I

h

->rReconnaissance Geology and 

Geochronology of the 

Precambrian of the 

Granite Mountains, Wyoming

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1055
Reconnaissance Geology and 

Geochronology of the 

Precambrian of the 

Granite Mountains, Wyoming

By ZELL E. PETERMAN and R. A. HILDRETH

GEOLOGICAL SURVEY PROFESSIONAL PAPER

Radiometric dating establishes ages of 2,860 m.y.

for an extensive metamorphic complex and 2,550 m.y.

for a major granite batholith in the Granite Mountains

1055

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1978
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Peterman, Zell E.

Reconnaissance geology and geochronology of the Precambrian of the Granite Mountains, Wyoming

(Geological Survey Professional Paper 1055)

Bibliography: p. 20

Supt.ofDocs.no.: 119.16:1055

1. Geology, Stratigraphic Precambrian. 2. Geology Wyoming Granite Mountains.

3. Radioactive dating. I. Hildreth, R. A., joint author. II. Title. III. Series.

QE653.P46 551.7'l'09787 77-608349

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

Washington, D.C. 20402 

Stock Number 024-001-03090-0
CONTENTS

Page

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

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

Regional Precambrian geology ........................................................... 3

Metamorphic complex ................................................................. 3

Granite ........................................................................................ 6

Diabase ........................................................................................ 9

Geochronology ................................................................................... 9

Page

Geochronology Continued

Metamorphic complex ................................................................. 9

Granite ........................................................................................ 12

Diabase dikes and nephrite veins ................................................ 14

Mineral ages ................................................................................ 15

Conclusions ........................................................................................ 18

References cited .................................................................................. 20

ILLUSTRATIONS

Page

FIGURE 1. Index map of exposed Precambrian in the Wyoming area ............................................................................................................... 1

2. Map of the geology of the Precambrian of the Granite Mountains ................................................................................................. 2

3. Diagram of modal quartz, plagioclase, and alkali feldspar .............................................................................................................. 5

4. Rb-Sr isochron plot for the metamorphic complex .......................................................................................................................... 10

5. Rb-Sr isochron plot for the granite rocks ........................................................................................................................................ 13

6. Chart of Rb-Sr and K-Ar mineral ages ............................................................................................................................................ 15

7. Chart of Rb-Sr and TMPb/TMPb ages of microcline .......................................................................................................................... 16

8. K-Ar and Rb-Sr ages of biotite from Precambrian W rocks ............................................................................................................ 18

TABLES

Page

TABLE 1. Descriptions and modal analyses of metamorphic rocks .................................................................................................................. 4

2. Descriptions and modal analyses of granitic rocks ........................................................................................................................... 7

3. Analytical data for metamorphic rocks ............................................................................................................................................ 11

4. Analytical data for granitic rocks ..................................................................................................................................................... 12

5. Analytical data for ^Ar/^Ar ages .................................................................................................................................................... 15

6. Conventional K-Ar ages ................................................................................................................................................................... 16

in
RECONNAISSANCE GEOLOGY AND GEOCHRONOLOGY OF THE 

PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

By ZELL E. PETERMAN and R. A. HILDRETH

ABSTRACT

The Precambrian of the western part of the Granite Mountains, 

Wyoming, contains a metamorphic complex of gneisses, schists, and 

amphibolites that were derived through amphibolite-grade metamorphism 

from a sedimentary-volcanic sequence perhaps similar to that exposed in 

the southeastern Wind River Mountains. Whole-rock Rb-Sr dating places 

the time of metamorphism at 2,860±80 million years. A high initial 

s7Sr/s6Sr ratio of 0.7048 suggests that either the protoliths or the source 

terrane of the sedimentary component is several hundred million years older 

than the time of metamorphism. Following an interval of 300±100 million 

years for which the geologic record is lacking or still undeciphered, the 

metamorphic complex was intruded by a batholith and satellite bodies of 

medium- to coarse-grained, generally massive biotite granite and related 

pegmatite and aplite. The main body of granite is dated at 2,55Q±60 million 

years by the Rb-Sr method. Limited data suggest that diabase dikes were 

emplaced and nephrite veins were formed only shortly after intrusion of the 

granite.

Emplacement of the granite at about 2,550 million years ago appears to 

be related to a major period of regional granitic plutonism in the 

Precambrian of southern and western Wyoming. Granites, in the strict 

sense, that are dated between 2,450 and 2,600 million years occur in the 

Teton Range, the Sierra Madre, the Medicine Bow Mountains and the 

Laramie Range. This episode of granitic plutonism occured some 50 to 100 

million years later than the major tonalitic to granitic plutonism in the 

Superior province of northern Minnesota and adjacent Ontario the 

nearest exposed Precambrian W terrane that is analogous to the Wyoming 

province. Initial 87Sr / 86Sr ratios of some of the Wyoming granites are higher 

than expected if the rocks had been derived from juvenile magmas and it is 

likely that older crustal rocks were involved to some degree in the 

generation of these granites.

Slightly to highly disturbed Rb-Sr and K-Ar mineral ages are obtained on 

rocks of the metamorphic complex and on the granite. These ages range 

from about 2,400 to 1,420 million years and are part of a regional pattern of 

lowered mineral ages of Precambrian W rocks of southern Wyoming. A 

major discontinuity in these mineral ages occurs along a line extending from 

the northern Laramie Range, through the northern part of the Granite 

Mountains, to the southeastern Wind River Mountains. North of this line, 

Rb-Sr and K-Ar biotite ages are 2,300 million years or greater, whereas to 

the south, the biotite ages decrease drastically over a short distance, to a 

common range of 1,600-1,400 million years. We suggest that these lowered 

ages represent regional cooling below the 300° C isotherm as a consequence 

of uplift and erosion of the large crustal block occurring south of the age 

discontinuity. In this interpretation, the westerly-trending age discontinuity 

would be a zone of major crustal dislocation that resulted from vertical 

tectonics in late Precambrian X or early Precambrian Y time.

INTRODUCTION

Precambrian igneous and metamorphic rocks are exposed 

in the Granite Mountains of central Wyoming a block of 

Precambrian basement that was uplifted during the

Laramide orogeny when vertical tectonics and thrust 

faulting resulted in the exposure of several major terranes of 

crystalline rock in Wyoming (fig. 1). The Granite Mountains 

uplift trends west-northwest and lies between the Wind 

River uplift on the west and the Laramie uplift on the east. 

The Phanerozoic and particularly the Cenozoic history of 

the Granite Mountains is complex, but a comprehensive 

account of this facet of the geology is given by Love (1970). 

Because of the economic importance of the Phanerozoic 

sedimentary rocks along the flanks and in basins adjacent to 

the Granite Mountains, geologic studies have emphasized 

these younger rocks. A few gold and sulfide occurences in 

Precambrian rocks were prospected and some were mined to 

a limited extent, but these did not prove sufficiently 

substantial to sustain operations. Nephrite (jade) occurs in 

Precambrian vein deposits and in Eocene conglomerates and 

Holocene alluvium derived therefrom; these deposits are 

important in a local gemstone trade.

Love (1970) reviewed the investigations of Precambrian 

rocks that had been made prior to 1965. Sherer (1969), in his 

study of nephrite deposits of central Wyoming, completed a

110" 108'

0 50 100 150 200 KILOMETERS 

j____I

FIGURE 1. Areas of exposed Precambrian in the Wyoming area (King and 

Beikman, 1974).

1
R.94 W

PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

108° R.93W. R.92W. R.gi W.

10

I

12 KILOMETERS

EXPLANATION

o O 

o> c ^ .* 

o> c

21J

TERTIARY

Moonstone Formation (Pliocene)

Split Rock Formation (Miocene) 

White River Formation (Oligocene) 

Wagon Bed Formation (Eocene)

Alkalic igneous rocks, 44 m.y. old (Eocene) (Pekerek °> 

and others, 1974)

MESOZOIC AND PALEOZOIC 

Sedimentary rocks undivided

PRECAMBRIAN W

Mafic dikes of basaltic composition

Medium- to coarse-grained biotite granite with numerous 

dikes of aplite and pegmatite

65

D1686.

Inequigranular biotite- quartz-feldspar augen gneiss

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

Fine- to medium-grained amphibolite. Contains numerous 

dikes and sills of granite and pegmatite

Fine-greined, banded epidote-rich gneiss

Dominantly medium grained, well-banded, biotite-quartzfeldspar gneiss of tonalitic to granitic composition. Interlayered with subordinate amounts of mica schist, 

amphibolite, epidote gneiss, serpentinite, and augen gneiss

Contact dashed where approximately located 

Fault dashed where approximately located 

Strike and dip of foliation and banding 

Strike and dip of horizontal foliation 

Sample locality and number

FIGURE 2. Reconnaissance geology of the Precambrian of the western part of the Granite Mountains. Base and Phanerozoic geology from

Love (1970, pi. 1). Sample numbers abbreviated for clarity.

detailed map of a small area of Precambrian gneisses in the 

western part of the Granite Mountains. Houston (1974) 

prepared reconnaissance maps of the Precambrian from 

several types of remote-sensing data. Stuckless and others 

(1977) have completed a preliminary geochemical and

petrologic study of the granite in the Granite Mountains. 

The present study emphasizes the regional geology and 

geochronology of the Precambrian rocks of the western part 

of the Granite Mountains. We completed reconnaissance 

mapping in the summers of 1968 and 1969 and collected
REGIONAL PRECAMBRIAN GEOLOGY

R.89 W. R.88 W.

samples for the geochronological study at that time.

Preliminary results of our geochronology study were 

previously summarized (Peterman and others, 1971). Other 

isotopic studies have emphasized the U-Th-Pb systematics in 

the granite and the metamorphic rocks. Rosholt and Bartel 

(1969) demonstrated that a large fraction of the uranium in 

the granite had been removed in relatively recent geologic 

time, the last 100 m.y. (million years), and suggested that the 

granite may have been a source for much or all of the 

uranium that forms the important economic deposits in 

adjacent Tertiary sandstones. Further work on the granite 

(Rosholt and others, 1973) and on gneisses of the 

metamorphic complex (NKomo and Rosholt, 1972) reinforced these earlier conclusions. This hypothesis conflicts 

with that of Love (1970) who, on the basis of geologic and 

geochemical data, concluded that the uranium in the 

Tertiary sandstones was probably derived by leaching of 

lacustrine tuff beds of the Pliocene Moonstone Formation.

We are especially grateful to Robert G. Coleman who 

introduced the senior author to the geology of the Granite 

Mountains, encouraged the study, and provided several 

samples for isotopic analyses. John Stacey assisted in the 

field work in the summer of 1968. The late William T. 

Henderson completed most of the chemical work attendant 

with the Rb and Sr analyses. G. T. Cebula and Jack Groen 

prepared all of the whole-rock and mineral samples. M. A. 

Lanphere and G.B. Dalrymple determined the 39 Ar/ 40Ar

ages and R. ¥. Marvin and H. H. Mehnert provided the 

conventional K-Ar ages. T. W. Stern analyzed zircon from 

the granite at Tincup Mountain. John Stuckless provided 

core samples from U.S. Geological Survey drill hole GM-1. 

J. David Love, Robert Houston, and John Stuckless 

provided many valuable comments on the manuscript. To all 

of these people, we express our sincere appreciation.

REGIONAL PRECAMBRIAN GEOLOGY

The Precambrian geology of the western part of the 

Granite Mountains (Tps. 29-31 N., Rs. 88-94 W.) is shown in 

figure 2. Three major terranes or lithologies are readily 

differentiated: (1) An assemblage of amphibolite-grade 

metamorphic rocks crops out in the northwestern and 

northern part of the area. (2) Granitic rocks, whence the 

name Granite Mountains, are part of a large batholith 

extending eastward beyond the map area. (3) Diabase dikes 

with dominant northeasterly trends intrude both the granitic 

and metamorphic rocks. The geologic maps prepared by 

Houston (1974) from various remote-sensing data and the 

reconnaissance mapping completed for this study are in 

excellent agreement.

METAMORPHIC COMPLEX

The metamorphic complex contains the oldest Precambrian rocks of the area and consists of a variety of schists and 

gneisses that attained amphibolite grade but locally have 

been retrograded. Foliation and compositional banding in 

this terrane trend northeasterly to easterly, and dips are 

generally in a southerly direction towards the granite. The 

metamorphic complex is divided into five major units based 

upon the dominant lithology in each (fig. 2): quartzofeldspathic gneiss; epidote-rich gneiss; amphibolite; 

biotite gneiss (interpreted to be metagraywacke); and augen 

gneiss. More detailed mapping would undoubtedly result in 

the recognition of a greater number of units. The relative 

ages of most of the metamorphic rocks are unknown. Some 

amphibolite and granitic gneiss are clearly intrusive but have 

been involved in the major period of folding and 

metamorphism. The major emphasis on the metamorphic 

rocks in the present study was on those cropping out in Tps. 

30 and 31 N., Rs. 92 and 93 W. Other areas were examined in 

less detail and to the extent necessary to identify the major 

lithologic types present. Little effort was devoted to the 

metamorphic rocks cropping out in T. 31 N., R. 88 W., and 

the mapping of Carey (1959) was used for the northern part 

of this township. A photogeologic map of the central part of 

the township was prepared by Houston (1974), and his 

ground check identified an occurrence of Precambrian 

iron-formation, a unit not encountered elsewhere in the area. 

It is noteworthy that Love (1970) reported the presence of 

fragments of banded iron-formation in the Eocene Wagon 

Bed Formation (T. 32 N., R. 86 W.) and suggested that 

Precambrian and Paleozoic rocks to the south and 

southwest should be examined as potential sources for these
PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING 

TABLE 1. Descriptions and modal analyses of metamorphic rocks

[Asterisk by sample number indicates modal analysis by point counting (500-600 points). Other modes are visual estimates. Sample localities illustrated in figure 2]

Sample Locality Description

*GM-35-68

*GM-3h-68

*GM-76-68

*GM-77-68

*GM-78-68

42°35'21" N. 

107°55'37" W.

42°35'04" N. 

107°54'55" W.

42°35'04" N. 

107°54'55" W.

42°33'58" N. 

107°52'18" W.

42°33'58" N. 

107°52'18" W.

*GM-98-68 42°32'38" N. 

107°12'40" W.

GM-103A-69 42°38'08" N. 

107°30'58" W.

*GM-104-69 42°38'46" N. 

107°30'58" W.

W2-CR-4(114) 42°36'40"N. 

107°46'10" W.

FCA-R-1 42°50'19" N. 

107°52'54" W.

Light-gray, fine- to medium-grained, xenoblastic, inequigranular granodioritic gneiss. Leucocratic bands from 2 to 10 

mm wide alternate with more biotite-rich bands from 1 to 10 mm wide. Plagioclase (An24,46 percent) is moderately 

serialized. Microcline (11 percent) is fresh and occurs mainly as grains interstitial to plagioclase and quartz (34 

percent), although a few larger porphyroblasts of microcline as much as 10 mm occur in the leucocratic bands. 

Epidote (1.1 percent) occurs as discrete subhedral grains in association with biotite (7.0 percent). Zircon, apatite, 

hornblende, altered allanite, and opaques are minor constituents.

Medium-gray, fine- to medium-grained, xenoblastic, inequigranular tonalitic gneiss. Alternating light and dark bands 

range from a few millimeters to several centimeters in thickness. Plagioclase (An27, 52 percent) is only slightly 

altered to sericite notably less so than in GM-35-68. Quartz (37 percent) locally forms composite ameboid lenses 

in the plane of foliation. A few grain boundaries show suturing. Biotite (8.1 percent) is completely unaltered and 

associated with epidote (1.0 percent) and trace amounts of allanite. Apatite, zircon, hornblende, and opaques are 

common accessory minerals.

Medium-gray, medium-grained, xenoblastic, inequigranular tonalitic gneiss. Banding is less distinctly developed than 

in sample GM-38-68 from the same locality. Composite grains of quartz (36 percent) and plagioclase (An27, 51 

percent) occur as eyes and discontinuous layers in the more biotite (10 percent) rich portions of the rock. 

Microcline (0.2 percent) occurs only as patch antiperthite in plagioclase. Plagioclase is somewhat saussuritized and 

biotite is locally partially chloritized and associated with epidote (1.2 percent). A few discrete grains of muscovite 

(1.7 percent) occur. Sphene, zircon, apatite, and opaques are accessory minerals.

Dark-gray, fine- to medium-grained, xenoblastic, inequigranular granodioritic gneiss with thin discontinuous 

leucocratic layers (as much as 5 mm) alternating with thicker (1 to several cm) mafic layers. Quartz (36 percent) 

occurs as both equant grains and as composite lenses elongated in the foliation plane defined by biotite (15 

percent). Plagioclase (An25 , 29 percent) is only slightly sericitized. Microcline (12 percent) occurs both as discrete 

grains and as patches in plagioclase. Blue-green hornblende (5.6 percent) is concentrated in the mafic layers. 

Epidote (1.8 percent), euhedral sphene (0.5 percent), and opaques (0.2 percent) are abundant trace minerals. 

Apatite, zircon, and allanite are present.

Light-gray, fine-grained, xenoblastic, inequigranular granitic gneiss with thin (1-2-mm) and continuous leucocratic 

layers imparting a distinct fine-scale banding. Plagioclase (An22, 30 percent) is only slightly altered to sericite. 

Microcline (30 percent) is a major constituent and locally is poikiloblastic with inclusions of quartz and 

plagioclase. Quartz (32 percent) locally occurs as composite lenses elongated in the extremely well developed 

foliation defined by the biotite (8.1 percent). Epidote, zircon, apatite, opaques, and chlorite are present in trace 

amounts.

Medium-gray, medium-grained, xenoblastic, inequigranular tonalitic gneiss. The rock has a well-developed foliation 

but is not banded. Plagioclase (An27, 39 percent) is only slightly sericitized and biotite (6.7 percent) is locally 

chloritized. Only trace amounts of microcline occur (0.2 percent). Quartz (39 percent) occurs in places as very 

elongate lenses as much as 10 mm in length and 3 mm in width. Epidote (0.4 percent) occurs as discrete grains in 

association with biotite. Apatite, zircon, allanite, and opaques are common trace minerals.

Dark-gray, fine-grained amphibolite intercalated with quartzofeldspathic gneiss. Amphibolite has well-developed 

nematoblastic texture. Hornblende and plagioclase in a ratio of about 2:1 dominate the mineralogy. Quartz is an 

important varietal mineral. Most of the plagioclase is extremely fresh but local areas within the section show 

intense saussuritization. Opaques, apatite, epidote, and white mica are minor phases.

Light-gray, fine-grained, xenoblastic, equigranular granodioritic gneiss. The rock is foliated but only faintly banded 

with sparse leucocratic layers as much as 5 mm in width alternating with massive bands from 2 to several cm in 

width. The foliation of the rock is defined by alinement of the hornblende (8.9 percent). Biotite is not present, a 

feature that distinguishes this rock from the other quartzofeldspathic gneisses described herein. Plagioclase (38 

percent), virtually unaltered, is more calcic (An37 ) than in the other gneisses, but the rock contains significant 

amounts of microcline (9.4 percent). Accessory minerals include sphene (0.7 percent), epidote (0.3 percent), and 

apatite (0.2 percent). Quartz is 43 percent.

Dark-gray, fine- to medium-grained amphibolite. Hornblende and plagioclase (An30) in approximately equal amounts 

constitute most of the rock. Quartz, biotite, sphene, apatite, opaques, epidote, and white mica are present in small 

amounts. Plagioclase is, for the most part, fresh, but locally it is saussuritized. This sample is mineralogically and 

texturally similar to GM-I03A-69 but is slightly coarser in grain size.

Medium-gray, medium- to coarse-grained, well-foliated granodioritic gneiss. Rock is not compositionally banded, but 

well-developed foliation is imparted by alined hornblende and biotite. Approximate mode: plagioclase, 40 percent; 

quartz, 30; hornblende, 15; biotite, 10; microcline, 5. Allanite, sphene, epidote, apatite, and zircon are accessory 

minerals. Altered allanite grains are commonly rimmed by epidote. White mica, epidote, chlorite, and carbonate 

are minor alteration minerals.
REGIONAL PRECAMBRIAN GEOLOGY

fragments. No outcrops of iron-formation had been identified in the region at that time.

The unit designated as quartzofeldspathic gneiss in figure 

2 contains a variety of biotite-quartz-feldspar gneisses that 

are interlayered with subordinate amounts of mica schist, 

amphibolite, augen gneiss, epidote gneiss, and serpentinite. 

Most of the samples (table 1) used for radiometric dating of 

the metamorphic complex are from this unit.

The gneisses are characteristically well foliated and form a 

layered sequence of alternating and compositionally distinct 

units. The thickness of the layers ranges from a few 

centimeters to several meters. Migmatitic varieties contain 

thin discontinuous layers of leucocratic phases as much as a 

centimeter thick with sparse porphyroblasts of microcline. 

Locally, very tight small-scale folding is present in the 

gneisses, and layers of amphibolite have been distended to 

form boudins and rotated blocks. The gneisses are intruded 

by numerous dikes and stringers of granitic material, some 

of which have been deformed and metamorphosed. Other 

dikes are undeformed and are apparently related to the 

younger granite (fig. 2) that intrudes the metamorphic 

complex. Some of the biotite augen gneiss that is interlayered in places with the finer grained quartzofeldspathic 

gneisses is similar to that shown as a separate unit (fig. 2).

The compositional range of the gneisses is shown on figure 

3. Using the nomenclature for igneous rocks (Streckeisen, 

1973) without any genetic implications, the gneisses range in 

composition from tonalitic to granitic. The granitic gneisses 

are subordinate in volume to the tonalitic and granodioritic 

gneisses. In T. 31 N., Rs. 92-93 W., Sherer (1969) identified 

three major lithologies of gneisses the dominant biotite 

gneiss, granite gneiss of uncertain relationship to the biotite 

gneiss, and quartzofeldspathic gneiss derived from granitic 

dikes and sills that are intrusive into both the biotite gneiss 

and granite gneiss. Gneisses in the granite field of figure 3 are 

mainly the granite gneiss and quartzofeldspathic gneiss of 

Sherer (1969).

Intense alteration adjacent to the zones of nephrite veins 

has produced a variety of epidote-rich gneisses similar to but 

generally coarser grained than the fine-grained epidote-rich 

gneiss of figure 2. Sherer (1969, 1972) concluded that the 

nephrite veins formed as a consequence of hydrous 

metasomatic alteration of amphibolite with iron, aluminum, 

and calcium being mobilized during this alteration to 

produce epidotization of the country rocks.

Much of the gneiss exposed in a series of low-lying hills 

centered in the area outlined by Tps. 30 and 31 N. and Rs. 91 

and 92 W. is pervasively altered. In the field, a major 

lithologic unit was called "green" gneiss because of its faint 

greenish-gray color. Thin sections show that the gneiss 

contains 30 to 50 percent quartz, but the feldspar, 

presumably plagioclase, has been completely altered to 

fine-grained, felted aggregates of white mica. Sherer (1969) 

described similar alteration and has identified the secondary 

mica as paragonite. In one sample, biotite has been totally

7

PLASIOCLASE ALKALI FELDSPAR

FIGURE 3. Samples of quartzofeldspathic gneiss (dots) plotted on a 

portion of ternary diagram of modal quartz, plagioclase, and alkali 

feldspar with the IUGS classification for plutonic rocks (Streckeisen, 

1973). Data are from Sherer (1969) and this report (table 1).

altered to chlorite that is dusted with fine-grained opaque 

inclusions, but in others the biotite is fresh, although it is 

extremely pale brown, suggestive of a low-iron content. 

Coarse muscovite occurs in one sample that transects the 

plagioclase relics and altered biotite. Epidote and zircon are 

trace constituents. Unaltered gneisses also occur in this area. 

Amphibolite and serpentinite layers are common but 

volumetrically small compared to the altered and unaltered 

quartzofeldspathic gneisses.

A variety of medium- to coarse-grained epidote-bearing 

gneisses occur throughout the metamorphic complex, but in 

T.31 N., Rs. 92 and 93 W., fine-grained epidote gneiss is 

mapped as a distinct unit (fig. 2). These rocks are extremely 

fine grained (0.1-0.5 mm), gray to pastel shades of green, 

yellow and pink gneisses that in outcrop resemble 

metarhyolite or fine-grained quartzite. They exhibit blocky 

fracturing and are extremely tough and brittle. The epidote 

gneisses are variable in composition but are characterized by 

quartz, epidote, and fresh to altered plagioclase. Some 

samples of the epidote gneiss contain significant amounts of 

actinolite both as poikiloblastic grains that include the other 

minerals and as fibrous aggregates. Compositional layering 

ranges from a few millimeters to several centimeters and 

results from variable epidote and actinolite concentrations. 

Granoblastic quartz-plagioclase layers alternate with 

strongly nematoblastic epidote- and actinolite-rich layers. 

One sample examined contains approximately equal 

amounts of epidote and quartz but no plagioclase. Sphene 

and apatite are ubiquitous accessory minerals. Aggregates of 

granular sphene are strung out in the direction of the 

foliation and are characteristically associated with epidote 

The origin of the epidote gneiss is problematic. Sherer (1969) 

related epidote-rich gneisses to alteration associated with the 

nephrite veins. The fine-grained epidote gneiss described 

here may have been derived from intermediate to silicic 

volcanic rocks, a speculation supported by its association 

with layered amphibolite, which is probably derived from a 

mafic volcanic rock.
PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

Dark-gray, fine- to medium-grained amphibolite (fig. 2) 

occurs as thin, discontinuous layers throughout the 

metamorphic complex and is the dominant lithology in a 

northeasterly trending belt in T. 31 N., R. 93 W. and in the 

northwestern part of T. 30 N., R. 92 W. In both areas, but 

particularly in the latter, quartzofeldspathic gneiss is abundantly interlayered with the amphibolite. In these areas the 

amphibolite is commonly banded with alternating mafic and 

very thin more leucocratic layers, and weathers to produce 

slabby or flaggy outcrops. Detailed petrographic studies of 

the amphibolite were not completed, but a few thin sections 

show it to consist dominantly of plagioclase and hornblende, 

with minor amounts of quartz. Epidote, sphene, and apatite 

are common accessory minerals and trace amounts of biotite 

are present in some samples. Some of the amphibolite within 

the quartzofeldspathic gneisses is dominated by hornblende 

with only small amounts of plagioclase. In T. 31 N., R. 93 

W., the amphibolite is pervaded by numerous dikes and sills 

of granite and pegmatite.

Dark-gray, fine-grained biotite gneiss (fig. 2) is poorly 

exposed in low-lying outcrops in the center of T. 31 N., R. 93 

W. The rock is strongly foliated with 20 to 30 percent biotite 

and approximately equal amounts of quartz and sodic 

plagioclase, much of which is untwinned. Apatite, epidote, 

zircon, and opaques are accessory minerals. The biotite 

occurs as subparallel grains as much as 1 mm in length in a 

granoblastic mosaic composed of 0.1-0.2-mm grains of 

quartz and feldspar. Compositional banding in outcrop is 

presumed to represent relic sedimentary layering and the 

rock is interpreted to be, on the basis of this and the 

composition, a metagraywacke. In comparison with the 

quartzofeldspathic gneisses (fig. 2), this biotite gneiss unit is 

characterized by its finer grain size, greater abundance of 

biotite, and absence of K-feldspar. The metagraywacke is 

intruded by numerous small dikes and stringers of granitic 

rock and pegmatite. A similar rock type is intercalated in 

places with the quartzofeldspathic gneiss and with the 

amphibolite.

Medium- to coarse-grained biotite augen gneiss (fig. 2) 

occurs in many places throughout the metamorphic complex 

and is distinguished as a sizable unit in T. 31 N., Rs. 92,93 W. 

The gneiss is strongly inequigranular with distinct augens as 

much as several centimeters in length. The augens are 

commonly composites of quartz, plagioclase, and 

poikiloblastic microcline. These are contained within a finer 

grained groundmass of mainly quartz, plagioclase, 

microcline, and biotite. Biotite is in lenticular aggregates and 

imparts a distinct but wavy foliation to the rock. Muscovite, 

epidote, sphene, apatite, zircon, opaques, and altered 

allanite are common accessory minerals. In places, the augen 

gneiss has been highly sheared and the augen have been 

stretched into long, recrystallized leucocratic streaks. Within 

the main outcrop area, the augen gneiss is extensively 

intruded by granitic and pegmatitic dikes that are undeformed.

GRANITE

The western part of a large granitic batholith crops out in 

the map area (fig. 2), where these rocks form a major part of 

the Precambrian. For purposes of discussion here, three 

masses of granite separated by metamorphic rocks are 

referred to as the granite at Long Creek Mountain (eastern 

part of T. 31 N., R.94 W. and the northwestern part of T. 31 

N., R. 93 W.), the granite at Tincup Mountain (a belt 

extending northeasterly across T. 31 N., R. 93 W.), and the 

granite of Lankin Dome (Tps. 29-30 N., Rs. 88-92 W.). The 

batholith extends eastward several kilometers beyond the 

limit of the map area. Excellent exposures of the granite 

occur in rounded to craggy knobs and hills that rise a few 

hundred meters above the plain of flat-lying Tertiary 

sedimentary rocks. The granite is clearly younger than the 

enclosing metamorphic rocks and both units are cut by 

diabase dikes. Samples that were used in the dating, a few of 

which are from outside the map area, are described in table 2.

The granite at Long Creek Mountain is a massive to 

foliated light-gray to reddish-gray biotite granite with 

inclusions and schlieren of biotite gneiss and amphibolite. 

Small dikes and stringers of pegmatite, aplite, and quartz are 

common throughout the body. At the north end of the mass, 

the granite becomes gneissic with faint banding, and an 

inclusion or screen of migmatite approximately 30 m thick 

was observed. The foliation in the gneissic phase strikes N. 

50° E., parallel to the regional structure of the metamorphic 

rocks, but dips approximately 35° to the northwest. The 

granite at Long Creek Mountain was not sampled for Rb-Sr 

dating because of pervasive weathering, but U-Pb dating of 

zircons suggests that the granite may be slightly older than 

the granites of Lankin Dome and Tincup Mountain (K. R. 

Ludwig and J. S. Stuckless, oral commun., 1976).

The granite at Tincup Mountain is a fine- to 

medium-grained, medium-gray, foliated muscovite-biotite 

granite that is cut by numerous dikes of pegmatite, aplite, 

and quartz. Some of the aplite dikes are unusually garnetiferous, and very coarse grained muscovite-bearing 

pegmatite is present locally.

The granite of Lankin Dome is the main body of granite 

and is typically massive and medium to coarse grained. 

Dikes of pegmatite ranging from several centimeters to a 

meter or more in width are common. Phenocrysts of 

microcline as much as several centimeters in length are 

dispersed throughout the granite at spacings of a meter or 

more. Isolated pegmatitic clots as much as several tens of 

centimeters long are also common. The major rock type 

exposed contains approximately equal amounts of quartz, 

plagioclase, and perthitic microcline, which form 90-95 

percent of the rock. Plagioclase composition ranges from 

albite to oligoclase, and the appropriate rock names are 

biotite granite and biotite alkali-feldspar granite, according 

to the IUGS classification (Streckeisen, 1973). The average 

composition of the biotite granite (Stuckless and others,
REGIONAL PRECAMBRIAN GEOLOGY 

TABLE 2. Descriptions and modal analyses of granitic rocks

[Asterisk by sample number indicates modal analysis by point counting (500 to 600 points). Other modes are visual estimates. Sample localities illustrated in figure 2]

Sample Locality Description

GM-55-68 42°32'03" N. 

107°53W W.

GM-56-68

GM-81-68

42°32'07" N. 

107°52'51" W.

42°33'04"N. 

107°43'47" W.

W2-CR-1(99) 42°4r50" N. 

107°19'46" W.

W2-CR-1(153) 42° 41'50" N. 

107°19'46" W.

*W2-CR-14(101) 42°32'35" N. 

107°39'15" W.

*W2-CR-14(157) 42°32'35" N. 

107°39'15" W.

*W2-CR-26(99) 42°33'46" N. 

107°22'09" W.

*W2-CR-26(165) 42°33'46" N. 

107°22'09" W.

*ZW-263 41°31'02" N. 

107°48'04" W.

*D1686 42°39'00" N. 

107°54'30" W.

Medium-grained, faintly foliated biotite granite collected from within 1.2 m of the contact with an 18-m-wide diabase 

dike. Contains approximately 40 percent plagioclase; 30 quartz; 20 microcline; 10 biotite; and trace amounts of 

epidote, white mica, chlorite, zircon, apatite, and opaques. Plagioclase is severely altered to white mica, and 

cryptoperthitic microcline is clouded with minute inclusions. Biotite is mainly fresh and appears to have been 

recrystallized, as cleavage lies at right angles to grain elongation in places. Epidote, commonly subhedral, generally 

occurs in association with the biotite. Alteration of the granite is apparently related to heating attendant with 

emplacement of the diabase.

Hand specimen and thin section of this sample was inadvertently lost. Rock is described in field notes as a strongly 

foliated and locally sheared porphyritic granite intruded by stringers and dikes of pegmatite and biotite granite. 

Sample was collected for its fresh biotite.

Light-greenish-gray, medium-grained epidotized granite. Contains approximately equal amounts of plagioclase 

(chessboard albite) and quartz (30-40 percent each), epidote (20 percent), and about 10 percent chlorite. Zircon and 

apatite are trace minerals. Epidote occurs both as composite aggregates and as discrete euhedral grains 

interspersed in quartz and feldspar. Anhedral to euhedral sphene is commonly in association with composite 

chlorite aggregates. Albite is locally poikilitic with numerous inclusions of quartz as well as epidote.

Pinkish-gray, coarse-grained granite. Quartz, plagioclase and microcline, in roughly equal amounts, constitute about 

90 percent of the rock. Chlorite, formed from biotite, is the next most abundant mineral. Epidote, sphene, apatite, 

zircon, opaques, and leucoxene occur in trace amounts. Plagioclase (albite or sodic oligoclase) shows faint zoning 

and is moderately to strongly saussuritized. Sphene, both as granular grains in chlorite and as discrete euhedral 

wedges, is strongly altered to leucoxene. Euhedral epidote grains occur in association with the mafic minerals. 

Microcline is fresh and commonly poikilitic with inclusions of quartz, plagioclase, and epidote. Composite grains 

of quartz are common with moderately sutured grain boundaries.

Texturally and mineralogically similar to sample W2-CR-1 (99) except not nearly as altered. Biotite, for the most part, is 

fresh with only a few grains showing alteration to chlorite. Plagioclase is somewhat altered to white mica but 

generally fresher than in W2-CR-1(99).

Light-gray, medium- to coarse-grained, inequigranular biotite granite. Quartz (29 percent) is strained and commonly 

occurs as composite grains with some suturing of grain boundaries. Microcline (33 percent) is fresh, strongly 

perthitic, and poikilitically includes quartz, plagioclase, and biotite. Plagioclase (Anio_15 ,31 percent) is moderately 

altered to white mica, faintly zoned, and locally shows bent twin lamellae. Biotite (1.4 percent) is partially altered to 

chlorite (1.1 percent) and granular sphene. Epidote (1.6 percent) occurs mainly as subhedral grains in association 

with the mafic minerals, and muscovite is present as discrete grains (1.6 percent). Sphene, zircon, apatite, and 

opaques are present in trace quantities.

Texturally and mineralogically similar to W2-CR-14(101). Modal analysis: quartz, 34 percent, microcline, 36; 

plagioclase, 26; biotite, 1.4; chlorite, 1.4; muscovite, 1.1; epidote, 0.4; and trace amounts of sphene, zircon, 

opaques, and white .mica.

Medium-grained, light-gray biotite granite. Quartz (26 percent) occurs as strained, composite grains with some sutured 

boundaries. Plagioclase (An :2_ 15 , 29 percent) is moderately saussuritized with some bent and broken twin 

lamellae and healed mortar structure around larger grains. Perthitic microcline (37 percent) is fresh and commonly 

poikilitic with inclusions of quartz, plagioclase, and biotite. Biotite (5.1 percent) is partially altered to chlorite (0.3 

percent) and traces of granular sphene. Epidote (1.5 percent) and opaques (0.2 percent) are commonly associated 

with the biotite aggregates. Sphene, zircon, apatite, white mica, and monazite (?) are present in trace amounts.

Generally similar to W2-CR-26(99). Modal analysis: plagioclase (zoned from An g to An 19 ,34 percent); microcline, 31; 

quartz, 30; biotite, 2.6; epidote, 1.6; muscovite, 0.2; chlorite, 0.2; and trace amounts of monazite(?), zircon, apatite, 

opaques, carbonate, sphene, and white mica.

Very light gray, coarse-grained, leucocratic porphyritic granite. Approximately 10 percent of the rock is composed of 

poikilitic microcline phenocrysts attaining lengths of 25 mm but more commonly about 10 mm. Groundmass is 

medium to coarse grained. Plagioclase (An8 , 32 percent) is altered to white mica and epidote and in places shows 

bent twin lamellae. Microcline (38 percent) is fresh and is poikilitic in both the groundmass and phenocryst phases 

with inclusions of quartz, plagioclase, and epidote. Quartz (28 percent) is strained and commonly forms composite 

grains. Biotite (0.3 percent), chlorite (0.2 percent), and epidote (0.3 percent) are commonly associated. Granular 

sphene in chlorite, zircon, and opaques are trace minerals. Muscovite (0.3 percent) occurs as discrete grains but 

commonly restricted to the plagioclase.

Light-gray, medium-grained, faintly foliated biotite granite. Quartz (27 percent) is strained with some sutured 

boundaries in composite grains. Plagioclase (albite or sodic oligoclase, 34 percent) is moderately altered to white 

mica and in places shows bent twin lamellae. Microcline (30 percent) is commonly poikilitic. Biotite (4.4 percent) is 

slightly chloritized (0.3 percent). Muscovite (2.9 percent) is a significant varietal mineral. Epidote, zircon, allanite, 

garnet, and opaques are trace constituents.
PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

TABLE 2. Descriptions and modal analyses of granitic rocks—Continued

Sample Locality Description

*DDH-1 42°31'00" N. Light-gray, medium-grained biotite granite with some iron staining. Plagioclase (albite or sodic oligoclase, 37 percent) 

107°38'30" W. is moderately saussuritized with bent twin lamellae locally. Quartz (37 percent) is in strained composite grains

showing some incipient mortar structure in places. Microcline (22 percent) is fresh and commonly poikilitic.

Subhedral grains of epidote (0.5 percent) are associated with slightly chloritized biotite (3.4 percent). Muscovite

(0.4 percent) occurs both as discrete grains and in the alteration products of plagioclase. Apatite, zircon, and

opaques are trace constituents.

DDH-2 42°32'55" N. Light-gray, medium-grained biotite granite. Approximately 95 percent of the rock comprises nearly equal amounts of 

107°49'14" W. quartz, plagioclase, and microcline. Biotite, somewhat chloritized, is the major varietal mineral. Plagioclase is

somewhat saussuritized. Epidote also occurs as discrete subhedral grains associated with biotite. A few grains of

monazite are present. Protoclasis is indicated by bent and broken twin lamellae in plagioclase and incipient mortar

structure.

Dl 14944 42° 10'27" N. Light-gray, medium-grained biotite granite collected from the abutment of the Kortes Dam (Rosholt and Bartel, 1969). 

Dl 14945 106°52'47" W. Hand specimens of these particular samples are not available but others from the area show marked similarity to

samples from the granite of Lankin Dome.

256179 42°18'28" N. Drill core of coarse-grained biotite granite from the Little Man mine. Described in Rosholt and Bartel (1969). 

106°50'46" W.

GM-1 42°32'35" N. Medium- to coarse-grained, leucocratic granite from USGS drill hole GM-1 (Stuckless and others, 1977). Descriptions 

107°39'15" W. of samples and modes are given by Stuckless and others (1976). Abundances of the major minerals, in percent, are 

listed below:

Sample Quartz Plagioctase Microcline Biotite

GM-1 (739).............................

(771).............................

(891) .............................

(1021) .............................

(1156).............................

...................... 29

...................... 37

...................... 30

...................... 39

...................... 39

43

30

20

37

59

23

31

45

21

0

1.8

.6

2.0

2.1

2.3

1977) is 31 percent quartz, 30 percent plagioclase, 34 percent 

microcline, and 5 percent biotite and chlorite, with alteration 

and accessory minerals constituting less than 2 percent.

Texturally, the biotite granite is allotriomorphic inequigranular. Perthitic microcline is poikilitic with inclusions 

of plagioclase, quartz and biotite. Plagioclase is altered to 

white mica and granular epidote. Epidote also occurs as 

subhedral grains in association with clusters of biotite and 

opaque minerals. Biotite shows all degrees of alteration to 

chlorite and granular sphene, which is partially altered to 

leucoxene. Quartz occurs as composite grains with marked 

undulatory extinction, and some sections show sutured 

grain boundaries within the quartz composites. Protoclastic 

textures are further evidenced by bent and broken twin 

lamellae in plagioclase. Zircon, apatite, allanite, monazite, 

and opaques are common accessory minerals.

Stuckless and others (1977) identified a second major 

phase of the granite that was encountered in a drill hole but is 

not exposed or at least was not recognized in outcrop. These 

authors described the rock as a leucocratic phase with an 

average modal composition of 38 percent quartz, 29 

plagioclase, 30 microcline, 1.2 biotite, 1.2 muscovite and 

minor amounts of epidote, garnet, opaques, and other 

accessory minerals. The leucocratic phase is significantly

lower in Th, Fe, Sr, and Rb than the biotite granite 

(Stuckless and others, 1977).

The granite is locally foliated at the western end of the 

batholith, where it is in contact with the metamorphic rocks 

and forms a mixed zone of granite and gneiss. The 

metamorphic rocks have been extensively intruded by 

numerous dikes and sills of pegmatite, aplite, and fine- to 

medium-grained granite. The metamorphic rocks adjacent 

to the granite at Long Creek Mountain and the granite at 

Tincup Mountain are pervaded in similar fashion by 

abundant granitic material.

In addition to the minor cataclasis and alteration that has 

occurred in the biotite granite, intense alteration has taken 

place along linear zones within the batholith. Here the 

granite has been strongly epidotized and converted to an 

extremely tough and brittle rock. The zones of epidotized 

granite stand above the unaltered granite as resistant ridges 

and are characterized by a blocky fracture pattern. Stuckless 

and others (1977) described this alteration and referred to 

the zones as being silicified and epidotized. Epidotization 

seems to be the dominant alteration in samples examined 

during the present study. Biotite and microcline, which are 

ubiquitous in the biotite granite, are sparse or lacking in the 

alteration zones. Plagioclase is altered and epidote has been
GEOCHRONOLOGY

introduced in significant quantities. The allochemical nature 

of the alteration is exemplified by the mineralogy and by the 

Rb and Sr contents. The biotite granite contains approximately 200 ppm Rb and 100 ppm Sr, whereas a sample 

of epidotized granite (GM-81-68, table 4) derived from the 

biotite granite has 2 ppm Rb and 700 ppm Sr. An epidote 

separate from this sample contains approximately 1,000 

ppm Sr. Significant amounts of material were added to and 

removed from the biotite granite to accomplish this alteration. It is likely that hydrothermal fluids of undefined nature 

and source rose along fractures within the granite to produce 

the extensive exchange of material and alteration that 

occurred. It is not known whether or not significant 

movement occurred along these fractures. As mentioned 

earlier, similar alteration has modified rocks of the 

metamorphic complex in places, especially in areas of 

nephrite mineralization.

DIABASE

Dikes of fine- to medium-grained diabase (fig. 2) intrude 

the granite and the metamorphic complex with a dominant 

east-northeast trend. The dikes range in width from less than 

a meter to several tens of meters. The dikes are chilled 

against the granite and metamorphic rocks and the wider 

dikes have baked the enclosing rocks, producing a contact 

zone that is commonly more resistant to weathering than 

either the diabase or the unaltered granite and gneiss. Sherer 

(1969) described variable degrees of alteration in the dikes, 

which he attributed to late-stage deuteric processes. The 

sample collected for dating in the present study is 

remarkably fresh, with olivine and pyroxene as mafic 

minerals and completely unaltered labradorite as the 

feldspar. The texture is subophitic with approximately equal 

amounts of plagioclase and pyroxene. Sparse grains of 

olivine are rimmed and veined with an opaque mineral.

GEOCHRONOLOGY

Most of the radiometric dating was done by the Rb-Sr 

method on whole-rock and mineral samples. Outcrops were 

sampled only where least weathered rock could be obtained; 

hence, the sampling is not representative of the actual 

lithologic abundances because of variable weathering 

susceptibilities of the rock types. A few core samples were 

obtained from holes that were sponsored by the Space and 

Missile Systems Organization (SAMSO) (Saucier, 1970) 

and by the U.S. Geological Survey (Stuckless and others, 

1976).

The greatest part of the Rb-Sr analytical work was 

completed from late 1968 through 1970, following 

procedures described by Peterman, Doe, and Bartel (1967). 

During this period, 26 analyses of the Eimer and Amend 

strontium were made, and an average 87Sr/ 86Sr of 

0.70797+0.00010 (1 standard deviation) was obtained. 

Duplicate analyses of rock samples completed about this 

same time indicate an uncertainty in the 87Sr/ 86Sr ratio of

±0.00026 (1 s.d.). This uncertainty is substantially greater 

than that of the standard and the increased variance is 

thought to be related to additional errors attendant in 

obtaining 0.2-0.5-gram aliquots of sample from material 

that contains minerals with drastically different 87Sr/ 86Sr 

values, that is, slight sample inhomogeneity problems. 

Duplicate 87Rb/ 86Sr values, also obtained on rock samples, 

indicate a coefficient of variation of ±1.3 percent of the ratio. 

These uncertainties are used in regressing the data by the 

method of Mclntyre and others (1966). Uncertainties in the 

ages and initial 87Sr/ 86Sr values obtained from the 

regressions are given at the 95-percent confidence level.

The isotopic and decay constants used in calculating the 

ages are those recommended by the IUGS Subcommission 

on Geochronology (Steiger and Jager, 1977). Ages herein 

quoted from published reports are recalculated using these 

constants where necessary.

METAMORPHIC COMPLEX

Rb and Sr data (table 3) when plotted on an isochron 

diagram (fig. 4) show scatter beyond experimental uncertainty, especially for samples with 87Rb/ 86Sr values of less 

than 0.8. However, the data can be separated into two 

groups on both statistical and geological grounds, and a 

reasonably precise isochron is obtained by eliminating 

samples FCA-R-1, CR-4(114), GM-103, and GM-98 from 

the regression. (Sample numbers are shortened for convenience here and in subsequent discussions.) The slope of 

the isochron based upon the remaining samples corresponds 

to an age of 2,860+80 m.y., and the initial 87Sr/ 86Sr is 

0.7048±0.0012. The data for this regression still show slight 

variance in excess of experimental uncertainty 

(MSWD=3.54), and a Model III fit is used. (See Mclntyre 

and others (1966) for explanations of the terminology and 

the statistical calculations.) NKomo and Rosholt (1972) 

determined whole-rock U-Th-Pb analyses on some of the 

same samples used in our study and obtained a 207Pb/ 206Pb 

age of 2,910+120 m.y.

Of the four samples that are excluded from the regression, 

GM-103 and CR-14(114) are amphibolites that are interlayered with the quartzofeldspathic gneisses. Samples 

FCA-R-1 and GM-98 are gneisses collected outside of the 

map area (fig. 2), in T. 33 N., R. 88 W., and T. 30 N., R. 95 

W., respectively. These are well-foliated but massive gneisses 

that have only faint or no compositional banding in outcrop. 

In contrast, all of the data that define the main isochron 

(solid line, fig. 4) were obtained from layered gneisses. A 

reference isochron (dashed line, fig. 4) is drawn parallel to 

the main isochron through the four samples that are 

excluded from the regression. This reference isochron 

intersects the ordinate at 0.7017, a value significantly lower 

than the initial 87Sr/ 86Sr of 0.7048 for the main isochron.

Coexistence of rocks with different primary 87Sr/ 86Sr 

ratios that have maintained an isotopic identity through 

periods of amphibolite-grade metamorphism has been
10 PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

1.000

0.960

0.920

0.880

oo

r-. 

00

0.840

0.800

0.760

0.720

012345678

87Rb/86Sr

FIGURE 4. Rb-Sr isochron plot for samples from the metamorphic complex. Rock types and modes are given in table 1. Solid circles, 

amphibolites (GM-103, CR-4( 114)) and massive gneisses (FCA-R-1); open circles with dots, layered gneisses. Solid isochron corresponds to 

an age of 2,860±80 m.y. with an initial 87Sr/ 86Sr value of 0.7048±0.0012. Dashed line is a reference isochron drawn parallel to the solid 

isochron but with an initial 87 Sr/ 86Sr value of 0.7017.

observed elsewhere. Subparallel isochrons for associated 

paragneisses and orthogneisses that were metamorphosed 

1,710 m.y. ago were determined by Hedge, Peterman, and 

Braddock (1967) on rocks of a metamorphic complex of 

north-central Colorado. The metasedimentary rocks have 

an initial 87Sr/ 86Sr of 0.7079 whereas the orthogneisses, 

including amphibolite, have a lower initial of 0.7021. 

Hofmann and Kohler (1973) observed similar differences 

between orthogneisses and diatexites gneisses in intermediate and advanced stages of anatexis of the 

Schwarzwald of southwest Germany. Thus, it is possible that 

the massive gneisses of the Granite Mountains have a

different origin than gneisses of the layered sequence and 

that primary differences in the isotopic composition of the 

common Sr were at least partially preserved during the 

metamorphism.

Inherent uncertainties in interpreting Rb-Sr isochron ages 

defined on whole-rock samples of metamorphic rocks may 

be considerably greater than in interpreting similar data for 

plutonic igneous rocks. The collinearity of data on the main 

isochron of figure 4 implies that the samples shared a 

uniform or nearly uniform composition of common Sr at 

2,860 m.y. ago. The major problem is interpreting the 

geologic significance of this age. We will not attempt to
GEOCHRONOLOGY 11

TABLE 3. Analytical data for metamorphic rocks

Sample No. 1 Rb (ppm) Sr(ppm) 87 Rb/ 86Sr "Sr/s&Sr

GM-35-68R

GM-38-68R

GM-76-68R

GM-77-68R

GM-78-68R

44.3

37.6

28.0

91.0

212

398

338

318

171

96.0

0.322 0.7 17<

.323 .7191

.255 .7145

1.550 .768(

6.55 .977:

GM-78-68Mi

GM-78-68P1

GM-78-68B1

GM-98-68R

GM-98-68P1

GM-98-68Bi

GM-103A-69R

GM-104-69R

W2-CR-4(114)R

FCA-R-1-69R

401

78.9

1,299

67.2

8.45

662

19.0

61.3

11.3

44.6

102

155

17.9

264

375

14.4

114

71.1

118

686

11.90

1.497

403

.739

.065

246

.479

2.50

.278

.188

1.1348

.8449

10.088

.7328

.7093

9.379

.7198

.8091

.7142

.7090

'Suffix letters on sample number indicates type of sample analyzed: R, whole rock; PI, plagioclase; 

Mi, microcline, Bi, biotite.

review all of the literature pertinent to this problem but only 

mention a few studies that have particular bearing on our 

interpretation.

Impure sedimentary rocks such as shales and 

graywackes, and volcanic rocks that are subjected to 

prograding metamorphism will undergo a series of physical 

and mineralogical changes until the highest 

pressure-temperature conditions are attained, at which time 

a stable mineral paragenesis may be approximated (Winkler, 

1974). A fluid phase may be particularly important in 

facilitating the mineralogic reactions and in modifying the 

compositions of the rocks through dewatering (Norris and 

Henley, 1976). Thus, ample opportunity exists for open 

system conditions to obtain, beginning with diagenesis, 

through the final stages of metamorphism. Indeed, a number 

of studies have shown that whole-rock Rb-Sr systems of 

sedimentary and volcanic rocks are highly susceptible to 

partial or complete resetting during postdepositional 

metamorphism (Pidgeon and Compston, 1965; Compston 

and others, 1966; Peterman, 1966; Turek and Stephenson, 

1966; Lanphere, 1968; Turek and Peterman, 1968; 

Gorokhov and others, 1970; Clauer and Bonhomme, 1970; 

Hofmann and Grauert, 1973; Lyon and others, 1973; 

Gebauer and Griinenfelder, 1974). Some of these studies 

show that only relatively low grades of metamorphism are 

required to disturb the Rb-Sr systems. The results obtained 

by Pidgeon and Compston (1965) are particularly important 

in interpreting and understanding whole-rock Rb-Sr ages of 

metamorphic rocks. They analyzed a series of metasedimentary rocks that circumscribe a granitic intrusion and increase 

in grade from the chlorite zone to migmatitic rocks of 

amphibolite facies. The whole-rock ages decrease toward 

and become concordant with the age of the granite in the 

cordierite-orthoclase and migmatitic zones. Hofmann and 

Grauert (1973) found similar systematics in a progressively 

metamorphosed contact zone within Belt Supergroup 

sedimentary rocks that are intruded by an early Tertiary

phase of the Idaho batholith. Their results demonstrated a 

systematic lowering of ages obtained on small whole-rock 

samples (1-2-cm-thick slabs), with some complexities, and 

the ages were totally reset in the sillimanite-muscovite zone.

The complex mineralogical changes and the disturbances 

in the Rb-Sr systems that occur during prograding 

metamorphism strongly suggest that whole-rock Rb-Sr 

isochrons obtained on intermediate to high-grade rocks can 

be assumed to approximate the time of metamorphism; that 

is, the time that these rocks attained their stable or 

near-stable mineral assemblages. Once the rocks have gone 

through a cycle of metamorphism that produced a relatively 

simple mineral suite, such as quartz + 

plagioclase+biotite±K-feldspar±garnet in a metagraywacke, 

the Rb-Sr whole-rock systems may remain closed or respond 

in a manner similar to those of granitic rocks during 

subsequent metamorphic events. Even in complexly banded 

gneisses, individual bands may retain their isotopic integrity 

through rather severe later metamorphism (Jager, 1970; 

Krogh and Davis, 1973; Grauert and others, 1974; Hanny 

and others, 1975; Steiger and others, 1976), although 

exceptions have been documented and the scale of compositional banding is a particularly critical factor (Grauert 

and Hall, 1974; Hanny and others, 1975). If severe shearing 

and recrystallization occurs during the later metamorphism, 

the Rb-Sr systems may be partially or totally reset (Zartman 

and Stern, 1967; Hanson and others, 1969; Hunziker, 1970; 

Turek and Peterman, 1971; Abbott, 1972; Sims and Peterman, 1976).

Although the effects of cataclasis are recognized locally in 

the metamorphic complex of the Granite Mountains such as 

in the augen gneiss (fig. 2), the samples collected for 

radiometric dating have not been sheared subsequent to 

their metamorphism (table 1). All of the samples were 

collected from bands that are uniform in composition over a 

half meter or more and none were taken in proximity to 

major lithologic boundaries. In view of these factors and the 

previous results obtained on studies of metamorphic rocks, 

we interpret the whole-rock Rb-Sr age of 2,860+80 m.y. as 

representing the time of major regional metamorphism. All 

of the Precambrian rocks of the area have been subjected to a 

late Precambrian X or early Precambrian Y cryptic thermal 

event that has disturbed the Rb-Sr mineral systems. This 

later thermal event does not seem to have affected the 

whole-rock Rb-Sr systems of the gneisses to any great 

extent, although some of the excess scatter on the isochron 

plot (fig. 4) may have resulted from this heating.

The initial 87Sr/ 86Sr value of 0.7048±0.0012 determined 

for the main isochron (fig. 4) suggests that the protoliths of 

the gneisses may have had a significant crustal history prior 

to the major metamorphism. The crustal residence time of 

the Sr can be estimated if we assume that the six gneisses 

defining the isochron, admittedly a small and biased 

sampling, are approximately representative of the gneiss
12 PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

terrane as a whole and that the Rb/ Sr ratios have not been 

drastically modified by the metamorphism. The mean 

87Rb/ 86Sr ratio of the tonalitic and granitic gneisses is 0.94. 

An Rb-Sr system with this ratio and an 87Sr/ 86Sr value of 

0.7048 at 2,860 m.y. ago would have separated from the 

mantle or mantle-like (in terms of Rb/Sr) source between 

3,200 and 3,300 m.y. ago. Geologic models can be invoked to 

agree with this isotopic speculation. A volcanic pile or a 

sedimentary sequence, or combination thereof, could have 

formed with mantle-like 87Sr/ 86Sr ratios a few hundred 

million years before the major metamorphic event. The 

presently observed initial ratio of 0.7048 would be the 

consequence of isochron rotation during the 

metamorphism. Alternatively, a sedimentary pile could have 

been deposited just shortly before the metamorphism, and 

the initial ratio of 0.7048 would have resulted through the 

derivation of these rocks from a crustal terrane that was 

several hundred million years older. The assumptions in this 

simplistic interpretation contain many uncertainties; 

nevertheless, a premetamorphic crustal residence time of 

several hundred million years is suggested for the Sr 

contained in the protoliths.

GRANITE

Twenty samples of granitic rocks were analyzed for Rb-Sr 

dating (fig. 5, table 4). Several of these samples are from 

granite bodies that lie outside of the map area (fig. 2). 

Samples 114944 and 114945 from the Seminoe Mountains, 

and 256179 from the Pedro Mountains were described 

briefly by Rosholt and Bartel (1969). These localities are 

40-50 km southeast of the southeastern corner of the area 

covered by figure 2. Samples W2-CR-1(99) and 

W2-CR-1(153) are from a SAMSO drill hole in the southern 

part of T. 32 N., R. 88 W.

All of the Rb-Sr data are plotted in figure 5 with the open 

circles representing core samples and the solid circles 

representing surface samples. Two core samples (DDH-1 

and DDH-2) were collected with a Winkie drill and are 

considered to be surface samples because of the shallow 

depth from which they were obtained.

A number of the data points depart from colinearity by 

considerably more than analytical error (fig. 5). Regression 

of all the points (20) results in a Model IV isochron with an 

age of 2,500±70 m.y. and initial 87Sr/ 86Sr ratio of 

0.7074±0.0045. By deleting all data points that deviate 

substantially from the isochron (samples GM-1(1156), 

GM-1(1021), ZW-263, GM-81, DDH-1, DDH-2, GM-55, 

and 256179), regression of the remaining data (12 points) 

results in a Model I fit with an age of 2,540±30 m.y. and an 

initial Sr ratio of 0.705I±0.0013. The statistical screening has 

some geologic meaning as suggested by the fact that it 

resulted in elimination of data for many of the surface 

samples.

The granite weathers more readily than do the gneisses of 

the metamorphic complex, and surface samples of the

granite commonly show some effect of weathering, such as 

iron staining. The scatter of data for some of the surface 

samples (fig. 5) indicates that weathering indeed has had a 

deleterious effect on the Rb-Sr systems. The whole-rock 

Rb-Sr systems are particularly susceptible to disturbance 

because the constituent minerals yield ages that are discordant with respect to the whole-rock ages. Thus, open-system 

behavior in the mineral Rb-Sr systems as a consequence of 

weathering can result in a shift of the whole-rock data point 

away from the isochron.

However, additional complexities are indicated because 

some of the fresh-appearing core samples also deviate from 

the isochron by more than experimental error. GM-1(1156) 

is a plagioclase-rich phase of the leucogranite and contains 

no microcline (table 2). Thus, the sample may have acted as a 

receptor or sink for radiogenic 87Sr that was mobilized 

during the cryptic thermal event that disturbed the mineral 

systems, and the position of the data point above the 

isochron (fig. 5) is consistent with this speculation. 

GM-1(1021) does not have an unusual modal composition, 

and the reason for its departure from the isochron is not 

known.

TABLE 4. Analytical data for granitic rocks

[Rband Sr concentrations for GM-81-68 R/Ep and GM-55-68R were determined by XRF analyses; 

all others were determined by isotope dilution]

Sample No. 1 Rb (ppm) Sr (ppm)

GM-55-68R

GM-56-68B1

GM-81-68R

GM-81-68Ep

W2-CR-1(99)R

126

404

2

2

136

272

19.2

724

1,090

254

1.35

69.4

.008

.005

1.545

0.7481

2.155

.7073

.7070

.7607

W2-CR-1(153)R

W2-CR-14(101)R

W2-CR-14(157)R

W2-CR-14(157)Mi

W2-CR-26(99)R

W2-CR-26(165)R

W2-CR-26(165)Mi

ZW-263R

ZW-263M1

D1686R

D1686M1

D1686P1

D1686Bi

D1686Mu

DDH-1R

DDH-2R

D114944R

D114944Mi

D114945R

D114945M1

256179R 

256179M1 

GM-1(739)R 

GM-1(771)R

GM-l(771)Mi 

GM-1(891)R 

GM-1(1021)R 

GM-1(1156)R

115

241

223

445

169

176

357

133

421

166

465

49.5

736

486

146

111

197

547

192

356

113

447

191

221

425

143

99.2

33.3

289

87.5

95.6

111

117

124

137

85.2

97.0

83.8

155

126

22.6

15.1

40.6

196

80.1

54.6

85.1

37.3

154

215

17.0

30.5

47.4

27.6

35.5

40.4

1.149

7.98

6.92

11.96

4.25

4.19

7.72

4.53

13.02

5.84

8.96

1.145

136.2

132.7

10.76

1.643

7.31

31.3

6.70

29.7

2.14

6.14

37.0

22.7

28.5

15.86

8.33

2.41

.7481

.9989

.9632

1.0760

.8603

.8606

.9483

.8606

1.0841

.9169

1.0100

.7612

5.277

5.098

1.0752

.7717

.9750

1.4977

.9491

1.4321

.7899

.8965

2.0844

1.5371

1.7006

1.2811

.9964

.8051

'Suffix letters on sample number indicate type of sample analyzed: R, whole rock; PI, plagioclase; 

Mi, microcline; Bi, biotite; Mu, muscovite; Ep, epidote.
GEOCHRONOLOGY 13

2.200

2.000 -

1.800 -

1.600 -

1.400 -

1.200 -

1.000 -

0.800 -

14(101 )X0 GM-H1021)-

Dl14944 

D114945 

D1686

32 36 40 44

87 Rb/86Sr

FIGURE 5. Rb-Sr isochron plot for samples of granite rocks. Open circles represent core samples; solid circles represent surface or near-surface 

samples. Regression of selected data points as outlined in the text gives an age of 2,550±60 m.y. with an initial 87Sr/ 86Sr of 0.7053+0.0056.

One further selection of data points can be made solely on 

the basis of geologic grounds. If some surface samples are 

isotopically disturbed by weathering, the data for all surface 

samples can appropriately be excluded from the regression. 

Similarly, data for samples that were not taken from the 

main body of granite (fig. 2), granite of Lankin Dome, can be 

deleted. The resultant regression, based only on data from 

cores (7 samples) gives an age of 2,550±60 m.y. with an initial 

87Sr/*6Sr ratio of 0.7053+0.0056. It should be emphasized 

that none of these screening procedures changes the 

whole-rock isochron age significantly. In the last regression, 

which is a Model I fit, the 95-percent confidence levels on 

both the age and intercept are increased significantly because 

of the smaller number of samples.

Rosholt, Zartman, and NKomo (1973) reported a 

whole-rock 207Pb/ 206Pb isochron age of 2,750±80 m.y. for 

the granite. They interpreted this as a maximum age because 

of loss of uranium from the rocks in the Cenozoic. Data 

obtained on additional samples of the granite have lowered 

the whole-rock 2°7Pb/ 206Pb age slightly, and it is in better 

agreement with the Rb-Sr age (Stuckless and others, 1975).

T. W. Stern (written commun., 1976) determined 

U-Th-Pb ages of zircon from the granite at Tincup Mountain (D1686):

U= 1,250 ppm 

Th= 1,363 ppm 

Pb=565 ppm

208pb /204pb=81.47

207pb/204pb=o.236

206Pb/204pb= 184.9

Ages based on a common Pb correction using a 2,500-m.y. 

composition are:

= 1,635 m.y. 

m.y. 

m.y. 

= 1,395 m.y.

Although the ages are discordant, they are in agreement with 

the Rb-Sr age of the granite if it is assumed that the data lie 

on a chord that intersects concordia at a reasonable lower 

value; that is, 50-100 m.y. K. R. Ludwigand J. S. Stuckless 

(written commun., 1976) have determined an age of 2,595±40 

m.y. on zircons from the granite of Lankin Dome and on 

similar granites that crop out east of the map area (fig. 2). 

Data for the zircon at Tincup Mountain plot on the same
14 PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

chord as their data. They report a slightly older age of 

2,640+20 m.y. for zircon from the granite at Long Creek 

Mountain.

On the basis of the Rb-Sr whole-rock data and the 

additional radiometric evidence from U-Th-Pb whole rock 

and zircon studies, we conclude that the granite at Tincup 

Mountain and the granite of Lankin Dome are coeval and 

were emplaced 2,550 m.y. ago. We also suggest that the 

granites represented by samples from drill hole W2-CR-1 

and the granites of the Pedro and Seminoe Mountains 

represented by samples 256179,114944 and 114945 (table 4) 

were emplaced at approximately this time.

The emplacement of these granites appears to be part of a 

major period of granitic plutonism, in the strict sense, in the 

Precambrian of western and southern Wyoming. (See 

summary of ages by Reed and Zartman, 1973.) Ages of 

major granitic bodies of this region are as follows:

million years

Granite Mountains:

Granites of Lankin Dome and Tincup Mountain

Whole rock Rb-Sr (this report) .................................. 2,550±60

Zircon U-Pb (Ludwig and Stuckless,

written commun., 1976) .......................................... 2,595±40

Wind River Mountains, Bears Ear pluton:

Zircon U-Pb (Naylor and others, 1970) ..................... 2,570±15

Teton Range, Mount Owen Quartz Monzonite:

Whole rock Rb-Sr (Reed and Zartman, 1973) ........... 2,440±75

Sierra Madre, Baggot Rocks Granite:

Whole rock Rb-Sr (Divis, 1976) ............................... 2,500±100

Medicine Bow Mountains, Baggot Rocks Granite:

Whole rock Rb-Sr (Hills and others, 1968) ................ 2,430±50

Laramie Range, granite:

Whole rock Rb-Sr (Hills and Armstrong, 1974) ........ 2,490+40

Whole rock Rb-Sr (Johnson and Hills, 1976) ............ 2,510±25

This period of granitic plutonism in southern and western 

Wyoming occurred slightly but distinctly later than tonalitic 

to granitic plutonism in the Superior Province of northern 

Minnesota and adjacent Ontario, where a number of 

intrusions are well dated at between 2,650 and 2,700 m.y. 

(Goldich, 1972). The dominantly granodioritic Louis Lake 

batholith of the Wind River Mountains, dated at 2,65Q±15 

m.y. by Naylor and others (1970), is the approximate 

temporal equivalent of the Precambrian W intrusions of 

northern Minnesota.

The 2,450- to 2,600-m.y.-old granitic intrusions of Wyoming also differ from the silicic intrusions of northern 

Minnesota in that their initial 87Sr/ 86Sr ratios are consistently higher. When normalized to a common value of 0.7080 for 

87Sr/ 86Sr of the Eimer and Amend standard, the initial ratios 

for the Wyoming granites are: (1) 0.7053 for granite of the 

Granite Mountains (this report), (2) 0.7022 for granite of the 

northern Laramie Range (Johnson and Hills, 1976), (3) 

0.7044 for granite of the southern Laramie Range (Hills and 

Armstrong, 1974), (4) 0.7046 for the Baggot Rocks Granite 

of the Medicine Bow Mountains (Hills and others, 1968), (5) 

0.7010 for the Baggot Rocks Granite of the Sierra Madre 

(Divis, 1976), and (6) 0.732 for the Mount Owen Quartz

Monzonite of the Teton Range (Reed and Zartman, 1973). 

These contrast with initial ratios consistently between 0.7000 

and 0.7010 for the Minnesota granitic rocks (Hanson and 

others, 1971; Prince and Hanson, 1972; Peterman and 

others, 1972; Jahn and Murthy, 1975).

To explain the unusually high initial 87Sr/ 86Sr ratio for the 

Mount Owen Quartz Monzonite, Reed and Zartman (1973) 

suggested that older crustal rocks were somehow involved in 

the genesis of the quartz monzonite, even though the country 

rocks did not have sufficiently high 87Sr/ 86Sr values at 2,500 

m.y. ago to account for the extremely high initial of the 

quartz monzonite. Whatever the exact mechanism may have 

been, it seems likely that older crustal rocks may have been 

involved to varying degrees in the genesis of the Wyoming 

granitic rocks.

Alteration of the granite as manifested by linear zones of 

epidotization has not been directly dated in the present 

study. However, some reasonable inferences are in order. 

Similar alteration in the metamorphic complex is associated 

with nephrite mineralization, which, as shown in the 

following section, is dated at 2,510 m.y. In addition, Rb and 

Sr analyses were completed on a whole-rock sample of 

strongly epidotized granite and epidote separated therefrom 

(table 4, GM-81-68). Nearly all of the Rb has been removed 

and Sr has been enriched by almost an order of magnitude 

during the alteration. The 87Sr/ 86Sr ratios of 0.7070 and 

0.7073 for the epidote and the whole-rock sample respectively are virtually identical and are indistinguishable from the 

initial 87Sr/ 86Sr ratio of the whole-rock isochron (fig. 5). If it 

is assumed that the Sr was derived locally, it had to have been 

separated from Rb only shortly after the granite crystallized. 

For example, if we take the average Rb/Sr value of the 

biotite granite as 1.9 and the isochron intercept at face value, 

the Sr in the epidotized granite would had to have been 

separated from a system with this mean Rb/ Sr value within a 

few tens of millions of years after the granite crystallized. 

More precise calculations are unwarranted because of the 

large uncertainty in the initial 87Sr/ 86Sr ratio of the granite. 

Based upon these calculations and the relationship of 

epidotization in other rocks with the nephrite mineralization, we conclude that this alteration occurred between 2,500 

and 2,600 m.y. ago.

DIABASE DIKES AND NEPHRITE VEINS

Conventional K-Ar ages of approximately 1,600 m.y. 

were previously reported (Peterman and others, 1971) for 

diabase and nephrite veins in the Granite Mountains. These 

ages are now known to be erroneous because of the difficulty 

in determining the potassium contents at the very low 

concentrations present approximately 0.03 percent K2O in 

labradorite from the diabase and 0.01 percent in the 

nephrite. G. B. Dalrymple and M. A. Lanphere (written 

commun., 1976) determined 39Ar/ 40Ar ages of approximately 2,600 m.y. on these same mineral separates (table 5). The 

diabase from which the separate of labradorite was obtained
GEOCHRONOLOGY 15

is described in an earlier section. The sample of nephrite was 

obtained from R. G. Coleman and was sawed from a piece of 

massive nephrite that occurred in a vein approximately 10 

cm wide. A 2-cm cube was crushed and a sized fraction of-40 

and +80 was prepared. The material was repeatedly washed 

with distilled water to remove the fines.

We conclude from these data that both the diabase and the 

nephrite veins were emplaced only shortly after the granite 

was intruded. Older mafic dikes that are metamorphosed 

occur in the metamorphic complex, but it is not known 

whether all of the younger diabase dikes are of the same age.

MINERAL AGES

Rb and Sr analyses completed on various mineral phases 

from samples of the granite and of the metamorphic rocks 

are given in tables 3 and 4. Microcline was the major mineral 

analyzed, but all of the essential minerals were analyzed 

from three samples (GM-78, GM-98, and D1686). Mineral 

ages, calculated from the whole-rock-mineral isochrons are 

illustrated in figure 6. Internally consistent mineral ages were 

obtained on only two samples. GM-98, a tonalitic gneiss, is 

essentially composed of plagioclase, quartz, and biotite. 

Data for plagioclase, biotite, and the whole rock define a 

2,440-m.y. isochron with an initial 87Sr/ 86Sr ratio of 0.7067. 

D1686, the granite at Tincup Mountain, yields a plagioclase, 

total rock, biotite, muscovite isochron of 2,300 m.y. with an 

intercept of 0.7203. Microcline from this sample deviates 

from the mineral isochron beyond experimental error with a 

microcline-total-rock age of 2,080 m.y. Minerals from 

GM-78, a granitic gneiss, are highly discordant as shown in 

figure 6. Microcline-whole-rock ages of the remaining 

samples range from approximately 1,500 to 2,100 m.y. 

Conventional K-Ar ages (table 6) were obtained on 

biotites 1,570 m.y. (GM-56), 1,780 m.y. (D1686), and 2,310 

m.y. (GM-98) and on hornblende 2,680 m.y. (FCA-R-1).

All of these data show clearly that the Rb-Sr and K-Ar 

mineral systems have been disturbed at some time subsequent to the emplacement of the granite at 2,550 m.y. ago. 

Rosholt and others (1973) observed a remarkable colinearity 

of Pb isotope data obtained on microcline separates from the

Metamorphic complex

GM-98

GM-78

FCA-R-1 

Granite

01686 

ZW-263 

GM-56 

CR-14 (157) 

CR-26 (165) 

GM-1 (771)

0114944

0114945

256179

Diabase GM-106 

Nephrite D1784

COn 

D--

Q~

FIGURE 6. Rb-Sr and K-Ar mineral ages. Rb-Sr mineral ages are 

calculated from the mineral-whole-rock line. Open symbols are Rb-Sr 

ages; stars, whole-rock isochrons; circles, plagioclase; squares, 

microcline; diamonds, biotite; square with cross, muscovite. Solid 

symbols are conventional K-Ar and ^Ar/^Ar ages: circle, labradorite; 

triangles, amphibole; diamond, biotite.

granitic rocks. They concluded that this secondary isochron 

resulted from uptake of radiogenic Pb by microcline from 

other phases in the granite as the consequence of a thermal 

event at 1,620±120 m.y. ago the age defined by the 

microcline isochron. Had the Pb in the mineral systems 

totally equilibrated at this time, the appropriate 

whole-rock-microcline isochrons would have given this 

same age. Complete isotopic homogenization was not 

attained, however, and ages calculated from the 

whole-rock-microcline points are variable, but greater than 

1,640 m.y. On the basis of regional variations in these ages, 

Rosholt and others (1973) suggested that the intensity of the 

thermal event increased from the western part of the Granite 

Mountains eastward to the Seminoe Mountains.

The mineral ages obtained in the present study support the 

interpretation of Rosholt and others (1973). Minerals from

TABLE 5. Analytical data for 39Ar/40Ar ages

[The 37Ar/ 39Ar value is corrected for 37 Ar decay using a halfrlife of 35.1 days. The percentages of 40Arrad, "Ar^and "Ar^ are calculated using 

assumptions in Dalrymple and Lanphere (1971). The uncertainties on the ages are estimates of analytical precision at the l.or level of 

confidence. Analytical techniques, methods of calculation, and the monitor mineral are described in Dalrymple and Lanphere (1971). 

Irradiations were made with a dose of approximately 4xl018 nvt. Sample localities are: D1784, lat 42°33'40" N., long I07°53'28" W. 

GM-106-69, lat 42°34'IO" N.,long 107°48'34" W.]

Sample

D1784 

(nephrite). 

Do ............

GM-106-69 

(labradorite).

«>Ar/ 3'Ar

235.3 

. 292.5

310.6

37Ar/ 3'Ar

322.1 

347.1

223.1

«Ar/ 3»Ar

0.1864 

.4151

.2451

J'

0.01122 

.01234

.01122

Percent

*°Ar 3*Ar 3'Ar

87.6 47.1 20.4 

67.6 22.8 22.0

82.4 24.8 14.1

Age (m.y.)

2,460±45 

2,560±45

2,650±130

'J is a function of the age of the monitor and of the integrated fast neutron flux (Dalrymple and Lanphere, 1971).
16 PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

IAUL.K o. — ^onvenuunai p±-/ir uges 

[Analysts: R. F. Marvin, H. H. Mehnert, and Violet Merritt. *40Ar refers to radiogenic "'Ar. The 

age uncertainty is given at the 2-cr level]

Sample K2O, percent *«°Ar *4»Ar *<0Ar/ <0K Age (m.y.)

GM-56-68 8.36 301.2 99 0.1426 1,570±50 

(biotite). 

FCA-R-1 .895 79.18 96 .350 2,680±160 

(hornblende). 

D1686 5.40 236.1 99 .177 1,780±40 

(biotite). 

GM-98-68 8.99 604.7 99 .272 2,310±55 

(biotite).

the westernmost sample (GM-98) give concordant ages of 

2,440 m.y. Progressing eastwards, D 1 686 minerals are nearly 

concordant at 2,300 m.y. Minerals from GM-78 are strongly 

discordant with the lowest age of 1,600 m.y. recorded by

hintitp Rintitp frntn fiM-Sfi icHatpH at 1 Af\(\ tn v Kv RK-Sr

>

2

•z. 2600

LU

0 

<

S. 2400

8CM~2s 

a.

g 2200

CM

* 

CJ

§ 2000

til _j 

0

MICROCLINE-\W

i 1

O— — — — • D1686

O— — — — — — — •GM-78

0 -W2-CR-14(157) ^ ——— ——— — — • W2-CR-26M65) 

O ———— 9ZW-263

O— — 0114944 

O — —— — 0114945

-

and 1,570 m.y. by K-Ar. Microcline from granitic rocks 

eastward to the Seminoe Mountains gives variably lowered 

ages that reach a lower limit of 1,500-1,600 m.y.

Some of the microcline separates analyzed by Rosholt and 

others (1973) and by NKomo and Rosholt (1972) were also 

analyzed in the present study. Two alternate comparisons of 

the results are given on figure 7. The 207Pb/ 206Pb age for each 

sample is calculated from the whole-rock and microcline tie 

line. Two Rb-Sr ages are given for each sample. The open 

circles (fig. 7) represent a comparison of 

microcline-whole-rock 207Pb/ 206Pb ages with the Rb-Sr ages 

as calculated from the appropriate whole-rock and 

microcline pairs. This follows conventional assumptions in 

the interpretation of Rb-Sr systematics of disturbed mineral 

systems (Lanphere and others, 1964). In effect, the Rb-Sr 

mineral isochron is assumed to rotate about the whole-rock 

point, and if the event is sufficiently strong to cause complete 

isotopic homogenization among the mineral phases, the 

resultant isochron will record the time of this disturbance. 

The solid circles (fig. 7) represent a comparison of 

microcline-whole-rock 207Pb/ 206Pb ages with the Rb-Sr ages 

as calculated on the basis of the microcline data points and 

the whole-rock isochron initial ratio of 0.7053. Brooks 

(1968) has documented a situation in which exchange of 

parent and daughter nuclides has occurred only between 

microcline and secondary alteration products of plagioclase. 

K-feldspar lost not only radiogenic 87Sr but also significant 

amounts of Rb and common Sr. Thus, in crystalline rocks 

that have undergone a mild thermal metamorphism without 

recrystallization, moderate isotopic disturbances of 

microcline may not involve exchange of common Sr with 

that of the host reservoir, that is, the whole-rock system.

Regardless of the method used for calculating the 

microcline ages, the correlation of the Rb-Sr and 

207Pb/ 206Pb ages as shown in figure 7 is reasonably good. The 

correlation is somewhat better if the Rb-Sr microcline ages

1600 1800 2000 2200 2400 

MICROCLINE Rb-Sr AGE, IN M.Y.

2600 2800

FIGURE 7.—Comparison of Rb-Sr and 207 Pb/ 206Pb ages of microcline. 

Open circles, Rb-Sr ages calculated from microcline-whole rock 

isochrons. Closed circles, Rb-Sr microcline ages calculated from the 

microcline-whole-rock initial 87Sr/ 86Sr value. Vertical broken bar 

represents the l,620±120-m.y. thermal event as defined by the Pb 

isotope data on microcline (Rosholt and others, 1973).

are computed on the basis of the isochron intercept. In both 

cases, with the exception of sample GM-78, the Pb-Pb ages 

are greater than the Rb-Sr ages. The general coherence of 

these ages is striking because resetting of the Rb-Sr 

microcline systems was accomplished by loss of radiogenic 

87Sr whereas the lead-isotope system records the disturbance 

by addition of radiogenic lead to the microcline. On the basis 

of a study in a contact metamorphic zone, Doe and Hart 

(1963) concluded that the lead-isotope system in feldspars 

was more easily affected than the Rb-Sr microcline and 

K-Ar hornblende systems but less easily than the K-Ar and 

Rb-Sr biotite systems.

The disturbance of mineral-age systems in rocks of the 

Granite Mountains is part of a regional pattern that is 

recognized in several areas of the Precambrian of Wyoming 

(Giletti and Gast, 1961; Hills and Armstrong, 1974; Reed 

and Zartman, 1973). The lowered mineral ages are generally 

interpreted as being the consequence of thermal events or 

episodes of metamorphism. Naylor, Steiger, and Wasserburg (1970) reported lowered Rb-Sr mineral ages in the 

southeastern Wind River Mountains and concluded that the 

rocks had been subjected to one or more events that occurred 

later than 2,000 m.y. ago. In this same area, biotites were 

dated at 2,200 and 1,430 m.y., whereas hornblende gave an 

older age of 2,620 m.y.—all by K-Ar (Bayley and others, 

1973). Reed and Zartman (1973) concluded that rocks of the 

Teton Range had been affected by two thermal events—one
GEOCHRONOLOGY 17

about 1,800 m.y. ago and a second between 1,300 and 1,500 

m.y. ago. Giletti and Gast (1961) also reported lowered 

mineral ages in the Teton and Gros Ventre Ranges, and 

Bassett and Giletti (1963) found similar lowered ages in the 

northwestern Wind River Mountains. Hills and Armstrong 

(1974) demonstrated the existence of a regional variation in 

K-Ar ages in Precambrian W rocks of the Laramie Range 

and of the Medicine Bow Mountains. They postulated the 

presence of a thermal or "geochronologic" front at the 

northernmost end of the Laramie Range, north of which 

micas yield 2,500 m.y. K-Ar ages, whereas micas from rocks 

of similar age farther south were lowered to between 1,400 

and 1,600 m.y. Hills and Armstrong (1974) suggested that 

the front strikes southwestward from the northern Laramie 

Range towards the Seminoe Mountains.

The existence of such a front seems to be supported by our 

mineral data from the Granite Mountains, and it can be 

fairly well defined by considering both Rb-Sr and K-Ar 

biotite ages from the Laramie Range, the Granite Mountains, and the Wind River Mountains (fig. 8). The front 

appears to trend westerly from the northern Laramie Range, 

inasmuch as mineral ages in the Seminoe Mountains and in 

all but the northwestern part of the Granite Mountains have 

been lowered. It can be projected farther west into the Wind 

River Mountains, where biotite ages are systematically 

lowered from north to south at the southeastern end of the 

range (fig. 8). The biotite that gives the 1,420-m.y. K-Ar age 

in the southeastern Wind River Mountains was obtained 

from a mylonitized granite (Bayley and others, 1973), and 

possibly the age dates the movement of a major shear zone. 

West of this locality, on the southern flank of the range, 

biotite from a basement core is dated at 1,840 m.y., whereas a 

whole-rock Rb-Sr determination clearly establishes the age 

as Precambrian W (Goldich and others, 1966).

The consistent lowering of K-Ar and Rb-Sr biotite ages in 

rocks of Precambrian W age in the region south of the front 

(fig. 8) is an impressive feature. Geologic evidence that might 

support the presence of a metamorphic event is lacking 

except in proximity to the major terrane of Precambrian X 

and Y rocks in southern Wyoming and Colorado. An 

alternative interpretation to that of a discrete metamorphic 

event is that the reset mineral ages reflect crustal uplift and 

cooling below temperatures at which the radiometric 

systems in the minerals attain closure or cease to diffuse 

daughter products (Armstrong, 1966; Wetherill, 1966; 

Harper, 1967). This interpretation would require differential 

uplift of large crustal blocks in late Precambrian X or early 

Precambrian Y time, with the obvious possibility of one or 

more second-order events or cooling episodes superimposed 

on the uplifted blocks, which would account for some of the 

variability in mineral ages as presently observed.

At the northern end of the Laramie Range, in the 

northwestern part of the Granite Mountains and in the 

southeastern part of the Wind River Mountains, the

gradient in Rb-Sr and K-Ar biotite ages is steep—ages are 

drastically lowered over distances of 10-20 km. These steep 

age gradients are more likely to have resulted from one or 

more zones of major vertical dislocation than from a thermal 

or metamorphic event somehow related to younger Precambrian orogenesis some 100-150 km to the south. A series of 

westerly trending faults is present in Phanerozoic rocks 

(Bayley and Muehlberger, 1968); these roughly coincide with 

or parallel the front defined by the biotite ages (fig. 8). Some 

or all of these may well have had precursors in the 

Precambrian. In the Granite Mountains, this hypothetical 

fault zone would lie near sample D1686 (fig. 2). Although a 

fault was not mapped, the existence of one is not precluded 

by our field study, as rocks with strongly developed vertical 

foliation occur near this locality. In the southeastern Wind 

River Mountains, several large Precambrian faults are 

mapped (Bayley and others, 1973) and, as mentioned 

previously, the K-Ar age of 1,420 m.y. is on biotite from a 

mylonitized granite apparently associated with one of the 

fault zones.

If the lowered mineral ages in Precambrian W rocks south 

of the front (fig. 8) are indeed the result of cooling as a 

consequence of regional uplift, estimates of temperatures 

and depths at which the presently exposed rocks resided 

prior to this uplift can be made. Biotite loses radiogenic Ar 

and Sr at temperatures in excess of about 300° C (Hanson 

and Gast, 1967; Hanson, 1971). Using estimates of average 

geothermal gradients that might have existed approximately 

1,500 m.y. ago (Hargraves, 1976), presently exposed 

Precambrian W rocks with lowered biotite ages would have 

been uplifted from depths of 11-13 km between 1,400 and 

1,600 m.y. ago. Local survival of amphibole ages such as of 

the nephrite in the Granite Mountains (table 5), and of 

hornblende in the Wind River Mountains from the same 

locality as the 2,210 m.y. biotite age (Bayley and others, 

1973), suggests temperatures of less than 500°C(Steigerand 

others, 1976) and depths of less than 18-22 km in the 

northern part of the block. K-Ar hornblende ages of 1,690 to 

1,820 m.y. from Precambrian W rocks of the central 

Laramie Range (Hills and Armstrong, 1974) suggest that 

these rocks cooled below about 500° C at this time and 

subsequently below 300° C between 1,400 and 1,600 m.y. 

ago, as indicated by the biotite ages. The depth calculations 

are not meant to be highly precise; they have large 

uncertainties because of the known variability of presentday geothermal gradients from one geologic province to 

another (Roy and others, 1968) and imprecise knowledge of 

the blocking temperature and diffusion characteristics of 

biotite. The actual geothermal gradients may in fact have 

been greater than average because of the high radioelement 

content of some of the constituent rocks such as in the 

Granite Mountains (Rosholt and others, 1973), and, more 

than likely, the depth estimates are maximum values.

In spite of these uncertainties, the temperature and depth
18 PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

110°

108 106° 104°

FIGURE 8.—K-Ar and Rb-Sr ages, in millions of years, of biotite from Precambrian W granitic and metamorphic rocks of southern Wyoming. 

Rb-Sr ages are italicized. Patterned areas are underlain by Precambrian X and Y rocks, here undifferentiated. The east-west-trending—line 

marks the front that separates biotite ages of 2,300 m.y. and older from those of 2,230 m.y. and younger. The inset shows ages plotted as a 

function of north or south distance from the dashed line. Data are from this study, Gilettiand Cast (1961), Goldichand others (1966), Hills 

and others (1968), Naylor, Steiger, and Wasserburg (1970), Bayley, Proctor, and Condie (1973), Hills and Armstrong (1974), and Divis 

(1976), and M. A. Lanphere (written commun., 1977). Precambrian outlines from Bayley and Muehlberger (1968).

(pressure) parameters suggested by the mineral ages correspond to conditions of "very low grade" to "low grade" 

metamorphism as defined by Winkler (1974). Thus, the lack 

of definitive geologic evidence for a thermal event is not 

surprising, although pervasive chloritization of biotite, 

saussuritization of plagioclase, and protoclastic textures in 

the granite of Lankin Dome (table 2) could be the result of 

the granite having been held at these metamorphic conditions for some 1,000 to 1,200 m.y. after crystallization.

If the foregoing interpretation has any basis in fact, the 

uplift of a large block of Precambrian W rocks in late 

Precambrian X or early Precambrian Y time has important 

implications to the regional tectonics of the area. Several 

kilometers, perhaps 10 km or more, of material would had to 

have been removed by erosion subsequent to or concomitant 

with the uplift. In Precambrian Y time, large sedimentary 

basins to the northwest, west, and southwest of southern 

Wyoming received voluminous amounts of sediments that 

formed the Belt Supergroup and correlative rocks (Stewart, 

1976). The Uinta Mountain Group would be a particularly 

attractive repository for material eroded from the uplifted 

block, and the source of the Uinta Mountain Group is 

thought to be toward the north (Hansen, 1965; Crittenden 

and Wallace, 1973).

CONCLUSIONS

Reconnaissance mapping of the western end of the 

Granite Mountains in central Wyoming has identified a 

metamorphic complex composed mainly of a layered 

sequence of quartzofeldspathic gneisses, amphibolites, and 

schists. Metagraywacke and amphibolite form sizable units 

at the westernmost end of the Granite Mountains. Tonalitic 

to granodioritic gneisses interlayered with lesser amounts of 

amphibolite, biotite schist, serpentinite, augen gneiss, and 

epidote gneiss form a sizable portion of the exposed 

metamorphic complex. The gneisses are well banded and 

range from layers of relatively uniform composition to 

migmatitic varieties. Interlayers of amphibolite are locally 

broken and distended to form boudins and rotated blocks 

within the gneisses.

Rocks of the metamorphic complex are at amphibolite 

grade and were probably derived from a sequence of 

interlayered mafic volcanic rocks and graywackes or silicic 

volcanic rocks. The sequence may be correlative with or 

compositionally analogous to the greenstone-graywacke 

terrane exposed in the southeastern Wind River Mountains. 

Here, Bayley, Proctor, and Condie (1973) have described a 

thick section of graywacke, greenstone, and iron-formation.
CONCLUSIONS

A migmatitic complex containing amphibolites and serpentinites is suggested to be the remnant of a mafic volcanic 

basement on which the sedimentary-volcanic sequence was 

deposited.

Compositional banding and foliation within the 

metamorphic complex commonly dip southeast and south 

toward the younger granite batholith. Other than compositional banding, primary structures in the metamorphic 

complex have been obliterated by metamorphism.

The metamorphic complex was intruded by a batholith 

and two satellite bodies of biotite granite. The granite is 

medium to coarse grained, generally massive but locally 

foliated near the margin and in the satellite bodies, and is cut 

by late-stage dikes of granite, aplite, and pegmatite. Rocks of 

the metamorphic complex are also pervasively intruded by 

these late-stage dikes especially in proximity to the main 

bodies of granite.

As a result of well-developed exfoliation, the granite 

typically forms rounded hills and knobs. In most areas, 

weathering has affected the granite to depths of several tens 

of meters or more and surface samples are characteristically 

stained with iron oxides. Strongly altered zones, generally 

linear and steeply dipping, occur within the granite; they 

contrast sharply with unaltered phases by virtue of a 

well-developed blocky fracturing and a greater resistance to 

weathering. The alteration has been allochemical in that 

substantial amounts of epidote have been introduced, biotite 

has been partially to -completely converted to chlorite and 

granular sphene, and microcline has been removed. In some 

of these alteration zones, the granite has been intensely 

silicified (Stuckless and others, 1977). Similar alteration 

occurs in the metamorphic complex where these zones are 

commonly associated with the nephrite mineralization.

Numerous diabase dikes with a dominant east-northeast 

trend intrude both the granite and the metamorphic 

complex. These dikes range in thickness from several meters 

to several tens of meters. They are chilled against the 

enclosing rocks and generally coarser in grain size toward 

the centers. The dikes show variable degrees of alteration 

(Sherer, 1969), although some are remarkably fresh; olivine, 

pyroxene, and labradorite are their essential minerals. It is 

not known whether more than one age of dikes is present. 

Within the metamorphic complex, some of the amphibolite 

bodies are discordant with the compositional banding; these 

are presumably metamorphosed mafic intrusions. The 

discordant amphibolites, however, are clearly distinguishable from the younger diabase dikes.

Veins of nephrite (jade) occur within the metamorphic 

complex, mainly at the western end of the area. These are of 

local economic interest and are mined on a small scale. The 

nephrite veins were not studied in detail, and the reader is 

referred to Sherer (1969) for a comprehensive account of 

these deposits.

Geochronologic studies have established a time 

framework into which the major Precambrian events of the

Granite Mountains can be placed. An Rb-Sr isochron 

obtained on whole-rock samples of gneisses gives an age of 

2,860+80 m.y. This age is interpreted as the time at which the 

rocks of the metamorphic complex attained 

amphibolite-grade metamorphism. An initial 87Sr/ 86Sr of 

0.7048±0.0012 is unusually high for juvenile material at 2,900 

m.y. ago and suggests that the Sr in the protoliths of the 

metamorphic complex had a significant pre-metamorphic 

crustal history. Estimations based upon this initial Sr ratio, 

the metamorphic age of 2,860 m.y., and the average Rb/Sr 

ratio of the gneisses suggest that the Sr may have separated 

from the mantle some 300 to 400 m.y. before metamorphism. 

Geologically this implies that the rocks themselves or the 

source terrane from which they were derived may be that old. 

Data obtained on four samples of amphibolite and massive 

tonalitic gneisses fall below the main isochron defined by the 

quartzofeldspathic gneisses and indicate an initial Sr ratio of 

approximately 0.7017. The amphibolites, at least, are likely 

to have been of volcanic origin; the lower initial Sr ratio 

would favor an interpretation that the layered quartzofeldspathic gneisses are partly or completely sedimentary 

in origin.

Twenty samples of granitic rock from the immediate area 

of study and from adjacent areas were analyzed for Rb and 

Sr. In general, data obtained from surface samples and on 

cores from shallow drill holes show excessive scatter on an 

isochron plot. Data for seven samples from deep drill holes 

define an isochron of 2,550±60 m.y. with an initial 87Sr/ 86Sr 

ratio of 0.7053+0.0056. This age is interpreted as dating the 

time of emplacement of the main body of granite in the 

Granite Mountains. Samples from other granite bodies in 

adjacent areas north of the map area and in the Seminoe 

Mountains provide data that plot on or close to the isochron, 

suggesting, at least provisionally, that these granites are 

approximately the same age. The Rb-Sr isochron age is in 

agreement with U-Pb data obtained on a zircon from the 

granite of Tincup Mountain and with zircon ages by K. R. 

Ludwig and J. S. Stuckless (written commun., 1976) on the 

main body.

A sample of nephrite and one of labradorite from a fresh 

diabase yielded 39Ar/ 40Ar ages of 2,510+35 and 2,650±130 

m.y. respectively. These data suggest that the diabase dikes 

were emplaced and the nephrite veins were formed only 

shortly after the granite was intruded. Ratios for 87Sr/ 86Sr 

determined on epidote and on a whole-rock sample from an 

epidotized zone in the granite also suggest that this alteration 

occurred within a few tens of millions of years after the 

granite was emplaced. Similar alteration in gneisses adjacent 

to nephrite mineralization, and the age of the nephrite, 

support this interpretation.

Rb-Sr and K-Ar ages of feldspars and micas from both the 

metamorphic complex and from the granite have been 

lowered to varying degrees. Within the western part of the 

Granite Mountains, mineral ages are increasingly lowered
20 PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

from west to east. This pattern in Rb-Sr and K-Ar ages is 

consistent with that observed earlier by Rosholt and others 

(1973) from the Pb-isotope systematics in microclines from 

the granite.

When the mineral ages, especially Rb-Sr and K-Ar biotite 

ages, of the Granite Mountains are considered in a regional 

context with those from the Laramie Range and from the 

Wind River Mountains, a regional pattern emerges that can 

best be explained by major vertical tectonics in late 

Precambrian X or early Precambrian Y time. A biotite-age 

discontinuity in Precambrian W rocks extends from the 

north end of the Laramie Range, through the Granite 

Mountains, to the southeast end of the Wind River 

Mountains. North of this discontinuity, biotite ages are 

2,300 m.y. or greater, whereas ages to the south decrease 

rapidly to between 1,400 and 1,600 m.y. This pattern could 

have been generated by vertical uplift of the southern block, 

south of the age discontinuity, between 1,400 and 1,600 m.y. 

ago. In this interpretation, the biotite ages register the time at 

which rocks at the presently exposed surface were uplifted 

and cooled through the 300° C isotherm. An uplift of several 

kilometers is required, with the corollary that several 

kilometers were removed by erosion at this time. This 

postulated uplift could have supplied detritus to the 

dominantly Precambrian Y Uinta Mountain Group.

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22 PRECAMBRIAN OF THE GRANITE MOUNTAINS, WYOMING

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