-1.7 to 1.8 Ga. 

10 20 KILOMETERS 

5 

EXPLANATION 

10 MILES 

LATE PROTEROZOIC 

Volcanic and sedimentary rocks of the Keweenawan Supergroup including the 

Jacobsville Sandstone 

EARLY PROTEROZOIC 

Metasedimentary and metavolcanic rocks of the Marquette Range Supergroup 

ARCHEAN 

Puritan Quartz Monzonite 

Metasedimentary and metavolcanic rocks 

of Late Archean age (Biotite schist, gneiss, and amphibolite) 

Augen gneiss, biotite gneiss, interlayered gneiss and amphibolite, and leucogranite 

dikes (includes Early and Late Archean rocks) 

CONTACT 

FAULT 

Figure Fl. Geologic map of the Marenisco-Watersmeet area, northern Michigan (modified from Sims and others, 

1984). 

52 Shorter Contributions to Isotope Research 
INTRODUCTION 

Previous isotopic dating (Peterman and others, 

1980) identified an Early Archean gneiss near Watersmeet 

in the western part of the upper peninsula of Michigan. 

The gneiss is in the core of a dome that was reactivated 

during the Penokean orogeny and that is encircled by 

Early Proterozoic supracrustal rocks-the Marquette 

Range Supergroup. Late Archean rocks crop out a few 

kilometers to the northwest of the dome and the intervening area composes the Great Lakes tectonic zone (Sims 

and others, 1980). 

Uraunium-lead (U-Pb) zircon data established a 

minimum age of 3. 4 Ga (3 .4 x 109 years) for a 

polymetamorphosed tonalitic augen gneiss and an age of 

2.6 Ga for less deformed leucogranite dikes that intrude 

the augen gneiss and associated units within the dome 

(Peterman and others, 1980). The effects of cataclasis and 

recrystallization that accompanied rejuvenation of the 

gneiss dome are recorded in Rb-Sr (rubidium-strontium) 

whole-rock and mineral ages of about 1. 75 Ga. Uranium31 

T 46N ONTONAGON COUNTY --------------- T 45N 

lead zircon data for the Early Archean gneiss were not 

readily interpretable in terms of either a single episodic 

disturbance or continuous diffusion lead-loss model, so 

that the minimum age of 3 .4 Ga was based on the oldest 207PbP06Pb age obtained. Additional isotopic dating of the 

major units in the Watersmeet dome was undertaken to 

better understand the discordance patterns exhibited by the 

U-Pb systems and to explore the possibility of Late Archean depositional overlap on the older gneiss basement during the formation of the Late Archean greenstone-granite 

belt that is to the northwest. 

ACKNOWLEDGMENTS 

The following people are gratefully acknowledged: G. 

T. Cebula for sample preparation, Ralph Christian for the 

microprobe analyses and back-scatter photographs, S. S. 

Goldich for abrading the zircon from sample M83, and 

Loretta K wak for the analytical work. The technical reviews 

of John Aleinikoff and Kenneth Ludwig were helpful. 

R 39W 

32 

2 KILOMETERS 

1 MILE 

EXPLANATION 

EARLY PROTEROZOIC 

Michigamme Formation 

ARCHEAN 

lnterlayered amphibolite and 

biotite gneiss (Late Archean) 

Biotite gneiss (Early Archean) 

Tonalitic augen gneiss (Early 

Archean) 

Archean rocks undivided 

FAULT -Approximately located 

LIMIT OF ROCK UNIT 

LIMIT OF OUTCROP AREA 

40 STRIKE AND DIP OF FOLIATION 

X Mll 2 SAMPLE LOCALITY 

5 SECTION NUMBER 

Figure F2. Geologic map of the northern part of the Watersmeet dome showing the distribution of the major 

lithologic units and the localities of samples (modified from Sims and others, 1984). 

A Protracted Archean History in the Watersmeet Gneiss Dome, Northern Michigan 53 
GEOLOGIC FRAMEWORK 

The geology of the Marenisco-Watersmeet area (fig. 

Fl) represents a remarkably long and complex Archean 

and Proterozoic history, and coordinated field and laboratory studies have established a chronologie framework of 

rock-forming, tectonic, and metamorphic events that span 

at least 2. 5 Ga of Precambrian time. Gneiss in the 

Watersmeet dome is part of an Early Archean sialic crust 

that is extensive in the Lake Superior region (Sims, 1980). 

Late Archean greenstone-granite terranes formed marginal 

to this gneiss terrane in northern Minnesota and northern 

Michigan about 2. 7 Ga. In the Marenisco-Watersmeet 

area (fig. F1), the Late Archean units are metasedimentary 

and metavolcanic rocks that are intruded by the 2. 65-Ga 

Puritan Quartz Monzonite (Sims and others, 1977). 

In Early Proterozoic time, the composite basement 

of Early and Late Archean rocks was covered by extensive 

sedimentary and volcanic strata of the Marquette Range 

Supergroup, which subsequently were folded and 

metamorphosed during the Penokean orogeny (about 1. 8 

Ga). Metamorphism and penetrative deformation during 

this event increased in intensity from the northwest to the 

southeast in the area (fig. F1). Locally, the older gneiss 

basement rose diapirically to penetrate the overlying supracrustal rocks. This deformation also produced two 

major antiformal folds in the central and northwestern part 

of the area with cores of Late Archean basement rocks. 

In Middle Proterozoic time, the region was covered by 

sedimentary and volcanic rocks of the 1 . 1-Ga Keweenawan Supergroup. 

In this report we are concerned primarily with the 

Archean units in the Watersmeet dome (fig. F2), and the 

history of these units during the formation of the Late 

Archean greenstone-granite belt to the northwest of the 

dome. Some depositional overlap of Late Archean rocks 

on the gneiss basement is inferred from field relations 

(Sims, 1980; Sims and others, 1980). However, much of 

the evidence of such overlap and of the structural fabric 

that may have been imposed on the older gneisses during 

the Late Archean event has been more or less obscured 

by penetrative deformation and metamorphism that accompanied the Penokean orogeny. 

Late Archean igneous activity is known to have extended into the gneiss terrane where it is manifested in 

the dome by the 2.6-Ga leucogranite dikes. Small bodies 

of granite northwest of the dome, previously called the 

granite near Thayer, are now interpreted on the basis of 

lithologic correlation to be satellite bodies of the main 

mass of Puritan Quartz Monzonite. Tonalite gneiss originally included with the granite near Thayer and dated at 

2. 75 Ga (sample M147, Peterman and others, 1980) is 

interlayered with amphibolite, forming a bimodal suite of 

gneiss that is thought to be of volcanic origin. 

54 Shorter Contributions to Isotope Research 

Field studies in the Watersmeet dome (Sims and 

others, 1984) have delineated two major lithologic units 

in addition to the augen gneiss and the leucogranite dikes 

(fig. F2). Biotite gneiss crops out over an area of about 

one-half square mile just west of the main body of augen 

gneiss. Foliation and layering in this unit dip moderately 

to steeply to the north. The unit consists mainly of alternating thinly banded (on a scale of a few millimeters to 

a few centimeters) and massive biotite gneiss (layers of 

a meter or more thick). Interlayered amphibolite and biotite gneiss constitute the second unit which crops out to 

the north and south of the belt of biotite gneiss and augen 

gneiss and at Brush Lake to the west (fig. F2). A minor 

but distinctive unit of very fine grained biotite schist is 

present as thin, elongate but discontinuous, interlayers in 

the biotite gneiss. All these units are cut by leucogranite 

dikes. 

GEOCHRONOLOGY 

Data for uranium-thorium-lead (table Fl) were obtained following a method modified from Krogh ( 1973) 

of HF dissolution in teflon-lined bombs. Analytical uncertainties (1 sigma) are estimated to be about ± 0. 7 percent 

for parent-daughter ratios and ± 0.05 percent for the 

measured 207PbP06Pb. Isotope ratios and decay constants 

used in the age calculations are those recommended by 

the International Union of Geological Sciences Subcommission on Geochronology (Steiger and Jager, 1977). 

Samples and Zircons 

Samples of about 15 kg (kilograms) were collected 

from the biotite gneiss and schist unit and the amphibolite 

and biotite gneiss unit in the Watersmeet dome (fig. F2). 

Sample M321 is from an outcrop of thin- to thick-banded 

biotite gneiss that has interlayers of fine-grained biotite 

schist. In hand specimen, M321 is strongly banded with 

thin, discontinuous leucocratic layers alternating with biotite-rich layers that range in width from a few millimeters 

to a centimeter or more. Sample M203 is from the biotite 

gneiss component of the amphibolite and biotite gneiss 

unit. The sample is a medium-grained biotite gneiss but 

has thin layers of hornblende-rich gneiss. 

Zircons were separated using a shaking table, heavy 

liquids, and a magnetic separator. Concentrates were sized 

on silk screens and final purification of splits ( 1-7 mg 

(milligrams)) for isotopic analyses was done by hand picking. Clerici solution was not used in any of the steps 

because of the previously observed difficulty of removing 

this material even by acid leaching from highly fractured 

zircons. 
Table Fl. Uranium, thorium, and lead data for zircons 

[Numbers in brackets are screen sizes for the fractions. NM, nonmagnetic fractions; ABR, abraded fraction; FLUX indicates sample decomposed 

by flux technique. Data for sample M83(100-l50) are from Peterman and others (1980) and are included here for comparison with data for 

sample M83(100-150)ABR. Isotopic composition of initial lead (Stacey and Kramers, 1975) is assumed to 204Pb: 206Pb: 207Pb: 208Pb=1 :11.42:13.25:31.45 for M83 and M321 series and 1:13.64:14.69:33.37 for M203 series] 

be 

Concentration Isotopic composition of lead Atom ratio 

(age, in 10 6 (parts per million) years) 

Sample No. u Th Pb 204pb 

M203: 

(100-150) 773.4 215.7 342.0 0.075 , 

(200-250) 607.5 204.9 303.1 0.044 

(200-250)NM 546.8 189.2 279.1 0.015 

M321: 

(+100) 1680.5 294.7 692.2 0.085 

(200-250) 1289.5 359.3 686.1 0.080 

(200-250)FLUX 1288.7 403.3 692.0 0.091 

(200-250)NM 1162.3 375.2 705.7 0.046 

(-400) 1209.5 544.7 580.7 0.086 

M83: 

(100-150) 520.2 170.2 347.8 0.010 

(100-150)ABR 516.1 203.4 398.2 0.008 

Zircons from sample M321 closely resemble those 

from the tonalitic augen gneiss (samples D1042 and M83, 

Peterman and others, 1980) in shape, crystal-face development, internal structure, and color. They are amber to 

dark brown and have relatively simple prismatic and 

pyramidal faces. Fine-scale zoning parallels the principal 

crystal faces. Euhedral apatite inclusions are present, fractures are common, and many grains have zones with disseminated fine opaque inclusions. 

A split of the 200-250 mesh fraction of sample 

M321 was examined under the electron microprobe, revealing the presence of two distinctive phases that are 

present both as sharply bounded zones and in domains 

in which the two types form a chaotic microbreccia, usually in the centers of the grains. In back-scatter radiation 

images, these phases appear as light and dark domains 

(fig. F3). Microprobe analyses (table F2) indicate that the 

light phase is uniform in composition, low in trace ele-

(atom percent) 

206pb 207pb 207pb 208pb 

206pb 207pb 208pb 233u 23Su 206pb 232Th 

74.79 13.58 11.56 0.3790 8.8351 0.1691 0.1614 

[2071] [2321] [2549] [3024] 

75.851 13.68 10.43 0.4363 10.4233 0.1733 0.1493 

[2334] [2473] [2589] [2812] 

77.53 13.86 8.59 0.4588 11.1564 0.1763 0.1343 

[2434] [2536] [2619] [2547] 

71.10 15.47 13.34 0.3356 9.4581 0.2044 0.2816 

[1865] [2384] [2862] [5015] 

68.50 17.76 13.66 0.4176 14.2240 0.2470 0.2391 

[2250] [2765] [3166] [4333] 

67.99 17.41 14.51 0.4176 13.9310 0.2420 0.2246 

[2249] [2745] [3133] [4095] 

71.46 19.81 8.69 0.5003 18.6684 0.2706 0.1529 

[2615] [3025] [3310] [2878] 

69.46 16.09 14.36 0.3818 11.4928 0.2183 0.1396 

[2085] [2564] [2968] [2641] 

71.06 20.63 8.30 o. 5511 21.9498 0.2889 0.1834 

[2830] [3182] [3412] [3404] 

70.32 21.04 8.63 0.6294 25.8702 0.2981 0.1844 

[3147] [3342] [3460] [3422] 

ments, and is essentially stoichiometric zircon in which 

the major oxides sum to 100 percent. The dark phase 

is more complex compositionally and contains an additional 9-10 percent of undetermined elements. Nothing 

was detected with the energy dispersive system in sufficient concentration to account for this deficit. The proportions of Zr:Hf:Si are nearly the same in both phases, but 

the dark phase contains appreciable Ca. In this phase, 

the P20 5 content is insufficient to explain the Ca as being 

contained in apatite inclusions or intergrowths. Thorium 

and uranium were not detected in the light phase ( detection limit is about 0.03 percent uo2 and 0.07 percent 

Th02). These elements are present in variable amounts 

in the dark phase, ranging from below the detection limit 

to 0.48 percent Th02 and to 0.33 percent U02 , with average values of 0.11 and 0.21, respectively. The sharply 

defined zones of light and dark phases (fig. F3) correspond to the well-developed zoning seen in transmitted 

A Protracted Archean History in the Watersmeet Gneiss Dome, Northern Michigan 55 
Figure F3. Back-scatter radiation photographs of zircons from sample M321 (200-250 mesh fraction). The "light" 

and "dark" zones correspond to the phases indicated in table F2. 

light. The chaotic domains appear as turbid zones in plane 

light and have uniform but speckled interference colors 

under crossed nicols. 

Some grains from sample M321 show radiating fractures emanating from their cores. Many of these fractures 

are filled with the dark phase in continuity with the dark 

phase in the regular zones and in the chaotic milieu. This 

feature gives the impression of expansion of the dark 

phase producing fracturing and then infilling of this material in the fractures. The fractures could have resulted 

from hydration and expansion of the dark phase, which 

also would account for the missing 9-10 percent in the 

analyses (table F2). Hydrated phases in zircons are common (see Speer, 1980), and Sommerauer (1979) has observed that the nonstoichiometric phases of complex zircons may contain as much as 10 percent H20. 

Zircons from sample M203 are lighter in color, optically more uniform, and generally appear to be less degraded than those from sample M321 . They show simple 

prismatic and pyramidal faces but are more elongate, on 

the average, than the M321 zircons. Fine zoning is present 

and all the grains have abundant fractures. Back-scatter 

56 Shorter Contributions to Isotope Research 

Table F2. Microprobe analyses for light and dark phases 

of zircon from the magnetic 200-250 mesh fraction of 

sample M321 (200-250)M 

[Minimum detection limit for minor elements is about 0.05 percent] 

Oxide Light phase Dark phase 

(3 analyses) (8 analyses) 

Si02 32.4 29.0 

zro2 66.0 56.5 

Hf0 2 1.03 1.08 

uo2 0.00 0.21 

Th02 o.oo 0.11 

CaO o.oo 1.85 

P2o5 0.01 0.14 

Y2o3 0.28 0.79 

La 2o3 0.08 0.08 

ce 2o3 o.oo 0.14 

Sum------- 99.8 89.9 
radiation images show no evidence of compositional zoning or the presence of distinct domains. In polarized light, 

the zircons are characterized by uniform high birefringence throughout the grains. Euhedral apatite inclusions are common. 

Compositionally distinct domains or zones within 

zircons are of particular interest because of the implications to the discordance mechanism in the U-Th-Pb systems (see summaries of pertinent literature by Speer, 

1980; Gebauer and Grunenfelder, 1979). Generally, the 

less pure phases are higher in uranium, have greater magnetic susceptibilities, and yield more discordant ages than 

the stoichiometric phases. 

Results 

Data for samples M203 and M321 are plotted with 

data for previously analyzed samples on figures F4 and 

F5, respectively. Three fractions of M203 are alined along 

a chord with intercepts at 2.64 and 0.63 Ga. These data 

clearly place the amphibolite and biotite gneiss unit in 

the Late Archean with the leucogranite (sample M45) and 

the bimodal gneisses (sample M147) northwest of the 

dome. Sample Mll2 (fig. F4) is also from the amphibolite and biotite gneiss unit (fig. F4), but the data point 

departs considerably from the chord defined by sample 

M203. For all three of these arrays, we are omitting the 

statistical uncertainties derived from the regression calculations. Because of the small sample population and the 

scatter in excess of analytical error, the statistical uncertainties are magnified to the extent that they have little 

geologic significance. As will be developed later, the scatter, including the deviation of sample Mll2, probably is 

related to the Early Proterozoic event at about 1.8 Ga 

reflected in the Rb-Sr ages, and, locally, in totally reset 

zircon ages (Peterman and others, 1980). 

Data for size and magnetic fractions of sample 

M321 (fig. F5) establish the biotite gneiss and schist unit 

as Early Archean and indistinguishable in age from the 

tonalitic augen gneiss. Excluding sample M83 

(200-270)NM, data for both units define a chord with 

intercepts at 3. 56 ± 0. 04 and 1. 25 ± 0. 09 Ga using Ludwig's (1982) regression program. Excessive scatter of the 

points is indicated by an MSWD (mean square of the 

weighted deviates) of 24. 

16 

Figure F4. Uranium-lead concordia plot for zircons from Late Archean rocks. Data for samples M45 

(leucogranite), M112 (amphibolite and gneiss unit), and M147 (bimodal gneiss unit) are from Peterman 

and others (1980). Mesh-size fraction of sample is indicated in parentheses. NM, nonmagnetic fraction. 

A Protracted Archean History in the Watersmeet Gneiss Dome, Northern Michigan 57 
0.6 

~M83{ 1 00-150) NM 

~ M83{ 1 00-150) N M-ABR- RM {Calculated) 

.... , '---01042{100-150) 

~ 01042{60-80) 

_/ \ M321{200-250)NM 

/ 01042{200-270) 

~M83{200-270)M 

\ .. M321{200-250) 

/ M321{200-250) FLUX 

/\M321{-400) 

~M321{+100) 

0.2 

OOL-------~---------1LO ________ _L ________ J20 ________ -L--------~3~0--------~------~40 

zo7Pb/z3su 

Figure FS. Uranium-lead concordia plot for zircons from Early Archean gneisses of the Watersmeet dome. 

Sample M321, biotite gneiss; samples M83, 01042, augen gneiss. Except for M83 (100-150)ABR, data are 

from this paper (table F1) and from Peterman and others (1980). Mesh-size fraction of sample is indicated 

in parentheses. NM, nomagnetic; ABR, abraded; FLUX, sample decomposed by flux technique. 

Sample M83 (100-150)NM-ABR was prepared by 

mechanically abrading (for example, Krogh, 1982) a split 

of a previously analyzed sample (table F1) to remove less 

competent and metamict material or outer zones that may 

have been enriched in uranium. A 6.7-mg sample was 

abraded for 20 minutes in a diamond-lined sphere-former, 

and 1. 6 mg of zircon was recovered for analysis. The 

procedure resulted in fragmentation of most of the crystals 

and substantial rounding of these fragments. A few survived as whole crystals but these were strongly frosted 

and comers were rounded. 

As shown by the position of the two points on figure F5, the abrading process removed material that was 

more discordant than the initial fraction. The higher U-Pb 

ages in the abraded material result from greater amounts 

of radiogenic 206Pb and 207Pb. The higher 208Pb is compensated by a higher thorium content, with the net result 

that the Th-Pb ages are nearly the same before and after 

abrading. The location of the material removed during the 

abrading is calculated and is shown on figure F5. 

These results suggest that the original fraction of 

sample M83 ( 1 00-150)NM contained grains or phases 

58 Shorter Contributions to Isotope Research 

within grains with discrete 207Pb/206Pb. Insofar as can be 

determined within the errors, these phases would have 

been alined along the 3.56- to 1.25-Ga chord. The results 

of this experiment contrast with those obtained in a leaching study previously done on a split of the same sample 

(Peterman and others, 1980). The zircons were given a 

mild wash in HCl and HN03 followed by partial dissolution in HF at atmospheric pressure. The radiogenic 207Pb/ 206Pb were found to be little changed in the leachates 

and residual material although a slight reverse discordance 

was induced in the residue from HF leaching and the uranium and thorium contents were substantially lower in the 

residue. 

The zircon data for the Early Archean units show 

the commonly observed relation between discordance and 

uranium content as illustrated in figure F6. A similar pattern exists for the Late Archean zircons (not shown). Departure of the points from the 3. ~a isochron is in the 

direction of 238U gain or 206Pb loss. The lowest value 

data point near the isochron is the HF-leached residue 

of sample M83 (100-150)NM (Peterman and others, 

1980). Development of this pattern through gain of 238U 
~ <( 

a: 

(.!) 

a: 

w 

a.. 

CJ) 

w 

...J 

0 

:; 

.0 

I 

0 

-.0 

0.. 

'-0 

0 

C\J 

2.8 

2.4 

2.0 

1.6 

1.2 

0.8 

0.4 

2 4 6 

238U (lo-6MOLES PER GRAM) 

8 

Figure F6. 206Pb versus 238U for zircons from Early Archean gneisses. Data are from this paper (table F1) and from 

Peterman and others (1980). A 3.6-Ga isochron is shown 

for reference. The departure of points from the isochron 

can be explained by loss of 206Pb in approximate proportion to the 238U content. 

would require an addition in an amount roughly proportional to that originally present. More likely, the pattern 

has resulted from loss of 206Pb as a consequence of radiation damage that is proportional to the amount of 238U, 

although some degree of uranium mobility cannot be ruled 

out. The general trend of the data array with the nearly 

condordant value for the HF-leached sample suggests that 

there may be a "threshold" concentration of uranium of 

about 1-2X 10-6 moles per gram of 238U below which 

the zircons would have remained closed in spite of multiple thermal events later in their history. 

Uranium-Lead Discordance Patterns 

The discordance patterns shown by data for the various rock types (figs. F4 and F5) cannot be explained by 

one-event episodic or continuous diffusion models of lead 

loss, because all the suites show excessive scatter greater 

than analytical error and the lower intercepts are not consistent with either of these models. Zircons from the Late 

Archean units are disposed about chords with lower intercepts in the interval of 0. 7 ± 0. 1 Ga, whereas the chord 

for the Early Archean zircons intersects concordia at 

1.25 ± 0.09 Ga. The latter age might be attributed to discordance imposed during an early phase of Keweenawan 

activity, but this is special pleading and does not explain 

the intercept of the Late Archean arrays. Extrapolation 

of the near-linear interval of Tilton's (1960) continuous 

diffusion curve from a primary age of 2.65 Ga would 

intersect concordia at 0.58 Ga, which is compatible with 

the value of 0. 7 ± 0.1 for the Late Archean arrays. However, extrapolation from a primary age of 3.60 Ga gives 

a lower intercept of 0.85 Ga, clearly lower than the value 

for the Early Archean zircons. Extension of radiationdamage dependent curves (Wasserburg, 1963) would intersect concordia at even lower values. 

In the ensuing discussion we will consider the effects of discordance produced by multiple disturbances. 

This will be done in the context of the known geologic 

framework as deduced from the field relations and the 

isotopic age data. Systematic lead loss or uranium gain 

during multiple disturbances can produce spurious chords 

with upper and lower intercepts that may not correspond 

to the ages of any of the events (see Gebauer and Grunenfelder, 1979). 

All the rock units have had the same post-Archean 

history except in local zones of high strain. In addition, 

the Early Archean gneisses were deformed and 

metamorphosed during the Late Archean event. In considering a multiepisodic model for the Watersmeet zircons, 

we will set the primary age and the time of disturbances, 

and consider the effects of varying degrees of discordance 

imposed during these events. Data other than the upper 

intercept of the chord (fig. F5) indicate that the Early 

Archean gneisses are about 3.5-3.6 Ga. A firm minimum 

age of 3. 46 Ga is provided by the least discordant fraction 

of sample M83 (table F1). Samarium-neodymium model 

ages, recalculated to revised chondrite constants of Jacobsen and Wasserburg (1980) are as follows: 3.50 Ga for 

augen gneiss sample Dl042 (McCulloch and Wasserburg 

( 1980); and 3. 78 Ga for augen gneiss sample M83 and 

3.43 for biotite gneiss sample D1395 (Futa, 1981). A 

lutetium-hafnium model age of 3.51 Ga is reported by 

Patchett and others ( 1981) for a fraction of M83 zircon. 

Collectively, these numbers suggest that the primary age 

of the gneisses is probably close to the upper concordia 

intercept of 3.56 Ga (fig. F5). 

The Late Archean event corresponds to the formation of the greenstone-granite belt northwest of the dome, 

intrusion of the Puritan Quartz Monzonite and satellite 

bodies at about 2.65 Ga, and deformation and metamorphism. Relict mineral assemblages in Late Archean metagraywackes immediately north of the dome indicate that 

that metamorphic event was at amphibolite grade (Sims 

and others, 1984). The major thermotectonic event at 1.7-

1. 8 Ga, attendant with formation of the dome, is recorded 

in whole-rock and mineral Rb-Sr ages and by concordant 

zircon of 1. 76 Ga from strongly cataclasized gneiss at 

the western end of the dome (Peterman and others, 1980). 

This event also was at amphibolite grade. The timing of 

more recent lead loss, as required by the zircon arrays, 

is more difficult to establish. The last regional uplift and 

cooling of the terrane was at about 0.5 Ga as indicated 

by apatite fission-track ages (R. Zimmermann, oral commun., 1980). We will assume that lead was lost at this 

time, perhaps related to a dilation process (Goldich and 

Mudrey, 1972). 

The effects of multiepisodic lead loss on zircon arrays are discussed by Wetherill (1956, 1963), Allegre and 

A Protracted Archean History in the Watersmeet Gneiss Dome, Northern Michigan 59 
others (1974), and Aftalion and van Breeman (1980). If 

near-linear arrays are produced by multiple events, the 

discordance imposed on a particular sample during one 

event must be related in some systematic fashion to that 

imposed during earlier or later events. Discordancy is expressed by the term R=(DIP)ai(DIP)b where D and Prefer 

to daughter and parent isotopes, and a and b refer to immediately after and immediately before the event, respectively. One method of relating R's is R 1 =R2~=Rl ... 

where the subscripts, 1, 2, and 3 . . . , are sequential 

events and x and y are exponents that describe the discordance relations. 

In evaluating the multiepisodic model for the Early 

Archean units, we assign the following values for the primary age and the subsequent events: T0 =3.60 Ga, 

T1=2.70 Ga, T2=1.76 Ga, and T3=0.50 Ga. In estimating relations between Rh R2 , and R3 , we note the correlation between uranium content and degree of . discordance 

(fig. F6). The time intervals of T0 -T1 and T1-T2 are of 

nearly the same duration. By assuming similar intensities 

of events at T1 and T2 and by ignoring slightly greater 

radiation doses in the interval T0 -T1 , we will define the 

:J 

00 

M 

0.6 

:o- 0.4 0.. 

<.0 

~ 

0.2 

relation of lead loss simply as R 1 = R2 . To complete the 

exercise, R3 is defined in terms of R1 and R2 for three 

different cases: 

Case 1: R1 =R2=R3 

Case 2: R1=R2=Rl 

Case 3: R1 =R2=R3°· 5 

The trajectories for these three cases are shown on figure 

F7 with the data points for the zircons. Most of the points 

are in a zone bounded by curves for case 1 and case 

2, and, within the range of the data set, the curvatures 

of these trajectories are relatively slight. Data for the zircons from Late Archean rocks also can be explained by 

this model for which T0 ranges from 2.60 to 2.80 Ga 

with T1 at 1. 76 Ga and T2 at 0.50 Ga. These arrays are 

best explained by a case 1 calculation in which R1 =R2 . 

Although not constituting proof, these calculations show 

that the present disposition of zircon data for these rocks 

is consistent with having evolved through multiple 

episodes of lead loss in a fairly systematic fashion, and 

R,=R2=R32 

R1=R2=R3 

--- R,=R2=R3o.5 

T0=3,600 Ma 

T1=2,700 Ma 

T2=1, 760 Ma 

T3=500 Ma 

o~------~--------_L ________ _L ________ ~ ________ L_ ________ L_ ______ ~--------~ 

0 10 20 30 40 

2o7Pb/23su 

Figure F7. Calculated trajectories for multiepisodic lead loss as discussed in the text. 

60 Shorter Contributions to Isotope Research 
that the different lower intercepts can be explained by 

this mechanism. 

Thorium-Lead Anomalies 

Several Th-Pb ages previously reported (Peterman 

and others, 1980) are remarkably close to the 207Pbt206Pb 

ages, rather than to the 206Pbt238U ages as would be 

expected by nonfractionating lead loss from one-phase 

systems (Steiger and Wasserburg, 1966). These ages now 

appear to represent a general anomaly in the Th-Pb systems further illustrated by spurious ages (impossibly old) 

for two fractions each of samples M321 and M203 (table 

F1). 

The Th-Pb anomaly was first detected in sample 

M321 (20{}-250) which gave an apparent Th-Pb age of 

4. 33 Ga. A second hand-picked part of this . sample was 

reanalyzed using the older flux method in case some phase 

was not completely decomposed in the bomb with HF. 

The uranium and uranogenic-lead contents agree within 

errors in both splits (table F1). The thorium content of 

the fluxed sample is higher, but the 208Pb content also 

is higher, with the net result that the Th-Pb age still is 

anomalously high although slightly lower, at 4.10 Ga. Because of the small size of the samples used, their exact 

identity cannot be verified even though the uranium-lead 

results reproduced adequately. The differences in the 

thorium-lead results are, therefore, attributed to inhomogeneous distribution of thorium and 208Pb in the two 

splits, rather than to analytical problems with the bomb 

decomposition. These anomalous Th-Pb ages prompted 

the microprobe analyses, described earlier, to search for 

a thorium-rich phase in the zircons. No thorium "hot 

spots" were found, although thorium seems to be variably 

enriched in the nonstoichiometric phase of sample M321 

(table F2). 

Several lines of evidence indicate that the spurious 

Th-Pb ages are related to phases or inclusions within the 

zircons. The presence of discrete grain impurities in the 

separates is ruled out, as the separates used for analyses 

were hand-picked, acid-washed, and then checked again 

for purity. Also, the magnitude of the thorium-age "anomaly" correlates with physical and magnetic properties of 

the separates. For example, the difference between the 

Th-Pb and the 206Pbt238U ages increases with increasing 

magnetic susceptibility for samples where such data 

exist-M83 (200-270), M321 (200-250), and M203 

(200-250)-with the Th-Pb age always being the greater 

of the two. Abrading of sample M83 (10<J-150)NM significantly reduced the difference in these two ages 

suggesting that the suspect phase is contained in the physically less-competent material. Leaching of this fraction 

with HCl and HN03 removed unsupported 208Pb, although 

the total mass removed was quite small (Peterman and 

others, 1980). 

Microcline from sample M321 was analyzed to determine if excess 208Pb may have been present in the rock 

during the 1. 7- to 1.8-Ga metamorphism and recrystallization. The results are: 

U (ppm)=0.27 206Pbt204Pb= 16.947 (16.831) 

Th (ppm)=0.86 207Pbt204Pb= 16.210 (16.198) 

Pb (ppm)=44.3 208Pbt204Pb=36.485 (36.373) 

The numbers in parentheses are the lead-isotope ratios 

corrected for growth of radiogenic lead since 1. 76 Ga, 

the time of metamorphism indicated by concordant zircon 

from sample M48 (Peterman and others, 1980). These 

data show that the lead in the feldspar equilibrated, partially or totally, with the rock-reservoir lead in the Early 

Proterozoic. 

Although the 208Pb/204Pb in the feldspar is substantially higher than that assumed (31.45) for making common lead corrections on the zircon data, the effect on 

the 232Th/208Pb ages is relatively small and is insufficient 

to explain the spurious values by equilibration with a high 

208Pb-reservoir at 1. 7-1.8 Ga or earlier. 

Collectively, these observations suggest that phases 

or inclusions in the zircons have preferentially lost 

thorium relative to 208Pb during one or more of the events 

that disturbed the U-Pb systems. The amount of thorium 

loss for some samples is considerable. For example, sample M321( + 100) with a Th-Pb age of 5.02 Ga, would 

require 586 ppm thorium, in contrast with the actual content of 295 ppm, to support the 208Pb at an age comparable to the 206PbP38U age. Although of considerable interest, we have not pursued this anomaly further. 

GEOCHRONOLOGIC IMPLICATIONS 

The new data reported here refine the temporal 

framework for units and events within this part of the 

Great Lakes tectonic zone. Isotopic ages cluster in intervals of 1.72-1.82 Ga, 2.6<J-2.75 Ga, and 3.5<J-3.60 Ga 

and record the major rock-forming and tectonic events in 

this zone (fig. F8). 

In northern Michigan and adjacent Wisconsin, Early 

Archean rocks have been identified thus far only in the 

Watersmeet dome, although the terrane is undoubtedly 

much more extensive in the sub-Proterozoic basement. 

Isotopic resolution at 3.5-3.6 Ga is poor because of the 

effects of multiple thermal and tectonic overprints as well 

as the simple time magnification of relatively small 

analytical uncertainties in the determinations. However, 

the additional zircon data for the biotite gneiss unit (sample M321) in the Watersmeet dome allows a less ambiguous designation of the primary age than was possible previously (Peterman and others, 1980). Even with the likelihood of discordance imposed by multiple episodes of lead 

A Protracted Archean History in the Watersmeet Gneiss Dome, Northern Michigan 61 
Michigamme and Copps Formations (Early Proterozoic) I I I I T I I I I I I 

Rb-Sr whole rock isochron1 • 

Puritan quartz monzonite (Late Archean) 

Rb-Sr whole rock isochron2 • 

Metagraywacke (Late Archean) 

Rb-Sr model age 1 ----- Gneiss and amphibolite unit (Late Archean) 

Rb-Sr whole rock and mineral isochron3 • U-Pb zircon discordia3 • 

Watersmeet dome 

Leucogranite dikes (Late Archean) 

U-Pb zircon discordia3 • 

lnterlayered amphibolite and gneiss (Late Archean) 

U-Pb zircon discordia4 • 

Biotite gneiss unit (Early Archean) 

Rb-Sr whole rock and mineral isochron3 • 

U-Pb "metamorphic" zircon3 • 

U-Pb zircon discordia4 • 

Tonalitic augen gneiss (Early Archean) 

Rb-Sr whole rock and mineral isochron3 • 

U-Pb zircon discordia4 • 

Nd-Sm model ages •• • 

Lu-Hf model age (zircon)s 

I I I I I I I I I I • I 

1 Sims and others (1984). 

2Sims and others {1977). 

3Peterman and others (1980). 

4This paper. 

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 

AGE (Ga) 

5McCulloch and Wasserburg (1980); Futa {1981 ). 

6Patchett and others {1981 ). 

Figure F8. Summary of radiometric ages in the Watersmeet-Marenisco area. The dashed bar indicates the 

uncertainty in the model age. 

loss, as discussed earlier in the paper, we can conclude 

with some confidence that the age of the augen gneiss 

and associated biotite gneiss is most likely to be about 

3.6Ga. 

Evidence of thermotectonic events in the interval 

from 3.6 Ga to the Late Archean has not been found. 

There may have been such events, but their isotopic or 

lithologic products have been so modified by Late Archean or Early Proterozoic overprinting that they are no 

longer recognizable, or this Early Archean terrane may 

indeed have had a long history (about 3.6--2.8 Ga) of 

thermal and tectonic stability. 

The Late Archean event, 2.60-2.75 Ga (fig. F8), 

was manifested in volcanism, partly submarine, clastic 

sedimentation in rapidly subsiding troughs, folding and 

metamorphism, and extensive granitic plutonism. In the 

Archean rocks southeast of Marenisco (fig. F1), the zircon 

age of 2. 75 Ga represents accumulation of a bimodal sequence of volcanic rocks now metamorphosed to interlayered felsic gneisses and amphibolites. A similar assemblage, but perhaps slightly younger, onlapped the 

older gneiss terrane, and this unit is dated by zircon from 

sample M203 at 2.64 Ga. The significance of the apparent 

62 Shorter Contributions to Isotope Research 

age difference between 2. 64 and 2. 7 5 Ga depends upon 

the influence of the 1. 76--Ga event on the discordia patterns as previously discussed. Although the apparent concordia-intercept values may be slightly lower than the real 

ages, the difference between the two rock sequences probably is real. Accumulating data for greenstone belts in 

the Superior Province indicate protracted volcanic histories in some areas (Davis and others, 1980; Nunes and 

Pyke, 1980; Nunes and Thurston, 1980; and Nunes and 

Wood, 1980), although in others extremely thick sequences were erupted over short intervals of time (Nunes 

and Jensen, 1980). The Puritan Quartz Monzonite, 

emplaced late in the cycle, is typical of the granitic components of Late Archean greenstone-granite belts. Leucogranite dikes in the Watersmeet dome are considered to 

be related to this period of granitic plutonism. 

The geologic record between the time of stabilization of the composite Archean crust and the inception of 

deposition of the Marquette Range Supergroup is lacking 

in this area. By analogy with northern Minnesota (Goldich 

and others, 1961), the Archean basement probably had 

stabilized by 2.5 Ga. The depositional interval of the Marquette Range Supergroup is not firmly established, but 
an upper age of about 1. 9 Ga commonly is accepted based 

largely on correlative volcanic rocks in Wisconsin (Van 

Schmus, 1980). In the Marenisco-Watersmeet area, these 

supracrustal rocks were deformed and metamorphosed 

during the Penokean orogeny. In the Watersmeet dome, 

ages between 1. 72 and 1. 76 Ga (fig. F8) represent diapiric 

uplift and locally concentrated shearing and cataclasis of 

the core rocks. Earlier formed structures in the Archean 

rocks were overprinted and locally obliterated by this tectonism. The 1. 72- to 1. 7~a Rb-Sr whole-rock and mineral ages and U-Pb ages of metamorphic zircon for rocks 

in the dome are slightly lower than the main phase of 

the Penokean orogeny as defined in central Wisconsin by 

the eruption of volcanic rocks and the emplacement of 

synorogenic intrusions between 1. 82 and 1. 86 Ga (Van 

Schmus, 1980). In Minnesota, igneous activity was continuous in the interval of 1.75-1.85 Ga (Goldich, 1981), 

and in central Wisconsin, late- to post-orogenic granites 

were emplaced at 1. 76 Ga. The penetrative deformation 

and diapiric uplift of the core of the Watersmeet dome 

appears to be synchronous with this anorogenic magmatic 

event. Apparently, Penokean stresses continued to be focused along the Great Lakes tectonic zone 60-100 Ma 

after synorogenic plutonism had ceased to the south. 

CONCLUSIONS 

Additional isotopic dating of zircons from gneisses 

in the core of the Watersmeet dome has refined the temporal framework of events that are recorded in this small 

inlier of Early and Late Archean rocks: 

1. Early Archean rocks in the dome are tonalitic 

augen gneiss and layered to massive biotite gneiss. Uranium-lead data for zircons from these units define an approximately linear relation on a concordia diagram with 

intercepts at 1.25 and 3.56 Ga. The upper intercept is 

thought to be close to the primary age. The near-linear 

array of points is interpreted to be the result of systematic 

lead loss in response to events at 2.7 Ga, 1.8 Ga, and 

during a younger event, perhaps 0.5 Ga. Consequently, 

the lower intercept of 1.25 Ga has no geologic significance. The zircons from these gneisses are composed of 

two discrete phases associated in fine zoning and in chaotic mixtures. One phase is stoichiometric zircon, and the 

other contains appreciable calcium and probably is hydrated. The nonstoichiometric phase has probably lost more 

radiogenic lead, relative to uranium, than the stoichiometric phase. 

2. Uranium-lead data for zircons from an interlayered gneiss and amphibolite unit in the dome establish 

the age as Late Archean (2.64 Ga) indicating that volcanic 

rocks were deposited at least locally on the older gneiss 

basement during the formation of the marginal Late Archean greenstone belt. All units in the dome are cut by 

leucogranite dikes dated at about 2.6 Ga. The Late Archean zircons also were affected by an event at about 1. 8 

Ga and at a more recent time. However, the upper intercept ages probably are close to the primary ages of the 

units. 

3. Thorium-lead ages of several fractions of Early 

and Late Archean zircons are impossibly old. Analytical 

problems and addition of 208Pb are ruled out as causative 

factors. More likely, the Th-Pb age anomalies are the result of inclusions or domains in the zircons that have preferentially lost thorium relative to 208Pb during one or more 

of the events that disturbed the U-Pb systems. 

REFERENCES CITED 

Aftalion, M., and van Breeman, 0., 1980, U-Pb zircon, monazite and Rb-Sr whole rock systematics of granitic gneiss 

and psammitic semi-pelitic host gneiss from Glenfinnan, 

northwestern Scotland: Contributions to Mineralogy and Petrology, v. 72, p. 87-98. 

Allegre, C. J. , Albarede, F., Grunenfelder, M., and Koppel, 

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creation of more concordant systems using an air abrasion 

A Protracted Archean History in the Watersmeet Gneiss Dome, Northern Michigan 63 
technique: Geochimica et Cosmochimica Acta, v. 46, p. 

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computer and HP-9862 plotter: U.S. Geological Survey 

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studies of Archean gneisses and lower Proterozoic rocks, 

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1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

AGE OF THE BASEMENT STAUROLITE-BIOTITE 

SCHIST OF NORTHEASTERN SOUTH DAKOTA 

FIGURES 

Chapter G 

1 . 2 By j. R. RICHARDS, D. M. BARON, and S. S. GOLDICH 

Abstract ..... 

Introduction . . . 

Acknowledgments 

CONTENTS 

Location of drill-hole site and description of core 

Analytical procedures 

Results ........ . 

Discussion . . . . . . . 

A geologic model 

Summary 

References cited . . . . 

Page 

66 

67 

67 

67 

69 

69 

69 

73 

74 

75 

Page 

G 1. Geologic map of the Precambrian rocks of east-central South Dakota 66 

02. Vertical section of core of staurolite-biotite schist . . . . . . 68 

G3. Rubidium-strontium isochron diagram . . . . . . . . . . . . . 70 

04. Plot of variations of modal sericite+ muscovite and of chlorite 71 

G5. Map showing structural trends of Archean rocks in northeastern South Dakota and adjacent parts of 

Minnesota and North Dakota . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 

G6. Diagram showing comparative radiometric ages of some Archean rocks from southern Ontario, northern 

Minnesota, and Bristol, South Dakota . . . . . . . . . . 72 

G7. Plot showing possible explanation of the rubidium-strontium data . . . . . . . . . . . . . . . . . 73 

TABLES Page 

G 1 . Modal analyses of staurolite-biotite schist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 

G2. Rubidium-strontium analytical data for whole-rock samples . . . . . . . . . . . . . . . . . . . . 68 

G3. Rubidium-strontium and potassium-argon analytical data and ages for biotite from sample 1738 70 

1

Research School of Earth Sciences, Australian National University, Canberra, Australia. 2

Johnson Division, Universal Oil Products, Inc., St. Paul, Minnesota, U.S.A. 

Age of the Basement Staurolite-Biotite Schist of Northeastern South Dakota 65 
Abstract 

Rubidium-strontium analyses of whole-rock samples of staurolite-biotite schist cored in a hole drilled 

near Bristol, South Dakota, indicate an Archean age 

(>2,500 million years) for the basement rock. Low-grade 

metamorphism has altered biotite to chlorite, staurolite 

to sericite, and garnet to chlorite and sericite. The northeasterly trend of the basement rocks in South Dakota, 

as determined from geophysical surveys, is alined with 

the structural trend of the Precambrian basement of Minnesota, and the rubidium-strontium age permits correla102° 

I 

I 

t----

1 

I 

I 

I 45° t-

, 

I 

I 

I 

I I 

, _____ J_ __ _ 

: I ~ I 

~..L ____ l ______ ... ..............,,... ..................... 

EXPLANATION 

PROTEROZOIC 

Sioux Quartzite, exposed in Sioux 

Falls area 

Silicic volcanics 

NEBRASKA 

INFERRED CONTACT 

INFERRED FAULT 

tion of the staurolite-biotite schist with Archean 

metasedimentary rocks in northern Minnesota and in the 

Rainy Lake district of Ontario, Canada. The new data 

suggest that a regional high-grade (amphibolite facies) 

metamorphism may have affected the Archean 

metasedimentary and metavolcanic rocks in a broad belt 

extending from Ontario southward into northeastern 

South Dakota. The regional metamorphism was followed 

by the emplacement of postkinematic granitic rocks that 

in northern Minnesota and southwestern Ontario have 

been dated at about 2,650 Ma. Similar 2,600-Ma postkinematic rocks are present in the Minnesota River Valley. 

Granite Falls 

1.80 z 

o 2.50 R DRILL HOLE-Showing age (Ga) 

PROTEROZOIC AND ARCHEAN 

~ Metamorphic belts, undivided 

50 100 KILOMETERS 

ARCHEAN 

Granite 

Granite and gneiss, exposed near 

M ilbank 

Granitic gneiss 

Mafic schist 

X 1

·

98 8 SURFACE SAMPLE 

50 MILES 

R Whole rock 

B Biotite 

Microcline 

z Zircon 

Figure Gl. Geologic map of the Precambrian rocks of east-central South Dakota (from Lidiak, 1971) showing location 

of drill-hole site near Bristol. Isotopic ages are from the present paper and from Goldich and others (1966, 1970). 

66 Shorter Contributions to Isotope Research 
Table Gl. Modal analyses of staurolite-biotite schist 

[Modes are volume percentages based on 300 or more points. Leaders(---), not found on point-count traverses; tr, less than 0.3 percent] 

Sample No.-------- 1613 1618 1628 1640 1663 

Quartz------------- 20 27 29 23 32.4 

Plagioclase-------- 41 10 7 22 18.1 

Biotite------------ 24 12 9 12 20.9 

Chlorite----------- 5 33 31 25 0.5 

Staurolite--------- --- 20.9 

Garnet------------- tr tr tr 

Opaque minerals---- 0.3 6 0.6 7 1.6 

Sericite----------- 9 5 23 6 5.3 

Muscovite---------- 0.4 5 2 

Potassium-feldspar- ---

Tourmaline--------- 0.3 1. 5 0.3 3 0.3 

Zircon------------- --- 0.5 

INTRODUCTION 

This paper presents the results of Rb-Sr (rubidiumstrontium) dating of staurolite-biotite schist and amphibolite cored in a hole drilled about 1 mi (mile) east of Bristol, northeastern South Dakota (fig. Gl). A magnetic 

anomaly found in a vertical-intensity magnetometer survey 

by the South Dakota Geological Survey (Petsch, 1965, 

1967) was investigated by the American Metal Climax 

Company3

, and two holes were drilled on the anomaly 

in 1964. One hole penetrated about 1 ,556 ft (feet) of glacial drift and Mesozoic strata before reaching Precambrian 

crystalline basement rock, of which 403 ft was cored. 

On expiration of its lease, the company transferred this 

core to the South Dakota Geological Survey. The core 

was studied by Baron (1971), and, during the course of 

this work, in 1970, he visited the Little Falls area in Minnesota to examine exposures of the Proterozoic staurolite 

schist that had been dated by the K-Ar (potassium-argon) 

method at about 1,600 Ma (million years) (Goldich and 

others, 1961, p. 104). By chance Baron and Goldich met 

on an outcrop in Little Falls and they agreed that 

radiometric dating of some samples from the Bristol core 

would be undertaken at Northern Illinois University in DeKalb, Ill. The work at Northern Illinois University was 

done by J. R. Richards (J.R.R.) who at the time was 

Visiting Professor of Geology on leave from the Australian National University. The preliminary age obtained 

from an isochron of six samples was 2,500 Ma, which 

indicated that the staurolite-biotite schist basement of 

northeastern South Dakota is of Archean age and is not 

correlative with the staurolite schist of the Little Falls 

area, Minnesota. 

Recently, the samples that were originally analyzed 

at Northern Illinois University and some additional sam3

Use of company and trade names is for descriptive purposes only 

and does not imply endorsement by the U.S. Geological Survey .. 

1667 1672 1680 1695 1707 1827 1830 1952 

35 35 40 35 19 21.7 32.3 31.4 

25 17 7 8 15 39.5 42.4 20.4 

29 13 22 36 18 17.8 22.3 7.4 

tr 27 0.2 3 2 0.4 1.5 19.6 

5 18 5 25 12.3 

0.5 2 1 

3 

2 

tr 

5 2 1 1.2 2.6 

1 4 3 13 6 16.4 

4 2 

2 4 3 0.4 0.3 

3 tr 

pies, including a biotite concentrate, were analyzed 

(J.R.R.) in the geochronologic laboratory of the Research 

School of Earth Sciences, Australian National University. 

Description of the core, modal analyses, and other 

mineralogical data are from Baron ( 1971). The final manuscript was prepared in Denver by S. S. Goldich. 

ACKNOWLEDGMENTS 

We thank W. Compston and I. McDougall for use 

of facilities at the Australian National University and D. 

Millar, R. Maier, and Z. Roksandic for assistance with 

the isotopic analyses. We are indebted to Professor V. 

J. Ansfield of the University of South Dakota and to the 

South Dakota Geological Survey for their support. K. R. 

Ludwig, C. E. Hedge, and Z. E. Peterman of the U.S. 

Geological Survey assisted materially in discussions and 

in critical reviews of the manuscript. 

LOCATION OF DRILL HOLE SITE AND 

DESCRIPTION OF CORE 

The drill hole 1 mi east of Bristol in Day County, 

S. Dak. (fig. Gl) is situated in the NE1/4NE1/4SW1/4 sec. 

29, T. 122 N., R. 57 W. (lat 45°20.9' N., long 97°42.3' 

W.). The basement schist was cored (diameter, ~ 1.5 in.) 

with approximately 95 percent recovery, and, on the average, about 6 in. (inches) of core was miss~ng from each 

box that contained 10-ft sections. For this reason, the 

depth below the surface of the midpoint of each samp~e 

taken from the core is given to the nearest foot, and this 

depth is used as the sample number (table Gl, fig. G2). 

The Bristol core is a fine-grained porphyroblastic 

staurolite-biotite-plagioclase-quartz schist with minor 

amounts of opaque minerals (magnetite-ilmenite), garnet, 

tourmaline, calcite, and accessory zircon and apatite. The 

Age of the Basement Staurolite-Biotite Schist of Northeastern South Dakota 67 
DEPTH 

(FEET) 

SURFACE 

"" 

(/) 

LU 

(/) 

~ <( 

z 

<( 

...J 

~ 0 

:E 

1556 

1600 

1700 

1800 

1900 

1959 

Top 

-1613 

-1618 

-1628 

-1640 

_1663 

-1667 

-1672 

-1680 

1-1695 

1-1707 

-1827 

-1830 

-1952 

Bottom 

"" 

s - s -

s -

1582 

1584 

1590 

s -1 

m -1 

e -1 

e -1 

Act- 1 

Hbl- 1 

s -1 

e - 1 

s -1 

s -1 

663 

707 

729 

7388 

786 

790 

830 

848 

888 

895 

(/) 

LU 

(/) 

>- ...J 

<( 

z 

<( 

U5 

.h 

c::: 

Figure G2. Vertical section of core of staurolite-biotite 

schist near Bristol, S. Dak. (fig. G1) showing position 

from which samples were taken. Numbers are those assigned to samples and represent depth, in feet. Degree 

of alteration of staurolite in rubidium-strontium samples: 

s <10 percent; m, -25 percent; e, 70-85 percent. Sample 17388, biotite concentrate; Act, actinolite schist; Hbl, 

hornblende schist. 

68 Shorter Contributions to Isotope Research 

name staurolite-biotite schist is used here for convenience. 

Throughout the length of the core the staurolite is altered 

in part or completely to sericite, biotite to chlorite, and 

garnet to chlorite or to sericite and chlorite. Modal 

analyses of 300 or more point counts of nine samples 

from the interval between 1,600 and 1,700 ft (fig. 02) 

show large differences in mineral composition. Although 

thin sections were examined, only a few modal analyses 

(table 01, fig. 02) were made of the core below 1, 700 

ft. Sampling of the core for Rb-Sr analyses, however, 

was more evenly distributed over the length of the core 

(fig. 02). 

Table G2. Rubidium-strontium analytical data for wholerock samples 

Concentration 

Sample (microgram/sram) Atomic ratio 

No. Rb Sr 87Rb/86sr 87sr/86sr 

1582 94.7 245.5 1.1178 0.7419 

1584 92.0 199.3 1,3374 0.7504 

1590 92.1 226.0 1.1810 0.7446 

1663 96.5 430,4 0.6483 0.7254 

1707 100.1 332.4 0.8719 0.7336 

1729 96.9 193.7 1.4518 0.7548 

1786 127.3 172.2 2.1484 0.7699 

1790 73.3 784.7 0.2698 0,7107 

1830 . 90.2 268.9 0.9712 0.7370 

1848 88.3 243.2 1.0511 0.7405 

1888 86.5 287.4 0.8715 o. 7339 

1895 91.1 225.7 1.1698 0.7447 

Examination with petrographic and binocular microscopes led to the conclusion that all the rock in the core 

was derived by metamorphism of sedimentary material, 

probably a graywacke-argillite sequence. The most prominent variations in rock type are a 6-in. layer of quartzite 

at a depth of 1,612 ft and two thin (less than 1 ft) amphibolitic layers. A thin actinolite-rich layer is at 1, 786 

ft, and a hornblende-rich layer at 1, 790 ft. Some calcite 

was found in both layers, and they probably were derived 

from calcareous sediments. The two amphibolitic layers 

were included in the Rb-Sr sampling (fig. 02). 

Biotite and staurolite give the rock a good schistosity, and, if the attitude of the core is taken as vertical 

from which departures probably were small, the foliation 

ranges from horizontal to a high dip of about 65°, which 

is characteristic of the greater part of the core. Porphyroblasts of biotite, as well as of staurolite, continued to 

grow after the formation of the foliation, and crystals of 

both minerals are oriented across the metamorphic structure. Kinked biotite also is present and indiCates a later 

period of deformation or shearing prior to the hydrothermal alteration of the biotite to chlorite. 
Quartz is always xenoblastic and averages about 

0.03 mm (millimeters) in diameter. Some quartzose veins 

are parallel to the foliation, but many are crosscutting. 

Like quartz, plagioclase is xenoblastic and averages about 

0.03 mm in diameter. The matrix plagioclase nearly always is untwinned. Some larger plagioclase grains in the 

small quartzose veins, which cut across the foliation, are 

twinned and these grains range in composition from An0 

to An20. On the average, quartz and sodic plagioclase 

compose 50 percent of the schist but the range is large 

(36-75 percent, table Gl). 

Biotite is an abundant mineral forming porphyroblasts that are subidioblastic and that range in length from 

1 to 2.5 mm. The finer grained matrix biotite (0.07-0.15 

mm) is xenoblastic. Quartz and zircon are the common 

inclusions in biotite. Biotite is altered to chlorite, and the 

abundance of the two minerals is complimentary; an apparent increase in chlorite is accompanied by a decrease 

in the percentage of biotite. 

Staurolite is a conspicuous mineral but ranges considerably in abundance. The porphyroblasts are 4-10 mm 

long; all show sieve texture (poikiloblastic) with inclusions of quartz, opaque minerals, garnet, and apatite. 

Commonly the inclusions have helicitic texture. The 

staurolite commonly is altered partially or completely to 

sericite. Garnet is present in porphyroblasts from 0.5 to 

2 mm in diameter. Like the staurolite, the garnet is 

poikiloblastic and commonly is helicitic. Partial alteration 

of garnet to chlorite and sericite is common, but fresh 

to slightly altered garnet is present in specimens in which 

the staurolite has been converted completely to sericitic 

pseudomorphs. 

In addition to the abundant fine-grained sericite, 

muscovite ( ---0.06 mm in diameter) is present in small 

amounts that could not be concentrated for purposes of 

radiometric dating. Potassium-feldspar is absent and could 

not be detected by staining, except for a small amount 

in deep sample 1952. Tourmaline is present in trace 

amounts throughout the core, but is more abundant in 

quartzose veins, one near the top at a depth of 1,648 

ft and another at the bottom at a depth of 1,957 ft. 

The staurolite schist represents a terrane that was 

metamorphosed regionally to the staurolite-almandine amphibolite grade. Subsequent superimposition of low-grade 

metamorphism resulted in an extensive greenschist-facies 

mineral assemblage: quartz-albite-biotite-chlorite-sericite. 

The intensive alteration of staurolite to sericite suggests 

some mobility of potassium and possibly other elements. 

Further discussion is deferred to a later section. 

ANALYTICAL PROCEDURES 

The analytical procedures for the isotopic analyses 

are similar to those developed over the years in a number 

of geochronology laboratories. A mixed 87Rb-84Sr spike 

was used. In the earlier work, some difficulty was exp~!rienced in dissolving the staurolite, but two dissolution procedures were found to be effective: prolonged refluxing 

during several days of a HN03-HF mixture, and the teflon pressure assembly (Krogh, 1973). The argon isotopic 

analyses were made by the method of McDougall (1966). 

Potassium was determined by flame photometry. The ages 

were calculated using decay constants and atomic abundance ratios recommended by the International Union of 

Geological Sciences Subcommission on Geochronology 

(Steiger and Jager, 1977). 

Linear regressions of the whole-rock Rb-Sr data 

were made in the U.S. Geological Survey laboratory in 

Denver using the program of Ludwig (1982). The 2-sigma 

errors of 1 percent for 87Rb/86Sr and 0. 02 percent for 87Sr/86Sr used in the regressions were supplied by the 

Australian National University laboratory (J .R.R.). 

RESULTS 

The Rb-Sr data (table G2) for all the whole-rock 

samples are plotted in the isochron diagram of figure G3. 

A regression of the data for 11 samples, excluding 

sample 1786, gives an age of 2,560 ±55 Ma, 

Ri =0. 7011 ± 0.0008 (model 3). If both amphibolite samples (1786 and 1790) are excluded, the 10 samples of 

staurolite-biotite schist give an age of 2,510±70 Ma, 

Ri=0.7020±0.0011 (model 3). The difference of 50 m.y. 

is within the analytical errors of the isochrons (fig. G3, 

lines A and B). A tie-line connecting the two amphibolite 

samples gives an approximate age of 2,180 Ma, 

Ri=0.7022 (fig. G3, line C). 

The Rb-Sr and K-Ar apparent ages (table G3) for 

biotite (sample 1738) are in good agreement at 2,440 Ma. 

The Rb-Sr age was calculated with Ri=0.7020 from the 

whole-rock isochron, but a lower value would not appreciably affect the age. The Rb-Sr and K-Ar ages, however, 

are lower than the isochron ages, and together with the 

amphibolite data clearly show that both systems (Rb-Sr 

and K-Ar) have been disturbed. 

DISCUSSION 

In addition to the metamorphism that produced the 

staurolite-biotite schist, there is evidence for two successive events. The Rb-Sr and K-Ar biotite ages of 2,440 

Ma suggest an event that preceded the low-grade 

metamorphism. The extensive alteration of staurolite, and, 

to a lesser extent of garnet, to sericite indicates the mobility of potassium during the low-grade metamorphism. Potassium was released during the alteration of biotite to 

chlorite, and this process would, of course, also release 

rubidium, so that the movement of rubidium must be conAge of the Basement Staurolite-Biotite Schist of Northeastern South Dakota 69 
0.75 

LINE AGE (Ma) 

A 

B 

c 

2,560±55 0. 7011 ±0.0008 

2,510±70 0.7020±0.0011 

2,1 80 0. 7022 

0.70L_ ______________ L_ ______________ L_ ______________ L_ ______________ L_ ____________ ~ 

0 0.5 1.0 1.5 2.0 2.5 

87Rbj86Sr 

Figure G3. Rubidium-strontium isochron diagram for whole-rock samples of staurolite-biotite schist (solid 

circles) and amphibolite (triangles). Line A includes all samples except 1786; line B, all samples except 

1786 and 1790; line C connects samples 1786 and 1790. 

Table G3. Rubidium-strontium and potassium-argon 

analytical data and ages for biotite from sample 1738 

[*Radiogenic + Ri=0.7020] 

Concentration 

(microgram/gram) 

Rb Sr 

317.3 11.33 

K 

(weight percent) 

7.350 

7.332 

Rubidium-strontium 

Atomic ratios 

112.1 4.659 

Potassium-argon 

*40Ar *40Ar x 100 

(1o-10 mole/gram) Total 40Ar 

657.4 99.9 

654.9 100 

Age (Ma) 

2,443±5o+ 

Age (Ma) 

2,438±52 

2,432±46 

sidered as a mechanism that could affect the Rb-Sr system. The alteration was not pervasive as can be seen in 

the plot (fig. 04) of the modal percentages of sericite+ muscovite and of chlorite. A similar pattern is noted 

70 Shorter Contributions to Isotope Research 

in the metasomatic replacement of staurolite by sericite 

(fig. 02). Locally the staurolite is extensively to completely altered, but in adjacent parts of the core it is unaltered. The alteration pattern suggests that the hydrothermal fluids were channeled by porosity that was formed 

during shearing of the staurolite-biotite schist. 

The 10 analyzed staurolite-biotite samples average 

93 ppm (parts per million) rubidium and range from 86 

to 110 ppm. Strontium ranges from 194 to 430 ppm with 

an average of 265 ppm. Biotite (sample 1738B) contains 

317 ppm rubidium and 11 ppm strontium (table 03). Biotite + chlorite range from 20 to 40 percent in the modes 

(table 0 1), and the rubidium in the staurolite-biotite schist 

can be accounted for almost entirely by the biotite. The 

samples for Rb-Sr analysis were selected (D. M. Baron) 

on the basis of the degree of apparent alteration. Seven 

of the samples show slight ( < 10 percent) alteration of 

staurolite to sericite, one has moderate ( ~25 percent) and 

two extensive (70-85 percent) alteration (fig. 02). The 

ages of the whole-rock samples, however, show little ap-
1 720 L__----L._ _ _L___J _ _____L_ _ _L___L _ ____j_____J 

0 10 20 30 40 

MODAL PERCENT 

Figure G4. Plot of variations of modal 

sericite+ muscovite (open circles) and of 

chlorite (solid circles) with depth (data in 

table G1). 

parent variation as is indicated by the close adherence 

of the data points to the 2, 51 0-Ma isochron (line B, fig. 

G3 ). A reasonable conclusion is that in the analyzed samples rubidium and any radiogenic 87Sr that had been 

formed prior to the time of the low-grade alteration were 

trapped rather uniformly in the sericite. This conclusion 

finds some support in the general correlation of the variations of sericite+ muscovite and those of chlorite (fig. 

G4). The low modal percentages of sericite and the much 

larger amounts of chlorite in parts of the core (see for 

example sample 1672, table G 1 , fig. G4), however, are 

deviations from the pattern and suggest that potassium and 

rubidium were redistributed within the schist. 

Model ages calculated for the actinolite schist (sample 1786) and the hornblende schist (sample 1790), using 

Ri=0.7020, are 2,190 Ma and 2,240 Ma, respectively. 

Using Ri = 0. 7011 (line A, fig. G 3) the model ages are 

2,220 Ma and 2,460 Ma, respectively. An explanation 

for the low ages of the amphibolite samples is rubidium 

gain during the hydrothermal alteration. The net gain in 

rubidium in the hornblendic layer was relatively small, 

but it was much larger in the actinolite layer that, on 

the basis of the common assignment of actinolite to lower 

grade than hornblende, may have been formed during the 

alteration. The time of alteration when potassium and 

rubidium became mobile, however, cannot be stated precisely. The time may have been about 2,250 Ma, or it 

may be related to a much younger event, in which case 

the apparent age represents a partial resetting of the Rb-Sr 

system. 

Staurolite schist in the Rice Bay area in the Rainy 

Lake district, Ontario (fig. G5) has been described by 

Harris (1974). The staurolite usually is identifiable only 

by the crystal forms; the white-mica pseudomorphs are 

from 2 to 10 mm long. Harris (1974) concluded that the 

staurolite was formed by metamorphism of pelitic rocks 

to the almandine-amphibolite facies and was altered by 

retrogressive metamorphism to white mica. Gamet was 

altered to chlorite. The metasedimentary rocks in the area, 

the Coutchiching and Seine Series of Lawson (1913), 

were intruded by postkinematic granite (Algoman of Lawson). The Rb-Sr isochron ages (fig. G6) for the Coutchiching metasedimentary, Keewatin meta volcanics, and Algoman granitic rocks are 2,560 Ma, 2,540 Ma, and 2,485 

Ma, respectively, but the U-Pb (uranium-lead) concordia 

age for zircon from the granite is 2,650 Ma. Rubidiumstrontium ages on muscovite from the granite range from 

2,545 to 2, 700 Ma and support the older U-Pb age. Peterman and others (1972) concluded that the Rb-Sr ages have 

been affected by younger metamorphic processes. The low 

Rb-Sr age for the Algoman granite is attributed to lowgrade metamorphism. 

The Newton Lake Formation (metasedimentary) and 

the Ely Greenstone (metavolcanic) of northern Minnesota 

(fig. G5) have been dated by Jahn and Murthy (1975). 

Postkinematic rocks in the Giants Range Granite have 

been dated by Catanzaro and Hanson ( 1971) and Prince 

and Hanson (1972) and in the Vermilion Granitic Complex by Peterman and others (1972). The Rb-Sr isochron 

ages for these various lithologic units are shown in figure 

G6. The Rb-Sr ages are lower than the U-Pb ages. 

The postkinematic granitic rocks set a limit on the 

time of metamorphism of the metasedimentary and 

metavolcanic rocks in northern Minnesota and in southwestern Ontario. Similar data are not available from 

northeastern South Dakota, but data are available from 

the Precambrian rocks in the Minnesota River Valley 

about 125 mi southeast of Bristol. A major metamorphic 

event has been dated by Rb-Sr and U-Pb age measurements at 2,600 Ma. Aplite and late postkinematic granite 

Age of the Basement Staurolite-Biotite Schist of Northeastern South Dakota 71 
SASKATCHEWAN 

.. __ _ I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

NORTH DAKOTA 

t--------

1 

I 

I 

I 

I 

I 

I 

I 

I 

SOUTH DAKOTA 

L-----

MANITOBA I 

--~! ____________ j ' h UNITED STATES \ \ 

\ 

Little Falls 0 

ST. PAULo 

ONTARIO 

., 

1 DO MILES 

EXPLANATION 

1 Rainy Lake district 

2 Vermilion Granitic Complex 

3 Giants Range Granite 

4 Newton Lake Formation 

5 Ely Greenstone 

6 Schist at Bristol 

< < Partial extent of Vermilion 

< Granitic Complex and Giants 

Range Granite 

Figure G5. Map showing structural trends of Archean rocks in northeastern South Dakota and adjacent parts of 

Minnesota and North Dakota. Trend lines are based on figure G1, regional magnetic and gravity trends, and on 

unpublished data. 

SOUTHERN ONTARJ01 

Algoman granite 

Algoman granite (U-Pb) 

Keewatin metavolcanics 

Coutchiching metasedimentary rocks 

Biotite (Rb-Sr, K-Ar range) 

NORTHERN MINNESOTA 

Vermilion Granitic Complex1 

Giants Range Granite2 

Giants Range Granite (U-Pb) 

Newton Lake Formation3 

Ely Greenstone3 

Biotite (Rb-Sr, K-Ar range) 

NORTHEASTERN SOUTH DAKOTA 

Schist at Bristol ( 10 samples) 

Schist at Bristol (11 samples) 

Biotite (Rb-Sr, K-Ar) 

1 Peterman and others (1972). 

2Prince and Hanson (1972). 

3Jahn and Murthy (1975). 

AGE (1 os YEARS) 

2.2 2.3 2.4 2.5 2.6 2. 7 

I I I I I I 

0.7015±9 

-----.6-

----- 0.7005±9 __..,_ 0.7014±11 

0.7005±12 

----- 0.7002±19 ............_ 

0.7009±2 

- 0.7002±3 

--+-- 0.7020±11 

----- 0.7011±8 

I I I I I I 

Figure G6. Comparative ages determined for Archean metasedimentary, 

metavolcanic, and postkinematic granitic rocks in the Rainy Lake district, 

Ontario; northern Minnesota, and core from Bristol, S. Dak. Solid circles, 

whole-rock rubidium-strontium ages; solid triangles, uranium-lead ages 

on zircon and sphene; lines give range for rubidium-strontium and potassium-argon ages on biotite. 

72 Shorter Contributions to Isotope Research 
give a Rb-Sr isochron age of 2,590 ± 40 Ma, 

Ri=0.7024±0.0011 (Goldich and Wooden, 1981, p. 91). 

The age determinations from the Minnesota River Valley 

and those from farther north compiled in figure G6 preclude the acceptance of the Rb-Sr whole-rock age of 

2,510 Ma as the time of formation of the staurolite-biotite 

schist. 

A Geologic Model 

The apparent contradiction in the radiometric ages 

and the stratigraphic relations determined geologically 

(fig. G6) is a well-known basic problem in the radiometric 

dating of metasedimentary rocks. The factors that controlled or influenced the measured ages are not well 

known, but considerable progress has been made. An 

early paper concerned with discordant mineral ages in 

Australian granites (Arriens and others, 1966) is signifi0.5 1.0 

cant because it cautioned of the possibility of open-rock 

conditions that might result from the migration of strontium, for example, as a result of "slight earth stress rather 

than reheating * * * ," incipient weathering, and lowtemperature alteration of plagioclase. Peterman and Hildreth (1978) have reviewed some of the problems of interpreting Rb-Sr ages determined on metasedimentary rocks 

and some of the extensive literature pertaining to the subject. They concluded that relatively low-grade metamor· 

phism may seriously disturb the Rb-Sr system. 

An interpretation of the Rb-Sr data based on information derived from previous geochronological studies in 

the Lake Superior region (see, for example, Goldich, 

1968, 1972) is shown in figure G7. Line A (fig. G7) 

is based on the lower six data points that have 87Rb/86Sr 

of less than 1.0. The approximate age is 2,620 Ma; 

Ri = 0. 7006. This age would relate the formation of the 

staurolite-biotite schist to the 2,600-Ma event, which in 

LINE AGE (Ma) Ri -

A 2,620 0.7006 

8 2,510 0.7020 

c 1,840 0.713 

1.5 2.0 2.5 

87Rbj86Sr 

Figure G7. Rubidium-strontium diagram showing a possible geologic interpretation of the analytical data 

from the Bristol core. Open circles, whole-rock samples of staurolite-biotite schist; open triangles, amphibolite. Line A includes the five lowermost points with 87Rb/86Sr of less than 1.0; line B is isochron 

B in figure G3; line C is a reference line from actinolite schist sample 1786 drawn through the cluster 

of points for staurolite-biotite schist samples, about at the median for line B. 

Age of the Basement Staurolite-Biotite Schist of Northeastern South Dakota 73 
the Lake Superior region was one of intense deformation, 

high-grade metamorphism, and magmatic activity. Line 

B is line B of figure G3, and the 2,510-Ma isochron, 

then, is a secondary metamorphic age with a higher initial 

ratio of 0. 7020. The whole-rock age is essentially a biotite 

age and possibly might be related to the uplift and stabilization of the region. Goldich and others (1970) considered 

this possibility for the Precambrian of the Minnesota River 

Valley. The K-Ar and Rb-Sr biotite ages of about 2,500 

Ma southeast of Granite Falls and ages of about 1, 750 

Ma at and northwest of Granite Falls may represent the 

time of stabilization of two Precambrian blocks, explaining the K-Ar and Rb-Sr ages on biotite, but open-system 

behavior for the staurolite-biotite schist is an inescapable 

requirement. 

Shearing and deformation of the schist during uplift 

following folding and metamorphism could account for 

the Rb-Sr whole-rock isochron age of 2,510 Ma. The lowgrade alteration of biotite to chlorite, staurolite to sericite, 

garnet to chlorite and sericite may have been caused by 

hydrothermal activity that accompanied the shearing; however, if the alteration took place 2,510 Ma ago, the 

2,440-Ma age on biotite is difficult to explain, and even 

more so is the apparent age of about 2,220 Ma for the 

actinolite schist. 

An alternative explanation is that the low-grade alteration was related to the 1 ,800-Ma (Penokean) event. 

Line C (fig. G7) from the actinolite schist (sample 1786) 

was projected through the cluster of points (samples 1582, 

1590, 1848, and 1895) and gives an age of 1 , 840 Ma 

with an initial ratio of about 0.713. This line, of course, 

is highly speculative and in no sense an isochron. Goldich 

and others (1970, p. 3690) pointed out that the 1 ,800-Ma 

event in the Minnesota River Valley "should not be 

equated with the Penokean orogeny to the north and northeast in the sense of a period of folding and metamorphism." A number of small plutons in the Archean of the 

Minnesota River Valley have been dated at about 1 , 800 

Ma. A U-Pb zircon age of 1 ,800 Ma was obtained by 

Catanzaro (1963) for a small adamellite pluton north of 

Granite Falls, Minn., about 115 mi southeast of Bristol, 

S. Oak. (fig. G1). 

In South Dakota, whole-rock samples of quartz latite cored in a drill hole in Kingsbury County and felsite 

porphyry from Sanborn County gave Rb-Sr ages of 1 , 700 

Ma (Goldich and others, 1966). The northwest-trending 

metamorphic belt (figs. G 1, G5) was delineated by Lidiak 

(1971) mainly on magnetic anomalies. He suggested, on 

the basis of the meager data available, that the belt can 

be traced by the pattern of gravity highs and lows in the 

buried basement of North Dakota and onto the shield at 

the Churchill-Superior boundary and is a Proterozoic deformation. Biotite concentrates from schist cuttings from 

a well in Walworth County and from granodiorite cored 

in a hole that penetrated the Sioux Quartzite in Davison 

7 4 Shorter Contributions to Isotope Research 

County, S. Oak. (fig. G 1) gave Penokean ages. The biotite ages, however, do not establish a Proterozoic age for 

the rocks that may date back to the Archean. 

The Bristol drill hole is well within a massive block 

of Archean rocks (fig. G 1). Potassium-argon and 

rubidium-strontium ages on biotite from the Bristol core 

are 2,440 Ma, and on biotite from adamellite cored in 

Marshall County, about 25 mi northwest of Bristol (fig. 

G1) 2,390 Ma. Biotite near Milbank was dated at 1,980 

Ma, and a biotite from metasedimentary rock south of 

Odessa (fig. G 1) gave an age of 2,180 Ma. The progression of numbers suggests the possibility that the biotite 

ages may have been influenced by the 1 ,800-Ma event. 

SUMMARY 

The Rb-Sr isochron age of 2,510 Ma is a minimum 

value for the time of formation of the staurolite-biotite 

schist cored near Bristol, S. Oak. The actual time probably was about 2,600 Ma during the major deformation 

accompanied by magmatic activity that affected the Lake 

Superior region. The minimum age, however, is significant because it lends considerable support to the concept 

that a massive block of Archean rocks underlies a large 

area in northeastern South Dakota. 

Structural trends in the basement rocks of northeastern South Dakota, delineated mainly on the basis of magnetic and gravity anomalies, continue in the buried basement rocks of adjacent parts of Minnesota and North 

Dakota. The radiometric age determinations, however, 

provide a basis for correlating the staurolite-biotite schist 

with Archean sequences in northern Minnesota and in 

southern Ontario. Whole-rock Rb-Sr isochron ages for 

metasedimentary and metavolcanic rocks both in the 

Rainy Lake district, Ontario, and in the Vermilion and 

Giants Range areas of northern Minnesota range from 

2,540 to 2,635 Ma, but the U-Pb ages on zircon and 

sphene date the postkinematic granites emplaced in the 

metasedimentary and metavolcanic rocks at 2,650 Ma. 

Low-grade metamorphism similar to that observed 

in the Bristol core has affected staurolite-bearing 

metasedimentary rocks in the Rainy Lake district, Ontario. 

Staurolite has not been reported from the metasedimentary 

rocks of the Vermilion Granite Complex and Giants 

Range Granite of northern Minnesota, but large areas of 

these rocks have attained amphibolite-facies grade, and 

high-grade (amphibolite facies) regional metamorphism 

possibly may have affected a belt of Archean rocks extending from southern Ontario across Minnesota into 

northeastern South Dakota. A similar regional metamorphism of m~ch younger Early Proterozoic age affected 

the Thomson Formation of Minnesota, extending from 

Duluth south into the Mille Lacs Lake area and westward 

into the Little Falls area where staurolite schist there resembles the staurolite-biotite schist of the Bristol core. 
Three samples of the staurolite schist from Little Falls 

and samples from the Thomson Formation of eastern Minnesota give an isochron age of 1,695 Ma, Ri=0.705 

(Keighin and others, 1972). 

REFERENCES CITED 

Arriens, P. A., Brooks, C., Bofinger, V. M., and Compston, 

W., 1966, The discordance of mineral ages in granitic rocks 

resulting from the redistribution of rubidium and strontium: 

J oumal of Geophysical Research, v. 71 , p. 4981-4994. 

Baron, D. M., 1971, The petrology of a Precambrian schist 

from Bristol, northeastern South Dakota: Vermillion, S. 

Dak., University of South Dakota, M.A. Thesis, 59 p. 

Catanzaro, E. J., 1963, Zircon ages in southwestern Minnesota: 

Journal of Geophysical Research, v. 68, p. 2045-2048. 

Catanzaro, E. J., and Hanson, G. N., 1971, U-Pb ages for 

sphene from early Precambrian igneous rocks in northeastem Minnesota-northwestern Ontario: Canadian Journal of 

Earth Sciences, v. 8, p. 1319-1324. 

Goldich, S. S. , 1968, Geochronology in the Lake Superior region: Canadian Journal of Earth Sciences, v. 5, p. 715-724. 

---1972, Geochronology in Minnesota, in Sims, P. K., and 

Morey, G. B., eds., Geology of Minnesota; A centennial 

volume: Minnesota Geological Survey, p. 27-37. 

Goldich, S. S., Hedge, C. E., and Stem, T. W., 1970, Age 

of the Morton and Montevideo gneisses and related rocks, 

southwestern Minnesota: Geological Society of America 

Bulletin, v. 81, p. 3671-3695. 

Goldich, S. S., Lidiak, E. G., Hedge, C. E., and Walthall, 

F. G., 1966, Geochronology of the midcontinent region, 

United States-Pt. 2, Northern area: Journal of Geophysical 

Research, v. 71, p. 5389-5408. 

Goldich, S. S., Nier, A. 0., Baadsgaard, H., Hoffman, J. H., 

and Krueger, H. W., 1961, The Precambrian geology and 

geochronology of Minnesota: Minnesota Geological Survey 

Bulletin 41, 193 p. 

Goldich, S. S. , and Wooden, J. L. , 1981 , Origin of the Morton 

Gneiss, southwestern Minnesota; Pt. 3, Geochronology, in 

Morey, G. B., and Hanson, G. N., eds., Selected studies 

of Archean gneisses and lower Proterozoic rocks, southern 

Canadian shield: Geological Society of America Special 

Paper 182, p. 77-94. 

Harris, F. R., 1974, Geology of the Rainy Lake area, district 

of Rainy River: Ontario Division of Mines, Geological Report 115, 94 p. 

Jahn, Bor-Ming, and Murthy, V. R., 1975, Rb-Sr ages of the 

Archean rocks from the Vermilion district, northeastern 

Minnesota: Geochimica et Cosmochimica Acta, v. 39, p. 

1679-1689. 

Keighin, C. W., Morey, G. B., and Goldich, S., S., 1972, 

East-central Minnesota, in Sims, P. K., and Morey, G. B., 

eds., Geology of Minnesota; A centennial volume: Minnesota Geological Survey, p. 240-255. 

Krogh, T. E., 1973, A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb 

for isotopic determinations: Geochimica et Cosmochimica 

Acta, v. 37, p. 485-494. 

Lawson, A. C., 1913, The Archean geology of Rainy Lake restudied: Geological Survey of Canada Memoir 40, 115 p. 

Lidiak, E. G., 1971, Buried Precambrian rocks of South Dakota: 

Geological Society of America Bulletin, v. 82, p. 

1411-1420. 

Ludwig, K. R., 1982, Programs for filing and X-Y plotting 

of isotopic and other data using an HP-9830 computer and 

HP-9862 plotter: U.S. Geological Survey Open-File Report 

82-385, 37 p. 

McDougall, I., 1966, Precision methods of potassium-argon 

isotopic age determinations on young rocks, in Methods and 

techniques in geophysics, v. 2: New York, Interscience, p. 

279-304. 

Peterman, Z. E., Goldich, S. S., Hedge, C. E., and Yardley, 

D. H., 1972, Geochronology of the Rainy Lake region, 

Minnesota-Ontario, in Doe, B. R., and Smith, D. K., eds., 

Studies in mineralogy and Precambrian geology: Geological 

Society of America Memoir 135, p. 193-215. 

Peterman, Z. E., and Hildreth, R. A., 1978, Reconnaissance 

geology and geochronology of the Precambrian of the Granite Mountains, Wyoming: U.S. Geological Survey Professional Paper 1055, 22 p. 

Petsch, B. C., 1965, Significant magnetic anomalies in South 

Dakota: South Dakota Academy of Science Proceedings, 

1965, v. 44, p. 52--62. 

Petsch, B. C., compiler, 1967, Vertical-intensity magnetic map 

of South Dakota--Ground magnetometers survey: South 

Dakota Geological Survey Mineral Resources Investigation 

Map 4, scale about 1 in.= 12 mi. 

Prince, L. A., and Hanson, G. N., 1972, Rb-Sr isochron ages 

for the Giants Range Granite, northeastern Minnesota, in 

Doe, B. R., and Smith, D. K., eds., Studies in mineralogy 

and Precambrian geology: Geological Society of America 

Memoir 135, p. 217-224. 

Steiger, R. H., and Jager, E., 1977, Subcommision on geochronology; convention on the use of decay constants in geoand cosmochronology: Earth and Planetary Science Letters, 

v. 36, p. 359-362. 

Age of the Basement Staurolite-Biotite Schist of Northeastern South Dakota 75 

FIGURE 

1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

APATITE FISSION-TRACK AGE FOR THE 

BULL DOMINGO BOULDER PIPE, 

CUSTER COUNTY, COLORADO 

Abstract .... 

Discussion . . . 

References cited 

Chapter H 

By W. N. SHARP and C. W. NAESER 

CONTENTS 

Page 

78 

78 

80 

Page 

HI. Index map of Silver Cliff and Rosita volcanic centers showing location of Bull Domingo pipe in relation 

to dominant volcanic and structural features . . . . . . . . . . . . . . . . . . . . . . . . . . 78 

TABLE Page 

HI. Apatite fission-track ages and analytical data for apatites from the Bull Domingo mineralized boulder 

pipe, Custer County, Colo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 

Apatite Fission-Track Age for the Bull Domingo Boulder Pipe, Custer County, Colorado 77 
Abstract 

Fission-track ages of apatites from Precambrian 

gneissic boulders in the Bull Domingo pipe in Custer 

County, Colorado, confirm an Oligocene age for the 

pipe. Although S. F. Emmons, as early as 1896, had related formation of the highly mineralized structure to 

Tertiary volcanism at Silver Cliff, the geologic setting of 

the pipe-entirely enclosed within Precambrian rocks, 

and lack of Tertiary clast in the pipe-as well as isotopic 

data for Cambrian or Precambrian leads in the pipe, had 

left an uncertainty regarding its age. 

27'30" 

I 

@)'BD 

GRABEN 

I I 

DISCUSSION 

Fission-track counts of apatites were used to settle 

the age uncertainty regarding a small, richly mineralized, 

breccia pipe, the Bull Domingo, that is associated closely 

with the eruptive centers at Silver Cliff and Rosita, Colo. 

(Sharp, 1978). The Bull Domingo mineralized pipe is totally within the Precambrian terrane of the south slope 

of the Wet Mountains, about 3 miles north of the towns 

of Silver Cliff and Westcliffe, and a mile or so from the 

limits of the Tertiary eruptive center at Silver Cliff (fig. 

22'30" 20' 1 05°17'30" 

I I 

-

-

I I 

6 KILOMETERS 

3 MILES 

EXPLANATION 

BW Ben West volcano 

@ BD Bull Domingo pipe 

JG Jay Gould pipes 

CG Upper Chloride Gulch vent 

G V Geyser Hill vent 

B P Bassick pipes 

Fault-Dashed where approximately located; dotted where 

buried; queried where position 

uncertain 

Figure H1. Index map of Silver Cliff and Rosita volcanic centers showing location of 

Bull Domingo pipe in relation to dominant volcanic and structural features. Custer 

County, Colo. 

78 Shorter Contributions to Isotope Research 
Table H1. Apatite fission-track ages and analytical data for apatites from the Bull Domingo mineralized boulder 

pipe, Custer County, Colo. 

[cm2 , square centimters; A.F=7.03X 10- 17yr- 1

; R, correlation coefficient; ppm, parts per million. Number of tracks counted are shown in parentheses] 

Sample Fossil tracis.s Induced tracks 

No. Lab. No. cm2x106 cm2x10 6 

BD-1 DF-2185 0.403 3.39 

(363) 

BD-3 DF-2186 0.411 4.04 

( 167) 

BD-4 DF-2187 0.531 4.70 

(378) 

BD-6 DF-2188 0.624 5.44 

(238) 

H1). The pipe was discovered in 1878 during the silver 

rush and mining episode at Silver Cliff. Mining at Bull 

Domingo followed the breccia-boulder pipe down to a 

depth of 550 feet or more with total value produced of 

about $1 million in silver and lead. The best geologic 

description of this pipe-the composition, the shape, ore 

characteristics, and thoughts on formation-are in "The 

Mines of Custer County," by S. F. Emmons (1896). Emmons related formation of the Bull Domingo to volcanism 

at Silver Cliff and considered the structure to be a product 

of hot fluids passing up through a fractured zone. He developed this model even though all clasts in the pipe are 

fragments of granite, gneisses, and syenite of the surrounding Precambrian and Cambrian terrane, and no Tertiary eruptive rocks are present in the explored parts of 

the pipe. 

An older Cambrian age for the Bull Domingo pipe 

is indicated not only by its position in Precambrian terrane 

and the character of the clasts, but also by a possible 

association with local alkalic and mafic-ultramafic intrusive complexes of Cambrian age and by lead isotopic data 

from galena in the pipe. A product of the alkalic to maficultramafic intrusive episodes is a series of red syenite 

dikes that are widely spaced and distributed in the region 

(Parker and Hildebrand, 1963). The Bull Domingo pipe 

pierces the Precambrian terrane at the site of a red syenite 

dike. Fragments of these rocks are found in the pipe. The 

eruptive energy for the pipe could have come from this 

alkalic intrusive episode. The intrusive complexes and the 

associated red syenite dikes are dated by rubidium-strontium, potassium-argon and fission tracks at about 500 Ma . 

(Olson and others, 1977). 

During the late 1950's and through the 1960's, interest in isotopic variations in rock and ore leads resulted 

Neutrons Age ±2o No. of u 

cm2xl0 15 (Ma) (Ma) grains R (ppm) 

4.59 32.6 2.2 6 0.993 21 

(1527) 

4.65 28.3 2.8 6 0.97 25 

(820) 

4.55 30.7 2.3 6 0.93 30 

(1675) 

4.52 31.0 2.7 5 0.91 35 

(1037) 

in determination of isotopic compositiOn of lead in the 

Bull Domingo pipe. Two galena samples from the Bull 

Domingo pipe were analyzed for lead isotopic compositions and reported by Delevaux and others ( 1966). The 

lead analyses were used to indicate a model age of 600 

Ma (J. C. Antweiler, R. S. Cannon, and A. P. Pierce, 

unpub. data). This model age and the geologic relations 

both supported an older age for the Bull Domingo as well 

as a possible genetic relationship to alkalic intrusive rocks 

within the Precambrian terrane close by. However, J. C. 

Antweiler and others (unpub. data) advanced an alternative interpretation of remobilization and redeposition of 

Precambrian lead during Tertiary time. 

In an effort to define a proper association of the 

Bull Domingo breccia pipe, fission-track ages were determined on apatite separated from gneissic boulders in the 

pipe. The reasoning, simply stated, was that apatite fission-track ages in these granitic to intermediate rocks of 

Precambrian age might have been reset at the time of 

pipe formation and would show a maximum age for that 

intrusion. The results of the fission-track analyses are 

given in table H1. The calculated ages from four populations of apatites range from 28.3 ± 2.3 Ma to 32.6 ± 2.2 

Ma and average 30.7 Ma; the ages are concordant within 

the limitations of analytical uncertainty. This age related 

nicely to the earliest eruptive action of the Ben West volcano at Silver Cliff (Sharp, 1978). 

The Bull Domingo pipe is a product of early explosive activity at the Silver Cliff volcanic center and is related specifically to the eruption of the Ben West volcano. 

The earlier conclusion by J. C. Antweiler and others 

(unpub. data), that the galena of the Bull Domingo is 

composed of Precambrian lead is explained if the lead 

in the Precambrian host rocks was remobilized by the 

local volcanic activity during the late Oligocene time. 

Apatite Fission-Track Age for the Bull Domingo Boulder Pipe, Custer County, Colorado 79 
REFERENCES CITED 

Delevaux, M. H., Pierce, A. P., and Antweiler, J. C., 1966, 

New isotopic measurements of Colorado ore-leads, in 

Geological Survey research 1966: U.S. Geological Survey 

Professional Paper 550-C, p. Cl78-Cl86. 

Emmons, S. F., 1896, The mines of Custer County, Colorado: 

U.S. Geological Survey 17th Annual Report, p. 405-472. 

Olson, J. C., Marvin, R. F., Parker, R. L., and Mehnert, H. 

H., 1977, Age and tectonic setting of lower Paleozoic alkalic and mafic rocks, carbonatites, and thorium veins in 

south-central Colorado: U.S. Geological Survey Journal of 

Research, v. 5, no. 6, p. 673-687. 

Parker, R. L., and Hildebrand, F. A., 1963, Preliminary report 

on alkalic intrusive rocks in the northern Wet Mountains, 

Colorado, in Geological Survey research 1962: U.S. 

Geological Survey Professional Paper 45~E. p. E8-El0. 

Sharp, W. N., 1978, Geologic map of the Silver Cliff and 

Rosita volcanic centers, Custer County, Colorado: U.S. 

Geological Survey Miscellaneous Investigations Map 

1-1081. 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

URANIUM-LEAD SYSTEMATICS OF A MIXED 

ZIRCON POPULATION-THE GRANITE AT 

YALE FARM, BERKSHIRE MASSIF, CONNECTICUT 

Chapter I 

By ROBERT E. ZARTMAN, LORETTA M. KWAK, and RALPH P. CHRISTIAN 

Abstract ..... 

Introduction . . . 

Acknowledgments 

Prior uranium-lead zircon dating efforts in the 

Berkshires . . . . . . . . . . . . . . . . . 

FIGURES 

CONTENTS 

Page 

82 

82 

82 

84 

Geologic setting of the granite at Yale Farm 

Analytical procedure ......... . 

Geochronologic results . . . . . . . . . 

Electron microprobe analysis of zircon 

References cited . . . . . . . . . . . . 

Page 

86 

86 

88 

94 

97 

Page 

11. Generalized structure map of the central Berkshire massif, Massachusetts and Connecticut 83 

I2. Concordia diagram for zircon from various localities in the Berkshire massif . . . . . . . 85 

13. Geologic map of Yale Farm and vicinity . . . . . . . . . . . . . . . . . . . . . . . . . . 87 

I4. Photomicrographs of representative grains of the three varieties of zircon from the granite at Yale Farm 

I5. Graph showing abundances of the three varieties of zircon from the granite at Yale Farm as a function 89 

of mesh size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 

I6. Concordia diagram for zircon and sphene from the granite at Yale Farm . . . . . . . . . . . . . 92 

I7. Photomicrograph, electron backscatter image, and zirconium and hafnium scanning maps of a select variety 

III zircon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 

TABLES 

Page 

11. Uranium-thorium-lead isotopic age of zircon from the Berkshire massif . . . 84 

I2. Uranium-thorium-lead isotopic data for S-2--67 standard zircon . . . . . . . . 88 

13. Uranium-thorium-lead isotopic ages of zircon from the granite at Yale Farm 91 

I4. Average value, standard deviation, and range in element concentration for representative grains of the 

different varieties of zircon . . . . . . . . . . . . . . 95 

I5. Electron microprobe analyses of a select variety III zircon . . . . . . . . . . . . . . . . . . . . . 97 

Uranium-Lead Systematics of a Mixed Zircon Population-Granite at Yale Farm, Connecticut 81 
Abstract 

late- to post-deformational granitic bodies in the 

Berkshire massif of western Massachusetts and Connecticut place a minimum age on major westward-directed 

thrust faulting. A detailed uranium-lead dating study of 

one such body, the granite at Yale Farm, reveals the 

presence of both inherited xenocrystic zircon and newly 

crystallized zircon. Varying proportions of these two 

components in different zircon size fractions establish 

a remarkably linear mixing array on a concordia diagram 

with upper and lower intercepts of 1,050 ±40 Ma and 

430±10 Ma, respectively. We associate the former age 

with the underlying Middle Proterozoic crystalline basement rocks of the massif, and the latter age-adjusted 

to 430-450 Ma when supplemented by uranium-lead 

sphene analyses-with the crystallization of the granite. 

The electron microprobe was used to chemically 

characterize the two components of zircon. Their hafnium and uranium contents proved to be the best discriminants in this regard-allowing us to classify individual 

grains and to map the interface between core and overgrowth of composite grains. Interestingly, the chemical 

interface did not always precisely match the optical interface and sometimes the optical interface was imperceptible. This observation leads us to the conclusion that the 

optical interface is of secondary origin and results largely 

from stresses arising from accumulating radiation dosages. 

INTRODUCTION 

The common presence of zircon as an accessory 

mineral in granitic rocks and its ability to withstand subsequent high-grade metamorphism has earned this mineral 

an unequaled reputation in geochronology. Information 

about primary age commonly can be obtained even though 

the host rock has been recrystallized pervasively. Indeed, 

the survival of preexisting zircon can persist beyond conditions of sillimanite-grade metamorphism and into the 

pressure-temperature field of anatectic melting. Under 

such extreme metamorphic conditions, the antecedent 

crystals generally are resorbed, and a new generation of 

zircon can form. In this latter instance, the igneous event 

that we wish to date actually may be recorded in the secondary component of zircon, whereas the primary component carries information about the source of the magma. 

The task for the geochronologist thus becomes one of resolving a complicated set of isotopic data obtained for 

polygenetic zircon suites into its geologically meaningful 

end members. 

A clue to a multiple-stage history sometimes is recorded morphologically in the zircon population either by 

a core and overgrowth relationship or by the mutual existence of two or more distinct varieties of the mineral. 

At other times, careful examination under the optical 

microscope fails to identify any trace of interfaces within 

82 Shorter Contributions to Isotope Research 

individual grains or inhomogeneities among the population. In both situations, however, unequivocal evidence 

for two (or more) generations of zircon formation has 

been found in the U-Pb (uranium-lead) isotopic systematics, which demand a mixture of components that have 

distinguishably different ages. A number of investigations 

detailing the experimental and interpretative techniques for 

deciphering the ages of such polygenetic zircon have been 

published during the past decade (excellent examples of 

such studies include Pidgeon and others, 1970; Koppel 

and Grunenfelder, 1971; Grauert and others, 1974; Pidgeon and Aftalion, 1978; Chase and others, 1978). Our 

purpose here in presenting a description of the mixed zircon population in the granite at Yale Farm, North Canaan, 

Conn. , is to document another particularly good example 

of this phenomenon. Unraveling the physical and chemical 

properties of this zircon permits the rather precise dating 

of a significant structural event in the Berkshire massif._ 

Our effort to date this granitic body (hereafter informally referred to as the granite at Yale Farm) represents 

only one of our attempts, albeit at present the most successful one, to provide structural control for the tectonic 

history of the Berkshire massif in Massachusetts and Connecticut. During the past several years we have accumulated a potpourri of zircon uranium-lead analyses from 

a variety of granitic rocks intruding the Middle Proterozoic basement rocks and the Late Proterozoic(?) to Cambrian(?) mantling metamorphic rocks of the Berkshire massif. Both the basement and mantling rocks are imbricately 

stacked and were transpot:fed westward over autochthonous quartzite and carbonate rock of Late Proterozoic(?) 

and early Paleozoic age of the North American shelf (fig. 

I 1). Although later metamorphic isograds attributed to the 

Acadian orogeny are superimposed on all these rocks 

(Ratcliffe and Harwood, 1975), a long-standing question 

not completely resolved by field relationships is the timing 

of major kinematic deformation. Constrained by the age 

of the _youngest stratigraphic unit (Middle Ordovician 

W alloomsac Formation) involved in the thrust faulting and 

by the regional Early to Middle Devonian Acadian 

metamorphic overprinting, this deformational event might 

be attributable either to the Late Ordovician Taconic 

orogeny or to an early phase of the Acadian orogeny. 

We believe that our dating of the late to post-deformational granite at Yale Farm as Late Ordovician or Early 

Silurian resolves this controversy in favor of the Taconic 

orogeny. 

ACKNOWLEDGMENTS 

We are indebted to N. M. Ratcliffe, D. S. Harwood, and the lateR. W. Schnabel of the U.S. Geological 

Survey, and S. A. Norton of the University of Maine 

for providing the rock samples that yielded the mineral 
10 

EXPLANATION 

D 

Devonian and Silurian rocks Autochthonous shelf sequence 

Unconformity-Decollement 

1-b.y.-old basement of the Berkshire massif 

Hawley-Rowe belt of eugeoclinal meta- and Housatonic Highlan'ds 

sedimentary rocks 

Hoosac belt 

20 KILOMETERS 

• • 

Synmetamorphic thrust faults of late 

Taconic age-Sawteeth on upper 

plate 

x3 

Sample locality 

Figure 11. Generalized structure map of the central Berkshire massif, Massachusetts and Connecticut. 

Modified from Ratcliffe and Hatch (1979). 

Uranium-Lead Systematics of a Mixed Zircon Population-Granite at Yale Farm, Connecticut 83 
Table 11. Uranium-thorium-lead isotopic ages of zircon from the Berkshire massif 

[Decay constants: 238U= 1.55125x w-10 yr-1; 235U=9.8485X w-10 yr-1; 232Th=4.9475X w- 11 yr-1

; 238UP35U= 137.88 Isotopic composition of common lead assumed to be 204Pb:206Pb:207Pb:208Pb = 1:18.0:15.6:38.0 for all samples except 3 for which it is assumed to be 1:17.0:15.5:36.8] 

Concentrations Isotopic composition of lead A~e, in millions of l':ears 

Sample <earts eer million) (atom eercent) 206pb 2o 'Pb 207pb 208pb 

No. Mesh size u Th Pb 20 ~Pb 2 o "Pb 20 7 Pb 208pb 2 ••u 2 • •u 2o 'Pb 232Th 

(-100+150) 2920 83.4 192.2 0.016 92.52 5.656 1.806 440 459 554 623 (-200+250) 2446 97.1 147.5 .014 92.62 5.552 1.816 405 423 523 446 

2 (-100+150) 211 25.4 19.4 .062 87.30 6.548 6.086 570 611 769 642 

(-200+250) 277 43.4 24.6 .028 88.44 6.093 5.442 560 600 754 560 

(-1 00+150) 290 48.2 48.9 .128 82.38 7.939 9.556 941 975 1051 1093 (-200+250) 351 55.0 53.5 .067 85.29 7.230 7.416 895 934 1029 1065 

(-100+150) 323 27.2 32.4 .111 85.33 7.154 7.404 597 635 774 840 

(-150+200) 298 24.7 29.0 • 108 85.62 7.072 7.198 582 619 757 813 

(-250+325) 276 21.6 24.6 • 117 85.62 7.078 7.190 534 568 705 698 (-400) 387 33.3 34.0 • 130 85.00 7.169 7. 705 522 553 683 635 

(-100+150) 417 49.8 47.1 .011 88.91 6.272 4.804 711 756 891 923 (-150+200) 372 39.2 36.1 .012 89.46 6.126 4.401 619 665 825 813 

(-100+150) 1162 169.5 102.6 .004 90.61 5.887 3. 498 573 611 754 457 (-200+250) 1311 203.8 94.5 .006 91 .59 5.558 2.844 477 498 597 276 (-325+400) 1315 247.7 80.7 .qo6 92.03 5.257 2. 708 410 418 460 184 

(-1 00+150) 119 25.2 10.1 .024 85.96 6.011 8.004 557 608 802 638 (-150+200) 153 18.9 12.3 .019 88.92 5.919 5.147 510 551 722 641 

(-200) 253 32.6 18.8 .068 87.83 6.018 6.088 467 473 503 457 

( -50+150) 443 129.1 41.6 .016 88.00 5.804 6.180 590 618 720 405 

(-150+250) 662 375.8 59.9 .013 83.36 5.397 11 .23 541 570 690 385 (-250) 983 652.7 76.0 .012 82.40 4.888 12.70 459 466 501 322 

DESCRIPTION OF SAMPLES 

Sample localities shown on figure 11. Quadrangles listed are names of U.S. Geological Survey 7-1/Z minute topographic quadrangle maps. 

1. Gray, medium-grained gneissic granite from the Hubbard River, Granville State Forest, West Granville quad.; lat 4Z0 03'ZO" N., long 7Z0 57'47" W. 

Z. Light-gray, medium-grained gneissic granite from roadcut on Route 8 overlooking Colebrook Reservoir, Tolland Center quad.; lat 4Z0 0Z'l0" N., long 73°03'Z6" W. 

3. Tectonic sliver of Washington(?) Gneiss west of Babb Hill, West Granville quad.; lat 4Z0 07'ZZ" N., long 7Z0 59'41" W. 

4. White, cataclastic muscovite granite (Ratcliffe and Hatch, 1979) from small quarry east of Cushman Brook, Becket quad.; lat 43°15'10" N., long 73°00'31" W. 

5. Light-gray, fine-grained, weakly foliated granite from the New Whiting quarry, Otis quad.; lat 4Z0 14'03"N., long 73°04'15" W. 

6. Gray, coarse-grained binary granite from the Winn quarry, Otis quad.; lat 4Z0 14'4Z" N., long 73°00'59" W. 

7. Gray, medium-grained, well-foliated biotite granite from the Williams quarry, Otis quad.; 1at 4Z0 13'55" N., long 73°01'ZZ"W. 

8. Gray, medium-grained, weakly-foliated binary granite from small quarry on Yale Farm, South Sandisfield quad.; lat 4Z0 0Z'll" N., long 73°13'48" W. 

separates analyzed in this study. Numerous helpful discussions and correspondences with these colleagues as the 

work progressed aided us substantially in understanding 

and interpreting the resultant data, and in bringing our 

ideas together into this paper. 

Just prior to completion of our manuscript, we 

learned of the research efforts by J. F. Sutter, N. M. 

Ratcliffe, and S. B. Mukasa using the 40 ArP9 Ar method 

to decipher the tectonic history of southwestern New England. Basically, their work points to a greater Taconic 

and diminished Acadian role in the metamorphism of the 

Berkshire massif. Although they now bring into question 

an Acadian assignment for the metamorphic isograds used 

to assign a minimum age to the major deformation, we 

otherwise find the conclusions of both studies to be in 

agreement and mutually supportive in emphasizing the 

Taconic orogeny. 

We gratefully acknowledge the following persons 

for technical assistance. Many of the preliminary uraniumlead analyses were performed by Margarita Gallego. G. 

T. Cebula and J. W. Groen assisted in the zircon and 

sphene separations. Barbara Lockett prepared the polished 

grain mounts for the electron microprobe. 

84 Shorter Contributions to Isotope Research 

PRIOR URANIUM-LEAD ZIRCON 

DATING EFFORTS IN THE BERKSHIRES 

The polygenetic nature of these zircons was first 

detected in a reconnaissance study of small granitic plutons intruding the southeastern flank of the Berkshire massif. The structural complexity of this terrane was not appreciated fully at the time, and the intimately associated 

"gneisses" and "schists", now recognized as tectonically 

imbricated Middle Proterozoic basement and Late Proterozoic(?) to Cambrian(?) mantling rocks, respectively, were 

conjectured to be a normal stratigraphic succession 

sandwiched between the more coherent basement core of 

the Berkshires on the west and the Proterozoic or Cambrian Hoosac Schist and its equivalent, the W aramaug Formation on the east. When the first zircon analyses yielded 

207Pb/206Pb ages of 523 to 769 Ma (sample localities 1-2, 

Table Il), they seemed to lend credence to the hypothesis 

that a major volcanoclastic unit comprised of massive 

flows and tuffs, the "gneiss", interbedded with clastic sediments, the "schist", conformably underlay the Hoosac 

Schist. According to this interpretation, the granitic bodies 

represented contemporaneous epizonal plutons associated 

with the volcanic centers. 
Some suspicion was aroused, however, by the tendency of the isotopic data to lie close to a reference chord 

drawn between 400 and 1 , 100 Ma on a concordia diagram 

(fig. 12). The upper intercept of that chord approximates 

the age of the Middle Proterozoic basement for the Berkshire massif (Ratcliffe and Zartman, 1976), whereas the 

lower intercept could represent either the Taconic or Acadian orogeny. Further doubt was raised by the analysis 

of a third sample (sample locality 3, table Il}-a gneissic 

unit interlayered with schist and hypothesized to be a volcanic flow rock. Contradictory evidence comes from its 

uranium-lead zircon ages, which are considerably older 

than those of samples 1 and 2. In fact, these determinations plot on the concordia diagram amid previously published data for two samples of Middle Proterozoic basement rocks (Tyringham and Washington Gneisses; 

Ratcliffe and Zartman, 1976). A revisit to locality 3 now 

convinces us that the dated rock is really a tectonic sliver 

of possible Washington Gneiss in fault contact with 

Hoosac Schist. With the demise of the "normal stratigraphic succession" theory and our inability to refine the 

ages of these granitic bodies, we turned our attention to 

~ 

00 

M 

0.11 

0.10 

0.09 

~ 0.08 <J:) 

0 

N 

0.07 

0.06 

6'0 

0 

::> 

00 

0 .12 

~ 0.08 

::o0... 

<J:) 

~ 

0.04 

the north and· west where detailed mapping was recognizing the true magnitude of the structures in the Berkshire 

massif. 

By carefully establishing and tracing out stratigraphic units within the basement and mantling rocks, as 

many as 12 separate tectonic slices were delineated across 

the massif. This westward-directed telescoping of the eastem margin on the North American continental crust produced an imbrication of stratigraphic units that m~de 

previous efforts to view the Berkshires as a regionally 

coherent anticlinal arch obsolete. The most conservative 

restoration of these stacked slices demand at least a 50-

percent shortening across the structure. Late-stage granites 

can be shown to intrude along several of the fault surfaces, and they present an opportunity for placing a 

further minimum age on the time of deformation. Much 

of this field and petrographic work, which provides the 

geologic context for the present study, has been published 

elsewhere (Ratcliffe, 1975; Harwood, 1975, 1979a; 

Ratcliffe and Zartman, 1976). 

Similar to the pattern we had observed to the southeast, our uranium-lead zircon data from the central and 

~0 0 

llb 0 w 

~y"JT3 ~ 400-1,1 00 Ma 

6' 0 / reference isochron 

00 / .,,., 

, 

0.04L---~-----L----~--~L---~-----L----~--~L----L----~----L---~----~----~ 

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 

207Pb/235U 

Figure 12. Concordia diagram for zircon from various localities in the Berkshire massif. (See figure 11 

for sample localities and table 11 for sample descriptions.) Also shown are data from the Middle Proterozoic 

Tyringham (T) and Washington (W) Gneisses (Ratcliffe and Zartman, 1976). 

Uranium-Lead Systematics of a Mixed Zircon Population-Granite at Yale Farm, Connecticut 85 
southwestern Berkshires fell close to the 400-

to 1 , 1 00-Ma reference line on the concordia diagram (see 

fig. 12, and sample localities 4--8; table I 1). However, 

with our better understanding of the stratigraphy, the 

anomalous nature of these results was apparent immediately when 207Pb/206Pb ages in excess of 700 Ma 

were found for granites invading along faults, which 

themselves displaced strata no older than latest Late 

Proterozoic, and, more probably, Cambrian in age. The 

demonstrable time-equivalence of these faults hosting the 

granites to the main transport of the Berkshire terrane 

across the Cambrian to Middle Ordovician shelf-facies 

rocks served to clinch the argument. For at least some 

of the granites, a local, anatectic origin could be postulated from field evidence, and distinct zircon overgrowths could be seen to enclose ovoidal, partially resorbed cores. It is from these preliminary attempts to determine radiometric age control for the granites that we 

decided to re-collect a large sample of the granite at Yale 

Farm for detailed study. Otherwise, we do not propose 

to use our preliminary data given in table 11 to add to 

existing stratigraphic and structural interpretation of the 

Berkshires. Neither precise enough radiometric results to 

resolve between a Taconic or Acadian age nor a correlation of intrusive units beyond that established through independent field evidence is implied for any of the other 

granites. 

GEOLOGIC SETTING OF THE 

GRANITE AT YALE FARM 

The local geologic setting of the granite at Yale 

Farm is shown in figure 13. A brief description of field 

relationships is necessary to document the critical role this 

igneous rock plays in establishing the time of thrust faulting and metamorphism in adjacent strata. The granite at 

Yale Farm occupies the largest pluton-having an area 

of about 0.3 km2 (square kilometers)-at the northwestern 

end of a linear trend of such bodies, which here cut across 

three of the stacked fault slices. Each of these slices contains a different lithologic assemblage, which, from lower 

to upper slice, consists of (1) Late Proterozoic or Cambrian Dalton Formation unconformably overlying Middle 

Proterozoic granitic gneiss, (2) Late Proterozoic and Cambrian Canaan Mountain Formation, and (3) Middle 

Proterozoic hornblende-biotite amphibolitic gneiss. Several kilometers to the west the lowermost of these slices 

overrides the Cambrian to Middle Ordovician autochthonous shelf carbonate rocks. Although by no means conclusively proved, stratigraphic and structural arguments can 

be made that successively higher slices have been transported from progressively further east during the period 

of major tectonic telescoping of the Berkshires. 

The granite is gray, fine to medium grained, and 

varies in texture from massive to slightly foliated except 

86 Shorter Contributions to Isotope Research 

where faulting and folding have imparted a stronger, local 

fabric on the rock. Compositionally, the bodies range 

from quartz monzonite to granite, contain two micas, and 

yield an accessory mineral suite of apatite, sphene, zircon, 

and opaque minerals. 

For our re-collection, about 50 kg (kilograms) of 

especially fresh, weakly foliated granite at Yale Farm was 

obtained from the small quarry site near the west margin 

of the pluton that had provided the preliminary sample. 

A strong microscopic mortar structure associated with the 

Haystack Mountain thrust and presumably superimposed 

on the granite by the last movement of the fault, was 

avoided entirely in our sampling. Poor outcrop exposures 

do not permit precise location of the pluton's contact with 

its wall rock, but at the present level of erosion, Middle 

Proterozoic gneisses surround most of the body. If the 

magma was transported some distance either vertically or 

along the Haystack Mountain fault surface before final 

crystallization, country rock representative of two or all 

three of the fault slices, and even the underlying autochthon could have contributed to its production. The uranium-lead isotopic systematics reveal that a component 

of zircon inherited from the Middle Proterozoic crystalline 

rocks unquestionably is present in the granite. The analytical results obtained for zircon and sphene of the re-collected granite of Yale Farm form the basis of this report. 

ANALYTICAL PROCEDURE 

A brief account of our analytical procedure is given 

in this section with particular emphasis on documentation 

of precision and accuracy. The sensitivity of the concordia 

intercept ages to only slight shifts in the position of the 

data points demands that special care be taken in carrying 

out all aspects of the chemistry and mass spectrometry. 

We include the results obtained on an interlaboratory zircon standard, which defines our reproducibility prevailing 

for the duration of the study. 

The sample size for individual analyses ranged from 

1 to 20 mg (milligrams), and weighings were made on 

an appropriate semimicro or micro balance to achieve an 

accuracy of ± 0.4 percent or better. A HF-HN03 hydrothermal digestion procedure for the zircon closely followed that reported by Krogh (1973). 235U and 23~h 

tracer solution was added to the sample prior to its digestion in a 15-mL (milliliter) teflon bomb, and 208Pb tracer 

solution was later added to an aliquot of the dissolved 

sample. Lead was purified by an anion-exchange column 

using an HBr elution technique as described by Manhes 

and others (1978), loaded with phosphoric acid and silica 

gel on a rhenium filament, and analyzed by a 12-inch, 

computer-controlled, solid-source mass spectrometer providing automated data reduction (Stacey and Hope, 1975). 

Uranium and thorium were purified by nitrate-form anion 
columns from the HBr elutriant, and were analyzed using 

a triple filament configuration in a 6--inch mass spectrometer (Tatsumoto, 1966). The chemical procedure for 

the sphene differed only in the digestion step, in which 

HF, HCl, and HC104 were substituted as the reactant 

acids. 

Total chemistry blanks range from 0.5 to 0.2 ng 

(nanograms) of lead depending mainly on the sample size, 

EXPLANATION 

GRANITE AND PEGMATITE 

g 

DALTON FORMATION 

£Zd Feldspathic granulite and schist 

Flaggy-bedded quartzite 

CANAAN MOUNTAIN FORMATION 

Feldspathic, micaceous granulite 

and garnet, sillimanite schist 

GNEISSES OF THE BERKSHIRE MASSIF 

Yhbg Hornblende-biotite gneiss 

Ygg Granite gneiss 

Yag Hornblende-biotite amphibolitic gneiss 

am Amphibolite 

THRUST FAULT-Sawteeth on upper 

2 KILOMETERS 

zZ 

} 

Q~ >--' gv.; 

a: a: 

ow 

a::!: 

wo 

c... ...... 

§:;~ 

u 

0 

N 

0 

a: 

I=! 

0 

g: 

w 

...... 

CJ 

CJ 

~ 

Figure 13. Geologic map of the granite at Yale Farm and vicinity. From Harwood (1979a). 

Uranium-Lead Systematics of a Mixed Zircon Population-Granite at Yale Farm, Connecticut 87 
Table 12. Uranium-thorium-lead isotopic ages of S-2--67 zircon standard 

[Decay constants: 238U= 1.55125x w-to yr- 1; 235U=9.8485x w-' 0 yr-1; 232Th=4.9475x to-" yr-1

; 238Ut235U= 137.88. Isotopic composition of common lead assumed to be 204Pb: 206Pb: 207Pb: 208Pb=l:l8.1:15.6:37.9] 

Approx. of lead Age, in millions of years Concentration Isotopic composition Date weight 

analyzed (milli- (parts per million) 

(mo./yr) grams) u Th Pb 20"Pb 

1/77 51 . 6 248.6 158.7 1 6. 7 3 0.0295 

10/77 42.9 245.6 1 61.6 16.61 0.0293 

11/7 7 3 4. 4 245.5 1 61 . 2 1 6. 4 6 0.0287 

2/78 32.4 247.6 1 62.9 17.03 0.0478 

2/78 32.7 246.4 161 . 4 16.61 0. 01 94 

12/79 3 4. 5 253. 1 1 63.6 16.76 0.0128 

12/80 10. 9 253. 1 163. 1 16.59 0. 01 01 

12/80 1 4. 2 249.3 162.3 16.56 0.0150 

7/81 9.8 24 9. 1 1 61 . 3 16.72 0. 01 55 

5/82 11.9 250.1 1 63.5 1 6. 82 0.0418 

2/83 27.0 252.7 163.7 16.76 0. 021 3 

6/83 9. 1 251.9 159.5 16.59 0.0185 

Average----------- 249.4 1 61.9 16.69 

Standard deviation- ±2.6 ±1. 5 ±0.15 

Coefficient of---- ±1. 0 ±0.9 ±0.9 

variation (percent) 

and represent less than 0.5 percent of the sample lead. 

After removing a nominal blank from each analysis, the 

remaining 204Pb was attributed to initial lead in the zircon 

that was assumed to have a model isotopic composition 

of 206PbP04Pb= 18.0, 207PbP04Pb= 15.6, and 208Pb/ 204Pb=38.0. In no instance was the correction for either 

the chemistry blank or the initial lead a significant factor 

of uncertainty in the zircon age calculations. The initial 

lead isotopic composition is a critical consideration in determining the sphene ages, and, accordingly, the lead 

isotope ratios were measured in a coexisting microcline 

and were corrected for 430 Ma of in-place radioactive 

decay. We then used this feldspar lead isotopic composition e06PbP04Pb = 18.46, 207PbF04Pb = 15.61 , 208Pb/ 204Pb=38.14), which differs slightly from the model 

isotopic composition, to compute the sphene ages. 

All tracers were calibrated periodically against 

gravimetrically prepared shelf solutions, and over the 

lifetimes of individual tracers, concentrations of uranium 

and thorium remained constant to within ± 0. 3 percent, 

and of lead to within ± 0. 2 percent with no systematic 

drifts noted. Also, an analysis of the USGS BCR-1 rock 

standard was performed during the course of the study, 

and uranium, thorium, and lead contents were found to 

be within 0.2 percent of the published values (Tatsumoto 

and others, 1972). In order to establish the overall accuracy in element concentration for a sample, we need to 

add the effects of weighing errors, chemical and isotopic 

inhomogeneity, and mass spectrometer fractionation. 

From experience gained on replicate analyses and interlaboratory comparisons, we can demonstrate a short-term 

accuracy spanning the duration of this study of about 

± 0.4 percent (2cr) for uranium, thorium, and lead contents, although over a longer period of time (that is, the 

88 Shorter Contributions to Isotope Research 

(atom percent) 206pb 2 0 7 p b 2o'Pb ~ 

2 o 6 Pb 207pb 2oePb 2 3. u 2 3 5 u 2 0 6 p b 232Th 

78.02 4.664 17.29 379.2 379.5 381 . 5 383.9 

78.03 4.665 17.27 381 . 1 381.4 383.3 374.2 

78.05 4.652 17.27 378.0 378.4 380.5 372.2 

77.28 4.892 1 7. 78 382. 1 382.0 381.9 376.0 

78.39 4.542 17.05 382.4 382.7 384.5 378.2 

78.65 4.454 16.88 377.7 378.2 381.5 378.3 

78.76 4.425 16.80 374.7 376.0 383.9 376.2 

78.54 4. 481 1 6. 9 6 378. 1 378.6 381.9 376.7 

78.52 4.490 16.98 381.8 382.0 383.4 382.7 

77.51 4. 81 3 17.64 375.4 376.0 379.8 372. 1 

78.36 4.557 17.07 376.1 376.5 378.6 375.0 

78.53 4. 5 31 1 6. 91 374.6 375.6 381 . 6 379.8 

378.4 378.9 381.8 377.1 

±2.9 ±2.6 ± 1. 7 ±3.7 

±0.8 ±0.7 ±0.4 ±0.9 

past 5 years) that value needs to be approximately doubled. A good indication of long-term reproducibility, 

which also should approximate accuracy, comes from a 

tabulation of results for our S-2---67 standard zircon (see 

table I2). 

Based on replicate analyses of various lead isotope 

standards, the following uncertainties (2cr) are assigned 

to the measured ratios: 206PbF04Pb ± 0.4 percent (1000), 

±2 percent (10,000), ± 10 percent (100,000); 206PbF07Pb 

±0.09 percent; 206Pb/208Pb ±0.13 percent. Although at 

first glance the isotopic data for S-2---67 standard zircon 

suggests a much greater uncertainty in replicate isotope 

ratios, this variability is due almost entirely to different 

blank and (or) initial lead contents among the analyzed 

fractions. After this factor is removed, the calculated ages 

attest to a much greater precision in measurement of the 

radiogenic isotopes. Especially for the 207Pb;2°6Pb age 

with its precision of 0.3 percent can we recognize an uncertainty in this ratio commensurate with that just quoted. 

A more complete treatment of error assignment to uranium-thorium-lead isotopic data as used in this paper is 

given by Ludwig (1979). 

GEOCHRONOLOGIC RESULTS 

Standard heavy liquid and magnetic mineral separation techniques were used to obtain >99 percent pure zircon and sphene concentrates, which were additionally 

handpicked of any remaining impurities to provide the 

sample material for analysis. The zircon was sized 

through silk screens into -50 + 100, - 100 + 150, 

-150+200, -200+250, -250+325, -325+400, and 
s: 

Dol 

:I 

c:· 

,... 

~ 

~ 

~ 

~ 3 

~ ;:;· 

"' a 

Dol 

~ ><. 

~ 

~ 

N §. 

:I 

l 

c 

i g· 

b 

;: 

:I 

;: 

~ 

-< Dol 

;-

"TT 

Dol 

__ 3 

~ 

0 

:I 

:I 

~ 

~ ;:;· 

s. 

..c = 

~ 

~ 

I 

I lla lib m 

Figure 14. Photomicrographs of representative grains of the three varieties of zircon from the granite at Yale Farm. I, Colorless, acicular, euhedral 

grains; II, (a) Clear, brilliant, euhedral to (b) dark-brown, opaque, anhedral crystallographically complex grains; Ill, Composite grains with distinct rims 

and cores. Largest grains are about 700 micrometers in long dimension. 
-400 mesh fractions, and each fraction was then weighed 

and examined under the binocular microscope. We were 

able to recognize three different morphological types of 

zircon: I, a colorless, euhedral variety comprised of simple, acicular crystals that increase in relative abundance 

with decreasing grain size; II, a crystallographically complex variety forming a continuum from clear, brilliant 

euhedral to dark-brown, opaque anhedral grains; and III, 

dominating all but the -400 mesh size fraction, composite grains with a distinct rim overgrowing a rounded and 

usually less turbid core. Initially, we thought that the extreme manifestations of variety II zircon represented two 

separate genetic groupings' but a more careful microscopic examination and subsequent electron microprobe 

analyses showed the morphological differences to be within a single population and to relate almost entirely to 

radioactivity. Apparently, the clarity of the zircon is a 

function of accumulated radiation dosage with the increased coloration and deteriorated crystallinity representing more advanced stages of metamictization. In order 

to facilitate discussion, the end-member components will 

be designated as follows: Ila, clear, low uranium, and 

lib, brown, high uranium subvarieties. The division between them is defined arbitarily on the basis of perceived 

color, and the two subvarieties are differentiated mainly 

for the purpose of making rough estimates of abundance. 

Photomicrographs illustrating representative grains of the 

three varieties of zircon are presented in figure 14. The 

approximate proportion of morphological types among the 

seven size fractions has been determined by point-counting grain mounts (fig. 15), although a number of grains 

leave doubt as to their classification under this simplistic 

scheme. The greatest uncertainty arose in the assignment 

to variety II of grains containing virtually no hint of an 

internal interface until examined under the microprobe, 

where their true composite nature was revealed. Here, 

again, we came to recognize the visual contrast between 

overgrowth and core as due to differences in their uranium 

content and accumulated radiation damage. 

Uranium-thorium-lead isotopic analyses were made 

on total splits of each of the seven size fractions of zircon, 

and the resultant data are given in table 13 and are plotted 

on a concordia diagram in figure 16. As we proceed with 

the discussion of the zircon isotopic systematics, a strong 

decoupling of the 238U .. 206Pb and 235U .. 207Pb and the 232Th .. 208Pb decay schemes soon will become apparent. 

Whereas the uranium and uranogenic lead isotopes give 

every indication of closed-system behavior, the thorium 

and thorogenic lead isotopes must have been affected by 

major postcrystallization open-system movement. We suspect that at least some of the thorium and resultant 

thorogenic lead is sited quite differently from the uranium 

and, thus, account for their independent migration. Attempts to identify such sites, either within the zircon lattice or in some separate, included phase, have not been 

90 Shorter Contributions to Isotope Research 

180 

160 

140 

120 

CJ) 

~ 

<t: 

a: 100 (!) 

::::i 

...J 

~ 

~ 

.._.: 

I 80 {!) 

w 

3: 

60 

40 

20 

0 

ZIRCON VARIETY 

-

Ill 

lib 

II a ~~?~?:~ - - I 

t-- -

t-- -

t-- -

-

-

~~~,tw~~' 

~i{~l '''~" ~~ ~,,,, '' ' ' -

/, 

~ %?/,?"i~:'ix~ 

-50 

+100 

-1 00 -1 50 -200 -250 -325 -400 

+1 50 +200 +250 +325 +400 

MESH-SIZE FRACTION 

Figure 15. Abundances of the three varieties of zircon 

from the granite at Yale Farm as a function of mesh 

size. 

successful yet, although, as is true of the -200 + 250 

mesh fraction, there is reason to expect the presence of 

a discrete thorium mineral. Quite frankly, we are puzzled 

by the discrepancy in the thorium-lead system, which invites concern about a correlative uranium-lead effect. 

Nevertheless, evidence for such an effect was not seen 

in the remarkably regular behavior of the uranium-lead 

systematics. 
Table 13. Uranium-thorium-lead isotopic ages of zircon and sphene from the granite at Yale Farm 

[Decay constants: 238U= l.55125X 10-10 yr -I; 235U=9.9585X w-to yr -I; 232Th=4.9475X 10-11 yr-1; 238U;2 35U= 137.88 Isotopic composition of 

common lead assumed to be 204Pb: 206Pb: 207Pb: 208Pb=l:l8.0:15,6:28.0, except for sphene where 204Pb:206Pb: 207Pb: 208Pb=l:l8.46:15.61:38.14 

as determined in coexisting micro line] 

Concentration Isotopic composition of lead Age, in millions of years 

206pb 207pb 207pb 208pb (parts per million) 

Sample Mesh size u Th Pb 20'+pb 

a (-50+100) 561.3 146.6 48.00 0.0100 

b (-100+150) 708.0 299.4 65.47 .0051 

c (-150+200) 838.2 398.6 75.76 .0076 

d (-200+250) 895.4 762.4 83.43 .0054 

e (-250+325) 944.2 519.8 79.15 .0050 

f (-325+400) 958.4 653.6 76.72 .0056 

g (-400) 1087.4 867.2 83.20 .0154 

h ( -50+150)' 

variety IIa 236.7 74.4 21.09 .0392 

i (-50+150), 

variety IIb 1513.2 315.9 189.22 .0052 

S-I Sphene I 22.8 29.4 3.81 .629 

S-II Sphene II 17.9 20.4 3.96 .821 

Most obvious in these total split data is the resemblance to a mixing array, which we were anticipating from 

the prior preliminary analyses of the granite at Yale Farm 

and elsewhere in the Berkshire massif. To a first approximation, the linear array can, in fact, be explained rather 

straightforwardly as a binary mixture of xenocrystic and 

igneous components. The uranium and thorium concentrations also support such two-component mixing with the 

less radioactive, xenocrystic zircon most abundant in the 

coarsest fractions, and the more radioactive, igneous zircon most abundant in the finest fractions. The high 

thorium content of the -200 + 250 mesh fraction is 

anomalous in this regard, but it might be explained by 

some undetected thorite or other high thorium phase included in that particular sample. In the coarser material, 

however, especially in the -50+ 100 and, probably, in 

the -100 + 150 mesh fractions, the presence of a third 

component that apparently was affected by some younger 

disturbance of its uranium-lead system rules out a completely binary behavior for the whole population. This 

third component appears to be related to rare but highly 

radioactive (> 3000 ppm U; ppm, parts per million) do-

(atom percent) 

206pb 207pb 208pb 23Bu 23Su 206pb 232Th 

88.25 5.825 5.918 541 584 754 409 

86.64 5.595 7.763 574 607 733 373 

85.86 5.496 8.636 556 585 699 358 

81.08 5.115 13.80 542 569 678 336 

84.77 5.216 10.01 511 533 628 337 

82.80 4.943 12.25 478 492 557 319 

80.81 4.795 14.38 446 451 474 299 

85.10 5.790 9.071 540 562 652 484 

90.38 6. 214 • 3.404 795 814 866 433 

48.31 11.88 39.18 443 445 459 442 

43.01 14.39 41.78 445 449 473 456 

mains in some of the large variety lib zircon, which we 

have avoided in the later analyses. 

The remarkably straight line defined by the 

-150+200, -200+250, -250+325, -325+400, and 

-400 niesh fractions does invite further consideration of 

those fractions as a true binary mixture. We see from 

figure I5 that the trend in apparent age for these five fractions correlates almost solely with the relative abundance 

of varieties I and III zircon. Such a correlation would 

be expected if-as seems reasonable-variety I is entirely 

related to the crystallization of the granite at Yale Farm 

and variety III records a polygenetic origin whereby the 

cores are partially resorbed zircon from the Middle 

Proterozoic crystalline basement and the overgrowths were 

added during granite crystallization. Barring any other 

complications in the isotopic systematics, the straight-line 

fit to the five data points will have a lower intercept with 

the concordia curve yielding the time of granite crystallization and an upper intercept representing the age of the 

xenocrystic component (Wetherill, 1956). Indeed, a leastsquares regression of the data does indicate linearity to 

within experimental uncertainty with lower and upper inUranium-Lead Systematics of a Mixed Zircon Population--Granite at Yale Farm, Connecticut 91 
0.11 

0.10 

0.09 

:::::> 

00 

C") 

N 

\l) 

~ 0 

N 

0.08 

0.07 

0.06 

0.05~----~-----L----~------L-----~----~----~------~----L-----~-----L----~ 

0.4 0.5 0.6 0.8 0.9 1.0 

Figure 16. Concordia diagram for zircon and sphene from the granite at Yale Farm. Letters a~h are those 

of samples listed in table 13. 

tercepts of 430± 10 Ma and 1,050±40 Ma, respectively. 

These results certainly are within the time constraints established from geologic field relationships, and, when 

considered together with the sphene uranium-lead ages 

presented here, provide the best radiometric age we have 

for the emplacement of the granite at Yale Farm. 

Nevertheless, the extreme sensitivity of such a concordia 

intercept age to but a slight rotation of the determining 

data points prompts some cautionary comment. 

Our experience has been that the most common 

complication to be superimposed on such a binary mixture 

of zircon arises from radiogenic lead loss accompanying 

near-surface dilatancy of the host rock. In New England 

this lead loss appears to take place quite recently-often 

indistinguishable from zero age at the resolution of concordia plots-and this phenomenon has the effect of moving data points toward the origin of the diagram. Furthermore, the amount of displacement usually correlates with 

the accumulated radiation damage in the zircon so that 

grains having the highest uranium content have the greatest percentage of lead loss. Accordingly, if any lead had 

92 Shorter Contributions to Isotope Research 

been lost during recent rock dilatancy, we would predict 

it to result in a downward translation combined with a 

counterclockwise rotation of our Yale Farm trend. That 

is, because of the increasing uranium content with the 

decreasing grain size, the finer grained fractions should 

show proportionately more displacement. One might argue 

that the coarser fractions contain a higher percentage of 

the older, inherited component of zircon and, therefore, 

really have had greater total radiation dosages. However, 

any such preexisting damage certainly was annealed at 

the time of granite emplacement. Also, we feel confident 

that the contribution of the highly radioactive third component to the -150 + 200 and finer total splits is negligible. Most important to our discussion here is that the consequence of a downward translation and (or) counterclockwise rotation of the array would be to reduce the 

lower intercept age. Because an Early Silurian to Late 

Ordovician age for the granite at Yale Farm is near the 

Middle Ordovician upper limit of its allowable stratigraphic range; the data points do not appear to have been 

moved significantly. The good agreement of the upper 
intercept age with that of the Middle Proterozoic basement 

further suggests that rotation of the linear array has not 

taken place. 

Thus, from these zircon data alone we favor the 

conclusion that the granite at Yale Farm is of late Taconic 

rather than Acadian age. The calculated 430 ± 10 Ma age 

should, in fact, be regarded as a minimum if the zircons 

have lost even a minor amount of postcrystallization 

radiogenic lead. In a further effort to establish the age 

of this granite, two samples of sphene-a more magnetic 

(sphene I) and a less magnetic fraction (sphene 11)-were 

analyzed (table 13). Our attempt to obtain a chemical distinction between the two samples by magnetically fractionating the abundant sphene was moderately successful, 

but the high common lead content of both fractions preclude a determination of precise 207PbP35U and, consequently, precise 207PbP06Pb ages. In order to reduce 

the uncertainty in making the common lead correction, 

we also analyzed the potassium feldspar from the granite 

as a means of ascertaining the isotopic composition of 

lead in the original magma. The resultant feldspar lead 

isotopic ratios e06PbP04Pb= 18.46; 207PbP04Pb= 15.61; 

208Pb;204Pb=38.14; Pb=32 ppm, U=0.06 ppm, Th=0.04 

ppm), corrected for almost negligible in place uranium 

and thorium decay is used in calculating the sphene ages 

given in table 13. Emphasis is placed on the closely agreeing 206PbP38Pb and 208PbP32Th ages, which are least susceptible to common lead correction, although no discrepancy not accommodated by analytical uncertainties exists 

even in the highly sensitive 207Pb/206Pb age. 

We take these sphene data as confirmation of a 

Taconic age for the granite at Yale Farm, and propose 

that it was emplaced sometime within the 430-450 Ma 

range delineated by our combined zircon and sphene results. The fact that the sphene ages slightly exceed the 

concordia lower intercept age obtained for the zircon may 

be meaningful, but we would be extremely cautious about 

making so exact an interpretation of these limited data. 

Although an experimental precision with potential of resolving Paleozoic ages at the ± 3-Ma level has been demonstrated by repeated analysis of our interlaboratory zircon 

standard, it is now likely to be geologic factors that prevent us from generally reaching such a goal. When we 

acknowledge that sillimanite-grade metamorphism has 

been superimposed on these rocks, an opportunity for serious disturbance to the isotopic systems being investigated 

certainly is present. The result of such metamorphism 

upon zircon isotopic systematics can be complex, especially if a new generation of zircon is produced, but the 

overall effect is usually some lowering of the primary parent -daughter as well as 207PbP06Pb ages toward that of 

the metamorphism. Either an increase or decrease in age 

might result from metamorphic recrystallization of sphene 

depending upon the relative losses or gains of uranium 

and radiogenic lead. In addition, newly grown zircon or 

sphene during a subsequent metamorphism would be expected to record the younger age of that event. Again, 

we stress that the rather tight clustering of ages for the 

granite argue against any drastic modification of the geochronologic systems in the case at hand. A small amount 

of disturbance cannot be ruled out, however, and, on the 

basis of our current data, we can assign no more precise 

an age than is implied by the 430- to 450-Ma range that 

we have quoted. 

In an attempt to shed light on the nature of the 

variety II zircon found most abundantly in the coarser 

mesh fractions, typical grains of the clearest, euhedral 

subvariety Ila and of the darkest brown, anhedral subvariety lib (hereafter simply called varieties Ila and llb) were 

handpicked carefully from the combined -50 + 100 and 

- 100 + 150 mesh fractions. Early in the study we anticipated that variety Ila formed predominantly when the 

granite crystallized, and that variety lib was inherited 

from the source rock of the granite. As such, these two 

analyses were seen as holding promise of extending the 

chemical and isotopic characterization of the mixture in 

the directions of both "pure" components. Consequently, 

several surprises did arise from the analytical data (see 

table 12) obtained for them. In hindsight, we do note (1) 

that variety Ila zircon frequently does have a chemical 

interface between a core and overgrowth under the microprobe even though that interface is optically imperceptible, and (2) that variety lib zircon is darker colored than 

the cores of most variety III zircon and itself shows few 

overgrowths. A further dilemma over our early interpretation arose when we realized that the two seemingly quite 

different morphologies were actually gradational into each 

other. We now know that both subvarieties entirely or 

in part derive from Middle Proterozoic source rock(s), although some questions about how their physical and 

chemical characters merge are left unanswered by these 

analyses. 

Perhaps not so unexpectedly in view of its observed 

gradation into composite grains, the variety Ila zircon reveals a 207Pb/206Pb age demanding that a significant contribution by an old, inherited component is included in 

it. Indeed, the position of the analysis on the concordia 

diagram permits only a slightly larger proportion of the 

younger component than is contained by the -50 + 100 

and - 100 + 150 mesh total splits. If we allow for the 

contribution to the total splits by the old variety lib zircon 

discussed below, no difference in relative abundance of 

end members can be demonstrated between varieties Ila 

and III zircon. What does serve to contrast them is their 

uranium and thorium concentrations, which are a factor 

of 2-3 lower in the handpicked clearest grains. Although 

this range in uranium and thorium content is geochemically interesting, we conclude that any genetic distinction 

between varieties Ila and III is more illusionary than real. 

Highly variable uranium content among individual grains 

Uranium-Lead Systematics of a Mixed Zircon Population-Granite at Yale Farm, Connecticut 93 
of otherwise homogeneous zircon populations is common 

in fission-track studies, and the concurrent effects of radiation dosage on crystal transparency, color, and morphology is well known. Thus, secondary rather than primary 

factors offer the best explanation of what, at their extreme 

development, appear to be profound varietal differences 

in zircon types. 

At first glance, the analysis of the dark-brown 

anhedral variety lib zircon seems to fit nicely with our 

anticipation that it was inherited largely from the source 

rock of the granite. The data point falls right on the 430-

to 1 ,050-Ma mixing line as defined by the five finer 

grained total split samples, and indicates a much greater 

proportion of the older component than is contained by 

any other fraction. Assuming that its contact with the 

magma had caused some episodic lead loss, the variety 

lib zircon might, in fact, qualify as the "pure" end member component to the mixing line if we ignored the uranium and thorium contents. However, judging from our 

isotopic and microprobe analyses, the variety lib zircon 

is much too radioactive to represent the postulated old 

end member. Although apparently sharing a similar Middle Proterozoic age with the variety III cores, the brown 

zircon with its lack of overgrowths and color distinction 

records a somewhat unique history. We propose that the 

cores of variety III zircon were incorporated into the 

magma of the granite at Yale Farm at an early stage of 

melting so that they first were partially resorbed and then 

overgrown as the magma accumulated, migrated upward, 

and finally solidified. On the other hand, the variety lib 

zircon was acquired as xenocrysts at a much later stage 

in the magmatic sequence and thereby almost completely 

escaped subsequent modification. The chemical and color 

distinctions between the zircon represent the different 

lithology but similar age of the country rock from which 

the two varieties were released. Of course, other interpretations that seek to reconcile the different uranium and 

thorium contents of the zircon wholly in terms of degree 

of reactivity with the magma are possible also. 

ELECTRON MICROPROBE ANALYSIS 

OF ZIRCON 

As a complement to the uranium-lead dating study, 

selected grains of different morphological types of zircon 

from the granite at Yale Farm were analyzed and characterized by the electron microprobe. Using this technique, 

we were able to identify distinct chemical domains, which 

correlate well with our interpretation of a polygenetic origin to the zircon population. In particular, the 

hypothesized Middle Proterozoic and the Lower Silurian 

or Upper Ordovician components can be recognized separately, whether present as individual or composite grains, 

based on their hafnium content. In this section we document the concentration and distribution of a few selected 

94 Shorter Contributions to Isotope Research 

elements, but offer no further genetic modeling for these 

data beyond that put forward to explain the uranium-lead 

systematics. 

Individual zircon grains representing each morphological type were probed to determine their chemical 

composition (table 14). The samples were analyzed at 20 

kilovolts and 10 nanoamps sample current with a de focused beam of 1~f.Lm (micrometer) diameter. Both natural minerals and synthetic glasses were used as standards, 

and data reduction was carried out by an atomic number, 

absorption, and fluorescence (ZAF) scheme. For varieties 

II and III composite grains, separate analyses made of 

cores and overgrowths show their different minor element 

concentrations. We note, first of all, that each analysis 

verifies a near approach to stoichiometric (Zr,Hf)Si04 for 

the zircon, and at the dimension of the probe beam, the 

absence of any other mineral species. The only elements 

in addition to Zr, Hf, and Si found were trace amounts 

of U, P, Ca, Th, and REE (rare-earth elements) sporadically indicated near their detection limits. These trace elements all could be crystallographically bound in the zircon 

lattice, and we tentatively make that assumption in the 

ensuing discussion. 

The most useful aspect of these data in table 14 

is in the systematic behavior of the two elements, hafnium 

and uranium. Almost certainly, the hafnium substitutes 

for zirconium in the basic mineral lattice, whereas the 

uranium is in some unknown but possibly correlative site 

to the hafnium. Immediately apparent is the higher average hafnium and uranium concentration in variety I grains 

and variety III overgrowths (1. 73 ± 0.36 and 2.05 ± 0.32 

percent Hf, and 1, 100 ± 500 and 1 , 000 ± 400 ppm U, respectively) than in variety III cores (1.21 ± 0.24 percent 

Hf, and 400± 300 ppm U). Such a dichotomous chemical 

characterization of the zircon runs parallel with the previously proposed two-component mixing model whereby 

one component formed during crystallization of the granite 

and the other comprises the xenocrystic contribution to 

the magma. Although significant fluctuation exists in the 

hafnium and, particularly, the uranium contents of both 

components, we are convinced that a statistical distinction 

can be drawn between the two generations of zircon. Furthermore, such probe analyses of variety IIa grains first 

disclosed the true composite nature of the grains by showing a chemical interface between core and overgrowth not 

detectable under the microscope. 

The intermediate hafnium and high uranium concentrations (1.60 ± 0.20 percent Hf, and 1,900 ± 900 ppm U) 

of variety lib zircon do not fit so well into a dichotomous 

scheme, but this discrepancy could be taken as additional 

evidence of heterogeneity within the xenocrystic component. A possible scenario supporting such heterogeneity 

to explain the rarity of this high radiaoctivity species as 

the cores of variety III zircon has been given already. 

With regard to variety lib zircon, we should mention also 
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g s 0 s 0 0 0 

8 ~ ~ 8 6 8 8 8 8 

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ........ ~ 0 0 0 0 0 0 

~ ~ ~ 8 

~ ~ ~ ~ ~ <""\ 0 N \.0 N 0 

1.1"'! ~ N.O 

"' <""\ 

~ "' 

C"i N ...-I N N 

0 0 0 0 0 

~ ~ ~ ~ ~ ll'\ N N <""\ N 0 0 0 0 0 

0 0 0 0 0 

~ CO 1"- \.0 \.0 1"- ~ __,. N \.0 0 ~ 0 0 N <""\ 

"' <""\ \.0 N <""\ 0 0 0 0 0 0 

~ ~ ~ 6 ~ 6 6 .1r ~ " ~ 0 0 0 0 0 0 

~ ~ g s o e o e e 

~ "' co co <""\ 0 

~ ~ '+: 0 <""\ <""\ 

co " <""\ 

8 ~ 8 8 ll'\ ll'\ 0 0 

0 ~ ...... 

~ ~ 0 * 0 3 ~ ...... 

N 0 0 0 0 0 0 <""\ 

<""\ <""> 0 ll'\ N 0 N 0 N 0 N QJ N 0 ,.C: 

>' U 0... :::l H 

the presence of a few grains with almost black domains 

having 3,000-5,000 ppm U. A brief HF acid leach shows 

these domains to be especially soluble, and we speculate 

that they may be the sites of postcrystallization radiogenic 

lead loss implied by the uranium-lead isotopic data for 

the -50 + 100 and - 1 00 + 150 total split analyses. Care 

was taken to exclude such grains with identifiable black 

domains from the handpicked sample of variety lib zircon 

analyzed subsequently. 

One final warning about the apparent correlative behavior of hafnium and uranium may be in order. Unlike 

the hafnium, which probably does represent rather 

straightforwardly the restricted Hf/Zr of the crystallization 

environment, uranium content is found to vary by at least 

an order of magnitude within a component and to have 

considerable overlap between components. Although noting that the overgrowths of variety III zircon generally 

do contain more hafnium and uranium than the cores, a 

direct coupling of the two elements need not exist. Thus, 

when we probed several unusual variety III grains that 

had a more turbid core than overgrowth, they did, in fact, 

have a higher uranium content in the core than the overgrowth while maintaining a normal hafnium relationship. 

Perhaps the most interesting use of the microprobe 

comes from the detailed study of the interface between 

two generations of zircon within a single variety III grain. 

We chose to work on a zircon that had an especially welldeveloped core and overgrowth relationship so as to facilitate an optical comparison with the microprobe analyses. 

A photomicrograph of the subject grain, its electron 

backscatter electron image, and zirconium and hafnium 

scanning maps are shown in figure 17. Partial chemical 

analyses of nine selected 1-jl.m diameter spots of the core 

(1-5) and overgrowth (6--9) are presented in table I5. 

Again we note the essential stoichiometry of the zircon. 

As we had come to expect from the reconnaissance 

survey, the contrast in hafnium and uranium content between the core and overgrowth are immediately apparent 

from the chemical data. Corresponding to the chemical 

distinction, an abrupt change in cathodoluminescence intensity of the probe spot also was observed at or near 

the interface between core and overgrowth. The core, depleted in hafnium, exhibited a bright luminescence, 

whereas the overgrowth, enriched in hafnium, had a faint 

cathode luminescence. Luminescence spectra were measured with a grating monochrometer, and, except for the 

intensity, all spectra were identical with strong components at 465 and 505 nanometers. At one end of the crystal, the intensity boundary did depart appreciably from 

the interface as defined optically and by the crack pattern 

on the backscatter image (see fig. I7 for comparison). 

Our explanation of this disparity is that the actual crack 

as seen in the photomicrograph and backscatter image is 

a secondary, stress-induced feature, which only approximately follows the chemical boundary. Thus, although 

Uranium-Lead Systematics of a Mixed Zircon Population-Granite at Yale Farm, Connecticut 95 
A 

Figure 17. Photomicrograph (A), electron backscatter image (8), and zirconium (C) and hafnium (D) scanning maps 

of a select variety Ill zircon. Grain is about 550 micrometers in long dimension. 

96 Shorter Contributions to Isotope Research 
Table 15. Electron microprobe analyses of a selected variety Ill zircon 

Core Over~rowth 

Analysis 1 2 3 4 s 6 7 8 9 

Zro2 66.6S 66.46 66.11 67.0S 64.92 66.16 6S.62 6S.34 66.02 

Hf02 1.06 1.03 .96 1.0S .99 1.92 1.79 2.19 1. 88 

Si02 32.88 32.2S 32.31 32.S6 31.98 31.64 31.64 31.62 32.31 

Cao2 .03 .01 .01 .oo .03 .OS .03 .oo .02 

Y203 .03 .01 .oo .01 .02 .OS .OS .03 .03 

Ce2o3 .01 .01 .oo .01 .oo .03 .oo .oo .oo 

P20S .01 .01 .01 .oo .01 .02 .01 .01 .oo 

uo2 .04 .03 .01 .01 .02 .13 .02 .10 .03 

Th02 .03 .oo .01 .01 .01 .os .'02 .oo .01 

Sum---- 100.24 99.81 99.42 100.70 97.98 99.0S 99.18 99.29 100.30 

1Numbered analyses are keyed to the indicated positions on the photomicrograph 

of the grain in figure 17. 

stresses associated with the different chemistry and (or) 

accumulated radiation damage probably is greatest right 

along the interface of the core and the overgrowth, fracturing may locally jut irregularly into one or the other 

of the domains. The more fractured and turbid character 

of overgrowths relative to cores of many variety III zircons (see figs. I4 and I7) could be another manifestation 

of this phenomenon-suggesting that greater internal 

stresses do accompany the higher uranium content. 

A corollary consideration in understanding the nature of the core to overgrowth relationship is the sharpness 

of the chemical gradient across the interface. In addition 

to the implications for stress concentration, the width of 

the chemical transition band holds information about diffusive distances during and after crystallization of the 

overgrowth. For example, if the transition is within the 

resolving capacity of the microprobe (2J.Lm), an argument 

could be made for insignificant diffusive movement of 

hafnium and uranium during the superimposed Acadian 

orogeny. An equivalent immobility of radiogenic lead 

would then explain the apparent post-Taconic closed-system behavior of most of the zircon population. On the 

other hand, a broad diffusion band certainly might raise 

the issue of some episode of substantial ion migration. 

Although the crack pattern prevents a precise delineation 

of this chemical gradient along much of the perimeter of 

the core, an excellent opportunity exists to make the 

measurement where the crack does depart from the 

luminescence boundary. Analysis 5, table I5, of the core 

and analysis 6 of the overgrowth represent immediately 

adjacent probe spots on either side of the luminescence 

boundary separated by no more than 4 J.Lm. These 

analyses do, in fact, bracket the full transition in hafnium 

and uranium contents, and thereby document an extremely 

abrupt gradient. 

At this early stage in our use of the electron microprobe to supplement geochronologic studies, we find that 

almost every grain offers some new insight and surprise. 

However, the sensitivity of the technique severely limits 

its application in making quantitative measurements of 

trace elements below the 0.1 percent (1 ,000 ppm) level. 

We were fortunate to find so great a "hafnium effect" 

in this suite of zircons. The higher uranium concentration 

of the overgrowths compared with that of the cores, which 

was predicted by our binary mixing model, also can be 

seen in the microprobe data. However, analytical uncertainty and probable real heterogeneity within and among 

grains prohibit their quantitative use to define end-member 

compositions. 

REFERENCES CITED 

Chase, R. B., Bickford, M. E., and Tripp, S. E., 1978, Rb-Sr 

and U-Pb isotopic studies of the northeastern Idaho batholith 

and border zone: Geological Society of America Bulletin, 

V. 89, p. 1325-1334. 

Grauert, B. W., Hanny, R., and Soptrajanova, G., 1974, Geochronology of a polymetamorphic and anatectic gneiss region-The Moldanubicum of area Lam-Degendorf, Eastern 

Bavaria, Germany: Contributions to Mineralogy and Petrology, v. 40, p. 105-130. 

Harwood, D. S., 1975, Fold-thrust tectonism in the southern 

Berkshire massif, Connecticut and Massachusetts, in 

Ratcliffe, N. M., ed., Guidebook for field trips in western 

Massachusetts, northern Connecticut, and adjacent areas of 

New York, New England Intercollegiate Geological Conference, 67th annual meeting, New York City College, p. 

122-143. 

---1979a, Geologic map of the South Sandisfield quadrangle, Massachusetts and Connecticut: U.S. Geological 

Survey Geologic Quadrangle Map GQ-1519. 

Uranium-Lead Systematics of a Mixed Zircon Population-Granite at Yale Farm, Connecticut 97 
---1979b, Bedrock geologic map of the Norfolk Quadrangle, Connecticut: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1518. 

Koppel, V. , and Grunenfelder, M. , 1971 , A study of inherited 

and newly formed zircons from paragneisses and granitised 

sediment of the Strona-Ceneri zone (Southern Alps): 

Schweizerische Mineralogische und Petrographische Mitteilungen, v. 51, p. 385--409. 

Krogh, T. E., 1973, A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb 

for isotopic age determination: Geochimica et Cosmochimica Acta, v. 37, p. 485--494. 

Ludwig, K. R., 1979, Calculation of uncertainties of U-Pb 

isotope data: Earth and Planetary Science Letters, v. 46, 

p. 212-220. 

Manhes, G., Minster, J. F., and Allegre, C. J., 1978, Comparative uranium-thorium-lead and rubidium-strontium study of 

the Saint Severin amphoterite; Consequences for early solar 

system chronology: Earth and Planetary Science Letters, v. 

39, p. 14-24. 

Pidgeon, R. T., and Aftalion, M., 1978, Cogenetic and inherited 

zircon U-Pb systems in granites; Paleozoic granites of Scotland and England: Geological Journal Special Issue 10, p. 

183-220. 

Pidgeon, R. T., Koppel, V., and Grunenfelder, M., 1970, U-Pb 

isotopic relationships in zircon suites from a para- and orthogneiss from the Ceneri zone, southern Switzerland: Contributions to Mineralogy and Petrology, v. 26, p. 1-11. 

Ratcliffe, N. M., 1975, Cross section of the Berkshire massif 

at 42° N.; Profile of a basement reactivation zone, in 

Ratcliffe, N. M., ed., Guidebook for field trips in western 

Massachusetts, northern Connecticut, and adjacent areas of 

New York: New England Intercollegiate Geological Conference, 67th annual meeting, New York City College, p. 

186-222. 

98 Shorter Contributions to Isotope Research 

Ratcliffe, N. M., and Harwood, D. S., 1975, Blastomylonites 

associated with recumbent folds and overthrusts at the westem edge of the Berkshire massif, Connecticut and Massachusetts-A preliminary report, [chap.] A, in Tectonic 

studies of the Berkshire massif, western Massachusetts, 

Connecticut, and Vermont: U.S. Geological Survey Professional Paper 888, p. 1-19. 

Ratcliffe, N. M., and Hatch, N. L., Jr., 1979, A traverse across 

the Taconide zone in the area of the Berkshire massif, westem Massachusetts, in Skehan, J. W., S.J. and Osberg, P. 

H. , eds. , The Caledonides in the U.S. A. , Geological excursions in the northeast Appalachians, Contributions to the 

International Geological Correlation Program, Project 27-

Caledonide Orogen: Weston, Mass., Weston Observatory, 

p. 175-224. 

Ratcliffe, N. M., and Zartman, R. E., 1976, Stratigraphy, 

isotopic age, and deformational history of basement and 

cover rocks of the Berkshire Massif, southwestern Massachusetts: Geological Society of America Memoir 148, p. 

373--412. 

Stacey, J. S., and Hope, J., 1975, A program for mass spectrometer control and data processing in isotope 

geology-Written in BASIC for an 8K Nova 1120 computer: U.S. Geological Survey Open-File Report 75-127, 

48 p. 

Tatsumoto, M., 1966, Isotopic composition of lead in volcanic 

rocks from Hawaii, lwo Jima, and Japan: Journal of 

Geophysical Research, v. 71, no. 6, p. 1721-1733. 

Tatsumoto, M., Knight, R. J., and Delevaux, M. H., 1972, 

Uranium, thorium, and lead concentrations in three silicate 

standards and a method of lead isotopic analysis, in Geological Survey research 1972: U.S. Geological Survey Professional Paper 800-D, p. Dlll-Dl15. 

Wetherill, G. W., 1956, Discordant uranium-lead ages, 1: 

American Geophysical Union Transactions, v. 37, p. 

32{}.--326. 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

FISSION-TRACK DATING OF SAMPLES OF THE 

ILLINOIS DRILL-HOLE CORE 

FIGURES 

Chapter J 

By ROBERT A. ZIMMERMANN 

CONTENTS 

Abstract ....................... . 

Introduction and geologic setting . . . . . . . . . . . 

Interpretation of apatite and zircon fission-track ages . 

Experimental procedures and results . 

Discussion . . . 

Conclusions . . 

References cited 

Page 

100 

100 

101 

101 

103 

106 

107 

11 . Plot of Apatite fission-track ages as a function of present temperature in the Illinois drill holes UPH-1 

Page 

and UPH-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 

12. Graphs showing projected track-length distributions of fossil and induced tracks in sample UPH-2-2185. . 104 

TABLES 

Page 

Jl. Fission-track age data for the Illinois drill core-samples . . . . . . . . . 102 

12. Hypotheses presenting the range of possible cooling and burial over time 106 

Fission-Track Dating of the Illinois Drill-Hole Core 99 
Abstract 

Drill holes UPH-1 and UPH-2 in Stephenson 

County, northern Illinois, penetrate to a depth of more 

than 1,640 meters, first through -630 meters of Paleozoic 

sedimentary section and then into underlying Precambrian granitic basement. Samples of apatite (five) and zircon 

(one) from various depths in the recovered Precambrian 

basement core were dated by the fission-track method. 

Because of the instability of tracks in these minerals at 

high temperatures, the ages define points in the thermal 

history of the samples. 

An apatite sample just beneath the basement unconformity (614 meters) presently at a temperature of 

-2ooc (Celsius) yielded an age of 132 ± 16 Ma (million 

years) at the 95-percent confidence level. Apatite ages 

decrease linearly with depth to 50± 8 Ma (95-percent 

confidence level) for the deepest sample (1 ,640 meters) 

at a temperature of -46°C. The age of the sample closest 

to the unconformity has been totally reset (as evidenced 

by comparative track-length measurements) from an age 

expected to be older than the oldest overlying sediments. The observation that non-zero ages persist 

through the total depth of the hole demonstrates that 

the resetting or closure temperature is higher than the 

highest temperature in the hole. The fission-track age 

of the zircon sample from just below the unconformity 

is older than the age of the sediments, and thus, the 

temperature was never higher than -160--180°C during 

the Phanerozoic. 

The range of temperatures sampled suggests a 

minimum of 26°C cooling. The extrapolation of the leastsquares regression line through the data to a zero age 

is at 56±2°C. This construction requires -36°C cooling 

for the upper sample. However, a geologically accurate 

model of track fading predicts a closure temperature of 

93 ± 18°C, requiring 73 ± 18°C of post--130 Ma cooling, 

mostly after 50 Ma. 

Consideration of a lower closure temperature 

(56°C) and a hypothetical large increase in geothermal 

gradient (to 36°C per kilometer at 130 Ma) requires the 

former existence and removal of >520 meters of cover. 

At the other extreme, a closure temperature of 93°C and 

the presently observed geothermal gradient of 23°C per 

kilometer require -3,000 meters of cover, about 2,000 

meters of which was removed post-50 Ma. These data 

suggest a history of sedimentation and uplift considerably different than that commonly proposed. 

INTRODUCTION AND GEOLOGIC SETTING 

The Illinois Deep Hole Project was an interdisciplinary study of the properties and characteristics of three 

deep holes (UPH-1 ,2,3) and their cores in Stephenson 

County, northern Illinois, near the town of Winslow, not 

far from the Wisconsin State line (see fig. E1, chap. E, 

for locations of drill holes). The geophysical, geochemical, and petrographic properties, as well as isotopic ages 

1 00 Shorter Contributions to Isotope Research 

of the cored and the surrounding rocks, were studied by 

other investigators. With the objective of studying the 

cooling history of the cored basement rocks in the temperature range from ~ 180°C to ~ 1 00°C, apatite and zircon 

mineral separates were dated as part of the project. The 

results obtained in this study have broader significance 

to the former sedimentary and subsequent uplift history 

of the region in general. 

The intrusion and crystallization of the granitic 

basement rocks between 1 ,430 Ma (rubidium-strontium) 

and 1,470 Ma (lead-lead) (Van Schmus and others, 1981) 

represent the earliest preserved geology in the region. The 

basement rocks show evidence of subsequent textural alteration and deformation (Lidiak and Denison, 1981). At 

some time prior to deposition of the Cambrian Mount 

Simon Sandstone, the Precambrian basement was uplifted 

and eroded, exposing a surface of moderate relief upon 

which Phanerozoic strata were later to be deposited. At 

the site of the drill holes, the sedimentary formations present start with the Mount Simon Sandstone, above the 

basement unconformity, and continue up to the GalenaDolomite of Middle Ordovician age. The Middle Ordovician Kress Member of the St. Peter Sandstone is above 

an unconformity of regional extent. This unconformity is 

increasingly lower in the section to the north, and is related to the early phases of formation of the Wisconsin 

arch. 

The drill holes are close to the axis of the Wisconsin 

arch near its bifurcation into the Mississippi River and 

the Kankakee arches, which bound the northern end of 

the Illinois structural basin. In its northern extension, the 

Wisconsin arch brings to the surface exposures of Precambrian basement in south-central Wisconsin. Within the 

Illinois and Michigan basins and along their flanks, rocks 

of Silurian through Pennsylvanian age are preserved. 

Isopach reconstructions suggest that previously some of 

these formations extended beyond the present structural 

basins. Paleocurrent measurements on basal-Pennsylvanian formations suggest continuity of sediment dispersal 

across interbasinal basement highs such as the Kankakee 

arch (Potter and Siever, 1956). Studies of coalification 

patterns in the Illinois Basin suggest the removal of ~970 

meters of Pennsylvanian or younger sedimentary rock 

along the northern edge of the Illinois Basin, about 60 

kilometers southeast of the drill sites (Dam berger, 1971). 

It has been assumed that this is part of a general uplift 

of the Great Lakes region in post-Pennsylvanian time 

(Eardley, 1962). The presence of Cretaceous sediments, 

both to the west and to the south at the head of the Mississippi Embayment, represent the youngest pre-Pleistocene 

stratigraphy in the immediate region. Because of the thin 

and unconsolidated nature of these deposits, little burial 

or subsequent uplift can be implied where they occur and 

their presence suggests limits to allowable amounts of 

post-Cretaceous erosion in the region in general and at 
the drill site in specific. 

Zartman and others (1967) have dated a number of 

dikes and sills in southern Illinois that yield Permian ages. 

There is some suggestion of intermittent igneous activity 

into the Cretaceous and that this activity possibly was related to the subsequent formation of the Mississippi Embayment. McGinnis and others (1976), in discussing the 

regional increase in Bouguer gravity field from northern 

into southern Illinois, suggested that the gravity increase 

is caused by an increase in basement density due to deep 

level intrusions, possibly related to the evolution in the 

Cretaceous of the Mississippi Embayment structure. These 

igneous events are mentioned because of their relationship 

to a possible regional increase of heat flow and hence 

geothermal gradient during the Mesozoic. The effect of 

such an increase would be to alter significantly the interpretation of the cooling history made from the apatite fission-track ages presented here. If in the past the geothermal gradient was higher than the presently observed gradient of 23°Cikm (degrees Celsius per kilometer) (Rahman 

and Roy, 1981), the cooling produced by the decrease 

in gradient must be taken into account in addition to that 

caused by erosion. 

INTERPRET AT ION OF APATITE AND 

ZIRCON FISSION-TRACK AGES 

Fission tracks are produced by the fissioning of uranium nuclei that leave a ~ 15-J.Lm path of structurally 

damaged material in the path of the fission fragments. 

The ability to etch, optically observe, and count the damage tracks is reduced or eliminated by thermal energy. 

Geological observations such as studies of fission-track 

ages of apatites in boreholes in areas of known geothermal 

history, such as by N aeser (1979), or of the fission-track 

age of zircons relative to better defined isotopic age systems, such as presented in Harrison and others (1979), 

provide one means of determining the kinetics of fissiontrack "fading." Another approach taken is to interpret laboratory studies of track fading and to extrapolate these 

results to geologic conditions of time and temperature. 

The interpretation of laboratory data as representing a 

first-order rate law for track fading with unique singular 

values for the activation energy and preexponential factor 

describing the temperature dependence of this rate has 

been advocated by Zimmermann and Gaines (1978). This 

interpretation is the basis of a mathematical model of track 

accumulation that accurately predicts the observed age 

trends for the previously mentioned borehole studies of 

Naeser (1979) (R. A. Zimmermann, unpub. data). The 

geological appropriateness of this approach is the basis 

for using this as the method for the interpretation of the 

apatite data in the present report. 

Apatite dates are here interpreted using the concept 

of effective closure temperature defined by Dodson ( 1973, 

p. 260) as "the temperature of the system at the time 

represented by its apparent age." The closure temperature 

is dependent upon the rate of sample cooling and decreases with decreasing cooling rate. Values for closure 

temperatures applicable to the present study were calculated from the relationships given by Dodson (1973) using 

the values of the kinetic constants given by Zimmermann 

(1977). Uncertainties in this closure temperature were calculated from equations derived from Dodson (1973). As 

an example, the closure temperature of apatite for a rate 

of cooling of 1°C/m.y., is about 100°C. The closure temperatures for zircon are known only relative to other 

isotopic age systems and probably fall in the range of 

150°-180°C (Harrison and others, 1979). 

EXPERIMENTAL PROCEDURES AND RESULTS 

For this study, samples were obtained from the 

cores retrieved from the UPH-1 and UPH-2 drill holes. 

These holes penetrated 605 m (1 ,955 ft) and 660 m (2, 179 

ft), respectively, of lower Paleozoic sedimentary rock before entering the granitic basement. The temperature at 

the basement contact was about 20°C. The temperature 

at the bottom of the UPH-2 hole was about 46°C. The 

last four digits of the sample numbers in table J 1 correspond to the depth, in feet, of the sample. 

Sufficient apatite and zircon for dating were obtained from ~ 500 grams of cored basement rock. Each 

sample was separated by crushing to finer than 60 mesh 

and separating apatite and zircon from the -60 + 200 mesh 

(246-74 micrometers) fraction by heavy liquid and magnetic separation. 

All samples were prepared for age determination by 

the external detector technique using procedures similar 

to those of Naeser (1976). Apatite samples were etched 

in 7-percent HN03 at 20°C for 30 or 40 seconds. Muscovite detectors were etched in 48-percent HF at 20°C for 

~ 15 minutes. Zircons were etched in an eutectic NaOHKOH mixture at 220°C for 2-8 hours. Only two grains 

in one of eight zircon samples were suitable for counting 

because of extremely high track densities and extreme 

grain corrosion. 

For thermal neutron irradiation, the samples were 

placed in an aluminum tube with the NBS (National Bureau of Standards) reference glass 963, covered with muscovite detectors, at the top and bottom of the irradiation 

package. The irradiation was carried out in the U.S. 

Geological Survey Triga reactor (Denver) in the lazy 

susan ring at a power level of 200 kW (kilowatts). The 

thermal neutron fluence for the standard glasses was calculated using the values reported for the NBS glass irradiated in the RT -3 site with copper foils as the primary 

standard (Carpenter and Reimer, 1974). The fluence for 

each sample was calculated by interpolation between values determined for the standards. 

Fission-Track Dating of the Illinois Drill-Hole Core 101 
Table )1. Fission-track age data for the Illinois drill-core samples 

[ps and pi represent the observed fossil and induced track densitites, respectively. To correct for the 2'1T geometry of the external detector configuration, 

the induced track densities (or raw counts) are multiplied by a factor of 2 in calculating the age. tr, tracks; cm2, square centimeter; <j>, thermal 

neutron fluence (calibrated by NBS-SRM 963, copper value); n, neutrons, thermal; Ma, million years; ppm, parts per million. Quoted uncertainty 

in the age represents the 95-percent confidence limits (95 pet CL). Ages were calculated using the following constants: A.d=l.55xto- 10yr- 1

; 

r=7 .03 X 10- 17yr-l; U-235/U-238=0.00725; A.r=580.2 barns] 

Number counted Time 

Sample Depth Mineral Fossil Induced ps pi 11\ !J.Ma Uranium 

No. (meters) dated tracks tracks Grains ( 10 6tr /em 2 ) (10 6tr/cm2 ) (1 0 15n/ em 2 ) Ma (95 pet CL) (ppm) 

UPH-1-2025 614 Apatite--- 543 602 10 

UPH-2-2185 662 --do------ 584 660 10 

UPH-2-2779 842 --do------ 611 740 10 

UPH-2-3194 968 --do------ 173 271 7 

UPH-2-4773 1446 --do------ 236 545 12 

UPH-2-5411 1640 --do------ 232 688 11 

UPH-1-2004 607 Zircon---- 153 22 2 

Samples were counted at ~2,5QOX in transmitted 

light. For age calculations the following constants were 

used: A.d= 1.55X 10yr- 1

; A.f=7.03X 10- 17 yr- 1; U-235/U238=0.00725; rrf=580.2 barns. Systematic differences 

exist in commonly used methods to calibrate neutron fluences and in values of A.f. The combination of values used 

here provides an accurate value for the fission-track age 

of the Oligocene Fish Canyon Tuff (Naeser and others, 

1981) that has been well dated by various K-Ar (potassium-argon) mineral systems. Ages were calculated ultimately by pooling counts from individual grains in a sample. The age uncertainties were calculated taking into account the uncertainties in the fossil and induced track 

counts, as well as the uncertainty of the neutron fluence, 

by combining these in the manner described by Johnson 

and others ( 1979, appendix 1). The uncertainties in the 

fossil and induced track counts were evaluated by using 

the Poisson uncertainties of the individually pooled data. 

The inclusion of a term involving the correlation of individual grain counts, as suggested by McGee and Johnson 

(1979), is inappropriate in this situation and was not used. 

The uncertainties given in table J 1 and shown as error 

bars in figure J1 represent 95-percent confidence intervals. 

In order to measure the projected track-length distribution of sample UPH-2-2185, a separate mount of 

grains annealed at 550°C for one hour was prepared and 

irradiated. Fossil and induced track lengths were measured 

at ~2,500X using an electronic digitizer in conjunction 

with a drawing tube attached to the microscope. Five 

hundred and thirteen fossil tracks and 531 induced tracks 

were measured to produce the distributions shown in figure J2. 

Before discussing the dating results, note that the 

average uranium concentration of all apatite grains measured was 17 ppm (parts per million). Other experimenters, working on these drill core samples, have observed 

1 02 Shorter Contributions to Isotope Research 

1.18 1.31 4.96 132 16 16 

1.54 1. 74 4.96 130 15 22 

1.66 2.02 4.80 118 14 26 

.606 .949 4.80 91 18 12 

.413 .954 4.80 62 10 12 

.414 1.23 4.96 50 8 15 

15.0 2.16 3.92 768 325 36 

extremely high concentrations of uranium in the wholerock chemistry. The concentrations found in these apatites 

suggest that the whole-rock values are not due to higher 

than normal concentrations in the apatite phase. Although 

the observed concentration can be biased by selection 

criteria, I believe that the grains counted are representative 

of the samples in general. The uranium concentration 

shown for the zircon sample is biased, in the extreme, 

to low concentrations. 

The dates of the five apatite samples range from 

132 ± 16 Ma for the sample closest to the basement unconformity at a depth of 614 m (at a temperature of 

~20°C) to 50± 8 Ma at a depth of 1,640 meters (at a 

temperature of ~46°C). The ages appear to decrease 

linearly with depth. A least-squares linear regression 

yields a slope for the present thermal gradient of 0.27°C/ 

Ma with a "zero age" intercept at a depth of 2,174 meters 

and present temperature 56 ± 2°C. The dating of zircon 

from sample UPH-1-2004, just below the unconformity, 

yielding an age of 768 ± 325 Ma (95-percent confidence 

level), is subjectively of marginal worth as shown by the 

large uncertainty associated with the age. It nonetheless 

has some merit in the general interpretation of the geothermal history at the drill site and therefore is included for 

discussion. 

A comparison of the projected track-length distribution for fossil and induced tracks in the apatite sample 

UPH-2-2185 reveals that the fossil distribution shows a 

shorter maximum and mean projected length than the induced distribution. Dakowski (1978) has shown that the 

projected track-length distribution for thermally unaffected 

(that is, unshortened) tracks is a sawtooth distribution with 

a linearly decreasing right shoulder; least-squares regression of this part of the distribution should produce the 

same maximum length intercept for both fossil and induced tracks. Sample UPH-2-2185 has maximum length 
en 

::> 

U5 

_J 

UJ 

u 

en 

20 

UJ 40 UJ 

a: 

(!) 

UJ 

0 

~ 

u..i 56 

~ 60 

~ a: 

UJ 

(l. 

~ 

UJ 

1-

80 

93 

Basement 

igneous 

rocks 

3000 

20 40 60 80 1 00 1 20 140 

APATITE FISSION-TRACK, IN MILLION YEARS 

Figure )1. Apatite fission-track ages as a function of depth and present temperature in the Illinois 

drill holes UPH-1 and UPH-2. Error bars (centered on X) enclose the 95-percent confidence intervals. 

Regression lirie through the data indicates cooling of 0.27°C/m.y. (with present geothermal gradient) 

and "zero" age at a depth of 2,174 m (7,174 ft). Dashed line suggests most recent cooling rate if 

the closure temperature is 93°C. 

intercepts of 12.6J.Lm for the fossil and 14.6J.Lm for the 

induced distribution suggesting some track shortening for 

the fossil tracks. 

Although a relationship exists between track shortening and track fading, various studies of this relationship 

have no general agreement (for example, Mark and 

others, 1973; Wagner, 1979; Green, 1981). Partial annealing of a large track population at some time in the past 

could have produced a double peaked or bimodal distribution (Wagner, 1979). A more likely effect would be a 

break in slope, with a steepening of the distribution at 

shorter length, on the right side of the sawtooth distribution. In the present example, although the fossil length 

statistics (mean and maximum projected length) all are 

smaller than those of the induced population, there is no 

evicic~~e of bimodality or of a significant nonlinearity on 

the right shoulder in the fossil distribution. This is interpreted as an argument against a mixed age (that is, the 

sample temperature was significantly higher than its closure temperature, completely removing pre-existing 

tracks) and for slow cooling through the temperature region of significant track fading such as to provide for 

the observed shortening of fossil versus induced tracks. 

DISCUSSION 

The unexpectedly young ages and additionally the 

presence of non-zero ages down the hole are significant. 

On the basis of the present geology, much different ages 

would be expected. The igneous nature and intrusive age 

of the basement rocks and the erosional surface developed 

at the basement unconformity clearly indicate that the 

basement was significantly uplifted and eroded before deposition of the Phanerozoic sedimentary sequence. Without subsequent reheating of that surface higher than the 

closure temperature for apatite, a pre-Mount Simon age 

for the apatite should be expected, reflecting that early 

uplift and cooling history. The observed apatite ages, 

therefore, require a reheating of the basement during the 

Paleozoic or Mesozoic and cooling during the Cenozoic. 

Two mechanisms for this reheating-formerly deeper burial and formerly higher geothermal gradients-will be discussed. 

The observation that the samples deeper into tht: 

basement have non-zero ages means that the temperatures 

of these samples also are less than the apatite closure temperature. Thus, the presence of tracks in a sample at 

~46°C (sample UPH-2-5411) means that sample UPHFission-Track Dating of the Illinois Drill-Hole Core 1 03 
fz 

LU 

u 

a: 

LU 

a.. 

z 

c:i 

15 

~ 10 

~ 

:::> 

z 

LU 

> 

i= 

::5 

LU 

a: 

5 

2 3 4 5 6 7 8 9 1 0 11 12 13 14 1 5 16 1 7 18 1 9 20 

PROJECTED TRACK LENGTH, IN MICROMETERS 

Figure )2. Projected track-length distribution of fossil (boundary of shaded area) and induced (light-grey) 

tracks in sample UPH-2-2185. The relative shapes and the regression-defined, right-shoulder intercept 

best show the shortening of the fossil tracks due, most likely, to slow cooling through the closure temperature rather than to partial resetting at lower temperatures. 

1-2025 at ~20°C can have been cooled an absolute minimum of ~26°C during the last 132 Ma. Assuming that 

the geothermal gradient has been constant over time, the 

slope of the line through the data suggests cooling at a 

rate of 0.27°C/Ma. The extrapolation of this decreasing 

age trend to "zero age" is at the depth of 2,174 m (7, 174 

ft) and a temperature of 56°C. Under the condition of 

a constant cooling rate, this extrapolation provides the 

geometric construction of the closure temperature. Therefore, if the cooling rate remained the same and the geothermal gradient has remained constant, a closure temperature of 56± 2°C would be indicated. This construct 

requires a revised estimate of ~ 36°C for the minimum 

post-Jurassic cooling. However, application of the firstorder loss model for track accumulation in apatite leads 

to the prediction of a closure temperature of 93 ± l8°C, 

thereby implying cooling on the order of 73 ± 18°C since 

the Jurassic. In addition, an increase in cooling rate after 

-SOMa (dashed line of fig. J1) is required. 

Two mechanisms can be invoked to account for preCretaceous heating and post-Jurassic cooling of the basement rocks. The first of these is burial by post -Ordovician 

sedimentary rocks later removed, possibly before but 

1 04 Shorter Contributions to Isotope Research 

mostly after -130 Ma. The second is the heating effect 

of a significantly higher geothermal gradient in the past, 

allowing for subsequent cooling as the gradient decreased 

to its present value and the closure temperature isotherm 

once again moved down deeper into the basement. 

What evidence is there for the following: 

1. Independently determined higher temperatures than 

presently exist in the basement or overlying sedimentary 

rocks, 

2. Formerly existent sedimentary units possibly still 

present elsewhere but missing in northern Illinois, and 

3. A higher geothermal gradient in the early Cenozoic? 

No independent and direct evidence is known to 

exist for formerly higher temperatures in the· rocks at the 

drill site. However, in the upper Mississippi Valley leadzinc district, about 100 km west of the drill sites, McLimans (1977) has reported fluid-inclusion homogenization 

temperatures of 7 5°-220°C in the ores. The age of these 

deposits is unknown relative to the thermal history at the 

drill sites. Cobb ( 1981) has determined homogenization 

temperatures of fluid inclusions in sphalerite in coal from 

the western shelf of the Illinois Basin, about 100 km 
southwest of the drill site, of from 75°C to 113°C. He 

also noted that there is no perceptible difference between 

the vitrinite reflectance of coals adjacent to or removed 

from the sphalerite veins, indicating thermal equilibrium 

between the sphalerite-forming solutions and the country 

rock. Cobb (1981) also calculated maximum temperatures 

of coalification on the western shelf for the Herrin (no. 

6) Coal, assuming burial at maximum temperature for 

50--75 m.y. of 70°C-80°C, on the basis of vitrinite reflectance values. 

Options are extremely limited in this region for attempting to attribute higher temperatures to formerly 

deeper burial. Because the youngest sedimentary unit 

(neglecting Pleistocene deposits) at the drill sites is the 

Middle Ordovician Galena Dolomite and Plattville Formation and the apatite ages suggest that the basement was 

cooling by ~ 130 Ma, the time of deposition of the "missing strata" would have had to have been in the Late 

Ordovician-130 Ma interval. Within the Illinois Basin, 

Pennsylvanian strata are unconformable on the underlying 

formations, which range progressively from Mississippian 

through Ordovician age from south to north (Willman and 

others, 1967). If this uplift and erosion in Late Mississippian time increased in magnitude north to the drill sites, 

burial as the cause of pre-130 Ma reheating could be attributed only to Pennsylvanian through ~ 130-Ma-old 

sedimentation. The Sandwich fault zone is between the 

northernmost Pennsylvanian sedimentary rocks and the 

drill sites. The cumulative displacement within the zone 

is ~242 m (800 ft), with a sense of movement down 

to the north. Movement apparently was major between 

post-Middle Silurian and pre-Pleistocene time (Kalata and 

others, 1978). This structure might have caused a delay 

in the uplift and erosion of pre-Pennsylvanian units to 

the north until well after their removal to the south. If 

so, additional burial depth could have been maintained 

into the Early Cretaceous making a simple uplift- and erosion-induced cooling model more viable. On the basis of 

coal rank in the northern Illinois Basin ( ~60 km southeast 

of the drill sites), Dam berger ( 1971) suggested about 970 

m of former burial. His estimate was, however, dependent 

upon the geothermal gradients at the time the coal attained 

maximum rank. Although no Upper Pennsylvanian or Permian strata generally have been recognized in the Illinois 

Basin or to the north, Kehn and others ( 1981) reported 

the presence of Permian strata preserved in a graben in 

western Kentucky. This late sedimentation, although presently localized, might suggest the possibility of more 

widespread post-Pennsylvanian sedimentation in the region than is preserved. 

On direct evidence, a case for an increased geothermal gradient in the region is difficult to make. This difficulty arises primarily because of the cryptic nature of the 

observable effects of such an increase and the inability 

to project back in time from present observations of geothermal gradients. However, an indirect case for this possibility can be made. 

McGinnis and others (1976) interpreted the increasingly positive regional Bouguer anomaly in southern 

Illinois as due to midcrustal level intrusives that increased 

crustal density there and that resulted in the subsidence 

of the Mississippi Embayment. The small Permian-age intrusions isotopically dated by Zartman and others ( 1967) 

in southern Illinois might be the surface expressions of 

these larger deep-seated parent bodies, closely dating them 

as well. 

If in the Permian, and perhaps through the Mesozoic, extra thermal energy was being transferred into and 

was heating the entire lithosphere along and extending 

beyond the axis of the Mississippi Embayment, the effect 

on the present observations of heat flow would be only 

minimal. Sleep (1971) showed that following a heating 

event of lithospheric dimensions, temperatures decrease 

exponentially with a time constant of about 50 Ma. The 

anomalous part of an increased geothermal gradient, therefore, would decay with a half-life of about 70 Ma. The 

measured geothermal gradient at the drill sites is 23°C/km. 

If a slightly lower geothermal gradient, say 18°C/km, actually would result from equilibrium heat flow, then a 

gradient of 36°C/km could have existed at 130 Ma and 

would have decayed to about 23°C/km by the present if 

the input of extra heat had ceased at or before 130 Ma. 

The value of 36°C/km in the following hypotheses is used 

as a maximum estimate of geothermal gradient in the Embayment rift environment because of its ability to decay 

to the present value in the time allowed while maintaining 

a regionally reasonable equilibrium value. 

Four hypotheses involving two sets of assumptions 

are suggested for the interpretation of the fission-track 

data. The first set involves the contradictory assumptions 

that the geothermal gradient has remained constant and 

that it has been decreasing exponentially since Early Cretaceous time. The second set of assumptions deals with 

the interpretation made for the closure temperature of apatite. Independent estimates of maximum temperatures, 

such as fluid-inclusion homogenization and vitrinite-reflectance temperatures in adjacent areas, are, in general, 

supportive of temperatures in the past close to the theoretically derived apatite closure temperature. These temperatures are somewhat lower than the theoretical closure 

temperature and are observed in areas where plentiful evidence of sedimentary accumulation exists. For these 

reasons, in addition to the moderate uncertainty associated 

with the estimate of the apatite closure temperature (that 

is, 93 ± l8°C) and the apparent extrapolation of the age 

trend to the present temperature of 56°C, the possibility 

of a lower closure temperature is considered also. 

The assumptions and their ramifications have been 

tabulated and are shown in table 12. The assumptions in 

Fission-Track Dating of the Illinois Drill-Hole Core 105 
Table )2. Hypotheses presenting the range of possible cooling and burial over time 

[Assumptions regarding the geothermal gradient over time and the closure temperature affect the interpretation of the apatite ages in terms of 

required amounts of former burial. The "temperatures at present burial depths" were calculated assuming a l5°C surface temperature and the 

appropriate geothermal gradient. °Cik:m, degrees Celsius per kilometer; Ma, million years] 

Hypothesis Assumptions Temperature Additional Temperature 

UPH-1-2025 Additional burial re- UPH-2-5411 Additional Additional 

Geothermal Closure At 130 Ma cooling to quired at at -so Ma cooling to burial regradient temperature for present the present 130 Ma for present the present quired at 50 

( 0 /C/km) (Tc) depth T130 Tc -Tuo 

Constant, 23 93°C 20°C 730 

2 Constant, 23 56°C 20°C 36° 

3 Decreasing 93°c 37°C 56° 

from 36 

at 150 Ma 

4 to 23 at 

present 56°<Tc<93° 37°c 19°-56° 

the four hypotheses result in calculation of different 

amounts of cooling, in addition to that caused by a decrease in the geothermal gradient where this is assumed. 

Differences in the timing of that cooling are observed 

also. Once cooling due to changes in geothermal gradient 

has been subtracted from the total cooling, the residual 

cooling can be explained only by the removal of overlying 

sedimentary rocks. The amount of removal can be calculated by dividing the residual cooling by the value of the 

geothermal gradient in effect at the time of passage 

through the closure temperature. In addition to changes 

brought about by the assumed decreasing gradients over 

time, assumptions must be made about the thermal conductivity of the now-removed sedimentary rocks. For 

simplicity rather than insight, the conductivity in any removed sedimentary formations is assumed to have sustained a 23°C/km gradient under the present heat flow 

regime. Lower conductivity in the removed section would 

result in the calculation of smaller amounts of removal 

and vice versa. 

In calculating hypothetical sample temperatures at 

the time of closure on the basis of present depths, a surface temperature of 15°C is assumed. The difference 

between these temperatures and the assumed closure temperature is then used to calculate the required additional 

burial. At 50 Ma, the exponentially decreasing gradient 

of hypotheses 3 and 4 would be ~26°C/km. This is the 

value used in the calculation of temperatures and additional burial for the bottom-hole sample, UPH-2-5411. 

Apparent in all four hypotheses is a significant 

amount of post-~ 130 Ma. erosion. This erosion ranges 

from a low of 528 m (hypotheses 4) to a high of 3,174 

(hypothesis 1). The effect of choosing a lower closure 

temperature with constant gradient (hypothesis 2) or the 

theoretically calculated closure temperature with an increased gradient (hypothesis 3) is to require approximately 

1 06 Shorter Contributions to Isotope Research 

(meters) depth T50 Tc-Tso Ma (meters) 

3,174 46°c 47° 2,043 

1,565 46°c 100 435 

1,556 58°C 35° 1,346 

528-1,556 58°C 0-35° 0-1,346 

the same net erosion. However, the effect of choosing 

the higher closure temperature (hypothesis 1 and 

hypothesis 3) is to require significantly more erosion post-

~ 50 Ma than with the minimum closure temperature of 

56°C which may require none at all (hypothesis 4). Any 

increased geothermal gradient, although not changing the 

depth at which the projected "zero age" occurs, would 

increase the temperature at that depth and hence the value 

assigned for the closure temperature in this construction. 

Therefore, the closure temperature of 56°C is inconsistent 

with the assumption of decreasing gradient and some post50-Ma erosion still would be indicated. 

The fission track age found for the zircon sample 

of 768 ± 325 (95-percent confidence level) suggests that 

its age is greater than 500 Ma, the age of the Mount 

Simon Sandstone. This age is taken as qualified support 

for a conclusion that this sample was not reheated to 

higher than 150° -180°C during the Phanerozoic. 

The younger than expected apatite fission-track ages 

at the Illinois drill sites are not unique in this region. 

Numerous apatite samples from the exposed Precambrian 

basement in central and northern Wisconsin and in northem Michigan also yield "young" fission-track ages, ranging from ~500 to 220 Ma (R. A. Zimmermann and C. 

W. Naeser, unpub. data). The general trend in these ages 

is for a decrease in age toward the present surface expression of the basement unconformity; the youngest ages are 

in southern Wisconsin. The Illinois drill hole results can, 

therefore, be viewed in the regional context of a progressive southward uplift and exposure of the Precambrian 

basement, which apparently has taken place at least since 

Permian time. 

CONCLUSIONS 

An inescapable conclusion of this apatite fissiontrack age study seems to be that there has been significant 
post-~ 130 Ma erosion around the site of the Illinois drill 

holes. Even under the assumptions of hypothesis 4 discussed earlier, which are the most difficult to justify 

geologically, the amount of burial required to reset the 

apatite ages is comparable to the burial implied by Damberger ( 1971) from coal rank in the northernmost exposures of Pennsylvanian rocks of the Illinois Basin. The 

time of regional erosion is not until post-~ 130 Ma and 

possibly pre-~50 Ma. If the post-Upper Ordovician 

through Mississippian sedimentary sequence had been removed in pre-Pennsylvanian time in northern Illinois, the 

implication is that Pennsylvanian rocks of considerable 

thickness (>500 m) extended 100 km or more north of 

their present limit. 

No compelling evidence exists from these data that 

requires a closure temperature as low as 56°C. An additional 2,000 ft of core, in retrospect, would have been 

conclusive in this respect. Independent geological calibration of the closure temperature and the apparent accuracy 

of the analytical approach used to calculate the closure 

temperature for these data suggest a closure temperature 

of 93 ± 18°C. Although the actual value may be somewhat 

lower, (toward the lower end of the calculated confidence 

interval), the amount of burial required with the constant 

or decreasing geothermal gradient is considerable. Additionally, a large amount of the uplift and erosion 

(maximum 1,346-2,043 m) would have taken place in 

the last 50 Ma. Maximum temperatures of 75°-l10°C in 

the Pennsylvanian coals and sphalerite veins suggest similar former burial depths where Pennsylvanian rocks are 

preserved. Whereas these occurrences allow only for the 

removal of overlying Pennsylvanian and younger rocks 

in the area of the drill holes, rocks of Ordovician age 

to as young as Jurassic (pre-130 Ma) could have been 

removed. The thickness of the post-St. Peters Sandstone 

through Mississippian section preserved in the Illinois 

Basin is a minimum of about 800 m. These formations 

are truncated progressively downsection, however, and 

then are overlain by Pennsylvanian rocks farther northward out of the Basin. The only way of retaining this 

thickness of Paleozoic rocks until ~ 130 Ma, so as to provide the greatest depth of burial in northern Illinois, is 

to require a reversal or cessation of this northerly uplift 

just north of the structural basin-perhaps the role played 

by the Sandwich Fault zone. Uplift and erosion after 

~ 130 Ma was then regional in extent, eroding and cooling 

the Pennsylvanian rocks in the Illinois basin, for which 

there is evidence, as well as the region to the north. 

This study suggests a burial history involving greater thicknesses or temperatures than generally is accepted 

for this region. Awareness of this revised burial history 

could be significant in understanding the conditions of 

genesis of lead-zinc mineralization in the region. Furthermore, resetting of the apatites in the basement raises the 

possibility that mtmmum temperatures for hydrocarbon 

generation could have been reached in the lower sedimentary units. 

REFERENCES CITED 

Carpenter, B. S., and Reimer, G. M., 1974, Calibrated glass 

standards for fission track use: National Bureau of Standards 

Special Publication 260-49, p. 16. 

Cobb, J. C., 1981, Geology and geochemistry of sphalerite in 

coal: Champaign, Ill., Illinois State Geological Survey, 

187 p. 

Dakowski, M., 1978, Length distribution of fission tracks in 

thick crystals: Nuclear Track Detection, v. 2, p. 181-189. 

Damberger, H. H. , 1971 , Coalification pattern of the Illinois 

Basin: Economic_Geology, v. 66, p. 488--494. 

Dodson, M. H., 1973, Closure temperature in cooling geochronological and petrological systems: Contributions to 

Mineralogy and Petrology, v. 40, p. 259-274. 

Eardley, A. J., 1962, Structural geology of North America (2d 

ed.): New York, Harper and Row, 743 p. 

Green, P. F. , 1981 , "Track-in-track" length measurements in 

annealed apatites: Nuclear Tracks, v. 5, p. 121-128. 

Harrison, T. M., Armstrong, R. E., Naeser, C. W., and 

Harakal, J. E. , 1979, Geochronology and thermal history 

of the Coast Plutonic Complex, near Prince Rupert, British 

Columbia: Canadian Journal of Earth Sciences, v. 16, p. 

400-410. 

Johnson, N. M., McGee, V. E., and Naeser, C. W., 1979, 

A practical method of estimating standard error of age in 

the fission track dating method: Nuclear Tracks, v. 3, p. 

93-99. 

Kehn, T. M., Beard, J. G., and Williamson, A. D., 1981, The 

Manzy Formation; A new stratigraphic unit of Permian age 

in western Kentucky: U.S. Geological Survey Open-File Report 81-790, 15 p. 

Kolata, D. R., Buschbach, T. C., and Treworgy, J. D., 1978, 

The Sandwich fault zone of northern Illinois: Illinois State 

Geological Survey Circular 505, 26 p. 

Lidiak, E. G. , and Denison, R. E. , 1981 , Petrography, chemistry, specific gravity and magnetic properties of silicic rocks, 

Illinois Deep Hole Project: EOS, v. 62, p. 388. 

Mark, E., Pahl, M., Purtscheller, F., and Mark, T. D., 1973, 

Thermische Auscheilung von Uranspaltspuren in Apatiten, 

Alterskorrekturen und Beitage ziir Geothermochronologie: 

Tschermaks Mineralogische und Petrologische Mitteilungen, 

v. 20, p. 131-154. 

McGee, V. E., and Johnson, N. M., 1979, Statistical treatment 

of experimental errors in the fission track dating method: 

Mathematical Geology, v. 11, p. 255-268. 

McGinnis, L. D., Heigold, P. C., Ervin, C. P., and Heidari, 

M., 1976, The gravity field and tectonics of Illinois: Illinois 

State Geological Survey Circular 494, 28 p. 

McLimans, R. K., 1977, Geological fluid inclusions and stable 

isotope studies of the upper Mississippi Valley zinc-lead district, southwest Wisconsin: College Station, Pa., The 

Pennsylvania State University Ph.D. thesis, 175 p. 

Fission-Track Dating of the Illinois Drill-Hole Core 1 07 
Naeser, C. W., 1976, Fission track dating: U.S. Geological Survey Open-File Report 76--190, 65 p. 

Naeser, C. W., 1979, Fission track dating and geologic annealing of fission tracks, in Jager, E., and Hunziker, J. C., 

eds., Lectures in isotope geology: New York, Springer-Verlag, 329 p. 

Naeser, C. W., Zimmermann, R. A., and Cebula, G. T., 1981, 

Fission track dating of apatite and zircon; An interlaboratory 

comparison: Nuclear Tracks, v. 5, p. 65-72. 

Potter, P. E., and Siever, R., 1956, Source of basal Pennsylvanian sediments in the eastern interior basin: Journal of Geology, v. 64,p. 225-244. 

Rahman, J. L., and Roy, R. F., 1981, Preliminary heat-flow 

measurement at the Illinois deep drill hole: EOS, v. 62, 

p. 388. 

Sleep, N. H. , 1971 , Thermal effects of the formation of Atlantic 

continental margins by continental break-up: Geophysical 

Journal of the Royal Astronomical Society, v. 24, p. 

325-350. 

1 08 Shorter Contributions to Isotope Research 

Van Schmus, W. R., Peterman, Z. E., and Doe, B. R., 1981, 

U-Pb and Rb-Sr geochronology of Precambrian granite from 

northern Illinois deep drill holes: EOS, v. 62, p. 389. 

Wagner, G. A. , 1979, Correction and interpretation of fission 

track age, in Jager, E., and Hunziker, J. C., eds., Lectures 

in isotope geology: New York, Springer-Verlag, 329 p. 

Willman, H. B., and others, 1967, Geologic map of Illinois; 

Urbana, Ill.: Illinois State Geological Survey, scale 

1:500,000. 

Zartman, R. E., Brock, M. R., Heyl, A. V., and Thomas, H. 

H., 1967, K-Ar and Rb-Sr ages of some alkalic intrusive 

rocks from central and eastern United States: American 

Journal of Science, v. 265, p. 848-870. 

Zimmermann, R. A. , 1977, The interpretation of apatite fission 

track ages with an application to the study of uplift since 

the Cretaceous in eastern North America: Philadelphia, Pa., 

University of Pennsylvania Ph.D. thesis, 126 p. 

Zimmermann, R. A., and Gaines, A. M., 1978, A new approach to the study of fission track fading: U.S. Geological 

Survey Open-File Report 78-701, p. 467-468. 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

OXYGEN ISOTOPIC CONSTRAINTS ON THE 

ORIGIN OF THE PRECAMBRIAN GRANITES FROM 

THE SOUTHERN WIND RIVER RANGE AND 

THE GRANITE MOUNTAINS, CENTRAL WYOMING 

Chapter K 

By K. K. CHEANG1 , D. B. WENNER1 , and j. S. STUCKLESS 

Abstract ......... . 

Introduction . . . . . . . . 

Laboratory procedures 

Geological setting . . . 

Oxygen isotope data--Granite Mountains 

Granitic rocks . . . 

Metamorphic rocks . . . 

Diabase dikes . . . . . . 

Hydrothermally altered rocks 

Oxygen isotope data-Louis Lake batholith . 

Relationships between 180/160 and magnetic 

susceptibility . . . . . . . . . . . . . . 

FIGURES 

CONTENTS 

Page 

111 

111 

112 

113 

115 

115 

116 

117 

117 

118 

119 

Chemical and mineralogical features 

Granites of the Granite Mountains 

Louis Lake batholith 

Discussion . . . . . . . . 

Comparison betwen the Granite Mountains 

and Louis Lake batholith . . . . . 

Comparison with Phanerozoic granites 

Postulated source rocks 

Conclusions . . . 

Acknowledgments 

References cited . 

K1. Index map of principal Precambrian exposures and location of uranium deposits in Tertiary rocks, 

Page 

121 

121 

122 

123 

123 

125 

125 

127 

127 

127 

Page 

Wyoming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 

K2. Map showing generalized Precambrian geology of the Granite Mountains region . . . . 113 

K3. Generalized geologic map of the southern Wind River Range, Wyo. . . . . . . . . . . 114 

K4. Histogram showing &180 values of various granitic units within the Granite Mountains 116 

K5. Histogram showing & 180 values of diabase dikes and metamorphic rocks from the Granite Mountains . 117 

K6. Vertical profile of drill-hole GM-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 

K7. Histogram showing &180 values of hydrothermally altered units . . . . . . . . . . . . . . . . . . 120 

K8. Three cycle semi-log plot of initial magnetic susceptibility versus &180 values of unaltered whole-rock 

samples from the Louis Lake batholith and the Granite Mountains . . . . . . . . . . . . . . 121 

K9. Histogram showing a comparison between the & 180 values of unaltered whole rocks of the Louis Lake 

batholith and granites from the Granite Mountains . . . . . . . . . . . . . . . . . . . . . . . 124 

1 Department of Geology, University of Georgia, Athens, GA 30602 

Oxygen Isotopic Constraints on the Origin of Precambrian Granites, Central Wyoming 1 09 
TABLES 

Page 

Kl. Whole-rock &1s0 data for samples from the Granite Mountains, Wyo. . . . . . . . . . . 115 

K2. & •so values of coexisting minerals from the Granite Mountains and Louis Lake batholith 116 

K3. Whole-rock & •so data for the hydrothermally altered rocks within the Granite Mountains 119 

K4. &1

s0 values of minerals from hydrothermally altered rocks within the Granite Mountains . 119 

K5. & •so whole-rock data for the Louis Lake batholith . . . . . . . . . . . . . . . . . . . . . 120 

K6. Initial magnetic susceptibility (X) and &180 data for the Granite Mountains and Lake Louis batholith . 121 

K7. Chemical and CIPW normative data for samples from the Louis Lake batholith . . . . . 122 

K8. Radium equivalent uranium, thorium, and potassium contents of the Louis Lake batholith 122 

K9. Comparison between the Granite Mountains and the Louis Lake batholith . . . . . . . . . 123 

K 10. Criteria used for distinguishing between I- and S-type Paleozoic granites from southeastern Australia . 126 

11 0 Shorter Contributions to Isotope Research 
Abstract 

Two contrasting types of approximately contemporaneous (-2.6 billion years old) granite batholiths 

from Wyoming were studied in order to elucidate their 

origins. and to provide a general understanding of why 

one is significantly more enriched in uranium and 

thorium than the other. The first, from the Granite 

Mountains, central Wyoming, comprises the granite of 

Long Creek Mountain and the volumetrically dominant 

granite of Lankin Dome. The granite of Lankin Dome 

consists of two major phases, a biotite granite and a 

leucocratic granite, both of which contain garnet, primary muscovite, and a few metasedimentary xenoliths. 

The second, the Louis Lake batholith is at the southern 

end of the Wind River Range, and comprises a 

hornblende-biotite granodiorite and a leucocratic biotite 

granite, both bearing sphene, hornblende, epidote and 

magnetite, and abundant hornblende-enriched rounded 

clots. 

An average of 32 analyses of the granite of Lankin 

Dome shows that it is peraluminous, with molar Ah03/ 

(Na20+K20+Ca0)=1.1, and high CIPW normative 

corundum=1.4. An average of 14 analyses of the Louis 

Lake batholith shows that it is metaluminous with Al20 3/ 

(Na20 + K20 + Ca0)=0.96 and low normative corundum=0.4. The two granites also differ in radioelement 

content with uranium, thorium, potassium and thoriumuranium ratios all higher in the granite of Lankin Dome. 

Average &180 values in parts per thousand (per mil) 

of the granite of Long Creek Mountain is 8.2 ± 0.3 per 

mil (N=4), and average &180 values of the two phases 

from the granite of Lankin Dome are as follows: biotite 

granite=8.5 ± 0.5 per mil (N =37), and leucocratic granite=8.4±0.3 per mil (N=12). The average &180 values for 

the Louis Lake batholith are: hornblende-biotite 

granodiorite=7.3 ± 0.3 per mil (N=13), and leucocratic biotite granite=7.7±0.1 per mil (N=6). Average &180 

quartz and &180 feldspar for the Granite Mountains are 

9.8 (N=15) and 7.8 per mil (N=8), respectively, whereas 

for the Louis Lake batholith, the average &180 quartz and 

&

180 feldspar values are 9.5 (N=6) and 7.5 per mil (N=5), 

respectively. Quartz-feldspar fractionation of pristine 

granites from both units in the Granite Mountains (7 

samples) and Louis Lake batholith (5 samples) give average values of 2.0, reflecting a close approach to hightemperature equilibrium. 

Three types of hydrothermally altered granites 

have been identified in the Granite Mountains. These, 

as well as their mean &180 values, include the progressively altered rock types: sericitized-chloritized-saussuritized granites (whole rock=7.1, N=17; quartz=8.9, 

N=6; feldspar=6.4, N=6); albitized granites (wholerock=6.3, N=6; quartz=8.1, N=3; and feldspar=5.6, 

N=3); silicified-epidotized granites (whole rock=4.7, 

N=26; quartz=5.6, N=4; feldspar=3.2, N=4). These rock 

types are all 180-depleted compared with the unaltered 

granites, which indicates an interaction between the 

granite and a low 180-fluid, probably meteoric water. 

The whole-rock, oxygen isotopic, mineralogical, 

chemical, and strontium isotopic data from the unaltered 

granites indicate that the magmas that formed the granites in the Granite Mountains probably were derived 

from a uranium-enriched source that had some 

peraluminous metasedimentary component, whereas the 

Louis Lake batholith was formed by partial melting of 

igneous source materials that are not strongly enriched 

in uranium. Thus, for the Granite Mountains, this 

metasedimentary protolith may be important in allowing 

the preconcentration of uranium and thorium. This difference in protolith and the possible presence of hydrothermally altered units may explain why granites from 

the Granite Mountains are spatially related to three 

major uranium districts, but the Louis Lake batholith is 

not. 

INTRODUCTION 

Oxygen isotopic studies combined with existing and 

new major- and trace-element chemical investigations 

were made of two contrasting types of approximately contemporaneous ( ~2.6 Ga (billion years)) granite batholiths 

from Wyoming: the granitic rocks of the Granite Mountains, and the Louis Lake batholith of the southern Wind 

River Range (fig. K1). 

Both the Granite Mountains and Louis Lake 

batholith lie within the southern extension of the Middle 

Rocky Mountain Province and form part of several Precambrian basement blocks uplifted during the Laramide 

orogeny. Most of the Precambrian blocks, including the 

Louis Lake batholith, trend north to northwest, but the 

Granite Mountains are exposed as a series of partly 

buried, discontinuous knobs and ridges forming an eastwest trending band of outcrops about 20 km (kilometers) 

wide and 110 km long, with an exposed area of roughly 

2,200 km2 (square kilometers). The Louis Lake batholith, 

which lies at the southern end of the Wind River Range, 

is about 50 km long and 40 km wide with an exposed 

area of 2,000 km2 (Bayley, 1965a, b). 

The granites of Granite Mountains are of interest 

because they have been proposed as the source for the 

uranium in the sandstone-hosted Tertiary deposits at the 

Gas Hills, Crooks Gap, and Shirley Basin uranium districts (Rosholt and Bartel, 1969). These three areas represent some of the largest uranium-producing districts in 

the United States. Each contains reserves plus production 

of ore (at 0.1 percent U30 8) of more than 1 million tons 

(Butler, 1972). 

The granite of Lankin Dome that forms most of 

the Granite Mountains (Stuckless and Peterman, 1977) initially was enriched in uranium when it formed, but present-day values are lowered to slightly above world average as a result of an average 20-percent loss between 

1 ,400-1,700 Ma (million years) and a more extensive loss 

(generally more than 70 percent) in the Cenozoic (Stuckless and Nkomo, 1978). The timing of uranium mineraliOxygen Isotopic Constraints on the Origin of Precambrian Granites, Central Wyoming 111 
I 

MONTANA I 

,----·-·------------·--------------\ 

4F 

IDAHO 

I 

I -----1 

UTAH 

I 

I 

WYOMING 

L _______ _ 0 

BIGHORN 

MTS 

0 

0 

0 

0 6. 

1 DO 200 KILOMETERS 

50 1 DO MILES 

0 

0 

0 

LARAMIE 

MTS 

I 

I 

\ SOUTH 

I 1 DAKOTA 

\ 

I 

I 

\ 

I 

I r-----

I 

I 

\ 

\NEBRASKA 

\ 

I 

I 

\ 

I 

I 

------J_ __ _ 

COLORADO 

Figure Kl. Index map of the principal Precambrian exposures (stippled areas) in Wyoming (from Stuckless and 

Nkomo, 1978) and the location of uranium deposits in Tertiary rocks. Solid circles, three producing districts with 

reserves plus production of ore (U30 8>0.1 percent) of more than 1 million tons; triangles, deposits with ore reserves 

plus production between 1,000 and 1,000,000 tons; open circles, deposits with reserves plus production between 

1 and 1,000 tons (Uranium data from Butler, 1972.) Dashed line encloses area of the Granite Mountains. 

zation in the sandstones (Ludwig, 1978, 1979) coincides 

with the period of Cenozoic uranium loss. The Granite 

Mountains were also the provenance for the sandstones 

that host the uranium ore (Seeland, 1976, 1978). 

The Louis Lake batholith represents a contrasting 

granite body in terms of mineralogy, major- and trace-element chemistry, and oxygen and strontium isotopic composition. It is provenance for Tertiary sandstones of central Wyoming (Seeland, 1978), but it is not associated 

with large uranium deposits (fig. Kl). We have made a 

study of these two contrasting batholiths principally in 

order to elucidate the nature of the source rocks for these 

two granitic terranes. Such a comparison ultimately should 

lead to a better understanding of the kinds of protoliths 

that are responsible for producing uranium-enriched granites. 

112 Shorter Contributions to Isotope Research 

Laboratory Procedures 

Oxygen isotope analyses were performed for 213 

whole-rock and mineral separates. This large number of 

analyses was necessary in order to provide adequate 

coverage of the exposed outcrop area, to elucidate the 

subtle differences among the different granitic units and 

phases, and to adequately define the various types of hydrothermally altered units. The number of samples of each 

unit analyzed was in proportion to the size of the unit, 

the degree of variability within each unit, and the availability of samples. Geologic relations pertinent to the interpretation of oxygen isotope and other data were noted 

in the field. 

Prior to analyses, all samples were checked for 

"freshness", and only unweathered samples were selected. 
45' 

EXPLANATION 

PRECAMBRIAN 

~-...:::. /~ Diabase dikes 

Granite of Lankin Dome 

Gran ite of Long Creek Mountain 

Metamorphic rock 

• WY-97 Sample locality and number 

30' 

WYOMING 

,.. Area of 

this figure 

10 20 KILOMETERS 

10 MILES 

Figure K2. Generalized Precambrian geology of the Granite Mountains region. Geology from Peterman and Hildreth 

(1978) and Stuckless and Nkomo (1978). 

All outer parts of whole-rock specimens were removed, 

and thin sections were made for each sample and checked 

under the polarizing microscope. The sample localities 

from the Granite Mountains are shown in figure K2; many 

of the samples are the same as those studied by Stuckless, 

Bunker, VanTrump, and Bush (1978, p. 6-13) and Peterman and Hildreth (1978, p. 4). Sample localities for the 

Louis Lake batholith are shown in figure K3. 

The relatively coarse-grained nature of the granites 

facilitated separation by handpicking. No attempt was 

made to separate potassium-feldspar and sadie plagioclase 

because their oxygen isotopic behavior is similar (O'Neil 

and Taylor, 1967) and their resistance to oxygen exchange 

is nearly equal. Quartz separates were purified by treating 

with cold hydrofluoric acid to remove any feldspar impurities. 

Extraction of oxygen from powdered whole-rock 

samples and mineral separates of quartz, feldspar, and biotite was achieved by reaction with fluorine in nickel vessels using the method described by Taylor and Epstein 

(1962). The oxygen liberated was converted to C02 gas 

by combustion with a resistance-heated graphite rod in 

the presence of a platinum catalyst. Oxygen analyses were 

performed on the C02 using a 90° -sector, magnetic-deflection isotope ratio mass spectrometer (McKinney type). 

The experimental data are reported in the standard 

B notation, with all data reported relative to SMOW (Standard Mean Ocean Water). In our laboratory, National Bureau of Standards standard reference material 28 is defined 

as 9. 61 per mil . Replicate analyses of 41 samples indicate 

a precision of ± 0. 2 per mil. Partitioning of 180 between 

coexisting minerals A and B is reported as ~A-B = BABB. 

Magnetic susceptibility measurements were made by 

Dr. B. B. Ellwood of the University of Georgia, on 

weighed samples using a susceptibility bridge. Data are 

reported as the initial susceptibility, x in G/Oe (gauss per 

oersted) calculated from: 

X scale reading of sample X mass of standard X Xstandard 

sample = scale reading of standard X mass of sample 

where the mass and scale reading of the standard are 

known. 

GEOLOGICAL SETTING 

The Precambrian geology of the Granite Mountains 

(fig. K2), has been described by Love (1970), Peterman 

and Hildreth (1978), Stuckless and Peterman ( 1977), 

Stuckless, Bunker, Bush, and others (1977), Stuckless, 

Bunker, VanTrump, and Bush (1978), and Stuckless, 

Bunker, Bush, and VanTrump (1981), Stuckless and 

Nkomo (1978), and Ludwig and Stuckless (1978). In 

brief, the three major Precambrian lithologies that are recognized among the rocks within the Granite Mountains 

area include: 

1. Amphibolite-grade metamorphic rocks (2860 ± 80 

Ma; Peterman and Hildreth, 1978), 

Oxygen Isotopic Constraints on the Origin of Precambrian Granites, Central Wyoming 113 
45' 

p£u 

p£be 

Bears Ears 

pluton 

QT 

109° 00' 

p£u~ 

10 

20 

WYOMING 

~Area of 

this figure 

45' 

0 Lander 

QT 

40 KILOMETERS 

20 MILES 

30' 

QT 

QT 

EXPlANATION 

r----

QT Quaternary and Tertiary strata 

Ml Mesozoic strata 

Pz Paleozoic strata 

p£u Precambrian rocks undivided 

p£m Precambrian metasedimentary rocks 

p£be Precambrian Bears Ears pluton 

Sample locality and number 

Figure K3. Generalized geologic map of the southern Wind River Range, Wyo. (from Naylor and others, 

1970) showing the location of the Precambrian Louis Lake batholith (stippled) and sample localities. 

114 Shorter Contributions to Isotope Research 
2. Intrusive peraluminous biotite gramtlc units 

(2640 ± 20 Ma, for the granite of Long Creek Mountain 

and 2595 ± 40 Ma for the granite of Lankin Dome, Ludwig and Stuckless, 1978), and 

3. Crosscutting diabase dikes ranging from less than 

I m wide to as much as several tens of meters wide 

(2650 ± 260 Ma; Peterman and Hildreth, 1978). 

The metamorphic rocks (about 10 percent of the 

total exposed outcrop) are schists and tonalitic to granitic 

or granodioritic gneisses. Present in lesser amounts are 

amphibolite gneisses intercalated with minor epidote- and 

biotite-rich horizons. 

The granite of Long Creek Mountain ( 10 percent 

of granite exposure) and the granite of Lankin Dome (90 

percent of granite outcrop) intrude the metamorphic units. 

The granite of Lankin Dome consists of a leucocratic 

phase (8 percent) and a biotitic phase (92 percent) and 

is transected by several aplites and pegmatites. Hydrothermally altered rocks include albitized granites, recognized 

only in drill cores (Stuckless and Peterman, 1977), and 

silicified-epidotized granites (2,325-2,560 Ma; Stuckless, 

Nkomo, and Doe, 1981) that form dikelike or tabular 

zones that crosscut the unaltered granites. 

The Louis Lake batholith (fig. K3), which intrudes 

Precambrian metagreywackes, schists and iron-formations, consists predominantly of a hornblende-biotite 

granodiorite (2,642 ± 9 Ma; Naylor and others, 1970)2 

with a volumetrically minor amount of leucocratic biotite 

granite (2562±75 Ma; Naylor and others, 1970)2 . The 

presence of xenoliths of hornblende-biotite granodiorite 

within the leucocratic biotite granite, combined with 

radiometric ages, suggests that the latter is slightly 

younger. Although aplite and pegmatite veins are common 

in the northern section of the Louis Lake batholith, no 

distinct hydrothermally altered zones have been recognized. 

OXYGEN ISOTOPE DATA-GRANITE 

MOUNTAINS 

Granitic Rocks 

Oxygen isotope data for the various gramttc units 

in the Granite Mountains are presented in table K1 and 

plotted on figure K4. These values are considered representative of the initial 8 180 signature of the primary 

magma because of the relatively little-altered nature of 

the rocks as shown in thin-section studies and the lack 

of isotopic reversals for quartz, feldspar, and biotite values in representative samples. The-narrow range of 8 180 

values (table K2) for quartz (9.3-10.8 per mil; N=15) 

and feldspar (7.2-8.2 per mil; N=8), and the mean 

2 Ages as recalculated by Ludwig and Stuckless (1978). 

Table Kl. Whole-rock &180 data for samples from the 

Granite Mountains, Wyo. 

[Values in per mil relative to Standard Mean Ocean Water; see text. 

For details of sample localities, see Stuckless, Bunker, VanTrump, and 

Bush (1978), and Peterman and Hildreth (1978). gdg, granodioritic 

gneiss; gg, granitic gneiss; tg, tonalitic gneiss; amqg, amphibolite intercalated with quartzofeldspathic gneiss] 

Sample No. Sample No. 

Biotite granite of Lankin Dome 

BBSW-1 9.2 IR-13 

BR-3 8.1 L-4 

BRQ-1 8.6 L-5 

DDH-3 10.2±0.2 LD-14 

DDH-5 9.0 LM-4 

DDH-8 8.6 LM-9 

GM-1-8.8 8.2 MS-20 

GM-1-67.0 8.2±0.3 PD-5 

GM-1-86.3 8.3 PD-16 

GM-1-125.8 8.0 PM-1-459 

GM-1-228.9 8.5 PM-1-491 

GM-1-266.7 8.6 PRSW-10 

GM-1-285.0 8.8 PRSW-11 

GM-1-374.0 8.4 SM-4 

GM-1-1195.8 8.3 SM-8 

GM-1-1278.5 8.9 SP-3 

GM-1-1342.5 8.9 W2-CR-1-134.5 

GM-2-1413.0 8.5 WY-11 

IR-6 8.6 

Leucocratic granite of Lankin Dome 

BBL-9 8.7 GM-1-851.9 

GM-1-581.9 8.6 GM-1-1022.2 

GM-1-739.0 8.6 GM-1-1325.0 

GM-1-773. 5 8.6 IR-12 

GM-1-814.5 8.3 PM-1-295.0 

GM-1-850.1 8.5 SP-6 

Granite of Long Creek Mountain 

BBL-13 

TCM-2 

GM-35-68(gdg) 

GM-38-68(tg) 

GM-76-68(tg) 

GM-77-68(gdg) 

GM-76-68(quartz) 

IR-10 

MS-8 

PM-1-1210.3 

PM-1-1350.7 

PRSW-8 

8.1 DDH-4 

8.2 MS-31 

Metamorphic units 

7.1 

7.8 

7.5±0.03 

7.0 

8.7 

GM-78-68(gg) 

GM-98-68(tg) 

GM-103A-69(amqg) 

GM-104-69(gdg) 

Diabase dikes 

5.0 

4.7 

4.2 

4.4 

2.8 

SD-5 

SD-14 

SRQ-2 

WY-9 (PD-5) 

8.4±0.03 

9.1 

8.8±0.2 

7.9 

9.1±0.3 

8.8 

8.7 

8.8 

8.0 

8.0 

8.5 

8.6 

8.,3±0.3 

8.6 

8.2 

7.8 

8.1 

7.8 

8.0 

8.3 

8.4 

8.4±0.03 

8.0 

7.9 

7.8 

8.5 

7.4 

4.8±0.3 

6.0 

5.3±0.3 

5.5 

5.3 

3.8 

2.1±0.1 

~quartz-feldspar of 2.0 suggest a close approach to hightemperature isotopic equilibrium. The latter mean quartzfeldspar value is within the limit of 1.5-2.0 for "normal" 

igneous plutonic rocks (Taylor and Epstein, 1962; O'Neil 

and Taylor, 1967). 

Oxygen Isotopic Constraints on the Origin of Precambrian Granites, Central Wyoming 115 
Table K2. 8180 values of coexisting minerals from the 

Granite Mountains and Louis Lake batholith, central 

Wyoming 

[Values in per mil relative to Standard Mean Ocean Water, see text. 

Leaders(---) indicate not analyzed] 

Sample No. Quartz Feldspar Biotite Rock 

Biotite granite of Lankin Dome 

GM-1-266.7 10.8 8.2 5.6 8.6 

L-5 1 0. 0 7. 9 8.8 

LM-9 10.0±0.15 7.9±0.13 8. 8 

MS-20 1 0. 0 8. 1 8.7 

PM-1-459 9.4±0.02 7.2 8.0 

WY-11 9. 4 7.5 3. 3 7.8 

GM-2-1413 9.7 8.5 

IR-27 9. 3 7. 9 

LD-14 9.8 

SP-6 9. 7 

SP-10 1 0. 0 

Leucocratic granite of Lankin Dome 

GM-1-773.5 9. 9 8.1 8.6 

GM-1-850.1 10.0 7. 9 8. 5 

Average----- 9.8 7.8 

Hornblende-biotite granodiorite of Louis Lake 

LLB-2 9.5 7.0 3. 0 7.4 

LLB-4 9.3 7. 1 7.3 

LLB-5 9.5 7.7 3. 7 8.0 

LLB-17 9. 3 7.8±0.2 3. 4 7.3 

LLB-16 9.8 7.9 7. 6 

LLB-19 9.8 6.7 

Average----- 9.5 7. 5 

Relatively fresh samples from the volumetrically 

dominant granite of Lankin Dome are characterized by 

a biotite granite that has a mean 8180=8.5 ±0.5 per mil 

(N = 37) and a leucocratic granite with mean 

8180=8.4±0.3 per mil (N=12). The granite of Long 

Creek Mountain has a mean isotopic value of 

8180 = 8. 2 ± 0. 3 per mil (N = 4) that is nearly identical to 

that of the granite of Lankin Dome. 

Whole-rock 8180 values for the various granitic 

units fall within relatively narrow ranges (table Kl). Typically, 8180 values for the biotite granites are within 7. 8-

9.1 per mil or within a 1.3 per mil range, with the exception of sample DDH-3, which has a value of 10.2 per 

mil. The leucocratic granites have values ranging from 

7.9-8.7 per mil or within a 0.8 per mil range. Hence, 

practically all the samples from the granite of Lankin 

Dome fall within 7. 8-9.1 per mil. The granite of Long 

Creek Mountain also falls within a narrow limit between 

7. 8 and 8. 5 per mil or 0. 7 per mil. Among unaltered 

samples, the whole-rock 8180 and quartz-feldspar values 

show no obvious variation with either geographic distribution or depth as reflected in one 426.7-m (1400-ft) drill 

core (GM-1; see fig. K6). Thus, the Granite Mountains 

appear to be quite isotopically uniform both in vertical 

and areal extent. 

116 Shorter Contributions to Isotope Research 

Metamorphic Rocks 

The rocks intruded by the granite consist of amphibolite-grade granodioritic gneisses, tonalitic gneisses, 

epidote-rich gneisses, and amphibolites. These rocks have 

a 8180 range of 4.8 to 7.8 per mil with an average of 

6.6 per mil (N=8). (See table Kl and fig. K5.) The 8180 

value of a quartz separate from sample GM-76-68 has 

a value of 8.7 per mil and a whole-rock 8180 value of 

7 .4 per mil, indicating a close approach to high temperature isotopic equilibrium. 

These Precambrian metamorphic units appear to 

have lower 8180 values than do equivalent lithologies 

from the Canadian Shield (Longstaffe and Schwarcz, 

4 GRANITE OF LONG CREEK MOUNTAIN 

2 

0 

6 GRANITE OF LANKIN DOME 

Leucocratic phase 

4 

(/) 

w 

...J 2 

a.. 

~ 

<( 

(/) 

LL. 0 

0 

a: 

~ 20 GRANITE OF LANKIN 

~ 

:::> 

z 

DOME 

Biotitic phase 

16 

12 

8 

4 

0 '--------

7 8 9 

?)180 (per mil) 

x =8.2 

R = 7.8-8.5 

n =4 

(]' =0.3 

x =8.4 

R = 7.9-8.7 

n = 12 

(]' =0.3 

x =8.5 

R = 7.8-9.1 

n = 37 

(]' =0.5 

10 

Figure K4. Histogram showing 180 values of various 

granitic units within the Granite Mountains. Vertical 

arrow positioned on the horizontal axis shows the mean 

oxygen isotopic composition. X, mean value, R, range 

of values; n, number of samples; a, standard deviation. 
2 

C/)1 

UJ 

...J 

c.. 0 ~ 

<{ 

Cf) 4 L.l.. 

0 

a: 3 UJ 

a:l 

~ 2 ::::> 

z 

DIABASE DIKES 

METAMORPHIC COUNTRY ROCKS 

x = 6 .6 

R = 4.8-7.8 

n = 8 

a =1 .1 

o~----------~----------~--------

2 3 4 5 

x =4.2 

R=2.1-5.5 

n = 9 

a =1.1 

6 7 8 

1)180 (per mil) 

Figure KS. Histogram showing &180 values of diabase dikes and metamorphic rocks from the Granite 

~ountains. Vertical arrow positioned on horizontal axis shows the mean oxygen isotopic composition. 

X, mean value; R, range of values; n, total number of samples; and cr, standard deviation. 

1977), possibly reflecting a more primitive protolith. Alternatively, these lower 8180 values also may reflect the 

higher grade of regional metamorphism, which in many 

studies has been observed to produce a systematic lowering of the 180/ 160 of whole rocks (Hoefs, 1980). 

Examination of the oxygen isotope data also 

suggests that the granites of the Granite Mountains were 

not derived directly from the metamorphic country rock 

through an anatectic process. All the metamorphic rock 

types have distinctly lower 8180 values (4.8-7.8 per mil; 

mean of 6.6 per mil) than the granite (mean=8.5 per mil). 

Because of the high temperatures involved in anatectic 

processes, the 180/ 160 partitioning between a protolith 

and granitic melt should be small (O'Neil and others, 

1977), implying that the source rocks for the granites of 

the Granite Mountains must themselves have been 180-enriched compared to the country rocks. 

Contacts between the metamorphic rocks and granites are fairly sharp and do not show any merging or intermingling boundaries. Locally, foliations in the metamorphic units parallel the granite contacts and generally wrap 

around the intrusives. All this evidence suggests forceful 

intrusion rather than in-situ derivation from the metamorphic rocks. Because the metamorphic complex is intruded 

by the granite, the source of the granite would appear 

to be from deeper levels in the crust, perhaps from different lithologies than those exposed at the surface. 

Diabase Dikes 

Diabase dikes from the Granite Mountains have a 

large variation in 8180 values ranging from 2.1 to 5. 5 

per mil, with an average of 4.2 per mil (table Kl and 

fig. K5). "Normal" basaltic or diabasic dikes have 8180 

values of about 5.5--6.0 per mil (Taylor, 1974; 

Longstaffe, McNutt, and Schwarcz, 1980). The lowered 

8180 values of the diabase dikes demonstrate that varying 

degrees of exchange with low 8180 waters have taken 

place in the dikes. These fluids may have been channeled 

along the same fracture zones that localized the hydrothermally altered granites. Chemical analyses of the mafic 

dikes (for three samples), show the presence of water 

ranging from 0.2 to 0.8 to 1.8 percent by weight (Stuckless and Peterman, 1977). Mineralogical studies show that 

intense saussuritization of calcium plagioclases, chloritization of biotites, and uralitization of augites, are present 

in most samples. 

HYDROTHERMALLY ALTERED ROCKS 

Stuckless, Bunker, Bush, poering, and Scott (1977) 

and Stuckless, Nkomo, and Doe (1981) recognized two 

types of hydrothermally-altered units in the Granite Mountains. These units include the albitized granite, which was 

observed only in those parts of drill cores that appear 

to be associated with fracture zones (fig. K6), and the 

silicified-epidotized granite, which is exposed as long, 

narrow, resistant ridges that cut across the granite along 

fracture planes and zones of weakness. A rock type not 

previously reported, the sericitized-chloritized-saussuritized granites, have a more subtle type of hydrothermal alteration. These altered granites have been 

recognized principally through the oxygen isotope studies. 

Petrographically, they are similar in mineralogy to the biotite and leucocratic granites, but have a higher, and more 

variable degree of sericitization and saussuritization of the 

feldspars and an intense chloritization of the biotites. 

These samples are also in many places restricted to highly 

fractured parts of drill cores (fig. K6). 

All the hydrothermally altered rocks have low 8180 

whole-rock values (table K3 and fig. K7), slightly deOxygen Isotopic Constraints on the Origin of Precambrian Granites, Central Wyoming 117 
DEPTH o 18Q 

(FEET) (per rnil) 

w 

!:: 

z 

~ 

a: 

(!} 

w 

1-

i= 

0 

Ill 

w 

!:: 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

~ 1000 a: 

(!} 

(.) 

i= 

~ 

a: 

(.) 1100 

0 

(.) 

::> 

w 

..J 

1200 

1300 

1400 

7.3 

8.6 

6.0 

8.3 

8.6 

8.3 

8.9 

8.8 

DEPTH 

(METERS) 

0· 

50 

100 

150 

200 

25 0 

300 

350 

400 

>==-

EXPLANATION 

~ BG+LG 

f.!m SCSG 

BB AG 

- Major fracture zones 

;::- High radioactivity 

pleted 8180 quartz values, and significantly lower 8180 

feldspar and 8180 biotite values; all ~quartz-feldspar and 

~quartz-biotite values deviate severely from normal (table 

K4). The water content (Stuckless and Peterman, 1977, 

appendix 1) also is higher for the sericitized-chloritized 

granites. The oxygen isotopic data for the hydrothermally 

altered rocks can be summarized from table K3 as follows: 

1. The sericitized-chloritized-saussuritized granite has a 

mean 8 180 whole-rock value of 7.1 per mil (N = 17), a 

mean 8180 quartz of 8. 9 per mil (N = 6), and a mean 

8180 of feldspar of 6.4 per mil (N=6). 

2. The albitized granite has a mean 8180 whole-rock 

value of 6.3 per mil (N =6), a mean 8180 quartz of 8.1 

per mil (N=3), and a mean 8180 of feldspar of 5.6 per 

mil (N=3). 

3. The silicified-epidotized granite has a mean wholerock value of 4.7 (N=26), a mean 8180 of quartz of 

5.6 per mil (N=4), and a mean 8180 of feldspar of 3.2 

per mil (N=4). 

In contrast, mean 8180 values for the whole-rock, 

quartz, and feldspar samples from the Granite Mountains 

(8.5, 9.85, and 7.85 per mil, respectively) and Louis Lake 

batholith (7 .4, 9.53, 7.52 per mil, respectively) are relatively 180-enriched. Thus, the 8180 values for the wholerock, quartz, and feldspar become progressively lowered 

in the sequence from unaltered granites to chloritized 

granites to albitized and finally, silicified-epidotized granites (tables K1, K2, K3, and K4, and figs. K4, K6, and 

K7) indicating an increasing degree of interaction between 

the granite and a low 180 fluid, probably meteoric water, 

as a function of time and (or) water to rock ratio. The 180-depleted, hydrothermally altered zones appear to have 

acted as ancient channelways for these low 180 fluids. 

These same sites also are associated with fracture zones 

that have a high radioactivity and anomalous uranium concentrations at depth as observed in drill core GM-1 (fig. 

K6), and thus may serve as sites where uranium become 

concentrated during the period of major uranium loss in 

the Cenozoic. 

OXYGEN ISOTOPE DATA-LOUIS 

LAKE BATHOLITH 

As is true for the Granite Mountains, whole-rock 

(table K5) and quartz 8180 data (table K2) from the Louis 

Lake batholith are considered to represent the primary 180; 160 of the magma for the following reasons: 

Figure K6. Vertical profile of drill-core GM-1 described by Stuckless, Bunker, Bush, Doering, and Scott (1977) showing the relationship between the 8180 values of the various granitic types, the major fracture zones and sections 

with high 'Y activity and uranium values. BG, biotite granite; LG, leucocratic granite; SCSG, sericitized-chloritized-saussuritized granite; AG, albitized granite. 

118 Shorter Contributions to Isotope Research 
Table K3. Whole-rock 8180 data for the hydrothermally 

altered rocks within the Granite Mountains 

[ ± reported for samples analyzed in duplicate except sample 

GM-1-1156.6 which was analyzed 5 times] 

Sample No. 

also 

(per mil) Sample No. 

also 

(per mil) 

Sericitized-chloritized-saussuritized granites 

BR-4 

GM-1-164.4 

GM-1-431. 9 

GM-1-502o2 

GM-1-651.7 

GM-1-842o2 

GM-1-922.7 

L-26 

PD-21 

GM-1-166.0 

GM-1-202o2 

GM-1-755.0 

GM-1-757.0 

BBL-3 

FR-3 

GR-3 

GR-4 

GR-5 

JC-2 

L-12 

L-18 

L-38 

LD-1 

LD-17 

LM-2 

MS-1 

WY-97 

6.8±0.1 

6.3 

7o6 

7o3 

6.0 

7.4 

7.3 

7o4±0.4 

7 0 7±0.3 

PD-22 

PM-1 2197.0 

SR-10 

SRQ-3 

WY-5 

WY-7 

WY-8 

WY-12 

Albitized granites 

6.9±0.2 

5.7 

7.5±0.1 

6.6±0.3 

GM-1-1156 o 6 

GM-2-934.4 

WY-6 

Silicified-epidotized granites 

6.5 

6.4 

6.2 

5.6 

4o1 

3.9 

5o2 

6.1 

5o1 

4.0 

3.3 

2o6 

2o6±0o2 

6.8 

MS-6 

MS-15 

PRNW-5 

PRNW-12 

PRSW-5 

SD-8 

SDRG-1a 

SDNE-11 

Sl1-5 

SM-11 

SRQ-7 

WY-10 

WY-13 

Episyenite 

7.1±0.1 

6.6 

6.6±0.4 

7 .6±0.2 

6.9 

7.3 

6.9 

7.1 

8. 7±0o3 

4.6 

6.8 

4.9 

6o1 

5.2 

4o0 

7o2±0.1 

3.3 

4.8 

3.3 

7.3 

4.7 

2.4 

3.6 

4.8 

1. Petrographic examination reveals little alteration. Although a few samples do show minor incipient sericitization in the cores of plagioclase, their 8180 values are not 

significantly different from the fresh, unaltered samples. 

2. 8 180 of quartz (9.3-9.8 per mil, N=6) and feldspar 

(7. 0--7.9 per mil, N = 5) show relatively restricted ranges 

of isotopic compositions. 

3. 8 180 relations among coexisting minerals for six samples have mean 8 180 values of quartz=9.5 per mil (N=6) 

and feldspar= 7. 5 per mil (N = 5) with average aquartzfeldspar=2.0 (N=6). The lack of a quartz-feldspar 

isotopic reversals and the approximation of aquartz-feldspar to "normal" values of 1.5-2.0 per mil (O'Neil and 

Taylor, 1967) would appear to indicate close approach 

to isotopic equilibrium. 

Oxygen isotope data for relatively fresh whole-rock 

samples from the Louis Lake batholith average 7. 3 ± 0. 3 

Table K4. 8180 values of minerals from hydrothermally 

altered zones within the Granite Mountains 

[Leaders ( ---) indicate not analyzed] 

alSo per mil (Standard Mean Ocean Water) 

Sample No. Quartz Feldspar Biotite Rock 

Sericitized-chloritized-saussuritized granites 

BR-4 9.7 6.6 6.8 

GM-1-164 8.4 5.8 3.9 6.3 

PM-1-2197 8.8 6.5 6.6 

~VY-7 8.7 6.4 7.3 

WY-8 9.0±0.3 6.4 1.3 6.9 

WY-12 8.9 6.7 2.0 7.1 

Albitized granites 

GM-1-166 7.8 5.8 6.9 

GM-1-755 9.7±0.05 6.8±0.06 7.5 

GM-2-934.4 6.7 4.1 4.6 

Silicified-epidotized granites 

JC-2 5.2 3.1 3.9 

MS-1 5.3±0.2 3.0 2.6 

PRNW-12 5.9 3.5 4.0 

SM-11 5.9±0.2 3o2±0.1 4.7 

Episyenite 

WY-97 8.3 5.9 6.8 

per mil (N = 13) for the hornblende-biotite granodiorite 

and 7. 7 ± 0.1 per mil (N = 5) for the leucocratic granitic 

phase (table K5). The leucocratic granite may represent 

a differentiate of the hornblende-biotite granodiorite because of its slightly higher 8 180 value, more felsic composition, mineralogy, and the presence of granodioritic 

xenoliths within this unit. 

A mafic xenolith within the hornblende-biotite 

granodiorite, interpreted to be a restite (sample LLB-7), 

has a 8 180 value of 7. 1 per mil, close to that of the 

hornblende-biotite granodiorite (mean=7 .3 per mil). 

RELATIONSHIPS BETWEEN 180/160 AND 

MAGNETIC SUSCEPTIBILITY 

Another parameter that may be used to characterize 

and elucidate the contrasting nature of granites is the magnetite content. Ishihara (1977), for example, suggested 

that the magnetite content of granites serves as a key constituent that defined the nature of the source rock. In particular, he noted that the presence or near absence of this 

mineral reflects the oxygen fugacity of the magma, which 

in tum is controlled fundamentally by the protolith; thus, 

granites forming from lower crustal sources have quite 

different oxygen fugacity conditions than those from 

metasedimentary protoliths. 

Oxygen Isotopic Constraints on the Origin of Precambrian Granites, Central Wyoming 119 
X =4.7 SILICIFIED-EPIDOTIZED GRANITES 

R = 2.4-7.3 

3 

2 

0 

(/) 

LI.J 

....1 X= 6.4 ALBITIZED GRANITES 

0.. 2 ~ 

<t: (J)1 

LL.. 

0 

~ OL-------~------------~----~ 

LI.J 

C:l 

~ 5 ::J SERICITIZED-CHLORITIZED-SAUSSURITIZED GRANITES 

z 

4 

3 

2 

X =7.1 

R = 6.0-7 .7 

n = 17 

(]' =0.5 

0 L_ ______ _J ____________ _J------------~--------------

2.25 3 4 5 6 7 8 

8 18Q (per mil) 

Figure K7. Histogram showing 8180 values of hydrothermally-altered units from the Granite Mountains. 

Vertical arrow positioned on the horizontal axis shows mean oxygen isotopic composition. X, mean 

value; R, range of values; n, total number of samples; and a, standard deviation. 

Table K5. 8180 whole-rock data for the Louis Lake 

batholith 

also 

Sample No. (per mil) Sample No. 

LLB-2 

LLB-3 

LLB-4 

LLB-5 

LLB-8 

LLB-9 

LLB-10 

LLB-11 

LLB-1 

LLB-6 

LLB-12 

Hornblende-biotite granodiorite 

7.4 

7.5 

7.3 

8.0±0.1 

7.4 

7.4 

7.4 

6.9 

LLB-15 

PRM-10 

PRM-11 

PRM-12 

PRM-13 

PRM-14 

LLB-7 (xenolith) 

Leucocratic biotite granite 

7.6 LLB-13 

7.6 LLB-14 

7.8 PRH-9 (dike) 

ciao 

(per mil) 

7.5 

7.6 

7.3±0.2 

7.5 

6.7±0.1 

6.8 

7.1 

7.9±0.1 

7.8 

7.1 

Magnetic susceptibility measurements provide an 

ideal method of determining the relative amounts of magnetite in a rock where quantitative volumetric determina120 Shorter Contributions to Isotope Research 

tions by optical means are difficult, as for example 

because of the small size and disseminated nature of magnetite grains, the general problem of identification of 

opaque minerals and the commonly observed intergrowth 

of magnetite with other mafic minerals. 

The relative amounts of magnetite for the granites 

of the Granite Mountains and Louis Lake batholith are 

expressed by the initial magnetic susceptibility, x. These 

data are compared with the results of Ellwood and Wenner 

( 1981), who have demonstrated that the low 180 granites 

of the southern Appalachian Piedmont, (5.5-8.2 per mil) 

have higher (> 1 x 10-4 G/Oe) magnetic susceptibilities 

than do the 180-enriched ones (7. 9-11.4 per mil). 

The initial magnetic susceptibility of 10 samples 

from the Granite Mountains and 8 from the Louis Lake 

batholith are reported in table K6 and are plotted on figure 

K8. The magnetic susceptibility measurements shown in 

figure K8 clearly indicate that the Louis Lake batholith 

has higher mean x values (4.5 X 10-4 G/Oe) than the Granite Mountain ( 1. 5 x 1 o-5 G/Oe). Petrographically, the 

Louis Lake batholith may be estimated to contain much 

more magnetite ( ~2 percent) than the Granite Mountains 

( ~0.3 percent). Ishihara's (1977) work suggested that the 
Table K6. Initial magnetic susceptibility (x) and &180 data 

for the Granite Mountains and the louis lake batholith 

Sample No, 

GM-1-773.5 

GM-1-1278,0 

GM-1-1342,1 

GM-2-1413,0 

LD-14 

LM-4 

LM-9 , 

MS-20 

PRSW-10 

WY-11 

LLB-6 

LLB-8 

LLB-9 

LLB-11 

LLB-13 

LLB-14 

PRM-11 

PRM-13 

Scalereading 

13 

10 

11 

10 

6 

15 

58, 58 

32 

35 

7 

Mass 

(grams) 

X 

(gauss/oersted) 1 

Granite Mountains 

25.5 ·1.12x1o-s 

22.6 9.69X10-G 

28.1 8.57x1o- 6 

32.8 6~68X10-G 

28.9 4.55Xl0-G 

31.4 1.05Xl0-s 

29.7 4.28X10~ 5 

35.7 1.96Xlo-s 

40.9 1.87Xl0-s 

15.9 9.64x1o- 6 

Average-- 1.42X10-s 

Standard deviation-- 1.11X10-s 

Louis Lake batholith 

256 24.6 2.28X10- 4 

1210 27.0 9.81x1o- 4 

330 23.6 3.06X10- 4 

1290 29.3 9.64x1o- 4 

54 26.3 4.50Xl0-s 

130 27.3 1.04X10- 4 

820 27.4 6.55X10- 4 

1130 31.7 7.81X10- 4 

Average-- 5.08X10- 4 

Standard deviation-- 3.83X10- 4 

01a 

per mil 

8.6 

8.9 

8.8 

8.5 

7.9 

9.1 

8.8 

8.7 

8.6 

7.8 

7.6 

7.4 

7.4 

6.9 

7.9 

7.8 

7.3 

6.7 

lAll magnetic data courtesy of B. B. Ellwood of the 

University of Georgia. 

higher magnetic susceptibility and magnetite content in 

the Louis Lake batholith can be ascribed to a higher oxygen fugacity during solidification of the magma. In such 

instances, most of the iron probably is consumed in the 

formation of iron oxides rather than being available for 

the formation of ferromagnesian minerals. In the Granite 

Mountains, the relatively low bulk magnetic susceptibility 

and magnetite content imply a low oxygen fugacity in 

the source region. However, near the end of crystallization, high oxygen fugacities may have occurred locally, 

resulting in the formation of some of the magnetite segregates observed in the granites (Stuckless and Peterman, 

1977). Thus, the combined higher magnetic susceptibility 

and lower 8180 values of the Louis Lake batholith relative 

to the Granite Mountains imply that these two granites 

were formed from different sources that had different oxygen fugacities. 

CHEMICAL AND MINERALOGICAL FEATURES 

Granites of the Granite Mountains 

Detailed whole-rock chemical analyses, petrographic studies, and uranium and thorium data are re5,5 6,5 7,5 8,5 9,5 10.5 11.5 

8 18Q (per mil) 

Figure K8. Three-cycle semilog plot <;>f initial m~gnetic 

susceptibility versus &180 for the Gramte Mountams a~d 

louis lake batholith. These data are contrasted w1th 

similar data (stippled area) for the Paleoz~ic age, l~te 

orogenic granites of the southern Appalachians stud1ed 

by Ellwood and Wenner (1981). n, number of samples. 

ported by Stuckless, Bunker, Bush, Doering, and Scott 

(1977), Stuckless, Bunker, VanTrump, and Bush (1978), 

Stuckless and Miesch ( 1981), and Stuckless and Peterman 

( 1977) for a large number of samples from the Granite 

Mountains. The bulk of the Granite Mountains pluton 

contains subequal amounts of quartz, plagioclase, and 

microcline; biotite (1-10 percent) is the dominant minor 

constituent. Magnetite and primary epidote are minor to 

trace constituents in all samples. Zircon and apatite are 

the only ubiquitous trace minerals. Muscovite is present 

in allleucocratic variants, and is locally abundant. 

Whole-rock chemical analyses reveal that the granites of the Granite Mountains are rich in normative corundum (1.0-1.9 percent), with a moderately high mean 

molecular Al20 3/(Na20 + K20 + CaO) value of 1.1, high 

Si02 content of 74-75 percent, high K20 content of 5.1 

percent, and a moderate Na20 content of 3.4-3.8 percent. 

The granite of Lankin Dome has an average uranium concentration of 8.1 ppm (parts per million), an extremely 

high average thorium concentration of 40.8 ppm and a 

very high mean thorium-to-uranium ratio of 14.6 (Stuckless and others, 1978) compared with the world averages 

for these elements in granites (Rogers and Adams, 1969a, 

b). However, Stuckless and Nkomo (1978), have shown 

that the granite of Lankin Dome lost most of its uranium 

(as much as 80 percent from even apparently fresh samples and from depths as great as 400 m) principally during 

the Cenozoic. Assuming that the thorium-lead system was 

closed, they calculated that the initial uranium content of 

the granite had to have once been 20-50 ppm. Although 

such high uranium contents are 5-12 times higher than 

are typical for granites (Rogers and Adams, 1969b), granites with 50 ppm uranium have been recovered from deep 

drill holes (Rosholt and Huestis, 1981). 

Oxygen Isotopic Constraints on the Origin of Precambrian Granites, Central Wyoming 121 
Table K7. Chemical and CJPW normative data for samples from the Louis Lake batholith, Wyoming 

[Leaders(---) indicate not determined. Where FeO is not determined, total iron is reported as Fe203 . Samples LLB-1,6, and 12 are leucocratic 

biotite granite. All other samples are hornblende-biotite granodiorite] 

Sample No. LLB-1 LLB-2 LLB-3 LLB-4 LLB-5 LLB-6 LLB-7 LLB-8 LLB-9 LLBll LLB12 LLB14 LLB15 PRM12 

Chemical composition (percent) 

Si02 

Al 2o3 

Fe 2o3 

FeO 

76.1 

13.9 

0.52 

0.20 

0.06 

0.83 

3.90 

4. 70 

0.50 

0.03 

0.05 

0.04 

0.01 

Total(-0) 100.84 

65.8 

16.1 

2.30 

l. 70 

1.50 

3.30 

4.30 

2.80 

0.65 

0.60 

0.26 

0.05 

0.01 

99.37 

65.2 

16.1 

l. 90 

2.20 

1. 70 

3.80 

4.40 

2.40 

0.63 

o. 66 

0.31 

0.05 

0.03 

99.38 

63.5 

16.0 

5.43 

l. 90 

4.22 

3.80 

2.40 

0.10 

0.71 

0.30 

0.06 

98.42 

65.4 

15.9 

5. 59 

1.60 

3.92 

4.30 

l. 99 

0.29 

0.65 

0.40 

0.06 

100.10 

75.3 

13.8 

1.47 

0.30 

0.90 

3.40 

5.45 

0.24 

0.06 

0.05 

0.01 

100.98 

58.8 

16.7 

7.08 

3.50 

5. 56 

4.10 

2.09 

0.49 

1.01 

0.40 

0.11 

99.84 

65.1 

14.8 

5.89 

1.10 

2. 90 

3. 20 

4. 52 

0.18 

0.91 

0.30 

0.04 

98.94 

72.6 

14.2 

2.31 

0.05 

1.83 

3.00 

5.00 

0.10 

o. 29 

0.10 

0.01 

99.49 

64.6 

16.1 

5.38 

1.80 

4. 22 

4.20 

2.24 

0.32 

0.60 

0.30 

0.06 

99.82 

71.6 

14.6 

2. 26 

o. 77 

2. 26 

3.60 

3. 74 

.0.22 

0.22 

0.10 

0.03 

99.40 

75.0 

13.4 

0.93 

0.20 

o. 95 

4.20 

3.66 

0.22 

0.06 

0.05 

0.01 

98.68 

64.8 

16.2 

5.56 

1.80 

4. 35 

4.30 

l. 96 

0.29 

o. 71 

0.30 

0.06 

100.33 

56.8 

12.3 

8.47 

7.67 

8.14 

2.80 

1. 61 

0.55 

0.86 

0.40 

0.17 

99.77 

Normative composition (weight percent) 

OR 

AB 

AN 

wo 

EN 

FS 

MT 

HM 

IL 

TN 

RU 

AP 

cc 

33.45 

1.02 

27.54 

32.73 

3. 70 

o.o 

0.15 

o.o 

0.68 

0.05 

0.06 

o.o 

o.o 

0.12 

0.02 

Total----- 99.360 

21.52 

0. 65 

16.65 

36.62 

14.70 

0.0 

3. 76 

0.33 

3.36 

0.0 

1.15 

o.o 

o.o 

0.62 

0.02 

99.36 

20.16 

0.17 

14.27 

37.46 

16.74 

o.o 

4. 26 

1.48 

2. 77 

o.o 

1.26 

o.o 

o.o 

o. 74 

0.07 

99.38 

21.52 

0.20 

14.41 

32.67 

19.28 

0.0 

4.81 

o.o 

o.o 

5. 52 

0.13 

o.o 

0.65 

o. 72 

o.o 

99.92 

23.09 

0.50 

11.75 

36.35 

16.82 

o.o 

3. 98 

0.0 

o.o 

5. 58 

0.13 

o.o 

0.58 

o. 95 

0.0 

99.73 

32.12 

o. 78 

31.89 

28.49 

4.10 

o.o 

o. 74 

o.o 

o.o 

1.46 

0.02 

o.o 

0.05 

0.12 

o.o 

99.77 

Table K8. Radium equivalent uranium, thorium, and potassium content of the Louis Lake batholith (Stuckless, 

Bunker, Bush, and VanTrump, 1981) 

Concentration 

(parts per million) 

Sample No. RaeU Th 

LLB-1 3.10 

LLB-2 2.80 

LLB-3 2. 70 

LLB-4 2.87 

LLB-5 5.72 

LLB-6 6. 55 

LLB-7 3.50 

LLB-8 8.38 

LLB-9 3. 70 

LLB-10 4.59 

LLB-11 5.57 

LLB-12 1.90 

LLB-13 8.21 

LLB-14 1. 78 

11.70 

15.60 

8.80 

14.70 

12.60 

23.5 

21.3 

63.7 

44.9 

45.7 

14.1 

15.2 

22.0 

11.4 

K (weight 

percent) Th/RaeU 

3.91 

2.27 

1.83 

1.86 

1.64 

4.43 

1.76 

3.73 

4.09 

3.95 

1.89 

3.11 

3.51 

3.06 

3. 77 

5.57 

3.26 

5.12 

2.20 

3.59 

6.09 

7.60 

12.1 

9.96 

2.53 

8.00 

2.68 

6.40 

LLB-15 3.01 9.8 1.67 3.26 

PRM-9 (LLB-21) 1.00 7.8 3.44 7.80 

PRM-10(LLB-16) 1.94 19.4 1.76 10.0 

PRM-ll(LLB-17) 2.63 15.1 2.19 5.74 

PRM-12(LLB-18) 2.23 4.2 2.19 1.88 

PRM-13(LLB-19) 2.21 7.9 1.86 3.57 

PRM-14 (LLB- 20) __ 1_. 8_2 ____ 12_._3 ____ 1 .'-2_8 ____ 6 ;_• 7_6 

Average----------- 3.63 19.1 2.64 5.61 

Range------------- 1.0-8.4 4.2-63.7 1.3-4.4 1.9-12.1 

Standard deviation 2.1 14.8 1.0 2.9 

122 Shorter Contributions to Isotope Research 

11.83 

o.o 

12.37 

34.75 

21.03 

o. 37 

8. 73 

0.0 

o.o 

7.09 

0.24 

2.18 

o.o 

0.95 

o.o 

99.53 

22.42 

0.09 

27.00 

27.37 

12.56 

o.o 

2.77 

o.o 

o.o 

5. 95 

0.08 

0.0 

0.87 

0. 72 

o.o 

99.83 

32.47 

o. 77 

29.70 

25.52 

8.47 

0.0 

0.13 

o.o 

o.o 

2.32 

0.02 

o.o 

0.28 

0.24 

o.o 

99.90 

20.87 

o.o 

13.26 

35.60 

18.59 

0.0 

4.49 

o.o 

o.o 

5. 39 

0.13 

0.36 

0.39 

o. 71 

o.o 

99.70 

30.82 

o. 77 

22.23 

30.65 

10.62 

0.0 

1. 93 

0.0 

0.0 

2.27 

0.07 

o.o 

0.19 

0.24 

o.o 

99.78 

34.83 

0.93 

21.92 

36.02 

4.45 

o.o 

0.51 

o.o 

o.o 

0.94 

0.02 

0.0 

0.05 

0.12 

o.o 

99.78 

Louis Lake Batholith 

21.17 

o.o 

11.54 

36.27 

19.05 

o.o 

4.47 

o.o 

o.o 

5.54 

0.13 

o. 36 

0.50 

o. 71 

0.0 

99.73 

11.28 

o.o 

9.54 

23.75 

16.28 

8.04 

19.15 

o.o 

o.o 

8.49 

0.36 

l. 64 

o.o 

0.95 

0.0 

99.47 

The bulk of the Louis Lake batholith typically contains hornblende, sphene, trace apatite as inclusions, 

abundant biotite (1-16 percent), epidote (3.7 percent), and 

magnetite (2 percent). 

Whole-rock chemistry shows that the Louis Lake 

batholith (table K7) is rich in normative diopside (0.2-1.0 

percent), has low A}z03/(Na20 + K20 + CaO) ratio 

(0.58-1.07), moderate Si02 values (57-76 percent), low 

K20 values (3.2 percent), and high Na20 (3.8 percent). 

The uranium content is slightly less than is typical of 

granite (3.6 versus 4.0 ppm), but about normal thorium 

content (19 ppm) and a mean Th/U of 5.6 (table K8). 

Preliminary evaluation of lead-isotope ratio (J. S. Stuckless and I. T. Nkomo, unpub. data) does not indicate 

any pronounced, post-crystallization loss of uranium for 

the Louis Lake batholith. 
Table K9. Comparison between diagnostic features of the granite of the Granite 

Mountains and the Louis Lake batholith 

[Leaders (---)indicate not present. Modal mineralogy in volume percent. Normative corundum, Si02, Na20, 

and K20 in weight percent. Uranium and thorium in parts per million. G/Oe, gauss per oersted] 

Granite of the 

Granite Mountains Louis Lake batholith 

Modal mineralogy 

Muscovite-------------------- 0.3 

Garnet----------------------- 0.4 

Magnetite-------------------- 0.3 

Epidote---------------------- 0.6 

Biotite---------------------- 1-5 

Hornblende-------------------

Sphene-----------------------

Plagioclase composition------ An14-19 

2.0 

3.7 

1-16 

4-6 

2.0 

An28 

Apatite occurrence----------- sparse discrete 

crystals. 

Small poikilitic 

inclusions. 

Chemistry 

Normative corundum----------- 1-1.9 

Al 2o3/Na20+K20+Cao (molar)--- 1.1 

Si02------------------------- 74-75 

Na 2o------------------------- 3.5-3.8 

K2o-------------------------- 5.1 

Uranium--------~------------- 8.1 

Thorium---------------------- 40.8 

Uranium (initially)---------- 20-50 

0.0-1.0 

0.96 

57-76 

2.8-4.4 

3.2 

3.6±2.0 

19±15 

4.8 

Isotopic and other features 

Age (million years)---------- 2600 

Hydrothermally-altered 

2642 

zones. Three major types-------- None recognized. 

Inclusions------------------- Few xenoliths Many hornblende-

(metasedimentary). bearing clots. 

o18o permil------------------ 8.5±0.5 7.4±0.4 

87sr; 86sr (initial)---------- 0.7053 0.7020 

x, G/Oe---------------------- 1.5x1o-s 4.5x1o- 4 

Inferred source characteristics 

Alumina saturation----------- Peraluminous------------- Metaluminous. 

Protolith-------------------- Hetasedimentary 

U-enriched. 

Metaigneous, not 

highly U-enriched. 

DISCUSSION 

Comparison Between the Granite Mountains 

and Louis Lake Batholith 

The various isotopic, mineralogical, field, and 

chemical characteristics of both the Granite Mountains and 

Louis Lake batholith are summarized in table K9. The 

most noticeable contrasting features include the following. 

1. The Granite Mountains consist of two mappable 

gramttc units of peraluminous biotite granite, the granite 

of Long Creek Mountain and the granite of Lankin Dome. 

Mean 8180 values for these units are 8. 2 ± 0. 3 per mil 

(N=4) for the former and 8.5 ± 0.3 per mil (N= 12) for 

a leucocratic ph-ase of the latter, respectively. The Louis 

Lake batholith is a metaluminous hornblende-biotite 

granodiorite with minor quantitites of a leucocratic biotite 

granitic phase. Mean 8180 values for these two units are 

7.3±0.3 per mil (N=13) and 7.7±0.1 per mil (N=5), 

respectively. Thus, based on estimates of the areal expoOxygen Isotopic Constraints on the Origin of Precambrian Granites, Central Wyoming 123 
6 

4 

2 

0 

7 

en 

LU 14 

...J 

Q.. 

~ 

<( 

12 en 

u.. 

0 

a: 10 

LU 

co 

~ 

:::J 8 

z 

6 

4 

2 

o~--------~------------

7 

8 

8 

LOUIS LAKE BATHOLITH 

x =7.4 

R = 6.7-7.9 

n = 18 

cr = 0 .3 

9 

9 

10 

GRANITE MOUNTAINS 

x =8.5 

R = 7.8-9.1 except 1 0.2 

n =53 

cr = 0.42 

10 

o 18Q (per mil) 

Figure K9 Histogram showing a comparison between the 8180 values of unaltered whole 

rocks of the louis lake batholith and granites from the Granite Mountains. Vertical arrow 

positioned on the horizontal axis shows the mean oxygen isotopic composition. 

sure of each granitic phase, the Granite Mountains have 

a distinctly higher weighted mean 8180 of 8.5 ±0.5 per 

mil compared with that of the Louis Lake batholith of 

7.4 ± 0.4 per mil (fig. K9). 

124 Shorter Contributions to Isotope Research 

2. Oxygen isotopic analyses made of feldspar and quartz 

from representative unaltered samples from both bodies 

(7 samples from the Granite Mountains and 5 from the 

Louis Lake batholith) give average ~quartz-feldspar val-
ues (2.0) that reflect a close approach to equilibrium, indicating that the whole-rock data essentially reflect the 180/ 160 of the primary magmas. 

3. The Granite Mountains have a higher ini!ial 87 Sr/86Sr 

(0.7053 ± .0056, Peterman and Hildreth, 1978) than the 

Louis Lake batholith (0. 7020 ± 0.001, Naylor and others, 

1970). 

4. The mean magnetic susceptibility of the Granite 

Mountains is distinctly lower ( 1. 5 x 1 o-s G/Oe) than that 

of the Louis Lake batholith (4.5X 10-4 G/Oe), which is 

consistent with the observed, greater magnetite content of 

the batholith. 

5. There are obvious differences in whole-rock chemistry, reflected principally by the high normative corundum 

and peraluminous nature of the granites of the Granite 

Mountains compared with the low normative corundum 

and metaluminous nature of the Louis Lake batholith. 

6. The granite of Lankin Dome was enriched initially 

in thorium (mean of 40.8 ppm) and uranium (calculated 

range of from 20 to 50 ppm) compared with rocks of 

the Louis Lake batholith, which has more nearly normal 

thorium (mean of 19 ppm) and uranium (mean of 3.6 

ppm) contents. 

7. The leucocratic phase of the granite of Lankin Dome 

is uniquely characterized by certain accessory minerals 

(muscovite, garnet) that are not observed in the Louis 

Lake batholith. In contrast, the mafic and accessory assemblage of the Louis Lake batholith is hornblende, biotite, sphene, and magnetite. 

8. The Granite Mountains contain only rare xenoliths 

(possibly of metasedimentary origin), whereas the Louis 

Lake batholith typically contains many rounded, maficrich hornblende-sphene-magnetite-epidote-biotite bearing 

xenoliths (possibly of metaigneous origin). 

9. The granites from the Granite Mountains preserve distinct oxygen isotopic evidence of hydrothermal alteration 

involving waters of probable meteoric origin. This alteration is manifested as pervasive, narrow lenticular zones 

of silicified-epidotized granite, and the more subtle albitized granite and sericitized-chloritized-saussuritized 

granite observed in drill cores. From least to most intensely altered these include: the sericitized-chloritizedsaussuritized granite (mean whole-rock 8180=7.1 per mil 

(N = 17)), the albitized granite (mean whole-rock 

8180 = 6. 3 per mil (N = 6)), and the silicified-epidotized 

granite (mean whole-rock 8180=4.7 per mil (N=26)). 

Isotopically depleted diabase dikes also have been identified in the Granite Mountains. No such altered rock 

types have been recognized in the Louis Lake batholith. 

Comparison With Phanerozoic Granites 

In recent years, considerable oxygen isotope data 

have been obtained for granitic rocks of different ages 

from different tectonic settings. Most detailed studies exist 

for the large Phanerozoic granitoid batholiths, including 

for example those from southeastern Australia (O'Neil and 

Chappell, 1977; and O'Neil and others, 1977), the southem Appalachian Piedmont (Wenner and Whitney, 1979; 

Wenner, 1981), the British Caledonian granitic suites 

(Harmon and Halliday, 1980), southwestern Nova Scotia 

(Longstaffe, Smith, and Muehlenbachs, 1980), the Peninsular Range of southern and Baja California (Taylor and 

Silver, 1978), and the Sierra Nevada of California (Masi 

and others, 1981). Among these, the most complete and 

detailed information comparable to what exists for the 

Granite Mountains and Louis Lake batholith is that from 

the southeastern Australian granitoids. 

For the southeastern Australian granites, O'Neil and 

Chappell (1977) and O'Neil and others (1977) have demonstrated that oxygen isotope data can provide one of the 

best criteria for characterizing granite protoliths. In particular, Chappell and White (1974), O'Neil and Chappell 

( 1977), and 0' Neil and others ( 1977) noted that granites 

that formed from source rocks of metaigneous origin 

(1-type granites) had differing isotopic, petrologic, chemical, and field characteristics compared with those of 

metasedimentary origin (S-type granites). These differences are summarized in table K10. 

Postulated Source Rocks 

The summary of contrasting features of the Granite 

Mountains and the Louis Lake batholith listed in table 

K9, and the criteria used for differentiating between the 

S- and 1-type granitoids of southeastern Australia shown 

in table K10 suggest that a number of geologic, 

mineralogic, and chemical features are similar. These features strongly imply that the Granite Mountains body was 

derived by partial melting of a peraluminous, uranium-enriched source material of sedimentary origin, whereas the 

Louis Lake batholith formed from a metaluminous, less 

uranium-enriched protolith that is probably of a deepseated igneous origin. 

However, a direct comparison of all of the various 

features serving to distinguish between the S- and !-origin 

of granites in the Paleozoic is not entirely applicable to 

the Archean granites of Wyoming, because certain fundamental differences do exist, primarily with the initial 

strontium and the oxygen isotopic data. Although differences between the initial strontium isotopic ratios of the 

Paleozoic and Archean granites may reflect their widely 

contrasting ages, the 180/160 discrepancies remain an 

enigma. 

The most obvious feature of the oxygen isotopic 

data is the absence of any significant 180-enrichment of 

the Granite Mountains, a body whose geologic setting, 

mineralogy, chemistry, and relatively radiogenic initial 

strontium isotopic composition all point to an origin from 

metasedimentary source rocks. This same feature has been 

Oxygen Isotopic Constraints on the Origin of Precambrian Granites, Central Wyoming 125 
Table K1 0. Criteria used for distinguishing between 1- and 5-type Paleozoic granites in southeastern Australia (Chappell and White; 1974, O'Neil and Chappell, 1977; O'Neil and others, 1977) 

Granites 

Properties S-type !-type 

Na 2o for felsic varieties 

with K2o >5 percent. <3.2 weight percent. >3.2 weight percent. 

Molar Al 2o3/(Na 20+K20+CaO)--- > 1.1 percent. <1.1 percent. 

Normative corundum----------- )1 weight percent. <1 weight percent. 

Si02------------------------- High percentage, narrow 

range of values. 

Broad spectrum of felsic 

to mafic. 

Mineral assemblage----------- Garnet, muscovite, 

apatite (discrete) 

Hornblende, sphene, epidote 

magnetite, apatite (inclusions) 

87sr/86sri (Berridale) 

batholith. )0.708 0.704-0.706 

Xenoliths-------------------- Rare, metasedimentary 

blocks. 

Many, hornblende-bearing 

clots with igneous texture. 

o18o values (per mil): 

New England batholith------

Berridale batholith--------

10.4-12.5 

9.9-10.5 

noted for many other large Archean granite bodies (Shieh 

and Schwarcz, 1977; Taylor, 1977; Longstaffe and others, 

1978). From limited studies made to date, virtually all 

large Archean granitic bodies appear to have 8180 values 

that are no more 180-enriched than the Granite Mountains. 

This oxygen isotopic difference between the Paleozoic and 

Archean peraluminous granites of metasedimentary origin 

points to fundamental differences between the source 

rocks involved in granite genesis during these two different periods. 

The most likely explanation for the relatively lower 

8180 values of the Granite Mountains compared with 

Phanerozoic granites of metasedimentary origin is that 

more detritus may have been present in the peraluminous, 

clay-rich metasediments of the Archean protolith. Such 

source rocks would be expected to have a lower overall 

8180 value and thus be capable of generating a granite 

through anatexis whose 180/ 160 signature is similar to that 

of the protolith. These source rocks still would be capable 

of producing a granitic batholith with the mineralogy, 

chemistry, and initial stronitum isotopic composition of 

the Granite Mountains. In such a model, the 

metasedimentary source rocks may be peraluminous, but 

the relative proportion of the detrital component to authigenic clay or metapelitic materials is important. 

This idea is supported by the observations of 

Longstaffe and Schwarcz ( 1977) and Shieh and Schwarcz 

(1978). They pointed out that Archean clastic sediments 

have distinctly lower mean 8 180 values (as much as 4 

126 Shorter Contributions to Isotope Research 

7.7-9.9 

7.9-9.4 

per mil) than those of Phanerozoic age. They also noted 

that a similar systematic difference is mirrored by Archean 

granites, and suggested that the lower 8 180 values of the 

metasedimentary protolith of Archean granites is due to 

higher proportions of unaltered felsic volcanic rock fragments or granitic detritus in the metagreywackes, and to 

lesser amounts of a weathered clay component. Likewise, 

Stuckless, Bunting, and Nkomo (1981) noted that Archean 

metasedimentary rocks typically may be less matured, in 

terms of chemical weathering and element separation, than 

their Phanerozoic equivalents. 

Similarly, the Louis Lake batholith has oxygen 

isotopic compositions (7 .4 ± 0.4 per mil) that are distinctly 

lower than those of the southeast Australian I-type granites (7.7-9.9 per mil; O'Neil and others, 1977), with virtually little overlap in the & 180 data. This relatively light 

oxygen again may reflect a fundamental difference in 

source-rock characteristics between igneous-protolith 

granites in the Archean and those in the Paleozoic of Australia. Alternatively, different processes may have brought 

about a difference in oxygen isotopic composition while 

leaving the mineralogy, chemistry, and magnetic and field 

data virtually identical. Differences in the initial strontium 

isotopic ratio between the Louis Lake batholith and Australian I-type granites again can be attributed to their contrasting ages. 

Another important fundamental difference between 

the 180/ 160 data of the Australian and Wyoming granites 

is the relatively extreme oxygen isotopic homogeneity of 
the latter. In fact, the range of oxygen isotopic compositions for such large granite bodies as the Granite Mountains and the Louis Lake batholith appears to be relatively 

limited compared with that of many of the Phanerozoic 

composite batholiths of comparable dimensions. These extremely uniform oxygen isotopic compositions for the two 

Wyoming granite batholiths may indicate that the source 

rocks were quite homogeneous in their 180/160. In contrast, for example, the granitoids that comprise the Sierra 

Nevada (Masi and others, 1981) and Peninsular Range 

batholiths (Taylor and Silver, 1978) have a wide range 

of 8180 values, initial strontium isotopic compositions, 

and mineralogical and chemical characteristics that have 

been attributed to a variety of source rocks. 

Alternatively, the extreme oxygen isotopic uniformity of the Granite Mountains and Louis Lake batholith 

could have been caused by convective processes operating 

in granite bodies of such large dimensions. For example, 

in the Fish Canyon magma system (J. A. Whitney, oral 

commun., 1981) the extreme homogeneity of this large 

magma body (at least 30,000 km3

) may be due to complete mixing by thermal convection. If such convective 

processes had operated in the granites of the Granite 

Mountains, then an isotopically heterogeneous source cannot be ruled out. 

CONCLUSIONS 

The contrasting primary features indicate two differing protoliths. From the data, the Granite Mountains are 

inferred to have formed from a peraluminous, 

metasedimentary source rock that previously was enriched 

in uranium and throium. Such a source rock is believed 

to have contained a greater detrital to authigenic component compared with most Phanerozoic sediments, because 

the 180/160 of the Granite Mountains is notably isotopically depleted compared with many chemically and 

mineralogically similar Phanerozoic granites thought to 

have formed from metasedimentary protoliths. Such a 

metasedimentary protolith thus may have been important 

in preconcentrating uranium and thorium. In contrast, interpretation of the data obtained from the Louis Lake 

batholith suggests that this magma formed from a 

metaigneous protolith that was not highly enriched in uranium or thorium. 

Uncertain is what, if any, significance the hydrothermal alteration event identified in the Granite Mountains may have had. The 180/160 lowering of progressively altered granite clearly points to interaction with 

waters of possible meteoric origin at an elevated temperature. Conceivably, such effects may have been responsible 

for preconditioning the granite for subsequent uranium 

loss, as this process appears to be restricted to the 

Archean granite body that clearly has lost much of its 

uranium at a subsequent time. However, the absence of 

any systematic 180/160 correlation in either whole-rock 

or ~quartz-feldspar values with samples that have had differing degrees of uranium loss, as for example was observed in drill-core GM-1 (Stuckless and Nkomo, 1978), 

strongly implies that uranium loss must have taken place 

at extremely low temperatures. By way of comparison, 

Wenner and Taylor (1976) estimated that temperatures as 

low as 50°-1 00°C produced systematic changes in both 

whole-rock 8180 and aquartz-feldspar values within the 

Proterozoic-age volcanic and plutonic rocks of southeastem Missouri. Temperatures lower than this range conceivably may have existed in the Granite. !vfoWitains during 

the major uranium-loss event in the Cenozoic. 

ACKNOWLEDGMENTS 

This work was supported by a U.S. National Science Foundation Grant No. EAR-7818127 to D. B. Wenner, J. A. Whitney, and J. C. Stormer, Jr., and a USGS 

Grant No. 14-08-0001--G-590 to Wenner and a Georgia 

Tech USDI Mining Fellowship No. 10-11-RH176-099 to 

Cheang. We especially wish to thank J. A. Whitney for 

guidance, encouragement and constructive comments during the course of this work. 

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Oxygen Isotopic Constraints on the Origin of Precambrian Granites, Central Wyoming 129 

FIGURES 

1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

ORIGIN OF THE LITHIUM-RICH BRINE, 

CLAYTON VALLEY, NEVADA 

Chapter L 

By JOSEPH R. DAVIS1 , IRVING FRIEDMAN, and j. D. GLEASON 

CONTENTS 

Abstract ..... . 

Introduction . . . . 

Geographic setting . 

Geology of the basin fill 

Subsurface hydrology 

Ground-water geochemistry 

Brine evolution 

Conclusions . . 

References cited . . 

Page 

132 

132 

132 

133 

135 

135 

135 

138 

138 

Page 

Ll. Major physiographic features of Clayton Valley, Nev. . . . . . . . . . . . . . . . . . . . . . . . 133 

L2. Well logs from Clayton Valley, Nev., and correlation with Searles Lake, Calif. . . . . . . . . . 134 

L3. Generalized cross section of Clayton Valley playa, showing structural position of the major tuff-bed 

aquifer and inferred directions of ground-water movement . . . . . . . . . . . . 136 

L4. Plot of molar chloride and sodium concentrations in water samples from Clayton Valley 137 

L5. Plot of molar chloride and lithium concentrations in water samples from Clayton Valley . 137 

L6. Plot of molar chloride and magnesium concentrations in water samples from Clayton Valley 137 

TABLE 

Page 

L 1. Partial solute chemistry of selected ground waters from Clayton Valley . . . . . . . . . . . . . . 136 

1

Present address: ARCO, P. 0. Box 2819, Dallas, TX 75221 

Origin of the Lithium-Rich Brine, Clayton Valley, Nevada 131 
Abstract 

The lithium-rich brines pumped from beneath 

Clayton Valley, Nevada, provide about one-third of the 

present United States requirements for lithium. Although 

the pre-Tertiary rocks in the vicinity do not contain 

anomalous amounts of lithium, the Tertiary volcanics 

that abound in the area could have been the origin of 

the lithium. Extensive diagenetic alteration of vitric material has taken place, and high lithium concentrations are 

associated with smectite in the Miocene Esmeralda Formation exposed east of the playa. The brines contain 

about 20 percent sodium chloride and as much as about 

320 parts per million of lithium. The deuterium contents 

of two samples of the brine are -97 and -113 per mil 

Vienna Standard Mean Ocean Water. The deuterium 

content of the brine is close to that of the recharge to 

the aquifers beneath the playa. Springs in the recharge 

area range in &D from -106 to -114. The brine results 

from solution by ground water of halite beds that are 

high in lithium. These salt beds probably were formed 

during the desiccation of a small saline lake that occupied the position of the present playa. 

INTRODUCTION 

In the first part of this century, demand for sodium 

and chloride, sodium carbonate, borates, and potash salts 

led to an extensive reconnaissance of playa lakes throughout the western United States. The playa in Clayton Valley near Silverpeak, Nev., was first studied by Dole 

(1912), who found high concentrations of sodium chloride 

in near-surface brines but made no mention of lithium. 

The lithium deposit at Silverpeak remained undiscovered 

until the late 1950's, when the Leprechaun Mining Company2 of Las Vegas, Nev., while searching for potassium 

brines, discovered high concentrations of lithium in the 

subsurface brines of the playa. In 1964, Foote Mineral 

Company acquired the property to develop the brine as 

a primary source of lithium. 

Foote Mineral Company pumps brine from depths 

of about 100-250 m (meters) into a series of evaporating 

ponds (Barrett and O'Neil, 1970). Solar evaporation results in the precipitation of sodium chloride and the concentration of lithium. Brines that initially contain about 

300 ppm (parts per million) lithium thus are concentrated 

to as much as 5,000 ppm lithium. After precipitation of 

the sodium chloride, lithium carbonate is precipitated in 

a processing plant. Production of Li2C03 from the 

Clayton Valley brine field began in 1967 and has continued to the present. The reported annual capacity of 

about 6,500 metric tons of Li2C03 represents about one2

Use of company names is for descriptive purposes only and does 

not imply endorsement by the U.S. Geological Survey. 

132 Shorter Contributions to Isotope Research 

third of the present United States requirements and is 

about 90 percent of all nonpegmatite lithium production 

in the United States. 

GEOGRAPHIC SETTING 

Clayton Valley is one of a group of intermediatesize valleys in west-central Nevada. It has a playa floor 

of about 100 km2 (square kilometers) that receives surface 

drainage from an area of about 1300 km2 . Figure Ll 

shows the location of the important physiographic features 

in the vicinity. The playa floor is surrounded by alluvial 

fan slopes and these, in tum, by mountain ranges, including the Silver Peak Range on the west and the Palmetto 

Mountains and Montezuma Range on the south and southeast. The W eepah Hills and Paymaster Ridge form mountain barriers on the north and the east, respectively. 

Altitudes range from 1,300 m on the playa floor to 2,880 

mat Piper Peak in the Silver Peak Range. 

There is no evidence of anomalous amounts of 

lithium in pre-Tertiary rocks. The Tertiary volcanic rocks, 

however, are considered likely to be involved in the origin 

of the lithium deposit because volcanism could have provided the heat energy and hydrothermal activity required 

to mobilize lithium from volcanic glass and other relatively unstable minerals. About 100 km3 (cubic kilometers) 

of lava erupted from the Silver Peak volcanic center in 

the western part of Clayton Valley about 6 million years 

ago (Robinson, 1972). 

East of Clayton Valley, more than 100 km3 of Tertiary ash-flow and air-fall tuff is exposed at Clayton Ridge 

and as far east as Montezuma Peak. These predominantly 

flat-lying, pumiceous rocks are interbedded with tuffaceous sediments between Clayton Ridge and Montezuma 

Peak; but at Montezuma Peak these rocks are altered considerably and dip at angles of as much as 30°. In the 

Montezuma Range, they are unconformably overlain by 

rhyolitic agglomerates. The source of these tuff sheets 

may have been a volcanic center to the east near Montezuma Peak, or to the south in the Montezuma Range, 

or the Palmetto Mountains, or Mount Jackson, or perhaps 

even the Silver Peak center to the west. 

Tertiary sedimentary rocks are exposed in the Silver 

Peak Range, in the Weepah Hills, and in the low hills 

east of the Clayton Valley playa. These rocks all are included in the Esmeralda Formation (Turner, 1900). The 

Esmeralda Formation consists of sandstone, shale, marl, 

breccia, and conglomerate, and is intercalated with volcanic rocks, although Turner (1900) excluded the major 

ash-flow units and other volcanic rocks in defining the 

formation. The rocks of the Esmeralda Formation in and 

around Clayton Valley apparently represent sedimentation 

in several discrete Miocene basins. The age of the lower 

part of the Esmeralda Formation in Clayton Valley is not 
38"00' 

AlcatraL 

Island 

cf .f 

c1 

20 

NEVADA 

40 KILOMETERS 

20 MILES 

Figure Ll. Major physiographic features in area of Clayton Valley, Nev. Modified from 

Albers and Stewart (1972). 

known, but an air-fall tuff in the uppermost unit of the 

Esmeralda Formation has a K-Ar (potassium-argon) age 

of 6. 9 ± 0.3 Ma (million years) (Robinson and others, 

1968). 

Extensive diagenetic alteration of vitric material to 

zeolites and clay minerals has taken place in the tuffaceous sandstone and shale of the Esmeralda Formation, 

and anomalously high lithium concentrations (as much as 

120 ppm lithium) accompany the alteration. However, 

lithium concentrations in these rocks generally are less 

than 200 ppm except where smectite is present, and the 

high lithium concentration associated with smectite is confined to the Esmeralda Formation exposed east of the 

playa. High lithium concentrations in these rocks is in 

hectorite, a lithium clay mineral thought to have been precipitated from alkaline lake waters (J. R. Davis, unpub. 

data). 

GEOLOGY OF THE BASIN FILL 

In mapping the Quaternary deposits of Clayton Valley, the playa surface, the surrounding alluvial fans and 

slopes, and the sand dunes are easily distinguished. A 

Origin of the Lithium-Rich Brine, Clayton Valley, Nevada 133 
large part of the playa surface has been changed by the 

construction of roads and evaporating ponds, but prior to 

this construction the northeastern part of the playa was 

characterized by a central zone of soft, moist silt and salt, 

an intermediate zone of silt and sand, and a marginal zone 

of either travertine or vegetated mounds of soft, puffy 

silt (Motts and Matz, 1970). The southwestern part of 

the playa is higher and dryer than the central zone of 

the northeastern part, and is similar to the marginal vegetated zone there. Alluvial fans are highly dissected and 

deeply entrenched in the southeastern part of the valley 

and less dissected elsewhere. Fan surfaces of at least two 

different ages can be distinguished in many parts of the 

valley. Sand dunes are in the southern part of the valley 

and are partially stabilized by vegetation. 

The subsurface stratigraphy of the basin fill is 

known from numerous wells drilled in the playa and adjacent alluvial slopes drilled by Foote Mineral Company 

between 1964 and 1978, and by the U.S. Geological Survey in 1912 and 1977. Poor sample recovery, contamination of cuttings, and the highly faulted nature of the playa 

sediments inhibited a clear understanding of the stratigraphy until Foote Mineral Company initiated an exploratory drilling program in 1976 using rotary drilling with 

reverse circulation and careful logging of the cuttings by 

well-site geologists. Much of the data collected since that 

time is still proprietary, but data from several wells are 

available (Davis and Vine, 1979), making the following 

summary possible. 

Essential to understanding the subsurface data is the 

concept of fluctuating pluvial and interpluvial climates 

40 

50 

60 

70 

80 

90 300 

1FM-4 

l I 

1 From Motts and Matz ( 1970). 

2Published with permission of Foote Mineral Company. 3From unpublished U.S. Geological Survey data. 4 From Smith (1976) 

2FX-6 

throughout the Pleistocene. During pluvial periods, which 

were cooler or wetter than today, shallow-water lacustrine 

conditions favored deposition of mud in the central part 

of the valley, merging laterally with fluvial and deltaic 

sands and muds and with well-sorted beach sands and 

gravels. During interpluvial periods, like the present, 

ephemeral lacustrine conditions favored deposition of 

muds, silt, sand, and evaporite minerals in the central 

part, grading laterally to alluvial deposits. The gravity 

data (Wilson, 1975) and subsurface stratigraphic correlations (fig. L2) suggest that the center of deposition and 

subsidence in Clayton Valley was in the northeastern arm 

of the playa throughout late Quaternary time. This center 

of deposition is bounded on the south and east by steeply 

dipping normal faults that parallel the two fault-scarp 

trends in alluvial gravels of late Quaternary age. The 

thickest clastic and chemical sedimentation was confined 

to the down dropped area north and west of these faults. 

The lacustrine sediment that was deposited near the 

center of pluvial lakes in Clayton Valley is generally 

green to black calcareous mud. About half of the mud, 

by weight, is smectite and illite, which are present in 

nearly equal amounts. Calcium carbonate generally makes 

up 10-20 percent of the mud, and koalinite, chlorite, 

sand- and silt-size volcaniclastic detritus, traces of woody 

organic material, and diatoms compose the rest. In the 

northeastern arm of the playa, thick layers of evaporite 

minerals are interbedded with lacustrine muds. These 

evaporite minerals probably were precipitated as the lake 

dried up during interpluvial periods. The correlation between the only radiometric date from the subsurface of 

EXPLANATION 

~Mud 

v{i}/GDJ Sand 

SEARLES 

LAKE 4 

~ Saline minerals 

f.2:Ig Halite 

illiiiiiiiJ Gypsum 

AGE. IN 

YEARS 

Figure L2. Well logs from Clayton Valley, Nev., and correlation with Searles Lake, Calif. 

134 Shorter Contributions to Isotope Research 
Clayton Valley (fig. L2) to nearby wells suggests that 

the pluvial stratigraphy in Clayton Valley possibly can 

be correlated with that in Searles Lake, Calif. (Smith, 

1979) and Lake Lahontan, Nev. (Benson, 1978). If the 

correlation of these salt and mud cycles is correct, the 

upper salt bed in well CV5-A was deposited since the 

last pluvial period ended about 10,500 years B.P. The 

underlying mud and salt sequence was deposited during 

the last pluvial period from 24,000 to 10,500 years B.P. 

(fig. L2). 

Subsurface Hydrology 

The structural and stratigraphic setting of Clayton 

Valley has contributed to a complex hydrologic system. 

The Paleozoic rocks in Clayton Valley contain much thinner sequences of carbonate rocks and probably do not 

form the effective fractured aquifers that they do 150 km 

to the southeast at the Nevada Test Site (Winograd and 

Thordarson, 1975). However, the upper Cenozoic rocks 

include alluvial sediments, bedded tuffs, and welded tuffs 

that act as aquifers. The chemical character of the ground 

water varies considerably within the basin, and three basic 

types can be identified: ( 1) cold, dilute ground water in 

the bedrock highlands; (2) thermal, saline ground water 

near the playa margin; and (3) cold, saline brines of the 

playa zone. 

The cold, dilute ground-water reservoir of the bedrock highlands is recharged by infiltration through the soil 

zones, alluvial channel bottoms, and bedrock fractures, 

but the thin alluvial cover and the low porosity and permeability of the clastic Paleozoic bedrock impedes deep 

circulation and large reservoir size. 

The thermal ground-water system obtains its heat 

from deep circulation or from contact with a shallow 

magma chamber. Circulation to at least 1-km-depth would 

be necessary to acquire this heat from contact with 

rocks heated due to the geothermal gradient, and the basin 

structure would require at least 500 m of this circulation 

into bedrock that is relatively impermeable except where 

fractured. However, the Quaternary cinder cone and Pliocene volcanics north of the playa suggest the presence 

of a magma chamber that could create a heat flux in excess of the normal geothermal gradient. 

The aquifers of the central playa zone contain cold, 

saline ground water. The recharge of these economically 

important aquifers is central to the topic of brine evolution 

and will be discussed. 

Groundwater Geochemistry 

The subsurface brines used for lithium production 

in Clayton Valley are sodium chloride brines of about 

20 percent salinity when they are first pumped from the 

playa (table L1). More dilute ground waters from the 

playa zone have similar proportions of major constituents, 

with salinities as low as 1 percent. Lithium seems to correlate well with concentration of these waters and shows 

no correlation to temperature; the thermal ground water 

has major-ion composition nearly identical to cold ground 

waters of similar concentration. 

All the ground waters from the vicinity of the playa 

are supersaturated with respect to calcite, the clay mineral 

hectorite, and the zeolite phillipsite. The brines also are 

supersaturated with respect to cristobalite and analcime, 

whereas some of the more dilute waters are saturated with 

respect to sepiolite. No natural brines analyzed from 

Clayton Valley are saturated with respect to gypsum or 

halite. 

The solute chemistry of the ground waters in 

Clayton Valley and the adjacent basins does not support 

the concept of subsurface drainage into Clayton Valley 

from the adjacent basins (Rush, 1968; Motts and Matz, 

1970). There certainly appears to be no lithium-enriched 

source waters in any of the other basins, which makes 

the possibility of inflow unimportant in considering the 

chemical evolution of the brines. 

Brine Evolution 

The most lithium-enriched brines are in the downfaulted northeastern arm of the playa. Brine is produced 

primarily from the southeast-dipping air-fall tuff bed or 

"ash aquifer." Although some brine previously was produced from the permeable salt beds overlying the ash 

aquifer, the practice was abandoned due to dissolution of 

the salt and collapse around the wells. The highest lithium 

concentrations are found in brines produced from the tuff 

where it abuts the faults and forms a structural trap for 

the dense brines (fig. L3). 

The lithium-enriched, sodium-chloride brines in 

Clayton Valley are typical of the sodium-sulfate-chloride 

brines that evolve during evaporation of inflow waters 

having approximately equal bicarbonate and calcium plus 

magnesium contents (Eugster and Jones, 1979). The halite 

and gypsum beds in the surface give direct evidence of 

high salinity levels during the interpluvial periods, but the 

isotopic composition of the brine suggests that it was not 

formed by evaporative concentration, and therefore the 

brine cannot be a residual pore water. 

Deuterium composition in a brine increases during 

evaporation due to differential fractionation of the lighter 

hydrogen isotope in the vapor phase. A careful study of 

this process has been made at Owens Lake, Calif., which 

is about 100 km to the southwest of Clayton Valley. At 

Owens Lake, the fractionation mechanism was shown to 

increase the deuterium values from about 80=-115 per 

mil on the inflow to 8D = + 3 per mil in evaporated brines 

of similar composition to those in Clayton Valley (FriedOrigin of the Lithium-Rich Brine, Clayton Valley, Nevada 135 
Table L1. Partial solute chemistry of selected ground waters from Clayton Valley 

[Leaders(---) indicate no data; temp. (

0

C), temperature in degrees Celsius; SMOW, Standard Mean Ocean Water] 

Sample Depth 

source (meters) 

CVl----------- 96 

CV2----------- 53 

CV3----------- 127 

CV4----------- 242 

cv5----------- 72 

CV5A---------- 218 

FM116---------

FM133---------

Minneosta-----

Coyote--------

Tarantula-----

Big-----------

McNamara------

Waterworks----

D Quaternary mud 

~ Quaternary conglomerate 

~ Tertiary Esmeralda Formation 

D Paleozoic bedrock 

Fault 

Concentration 

Temp. (parts per million) 

(oC) Na Cl Li 

Wells 

22.0 7,073 11,788 27 

21.0 7,094 10,837 29 

33 9,775 15,640 42 

36 12,700 20,500 58 

22 5,900 9,850 26 

19.5 82,375 132,860 283 

19.5 63,750 100,000 230 

19.5 46,930 69,400 180 

Springs 

60 100 .05 

57 75 .05 

105 120 .05 

52 55 .05 

31 65 .05 

350 740 0.69 

I·;::·.:·::·:·:· .. , Sand 

B Tra,vertine 

I I Tuff 

a Salt 

40pprn• Approximate lithium concentration (parts per million) 

Molar Molar 00 (per 

Na/Cl Li/Cl mil SMOW) 

0.923 0.012 

1. 0097 0.014 

0.952 0.014 

0.954 0.014 

0.9255 0.013 

0.9435 0 •. 011 

0.9083 0.013 -97 

0.9826 0.012 -113 

0.8966 -109 

1.1905 -109 

1. 3529 -111 

1.4375 -114 

0.6842 -106 

0.7204 

2 KilOMETERS 

1 MILE 

DEPTH 

METERS 

100 

200 

3110 

400 

Figure L3. Generalized cross section of Clayton Valley playa, showing structural position of the major tuff-bed 

aquifer and inferred directions of ground-water movement. 

136 Shorter Contributions to Isotope Research 
man and others, 1976). However, in Clayton Valley, the 

brines have deuterium compositions approximately equal 

to the inflow (table L1), indicating that the brine was 

not produced by evaporative concentration of the inflow 

waters. Furthermore, the isotopic compositions of the two 

brine samples from Clayton Valley show considerable variation (80=-97 per mil and 80=-113 per mil). This variation probably is due to seepage of small amounts of the 

isotopically heavy waters in the evaporating ponds into 

the aquifer near well FM 116. Well FM 113 is located 

away from the ponds, and the deuterium concentration 

from that well (80=-115 per mil) is similar to that of 

potential ground-water recharge emanating in the Silver 

Peak Range to the west or in the Palmetto Mountains 

to the south. 

Further study of the geochemistry of the hydrologic 

system also suggests that the brine is produced primarily 

from dissolution of halite. Dilute inflow in the basin varies in sodium and chloride concentrations, having molar 

Na:Cl that range from 0.6 to 1.4 (table L1), whereas 

ground waters and brine from the playa have molar Na:Cl 

near unity regardless of concentration. Given the large 

amount of variation in the Na:Cl of the inflow waters, 

it is difficult to conceive of any process other than dissolution of halite that always would produce an Na:Cl so near 

unity. 

Plots of molar chloride concentrations against various cation concentrations (figs. L4-L6) give added insight 

to the problem of brine evolution. Figure L4 demonstrates 

how the Clayton Valley brines and more dilute ground 

waters from the playa plot on a trend of equal molar proportions of sodium and chloride. Figures L5 and L6 show 

+ .. 

z 

~ 

(.!) 

0 

..J 

4.0 

3.5 

3.0 

2.5 / 

/ 

2.0 

1.5 

1.0 

0.5 

/-?/ 

//A..,Path of evaporation or 

/ halite dissolution 

0 

8 

0 

0 

00 

0 oo 

& 

0 

0 

0 0 

o~~--~--~--~--~--~--~L_~L_ __ L_~ 

-o.1 0 -o.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 

LOG Me 1

-

Figure L4. Plot of molar (M) chloride (CI) and sodium 

(Na) concentration in water samples from Clayton Valley .. 

2.0 

1.5 ~.!b.Efll.!l!!.eyJ!~.!!!!Lo.rr~ 00 

c: 0 

c:tf\ .g, 0 

~I 7/ .g / 

1.0 :9-1 ·0<::-// 

0 ::I 

0 Ul ~/ (I) el f,.'b-/ (I) 

~I ~0/ cP ::c ,:.'b- '/ 00 0.5 !!l ~I !1..0/ 0 co ~I o/ 

+_ ~ "Vii ~// 

i .iii al ~'b-/ 

/ 

0 0 ..c. I / 

(.!) -5 ~I / 

/ 0 0 co ~-..v / ..J ~ 

-0.5 

-1.0 0 

-1.5 

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 

LOG Me,-

Figure L5. Plot of molar {M) chloride (CI) and lithium (Li) 

concentrations in water samples from Clayton Valley. 

2.0 

_P.!_t!!_~.!!'!!.!~..2i.!_S£1~~~ 

c:1' c:l 0 0 

1.5 .gl .gl "11 21 ~I ~oj/ ~I ·c. I 

.!!?I ·c::;l f,.'b-/ 0 + ~0/ 

+ 1.0 "CI el ,:.'b-/ 0 

"" !!ll c. I !1.0 / 0 

:E .~I ~I o/ 00 

~ ~/ 

~I -~I ~~/ 0 o 0 (.!) -I ~I 0 

0 0.5 /0 0 01 -I ...J 

~I ~I 00 

~I ~I 0 0 

~,J., 0 

0 0 

-o.5 

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 

LOG Me 1-

Figure L6. Plot of molar (M) chloride (CI) and magnesium 

(Mg) concentrations in water samples from Clayton Valley. 

that the lithium and magnesium increase in proportion to 

chloride concentration, although magnesium shows much 

more variation than lithium. These trends indicate that the 

lithium, and to a lesser extent magnesium, were concentrated with chloride by the same evaporative processes 

that led to halite precipitation. 

Microscopic examination of halite and gypsum crystals reveals fluid inclusions as well as fine-grained detrital 

material incorporated in many crystals. Roedder and Bassett ( 1981) reported that bedded halite may contain several 

Origin of the Lithium-Rich Brine, Clayton Valley, Nevada 137 
weight percent water or brine in the form of fluid inclusions, intergranular fluids, and hydrous minerals. Fluid 

in the inclusions or intergranular pores would be a 

lithium-enriched bittern, and release of this fluid during 

halite dissolution could be modeled as a closed system. 

The linear trend of the lithium-chloride plot (fig. L5) supports the closed-system model, whereas the deviation from 

the linear trend in the magnesium-chloride plot (fig. L6) 

suggests that the magnesium concentration in the brine 

has been altered, probably by interaction with hydrous 

min~rals or by cation-exchange with clays. 

CONCLUSIONS 

The playa sediments in Clayton Valley were deposited in a lithium-enriched environment. The lack of evidence for large pluvial lakes in Clayton Valley and the 

general character of the subsurface sediments suggests that 

the depositional environment probably was a playa or 

shallow lake that evaporated to total or near dryness in 

some seasons or years. This process concentrated dilute 

inflow with low lithium concentrations many times. The 

mechanism that we propose for the formation of the present lithium brine is that dilute, lithium-containing, sodium-chloride waters were concentrated by evaporation on 

a playa during interpluvial periods. Halite was deposited 

with a bittern fluid enriched in lithium. During the last 

10,000 years, meteroric water has penetrated the playa 

sediments, dissolving halite, and producing a brine similar 

in concentration and chemistry to an evaporative brine. 

REFERENCES CITED 

Albers, J. P., and Stewart, J. H., 1972, Geology and mineral 

deposits of Esmeralda County, Nevada: Nevada Bureau of 

Mines and Geology Bulletin 78, 80 p. 

Barrett, W. T., and O'Neill, B. J., Jr., 1970, Recovery of 

lithium from saline brines using solar evaporation, in Rau, 

J. L., and Dellwig, L. E., eds., Third symposium on salt: 

Northern Ohio Geological Society, v. 2, p. 47-50. 

Benson, L. V., 1978, Fluctuation in the level of pluvial Lake 

Lahontan during the last 40,000 years: Quaternary Research, 

V. 9, p. 3~318. 

138 Shorter Contributions to Isotope Research 

Davis, J. R., and Vine, J. D., 1979, Stratigraphic and tectonic 

setting of the lithium brine field, Clayton Valley, Nevada, 

in Newman, G. W., and Goode, H. D., eds., 1979 Basin 

and range symposium: Denver, Colo., Rocky Mountain Association of Geologists, p. 421--430. 

Dole, R. B., 1912, Exploration of salines in Silver Peak Marsh, 

Nevada: U.S. Geological Survey Bulletin 530-R, p. 1-18. 

Eugster, H. P., and Jones, B. F., 1979, Behavior of major solutes during closed-basin brine evolution: American Journal 

of Science, v. 279, p. 609--631. 

Friedman, Irving, Smith, G.l., and Hardcastle, K. G., 1976, 

Studies of Quaternary saline lakes-11; Isotopic and compositional changes during desiccation of brines in Owens Lake, 

California, 1969-1971: Geochimica et Cosmochimica Acta, 

v. 40, p. 501-511. 

Motts, W. S., and Matz, D. B., 1970, Reconnaissance geology 

of Clayton Playa, Nevada, Chapter 6, in Motts, W. S., 

ed., Geology and hydrology of selected playas in the westem United States: U.S. Air Force Cambridge Research Laboratories, Hauscom Field, Bedford, Mass., Final report, 

Contract No. AFL 19 (628}-2486, p. 214-236. 

Robinson, P. T., 1972, Petrology of the potassic Silver Peak 

Volcanic center, Western Nevada: Geological Society of 

America Bulletin, v. 83, p. 1693-1708. 

Robinson, P. T., McKee, E. H., and Moiola, R. J., 1968, Cenozoic volcanism and sedimentation, Silver Peak region, 

western Nevada and adjacent California: Geological Society 

of America Memoir 116, p. 577--611. 

Roedder, Edwin, and Bassett, R. L., 1981, Problems in determination of the water content of rock-salt samples and its significance in nuclear-waste storage siting: Geology, v. 9, p. 

525-530. 

Rush, F. E., 1968, Water-resources appraisal of Clayton Valley, 

Stonewall Flat area, Nevada and California: Nevada Department of Conservation and Natural Resources Water Resources Reconnaissance Series Report 45, 54 p. 

Smith, G. 1., 1979, Subsurface stratigraphy and geochemistry 

of late Quaternary evaporites, Searles Lake, California: U.S. 

Geological Survey Professional Paper 1043, 130 p. 

Turner, H. W., 1900, The Esmeralda Formation, a fresh water 

lake deposit: U.S. Geological Survey Annual Report 21, 

part 2, p. 191-208. 

Wilson, C. W., 1975, Bouguer gravity map of Clayton Valley, 

Nevada: U.S. Geological Survey Open-File report 75-333. 

Winograd, I. J., and Thordarson, W., 1975, Hydrogeologic and 

hydrochemical framework, south-central Great Basin, 

Nevada--California, with special reference to the Nevada 

Test Site: U.S. Geological Survey Professional Paper 

712--C, 120 p. 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

CHEMICAL AND STABLE ISOTOPE COMPOSITIONS 

OF ANDROGENIC GRANITE FROM 

STEPHENSON COUNTY, ILLINOIS 

FIGURES 

Abstract ..... 

Introduction . . . 

Acknowledgments 

Analytical techniques 

Petrographic descriptions . 

Chemical compositions 

Oxygen isotope compositions 

Chapter M 

By T. KURTIS KYSER 

CONTENTS 

Significance of &180 values of altered samples . 

Hydrogen isotopic compositions of biotites 

Conclusions . . 

References cited . . . . . . . . 

Page 

140 

140 

140 

140 

141 

141 

141 

146 

147 

149 

149 

Page 

Ml. Ternary diagrams of chemical compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 

M2. Ternary diagram showing comparison of normative compositions of samples from present study with 

those from Wolf River batholith, Wisconsin . . . . . . . . . . . 145 

M3. Plot of whole-rock &180 values as function of depth for each core 146 

M4. Plot of oxygen isotope fractionations of quartz-mineral pairs . . . . 148 

TABLES 

M 1. Chemical compositions . . . . . . . . . . . . . . . . . . . . . . . 

M2. Calculated norms, Al20 3/(Na20 + K20 + CaO) and modal analyses . 

M3. & 180 values of samples . . . . . . . . . . . . . . 

M4. &D values and water content of selected biotites ........ . 

Chemical and Stable Isotope Compositions of Anorogenic Granite, Stephenson County, Illinois 

Page 

142 

142 

147 

148 

139 
Abstract 

Oxygen isotope and chemical compositions were 

measured in samples of granite from the Precambrian 

basement of Illinois. Samples were selected from three 

cores, each of which penetrated an unconformity between Paleozoic sedimentary rocks and the granitic basement at a depth of 2,000-2,200 feet. The chemical and 

isotopic compositions indicate that granite near the unconformity is altered and is enriched in 180 and in many 

incompatible elements relative to deeper samples. The 

variable 8180 values of feldspars and biotites and 80 values of the biotites suggest that samples near the unconformity may have interacted at temperatures greater than 

300°C with formation waters typical of the Illinois basin. 

Chemical compositions and 8180 values of 7.0 per mil 

for unaltered samples suggest that the Illinois granites 

may have differentiated from magmas that formed from 

melting of the upper mantle or primitive lower crust. · 

INTRODUCTION 

Samples of granite from three drill holes in Stephenson County, Ill., yield whole-rock Rb-Sr (rubidium-strontium) and U-Pb (uranium-lead) ages of 1,400--1,500 Ma 

(chap. E, this volume). Although rubidium-strontium mineral ages (chap. E, this volume) and Pb-Th-U (leadthorium-uranium) systematics (Doe and others, 1983) 

suggest that there have been younger disturbances in some 

samples, the emplacement age indicates that these samples 

probably are part of a 1 ,500-Ma event that produced 

anorogenic granites over large areas of North America 

(Hoppe and others, 1983). The core samples are enriched 

in light rare-earth elements (Schulz, 1981), uranium, 

thorium (Schulz, 1981; Doe and others, 1983), fluorine, 

and other incompatible elements (chap. E, this volume; 

present report) but their low 8 180 values of 7.0-8.3 per 

mil (Shieh, 1983; Kombrink and Clayton, 1983) indicate 

that the granites probably did not form from the anatexis 

of typical sedimentary or metasedimentary rocks (for example, O'Neil and Chappell, 1977; O'Neil and others, 

1977). A whole-rock 87Sr/86Sr initial value of 

0. 7026 ± 0. 0041 for the granite and a calculated initial 

Rb/Sr of 0.029--0.25 for the source suggest that the 

Illinois granite could have been derived from the partial 

melting of the mantle or lower crust (chap. E, this volume). 

The Precambrian granitic basement in Stephenson 

County, Ill., is unconformably overlain by 2,000-2,200 

ft (feet) of lower Paleozoic sedimentary rocks (Hinze and 

others, 1981). Uranium, lead, and thorium isotope systematics of the granite (Doe and others, 1983) suggest 

that there may have been an interaction between the granite near the unconformity and a chloride brine about 200-

800 Ma. This study uses petrographic observations and 

140 Shorter Contributions to Isotope Research 

chemical and stable isotopic compositions of samples from 

the three drill holes UPH-1, UPH-2, and UPH-3 (see 

figure E1, chap. E, this volume) to characterize the alteration of the granite and to place constraints on the history 

and origin of the granite. 

ACKNOWLEDGMENTS 

The author is indebted to Z. E. Peterman and S. 

S. Goldich for their encouragement and advice and to T. 

Bell for the petrographic and modal analyses. The analytical expertise of J. Gleason and M. Huebner is greatly 

appreciated as is the scientific advice of I. Friedman. The 

author benefited greatly from the reviews of J. L. Hannah, 

J. F. Whelan, and J. R. O'Neil. 

ANALYTICAL TECHNIQUES 

Several samples from each core were selected to 

determine the range in the chemical and stable isotopic 

compositions of the granites. The abundances of most of 

the elements were determined using standard X-ray 

fluorescence spectroscopy. Fluorine contents were measured with an ion selective electrode, sulfur concentrations 

were determined with an IR analyzer, C02 quantities were 

measured coulometrically, and water and Fe+ 2 were 

analyzed by standard techniques. The average 2<T error 

for all elements is ± 2 percent of the amount present. 

Oxygen was extracted from whole-rock powders 

and mineral separates using the BrF5 technique of Clayton 

and Mayeda (1963). Mineral separates were purified by 

use of heavy liquids and handpicking, and X-ray diffraction analysis indicates that the separates were more than 

95 percent pure. 8180 values are expressed in units of 

per mil relative to the SMOW (Standard Mean Ocean 

Water) standard: 

The value of NBS-28 (National Bureau of Standards) is * 9. 6 in the Denver laboratory. Hydrogen isotopic compositions were determined by fusing the mineral in vacuum and converting the water to hydrogen (Friedman, 

1953; Godfrey, 1962). 8D values also are reported in units 

of per mil relative to SMOW. 8180 values are precise 

to ± 0.2 (2<r) whereas the reproducibility of the 8D values 

is ± 2.0 (2<r). 
PETROGRAPHIC DESCRIPTIONS 

The samples in this study are holocrystalline and 

have xenomorphic or hypidomorphic textures. Samples 

closest to the unconformity tend to be finer grained than 

those from deeper parts of the cores. Biotite, fluorite, and 

zircon are accessory minerals present in most of the samples. The late-stage appearance of biotite in most samples 

suggests that the original water content of the magma may 

have been less than than 1. 2 weight percent (Maaloe and 

Wyllie, 1975). Microcline is perthitic and the plagioclase 

is usually an9 to an12 . Most of the samples contain a few 

short hairline fractures. 

Samples from core of drill hole UPH-1, all of 

which are near the unconformity, contain highly 

sericitized plagioclase and iron-oxide stain. Calcite veins 

cut through many of the samples from UPH-1 and 

twinned biotites and plagioclases commonly are bent. The 

edges of some biotites are altered slightly to chlorite. The 

preponderance of altered plagioclase and the presence of 

calcite veins indicate that these samples have interacted 

with an aqueous fluid. 

Only a few of the samples from cores of drill holes 

of UPH-2 and UPH-3 but the plagioclases from deeper 

parts usually are less altered. Muscovite and chlorite are 

common accessory and alteration minerals, and calcite 

veins usually are absent. With the exception of the moderately altered samples close to the unconformity, most of 

the granite in cores of UPH-2 and UPH-3 has been 

affected by only minor alteration events. 

CHEMICAL COMPOSITION 

Chemical analyses of samples from cores of 

UPH-1, UPH-2 and UPH-3 are given in table Ml. Variations in the composition of samples from UPH-3 are small 

relative to those measured in samples from UPH-2 despite 

the close proximity of these two holes (less than 1 kilometer apart). Hole UPH-1, which is 8 km (kilometers) north 

of holes UPH-2 and UPH-3, had a short core and many 

of the samples of the core were taken from nearly the 

same depth (sample 2064C is only 3 inches deeper than 

sample 2064B; 2071F and 2071G are only 6 inches apart). 

Differences among the compositions of those samples 

from UPH-1 that are separated by only a few inches commonly are as large as differences among samples from 

UPH-3 that are separated by several thousand feet. The 

range in the chemical compositions of samples from 

UPH-1 may reflect local differences in the degree of alteration. 

There are subtle differences between the chemical 

compositions of samples from UPH-1 and UPH-3. For 

example, the content of P20 5 , Ti02 , F, C02 and H20+ 

in samples from UPH-1 are consistently higher than those 

of samples from UPH-3 (table M1). Samples from 

UPH-1 also have higher concentrations of MgO but lower 

concentrations of Na20 than do samples from UPH-3 

(fig. M1). The enrichment of incompatible elements in 

samples from core of hole UPH-1 relative to those from 

core of hole UPH-3 probably is the result of the pervasive 

alteration in UPH-1 . 

Calculated norms, A}z03/(Na20 + K20 + CaO), and 

modal anlayses are given in table M2. The presence of 

only small amounts of modal muscovite and calculated 

A}z03/(Na20 + K20 + CaO) of about 1.0 indicate that the 

samples are only mildly peraluminous. Most of the samples plot to the orthoclase-rich side of the minimum melting compositions in the quartz-albite-orthoclase system 

(fig. M2). Comparison of the normative compositions of 

the Illinois granites with the minimum melting compositions and with the results of experimental work in the 

granite system compiled by Anderson and Cullers (1978) 

indicates that the Illinois granites probably crystallized at 

a low pressure (for example <2 kilobars). 

The normative compositions of the Illinois granites 

are comparable with those of Archean granites from Finland (Salvolahti, 1960; Harme, 1965) and with the 1,400-

to 1 ,500-Ma granites from the Wolf River batholith, Wisconsin (Anderson and Cullers, 1978). The compositions 

of most of the samples from the Illinois cores plot within 

the field of the differentiated plutons from Wisconsin (fig. 

M2). Anderson and Cullers ( 1978) estimated that the differentiated granites from the Wolf River batholith crystallized at a pressure of 0. 5-1.0 kilo bars. Anderson (1980) 

calculated from the compositions of coexisting irontitanium oxides that granites from the Wolf River 

batholith equilibrated slightly below the QFM buffer curve 

at subsolidus temperatures between 500° and 635°C. Plagioclase-liquid and coexisting feldspar geothermometers 

yielded solidus temperatures of about 700°C for granites 

of the Wolf River batholith. Inasmuch as the ages and 

chemical compositions of the granites from the Illinois 

cores are comparable with those of the differentiated 

batholiths from Wisconsin, the equilibration temperatures 

and pressures of the two probably were similar. 

OXYGEN ISOTOPE COMPOSITIONS 

8 180 values of whole-rock powders and mineral 

separates are given in table M3. The oxygen isotopic composition of quartz is constant relative to that of the other 

minerals because quartz is the mineral most resistant to 

alteration. The 8180 values of sodium-plagioclase and 

Chemical and Stable Isotope Compositions of Anorogenic Granite, Stephenson County, Illinois 141 
Table Ml. Chemical compositions of Illinois drill core samples 

[Sample number comprises drill-hole number (UPH-1, -2, or -3) and the depth (in feet) of core at which sample was taken; letter suffix indicates 

Sample 

No.-------

Si02 

Al 2o3 

Fe2o3 

FeO 

HgO 

CaO 

Na 2o 

K20 

H2o+ 

H20-

Ti02 

P2o5 

HnO 

Zro2 

Co2 

F 

BaO 

UPH-1 

2004B 

70.6 

13.6 

0.81 

1.87 

0.90 

1.46 

2.30 

6.43 

0.56 

0.09 

0.57 

0.13 

0.02 

0.04 

0.50 

0.20 

o.oo 

0.10 

UPH-1 

2025B 

71.8 

13.3 

0.69 

1.93 

0.78 

1.23 

2.29 

6.35 

0.63 

0.04 

0.42 

O.ll 

0.02 

0.04 

0.25 

0.23 

0.04 

0.08 

UPH-1 

2064B 

73.0 

12.4 

0.98 

2.03 

0.82 

1.17 

2.27 

5.45 

0.70 

0.06 

0.48 

0.13 

0.01 

0.04 

0.07 

0.23 

0.03 

0.06 

UPH-1 

2064C 

71.3 

14.0 

0.91 

1.33 

0.78 

0.92 

2.53 

7.44 

0.62 

0.07 

0.30 

0.08 

0.02 

0.03 

0.15 

0.16 

0.29 

0.10 

UPH-1 

2071F 

69.9 

13.1 

0.92 

2.60 

0. 79 

1. 75 

2.56 

5.49 

0.55 

0.07 

0.59 

0.17 

0.05 

0.05 

0.12 

0.31 

0.18 

0.06 

UPH-1 

2071G 

73.1 

12.9 

0.75 

1.80 

0.62 

1.45 

2.77 

5.75 

0.46 

0.05 

0.44 

0.11 

0.03 

0.03 

0.11 

0.21 

0.02 

0.08 

UPH-2 

2185D 

70.3 

13.8 

0.84 

2.26 

0.90 

1.43 

2.65 

5.76 

o. 72 

0.05 

0.54 

0.14 

0.03 

0.04 

0.10 

0.35 

0.03 

0.07 

UPH-2 

2358D 

71.7 

12.9 

0.98 

2.23 

0.62 

1. 63 

2.49 

5.66 

0.44 

0.04 

0.57 

0.14 

0.05 

0.04 

0.07 

0.28 

0.05 

0.07 

UPH-2 

2779D 

71.5 

14.7 

0.50 

0.90 

0.30 

0.85 

2.70 

7.26 

0.37 

0.05 

0.20 

0.04 

0.01 

0.02 

0.14 

0.12 

o.oo 

0.10 

UPH-2 

3194D 

73.9 

13.1 

0.49 

1.49 

0.38 

1.17 

3.34 

4.60 

0.47 

0.03 

0.27 

0.05 

0.01 

0.03 

0.14 

0.30 

0.01 

0.04 

UPH-2 

3678D 

76.5 

12.1 

0.13 

0.72 

0.15 

0.63 

2.38 

6.33 

0.22 

0.03 

0.08 

0.04 

0.01 

0.01 

0.09 

0.12 

0.02 

0.01 

UPH-2 

4204D 

75.7 

12.6 

0.29 

1.18 

0.12 

0.70 

2.76 

6.15 

0.33 

0.03 

0.13 

0.04 

0.01 

0.01 

0.06 

0.32 

0.00 

0.03 

UPH-2 

4405 

70.4 

14.6 

0.03 

1.71 

0.17 

0.93 

2.95 

7.28 

0.42 

0.07 

0.21 

0.04 

0.03 

0.03 

0.07 

0.45 

0.14 

0.06 

Total---- 100.10 100.13 99.88 100.96 99.13 100.59 99.86 99.84 99.71 99.69 99.62 100.33 99.40 

(less 0) 

Table M2. CIPW norms, AI 0~(Na 0 + K20 + CaO), and modal analyses of samples from the Illinois deep hole cores 

[nd, not detected; tr, trace; leaders (---) indicate not present. Trace liomblende present in samples UPH-1 2064C and UPH-1 2071F; trace monazite 

Sample UPH-1 UPH-1 UPH-1 UPH-1 UPH-1 UPH-1 UPH-2 UPH-2 UPH-2 UPH-2 UPH-2 UPH-2 UPH-2 

No.------- 2004B 2025B 2064B 2064C 2071F 2071G 2185D 2358D 2779D 3194D 3678D 4204D 4405 

Q 

or 

ab 

an 

hy 

mt 

il 

ap 

c 

z 

fr 

pr 

cc 

mg 

29.5 

38.0 

19.5 

2.04 

4.10 

1.17 

1.08 

0.31 

2.11 

0.06 

0.39 

1.14 

1.02 

Quartz------ 24.0 

Microcline-- 38.2 

Plagioclase- 19.0 

Biotite------ 9.66 

Muscovite---- nd 

Opaque 1.66 

minerals---

Apatite------ 0.16 

Zircon------- 0.16 

Fluorite----- 1.00 

Chlorite----- 0.16 

Sericite----- 5.66 

Carbonate---- 0.32 

30.9 

37.5 

19.4 

2.33 

4.17 

1.00 

0.80 

0.26 

1.80 

0.06 

0.45 

0.08 

0.57 

1.03 

19.0 

61.5 

12.2 

2.50 

nd 

0.65 

tr 

tr 

0.32 

0.32 

3.50 

tr 

35.5 

32.3 

19.2 

3.03 

4.22 

1.42 

0.91 

0.31 

1.66 

0.06 

0.45 

0.06 

0.16 

1.05 

41.2 

16.0 

21.0 

3.50 

nd 

0.98 

0.32 

0.48 

0.16 

0.64 

15.50 

0.32 

142 Shorter Contributions to Isotope Research 

25.5 

43.6 

21.2 

2.14 

2.56 

1.31 

0.57 

0.19 

0.99 

0.04 

0.31 

0.54 

0.34 

1.01 

31.7 

40.5 

14.8 

6.66 

nd 

0.65 

0.16 

o.oo 

0.85 

1.00 

2.66 

nd 

29.8 

32.8 

21.9 

4.81 

4.78 

1.35 

1.13 

0.41 

1. 21 

0.08 

0.61 

0.34 

0.28 

0.98 

18.0 

28.5 

34.5 

4.00 

nd 

0.16 

nd 

0.16 

0.16 

1.16 

12.16 

nd 

31.1 

33.8 

23.3 

4.44 

3.50 

1.09 

0.83 

0.26 

0.48 

0.04 

0.41 

0.04 

0.25 

0.96 

27.7 

29.8 

19.3 

7.66 

0.32 

1.33 

0.32 

0.48 

tr 

0.83 

11.83 

tr 

29.0 

34.1 

22.5 

3.21 

4.80 

1.22 

1.03 

0.33 

2.04 

0.06 

0.70 

0.06 

0.23 

1.05 

28.2 

28.8 

28.3 

3.83 

0.84 

1.00 

nd 

tr 

0.66 

0.16 

8.16 

nd 

31.5 

33.5 

21.1 

4.91 

3.89 

1.42 

1.09 

0.33 

0.88 

0.06 

0.55 

0.09 

0.16 

0.98 

19.8 

43.0 

18.2 

7.33 

nd 

1.16 

nd 

tr 

0.50 

0.50 

9.00 

nd 

26.2 

43.0 

22.9 

2.41 

1.68 

0.73 

0.38 

0.10 

1.53 

0.03 

0.24 

0.32 

1.06 

35.3 

33.0 

19.0 

0.50 

1.50 

0.50 

nd 

0.33 

nd 

1.66 

7.66 

0.50 

Normative composition (weight 

34.5 

27.3 

28.3 

2.51 

2.84 

0.71 

0.52 

0.12 

1.72 

0.05 

0.61 

0.02 

0.32 

1.04 

37.2 

37.6 

20.2 

1.47 

1.44 

0.19 

0.15 

0.10 

0.90 

0.02 

0.24 

0.04 

0.21 

1.02 

34.8 

36.2 

23.3 

0.57 

2.02 

0.42 

0.25 

0.09 

1.19 

0.02 

0.65 

0.14 

1.04 

23.8 

43.3 

25.1 

0.76 

2.98 

0.04 

0.40 

0.09 

1.60 

0.05 

0.92 

0.26 

0.16 

1.01 

Modal analyses (modes determined 

45.7 

17.7 

23.0 

2.66 

1.33 

1.50 

nd 

0.16 

0.16 

2.33 

5.16 

nd 

29.7 

50.3 

16.7 

0.50 

0.33 

0.50 

nd 

tr 

nd 

tr 

1.83 

tr 

39.8 

32.2 

24.3 

1.00 

1.66 

tr 

nd 

0.16 

0.32 

0.50 

tr 

nd 

30.2 

32.7 

28.7 

4.50 

3.33 

tr 

nd 

0.33 

tr 

nd 

0.33 

nd 
exact split of core; totals include correction for excess oxygen. Analysts: J. S. Wahlberg, J. Taggert, J. Baker] 

Sample UPH-2 

No.------- 4773D 

Si02 69.9 

Al2o3 14.3 

Fe 2o3 0.99 

FeO 2.37 

MgO 0.60 

CaO 2.01 

Na 2o 3.43 

K2o 4.90 

H2o+ 0.52 

H2o 0.02 

Ti02 0.63 

P2o5 o.1s 

MnO 0.06 

zro2 0.04 

Co2 0.10 

F 0.49 

0.11 

BaO 0.06 

UPH-2 

5411D 

72.9 

13.4 

0.43 

1.69 

0.28 

1.28 

2.80 

5.80 

0.41 

o.oo 

0.29 

o.os 

0.03 

0.04 

0.06 

0.42 

0.03 

o.os 

UPH-3 

2209H 

74.4 

13.1 

0.33 

1. 25 

0.13 

0.90 

2.75 

6.28 

0.47 

0.01 

0.12 

0.04 

0.03 

0.03 

0.13 

0.66 

0.07 

0.01 

UPH-3 

2290C 

75.0 

12.7 

0.46 

1.13 

0.19 

0.92 

2.75 

5.90 

o.so 

o.oo 

0.14 

0.04 

0.01 

0.04 

0.02 

0.62 

0.02 

0.01 

UPH-3 

2397E 

74.5 

12.7 

1.35 

1. 23 

0.14 

0.95 

2.81 

5.93 

0.40 

0.08 

0.15 

0.04 

0.01 

0.03 

0.04 

0.57 

0.08 

0.01 

UPH-3 

2422E 

73.8 

13.1 

0.44 

1.08 

0.15 

0.90 

3.10 

5.47 

0.35 

0.09 

0.12 

0.04 

0.01 

0.02 

0.04 

o.ss 

0.01 

0.02 

UPH-3 

3333E 

75.2 

12.5 

0.42 

1.38 

0.19 

1.08 

2.64 

5.62 

0.47 

o.oo 

0.22 

0.04 

0.02 

0.03 

0.05 

0.44 

0.01 

0.03 

UPH-3 

3514F 

75.5 

12.5 

0.43 

1.16 

0.15 

0.99 

3.02 

5.40 

0.49 

o.oo 

0.15 

0.04 

0.01 

0.04 

0.12 

0.43 

0.02 

0.02 

UPH-3 

3880E 

72.4 

13.5 

0.46 

1.44 

0.35 

1.24 

2.86 

5.88 

0.39 

0.08 

0.28 

0.08 

0.01 

0.03 

0.11 

0.32 

0.02 

0.06 

UPH-3 UPH-3 

4059C 4609E 

74.1 74.1 

12.6 13.1 

0.61 0.44 

1.90 1.62 

0.36 0.23 

1.39 1.08 

2.67 2. 72 

5.26 6.07 

0.36 0.39 

0.03 0.04 

0.39 0.24 

0.08 0.04 

0.04 0.03 

0.04 0.04 

0.07 0.06 

0.39 0.45 

o.oo 

0.06 

0.01 

0.04 

UPH-3 

4689G 

75.6 

12.1 

0.40 

1.62 

0.22 

1.11 

2.76 

4.92 

0.45 

0.03 

0.23 

0.04 

0.03 

0.03 

0.06 

0.48 

0.02 

0.03 

UPH-3 

5056E 

72.9 

13.4 

0.40 

1.60 

0.26 

1.13 

2.76 

6.32 

0.43 

0.03 

0.26 

0.04 

0.03 

0.04 

0.06 

0.36 

0.05 

0.06 

Total---- 100.37 99.78 100.43 100.19 100.78 100.08 100.16 100.29 99.39 100.19 100.51 99.93 99.98 

(less O) 

present in samples UPH-2 2358D, UPH-2 5411D, UPH-3 2209H, UPH-3 3333E, and UPH-3 4609] 

Sample UPH-2 UPH-2 UPH-3 UPH-3 UPH-3 UPH-3 UPH-3 UPH-3 UPH-3 UPH-3 UPH-3 UPH-3 UPH-3 

No.------- 4773D 5411D 2209H 2290C 2397E 2422E 3333E 3514F 3880E 4059C 4609E 4689G 5056E 

percent) 

Q 

or 

ab 

an 

hy 

mt 

il 

ap 

c 

z 

fr 

pr 

cc 

mg 

26.9 

28.9 

28.9 

4.96 

4.02 

1.43 

1.00 

1.35 

1.52 

0.06 

0.98 

0.21 

0.23 

0.98 

by point counting) 

Quartz------ 26.2 

Microcline-- 40.3 

Plagioclase- 23.1 

Biotite------ 1.83 

Muscovite---- 0.65 

Opaque tr 

minerals---

Apatite------ nd 

Zircon------- tr 

Fluorite----- 0.32 

Chlorite----- 0.16 

Sericite----- 7.00 

Carbonate---- nd 

31.9 

34.3 

23.7 

2.70 

2.97 

0.63 

0.55 

0.12 

1. 53 

0.06 

0.86 

0.06 

0.14 

1.01 

30.2 

33.9 

25.3 

2.50 

0.63 

o. so 

0.16 

0.16 

0.16 

0.16 

5.83 

nd 

33.4 

36.9 

23.2 

1. 76 

0.48 

0.23 

0.09 

1.77 

0.04 

1.18 

0.13 

0.25 

1.01 

39.8 

30.0 

21.0 

1.16 

6.16 

nd 

nd 

tr 

0.16 

1.50 

nd 

nd 

35.4 

34.8 

23.2 

1.87 

0.67 

0.27 

0.10 

1. 79 

0.04 

1.21 

0.04 

0.04 

1.01 

38.5 

29.7 

21.5 

3.16 

5.16 

0.16 

nd 

0.50 

1.00 

0,.33 

nd 

nd 

34.6 

34.8 

23.6 

0.07 

1.09 

1.94 

0.28 

0.09 

1.62 

0.06 

1.16 

0.15 

0.09 

1.00 

32.0 

39.3 

27.8 

2.50 

1.00 

1.00 

nd 

tr 

o.so 

0.83 

nd 

nd 

35.0 

32.3 

26.2 

1.79 

0.64 

0.23 

0.10 

2.08 

0.03 

1.12 

0.02 

0.09 

1.04 

42.7 

32.2 

21.8 

1.00 

1.50 

nd 

nd 

tr 

0.16 

0.66 

tr 

nd 

36.4 

33.2 

22.3 

1.64 

2.31 

0.61 

0.42 

0.10 

1.47 

0.05 

0.90 

0.02 

0.11 

1.01 

29.8 

32.3 

26.2 

nd 

1.66 

0.66 

nd 

0.33 

1.33 

0.66 

0.83 

0.33 

35.9 

31.8 

25.5 

0.80 

1.87 

0.62 

0.28 

0.09 

1.39 

0.06 

0.87 

0.04 

0.27 

0.99 

35.2 

35.3 

23.7 

1.00 

2.16 

0.16 

nd 

tr 

0.66 

1.33 

0.48 

nd 

31.0 

35.0 

24.4 

2. 77 

1.65 

0.67 

0.54 

0.19 

1.43 

0.05 

0.65 

0.06 

0.25 

1.01 

30.8 

43.2 

16.5 

6.83 

0.16 

tr 

nd 

tr 

tr 

tr 

tr 

nd 

35.3 

31.0 

22.6 

3.23 

3.31 

0.88 

0.74 

0.19 

1.33 

0.06 

o. 79 

0.16 

1.00 

32.0 

38.3 

23.3 

3.33 

tr 

1.50 

tr 

tr 

0.16 

tr 

1.34 

nd 

32.9 

35.7 

22.9 

1.51 

2.81 

0.64 

0.45 

0.09 

1.49 

0.06 

0.91 

0.02 

0.14 

1.01 

32.2 

39.5 

16.0 

8.16 

0.83 

1.50 

nd 

0.32 

0.32 

0.32 

0.64 

tr 

38.7 

29.1 

23.4 

1.43 

2.83 

0.58 

0.44 

0.10 

1.71 

o.os 

0.98 

0.04 

0.14 

1.02 

30.6 

43.6 

14.6 

7.16 

0.64 

0.50 

tr 

0.16 

0.64 

tr 

1.60 

nd 

30.2 

37.4 

23.4 

2.47 

2.78 

0.58 

0.49 

0.10 

1.12 

0.06 

0.73 

0.09 

0.14 

1.00 

28.8 

36.1 

20.8 

7.00 

0.16 

1.00 

tr 

tr 

0.32 

0.16 

5.33 

nd 

Chemical and Stable Isotope Compositions of Anorogenic Granite, Stephenson County, Illinois 143 
A 

B 

0 • 

0. 

D. 0 D. ••• 

0 

0 

DRILL HOLE SAMPLES 

• UP H -1 

0 UP H -2 

D. UP H -3 

Figure M1. Ternary diagram showing composition of various constituents in samples of core from drill holes UPH-1, 

UPH-2, and UPH-3. A, Relationship of contents of Na20, K20, and CaO. Note that samples from UPH-1 are depleted 

in Na2 0 relative to those from UPH-3. 8, Relationship of Na2 0 + K20, FeO +0.9Fe2 0 3 + MnO, and MgO. Note that 

samples from UPH-1 are enriched in MgO but are depleted in alkalis relative to samples from UPH-3. 

144 Shorter Contributions to Isotope Research 
Q 

500---

-----------x2000----------x--

WRP 

EXPLANATION 

Drill-hole samples 

• UPH-1 

D UPH-2 

t:. UPH-3 

* Precambrian granites from Finland 

X Minimum melt compositions at different 

water pressures in the granite system. 

(Tuttle and Bowen, 1 958) 

WRP 

undifferentated 

Figure M2. Comparison of the normative compositions of samples from present study with field of differentiated and undifferentiated plutons (WRP) from the Wolf River batholith, Wisconsin (Anderson and Cullers, 

1978), and Precambrian granites from Finland (Savolahti, 1%0; Harme, 1965). Dashed lines indicate water 

pressure, in bars, for minimum melt compositions in the granite system (Tuttle and Bowen, 1958). 

microcline usually are identical, indicating that they are 

in isotopic equilibrium (O'Neil and Taylor, 1967). Samples closest to the unconformity are more 180-rich than 

are those from deeper parts of the cores which have a 

constant 8180 value of 7.0 (fig. M3). 

The low 8180 values of the deeper samples indicate 

that the source of the granites cannot be sediments, 

metasediments, or altered continental crust. The relatively 

high 8180 value of the samples nearest to the unconformity can be attributed to the same alteration events as 

those suggested by the petrographic and chemical 

analyses. 

The fractionation of oxygen isotopes between quartz 

and any other mineral phase can be expressed as: 

1000lna(Q-min)=A + B!T2 (Bigeleisen and Mayer, 1947). 

A and B are constants for each quartz-mineral pair and 

are determined by combining experimental result~ .-with 

measurements of the 180 fractionations in natural systems 

(Bottinga and Javoy, 1975). 1000 lna(Q-min) is approximately equal to the difference between the 8180 value 

of quartz and another mineral (that is, A180 Q-min). A 

graphical method for evaluating the concordancy of 

equilibration temperatures of several coexisting minerals 

involves a plot of 1000lna(Q-min)-A against B (Javoy and 

others, 1970). Achievement of isotopic equilibrium is represented by a straight line of slope liP that passes through 

the origin and A 180 for every quartz-mineral pair. 

Quartz-mineral fractionations of samples from the 

Illinois cores are plotted against B in figure M4. The values of A and B for each quartz-mineral pair are those 

Chemical and Stable Isotope Compositions of Anorogenic Granite, Stephenson County, Illinois 145 
2050 

2100 

2000 

1-

UJ 

UJ 

u... 2500 

~ 

:I 

1-

a... 

UJ 

0 3000 

3500 

4000 

4500 

5000 

5500 

A 

B 

6 

Uncertainty 

±2a 

Uncertainty 

±2a 

0 

0 

I 

o UPH-1 

6 --------0 

/:!,............ ---- o<!{"""--

L!,.'l( 

\ 

\ 

\ 

\ 

D 

/ 

/ / / / 

D/ 

I 

/:!,. I 

I \Mt 

00 

' \ /:!,. I /)/ 

/ /:!,. 

/ /:!,.; 

\/ 

\ D UPH-2 

\ 6 UPH-3 \ 

\ 

D 

7 8 

WHOLE-ROCK 8 lSo 

Figure M3. Whole-rock 8180 values plotted as a function 

of depth for each core. A, Data for drill hole UPH-1; B, 

data for drill holes UPH-2 (dashed line) and UPH-3 (solid 

line). Depth (in feet) to the unconformity in each core is 

as follows: UPH-1=1,975; UPH-2=2,172; and UPH3=2,172. Note that core from UPH-1 is much shorter than 

core from UPH-2 and UPH-3. 

determined by Bottinga and Javoy (1975). Shown for 

comparison are concordant slopes for equilibration temperatures of 500°, 600°, and 700°C. 

146 Shorter Contributions to Isotope Research 

The quartz-feldspar fractionations in some of the 

samples from UPH-1 are too small for reasonable equilibration temperatures (table M3; fig. M4). Small quartzfeldspar fractionations are probably the result of changes 

in the 8180 value of the feldspar rather than of the quartz 

because feldspar exchanges oxygen more readily with 

most aqueous fluids than does quartz (O'Neil and Taylor, 

1967; Clayton and others, 1968; Forester and Taylor, 

1972). The enrichment of the feldspars in 180 probably 

is related to the alteration events that produced the pervasive sericite. Quartz-biotite fractionations in UPH-1 correspond to equilibration temperatures of between 500° and 

600°C. Comparison of the 8180 value of the biotites in 

samples from UPH-1 with those from the deeper, less 

altered samples from UPH-3 (fig. M4) suggests that the 

biotites in samples from UPH-1 have become enriched 

in 180 as a result of the alteration events so that the original quartz-biotite fractionations were larger. Magnetite 

from a sample from UPH-1 also is enriched in 180 relative to magnetite from deeper samples so that even the 

magnetite near the unconformity has become slightly enriched in 180 as a result of the alteration. 

The 180 fractionations among the minerals of the 

sample from core of UPH-2 and the samples from core 

of UPH-3 (fig. M4) yield nearly concordant quartz-feldspar-magnetite equilibration temperatures of 500 ± 60°C 

but lower quartz-biotite and quartz-muscovite equilibration 

temperatures. The concordant quartz-feldspar-magnetite 

equilibration temperatures compare favorably with the 

iron-titanium oxide subsolidus temperatures of samples 

from the Wolf River batholith (Anderson, 1980). The 

large fractionations between biotite or muscovite and the 

other minerals from deep samples may be the result of 

the continued exchange of oxygen isotopes between 

hydrous phases and other minerals at low temperatures. 

Local equilibrium between hydrous phases and adjacent 

minerals such as feldspar may be achieved at low temperature but the overall 8180 of the feldspar will not be 

affected because the percentage of the total amount of 

feldspar that has exchanged with the biotite is small. 

Therefore, the 180 fractionation between biotite and feldspar will approach that appropriate for the "closure" temperature for biotite. The lowest equilibration temperature 

of the oxygen isotopes in biotite and feldspar in the unaltered samples from cores of holes UPH-2 and UPH-3 

is 395°C. 

SIGNIFICANCE OF 8180 VALUES OF 

ALTERED SAMPLES 

The range in the 8 180 values of feldspars and biotites from samples near the unconformity (table M3) could 

have resulted from ( 1) variable degrees of exchange of 

oxygen isotopes between these minerals and the same 
Table M3. 8180 values of samples from the Illinois deep hole cores 

[Leaders ( ---) indicate not measured] 

Sample Whole Potassium 

No. rock Quartz Plagioclase feldspar Biotite Muscovite Magnetite 

Drill hole UPH-1 

2004B 8.1 8.2 8.1 8.0 3.6 2.0 

2025B 8.0 8.0 7.2 7.3 

2064B 7.4 7.9 8.3 8.1 2.8 

2064C 8.4 8.5 7.4 

2071F 7.5 8.2 7.1 7.7 3.1 

2071G 7.8 

Drill hole UPH-2 

2185D 8.3 

2358D 7.3 

2779D 7.5 

3194D 7.1 

3678D 6.9 

4204D 7.1 

4405 6.9 

4773D 6.7 

5411D 6.9 8.5 6.9 1.4 0.5 

Drill hole UPH-3 

2209H 7.9 8.4 

2290C 7.1 

2397E 7.4 8.5 

2422E 7.3 

3333E 6.8 8.5 

3514F 6.9 8.6 

3880E 7.3 

4059C 7.2 

4609E 7.1 8.0 

4689G 7.0 8.6 7.2 

5056E 6.8 8.5 

aqueous fluid, (2) exchange with the same fluid but at 

different temperatures, (3) interaction with fluids having 

a range of 8180 values, or (4) exchange with fluids having 

variable chemical compositions that produce different 

rates of exchange by altering the mechanism of the exchange reaction (Cole and Ohmoto, 1976). The rate of 

diffusion of oxygen in feldspar usually is faster than that 

in micas (Giletti and Anderson, 1975; Giletti and others, 

1978) so that partial exchange should affect the 8180 

value of feldspar more than that of biotite. Inasmuch as 

the most 180-rich feldspar is not from the same sample 

as the most 180-rich biotite, the 8180 variations in the 

feldspars and biotites are not likely to be only a consequence of different degrees of exchange with the same fluid. 

variations in the differences between the 8180 values of 

coexisting feldspar and biotite from samples near the unconformity are more likely the result of different rates 

of exchange of these minerals with fluids having variable 

chemical and isotopic compositions. 

Doe and others ( 1983) suggested that granite near 

the unconformity may have reacted with a chloride brine 

7.2 4.6 

7.4 4.3 

6.5 1. 5 0.6 

6.7 1.4 

6.3 1. 6 0.2 

7.2 1.8 4.8 -0.3 

7.2 1.8 -0.9 

200-800 Ma ago. Clayton and others ( 1966) measured 

8180 values of from --4 to + 5 for chloride-rich formation 

waters from deep wells in the Paleozoic sedimentary rocks 

of the Illinois basin. The formation waters have the appropriate age and location to be the aqueous fluids that may 

have interacted with the granite. Temperatures of 150°-

4000C are inferred to produce feldspars of 8. 3-7 .1 per 

mil from the Illinois formation waters (O'Neil and Taylor, 

1967). 

HYDROGEN ISOTOPIC COMPOSITION 

OF BIOTITES 

8D values and water contents of biotites from deep 

samples and from samples near the unconformity are 

given in table M4. The 8D values of the granite are similar to those measured in unaltered basic lavas (T. K. 

Kyser and J. R. O'Neil, 1984), phlogopites from ultramafic xenoliths (Boettcher and O'Neil, 1980), and 

younger granites (for example, Baur and others, 1978). 

Chemical and Stable Isotope Compositions of Anorogenic Granite, Stephenson County, Illinois 147 
8 DRILL HOLE 

6 

<:C <:C 

+I +I c c 2397EL). "§ ·e 

I I 4689GL). ~ 0 

~ 4 ~ 

E -= 0 0 0 0 0 0 - - 2 

0 

0 :a 2~ B(Xl06) ~ 4 ~ 6 0 ... 

(Q 2~ ~ 4 ~ 6 c. B(Xl06) ·:;: ·g ·;; c. ·:;: ·g ·;; II) 0 G) II) 0 G) 

"0 u iii c: "0 u iii c: a; II) C) a; II) C) 

LL. ::I (Q LL. ::I (Q 

~ ~ ~ ~ 

figure M4. Oxygen isotope fractionations between quartz and other minerals in each core as a function 

of the constant 8 (see text for discussion). Concordant equilibrium fractionations for 500°, 600°, and 700oC 

are shown for comparison. 

Table M4 80 values and water contents of 

selected biotites from the Illinois deep hole cores 

Drill Water 

hole Sample (weight OD 

No. No. percent) (per mil) 

UPH-1 2004B 2.6 -71 

UPH-2 5411D 2.2 -73 

UPH-3 2209H 2.1 -71 

UPH-3 5056E 2.5 -74 

These similarities suggest that the Illinois granites may 

have been derived from deep sources that have maintained 

a constant hydrogen isotopic composition. 

The hydrogen isotopic compositions of these samples are the same despite the large variations in their 8180 

148 Shorter Contributions to Isotope Research 

values (table M3). The rate of exchange of hydrogen 

isotopes between minerals and water is much faster than 

the rate of exchange of oxygen isotopes (O'Neil and 

Kharaka, 1976), so that biotites that are enriched in 180 

because of alteration events probably will have 8D values 

that reflect equilibration with the fluid. Extrapolated fractionation factors for biotite and water (Suzuoki and Epstein, 1976) suggest that biotite is depleted in deuterium 

by about 50 per mil relative to water at 400°C. Assuming 

that the 8D value of biotite from samples near the unconformity equilibrated with a fluid at temperatures of about 

400°C, the 8D value of the fluid would have been about 

-20. If the fluid equilibrated with the biotite at 150°C, 

the 8D value of the fluid would have been about + 30. 

The 8D values of formation waters from deep wells in 

the Illinois basin range between -46 and + 7 (Clayton 

and others, 1966) suggesting that the biotites and formation waters could have equilibrated only at temperatures 

higher than about 300°C. 
CONCLUSIONS 

Chemical and normative compositions of the Precambrian granites from the Illinois Deep Hole Project 

suggest that the granites are differentiated batholiths that 

crystallized at pressures less than about 2 kilobars. Differences in the chemical compositions between shallow 

samples near the unconformity and deep samples can be 

attributed to the interaction of the shallow samples with 

aqueous fluids. Alteration of samples near the unconformity has resulted in enrichments of K20, P20 5 , H20 +, 

C02, F, and 180 in addition to increases in the modal 

amounts of sericite and calcite. The 8180 values of quartz 

are nearly identical in every sample, but feldspar and biotites from altered samples are more 180-rich than are these 

minerals in unaltered samples. Oxygen and hydrogen 

isotopic compositions of biotites and 8180 values of feldspars from altered samples suggest that formation waters 

in the Illinois basin are likely candidates for the aqueous 

fluids that produced the alteration. Moreover, the chemical and isotopic compositions and temperatures of these 

fluids probably were quite variable. 

Oxygen isotope compositions of quartz, feldspar, 

and magnetite from unaltered samples yield concordant 

equilibration temperatures of 500 ± 60°C. Quartz-biotite 

and quartz-muscovite equilibration temperatures are substantially lower at 380°C, probably because hydrous minerals continued to exchange oxygen with quartz and feldspar at lower temperatures. The 8180 values of most of 

the unaltered samples are about 7.0, which indicates that 

the source for these anorogenic granites must be the upper 

mantle or unaltered primitive crustal material. Initial 

87 Sr/86Sr of 0. 7029 ± 0. 0041 do not eliminate these possibilities (chap. E, this volume). 

REFERENCES CITED 

Anderson, J. L. , 1980, Mineral equilibria and crystallization 

conditions in the late Precambrian Wolf River rapakivi massif, Wisconsin: American Journal of Science, v. 280, p. 

289-332. 

Anderson, J. L., and Cullers, R. L., 1978, Geochemistry and 

evolution of the Wolf River batholith, a late Precambrian 

rapakivi massif in northern Wisconsin, USA: Precambrian 

Research, v. 7, p. 287-324. 

Baur, W. H., Friedman, 1., and O'Neil, J. R., 1978, Hydrogen, 

in Wedepohl, K. H., ed., Handbook of geochemistry: New 

York, Springer-Verlag, v. 2, p. 1B1-1L4. 

Bigeleisen, J., and Mayer, M.G., 1947, Calculation of equilibrium constants for isotopic exchange reactions: Journal of 

Chemical Physics, v. 15, p. 261-267. 

Boettcher, A. L., and O'Neil, J. R., 1980, Stable isotope, 

chemical, and petrographic studies of high-pressure amphiboles and micas; evidence for metasomatism in the mantle source regions of alkali basalts and kimberlites: American Journal of Science, v. 280A, p. 594--621. 

Bottinga, Y. , and J avoy, M. , 197 5, Oxygen isotope partitioning 

among the minerals in igneous and metamorphic rocks: Review of Geophysics and Space Physics, v. 13, no. 2, p. 

403-418. 

Clayton, R. N., and Mayeda, T:· K., 1963, The use of bromine 

pentafluoride in the extraction of oxygen from oxides and 

silicates for isotopic analysis: Geochimica et Cosmochimica 

Acta, v. 27, p. 43-52. 

Clayton, R.N., Muffler, L.J.P., and White, D. E., 1968, Oxygen isotope study of calcite and silicates of the River Ranch 

no. 1 well, Salton Sea geothermal field, California: American Journal of Science, v. 266, p. 968-979. 

Clayton, R. N., Friedman, 1., Graf, D. L., Mayeda, T. K., 

Meents, W. F., and Shimp, N. F., 1966, The origin of 

saline formation waters-[pt.] 1. Isotopic composition: Journal of Geophysical Research, v. 71, p. 3869-3882. 

Cole, D. R., and Ohmoto, H., 1976, Effect of NaCl on the 

rate of oxygen isotopic exchange reactions between rocks 

and water [abs]: Geological Society of America Abstracts 

with Programs, v. 8, p. 817. 

Doe, B. R., Stuckless, J. S., and Delevaux, M. H., 1983, The 

possible bearing of the granite of the UPH Deep Drill Holes, 

Northern Illinois, on the origin of Mississippi Valley ore 

deposits: Journal of Geophysical Research, v. 88, p. 7335-

7345. 

Forester, R. W., and Taylor, H. P., Jr., 1972, Oxygen and 

hydrogen isotope data on the interaction of meteoric ground 

waters with a gabbro-diorite stock, San Juan Mountains, 

Colorado: International Geological Congress, 24th, 

Montreal, Proceedings, sec. 10, p. 254-263. 

Friedman, I., 1953, Deuterium content of natural waters and 

other substances: Geochimica et Cosmochimica Acta, v. 4, 

p. 89-103. 

Giletti, B. J., and Anderson, T. F., 1975, Studies in diffusion; 

II, Oxygen in phlogopite mica: Earth and Planetary Science 

Letters, v. 28, p. 225-233. 

Giletti, B. J., Semet, M. P., and Yund, R. A., 1978, Studies 

in diffusion; III, Oxygen in feldspars-An ion microprobe 

determination: Geochimica et Cosmochimica Acta, v. 42, 

p. 45-58. 

Godfrey, J. D., 1962, The deuterium content of hydrous minerals from the east-central Sierra Nevada and Yosemite National Park: Geochimica et Cosmochimica Acta, v. 26, p. 

1215-1245. 

Harme, M., 1965, On the potassic migmatites of Southern Finland: Bulletin de la Commission Geologique de Finlande, 

no. 219, 43 p. 

Hinze, W. J., Haimso, B. C., and Van Schmus, W. R., 1981, 

The Illinois deep hole project-An overview [abs.]: EOS, 

v. 62, no. 17, p. 387. 

Hoppe, W. J., Montgomery, C. W., and Van Schmus, W. R., 

1983, Age and significance of Precambrian basement samples from northern Illinois and adjacent states: Journal of 

Geophysical Research, v. 88, p. 727fr7286. 

Javoy, M., Fourcade, S., and Allegre, C. J., 1970, Graphical 

method for examination of 180/160 fractionations in silicate 

rocks: Earth and Planetary Science Letters, v. 10, p. 12-16. 

Chemical and Stable Isotope Compositions of Anorogenic Granite, Stephenson County, Illinois 149 
Kombrink, M., and Clayton, R. N., 1983, Oxygen isotopic 

compositions of minerals from Illinois Deep Drill Hole granites: Journal of Geophysical Research, v. 88, p. 7305-7306. 

Kyser, T. K., and O'Neil, J. R., 1984, Hydrogen isotope systematics of submarine basalts: Geochimica et Cosmochimica 

Acta, v. 48, p. 2123-2133. 

Maaloe, S. , and Wyllie, P. J. , 197 5, Water content of a granite 

magma deduced from the sequence of crystallization determined experimentally with water-undersaturated conditions: 

Contributions to Mineralogy and Petrology, v. 52, p. 

175-191. 

O'Neil, J. R., and Chappell, B. W., 1977, Oxygen and hydrogen isotope relations in the Berridale batholith: Journal of 

the Geological Society of London, v. 133, p. 559-571. 

O'Neil, J. R., and Kharaka, Y. K., 1976, Hydrogen and oxygen 

isotope exchange reactions between clay minerals and water: 

Geochimica et Cosmochimica Acta, v. 40, p. 241-246. 

O'Neil, J. R., Shaw, S. E., and Flood, R. H., 1977, Oxygen 

and hydrogen isotope compositions as indicators of granite 

genesis in the New England batholith: Contributions to 

Mineralogy and Petrology, v. 62, p. 313-328. 

O'Neil, J. R., and Taylor, H. P., Jr., 1967, The oxygen isotope 

and cation exchange chemistry of feldspars: American 

Mineralogist. v. 52, p. 1414-1437. 

150 Shorter Contributions to Isotope Research 

Savolahti, A., 1960, The Ahvenisto massif in Finland: Bulletin 

de Ia Commission Geologique de Finlande, no. 174, 96 p. 

Schulz, K. J., 1981, Trace element geochemistry of Precambrian 

granite from northern Illinois deep drill hole UPH-3 [abs.]: 

Geological Society of America Abstracts with Programs, v. 

13, p. 549. 

Shieh, Y., 1983, Oxygen isotope study of Precambrian granites 

from the Illinois Deep Drill Hole project: Journal of 

Geophysical Research, v. 88, p. 7300-7304 

Stuckless, J. S., Doe, B. R., and Delevaux, M., 1981, U-Th-Pb 

isotope systematics and uranium distribution within granite 

recovered from drill-hole UPH-3, northern Illinois [abs.]: 

Geological Society of America Abstracts with Programs, v. 

13, p. 562. 

Suzuoki, T., and Epstein, S., 1976, Hydrogen isotope fractionation between OR-bearing minerals and water: Geochimica 

et Cosmochimica Acta, v. 40, p. 1229-1240. 

Tuttle, 0. F., and Bowen, N. L., 1958, Origin of granite in 

the light of experimental studies in the system NaA1Si30sKAlSi30s-SiO:rH20: Geological Society of America 

Memoir 74, 153 p. 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

NEGATIVE 8180 VALUES FOUND FOR 

EASTERN NEVADA PLUTONIC ROCKS 

DEFORMED BY STRESSES RESULTING FROM 

POST-CRYSTALLIZATION MOVEMENT ALONG 

SPATIALLY RELATED THRUST FAULTS 

Chapter N 

By DONALD E. LEE, IRVING FRIEDMAN, and JIM D. GLEASON 

CONTENTS 

Page 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 

Recent data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 

Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 155 

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 

fiGURE Page 

Nl. Map of part of eastern Nevada and western Utah, showing sample localities .......... . 

TABLE 

N 1. Summary of 8180 values and potassium-argon age data for selected plutonic rocks and minerals of 

eastern White Pine County, Nev. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

153 

Page 

154 

Negative &180 values for Nevada Plutonic Rocks Deformed by Post-Crystallization Movement 151 
Abstract 

Four plutons tens of kilometers apart in eastern 

White Pine County, Nevada, have been deformed or 

even cataclasized by post-crystallization movement along 

spatially related thrust faults. This deformation has resulted in the reduction of both the 180/160 and the apparent potassium-argon ages of the affected rocks. In 

each case there apparently was oxygen isotope exchange 

between the rock and meteoric waters at about the same 

time as the constituent micas were degassed by stresses 

resulting from movement along the thrust faults. 

Modification of the oxygen isotope composition of 

these deformed or cataclasized intrusive rocks was more 

spotty than pervasive. Also, in the Young Canyon-Kious 

basin area of the southern Snake Range (the "type" 

area), cataclasis did not change the rubidium-strontium 

systematics of the intrusive. Quartzites in the study area 

have "normal" 8180 values, even though collected just 

beneath the sole of the Snake Range decollement. However, metamorphic micas recovered from such quartzites 

were degassed at the time of latest movement along the 

overlying thrust fault. 

We attempt to explain how the oxygen isotope exchange between meteoric waters and the deformed or 

cataclasized plutons may have been effected. 

INTRODUCTION 

Oligocene (32 ± 1.5 Ma; chap. P, this volume) 

plutonic rocks of the Young Canyon-Kious basin area, 

southern Snake Range, Nevada (fig. N1) have been cataclasized by post-crystallization movement along the overlying Snake Range decollement (Lee and Van Loenen, 

1971, p. 6, 40). A small amount of secondary muscovite 

formed during cataclasis was dated (17-18 Ma) by the 

K-Ar (potassium-argon) method to fix the time of deformation (Lee and others, 1970). These same writers also 

described spuriously young K-Ar age results for micas 

present in relatively undeformed Cretaceous (chap. P, this 

volume) and Jurassic (Lee and others, 1968) intrusive 

rocks of the southern Snake Range and concluded that 

the modified K-Ar ages are a result of partial degassing 

of the micas by stresses related to late movement on the 

overlying Snake Range decollement. 

These same deformed igneous rocks of the Young 

Canyon-Kious basin area have quite low 8180 values (to 

-2.6 per mil), apparently resulting from oxygen isotope 

exchange between the plutonic rock and meteoric waters 

at the time of cataclasis (Lee and others, 1982). The undeformed plutonic rocks of the southern Snake Range were 

found to have 8180 values in the "normal" range of + 9 

to + 13, even though, as just noted, some of the constituent micas gave spuriously low K-Ar age results. 

In other words, micas recovered from the cataclasized rocks of the Young Canyon-Kious basin area, 

152 Shorter Contributions to Isotope Research 

southern Snake Range, Nev., give spuriously low (17-18 

Ma) age results, and the rocks themselves have extremely 

low (to -2.6 per mil) 8180 values. The K-Ar ages and 

the 8 180 values apparently were modified at the same 

time; that is, when the rock was cataclasized as a result 

of post-crystallization movement on the overlying Snake 

Range decollement. The purpose of this paper is to present negative 8180 values for three other deformed plutons 

in eastern White Pine County, Nev. At each locality there 

apparently was oxygen isotope exchange between plutonic 

rocks and meteoric waters during post-crystallization 

movement along a spatially related thrust fault. 

RECENT DATA 

Nelson (1966, p. 938) stated: "The rocks in the 

easternmost part of the Kern Mountain pluton have been 

subjected to cataclasis and shearing which, as determined 

from the orientation of the shear planes, is believed to 

be related to the eastward-directed decollement thrusting 

* * *." Although in the eastern part of the Kern Mountains the main decollement has been completely eroded 

away (R. K. Hose, written commun., 1978; also, see map 

of Hose and Blake, 1976), the cataclasis described by 

Nelson is practically identical to that described by Lee 

and Van Loenen (1971, p. 6, 40) for the Young CanyonKious basin area of the southern Snake Range, about 83 

km (kilometers) south of the Kern Mountains (fig. N1). 

The main intrusive in the Kern Mountains crystallized about 75 Ma ago (chap. D, this volume). Muscovite 

recovered from a sample (no. 564) of the cataclasized 

intrusive of Kern Mountains gave a K-Ar age of 

24.7 ± 1.1 Ma (Lee and others, 1980), appreciably 

younger even than an Oligocene (31 m.y.) event postulated by Best and others ( 197 4). Rock sample 564 has 

a 8180 value of -5.4 per mil. Thus in the Kern Mountains, as in the Young Canyon-Kious basin area of the 

southern Snake Range, post-crystallization movement on 

an overlying thrust fault has reduced both the apparent 

K-Ar age and the 180/160 of the resulting cataclasized 

pluton. 

Samples 383 and 384 were collected from a quartzmonzonite pluton in the Warm Springs area of the northem Egan Range, Nev. (fig. N1). This pluton has been 

sheared and deformed where these samples were collected, and Lee and others (1980, p. 26) attributed the 

shearing and deformation to stresses related to movement 

on an overlying (eroded) thrust fault shown on the map 

and cross section E-E' of Fritz (1968). We have determined 8180 values of -0.9 and -3.0 per mil for rock 

samples 383 and 384, respectively, and values of -7.2 

and -7.1 per mil for muscovites 383 and 384, respectively. Splits of these same muscovite fractions gave K-Ar 

ages of 25.2±0.7 and 36.2± 1.0 Ma, respectively, but 
1W45' 

• 386 

>- 39° 45' >- LU 

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LU ® > 

0 

ra_ 

LU 

r- (!) 

30' (j) 

z 

a: 

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15' 

NEVADA 

Area of ..L 

this figure 

30' 15' 

10 20 KILOMETERS 

10 MILES 

I 

I 

I 

564. 

lrI~ IS 

f-

~10 u z co ::J <( 

81=5 

~I---

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-J 

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:::::. 

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Figure Nl. Map of part of eastern Nevada and western Utah, showing sample localities. The 250~m contour 

lines are not shown in the southwestern part of the map area. Samples 184-MW-61 and 188-MW-61 are from 

the Young Canyon-Kious basin area. 

Negative 8180 values for Nevada Plutonic Rocks Deformed by Post-Crystallization Movement 153 
Table Nl. Summary of &180 values and potassium-argon age data for selected plutonic rocks and minerals of eastern 

White Pine County, Nev. 

[Latitude and longitude are given in degrees CO), minutes('), and seconds(")] 

Sample 

Number 

Latitude, 

north 

Longitude, 

west 

Muscovite 

o18o value, per mil K-Ar age (Ma) 

Intrusive (Oligocene, 32 Ma) Young Canyon-Kious basin area, southern Snake Range 

184-MW-61 

1188-MW-61 

110 

38°57' 58" 

38°57'50" 

38°57'02" 

114°10' 50" 

114°09, 32" 

114°08' 16" 

-2.6 (rock); -6.6 (microcline)--- 17.0±0.8 

-0.9 (rock)---------------------- 18.5±1.0 

-1.1 (rock)---------------------- Not determined. 

Intrusive (Cretaceous, 70 Ma), Kern Mountains 

564 114°03'35" -5.4 (rock) 24. 7±1.1 

Intrusive (age unknown), Warm Springs area northern Egan Range 

383 

384 

39°38'05" 

39°37'40" 

114°-53, 20" 

114°52'55" 

-0.9 (rock); -7.2 (muscovite)---- 25.2±0.7 

-3.0 (rock); -7.1 (muscovit~)---- 36.2±1.0 

Intrusive (age unknown), Big Rock area, Cherry Creek district, northern Egan Range 

386 114°53'25" -0.4 (rock)---------------------- 34.9±1.0 

1Sample 188-MW-61 is the same as number 105 in Lee and others (1982, table 1). 

Lee and others (1980, p. 26) regarded these ages as spuriously young, and suggested that the intrusive may have 

crystallized at a much earlier time. To our knowledge this 

pluton has not yet been dated by either Rb-Sr (rubidiumstrontium) or U-Th-Pb (uranium-thorium-lead) isotope age 

work. 

The intrusive at Warm Springs clearly seems to 

have been deformed no earlier than 36 Ma ago in the 

area of sample sites 383 and 384 as a result of movement 

on the overlying thrust fault, and the 180/ 160 of the rock 

seems to have been modified at the time of deformation. 

The age of this pluton has yet to be determined. 

Sample 386 is from the "Big Rock" area of intrusive 

outcrop in the southern part of the Cherry Creek district 

of the northern Egan Range. The intrusive is cut by shears 

in the area of the sample site, which is about 100 m 

(meters) east of a fault within the intrusive that dips about 

30° east-northeast (see the map of Adair, 1961). The alteration and deformation of parts of this intrusive were recognized clearly by Adair (1961, p. 43), who stated: "The 

fact that thrust faults exist in and around this igneous body 

can hardly be questioned." Rock sample 386 has a 8180 

value of --{).4, and the muscovite recovered from this rock 

gave a K-Ar age of 34.9 ± 1.0 Ma, regarded as spuriously 

young by Lee and others (1980, p. 26). The 180/ 160 of 

this intrusive rock appears to have been modified during 

a deformation that took place within the last 34.9 Ma, 

but the time of crystallization of this pluton is as yet undetermined. 

154 Shorter Contributions to Isotope Research 

From the data discussed here and summarized in 

table N1, we conclude that the 180/ 160 of each of these 

rocks and minerals was modified during deformation or 

even cataclasis resulting from post-crystallization movement along a spatially related thrust fault. The times of 

cataclasis are indicated by the K-Ar results listed. Where 

the rocks are only sheared and deformed but not cataclasized, as in the northern Egan Range (samples 383, 

384, and 386), degassing of the dated muscovites may 

not have been complete, and the K-Ar results indicate 

a maximum age for the time of deformation. 

Beyond our conclusion that there was oxygen 

isotope exchange between plutonic rocks and meteoric 

waters in the tectonic environments described, any explanation of the exact process that produced these low 8180 

values must be reconciled with three additional lines of 

evidence. First, in the Young Canyon-Kious basin area, 

the "type" area, where relations are best exemplified and 

most intensively studied, masses of relatively undeformed 

intrusive are suspended in the more contorted rock, as 

described in detail by Lee and Van Loenen (1971, p. 6, 

40). In general, the most cataclasized samples tend to 

have the lowest 8180 values (Lee and others, 1982). In 

other words, the modification of the oxygen isotope composition of these rocks was more spotty than pervasive. 

The cataclasis of the intrusive in Young CanyonKious basin intrusive did not change the Rb-Sr systematics of that rock. Three points based on 87Sr/86Sr and 87Rb/86Sr data for one sample of the undeformed intrusive 
and two samples of its cataclasized equivalents fall near 

a straight line that indicates an initial 87 Sr/86Sr of 0. 71215 

at an age of 32 ± 1.5 Ma (chap. P, this volume). 

A final line of evidence is provided by our unpublished oxygen isotope data for 23 quartzite samples of 

late Precambrian-Early Cambrian age collected within the 

area of figure N 1 . The field settings are described and 

petrographic and K-Ar age data for all these quartzites 

are included in a study by Lee and others (1980). Some 

of these quartzites, collected just beneath the sole of the 

Snake Range decollement have a strong directional element obviously formed during late movement on the decollement. Micas recovered from these schistose quartzites 

gave K-Ar ages similar to those listed in table N1, and 

were interpreted by Lee and others ( 1980) to represent 

a maximum age for the most recent movement on the 

decollement. However, none of these 23 quartzite samples 

has a 8180 value of less than 9.7 per mil, and the 8180 

values determined for these rocks and their constituent 

micas are not related to distance below the decollement 

in any obvious way. 

We note that three samples of Cambrian Pole Canyon Limestone exposed in contact with the cataclasized 

intrusive in the Young Canyon-Kious basin area have relatively low 8180 values (Lee and others, 1982). These low 

8180 values may be due to some kind of contact action, 

or they might result from isotopic exchange between the 

limestone and either the cataclasized intrusive or meteoric 

waters during movement on the overlying thrust fault. We 

have not yet investigated limestone exposed beneath the 

sole of a thrust fault but remote from any intrusive rock. 

SUMMARY AND CONCLUSIONS 

In the area of figure N I the constituent micas of 

various plutonic rocks and of upper Precambrian-Lower 

Cambrian quartzites were degassed by stresses resulting 

from movement along spatially related thrust faults. At 

or about the same time the 180/ 160 ratios of these plutonic 

rocks were lowered, apparently as a result of oxygen 

isotope exchange with meteoric waters. However, there 

was no obvious change in the 180/ 160 ratio of any of 

the quartzites, even those exposed just beneath the sole 

of the Snake Range decollement, which have a pronounced metamorphic fabric obviously formed during 

movement on the overlying thrust. Thus in the rocks discussed here the process leading to lowered 8180 values 

would seem to have been operative only within the deformed plutons. 

Such low 8180 values as those listed in table N1 

were reported by Taylor ( 1968) for a few granophyres 

or shallow granites from the Scottish Hebrides, the Skaergaard intrusion, and the Seychelles Islands. Taylor (1968, 

p. 39) stated, "The only obvious manner in which such 

low 8-values could arise is by mtxmg or exchange between the magmas and low- 180 meteoric ground waters." 

Later such isotopic exchange between magma and ground 

water low in 180 was shown for the rhyolites of the Y ellowstone Plateau (Friedman and others, 1974), and for 

the ash-flow sheets of southern Nevada (Lipman and 

Friedman, 1975). 

The plutonic rocks described in the present study 

(table N1), however, were crystallized before deformation, and their undeformed equivalents have "normal" 

8180 values. Thus mixing of meteoric water with magma 

cannot have produced the low 8180 values listed. Inasmuch as the process leading to lowered 180 values was 

operative only within the deformed plutons, either 

meteoric waters were selectively commingled with the affected intrusive rocks, or meteoric waters already in the 

intrusive environment were activated, by stresses resulting 

from movement on the thrust fault. 

In the Young Canyon-Kious basin pluton, at least, 

the system does not appear to have been open to large 

amounts of meteoric waters. Modification of the oxygen 

isotope composition of the cataclasized rocks apparently 

was more spotty than pervasive, and the cataclasis had 

no apparent effect on the Rb-Sr systematics of this intrusive. 

If the quartzites were well cemented with silica before metamorphism, porosities may have been reduced to 

the point at which little meteoric pore water was available 

for isotopic exchange. On the other hand, the 

polymineralic granitoid rocks contained many grain surfaces that might have held low- 180 water. Activation of 

such water during the heating associated with cataclasis 

may have been enough to cause the spotty reduction of 

8180 values best exposed in the Young Canyon-Kious 

basin area. 

REFERENCES CITED 

Adair, D. H., 1961 , Geology of the Cherry Creek district, 

Nevada: Utah University M.S. thesis, 125 p. 

Best, M. G., Armstrong, R. L., Graustein, W. C., Embree, 

G. F., and Ahlborn, R. C., 1974, Mica granite of the Kern 

Mountains pluton, eastern White Pine County, NevadaMobilized basement of the Cordilleran miogeosyncline?: 

Geological Society of America Bulletin, v. 85, no. 8, p. 

1277-1286. 

Friedman, 1., Lipman, P. W., Obradovich, J. D., Gleason, J. 

M. , and Christiansen, R. L. , 197 4, Meteoric water in magmas: Science, v. 184, p. 1069-1072. 

Fritz, W. H., 1968, Geologic map and sections of the southern 

Cherry Creek and northern Egan Ranges, White Pine 

County, Nevada: Nevada Bureau of Mines Map 35, scale 

1:62,500. 

Negative 8180 values for Nevada Plutonic Rocks Deformed by Post-Crystallization Movement 155 
Hose, R. K., Blake, M. C., 1976, Geology of White Pine 

County, Nevada: Nevada Bureau of Mines and Geology 

Bulletin 85, pt. 1, p. 1-35. 

Lee, D. E., Friedman, 1., and Gleason, J. D., 1982, Oxygen 

isotope composition of granitoid and sedimentary rocks of 

the southern Snake Range, White Pine county, Nevada: 

Contributions to Mineralogy and Petrology, v. 79, p. 

150-158. 

Lee, D. E., Marvin, R. F., and Mehnert, H. H., 1980, A 

radiometric age study of Mesozoic-Cenozoic metamorphism 

in eastern White Pine County, Nevada, and nearby Utah: 

U.S. Geological Survey Professional Paper 1158-C, p. 

17-28. 

Lee, D. E., Marvin, R. F., Stern, T. W., and Peterman, Z. 

E., 1970, Modification of potassium-argon ages by Tertiary 

thrusting in the Snake Range, White Pine County, Nevada, 

in Geological Survey research 1970: U.S. Geological Survey Professional Paper 700-D, p. D92-D 102. 

156 Shorter Contributions to Isotope Research 

Lee, D. E., Stern, T. W., Mays, R. E., and Van Loenen, R. 

E., 1968, Accessory zircon from granitoid rocks of the 

Mount Wheeler mine area, Nevada, in Geological Survey 

research, 1968: U.S. Geological Survey Professional Paper 

600-D, p. D197-D203. 

Lee, D. E., and Van Loenen, R. E., 1971, Hybrid granitoid 

rocks of the southern Snake Range, Nevada: U.S. Geological Survey Professional Paper 668, 48 p. 

Lipman, P. W., and Friedman, 1., 1975, Interaction of meteoric 

water with magma-An oxygen-isotope study of ash-flow 

sheets from southern Nevada: Geological Society of 

America Bulletin, v. 86, p. 695-702. 

Nelson, R. B., 1966, Structural development of northernmost 

Snake Range, Kern Mountains, and Deep Creek Range, 

Nevada and Utah: American Association of Petroleum 

Geologists Bulletin, v. 50, no. 5, p. 921-951. 

Taylor, H. P., Jr., 1968, The oxygen isotope geochemistry of 

igneous rocks: contributions to Mineralogy and Petrology, 

V. 19, p. 1-71. 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

PRELIMINARY HYDROGEN, OXYGEN AND SULFUR 

ISOTOPIC STUDY OF THE XIHUASHAN 

QUARTZ-WOLFRAMITE DEPOSIT, CHINA 

Chapter 0 

By ROBERT 0. RYE, T. P. DING1 , JOSEPH F. WHELAN, and GARY P. LANDIS 

CONTENTS 

FIGURES 

Abstract .......... . 

Introduction . . . . . . . . . 

General geologic framework . 

Geology of the Xihuashan deposit 

Stable isotope studies . . . 

Sulfur isotope studies . . 

Oxygen isotope studies . 

Hydrogen isotope studies 

Conclusions . . 

References cited . . . 

Page 

158 

158 

158 

159 

162 

162 

162 

168 

168 

169 

Page 

01. Index map showing outline of Nanling tungsten province and location of Xihuashan . . . . . . . 159 

02. Simplified geologic map of the Xihuashan deposit showing veins and projected sampling sites 160 

03. Diagrammatic north-south cross section of the Xihuashan deposit showing first and second phases of 

granite and relationship of quartz-wolframite veins to the margin of the stock . . . . . . . . 161 

04. 8180 val1.1es of samples collected from traverses across 247 vein at 270 level and across 299 vein 

at 620 and 632 levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 

05. Cross section of the 299 vein showing spacial distribution of 8180 values for quartz, wolframite, potassium-feldspar and muscovite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 

TABLES Page 

01. 834S of sulfide minerals in samples from the quartz-wolframite veins at Xihuashan . . . . . . . 162 

02. 8180 and 8D values of minerals and whole rocks from granite and hornfels . . . . . . . . . . . 163 

03. 8180 and 8D values of minerals and inclusion fluids from veins and greisens from the Xihuashan deposit 164 

04. Temperatures calculated from 8180 data for granite, greisen, and veins . . . . . . . . . . . . . . 168 

1

Institute of Geology of Mineral Deposits, Chinese Academy of 

Geological Sciences, Peking, People's Republic of China. 

Hydrogen, Oxygen, and Sulfur Isotope Systematics, Xihuashan Quartz-Wolframite Deposit, China 157 
Abstract 

The Xihuashan quartz-wolframite deposit in southeastern China consists of more than 600 steeply dipping 

quartz veins in the upper margins of a complex Mesozoic biotite granite stock that intrudes Cambrian pelitic 

rocks. Mineralization is confined to veins in the stock 

and individual veins show a pronounced zoning in both 

vein and alteration assemblages. 

The &180 data on the stock indicate postcrystallization exchange with 180-depleted meteoric water. Wholerock and quartz &180 values suggest that the biotite granite may have some 5-type characteristics, although alteration of the stock in the mine area makes determination 

of primary compositions difficult. The &180 data on 

quartz, wolframite, sericitized potassium-feldspar, and 

muscovite from the veins and associated greisen envelopes indicate deposition from fluids that had a remarkably narrow range of &180Hp and temperature 

throughout vein formation and greisenization. 

Preliminary measurements of filling temperature of 

fluid inclusions in quartz indicate that depositional temperatures were 250 ± 50°C and that at times the fluids 

boiled; at 250°C, the &180Hp of the hydrothermal fluids 

can be calculated to have been 1.0±1.6 per mil. These 

values are low enough to imply that the fluids contained 

a substantial component of highly exchanged meteoric 

water. Most of the &DH 0 values of the fluids are in the 

range typical of magm~tic fluids and therefore cannot 

be used to distinguish magmatic- and meteoric-water 

components without a more detailed study. Sulfur 

isotopic data on sulfides, however, indicate that sulfide 

sulfur was derived from a deep-seated igneous source. 

Many features of the geology, the mineralogy, the 

paragenesis, and the geochemical environment of ore 

deposition of the Xihuashan deposit appear to be similar 

to those of other quartz-wolframite deposits that have 

been studied in greater detail and imply fundamental 

processes common to the origin of such deposits. 

INTRODUCTION 

Recent studies of vein-type quartz-wolframite deposits ranging in age from late Precambrian to Tertiary 

indicate many similarities in geologic environment, 

mineralogy, paragenesis, alteration assemblages and environment of ore deposition (see summary by Casadevall 

and Rye, 1980). Such deposits are typically tungsten ore 

bodies, with only minor sulfides and possibly some tin 

mineralization, and are present in quartz veins that are 

spatially associated with granites that have intruded pelitic 

rocks. Their sulfur invariably is derived from deep-seated 

sources even though meteoric waters often compose a substantial component of their hydrothermal fluids. The similarity of these different quartz-wolframite deposits indicates that fundamental to their origin is the history of 

158 Shorter Contributions to Isotope Research 

a meteoric ± magmatic water hydrothermal system associated with a granite magma that intrudes certain pelitic 

rock suites. 

The purpose of this study is to provide preliminary 

stable isotope data on the famous Xihuashan deposit of 

China for comparison with other quartz-wolframite deposits that have been studied in detail, as well as to constrain discussions about the origin of the deposit. Most 

of this study was conducted while T. P. Ding was a guest 

at the U.S. Geological Survey (USGS) laboratories in 

Denver, Colo., and is part of an ongoing investigation 

by the USGS to define the environment of quartz-wolframite deposition. Acknowledgements are made to C. H. 

Wang and Y. Z. Zhou for their help to T. P. Ding in 

field observations and sample collections at Xihuashan. 

GENERAL GEOLOGIC FRAMEWORK 

The Xihuashan tungsten deposit is about 9 km (kilometers) northwest of Dayu in the southern part of Jiangxi 

province in southeastern China. Tin was discovered in the 

area as early as the Song dynasty (A.D. 960-1279). The 

discovery of tungsten dates from 1908. The deposit has 

been one of the great wolframite producers of the world, 

having a daily production of as much as 3,000 tons of 

ore containing more than 1 percent W03 . In addition to 

tungsten, the deposit contains tin, molybdenum, bismuth, 

and rare-earth elements. The following description of the 

district is based on reports of many geologists who have 

contributed to an understanding of the area and which 

are summarized by Xu and Ding (1941, 1943); Wang and 

Zhou (in press), and Wu and Mei (in press). 

The Xihuashan deposit is in the Nanling tungsten 

metallogenic province (fig. 01). The deposits in this province are closely associated with Mesozoic granitic rocks 

that intrude folded and faulted Cambrian to Silurian clastic 

metasedimentary rocks and Devonian limestones. There 

are three types of tungsten deposits in the province: 

tungsten-molybdenum-beryllium bearing porphyries, 

scheelite-sulfide skarns, and quartz-wolframite veins. The 

quartz-wolframite deposits are economically the most important and Xihuashan is one of the most important examples of these in the province. 

The Xihuashan deposit is in the southwestern part 

of the Xihuashan biotite-granite stock that has biotite 

K-Ar (potassium argon) ages of 138-155 Ma (Li and 

others, 1964), is exposed over an area of 20 km2 (square 

kilometers) and is composed of five sequentially intruded 

phases, all of which have been mineralized (Wu and Mei, 

in press). The stock intrudes slightly metamorphosed 

arenaceous and argillaceous Cambrian rocks that have 

been metamorphosed to hornfels in contact zones adjacent 

to the stock. 
0 Shijiazhuang 

0 TAIYUAN 

CHIN A Hefei 0 

0 WUHAN 

0 NANCHANG 

oCHANGSHA 

Chenzian 0 o Ganzhou 

Xihuashan deposit~ 

Shaoguan°~u 

NANLING TUNGSTEN 

PROVINCE 

'-----"----- River 

FUZHOU0 

500 1 000 KILOMETERS 

500 MILES 

Figure 01. Index map showing location of Nanling tungsten province and Xihuashan, 

China. 

GEOLOGY OF THE XIHUASHAN DEPOSIT 

The Xihuashan deposit is the most significant of 

six deposits that are defined on the basis of groups of 

vertical veins within the stock. The deposit consists of 

subparallel, en-echelon veins with an east-west trend. The 

veins are related to a well-defined joint pattern in the 

upper part of the stock. A diagrammatic cross section 

showing the relationship of the veins to the stock is shown 

in figure 03. 

The veins range in thickness from a few centimeters 

to nearly 4 m (meters), with an average interval of about 

4 m between the margins of major veins. The length of 

individual veins ranges from less than 100 m to more than 

I ,000 m and the average vertical extent is 200-300 m. 

Mineralization was by fracture filling. The major 

mineral is quartz with wolframite, potassium-feldspar, 

beryl, calcite, muscovite, and fluorite. Minor amounts of 

topaz, molybdenite, cassiterite, arsenopyrite, and basemetal sulfides are present also. Quartz, comprising more 

than 90 percent of vein material, usually is massive; vugs 

are rare. Wolframite (FeO-MnO mole ratio of 1.1 to 1.6), 

however, is present as individual black crystals that may 

Hydrogen, Oxygen, and Sulfur Isotope Systematics, Xihuashan Quartz-Wolframite Deposit, China 159 
Figure 02 (above and facing page). Simplified geologic map of the Xihuashan deposit (modified from a map in 

the "Guidebook for field excursions," Symposium on Tungsten Geology, Jiangxi, China, (October 12-21, 1981) showing veins and projected sampling sites). Letter prefixes have been omitted from sample numbers to save space. 

160 Shorter Contributions to Isotope Research 
102 VEIN 

x05 

NORTH 

EXPLANATION 

JURASSIC 

UNIT 5-Granophyre 

be as large as several centimeters. Potassium-feldspar is 

present as irregularly shaped masses that usually are partially sericitized. 

UNIT 4-Fine-grained two-mica granite 

The veins are characterized by a striking vertical 

zonation that extends from the deeper and inner levels 

of the veins toward their upper and outer margins. From 

the deeper levels, barren quartz veins grade into a quartz 

potassium-feldspar-sulfide assemblage, followed by a 

quartz-wolframite assemblage, and then to a quartz-wolframite-beryl assemblage that caps the veins. This zoning 

in the veins is accompanied by a zoning in wall-rock alteration. The deepest levels contain a sodium-rich feldspar 

alteration zone that grades into a zone of intense potassium-feldspar alteration, then into a zone of silification, 

and, finally, into a zone of greisen which involves intense 

development of muscovite and coincides with the richest 

ore in veins. Wolframite mineralization is closely related 

to the zones of greisen and silicification and is absent 

in the potassium-feldspar zone. Wolframite mineralization 

also is limited usually to veins in the stock and rarely 

UNIT 3-Fine-grained porphyritic biotite 

granite 

UNIT 2-Medium-grained porphyritic biotite 

granite 

UNIT 1-Medium-grained biotite granite 

CAMBRIAN-Quartzite, pelitic sedimentary rocks 

CONTACT 

FAULT 

QUARTZ VEIN 

SAMPLE LOCALITY 

/ / / 

/ 

/ 

/ --------

EXPLANATION 

JURASSIC 

UNIT 2-Medium-grained porphyritic biotite granite 

UNIT 1-Medium-grained biotite granite 

CAMBRIAN-Quartzite, pelitic sedimentary 

rocks 

~ ~ VEIN 

CONTACT-Dashed where inferred 

SOUTH 

Figure 03. Diagrammatic north-south cross section of the Xihuashan deposit showing first and second phases 

of granite and relationship of quartz-wolframite veins to the margin of the stock. 

Hydrogen, Oxygen, and Sulfur Isotope Systematics, Xihuashan Quartz-Wolframite Deposit, China 161 
extends into veins in the country rock. The formation of 

wolframite may be closely related to wall-rock alteration, 

and this association should be studied in future investigations of the area. Wolframite mineralization is limited to 

a 200- to 300-m zone that extends to about 200 m below 

the roof of the granite. (See fig. 05.) 

The zoning and the general lack of vugs coupled 

with the complex nature of individual veins makes detailed paragenetic studies difficult. However, quartz 

mineralization clearly spans the major part of the 

paragenetic sequence and brackets wolframite mineralization. 

STABLE ISOTOPE STUDIES 

Most vein and greisen samples were collected from 

the 299 vein, which is one of the major veins in the northem part of the deposit. Samples also were collected from 

the 247 vein in the southern part of the deposit and from 

a few other places in the central part of the deposit, as 

indicated in the tables. Samples of the granite were taken 

from numerous underground and surface sites throughout 

the deposit. Although sample coverage of the 299 vein 

is fairly representative of temporal and spatial variations 

in that vein, it is only one of more than 600 veins in 

the deposit. To what extent the samples are representative 

of the entire deposit is not certain. The isotope results 

are summarized in tables 01-03 and those from vein 299 

are plotted in figures 04 and 05. Samples were analyzed 

by conventional techniques and are reported according to 

conventional standards as summarized by Rye and Sawkins ( 197 4) and Thode and others ( 1961). 

Sulfur Isotope Studies 

Sulfide minerals are not abundant in the Xihuashan 

deposit and are confined to the deeper levels of the veins; 

only six samples were analyzed for sulfur isotopic composition. Sulfate minerals have not been described in the 

deposit and the hydrothermal system appears to have been 

sulfide-dominant. The 834S values of molybdenite, pyrite, 

and chalcopyrite range from only 1.0 to --0.7 per mil. 

Although no mineral pairs were analyzed, the narrow 

range of values and the trend of the 34S enrichment among 

molybdenite, pyrite, and chalcopyrite suggest that the sulfides precipitated under equilibrium conditions that were 

extremely uniform over the sampled area. 

The 834S values of the sulfides are typical of vein 

deposits associated with felsic intrusions in which the 

hydrothermal fluids are H2S-dominant (Ohmoto, 1972). 

Under such conditions the 834s:ts of the hydrothermal fluid 

is slightly less than that of pyrite or about zero per mil 

(Ohmoto and Rye, 1979). The sulfur isotope geochemistry 

of the main-stage sulfides in the deposit is typical of all 

quartz-wolframite deposits studied to date. (See Landis 

and Rye, 1974; Kelly and Rye, 1979; Casadevall and 

Rye, 1980.) The sulfur involved in the Xihuashan deposit 

probably is of deep-seated igneous origin, either directly 

from a magma or by leaching of sulfur from igneous sulfides by meteoric-water hydrothermal fluids. 

Oxygen Isotope Studies 

More than 125 analyses were made on representative samples of granite, hornfels, veins, and greisen 

(tables 02 and 03). The following 8180 values were 

found in the granite: quartz, from 11.3 to 13.2 per mil; 

plagioclase, from 6.8 to 9. 7 per mil; potassium-feldspar, 

from 5.8 to 8.8 per mil; muscovite, 7.7 permil; biotite, 

6.5 per mil; and whole rock from 7.7 to 10.7 per mil. 

Temperatures calculated from the quartz-plagioclase, quartz-potassium-feldspar, quartz-muscovite, and 

quartz-biotite 180 fractionations in the granite are too low 

to represent primary magmatic crystallization temperatures 

(table 04). Thus, post-crystallization exchange with 

Table 01. The 8345 values (in per mil) of sulfide minerals in samples from quartzwolframite veins from the Xihuashan ore deposit 

[Samples were analyzed by T. P. Ding while a guest at the laboratory of Dr. A. G. Thode, McMaster 

University, Hamilton, Ontario, Canada. Leaders (---) indicate not present or not analyzed. CDT, Canon 

Diablo troilite] 

o34s (CDT) 

Sample Molyb- Chalconumber denite Pyrite pyrite Sampling site location 

Xi 007 0.7 620 level, 299 vein, north wall. 

Xi 022 0.4 538 level, 299 vein. 

Xi 020 -0.7 Do. 

Xi 037 -0.3 431 level, 71 vein. 

Xi 038 1.0 Do. 

Xi 062 0.3 270 level, 247 vein. 

162 Shorter Contributions to Isotope Research 
Table 02. &180 and &D values (in per mil) of minerals and whole rocks from granite and hornfels 

[Q, quartz; Mus, muscovite; PI, plagioclase, 20 percent anorthite; Kf, potassium feldspar; Bio, biotite; SMOW, Standard Mean Ocean Water. 

Leaders ( ---) indicate not present or not analyzed] 

Sample o18o (SMOW) 6D (SMOW) 

No. Q Pl Kf Mus Bio Whole rock Mus Bio Sampling site and granite phase 

Granite 

Xi 018 ll. 8 7.3 8.9 538 level, 1.5 m from 299 

vein; Jg2. 

Xi 025 ll.8 7.3 9.1 670 level, 1 m from 299 

vein; Jg2. 

Xi 033 12.2 6.7 8.6 431 level, on contact; 

Jg2. 

Xi 034 12.3 6.6 7.7 431 level, 1 m from 

contact; Jg2. 

Xi 044 12.0 6.1 8.0 431 level, on contact; Jg2. 

Xi 045 13.2 6.3 431 level, 2 m from 

134 vein; Jg2. 

Xi 065 11.5 7.8 7.7 10.7 -79 270 level, 1 m from 

247 vein; Jg2. 

Xir 01 12.0 9.7 6.5 10.3 -96 594 level, 5 m from 

62 vein; Jg2. 

Xir 02 11.5 6.2 8.1 Surface; Jg3. 

Xir 03 ll. 9 8.8 Surface; Jg4. 

Xir 04 12.0 5.8 Surface; JgS. 

Xir OS 11.7 7.0 8.8 431 level, 3 m from 

62 vein; Jg2. 

Xir 06 11.3 6.8 8.6 632 level, 3 m from 

61 vein; Jg2. 

Hornfels 

Xi 032 9.8 

Xi 043 9.0 

Xi 031 8.5 

Xi 042 12.3 

Xi 041 9.4 

Xi 040 ll.2 

Xi 039 10.3 

180-depleted waters has taken place among the biotite, 

muscovite, and feldspars in the granite. This fact is substantiated by petrographic evidence of incipient sericitization of feldspars in the granite. 

In view of these 180 depletions, the whole-rock 

8180 values must be considered minimum indicators of 

the initial magmatic values. A better indication of the primary magmatic values is given by the 8180 of quartz, 

which is least susceptible to postmagmatic exchange. 

These values of 11.3-13.2 per mil are slightly greater 

than those of "normal" igneous granites (Taylor, 1968) 

and are in the lower part of the range recorded for S-type 

granites (O'Neil and Chappell, 1977). 

S-type granites usually are highly silicic, two-mica 

granites that are characterized by normative corundum, 

high initial strontium ratios, and by 8180 values of greater 

than 9 per mil. These granites presumably are derived 

from the anatexsis of shallow sedimentary rocks. There 

appears to be a strong positive correlation between tin 

431 level, on contact with Jg2. 

431 level, 0.5 m from contact. 

431 level, 1 m from contact. 

431 level, 5 m from contact. 

431 level, 15 m from contact. 

431 level, 25 m from contact. 

431 level, 30 m from contact. 

or tin-tungsten mineralization and S-type granites (Chappell and White, 1974; Kelly and Rye, 1979). This relationship, however, does not appear to hold for deposits 

that contain only tungsten (Casadevali and Rye, 1980). 

The various phases of the Xihuashan granites are highly 

siliceous, biotite± muscovite granites with some excess 

aluminum (Wang and Zhou, in press). However, because 

the Xihuashan granites have been so altered, whether or 

not they are S-type is unclear. 

The hornfels adjacent to the granite stock has 

whole-rock 8 180 values of 8.5 to 12.3 per mil (table 02). 

A traverse across the granite-hornfels contact at the 431 

level near the west end of the 71 vein indicates that the 

hornfels were not noticeably affected by exchange with 180-depleted fluids, except possibly within 1 m from the 

contact (table 02). 

The range of 8180 values for most minerals in the 

veins generally is less than 3 per mil: quartz ranges from 

9. 7 to 12.9 per mil; potassium-feldspar from 4. 9 to 7. 3 

Hydrogen, Oxygen, and Sulfur Isotope Systematics, Xihuashan Quartz-Wolframite Deposit, China 163 
Table 03 8180 and 80 values of minerals and inclusion fluids of veins and greisen from the Xihuashan deposit 

[Q, quartz; Kf, potassium-feldspar; W, wolframite; Mus, muscovite; SMOW, Standard Mean Ocean Water. Leaders (---) indicate not present or 

not analyzed] 

c18 0(SMOW) 

Sample 

No. Q Kf w Mus Muse 

Xi 001 11.6 4.9 5.1 

002 11.5 6.0 

003 11.2 5.4 

004 11.2 5.9 

005 11.0 5.8 

006 11.7 

008 12.9 7.3 5.9 

009 11.8 5.7 

010 12.2 5.7 5.6 

011 11.7 6.2 6.2 

012 9.7 5.7 5.5 

013 12.2 5.4 

014 12.3 5.8 

015 11.4 5.7 

016 11.4 

017 10.5 

019 9.9 6.3 5.8 -84 

020 10.4 4.8 

021 11.4 5.5 

022 11.2 

023 10.5 6.1 5.0 

024 4.2 

026 12.0 

027 11.2 5.8 

028 12.1 5.1 

029 10.0 

030 11.0 7.0 

049 -79 

051 11.4 5.4 6.7 -111 

054 11.9 6.2 8.4 

055 12.0 5.8 

056 11.9 4.8 

057 12.2 5.0 

058 12.2 4.9 

059 12.4 5.4 5.3 

060 12.4 5.1 

061 12.4 5.5 

062 12.4 

063 ll.8 8.1 -75 

064 11.8 7.9 -71 

066 12.0 7.2 

per mil; muscovite from 5.8 to 8.4 per mil; and wolframite from 4.8 to 6.2 per mil (table 03). The veins are 

dense and, because of the absence of vugs and crosscutting relationships, do not lend themselves readily to detailed paragenetic studies. However, an attempt was made 

164 Shorter Contributions to Isotope Research 

cD(SMOW) 

Kf Fluid inclualteration sions in Q Sampling site 

299 vein, 538 level. 

299 vein, 620 level. 

Do 

-59 Do. 

Do. 

-61 Do. 

Do. 

Do. 

-73 Do. 

Do. 

Do. 

-56 299 vein, 594 level. 

Do. 

299 vein, 538 level. 

Greisen near 015. 

Do. 

-73 299 vein, 538 level. 

Do. 

-60 Do. 

-67 Do. 

-74 299 vein, 670 level. 

Greisen near 023. 

-78 299 vein, 632 level. 

Do. 

Do. 

Do. 

Greisen near Xi 029, 

102 vein. 

Do. 

-62 Do. 

247 vein, 270 level. 

Do. 

Do. 

Do. 

Do. 

Do. 

-75 Do. 

Do. 

Do. 

Greis en near 054. 

Do. 

-62 247 vein, 270 level. 

to get some impression of the time-space variation of 8180 

in quartz and wolframite in the 299 and 247 veins that 

could be related to variations in temperature and (or) 

8180Hp of the fluids. The results of traverses across different levels of the veins which range in thickness from 
EXPLANATION 

QUARTZ 

o In quartz-wolframite 

vein 

«> In greisen 

• In granite 

MUSCOVITE 

o In greisen 

• In granite 

6 WOLFRAMITE-In quartzwolframite vein 

~o----o----o"""-o 

299 VEIN 

632 level 

~-----------o---- ------o--

_o __ 0" 0 

299 VEIN 

620 level 

299 VEIN 

620 level 

-----«P- 0 _ 0 _ 0 -o-o-o-o-o 

------------c----o 

100 50 

South 

6-6 

'6~6-6- 6,6,6 

0 

DISTANCE, IN CENTIMETERS 

South 

wall of 

vein 

50 

North 

247 VEIN 

270 level 

B 18Q (per mil) 

12 

8 

4 

0 

12 

8 

4 

0 

12 

8 

4 

0 

12 

8 

4 

0 

100 

Figure 04. 8180 values of samples collected from traverse across the 247 vein at the 270 

level and across the 299 vein at the 620 and 632 levels. Data plotted at distances north and 

south from a plane representing the south wall of each vein. 

Hydrogen, Oxygen, and Sulfur Isotope Systematics, Xihuashan Quartz-Wolframite Deposit, China 165 
... WEST 0'1 

0'1 METERS 

900 

fll 

:::r 

0 

:s. 

~ 

n 

0 

a 

.. BOO s.: 

s. 

s· 

= "' 0 

~ i 700 

::111:1 

I'D 

"' I'D 

Dol 

~ :::r 

600 

500 

400 

100 200 300 METERS 

0 300 60 0 FEET 

EXPLANATION 

HIGH GRADE ORE ZONE 

JURASSIC 

UNIT 1-Medium-grained porphyritic 

biotite granite 

UNIT2 -Medium-grained biotite granite 

FAULT ZONE 

X Xi01 

11.4 

5.5 

CONTACT BETWEEN GRANITE PHASES 

SAMPLE LOCALITY 

818Q VALUE, IN PER MIL 

Quartz 

Wolframite 

Figure 05. Cross section of the 299 vein showing sample localities and spatial distribution of 8180 values for quartz and 

wolframite. 

EAST 
about 30 em to 2 m are shown in figure 04. There are 

no consistent variations in either a1s0 values or quartzwolframite 1

s0 fractionations. 

In order to get some impression of possible spatial 

variations in the a1

s0 of the quartz and wolframite, the 

data for the 299 vein are plotted in figure 05. Only the 

a1s0 values of wolframite show evidence of a spatial variation in that they tend to decrease toward the top, sides, 

and outer margins of the vein. This spatial variation in 180 leads to a slight increase in quartz-wolframite fractionation toward the margins of the veins, suggesting a 

possible spatial decrease in temperature. The narrow range 

of a1

s0 values for wolframite seems to be a characteristic 

feature of quartz-wolframite deposits and indicates that 

wolframite deposition takes place under restricted 

geochemical conditions in a low water-rock hydrothermal 

system during a specific phase in the evolution of an igneous body at individual deposits (Landis and Rye, 1974; 

Kelly and Rye, 1979; Casadevall and Rye, 1980). 

The interpretation of the a1

s0 data on the vein minerals is hampered by lack of detailed temperature information that can be related to specific a1

s0 analyses, and 

by uncertainty in the 180 quartz-water fractionation 

curves. The 3. 2 per mil range in a1

s0 quartz could be 

accounted for by a 60°-1 00°C temperature variation in 

the 200°- to 400°C-temperature interval. Because some of 

the range in 1s0 quartz must have been due to at least 

minor variations in the a1

s0H2o of the fluid, the actual 

temperature variation during quartz mineralization probably was less than 60°-100°C. 

Initial filling temperature measurements of fluid inclusions in quartz samples Xi 006, Xi 021, and Xi 060 

range from about 200° to 300°C. Filling-temperature 

studies on these samples were hampered by the presence 

of a large number of secondary inclusions, by neckingdown of inclusions, and by variable liquid to vapor ratios 

in the primary inclusions that give higher temperatures 

and indicate that, at times, the fluids were boiling. These 

results are consistent with those obtained by Lu and others 

(1977, 1982). If we assume that the fluid temperature was 

250°C, the calculated a180H 0 of the fluids was 1.0 ± 1.6 2 

per mil using the curve of Taylor (1974). The extrapolated 

curve of Bottinga and Javoy (1973) results in values about 

1 per mil smaller; the curves of Clayton and others ( 1972) 

and Matsuhisa and others ( 1979) result in values about 

1 per mil larger. Fluids derived from magmas have 

a1s0H2o values determined by the composition of the 

magma, usually about 6-8 per mil (Taylor, 1974). These 

initial a1

s0 values could be decreased by lower temperature exchange with previously crystallized igneous rock. 

At Xihuashan, the a1

s0 values for the quartz in the granite, greisen, and veins are similar, suggesting that the 

hydrothermal fluids were in equilibrium with the host 

rocks at the temperatures prevailing during ore deposition. 

A similar situation was observed at Panasqueira (Kelly 

and Rye, 1979). One could argue that the fluids were 

magmatic in origin, and the low a1

s0H2o resulted from 

low-temperature exchange with the granite. This argument, however, would require nearly complete exchange 

between the magmatic fluids and the granite at the site 

of deposition-a condition that is unlikely to have existed 

in an open vein system. The calculated a1

s0H2o values 

seem likely to have been sufficiently low to indicate a 

significant component of highly exchanged meteoric water 

in the hydrothermal system. Only if the temperatures of 

mineralization were near 400°C could the fluids have had 

a1s0H2o values large enough to argue for a predominantly 

magmatic origin. 

An attempt was made to determine depositional 

temperatures from coexisting quartz and muscovite, and 

quartz and wolframite in the veins and greisen. There is 

no certainty that these minerals were deposited contemporaneously or in isotopic equilibrium. Furthermore, calculated temperatures can vary by as much as 160°C depending upon which fractionation curve is used. The 

quartz-muscovite curves derived from the experimental 

muscovite-water curve of O'Neil and Taylor (1969) and 

the quartz-water curve of Clayton and others (1972) and 

Matsuhisa and others (1979) give temperatures ranging 

from 246° to 356°C and 210° to 296°C, respectively (table 

04), whereas the extrapolated curve of Botting a and 

Javoy (1973) gives temperatures ranging from 371° to 

459°C. Quartz-wolframite fractionations range from 4.2 

to 7.2 per mil, but unfortunately the empirical equation 

for quartz-wolframite 1

s0 fractionations quoted in Landis 

and Rye (1974) is strongly dependent upon composition 

of the wolframites and needs to be calibrated experimentally (G. P. Landis and R. 0. Rye, unpub. data). 

The question of temperature needs to be resolved 

by detailed fluid-inclusion studies. The impression that we 

have at this stage is that depositional temperatures averaged ~ 250 ± 50°C. If these are the depositional temperatures, the Xihuashan hydrothermal fluids probably had a 

significant component of highly exchanged meteoric 

water. This conclusion is consistent with the a1

s0 data 

from the granite that require postcrystallization exchange 

with 1

s0-depleted waters, although the fact that postcrystallization exchange with meteoric water took place with 

the granite does not necessarily require that hydrothermal 

fluids were of meteoric-water origin. 

The a1

s0 values of quartz and muscovite from the 

greisen adjacent to the veins are 4.2-11.9 per mil and 

7.0--8.1 per mil (table 03), respectively. These values, 

with the exception of the 4.2 per mil for sample Xi 24, 

are consistent with those in the veins. The anomalously 

low value for Xi 24 indicates deposition from or exchange 

with relatively unexchanged meteoric waters in the upper 

part of the vein system. 

Hydrogen, Oxygen, and Sulfur Isotope Systematics, Xihuashan Quartz-Wolframite Deposit, China 167 
Table 04 Temperatures (in degrees Celsius) calculated 

from &180 data of coexisting minerals in granite, griesen, 

and veins 

[Leaders ( ---) indicate mineral pairs not present or not analyzed] 

Sample llQuartz-muscovite 

No, llQ-Pl 1 llQ-Kf 1 (I) ( 2) ( 3) llQ-Bio 

Granite 

Xi 018 239 

Xi 025 191 

Xi 033 147 

Xi 034 139 

Xi 044 132 

Xi 045 )102 

Xi 065 291 435 

Xir 01 443 305 

Xir 02 155 

Xir 03 285 

Xir 04 122 

Xir 05 228 

Xir 06 239 

Veins 

Xi 019 (+) 410 294 248 

Xi 051 (4) 370 246 210 

Xi 054 (4) 455 358 296 

Greisen 

Xi 030 418 301 262 

Xi 063 449 334 288 

Xi 064 426 314 264 

1Temperatures based on extrapolated curve of Bottinga and 

Javoy (1973). 

2Temperatures based on experimental quartz-water curves of 

Clayton and others (1972), and muscovite-water curve of O'Neil and 

Taylor (1967), 

3Temperatures based on curves of Matsuhisa and others (1979), 

and O'Neil and Taylor (1969). 

4Quartz-potassium feldspar temperatures in veins were not 

determined because feldspar clearly is seriticized. 

Hydrogen Isotope Studies 

Hydrogen isotope analyses were made on hydrous 

minerals from granite, greisen, and veins and on inclusion 

fluids from vein quartz. Suitable samples of wolframite 

were not available for fluid-inclusion analyses. 

One sample each of muscovite and biotite from the 

granite have 8D values of -79 and -96 per mil, respectively. Using the equations of Suzuoki and Epstein 

(1976), these values correspond to 8DH2o values of about 

-65 and -69 per mil, respectively, at magmatic conditions 

and are consistent with those for magmatic fluids (Taylor, 

197 4). However, because the 8180 data suggest that some 

postcrystallization exchange with 180-depleted waters has 

taken place in the primary hydrous minerals in the granite, 

the 8D values also should reflect exchange with these later 

waters. 

168 Shorter Contributions to Isotope Research 

Muscovite in the greisen and the veins has 8D values ranging from -71 to -111 per mil (table 03) corresponding to 8DH2

o values of from -33 to -73 per mil 

at 250°C. These values agree reasonably well with the 

8DH2o values of inclusion fluids in quartz that range from 

-56 to -78 per mil, considering that there is great deal 

of uncertainty in the low temperature extrapolation of the 

muscovite-water D fractionation curve of Suzuoki and 

Epstein (1976). 

The potassium-feldspars in the veins typically show 

incipient alteration to sericite. The 8D values of four of 

these samples range from -62 to -74 per mil. These values overlap the lower range of values for muscovite in 

the veins and greisen, suggesting that the presumably later 

feldspar alteration was produced by fluids that had a less 

negative 8DH2o or lower temperatures than the veingreisen muscovite fluids. More detailed paragenetic information and analyses of additional minerals might allow 

recognition of variations in 8DH2o among different generations of fluids that could be used to sort out the detailed 

water history of the deposit. 

CONCLUSIONS 

Perhaps the most significant result of this study is 

the extremely narrow range of 8180 values for various 

minerals in the Xihuashan deposit indicating that mineralization took place from fluids that had a limited temperature and 8180H20 range. The calculated 8180H20 values 

are low enough to suggest a highly exchanged meteoricwater component in the hydrothermal fluids. This conclusion is consistent with the 8180 data on the stock which 

indicate that the stock underwent exchange with 180-depleted fluids after crystallization. The fact that the 8180 

values of the vein minerals are so uniform indicates that 

the the hydrothermal fluids, regardless of their origin, had 

a remarkably uniform exchange history with the silicate 

wall rocks. The sulfide sulfur from the lower part of the 

veins was derived from deep-seated sources. 

Initial fluid-inclusion data, when combined with 

8180 data on the minerals' suggest that the temperature 

of mineralization of quartz that obtained during most of 

the paragenesis probably was between about 200° and 

300°C. At times the fluids were boiling. 

Many aspects of the stable isotope geochemistry of 

the Xihuashan deposit appear similar to those of other 

quartz-wolframite deposits that have been studied in detail. The results of this study provide evidence that the 

Xihuashan deposit, like many other quartz-wolframite deposits, formed under a narrow range of conditions when 

a rock-dominated hydrothermal system formed during a 

specific phase in the evolution of a composite granite that 

intruded a pelitic sedimentary rock suite. 
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Central Geological Survey, Department of Economy, 

[China], A, no. 60, p. 1-21. 

---1943, Geological records of tungsten ore deposits in 

southern Jiangxi: Geological Report of Central Geological 

Survey, Department of Economy, [China], no. 17, p. 

170--191. 

Hydrogen, Oxygen, and Sulfur Isotope Systematics, Xihuashan Quartz-Wolframite Deposit, China 169 

1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

THE STRONTIUM ISOTOPE COMPOSITION OF 

GRANITOID ROCKS OF THE 

SOUTHERN SNAKE RANGE, NEVADA 

FIGURES 

Chapter P 

By DONALD E. LEE, RONALD W. KISTLER, and ALLEN C. ROBINSON 

CONTENTS 

Abstract ............... . 

Introduction . . . . . . . . . . . . . . . . . . . . . 

Intrusive rock of the Pole Canyon-Can Young Canyon area 

Intrusive rock of the Snake Creek-Williams Canyon area 

Intrusive rocks of the Young Canyon-Kious Basin area 

Tertiary aplite . . . . . . . . . . 

Osceola stock . . . . . . . . . . . 

Intrusive rock of Willard Creek . . 

Intrusive rock of Lexington Creek 

Sedimentary rocks . . . . 

Summary and conclusions 

References cited . . . . . 

Page 

172 

172 

172 

174 

176 

176 

176 

177 

177 

177 

177 

178 

Page 

Pl. Map showing granitoid rocks of the southern Snake Range, Nevada . . . . . . . . . . . . . . . 172 

P2. Strontium evolution diagram for the muscovite-phenocrystic two-mica granite of the Pole Canyon-Can 

Young Canyon area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 

P3. Strontium evolution diagram for the Snake Creek-Williams Canyon pluton . . . . . . . . . 175 

P4. Strontium evolution diagram for the intrusive rock of the Young Canyon-Kious Basin area 176 

P5. Strontium evolution diagram for the Osceola stock . . . . . . . . . . . . . . . . . . . . . . 177 

TABLE 

Page 

Pl. Rubidium-strontium data for intrusive and sedimentary rocks of the southern Snake Range, White Pine 

County, Nevada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 

Strontium Isotope Composition of Granitoid Rock, Southern Snake Range, Nevada 171 
Abstract 

Six discrete granitoid plutons are exposed within 

a 900 square-kilometer area in the southern Snake Range 

of eastern White Pine County, Nev. These granitoid 

rocks range in age from Jurassic to Tertiary. Initial 87Sr/ 86Sr range from 0.70714 to about 0.71574, 8180 values 

from -2.6 to 13.2 per mil, and Si02 contents from 63 

to 76 weight percent. The petrologic types include calcalkalic rocks and two different types of two-mica granite. 

The younger granitoids were emplaced at progressively 

more shallow depths, and generally crystallized from increasingly evolved magmas. All intrusive types present 

in the southern Snake Range are present elsewhere in 

the eastern Great Basin, but the southern Snake Range 

is distinguished by the grouping of all these intrusive 

types within a relatively small area. The chronology of 

the igneous events in the study area is remarkably similar 

to that found in the Ruby Mountains of Elko County, 

Nevada, about 200 km to the northwest. 

INTRODUCTION 

Several granitoid masses are exposed within an area 

of 900 km2 (square kilometers) in the southern Snake 

Range, White Pine County, Nev. (fig. Pl). Previous 

studies (Lee and Van Loenen, 1971 , and references cited 

therein) have shown that these rocks range in age from 

Jurassic to Tertiary and in composition from granodiorite 

(63 percent Si02) to granite (76 percent Si02). A similarly 

large range of &180 values (-2.6 to 13.2 per mil) was 

found in these plutons (Lee and others, 1982). Among 

the intrusive types recognized are two different kinds of 

two-mica granites (Lee, Kistler, Friedman, and Van 

Loenen, 1981). The strontium isotope data presented here 

supplement previously published data in a way that greatly 

increases our understanding of the igneous history of this 

part of eastern Nevada. 

Several works on the regional variations in strontium and oxygen isotopes of Cordilleran granitoids have 

been completed in the last several years (Masi and others, 

1976, 1981; Taylor and Silver, 1978). In addition to 

studies of oxygen isotope variation in granitoids in the 

Great Basin (Lee, Friedman, and Gleason, 1981), and 

oxygen isotope variations in granitoids generated from diverse source materials (O'Neil and Chappell, 1977), these 

works provide a data base for further study of the source 

materials of granitoid rocks in general. 

INTRUSIVE ROCK OF THE POLE 

CANYON-CAN YOUNG CANYON AREA 

The intrusive rock of the Pole Canyon-Can Young 

Canyon area is an unusual two-mica granite that was described in detail by Lee and Van Loenen ( 1971 , p. 5, 

38, 39). This rock is distinguished in part by the presence 

1 72 Shorter Contributions to Isotope Research 

39"00' 

15' 

~77-MW-60 

Osceola 

stock 

1 .-./] Willard Creek 

295-DL-64 U intrusive 

'() Tertiary aplitejd 

284-0L-63 

59 Pole CanyonCan Young~ 

Canyon area.---......_'\_______/ 

x2 

Canyon area 

Area of 

this 

figure 

Young CanyonKious Basin area 

Intrusive of-c!J.GR4J 

Lexington Creek X X 

GR48 

38°50' '------------'----------_j 

10 KILOMETERS 

5 MILES 

Figure P1. Map showing granitoid rocks of the southern 

Snake Range, Nevada. Outlines of intrusives modified 

from the map of Hose and Blake (1976). Shaded area 

shows intrusive that has been sheared and brecciated 

by the Snake Range decollement. Only selected sample 

localities are shown. Sampling sites 5, 12, 15, 31, 57, 61, 

66, 79 and 83 (table P1) are present in sequence between 

sample localities 2 and 86. 

of muscovite phenocrysts as large as 2 em (centimeters) 

across that contain small inclusions of euhedral biotite. 

It is not unusual to find as many as 20 biotite euhedra 

within a single large grain of muscovite. Localities, chemical and modal analyses, and norms for samples 89, 92, 

93, and 95 (table Pl) and for five additional samples of 

this pluton (Lee and Van Loenen, 1971) show that these 

samples contain 2.8-3.4 percent normative corundum, and 

that each is strongly peraluminous (molecular 

Al203> >[CaO + Na20 + K20]). 

The biotites in these intrusive rocks contain only 

about 6. 0 percent MgO, leading Lee and Van Loenen 

(1970) to suggest that the magmas may have had relatively low oxygen fugacities. This inference is further substantiated by the complete absence of opaque oxides from 

these rocks. Indeed, the only accessory minerals present 

are monazite, an allanite-like mineral, and equant apatite 

containing many tiny inclusions of acicular zircon (see 

chap. D, fig. D2, this volume). Based mainly on field 

relations, Lee and Van Loenen (1971) concluded that this 

distinctive rock resulted from assimilation of the latest 

Precambrian Osceola Argillite of Misch and Hazzard 

(1962). More recently Lee Kistler, Friedman, and Van 

Loenen (1981) described two similar two-mica granite 
Table Pl. Rubidium-strontium data for intrusive and sedimentary rocks of the southern 

Snake Range, White Pine County, Nev. 

Sample 

No. 

89 

92 

93 

95 

1226-MW-61 

PCA-P1 

RK 

2 

5 

12 

15 

27 

28 

31 

57 

59 

61 

66 

79 

83 

98 

103 

110 

1284-DL-63 

177-MW-60 

334-DL-66 

371-DL-66 

295-DL-64 

GR47 

GR48 

Ar3 

Ar4 

Q2 

S6 

S7 

p 

Concentration 

([!arts 2er million) 

Type Rb Sr 

Intrusive, Pole Canyon-Can Young 

Two-mica granite--------- 185 327 

---do.------------------- 178 358 

---do.------------------- 164 358 

---do.------------------- 165 351 

Muscovite---------------- 582 20.04 

Aplite-pegmatite--------- 279 18.1 

Microcline megacryst----- 738 31.7 

Intrusive, Snake Creek-Williams 

Intrusive---------------- 231 106 

---do.------------------- 219 119 

---do.------------------- 153 224 

---do.------------------- 148 329 

---do.------------------- 136 421 

---do.------------------- 144 415 

---do.------------------- 121 491 

---do.------------------- 101 511 

---do.------------------- 151 420 

---do.------------------- 83.8 713 

---do.------------------- 86.2 676 

---do.------------------- 82.3 716 

---do.------------------- 100 678 

Intrusive, Young Canyon-Kious 

Intrusive---------------- 238 68 

Cataclastic intrusive---- 173 235 

---do.------------------- 109 409 

Tertiary aplite 

Aplite------------------- 529 8.88 

Intrusive, Osceola 

Intrusive----------------

---do.-------------------

---do.-------------------

92.3 

200 

102 

482 

87 

528 

Intrusive, Willard Creek 

Two-mica granite--------- 152 164 

Intrusive, Lexington Creek 

Two-mica granite--------- 101 

---do.------------------- 113 

610 

671 

2sedimentary rocks 

pCo---------------------- 148 

pCo---------------------- 220 

CpCm--------------------- 113 

Cp----------------------- 338 

Cp----------------------- 217 

Cpc----------------------

51.9 

28.7 

28.9 

28.4 

45.9 

Rb/Sr 

Canyon 

0.566 

.497 

.458 

.470 

29.04 

15.4 

23.3 

Canyon 

2.18 

1.84 

.683 

.450 

.323 

.347 

.246 

.198 

.360 

.118 

.128 

.115 

.147 

basin 

3.49 

.786 

.267 

59.6 

0.192 

2.3 

.193 

0.927 

0.165 

.168 

2.85 

7.67 

3.91 

11.9 

4.72 

87Rb/86sr 87sr;86sr 

1.64 0.71823 

1.44 .71841 

1.33 • 71528 

1.36 .71753 

84.9 .8123 

44.9 .76570 

68.3 .79243 

6.31 o. 72115 

5.33 .71875 

1.98 • 71134 

1.30 .71017 

.935 .70991 

1.00 .70994 

.713 .70883 

.573 .70838 

1.040 • 71025 

.340 .70781 

.370 .70786 

.333 • 70775 

.427 .70832 

10.12 0.71748 

2.13 .71307 

.772 .71251 

173 0.7821 

0.554 0.70864 

6.66 • 72119 

.559 .70854 

2.68 o. 71432 

0.479 o. 71191 

.488 • 71203 

8.30 0.76454 

22.34 .80317 

11.41 .79620 

34.75 .80272 

13.77 .79023 

• 70911 

1Data from Lee and others (1970, table 3). z. E. Peterman, R. A. Hildreth, and 

W. T. Henderson, analysts. 2pCo, Precambrian Osceola Argillite of Misch and Hazzard (1962); CpCm, Precambrian and 

Cambrian Prospect Mountain Quartzite; Cp, Cambrian Pioche Shale; Cpc, Cambrian Pole Canyon 

Limestone. 

Strontium Isotope Composition of Granitoid Rock, Southern Snake Range, Nevada 173 
plutons in the Kern Mountains and the Toana Range of 

eastern Nevada and concluded that all three probably resulted from anatexis of upper Precambrian argillites under 

conditions of relatively low oxygen fugacity, along a line 

that coincides roughly with the westward disappearance 

of a continental basement as indicated by the lead isotope 

data of Stacey and Zartman (1978). However we should 

note that our unpublished strontium data indicate that the 

edge of the continental basement is farther to the west. 

Granites have been classified according to source 

materials by Chappell and White (1974). In the scheme, 

granites inferred to be derived from predominantly sedimentary (weathered) protoliths are S-types, whereas those 

derived from igneous source materials are I-types. The 

chemical, mineralogical, and isotopic criteria used to distinguish between S- and I-type granites have been summarized by Chappell and White (1974), Beckinsale 

(1979), Furgeson and others (1980), and O'Neil and 

Chappell ( 1977). The peraluminous nature of the Pole 

Canyon-Can Young Canyon pluton, as well as its content 

of muscovite and monazite, are indicative of an S-type . Th ~t8 • grantte. e u 0 values of mne samples of this rock 

(10.6-12.1 per mil) support the idea that this intrusion 

was derived from an evolved protolith such as the Osceola 

Argillite. Indeed, six samples of this argilllite gave 8180 

values that ranged from 11.6 to 12.5 per mil (Lee and 

others, 1982). 

Rubidium-strontium data for four whole-rock samples, a muscovite, a microcline megacryst, and an aplitepegmatite are listed in table Pl. The scatter in the 

87Sr/86Sr values for the four whole-rock specimens is not 

unusual for muscovite granites. We therefore averaged the 

87Rb/86Sr and 87Sr/86Sr values for the whole rocks to 1.44 

and 0. 71736, respectively. A regression (York, 1969) of 

this value for the whole-rock specimens, the muscovite, 

the microcline, and the aplite-pegmatite yields an age of 

79. 1 ± 0. 5 Ma with an initial 87 Sr/86Sr of 

0. 71574 ± 0.00003. Other isotope data (chap. D, this volume) support these conclusions and reaffirm the great 

similarities between this two-mica granite and that of the 

Kern Mountains, ·about 83 km to the north. The initial 

87Sr/86Sr determined (0. 71574) is well within the range 

representative of S-type granites. For comparison, a twopoint isochron diagram for the muscovite-phenocrystic 

two-mica granite of the Kern Mountains is included in 

figure P2. 

INTRUSIVE ROCK OF THE SNAKE 

CREEK-WILLIAMS CANYON AREA 

The Jurassic (Lee and others, 1968; Lee, Stem, and 

Marvin, 1981) intrusive of the Snake Creek-Williams 

Canyon area is separated from the just-described Upper 

Cretaceous two-mica granite of the Pole Canyon-Can 

174 Shorter Contributions to Isotope Research 

0.7800 

J:i 

\0 

~ 0.7600 

;!> 

00 

0.7400 o--o Pole Canyon-Can Young Canyon, 

southern Snake Range (table P1 ). 

t=79.1±0.5 Ma 

Tj=0.71574±0.00003 

0.7200 •----• Kern Mountains (Lee and others, 

1 980. table 8) 

Granite of t=71 ±6 Ma 

Kern Mountains ri=0.719 

87Rbj86Sr 

Figure P2. Strontium evolution diagram for the muscovite-phenocrystic two-mica granite of the Pole Canyonc.an Young Canyon area (fig. P1 ). The two-point isochron 

~1agram f~>r a pr~cti~ally identical muscovite-phenocrystlc two-m1ca gramte m the Kern Mountains is shown for 

c.om~aris~n. (lee and others, 1980, fi_g. 11, table 8). t, 

t1me m m1ll1ons of years; ri, initial 87SrFSr. 

Young Canyon area only by a septum of sedimentary 

rocks about 2 km long and 200 m (meters) wide (Lee 

and Van Loenen, 1971, p. 1). This Jurassic intrusive is 

undeformed, probably has not been eroded to a depth of 

much more than 300 m, and is well-exposed in contact 

with quartzite, shale, and limestone. Within a horizontal 

distance of 5 km, the intrusive grades from quartz monzonite (76 percent Si02 , 0.5 percent CaO) where the host 

rock is quartzite to granodiorite (63 percent Si02 , 4.5 percent CaO) where the host rock is limestone. Other major 

elements vary as would be expected in a normal differentiation sequence. Most minor elements show similar variations, the notable exceptions being the rare earths (Lee 

and Van Loenen, 1971, p. 24, 25). Cain (1974) showed 

that the distribution of rare-earth elements in this pluton 

can be attributed to their uptake in accessory minerals, 

which contain 80-98 percent of the rare earths (except 

europium) present in the rock. 

Chemical analyses of this intrusive rock cluster 

within and near Bowen's (1937) thermal valley on the 

liquidus surface in the system NaA1Si30 8-KA1Si30 8-Si02 

(Lee and Van Loenen, 1971, p. 32, 33). As emphasized 

by Tuttle and Bowen (1958), such a concentration of 

points is explained readily by liquid-crystal equilibria, but 

would be a remarkably fortuitous result of any mechanism 

not involving a magma. Based on a number of mineralogi-
cal and geochemical features summarized by Lee and Van 

Loenen (1971 , p. 44, 45) we have concluded that the 

crystallizing magma was contaminated by reaction with 

country rock. In other words, the field and laboratory 

studies cited indicate that the intrusive rock of the Snake 

Creek-Williams Canyon area evolved through combined 

assimilation-fractional crystallization. 

The results of recent oxygen isotope work (Lee, 

Friedman, and Gleason, 1982) also support the idea that 

both assimilation and fractional crystallization were involved in the development of the Snake Creek-Williams Canyon pluton. Whole-rock 8180 values for 25 samples of 

this pluton range from 10.2 to 12.2 per mil, indicating 

that the magma was derived from an evolved (sedimentary) protolith. Moreover, there is a rather systematic increase in 8180 values from the most mafic ( 63 percent 

Si02) to the most felsic (76 percent .Si02) parts of the 

exposure. Enrichment of 8180 in the felsic rock is consistent with a fractional crystallization model because most 

early crystallizing minerals (such as biotite, magnetite, 

epidote, and sphene) are 180-poor (Taylor, 1968). 

Rubidium-strontium data for 13 samples of the 

Snake Creek-Williams Canyon pluton are listed in table 

Pl. Ten samples fit an isochron with an indicated age 

of 155.2 ± 4 Ma and an initial 87Sr/86Sr of 

0. 70714 ± 0.00006 (fig. P3). 

Samples 27, 28, and 59 (table Pl, fig. P3) plot 

above the ten-point isochron just noted. They are displaced toward the "field" of the whole-rock samples from 

the two-mica granite of the Pole Canyon-Can Young Canyon area (table PI, fig. P2). This displacement is of particular interest because the Snake Creek-Williams Canyon 

pluton is in contact with the Osceola Argillite only in 

the area of samples 27, 28, and 59 (see Lee and Van 

Loenen, 1971, pl. 1), and as noted, there is ample evidence that the Pole Canyon-Can Young Canyon pluton 

formed through anatexis of this argillite and related sediments. The apatite-zircon relationship already noted is 

also of special interest here. In discussing the petrography 

and petrology of the Pole Canyon-Can Young Canyon 

pluton, Lee and Van Loenen (1971 , p. 39) stated: "One 

of the most striking mineralogical distinctions of these 

rocks is the apatite-zircon relationship mentioned earlier. 

The apatite has a stubby habit, and almost all of the zircon 

in these rocks is present as tiny needlelike inclusions within this apatite. This is different from anything observed 

in the Snake Creek-Williams Canyon area, except in one 

small area north of Williams Canyon on the west side 

of the range, where (locs. 27, 28, and 59, pl. 1) the 

intrusive is exposed in contact with the Osceola Argillite. 

In this restricted area, the apatite carries a few grains of 

acicular zircon * * *." 

Thus, rubidium-strontium results tend to confirm 

previous conclusions based on field, chemical, mineralogical and oxygen isotope data. Although the ten-point iso0.723 

0.721 0/ 

0.719 / 0 

0.717 

U3 08 0.715 "">:;-

~ 0.713 00 

0.711 "'0/0 

0.709 07 

o· t=1 55.2±4Ma c9' 

0.707 r;=Q. 707145±0.00006 

0.705 

0 2 3 4 5 6 

87Rbf86Sr 

Figure P3. Strontium evolution diagram for the Snake 

Creek-Williams Canyon pluton (open circles). Samples 

27, 28, 59 (solid circles) were collected from the extreme 

northwestern part of the pluton (fig. P1), where intrusive is in contact with the Osceola Argillite, and were 

not used in calculating the isochron. Sample 295-DL-64 

(open triangle) is from the Willard Creek intrusive. t, 

time in millions of years; ri, initial 87Sr!*'Sr. 

7 

chron supports a fractional crystallization model, the displacement of the points for samples 27, 28, and 59 is 

evidence that the country rock had some influence on the 

crystallizing intrusive in the area north of Williams Canyon, where the pluton is in contact with Osceola Argillite. 

The 155.2 ± 4 Ma rubidium-strontium age (fig. P3) 

for the Snake Creek-Williams Canyon pluton is in good 

agreement with the uranium-thorium-lead isotopic ages 

determined for zircon from this same pluton (Lee and 

others, 1968; Lee, Stem, and Marvin, 1981). The initial 87Sr/86Sr (0.70714±0.00006) suggests a sialic protolith 

(Kistler and Peterman, 1973), in agreement with our 8180 

data. However, some of the chemical characteristics of 

this intrusion are ambiguous as indicators of the type of 

magma protolith. Thus, Na20 is mostly >3.2 percent, 

and some of the larger xenoliths in the mafic parts of 

the intrusive contain hornblende (1-type granite). Normative corundum is both greater than 1 percent (S-type) and 

less than 1 percent (I-type). Perhaps the most striking ambiguity is apparent from study of the accessory minerals 

(Lee and Van Loenen, 1971, fig. 10). The most mafic 

parts of the intrusive (63 percent Si02) contain 2 weightpercent epidote, 1 percent sphene, 0.5 percent magnetite, 

and well-developed allanite, apatite, and zircon. As Si02 

increases, the amounts and types of accessory minerals 

change gradually, until the most felsic parts of the intrusive (76 percent Si02) contain only ilmenite, garnet, 

monazite, and trace amounts of apatite and zircon. The 

entire suite of accessory minerals comprises only about 

0. 1 weight percent of the most felsic parts of the intrusion. 

Strontium Isotope Composition of Granitoid Rock, Southern Snake Range, Nevada 175 
INTRUSIVE ROCK OF THE YOUNG 

CANYON-KIOUS BASIN AREA 

Most of the granitoid rock exposed in the Young 

Canyon-Kious basin area (fig. Pl) has been deformed 

cataclastically as a result of stresses related to late movement on the Snake Range decollement. The deformed and 

undeformed parts of this granitoid rock are separated by 

a fault. Concentrates of secondary muscovite recovered 

from the cataclastic part of the intrusion have K-Ar ages 

of 17-18 Ma, presumbably indicating the time of deformation and thus of the most recent movement on the 

Snake Range decollement (Lee and others, 1970). 

The 8180 values determined for three undeformed 

samples (8.7, 9.8, and 10.0 per mil) are relatively high 

compared with 8180 values determined for 115 plutons 

from the Basin and Range Province (Lee and others, 

1982). However, most samples of the cataclastic part of 

this intrusive have very low (to -2.6 per mil) 8180 values. 

In general, the most intensely deformed samples usually 

have lowest 8 180 values, leading Lee, Friedman, and 

Gleason (chap. N, this volume) to conclude that this intrusive underwent oxygen isotope exchange with meteoric 

water during cataclasis. 

Rubidium-strontium data for undeformed sample 98 

(8 180 of + 10.0 per mil) and for cataclasized samples 

103 and 110 (8 180 of +0.1 and -1.1, respectively) are 

listed in table Pl. These data gave an initial 87Sr/86Sr 

of 0.71203 ±0.00010 at an age of 37.4± 1.5 Ma (fig. 

P4). The fact that both undeformed and deformed samples 

fit the isochron strongly suggests that cataclasis did not 

affect the rubidium-strontium systematics of this rock, 

even though it led to modification of both K-Ar ages and 

1801'60. 

Our results clearly show that there was Jurassic, 

Cretaceous, and Oligocene intrusive activity within the 

relatively restricted area of figure Pl. The high initial 

87Sr/86Sr of the Oligocene phase indicates derivation from 

sedimentary materials, an interpretation consistent with 

the relatively high 8180 values found in the undeformed 

part of this intrusion. 

TERTIARY APLITE 

A wedge-shaped mass of Tertiary aplite separates 

cataclastic from undeformed intrusive rocks in the northwestern corner of Kious basin (fig. Pl; Lee and Van 

Loenen, 1971). This aplite body is in fault contact with 

the adjacent Tertiary (37 Ma) intrusive rocks, possibly 

indicating that it represents late-stage felsic material that 

was squeezed into place by stresses resulting from movement on the Snake Range de' collement (Lee and Van 

Loenen, 1971). According to this interpretation, the aplite 

is related genetically to both the cataclastic and the undeformed :intrusive with which it is in fault contact. 

176 Shorter Contributions to Isotope Research 

Rubidium-strontium data for sample 284-DL-63 

(table Pl), collected from an outcrop of aplite exposed 

about 5 km northwest of the wedge-shaped mass of Tertiary aplite just referred to, yields an apparent age of 28.5 

Ma if it had the same initial strontium isotopic composition as did the intrusive in Young Canyon-Kious basin. 

This 28.5-Ma age indicates that the aplite phase is 

younger and is not related to the main intrusive mass of 

the Young Canyon-Kious basin area. 

0.7170 

0.7160 

eli 

~ 0.7150 

';::;-

~ 0.7140 

0.7130 t:: 37.4 Ma 

r; = 0.71 203±0.00010 

0. 71 1 0 L____L_L__l__jL__l__j _ ___j___j _ ___L___j _ ___L__ 

0 2 3 4 5 6 8 9 10 11 12 

87Rbf86Sr 

Figure P4. Strontium evolution diagram for the intrusive 

rock of the Young Canyon-Kious basin area (fig. P1), 

based on one sample of undeformed rock (open circle) 

and two samples of cataclasized rock (open squares). t, 

time in millions of years; ri, initial 87Sr/86Sr. 

OSCEOLA STOCK 

The area of intrusive rock referred to as the Osceola 

stock (fig. Pl) by Armstrong (1966) has not been studied 

nearly as intensively as has the Snake Creek-Williams 

Canyon pluton, but preliminary work has indicated that 

the two intrusives are similar. Both plutons become more 

felsic from east to west, and both have relatively high 

8180 values (Lee and others, 1982). Rubidium-strontium 

results for the two plutons are similar (table Pl), and the 

points for three samples of the Osceola stock would plot 

close to the Rb-Sr isochron for the Snake Creek-Williams 

Canyon pluton (fig. P3). However, taken alone, the data 

for the Osceola stock (fig. P5) give an age (145 Ma) 

slightly younger than the 155.4±4 Ma determined (fig. 

P3) for the Snake Creek-Williams Canyon pluton. Data 

for the three samples of the Osceola stock give an initial 

87 Sr/86Sr of about 0. 707 5, slightly greater than the 

0. 70714 ± 0.00006 value obtained for the Snake CreekWilliams Canyon pluton. The results of U-Pb (uraniumlead) age work on a zircon fraction recovered from sample 

177-MW-60 (chap. D, this volume) are in fair agreement 

with these Rb-Sr results for the Osceola stock. 
0.721 

0.719 

0.717 

:l> 0.715 00 

";:::-

~ 0.713 00 

0.711 

0.709 

0.707 

0. 705 L.__--L._ __ ...__ _ ____L __ _,__ _ ___, __ __._ __ 

0 2 3 4 5 6 7 

87Rbf86Sr 

Figure PS. Strontium evolution diagram for the Osceola 

stock (fig P1). Sample 295-Dl--64 (open triangle), from 

the Willard Creek mass, is not comagnatic with the Osceola stock (open circles). t, time in millions of years; 

rj, initial 87Sr;B6Sr. 

INTRUSIVE ROCKS OF WILLARD CREEK 

The equigranular two-mica granite of the Willard 

Creek area (fig. P1) has been regarded as a felsic phase 

of the Osceola stock (Lee, Marvin, Stem, and Peterman, 

1970; Lee, Kistler, Friedman, and Van Loenen, 1981; 

Lee, Friedman, and Gleason, 1982). However, Rb-Sr results for sample 295-DL-64 (table P1) show that the Willard Creek mass is not comagmatic with either the Osceola 

stock (fig. P5) or the intrusive rock of Snake Creek-Williams Canyon (fig. P3). Muscovite recovered from sample 

295-DL-64 gave a K-Ar age of 151 ±4 Ma. (Lee, Marvin, Stem, and Peterman, 1970, table 1), similar to the 

Rb-Sr ages determined for the Osceola stock ( ~ 145 Ma) 

and the Snake Creek-Williams Canyon pluton (155.4 ± 4 

Ma). The data in table P1 gave an initial 87Sr/86Sr of 

0.70857 for the Willard Creek mass at an age of 151 

Ma. Although this value is somewhat higher than the initial ratios determined for the intrusive masses of the 

Osceola and Snake Creek-Williams Canyon areas, it is 

appreciably lower than the values (0.7111 to 0.7250) determined for four other two-mica granites in northeastern 

Nevada (Lee, Kistler, Friedman, and Van Loenen, 1981), 

which would indicate that the Willard Creek mass was 

generated from less radiogenic source materials than were 

the other two-mica granites in the area. On the other hand, 

the 8180 value of sample 295-DL-64 was found to be 

13.2 per mil (Lee, Friedman, and Gleason, 1981), and 

such a high value suggests that sedimentary materials 

were an important component in the source region of this 

two-mica granite. 

INTRUSIVE ROCK OF LEXINGTON CREEK 

The map of Whitebread ( 1969) shows the Lexington 

Creek pluton cut by a thrust fault; Lee, and others (1980) 

suggested that the foliation noted by Whitebread and by 

Drewes ( 1958) may have resulted from movement on the 

thrust before the pluton was crystallized completely. The 

original mineralogy of this pluton has been obscured 

somewhat by deuteric alteration that may have accompanied formation of the foliation. However, the unaltered 

intrusive apparently was a two-mica granite in which the 

biotite and muscovite were present as discrete crystals. 

This texture contrasts with that of the Pole Canyon-Can 

Young Canyon pluton, which has biotite euhedra present 

within muscovite phenocrysts. 

Analysis of the zircon recovered from sample 378 

suggests that Lexington Creek pluton is Cretaceous in age 

and was derived at least in part from a sedimentary source 

(chap. D, this volume). Muscovites recovered from three 

samples gave Late Cretaceous K-Ar ages ranging from 

77.9 to 86.3 Ma (Lee and others, 1980). The 86.3-Ma 

age is assumed to be most nearly representative of the 

time of emplacement of this pluton because there may 

have been partial degassing of the analyzed micas as a 

result of heat stress related to late movement along the 

thrust fault that truncates the intrusive. Using 86 Ma as 

the emplacement age of this pluton results in a calculated 

(

87Sr/86Sr) of 0. 7114. 

SEDIMENTARY ROCKS 

Rubidium-strontium data are listed (table P1) for six 

samples of sedimentary rocks collected in the study area. 

Localities, 8180 values, and chemical analyses of these 

rocks are given by Lee and Van Loenen (1971) and Lee 

and others (1982). Although the 87Sr/86Sr of these samples 

range from 0.70911 to 0.80317, most of the clastic rocks 

have ratios of about 0. 8000. Some ratios may have been 

modified during the complex igneous and metamorphic 

history of this area (Lee and others, 1980). 

SUMMARY AND CONCLUSIONS 

Although a great deal of information has been accumulated on the intrusive rocks exposed in the area of 

figure P1 during the last 20 years, it has remained for 

the rubidium-strontium data presented here to resolve 

many of the questions regarding the complex history of 

these granitoid rocks. The magmatic activity of the area 

began with the emplacement of three plutons of Middle 

Jurassic age. These are the equigranular two-mica granite 

of the Willard Creek area and the calc-alkalic intrusives 

of the Osceola and Snake Creek-Williams Canyon areas. 

The calc-alkalic masses of the Osceola and Snake Creek 

areas had initial 87Sr/86Sr of 0. 7075 and 

0.707145 ± .00006, respectively. These are the lowest initial ratios determined for any of the intrusive rocks of 

the southern Snake Range, and they indicate source materials possibly in the lower continental crust (Kistler and 

Peterman, 1973). The initial ratio of the Willard Creek 

Strontium Isotope Composition of Granitoid Rock, Southern Snake Range, Nevada 177 
pluton was somewhat higher at 0. 70857, but this value 

is low compared with those of younger two-mica granites 

of the area. 

After a period of quiescence lasting about 75 m. 

y. intrusive activity resumed with the emplacement of the 

Upper Cretaceous two-mica granites of the Pole ___ CanyonCan Young Canyon and the Lexington Creek areas. 

Although these two-mica granites are only 10 km apart 

and each crystallized from a relatively evolved magma, 

they are different rock types (Lee, Kistler, Friedman, and 

Van Loenen, 1981). For example, the Lexington Creek 

mass contains discrete grains of both muscovite and biotite, whereas all the biotite in the Pole Canyon-Can Young 

Canyon pluton is present as euhedra within large phenocrysts of muscovite. 

The next period of quiescence lasted almost 50 

m. y. and ended with the emplacement of the 37-Ma-old 

calc-alkalic mass of the Young Canyon-Kious basin area, 

and the penecontemporaneous formation of an aplite phase 

in the same general area. These intrusive phases had a 

relatively high initial ratio (

87 Sr/86Sr= 0. 71203). 

Field studies indicate that none of the intrusive masses exposed in the southern Snake Range has been eroded 

to a depth of more than about 300 m. Thus, the tops 

of these intrusives are present at about the same level, 

even though they range in age from Jurassic to Oligocene. 

Furthermore, initial 87Sr/86Sr tend to increase with decreasing age. These observations suggest that the younger 

masses were emplaced at progressively more shallow 

depths, and crystallized from generally more evolved 

magmas. In addition, all the plutons of the southern Snake 

Range are exposed at or beneath the level of the Snake 

Range decollement. Indeed, the Young Canyon-Kious 

Basin and the Lexington Creek intrusive masses were affected by late movement on this structure (Lee, Marvin, 

Stem, and Peterman, 1970; Lee, Marvin, and Mehnert, 

1980). 

There is evidence to indicate that the spatial relationships between the intrusive rocks and the thrust 

faults is not unusual in the eastern Great Basin (chap. 

N, this volume). There also appears to be a confluence 

of regional igneous types in the southern Snake Range. 

We have noted the similarities between the distinctive 

muscovite-phenocrystic two-mica granite of the Pole Canyon-Can Young Canyon area and that of the Kern Mountains, about 83 km to the north. Another similar pluton 

is present farther north in the Toana Range of Elko 

County, Nev., and extends the north-south trend defined 

by these distinctive granites to about 225 km (Lee, Kistler, Friedman, and Van Loenen, 1981). The Middle 

Jurassic calc-alkalic plutons in the Osceola and the Snake 

Creek-Williams Canyon areas are similar in age and petrology to the Notch Peak intrusive mass (169 ± 3 Ma) of 

the House Range in western Utah (chap. D, this volume) 

178 Shorter Contributions to Isotope Research 

and to the southern part of the intrusive (152 Ma) outcrop 

of the Gold Hill, Utah, area (Stacey and Zartman, 1978). 

The middle Tertiary igneous event apparent in the Young 

Canyon-Kious basin area is represented by the Oligocene 

(38 Ma) pluton that makes up the northern part of the 

intrusive outcrop of the Gold Hill, Utah, area (Stacey and 

Zartman, 1978). 

Finally, we note that the sequence of igneous events 

described here is remarkably similar to that found in the 

Ruby Mountains of Elko County, about 200 km northwest 

of the area of figure Pl. Jurassic (160 ± 2 Ma), Cretaceous 

(83 ± 1.3 Ma) and Tertiary (31.9 ± 1.2 Ma) intrusive rocks 

all are present in the Ruby Mountains (Kistler and others, 

1981). 

REFERENCES CITED 

Armstrong, R. L., 1966, K-Ar dating using neutron activation 

for Ar analysis: Geochimica et Cosmochimica Acta, v. 30, 

no. 5, p. 565--600. 

Beckinsale, R. D., 1979, Granite magmatism in the tin belt 

of southeast Asia, in Atherton, M.P., and Tarney, J., eds., 

Origin of granite batholiths; geochemical evidence: Kent, 

United Kingdom, Shiva Publishing, Ltd., 148 p. 

Bowen, N. L., 1937, Recent high-temperature research on silicates and its significance in igneous geology: American 

Journal of Science, v. 33, no. 193, p. 1-21. 

Cain, D., 1974, The determination and interpretation of rare 

earth abundances in hybrid granitoid rocks of the southern 

Snake Range, Nevada: Golden, Colo. Colorado School of 

Mines, M.S. thesis, 94 p. 

Chappell, B. W., and White, A.J.R., 1974, Two contrasting 

granite types: Pacific Geology, v. 8, p. 173-174. 

Drewes, H., 1958, Structural geology of the southern Snake 

Range, Nevada: Geological Society of America Bulletin, v. 

69, no. 2, p. 221-239. 

Furgeson, J., Chappell, B. W., and Goleby, A. B., 1980, 

Granitoids in the Pine Creek geosyncline: Vienna, International Atomic Energy Agency, p. 73-89. 

Hose, R. K., and Blake, M. C., 1976, Geology of White Pine 

County, Nevada: Nevada Bureau of Mines and Geology 

Bulletin 85, pt. 1, p. 1-35. 

Kistler, R. W., Ghent, E. D., and O'Neil, J. R., 1981, Petrogenesis of garnet two-mica granites in the Ruby Mountains, 

Nevada: Journal of Geophysical Research, v. 86, no. Bll, 

p. 10591-10607. 

Kistler, R. W., and Peterman, Z. E., 1973, Variations in Sr, 

Rb, K, Na and initial 87Sr/86Sr in Mesozoic granitic rocks 

and intruded wall rocks in central California: Geological 

Society of America Bulletin, v. 84, p. 3489-3512. 

Lee, D. E., Friedman, 1., and Gleason, J.D., 1981, Map showing the oxygen isotope composition of granitoid rocks of 

the Basin-Range Province: U.S. Geological Survey Miscellaneous Field Studies Map MF-1305, scale 1:3,168,000. 

---1982, The oxygen isotope composition of granitoid and 

sedimentary rocks of the southern Snake Range, Nevada: 

Contributions to Mineralogy and Petrology, v. 79, p. 

150--158. 
Lee, D. E., Kistler, R. W., Friedman, 1., and Van Loenen, 

R. E., 1981, Two-mica granites of northeastern Nevada: 

Journal of Geophysical Research, v. 86, no. B 11, p. 

10607-10616. 

Lee, D. E., Marvin, R. F., and Mehnert, H. H., 1980, A 

radiometric age study of Mesozoic-Cenozoic metamorphism 

in eastern White Pine County, Nevada, and nearby Utah, 

in Geologic studies in White Pine County, Nevada: U.S. 

Geological Survey Professional Paper 1158-C, p. 17-28. 

Lee, D. E., Marvin, R. F., Stem, T. W., and Peterman, Z. 

E., 1970, Modification of potassium-argon ages by Tertiary 

thrusting in the Snake Range, White Pine County, Nevada, 

in Geological Survey research 1970: U.S. Geological Survey Professional Paper 700--D, p. D92-D 102. 

Lee, D. E., Stem, T. W., and Marvin, R. F., 1981, Uraniumthorium-lead isotopic ages of zircon from the southern 

Snake Range, Nevada: Isochron/West, no. 31, p. 25. 

Lee, D. E., Stem, T. W., Mays, R. E., and Van Loenen, R. 

E., 1968, Accessory zircon from granitoid rocks of the 

Mount Wheeler mine area, Nevada, in Geological Survey 

research 1968: U.S. Geological Survey Professional Paper 

600-D, p. D197-D203. 

Lee, D. E., and Van Loenen, R. E., 1970, Biotites from hybrid 

granitoid rocks of the southern Snake Range, Nevada, in 

Geological Survey research 1970: U.S. Geological Survey 

Professional Paper 700--D, p. D196-D206. 

---1971, Hybrid granitoid rocks of the southern Snake 

Range, Nevada: U.S. Geological Survey Professional Paper 

668, 48 p. 

Masi, U., O'Neil, J. R., and Kistler, R. W., 1976, Stable 

isotope systematics in Mesozoic granites near the San Andreas and Garlock fault systems, California [abs.]: Geological Society of America, Abstracts with Programs, v. 8, p. 

998. 

---1981, Stable isotope systematics in Mesozoic granites of 

central and northern California and southwestern Oregon: 

Contributions to Mineralogy and Petrology, v. 76, p. 

115-126. 

Misch, P., and Hazzard, J. C., 1962, Stratigraphy and 

matamorphism of Late Precambrian rocks in central northeastern Nevada and adjacent Utah: American Association 

of Petroleum Geologists Bulletin, v. 46, no. 3., pt. 1, p. 

289-343. 

O'Neil, J. R., and Chappell, B. C., 1977, Oxygen and hydrogen 

isotope relations in the Berridale batholith: Journal of the 

Geological Society of London, v. 133, p. 559-571. 

Stacey, J. S., and Zartman, R. E., 1978, A lead and strontium 

isotopic study of igneous rocks and ores from the Gold Hill 

mining district: Utah Geology, v. 5, p. 1-15. 

Taylor, H. P., Jr., 1968, The oxygen isotope geochemistry of 

igneous rocks: Contributions to Mineralogy and Petrology, 

v. 19, p. 1-71. 

Taylor, H. P., and Silver, L. T., 1978, Oxygen isotope relationships in plutonic igneous rocks of the Peninsula Ranges 

batholith, southern and Baja, California, in Short papers of 

the Fourth International Conference, geochronology, cosmochronology and isotope geology: U.S. Geological Survey 

Open-File Report 78-701, p. 423-426. 

Tuttle, 0. F., and Bowen, N. L., 1958, Origin of granite in 

the light of experimental studies in the system NaA1Si30sKA1Si30s-H20: Geological Society of America Me~oir 74, 

153 p. 

Whitebread, D. H., 1969, Geologic map of the Wheeler Peak 

and Garrison quadrangles, White Pine County, Nevada, and 

Milford County, Utah: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-578, scale 1:48,000. 

York, D., 1969, Least-square fitting of a straight line with correlated errors: Earth and Planetary Science Letters, v. 5, p. 

320-324. 

Strontium Isotope Composition of Granitoid Rock, Southern Snake Range, Nevada 179 

1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

LEAD-ISOTOPE EVIDENCE FOR A PRE-GRENVILLE 

CRUST UNDER THE PIEDMONT OF GEORGIA 

FIGURES 

Chapter Q 

By j. S. STUCKLESS, D. B. WENNER\ and I. T. NKOMO 

Abstract ...... . 

Introduction . . . . . 

Analytical procedures 

Results and discussion . 

Geochronology . . 

Protolith and magmatic 

Summary and conclusions 

References cited . . . . . . 

CONTENTS 

considerations 

Page 

183 

183 

184 

186 

186 

194 

197 

198 

Page 

Q 1. Generalized geologic map showing location of the Elberton Granite batholith, Georgia . . . . . 185 

Q2. Lead-lead diagram of whole-rock and feldspar samples from the Elberton Granite batholith, Georgia . 186 

Q3. Thorium-lead diagram of whole-rock and feldspar samples from the Elberton Granite batholith, Georgia 187 

Q4. Uranium-lead diagram of whole-rock and feldspar samples from the Elberton Granite batholith, Georgia 188 

Q5. Lead-lead diagram for whole-rock samples from the Elberton Granite batholith corrected for the growth 

of radiogenic lead from 320 Ma to the present . . . . . . . . . . . . . . . . . . . . . . . . 190 

Q6. Initial 87Sr/86Sr versus 8180 values for the Elberton Granite batholith with trends for data from the 

Inner Piedmont and Charlotte-Carolina slate belt . . . . . . . . . . . . . . . . . . . . . . . . 191 

Q7. Initial 206PbP04Pb versus initial 208PbP04Pb ratios for whole-rock samples from the Elberton Granite 

batholith, Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 

Q8. Oxygen isotope compositions versus initial 206Pbt204Pb ratios for samples from the Elberton Granite 

batholith, Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 

Q9. Initial isotopic ratios versus reciprocal of concentrations in the systems 206Pbt204Pb and 87Sr/86Sr . 194 

Q10. Concordia diagram for whole-rock samples from the Elberton Granite batholith, Georgia . . 196 

Q11. Plot of 206PbP04Pb versus Th!U for the protolith of the Elberton Granite batholith, Georgia . . 198 

1

Department of Geology, University of Georgia, Athens, GA 30602 

Lead-Isotope Evidence for a Pre-Grenville Crust Under the Piedmont of Georgia 181 
TABLES 

Page 

Q1. Uranium, thorium, and lead concentrations and the isotopic compositiOn of lead for whole-rock and 

potassium feldspar separates from the Elberton Granite batholith, Georgia . . . . . . . . . . . 184 

Q2. Oxygen isotope composition and lead isotope ratios corrected for the growth of radiogenic lead from 

320 Ma to the present . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 

Q3. Thorium-lead, uranium-lead, and thorium-uranium ratios just prior to generation of the granite and at 

present with percent change for the Elberton Granite batholith, Georgia . . . . . . . . . . . . 189 

Q4. Calculated 238UP04Pb of the source regions of some granitic rocks . . . . . . . . . . . . . . . . 197 

Q5. Interpreted features of the protoliths for granites in the Inner Piedmont and Charlotte-Carolina slate 

belts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 

' 

182 Shorter Contributions to Isotope Research 
Abstract 

Isotopic data for the Elberton Granite batholith, 

Georgia, do not yield isochrons in the uranium-lead, 

thorium-lead, or lead-lead systems except for samples 

that have essentially identical 8180 values. Three-point 

isochrons in the uranium-lead and thorium-lead systems 

yield ages that are compatible with those reported for 

zircons. Slopes of feldspar and whole-rock pairs in the 

lead-lead system also are compatible with a 320-millionyear age for crystallization. 

Data in the lead-lead system, corrected to the age 

of intrusion, yield an excellent linear array that is interpreted to be a secondary isochron that retained the age 

of the protolith. This age is interpreted to be 1,720 ± 30 

million years. Although this age is much older than that 

obtained for Grenville-age basement rocks in the southeastern United States, at least three other studies have 

suggested that rocks of this age may exist. 

The Elberton Granite batholith is different from 

many isotopically studied granites in that only one of 

14 whole-rock samples has any evidence of recent uranium mobility in response to near-surface effects. like 

many other granites, the granite from the Elberton 

batholith has a wide range and high mean for thoriumuranium values. However, the data strongly suggest that 

this feature developed at the time of intrusion rather 

than through recent uranium mobilization in the nearsurface environment. 

The protolith for the Elberton batholith is interpreted to be a complex mixture of two heterogenous 

sources consisting of rocks of mafic and intermediate 

composition with the latter having a significant pelitic 

component. Interpretation of the compositions of the 

protoliths presents an enigma in that the presumed 

mafic end member had a higher 206PbF04Pb value at the 

time of granite formation than did the more intermediate 

end member. This anomalous feature may reflect a traceelement difference between the two sources or may 

suggest an early history of metamorphism of the two 

sources that more strongly affected the intermediate 

end-member protolith. These two end-member sources 

appear to be consistent with other geochemical and 

geophysical data that suggest that a mafic protolith underlies both the Charlotte belt and the Carolina slate belt 

and that an intermediate end member underlies the 

Inner Piedmont belt. 

INTRODUCTION 

The Elberton batholith consists uniformly of a finegrained biotite granite that is exposed over an area of 

~500 km2 (square kilometers) in northeastern Georgia. 

This granite body represents one of a series of discrete 

late orogenic granitic plutons that are exposed throughout 

the southern Piedmont province. 

The batholith is intruded along the eastern flank of 

older (the metamorphic age is estimated at ~380-420 Ma 

(million years) by Dallmeyer, 1978) high-grade, 

polydeformed metasedimentary and metavolcanic rocks 

comprising the Inner Piedmont belt (fig. Q1). Immediately 

to the southeast of the batholith, a major, regionally extensive pre-Elberton-age ductile shear zone called the 

Middleton-Lowndesville fault zone, separates the Inner 

Piedmont belt from other major lithotectonic belts, such 

as the Charlotte and Carolina slate belts (Whitney and 

Stormer, 1980). The latter two belts represent a series 

of variably metamorphosed and deformed volcanic, volcaniclastic, and volcanic sedimentary units that may in 

part constitute an ancient protoisland-arc system (Whitney 

and others, 1978). 

The Elberton Granite magma was intruded at an estimated depth of from 12 to 15 km (Stormer and others, 

1980) about 320 Ma (Ross and Bickford, 1980). Subsequently, rapid uplift produced argon retention ages of 

hornblende at ~270 Ma and of biotite at ~240 Ma 

(Dallmeyer and others, 1981). Uplift probably was completed during the Triassic when diabase dikes were intruded and quenched against existing units about 200 Ma. 

The regional pattern of oxygen and strontium 

isotopic compositions of the late orogenic granitic plutons 

of the southern Piedmont (Wenner, 1981) combined with 

seismic (Cook and others, 1979) and gravity (Long, 1979) 

data have been interpreted as indicating that thinning older 

continental crustal material may underlie the Elberton 

batholith (see for example, the tectonic models by Whitney and others, 1980). Accordingly, the Elberton batholith 

essentially may be rooted to its protolith (Ellwood and 

Whitney, 1980), which, according to strontium- and oxygen-isotope data (Wenner, 1981), may consist of gradational subcrustal sequences of continental crustal to 

subcrustal mafic island-arc materials. This suggestion is 

consistent with the earlier reported zircon age of 450 Ma 

(Grunenfelder and Silver, 1958) from the Elberton Granite 

batholith, which may represent an inherited older component. 

Detailed petrological and chemical studies (Ellwood 

and others, 1980; Stormer and others, 1980), as well as 

oxygen and strontium isotopic studies (Wenner, 1981) reveal that the Elberton batholith is essentially homogenous 

in its texture, major-mineral content, and major-element 

chemistry, but is extremely heterogeneous in certain dispersed trace elements (for example, Rb, Sr, Ba) and 180/ 160 and initial 87Sr/86Sr. These trace-element and isotopic 

heterogeneities observed throughout the batholith are interpreted as representing the preservation of magmatically 

produced stripes resulting from viscous laminar flowage. 

Lead-Isotope Evidence for a Pre-Grenville Crust Under the Piedmont of Georgia 183 
Partial fusion of a lithologically variable protolith could 

produce a magma that would be essentially homogeneous 

in its major-element chemistry, yet would be heterogeneous in certain trace elements and isotopic ratios (Whitney 

and others, 1980). 

The variability in initial strontium isotopic compositions noted for the Elberton batholith (Ellwood and others, 

1980; Dallmeyer and others, 1981) has presented difficulties in obtaining accurate Rb-Sr (rubidium-strontium) 

whole-rock ages. Analysis of 13 whole-rock samples, in 

fact, reveals the presence of two generally parallel isochrons (Ellwood and others, 1980). One gives a generally 

consistent age of 350 ± 11 Ma, but this appears to be 

somewhat older than the U-Pb (uranium-lead) zircon intercept age of 320 ± 20 Ma (Ross and Bickford, 1980). Because of apparent problems in the Rb-Sr systematics, the 

zircon age is presumed to be a more accurate indication 

of the crystallization age of the Elberton granite batholith. 

ANALYTICAL PROCEDURES 

Uranium, thorium, and lead concentrations and the 

isotopic composition of lead were determined for 14 

whole-rock and 5 potassium feldspar separates by isotope 

dilution and mass spectrometry (table Q1). Whole-rock 

analyses were made on about 1-g (gram) splits of finely 

pulverized material. Feldspar analyses utilized about 1-g 

of sample and were washed in warm HN03 prior to dissolution. Chemical procedures generally are those described by Tatsumoto and others ( 1972) except that final 

purification of lead was accomplished by electroplating 

(Barnes and others, 1973), and compositions and concentrations were determined on a single dissolution followed 

by liquid aliquoting. 

Mass spectrometric analysis was done on an NBStype (National Bureau of Standards) 12-inch radius-ofcurvature machine. Lead compositions and concentrations 

were obtained by use of the silica gel-H3P04 technique 

and a triple filament mode of ionization. Mass spectrometric results with more than 0.1 percent variation were rejected, and the sample was reanalyzed completely. Results 

reported in table Q1 are precise to ± 0.1 percent (2<T) 

for compositions and ± 0. 2 percent (2<T) for concentrations. 

Regression of best-fit lines was accomplished using 

the methods of York ( 1969). Age calculations for secondary isochrons were made with the equations of Rosholt 

and others (1973). Isotopic and decay constants used are 

those recommended by the International Union of Geological Sciences Subcommission on Geochronology (Steiger 

and Jager, 1977). 

Table Q1. Uranium, thorium, and lead concentrations (in parts per million) and 

the isotopic composition of lead for whole-rock and potassium-feldspar (Kf) samples 

from the Elberton Granite batholith, Georgia 

Concentration Atomic ratio 

Sample No. u Th Pb 20 6Pb/ 204pb 207Pb/204pb 208Pb/ 204pb 

EB-2 5.47 46.23 31.24 18.916 15.668 39.774 

EB-2a 4.57 46.48 28.91 18.786 15.652 39.765 

EB-8 6.96 42.92 29.02 18.896 15.667 39.700 

EB-9 3.84 20.72 22.39 19.031 15.680 39.425 

EB-12 1.95 36.63 27.67 18.543 15.651 39.454 

EB-12Kf 0.33 0.74 58.03 18.425 15.645 38.265 

EW-13 1.52 45.83 25.98 18.526 15.567 40.106 

EW-17 1.91 45.84 25.78 18.570 15.649 40.090 

EW-21 4.35 45.58 34.45 18.420 15.619 39.498 

EW-21a 2.46 35.40 36.51 18.396 15.616 39.100 

EW-21aKf 0.71 1.02 87.15 18.285 15.610 38.564 

EW-23 1.17 50.16 24.69 18.488 15.650 40.273 

EW-23a 2.73 39.91 25.01 18.510 15.636 39.661 

EW-23aK.f 0.12 0.42 59.58 18.333 15.627 38.314 

EW-29 3.34 28.07 21.97 18.996 15.688 39.632 

EW-29a 5.93 21.19 21.45 18.621 15.615 39.073 

EW-29aKf 0.63 1.46 53.68 18.485 15.607 38.234 

EW-42 6.12 34.85 33.35 18.802 15.651 39.391 

184 Shorter Contributions to Isotope Research 
83°15' 83°00' 82°45' 

34°15' r------------------------,-------------,...._--------::., 

0 

Area of 

this figure 

(] () 

0 

0 

0 

10 20 KILOMETERS 

10 MILES 

33°45' '------------4!::....---____________ _l_ _____________________ _J 

Figure Q1. Generalized geologic map (modified from Whitney and Stormer, 1980) showing location of the Elber· 

ton Granite batholith (stippled where exposed), Georgia, the relationship of the granite to the Inner Piedmont 

and Charlotte·Carolina slate belts, and sample localities. Contact dashed where uncertain. 

Lead·lsotope Evidence for a Pre·Grenville Crust Under the Piedmont of Georgia 185 
Oxygen isotope data (table Q2) were obtained following the fluorine extraction procedure described by 

Taylor and Epstein (1962). Analyses were made on a VG 

Micromass2 model 602C modified with a PSC data acquisition and control system, 180/ 160 data are reported 

relative to SMOW (Standard Mean Ocean Water), and 

are calibrated relative to the NBS-28 standard that has 

a defined value of 9.61 per mil (parts per thousand). 

2Use of trade names is for descriptive purposes only and does not 

imply endorsement by the U.S. Geological Survey. 

15.66 

15.62 

15.58 

15.54 

RESULTS AND DISCUSSION 

Geochronology 

The Pb-Pb, Th-Pb, and U-Pb whole-rock systems 

do not yield isochrons within the limits of analytical error 

(figs. Q2, Q3, and Q4). These results are similar to the 

Rb-Sr whole-rock results for which scatter has been attributed to variations in the initial 87 Sr/86Sr (Whitney and 

others, 1980). Four samples that have 11~0 values of 

7.6-7.7 per mil, which apparently are free of contamination from the country rock, yield an Rb-Sr isochron of 

319 ± 26 Ma which is in good agreement with the zircon 

17.9 18.1 18.3 18.5 18.7 18.9 19.1 

206 Pbj204 Pb 

Figure Q2. Lead-lead diagram for whole-rock and feldspar samples (suffix Kf) from the Elberton Granite 

batholith, Georgia. Slopes for two-point isochrons (from top to bottom) correspond to ages of 234, 234, 

373, and 561 Ma. Symbol size corresponds to ±2<T. Prefix letters EB and EW have been omitted from 

sample numbers to save space. 

186 Shorter Contributions to Isotope Research 
age of 320±20 Ma (Ross and Bickford, 1980). Three 

samples that have similar 8180 values of 7.3-7.4 per mil 

yield Th-Pb and U-Pb isochrons of 331 ±46 Ma and 

325 ± 19 Ma, respectively (figs. Q3 and Q4). 

The correspondence of the U-Pb and Th-Pb ages 

is somewhat surprising in view of the well-documented 

mobility of uranium in many other whole-rock granite systems (Rosholt and others, 1973; Manton, 1973; Stuckless 

and Nkomo, 1978; Nkomo and others, 1978; and Stuckless, Bunting, and Nkomo, 1981; Stuckless, Nkomo, and 

Doe, 1981). If the U-Pb and Th-Pb whole-rock systems 

have been closed for all samples, then the lead compositions at the time of crystallization can be calculated. 

..0 

0.. 

~ 

0 

40 

~ 39 0.. 

00 

0 

C\1 

38 

Feldspar data (table Q1) are consistent with results 

reported for potassium feldspars from granites for which 

closed-system behavior for lead can be documented 

(Rosholt and others, 1973; Ludwig and Silver, 1977; 

Stuckless and Nkomo, 1978). Slopes of feldspar and 

whole-rock pairs in the Pb-Pb system (fig. Q2) approximate an age of 320 Ma, but all feldspars (corrected for 

the in-place accumulation of radiogenic lead) are much 

more radiogenic than the calculated initial leads for the 

whole-rock samples (table Q3). Furthermore, feldspar 

whole-rock pairs in the Th-Pb and U-Pb systems (figs. 

Q3 and Q4) do not yield lines whose slopes correspond 

to 320 Ma. 

37 o~--~----~20----~----4~0----~----6~0----~----8L0----~---1~0LO ____ L_ ___ 1~2-0 ____ L_ ___ 1~40----~ 

232Thj204Pb 

Figure QJ. Thorium-lead diagram for whole-rock and feldspar (suffix Kf) samples from the Elberton Granite 

batholith, Georgia. Regressed line (through points EB-2, EW-13, and W-17) has a slope of 0.01652±0.0021 and 

an intercept of 38.129±9,269 (at the 95-percent confidence level) that corresponds to an age of 331 ±46 Ma. 

Symbol size corresponds to ± 2cr. Prefix letters EB and EW have been omitted from sample numbers to save 

space. 

Lead-Isotope Evidence for a Pre-Grenville Crust Under the Piedmont of Georgia 187 
Several studies (for example, Stuckless, Nkomo, 

and Doe, 1981) have suggested that whole-rock systems 

may remain closed with respect to lead because potassium 

feldspar serves as a trap for mobilized radiogenic lead. 

Feldspar from the Elberton Granite batholith is similar to 

other feldspars with a documented excess of radiogenic 

lead (Ludwig and Silver, 1977) in that it contains abundant microinclusions and alteration products. Thus feldspar data for four samples (EB-12, EW-21a, EW-23a, 

and EW-29a) seem to support the conclusion reached for 

three other samples (EB-2, EW-13, and EW-17) that 

lead at least has followed closed-system behavior, on a 

whole-rock scale, from the time of intrusion to the present. Calculation of initial lead compositions at the time 

19.1 

18.9 

18.7 

..0 

0.. 

"'" 0 

N 

-..0 

0.. 

1

/rt" ~ 

0 

N 

12 

L3a 18.5 t/ 13 

23 

t 21 a 

18.3 

L1 

of intrusion on the basis of present-day uranium and lead 

concentrations is therefore reasonable with the caveat that 

some scatter may result from gain or loss of either element. 

Calculated initial 206PbF04Pb and 207Pb/204Pb values 

are colinear within the limits of analytical error (fig. Q5) 

without consideration of errors caused by age assumptions, concentration errors, or open-system behavior by 

either uranium or lead. The resulting line can be interpreted as a mixing line or a secondary isochron. Several 

arguments suggest that even if the linear array is the result 

of mixing of two source regions, the source regions are 

related genetically and thus the slope of the line has age 

significance. 

t 29 

fs 

f 2a 

f 42 

Figure Q4. Uranium-lead diagram for whole-rock and feldspar (suffix Kf) samples from the Elberton Granite batholith, Georgia. Regressed line (through points EW-13, EW-17, and EB-2) has a slope of 

0.051788±0.003054 and an intercept of 18.325±0.023 at the 95-percent confidence level that corresponds 

to an age of 325 ± 19 Ma. Symbol size represents ± 2cr errors. Prefix letters EB and EW have been omitted 

from sample numnbers to save space. 

188 Shorter Contributions to Isotope Research 
Table Q2. Oxygen isotope compositions (in per mil) 

and lead isotope ratios corrected for the growth of 

radiogenic lead from 320 Ma to the present for for 

whole-rock and potassium feldspar (Kf) samples of the 

Elberton Granite batholith, Georgia 

[Leaders ( ---) indicate not analyzed] 

Sample No. 

EB-2 

EB-2a 

EB-8 

EB-9 

EB-12 

EB-12Kf 

EW-13 

EW-17 

EW-21 

EW-21a 

EW-21aKf 

EW-23 

EW-23a 

EW-23aKf 

EW-29 

EW-29a 

EW-29aKf 

EW-42 

7.4 

7.8 

6.6 

8.7 

7.4 

7.3 

8.7 

7.1 

6.9 

7.7 

18.336 

18.263 

18.102 

18.464 

18.312 

18.407 

18.332 

18.325 

18.006 

18.176 

18.259 

18.331 

18.151 

18.327 

18.492 

17.718 

18.447 

18.178 

15.637 

15.624 

15.625 

15.650 

15.639 

15.644 

15.647 

15.636 

15.597 

15.604 

15.609 

15.642 

15.617 

15.627 

15.661 

15.567 

15.605 

15.619 

38.185 

38.042 

38.114 

38.434 

38.046 

38.252 

38.213 

38.182 

38.093 

38.076 

38.552 

38.090 

37.960 

38.307 

38.261 

38.027 

38.206 

38.276 

Derivation of the Elberton Granite batholith through 

the mixing of magma from two geochemically distinct 

sources is highly probable because of its unique location 

along the boundary of two geologic provinces (fig. Q1). 

Whitney and Wenner (1980) and Wenner (1981) have 

summarized data that show chemical and isotopic differences between the post-metamorphic, late-orogenic granites of the Inner Piedmont region and those of the Charlotte-Carolina slate belts. The former yield characteristics 

typical of evolved granites derived from a crustal source. 

Some are strongly peraluminous, all have relatively high 

8180 values' and the few that have been examined have 

initial 87Sr/86Sr values greater than 0.710. Granites within 

the Charlotte-Carolina slate belts generally are metaluminous, have distinctly lower 8180 values and have lower 

initial 87 Sr/86Sr values. The Elberton batholith has characteristics between these extremes as shown for oxygen and 

strontium isotope data on figure Q6. 

If the Elberton batholith formed by mixing of 

magma from two source regions that had a common heritage, the two source regions would have the same common 

lead. Granites derived froni either or both of these source 

regions would retain characteristics of the lead common 

to both source regions. Thus, at the time of intrusion (assuming no contamination for extraneous leads) all lead 

compositions should plot along a line starting at the common or initial lead composition and extending to the lead 

from the domain of highest U/Pb. 

Table QJ. Thorium-lead, uranium-lead, and thorium-uranium just prior to generation of the granite(initial) and at 

present, and percent change for whole-rock samples of the Elberton Granite batholith, Georgia 

232Th/204pb Percent 23Bu/ 204pb Percent Th/U Percent 

Sample No. Initial Present change Initial Present change Initial Present change 

EB-2 39.36 99.59 153.0 10.53 11.40 8.26 3.67 8.45 130.3 

EB--..2a 37.41 108.0 188.7 10.24 10.27 0.29 3.58 10.17 184.1 

EB-8 38.39 99.40 158.9 9.59 15.60 62.7 3.93 6.17 57.0 

EB-9 42.76 62.09 45.2 11.03 11.14 1.0 3.81 5.40 41.7 

EB-12 37.47 88.25 135.5 10.43 4.55 -56.4 3.53 18.78 432.0 

EW-13 39.75 118.6 198.4 10.51 3.81 -63.8 3.71 10.14 173.3 

EW-17 39.32 119.6 204.2 10.48 4.82 -54.0 3.69 24.00 552.2 

EW-21 38.10 88.07 131.2 9.20 8.13 -22.5 4.07 10.48 157.5 

EW-21a 37.38 64.17 69.4 9.89 4.32 -56.3 3.76 14.39 282.7 

EW-23 38.06 136.8 259.4 10.51 3.09 -70.6 3.55 42.87 1108 

EW23a 36.29 106.6 193.7 9.78 7.06 -27.8 3.64 14.62 301.7 

EW-29 40.40 85.93 112.7 11.16 9.89 -11.38 3.55 8.40 136.6 

EW-29a 37.20 65.55 76.2 8.04 17.76 120.9 4.55 3.57 -21.5 

EW-42 40.61 69.84 72.0 9.97 11.86 19.0 4.00 5.69 42.3 

Average 38.79 93.8 142.3 10.10 8.84 -10.8 3.79 13.08 255.5 

1o ±1. 70 ±23.1 ±62.6 ±0.80 ±4.54 ±53.4 ±0.27 ±10.24 ±291.8 

Stacey and 36.84 9.74 3.78 

Kramers 

(1975). 

Lead-Isotope Evidence for a Pre-Grenville Crust Under the Piedmont of Georgia 189 
A line formed by the m1xmg of two unrelated 

sources also would extend from an extreme low value 

to an extreme high value; however, if linearity for the 

leads is perfect, then linearity for other variables should 

be as good except under highly fortuitous circumstances. 

Correlation between initial thorogenic and uranogenic 

leads is good for the Elberton (fig. Q7) but not as good 

as that for the two uranogenic leads (fig. Q4), which implies more than two Th/U relative to Th/Pb or U/Pb in 

the source region. Similarly the correlation between initial 

uranogenic lead composition and oxygen isotope composiCll 

~ 15.62 0 

N 

(") 

1-

<! 

.D 

0.. 

<:!" 

0 

C\1 

........ 

.D 

e- 15.58 0 

C\1 

tions (fig. Q8) or initial strontium composition and oxygen isotope compositions (fig. Q6) implies more than two 

end members in the source region. In fact, data in table 

Q3 show that five different samples represent extreme 

compositions in terms of initial U/Pb, Th/Pb, and Th/U 

compositions and only one of these represents the extreme 

in terms of oxygen isotope composition (table Q2). Furthermore, neither the lead isotope compositions or uranium or thorium concentrations yield geographic patterns 

similar to those observed for certain dispersed trace elements (Stormer and others, 1980). 

206Pbf204Pb AT 320 Ma 

Figure Q5. Lead-lead isochron for whole-rock samples from the Elberton Granite batholith, Georgia, corrected for the growth of radiogenic lead from 320 Ma to the present. Regressed line yields a slope of 

0.11734±0.01951 at the 95-percent level of confidence which corresponds to an age of 1,750±280/350 Ma. 

Symbol size corresponds to ±2rr assuming analytical error only. Prefix letters EB and EW have been omitted 

from sample numbers to save space. 

190 Shorter Contributions to Isotope Research 
A final test of a mtxmg model can be made by 

plotting 206PbP04Pb versus one over the concentrations of 

lead or 87Sr/86Sr versus one over the concentration of 

strontium (Lancelot and Allegre, 1974). Figure Q9 shows 

that neither of these plots is linear, although the lead plot 

yields a better correlation than the one for strontium. Thus 

there is no evidence that a simple mixing of two end 

members can explain the data. 

Variations about trends such as those observed in 

figures Q6, Q7, Q8, and Q9 can be caused by mixing 

of two inhomogenous sources. Similar variations about 

a linear trend would also be expected in the initial leadlead system except for the case where all composition 

11 

10 

8 

7 

Charlotte-Carolina slate belts trend 

6 

0.702 0.704 0.706 0.708 0.710 

evolved from a single common lead. An exact determination of the composition of the common lead is not possible, but a reasonable estimation can be made ·from the 

regressed line for the lead-lead system and the lead-evolution model of Stacey and Kramers (1975). This method 

requires that the approximate age of the common lead 

be known. This age can be estimated by assuming that 

the array on figure Q5 is that of a secondary isochron. 

The slope of this isochron corresponds to an age of 1, 750 

(within an error of either + 280 or -350) Ma. The intersection of the secondary isochron with a chord that intersects the model growth curve of Stacey and Kramers 

(1975) at 3,700 Ma and 1,750 Ma yields the common 

lead composition. 

Inner Piedmont trend 

0.712 0.714 0.716 0.718 0.720 

INITIAL 87Srj86Sr 

Figure Q6. Initial 87Srf6Sr versus &180 values for the Elberton Granite batholith (Whitney and others, 

1980) with trends for data from the Inner Piedmont and Charlotte-Carolina slate belts (Wenner, 1981). 

Square is the average for all Elberton Granite batholith data; solid circles are individual data points. 

Lead-Isotope Evidence for a Pre-Grenville Crust Under the Piedmont of Georgia 191 
If the assumptions that yielded the secondary isochron and common lead composition are correct, the data 

can be reduced to a radiogenic component and a concordia 

diagram can be constructed. The resulting diagram should 

yield a linear array of points (within the limits of analytical error) with a lower intercept equal to the time of intrusion and an upper intercept equal to the time when the 

leads separated from a common source. The predicted 

linear array and lower intercept (290 ± 90 Ma) are observed (fig. QIO), and we conclude that the internal consistencies of the model warrant interpretation of the upper 

intercept age (I, 720 ± 30 Ma) as the age of the source 

region. This age applies to either a single heterogeneous 

source region or two heterogeneous source regions that 

separated from a common source about 1 , 720 Ma ago 

and remained closed with respect to uranium and lead 

mobility until the formation of the granite. 

38.5 T I I 

-

38.3 -

«:l I- ~ 

0 

N 

M 

1- -t <( 38.1 I-- ..0 

0.. 

"" 

~ i. ::c0.. 

00 

0 

N 

37.9 -

I 

The calculated age of the source region seems very 

old relative to reported ages for the Grenville-age basement rocks of the Appalachian and Piedmont regions; 

however, a few studies have suggested that older crustal 

rocks did exist in the area prior to the Grenville orogeny. 

Zircons from crystalline rocks of central Virginia suggest 

a precursor as old as 1,750 Ma (Davis, 1974), and detrital 

zircons from a lower Paleozoic sandstone in Florida yield 

a minimum age of 1,850 Ma (Odom and Brown, 1976). 

Because rocks of such antiquity were unknown in the 

southeastern United States, the authors concluded that the 

source for the zircons was the Guinean shield. Recent 

work on zircons by T. W. Stem (oral commun. , 1981) 

suggests that the basement terrane within the Grandfather 

window of North Carolina also may have a precursor of 

Middle Proterozoic age. 

I I I 

+. -

-t t -

-

i * 

1 

t.+T-' -t -

-

-t. -

37.7~----~'------~'------~'------~~-----L-'-----L-'----~'L-----~'------~'----~ 17.6 17.8 18.0 18.2 18.4 18.6 

206Pbj204Pb AT 320 Ma 

Figure Q7. Initial 206PbP04Pb versus initial 208PbP04Pb for whole-rock samples from the Elberton Granite 

batholith, Georgia. Symbol sizes correspond to ±2a. Prefix letters EB and EW have been omitted from 

sample numbers to save space. 

192 Shorter Contributions to Isotope Research 
Concordia diagrams can be constructed by correcting the data for possible Grenville-age common leads. We 

evaluated the data using both a Grenville-age lead that 

would be co linear with the data array on figure Q5, and 

an average Grenville-age lead as calculated by the Stacey 

and Kramers ( 197 5) oro gene growth curve. Neither model 

provides consequences that are as attractive as those that 

result from the model based on a 1,750-Ma common lead. 

A Grenville-age lead that is colinear with the initial 

lead-lead data can be calculated from the intersection of 

the lead-lead line (fig. Q5) and a line through the Stacey 

and Kramers (1975) values for average common leads at 

3.7 and 1.0 Ga. This point yields slightly less radiogenic 

values e06Pb/204Pb= 17.001 versus 17.066, and 206Pb/ 

204Pb= 15.481 versus 15.509) than that reported for an 

average growth curve. If corrected for the less radiogenic 

18.6 

ro 

::2: 18.4 0 

N 

M 

1-

<( 

.D 

0.. 

"<!" 

0 

N 

::o0.. 18.2 

"' 

~ 

18.0 

Grenville-age lead, concordia intercepts correspond to 

ages of 1, 705 ± 60 and 295 ± 75 Ma with a range in 238U/ 

204Pb of from 6. 45 to 13.4 for the source region of the 

granite. These intercept ages are not markedly different 

from those obtained by use of a 1, 750 Ma common lead, 

but the upper intercept age contradicts the presumed age 

of the source-area common lead. Furthermore, this model 

requires that most samples gain considerable uranium relative to lead at the time of granite genesis, which makes 

interpretation of the high Th/U more difficult. 

Correction of the data for an average Grenville-age 

common lead results in intercept ages of 1,430 ± 140 and 

360 ± 200 Ma. These ages are in fair agreement with the 

known age of intrusion and assumed source age, but again 

the concordia plot shows that most samples would have 

gained a considerable amount of uranium relative to lead. 

t-12 

-+, +.-W-42 

W-23a 

---t;_a 

17.8L-__ _L ____ L_ __ _L ____ L_ __ _L ____ L_ __ _L ____ L_ __ _L ____ L_ __ _L ____ L_ __ -L----L---~ 

6.0 6.4 6.8 7.2 7.6 

1)18Q 

8.0 8.4 8.8 

Figure Q8. Oxygen isotope composition versus initial 206PbP04Pb for whole-rock samples from the Elberton Granite batholith, Georgia. Symbol sizes correspond to ±2a. 

Lead-Isotope Evidence for a Pre-Grenville Crust Under the Piedmont of Georgia 193 
Calculated 238UP04Pb for the source region range from 

5. 86 to 12. 85 and correspond closely to lower and upper 

crustal values ( 5. 89 and 12.24) of Doe and Zartman 

( 1979), but this would require the lower crust to have 

high 8180 and high 87Sr/86Sr values, and the upper crust 

to have low 8180 and low 87Sr/86Sr values. We therefore 

conclude that a Grenville-age source region is inconsistent 

with the data. 

Protolith and Magmatic Considerations 

The Th!U observed for the Elberton Granite 

batholith range from 3.6 to 42.9 and average 13.1 (table 

Q3). Both the range and average are much larger than 

the range of 3 to 5 and average of about 4.5 cited as 

typical for granitic rocks (Rogers and Adams, 1969). In 

other granites, large and variable ratios have been interpreted to be the result of recent uranium loss in response 

to near-surface conditions (Stuckless and Nkomo, 1978). 

The data as modeled (fig. Q10 and table Q3) do not support a similar interpretation for the Elberton Granite 

batholith because open-system behavior such as gain or 

loss of uranium or lead for the whole-rock samples would 

prohibit generation of linear arrays on figures Q5 and 

Q10. 

The data support the hypothesis of Wenner and 

Spaulding (1980) that the large and variable Th!U resulted 

in large part during the magmatic phase of the granite's 

history. These authors pointed out that the granite crystallized under conditions of high oxygen fugacity and in the 

presence of a free vapor phase such that separation of 

hexavalent uranium and tetravalent thorium could take 

place. They also reported that the presumed top of the 

batholith is the least depleted in uranium, and that latestage pegmatites are strongly enriched in uranium relative 

to thorium. 

Exact Th!U within the source region for the Elberton Granite batholith cannot be calculated because the 

common 208PbP04Pb is not known. However, the calculated common 206Pb/204Pb and 207PbP04Pb are very similar to the values reported by Stacey and Kramers (1975) 

for average terrestrial lead, and thus the average terrestrial 

208PbP04Pb at 1 , 720 ± 20 Ma ago probably provides a 

close estimate of that in the source region at 1, 720 Ma 

ago. This assumption and preceding assumptions about 

closed-system behavior permit calculation of sourceregion Th!Pb and Th!U at the time of granite generation 

(table Q3). The calculations show that these ratios, as 

well as the U/Pb, were fairly constant and had mean values that are similar to terrestrial averages proposed by 

Stacey and Kramers (1975). The calculations also show 

a consistant and marked increase in Th!Pb between those 

in the source region and those currently observed in the 

granite, an increase in the Th!U for all but one sample, 

and a general decrease in the U/Pb. 

Confirmation, within the limits of the modeled data. 

that the anomalous Th!U were generated at the time of 

magmatism can be seen on figure Q 10. Displacement of 

data points by the preferential movement of uranium or 

lead will be along a chord that joins concordia at the 

age of disturbance and the position of the data point prior 

to disturbance. Lead gain or uranium loss will move data 

points away from the point on concordia that corresponds 

to the age of disturbance. The pattern on concordia for 

the Elberton Granite is best interpreted in terms of uranium movement because the points that plot above concordia are those with the largest Th!U (the sample with 

the largest Th!U, 42.87, plots furthest above concordia) 

and the points that plot below concordia are those with 

the smallest Th!U (the sample with the smallest Th!U, 

3.57, plots furthest below concordia). 

Figure Q 10 also permits evaluation of the assumed 

immobility of uranium and lead in the recent, near-surface 

history of the granite. All but one data point lies on the 

chord that connects the age of the granite with the age 

of the source region. One data point (21a) lies slightly 

above this chord and may therefore reflect recent uranium 

loss (or lead gain, assuming an appropriate composition 

for the acquired lead). Projection of this point back towards a time of uranium loss of 0 Ma suggests that data 

point 21a should plot near point 21 (fig. Q10). 

FJ!ure Q9 (facin~~age). Initial isotopic ratios versus reciprocal of concentration for the systems 2 PBP04Pb and Sr/86Sr. Strontium data from Whitney and others (1980). Symbols correspond 

to ±2u assuming 1 percent for the strontium concentration error. Sample identification numbers 

for the strontium diagram are from table 1 of Whitney and others (1980). Prefix letters EB and 

EW have been omitted from sample numbers to save space. 

194 Shorter Contributions to Isotope Research 
18.6 I I I I I I I I I I 

f- t 29 -

t9 

18.4 - -

+2 t 12 13tt17 t23 -

co t2a ~ 

0 18.2 -

+21a t 42 - N 

C") t 23a I-

<( 

+a .D -

c. 

~ 

~ 

::o- 18.0 - t 21 c. - 'Cl 

0 

N 

-

17.8 - -

+ 29a -

A 

17.6 I I I I I I I I I l I I 

0.025 0.029 0.03:3 0.037 0.041 0.045 0.049 

1/Pb AT 320 Ma 

0.713 I I I I 

to 

T I I 

f- -

0.711 f-- -

L -

co t J+, ~ 0.709 - - 0 

N -t C") 

I-

+12 

<( -

+2 

-

J5 

'Cl 

00 

';::;- 14 ;[! 0. 707 - -

00 

- +5 t1 -

0.705 -

t5 tii+. -

13++17 +16 

8 

- -

B 

0.703 I I I I I I I I I I I 

0.0020 0.0030 0.0040 0.0050 0.0060 0.0070 0.0080 

1/Sr 

Lead-Isotope Evidence for a Pre-Grenville Crust Under the Piedmont of Georgia 195 
;::) 

00 

M 

N 

::o0.. 

-c 

0 

N 

1.0 

0.8 

0.6 

0.4 

0.2 

4 8 12 16 

207 Pbj23E· U 

Figure Q10. Concordia diagram for whole-rock sam.eles from the Elberton Granite, Georgia. Data are 

corrected for a common lead composition of 206Pb/2 Pb==15.721 and 207PbP04Pb=15.332. Regressed line 

intersects concordia at 1,720 ± 30 and 290 ± 90 Ma at the 95-percent confidence level. Symbol size corresponds to an estimated ±2rr, which includes an error magnification estimate for the choice of commonlead composition. Prefix letters EB and EW have been omitted from sample numbers to save space. 

The isotopic data and the model age of the source 

region provide some constraints on the protolith of the 

Elberton Granite batholith. Table Q3 shows ranges in values for 238UF04Pb, 232ThF04Pb, and Th/U ratios that may 

have existed in the source region prior to generation of 

the granite. This range of values is at the low end of 

the range reported for the source region of other granites 

(table Q4) which is reasonable in that most of the granites 

listed in table Q4 are more evolved in terms of major-element chemistry than the Elberton Granite. 

Published data for the Elberton batholith (Whitney 

and others, 1980) yield a range in initial 87Sr/86Sr ratios 

at 320 Ma from about 0.704 to almost 0.713. Assuming 

an age for the source region of 1750 Ma and an initial 

87Sr/86Sr of 0. 702, calculated 87Rb/86Sr range from 0.10 

to 0.54 or Rb/Sr range from 0.29 to 1.53. These ratios, 

as well as those listed in table Q3, are reasonable for 

mafic to intermediate proto lith materials. 

196 Shorter Contributions to Isotope Research 

Table Q5 lists features for the protoliths for granites 

in the Inner Piedmont and Charlotte-Carolina slate belts 

as they presumably correlate with the parameters measured for the Elberton Granite batholith .. The only anomalous feature of a mixed protolith for the Elberton is the 

negative correlation between initial 206PbF04Pb or initial 

208Pbt204Pb and 8180 or initial 87Sr/86Sr. At least two explanations are possible. The protolith beneath the Charlotte-Carolina slate belt has a component whose trace-elemenlt geochemistry is similar to that of alkali basalts such 

that uranium and thorium are enriched relative to lead, 

but that rubidium is not enriched relative to strontium. 

Altematively, the protolith under the Inner Piedmont belt 

may have been more highly metamorphosed during its 

earli1~st history than that beneath the Charlotte-Carolina 

slate belt such that the U/Pb ratio was reduced greatly. 

A similar mechanism has been proposed to change the 

U/Pb ratio in the source region of other granites (Stacey 

and Zartman, 1978; Doe and Delevaux, 1980, table 4). 
Table Q4. Calculated 238UP04Pb of the source regions of some granitic rocks 

Granite 

23au;204pb age (Ma) Source and (or) identification Reference 

8.04 to 11.16 320 Elberton granite batholith, eastern Georgia-------------- This study. 

7.4 2,600 Icarus pluton, northeastern Minnesota-------------------- Arth and Hanson (1975). 

7.84 to 8.12 2,950 Granite gneiss of the Pilbara Block, western Australia--- Oversby (1975). 

11.64 2,670 Owl Creek Mountains, Wyoming----------------------------- Nkomo and others (1978). 

11.47 2,350 Mount Boreas type granite, western Australia------------- Stuckless, Bunting, and Nkomo (1981). 

11.26 2,675 Syenitic granite, western Australia---------------------- Do. 

12.01 2,600 Granite Mountains, Wyoming------------------------------- Stuckless and Nkomo (1978). 

112.7, 7.4 38 Northern Gold ~ill, Utah--------------------------------- Stacey and Zartman (1978). 

111.7, 12.1 152 Southern Gold Hill, Utah--------------------------------- Do. 

110.3, (9.0 1,850 Section 28 granite, Minnesota---------------------------- Doe and Delevaux (1980). 

114.5, 4.5 2,600 Sacred Heart granite, Minnesota-------------------------- Do. 

1Reported as a two-stage ratio, value for earliest stage listed first. 

Oxygen, strontium, and major-element data for the 

postmetamorphic granites strongly suggest that the protolith for the Inner Piedmont granites had a large pelitic 

component; therefore, an explanation for low U/Pb by 

uranium loss is attractive. Furthermore, the data for the 

Elberton Granite batholith have two features that have 

been observed in other areas for which a lowering of the 

U/Pb by metamorphism has been proposed. In the Gold 

Hill plutons, Utah, there is a negative correlation between 

initial (

87Sr/86Sr)i and e06PbF04Pb)i (Stacey and Zartman, 

1978). For the Section 28 granite, Minnesota, Doe and 

Delevaux (1980) noted both a decrease in the U/Pb and 

an increase in the Th/U as a result of metamorphism. 

Data for the Elberton show a negative correlation between 

initial Th/U and initial 206Pb/204Pb (fig. Qll) that could 

be explained by a preferential loss of uranium from one 

source region shortly after it evolved. 

SUMMARY AND CONCLUSIONS 

Data presented in this study support the hypothesis 

that the Elberton Granite batholith formed by the partial 

melting of two distinct protoliths (Stormer and others, 

1980; Wenner, 1981) and further suggest that these protoliths were at least heterogeneous with respect to several 

radioelements. Several isotopic parameters are correlated 

with each other, particularly if corrected to their values 

at the time of intrusion. This suggests mixing of two end 

members. 

Table QS. Interpreted features of the protoliths for 

granites in the Inner Piedmont and Charlotte-Carolina 

Slate belts 

Parameter 

Inner 

Piedmont belt 

Charlotte-Carolina 

slate belts 

Molar Al/(Na+K+Ca)------- High--------------- Low. 

Mafic content------------ Low---------------- High. 

0180--------------------- High--------------- Low. 

Initial 87sr; 86sr-------- High--------------- Low. 

Initial 20 6Pb/20 4Pb------ Low---------------- High. 

Initial 208pbf20 4Pb------ Low---------------- High. 

Initial Th/U------------- High--------------- Low. 

The Elberton magma did not mix isotopically at the 

time of intrusion. As a result of this unique magmatic 

history, the granite preserved the age of the source region. 

None of the correlated isotopic parameters, including 

isotopic ratios relative to the reciprocal of concentrations 

(such as 206PbP04Pb versus 1/Pb, Lancelot and Allegre, 

1974) are colinear within the limits of experimental error 

except for 206PbP04Pb and 207PbP04 This fact can be 

explained by the mixing of two heterogeneous source regions that had the same common lead at the time the 

source regions formed. With this assumption, the lead 

data can be interpreted to yield an age for the protolith 

of 1,720±30 Ma. 

Lead-Isotope Evidence for a Pre-Grenville Crust Under the Piedmont of Georgia 197 
ctl 

~ 18.2 0 

N 

M 

1-

<{ 

..0 

0... 

'<:!" 

0 

N 

::c0... 

~ 18.0 0 

N 

INITIAL Th/U 

Figure Q11. Plot of 206PbF04Pb versus Th/U for the protolith of the Elberton Granite batholith, Georgia 

at the time of granite formation. Symbol size corresponds to ±2rr. Prefix letters EB and EW have been 

omitted from sample numbers to save space. 

The Elberton is similar to granites that have been 

identified as favorable source rocks for uranium (for example Stuckless and Nkomo, 1978) in that Th/U for the 

granite have a wide range of values and an anomolously 

large mean value. However, for granites that are associated with low-temperature uranium deposits, the large 

range and mean are thought to have originated by uranium 

loss in the near-surface environment. Isotopic data for the 

Elberton Granite indicate that the anomalous Th/U were 

generated as part of the magmatic process and that uranium has been remarkably immobile since the time of 

intrusion. This conclusion is supported by the enrichment. 

of uranium in the upper parts of the intrusion and the 

existence of uraniferous pegmatites. Thus, the high Th/U 

probably are due to uranium loss, but this loss took place 

at the same time as the loss of volatiles from the magma. 

198 Shorter Contributions to Isotope Research 

REFERENCES CITED 

Arth, J. G., and Hanson, G. N., 1975, Geochemistry and origin 

of the early Precambrian crust of northeastern Minnesota: 

Geochimica et Cosmochimica Acta, v. 39, p. 325-362. 

Barnes, I. L., Murphy, T. J., Gramlich, J. W., and Shields, 

W. R., 1973, Lead separation by anodic deposition and 

isotope ratio mass spectrometry of microgram and smaller 

samples: Analytical Chemistry, v. 45, p. 1881-1884. 

Cook, F. A., Albaugh, D. S., Brown, L. D., Kaufman, S., 

Oliver, J. E., and Hatcher, R. D., Jr., 1979, Thin skinned 

tectonics in the crystalline Southern Appalachians; COCORP 

seismic-reflection profiling of the Blue Ridge and Piedmont: 

Geology, v. 7, p. 563-567. 

Davis, R. G. , 197 4, Pre-Grenville ages of basement rocks in 

central Virginia; A model for interpretation of zircon ages: 

Blacksburg, Va. Virginia Polytechnic Institute, M.A. thesis, 

46 p. 
Dallmeyer, R. D., 1978, 4() ArP9 Ar incremental release ages of 

hornblende and biotite across the Georgia Inner Piedmont; 

Their bearing on late Paleozoic-early Mesozoic tectonothermal history: American Journal of Science, v. 278, p. 

124-149. 

Dallmeyer, R. D., Hess, J. R., and Whitney, J. A., 1981, Postmagmatic cooling of the Elberton Granite; Bearing on the 

late Paleozoic tectonothermal history of the Georgia Inner 

Piedmont: Journal of Geology, v. 89, p. 585-600. 

Doe, B. R., and Zartman, R. E., 1979, Plumbotectonics I, The 

Phanerozoic, in H. Barnes, ed., Geochemistry of hydrothermal ore deposits: New York, John Wiley and Sons, p. 

22-70 

Doe, B. R., and Delevaux, M. H., 1980, Lead-isotope investigations in the Minnesota River Valley-Late-tectonic and 

post-tectonic granites: Geological Society of America Special Paper 182, p. 105-112. 

Ellwood, B. B., Whitney, J. A., Wenner, D. B., Mose, D., 

and Amerigan, C., 1980, Age, paleomagnetism, and tectonic significance of the Elberton Granite, northeast Georgia 

Piedmont: Journal of Geophysical Research, v. 85, p. 

6521-6533. 

Ellwood, B. B., and Whitney, J. A., 1980, Magnetic fabric 

of the Elberton Granite: Journal of Geophysical Research, 

V. 85, p. 1481-1486. 

Grunenfelder, M., and Silver, L. T., 1958, Radioactive age dating and its petrological implications for some Georgia granites [abs.]: Geological Society of America Bulletin, v. 69, 

p. 1574. 

Lancelot, J. R., and Allegre, C. J., 1974, Origin of carbonatitic 

magma in the light of the Pb-U-Th isotope system: Earth 

and Planetary Science Letters, v. 22, p. 233-238. 

Long, L. T., 1979, The Carolina slate belt-Evidence of a continental rift zone: Geology, v. 7, p. 180-184. 

Ludwig, K. R., and Silver, L. T., 1977, Lead isotope inhomogeneities in Precambrian igneous K-feldspars: 

Geochimica et Cosmochimica Acta, v. 41, p. 1457-1471. 

Manton, W. 1., 1973, Whole rock Th-Pb age for the Masuke 

and Dembe-Dwula complexes, Rhodesia: Earth and Planetary Science Letters, v. 19, p. 83-89. 

Nkomo, I. T., Stuckless, J. S., Thaden, R. E., and Rosholt, 

J. N., 1978, Petrology and uranium mobility of an early 

Precambrian granite from the Owl Creek Mountains: Wyoming Geological Association 30th Annual Field Conference 

Guidebook, p. 335-348. 

Odom, A. L., and Brown, J. F., 1976, Detrital zircons from 

Ordovician sediments in Florida [abs.]: Geological Society 

of America Abstracts with Programs, v. 8, p. 237-238. 

Oversby, V. M., 1975, Isotopic ages and geochemistry of Archean acid igneous rock for the Pilbara, Western Australia: 

Geochimica et Cosmochimica Acta, v. 40, p. 817-829. 

Rogers, J.J.W., and Adams, J.A.S., 1969, Thorium, in 

Wedepohl, K. H., ed., Handbook of geochemistry, v. 12, 

no. 4: Berlin, Springer-Verlag, p. 90-1 to 90-0. 

Rosholt, J. N., Zartman, R. E., and Nkomo, I. T., 1973, Lead 

isotope systematics and uranium depletion in the Granite 

Mountains, Wyoming: Geological Society of America Bulletin, v. 84, p. 989-1002. 

Ross, C. R., II, and Bickford, M. E., 1980, The U-Pb age 

of zircons from the Elberton granite, Piedmont of Georgia, 

in Stormer, J. C., Jr., and Whitney, J. A., eds., Guidebook 

19, Geological, geochemical, and geophysical studies of the 

Elberton batholith, eastern Georgia: A guidebook to accompany the 15th annual Georgia Geological Society field trip, 

p. 52-62. 

Stacey, J. S., and Kramers, J. S., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth 

and Planetary Science Letters, v. 26, p. 207-221. 

Stacey, J. S., and Zartman, R. E., 1978, A lead and strontium 

isotopic study of igneous rocks and ores from the Gold Hill 

Mining District, Utah: Utah Geology, v. 5, p. 1-15. 

Steiger, R. H., and Jager, E., 1977, Subcommission on geochronology; Convention on the use of decay constants in geoand cosmochronology: Earth and Planetary Science Letters, 

v. 36, p. 359-362. 

Stormer, J. C., Jr., Whitney, J. A., and Hess, J. R., 1980, 

Petrology and geochemistry of the Elberton Granite, in 

Stormer, J. C., Jr., and Whitney, J. A. eds., Guidebook 

19, Geological, geochemical and geophysical studies of the 

Elberton batholith, eastern Georgia: A guidebook to accompany the 15th annual Georgia Geological Society field trip, 

p. 10-30. 

Stuckless, J. S., Bunting, J. A., and Nkomo, I. T., 1981, U-ThPb systematics of some granitoids from the northeastern Yilgam Block, Western Australia and implications for uranium 

source rock potential: Journal of the Geological Society of 

Australia, v. 28, p. 365-375. 

Stuckless, J. S., and Nkomo, I. T., 1978, Uranium-lead isotope 

systematics in uraniferous alkali-rich granites from the Granite Mountains, Wyoming; implications for uranium source 

rocks: Economic Geology, v. 43, no. 3, p. 427-441. 

Stuckless, J. S., Nkomo, I. T., and Doe, B. R., 1981, U-Th-Pb 

systematics in hydrothermally altered granites from the 

Granite Mountains, Wyoming: Geochimica et Cosmochimica Acta, v. 45, p. 635-645. 

Tatsumoto, M., Knight, R. J., and Delevaux, M. H., 1972, 

Uranium, thorium, and lead concentrations in three silicate 

standards and a method of lead isotopic analysis, in Geological Survey research 1972: U.S. Geological Survey Professional Paper 800-D, p. 111-115. 

Taylor, H. P., Jr., and Epstein, S., 1962, Relationship between 

o16;0 16 ratios in coexisting minerals of igneous and 

metamorphic rocks part I; Principles and experimental results: Geological Society of America Bulletin, v. 73, p. 

461-480. 

Lead-Isotope Evidence for a Pre-Grenville Crust Under the Piedmont of Georgia 199 
Wenner, D. B., 1981, Oxygen isotopic compositions of the late 

orogenic granites in the southern Piedmont of the Appalachian Mountains, U.S. A. , and their relationship to subcrustal 

structures and lithologies: Earth and Planetary Science Letters, v. 54, p. 186-199. 

Wenner, D. B., and Spaulding, J.D., 1980, U-Th geochemistry 

of the Elberton pluton, in Stormer, J. C., Jr., and Whitney, 

J. A. , eds. , Guidebook 19, Geological, geochemical, and 

geophysical studies of the Elberton batholith, eastern Georgia: A guidebook to accompany the 15th annual Georgia 

Geological Society field trip, p. 44-51. 

Whitney, J. A., Hess, J. R., and Mose, D., 1980, Geochronology and cooling history of the Elberton Granite, in Stormer, 

J. C., Jr., and Whitney, J. A., eds., Guidebook 19, Geological, geochemical, and geophysical studies of the Elberton 

batholith, eastern Georgia: A guidebook to accompany the 

15th annual Georgia Geological Society field trip, p. 63-69. 

200 Shorter Contributions to Isotope Research 

Whitney, J. A., Paris, T. A., Carpenter, R. H., and Hartley, 

M. T., III, 1978, Volcanic evolution of the southern slate 

belt of Georgia and South Carolina; A primitive oceanic 

island arc: Journal of Geology, v. 86, p. 173-192. 

Whitney, J. A., and Stormer, J. C., Jr., 1980, Road log for 

the 1980 Georgia Geological Society field trip, in Stormer, 

J. C., Jr., and Whitney, J. A., eds., Guidebook 19, Geological, geochemical, and geophysical studies of the Elberton 

batholith, eastern Georgia: A guidebook to accompany the 

15th annual Georgia Geologial Society field trip, p. 

108-134. 

Whitney, J. A., and Wenner, D. B., 1980, Petrology and structural setting of post-metamorphic granites of Georgia, in 

Frey, R. W., ed., Excursions in southeastern geology: 

American Geological Institute, v. 2, p. 351-378. 

York, Derek, 1969, Least-squares fitting of a straight line with 

correlated errors: Earth and Planetary Science Letters, v. 

5, p. 320-324. 
FIGURES 

1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

SOIL-GAS HELIUM DISTRIBUTION AT THE 

SHADOW MOUNTAIN COLLAPSE, 

CAMERON URANIUM DISTRICT, ARIZONA 

Chapter R 

By C. G. BOWLES, G. M. REIMER, j. M. BEEN, and D. G. MURREY 

CONTENTS 

Abstract .... 

Introduction . . 

Geologic setting 

Equipment and sampling procedure 

Helium distribution 

Discussion . . . 

Conclusion . . . 

References cited 

Page 

202 

202 

203 

207 

209 

210 

211 

211 

Page 

Rl. Index map showing location of helium study area, Cameron, Ariz. . . . . . . . . . . . . . . . . 202 

R2. Map showing geology of the helium study area, Shadow Mountain Collapse, Cameron uranium district, 

Arizona . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 

R3. Map showing topography and location of sample sites at the Shadow Mountain collapse 206 

R4. Diagrammatic section showing stratigraphy at the Shadow Mountain collapse . . . . . . . 207 

R5. Map showing high and low ratios of yellow-green reflectance to infrared (IR) radiation, Cameron uranium 

district . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 

R6. Histogram showing frequency distribution of soil-gas helium in 82 samples from the Shadow Mountain 

collapse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 

R7. Map showing distribution of soil-gas helium at Shadow Mountain collapse . . . . . . . . . . . . 210 

Soil-Gas Helium Distribution, Shadow Mountain Collapse, Arizona 201 
Abstract 

Soil-gas helium at the Shadow Mountain collapse 

ranged from 9 to 288 ppb (parts per billion) helium (with 

respect to air) with a mean helium concentration of 90 

ppb. High helium values (110-180 ppb He) are distributed in a circular or annular pattern around the margin 

of the collapse. The pattern suggests a structural control 

for helium and its uranium parent analogous to the ringfracture control for uranium ore deposits in breccia 

pipes of the Grand Canyon. The soil-gas helium 

anomalies provide favorable targets for uranium exploration. 

® 

INTRODUCTION 

A soil-gas helium survey of the Shadow Mountain 

collapse in the northern part of the Cameron uranium district of Arizona (fig. R 1) was made on October 17, 1980. 

The purpose of the survey was to determine whether or 

not helium detection methods can contribute to the evaluation of uranium potential within the district's collapse 

structures. 

Helium detection is a promising technique for the 

discovery of deeply buried uranium deposits; however9 

many factors can alter the concentration and distribution 

1W30' 

35°30' L_ __ ..l__ _________ ___j( ____ _L_ _____________ ----"(IL_-----------'-----' 

20 40 KILOMETERS 

20 MILES 

Figure Rl. Index map showing location of helium study area, Cameron, Ariz. 

202 Shorter Contributions to Isotope Research 
of the helium. Helium is formed from alpha particles 

emitted during radioactive disintegration of the uranium 

and thorium decay series. Helium released by the rocks 

may be dissolved in the ground water, or it may migrate 

as a free gas. An upward leakage of free-gas helium to 

the soil zone is facilitated by the permeability of fractured 

and brecciated rock. For this reason a soil-gas helium survey may detect deeply buried uranium deposits within collapse structures. 

Past speculation on the potential for economic veintype uranium deposits within the Shadow Mountain collapse and other structures in the Cameron district was 

stimulated by the production of uranium and copper from 

the Riverview breccia pipe within the district. In addition, 

uranium and copper have been produced from the Orphan 

breccia pipe and other pipes of the Grand Canyon district 

to the west. Kerr (1958) called attention to uranium 

mineralization in the Shadow Mountain collapse, and later 

Barrington and Kerr (1963, p. 1254) reported that "outcrops within the collapse area display anomalous radioactivity, and [that] subeconomic uranium mineralization was 

found in drilling." A small uranium deposit in the 

Shinarump Member of the Chinle Formation of Triassic 

age is located near the north margin of the collapse. The 

deposit produced about 25 tons of "no pay ore" (D. N. 

Magleby, written commun., 1960). In addition, Barrington and Kerr (1963) reported radioactivity anomalies 

associated with the western fault of a graben that extends 

northward from Shadow Mountain toward the collapse. 

GEOLOGIC SETTING 

The Shadow Mountain collapse is in the outcrop 

of the lowermost Shinarump Member of the Chinle Formation of Triassic age (fig. R2). The overlying Petrified 

Forest Member crops out less than 1 km (kilometer) east 

of the collapse, and an outlier of this member forms the 

rim of the collapse structure and dips inward at 5°-18° 

(Cooley and others, 1969). Outside the collapse, the 

Chinle Formation dips 2°--4° eastward. The collapse forms 

a topographic bowl measuring 2.5 km across (fig. R3). 

The rim of the bowl rises as much as 35 m (meters) above 

the nearly flat inner floor. Intermittent streams flow eastward to southeastward across the structure through gaps 

incised into the west and north sides of the bowl and 

then merge to exit as a single stream at the southeast 

side. Alluvium of Quaternary age covers the bottom of 

the bowl as well as the drainages that dissect the rim. 

Horizontally bedded gravels of Quaternary(?) age rest unconformably upon a segment of the rim at the west side 

of the structure (Barrington and Kerr, 1963). These 

gravels were deposited in an earlier course of a stream 

that now flows through a gap in the rim, about 1 ,000 

m to the south. During deposition of the gravels the topographic bowl of the collapse was occupied by strata of 

the Petrified Forest Member. The member is more resistant to erosion where it underlies the course of the ancestral stream. This fact and the abundance of fine-grained 

interstitial and veinlet calcite, described by Barrington and 

Kerr (1963), suggest that calcite cementation took place 

where bicarbonate- and carbon dioxide-bearing solutions 

may have ascended through the collapse structure to discharge into the drainage of the paleostream. 

The vertical extent of the collapse structure is not 

known . Subsurface information within the collapse is limited to that obtained from a well drilled to a depth of 

96 m. The drill hole penetrated conglomerate of the 

Shinarump Member at depths ranging from 76 to 91 m. 

The overlying Petrified Forest Member, which is draped 

into the collapse, may have subsided as much as 120 m 

at the center of the structure. A collapse of this magnitude 

requires that the structure extend at least into the Kaibab 

Limestone of Permian age, the highest stratigraphic unit 

(fig. R4) in which dissolution of rock could be on a scale 

necessary for the accommodation of the large volume of 

displaced rock. Within the buried 1,1 00--m-thick section 

of sedimentary rock underlying the Chinle are other carbonate strata, and they also may have been dissolved (fig. 

R4). These strata are present in the Muav Limestone of 

Cambrian age, the Temple Butte Limestone of Devonian 

age, the Redwall Limestone of Mississippian age, the 

Supai Group of Permian and Pennsylvanian age, and the 

Toroweap Formation of Permian age. 

The Shadow Mountain collapse is at the north end 

of a graben, a structural relationship that suggests a genetic relationship. Within the Cameron district, faults and 

grabens are common in the vicinity of collapse structures 

and breccia pipes. The collapse structures are not present 

within the graben blocks; instead, the collapses are in adjacent fault blocks where solutions ascending along sympathetic tensional faults and fractures flow away from the 

graben, or the collapses are at the ends of the grabens. 

Both the breccia pipe at the Riverview uranium mine 

(Haines and Bowles, 1976) and the Shadow Mountain collapse are located where fractures in the monoclinal flexures at the ends of the grabens might readily transmit 

ascending solutions. The graben associated with the 

Shadow Mountain collapse extends southward 5 km to 

the Shadow Mountain volcanic cinder cone (fig. R2). This 

graben is as much as 390 m wide and its faults have 

a minimum throw of 20 m, as indicated by the topographic relief above the alluvium-covered downdropped 

block (Condit, 1974). The graben may have formed, at 

least in . part, as a result of crustal dilation accompanying 

the development of a deep-seated magma chamber. According to Condit (1974), the graben formed before eruption of a lava flow that has been dated by Damon and 

others (1974) as Quaternary in age. Structural adjustments 

at the graben occurred penecontemporaneously with, or 

later than, this eruption. The northernmost basaltic cinder 

Soil-Gas Helium Distribution, Shadow Mountain Collapse, Arizona 203 
1W30' 

36°07'30" 

5' 

2'30' 

55' 

lies X 

X X 

4 KILOMETERS 

Figure R2 (above and facing page). Map showing geology of the Helium study area, 

Shadow Mountain collapse, Cameron uranium district. Geology modified from 

Chenoweth and Magleby (1960, revised 1971) and Haynes and Hackman (1978). 

204 Shorter Contributions to Isotope Research 
>-

a: 

<( 

z 

ffi 

t-

~ 0 

EXPLANATION 

ALLUVIUM 

GRAVEL 

EXTRUSIVE IGNEOUS ROCKS-Lava and cinders 

DINOSAUR CANYON MEMBER OF MOENAVE 

FORMATION 

~ CHINLE FORMATION 

~ OWL ROCK MEMBER (/) 

~ ~~ PETRIFIED FOREST MEMBER 

SHINARUMP MEMBER 

MOENKOPI FORMATION 

z 

~ { 0 KAIBAB LIMESTONE 

Q... 

---GEOLOGIC CONTACT-Dashed where approximately 

located 

-....,.--FAULT-Dashed where approximately located; bar and 

ball on downthrown side 

_!!!__ STRIKE AND DIP OF BEDS 

~ URANIUM MINE 

X URANIUM PROSPECT 

cones and associated dikes and flows of the Shadow 

Mountain volcanic field form a north-trending ridge that 

is less than 1 km south-southwest of the collapse and that 

is 750 m west of the graben. The proximity of the Shadow 

Mountain eruptive center to the collapse and the existence 

of an interconnective graben suggest that hydrothermal 

solutions probably have flowed within the collapse structure. 

The Shadow Mountain collapse is considered to 

have formed as a result of the dissolution of carbonate 

rock by either hydrothermal or meteoric ground water, 

although at one time the collapse was thought to be a 

diatreme (Kerr, 1958). The diatreme hypothesis was replaced by a hydrothermal solution hypothesis by Barrington and Kerr (1963), who believed that all collapses 

in the Cameron district resulted from hydrothermal dissolution of carbonate rocks. As just discussed here, the 

hydrothermal solution origin for the Shadow Mountain 

collapse draws strong support from the proximity of the 

collapse to the Shadow Mountain eruptive center of late 

Tertiary(?) and Quaternary age. However, the possibility 

exists that the initial collapse at Shadow Mountain may 

have resulted from limestone dissolution by meteoric 

ground water before the commencement of volcanic activity in the area. 

Hydrologic conditions favorable for the formation 

of solution collapse structures existed prior to volcanism 

at the Shadow Mountain eruptive center. By late Miocene(?) or early Pliocene time, renewed uplift of the 

Kaibab Plateau had blocked the flow of an ancestral river 

into the Grand Canyon (Hunt, 1969). Unpublished data 

by Bowles suggests that a lake formed in the upper part 

of Marble Canyon and recharged the Redwall Limestone 

and Coconino Sandstone aquifers. According to this 

hypothesis, the ground water migrated toward the Little 

Colorado River and, driven by artesian pressure, ascended 

along faults and fractures to discharge into the surface 

drainage system. In the Cameron uranium district, limestone dissolution by ground water ascending from the 

Coconino Sandstone through the Toroweap Formation and 

Kaibab Limestone probably was instrumental in the formation of many collapse structures, including the Shadow 

Mountain collapse. This stage of solution and collapse 

possibly overlapped the late Pliocene(?) to Pleistocene 

episodic volcanic and hydrothermal activity at Shadow 

Mountain. 

Even though we favor our hypothesis for the origin 

of the Shadow Mountain collapse, we recognize a possibility that solution and collapse may have commenced in 

the more deeply buried Paleozoic rocks. The possible 

deeper origin for the Shadow Mountain collapse is supported by evidence from the Lookout collapse of the 

Cameron district (Barrington and Kerr, 1963). Ring fractures of the Lookout structure are present within the 

Coconino Sandstone; therefore the structure must bottom 

lower in the stratigraphic section, probably within the 

Redwall Limestone where dissolution and collapse is common (Bowles, 1977). 

Even if the Shadow Mountain collapse is a deepseated structure, uranium deposits comparable to those at 

the Orphan mine are not likely to be present at depth 

within the Supai Group. The proximity of the Shadow 

Mountain collapse to the graben suggests a structural control for dissolution and collapse, which also implies a late 

Cenozoic age for the collapse structure. If this implication 

is true, hydrologic and hydrochemical conditions at the 

Shadow Mountain structure differed from those present 

at the Orphan breccia pipe when Late Jurassic uranium 

mineralization took place in the pipe structure within the 

Supai Group (Gomitz and Kerr, 1970). 

If uranium deposits are present at the Shadow 

Mountain collapse, their origin probably is related to the 

mineralization of the Cameron uranium district. The origin 

and age of primary uranium in the Chinle Formation is 

uncertain. Tetravalent uranium is present as uraninite associated with pyrite and marcasite in carbonaceous logs, 

indicating that a reducing environment effected mineralization and preservation of the deposits (Austin, 1964). 

According to Austin (1964, p. 89), acid and sulfate solutions formed by the oxidation of pyrite and marcasite were 

responsible "for the bleaching of the clay, the general yellowish tone of the ore zones, the alteration of montmorillonite to illite or kaolinite, and the solution of uranium 

from primary uraninite in fossil logs and its movement 

to disseminated secondary deposits." Uranium has been 

redistributed as recently as the Pleistocene. 

The bleaching and alteration associated with the ore 

deposits of the Cameron district have effected color 

Soil-Gas Helium Distribution, Shadow Mountain Collapse, Arizona 205 
Base from US Geo logical Survey 

Moa Ave SW. 1955, 124.000 

1 

EXPLANATION 

4 KILOMETERS 

2 MILES 

• Soil-gas sample; helium amount 

above air, in parts per billion 

Figure R3. Map showing topography and location of sample sites at the Shadow Mountain collapse, Cameron uranium district, Arizona. 

changes in a belt that trends north-south through the district. Landsat (ERTS-1) data prepared by Spirakis and 

Condit ( 1975) revealed significant differences in the reflectance of yellow-green light from the ground surface. 

Figure R5, derived from a stretch-ratioed false-color 

image (Spirakis and Condit, 1975), shows a zone characterized by relatively low reflectance of yellow-green light 

as compared with infrared (IR), and to the east a zone 

characterized by relatively high reflectance of yellowgreen light. On the image, the Shadow Mountain collapse 

is within the zone of low yellow-green reflectance; however, the image shows some high yellow-green reflectance 

at the margin of the collapse. Uranium deposits generally 

are within the eastern zone of high yellow-green reflectance. This zonation is suggestive of alteration associated 

206 Shorter Contributions to Isotope Research 

with the downdip migration of a redox front that has progressed eastward across the Shadow Mountain study area. 

A transitory enrichment of uranium in the Chinle 

Formation would be expected during the advance of a 

redox front. Local retention of the uranium during the 

ensuing encroachment of oxygen-bearing ground water 

would be possible under favorable geologic and hydrologic conditions. The transmission of reducing gases and 

hydrothermal solutions of volcanic origin by the Shadow 

Mountain collapse structure could create a favorable site 

for the reduction, precipitation, and preservation of uranium in the Chinle Formation. 

In summary, field evidence in the Cameron district 

suggests that solution collapse at many of the structures, 

including the Shadow Mountain collapse, is of mid- to 
QUATERNARY 

TRIASSIC 

1 ln. = 200 meters 

METERS 

0 

100 

200 

PERMIAN 

PENNSYLVANIAN 

MISSISSIPPIAN 

DEVONIAN 

CAMBRIAN 

0 

LU 

(j) 

0 

c.. 

X 

LU 

0 

LU 

a: 

:::> 

Cil 

ALLUVIAL DEPOSITS-Silt. sand and gravel 

CHINLE FORMATION-Petrified Forest Member: Siltstone. mudstone, and lenticular sandstones and conglomerate. Shinarump Member: Conglomerate, sandstone and mudstone 

MOENKOPI FORMATION-Sandstone and siltstone. minor gypsim (90 meters thick) 

KAIBAB LIMESTONE- Limestone with abundant chert molecules and sandy dolomite (130 meters thick) 

TOROWEAP FORMATION-Cherty limestone. crossbedded and gnarly-bedded sandstone and siltstone (1 0 

meters th ick) 

COCONINO SANDSTONE-Crossbedded eolian sandstone (200 meters thick) 

HERMIT SHALE-Shale, siltstone, and lenticular sandstone (30 meters thick) 

SUPAI GROUP-Sandstone, siltstone. 'limestone, and mudstone (240 meters thick) 

REDWALL LIMESTONE- Limestone and dolomite (140 meters thick) 

TEMPLE BUTIE LIMESTONE-In channels (1 0 meters thick) 

MUAV LIMESTONE- Thin-bedded limestone containing interbedded mudstone and siltstone (90 meters thick) 

BRIGHT ANGEL SHALE-Shaly mudstone and sparse sandstone (140 meters thick) 

TAPEATS SANDSTONE-Sandstone. coarse to medium grained (60 meters thick) 

Figure R4. Diagrammatic section showing stratigraphy at the Shadow Mountain collapse, Arizona. 

Soil gas at a 1-m depth was tapped by hollow 

late-Cenozoic age. Collapse in these structures probably 

is limited to formations above the Coconino Sandstone, 

but the structures may extend downward to the Redwall 

Limestone. Strata of the Shinarump and Petrified Forest 

Members of the Chinle Formation are the most favorable 

hosts for uranium deposits within the Shadow Mountain 

collapse, but a reducing environment favorable for uranium deposition may have existed throughout the vertical 

extent of the structure during volcanic eruptions at 

Shadow Mountain; therefore, uranium may be present in 

deeply buried rocks of Paleozoic age. 

EQUIPMENT AND SAMPLING PROCEDURE 

probes equipped with rubber septum holders, and the gas 

was collected in 1 O-cm3 (cubic centimeters) hypodermic 

syringes. Soil-gas samples within the collapse and the 

nearby graben were taken at 0.2-mile (0.32-km) intervals 

from sites located by vehicle odometer distances and topographic control. Later the same day, seven additional samples were obtained from sites spaced at 0 .1-mile 

(0.16-km) intervals. These additional samples were collected in the south part of the collapse in order to obtain 

greater sample control near one sampling site at which 

Soil-Gas Helium Distribution, Shadow Mountain Collapse, Arizona 207 
111 °30' 

36°07'30" 

5' 

2'30' 

55' 

lies 

208 Shorter Contributions to Isotope Research 

~ 

X 

X X 

0 
a high concentration of soil-gas helium was detected. 

Samples also were collected outside the collapse to determine the regional helium background. The sampling sites 

for determination of the background helium concentration 

were located at 0.5-mile (0.8-km) intervals along two 

2-mile-(3. 2-km-) long traverses extending east and northwest of the collapse. 

The dissolved-helium concentration in ground water 

within the alluvium was determined for one water sample 

obtained from a shallow well in the southeast part of the 

collapse structure. The water was collected in a 1-liter 

plastic bottle that had a rubber septum fitted into a needle 

guide that, in tum, was incorporated into the bottle cap. 

The bottle was filled three-fourths full with the sample 

water, leaving the remaining space occupied by air. The 

bottle was then capped and vigorously shaken for 30 seconds to degas the water. After allowing the bottle to stand 

for 2 minutes, 10 cm3 of air was injected from a hypodermic syringe into the bottle to insure a positive pressure, 

thus enabling withdrawal of a gas sample into the syringe. 

>-

a: 

< z 

a: 

~ 

~ 

EXPLANATION 

HIGH RATIO-Yellow-green reflectance, infrared radiation 

LOW RATIO-Yellow-green reflectance, infrared radiation 

REFLECTANCE IMAGE- Density of picture element 

(pixel) for high ratios band 4/band 7 indicates relative intensity of yellow-green light 

ALLUVIUM 

GRAVEL 

EXTRUSIVE IGNEOUS ROCKS- Lava and cinders 

DINOSAUR CANYON MEMBER OF MOENAVE 

FORMATION 

~ CHINLE FORMATION 

~ OWL ROCK MEMBER 

"' 

~ ~s PETRIFIED FOREST MEMBER 

SHINARUMP MEMBER 

MOENKOPI FORMATION 

z 

~ { ~ KAIBAB LIMESTONE 

~ ----GEOLOGIC CONTACT-Dashed where approximately 

located 

--.,..--FAULT-Dashed where approximately located; bar and 

ball on downthrown side 

_!f!_ STRIKE AND DIP OF BEDS 

'X' URANIUM MINE 

URANIUM PROSPECT 

25 

~ 20 _J 

a... 

~ 

;75 15 

u... 

0 

ffi 10 OJ 

~ 

:::> 

z 5 

10 30 50 70 90 11 0 130 150 

HELIUM, IN PARTS PER BILLION 

170 

~ 

>170 

Figure R6. Frequency distribution of soil-gas helium in 

82 samples from the Shadow Mountain collapse, 

Arizona. No samples contained less than 0 ppb helium 

with respect to ambient air. 

The analytical equipment used for helium analysis 

was the mobile helium analyzer developed by the U.S. 

Geological Survey. This unit consists of a small mass 

spectrometer mounted in a four-wheel-drive, four-door 

pickup truck (Reimer, 1976). The equipment can make 

a helium analysis in the field with a sensitivity of 10 

ppb (parts per billion) and with equal precision. 

Helium analyses of the soil-gas samples are reported 

for convenience as parts per billion (ppb) of helium with 

respect to air that contains 5,240 ppb helium (Glueckauf, 

1946). Helium analysis of the water sample is reported 

in cm3 He/cm3 H20. Water in equilibrium with air at 

standard temperature and pressure contains 45 x 1 o-9 cm3 

He/cm3 H20. 

HELIUM DISTRIBUTION 

The helium concentration in 82 soil-gas samples 

collected at or near the Shadow Mountain collapse ranged 

from 9 to 288 ppb, and the mean helium concentration 

for all samples was 90 ppb. A frequency distribution (fig. 

R6) for the helium samples is positively skewed; however, 

about one-fourth of the samples are fairly uniformly distributed among the four class intervals that have less than 

70 ppb He. The helium background for the area, as determined from the 14 samples collected away from the strucfrom a reconnaissance study of helium in soil gas in the 

Powder River Basin (Reimer and others, 1980). The average helium soil-gas concentraion for ~838 samples collected for that study averaged 0 ppb. 

Figure R5 (above and facing page). Map showing high and low ratios of yellow-green reflectance to infrared 

(IR) radiation in the northern part of the Cameron uranium district. Reflectance data modified from stretchedratioed false-color images (L-1, formerly ERTS-1, band 4/band 7 and band 7/band 4) by Spirakis and Condit 

(1975, fig. R6). Geology modified from Chenoweth and Magleby (1960, revised 1971), and Haynes and Hackman 

(1978). 

Soil-Gas Helium Distribution, Shadow Mountain Collapse, Arizona 209 
EXPLANATION 

Map Helium above air, in 

No. parts per bi l lion 

0-9 

10-29 

30-49 

50-69 

70-89 

90-109 

110-129 

130-149 

150-169 

170-189 22 

2 2 22, 

· ·~ 

4 KILOMETERS 

2 MILES 

Figure R7. Map showing distribution of soil-gas helium at Shadow Mountain collapse, Arizona. Contours 

dashed where inferred. 

Soil-gas helium is present in greater than average 

concentrations around the periphery of the structure and 

within its south half (fig. R7). The highest concentration 

of soil-gas helium (288 ppb He) was detected in a sample 

from the south part of the collapse, but similarly high 

helium concentrations were not present in seven additional 

samples collected on a grid pattern from nearby sites 

spaced 0.1 mile apart. A northeasterly trend appears on 

the helium distribution within the structure. Low concentrations of helium-less than 60 ppb He-are common 

just outside the structure, and similarly low helium concentrations also are present in four samples from within 

the graben that is south of the collapse. Ground water 

collected from a shallow well in the southeast part of the 

collapse contained 85x 10-9

cm3He/cm3H20 , which is not 

unusual. This weakly alkaline water (7. 3 pH) had 9-10 

ppb uranium in solution. 

210 Shorter Contributions to Isotope Research 

DISCUSSION 

The high helium background in soil-gas samples 

collected away from the Shadow Mountain collapse probably is derived from uranium deposited during an episode 

of mineralization that took place in the Chinle Formation 

throughout the Cameron uranium district. This uranium 

may be present in the hexavalent state in secondary minerals formed within the oxidizing environment behind the 

advancing redox front. 

At the Shadow Mountain collapse, the distribution 

of soil-gas helium suggests a structural control for uranium mineralization. High helium concentrations at the 

margin of the collapse form a ring-shaped pattern similar 

to that of the distribution of uranium minerals at the margin of breccia pipes in the Grand Canyon district. In addition, within the south half of the collapse is a northeast-
erly trending zone in which helium concentration in the 

soil is high. A computer-derived plot (fig. R7) (Reimer 

and Dean, 1979) of the soil-gas helium distribution in 

this zone resembles an irregular oval-shaped ring that 

shares a common segment with the helium anomalies at 

the south margin of the structure. This helium distribution 

suggests a possible analogy with the small secondary collapses that are present within the breccia pipe at the Orphan mine where ring fractures associated with the 

secondary structures commonly are mineralized with uranium. Solution collapse at the Shadow Mountain structure 

may have been more extensive and persistent at the south 

and southwest parts of the structure where steeper dips 

( 18°) were mapped at the margin of the collapse by 

Cooley and others (1969). However, this suggestion 

should be considered only as a working hypothesis. The 

ring-shaped patterns for the high helium concentrations 

are suggestive of either direct or indirect fracture control 

for the helium distribution. Fractures may control the escape of gaseous helium, or the fractures may have controlled uranium mineralization that now generates the helium. 

Extremely low concentrations of helium in soils 

sampled just outside the structure may reflect the leaching 

of uranium by an acidic solution. Such an acidic solution 

may have formed at the margin of the structure by the 

mixing of oxygenated ground water in the Chinle Formation with sulfurous hydrothermal waters transmitted 

through the collapse. 

Helium values are low for samples of soil gas collected within the graben south of the collapse. The low 

values are not inconsistent with the uranium that is present 

along the west fault of the graben reported by Barrington 

and Kerr (1963). Parallel normal faults and fractures at 

the margins of the grabens form conduits to carry ascending solutions and gases away from the graben fault blocks. 

As a result, a favorable environment for uranium mineralization could have existed along faults and fractures along 

the west margin of the graben where a downdip flow of 

uranium-bearing ground water might have first come in 

contact with reductants in escaping gases or hydrothermal 

solutions and then precipitated uranium. 

CONCLUSION 

Geologic considerations, including the mineralization of the Chinle Formation throughout the Cameron district, suggest that the high concentrations of helium detected by the survey probably are derived from uranium 

deposits in the Chinle. The high concentrations of soil-gas 

helium at the south and west margin of the collapse and 

within the south half of the structure indicate the most 

favorable target areas for uranium exploration. The helium 

data alone resolve neither the question of the possible 

mineralization of rocks underlying the Kaibab Limestone, 

nor the question of the effectiveness of a soil-gas survey 

for detecting deeply buried deposits within a collapse 

structure. Deep exploratory drilling, preferably within the 

favorable target areas outlined by the helium survey, 

might resolve both questions. The helium survey does reveal the outline of the collapse structure and may be a 

useful technique in locating buried, eroded, or otherwise 

unrecognizable collapse structures. 

REFERENCES CITED 

Austin, S. R., 1964, Mineralogy of the Cameron area, Coconino 

County, Arizona: U.S. Atomic Energy Commission Report 

RME-99, 99 p. 

Barrington, J., and Kerr, P. F., 1963, Collapse features and 

silica plugs near Cameron, Arizona: Geological Society of 

America Bulletin, v. 74, no. 10, p. 1237-1258. 

Bowles, C. G., 1977, Economic implications of a new 

hypothesis of origin of uranium- and copper-bearing breccia 

pipes, Grand Canyon, Arizona, in Short papers of the U.S 

Geological Survey uranium-thorium symposium, 1977: U.S. 

Geological Survey Circular 753, p. 25-27. 

Chenoweth, W. L., and Magleby, D. N., 1960, revised 1971 , 

Mine location map, Cameron uranium area, Coconino 

County, Arizona: U.S. Atomic Energy Commission Preliminary Map 20. 

Condit, C. D., 1974, Geology of Shadow Mountain, Arizona, 

in Geology of northern Arizona with notes on archaeology 

and paleoclimate; Part 11-Area studies and field guides: 

Geological Society of America, Rocky Mountain section 

meeting, Flagstaff, Arizona, 1974, p. 454--463. 

Cooley, M. E., Harshbarger, J. W., Akers, J. P., and Hardt, 

W. F., 1969, Regional hydrogeology of the Navajo and 

Hopi Indian Reservations, Arizona, New Mexico, and Utah: 

U.S. Geological Survey Professional Paper 521-A, 61 p. 

Damon, P. E., Shafiqullah, M., and Leventhal, J. S., 1974, 

K-Ar chronology for the San Francisco volcanic field and 

rate of erosion of the Little Colorado River, in Geology 

of northern Arizona with notes on archaeology and paleoclimate; Part I-Regional studies: Geological Society of 

America, Rocky Mountain section meeting, Flagstaff, 

Arizona, 1974, p. 221-235. 

Glueckauf, E., 1946, A microanalysis of helium and neon contents in air: London, Proceedings of the Royal Society, v. 

185, p. 98-119. 

Gornitz, V., and Kerr, P. F., 1970, Uranium mineralization and 

alteration, Orphan mine, Grand Canyon, Arizona: Economic 

Geology, v. 65, no. 7, p. 751-768. 

Haines, D. V., and Bowles, C. G., 1976, Preliminary geologic 

map and sections of the Wupatki NE quadrangle, Coconino 

County, Arizona: U.S. Geological Survey Open-File Report 

76-703. 

Soil-Gas Helium Distribution, Shadow Mountain Collapse, Arizona 211 
Haynes, D. D., and Hackman, R. J., 1978, Geology, structure, 

and uranium deposits of the Marble Canyon 1 ox 2° quadrangle, Arizona: U.S. Geological Survey Miscellaneous Investigations Map 1-1003. 

Hunt, C. B., 1969, Geologic history of the Colorado River: 

U.S. Geological Survey Professional Paper 669-C, p. 

59-130. 

Kerr, P. F., 1958, Uranium emplacement in the Colorado 

Plateau: Geological Society of America Bulletin, v. 69, no. 

9, p. 1075-1112. 

Reimer, G. M., 1976, Design and assembly of a portable helium 

detector for evaluation as a uranium exploration instrument: 

U.S . Geological Survey Open-File Report 76-398, 18 p. 

212 Shorter Contributions to Isotope Research 

Reimer, G. M., and Dean, V. C., 1979, Computer program 

for the Hewlett-Packard-9825A to contour and plot data 

collected from helium surveys: U.S. Geological Survey 

Open-File Report 79-865, 15 p. 

Reimer, G. M., Murrey, D. G., and Been, J. M., 1980, Helium 

soil-gas concentrations in the Torrington, Newcastle, Gillette, and Ekalaka 1°X2° quadrangles; Data from a reconnaissance survey: U.S. Geological Survey Open-File Report 

80-452. 

Spirakis, C. S., and Condit, C. D., 1975, Preliminary report 

on the use of LANDSAT-I (ERTS-1) reflectance data in 

locating alteration zones associated with uranium mineralization near Cameron, Arizona: U.S. Geological Survey OpenFile Report 75-416, 20 p. 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

AN AUTOMATED HELIUM ANALYSIS STATION 

FIGURES 

ChapterS 

By IRVING FRIEDMAN, JOE JURCEKA, WILLIS DOERING, 

WILLIAM LONG, and DON McNAIR 

Abstract ..... 

Introduction . . . 

Acknowledgments 

Description 

Sampling shower 

Sampling system 

Analytical system 

CONTENTS 

Page 

214 

214 

214 

214 

214 

214 

216 

Page 

S 1. Photograph showing underground enclosure on the Miller property . 214 

S2. Diagram of sampling shower . . . . . . . . . . . . . . . . . . . . 215 

S3. Photograph of interior of enclosure showing shower . . . . . . . . 215 

S4. Plot of the helium reading versus time after an abrupt change in the helium abundance of water entering 

the sample shower . . . . . . . . . . . . . . . . 215 

S5. Diagram of automated sample inlet system . . . . . . . . . . . . 216 

S6. Photograph showing automated sample inlet system . . . . . . . 216 

S7. Photograph showing system, mass spectrometer and data recorder 217 

S8. Example of a typical recorder chart . . . . . . . . . . . . . . . 217 

S9. Comparison of the data plots secured by three different techniques 218 

An Automated Helium Analysis Station 213 
Abstract 

An automated device for the analysis of helium in 

well water has operated with minimum maintenance at 

a remote site for more than 5 years. This equipment 

makes an analysis once an hour and telemeters the data 

every 3 hours via satellite. A comparison of the telemetered data with data recorded onsite and with data secured by the analysis in Denver, Colo., of water samples 

collected daily from the well shows the three sets of data 

to be compatible. 

INTRODUCTION 

One of the objectives of the helium sniffer earthquake prediction program was to develop an automated 

station that could sample, analyze, and telemeter data, 

and that would operate with a minimum of supervision. 

The following description of the "automated Miller well" 

is of a system that successfully has extracted and analyzed 

helium from water for 5 years at a site remote from 

sophisticated facilities with infrequent (every 2-3 months) 

visits by technical personnel. Maintenance of components 

has been limited to occasional replacement of the mass 

spectrometer filament and replenishment of rotary pump 

oil by Mr. Miller and his family, the owner of the property, and by an annual cleanup of some of the valves 

that have become contaminated by the corrosive gases 

given off by the well waters. For the first year the data 

were recorded by a chart recorder at the site. At the beginning of the second year this recording mode was supplemented by telemetering of the data via GOES satellite 

to the receiving station in Wallops Island, Va. and then 

by phone line to Denver, Colo. 

The 192-foot (58-meter) well is located on the Miller property 5 .1 miles (8. 2 kilometers) north of Gardiner, 

Figure 51. Underground enclosure on the Miller 

property. 

214 Shorter Contributions to Isotope Research 

Mont. on highway 89. An enclosure was constructed over 

the well to protect the monitoring equipment (fig. S 1). 

ACKNOWLEDGMENTS 

We are particularly indebted to Mr. and Mrs. 

Richard Miller for the use of their well, for their kind 

permission to construct an enclosure on their property, 

and for their continued . help and hospitality. They, together with Mauray and Paul Miller were of great help 

in observing instrument performance, making repairs 

when necessary, collecting daily water samples for analysis in Denver, and, in general taking a continued interest 

in the success of the work. 

The telemeter system, as well as the solid-state timing system, was constructed by Don McNair, aided by 

Willis Doering who also was responsible for the analysis 

of samples in Denver. The shower was constructed by 

William Long. Joe Jurceka was responsible for the construction of the sampling system. 

Mr. Ted Denton was instrumental in first-model design and development and was the initiator of the constant-pressure device. His help in the initial stages of the 

project are gratefully acknowledged. 

DESCRIPTION 

Sampling Shower 

The gas sample that is analyzed is secured by spraying the well water continuously into a small closed space. 

The gas in the water equilibrates with the air in the enclosure, and it is this equilibrated air that is sampled once 

an hour for its helium content. Figure S2 is a sketch of 

the sampling "shower"; figure S3 is a photograph of the 

shower. 

The shower enclosure is vented to ambient air 

through a small opening. The well is pumped continuously at 1 V2 gallons/minute by a downhole pump located 

180 ft from the ground surface, and about 170 ft from 

the water surface. The response time of the sampling 

shower was measured by changing the inflow water to 

a source quite low in helium, allowing the system to reach 

a steady state, and then changing the inflow again to water 

with a high helium content. Figure S4 is a plot of the 

response of the system and shows that the system will 

respond to a change in helium abundance of the input 

water with a response time (63 percent of the final value) 

of 3.6 hours. 

Sampling System 

The valving system for gas sampling shown in figures S5 and S6 uses inexpensive solenoid-operated stainless steel valves for switching gases. Once an hour a small 
Sample gasoutlet 

Shower 

head 

"0"-ring 

seal 

-75 centimeter high 

X1 5 centimeter diameter 

Figure 52. Diagram of sampling shower. 

sample of the gas in the shower is pumped by a stainless 

steel bellows pump into a small volume. This volume is 

then opened to the mass-spectrometer inlet system through 

Figure 53. Interior of enclosure showing shower (tall cylinder in center of photograph). 

(f) 

a: 

:::> 

0 

::r: 

z 

10 

1.0 

010 

0 

I 

0 

I 

0 

I 

0 

63 pe<Oent; t;me 3.6 hou"/ 

/0 

0 

0.1L-----~L-----~------~------~------~ 

0 w ~ w w 100 

HELIUM RESPONSE (PERCENT FINAL VALUE) 

Figure 54. Plot of the helium reading versus time after 

an abrupt change in the helium abundance of water entering the sample shower. 

an adjustable leak valve that allows a small amount of 

gas to flow into the mass spectrometer. In order to maintain a constant pressure of the gas flowing into the spectrometer, a vertically mounted glass hypodermic syringe 

is connected to the system. The gas pressure is sufficient 

to raise the plunger of the syringe, and the weight of 

the plunger in the almost frictionless barrel maintains a 

constant pressure on the gas flowing into the spectrometer. The ingenious glass-syringe constant-pressure device 

was suggested by Mr. Ted Denton. Gas leakage between 

the plunger and barrel of the unlubricated syringe is small 

and causes no problem. 

An Automated Helium Analysis Station 215 
To high-pressure gas 

cylinder regulator valve 

.---___JL----, Diaphragm Pressure-reducing 

pump valve 

Glass 

hypodermic 

syringe 

(Normally open) 

To mechanical 

vacuum pump 

Adjustible 

leak valve 

Figure 55. Diagram of automated sample inlet system. See figure 56 for approximate scale. 

Figure 56. Automated sample inlet system. 

Analytical System 

The gas that flows past the variable leak valve 

(Granville Phillips, model 203) flows into a Dupont In216 Shorter Contributions to Isotope Research 

struments model 24-120B helium leak detector 1

• The only 

modifications that have been made to this instrument are 

( 1) the installation of a fuse in the primary of the Penning 

gauge high-voltage transformer, and (2) the installation 

of a time-delay relay in the primary of the Penning gauge 

and the filament current circuits. The time-delay modification is necessary because power failures in the area are 

frequent, usually several per week and often as frequent 

as five per day. The power interruptions last from seconds 

to hours. When the power is off for more than a few 

minutes, the vacuum in the mass spectrometer is degraded 

by the gas continuously leaking by the glass syringe in 

the sample system, or by the fact that the power interruption may take place at a time when gas is being admitted 

into the spectrometer during a sample cycle. When the 

power is restored, normally the vacuum gauge and filament are actuated immediately, resulting in a burned-out 

filament and a high current flow in the vacuum gauge 

circuit and overheating of the Penning gauge transformer. 

The installation of a time-delay circuit of about 15 minutes between the time that power is restored (and the vacuum pumps restarted) and the time the Penning gauge 

and filament circuits are actuated has taken care of the 

problem. 

The equipment is shown in figures S3, S6, and S7. 

1

Use of trade and company names is for descriptive purposes only 

and does not imply endorsement by the U. S. Geological Survey. 
Figure 57. Inlet system, mass spectrometers and data 

recorder. 

The output of the mass spectrometer amplifier is 

40 volts above ground. We have introduced an isolation 

circuit so that the output from the instrument is referred 

to ground before it is fed into the telemetering platform 

for the GOES satellite transmitter. 

The LeBarge convertible data-collection platform 

interrogates the spectrometer, and stores the data. It transmits the stored data to the satellite every 3 hours, during 

an assigned time slot. In order to keep alive the timing 

and other program functions during power interruptions, 

the platform is powered by a lead-acid storage battery 

that is charged continuously from the power line. Power 

;----fAir 

0 0 .1 0.2 0 .3 0.5 0.6 

interruptions of as many as several days can be accommodated by this system. 

In order to calibrate the signal produced by the sample gas flowing into the mass spectrometer, we allow two 

reference gases (air and air spiked with helium) to flow 

into the spectrometer shortly before and after the sample 

gas. The sampling cycle takes 12 minutes and is as follows. 

1. Sample system evacuated. 

2. Sample system filled with reference gas from a cylinder (air spiked with helium). 

3. Reference gas opened to spectrometer. 

4. Reference gas pumped out of sample system. 

5. Sample gas pumped into sample system from 

shower. 

6. Sample gas opened to spectrometer and analyzed. 

7. Sample gas pumped out of system. 

8. Reference gas again admitted to system. 

9. Reference gas again analyzed. 

10. Reference gas pumped out. 

11. Air pumped into sample system. 

12. Air pumped opened to spectrometer and analyzed. 

13. Air pumped out. 

14. System evacuated and kept under vacuum until next 

cycle in 48 minutes. 

The sample gas is bracketed by samples of the reference gas and the whole cycle completed by an analysis 

of air, (5.24 part per million helium). An example of 

the recorder chart is given in figure S8. 

Figure S9 is a comparison of the data as read from 

the onsite recorder chart, the satellite telemetered data, 

and the data secured by the analysis of water samples 

collected daily by the Millers and mailed to the U.S. 

Geological Survey laboratory in Denver, Colo., for analysis. All three techniques yield comparable data. 

0.7 0.8 0.9 1.0 

Figure 58. An example of a typical recorder chart. 

An Automated Helium Analysis Station 217 
a: 

UJ 200 I-

:::i 

:::i 

....J 

~ 

a: 

UJ 

a.. 

z 

0 

:::i 

....J 

~ 100 a: 

UJ 

a.. 

(f) 

Ia: 

<( 

a.. 

z 

~ 

::J 

:::i 

UJ 0 I 

200 

0 

0 

X 

z 

0 

:::i 

::::! 

~ 

a: 

UJ 

a.. 100 en 

Ia: 

<( 

a.. 

~ 

~ 

::J 

:::i 

UJ 

I 

0 

1 38 74 

Figure 59. 

111 

MILLER STATION 300 

Water samples collected daily and 

shipped to Denver for analysis 

147 184 

1980 

220 257 293 

Comparison of the data plots secured by three different techniques. 

218 Shorter Contributions to Isotope Research 

330 366 
1986 

U.S. GEOLOGICAL SURVEY BULLETIN 1622 

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

CONSTRAINTS ON TIME-EFFICIENT DATA-TAKING 

STRATEGIES FOR SINGLE-COLLECTOR, 

ISOTOPE-RATIO MASS SPECTROMETERS 

Chapter T 

By KENNETH R. LUDWIG 

CONTENTS 

Page 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 

Precision of isotope ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 

Real-beam considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 

Time-Efficient Data-Taking Strategies for Single-Collector, Isotope-Ratio Mass Spectrometers 219 
Abstract 

To a reasonable approximation, considerations of 

ion-counting statistics and dark noise lead to simple algorithms for calculating the most time-efficient allocation 

of peak-top and background integration times, where 

the goal is defined as obtaining the most precise data 

in a given amount of time. 

INTRODUCTION 

In single-collector, isotope-ratio mass spectrometry, 

decisions regarding the proper allocation of integration 

times for different isotopes, and for the ratio of 

background integration time to peak-top integration time, 

generally have been made either by a relatively inflexible 

computer algorithm or by the operator, using semi-intuitive decision procedures. Such procedures may not be unreasonable for relatively unstable ion-beams, but for the 

highly stable beams that are now attained by most laboratories for many elements, will not in general yield the 

most precise possible data for a given amount of datataking time. 

PRECISION OF ISOTOPE RATIOS 

In the following discussion, I will assume a datataking pattern where n cycles of peak switching and peaktop integration are performed over each of I isotopes, and 

the resulting background-corrected matrix is corrected in 

some interpolative manner to obtain estimates of the ratios 

of the I isotopes. Background data are assumed to be collected for each block of n peak-switching cycles, and 

taken either at independent positions in the mass spectrum 

for each isotope (background-type 1), or at one common 

position in the mass spectrum and applied to all peaks 

(background-type 2). Three types of ion-beam fluctuation 

will be considered. 

Type I.-Instability arising from the process of 

ionization itself; that is, an unstable beam due to factors 

intrinsic to the sample load. Accurately interpolable variations (for example, linear decay) do not concern us, however. 

Type 2.-Noise that is independent of the ionbeam and constant in magnitude (that is, "dark noise"; 

for Faraday cup collectors, arising mainly from thermal 

noise in the feedback resistor). 

Type 3.-Noise due to the finite number of ions 

arriving at the collector; thus giving counting-statistics 

limitations. 

Uncertainty for perfectly stable beams: 

The predicted uncertainty in the average intensity 

of a particular peak during a block of n cycles is then 

given by 

220 Shorter Contributions to Isotope Research 

I;=IOOVA+B+C (1) 

where I;=the uncertainty at one standard deviation, in 

percent, of the average peak intensity of peak i. 

A=fractional variance1 due to dark noise on the peak= 

B=fractional variance1 due to background uncertainty for 

ith peak= 

C =fractional variance 1 due to counting statistics= 

I 

SnP;T; 

N=dark noise, in amperes/second 

n=number of peak-switching cycles 

P;=ion-beam intensity for ith peak, in amperes 

T;=integration time for ith peak per peak-switching cycle 

Z;= background integration-time for background of ith 

peak 

S=number of singly charged ions per second per ampere=6.2X 10 18 

We assume here that no ions arrive at the collector at 

the background positions. 

It follows that for perfectly stable beams, the percent uncertainty (IR) in the ratio of any two isotopes (i,j) 

must be 

I =IOOVA'+B'+C' RiJ (2) 

where 

[ B'=N2] (-1 +-1 )P/Z; P/Zj 

[background-type 1] 

1Variance divided by the square of the mean of the value measured; 

for example cr2

x/X2 . 
orB'=- -'- N2 (P-P-)1 

2 

ZiJ P;Pj [backgound-type 2] 

and A', B', and C' have significances similar to A, B, 

and C. 

We are concerned here simply with finding the 

ratios of Z;lnT; and T;!Tj that will yield the least uncertainty in the estimate of the mean of P;!Pj, for a given 

total time spent taking data. In the first case, we constrain 

Z; + nT; (background-type 1) or ZiJ + n (T; + T) (background-type 2) and differentiate the rearranged version of 

equation (1) to find 

[background-type 1] (3) 

where ~;is the ratio of Z;lnT; giving the lowest I;; or 

(

P-P-) 2 

(1 +lip;) ;;P; 

-+- +- -+-

( 

1 PiJ ) 1 ( 1 PiJ) 

P? P/ SN2 P; pj 

[background--type 2] ( 4) 

where ~ij is the ratio of Z;'J·InT; giving the lowest I.2

R ., 

I) 

and P;f::::::.T;!Tj. 

To find the optimum T;!Tj, we constrain T; + Tj (sum of 

peak-top integration time for the two isotopes) and differentiate the rearranged version of equation (2) to find 

N2/P2+ l!(SP;) 

(5) 

where PiJ is the ratio of T;!Tj giving the lowest I 2ij· 

Equation (3) for ~i approaches unity for small ion 

beams using a Faraday cup collector (relatively large dark 

noise), becomes somewhat less than one for large ion 

beams using a Faraday cup collector, and becomes much 

less than one for ion-counting techniques where N is insignificant. Equation (4) follows the same trend, except 

is reduced in all cases as the isotope ratios approach unity. 

Equation (5) for p;j approaches P;IPj for small ion beams 

using a Faraday cup collector, and approaches v'P;!Pj 

either for large ion beams using a Faraday cup collector 

or for ion-counting techniques where N is insignificant. 

REAL-BEAM CONSIDERATIONS 

Real ion beams may, of course, have a significant 

noninterpolable component of type-1 noise (although 

perhaps less often than is generally assumed), which in 

general will be proportional to the ion-beam intensity. 

Such noise will reduce the optimum ratio of background 

time to peak time, (~;) but in most cases probably will 

not greatly affect Pij· The exception is the case where 

the period of the major component of the noise is significantly less than the integration times on the peaks, and 

the magnitude of this noise is greater than the contribution 

of dark noise and ion-counting statistics. In this case, the 

optimum p;j would tend towards unity. Actual examples 

of such noise, however, do not seem to be common for 

well- prepared samples. 

The most effective strategy of dealing with most 

varieties of type-1 noise is probably the intuitively obvious one of reducing the cycle time (=I[D;+T;J, where 

D; is the delay time after peak-switching and before integration begins), in order both to increase the accuracy of 

interpolation and to increase the number of calculated 

ratios. In practice, such a strategy implies that values of 

~; and p;j selected using equations (3) through (5) should 

be combined with cycle times just long enough that delay 

times (to allow magnet switching) do not become a major 

fraction of the cycle times, and thus constitute a limiting 

factor on the theoretical efficiency of the cycles. Personal 

experience indicates that with cycle times of as much as 

20 seconds or so, linear interpolation of stable-appearing 

ion beams of less than about 10-12 amperes commonly 

gives isotope-ratio precisions limited only by type-2 plus 

type-3 noise, and that type-2 plus type-3 noise alone limits 

the precisions of ratios even from many runs with ion 

beams of greater than 10-11 amperes. Thus, the application of the simple concepts discussed here is of more than 

theoretical interest. The reader can calculate quickly the 

advantage of using equations (3) through (5) by substituting into equation (2) values from equations (3) through 

(5) and comparing the results to those using some standard 

set of P;, T;, and Z;. A typical value of N for a 10 11 

Q feedback resistor is 5 x 10-16 amperes/second. 

Time-Efficient Data-Taking Strategies for Single-Collector, Isotope-Ratio Mass Spectrometers 221 

*U.S. GOVERNMENT PRINTING OFFICE: 1986-676-047:46001 



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