GEOLOGICAL SURVEY PROFESSIONAL PAPER 1.199 A-E
Shorter Contributions to Isotope Research 

in the Western United States, 1980

Archean Gneisses in the Little Rocky Mountains, Montana 

By ZELL E. PETERMAN

Radioelement Distribution in a 3.06-Kilometer Drill Hole in Precambrian 

Crystalline Rocks, Wind River Mountains, Wyoming 

By CARL M. BUNKER and CHARLES A. BUSH

Ages of Igneous Rocks in the South Park-Breckenridge Region, Colorado, 

and their Relation to the Tectonic History of the Front Range Uplift

By BRUCE BRYANT, RICHARD F. MARVIN, 

CHARLES W. NAESER, and HARALD H. MEHNERT

Potassium-Argon and Fission-Track Zircon Ages of Cerro Toledo Rhyolite

Tephra in the Jemez Mountains, New Mexico

By G. A. IZETT, J. D. OBRADOVICH, C. W. NAESER, and G. T. CEBULA

Fission-Track Dating of the Climax and Gold Meadows Stocks, Nye

County, Nevada

By C. W. NAESER and FLORIAN MALDONADO

GEOLOGICAL SURVEY PROFESSIONAL PAPER1199A-E

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1981
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Doyle G. Frederick, Acting Director

Catalog No. 81-600048

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, B.C. 20402
CONTENTS

[Letters designate the chapters]

Page

(A) Archean gneisses in the Little Rocky Mountains, Montana, by Zell E. Peterman 1

(B) Radioelement distribution in a 3.06-kilometer drill hole in Precambrian

crystalline rocks, Wind River Mountains, Wyoming, by Carl M. Bunker

and Charles A. Bush................................................................................................................ 7

(C) Ages of igneous rocks in the South Park-Breckenridge region, Colorado, and 

their relation to the tectonic history of the Front Range uplift, by Bruce 

Bryant, Richard F. Marvin, Charles W. Naeser, and Harald H. Mehnert............. 15

(D) Potassium-argon and fission-track zircon ages of Cerro Toledo Rhyolite

tephra in the Jemez Mountains, New Mexico, by G. A. Izett, J. D. Obradovich,

C. W. Naeser, and G. T. Cebula .......................................................................................... 37

(E) Fission-track dating of the Climax and Gold Meadows stocks, Nye County,

Nevada, by C. W. Naeser and Florian Maldonado ...................................................... 45

SYMBOLS AND ABBREVIATIONS 

USED IN THIS VOLUME

°C 

CIPW

cm2

g

IUGS

km

m

mm

m.y.

n

ppm

RaeU

yr

Xf

Xg

Xe

Pi 

p. 

a

Degrees Celsius

Cross, Iddings, Pirsson, and Washington (igneous rock

classification system) 

Square centimeters 

Grams

International Union of Geological Sciences 

Kilometers 

Meters 

Millimeters 

Million years 

Neutrons 

Parts per million 

Radium-equivalent uranium 

Years

Decay constant for spontaneous fission of 238U 

Decay constant based on beta-particle emission 

Decay constant based on orbital electron capture 

Microcalories per gram-year 

Micrometers (10~6 m) 

Induced fission-track density 

Spontaneous (fossil) fission-track density 

Standard deviation 

Neutron flux

Archean Gneisses in the 

Little Rocky Mountains, 

Montana

By ZELL E. PETERMAN

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

IN THE WESTERN UNITED STATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1199-A
CONTENTS

Page

Abstract...................................................................................................................................................... 1

Introduction.............................................................................................................................................. 1

Acknowledgments ................................................................................................................................ 1

Samples and analytical methods ...................................................................................................... 2

Results ........................................................................................................................................................ 3

Regional considerations........................................................................................................................ 3

References cited........................................................................................................................................ 6

ILLUSTRATIONS

Page

FIGURE 1. Generalized geologic map of the Little Rocky Mountains, Montana .............. 2

2. Rb-Sr isochron plot for samples of gneiss and amphibolite from the Little

Rocky Mountains ......................................................................................................... 4

3. Map showing regional Precambrian framework of north-central United

States and adjacent Canada.................................................................................. 5

TABLES

Page

TABLE 1. Rb-Sr analyses of Precambrian rocks from the Little Rocky Mountains .... 3
SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES

ARCHEAN GNEISSES IN THE LITTLE ROCKY MOUNTAINS, MONTANA

By ZELL E. PETERMAN

ABSTRACT

Rb-Sr dating of a layered complex of tonalitic and granitic 

gneisses and amphibolite in the Little Rocky Mountains of northcentral Montana defines their age as late Archean. Previous K-Ar 

ages of 1,710 and 1,750 m.y. on hornblende and biotite, 

respectively, reflect a widespread resetting of K-Ar and Rb-Sr 

mineral ages in the basement of the Williston basin and of the 

western Canada sedimentary basin. Although exhibiting some 

scatter, the Rb-Sr data are consistent with an age of at least 2,550 

m.y., and consideration of reasonable initial 87Sr/86Sr ratios 

indicates that the gneisses may be substantially older. Even 

though the Rb-Sr data do not provide an exact age, an Archean 

age is firmly established. This age, coupled with determinations 

on basement cores from southeastern Montana and northeastern 

Wyoming, extends the Archean terrane of the Wyoming age 

province well into the buried basement towards the Williston 

basin. Continuity of the Archean of the Superior province and the 

Wyoming age province in the subsurface of the Dakotas and 

eastern Montana remains to be demonstrated, but the gap is 

narrowed considerably by the data reported here.

INTRODUCTION

The Little Rocky Mountains of north-central 

Montana are well known for excellent sections of 

Paleozoic and Mesozoic sedimentary rocks exposed 

in a domical uplift associated with several Tertiary 

syenite porphyry stocks (fig. 1). Precambrian rocks 

are also exposed in the area but as blocks within and 

marginal to the intrusions. These blocks consist of 

biotite schist and gneiss, quartzite, and amphibolite 

that are thought to represent a sequence of 

sedimentary and volcanic rocks metamorphosed to 

amphibolite grade (Knechtel, 1959). Hearn and others 

(1977) report K-Ar ages of 58 to 66 m.y. for the syenite 

and assign it to the Paleocene. The syenite was 

locally fractured, silicified, and altered during an 

episode of mineralization that produced gold-silver 

deposits in the area.

Burwash, Baadsgaard, and Peterman (1962) 

determined K-Ar ages of 1,710 and 1,750 m.y. on

hornblende and biotite respectively for Precambrian 

rocks just north of Zortman (fig. 1). These ages are in 

the common range of K-Ar ages of basement rocks of 

the western Canada sedimentary basin and of the 

Williston basin of western north Dakota and eastern 

Montana. The concordance of the ages from the 

Little Rocky Mountains suggests that a specific 

thermal or metamorphic event is being recorded by 

the K-Ar systems. That these systems were 

apparently little affected by the Tertiary intrusion is 

surprising in view of the association of the syenite 

and the Precambrian rocks. The syenite intrudes 

Precambrian and possibly Cambrian rocks but is in 

fault contact with younger rocks (Knechtel, 1959). 

Considering the relatively small areas of 

Precambrian rocks, the thermal effects must have 

been significant but not sufficient to reset the K-Ar 

systems.

The K-Ar determinations provide a minimum age 

for the Precambrian rocks of the Little Rocky 

Mountains and identify them as Precambrian X or 

older. The nearest Archean rocks crop out in the 

Little Belt Mountains approximately 200 km 

southwest of the Little Rocky Mountains; they have 

been dated by Catanzaro and Kulp (1964), and are 

part of the major Archean terrane of the Wyoming 

age province. The extent of Archean rocks in the 

buried basement of northern and eastern Montana is 

poorly defined. Gneisses exposed in the Little Rocky 

Mountains provide indirect access to this otherwise 

buried basement, and the results obtained from RbSr dating of these gneisses are reported here.

ACKNOWLEDGMENTS

The technical support of G. T. Cebula and J. W. 

Groen in the careful preparation of the rock samples 

and of Kiyoto Futa in the analytical work is 

gratefully acknowledged.
SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

108°30'

48°00'-

10 KILOMETERS

I

EXPLANATION 

Tertiary syenite 

Mesozoic sedimentary rocks 

Paleozoic sedimentary rocks 

Archean metamorphic rocks

Mine

Contact

Fault

Sample locality

FIGURE 1. Generalized geology of the Little Rocky Mountains, Montana (adapted from Knechtel, 1959).

SAMPLES AND ANALYTICAL METHODS

Rb and Sr analyses were completed following 

procedures slightly modified from those described 

earlier (Peterman , and others, 1967). Ages are calculated using the isotopic and decay constants recommended by the International Union of Geological 

Sciences Subcommission on Geochronology (Steiger 

and Jager, 1977).

Seven samples were collected from an exposure of 

Precambrian rocks in a blasted cut along the old 

haulage road from Zortman to the Ruby Gulch gold

mine (fig. 1). A layered complex of gneiss and 

amphibolite has nearly horizontal banding and 

foliation at this locality. Fine-grained biotite gneiss is 

in bands from several centimeters to several tens of 

centimeters thick. Biotite-hornblende gneiss occurs 

in layers as much as a meter thick. Amphibolite 

layers range from a few centimeters to a meter or 

more, and thinner layers are commonly boudined 

and tightly folded.

Samples LRM-1, -5, and -6 are light- to mediumgray, fine-grained biotite granite gneiss. A well 

developed foliation is defined by biotite alinement 

and by thin (up to 2 mm) lenses of composite quartz.
ARCHEAN GNEISSES IN THE LITTLE ROCKY MOUNTAINS, MONTANA

Quartz, plagioclase, and microcline are present in 

nearly equal amounts, and biotite forms about 5

percent of the gneiss. Samples LRM-3 and -4 are 

biotite-hornblende gneiss of tonalitic to granodioritic 

composition. Poikiloblastic hornblende grains (as much 

as 2 mm long) are present in fine-grained,, well-foliated 

biotite gneiss with abundant quartz and plagioclase but 

only sparse microcline. Both samples contain composite 

augen of quartz, plagioclase, and microcline. LRM-4 

contains single plagioclase grains (as much as 3 mm in 

diameter) that are commonly surrounded by healed 

mortar structure. Sample LRM-7 is a biotite granite 

gneiss similar to LRM-1 but with composite augen of 

quartz and feldspar. LRM-2 is a dark-gray, fine-grained 

amphibolite with abundant and nearly equal amounts 

of hornblende and plagioclase, and minor quartz.

Accessory minerals in the amphibolite aresphene, 

apatite, and altered allanite. The felsic gneisses 

.contain accessory sphene, apatite, zircon, pyrite, 

magnetite-ilmenite, and altered allanite. Plagioclase 

is slightly to moderately sericitized. Biotite is 

generally fresh with only local chloritization. 

Hornblende is commonly altered along cleavages. 

Minor carbonate occurs in most of the samples. 

Sphene is partially altered to leucoxene on grain 

margins and along fractures.

The primary mineral assemblages indicate 

metamorphism in the amphibolite facies. Some of 

the alteration may be related to the intrusion of the 

syenite, but the iron staining is probably due to 

weathering.

RESULTS

The felsic gneisses have similar Rb and Sr 

contents (table 1) and, therefore, a limited range in 

Rb/Sr ratios that precludes clear definition of a Rb-Sr 

isochron by these data alone. The amphibolite has a 

much lower Rb/Sr ratio, and a reference isochron 

corresponding to an age of 2,550 m.y. is drawn 

through this point and the points for the felsic 

gneisses (fig. 2). The points scatter well outside of 

analytical error, and the data do not warrant a more 

sophisticated regression. The reference isochron has 

to be drawn through a relatively high initial 

87gr/86gr ratio of 0.706 in order to accomodate the 

data point for the amphibolite. Model ages can be 

calculated for the felsic gneisses using an initial-Sr 

ratio of 0.071, a value more common for rocks of late 

Archean age. These model ages range from 2,920 to 

2,620 m.y., and the average value of 2,770 m.y. would 

correspond approximately to an average 

"wholerock" composite age for the felsic gneisses.

TABLE 1. Rb-Sr analyses of samples of Precambrian rocks 

from the Little Rocky Mountains, Montana

[Analyst: Kiyoto Futa]

Sample Rb (ppm) Sr (ppm) 87 Rb/ 86Sr

LRM-lJ

LRM-r

LRM-2

LRM-3

LRM-4

LRM-5

LRM-6

LRM-7

124

123

10.7

114

98.7

124

110

103

219

219

269

285

220

232

258

181

1.655

1.630

.116

1.159

1.306

1.555

1.241

1.649

0.7636

.7635

.7105

.7486

.7563

.7617

.7516

.7660

*Normalized to 86Sr/ 88Sr = 0.1194. 

^"Duplicate analyses.

Although the Rb-Sr systematics do not provide an 

especially accurate determination of the age, the 

data clearly establish the gneisses as Archean. The 

scatter of data points is not surprising in view of the 

metamorphic event at 1,700 to 1,800 m.y., the contact 

effects of the Paleocene syenite, and recent 

weathering. If the whole-rock Rb-Sr systems have 

been modified by any or all of these events, the effect 

would most likely have been to lower the age through 

loss of radiogenic Sr, gain of Rb, or internal 

redistribution of both elements. Consequently, the 

Rb-Sr systematics provide a firm minimum age for 

the gneisses.

REGIONAL CONSIDERATIONS

Most of the Precambrian of the United States is 

concealed beneath Phanerozoic cover (see King, 1976, 

fig. 1, p. 4-5) and not readily accessible for geologic 

studies. Significant advances in understanding the 

Precambrian history of the buried basement have 

been made through rediometric dating of basement 

samples obtained in drilling, largely in petroleum 

exploration. A major dating study of the basement of 

the U.S. culminated in a series of four reports 

(Goldich, Lidiak, and others, 1966; Goldich, 

Muehlberger, and others, 1966; Muehlberger and 

others, 1966; and Lidiak and others, 1966) followed 

by a regional synthesis for North America 

(Muehlberger and others, 1967). Very few ages of 

basement rocks have been reported since 1966.

A goal of the dating studies is the delineation of 

age provinces in the basement. Many of these are 

subsurface extensions of the geologic and age 

provinces of the Canadian shield; others appear to be 

unique to the subsurface. The extent of these 

provinces, their age and lithologic character, and 

their mutual relationships are fundamental
SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

g 0.740

LRM-7 

LRM-1 -z 

LRM-5;

'— LRM-6 

-LRH-3

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

FIGURE 2. Rb-Sr isochron plot for samples of gneiss and amphibolite from the Little Rocky Mountains, Montana. The slope of 

the isochron corresponds to an age of 2,550 m.y., and the 

initial 87Sr/86Sr intercept is 0.706.

problems that have not been fully resolved in the 

buried basement nor in many exposed Precambrian 

terranes for that matter.

The basement of eastern Montana, eastern 

Wyoming, North Dakota, and South Dakota is of 

particular interest because it lies between two major 

exposed Archean terranes: the Wyoming age 

province on the west and the Superior province on 

the east (fig. 3). Three major problems are (1) whether 

or not Archean crust is continuous in the subsurface 

between the exposed Archean terranes, (2) the extent 

of lower Proterozoic (Precambrian X) supracrustals 

in the subsurface, and (3) the nature of the event that 

produced the 1,600- to 1,800-m.y. K-Ar and Rb-Sr 

mineral ages and the amount of intrusive rock 

emplaced during this event. Only a few dated 

samples from the basement provide clear evidence of 

Archean rocks (fig. 3). In the eastern Dakotas, 

proximity to the exposed Archean rocks of 

Minnesota and Manitoba and geophysical data 

coupled with basement lithologies provide 

additional evidence for an Archean basement in this 

region (Muehlberger and others, 1967; Lidiak, 1971).

In the Black Hills, a key area in the western part of 

this region (fig. 3), folded and metamorphosed lower 

Proterozoic supracrustals lie on Archean basement. 

The deformation and intrusive event in this area 

between 1,600 and 1,800 m.y. was named the Black 

Hills orogeny (Goldich, Lidiak, and others, 1966). The 

Harney Peak Granite associated with this event is 

dated at 1,710 m.y. (Riley, 1970). Zartman and Stern

(1967) established the Little Elk Granite in the 

northeastern Black Hills as Archean basement. RbSr systems in whole rocks and minerals were reset by 

cataclasis and recrystallization during the Black 

Hills orogeny, but the U-Pb systems in zircon retained the Archean age. Archean rocks were also 

discovered in the western Black Hills, where Ratte 

and Zartman (1970) obtained 2,500-m.y. ages on 

granite and pegmatite from the core of a gneiss dome 

that penetrates supracrustals at Bear Mountain.

The extent of lower Proterozoic supracrustals in 

the basement to the west and north of the Black Hills 

is largely conjectural. However, Lidiak (1971), in his 

construction of a geologic map of the basement of 

South Dakota, interprets gravity and aeromagnetic 

anomalies in the central and western part of the state 

as reflecting 1,600- to l,800-m.y.-old structural elements that trend north to northwest. On a more 

regional scale, Camfield and Gough (1977) have 

delineated a conductive zone in the basement that 

skirts the east side of the Black Hills and extends 

north to the Canadian shield and south to the 

Laramie Range of southeastern Wyoming. The origin of the anomaly is problematic, but Camfield and 

Gough (1977) note a correspondence or alinement 

with highly sheared and deformed rocks in both 

Saskatchewan and southeastern Wyoming, and they 

suggest that the zone may be a suture related to plate 

collision in the Proterozoic. The southernmost part of 

the anomaly is more or less coincident with part of 

the Colorado lineament described by Warner (1978). 

Warner presents abundant evidence for a Precambrian wrench-fault system extending from northern 

Arizona northeasterly to the southwestern tip of 

Lake Superior and postulates that the fault was 

approximately marginal to a continental plate in the 

early Proterozoic.

The present study of the Precambrian of the Little 

Rocky Mountains does not bear on the extent of the 

lower Proterozoic in the basement or on the nature of 

the erogenic event, but it does document an extension of the Archean into north-central Montana. 

Further evidence for this extension is provided by 

data for two basement cores obtained in U.S. Geological Survey drill holes that have been dated by the 

Rb-Sr method (Peterman, unpub. data, 1978). A hole 

in northeastern Wyoming (fig. 3) penetrated a banded gneiss complex that gives an isochron age of 2,640 

m.y. A hole to the north in Montana encountered a 

porphyriticbiotite granite that gives a model Rb-Sr age 

of approximately 2,800 m.y. These data, coupled with 

data for the Little Rocky Mountains, extend the 

Archean basement well into the subsurface towards 

the Wiliston basin. Whether or not the Archean was 

continuous between the Wyoming age province and
ARCHEAN GNEISSES IN THE LITTLE ROCKY MOUNTAINS, MONTANA

: r - - i • ^-L L v ' y< < < i v ^ •> * < > * y-r*

/ < r r 7<''w^ 1 <v// <

7 v s/ t-/L u r^^ r </< //-7-v

SASKATCHEWAN

Lxi v >'iv^ j,/\

| s\ -3 A v ^ V ^

-__ _ _CANADA

UNITED (STATES

Be/t Mountains

MONTANA

EXPLANATION

PRECAMBRIAN Y

Belt Supergroup of Montana, Keweenawan Supergroup 

of Minnesota, and Sioux Quartzite of Minnesota and 

South Dakota

PRECAMBRIAN X

Penokean granites of Minnesota, and Churchill Province of 

Canada (in part Archean)

Anjmikie Group of Minnesota, and metasedimentary and metavolcanic 

rocks of the Black Hills (including the Harney Peak Granite)

ARCHEAN

Superior Province of Canada and Archean rocks of 

Minnesota and of the Wyoming age province

Subsurface samples dated as Archean, and two Archean 

outcrop areas in the Black Hills (the Little Elk Granite and 

the granite gneiss at Bear Mountain)

FIGURE 3. Regional Precambrian framework of the north-central United States and adjacent Canada. Adapted from the "Basement 

Map of North America" (Flawn, 1967). Age data for basement samples are from Burwash and others (1962); Peterman and 

Hedge (1964); Goldich, Lidiak, and others (1966); Peterman (unpub. data, 1978). Archean ages in the Black Hills are from 

Zartman and Stern (1967) and Ratte and Zartman (1970).
SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

the Superior province will have to be determined by 

additional dating of basement samples by methods 

that are capable of penetrating the effects of the 

1,600- to 1,800-m.y. event.

REFERENCES CITED

Burwash, R. A., Baadsgaard, H., and Peterman, Z. E., 1962, 

Precambrian K-Ar dates from the western Canada sedimentary basin: Journal of Geophysical Research, v. 67, p. 1617- 

1625.

Camfield, P. A., and Gough, D. I., 1977, A possible Proterozoic 

plate boundary in North America: Canadian Journal of Earth 

Sciences., v. 14, p. 1229-1238.

Catanzaro, E. J., and Kulp, J. L., 1964, Discordant zircons from 

the Little Belt (Montana), Beartooth (Montana) and Santa 

Catalina (Arizona) Mountains: Geochimica et Cosmochimica 

Acta, v. 28, p. 87-124.

Flawn, P. T., 1967, Basement map of North America: U.S. Geological Survey.

Goldich, S. S., Lidiak, E. G., Hedge, C. E., and Walthall, F. G., 

1966, Geochronology of the midcontinent region, United 

States, 2., Northern area: Journal of Geophysical Research, 

v. 71, p. 5389-5408.

Goldich, S. S., Muehlberger, W. R., Lidiak, E. G., and Hedge, 

C. E., 1966, Geochronology of the midcontinent region, United 

States, 1., Scope, methods, and principles: Journal of Geophysical Research, v. 71, p. 5375-5388.

Hearn, B. C., Jr., Marvin, R. F., Zartman, R. E., and Naeser, 

C. W., 1977, Geochronology of igneous activity in the northcentral alkalic province (abs.): Geological Society of America 

Abstracts with Programs, v. 9, no. 6, p. 732.

King, P. B., 1976, Precambrian geology of the United States; 

an explanatory text to accompany the geologic map of the 

United States: U.S. Geological Survey Professional Paper 

902, 85 p.

Knechtel, M. W., 1959, Stratigraphy of the Little Rocky Mountains 

and encircling foothills, Montana: U.S. Geological Survey 

Bulletin 1072-N, p. 723-752.

Lidiak, E. G., 1971, Buried Precambrian rocks of South Dakota: 

Geological Society of America Bulletin v. 82, p. 1411-1420.

Lidiak, E. G., Marvin, R. F., Thomas, H. H., and Bass, M. N., 

1966, Geochronology of the midcontinent region, United 

States, 4., Eastern region: Journal of Geophysical Research, 

v. 71, p. 5427-5438.

Muehlberger, W. R., Denison, R. E., and Lidiak, E. G., 1967, Basement rocks in continental interior of United States: American 

Association of Petroleum Geologists Bulletin, v. 51, p. 2351- 

2380.

Muehlberger, W. R., Hedge, C. E., Denison, R. E., and Marvin, 

R. F., 1966, Geochronology of the midcontinent region, United 

States, 3., Southern area: Journal of Geophysical Research, 

v. 71, p. 5409-5426.

Peterman, Z. E., and Hedge, C. E., 1964, Age of basement rocks 

from the Williston basin of North Dakota and adjacent areas, 

in Geological Survey research 1963: U.S. Geological Survey 

Professional Paper 475-D, p. D100-D104.

Peterman, Z. E., Doe, B. R., and Bartel, Ardith, 1967, Data on 

the rock GSP-1 (granodiorite) and the isotope-dilution method 

of analysis for Rb and Sr, in Geological Survey research 1967: 

U.S. Geological Survey Professional Paper 575-B, p. B181- 

B186.

Ratte, J. C., and Zartman, R. E., 1970, Bear Mountain gneiss 

dome, Black Hills, South Dakota Age and structure (abs.): 

Geological Society of America Abstracts with Programs, v. 2, 

no. 5, p. 345.

Riley, G. H., 1970, Isotopic discrepancies in zoned pegmatites, 

Black Hills, South Dakota: Geochimica et Cosmochimica Acta, 

v. 34, p. 713-725.

Steiger, R. H., and Jager, E., 1977, Subcommission on geochrcnology Convention and use of decay constants in geo- and 

cosmochronology: Earth and Planetary Science Letters, v. 36, 

p.359-362.

Warner, L. A., 1978, The Colorado lineament A middle Precambrian wrench fault system: Geological Society of America 

Bulletin, v. 89, p. 161-171.

Zartman, R. E., and Stern, T. W., 1967, Isotopic age and geologic 

relationships of the Little Elk Granite, northern Black Hills, 

South Dakota: U.S. Geological Survey Professional Paper 

575-D, p. D157-D163.
Radioelement Distribution 

in a 3.06-Kilometer Drill Hole 

in Precambrian Crystalline Rocks, 

Wind River Mountains, Wyoming

By CARL M. BUNKER and CHARLES A. BUSH

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

IN THE WESTERN UNITED STATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1199-B
CONTENTS

Page

Abstract.......................................................................................^^ 7

Introduction........................................................................................................................... ................ 7

Analytical method .................................................................................................................................. 8

Results ........................................................................................................................................................ 8

Conclusions .............................................................................................................................................. 14

References cited........................................................................................................................................ 14

ILLUSTRATIONS

Page

FIGURE 4. Map showing location of 3.06-kilometer-deep drill hole in the Wind

River Mountains, Wyoming ...................................................................................... 7

5. Plots of radioelement concentrations and ratios and radiogenic heat

in 3.06-km-deep drill hole...................................................................................... H

6. Histograms showing frequency distribution of radioelement

concentrations and ratios and radiogenic heat .................................................... 13

TABLES

TABLE 2.

3.

Summary of radioelement and radiogenic heat analyses on 299 

core samples.. ...........................................................................................................

Averages of radioelement contents and ratios and radiogenic heat for 

samples from drill hole, and summary of published averages for 

igneous rocks ..............................................................................................................

Page 

9

12
SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES

RADIOELEMENT DISTRIBUTION IN A 3.06-KILOMETER

DRILL HOLE IN PRECAMBRIAN CRYSTALLINE ROCKS,

WIND RIVER MOUNTAINS, WYOMING

By C. M. BUNKER and C. A. BUSH

ABSTRACT

Uranium, thorium, and potassium contents were measured in 

229 samples of cuttings from a drill hole that penetrates 3.06 

kilometers of Precambrian crystalline rock on the southwestern 

flank of the Wind River Mountains, Wyoming. Thorium and 

potassium contents are similar to reported averages for intermediate to silicic igneous rock, and uranium content is in the 

range of averages for mafic to intermediate rock; these data 

indicate that the rock is deficient in uranium. The radioelement 

data support geologic and geophysical data which indicate that 

the drill hole penetrates a thick section of layered, fractured, 

heterogeneous rock.

INTRODUCTION

A 3.06-kilometer exploratory borehole was drilled 

into Precambrian crystalline rocks on the southwestern flank of the Wind River Mountains, Wyoming, to obtain geologic and geophysical data. The 

hole is in sec. 2, T. 32 N., R. 197 W. (lat 42°45'30" N., 

long 109°31'30" W.) about 23 km southeast of Pinedale, Wyo.; the collar elevation is 2219 m above mean 

sea level (fig. 4).

The Wind River Mountains are a broad asymmetrical anticline exposing Precambrian crystalline 

rocks in the core. Several kilometers of rock may 

have been eroded above the existing surface at the 

drill-hole site. The flank of the Wind River Mountains overthrusts the Green River Basin to the 

southwest; the thrust dips gently to the northeast. 

Rocks exposed at the surface near the drill hole and 

all core samples show nearly horizontal foliation.

Petrographic descriptions of cores taken from 

depths 50 to 300 m apart have been reported by 

Ebens and Smithson (1966). The rock cores are 

granitic (quartzo-feldspathic) in general character, 

but they are very heterogeneous. Composition ranges from quartz-dioritic to granodioritic to quartzmonzonitic gneiss; texture ranges from fine grained 

to porphyroblastic. Neither composition nor texture

shows any relationship to depth, although the mineralogy suggests an increase in grade of metamorphism with depth. Ebens and Smithson (1966) concluded that the hole penetrates a heterogeneous 

layered sequence of metamorphic rocks.

A seismic velocity survey and sonic, caliper, and 

neutron logs of the drill hole indicate that the rock is 

fractured throughout the depth of the hole (Smithson 

and Ebens, 1971). The fracturing is very likely 

related to the proximity of the drill hole to the thrust 

fault. A major fracture zone with cracks exists to a 

depth of 460 m; a sharp increase in velocity at that 

depth is interpreted as being related to the absence of 

cracks below that depth. Other highly fractured 

zones are indicated at 1,642 m and from 2,665 to at 

least 2,700 m. Relatively unfractured zones occur 

from about 2,000 to 2,650 m, which is also a zone of 

relatively high rock density, and from 2,865 to 2,890 

m. A gamma-ray log of the hole shows general 

changes in radiation intensity which are probably

1

44°

42°

Fl

12° 110° 108° 106° 104°

I

_~r\

-

IDAHO

MONTANA -I r n

WYOMING

Pinedale |lii«v /Wind River Mountains

"il

D

L

i I i ,COLORAD(

/5

O

I

o

!-

i

C/5

3, ""

CURE 4. Location of 3.06-kilometer-deep drill hole in the Wind 

River Mountains, Wyoming.

7
8 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

related to changes in rock type. The gamma-ray log 

also shows many zones ranging in thickness from 

about 0.3 to 15 m in which the radioelement content 

is greater than that of the host rock. These relatively 

thin zones of anomalous radioactivity are not apparently related to anomalies on the neutron log; 

however, the radioactivity anomalies can be interpreted as indications of either mineralogic differences in the host rock or enrichment of radioelement 

contents in at least some of the fracture zones. These 

geophysical data confirm that the rock is heterogeneous both in composition and physical properties, as 

was previously determined from a few core samples. 

In addition to the 19 cores that were obtained, drill 

cuttings were collected from virtually the entire 

depth of the hole. Ninety-nine of these samples were 

used for measurements of radiogenic heat production (Lachenbruch and Bunker, 1971); those plus an 

additional 130 samples were used in the present 

investigation. Most of the samples represent about 3 

m of rock penetrated by the drill hole; a few represent 

1.5-m intervals. The distance between samples range 

from 0 to 36 m, but most of the sampled intervals are 

3.3 to 9.8 m apart.

The principal objective of the radioelement analyses was to determine the distribution of uranium, 

thorium, potassium, and the associated radiogenic 

heat through a thick vertical section of Precambrian 

crystalline rocks. We also plan to compare these data 

with radioelement analyses of Precambrian granites 

in central Wyoming.

ANALYTICAL METHOD

Radioelement (K, RaeU, and Th) contents of the 

samples were measured by gamma-ray spectrometry. Approximately 600 g of the material were sealed 

in 15-cm-diameter plastic containers. The containers 

were placed on a sodium iodide crystal 12.5 cm in 

diameter and 10 cm thick. The gamma radiation 

penetrating the crystal was sorted according to 

energy by the associated electronic devices, and the 

resulting spectra were stored in a 512-channel memory. The spectra were interpreted with the aid of a 

linear-least-squares computer method which matches the spectrum from a sample to a library of 

radioelement standards; the computer method for 

determining concentrations is a modification of a 

program written by Schonfeld (1966). Standards 

used to reduce the data include the U8G8 standard 

rocks, New Brunswick Laboratories standards, and

several samples for which uranium and thorium 

concentrations have been determined by isotope 

dilution and mass or alpha spectrometry.

Uranium contents were measured indirectly by 

measuring the 226Ra daughters (214Bi and 214Pb) to 

obtain radium-equivalent uranium (RaeU) values. 

Radium-equivalent uranium is the amount of uranium required for secular isotopic equilibrium with 

the 226Ra and its daughters measured in a sample. 

Isotopic equilibrium between these daughters and 

226Ra was accomplished by allowing the sealed 

sample containers to sit for at least 21 days prior to 

the analyses. All uranium concentrations measured 

in the drill-hole samples are radium-equivalent values.

Although thorium is also measured from daughter 

products (212Bi, 212Pb, and 208T1), isotopic disequilibrium is improbable because the daughter 

products measured have such short half-lives that 

their concentration directly reflects that of thorium. 

Potassium is determined from the 40K isotope, which 

is radioactive and directly proportional to the total 

potassium.

All the radioelement data (table 2) reported in this 

paper are based on replicate analyses. The coefficient of variation for the accuracy of these data, 

when compared to isotope-dilution and flame-photometry analyses, is about ±2 percent for RaeU and Th 

and about ±1 percent for K. These percentages are in 

addition to minimum standard deviations of about 

0.05 ppm for RaeU and Th and 0.03 percent for K.

RESULTS

The highly variable character of the distribution of 

the radioelement contents and ratios in the rock 

penetrated by the drill hole (fig. 5) reflects the 

layering, fracturing, and heterogeneity in mineral 

content and rock types that were observed by examination of core samples or interpreted from geophysical logs. An exception to the highly variable 

radioelement distribution is the depth interval from 

about 2,040 to 2,650 m, in which the uranium and 

potassium contents are nearly constant, thereby 

indicating a fairly homogenous layer of rock. This is 

a relatively unfractured zone in which the transport 

and redistribution of radioelements is less likely 

than in the highly fractured zones. We believe that 

the measured radioelement contents in the 2,040- to 

2,650-m interval are the intrinsic values of the rock, 

which has been described as foliated porphyroblastic quartz monzonite (Ebens and Smithson, 1966).
RADIOELEMENT DISTRIBUTION, PRECAMBRIAN CRYSTALLINE ROCKS, WIND RIVER MOUNTAINS, WYOMING 9

TABLE 2.—Summary of radioelement and radiogenic heat analyses on 229 core samples from a drill hole in Wind River 

Mountains, Wyoming

TABLE 2.—Summary of radioelement and radiogenic heat analyses on 229 core samples from a drill hole in Wind River 

Mountains, Wyoming—Continued

Sample

depth 

(meters)

6.1- 9.1

18.3- 21.3

36.6- 39.6

45.7- 48.8

51.8- 54.9

61. n- 64.0

67.1- 7(1.1

76.2- 79.2

85.3- 88.4

97.5- ion. 6

106.7- 109.7

112.8- 115.8

121.9- 125.0

131.1- 134.1

140.2- 143.3

152.4- 155.4

161.5- 164.6

167.6- 170.7

176.8- 179.8

185.9- 189.0

204.2- 207.3

213.4- 216.4

222.5- 225.6

231.6- 234.7

240.8- 243.8

249.9- 253.0

259.1- 262.1

268.2- 271.3

277.4- 280.4

283.5- 286.5

289.6- 292.6

295.7- 298.7

304.8- 307.8

313.9- 317.0

320. n- 323.1

329.2- 332.2

338.3- 341.4

344.4- 347.5

350.5- 353.6

351. 7- 362.7

368.8- 371.9

381.0- 384.0

387.1- 390.1

396.2- 390.3

405.4- 408.4

414.5- 417.6

423.7- 426.7

432.8- 435. 0

442.0- 445.0

448.1- 451.1

454.2- 457.2

460.2- 463.3

493.8- 496.8

521.2- 524.3

545.6- 54R.6

554.7- 557.8

579.1- 582.2

591.3- 594.4

597.4- 600.5

609.6- 612.6

618.7- 621.8

624.8- 677.9

634.0- 637.0

646.2- 649.2

658.4- 661.4

RaeTI

(ppm)

0.42

0.84

1.19

1.15

1.07

2.?3

1.15

1.16

1.74

1.78

1.50

1.25

1.40

0.90

1.43

1.06

1.84

2.06

1.31

1.48

2.70

2.56

2.13

2.02

1.76

5.04

12.94

3.17

2.88

1.55

3.29

2.74

3.87

2.48

3.81

2.09

2.71

2.62

2.25

2.83

3.64

2.96

2.29

2.62

2.27

2.35

2.57

2.22

3.73

2.37

2.19

1.95

2.15

1.81

1.30

1.7?

2.23

1.52

2.45

1.38

2.18

7.48

3.00

1.80

1.41

Th

(ppm)

5.95

15.60

28.53

18.04

21.32

24.09

15.78

78.27

72.18

52.11

43.32

32.91

30.98

28.69

34.90

8.10

9.29

9.68

9.38

15.95

19.60

17.85

12.74

21.81

27.26

21.58

33.24

21.84

26.78

12.62

31.25

21.48

11.96

10.41

14.20

10.76

20.28

21.79

19.87

19.01

21.72

15.82

13.86

16.82

16.12

16.57

18.00

16.00

20.54

15.04

16.74

14.99

16.06

14.04

7.47

11.88

a. 32

8.04

21.73

10.10

16.90

26.14

20.06

10. SO

11.00

r

(percent)

4.94

3.45

3.92

4.09

3.77

3.62

4.31

5.23

4.53

4.76

4.35

4.56

4.11

3.81

3.64

3.55

2.38

1.67

3.40

2.52

3.43

3.26

2.75

3.58

3.42

3.16

2.79

2.64

3.55

4.82

3.60

3.29

2.78

2.19

2.66

2.03

3.60

3.73

3.11

3.07

3.27

2.81

2.13

2.85

3.32

2.98

2.86

3.05

3.69

3.55

3.69

3.26

3.31

3.31

2.78

3.10

1.00

2.73

3.29

2.86

2.71

3.95

3.42

3.41

3.00

Heat

(Meal/

g-yr)

2.83

4.66

7.63

5.55

6.06

7.42

5.16

17.91

16.93

13.01

10.93

8.73

8.33

7.42

9.01

3.35

3.84

3.89

3.75

4.95

6.82

6.32

4.85

6.80

7.66

8.85

16.85

7.39

8.42

4.96

9.62

7.18

5.97

4.48

6.34

4.23

7.01

7.28

6.46

6.70

7.88

6.08

5.02

6.05

5.78

5.83

6.25

5.6A

7.83

5.90

5.94

5.30

5.68

5.02

3.19

4.47

4.03

3.45

7.02

3.82

5.70

8.10

7.13

4.35

4.04

Th

Raell

14.17

18.57

23.97

15.69

19.93

10.80

13.72

6 .47

41.48

29 . 28

28.88

26.33

22.13

31.88

24.41

7.64

5.05

4.70

7.16

10.78

7.26

6.97

5.98

10.80

15.49

4.28

2.57

6.89

9.30

8.14

P. 50

7.84

3.0Q

4.20

3.73

5.15

7.48

8.32

8.83

6.72

5.07

5.34

6.05

6.42

7.10

7.05

7.00

7.21

5.51

6.77

7.64

7.69

7.47

7.76

5.75

6.91

4.18

5.20

8.87

7.38

7.75

10.54

6.60

5.88

7.80

Raell

KxlO'4

0.09

0.24

0.30

0.28

0.28

0.62

0.27

0.22

0.38

0.37

0.34

0.27

0.34

0.24

0.39

0.30

0.77

1.23

0.39

0.59

0.79

0.79

0.77

0.56

0.51

1.59

4.64

1.20

0.81

0.32

0.91

0.83

1.39

1.13

1.43

1.03

0.75

0.70

0.72

0.92

1.11

1.05

1.08

0.92

0.68

0.79

0.90

0.73

1.01

0.67

0.59

0.60

0.65

0.55

0.47

0.55

1.12

0.56

0.74

0.48

0.80

0.63

0.88

0.53

0.47

Th

KxlO'4

1.20

4.52

7.28

4.41

5.66

6.65

3.66

14.97

15.93

10.95

9.06

7.22

7.54

7.53

9.59

2.28

3.QO

5.80

2.76

6.33

5.71

5.48 .

4.63

6.09

7.97

6.83

11.91

8.27

7.54

2.62

8.68

6.53

4.30

4.75

5.34

5.30

5.63

5.84

6.3Q

6.70

6.64

5.63

6.51

5.90

4.86

5.56

6.20

5.25

5.57

4.52

4.54

4.60

4.85

4.24

7.60

3.83

4.68

2.95

6.60

3.56

6.24

6.62

s.87

3.11

3.67

Sample

depth 

(meters)

664.5- 667.5

679.7- 682.8

685.8- 688.8

694.9- 698.0

701.0- 704.1

713.2- 716.3

728.5- 731.5

740.7- 743.7

752.9- 755.9

759.0- 762.0

762.0- 765.0

795.5- 798.6

823.0- 826.0

847.3- 850.4

883.9- 887.0

914.4- 917.4

938.8- 941.8

975.4- 978.4

984.5- 987.6

993.6- 996.7

1005.8-1008.9

1015.0-1018.0

1033.3-1036.3

1048.5-1051.6

1063.8-1066.8

1094.2-1097.3

1130.8-1133.9

1155.2-1158.2

1164.3-1167.4

1170.4-1173.5

1182.6-1185.7

1191.8-1194.8

1213.1-1216.2

1225.3-1228.3

1240.5-1243.6

1249-7-1252.7

1283.2-1286.3

1313.7-1316.7

1325.9-1328.9

1338.1-1341.1

1344.2-1347.2

1353.3-1356.4

1356.4-1359.4

1368.6-1371.6

1380.7-1383.8

1402.1-1405.1

1432.6-1435.6

1463.0-1466.1

1499.6-1502.7

1527.0-1530.1

1557.5-1560.6

1566.7-1569.7

1572.8-1575.8

1581.9-1585.0

1588.0-1591.1

1597.2-1600.2

1615.4-1618.5

1645.9-1649.0

1682.5-1685.5

1703.8-1706.9

1713.0-1716.0

1743.5-1746.5

1764.8-1767.8

1801.4-1804.4

1831.8-1834.9

RaeU

(ppm)

1.98

2.02

2.04

2.39

2.09

1.96

1.86

1.57

1.59

1.75

1.58

1.80

2.09

1.90

3.19

1.94

1.79

2.37

2.05

2.19

3.44

2.19

1.97

2.55

2.81

2.40

2.65

2.79

1.51

1.84

2.34

3.30

2.82

1.93

2.27

2.66

2.42

2.01

2.82

4.06

3.68

3.15

3.01

3.15

1.66

2.07

1.61

2.37

2.77

1.92

2.16

3.43

1.88

4.09

2.75

1.76

1.90

1.64

1.46

1.87

1.62

2.74

1.85

1.98

2.44

Th

(ppm)

13.87

17.96

24.75

24.37

21.62

10.92

20.57

11.05

20.86

15.67

13.64

12.59

12.53

10.14

14.61

10.15

11.56

13.74

14.99

14.11

16.42

14.46

6.71

10.31

11.96

11.54

17.36

20.26

12.38

14.16

18.51

14.93

16.03

8.56

9.92

14.30

11.12

14.75

12.83

20.74

27.49

25.37

20.29

12.20

11.90

12.57

12.19

11.91

10.26

9.75

14.67

12.20

16.51

44.90

30.91

18.73

12.89

16.13

17.15

15.08

11.82

23.65

13.56

13.52

12.69

K

(percent)

2.96

3.64

3.58

3.82

3.61

3.12

3.59

2.62

3.45

3.10

3.11

4.14

3.47

3.01

3.57

2.72

3.19

3.91

3.58

2.85

3.37

3.58

4.35

3.48

3.23

3.00

3.18

3.35

3.43

3.51

3.44

3.14

3.38

2.10

2.67

3.10

2.85

2.85

3.07

3.50

3.86

3.96

3.68

3.35

2.70

2.91

2.82

2.28

2.49

2.88

2.67

2.65

2.51

2.56

2.60

3.29

2.60

3.15

2.92

2.46

2.65

2.55

2.90

2.70

2.67

Heat

</ical/

g-yr)

5.02

6.05

7.41

7.65

6.82

4.46

6.44

4.06

6.26

5.25

4.72

4.95

4.97

4.23

6.21

4.18

4.48

5.53

5.46

5.19

6.71

5.46

3.95

4.86

5.32

4.87

6.27

6.99

4.50

5.12

6.34

6.24

6.18

3.69

4.36

5.64

4.76

5.19

5.45

8.06

9.23

8.44

7.2.5

5.64

4.32

4.81

4.37

4.73

4.75

4.13

5.23

5.66

5.35

12.66

8.89

5.92

4.67

5.27

5.28

5.05

4.26

7.42

4.85

4.88

5.03

Th

RaeU

7.01

8.89

12.13

10.20

10.34

5.57

11.06

7.04

13.12

8.95

8.63

6.99

6.00

5.34

4.58

5.23

6.46

5.80

7.31

6.44

4.77

6.60

3.41

4.04

4.26

4.81

6.55

7.26

8.20

7.70

7.91

4.52

5.68

4.44

4.37

5.38

4.60

7.34

4.55

5.11

7.47

8.05

6.74

3.87

7.17

6.07

7.57

5.03

3.70

5.08

6.79

3.56

8.78

10.98

11.24

10.64

6.78

9.84

11.75

8.06

7.30

8.63

7.33

6.83

5.20

RaeU

KxlO'4

0.67

0.55

0.57

0.63

0.58

0.63

0.52

0.60

0.46

0.56

0.51

0.43

0.60

0.63

0.89

0.71

0.56

0.61

0.57

0.77

1.02

0.61

0.45

0.73

0..87

0.80

0.83

0.83

0.44

0.52

0.68

1.05

0.83

0.92

0.85

0.86

0.85

0.71

0.92

1.16

0.95

0.80

0.82

0.94

0.61

0.71

0.57

1.04

1.11

0.67

0.81

1.29

0.75

1.60

1.06

0.53

0.73

0.52

0.50

0.76

0.61

1.07

0.64

0.73

0.93

Th

KxlO'4

4.69

4.93

6.91

6.38

5.99

3.50

5.73

4.22

6.05

5.05

4.39

3.04

3.61

3.37

4.09

3.73

3.62

3.51

4.19

4.95

4.87

4.04

1.54

2.96

3.70

3.85

5.46

6.05

3.61

4.03

5.38

4.75

4.74

4.08

3.72

4.61

3.90

5.18

4.18

5.93

7.12

6.41

5.51

3.64

4.41

4.32

4.32

5.22

4.12

3.39

5.49

4.60

6.58

17.54

11.89

5.69

4.96

5.12

5.87

6.13

4.46

9.27

4.68

5.01

4.84
10 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

TABLE 2. Summary of radioelement and radiogenic heat analyses on 229 core samples from a drill hole in Wind River

Mountains, Wyoming — Continued

Sample

depth

(meters)

1859.3-1862.3

1868.4-1871.5

1874.5-1877.6

1883.7-1886.7

1889.8-1892.8

1895.9-1898.9

1902.0-1905.0

1914.1-1917.2

1923.3-1926.3

1932.4-1935.5

1941.6-1944.6

1947.7-1950.7

1950.7-1953.8

1959.9-1962.9

1969.0-1972.1

1978.2-1981.2

1984.2-1987.3

1993.4-1994.9

2002.5-2004.1

2011.7-2013.2

2020.8-2022.3

2026.9-2030.0

2037.6-2039.1

2045.2-2046.7

2062.0-2063.5

2072.6-2075.7

2094.0-2097.0

2106.2-2107.7

2110.7-2112.3

2121.4-2124.5

2130.6-2133.6

2139.7-2142.7

2148.8-2151.9

2158.0-2161.0

2167.1-2170.2

2176.3-2179.3

2188.5-2191.5 

2200.7-2203.7

2206.8-2209.8 

2218.9-2222.0

2225.0-2228.1 

2237.2-2240.3

2249.4-2252.5

2258.6-2261.6 

2267.7-2270.8

2279.9-2283.0

2289.0-2292.1

2319.5-2322.6 

2350.0-2353.1

2383.5-2385.1

2411.0-2414.0

2435.4-2438.4 

2441.4-2444.5

2465.8-2468.9 

2505.5-2508.5

2535.9-2539.0 

2560.3-2563.4

2584.7-2587.8 

2593.8-2596.9

2621.3-2624.3

2654.8-2657.9

2664.0-2667.0

2673.1-2676.1

2682.2-2685.3

2694.4-2697.5

RaeU

(ppm)

1.47

1.40

1.60

1.80

2.11

1.38

0.77

1.51

1.05

1.68

1.20

1.33

0.91

1.06

2.21

1.83

1.39

1.81

2.08

1.22

1.36

1.31

1.23

1.15

1.27

1.20

1.01

1.16

1.13

1.05

1.56

1.15

1.06

2.88

1.62

1.07

1.28 

1.03

0.83 

0.88

0.91 

1.06

1.05

1.20 

0.99

0.99

1.09

1.05 

1.11

1.16

O.Q3

1.10 

0.84

1.00 

1.07

1.10 

1.11

1.09 

1.19

1.06

1.18

1.81

1.71

1.62

2.04

Th

(ppm)

12.58

12.99

17.12

13.75

14.25

9.23

4.57

8.57

11.67

22.02

15.98

20.35

12.08

35.42

38.03

21.12

16.64

21.89

25.65

10.34

15.71

12.12

13.78

15.85

16.92

17.84

12.50

21.23

17.47

19.61

18.31

18.49

13.24

30.30

20.49

14.11

15.76 

16.44

11.79 

15.67

12.91 

14.74

15.16

15.08 

14.64

15.75

14.22

19.93 

16.52

17.18

14.35

13.65 

11.75

11.62 

15.55

15.76 

15.55

14.14 

15.13

14.76

17.76

26.43

26.33

26.55

27.75

K Heat

(per- fyical/

cent) g-yr)

2.91 4.37

3.05 4.44

2.60 5.29

2.43 4.72

2.43 5.05

2.70 3.58

2.51 2.15

2.34 3.45

2.30 3.72

2.11 6.20

2.07 4.63

1.87 5.55

1.82 3.57

2.92 8.65

2.87 9.99

2.90 6.34

2.71 5.07

2.58 6.40

2.47 7.32

2.17 3.54

2.47 4.80

2.74 4.12

3.23 4.53

3.22 4.88

3.16 5.16

3.30 5.33

3.29 4.13

3.26 5.97

3.30 5.21

3.10 5.53

3.20 5.66

3.41 5.46

3.53 4.37

3.44 9.09

3.09 6.11

3.06 4.43

2.98 4.89 

3.39 4.96

1.58 3.39 

3.23 4.65

3.28 4.13 

3.46 4.66

3.52 4.75

3.43 4.82 

3.38 4.56

3.48 4.81

3.12 4.48

3.43 5.68 

3.51 5.06

2.65 5.00

3.25 4.43

3.37 4.44 

3.38 3.88

3.54 4.01 

3.36 4.80

3.20 4.82 

3.30 4.81

3.00 4.43 

3.38 4.81

3.38 4.64

1.35 4.78

2.53 7.29

2.41 7.16

2.85 7.26

2.97 7.84

Th

 

RaeU

8.56

9.28

10.70

7.64

6.75

6.69

5.94

5.68

11.11

13.11

13.32

15.30

13.27

33.42

17.21

11.54

11.97

12.09

12.33

8.48

11.55

9.25

11.20

13.78

13.32

14.87

12.38

18.30

15.46

18.68

11.74

16.08

12.49

10.52

12.65

13.19

12.31 

15.96

14.20 

17.81

14.19 

13.91

14.44

12.57 

14.79

15.91

13.05

18.98 

14.88

14.81

15.43

12.41 

13.99

11.62 

14.53

14.33 

14.01

12.97 

12.71

13. "2

15.05

14.60

15.40

16.39

13.60

RaeU

 

KxlO'4

0.51

0.46

0.62

0.74

0.87

0.51

0.31

0.65

0.46

0.80

0.58

0.71

0.50

0.36

0.77

0.63

0.51

0.70

0.84

0.56

0.55

0.48

0.38

0.36

0.40

0.36

0.31

0.36

0.34

0.34

0.49

0.34

0.30

0.84

0.52

0.35

0.43 

0.30

0.53 

0.27

0.28 

0.31

0.30

0.35 

0.29

0.28

0.35

0.31 

0.32

0.44

0.29

0.33 

0.25

0.28 

0.32

0.34 

0.34

0.36 

0.35

0.31

0.87

0.72

0.71

0.57

0.69

Th

 

KxlO-4

4.32

4.26

6.58

5.66

5.86

3.42

1.82

3.66

5.07

10.44

7-72

10.88

6.64

12.13

13.25

7.28

6.14

8.48

10.38

4.76

6.36

4.42

4.27

4-92

5.35

5.41

3.80

6.51

5.29

6.33

5.7?

5.42

3.75

8.81

6.63

4.61

5.29 

4.85

7.46 

4.85

3.94 

4.26

4-31

4.40 

4.33

4.53

4.56

5.81 

4.71

6.48

4.42

4.05 

3.48

3.28 

4.63

4.93 

4.71

4.71 

4-48

4.37

13.16

10.45

10-93

9.32

9.34

TABLE 2. Summary of radioelement and radiogenic heat analyses on 229 core samples from a drill hole in Wind River

Mountains, Wyoming — Continued

! Sample RaeU Th K Heat Th RaeU Th

depth (ppm) (ppm) (per- fytcal/ 

1 (meters) cent) g-yr) RaeU KxlO"4 KxlO' 4

1

12700.5-2703.6 2.45 27.96 2.72 8.11 11.41 0.90 10.28

^2715. 8-2718. 8 2.41 25.82 2.72 7.66 10.71 0.89 9.49

2724.9-2728.0 2.14 28.33 2.76 7.97 13.24 0.78 10.26

2734.1-2737.1 2.30 20.48 2.36 6.41 8.90 0.97 8.68

2746.2-2749.3 1.64 18.53 2.64 5.62 11.30 0.62 7.02

2767.6-2770.6 1.99 24.57 2.67 7.09 12.35 0.75 9.20

2776.7-2779.8 1.87 23.24 2.68 6.74 12.43 0.70 8.67

2788.9-2792.0 2.37 18.78 2.69 6.21 7.92 0.88 6.98

2798.1-2801.1 2.91 17.17 2.86 6.33 5.90 1.02 6.00

2807.2-2810.3 1.58 8.14 3.11 3.62 5.15 0.51 2.62

2816.4-2819.4 2.33 12.79 2.78 5.01 5.49 0.84 4.60

2828.5-2831.6 1.12 7.87 2.81 3.15 7.03 0.40 2.80

2840.7-2843.8 1.18 8.02 3.09 3.30 6.80 0.38 2.60

2846.8-2849.9 1.22 9.60 3.05 3.63 7.87 0.40 3.15

2862.1-2865.1 0.91 6.48 3.04 2.78 7.12 0.30 2.13

2874.3-2877.3 1.19 13.13 2.87 4.27 11.03 0.41 4.57

2886.5-2889.5 1.15 16.06 3.06 4.88 13.97 0.38 5.25

2898.6-2901.7 1.22 16.16 3.05 4.95 13.25 0.40 5,. 30

2907.8-2910.8 13.86 23.05 3.74 15.74 1.66 3.71 6.16

2920.0-2923.0 6.16 21.71 3.21 9.71 3.52 1.92 6.76

2932.2-2935.2 2.50 22.21 2.96 7.07 8.88 0.84 7.50

2941.3-2944.4 1.90 21.40 3.06 6.49 11.26 0.62 6.99

2950.5-2953.5 1.72 16.76 2.88 5.39 9.74 0.60 5.82

2962.7-2965.7 2.08 18.22 2.86 5.93 8.76 0.73 6.37

2974.8-2977.9 1.71 15.54 2.63 5.07 9.09 0.65 5.91

2984.0-2987.0 2.31 13.90 2.43 5.12 6.02 0.95 5.72

2993.1-2996.2 1.87 14.14 2.86 4.97 7.56 0.65 4.94

3002.3-3005.3 1.56 18.48 2.83 5.60 11.85 0.55 6.53

3014.5-3017.5 1.52 17.77 2.87 5.44 11.69 0.53 6.19

3023.6-3026.7 2.56 23.03 2.54 7.16 9.00 1.01 9.07

3035.8-3038.9 2.16 13.02 2.46 4.84 6.03 0.88 5.29

3045.0-3048.0 1.73 10.52 2.42 4.02 6.08 0.71 4.35

3051.0-3054.1 1.71 9.89 2.65 3.94 5.78 0.65 3.73

3060.2-3063.2 2.66 16.89 2.57 6.01 6.35 1.04 6.57

The thorium content for all samples ranges from

4.57 to 78.3 ppm; the mean and standard deviation is

17.6±8.76 ppm, which is very near the average for

silicic igneous rocks (table 3). Most of the thorium

analyses are within the range of averages reported

for intermediate and silicic igneous rocks. The thorium content in the depth interval from 2,040 to 2,650

m is limited to a narrower range than that of all

samples, but the mean content (16.ltiJ3.38 ppm) is not

significantly different. Virtually all the samples in

which the thorium content is greater than three

standard deviations from the mean are from a highly

fractured zone near the top of the drill hole.

The potassium content ranges from 1.35 to 5.23

percent; the mean and standard deviation is 3.09±

0.58 percent. The potassium content in most of the

samples is within the range commonly measured in

rock types ranging from granodiorite to quartz monzonite. The mean and standard deviation of the

potassium analyses in the interval from 2,040 to

9 fi50 m is 3 94+ft 34 nerrent.
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———————————— \r ————————————————————————— "' 

PARTS PER MILLION PERCENT 

FIGURE 5.—Radioelement concentrations and ratios and radiogenic heat in 3.06-m-deep drill hole. Long tick marks indicate p

scale limit. Sample depths are at midpoint of sampled interval.
12 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

TABLE 3. Averages of radioelement contents and ratios and radiogenic heat for samples from a 3.06-kilometer drillhole, Wind River 

Mountains, Wyoming, and summary of published averages for igneous rocks

[Drill hole data reported as average ±1 standard deviation]

Constituent

All samples 2040- to 2650-m interval Average in igneous rocks

Average Range Average Range Continental Mafic Intercrust mediate

Silicic

RaeU (ppm)

Th (ppm) •

K (percent) •

2.0211.33 0.42-13.86 1.1510.33 0.83- 2.88

17.6 ±8.8 4.57-78.3 16.1 ±3.4 11.6 -30.3

3.09±0.58 1.35- 5.23 3.24±0.34 1.58- 3.54

2.8

'0.9

'6-10 2.7

'2.0-2.6

1.5-9.3

d !.6-2.6 d0.6-0.75 d2. 7-3.0

4.7 

4.75

'20 

J 18

Th/RaeU———————— 10.3 ±6.7 1.66-67.5 14.3 ±2.0 10.5 -19.0

RaeU/KxlO~4 ————— 0.68±0.44 0.09- 4.64 0.36±0.10 0.25- 0.84

Th/KxlO~4 —————— 5.76±2.43 1.20-15.9 5.01±1.12 3.28- 8.81

3.5-4 <3 4.1

'l V6 g0.7-l

: 3.3 ^2.8 g2.8-3.4

4.5 

4.0

1.29 

5.0

Heat (ycal/g-yr)- 5.84±2.20 1.93-17.9 4.93±0.90 3.39- 9.09 g3.7-4.6 gi.4 g 3.9-4.6 g7.5-8.5

Rogers and Adams (1969b).

^Heier and Rogers (1963).

'Rogers and Adams (1969a).

Cocco and others (1970).

Clark, Peterman, and Heier (1966). 

Z. E. Peterman (written commun., 1963) 

Calculated from published values.

The uranium (RaeU) content of all samples from 

the exploratory hole ranges from 0.42 to 13.86 ppm. 

Most of the uranium analyses are within the range of 

averages for mafic to intermediate rocks. The mean 

and standard deviation is 2.02*1.33 ppm, which is 

near the average for diorite and quartz diorite (Clark 

and others, 1966). The mean and standard deviation 

of the uranium content for the depth interval from 

2,040 to 2,650 m is 1.15±0.33 ppm, which is close to 

the average for mafic igneous rocks (table 3). The 

uranium content in this 61 Om interval is less than 

the content usually measured in quartz monzonite. A 

histogram of the data (fig. 6) shows that low uranium 

content is not unique to this interval, but most of the 

samples have higher content.

The Th/RaeU ratio ranges from 1.66 to 67.5. A 

histogram of the data (fig. 6) indicates a bimodal 

distribution with the two groups separated at a ratio 

of about 10.0. The data from the 2,040- to 2,650-m 

depth interval are limited to and contribute greatly 

to the group of higher ratios. The average Th/U ratio 

in most rocks ranges from 3 to 5; virtually all samples 

from the drill hole have ratios greater than normal. 

These abnormally high ratios are indicative of either 

uranium depletion or excess thorium.

Average U/KX10-4 ratios range from 0.6 in mafic

rocks to 1.3 in silicic igneous rocks. The mean and 

standard deviation for all samples from the exploratory hole is 0.68±0.44, and for the 2,040- to 2,650-m 

depth interval they are 0.36*0.10. The ratios from the 

drill hole samples indicate either that the rock types 

are in the mafic to intermediate classification, that 

uranium is depleted, or that potassium is in excess. 

Average Th/KX10-4 ratios range from 2.8 in mafic 

rocks to 5.0 in silicic igneous rocks. Mean and 

standard deviations for the drill hole data are 5.76+± 

2.43 for all samples, and 5.01±1.12 for the 2,040- to 

2,650-m depth interval; a histogram of the data 

indicates no significant difference in the two sets of 

data. The means are not greatly different from the 

average for silicic igneous rocks, although the ratios 

in the small percentage of the samples that contain 

anomalously large amounts of thorium are significantly greater than normal.

A histogram of the radiogenic heat data (fig. 6) 

indicates no major difference between the distribution for all samples and that for the 2,040- to 2,650-m 

depth interval. The means and standard deviations 

for these two groups of samples are 5.84^2.20 and 

4.93*0.90, respectively. Most of the radiogenic heat 

values are similar to those calculated in intermediate 

to silicic igneous rocks.
RADIOELEMENT DISTRIBUTION, PRECAMBRIAN CRYSTALLINE ROCKS, WIND RIVER MOUNTAINS, WYOMING 13

20 r- 20 r

15

10

•tfOOCMtOO^OOCM tO O O

CM CM CM CO CO ^

RaeU, IN PARTS PER MILLION

15

10

JJ

Th/RaeU

i\> 

15

10

5

LU n

-

-

_

M

—

^- ii

in

Pi

—

m

—

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iTU^-^ „ n p. ^COCNCOO^COCNCOOO 

= «-«-CNCNCNOOC»>^-^- ^> A

< Th,IN PARTS PER MILLION

20 i15

10

o o o o

*d- tO 00 O CM CD co oo

(RaeU/K)X10-4

15

10

og^o'tfoocsi^o^'oooo

«~ i— CNCNCNMCO^ ^ -y in in

A

K, IN PERCENT

20

15

10

°. °. °.

«- CN CO

pop

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O O O O O 

00 O) C) r-' r^

(Th/K)X10-4

20 r15

10

O O

CN co

pop p

O «— OJ CN '"'"'" ^

HEAT, IN M cal/g-YEAR

FIGURE 6. Frequency distribution of radioelement concentrations and ratios and radiogenic heat. Open bars are all 

229 samples; shaded bars are 37 samples at depth interval 2,040-2,650 m plotted as a percent of the total.
14 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

CONCLUSIONS

The variable distributions of the uranium, 

thorium, and potassium contents throughout the 

depth of the drill hole substantiate observations of 

similar variations in rock type and composition 

determined from petrographic examinations of a few 

core samples from the hole. The radioelement data 

and geophysical logs of the hole indicate that the 

drill hole penetrates a thick section of layered, 

fractured, heterogeneous rock.

Thorium and potassium analyses indicate that the 

rock types range from intermediate to silicic and that 

virtually all the samples are more silicic than 

granodiorite, based on published average 

radioelement contents for those rock classifications.

Radium equivalent uranium analyses indicate 

that the uranium content in most of the samples is 

less than the average reported for granodiorite. The 

amount of uranium measured may be less than the 

amount present because of a deficiency of daughter 

products in the uranium decay series. If the uranium 

is not in equilibrium, the disequilibrium is probably 

long lived and probably occurs among the isotopes 

238U, 234U, and 230Th. If the uranium series is in 

equilibrium, abnormally low uranium content in the 

original material from which the rock was formed is 

indicated. We favor the possibility that the uranium 

was mobilized and removed, probably at the time of 

metamorphism.

Uranium content is lower in relatively unfractured 

sections, for example, in the depth interval from 

2,040 to 2,650 m, than it is in highly fractured 

sections of the hole. The uranium content in the 

unfractured sections may represent the content of 

that rock type prior to the occurrence of the overthrust and its associated fracturing; if so, the higher

uranium content in the fractured zones may represent enrichment through migration of uranium from 

other areas in the Wind River Mountains. If the 

uranium loss in the unfractured rock occurred during 

or after the thrusting, the uranium may have migrated vertically and redeposited in the fractures.

REFERENCES CITED

Clark, S. P., Jr., Peterman, Z. E., and Heier, K. S., 1966, Abundances of uranium, thorium, and potassium, sec. 24 of Handbook of physical constants (revised ed.): Geological Society 

of America Memoir 97, p. 521-541.

Cocco, G., Fanfani, L., Zanazzi, P. F., Heier, K. S., and Billings, 

G. K., 1970, Potassium, chapter 19 of Wedepohl, K. H., ed., 

Handbook of geochemistry, v. 2, no. 2: New York, SpringerVerlag, p. Al-Nl.

Ebens, R. J., and Smithson, S. B., 1966, Petrography of Precambrian rocks from a 3.05-kilometer-deep borehole, Wind 

River Mountains, Wyoming: University of Wyoming Contributions to Geology, v. 5, p. 31-38.

Heier, K. S., and Rogers, J. J. W., 1963, Radiometric determination 

of thorium, uranium, and potassium in basalts and in two 

magmatic differentiation series: Geochimica et Cosmochimica Acta, v. 27, no. 2, p. 137-154.

Lachenbruch, A. H., and Bunker, C. M., 1971, Vertical gradients 

of heat production in the continental crust, 2. Some estimates 

from borehole data: Journal of Geophysical Research, v. 76, 

no. 17, p. 3852-3860.

Rogers, J. J. W., and Adams, J. A. S., 1969a, Thorium, chapter 

90 of Wedepohl, K. H., ed., Handbook of geochemistry, v. 2, 

no. 4: New York, Springer-Verlag, p. Bl-Ol.

___1969b, Uranium, chapter 92 of Wedepohl, K. H., ed., Handbook of geochemistry, v. 2, no. 4: New York, Springer-Verlag, 

p. Bl-Ol.

Schonfeld, Ernest, 1966, Alpha M An improved computer program for determining radioisotopes by least-squares resolution of the gamma-ray spectra: Oak Ridge, Tenn., U.S. National Laboratory (Publications), ORNL-3975, 43 p.

Smithson, S. B., and Ebens, R. J., 1971, Interpretation of data 

from a 3.05-kilometer borehole in Precambrian crystalline 

rocks, Wind River mountains, Wyoming: Journal of Geophysical Research, v. 76, p. 7079-7087.
Ages of Igneous Rocks in the South ParkBreckenridge Region, Colorado, and 

their Relation to the Tectonic History 

of the Front Range Uplift

By BRUCE BRYANT, RICHARD F. MARVIN, CHARLES W. NAESER, 

and HARALD H. MEHNERT

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

IN THE WESTERN UNITED STATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1199-C
CONTENTS

Page

Abstract...................................................................................................................................................... 15

Introduction .............................................................................................................................................. 15

Outline of the geology of the South Park-Breckenridge region.............................................. 17

Fission-track dating procedure .......................................................................................................... 17

Age of the South Park Formation .................................................................................................... 17

Intrusive rocks..-..................................................................................................-....---.- ------- 19

Sample localities........................................................................................... ................................ 21

Age of the intrusive rocks............................................................................................................ 21

Extrusive rocks ........................................................................................................................................ 23

Age of the main deformation in South Park and along the margin of the Front

Range uplift...................................................................................................................................... 24

Implications of ages for the extent of the Eocene erosion surface........................................ 24

References cited...................................................................................................................................... 25

ILLUSTRATIONS

Page

FIGURE 7. Index map of central and western Colorado .......................................................... 16

8. Geologic map of the South Park-Breckenridge region ........................................ 18

9. Graphical representation of mineral ages................................... .......................... 19

10. Normative quartz-plagioclase-orthoclase diagram for intrusive rocks

of the South Park-Breckenridge region ................................................................ 21

TABLES

Page

TABLE 4. K-Ar and fission-track ages of igneous rocks in the South ParkBreckenridge region .................................................................................................. 20

5. Comparative classifications of intrusive rocks in the South ParkBreckenridge region...................................................................................................... 21

6. Location and description of dated and analyzed samples ................................ 28

7. Analytical data for K-Ar ages of igneous rocks in the South ParkBreckenridge region...................................................................................................... 32

8. Analytical data for fission-track ages of igneous rocks in the South

Park-Breckenridge region............................................................................................ 33

9. New chemical analyses and norms of Tertiary igneous rocks from the

South Park-Breckenridge region .............................................................................. 34
SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES

AGES OF IGNEOUS ROCKS IN THE SOUTH PARK—BRECKENRIDGE

REGION, COLORADO, AND THEIR RELATION TO THE TECTONIC

HISTORY OF THE FRONT RANGE UPLIFT

By BRUCE BRYANT, RICHARD F. MARVIN, CHARLES W. NAESER, and HARALD H. MEHNERT

ABSTRACT

Potassium-argon ages of biotite and fission-track ages of 

zircon, sphene, and apatite show that porphyries of the Colorado 

mineral belt between the Breckenridge and Tarryall districts are 

predominantly of late Eocene and early Oligocene age, ranging 

from 35 to 42 m.y. (million years) old. A discrepancy between 

biotite ages of 44 to 50 m.y. and zircon, sphene, and apatite ages of 

35-42 m.y. in some rocks may be due to excess argon derived from 

Precambrian basement rocks rather than to later regional heating 

or slow cooling. Concordant ages of biotite and zircon were found 

in one sample of porphyry at 41 m.y. The existence of coeval 

extrusive rocks is indicated by a 40-m.y.-old andesite in a 

paleov alley fill in southwestern South Park.

Radiometric ages show that sedimentary and volcanic rocks 

were deposited in the South Park basin throughout the Paleocene. 

The main phase of the Laramide orogeny in South Park took place 

in the early Eocene, when the basin fill was folded, faulted, and 

overridden by the west margin of the Front Range uplift and 

before intrusion of the porphyries.

INTRODUCTION

The South Park-Breckenridge region lies at the 

intersection of the northeast-trending Colorado 

mineral belt and the north-northwest-trending western margin of the Front Range uplift (fig. 7). The 

Colorado mineral belt, characterized by many Late 

Cretaceous and Tertiary intrusive rocks and associated mineral deposits, was recognized by Spurr 

and Garrey (1908) and described in some detail by 

Lovering and Goddard (1950). Lovering and Goddard had no way of determining the ages of the 

intrusive rocks, but determined relative ages in part 

on the assumption that petrographically similar 

rocks were of similar age. They concluded that 

intrusion of the igneous rocks of the belt occurred

over a long period of time and progressed from 

southwest to northeast.

More recently, isotopic dating has furnished ages 

of the intrusive porphyries of the mineral belt. K-Ar 

ages of minerals from the porphyries in the northeastern part of the belt and in the Leadville district 

adjacent to the Sawatch Range are 72 to 64 m.y., or 

Late Cretaceous to earliest Paleocene (Hart, 1960; 

1964; Pearson and others, 1962). Further isotopic 

dating revealed that a few of the intrusives are 26 to 

38 m.y. old (Oligocene), principally the ones associated with molybdenum deposits (McDowell, 1971; 

Taylor and others, 1968; Schassberger, 1972). Farther southwest along the mineral belt, isotopic dating of rocks in the Aspen region showed that small 

intrusives in the mining district are 67 to 72 m.y. old 

(Late Cretaceous) but that the larger igneous complexes of the Elk Mountains are 29 to 34 m.y. old 

(Oligocene) (Obradovich and others, 1969). This 

latter age range is similar to that of the bulk of the 

volcanic field of the San Juan Mountains in southwestern Colorado (Lipman and others, 1970). Relatively few ages in the range 45-55 m.y. have been 

obtained from intrusive rocks in the mineral belt, but 

some have been reported.

Intrusive rocks along a 20-km segment of the 

mineral belt in the South Park-Breckenridge region 

have been determined by us to be late Eocene to early 

Oligocene in age, 41 to 35 m.y. old. The relation of 

these rocks to the fault at the west margin of the 

Front Range uplift indicates that these intrusions, 

occurred after Laramide uplift of the Front Range 

block.

15
16 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

108° 106° 104°

50 100 KILOMETERS

Area of figure

COLORADO

EXPLANATION

Tertiary volcanic rocks

Tertiary and Upper Cretaceous intrusive rocks

Phanerozoic sedimentary rocks

Precambrian rocks 

. Thrust fault, teeth on upper plate 

• Fault

Outline of Colorado mineral belt

FIGURE 7. Index map of central and western Colorado showing the location of the South Park-Breckenridge area outlined and major 

tectonic features associated with the Front Range uplift. Simplified from King and Beikman (1974).
AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 17

OUTLINE OF THE GEOLOGY OF THE 

SOUTH PARK-BRECKENRIDGE REGION

A continuous downfolded and faulted belt of Cretaceous and older sedimentary rocks, 250 km long, 

borders the Front Range on the west and separates it 

from the Park, Gore, Tenmile, and Mosquito Ranges 

farther west (fig. 7). Mountains formed by Tertiary 

igneous rock divide this belt into three broad basins: 

North, Middle, and South Parks. Along 150km of the 

west margin of the Front Range uplift, Precambrian 

rocks are thrust westward over sedimentary rocks of 

the basins along the Williams Range fault in Middle 

Park and the Elkhorn fault in South Park (fig. 8). In 

the South Park-Breckenridge region, Tertiary intrusives of the Colorado mineral belt interrupt the belt 

of sedimentary rock west of the Front Range uplift 

and separate Middle Park from South Park.

In South Park, Tertiary sedimentary and volcanic 

rocks as much as 3,000 m thick form the South Park 

Formation (Sawatzky, 1967; Wyant and Barker, 

1976). The South Park Formation overlies the Upper 

Cretaceous Laramie Formation, Fox Hills 

Sandstone, and Pierre Shale. At the base of the South 

Park Formation are andesitic flows and breccias and 

tuffaceous sandstone and conglomerate of the 

Reinecker Ridge Volcanic Member (Wyant and 

Barker, 1976). These volcanic and volcaniclastic 

rocks are overlain by a conglomerate rich in igneous 

clasts in the basal part. Stratigraphically higher, the 

conglomerate also contains sedimentary clasts from 

Paleozoic and Mesozoic rocks that crop out on the 

west side of the South Park basin. Beds of sandstone, 

mudstone, and tuff interlayered with the 

conglomerate make up a large proportion of the 

upper part of the unit. The Link Spring Tuff Member, 

about 2,000 m above the base of the South Park 

Formation, consists of tuff, tuffaceous conglomerate, 

and andesite as much as 200 m thick. The upper part 

of the South Park Formation is composed of arkose, conglomerate, mudstone, and tuff and contains beds of 

boulder conglomerate in its highest part. Boulders in 

the conglomerate are as much as 3 m in diameter and are 

composed mainly of Precambrian granitic rock like that exposed along the margin of the Front Range uplift east of the 

Elkhorn fault. Although these rocks rest unconfromably on 

Cretaceous rocks, the principal deformation in the South 

Park basin occurred after deposition of the South Park Formation, because the main folds and faults deform both the 

Cretaceous and Tertiary rocks.

Volcanic rocks in the South Park Formation are 

thicker in the western part of South Park (fig. 8) and

must have been derived from the area southwest of 

the intrusive rocks we have dated in this study, 

probably from near Leadville or east of Leadville, 

where igneous intrusive rocks of Paleocene age are 

exposed (Pearson and others, 1962; Young, 1972).

FISSION-TRACK DATING PROCEDURE

Sphene and zircon separates were dated by the 

external detection method, using muscovite as the 

detector. A geometry factor of 0.5 was used to 

determine the induced track density for the age 

equation. The apatite ages were determined using 

the population method. The defect density in most of 

the apatite concentrates was too high to permit the 

use of the external detection method. The neutron 

dose was determined by counting tracks in a muscovite detector, which covered National Bureau of 

Standards glass SRM 962 during its irradiation. 

This muscovite-glass pair has been calibrated using 

the NBS copper flux values. The errors on the 

apatite, sphene, and zircon (less than 4 grains) were 

calculated by combining the standard deviations 

(number of counts) for the induced and fossil counts. 

The standard deviations for samples in which five or 

more sphene or zircon grains were counted were 

calculated using the procedure outlined by Naeser 

and others (1978).

AGE OF THE SOUTH PARK FORMATION

The South Park Formation (Denver Formation of 

Stark and others, 1949) contains leaves indicative of 

a Paleocene age (Stark and others, 1949; Brown, 

1962). The localities specified in these publications 

are all in the lower part of the South Park Formation, 

below the Link Springs Tuff Member. Sawatsky 

(1967) obtained a late Paleocene or early Eocene 

K/Ar whole-rock age of 56±2.6 m.y. from the lowest 

exposed flow in the Reinecker Ridge Volcanic Member at Devils Gap (fig. 8), the type locality of the 

Reinecker Ridge Volcanic Member (Wyant and Barker, 1976). That date suggested that much of the 

.South Park Formation might be significantly younger than the comparable lower Tertiary deposits of 

the Denver Formation on the east side of the Front 

Range uplift.

Our studies of the South Park Formation, however, 

show that the epoch of sedimentation in the South 

Park basin was roughly comparable to that of the 

Denver Formation in the Denver basin, as earlier
18 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

106°

15 KILOMETERS

_J

EXPLANATION

Tertiary ^ 

intrusive rocks

Tertiary volcanic rocks: 

Twm, Wall Mountain Tuff; 

Tan, andesite

Tsp, Paleocene South Park Formation: 

Tsl, Link Springs Tuff Member; 

Tsr, Reineker Ridge Volcanic Member

Other Tertiary and Mesozoic and Paleozoic 

sedimentary rocks

Precambrian rocks

Thrust fault, teeth on upper plate, dashed 

where inferred

Fault, bar and ball on downthrown side, 

dashed where inferred

Axis of syncline

Locality of dated sample, listed in tables 4 and 6

Locality of sample dated by Sawatzky (1967)

FIGURE 8.-Geologic map of the South Park-Breckenridge region showing location of samples from which minerals were dated 

(numbers indicate locality). Simplified from Bryant and others (1980); Tweto and others (1978), and Scott and others (1978).
AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 19

workers believed. The age of samples from the base of 

the South Park Formation (localities 12 and 13, table 

4 and fig. 8) is early Paleocene. The sample at locality 

13 was taken about 200 m above the base of the 

Reinecker Ridge Volcanic Member and thus is from a 

horizon higher than that corresponding to Sawatzky's sample. According to Sawatzky's description 

of his dated sample and our observations at his 

sample locality, the rock there is altered. (See field 

no. 323, table 9.) Consequently, his whole-rock age 

may be too young. The sample from locality 12 is a 

crystal tuff from about 3 m above the base of the 

South Park Formation in an area where rocks younger than the Reinecker Ridge Volcanic Member form 

the base of the formation. The rock at this locality 

probably is stratigraphically equivalent to rocks 

directly above that member. The biotite ages from 

localities 12, 11, and 10 are progressively younger 

and fit the stratigraphy. Fission-track ages of coexisting zircons from these localities are in good 

agreement with the biotite ages in great contrast to 

some of the biotite and zircon ages for the younger 

intrusives (fig. 9, table 4). These data show that 

deposition of the South Park Formation occupied the 

entire Paleocene. Some of the lowest beds may be of 

latest Cretaceous age, and the uppermost beds, 

which were not dated, might be as young as earliest 

Eocene.

INTRUSIVE ROCKS

In intrusive bodies of the Breckenridge area, Ransome (1911) and Levering (1934) recognized three 

main rock types: a quartz monzonite porphyry distinguished by phenocrysts of potassic feldspar as 

much as several centimeters long; a monzonite porphyry characterized by smaller phenocrysts of hornblende, biotite, and plagioclase and by a lack of 

quartz phenocrysts; and an intermediate type they 

called quartz monzonite. Ransome (1911) found no 

contacts between the rock types and believed that 

they were closely related in age and origin. Levering 

(1934) found a few exposures where the quartz 

monzonite porphyry cuts monzonite and quartz monzonite, and he inferred, assuming a normal sequence 

of magmatic differentiation, that the porphyritic 

quartz monzonite was younger than the other two 

rock types. In some intrusives, both Levering and we 

have noted gradations between the monzonite and 

quartz monzonite. The mapping of those two rock 

types may be rather arbitrary in many places.

Pride and Robinson (1978) determined a sequence 

of intrusion in the area of the Wirepatch Mine, in the

10

11

12

13

GEOLOGIC AGE

CRETACEOUS PALEOCENE

EOCENE

OLIGOCENE

70 60 50 40 30 

AGE IN MILLIONS OF YEARS

EXPLANATION 

• K-Ar biotite age

Fission-track ages 

A Zircon A Sphene O Apatite

FIGURE 9. Graphical representation of mineral ages from rocks 

in the South Park-Breckenridge region. Horizontal bars show 

range of uncertainty (2 sigma) for each determination. Two lines 

between epochs delineate range of uncertainly of absolute age 

of boundary (J. D. Obradovich and G.A. Izett, written commun., 

1979).

eastern part of the Breckenridge district, in which 

quartz monzonite porphyry is younger than monzonite poyphyry and also includes a younger rhyodacite porphyry and intrusive breccia. They cite an 

age of 41.4 m.y. for the rhyodacite porphyry but do 

not say how that age was determined.

In the Tarryall district, south of Breckenridge, 

Muilenburg (1925) mapped the south end of a large 

sill (the Bald Mountain sill, fig. 8) as quartz monzonite porphyry, because quartz phenocrysts were
20 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

TABLE 4. — K-Ar and fission-track ages of igneous rocks in the 

South Park-Breckenridge region

[Localities and sample descriptions in table 6; analytical data in tables 7 and 8 in 

appendix. Leaders (......) indicate no determination made.]

Loc. Field K-Ar FissionNo. No. biotite age track age

Mineral 

dated

Intrusive rocks

1 223 44.8±1.5 35. 1± 3.6 

43.1±1.0 

1 44.0±1.0

2 222 50.6±1.7 38. 3± 1.8 

45.5±1.1 36. 6± 2.0 

2 49.1±1.2 36. 2± 1.6 

466 —————— 38. 2± 1.8

3 74 44.0±1.5 40. 4± 1.7 

37. 9± 1.6 

42. 7± 8.2

4 233 —————— 39. 9± 2.0

35. 5± 1.4 

41.1±10

5 503 3 40.7±1.4 40. 3± 1.8 

40. 6± 5.8 

503a 43.2±1.5 ——————

6 502 —————— 41. 5± 1.8

7

T>Q 17 £.+ 1 <;

4 56. 4± 7.4

Zircon.

Do. 

Do. 

Do. 

Do.

Sphene. 

Zircon. 

Apatite.

Sphene . 

Zircon. 

Apatite.

Zircon. 

Apatite.

Zircon.

Do. 

Apatite.

Extrusive rocks

8 430a 37.8±0.9 41. 5± 1.8 

42.8+ 5.1

9 420 —————— 36. 4± 1.9

Zircon. 

Apatite.

Zircon.

South Park Formation

10 W-l-75 56.3±1.3 55. 5± 2.7 

66.5±11.6

11 150 59.7±2.0 54. 7± 4.8 

66.4±12

12 444 65.5±1.6 64. 1± 3.7 

59. 3± 8.1

n

OOC £f\ Q+ O O

63. 3± 8.5

Zircon. 

Apatite.

Zircon. 

Apatite.

Zircon. 

Apatite.

Zircon. 

Apatite.

Contains minor hornblende. 

Contains some hornblende. 

3 Contains minor hornblende and chlorite. 

Has many defects; age only approximate.

visible in the rock there. The north end of this sill was 

called monzonite porphyry by Ransome (1911). Muilenburg called an intermediate rock, containing less 

quartz, Silverheels Quartz Monzonite Porphyry, although he pointed out that it was impossible to 

consistently differentiate the two rock types. A third, 

more mafic rock type, he designated diorite porphyry. In the same region, Singewald (1942; 1951) 

mapped a monzonite porphyry containing little or no 

visible quartz, a quartz monzonite porphyry, and 

Lincoln Porphyry, characterized by large phenocrysts of potassic feldspar and resembling the quartz 

monzonite porphyry of Ransome (1911) in the Breckenridge district. Singewald (1942) also pointed out 

the difficulty of distinguishing between the first two 

map units. On his maps he designated the rock in the 

sill on Bald Mountain southwest of Breckenridge as 

quartz monzonite and diorite (1942) and as monzonite porphyry (1951).

Stark and others (1949) gave a formal name, Esche 

Porphyry, to the igneous rocks, including the south 

end of the Bald Mountain sill, on the northwest side 

of South Park near Como, and they described the 

Esche Porphyry as quartz monzonite porphyry and 

quartz diorite porphyry. They thought that the igneous rocks were emplaced contemporaneously with 

deposition of the South Park Formation, although 

they added that "at least some intrusions are later 

than the large north-trending faults of South Park."

This review of the rock names applied to the 

intrusive rocks of the South Park-Breckenridge region illustrates the lack of precision of field and 

petrographic determinations of the rock types. Chemical analyses (table 9; Ransome 1911; Muilenburg, 

1925; Phair and Jenkins, 1975) show that these rocks 

range from rather potassic diorite to quartz monzonite (fig. 10). Most of the rocks are granodiorite to 

quartz monzonite, even though the term monzonite 

has been applied to some of the larger bodies, such as 

the Bald Mountain sill (localities 4 and 5, fig. 8).

Bryant and others (1980) generally followed the 

classification of Ransome (1911), because it seemed 

the most practical for purposes of field mapping 

(table 5).

The intrusive rocks occur as sills, plugs, and dikes, 

and they form the high peaks separating Middle 

Park from South Park. For example, Mt. Guyot (4,075 

m) is a plug of the quartz monzonite (fig. 8, locality 3). 

Heat from this intrusion has metamorphosed Pierre 

Shale and a slice of Dakota Sandstone in the hanging wall of the Williams Range fault (fig. 8). Bald 

Mountain (4,170 m) is a sill of monzonite porphyry as 

much as 1 km thick; this sill cuts branches of the
AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 

TABLE 5.—Comparative classifications of intrusive rocks in the South Park-Breckenridge region

21

Ransome (1911) Bryant and others (1980) Rock types based on 

chemical analysis

Localities 

dated

Ouartz monzonite porphyry Porphyritic quartz monzonite Quartz monzonite—

Quartz monzonite—— 

Monzonite porphyryQuartz monzoniteMonzonite—————-

Granodiorite———————————— 

Ouartz monzonite (7), granodiorite (2, 4, 5), and very 

minor potassic diorite.

1 

3

2, 4, 5, 

6, 7

Boreas Pass-South Park fault zone (fig. 8, localities 4 

and 5). A small plug of monzonite porphyry located 

at the junction of the Middle and South Forks of the 

Swan River is intrusive into and has metamorphosed brecciated Precambrian rock along the Williams Range fault zone (fig. 8, locality 2).

The intrusive bodies predate some fault movements. A sill in the Benton Group west of Kenosha 

Pass is cut by faults that, if they are vertical, have 

displacements as much as 200 m (fig. 8). The west 

margin of the Bald Mountain sill is offset by faults. 

Levering (1934) shows numerous faults, with displacements as great as 200 m, that cut the intrusive 

rocks near Breckenridge.

FIGURE 10.—Normative quartz-plagioclase-orthoclase diagram 

for intrusive rocks of the South Park-Breckenridge area. Based 

on chemical analyses from Ransome (1911), Muilenburg(1925), 

Phair and Jenkins (1975), and table 8, this paper. Circled points 

are analyses of rocks dated in this study; numbers indicate 

locality (fig. 8). Squares are averages for rock types from 

Nockolds (1954); D, diorite; M, monzonite; GD, granodiorite; 

QM, quartz monzonite.

SAMPLE LOCALITIES

We sampled each of the three major rock types for 

age determinations (table 5; figs. 8 and 9). One 

sample each of monzonite and quartz monzonite are 

from intrusives at or near the Williams Range fault. 

Locality 1 is in a sill of porphyritic quartz monzonite 

west of the Williams Range fault. Locality 2 (fig. 8) is 

in a small monzonite intrusive in the Pierre Shale 

along the fault zone. Dikes from the intrusive cut the 

fault zone, and brecciated Precambrian rock in the 

fault zone has been contact metamorphosed. Locality 3 is on the northeast margin of the Mt. Guyot plug 

near metamorphosed Pierre Shale and Dakota Sandstone west of the Williams Range fault. Localities 4 

and 5 are in monzonite porphyry from the southeast 

margin of the Bald Mountain sill in the area where it 

cuts branches of the Boreas Pass-South Park fault 

zone. Locality 6 is in a thin sill of monzonite in the 

Pierre Shale. Thin section study indicates that the 

rock at locality 6 is similar to that at localities 4 and 5 

but is much more altered.

AGE OF THE INTRUSIVE ROCKS

K-Ar and fission-track ages of minerals are shown 

in fig. 9 and table 4; analytical data are listed in 

tables 7 and 8 in the appendix.

Biotite ages from localities 1 through 3 are 44 to 55 

m.y., but fission-track ages of zircon from these same 

localities are 35-38 m.y. Thus there are discrepancies 

of 9 m.y., 8 to 13 m.y., and 6 m.y. between the K-Ar 

ages of the biotite and the fission-track ages of the 

zircon in samples from localities 1, 2, and 3 respectively. Ages were determined for biotite from three 

different separates of the samples from localities 1 

and 2 in an attempt to resolve these discrepencies. To 

confirm the zircon ages, a second determination was 

made from the original separate from the sample of 

locality 2, and a third was made on a new separate 

from the same sample.

To further test the validity of the discordant ages 

for samples from locality 2, we dated a zircon from
22 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

contact-metamorphosed brecciated Precambrian rock 

(locality 2, sample 466) adjacent to the intrusive. The 

age of 38.2 m.y. is the same as that obtained from 

zircons from the intrusive rock. Fission-track ages of 

apatite (Bryant and Naeser, 1980) from Precambrian 

rocks 1-2 km from the exposed plutons are older than 

the zircon ages from the intrusive rocks; thus the 

discrepancy between zircon and biotite ages cannot 

be due to regional heating or cooling 35-38 m.y. ago.

Excellent agreement of ages of about 41 m.y. was 

obtained from biotite, zircon, and apatite from the 

Bald Mountain sill at locality 5 (table 4). A second 

biotite determination from locality 5 was made on an 

inclusion, and it agrees well with those from the sill, 

within the respective limits of analytical uncertainty. The sample from locality 4 is from the same sill, 

about 2km from locality 5. The two samples are 

petrographically similar (table 6), except that the 

rock at locality 4 is somewhat more altered than that 

at 5. Ages of sphene and apatite from locality 4 agree 

with the ages from locality 5, but the zircon is about 5 

m.y. younger at 4 than at 5. We conclude that the 

Bald Mountain sill was emplaced about 41 m.y. ago 

in the late Eocene. A small sill at locality 6, 6 km to 

the southeast, has the same age.

The general agreement of ages of apatite, sphene, 

and zircon from the intrusive rocks indicates that the 

rock cooled quickly, for the annealing temperature of 

apatite is much lower than that of sphene and zircon 

(Naeser and Faul, 1969). A notable exception is from 

locality 7, where the apatite age of 56.4 m.y. is older 

than the zircon age of 37.6 m.y. In that sample, 

however, the apatite has many crystal defects that 

may have been counted as fission tracks.

The authors have found, in many previous investigations where both fission-track and K-Ar ages 

were determined, that agreement of K-Ar and fissiontrack ages is usually good, except where a thermal 

event has caused loss of radiogenic argon and (or) 

annealing of fission tracks (Cunningham and others, 

1977; Naeser and others, 1977). Following such an 

event, the fission-track ages of sphene are usually 

similar to the K-Ar age of biotite, which, in turn, is 

usually greater than the fission-track age of zircon, 

which is greater than the fission-track age of apatite 

(Harrison and others, 1979). In the present study, 

however, the K-Ar and fission-track ages for minerals from localities 1 through 3 indicate a rather 

unusual situation. The K-Ar biotite ages are significantly older than the fission-track zircon ages of 

sphene, zircon, and apatite. The fission-track ages of 

these three minerals are concordant. It seems illogical that a thermal event, after emplacement, 

could anneal fission tracks in the zircons without

resetting the apatite ages in nearby Precambrian 

country rock. Furthermore, it seems unreasonable 

that 5 to 10 m.y. could have elapsed between the time 

biotite became a closed system, and the time that 

zircon fission tracks were annealed, without producing marked discrepancies in fission-track ages of 

sphene, zircon, and apatite.

Based on the information given above, the most 

logical explanation is that there is excess radiogenic 

argon in the biotite. It appears that the biotite from 

locality 1 has a nearly constant amount of excess 

argon distributed throughout the biotite grains, resulting in good agreement for triplicate argon determinations (table 7). Even the third biotite separate, 

which has a discernible hornblende impurity (indicated by the K2 O content, table 7, appendix), has a 

K-Ar age concordant with those of the two clean 

biotite separates. The ages of biotite from locality 2, 

however, indicate that excess argon varies in amount among individual biotite grains. Again, a 

significant amount of impurity in the third biotite 

sample run from locality 2 did not produce a biotite 

age greater than that determined for one of the 

clean samples. Therefore, excess argon does not 

appear to be concentrated in the impurities, but must 

be partitioned among the mafic minerals in an 

approximately even manner.

In line with this reasoning, the fission-track ages 

are accepted as the ages of emplacement of the 

plutons at localities 1 through 3, which are 35, 37, 

and 38 m.y., respectively, or early Oligocene.

The Montezuma stock, northeast of Breckenridge 

(fig. 5), has a biotite K-Ar age, a Rb-Sr biotite wholerock age, and zircon and apatite fission-track ages 

that are all concordant at 39 m.y. (McDowell, 1971; 

Simmons and Hedge, 1978; A. A. Bookstrom and C. 

W. Naeser, written communication, 1979). A zircon 

fission-track age of 35 m.y. has been obtained from a 

satellitic intrusion just to the south (Cunningham 

and others, 1977). These ages are similar to those 

obtained from rocks in the South Park-Breckenridge 

region.

K-Ar ages of 43-45 m.y. cannot be dismissed, 

however, for some intrusive rocks in this region. 

Simmons and Hedge (1978) obtained a Rb-Sr biotite 

whole-rock age of 45 m.y. from a sill of porphyritic 

quartz monzonite at Swan Mountain, north of Breckenridge. V. E. Surface of Climax Molybdenum Co. 

obtained biotite K-Ar ages of 35-47 m.y. from quartz 

monzonite porphyry of the Humbug stock in the 

Tenmile Range, about 10-km west of Breckenridge 

(Marvin and others, 1974). In the Humbug stock, 

fission-track ages of zircon also are younger than the
AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 23

biotite K-Ar ages (M. A. Kuntz and C. W. Naeser, oral 

communication, 1978).

Fission-track ages of zircon and sphene are concordant with K-Ar and Rb-Sr ages of biotite in the 

Twin Lakes stock in the Sawatch Range (fig. 7) 

(Moorbath and others, 1967; Obradovich and others, 

1969; Marvin and others, 1974; and Marvin and 

Dobson, 1979). The main part of the Twin Lakes 

stock was intruded about 45 m.y. ago. A younger 

granitic intrusion that cuts the main Twin Lakes 

stock (Wilshire, 1969) yields zircon and sphene ages 

of about 35 m.y., about 10 m.y. younger than the 

main part of the Twin Lakes stock. This age is 

similar to that of a stock intruding the Grizzly Peak 

cauldron in the Sawatch Range about 10 km farther 

west (Obradovich and others, 1969). Apatite fissiontrack ages from the Twin Lakes stock, the younger 

intrusion that cuts the Twin Lakes stock, and the 

Precambrian rocks on the west margin of the Sawatch Range all are less than 29 m.y. old and are all 

related to later uplift and cooling of the mountain 

block (Bryant and Naeser, 1980).

Relatively few ages in the range 45-55 m.y. have 

been obtained on rocks in the Colorado mineral belt, 

but a few are reported from the northeast end of the 

mineral belt (Marvin and others, 1974).

The number of intrusive rocks in the mineral belt 

that have been dated as Eocene suggests that the 

former idea of distinct Laramide and mid-Tertiary 

intrusive events is no longer acceptable.

It also is now clear that neither proximity nor 

similarity in chemistry and (or) mineralogy implies contemporaneity of intrusions. This point is well illustrated 

by the difference in the ages of the Lincoln Porphyry 

(64 m.y.) near Leadville (Pearson and others, 1962; 

McDowell, 1971) and the petrographically and texturally similar coarse-grained porphyritic quartz 

monzonite of the Breckenridge district at locality 1, 

which has an age of about 35 m.y. The latter was 

correlated with the Lincoln Porphyry by Ransome 

(1911) and Levering (1934).

In some areas of the mineral belt in the Front and 

Sawatch Ranges, rhyolites in small bodies and 

granites in larger bodies have been dated as being as 

young as late Oligocene or early Miocene, (Taylor 

and others, 1968; Naeser and others, 1973; Schassberger, 1972; Van Alstine, 1969; Limbach, 1975). Since 

dikes and plugs of rhyolite commonly are altered and, 

consequently, not datable by some methods, the distribution of these younger, more silicic rocks is 

imperfectly known. However, they are volumetrically 

less significant than the early Oligocene and late 

Eocene intrusives.

Traditional concepts of the igneous history of

the mineral belt, involving either an orderly progression of intrusive events from south west to northeast or two well-defined intrusive events, seem to be 

yielding to the idea of a more continuous period of 

intrusion spanning the period from 70 to 25 m.y. ago. 

Geophysical studies suggest that the mineral belt is 

underlain by a batholith (Tweto and Case, 1972; 

Isaacson and Smithson, 1976), which may be a 

composite batholith with a long and complex history. In some areas, however, such as the San Juan 

Mountains, most of the magma that formed the top 

of the batholith apparently was emplaced during 

Oligocene time.

EXTRUSIVE ROCKS

In the Antero syncline, in the southwestern part 

of South Park, andesite flow breccia apparently 

occurs in a northwest-trending paleovalley (fig. 8). 

The Wall Mountain Tuff, about 36 m.y. old, occurs in 

shallow channels carved into the andesite. These 

volcanic rocks are overlain by the Antero Formation 

of Oligocene age and the Wagontongue Formation of 

Miocene age. Much of the folding of the syncline is 

younger than the Wagontongue Formation. The age 

of the andesite flow breccia (locality 8, table 4 and fig.

8) is about 40 m.y. (average of biotite K-Ar age of 38 

m.y. and zircon and apatite fission-track ages of 42 

m.y.). This age indicates that the flow is contemporaneous with some of the intrusive rocks discussed 

above but is older than the Wall Mountain Tuff and 

the stratigraphically higher Buffalo Peaks Andesite 

and Badger Creek Tuff (Epis and Chapin, 1974; 

Sanders and others, 1976). The intrusive bodies 

nearest to locality 8 are 10 km to the northwest, but 

they have not been dated. Stark and others (1949) 

classified these intrusives as quartz monzonite. Chemical analysis of the andesite flow breccia (field no. 

430a, table 9) shows that it is much more siliceous 

than is apparent in outcrop or thin section and is a 

quartz latite. The nearby intrusives therefore are a 

reasonable source for the flow breccia.

Southeast of Kenosha Pass (fig. 8), an andesitic 

ash flow (field No. 430a, mapped as andesite but 

shown to be a rhyodacite by chemical analysis (table

9)) occupies a southeast-trending paleovalley, the 

bottom of which is 50 to 150 m below a widespread 

erosion surface on Precambrian rocks east of South 

Park. This surface, known as the Elkhorn surface, 

has been correlated with a widespread erosion surface of Eocene age in the region to the south (Scott, 

1975; Epis and Chapin, 1975). A fission-track age of 

36 m.y. on zircon from the volcanic rock in the 

paleovalley is similar to ages obtained from the
24 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

intrusive rocks to the west. The trend of the paleovalley, however, suggests that the ash flow in it is an 

outflow from a vent in the area near the Montezuma 

stock, where the intrusives are also of that age.

AGE OF MAIN DEFORMATION IN SOUTH

PARK AND ALONG THE MARGIN OF

THE FRONT RANGE UPLIFT

Our data on the age of rocks in the South 

Park-Breckenridge region put some constraints on 

the timing of deformation in the region. The Boreas 

Pass-South Park fault zone, northwest of Como, is 

cut by the Bald Mountain sill (localities 4 and 5, fig 

8). Latest major movements on this fault zone occurred after deposition of the South Park Formation in 

the Paleocene and before emplacement of that sill 

about 41 m.y. ago. This fault zone also had large preLaramide movements and formed the front of a late 

Paleozoic range. Boulder conglomerates in the uppermost part of the South Park Formation, adjacent 

to the Elkhorn fault, were derived from source areas 

east of the Elkhorn fault. They indicate that uplift 

and probably faulting along the west margin of the 

Front Range occurred during the earliest Eocene.

Ages of rocks from localities 2 and 3, farther 

north, put a limit of middle Eocene on the minimum 

age of movement of the Williams Range fault. A 

connection between the Williams Range and Elkhorn faults in the northern part of South Park is not 

exposed and is inferred on the basis of geophysical 

data (Barker and Wyant, 1976). In the absence of a 

known connection with the Elkhorn fault, no inference can be made about the earliest possible time of 

movement on the Williams Range fault except at its 

northernmost exposures 90 km to the northwest (fig. 

7). There, earliest movement was very Late Cretaceous, before deposition of the Middle Park Formation, a basin fill analogous to the South Park 

Formation. (Izett and Barclay, 1973). However, to the 

east, the Middle Park Formation is folded with the 

Cretaceous rocks and cut by faults (Izett 1968; Taylor, 1975), indicating a time relation between early 

Tertiary deposition and deformation similar to that 

in South Park.

In Paleocene igneous rocks along the east side of 

the Front Range uplift between Golden and Lyons, 

Hoblitt and Larson (1975) found paleomagnetic evidence that Laramide deformation there was older to 

the north than to the south. Perhaps this relationship is also true along the west side of the Front 

Range.

Between Denver and Colorado Springs, several 

faults cut the Upper Cretaceous and Paleocene Denver Formation and the Paleocene and Eocene Dawson Arkose at the eastern margin of the Front Range 

(fig. 7). Because the Denver and the Dawson are 

mostly structurally concordant with the underlying 

Cretaceous rocks, the structural relief at the present 

mountain front is due to post-Paleocene faulting. 

Soister (1978) and Soister and Tschudy (1978) have 

recently found that some of the Dawson Arkose 

above a well-developed soil horizon is of Eocene age, 

but as yet it has not been possible to map this soil 

zone and determine relations between rocks above 

and below it along the mountain front. However, 

north of Colorado Springs, lower Oligocene rocks 

(Wall Mountain Tuff; Castle Rock Conglomerate) 

that occur on the highest hills on the margin of the 

Great Plains can be projected westward to the range 

front. Then, if we assume that a well-developed 

surface on the crest of the Rampart Range, just west 

of the margin of the Front Range, is approximately 

at the level of an erosion surface of Eocene age (Scott, 

1975) and about on grade with deposits laid down in 

the early Oligocene, we can infer about 600m of offset 

by post-Oligocene movement along the range front 

fault. Cross sections and stratigraphic relations 

along the mountain front in that area suggest postPaleocene but pre-Oligocene displacement of as much 

as 3 km along the same fault. Much of this displacement may have been contemporaneous with the 

Eocene deformation of the South Park Formation on 

the west side of the Front Range uplift.

IMPLICATIONS OF AGES FOR THE

EXTENT OF THE EOCENE EROSION

SURFACE

The highest peaks between South Park and 

Middle Park are composed of intrusive igneous rocks 

of latest Eocene and earliest Oligocene age, which 

crop out at altitudes as great as 4,170 m, but no extrusive volcanic rocks are known from that area. Extrusive 

rocks of this age were deposited in shallow valleys in the 

Elkhorn surface on the east side of South Park, 

and these are preserved at 2,900-m altitude. The 

Elkhorn surface correlates with the Eocene erosion 

surface that is so widespread to the south and east 

(Scott, 1975; Epis and Chapin, 1975). If we assume a 

minimum cover of 500 to 1,000 m on the latest Eocene 

and earliest Oligocene intrusive rocks at the time of 

intrusion, a postulated Eocene erosion surface west 

of South Park would presently be at an altitude of 

about 5,000 m. If such an erosional surface did exist 

west of South Park in the area underlain by intrusive 

rocks, substantial vertical displacement of that area 

since the middle Tertiary is required. Bryant and
AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 25

others (1980), Stark and others (1949), and Barker 

and Wyant (1976) have not detected any major fault 

bounding the west side of the park; the mapped faults 

trend north-northwesterly and cross the high country of the continental divide underlain by the intrusive rocks. Perhaps the deformation that produced this apparent vertical offset was accomplished 

by warping rather than high-angle faulting, or 

alternatively, there may have been high mountains 

in Eocene time rising above the broad lowland 

represented by the Elkhorn surface.

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Kremmling quadrangle, Grand Co., Colorado: U.S. Geological 

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King, P. B., and Beikman, H. M., 1974, Geologic map of the 

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Lipman, P. W., Steven, T. A., and Mehnert, H. H., 1970, Volcanic 

history of the San Juan Mountains, Colorado, as indicated 

by potassium-argon dating: Geological Society America Bulletin, v. 81, p. 2329-2352.

Levering, T. S., 1934, Geology and ore deposits of the Breckenridge mining district, Colorado: U.S. Geological Survey Professional Paper 176, 64 p.

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western United States: Isochron/West, no. 2, p. 1-16.

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32 p.

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1974, Summary of radiometric age determination on Mesozoic 

and Cenozoic igneous rocks and uranium and base metal 

deposits in Colorado: Isochron/West, no. 11., 41 p.

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Muilenburg, G. A., 1925, Geology of the Tarryall district, Park 

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64 p.

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in apatite and sphene: Journal of Geophysical Research, 

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Naeser, C. W., Hurford, A. J., and Gleadow, A. O. W., 1977, 

Reply to comment by G. A. Wagner on Fission-track dating 

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fission-track ages from some middle Tertiary igneous rocks in 

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Abstracts with Programs, v. 5, no. 6, p. 498.

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the fission-track dating method, in Zartman, R. E., ed., 

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Obradovich, J. D., Mutschler, F. E., and Bryant, Bruce, 1969, 

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26 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

Pearson, R. C., Tweto, Ogden, Stern, T. W., and Thomas, H. H., 

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Survey Professional Paper 450-C, p. C78-C80.

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TABLES 6-9
28 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

TABLE 6.—Location and description of dated and analyzed

Loc. 

No.

Field 

No.

North 

Latitude

West 

Longitude

Locality description Ouadrangle Rock type

Intrusive rocks

1 223 39°30'19" 105°57'47" Knob with spot elevation 11,212, 

500 m NE. of Brewery Hill. 

Sill(?) in Pierre Shale.

Keystone—— Porphyritic hornblendebiotite quartz 

monzonite.

2 222 39°29'33" 105°56'31" Small cliff 300 m S. 22° E. of spot Boreas Pass 

elevation 9977 on sharp corner on 

Middle Fork Swan River road. 

Small plug in Pierre Shale and 

Williams Range fault.

Pyroxene-bearing 

hornblende-biotite 

granodiorite.

466 39°29'31' 105 056'14" 3291-m altitude on ridge between 

Middle and South Forks Swan River. 

Williams Range fault zone.

———do——- Brecciated and contact 

metamorphosed gneiss.

74 39°27'34" 105°55'44" Small cliff at 3672-m altitude on E. 

shoulder of Mt. Guyot 1 km N. 80° 

W. of Georgia Pass. Stock in 

Pierre Shale 120 m from Williams 

Range fault.

———do——— Biotite quartz 

monzonite.

233

503

503a

39°21'37" 105°56'12" Cut in abandoned railroad grade 500 Como-———— 

m N. 81° W. of BM 10,576 at Halfway Gulch. Large irregular sill 

in Morrison Formation to lower 

Pierre Shale.

39°20'40" 105°55'37" Cut in old railroad grade and road ———do———

to Boreas Pass. 300 m S. 24° E.

of Peabodys site, 

as sample 4.

Same intrusive

Porphyritic biotite 

granodiorite.

Porphyritic hornblendebiotite quartz 

monzonite.

Biotite-rich xenolith—

502 39°17'54" 105°53'50" Roadcut on U.S. Highway 285, 2 km 

S. 15° W. of roundhouse in Como. 

Probable sill in Pierre Shale.

———do——— Altered porphyritic 

quartz monzonite.

328 39°25'19" Jefferson Hill, 3100-m altitude on 

southwest side. Sill in the 

Benton Group.

Jefferson— Hornblende-biotite 

andesite porphyry.

Extrusive rocks

8 430a

9 420

39 000'48"

39°22'44"

105°53'55"

105°43'38"

Sec. 9, T. 12 S., R. 75 W., 550 m 

E. of SW. corner and 60 m N. 

of section line. Andesite 

underlying Wall Mountain Tuff.

Sec. 1, T. 8 S., R. 75 W., on 

centerline of SW 1/4 and 300 m 

N. of S. boundary. Fills 

paleovalley in Elkhorn surface.

Garo- Ouartz-bearing andesite 

flow breccia.

Mt. Logan— Andesitic ash flow tuff
AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 

samples frot.^ the South Park-Breckenridge region

29

Loc. 

No.

Field 

.No.

Sample description Accessory 

minerals

Chemical analysis 

of similar rock

Intrusive rocks

223 Light-gray rock containing quartz phenocrvsts to 0.5 mm, 

biotite to 2 mm, plagioclase to 0.5 mm, and widely 

scattered potassic feldspar to 1 cm. Euhedral sodic 

andesine and calcic oligoclase with normal zoning, somewhat 

resorbed euhedral quartz, euhedral biotite, and small, 

euhedral, light-green, locally chloritized amphibole 0.2 

to 0.3 mm. A matrix of quartz and potassic feldspar with 

a grain size of 0.05 to 0.1 mm.

222 Euhedral to subhedral andesine to labradorite to 1.5 mm 

long; anhedral, partly interstitial quartz to 0.2 mm 

in diameter; interstitial potassic feldspar; very light 

green anhedral monoclinic pyroxene intergrown with biotite; 

light-green euhedral to subhedral amphibole to 1 mm long 

partly intergrown with biotite and pyroxene; and anhedral 

to subhedral biotite to 1 mm long.

466 Fragments of quartz as large as 4 mm in diameter, fractured, 

partly altered garnet and feldspar in a fine-grained matrix 

of quartz and feldspar, and postkinematic brown biotite.

74 Subhedral to euhedral sodic andesine to 1.5 mm long with 

normal and oscillatory zoning, anhedral cryptoperthitic 

potassic feldspar to 3 mm, interstitial quartz 0.3 to 1 mm, 

subhedral brown biotite 0.5 mm (average) to 1.2 mm (maximum), 

and a few grains of olive-green hornblende 0.1 to 0.6 mm 

long, partly altered to biotite.

233 Euhedral phenocrysts of sodic andesine to 2 mm long, partly 

altered to sericite and carbonate; euhedral to anhedral 

resorbed phenocrysts of quartz to 1.5 mm; euhedral, partly 

chloritized phenocrysts of brown biotite to 1 mm in a matrix 

of quartz and feldspar with a grain size less than 0.05 mm.

503 Partly resorbed phenocrysts of andesine as long as 2 mm, 

somewhat resorbed euhedral quartz to 1 mm, euhedral 

biotite to 1.4 mm, euhedral light-green hornblende to 

0.6 mm, and sanidine to 3 mm in a matrix of quartz 

and feldspar with a grain size of 0.02 mm.

502 Phenocrysts of partly resorbed quartz to 3 mm; plagioclase as 

much as 1.3 mm in diameter altered to albite and carbonate; 

and chlorite after biotite and hornblende in a fine-grained 

matrix of quartz, altered feldspar, chlorite, and carbonate.

328 Euhedral phenocrysts of biotite to 2 mm, phenocrysts and 

glomerophenocrysts of andesine with oscillatory zoning 

in a sodic trend to 5 mm in diameter, and hornblende 

and monoclinic pyroxene in a very fine grained matrix of 

feldspar and quartz.

Sphene, apatite, 

zircon, and 

opaques.

Ransome (1911, 

p. 45, No. 1).

Opaque mineral, Table 9.

apatite, and 

zircon.

Epidote, sericite, zircon, 

and opaque 

minerals.

Opaque minerals, Ransome (1911, 

apatite, and p. 58). 

zircon.

Not available.

———do———————— Muilenburg (1925, 

p. 36).

Apatite, allanite, zircon, 

and opaque 

minerals. Some 

carbonate from 

alteration of 

plagioclase.

Apatite, 

allanite, 

and opaque 

minerals.

Opaque minerals, 

apatite, and 

zircon.

Not available.

Do.

Table 9.

Extrusive rocks

8 430a Phenocrysts and glomerophenocrysts of andesine to 5 mm 

in diameter, of biotite to 2 mm, hornblende to 1 mm, and 

quartz as fragments as much as 2.5 mm in diameter, all 

set in a fine-grained matrix.

Opaque minerals, Table 9. 

apatite, and 

zircon.

420 Euhedral to anhedral cyrstals and crystal fragments of 

andesine, quartz, biotite, and green hornblende in a 

fine-grained matrix of devitrified glass.

———do———————— See sample No. 

421, table 9.
30 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

TABLE 6.—Location and description of dated and analyzed

Loc. Field North West 

No. No. Latitude Longitude

Locality description Quadrangle Rock type

South Park Formation

10 W-l-75 39°13'49" 105°48'31" T. 9 S., R. 75 W., 120 m 

S. of NE. corner sec. 31. 

About 1600 tn above base of 

South Park Formation.

Elkhorn——— Hornblende-biotitequartz-plagioclase 

crystal tuff.

11 150

12 444

39°15'27" 105 049'32" Borrow pit on Elkhorn road 1020 m 

SW. of BM 9405 on Elkhorn Road. 

Link Springs Tuff Member of the 

South Park Formation.

39°08'58" 105°48'55" T. 10 S., R. 75 W., 200m

S. 51° E. from center sec. 30. 

Base of South Park Formation.

Milligan Hornblende-biotiteLakes. plagioclase-quartz 

crystal tuff.

Elkhorn——— Felsic crystal tuff———

13 325 39°17'51" 105°5riO" T. 9 S., R. 76 W., 120 m N. 60° E. Milligan 

of center of sec. 2. Unaltered Lakes, 

lens in andesite about 160 m above 

base of formation.

Biotite-hornblende 

andesite flow.

Analyzed samples closely related to rocks dated in the study

14 323

15 421

105°55'01" Stratigraphically lowest outcrop at 

2993-m altitude 17.2 m N. 37° E. of 

spring. Sawatzky (1^67) gives K-Ar 

whole-rock age from this outcrop.

105°42'07" 2993-m altitude 167 m N. 70° E. of 

iunction on Lost Park Road marked 

9733 on map. Same unit as dated 

rock from locality 9.

Como- AndesiteObservatory Andesitic ash flow— 

Rock.
AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 

samples from the South Park-Breckenridge region—Continued

31

Loc. 

No.

Field 

No.

Sample description Accessory 

minerals

Chemical analysis 

of similar rock

South Park Formation

Apatite, zircon, 

and opaque 

minerals.

10 W-l-75 Fragments of quartz 0.1 to 1.5 mm, andesine 0.1 to 1.5 mm, 

biotite to 1 mm, and hornblende to 0.4 mm in a matrix of 

devitrified glass containing some pumice fragments. One 

fragment of feldspathic sandstone was seen. X-ray study 

indicates the glass has been converted to clinoptilolite.

11 150 Biotite to 0.5 mm in diameter, dark-green to brownish-green 

hornblende to 0.3 mm, andesine 0.2 to 0.5 mm, quartz, and 

partly devitirified glass. X-ray study shows the glass 

has been replaced by clinoptilolite.

12 444 Crystal fragments of quartz 0.1 to 1 mm, sodic andesine

0.1 to 1 mm, biotite to 1 mm, and green to brown hornblende 

in a matrix of pumice and devitrified glass. X-ray study 

indicates much of the devitrified glass is clinoptilotite.

13 325 Euhedral, dark-greenish-brown hornblende 0.3 to 3 mm, Epidote, opaque, 

andesine 0.2 to 2 mm with delicate oscillatory zoning, and apatite, 

biotite to 1 mm, anhedral quartz to 0.5 mm, and light-green 

monoclinic pyroxene in a matrix of devitrified glass. 

Quartz in amygdules.

———do————————

Table 9.

Do.

no.

Do.

Analyzed samples closely related to rocks dated in this study

14 323 Euhedral phenocrysts of reddish-brown hornblende 0.15 to

1.5 mm long, partly altered to greenish-brown hornblende, 

and of altered plagioclase 0.15 to 2 mm. Matrix of altered 

glass contains a few aggregates of quartz.

15 421 Fragments of andesine to 2 mm, quartz to 1.5 mm, euhedral 

to subhedral biotite to 1.5 mm, and greenish-brown 

hornblende with a partly zeolitized glassy matrix.

Opaque minerals Table 9. 

and apatite.

Zircon, apatite, Do. 

opaque

minerals and 

carbonate.
32 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES 

TABLE 7.—Analytical data for K-Ar ages of igneous rocks in the South Park-Breckenridge region

[All determinations made on biotite; some additional minerals intermixed1 as noted below. 40K At=0.581X10"10/yr. 

X/£=4.962X10~"Vyr, Atomic abundance 40K=1.167X10~4 atom/atom K. Potassium determinations made with an Li internal 

standard. Analysts: R. F. Marvin, H. H. Mehnert, and Violet Merritt]

Loc. 

No.

1

2

3

5

8

9

10

11

Field 

No.

223

222

74

503

503a

430a

W-l-75

150

444

K20 

(percent

8

9

1 8

8

8

2 6

8

3 2

7

7

3

3

8

8

.71,

.01,

.17,

.42,

.70,

.07,

.57,

.57,

.79,

.66,

.90,

.99,

.11,

.26,

8.

8.

8.

8.

8.

6.

8.

2.

7.

7.

3.

3.

8.

8.

Radiogenic ^Ar Age

:) Moles/g Percent of 

(xlO~ 10 ) total Ar

69

93

20

42

65

10

61

57

76

64

95

90

07

30

5

5

5

6

5

4

5

1

4

4

3

7

7

.689

.634

.252

.214

.754

.361

.511

.524

.896

.211

.238

.075

.952

85

74

72

87

87

87

89

50

78

77

45

80

92

44.

43.

44.

50.

45.

49.

44.

40.

43.

37.

56.

59.

65.

(m.y.) 

±2a

8±1

1±1

0±1

6±1

5±1

1±1

0±1

7±1

2±1

8±1

3±1

7±2

5±2

.5

.5

.5

.7

.5

.6

.5

.4

.5

.3

.9

.0

.2

1 Biotite mixed with minor hornblende.

2 Biotite mixed with hornblende.

Biotite mixed with minor chlorite and hornblende.
AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 

TABLE 8.—Analytical data for fission-track ages of igneous rocks in the South Park-Breckenridge region

[Constants: .'Xf=7.03X10""yr"1 - Analyst C. W. Naeser. Number of tracks counted is given in parentheses]

33

Loc. 

No.

1

2

3

4

5

6

7

8

9

10

11

12

13

Field 

No.

223

222 

466

74

233

503

502

328

430a

420

W-l-75

150

444

325

Lab 

No. 

DF592

817 

817 

841 

1504

597 

588 

509

598 

594 

595

1802 

1803

1800

1345 

1341

1322 

1306

2337

1320 

1304

590 

591

1323 

1307

1321 

1305

Mineral

Zircon

—— do— 

—— do~ 

—— do— 

—— do—

Sphene 

Zircon 

Apatite

Sphene 

Zircon 

Apatite

Zircon 

Apatite

Zircon

—— doApatite2

Zircon 

Apatite

Zircon

—— doApatite

Zircon 

Apatite

Zircon 

Apatite

Zircon 

Apatite

Fossil- track 

density1 (p g ) 

(106 tracks/ cm2 )

10.4

6.08 

3.39 

5.22 

5.55

5.93 

3.61 

.243

2.10 

5.56 

.144

7.76 

.159

4.24

4.09 

.108

3.62 

.115

4.75

6.35 

.149

8.12 

.162

10.30 

.102

5.12 

.110

(723)

(1188) 

(392) 

(628) 

(719)

(1456) 

(852) 

(113)

(670) 

(1055) 

(134)

(1149) 

(331)

(610)

(910) 

(450)

(787) 

(479)

(1122)

(912) 

(275)

(1240) 

(337)

(1045) 

(427)

(1208) 

(457)

Induced- track 

density1 (p^) 

(106 tracks/ cm2 )

20.4

9.00 

5.32 

8.44 

8.76

11.12 

6.87 

.387

3.92 

10.68 

.238

12.30 

.239

6.59

8.32 

.116

5.41 

.160

8.80

7.16 

.136

10.38 

.164

9.82 

.102

5.25 

.104

(700)

(943) 

(308) 

(508) 

(568)

(1364) 

(795) 

(118)

(626) 

(1014) 

(220)

(911) 

(497)

(474)

(924) 

(485)

(589) 

(667)

(1039)

(514) 

(251)

(793) 

(342)

(500) 

(425)

(620) 

(434)

Neutron Age (±2o) 

flux ($) (m.y.) 

(1015 n/cm2 )

1.09

1.02 

.965 

.980 

1.01

1.27 

1.18 

1.13

1.25 

1.14 

1.13

1.07 

.997

1.08

1.28 

1.02

1.04 

1.00

1.13

1.05 

1.02

1.17 

1.13

1.03 

.990

1.05 

1.01

35. 1± 3.6

38. 3± 1.8 

36. 6± 2.0 

36. 2± 1.6 

38. 2± 1.8

40. 4± 1.7 

37. 9± 1.6 

42. 7± 8.2

39. 9± 2.0 

35. 5± 1.4 

41.1±10.

40. 3± 1.8 

40. 6± 5.8

41. 5± 1.8

37. 6± 1.5 

56. 4± 7.4

41. 5± 1.8 

42. 8± 5.1

36. 4± 1.9

55. 5± 2.7 

66.5±11.6

54. 7± 4.8 

66.4±12.

64. 1± 3.7 

59. 3± 8.1

60. 9± 3.2 

63. 3± 8.5

Number 

of 

grains

3

19 

6 

6 

6

6 

6 

50

6 

6 

50

6 

50

6

6 

100

6 

100

6

6 

100

6 

50

4 

100

6 

100

Uranium 

content 

(ppm)

570

290 

290 

275 

250

280 

190 

11

100 

300 

6. a

330 

6.9

180

190 

3.3

150 

4.6

220

200 

3.8

280 

4.7

270 

3.0

140 

3.0

1 Numbers in parentheses show total number of tracks counted in each determination.

2 Sample has many defects.
34 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

TABLE 9.— New chemical analyses and norms of Tertiary igneous rocks from South Park-Breckenridge region

[Major oxides determined by Hezekiah Smith, U.S. Geological Survey, 1976, using rapid methods described by Shapiro (1975, p. 43-45). Minor elements determined by 

semiquantitative spectrographic methods by Leung Mei, U.S. Geological Survey, 1976. CIPW norms calculated on the basis of major-oxide analyses recalculated to 

100 percent after deduction of volatiles. The standard deviation of any single answer should be taken as plus 50 percent and minus 33 percent. Looked for but not 

found: Ag, As, Au, Bi, Cd, Dy, Er, Gd, Ge, Hf, Ho, In, Ir, Li, Lu, Mo, Os, Pd, Pr, Pt, Re, Rh, Ru, Sb, Sm, Sn, Ta, Tb, Th, Tl, Tm, U, W]

Paleocene volcanic

Field No 

Lab No.-

323

W J. 7 i_/ J*T

325 

W-191538

rocks

444 150 

W-191539 W-191533

t.

W-l-75 

W- 19 1540

Major oxides

O £ f\

A1 2°3 —— 

T?«» n _.

i?af"\_____.

MgO ————

P a /"t__— __.

Na20 —— -

*>i/ 

H n+

H n~ ___.

J.HJ*\

u n — . P2°5 

MnO ———— •

ft\ —— .

Sum

rt * f\

Fe2°3

r evj 

u~^s\ MfcO ———— •

\jSL\J

MM f\

K

f\

*r*r\ ____.

P2°5

—— 55.9

—— 16 . 7

—— 3.9

—— 3.2

3

1

-—— 5.4

——— 3 . 5

——— 2.1

——— 3.1 

——— 1.9

——— . 74 

——— .40

— ns

____ no

—— .04

100

Maior

____ Rfl fil

—— 4.09 

3 oe.

3 C£

——— 5.66 

3 (.1

—— 2.20

___ 7ft

—— .42

____ i *

62.9 

15.6 

2.1 

3.2 

1.9

4.6 

4.1 

2.8 

.80 

.40

.59 

.34 

.14 

.01 

.04

100

66.3 

15.5 

3.4 

1.6 

1.5

3.8 

2.8 

1.3 

2.0 

2.4

.39 

.18 

.06 

.02 

.02

101

oxides recalculated to

64.01 

15.87 

2.14 

3.26 

1.93 

4.68

4.17 

2.85 

.60 

.35

1 A

68.47 

16.01 

3.51 

1.65 

1.55 

3.92

2.89 

1.34 

.40 

.19 

n£

67.0 

14.6 

1.7 

1.4 

1.6

4.5 

2.4 

1.9 

2.8 

1.5

.47 

.33 

.08 

.01 

.04

100

67.8 

16.1 

1.7 

.88 

.96

4.5 

2.8 

1.7 

2.1 

1.1

.35 

.16 

.05 

.03 

.01

100

100 percent after

69.61 

15.21 

1.77 

1.46 

1.67 

4.69

2.50 

1.98 

.49 

.34 

na

69.90 

16.60 

1.75 

.91 

.99 

4.64

2.89 

1.75 

.36 

.16 

ns

Eocene and 

Intrusive rocks

222 

W-191535

(percent)

58.1 

16.4 

3.6 

4.0 

3.3

5.8 

3.2 

3.2 

.94 

.12

.99 

.48 

.12 

.05 

.07

100

deduction of

58.57 

16.53 

3.63 

4.03 

3.33 

5.85

3.23 

3.23 

1.00 

.48

T>

328 

W-191536

63.3 

16.7 

3.1 

1.2 

1.4

3.5 

3.5 

4.7 

.77 

.78

.72 

.32 

.07 

.03 

.08

100

volatiles

64.26 

16.95 

3.15 

1.22 

1.42 

3.55

3.55 

4.77 

.73 

.32 

0.7

Oligocene 

Volcanic

421 

W-191541

66.3 

15.5 

1.9 

2.3 

1.0

3.4 

3.4 

3.9 

.80 

.49

.79 

.36 

.05 

.01 

.10

100

(percent)

66.97 

15.76 

1.92 

2.32 

1.01 

3.43

3.43 

3.94 

.80 

.36 

ns

rocks

430A 

W-191542

69.6 

15.1 

2.0 

1.1 

.72

3.1 

3.1 

3.4 

.79 

.28

.48 

.20 

.04 

.02 

.05

100

70.27 

15.35 

2.03 

1.12 

.73 

3.15

3.15 

3.46 

.49 

.28 

OA
AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 35 

TABLE 9.—New chemical analyses and norms of Tertiary igneous rocks from South Park-Breckenridge region—Continued

Paleocene volcanic rocks

Field No.

Lab No.—

323

" J- 7 LJ -f*T

325 

W-191538

444 

W-191539

150 

W-191533

W-l-75 

¥-191540

Eocene and 

Intrusive rocks

222 

W-191535

328 

W-191536

Oligocene 

Volcanic rocks

421 

W-191541

430A 

W-191542

CIPW Norms

0 ——————

Or—- ——— 

Ab —————

nil

Hy-en ———

Hy-fs ———

Di-en ———

Di-fs ———

Vft-___ __

Hm ————— 

11 —————

Ap —————

Ba —————

Be —————

Ce —————

f***.

C-r——— ——

Cxi —————

Eu —————

f* _

La —————

Vf—

Nb —————

Nd —————

Ni —————

Pb —————

G*«

Sr —————

V ——————

Y ——————

TTl_

•7**

J J. » \t J

—— 24.80

1.74

—— .23

.17 

.03

5.93

1.47

.99

<4.4

—— 900

__ QO

—— <130

—— 14

17

—— 20

2

f

—— 20

— — oft

—— 1100

—— <4.4 

—— <65

11

—— <14

14

—— 1200

—— 100

15

2.5

___ ft 7

—— 07

16.35

16.84 

35.30 

16.17

3.70 

2.68 

2.00 

1.11 

.81

3.10

1.14 

.82

<4.4 

1100 

1.3 

130 

12

57 

17 

2.2 

18 

59

1100 

12 

<65 

20 

14

18 

940 

110 

25 

3.3

50 

tin

36.32 

3.11 

7.93 

24.47 

18.25

3.86

4.36 

.50 

.76 

.44

Minor

<4.4 

970 

1.2 

75 

5.7

6.9 

5.7 

<2.1 

14 

35

540 

4.6 

<65 

<6.5 

15

12 

1200 

40 

13 

2.3

39 

i on

35.87 

1.25 

11.70 

21.16 

21.01

4.15 

.56

2.57

.93 

.81

elements

<4.4 

1100 

1.4 

88 

6.0

6.9 

6.4 

<2.5 

13 

33

540 

5.0 

<65 

<6.5 

<14

9.9 

1100 

37 

26 

3.1

49 

onn

35.44 

1.91 

10.36 

24.43 

21.94

2.46

2.05 

.34 

.69 

.39

(parts per

<4.4 

910 

1.3 

95 

5.8

7.5 

11 

2.4 

18 

44

430 

<4.4 

<65 

>6.5 

18

7.9 

1500 

41 

12 

1.3

36 

i on

11.01

19.06 

27.30 

21.11

6.94 

2.50 

1.98 

1.34 

.48

5.26

1.98 

1.15

million)

<4.4 

1200 

1.9 

150 

18

46 

18 

3.0 

26 

70

1300 

9.8 

67 

21 

22

21 

980 

120 

20 

3.4

98 

nn

16.52 

.26 

28.19 

30.06 

15.50

3.54

2.04 

1.74 

1.39 

.77

<4.4 

1600 

2.9 

210 

8.7

6.9 

11 

3.8 

24 

100

750 

19 

84 

8.4 

30

15 

770 

70 

25 

4.7

38 

Aon

23.42 

.47 

23.28 

29.06 

14.66

2.52 

1.46

2.78

1.52 

.86

<5.2 

1100 

2.9 

180 

11

19 

26 

3.3 

22 

97

520 

17 

89 

10 

17

15 

870 

120 

29 

3.8

56 

oon

31.43 

1.18 

20.43 

26.67 

14.31

1.82

2.32 

.43 

.93 

.48

<4.4 

1200 

2.2 

200 

7.1

5.6 

5.9 

2.8 

18 

100

420 

7.2 

82 

<6.5 

18

9.1 

750 

53 

16 

2.1

44

17O

Potassium-Argon and Fission-Track Zircon 

Ages of Cerro Toledo Rhyolite Tephra 

in the Jemez Mountains, New Mexico

By G. A. IZETT, J. D. OBRADOVICH, C. W. NAESER, and G. T. CEBULA

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

IN THE WESTERN UNITED STATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1199-D
CONTENTS

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

Introduction .......

Methodology.......

Discussion...........

References cited.

Page

37

37

38

41

43

ILLUSTRATIONS

FIGURE 11, Map of western United States showing distribution of lower Pleistocene 

ash beds derived from pyroclastic eruptions from the Jemez 

Mountains area............................................................................................................

12. Diagram showing nomenclature of lower Pleistocene pyroclastic units

of the Jemez Mountains area...................................................................................

13. Photograph showing sampled interval of Cerro Toledo Rhyolite between 

the Otowi and Tshirege Members of the Bandelier Tuff in Pueblo 

Canyon, Los Alamos County, New Mexico .......................................................

14. K-Ar isochron plot for three minerals from sample 77G55 of a pumice

unit in Cerro Toledo Rhyolite .................................................................................

Page

38

40

40

42

TABLES

TABLE 10. Description of localities of Jemez Mountains-derived tephra deposits

shown in figure 11 ..................................................................................................

11. Reported and recalculated K-Ar ages of the Otowi and Tshirege

Members of the Bandelier Tuff of north-central New Mexico ................

12. K-Ar ages of minerals from pumice units of the Cerro Toledo Rhyolite

13. Zircon fission-track age determinations for sample 77G55, from an

airfall pumice unit of the Cerro Toledo Rhyolite ........................................

Page

39

40

42

42
SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES

POTASSIUM-ARGON AND FISSION-TRACK ZIRCON AGES OF CERRO 

TOLEDO RHYOLITE TEPHRA IN THE JEMEZ MOUNTAINS,

NEW MEXICO

By G. A. IZETT, J. D. OBRADOVICH, C. W. NAESER, and G. T. CEBULA

ABSTRACT

Pumice units of the Cerro Toledo Rhyolite of early Pleistocene 

age in Pueblo Canyon in the eastern part of the Jemez Mountains 

of north-central New Mexico lie stratigraphically between the 

Otowi (lower) andTshirege (upper) Members of the Bandelier Tuff. 

The K-Ar ages of sanidine, plagioclase, and hornblende from a 

lower unit of air-fall pumice of the Cerro Toledo are 1.46±0.03 m.y., 

1.50±0.03 m.y., and 1.58±0.11 m.y., respectively, based on the newly 

recommended decay constants for 40K, A, K-Ar isochron age for the 

three minerals is 1.47±0.04 m.y. The K-Ar age of sanidine from the 

uppermost pumice unit of the Cerro Toledo is 1.23±0.02 

m.y. These K-Ar ages are stratigraphically compatible with K-Ar 

ages of the lower and upper members of the Bandelier Tuff 

determined by G. B. Dalrymple in 1968. Zircon fission-track ages 

of the lower unit of the Cerro Toledo are 1.39 ±0.11 m.y. and 

1.46 iO. 12 m.y.The isotopic ages here reported for the Cerro Toledo 

Rhyolite coupled with those determined by Dalrymple for the 

Bandelier Tuff provide the basis for dating their downwind tephra 

correlatives in the southern High Plains of Kansas, New Mexico, 

and Texas.

INTRODUCTION

At several localities in the southern High Plains 

(fig. 11, table 10), such as the Borchers Ranch locality 

(Hibbard, 1941) in Meade County, Kans., deposits of 

volcanic ash occur (9 m stratigraphically above the 

type B Pearlette ash) that have mineralogical and 

chemical affinities with tephra of early Pleistocene 

age in the Jemez Mountains, north-central New 

Mexico. Elsewhere in the southern High Plains of 

New Mexico and Texas (fig. 11), volcanic ash beds 

tentatively correlate with two large pyroclastic units, 

the Guaje and Tsankawi Pumice beds (of the Otowi 

and Tshirege Members, respectively, of the Bandelier Tuff) (Izett and others, 1972; G. A. Izett, unpub. 

data, 1968-1979); but until the present, downwind 

equivalents of the Cerro Toledo Rhyolite, which lies 

between these two units, have not been recognized.

If, in the future, these deposits of volcanic ash can be 

correlated with certainty with their suspected sourcearea tephra units, then they will have the potential of 

increasing the stratigraphic resolving power of the 

established sequence of ash beds and increasing the 

stratigraphic usefulness of associated fossil land 

mammals. The usefulness of these potential marker 

volcanic-ash beds may be further enhanced if reliable K-Ar ages can be assigned to their source-area 

equivalents. It is the purpose of this report to present 

K-Ar ages of the Cerro Toledo Rhyolite tephra units, 

which previously have not been dated but whose 

stratigraphic position between the Otowi and Tshirege Members of the Bandelier Tuff is well established.

The samples chosen for K-Ar age determinations 

were collected from the eastern Jemez Mountains 

(fig. 12) in north-central New Mexico. Large samples 

(77G55 and 77G56) were taken from two air-fall 

pumice units (fig. 13) of the Cerro Toledo Rhyolite of 

early pleistocene age in Pueblo Canyon east of Los 

Alamos, N. Mex. Smith, Bailey, and Ross (1970) did 

not map the deposits of Cerro Toledo Rhyolite at the 

sample locality in Pueblo Canyon, owing to their 

small areal extent (R. L. Smith, oral commun., 1977). 

According to Smith (oral commun., 1977) and Smith, 

Bailey, and Ross (1970), the pumice units sampled 

are pyroclastics associated with the emplacement of 

rhyolite domes. These domes were emplaced following the eruption of the lower Bandelier Tuff 

(Otowi Member) of early Pleistocene age and the 

subsequent collapse that formed the Toledo caldera.

At the sample locality, the two units sampled are 

the uppermost of several discrete air-fall pumice beds 

interlayered with tuffaceous sediments that lie in

37
38 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

90°

FIGURE 11.—Map of Western United States showing the distribution of lower Pleistocene volcanic-ash beds derived from pyroclastic 

eruptions from the Jemez Mountains area, New Mexico. Volcanic-ash-bed localities: Guaje ash, open circle; Cerro Toledo ash, filled 

triangle; Jemez Mountains-derived ash suspected to be Cerro Toledo ash, open triangle; and Tsankawi ash, filled circle. Correlation of 

volcanic-ash beds here provisionally assigned to the Cerro Toledo and Tsankawi ashes with their suspected source-data tephra is 

tentative until more mineralogical and chemical data are available. Localities are described in table 10.

stratigraphic succession above the Guaje Pumice 

bed and overlying ash flows of the Otowi Member 

and below the Tsankawi Pumice Bed and overlying 

ash flows of the Tshirege Member. Figure 12 shows 

the nomenclature for lower Pleistocene pyroclastic 

units of the Jemez Mountains, N. Mex. K-Ar sanidine 

ages of samples of the Bandelier Tuff were determined by G. B. Dalrymple and reported in a paper by 

Doell and others (1968, p. 238). Dalrymple's K-Ar 

sanidine ages are given in table 11, as well as those 

ages recalculated using the new decay constants

recently recommended by the IUGS Subcommission 

on Geochronology (Steiger and Jager, 1977).

METHODOLOGY

Mineral separations were made on the two samples from two of the air-fall pumice units of the Cerro 

Toledo Rhyolite by G. T. Cebula, M. G. Sawlan, and 

J. W. Groen of the U.S. Geological Survey. About 28 

kg of sample 77G55 was used to obtain 39 g of calcic 

albite, 10 g of sanidine, and 0.04 g of zircon. About 2 

kg of sample 77G55 was used to obtain about 10 g of
POTASSIUM-ARGON, FISSION-TRACK ZIRCON AGES OF CERRO TOLEDO RHYOLITE TEPHRA, NEW MEXICO 39

TABLE 10.—Description of localities ofJemez Mountains-derived tephra deposits shown in figure 11.

[Leaders (——) indicate data unavailable]

Tephra 

No. bed

Approx.

thickness

(m)

Locality description Collector Sample Nos.

1. Tsankawi2. ——do—

1 In Pleistocene deposits in roadcut along U.S. Highway 64 

about 0.98 km southwest of junction with New Mexico 

State Highway 96, about 1.46 km southwest of the church 

at Ranches de Taos in the Taos Southwest 7 1/2-minute 

minute quad., Taos County, N. Mex.

1.5 In Pleistocene deposits near center of sec. 26, T. 7 N., 

R. IE., in Dalles 7 1/2-minute quad., Valencia County, 

N. Mex.

3. Cerro Toledo(?)- 1

4. Tsankawi(?) —

5. Cerro Toledo——— 1.2

7. ——do———————— 1.4

8. ——do———————— 4.5

9. Guaje———————— 1.5

10. Cerro Toledo(?)-

12. Guaje13. ——do————————

14. ——do—————-

15. ——do—————

About 2 m above base of 35-m-thick Pleistocene sequence 

(Tule Formation) along Rock Creek about 3 km north 

of Rock Creek Store in Cope Creek 7 l/2-minute quad., 

Briscoe County, Tex.

West of Channing, Tex., east of Rita Blanca Creek In 

NE 1/4 sec. 18, block 49, above windmill marked with 

altitude 3,555 feet in Channing NW 7 1/2-minute quad., 

Hartley County, Tex. Sample 77G28 is from second 

tephra bed about 3 m higher, above intervening sandstone.

Pumice lapilli bed in abandoned pumice pit in SE 1/4 

NE 1/4 sec. 24, T. 16 N., R. 8 E., Turquoise Hill 

7 1/2-minute quad., Santa Fe County, N. Mex.

Abandoned tephra pit about 13 km south of U.S. Highway 

66 about 12.8 km west of junction with U.S. Highway 

84, Guadalupe County, N. Mex.

Pumice gravel bed about 4.8 km southeast of San Felipe 

Pueblo in NE 1/4 NE 1/4 sec. 33, T. 14 N., R. 5 E., 

San Felipe 7 1/2 minute quad. , Santa Fe County, N. Mex.

Pumice gravel at southwest corner of sec. 26, T. 4 S., 

R. IE., San Antonio 15-minute quad., Socorro County,

N. Mex.

Ash and pumice lapilli bed In roadcut along Edith Blvd. 

near Albuquerque, N. Mex., NW 1/4 SE 1/4 SW 1/4 sec. 

22, T. 11 N., R. E., Alameda 7 1/2-mlnute quad., 

Bernalillo County, N. Mex.

Pleistocene channel deposit at north edge of New Mexico 

Institute of Mining Technology at Socorro, N. Mex. 

Tephra deposits now destroyed.

About 9.1m above Pearlette type B ash (Pliocene) in 

Pleistocene sediments In Borchers Ranch area, SE 1/4 

sec. 16, T. 31 S., R. 28 W., Irish Flats NE 7 1/2- 

minute quad., Meade County, Kans. Ash bed Incorrectly 

called type S Pearlette by some.

Roadcut along Texas State Highway 193 in NW 1/4 NE 1/4 

sec. 1, block 3, of Eastland County school lands In 

Floydada SE 7 1/2-minute quad., Crosby County, Tex.

.6 Roadcut along U.S. Highway 60 about 3.85 km west of

Pecos River bridge In SE 1/4 NE 1/4 sec. 32, T. 3 N., 

R. 25 E., Fort Sumner 7 l/2-mlnute quad., DeBaca 

County, N. Mex.

Pumice lapilli bed In Arroyo Hondo at south edge of 

Santa Fe 7 1/2-mlnute quad., Santa Fe County, N. Mex.

Roadcut of Farm Road 835 along south wall of Yellowhouse Canyon just north of junction with Farm Road 

3020 in Buffalo Springs Lake 7 1/2-mlnute quad., 

Lubbock County, Tex.

G. R. Scott———— 77W29

R. E. Wilcox——— 66W69

G. A. Izett———— 68G63, 75G11 

75G32, 77G103, 

77G104, 76G20

__do——————— 77G27, 77G28

R. E. Wilcox——— 65W123

R. H. Weber———— 66W162

R. E. Wilcox——— 65W121

R. H. Weber——— 66W156

R. E. Wilcox, 65W116 

P. W. Lambert.

R. H. Weber———— 66W160

C. W. Hibbard—— 73W28 

G. A. Izett——— 74G42, 74G49, 

74G51, 78G140

G. A. Izett———— 70G17

——do———————— 74G30

R. E. Wilcox——— 65W124 

G. A. Izett———— 75G3

G. A. Izett——— 75G28
40

TABLE 1 1 . — Reported and recalculated K-Ar ages of the Otowi and 

Tshirege Members of the Bandelier Tuff of north-central 

New Mexico

Unit

Tuff Bandelier 

Member Tshirege 

Otowi Member

Subunit

Ash flows

Tsankawi Pumice Bed

Ash flows

Guaje Pumice Bed

K-Ar age (m.y.)

Reported by Recalculated

others (1968) 1 constants2

1.02±0.04 1.05±0.04 

1.06±0.03 1.09±0.03

1.09±0.03 1.12±0.03

1.48±0.09 1.52±0.09

1.37±0.04 1.40±0.04

1 HU K/K -1 IQvirT^ • X - ' t* n 1~ a 1 ~ * E ~

2 ^K/l^^^l. 167x10-* ;Xe H 

Xe =4.962xlO~10 yr~1 (Steiger and Jager, 1977).

; X e =4.72xlO~10yr

BANDELIER TUFF

BANDELIER TUFF

JEMEZ MOUNTAINS AREA, 

NEW MEXICO

TSHIREGE 

MEMBER

ASH FLOW 

UNITS

TSANKAWI 

PUMICE 

BED

CERRO TOLEDO 

RHYOLITE

OTOWI 

MEMBER

ASH FLOW 

UNITS

GUAJE 

PUMICE 

BED

FIGURE 12.—Diagram showing the nomenclature of lower Pleistocene pyroclastic units of the Jemez Mountains 

area, north-central New Mexico. Modified from Smith, Bailey, and Ross 

(1970).

FIGURE 13.—View in Pueblo Canyon, NWV4NWV4 sec. 13, T. 6 E., R. 

19 N., Guaje Mountain TV^-minute quadrangle, Los Alamos 

County, N. Mex., showing the stratigraphic succession where 

samples 77G55 and 77G56, used for K-Ar age determinations, 

were collected from the Cerro Toledo Rhyolite of early Pleistocene age. Topmost part (covered) of the lower Bandelier Tuff 

(Otowi Member) is in lower part of photograph; the Otowi 

Member is overlain by a succession of air-fall pumice units of the 

Cerro Toledo Rhyolite and the Tshirege Member of the Bandelier Tuff.

sanidine. The separation procedure for sample 77G55 

consisted of screening the raw sample with a 4.75- 

mm (4-mesh) sieve and collecting only those 4.75-mm 

or larger pumice lumps that floated in water. Only 

the water-floatable pumice lumps were used for the 

mineral separations to insure that non-pumiceous 

accidental lithic fragments would not contribute
POTASSIUM-ARGON, FISSION-TRACK ZIRCON AGES OF CERRO TOLEDO RHYOLITE TEPHRA, NEW MEXICO 41

material to the mineral separations. The pumice 

lumps were ultrasonically scrubbed, dried at about 

50°C, and crushed and pulverized so that the sample 

would pass through a 300-^m (50-meshj sieve. About 

2 kg of sample was lost during sample preparation. 

Plagioclase was recovered using a bromoform and 

acetone mixture cut to appropriate specific gravity 

from the 300-tO lOO-j-an size range (-50 to +150 mesh), 

whereas the hornblende, sanidine, and zircon were 

recovered from the less-than-300-/mi (-50 mesh) size 

range owing to their small size and scarcity. The 

purity of the mineral separates was improved by 

using a magnetic separator set at appropriate current, forward tilt, and side tilt. Sanidine was recovered from sample 77G56 using the same procedures as for 77G55. Because many of the mineral 

grains had glass welded to their edges, the mineral 

grains were etched with hydrofluoric acid (24 percent) to remove the glass, using the following etch 

times:

77G56 sanidine................... 2 minutes

77G55 sanidine .................. 3 minutes

77G55 plagioclase.............. 3 minutes

77G55 hornblende.............. 6 minutes

Following the hydrofluoric acid etch, the minerals 

were ultrasonically scrubbed for about 5 minutes and 

sized as follows:

sanidine (77G56).......................... 300 to 10&Mn (-50 to +140 mesh)

sanidine (77G55)........................ 300 to 100 A*m (-50 to +150 mesh)

plagioclase (77G55)........................ 300 to 106 j«n (-50 to +140 mesh)

hornblende (77G55).................................................... =300 >im (-50 mesh)

After the samples were sized, they were split, using a 

Jones-type microsplitter1 , into fractions for argon 

and potassium analysis. Argon was collected and 

argon isotopic ratio determined from melted splits 

of the samples.

The potassium content of splits of the minerals 

used for argon analysis was determined by three 

methods—isotope dilution, flame photometry, and 

electron microprobe (table 12), although only the 

potassium contents as determined by isotope 

dilution were used to calculate the ages. Isotope 

dilution analysis of the samples was done by J. D. 

Obradovich and Kiyoto Futa on all samples. Flame

'Use of trade names in this report is for descriptive purposes only and does not 

constitute endorsement by the U.S. Geological Survey.

photometry was done by G. A. Izett and Wayne 

Montjoy to compare the results (table 12) with those 

done by isotope dilution.

The potassium content of the samples was 

determined by G. A. Izett using an Applied 

Research Laboratories EMX model electron 

microprobe to compare the results by this method 

with the isotope dilution results. Splits of the 

sami les were mounted in epoxy in holes drilled in an 

aluminum wafer. The potassium content was 

determined by analyzing the specimens using 15 

kilovolts operating voltage and 15 nanoamperes 

sample current measured on benitoite. A fixed count 

of beam current was used with a counting period of 

about 15 seconds. The analytical procedure consisted of 

analyzing 10 grains of each of four standard feldspar 

samples. A computer program linked to the electron 

microprobe (1) calculated a mean value of the number 

of counts and its associated standard deviation for 

potassium in the standard feldspars and (2) determined 

a least-squares curve relating the number of counts to 

weight percent potassium. Ten grains of each of the 

samples of unknown potassium content were analyzed, 

and their mean counts were compared by the computer 

with the curve relating counts to weight percent of the 

standard feldspars. The potassium contents determined 

in this way for the different runs were averaged to give 

the potassium content of each sample, and the results 

are in fairly good agreement with the results from 

isotope diluton (table 12).

DISCUSSION

The K-Ar age determinations (table 12) made on 

pumice units of the Cerro Toledo Rhyolite of Pleistocene age are compatible, within their analytical 

uncertainty, with K-Ar ages of underlying and overlying units of the Bandelier Tuff determined by G. B. 

Dalrymple (reported in Doell and others, 1968; and 

table 10). The results suggest that the lower pumice 

unit of the Cerro Toledo, herein dated 1.47±0.04 m.y. 

(77G55), is very close in age to the Otowi Member of 

the Bandelier Tuff as determined by G. B. Dalrymple 

(1.45 m.y.; average of 3 determinations (weighted 

mean)) and cannot be separated from it based on the 

K-Ar age determinations at hand. The uppermost 

pumice unit of the Cerro Toledo Rhyolite (77G56) has 

a K-Ar age (1.23+0.02 m^f.) significantly younger
42 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

TABLE 12.—K-Ar ages of minerals from pumice units of the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico

Sample

77G56

77G55

77G55

77G55

Mineral

Sanidine ———

—— do —— — —

PlagioclaseHornblende —

Lab No. 

ntfA_

3502

3500

3499

3497

K20

C 1 )

7.21

8.36

1.98

.56

(percent)

( 2 )

7.33

8.62

2.01

.56

( 3 )

7.22

_ ——

1.98

_. — _

Sample 

weight

(g)

3.9867

1.0727

12.5853

4.2865

Moles Percent K-Ar age4 

radiogenic radiogenic (m.v.)

"OAr 4 °Ar

5.

1.

5.

5.

llxlO"11

89xlO"u

36xlO"n

50xlO~12

56.8

24.5

28.3

6.1

1.

1.

1.

1.

23±0.02

46±0.03

50±0.03

58±0. 11

Potassium determined by J. D. Obradovich and K. Futa by the isotope dilution technique.

2 Potassium determined by G. A. Izett using the electron microprobe.

3 Potassium determined by G. A. Izett and W. Mountjoy by flame photometry technique.

4 Decay constants: Ag = 4.962xlO~10 yr"1 ; A e +* e ' - 0.581xlO~10 yr"1 .

40 K abundance: 4 °K=1.167x10"^ atom/atom K. Precision of age determinations given at the 

1-sigma level. K-Ar ages calculated using the potassium content as determined by isotope 

dilution.

TABLE 13.—Zircon fission-track age determination for sample 77G55, from an airfall pumice unit of the Cerro Toledo Rhyolite, Jemez

Mountains, New Mexico

Lab No. Spontaneous-track density1 Induced-track density1 

DF- (P g ) (106 tracks/cm2 ) (p ± ) (106 tracks/cm2 )

1646 

1646A

0.423 

.374

(45) 

(71)

19. 

7.

14 

55

(1019) 

(717)

Neutron flux 

(<f>)(1015 n/cm2 )

1.05 

.493

Fission-track 

age2 (m.y.)

1. 

1.

39±0. 

46±0.

11 

12

U 

(ppm)

520 

440

Total number of tracks counted shown in parentheses. 

2 Error is ±2 sigma.

than the K-Ar age of the underlying pumice unit 

(77G55) of the Cerro Toledo (1.47±0.04 m.y.), and 

significantly older than K-Ar ages of the overlying 

Tshirege Member of the Bandelier Tuff as dated by 

G. B. Dalrymple (about 1.09 m.y.; average of 3 

determinations (weighted mean)).

The K-Ar ages of sanidine (1.46±0.03 m.y.), plagioclase (1.50+0.03 m.y.), and hornblende (1.58±0.11 

m.y.) of sample 77G55 are in agreement with each other 

within their analytical uncertainty at the 2-a 

level. Because of the small amount of hornblende 

recovered through mineral separations from the 

phenocryst-poor rhyolite (77G55), and because of the 

low potassium content of the hornblende, the K-Ar 

age of the hornblende has a fairly high uncertainty. 

A K-Ar isochron plot of the analytical data for the 

three minerals from sample 77G55 is shown in figure 

14, and the age calculated from the data is 1.47±0.04 

m.y. The analytical precision estimate for this isochron age was calculated using a modified York 

linear regression. The minerals contain no analytically significant amount of inherited argon, inasmuch as the 40Ar/36Ar intercept is about 297.

Two fission-track age determinations (table 13) 

were made by C. W. Naeser on zircon separated from 

sample 77G55 of the Cerro Toledo. The calculated 

agesarel.39±0.11m.y. andl.46±0.12 m.y. (2r), and 

these ages are concordant with the K-Ar ages within 

their analytical uncertainty. The fission-track age of

FIGURE 14.—K-Ar isochron plot for three minerals from sample 

77G55 of a pumice unit in Cerro Toledo Rhyolite in the eastern 

Jemez Mountains, N. Mex.
POTASSIUM-ARGON, FISSION-TRACK ZIRCON AGES OF CERRO TOLEDO RHYOLITE TEPHRA, NEW MEXICO 43

1.46 m.y. is perhaps the better of the two, inasmuch 

as it was made following re-irradiation of the original sample, which had too high an induced-track 

density for optimum counting conditions.

REFERENCES CITED

Doell, R. R., Dalrymple, G. B., Smith, R. L., and Bailey, R. A., 

1968, Paleomagnetism, potassium-argon ages, and geology of 

rhyolites and associated rocks of the Valles Caldera, New 

Mexico, in Studies in volcanology—A memoir in honor of 

Howel Williams: Geological Society of America Memoir 116, 

p. 211-248.

Hibbard, C. W., 1941, The Borchers fauna, a new Pleistocene 

interglacial fauna from Meade County, Kansas: University 

of Kansas Science Bulletin 38, pt. 7, p. 197-220.

Izett, G. A., Wilcox, R. E., and Borchardt, G. A., 1972, Correlation 

of a volcanic ash bed in Pleistocene deposits near Mount 

Blanco, Texas, with the Guaje Pumice bed of the Jemez 

Mountains, New Mexico: Quaternary Research, v. 2, no. 4, 

p. 554-578.

Smith, R. L., Bailey, R. A., and Ross, C. S., 1970, Geologic 

map of the Jemez Mountains, New Mexico: U.S. Geological 

Survey Miscellaneous Investigations Map 1-571, scale 

1:250,000.

Steiger, R. H., and JSger, E., 1977, Subcommission on Geochronology: Earth and Planetary Science Letters, v. 36, p. 359-362.

Fission-Track Dating of the 

Climax and Gold Meadows Stocks, 

Nye County, Nevada

By G. W. NAESERa^FLORIANMALDONADO

SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH 

IN THE WESTERN UNITED STATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1199-E
CONTENTS

———————————— Page

Abstract.................................................................................................................._^ 45

Results and interpretation .................................................................................................................. 45

Other ages.................................................................................................................................................. 46

Analytical methods................................................................................................................................ 46

Acknowledgments .................................................................................................................................. 46

References cited........................................................................................................................................ 47

TABLES

Page

TABLE 14. Fission-track data for samples from the Climax and Gold

Meadows stocks, Nye County, Nevada .............................................................. 46

15. Sample localities in the Climax and Gold Meadows stocks,

Nye County, Nevada................................................................................................... 46
SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES

FISSION-TRACK DATING OF THE CLIMAX AND GOLD MEADOWS

STOCKS, NYE COUNTY, NEVADA

C. W. NAESER and FLORIAN MALDONADO

ABSTRACT

Fission-track ages indicate an age of 101 million years for the 

Climax stock and a minimum age of 93.6 million years for the Gold 

Meadows stock, both at the Nevada Test Site, Nye County, 

Nevada. Younger fission-track ages for some of the apatite 

concentrates suggest that the stocks have been within 4 kilometers of the surface since late Paleocene time.

RESULTS AND INTERPRETATION

The Climax stock, Nye County, Nevada (Maldonado, 1977), has been sampled in five locations for 

fission-track dating. Apatite from five of the samples 

was dated, as were zircon from two and sphene from 

one. A sample of the Gold Meadows stock, Nye 

County, Nevada, was also dated using apatite and 

zircon. The purpose of this study was to determine 

the thermal history of the respective stocks. Because 

apatite records the rock's most recent cooling regime 

(Naeser and Forbes, 1976), it is a very useful indicator of tectonic history.

Four of the five samples collected from the Climax 

stock (Maldonado, 1977) are outcrop samples (tables 

14 and 15, Nos. 2-5). The fifth sample (table 14, No.l) 

is a drill core from a depth of 536 m in drill hole 

U15BGZ. Apatite from the four surface samples gave 

concordant fission-track ages, with an average of 

101 ±3.2 m.y. (± one standard error (a) of the mean). 

This age is in excellent agreement with the stock's 

sphene and zircon ages, which give an average of 

101 ±1.2 m.y. However, the apatite from the drill core 

gave a discordant age, 78.6±3.4 m.y.

The concordance of the fission-track ages of the 

surface samples means that the rock now exposed at 

the surface has never been above 100°C in the last 

100 m.y.; these concordant ages also indicate the

probable time of emplacement of the stock. Assuming that the geothermal gradient for this area 

was the same during the late Cretaceous as it 

presently is, about 25°C/km (Roy and others, 1968), 

we can state that this stock was intruded at a depth 

of less than 4 km. The discordant age of the drill-core 

apatite points to a somewhat different thermal history for the buried parts of the stock. There are two 

possible interpretations: The first is that the stock 

was emplaced at such a depth in the crust that the 

100°C isotherm was between the rock presently at 

the surface and the rock at a depth of 500 m. This 

placement would have permitted the "surface" samples to cool to less than 100°C while the lower 

samples remained above 100°C for an extended time. 

The second interpretation is that the accumulation 

of sedimentary or volcanic materials buried the stock 

to such a depth that partial annealing of the apatite 

took place at some time after intrusion. It is not 

possible to distinguish between these two possibilities using the present data. If it is assumed that 

sample 1 (core) cooled through the 100°C isotherm 79 

m.y. ago, an average uplift rate of 0.05 mm/yr can be 

calculated. Most of that uplift would have taken 

place prior to the deposition of the Belted Range Tuff 

on the Climax stock 13 m.y. ago.

The Gold Meadows stock, Nye County, Nevada 

(Snyder, 1977), was sampled at one surface site (tables 1 

and 2, no. 6). The apatite and zircon from this 

sample have discordant fission-track ages: 55.6±2.8 

m.y. and 93.6±4.3 m.y., respectively. These ages 

suggest a slightly more complex history for this 

stock than for the Climax stock. Apparently the Gold 

Meadows stock either was emplaced at a deeper level 

in the crust or was, at some later time, buried to a

45
46 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES

TABLE 14.—Fission-track data for samples from the Climax and Gold Meadows stocks, Nye County, Nevada

[Decay constant Xt=7.03X10"'V1 ]

Sample 

No.

Analysis 

No. Mineral

Spontaneous-track

density"1 (p s ) 

(10 6 tracks/cm2 }

Induced-track

density 1 (P-J)

(Id 6 tracks/cm2 )

Neutron

flux (0)

(10 15 n/cm 2 )

Fissiontrack age 

(m.y.) ±2a

Uranium

content

(ppm)

Climax stock

11

1

2

3

4

5

5

DF-1637

DF-1638

DF-1639

DF-1640

DF-1641

DF-1642

DF-1645

DF-1644

Apatite

Sphene

Zircon

Apatite

Apatite

Apatite

Apatite

Zircon

0.287

19.79

10.99

.431

.327

.372

.349

17.36

(1195)

(733)

(1628)

(1794)

(1362)

(1552)

(1454)

(1366)

0.243

43.84

7.21

.307

.203

.248

.212

11.44

(1013)

(812)

(534)

(1280)

(845)

(1032)

(885)

(450)

1,

3,

1,

1.

1.

1.

1.

1.

,12

,87

,12

,12

,11

,10

,10

,11

78.

104

101

93.

106

98.

107

100

6± 6.7

±11

± 6.4

3± 6.8

± 9

3± 7.9

± 9

± 6.4

6.3

330

190

7.9

5.3

6.5

5.6

300

Gold Meadows stock

6 DF-1646 

7 DF-1647

Apatite 

Zircon

0. 

13.

,163 

,71

(681) 

(1904)

0. 

9.

,193 

,58

(803) 

(665)

1.10 

1.10

55. 6± 

93. 6±

5.6 

8.5

5.0 

250

Numbers in parentheses show total number of tracks counted in each determination,

TABLE 15.—Sample localities in the Climax and Gold meadows 

stocks, Nye County, Nevada

Sample North West Site 

No. latitude longitude description Quadrangle

Granodiorite from Climax stock

1

2

3

4

5

370-14. 04"

37°14'23"

37°14'23"

37°14'18"

37°13'32"

116 0 03'27"

116 0 03'16"

116°03'50"

116°03'54"

116°03'40"

From drill

hole

U15bG2;

536m below

collar.

Surface outcrop

—— do. ———

—— do. ———

—— do. ———

Oak Spring

Do.

Do.

Do.

Do.

Quartz monzonite from Gold Meadows stock

37°13'52" 116°12'28" Surface out- Rainier Mesa 

crop

deeper level. It has a calculated uplift rate of about 

0.07 mm/yr for the last 56 m.y. The zircon from the 

Gold meadows stock is apparently younger than the 

sphene and zircon from the Climax stock. This 

younger age could be the result of a deeper level of 

intrusion and (or) later burial of the Gold Meadows

stock, or of the emplacement of the stock about 7 m.y. 

after the emplacement of the Climax stock.

OTHER AGES

Marvin and others (1970) reported six biotite K-Ar 

ages from the Climax stock and one from the Gold 

Meadows stock. The six biotite ages from the Climax 

stock range from 89 to 97 m.y. Using the IUGS 

constants for 40K, we recalculated the ages, Obtaining 91 to 100 m.y. The oldest age is very close to the 

average fission-track age for the Climax stock. The 

91.8-m.y. K-Ar biotite age for the Gold Meadows 

stock recalculates to 94.3 m.y., which is in very good 

agreement with the zircon age of 93.6 m.y.

ANALYTICAL METHODS

The apatites were dated by the population method 

(Naeser, 1976); they were etched in 7 percent HNO3 

for 25 seconds at room temperature. The zircon and 

sphene were dated by the external-detector method 

(Naeser, 1976). The zircon was etched in a KOHNaOH eutectic melt (Gleadow and others, 1976), and 

the sphene, in 5(X M NaOH at 140°C (Naeser, 1976).

ACKNOWLEDGMENTS

We wish to thank G. T. Cebula of the U.S. Geological Survey for his excellent mineral separations on 

these rocks.
FISSION-TRACK DATING, CLIMAX AND GOLD MEADOWS STOCKS, NYE COUNTY, NEVADA 47

REFERENCES CITED

Barnes, Harley, Houser, F. N. and Poole, F. G., 1963, Geologic map 

of the Oak Spring quadrangle, Nye County, Nevada: U.S. 

Geological Survey Geologic Quadrangle Map GQ-214.

Gibbons, A. B. Hinrichs, E. N. Hansen, W. R., and Lemke, 

R. W., 1963, Geology of the Rainier Mesa quadrangle, Nye 

county, Nevada: U.S. Geological Survey Geologic Quadrangle 

Map GQ-215.

Gleadow, A. J. W., Hurford, A. J., and Quaife, R. D., 1976, 

Fission-track dating of zircon—Improved etching techniques: 

Earth and Planetary Science Letters, v. 33, p. 273-276.

Maldonado, Florian, 1977, Summary of the geology and physical 

properties of the Climax stock, Nevada Test Site: U.S. Geological Survey Open-File Report 77-356, 27 p.

Marvin, R. F., Byers, F. M., Jr., Mehnert, H. H., Orkild, P. P.,

and Stern, T. W., 1970, Radiometric ages and stratigraphic 

sequence of volcanic and plutonic rocks, southern Nye and 

western Lincoln Counties, Nevada: Geological Survey of 

America Bulletin, v. 81, p. 2657-2676.

Naeser, C. W., 1976, Fission-track dating: U.S. Geological Survey 

Open-File Report 76-190, 68 p.

Naeser, C. W. and Forbes, R. B., 1976, Variation of fission-track 

ages with depth in two deep drill holes: EOS, American Geophysical Union Transactions, v. 57, p. 353.

Roy, R. F., Decker, E. R., Blackwell, D. D., and Birch, Francis, 

1968, Heat flow in the United States: Journal of Geophysical 

Research, v. 73, p. 5207-5222.

Snyder, R. P., 1977, Geology of the Gold Meadows stock, Nevada 

Test Site: Available only from U.S. Department of Commerce, 

National Technical Information Service, Springfield, VA 

22161, as U.S. Geological Survey report USGS-474-179,13 p.

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