Precambrian Geology of 

the Needle Mountains, 

Southwestern Colorado

GEOLOGICAL SURVEY PROFESSIONAL PAPER 644-A
Precambrian Geology of 

the Needle Mountains, 

Southwestern Colorado

By FRED BARKER

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 644-A

A restudy of the Precambrian metamorphic 

and igneous rocks of the Needle Mountains 

ublift

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

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

Washington, D.C. 20402 - Price 55 cents (paper cover)
CONTENTS

Abstract .-----------_---_________________________

Introduction- ______________________________________

Location and access____________________________.

Physical features.______________________________

Previous work__________________________________

Acknowledgments__ ___________________________

Geologic sequence-_________________________________

Layered rock formations..___________________________

Vallecito Conglomerate._________________________

Irving Formation.______________________________

Name and correlation-______________________

Southeastern Needle Mountains._____________

Western Needle Mountains__________________

Northern Needle Mountains _________________

Stratigraphic thickness._____________________

Rocks of Weminuche Creek._____________________

Uncompahgre Formation._______________________

Basal conglomerate.________________________

Quartzite._________________________________

Slates, phyllites, and schists__________________

Thickness_ _ ______________________________

Origin.____________________________________

The sub-Uncompahgre unconformity and phyllonite 

Igneous rocks._____________________________________

Types and age relations________________________

Twilight Gneiss_______________________________

Tenmile Granite..______________________________

Whitehead Granite____------__________________

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Igneous rocks Continued)

Bakers Bridge Granite_________-------_---------

Quartz diorite of Pine River____-_____-___________

Eolus Granite. _________________________________

Electra Lake Gabbro _________________________

Trimble Granite _ _____________________________

Minor intrusive rocks. __________________________

Metadacite. _______________________________

Rhyolite porphyry dikes __ _________________

Aplite and pegmatite-_-_-__----------------_

Mafic quartz syenite of Ute Creek. ___________

Biotite quartz monzonite near Lake Silex______

Lamprophyre dikes. ________________________

Diabase dikes. _____________________________

Structural geology. _________________________________

Folds of Vallecito Conglomerate and Irving FormaTrends in the Irving Formation. _________

The Uncompahgran disturbance. _ _______ _

Folds of the Uncompahgre Formation _____

Faults. ________________-_-_____-___----

Metamorphism. ____________________________

Metamorphism of the Uncompahgre pelites 

Regional metamorphism. ____________

Thermal metamorphism_ ____________

Oxidation of detrital magnetite to hematite. 

References cited_ ___________________________

Index. ____________________________________

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ILLUSTRATIONS

 Page

FIGURE 1. Map showing areas of previous geologic investigations_____________--_-___---------___------_--------- A3

2. Geologic sketch map of Precambrian rocks_____________________-____--------_-_-----------_------- 6

3. Schematic section of Precambrian rocks______________________________-____--_----------_-__-------- 8

4. Map of the Needle Mountain area, showing geographic place names and topographic features------------ 9

5-10. Photographs:

5. Cross-stratification in the Vallecito Conglomerate-______------____-------_-----------_------- 10

6. Metamorphosed pillow lava, Irving Formation_____________-___-_---_---------------_-------- 10

7. Eastern Grenadier Range_______________________________-__-____-_--__----_-----_--------- 12

8. Banded chiastolite-garnet-staurolite schist in the Uncompahgre Formation ______________________ 13

9. Chiastolite schist in the Uncompahgre Formation____________---_---__-----__-----_---------- 14

10. Basal contact of the Uncompahgre Formation______________-_-_-__-----__--_---------------- 15

11. Ternary plots of normative minerals in Precambrian granitic rocks of the Needle Mountains._____________ 17

12. Plots of Na^O versus K20 and of Na20 versus CaO for Precambrian granitic rocks of the Needle Mountains. _ 19

13. Photograph of lower Flint Fork___________________________.________________--__ 25

14. Photograph of Eolus Granite outcrop____---_-___________________--_-______-_---__----------------- 26

TABLES

 Page 

TABLE 1. Description of the Precambrian intrusive rocks of the Needle Mountains.._________________--.--------- A18

2. Chemical analyses and CIPW norms of Precambrian granitic rocks and their inclusions of the Needle

_______.......___________ __ __ ___________________ 20

m

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO

By FEED BARKER

ABSTRACT

The Needle Mountains of southwestern Colorado are an uplifted mass of Precambrian rocks. Paleozoic and Mesozoic sedimentary rocks flank the uplift on the southeast, south and west; 

and Tertiary volcanic rocks flank it on the north and northeast. 

The Needle Mountains uplift extends from Weminuche Greek on 

the east to Electra Lake and U.S. Highway 550 on the west, and 

from Vallecito Reservoir on the south to Silverton on the north.

The Precanibrian rocks of the Needle Mountains have been 

restudied, and the results have led to a revision of the geologic 

history presented by Whitman Cross and associates in 1905-10. 

Field relations, petrographic descriptions, and isotopic age determinations indicate that these rocks were formed in two cycles of 

deposition, folding, metamorphism, and injection of plutonic 

rocks. The first cycle was approximately 1,800 to 1,700 m.y. 

(million years) ago, and the second, about 1,650 to 1,450 m.y. 

ago.

The older cycle consisted of:

1. Deposition of the Vallecito Conglomerate, a quartzose rock 

whose matrix is quartz sericite, and hematite and whose 

clasts are quartzite, quartz, red and black jasper, chert, 

and minor argillite and volcanic rocks. The conglomerate 

is at least 2,400 feet thick and perhaps 5,000 feet thick, 

and its base and the underlying rocks are not exposed.

2. Volcanism, probably in a eugeosynclinal environment, during 

which great but unknown thicknesses of olivine basalt, 

tholeiitic basialt, and rocks of intermediate to acidic compositions accumulated. These rocks and interlayered graywacke, quartzite, impure sandstone, siltstone, and clay 

make up the Irving Formation.

3. Formation of the now foliated, partly lineated, and metamorphosed Twilight Gneiss either as a complex of sills of porphyry in layered basalt and more acidic lavas and tuffs 

or as an extrusive accumulation of lavas and tuffs of both 

silicic and basaltic compositions.

4. Intense folding, in part isoclinal; folds trend generally 

north to northeast.

5. Accompanying metamorphism of all the rocks to the amphibolite facies.

6. Intrusion of the postkinematic Tenmile Granite, which is 

mostly biotite quartz monzonite, and of the Bakers Bridge 

Granite, which is alkali hornblende granite, about 1,720 

m.y. ago.

7. Profound erosion. 

The younger cycle consisted of:

8. Deposition of the Uncompahgre Formation as quartz sands,

aluminous muds that partly were carbonaceous and sulfurous, and minor silt and conglomerate. The upper layers 

are not preserved and the overlying rocks are unknown, 

but a postmetamorphism thickness of about 8,000 feet is 

now present.

9. The Uncompahgran disturbance, predominantly isoclinal 

folding along easterly to southeasterly trends. The quartzite layers show parallel folds, and the pelitic layers show 

similar folds.

10. Metamorphism to slate and phyllite grade in the west. Metamorphism of thermal type possibly caused by underlying 

Eolus Granite is superimposed in the east, with nondirectional fabrics and such minerals as chiastolite, gTrnet, 

and staurolite.

11. Intrusion of the postkinematic Eolus Granite as two large 

composite masses, each more than 100 square mi>s in 

area, and each mostly biotite-hornblende quartz di^rite; 

and intrusion of the Electra Lake Gabbro; both about 

1,460 m.y. ago.

12. Intrusion of the Trimble Granite into the Eolus Granite 

and of minor dikes and stocks of various basic and acidic 

rocks into the Irving Formation, Vallecito Conglomerate, 

Bakers Bridge Granite, and Uncompahgre Formation.

13. Deep erosion and deposition of Cambrian and younger

sediments.

Chemical analyses show that the Twilight Gneiss is sodic 

rather than potassic; that the Tenmile, Bakers Bridge, and 

Trimble Granites lie in or close to the granite minimum malting 

trough; and that the Eolus Granite and the associated cmartz 

diorite of Pine River show a trend from a relatively calcic composition to the granite minimum.

INTRODUCTION

The rugged Needle Mountains of Colorado lie in 

the southwestern part of the much larger San Juan 

Mountains and form the only extensive exposure, r.bout 

425 square miles, of Precanibrian rocks in this part of 

Colorado. Tertiary volcanic rocks bound the Precanibrian on the north and east, and Paleozoic and 

Mesozoic rocks bound it on the south and west. The 

U.S. Geological Survey is investigating the geolo/ry of 

the 1° by 2° Durango quadrangle, which include? the 

Needle Mountains, and I have studied the geology of 

the Precambrian rocks as a part of this larger effort.

Al
A2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Whitman Cross and his associates (see p. A3) mapped 

the geology of the western two-thirds of the Needle 

Mountains at a scale of 1:62,500, and of the eastern onethird at 1:250,000. One feature of these maps is controversial Cross inferred that many of the Precambrian formations lie in fault contact with each other. 

I have restudied the area mapped by Cross, with 

emphasis on the interrelations of the various Precambrian formations, and have remapped the geology 

of the eastern one-third of this area, covered previously 

by reconnaissance only.

LOCATION AND ACCESS

The Needle Mountains lie from 20 miles north to 40 

miles northeast of Durango, Colo., and 10 miles southwest to 25 miles southeast of Silverton, Colo. (figs. 1, 

4) or roughly between lat 37°25' and 37°45' N. and long 

107°25'andl07°50'W.

The western part of the Needle Mountains is accessible in the summer by the Denver and Rio Grande 

Western narrow-gage railroad, which passes through 

the Animas Canyon; by State Highway 550 to the west; 

and by the gravel roads along the southern and eastern 

flanks of the range. Most of the Needle Mountains lies 

in the San Juan Primitive Area and, therefore, is accessible only by trail, with travel limited to nonmotorized means. The now abandoned mining town of Beartown, on the northeast side of the range near the head 

of Bear Creek, is accessible to four-wheel-drive vehicles 

via a side road from the old Silverton-Stony Pass-Rio 

Grande wagon road.

PHYSICAL FEATURES

The Needle Mountains are some of the most rugged 

mountains in Colorado; and many sharp peaks 13,000 to 

14,000 feet high are cut by precipitous canyons. The 

mountains are of two main types: those of quartzite, as 

in the north to northeast, which are mostly in rows 

parallel to the east to southeast strikes of the more 

resistant layers; and those of granite, amphibolite, and 

other rocks, which are randomly arranged and have an 

irregular drainage pattern. The predominant drainage 

of the Needle Mountains is southward (fig. 4), through 

deep canyons cut by the Pine River, Vallecito Creek, 

the Animas River, and Lime Creek.

PREVIOUS WORK

The Precambrian rocks of the Needle Mountains were 

examined by geologists prior to 1890. (See, for example, 

Endlich, 1876; Comstock, 1883; Lakes, 1889.) A significant conclusion by Emmons (1890, p. 257) was that 

over 10,000 feet of closely folded quartzites, conglomerates, and slates of Precambrian age (Algorkian) occur near Ouray. Emmons further concluded that these 

rocks of the Grenadier Range, then called the Quartzite 

Peaks, probably are of the same series. Van Hise (1892, 

p. 319-323) observed the rocks of the Animgs Canyon 

between Silverton and Needleton, and tentatively concluded that the clastic Uncompahgre Formation is 

younger than the crystalline Irving Formation and 

Tenmile Granite. He noted the marked discordance in 

structural attitudes of the Uncompahgre Formation 

and of the Irving Formation and Tenmile Granite.

Intensive geologic investigations in the Neeile Mountains were made by Whitman Cross and his associates. 

The results of their investigations include the Silverton 

folio (Cross, Howe, and Ransome, 1905), the Needle 

Mountains folio (Cross, Howe, Irving, and Emmons, 

1905), and the Engineer Mountain folio (Cross and 

Hole, 1910). This work also led to the summary publications of Cross and Larsen (1935) and of Larsen and 

Cross (1956). A review paper by Hinds (1936) discusses the younger Precambrian rocks of the San Juan 

Mountains. Cross, Howe, and Irving (1907), Kelley 

(1946), Luedke and Burbank (1962), and Burbank and 

Luedke (1964) described the Uncompahgre Formation 

of the type locality. Varnes (1963, p. A7-A8) described 

the Precambrian rocks of the South Silverton mining 

area. Figure 1 shows the areas covered in the pertinent 

field investigations.

ACKNOWLEDGMENTS

I am pleased to acknowledge the close cooperation 

and informative discussions of Professors M. E. Bickford, George Wetherill, and L. T. Silver, wl ose radiometric age determinations constitute a valuable contribution. R. E. Zartman of the Geological Survey determined potassium-argon ages of two minerals. W. J. 

Hail, P. W. Lipman, R. G. Luedke, and T. A. Steven, 

all members of the Geological Survey who ar<>, studying 

the geology of the Durango quadrangle, have helped 

me in many ways. T. K. Hinderman, geologic field assistant, worked capably during the wet summer of 1965.

GEOLOGIC SEQUENCE

The Precambrian rocks of the Needle Mour tains have 

a complex history, including two major Prwambrian 

tectonic events, which consisted of:

1. Deposition of the Vallecito Conglomerate, a quartzose conglomerate at least 2,400 feet thicv and perhaps 5,000 feet thick. The base and the underlying 

rocks are not exposed.

2. Deposition of the Irving Formation, now metamorphosed volcanic and sedimentary rocks of great
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A3

OURAY 

7V2-MINUTE 

QUADRANGLE

Luedke and 

Burbank (1962)

IRONTON 

7 Vi-MINUTE 

QUADRANGLE

Burbank and 

Luedke (1964)

38°07'30"

SILVERTON 

QUADRANGLE

Cross, Ho we, and 

Ransome (1905)

ENGINEER MOUNTAIN 

QUADRANGLE

Cross and Hole 

(1910)

/NEEDLE MOUNTAINS 

QUADRANGLE

Cross, Howe, Irving, 

and Emmons (1905)

MOUNTAINS

38°00'

108°00r

10 

_ I

15 MILES Vallecito 

Reservoir

FIGUBE 1. Map showing areas of previous, detailed geologic investigations of Precambrian rocks in the western San Juan Mountains. Geologic maps by Cross and Larsen 

(1935) and Larsen and Cross (1956) show the entire San Juan Mountains.

but unknown thickness. Twilight Gneiss formed as 

flows or sills of dacite to quartz latite. 

3. Folding along general north to northeast trends, 

metamorphism to high rank, and posttectonic intrusion of the 1,720 million-year-old Bakers Bridge 

Granite, which is massive alkali hornblende granite, and Tenmile Granite, which is largely massive 

biotite quartz monzonite.

4. Deep erosion.

5. Deposition of the Uncompahgre Formation, now 

quartzite, slate and schist, siltstone, and conglomerate. The aggregate thickness is at least 8,000 

feet, and the upper layers are not preserved.

6. The Uncompahgran disturbance, involving isoclinal 

folding along east to southeast trends, metamorphism to low to medium rank, and development of
A4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

a phyllonite along part of the unconformity below 

the Uncompahgre Formation.

7. Intrusion of the Eolus Granite batholith (biotitehornblende and biotite quartz monzonite with 

minor biotite-hornblende-augite quartz diorite) 

and of the Electra Lake Gabbro and Trimble Granite stocks.

8. Deep erosion followed by deposition of Cambrian 

and younger sediments.

Though rocks below the Vallecito Conglomerate are 

not exposed, clasts in the conglomerate indicate that the 

source terrane included much massive quartz either 

quartzite or veins or both red and black jasper, chert, 

argillite, iron-formation, and epidote-rich greenschist.

The rocks of the Needle Mountains Precambrian are 

shown on the geologic sketch map in figure 2, and their 

relations are given in a schematic cross section in figure 

3. The geographic place names and topographic features 

cited in this report are shown in figure 4.

LAYERED ROCK FORMATIONS 

VALLECITO CONGLOMERATE

The Vallecito Conglomerate was named by Cross, 

Howe, Irving, and Emmons (1905, p. 3) for the type 

locality in the gorge of Vallecito Creek in the southeastern part of the Needle Mountains 15-minute quadrangle and in the northeastern part of the Vallecito 

Reservoir 7^-minute quadrangle. The formation is 

found only in the southeastern part of the Needle Mountains (fig, 2), where, for the most part, it is superbly 

exposed in steep canyon walls and cliffs, particularly 

along the Pine River (Larson and Cross, 1956, figs. 

3,4).

The Vallecito Conglomerate is metaconglomerate and 

pebbly quartzite. Its clasts are subangular to rounded 

fragments of milky quartz, red and black jasper, chert, 

gray argillite, ferruginous quartzite, iron-formation, 

intermediate volcanic rocks, epidote-rich greenschist, 

and muscovite schist. This formation contains randomly 

distributed beds of coarse-grained quartzite 1 to 18 

inches thick, and a few much thicker beds. The clasts 

range in maximum dimension from less than 1 mm to 

about 1 foot, but ones larger than 4 inches are rare. 

Those 1 to 4 inches in size are most common. The matrix 

is a very poorly sorted aggregate of quartz and sericite, 

with minor hematite, leucoxene, garnet, albite, biotite, 

and epidote. The matrix is mostly gray but is locally 

pink to purple, and the grains are largely y8 to 1 mm 

in size. The original sediment was wholly recrystallized 

during metamorphism as shown by the following: (1) 

Quartz in the matrix and in the various types of clasts 

in which it occurs has grown into polygranular mosaic 

fabrics; (2) the sericite is mostly well alined and gives a

schistose aspect to the matrix; (3) the few garnets all 

lie enclosed in sericite and may be metamorphic rather 

than detrital; and (4) epidote grains only rarely show 

rounded outlines, a feature which suggest? detrital 

origin and later recrystallization. Many of the quartz 

clasts contain 1- to 2-mm grains that are dispersed 

amongst %- to 1-mm grains. These larger grains constitute 10-20 percent of these clasts. This fabric suggests that these clasts were derived from quartzite or 

from fine-grained conglomerate that contair^d larger 

detrital grains set in a finer matrix. Bands o£ hematite 

1-3 mm thick are abundant in much of the formation. 

Ovoid aggregates 1 to 3 mm in diameter cf sericite, 

sericite plus quartz plus albite and of sericite plus epidote plus biotite plus quartz are also present. These 

aggregates are interpreted as metamorphosed grains of 

feldspar or of relatively fine grained qur,rtzo-feldspathic rocks. These rocks were probably volcanic rocks 

of intermediate to rhyolitic compositions in which original potassic feldspar has been hydrolyzed to sericite 

and original plagioclase has been converted to albite 

and epidote.

In outcrop the Vallecito Conglomerate is mostly gray 

but is locally pink or purple. It is thin to ve^y thickly 

bedded and shows well-developed cross-stratification 

(fig. 5) of the trough type of McKee and Weir (1953, p. 

387) or the pi-type of Alien (1963, p. 109), except that 

the cross-strata are lithologically heterogeneous hematite laminae are interlayered with pebble conglomerate. 

Joints in the rock cut clasts and matrix alike. A few 

folds having amplitudes of one hundred to several 

hundred feet were found, but smaller folds having 

amplitudes of a few inches to several feet are extremely 

rare.

The southernmost exposure of the Vallecito Conglomerate on the west side of the Vallecito Creek valley 

consists of impure, sparsely pebbly quartzite. This 

quartzite layer (1) has an apparent thickness of about 

1,500 feet, (2) shows rare west-facing cross-stratification, (3) is gray to blue gray, massive, fine to coarse 

grained, and (4) contains 10-15 percent muscovite, 

microcline, biotite, andalusite, and magnetite. Andalusite occurs as cores in grains of muscovite and may have 

formed in the regional metamorphism 1,720 to 1,780 

m.y. ago. This quartzite unit has a gradatioral contact 

with the conglomerate to the east which it apparently 

overlies. It underlies the Irving Formation to the north 

and is overlain by Paleozoic rocks on the west and by 

alluvia] gravel to the south.

The exposed thickness of the Vallecito Con glomerate 

remains uncertain. At Granite Peaks Ranch in the Pine 

River valley, where there is good top-and-bottom control by cross-stratified beds, the formation is more than
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A5

2,400 feet thick. Larsen and Cross (1956, p. 24) reported 

"an estimated exposed thickness of more than 3,000 feet 

west of Pine River." The Vallecito Conglomerate, however, may well be more than 5,000 feet thick, and an 

unknown additional thickness to the south is covered 

by younger rocks.

The origin of such a thick mass of conglomerate and 

pebbly quartzite that is relatively homogeneous as to 

types of clasts and matrix is problematical. The source 

terrane included quartzose and ferruginous sedimentary 

rocks, intermediate to basic volcanic or metavolcanic 

rocks, and argillite and schist. Poorly sorted crossstrata, commonly containing pebbles as large as 2 inches 

in diameter, imply fluvial deposition. The Vallecito 

Conglomerate may be deltaic, deposited perhaps in a 

technically controlled basin.

The Vallecito Conglomerate underlies the Irving 

Formation, as indicated by cross-stratification in the 

conglomerate near the contact, which is mostly steep 

to vertical, both along Vallecito Creek and to the northeast on the broad ridge west of Emerald Lake. West of 

Vallecito Creek bedding surfaces in the conglomerate 

lie at angles of 10°-30° to the Vallecito-Irving contact, 

which is not well exposed; whether this discordance is 

due to local tilting and erosion of the Vallecito before 

deposition of the Irving or is due to folding along the 

contact could not be determined. However, there is no 

evidence that the discordance is due to faulting. West 

of Emerald Lake the Vallecito-Irving contact is clearly 

exposed, and the two formations are conformable.

IRVING FORMATION 

NAME AND CORRELATION

The name Irving Greenstone was applied by Cross 

and Ho we (Cross, Howe, Irving, and Emmons, 1905, 

p. 2) to the amphibolite, biotite schist, metagraywacke 

and biotite-bearing metavolcanics, metamorphosed plutonic rocks, micaceous metasiltstone, quartzite, and 

iron-formation of the southeastern Needle Mountains, 

underlying Irving Peak, and extending eastward to 

Lake Fork of the Pine River and southward along Vallecito Creek. Because it has so little true greenstone this 

formation is here renamed the Irving Formation. The 

areas in the western and northern parts of the Needle 

Mountains that are designated Irving Formation in 

figure 2 were called Archean schist and gneiss by Cross 

and Howe (Cross, Howe, and Ransome, 1905, p. 2-3; 

Cross, Howe, Irving, and Emmons, 1905, p. 1-2; Cross 

and Hole, 1910, p. 2-3); the rocks are largely amphibolite, plagioclase-quartz-biotite gneiss, and metagraywacke or intermediate metavolcanic rocks. From their 

lithologic similarity to the type Irving rocks, their

356-028 O 69 2

common but not ubiquitous retrograde metamorphism, 

their structural trends, and their position under the 

major unconformity, I have grouped all these rocks 

under the name Irving Formation. The accuracy of this 

correlation, however, awaits testing by radioimtric 

dating.

SOUTHEASTERN NEEDLE MOUNTAINS

The Irving Formation in the southeastern Needle 

Mountains contains:

1. Much amphibolite, which is composed of green or 

blue-green hornblende, andesine or calcic oMgoclase, epidote or biotite, and ilmenite or magnetite, 

and presumably is metabasalt and meta-andesite.

2. Biotite-bearing and epidote-bearing schists and 

gneisses that contain such assemblages as epidotequartz-actinolite-magnetite-plagioclase, quartzbiotite-albite-epidote, hornbleiide-biotite-albitemagnetite, albite-hornblende-biotite, and sericitequartz-chlorite-oligoclase; some of these rocks 

probably were derived from volcanic rocks of intermediate compositions and some probably from 

graywackes.

3. Metamorphosed feldspathic plutonic rocks, of which 

andesine quartz-hornblende-biotite gneiss and albite-hornblende-epidote-chlorite-biotite rocks are 

typical.

4. Meta-andesite that exhibits relict outlines of phenocrysts of plagioclase and a mafic mineral (probably 

clinopyroxene or hornblende); these original feldspar phenocrysts are now sericite and epidote, the 

mafic mineral phenocrysts are ragged blue-green 

hornblende, and the matrix is hornblende, oligoclase, epidote, sericite, ilmenite, and chlorite.

5. Quartzite and feldspathic quartzite.

6. Metasiltstone that is composed of quartz, muscovite, 

and biotite.

7. Banded iron-formation that consists of the as^e,mblage quartz-magnetite-biotite-garnet and of various hornblende-mica-epidote-quartz-magnetite assemblages, according to J. E. Gair and Harry 

Klemic (unpub. data).

8. Muscovite-quartz-biotite schist.

Most foliation and compositional layering in this area 

trend north and dip steeply.

Much of the Irving Formation here shows mild retrograde metamorphism in which plagioclase is partially 

altered to sericite and epidote; hornblende to epidote, 

chlorite, calcite, and perhaps albite; and biotite to chlorite. A pelitic schist on the northwest side of Irving 

Peak collected just south of the Irving-Uncompahgre 

unconformity consists of pseudomorphs of sericite and 

chloritoid after andalusite( ?). These pseudomorphs are
A6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

107°30'

~#pm : ... 

:. *(. >* . . . .%  :-. \\ r \ \\

FIGURE 2. Geologic sketch map of Precambrian rocks of the Needle Mountains, southwestern Colorado.

set in a matrix of quartz, chlorate, chloritoid, garnet, 

and ilmenite.

A distinctive, 40- to 50-foot-thick gray conglomerate 

lies several hundred feet above the. base of the Irving 

Formation at Vallecito Creek. This rock contains subangular to subromided granules to cobbles of both 

polygranular quartz 'and metavolcanic rock of the 

assemblage hornblende-plagioclase-magnetite-quartz, in

which the plagioclase is untwinned oligoclase or andesine. The matrix is comprised of quartz, plagioclase, 

hornblende, and magnetite. About 1.1 miles south-southwest this layer lies directly on the Vallecito Conglomerate, and farther 'southwestward it is absent. This 

relationship suggests that in this locality the Irving is 

a wedge-shaped deposit overlying the Vallecito 

Conglomerate.
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A7

EXPLANATION

Trimble Granite

Electra Lake Gabbro Eolus Granite

Quartz diorite of Pine River

Uncompahgre Formation

Darker pattern shftws slate, phylhtf, and schist 

Stipp/e pattern shown guart'ite and conglomerate

Tenmile Granite Bafeers Bridge Granite

Showing trend of foliation ami banding

Showing trend of foliation rind bunding

Vallecito Conglomerate

Contact 

Fault

The Irving Formation in the southeastern Needle 

Mountains is cut by scattered thin dikes, mostly 1-20 

feet thick, of fine-grained leucocratic quartz monzonite 

or granite. Microcline, quartz, and albite or oligoclase 

are the principal mineral constituents. These dikes are 

homogeneous and massive in texture and are posttectonic. They are described below on page A28.

WESTERN NEEDLE MOUNTAINS

The Irving Formation in the western Needle Mountains is well exposed for about 16 miles along the canyon 

of the Animas River, from near Rockwood northward 

and northeastward to Snowdon Peak and Balsam Lake. 

(See Cross, Howe, Irving, and Emmons, 1905; Cross 

and Hole, 1910.) The formation consists largely of darkgray well-foliated ampliibolites consisting of green 

hornblende and andesine and partly of biotite, garnet, 

or epidote. In the south half of this terraiie and near 

the Twilight Gneiss the ampliibolites are interlayered

with medium-grained gray to pinkish-gray commonly 

banded plagioclase-quartz-biotite gneiss. The gneiss 

forms from a few percent to the bulk of individual outcrops. Augen are conspicuous in parts of this gr?,iss. 

Layers of this gneiss range in thickness from 6 mm to 

several tens of feet. A widespread lineation of clustered 

biotite grains in the gneiss invariably is parallel to the 

lineation in adjacent ampliibolites that is given by parallel prisms of hornblende. Contacts between amphibolite 

and gneiss typically are parallel to the foliations but 

crosscutting relations are common in which thin dikes 

extend out from conformable layers of gneiss and cut 

the amphibolite at high angles to the foliation. The 

type of occurrence in which dikes crosscut the gneiss 

grades into a breccia with a matrix of plagioclasequartz-biotite gneiss enclosing broken slabs of amphibolite; in some places marked rotation and bendirg of 

the fragments of amphibolite has occurred. Other rock 

types in this area of Irving Formation are horiiblendeplagioclase-quartz-biotite gneiss and plagioclase-quartzbiotite-garnet gneiss. Retrograde effects here are mild 

and present only locally; they include partial sericitization of plagioclase and alteration of garnet to biotite 

along fractures and margins of grains.

Amphibolite lies north of the main mass of Ter mile 

Granite but is cut by dikes and sills of that granite. 

This rock is fresh, banded in outcrop, and dark grr,yish 

green. It shows a pronounced lineation of acicular hornblende grains that are %-l mm in size. This amphibolite 

is about 70 percent green hornblende, 25 percent zoned, 

rarely twinned, calcic oligoclase to sodic andesine, and 

5 percent colorless clinopyroxene that shows a reaction 

texture with hornblende and plagioclase. The clinopyroxene and plagioclase occur in the rock as disseminated lenses about 1 mm thick and 5-10 mm long, and 

their long axes are parallel to the hornblende lineation. 

This clinopyroxene may be of regional metainorphic 

origin or it may be a contact product of the Tenmile 

Granite. The linear disposition of the clinopyroreneplagioclase lenses suggests the former. Analyses by C. L. 

Parker of this amphibolite, collected near the railroad 

tracks 200 yards north of the Tenmile Granite, gave:

Chemical analysis 

SiOs ... .......................... 51.66

A12O3 -- -------- ---------- 12.77

FesOs. ------ -.--_.-....------- 2.04

FeO ----___-------___-_-_-. 8.01

MgO- --------- ..-- -- -- 8.44

CaO--- ----- ------------- 11.41

NajO. ----. --_. - -- 3.31

H2O -

TiOs..

P2O5 -

MnO-.

CO2--

CL ___

P. _..-

1.26 

.00 

.35 

.09 

.17 

.08 

.01 

.03

Subtotal--...-----------.-- 99.87

Less O.._------------------ .01

Total----.. -------- 99.S

ClPWnorm 

Orthoclase-------------------- 1.536

Albite ....... . 27.934

Anorthite.-.----- --.- -- 19.258

Halite -------------------- .016

Wollastonite ------------ 15.111

Enstatite---------------------- 14.018

Ferrosilite ------.---.--.----- 8.511

Forsterite.------._.----_-_--.-- 4.906

Fayalite.----------------------- 3.283

Magnetite..-._._..-.... ..- 2.958

Ilmenite----------------------- .665

Apatite. -__ - -- .213

Fluorite-. _---_----------_----- .053

Calcite-----__-_-_-_-._.._--. .136

Total----..--.---------.- 98.600

Salic.. _ -_- _. 48.745 

Femic -------------- 49.855
A8 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

://>; .-'. Electra' La ke Qa bb'roS\V :' : >'.

 ! ': . .Cpriglpmera't'e ( -. :

'x-;-Trfmble 

 'Granite'

FIGUBE 3. Schematic cross section of Precambrian rocks of the Needle Mountains. Map patterns are the same ar in figure 2.

This amphibolite, containing more than 8 percent 

normative olivine, probably is a metamorphosed olivine 

tholeiite.

Trends of foliation and compositional layering of the 

Irving Formation in the western Needle Mountains are 

mostly north-northeast to east-northeast, with steep 

dips to the southeast or northwest; a few east and northnorthwesterly trends were found. Most lineations are 

at angles less than 30°, and they plunge northeast or 

southwest.

NORTHERN NEEDLE MOUNTAINS

The Irving Formation in the northern Needle Mountains is superbly exposed in the canyon of the Animas 

River south of Silverton, well exposed eastward in the 

vicinity of Highland Mary Lakes and at the headwaters 

of Deep and and Quartzite Creeks, and poorly exposed 

near Beartown.

This formation in the Animas Canyon consists of 

interlayered fine-grained dark-gray to grayish-green 

banded amphibolite and various fine-grained gray to 

green plagioclase-quartz-biotite-bearing gneisses and

minor sericite-biotite-clllorite schists. Amphibolite is the 

more abundant rock type; typically it consists of bluegreen hornblende, untwinned calcic oligoclase or andesine, and epidote, but varieties with biotite, chlorite, or 

quartz are not uncommon. Lenses of quartz pud epidote 

and feldspar-rich bands are scattered through much of 

the amphibolite. Most of the plagioclase-quartz-biotite 

gneiss also contains one to several of the following minerals : Chlorite, epidote, microcline, garnet, crlcite, magnetite, and pyrite. One specimen contains scattered %- 

to 2 mm aggregates of quartz, plagioclase, and epidote 

with chlorite, and the individual grains are y^-y^ 1TUn 

in size. These aggregates may be recrystallized phenocrysts. Much of the disseminated chlorite, epidote, and 

calcite was formed during retrograde metamorphism; 

at least some of the biotite was also formed th^n, because 

it forms partial pseudomorphs after garnet.

Eastward, around Highland Mary Lakes and the 

southern headwaters of Deep Creek and rlong Bear 

Creek, the proportion of amphibolite in this formation 

decreases, and fine- to medium-grained gra7 well-foliated partly banded plagioclase-quartz-biotite gneiss is
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO

107°45' 30'

AQ

107°15'

FIGURE 4. Map of the Needle Mountains area, showing geographic place names and topographic features.

the preponderant rocli type. The gneiss contains both 

feldspathic and biotitie layers, which are 1-20 mm thick 

and intensely contorted. This gneiss is of quartz dioritic or granodioritic composition, and most probably is 

metamorphosed igneous rock. An origin from volcanic, 

hypabyssal, or plutonic parent rock could not be determined in this study, but the interlayered amphibolite is

at least partly metavolcanic, as shown by pillow structure near Highland Mary Lakes (fig. 6). Contacts between the plagioclase-quartz-biotite gneiss and the 

amphibolite are mostly conformable but are locally 

crosscutting, as at Bear Creek. Biotite schist and muscovite schist also are interlayered in some places with the 

gneiss and amphibolite. Retrograde metamorphism here
A10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

-^K*-. X! - T^' *

 7SV-. . ' -Jf; 

»

.«.«<* ....*£

FIGURE 5. Cross-stratification in 'the Vallecito Conglomerate, 

Vallecito Creek Trail about 0.5 mile north of Vallecito 

Campground.

is similar to that along the Animas River, as sericite, 

chlorite, epidote, and calcite have formed.

The attitude of foliation and the compositional banding are parallel in this northern area of the Irving Formation ; the formation strikes mostly east-northeast and 

dips very steeply, although flat dips were noted along 

the Animas River near its junction with Sultan Creek. 

Lineations of fold axes and of parallelism of elongate 

grains, observed at several places in this terrane, lie at 

both low and very steep angles.

STRATIGRAPHIC THICKNESS

The stratigraphic thickness of the Irving Formation 

could not be determined, mostly because of the lack of 

prominent marker horizons, but the Irving probably is 

at least several thousand feet thick in each of its three 

areas of exposure.

BOCKS OF WEMINUCHE CHEEK

A small exposure of Precambrian interlayered quartzite and mica schist with associated sheared granite and 

massive pyroxenite lies in the eastern part of the area 

near the head of Weminuche Creek, at the northeastern 

corner of the eastern mass of Eolus Granite (fig. 2). 

These rocks are mapped in figure 2 with the Irving Formation. These rocks are intruded by the Eolus Granite, 

are unconformably overlain by Tertiary volcanic rocks

^tt

^ ,

 » ^-: ^» > * ^T*"~ ^A -

- ^Pl"

^

 t'

'» ,^V

i ' , ^^

^ -TT-i ! »*-

FIGURE 6. Metamorphosed pillow lava, Irving Formation, Highland Mary Lakes.

on the north, and are in fault contact with Mesozoic 

rocks to the east. The quartzite is vitreous, gray, blue, 

and red. Bedding at about N. 15° E. with steep dips is 

visible in only a few outcrops. This rock is massive and 

so thoroughly recrystallized that no traces remain of 

original grain outlines. The grains are 1-6 mm in size 

and consist mainly of quartz but include 1-2 percent of 

muscovite and traces of rutile, hematite, and zircon. The 

schist is largely muscovite, biotite, and quartz; its original garnet has been altered to limonite and a colorless 

material of very low birefringence. Poor to fair exposures indicate that layers of quartzite and schist several 

feet to about 100 feet in thickness are interlayered and 

possibly isoclinally folded. Pyroxenite, sheared pink 

Eolus(?) Granite, and quartzite lie in roughly northtrending bands in the eastern part of this Precambrian 

mass. The pyroxenite is greenish brown, massive, and 

medium to coarse grained and consists mostly of 

twinned clinopyroxene and minor green hornblende and 

very fine grained alteration products.

The position of these rocks in the stratigraphic sequence is not known; however, the nature of the quartzite suggests affinities with the Vallecito or Irving 

Formation.

UNCOMPAHGRE FORMATION

The thick sequence of Precambrian interlayered 

quartzite and slate in the canyon of the Uncompahgre
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO All

River above Ouray was named the Uncompahgre Formation by Cross and Howe (Cross, Howe, and Ransome, 1905, p. 3). This section, whose basal and uppermost parts are not exposed, was described by Kelley 

(1946, p. 296-297, 306), who stated that about 8,700 

feet of strata are exposed, and it later was shown on the 

geologic quadrangle maps of Luedke and Burbank 

(1962) and Burbank and Luedke (1964). The quartzite 

and slate of the Needle Mountains area were correlated 

on the basis of their lithological similarity with those of 

the Uncompahgre Canyon by Cross and Howe (Cross, 

Howe, and Ransome, 1905, p. 3; Cross and others, 1907). 

This correlation has not been verified by modern geochronologic methods, but I see no reason to doubt it.

The Uncompahgre Formation in the Needle Mountains area is exposed in a great arc, about 22 miles long, 

that extends eastward from Lime Creek (Cross and 

Hole, 1910) across the north end of the West Needle 

Mountains and the Animas Canyon, along the Grenadier 

Range and the peaks that lie to the north (Cross, Howe, 

Irving, and Emmons, 1905), and southeastward across 

Vallecito Creek to the upper drainages of Ute Creek and 

Flint Fork and to the high ridge between the Lake Fork 

and Flint Fork of Pine River. This formation ends at its 

intrusive contact with the Pine River batholith of Eolus 

Granite. The Uncompahgre Formation unconformably 

overlies older rocks and occurs in a synclinorium here 

termed the "Uncompahgre synclinorium." Similarly, 

the episode of folding and metamorphism that affected 

the Uncompahgre Formation is here named the "Uncompahgran disturbance."

The Uncompahgre Formation is largely quartzite and 

pelitic rock (slate and schist) ; siltstone and conglomerate are present in very minor amounts.

BASAL CONGLOMERATE

Conglomerate of varying thickness lies at the base of 

this formation at several localities: in the vicinity of 

Showdon Peak where it is 8 feet thick (fig. 10) ; at the 

junction of the Animas River and Whitehead Gulch 

where interlayered conglomerate and pebbly quartzite 

are about iy2 feet thick; on the west rim of the canyon 

of the Animas River east-southeast of Molas Lake 

where 2i£-31/£ feet of conglomerate contain about onethird pebbly quartzite; and along the north side of the 

valley of Tenmile Creek the conglomerate is as much as 

20 feet thick. These bodies are lenticular; for example, 

on the west slope of the Animas Canyon, between the 

Snowdon Peak and Tenmile Creek localities, there is 

no conglomerate at the base of the Uncompahgre Formation. Many exposures of this conglomerate and of 

the overlying quartzite show cross-stratified layers 

whose tops invariably face away from the unconformity.

Lithologically the basal conglomerate is similar to 

the Vallecito Conglomerate, and they may have been 

derived from the same source rocks. The clasts rarely 

exceed 4 inches in maximum dimension, and most are 

i/^-l^ inches. Subrounded to rounded quartz clasts are 

most abundant and form 70-95 percent of the total; 

some show fabrics similar to those of metamorphosed 

quartzites, but most are so thoroughly recrystallized 

that their origin cannot be ascertained. Jasper clasts 

occur in all the conglomerate and pebbly quartzite; 

both red and dark-gray types are found, and the latter 

is more abundant. Most of the conglomerate contains 

gray chert fragments, which are recrystallized to mosaic 

fabrics of y50- to %0-mm grains. Subangular tabular 

fragments of dark-gray argillite were observed in the 

conglomerate west of Snowdon Peak. The matrices of 

these conglomerates are mostly quartzose with minor 

interstitial sericite, but some is sericitic with minor 

quartz. Minor amounts of hematite, limonite, ilmenite, 

leucoxene, tourmaline, and zircon are present in the 

matrix of most specimens. Grain size of the quartz in 

the matrix commonly ranges from about y2G to y2 mm, 

and that of the sericite from %0 o to %0 mm- This basal 

conglomerate was sampled and analyzed for gold but 

gave negative results (Barker, 1969a).

QUARTZITE

The quartzite layers of the Uncompahgre Formation 

are indistinguishable from each other in outcrop, and 

they are described here as a group. The quartzite layers 

are very resistant to erosion and form most of the spectacular peaks of the northern Needle Mountains, of 

which the Grenadier Range (fig. 7) is the most prominent. These rocks are light gray to pale blue, commonly 

show buff-colored joint surfaces in weathered outcrops, 

and are thin to thick bedded. They contain laminae of 

hematite 1-3 mm thick and show trough-type crossstratification in many outcrops. Rare interlayered beds 

of pebbly quartzite and quartz pebble conglomerate 5- 

10 cm thick also occur. They are 95-99 percent quartz; 

the remainder is hematite, which formed from detrital 

magnetibe during metamorphism, and interstitial sericite. Individual quartz grains in much of the quartzite 

range from yw to 2 mm, but some quartzite contains 

original detrital grains as large as 3 cm. In thin section 

the quartz grains show a polygonal mosaic texture in 

which original grain outlines are obliterated and in 

which grain growth was minor. Some specimens consist 

of very irregularly shaped grains 3 to 15 mm in maximum dimension in which the outlines of original, 

smaller grains are indicated by strung-out grains of 

hematite and aggregates of sericite. All quartz grains are
A12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

FIGURE 7. View of the eastern Grenadier Range, from the northeast, showing, from left to right, The Guardian, Mount Silex, 

and Storm King Peak.

characterized by moderate to pronounced undulatory 

extinction.

Layers of siltstone are found locally in the Uncompahgre Formation. They are mostly a few inches to a 

few feet thick and buff to red to gray, and they consist 

largely of quartz and sericite.

The origin of the sericite in the quartzite layers is 

problematical. Three possible sources are: Coatings of 

clay on original detrital grains; clay from the interlayered shale beds that was flushed into the original 

pores sometime after deposition and before diagenesis 

and metamorphism; and hydrolysis of detrital potassic 

feldspar during metamorphism. I favor the third hypothesis but cannot rule out the first and second.

SLATES, PHYLLITES, AND SCHISTS

Pelitic rocks form about 25 percent of the Uncompahgre Formation, as shown in figure 2. Argillaceous 

slates and schists are preponderant, but interlayers of 

siltstone and fine-grained sandstone are present in many 

of the pelite layers, especially the one that is under Rock 

Lake. The pelites are described in this report from west 

to east and southeast, which also corresponds to the direction of increasing grade of metamorphism.

The westernmost exposures of the Uncompahgre pelites in the Needle Mountain uplift are at Lime Creek. 

These rocks are banded gray, green, and black extremely 

fine grained slates. They consist of sericite-quartz assemblages with carbon and pyrite as common accessories. 

Pyrite forms lenses parallel to bedding and occurs also 

along postcleavage joints, suggesting relatively late 

growth of this mineral. The banding is in units 1-5 mm 

thick, and the grain size in individual bands changes 

from %o f4oo mm a* *ne bottom to %oo %oo *&*& ®k the 

top. These bands may be varves. Slaty cleavage is well 

developed and apparently is parallel to axial planes of 

minor folds. Intense folding is evident from many outcrops, as the bedding is at low to high angles to the 

slaty cleavage. A specimen of gray siltstone about 20 

mm thick from a slate layer at Lime Creek consists of 

%o- to %0-mm quartz grains and 15-20 percent of interstitial sericite; bedding is faintly visible because of 

slight variations of the sericite content.

The slate layers in the canyon of the Animas River 

mostly are of the typical gray to dark-gray commonly 

carbonaceous varieties. However, the northernmost 

layer (fig. 2) is largely purple hematitic slate and siltstone with poikiloblasts of andalusite, the westernmost 

occurrence found of this mineral. The next layer to the 

south, whose westernmost part is greatly thickened by 

folding, is pale-green, gray, and grayish-green slate and 

banded siltstone of quartz, sericite, and chlorite that locally contains biotite and plagioclase. The latter rock 

type suggests provenance from intermediate or basic 

volcanic rocks.

To the east the pelitic layers from the south side of 

White Dome to Trinity Lake are mostly dark-gray to 

black fissile banded slate of the assemblage quartz-sericite-carbon, and they commonly contain minor pyrite 

and (or) ilmenite. This slate has interlayers of muscovite-chloritoid-quartz-garnet phyllite, muscovitechlorite-quartz-andalusite phyllite, muscovite-chloritechloritoid-quartz-ilmenite-carbon slate, quartz-sericite 

siltstone of i£0 -mm grain size, and similar rocks. The 

metamorphic grade thus has increased from the westward to an area where chloritoid and garnet appear. 

This study, however, is not sufficiently detailed to determine the order of appearance of these two minerals.

The lower part of the layer that lies south of Storm 

King Peak and The Guardian is dark-gray and greenish-gray slate, in part chlorite bearing. The upper part 

is mostly micaceous siltstone and fine-grained sandstone, in part contains both muscovite and biotite, and 

is locally cross-stratified.

The pelite layers in the vicinty of Beartown and 

south to the head of Vallecito Creek are finely banded, 

gray to black slates with typical assemblages of quartzsericite-biotite-carbon and sericite-chlorite-quartz-biotite-carbon. Pyrite, ilmenite, albite, and tourmaline are 

accessories. Biotite here is in randomly oriented ragged anhedrons that indiscriminately cut across bands
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A13

of well-oriented sericite; this ragged biotite is believed 

to be of postkineniatic crystallization and of thermal 

metamorphic origin, as further discussed on page 

A31-A32.

South of the latitude of Mount Nebo along Vallecito 

Creek the slates and phyllites contain randomly oriented 

metacrysts of gray chiastolite 1-2 inches long that show 

the usual regularly arranged carbonaceous inclusions. 

The metamorphic grade increases rapidly to the south, 

for the pelite layer that crosses under Vallecito Creek 

about 0.4 mile north of the junction of Vallecito and 

Rock Creeks contains the assemblage biotite-chiastolitemuscovite-quartz and the following sets of additional 

phases: chloritoid-garnet-plagioclase-staurolite, chloritoid-garnet-plagioclase, and chloritoid-plagioclasestaurolite. Compositional banding is common in these 

schists, as shown in figure 8. The plagioclase is calcic 

albite and sodic oligoclase. Colorless to gray-green to 

blue-green tourmaline is a common accessory. The trio 

graphite-ilmenite-pyrite also are accessory and are stable together. Pale-green chlorite forms ragged partial 

to complete pseudomorphs after biotite but is not a stable phase. Sericite, of much finer grain than stable muscovite, is the other secondary mineral. Biotite is pleochroic straw yellow to medium brown. Chiastolite is 

pink in hand specimen, the only occurrence of this color 

variation in the Needle Mountains. Chloritoid is pale

\

FIGURE 8. Banded chiastolite-garnet-staurolite schist in the 

Uneompahgre Formation, 0.4 mile north of the junction of 

Rock and Vallecito Creeks 

356-028 O 69 3

violet and shows no visible pleochroism. Garnet is blood 

red in hand specimen and pale pink in thin section. 

Staurolite has Z = pale yellow rather than golden yellow. The matrices of these schists, which are mostly 

quartz, muscovite, and plagioclase, range in grain size 

from y40 to yw mm, whereas the metacrysts of biotite, 

chloritoid, garnet, and plagioclase are Vio^i mm, 

those of staurolite 1-10 mm, and those of andalusite 

5-50 mm. These rocks are banded, as is typical of the 

Uneompahgre pelite, but they definitely are not schistose. Even the biotite and muscovite are randomly oriented, and the rocks are granoblastic and porphyroblastic. The fabrics are those of hornfels rather than of regionally metamorphosed pelites. Chloritoid was not observed east or south of this locality.

East to southeast and south of the junction of Vallecito and Rock Creeks the layers of Uneompahgre schist 

mostly have similar granoblastic and porphyroblastic 

fabrics, but rocks retaining their well-defined schistosity and finer grain are not uncommon. Chiastolite schists 

sampled along the northeast slope of Rock Creek valley, 

1.1 miles east-southeast of the Vallecito-Rock Creek 

junction, contain rare staurolite and no chloritoid. Staurolite was not observed east or south of this locality. 

Schists from here to the head of Rock Creek and near 

the divide with Flint Fork contain prominent poikiloblasts and knots of chlorite along with disseminated 

octahedral magnetite. These rocks contain only traces 

of biotite, or none at all, and biotite may have reacted 

late in the metamorphic process to produce this chlorite 

and magnetite. The biotite was not analyzed but probably is more ferruginous than the chlorite. Locally high 

activities of IT2O, perhaps of magmatic generation as 

discussed subsequently, are believed to have driven this 

reaction. Partial to complete alteration of chiastolite to 

sericite also has taken place in these rocks. Schist collected 1.4 miles south of the chiastolite-chloritoidgarnet-staurolite occurrence of Vallecito Creek, from 

near the 13,250-foot summit ridge between Vallecito and 

Rock Creeks, is especially notable because it contains 

much secondary sericite and chlorite.

The pelitic schists at the head of Rock Creek and 

north across the Continental Divide to the head of Middle Ute Creek are gray to black banded fine-grained 

rocks with the following assemblages: muscovitequartz, biotite-chlorite-garnet-muscovite-quartz, biotitechiastolite-garnet-magnetite-muscovite-quartz, and biotite-chiastolite-muscovite-quartz. Each assemblage contains graphite, tourmaline, and pyrite as accessory 

minerals. The chlorite in the second assemblage is 

in ragged tablets, similar in habit to the biotite in this 

rock, and apparently is in equilibrium. The chiastolite 

metacrysts are 1-2 inches long (fig. 9) and gray, and
A14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

FIGURE 9. Chiastolite schist in the Uncompahgre Formation, 

0.35 mile south of Twin Lakes, Middle Ute Creek drainage.

most show some secondary very fine grained sericite, 

quartz, and chlorite. Quartz-biotite-muscovite schist, 

which apparently was deposited as silt or very fine sand, 

is interlayered with the chiastolite schists.

The Uncompahgre schists in the headwaters of the 

Flint Fork of Pine Kiver contain andalusite and chlorite 

as stable phases, with typical assemblages: andalusite 

(poikiloblastic, not carbonaceous)-biotite-chloritegarnet-muscovite-quartz, and andalusite-biotite-chlorite-muscovite-plagioclase (albite-oligoclase) -quartz. 

Graphite is a common accessory.

Within about i/^-l mile of the Pine River batholith of 

Eolus Granite the Uncompahgre schists show contact 

metamorphic effects. Sillimanite occurs in place of andalusite, plagioclase is slightly more calcic, pinite occurs as pseudomorphs after cordierite(?), and microcline is present but chlorite absent within a few hundred feet of the contact. Typical 'assemblages are 

biotite-muscovite-pinite-quartz-sillimanite, biot'te-muscovite-plagioclase-quartz-sillimanit©, and biotitegarnet-microcline-plagioclase.

Lenses of quartz are scattered throughout the Uncompahgre pelite. Most are but a fraction of an inch 

thick, and many are contorted by folding. Most are 

parallel to bedding, but oblique ones are not uncommon. These lenses may be metamorphosed chert nodules or lenses or quartz veins.

The pyritic pelites of the Uncompahgre Formation 

were analyzed for 30 metals (Barker, 1969a), including gold, and were found to contain generally less metal 

than the average shale.

A study of carbon isotopes in the slates and schists of 

this formation by Barker and Friedman (1969) shows 

that the carbon is isotopically light, with delta C13 

values ranging from about 23 to 31 per mil. The 

carbon most probably is of photosynthetic origin.

The ground water that issues from the pyrite-bearing pelite of the Uncompahgre Formation commonly 

leaves a gelatinous coating of ferric hydroxide on 

stream boulders. Pyrite is unstable in aerated ground 

waters and oxidizes (Hem, 1960, p. 71) to give a ferrous 

sulfate water. The ferrous iron is further oxidized at 

the surface and precipitates largely as ferric hydroxide. 

Springs of iron sulfate-bearing water issue from the 

great slab of schist that underlies the north-facing 

mounta: n slope southeast of the junction of Rock and 

Vallecito Creeks and from the underlying quartzite. 

The waters have killed trees, stained cliffs of quartzite 

and boulders in Rock Creek below, and caused fish 

to leave the lower part of Rock Creek. A 2-quart sample 

of water issuing from one of the springs on this slope 

was collected and analyzed by G. F. Scarbro and M. R. 

Midgett, with the following results:

Parts per Equivalent 

million per million

SiO2 ------------------------------

Al

Fe__________________________________

Mn_ _______________________________

Ca______________ ___________________

Mg_______________________-_-_____-_

Sr _ ____________ _________________

Na_ ___________ __ ___________ _____

K____ _ _______ _ ________________ _

Li _________________________________

Cu______________ ____ _ 

Zn

Acidity (as H2SO4)

Total cations

Sulfate (SO4) _ _ _ _ _ _ _ _ ___

Chloride (Cl)_____________----___-___

Fluoride (F)

Nitrate (NO3)--_- __ _ _ _ - --

Phosphate (PO4) __ __ _ _ 

Total anions __ _ _

Hardness as CaCO3

Total. __ _____ ___ ___ _

Specific conductance (jumbos at 25° C)

pH__________.______-______-_ 

14

18

12

12

77

114

40

32

8

12

13

15

11

__ 1,270

1.8

1. 7

. 1

.00

__ 1,530

661

__ 661

2, 440

2.

2 00

64

44

3 84

9 38

01

14

02

02

00

04

10 91

27.44

26.44

.05

.09

. 00

. 00

26.58

6
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A15

The sample was not analyzed for bicarbonate (HCO3 ) 

or for carbonate (CO3 ).

THICKNESS

The thickness of the Uncompahgre Formation cannot be measured precisely because the quartzite layers 

locally show minor folding and the pelitic layers almost 

everywhere show complex minor folding. A rough 

estimate can be made from the relatively simple section 

that is exposed from Balsam Lake to the 12,985-foot 

summit about 1.4 miles northeast of the junction of Vallecito and Rock Creeks. Here, as elsewhere, the uppermost part of the formation is not preserved. This section is perhaps 8,000 feet thick and consists of the 

following units:

4th slate layer, 50-100 ft (estimated thickness); 

4th quartzite layer, 1,000 ft ; 

3d slate layer, 200-300 ft; 

Quartzite of Stormy Gulch, 800 ft; 

Slate of Trinity Lake, 200-400 ft; 

Quartzite of Storm King Peak, 2,000 ft; 

Slate and siltstone of Lake Silex, 400-500 ft; 

Quartzite at Balsam Lake, 3,000 ft; 

Unconformity.

The position of the measured section of the Uncompahgre River south of Ouray relative to this one is not 

known, for no individual unit can be recognized as 

common to both areas.

ORIGIN

The following facts bear upon the orig: n of the Uncompahgre Formation: (1) Compositions of the original 

sediments of this formation are essentially bimodal 

highly quartzose sands alternate with aluminous clays 

and form 80-90 percent of the formation; (2) clasts in 

the basal conglomerate are mostly quartz, jasper, and 

argillite; the granitic and metavolcanic rock types that 

lie below the Uncompahgre unconformity elsewhere are 

not found; and (3) the formation is more than 8,000 

feet thick. The bimodal nature of the sediments is most 

easily explained if one postulates a source area of 

quartzite, argillite, jasper, and iron-formation or jaspilite. This postulate appears reasonable when one considers that similar clasts occur in both the basal 

Uncompahgre conglomerate and the older Vallecito 

Conglomerate. Thus, I suggest that the Vallecito Conglomerate and the Uncompahgre Formation had the 

same source rocks and that some of the Vallecito or 

other first-cycle sediments exposed in the area may have 

been recycled into the Uncompahgran trough. The

thickness and known extent of the Uncompahgre 

Formation necessitate a relatively extensive source area.

THE SUB-UNCOMPAHGBE UNCONFORMITY AND 

PHYLLONITE

A phyllonite lies along a part of the unconformity 

between the Uncompahgre Formation and the Irving 

Formation and Twilight Gneiss. This unconformity, of 

course, was deformed during the younger Uncompahgran disturbance and everywhere is now steeply dipping, except south and west of Snowdon Peak (fig. 10) 

and south of Balsam Lake. (See Cross, Howe, Irving, 

and Emmons, 1905, geol. map; fig. 4, this report.) The 

angles between the banding or foliation of the Irving 

Formation and of the Twilight Gneiss and the bedding 

of the Uncompahgre Formation range from a few 

degrees to almost 90°; greater and more abrupt differences occur along the south margin of the Uncompahgre 

Formation than along the north margin.

An unusual rock, which apparently is a phyllonite 

and can be classified as quartz sericite schist, lies in 

gradational to abrupt contact with the Irving Formation and Twilight Gneiss at and west of the Animas 

River at both the north and south margins of the Un-

"r" :>r* -

IT - *

^«i -a^W* . : ^-

FIGURE 10. Basal contact of the Uncompahgre Formation, about 

0.4 mile southwest of Snowdon Peak. Man (center) stands in 

front of the 8-foot-thick basal conglomerate, which is overlain 

by quartzite and underlain by phyllonite. Broken Twilight 

Gneiss and amphibolite, both sheared and altered, lie at the 

left.
A16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

compahgre Formation. This rock unit ranges from 4 

to about 40 feet thick and consists of about 10 to 50 

percent of y±- to 4-mm subrounded to rounded single 

grains and polygranular aggregates of quartz set in a 

schistose matrix of sericite with minor hematite, limonite, and leucoxene. The schistosity in all outcrops observed is parallel to the bedding of the adjacent Uncompahgre Formation. The phyllonite probably was derived from rocks of granitic composition, in which the 

original quartz grains were not comminuted but were 

first severely strained and then recrystallized and in 

which much of the original ferrous iron was oxidized. 

A granite dike in the Irving Formation 75 feet north 

of the unconformity and immediately east of the Animas River contains similarly strained and recrystallized 

quartz. Similar quartz-sericite schist which also probably is phyllonite and 50 to 75 feet thick lies along the 

unconformity west of Moon Lakes; this is the only outcrop of this type of rock found east of the canyon of 

the Animas River.

The unconformity also was observed northwest of 

Irving Peak and at Beartown (See Cross, Howe, Irving, 

and Emmons, 1905, geol. map) ; in both places the phyllonite is absent and the Uncompahgre quartzite lies directly on the Irving Formation. Northwest of Irving 

Peak, sheared and recrystallized quartzite of the Uncompahgre Formation lies on a quartz-sericite schist 

containing chloritoid, garnet, chlorite, and ilmenite and 

having schistosity almost perpendicular to the bedding 

of the quartzite. At the Beartown locality massive 

quartzite of the Uncompahgre Formation lies on 

sheared and closely fractured fine-grained feldsparquartz-biotite gneiss that has been largely altered to 

sericite, clay minerals, calcite, and limonite.

Both types of occurrence, in which phyllonite or 

sheared quartzite or gneiss is present, indicate relative 

movement of unknown but considerable magnitude 

along the unconformity and suggest that the massive 

basal quartzite and conglomerate of the Uncompahgre 

Formation greatly exceeded in competence the Irving 

Formation and the Twilight Gneiss. Thus, in a sense, 

at least part of this unconformity is a thrust fault. 

Mineral transformations in the Irving and Twilight 

rocks that produced sericite and other retrograde minerals apparently were a major contributor to the low 

competence of these rocks. The phyllonite formed during the Uncompahgran disturbance and is synchronous 

with the folding and metamorphism of the Uncompahgre Formation.

IGNEOUS ROCKS 

TYPES AND AGE RELATIONS

The various types of Precambrian intrusive rocks of 

the Needle Mountains, except for the Bakers Bridge 

Granite and the Electra Lake Gabbro, were named by 

Cross, Howe, Irving, and Emmons, (1905); all except 

the Bakers Bridge were described briefly and their 

general age relations given. Cross' names are used here 

with little or no modification, and the reader should 

keep in mind that most of these rocks that are termed 

"granite" actually are quartz diorite and other less 

potassic types. Pertinent information on these rocks is 

summarized in table 1, and more detailed descriptions 

are given below under individual headings. Chemical 

analyses and CIPW norms of most of the intrusive 

types are presented in table 2, and ternary plots of 

normative Ab-Or-An, Ab-Or-Qz, and (Ab + An)-OrQz are shown in figure 11. Plots of the ratios of Na2O 

versus K2O and of Na2O versus CaO are shown in figure 12.

TWILIGHT GNEISS

Cross and Howe (Cross, Howe, Irving, and Emmons, 1905, p. 7) named their Twilight Granite from 

the extensive exposures at Twilight Peak, in the 

West Needle Mountains. This unit underlies the central and western West Needle Mountains, the canyons of Lime and Cascade Creeks, Potato Hill and the 

area to the south, the area east and southeast of Columbine Lake, and the Animas Canyon in the vicinity of 

Cascade Siding and Teft Spur -an area of about 60 

square miles. This rock is a gneiss of trondhjemitic to 

quartz monzonitic composition and is here renamed the 

Twilight Gneiss.

The Twilight Gneiss has a metamorphic fabric in 

that it is well foliated and commonly exhibits a lineation 

of strung-out clusters of biotite. It is homogeneous in 

outcrop except for ubiquitous and locally abundant 

slabby interlayers of amphibolite and for very subordinate interlayers of other rocks. Hornblende prisms 

in the amphibolite interlayers are lineated parallel to 

the biotite-cluster lineation of the granite, indicating 

that these rocks were crystallized together.

The Twilight Gneiss is light to medium gray, partly 

with pink, green, or red tones. It is fine to medium 

grained, and many specimens exhibit bimodal grain 

size; a quartzo-feldspathic matrix of mostly %0 - to 14- 

mm grains encloses quartz "eyes", plagioclase anhedra, 

biotite tablets, and rare microcline anhedra of %- to 

3-mm size. The quartz ovoids or "eyes" give
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO

Qz Qz

A17

D,,c Rhyolite porphyry dike of Vallecito Creek 

DBB Rhyolite porphyry dike in Bakers Bridge

Granite

+ Trimble Granite 

o Eolus Granite 

OQ Quartz diorite of Pine River 

x Bakers Bridge Granite, including quartz 

monzonite southeast of Bakers Bridge 

n Tenmile Granite

lnclusion(?) in Twilight Gneiss

Twilight Gneiss

Ab

FIGURE 11. Ternary plots of normative minerals in Precambrian granitic rocks of the Needle Mountains. Superscripts are percentages of normative salic minerals. C shows the minimum melting trough after Yoder, Stewart, and Smith (1957) and Kleeman 

(1965).

many specimens an unusual appearance; they are polygranular, and adjacent grains differ only slightly in 

optic orientation suggesting origin from single grains. 

Some ovoids are straight-sided, blocky aggregates that 

may be re-crystallized, dipyramidal grains. The size 

distribution and quartz "eyes" may well indicate genesis 

from a porphyry.

The Twilight Gneiss ranges in composition from 

quartz diorite to quartz monzonite, and much of the 

light-colored oligoclase-bearing rock deserves the name 

trondhjemite. Oligoclase or sodic andesine and quartz 

comprise the bulk of most specimens; microcline ranges

in content from several to about 25 percent. Biotite 

forms 5-10 percent of all specimens, and accessory ninerals include hornblende epidote, garnet, opaque oxides, 

and muscovite. Hornblende and muscovite were not seen 

in the same thin section. Microcline is fresh, but plaqrioclase of most specimens shows faded albite twinning and 

moderate to heavy alteration to sericite, clay, and epidote, and clear rims of albite are not common. The hornblende is blue green to green, but the biotite vr.ries 

widely in color, from golden brown to dark green to 

olive brown.
TABLE 1. Description of the Precambrian intrusive rocks of the Needle Mountains

Formation

abandoned in this report) .

River batholith of Larsen 

and Cross.

granitic dikes.

Named by

Irving, and 

Emmons(1905).

-....do.-.--.-.-...

.....do---...-.....

report.

Irving and 

Emmons(1905).

Irving, and 

Emmons(1905).

Rock type

quartz monzonite, 

trondhjemite.

Mostly quartz monzonite,

partly trondhjemite.

monzonite.

diorite.

monzonite granite.

sthene gabbro, diorite.

alaskite.

Fabric

Mostly massive,

partly foliated.

foliated.

...-do.--.----.

 ..do.---.......

.....do.--...--...

....do-- ......

.....do..-.... .

Orogenic type 1

synkinematic. 

Mostly postkinematic, partly 

synkinematic.

--.-do.---.------.

. .do---.. -.

---do--. -_ 

-...do--- ..- 

..... do-. ..

. do---.... .-

Geologic relations

slabs of amphibolite; ovoid quartz grains in m

specimens may be relics from an original porph

ritic fabric. 

Older lineated metamorphosed trondhjemite similar 

Twilight Gneiss; younger massive strained qua

mon^onite similar to Tenmile Granite; dike of 

younger type is truncated by Uncompahgre u

conformity.

ment of foliation in the Uncompahgran disturban

not changed since the rock was first forme i.

forerunner of the Eolus batholith.

rocks; compound batholith; original fabrics w

preserved.

serve i; not in contact with Eolus Granite, so relat

age relations unknown.

plagioclase and mafic minerals.

the Trving Formation in Vallecito Creek.

1 Refers to cycle during which body was emplaced.

2 U-Pb ages determined on zircons, Silver and Barker (1968); Rb-Sr whole-rock ages from Bickford, Barker, Wetherill, and Lee-Hu (1968); K-Ar age by R. E. Zartman (wri
PRECAMBRIAN GEOLOGY OP THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A19

DBB x +

2345 

PERCENTAGE OF KoO

4 

3 

2 

1 

0 1234567 

PERCENTAGE OF CaO

FIGURE 12. Plots of Na-O versus K2O and of Na2O versus CaO, 

in weight percent, for Precambrian granitic rocks of the Needle 

Mountains. Symbols are the same as in figure 11.

Chemical analyses and CIPW norms of five specimens are given in table 2. Relative proportions of Na2O 

and K2O and of Na2O and CaO are plotted in figure 12, 

and three ternary combinations of the normative 

feldspars and quartz are plotted in figure 11. These 

rocks are relatively sodic, having low contents of both 

K2O and CaO. All show normative corundum, as do 

many others of the Needle Mountains granitic rocks. 

Three specimens, BSJ-13, BSJ-17, and LS-T2, are 

trondhjemitic, whereas BSJ-48 is a quartz monzonite. 

BSJ-113, which shows excellent "eyes" of quartz, is intermediate and is a leucogranodiorite. Thus, the five 

analyses, though insufficient in number for the purpose, do suggest a trend from trondhjemite to quartz 

monzonite and toward granite, as expressed in the terminology of plutonic rocks.

The age of the Twilight Gneiss, as determined by the 

uranium-lead isotopic method (Silver and BarVer, 

1968), is 1,780±20 m.y. A rubidium-strontium isochron 

of 12 samples of Twilight Gneiss and interlayen of 

albite gneiss and amphibolite gives an age of 1.805 ±35 

m.y. and an initial Sr^/Sr86 ratio of 0.7015 (Fred 

Barker, Z. E. Peter-man, and K. A. Hildreth, unpub. 

data). These two determinations are thus well within 

limits of error.

Slabby interlayers of amphibolite are found throughout the Twilight Gneiss. Locally, as at the hairpin turn 

on U.S. Highway 550 one-half mile east of Mill Creek, 

the slabby interlayers are brecciated, and a wesldy 

foliated to massive, variably gray-colored matrix of 

Twilight contains irregularly shaped fragments of amphibolite, which range in size from a fraction of an inch 

to 3 feet and in shape from irregular and angular to 

elongate and lenticular. Two samples from amphibcUte 

interlayers, BSJ-18 from a thick layer and BSJ-114 

from a 2-foot fragment, were chemically analyzed 

(table 2). These two compositions are equivalent to 

tholeiitic basalt; they show normative quartz and 

hypersthene and are free of normative olivine and 

nepheline. The similarity of the two analyses might 

suggest little or no change in composition during intrusion and metamorphism of the Twilight, except of 

course for hydration.

The northwestern part of the Twilight Gneiss contains scattered interlayers of oligoclase-quartz-muscovite-biotite-garnet gneiss. These are mostly lenticular, 

are several inches to several feet thick, have their foliation parallel to that of the Twilight Gneiss, and only 

locally are crosscut by the gneiss. These interlayers 

possibly are metamorphosed graywacke.

Along U.S. Highway 550 several tenths of a mile 

east of Mill Creek the Twilight Gneiss contains scattered slabby masses of fine-grained pinkish-gray albitequartz-chlorite-microcline-biotite gneiss. These masses 

show a faint to fair banding that resembles flow band'ng 

in acidic volcanic rocks. The slabs are a few feet to 

about 20 feet thick, and both their contacts and tl Qir 

foliation are parallel to the foliation of the enclosing 

Twilight Gneiss. This albite gneiss shows chevron fcMs 

with axes plunging 60°-65° NE., which is roughly rormal to the lineation of the Twilight Gneiss and suggests that these slabs may be inclusions. The all ite 

gneiss contains only 7-10 percent of dark minerals, has 

an average grain size of about one-half mm, and is 

relatively homogeneous in outcrop. Chemical analyses 

and norms of specimens from two of these inclusions (?) 

are given in table 2, and these two rocks are represented 

in figures 11 and 12. These rocks, much alike in composi-
A20 .SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 2. Chemical analyses and CIPW norms of Precambrian

[0, looked for but not detected; ....._, not looked for. Analyst C. L. Parker: BSJ-13, 

Rapid rock analyses by P. L. D. Elmore, S. D. Botts, Gillison Chloe, J. Glenn, Lowell 

209, 210, 214, 216-218; CO-199]

Twilight Gneiss

Field No....... ....... .--- BSJ-13 BSJ-17 BSJ-48 BSJ-113 LS-T2

Amphibolite of 

Twilight Gneiss

BSJ-18 BSJ-114

Metarhyodacite of 

Twilight Gneiss

BSJ-214 BSJ-216

Tenmile 

Granite

BSJ-19

Whitehead Granite

BSJ-217 BSJ-66a

Chemical anal

SiO2 -----------

AlsOs -

FeO ...-._-- --

MgO...... .. ..........

CaO. ............. ....

NajO.... .............

KjO...... .......... ...

HjO+.. . ...... ......

HjO....... ............

Ti02.~~ .- -

P2 05 .. ------- -----

MnO ...............

C02 -------------

Cl .... ..... .... ....

V. ....................

Total.. .........

n

OQ

 14.07

-. 1.32

----- 3.13

...... .75

-_.- 2.26

.. . 4.11

----- 1.26

...... .83

...... .08

...... .27

...... .08

...... .08

------ .01

00

------ .05

QQ f(Q

...... .02

 99.57

75.11 

12.98 

.77 

1.73 

.33 

1.60 

4.81 

1.23 

.60 

.04 

.13 

.02 

.07 

.16 

.01 

.06 

99.65 

.03 

99.62

74.38 

13.22 

.36 

1.82 

.39 

1.90 

3.16 

3.64 

.52 

.01 

.13 

.03 

.05 

.09 

.01 

.05 

99.76 

.02 

99.74

74.67 

13.15 

.61 

1.76 

.53 

1.41 

3.82 

2.82 

.56 

.03 

.13 

.03 

.06 

.04 

.01 

.09 

99.72 

.04 

99.68

71.21 

13.91 

.36 

3.69 

1.56 

1.44 

4.27 

1.34 

1.12 

.06 

.26 

.08 

.09 

.18 

.00 

.07 

99.64 

.03 

99.61

50.31 

13.26 

3.38 

11.84 

5.16 

8.91 

2.28 

.88 

1.52 

.06 

1.60 

.15 

.24 

.09 

.02 

.06 

99.76 

.03 

99.73

51.55 

13.53 

3.31 

11.25 

5.28 

8.94 

2.03 

.55 

1.79 

.03 

1.35 

.14 

.24 

.01 

.02 

.07

.03 .

100.06

74.1 

13.3 

1.3 

1.7 

.44 

1.5 

4.1 

1.8 

1.2 

.00 

.15 

.00 

.23 

.08 

.004 

.050

100

75.2 

13.0 

.62 

1.8 

.33 

1.2 

4.5 

1.8 

1.2 

.00 

.13 

.00 

.19 

<.05 

<.001 

.030

100

75.91 

12.94 

.34 

.61 

.20 

1.17 

3.41 

4.09 

.41 

.03 

.09 

.01 

.02 

.37 

.01 

.02 

99.63 -

.01 .

99.62

7f.7 

IS. 3 

.79 

2 1

.'65 

.81 

?. 1 

ff.3 

1.2 

.05 

.15 

.08 

.06 

.08 

.004 

.078

9S

74.5 

14.0 

.17 

.36 

.17 

.31 

3.2 

5.8 

.51 

.05 

.02 

.02 

.03 

.05 

.005 

.028

99

CIPW

Quartz...- ...

Corundum. ..._._......

Orthoclase. ............

Albite.... ..............

Anorthite. ............

Halite............ ...

Wollastonite _ ---------

Enstatite _ ...........

Ferrosilite _ ...........

Magnetite. .............

Ilmenite. ..............

Apatite. .........

Fluorite ...... ..--.. .

Calcite... ..............

----- 35.01

...... 2.18

------ 7.45

----- 34.78

----- 10.29

----- .00

...... .00

...-- 1.87

------ 4.36

------ 1.91

...... .51

...... .19

--- .10

.02

38.12

1.42

7.27

40.63

6.36

.02

.00

.82

2.46

1.12

.25

.05

.12

.36

36.62

1.05

21.51

26.66

8.31

.02

.00

.97

2.92

.52

.25

.07

.10

.21

37.18

1.67

16.66

32.25

5.90

.02

.00

1.32

2.63

.88

.25

.07

.18

.09

33.93

3.60

7.92

36.13

5.00

.00

.00

3.89

6.22

.52

.49

.19

.14

.41

4.27

.00

5.20

19. 15

23.43

.03

7.86

12.85

16.75

4.90

3.04

.36

11

.20

7.58

.00

3.25

17.03

26.26

.03

6.95

13.15

16.14

4.80

2.56

.33

.13

.02

38.86

2.20

10.64

34.69

6.57

.00

.00

1.10

2.23

1.88

.29

.00

.10

.18

37.83

1.55

10.64

38.08

5.73

.00

.00

.82

2.93

.90

.25

.00

.06

.00

38.44

1.72

24.17

28.78

3.26

.02

.00

.50

.73

.49

.17

.02

.04

.84

39.61

3 73

19.50

26 23

2.46

.00

.00

1 62

S.07

1,14

.29

.19

.15

.18

32.81

2.13

34.27

27.08

.88

.00

.00

.42

.54

.25

.04

.05

.06

.11

SAMPLE LOCALITIES

BSJ-13: U.S. Highway 550, 9,920 ft alt, 0.20 mile SW. of Mill Creek. 

BSJ-17: Lime Creek road, 9,200 ft alt, 0.6 mile N. of Lime Creek Campground. 

BSJ-48: Purgatory trail in canyon of Cascade Creek at 8,040 ft alt, 0.35 mile from the Animas River. 

BSJ-113: Lime Creek road, 9,100 ft alt, 0.05 mile N. of Lime Creek Campground. 

LS-T2: U.S. Highway 550,10,260 ft alt, 1.1 mile S. of Coal Bank Summit. 

BSJ-18: Lime Creek road, 9,470 ft alt, 0.15 mile N. of La Plata-San Juan County line. 

BSJ-114: Same as BSJ-113.

BSJ-214: U.S. Highway 550, 9,880 ft alt, 0.15 mile SW. of Mill Creek. 

BSJ-216: U.S. Highway 550, 9,930 ft alt, 0.20 mile SW. of Mill Creek. 

BSJ-19: Animas River canyon, at railroad track about 0.4 mile N. of bench mark 8756. 

BSJ-217: Animas River Canyon, 0.11 mile S. of mouth of Sultan Creek, about 30 ft E. of railroad track.

BSJ-66a: East of Animas River canyon, at 12,000 ft alt, 0.51 mile N. and 0.53 mile W. of SE. corner of Silverton 7J£minute quadrangle (lat 37° 45' N., lo-?g 107° 37' W.). 

BSJ-28: U.S. Highway 550, 7, 080 ft alt, 0.6 mile S. of Shalona Lake.

tion, probably are metarhyodacites with low color indices. Their composition, incidentally, is much like that 

of some of the Twilight Gneiss. Uranium-lead isotopic 

analyses of zircon (L. T. Silver, written commun., Nov. 

1967) from this gneiss plot on a concordia diagram just 

to the right of the well-defined chord of the Twilight, 

and thus the albite gneiss appears to be slightly older. 

Small pegmatites and aplites of pale-orange microcline, quartz, and muscovite are irregularly dispersed 

in the Twilight Gneiss but are more abundant in the 

northwestern part than elsewhere. The pegmatite dikes 

range in thickness from about 1 to 30 feet, and the 

aplites from about 1 to 12 inches. Both types commonly 

are closely fractured and strained and probably formed 

before the Uncompahgran disturbance.

The contact with amphibolites of the Irving Formation on the southeast side of the Twilight Gneiss consists of a zone about 1,500 feet thick that includes 40-60 

percent of tabular masses of amphibolite in foliated 

quartz diorite. This contact is especially well exposed in 

cliffs along the Animas River between the mouths of 

Little Cascade and Grasshopper Creeks. The smaller 

slabs of amphibolite, less than about 2 feet thick, are 

broken and pulled apart, and massive plagioclasequartz aplite is in the fractures, yet the slabs show only 

slight rotation. (See fig. 11 of Cross, Ho we, Irving, and 

Emmons, 1905.) The larger amphibolite layers are cut 

by dikes of the aplite. Because the foliated interlayers 

of quartz diorite grade into the massive plagioclasequartz dikes, I infer that metamorphic conditions here
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO

granitic rocks and their inclusions from the Needle Mountains

17-19, 48,113,114; LS-T2, El. Analyst Edythe Engleman: BSJ-92-94,133, 141,158, 208. 

Artis, H. Smith, J. Kelsey, P. Aruscavage, E. Gray, and J. Budinsky: BSJ-28,53,66a,

A21

Quartz monBakers Bridge Granite zonite SE of 

Bakers Bridge

BSJ-28 BSJ-53 BSJ-210 BSJ-209 BSJ-93

Eolus Granite

Quartz Trimble 

diorite of Granite 

Pine River

BSJ-94 BSJ-141 BSJ-158 BSJ-208 BSJ-92 BSJ-133

Rhyolite AlasVite 

porphyry dikes dike

CO-199 BSJ-218 LS-E1

yses (in percent)

73.16

13.48

.63

1.40

.19

.86

3.35

5.73

.47

.04

.19

.02

.05

.16

.02

.09

99.84

.04

99.80

74.26

12.45

1.03

1.44

.15

.80

3.17

5.46

.41

.05

.26

.02

.07

.25

.01

.03

99.86

.01

99.85

74.6

12.2

.76

1.4

.27

1.5

3.1

4.7

.85

.00

.20

.00

.25

.05

.015

.15

100

76.0

11.7

.53

.88

.16

1.0

3.2

4.7

1.1

.00

.11

.00

.11

<.05

.007

.32

99

63.97

14.51

1.46

5.04

1.49

3.56

3.21

4.00

.62

.03

1.11

.33

.11

.07

.04

.20

99.75

.09

99! 66

65.86

15.00

1.52

3.60

1.22

2.18

2.93

5.11

.81

.14

.76

.23

.08

.02

.02

.12

99.60

.05

99.55

65.09

14.50

2.79

3.86

1.42

2.86

2.93

3.97

.70

.08

1.02

.32

.10

.00

.02

.17

99.83

.07

99.76

72.50

14.68

.43

1.39

.29

.72

2.96

5.99

.46

.08

.19

.04

.03

.05

.00

.13

QQ »y. Od trr

.05

99.89

61.60

14.00

2.17

7.18

1.77

3.91

3.29

2.75

1.11

.06

1.34

.33

.16

.06

.02

.09

99.84

.04

99.80

55.48

16.20

2.70

7.02

3.60

6.80

3.09

1.98

.70

.13

1.30

.48

.15

.00

.04

.14

99.81

.07

99.74

70.99

14.51

.65

1.57

.66

1.17

3.19

5.60

.67

.05

.27

.08

.04

.22

.01

.12

99.80

.05

99.75

72.9

12.9

.58

2.2

.29

1.1

3.2

5.0

1.2

.00

.20

.00

.35

.09

.005

.13

100

72.4

14.6

.90

.64

.39

1.0

4.1

4.0

1.2

.02

.22

.03

.26

.11

.002

.088

100

73.28

14.00

.41

.81

.38

.87

3.13

5.83

.50

.06

.10

.03

.05

.28

.01

.02

99.76

.01

99.75

Norms

29.66

.89

33.86

28.20

2.47

.03

.00

.47

1.83

.91

.36

.05

.18

.36

33.19

.59

32.26

26.75

2.05

.02

.00

.37

1.49

1.49

.49

.05

.06

.57

34.83

.00

27.77

26.08

5.57

.03

.19

.67

2.08

1.10

.38

.00

.31

.11

37.54

.40

27.77

27.00

2.62

.02

.00

.40

1.20

.77

.21

.00

.66

.00

18.71

.00

23.64

27.16

13.37

.00

.71

3.71

6.42

2.12

2.11

.78

.00

.16

21.54

1.28

30.20

24.79

9.19

.00

.00

3.04

4.25

2.20

1.44

.55

.00

.04

24.02

.95

23.46

24.79

12.10

.00

.00

3.54

3.28

4.05

1.94

.76

.00

.00

29.75

2.23

35. 40

25.05

2.99

.00

.00

.72

1.94

.62

.36

.10

.00

.11

18.04

.00

16.25

27.84

15.31

.00

.65

4.41

9.48

3.15

2.54

.78

.00

.14

8.66

.00

11.7

26.15

24.49

.00

2.55

8.97

8.79

3.91

2.47

1.14

.00

.00

27.44

1.78

33.09

26.99

3.89

.00

.00

1.64

1.97

.94

.51

.19

.00

.50

31.30

.80

29.55

27.00

3.94

.02

.00

.72

3.88

.84

.38

.00

.27

.20

30.93

2.27

23.64

34.69

3.42

.00

.00

.97

.55

1.31

.42

.07

.18

.25

30.80

1.74

34.45

26.41

2.21

.02

.00

.95

1.08

.59

.19

.07

.04

.64

BSJ-53: Animas River canyon, at railroad track 0.5 mile E. of Rockwood.

BSJ-210: NWKSW}£ sec. 18, T. 37 N., R. 8 W., 200 ft E. of small unnamed pond at 7,080 ft alt.

BSJ-209: SWMSE^, sec. 19, T. 37 N., R. 8 W., 400 ft NW. of Annisi Ranch.

BSJ-92: South Canyon Creek, alt 10,100 ft (\% mile SW. of Divide Lakes).

BSJ-93: Pine River trail, 9, 200 ft alt, about 0. 2 mile S. of Flint Fork.

BSJ-94: Pine River-Sand Creek divide, 0.6 mile SW. of Graham Peak, 11,910 ft alt.

BSJ-141: Pine River trail, 8,700 ft alt, 0.75 mile below Falls Creek.

BSJ-158: Slope S. of Sunlight Creek at 10,270 ft alt, 0.38 mile W. of Vallecito Creek.

BSJ-208: At road in Tank Creek, 10, 300 ft alt.

LS-E1: SWMNEM sec. 12, T. 38 N., R. 9 W., shore of Electra Lake.

BSJ-133: 150ft W. of Trimble Pass.

CO-199: SEMNEJi sec. 13, T. 37 N., R. 9 W., 7, 200 ft alt, 750 ft W. of the Animas River.

BSJ-218: Vallecito Creek trail, 150yds S. of Quartz Mill Bridges.

were so intense that the Twilight Gneiss partly melted 

to plagioclase-quartz aplite, which then intruded the 

amphibolite layers along tensional fractures.

The origin of the Twilight Gneiss deserves comment. 

This rock has undergone amphibolite-facies metamorphism, and most of the original fabric has been obliterated. The quartz ovoids or "eyes" in much of it, however, 

may be relict phenocrysts. The rock is generally similar 

in appearance in all its exposures; yet it ranges in composition from quartz diorite to quartz monzonite. The 

tabular interlayers of metamorphosed tholeiitic basalt 

and other, much less abundant rocks are generally 

parallel to the foliation of the granite; yet crosscutting 

relations on a scale of a few inches are common. The 

proportion of these slabby interlayers ranges from

about 1 percent to about 80 percent in zones 50 to 200 

feet thick; and at the contact of the Twilight Gneiss 

with the Irving Formation the proportion of amphibolite increases irregularly southeastward through a 1,£00- 

foot-thick zone to almost 100 percent.

In view of these facts, two leading hypotheses for 

the origin of the Twilight Gneiss are: (1) A complex 

of sills of porphyritic dacite to quartz latite intruded a 

flat or gently dipping pile of basalt layers, a part, of the 

Irving Formation, and metamorphism and folding of 

both produced what is now called Twilight Gneiss; or 

(2) a closely interlayered volcanic sequence of basalt 

and silicic extrusives, the latter including much crystal 

tuff, was deposited in close association with the Irving 

Formation, perhaps under marine conditions, and later
A22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

folded and metamorphosed. In both hypotheses I assume that magma of partly trondhjemitic type and 

partly more potassic type was generated in a eugeosynclinal environment, as is not uncommon. The possible 

slightly older age of the interlayers of albite gneiss in 

the Twilight Gneiss suggests that the first hypothesis is 

more likely, but the available data are certainly insufficient to rule out the second hypothesis. The fact that 

the trondhjemitic gneiss, albite gneiss, and amphibolite 

of the Twilight lie on the same isochron, however, tends 

to support the second hypothesis. Another, and less 

attractive hypothesis, is that the Twilight Gneiss is 

metamorphosed graywacke, but this possibility is not 

supported by the rubidium-strontium data.

TENMILE GRANITE

Cross and Howe (Cross, Howe, Irving, and Emmons, 1905, p. 7) named the Tenmile Granite from the 

exposures along the lower part of Tenmile Creek. This 

unit is a stock of very rough oval map pattern; the major axis trends east-northeast and is about 4.2 miles 

long, and the minor axis trends west-southwest and is 

about 2.2 miles long. The granite intrudes the Irving 

Formation. The main mass of Tenmile Granite lies no 

closer than about 0.7 mile to the nearest exposure of the 

Uncompahgran unconformity, but dikes of it are 

truncated by that unconformity.

In their petrographic description Cross and Howe 

(Cross, Howe, Irving, and Enimons, 1905, p. 8) stated:

The Tenmile rock as a whole is a rather coarse light-pink or 

gray granite, but there is an intricate mingling of granite of 

other textures, shades and colors, ranging from coarse pink 

pegmatites to fine dark-gray gneissoid rocks rich in biotite, that 

resemble some mica-diorites. The association of these rocks is 

most intimate, but notwithstanding the diversity of their megascopic characters, their mineralogical compositions show no 

marked differences.

These authors further stated that intrusion of the 

darker varieties followed very closely after intrusions 

of the lighter granite and gave as evidence the very diffuse contacts between the two types at some places. They 

interpreted that the older, lighter rock here first crystallized to an extent that it behaved coherently and then 

was both brecciated and disintegrated at its margins by 

bhe younger, darker intrusive.

The Tenmile Granite typically is pink to light gray 

to brownish gray, medium to coarse grained, and mostly 

homogeneous in outcrop. It locally contains xenoliths of 

biotite amphibolite and in some outcrops shows a faint 

to moderately well defined foliation which usually 

strikes east-west and dips vertically. Many granitic 

dikes cut the main mass of the Tenmile. In this section 

the Tenmile Granite is seen to be a severely strained

quartz monzonite in which the fresh well-thinned microcline shows marked undulatory extinction. The sodic 

oligoclase has albite twinning with fractures and pink 

bands, shows undulatory extinction, and is patchily 

altered to sericite and a brownish-gray opaque substance. The original quartz grains, whose outlines are 

preserved, have recrystallized to aggregates of much 

finer (Vio-1/^ mm) grains. The original biotite has been 

altered to chlorite, sericite, and a clay mineral. Epidote, 

garnet, allanite, apatite, and zircon are accessory minerals. The deformation and alteration are judged to 

have occurred during the Uncompahgran disturbance.

A chemical analysis of typical Tenmile Granite is 

given in table 2 and is represented in figures 11 and 12.

Radiometric age determinations on the light-colored 

variant of Tenmile Granite are 1,720 ±20 m.y. by the 

uranium-lead method on zircons (Silver and Barker, 

1968) and 1,724±55 m.y. by the rubidium-strontium 

whole-rock method (Bickford and others, 1968). These 

two results show a remarkably good agreement.

WHITEHEAD GRANITE

Cross and Howe (Cross, Howe, Irving, and Emmons, 

1905, p. 8) named the Whitehead Granite from exposures of several small stocks and a multitude of dikes 

in and about Whitehead Gulch, a tributary to the Animas River about 4 miles south of Silverton. They classified all the granitic dikes and small stocks ir the Irving 

Formation north of the Uncompahgre Formation as 

Whitehead Granite. Their petrographic description of 

it, as a pink or light-red granite containing only "a 

little oligoclase," grossly applies to the stocks and to 

some, but not all, of the dikes.

In this study of Cross and Howe's Whitehead Granite 

two rock types were found. An earlier type, in generally 

concordant and lenticular bodies, is a strongly foliated 

or lineated, fine- to medium-grained quartz monzonite, 

granodiorite, and light-colored quartz diorite; the oligoclase-albite is extensively altered to sericite f.nd a brown 

clay, and it contains small amounts of chlorite and 

epidote as well as biotite. A later type is a massive 

medium- to coarse-grained biotite quartz monzonite. In 

view of the dual nature of the formation, I abandon the 

name Whitehead Granite.

The earlier type of dike underwent first deformation 

and metamorphism with the Irving Formation and 

later retrograde metamorphism apparently in the Uncompahgran disturbance. The second type was intruded 

after metamorphism of the Irving and was later affected 

by the Uncompahgran disturbance. A sample of the 

earlier type gave a uranium-lead age on zircons of 

1,780±20 m.y. (Silver and Barker, 1968), and a sample
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A23

of the later type gave a uranium-lead age on zircons of 

1,720±20 m.y. (Silver and Barker, 1968) and a 

rubidium-strontium whole-rock isochron of 1,695 ±65 

m.y. (Bickford and others, 1968).

A gneissic quartz monozonite dike of the earlier 

type was chemically and spectrographically analyzed. 

In chemical composition (table 2) and trace-element 

content determined spectrographically (Barker, unpub. 

data), the dike is very similar to the Twilight Gneiss. 

Thus, this earlier type of dike, included in Wllitehead 

Granite by Cross and Ho we (Cross, Ho we. Trviiig, and 

Emmons, 1905), is of the same age, petrographic and 

chemical type, and geologic occurrence as the Twilight 

Gneiss.

The younger dikes are similar to the several stocks 

of biotite quartz monzonite shown by Cross and Howe 

(Cross,Howe, and Ransome, 1905; Cross, Howe, Irving, 

and Emmons, 1905), and the largest, dike is shown in 

figure 2. One of the dikes that cuts the Irving Formation and is truncated by the sub-Uncompahgre 

unconformity was sampled 75 feet north of that unconformity. This rock, which gave a uranium-lead age of 

1,720 m.y., is greenish gray with flecks of orange microcline, medium to coarse grained, and massive. In thin 

section, the oligoclase-albite is seen to have bent and 

broken albite twinning, and the microcline has extreme 

undulatory extinction. Apparently original quartz 

grains were recrystallized (or annealed) to aggregates 

of many smaller, polygonal grains with sharp extinction, and original mafic minerals were completely 

altered to aggregates and veinlets of brown opaque 

material, epidote, and chlorite. Some of the plagioclase 

and microcline have mortar structure. This dike is a 

quartz monzonite, similar in bulk composition to the 

Tenmile Granite.

The stock shown as Tenmile Granite in figure 2 about 

1.5 miles north of the Uncompahgre synclinorium consists of massive quartz monzonite. The quartz in this 

rock is recrystallized to aggregates of smaller grains. 

The original oligoclase is albitized, heavily dusted with 

clay, and partly replaced by 1- to 2-mm grains of 

muscovite; the original biotite is now a mass of finegrained chlorite. Irregular fractures cut across the rock, 

and the rock along the fractures contains crushed feldspars and much secondary chlorite. At this distance 

from the sub-Uncompahgre unconformity and phyllonite, the bulk of the feldspars in the rock are not conspicuously strained or fractured. In a chemical analysis 

this rock (table 2, sample BSJ-66a) is similar to the 

Tenmile Granite except for its anomalously low content 

of CaO, 0.31 percent, and its relatively high content of 

K2O, 5.8 percent. These variations are believed to result 

from alteration during the Uncompahgran disturbance.

These dikes and stocks of the Wllitehead Granite are 

similar to the Tenmile Granite to the south in their 

geologic occurrence, radiometric age, chemistry, and 

petrography. These features indicate a common origin. 

Cross and Howe (Cross, Howe, Irving, and Emmons, 

1905, p. 8) suggested that the Whitehead and Tenmile 

Granites were contemporaneous and "derived from the 

same source."

BAKERS BRIDGE GRANITE

The stock of massive pale-red medium- to coarsegrained granite and alaskite that is exposed in the 

Animas River valley at Bakers Bridge, Shalona Lake, 

Rock wood, and vicinity is here named the Bakers 

Bridge Granite. Its type locality is at Bakers Bridge 

in the Hermosa 71/£-niinute quadrangle. About 3 square 

miles of this granite is exposed. It passes under Paleozoic rocks to the west and alluvium to the south.

Most of the Bakers Bridge Granite is seriate inequigranular, coarse grained, massive, and homogeneous. 

Grains 5-15 mm in size are predominant, but some 

varieties have fine-grained matrices and plagioclasemantled microcline phenocrysts 15-20 mm in maxirium 

dimension, and others have relatively even-grained 

fabrics with 1-5 mm grains. Swarms of inclusion^ of 

amphibolite, presumably from the Irving Formation, 

occur in the granite east and northeast of Rockwood. 

The Bakers Bridge Granite is composed mostly of 

perthite and quartz and contains less than 10 percent 

each of plagioclase and dark minerals in most of the 

stock. Allanite and fluorite are accessory minerals.

In thin section the rock's characteristic brick-red 

microcline is seen to be a perthite of both flame and 

blocky types containing 20-50 percent albite; the microcline shows excellent grid twinning, whereas only the 

blocky albite shows albite twinning. Myrmekite lies at a 

few of the microcline-microcline and quartz-microcline 

boundaries. Quartz is present both as single grains and 

as aggregates of several to a dozen small grains. The 

aggregates probably formed by recrystallization of original, larger grains; both simple grains and aggregates 

show pronounced undulatory extinction. The quartz 

ranges from light gray to pale blue to pale violet-. 

Plagioclase is mostly anhedral, but subhedrons, both 

stubby and lathlike, are not uncommon. Its composition 

is calcic to median andesine; clear rims of sodic albite 

surround many grains. Grains with faded, peri aps 

original, twinning and grains with sharp twinning that 

may be secondary may be seen in the same thin section; 

the cores of most grains have been irregularly 

altered to clay and sericite, and these areas may 

originally have been more calcic than albite. The 

ferromagnesian minerals in the Bakers Bridge
A24 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Granite are iron rich, for these rocks contain only 

0.15-0.27 percent MgO, as shown in table 2. The only 

original mafic silicate is a hornblende with X=grayish 

green and Z brownish green, and only one partially 

preserved hornblende grain was seen. The major alteration products of this brownish-green hornblende are 

biotite with Z=medium to dark green, hornblende with 

Z=blue-green, and tabular ilmenite that in polished 

sections is seen to contain finely disseminated hematite. 

The biotite along with a little of the primary hornblende, was altered to ferristilpnomelane that is pleochroic golden yellow to various red-brown tints. The 

ilmenite-hematite has been altered to leucoxene and 

ilmenite. Ilmenite also occurs separately from these 

mafic silicates, in what probably are primary grains of 

euhedral zircon. Other alteration minerals include epidote and chlorite associated with biotite, talc and hornblende, and sphene as rims on ilmenite. The major 

sequence is:

brownish-green hornblende ^biotite -fblue-green hornblende -f- ilmenite, 

and: 

brownish-green hornblende -f- biotite »stilpnomelane.

Whether any or all of this alteration was coincident 

with the Uncompahgran disturbance is problematical.

Most of the granite contains only 3-8 percent of dark 

minerals. A mafic variety comprised of about 60 percent microcline perthite, 25 percent ferromagnesian 

silicates, 10 percent quartz, 5 percent magnetite-ilmenite with rims of sphene, and 1 percent albite forms a 

20-foot-thick segregation 150 feet south of the northern 

contact of the stock along the tracks of the Denver and 

Rio Grande railroad. The contacts of this mafic variety 

are gradational over 2-4 inches with the quartzose alaskitic granite. The mafic silicates, whether originally 

hornblende or clinopyroxene, are now aggregates of 

actinolite, epidote, anthophyllite, chlorite, and stilpnomelane.

Chemical analyses of three specimens of Bakers 

Bridge Granite are given in table 2. BSJ-28 and BSJ53 are of the seriate inequigranular variety, and BSJ210 is of the type with blocky microcline phenocrysts 

set in a fine-grained matrix. All three are very similar 

in composition except that BSJ-210 has about 0.7 to 

1.0 percent less of K2O. This granite is characterized 

by very low proportions of CaO, as shown in figure 12. 

In figure 11 the normative minerals of Bakers Bridge 

Granite plot closer, as a group, to the granite minimum 

than any other granitic rock of the Needle Mountains.

Silver and Barker (1968) reported a uranium-lead 

age for zircons from the Bakers Bridge Granite of 

1,720±20 m.y., and Bickford, Barker, Wetherill, and

Lee-Hu (1968) obtained a whole-rock rubidium-strontium isochron of 1,692±78 m.y.

The southeastern part of the area of Bakers Bridge 

Granite includes a crosscutting pink massive mostly 

medium grained seriate inequigranular quartz monzonite. Whether or not this rock is cogenetic with the 

Bukers Bridge Granite is problematical. Thi^ rock consists of about 40 percent microcline microperthite, 30 

percent quartz, 25 percent calcic albite, 5 percent biotite 

that has been mostly altered to chlorite, and traces of 

muscovite and fluorite. A chemical analysis of it (sample BSJ-209, table 2) is almost identical with that of 

a sample of Bakers Bridge Granite (BSJ-210), the 

variety containing large phenocrysts of microcline in a 

fined-grained groundmass. An age determination of 

uranium and lead isotopes in two samples of the zircon 

from BSJ-209 gives an apparent age of 1,631 m.y. 

(L. T. Silver, written commun., Nov. 1967). There is a 

possibility, which will not be discussed here, that this 

zircon has been disturbed, perhaps by Eolus Granite 

lying to the east.

QUARTZ DIORITE OF PINE RIVE^

A mass of quartz diorite about 4 square miles in 

area lies northwest of upper Pine River at and south 

of the latitude of Divide Lakes. Larsen and Cross (1956, 

p. 29-30) briefly described this body anc1 called it 

"Granodiorite of the Pine River batholith.r However, 

potassic feldspar is developed only along tl °, margins 

of this body, and the genetic relationship of this body 

to the Eolus-Pine River batholith is probrble rather 

than proved, so this mass is here called the quartz diorite of Pine River. The quartz diorite mass is oval in 

plan, and its axes are about 2.5 and 1.8 miles long. It lies 

entirely within the Pine River batholith of Eolus Granite, and it is older than the Eolus, as shown by crosscutting relations and xenoliths. Several bodies of similar 

rock lie in Eolus Granite west of Vallecitc Creek, as 

reported by Larsen and Cross (1956, p. 29).

In hand specimen the quartz diorite of Pine River is 

gray to brownish gray, massive, medium to coarse 

grained, and homogeneous except for the marginal variety that contains scattered dents de cheval of microcline microperthite. The rock consists of approximately 

55-60 percent andesine, 15-20 percent quartz, 15 percent biotite, 7-8 percent hornblende, 2-3 percent, clinopyroxene, and 1-2 percent total of magnetite-ilmenite, 

apatite, and sphene. The plagioclase is fresh except for 

spotty sericitization, shows both albite and pericline 

twinning, and occurs as blocky, lathlike, and irregularly shaped grains. Quartz shows only mild undulatory 

extinction. Biotite is in fresh irregularly shaped anhedra; it is pleochroic from pale golden brown to deep
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A25

red brown. Hornblende in formless anhedra is fresh, 

commonly is twinned, has Z=green, and forms thick 

rims on ragged cores of colorless clinopyroxene. The 

quartz diorite of Pine River is a relatively unstrained 

rock and clearly is younger than the Uncompahgran 

disturbance. A chemical analysis of it is given in table 2. 

On the plots of normative quartz and feldspar in figure 

11 and on the Na2O-K2O and Na2O-CaO diagrams in 

figure 12, the quartz diorite is seen to lie along the trend 

of the Eolus Granite, which is highly suggestive that the 

quartz diorite was the first major intrusive of the Eolus 

batholith.

EOLUS GRANITE

The Eolus Granite is the most widespread plutonic 

rock in the Needle Mountains and forms two composite 

batholiths. It was named by Cross and Howe (Cross, 

Howe, Irving, and Emmons, 1905, p. 8) from 14,084- 

foot-high Mount Eolus (aptly named after the ancient 

Greek god of the winds), which is the second highest 

peak in the Needle Mountains uplift.

About 100 square miles of Eolus Granite is exposed 

in the southern Needle Mountains between Vallecito 

Creek and the east rim of the canyon of the Animas 

River; superb exposures are in the drainage basins of 

the Florida River and Johnson, Needle, and Sunlight 

Creeks. This mass extends southward under the Paleozoic sedimentary rocks for an unknown distance. Another mass of this granite, almost as large, underlies 

most of the drainage basin of the Pine River above the 

Lake Fork (fig. 13) and extends eastward from the 

area shown in figure 2 into the upper reaches of Sand, 

Little Sand, and Weminuche Creeks. This body extends 

southeastward under Paleozoic and Mesozoic sedimentary rocks and is exposed along the Piedra River and 

lower Weminuche Creek about 8 miles from the main 

mass (Larsen and Cross, 1956, pi. 1). The two intrusives 

are here called the western and eastern bodies, respectively. Larsen and Cross (p. 29-31) called both of these 

bodies the Pine River batholith, which practice is not 

followed here, and described several varieties of the 

granitic rocks in them. A small mass of Eolus Granite 

cuts massive quartzite of the Uncompahgre Formation 

along Leviathan Creek. Dikes of Eolus-like granite and 

rhyolite porphyry cut schists of the Uncompahgre Formation in the Flint Fork drainage and metavolcanic 

rocks of the Irving Formation along Vallecito Creek. 

However, apart from these occurrences, minor intrusions of Eolus type are rare. The name Eolus Granite 

is here used to include all variants of these intrusive 

masses. Several of these variants are briefly described 

here, but it is to be kept in mind that this study is of a 

reconnaissance nature.

The bulk of both the western and the eastern bodies

FIGURE 13. Southward view of lower Flint Fork, showing Popes 

Nose. Entire area shown is underlain by Precambrian Eolus 

Granite except the slope in the middle skyline, which consists 

of Tertiary volcanic rocks.

of Eolus Granite is fresh mostly massive gray to pink 

to brick-red biotite-hornblende quartz monzonite. Conspicuous y2-~ to 1%-inch grains of pink blocky carlsbadtwinned microcline microperthite (fig. 14) are set in a 

medium- to coarse-grained matrix of andesine, quartz, 

biotite, and hornblende. The andesine is blocky and 

median to sodic. The quartz is gray to pale-blue and 

commonly is in clusters of equant grains. The biotite is 

tabular to irregularly shaped and is pleochroic from 

pale golden or olive brown to dark brown. The hornblende is in ragged anhedral grains with pleochroism 

X = pale green-brown, Y = light brown, and Z = green. 

Accessory minerals are ilmenite rimmed with sphene, 

and, in minor proportions, zircon, apatite, and allanite. 

Typically, 12-20 percent of hornblende and biotite is 

present, */£-% of it biotite. This rock is homogeneous 

except in the zones of inclusions in the eastern body, 

where it grades into magnetite-bearing granodiorite. 

The inclusions are found miles from the walls in swarms 

of lenticular individuals that are 1-3 inches thick and 

3-8 inches in diameter. They consist of the same minerals 

as the enclosing granodiorite but have greater proportions of biotite, hornblende, ilmenite, magnetite, and 

other accessory minerals. These inclusions apparently 

were equilibrated with the magma; their origin, however, is unknown. Parts of the quartz monzonite and
A26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

FIGURE 14. Eolus Granite. Variety shown is coarse biotitehornblende quartz monzonite with blocky microcline microperthite grains % to 1% inches long, and with several dark 

inclusions of biotite-hornblende gneiss.

granodiorite show a foliation of the microcline subhedra: In the western body along parts of its margin, 

and in the eastern body in scattered areas in its interior, 

especially in the zones of inclusions, but rarely along its 

margin.

The biotite-hornblende quartz monzonite was intruded by biotite quartz monzonite and biotite granite, 

especially in the Florida River drainage. The biotite 

quartz monzonite is generally similar to the predominant quartz monzonite except that hornblende is absent; 

the microcline is tabular rather than blocky and is 

poikilitic with V10- to i/^-mm grains of plagioclase and 

quartz; little or no ilmenite and sphene are present; and 

the rock contains only 5-10 percent of biotite. Biotite 

granite crops out in the canyon of the Florida River 

and to the east near Stump Canyon Lakes. In 

the Florida River area the granite is a deep-pink 

homogeneous massive and fine- to medium-grained 

rock. In the Stump Canyon Lakes area the rock is also 

deep pink, homogeneous, and massive, but it consists of 

about 60 percent phenocrysts of tabular to blocky microcline, 5-18 mm in size, set in a medium-grained matrix 

of quartz, calcic oligoclase, and biotite. A brick-red 

microcline-rich biotite granite also intruded the eastern 

mass of Eolus Granite (Larsen and Cross, 1956, p. 30).

The two masses of Eolus Granite also contain dark 

variants. The western body at Mountain View Crest

and southwestward to Tank Creek contains a granodiorite with 30-35 percent hornblende and biotite combined. The minerals in this rock are very similar to 

those of the predominant biotite-hornblende quartz 

monzonite, but the proportions are different. It is not 

known if this mafic granodiorite is cut by the quartz 

monzonite or not, but its time of crystallization as determined by isotopic methods is indistinguishable from 

that of the quartz monzonite, as discussed below. 

The eastern mass at Flag Mountain contains a dark 

granodiorite; this is a gray massive rock in which 10-20- 

mm blocky microcline phenocrysts are set in a mediumgrained matrix. This dark granodiorite is cut by the 

biotite-hornblende quartz monzonite. An estimated 

mode of this grandiorite is 45-50 percent andesine, 15 

percent each biotite and quartz, 10 percent hornblende, 

8 percent microcline, 3 percent ilmenite, and 1 percent 

sphene.

Five chemical analyses and CIPW norms of three 

varieties of Eolus Granite are given in table 2. These 

varieties are the biotite-hornblende quartz monzonite, 

BS J-93, -94, and -141; the dark variant of Tank Creek 

that contains much hornblende and biotite, BSJ-208; 

and the hornblende-free biotite granite, BSJ-158. These 

five norms, together with that of the cogenetic quartz 

diorite of Pine River, show in figure 11 a well-marked 

trend from a composition rich in normative plagioclase 

to one in the theoretical minimum melting trough.

The eastern contact of the western body of Eolus 

Granite is shown by Cross and Howe (Cross, Howe, 

Irving, and Emmons, 1905) as a fault. This contact was 

examined in detail by me west of Vallecito Creek in the 

vicinity of lat. 37°30', and was found to be intrusive. 

Amphibolite of the Irving Formation is brecciated by 

the granite, and jumbled fragments of the Irving from 

a few inches in size to slabs 4 feet thick and 30 feet long 

in a matrix of hybrid granite form a marginal zone 

100-200 feet thick. The Irving Formation along this 

contact contains many dikes of Bolus-like granite and 

rhyolite porphyry in a zone extending ^-l mile east 

of the batholith.

The eastern body of Eolus Granite sharply crosscuts 

quartzite and schist of the Uncompahgre Formation as 

well as the Vallecito Conglomerate and Irving Formation. Furthermore, this contact of the granite with the 

Vallecito is linear (fig. 2), possibly owing to intrusion 

along a fault.

Radiometric age measurements have been made on 

both masses of Eolus Granite. Silver and Barker (1968) 

reported an age of the western body, by uranium-lead 

isotopic methods, of about 1,460 ±20 m.y. Bickford, 

Barker, Wetherill, and Lee-Hu (1968) obtained a 

rubidium-strontium whole-rock isochron from samples
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A27

of both bodies of 1,466 ±27 m.y. and mineral isochrons 

of approximately 1.400 m.y. E. E. Zartmaii (written 

commun., Jan. 1966) made potassium-argon age determinations on two minerals from the eastern batholith 

of the Eolus: the hornblende age is 1,460±70 m.y., and 

the biotite age, 1,390 ± 40 m.y. Thus one may conclude 

that these batholiths were emplacecl about 1,460 m.y. 

ago and that they experienced a minor thermal disturbance 60 or 70 m.y. later.

ELECTBA LAKE GABBBO

The mass of gabbroic and related rocks that has intruded the Twilight Gneiss and Irving Formation at 

the northern half of Electra Lake and extends southeastward to the Ardmas Elver is here named the 

Electra Lake Gabbro. The type locality is 011 the east 

shore of the north half of the lake. This intrusive was 

first mapped and briefly described by Cross (Cross and 

Hole, 1910, p. 3 4). It is roughly triangular in plan, 

about 2i/£ miles along its southern, east-trending base 

and about 214 miles along its western, north-trending 

side. Paleozoic sedimentary rocks overlie it on the west, 

so its extent in that direction is unknown. The Electra 

Lake Gabbro sharply crosscuts its wallrocks; in some 

places, as along the Animas River, the contact is nearly 

planar and vertical; at other places many small dikes 

of gabbro intrude the wallrock near an irregular main 

contact. Dikes of pink to red alaskite and granophyre, 

quartzo-feldspathic pegmatite, and hornblende-bearing 

pegmatite cut the gabbroic rocks and apparently are 

genetically related to it, as first suggested by Cross 

(Cross and Hole, 1910, p. 4). Bickford, Barker, Wetherill, and Lee-Hu (1968) reported a rubidium-strontium whole-rock isochron for this gabbro of 1,454 ±50 

m.y. One of these dikes, of mixed alaskite and granophyre, gives a uranium-lead age of 1,460±20 m.y. (Silver and Barker, 1968), which is essentially identical 

with that of the gabbro.

The Electra Lake Gabbro is markedly heterogeneous. 

Some individual outcrops 100 feet in maximum dimension contain gabbro, hypersthene gabbro, augite-biotite 

diorite, and even quartz diorite that bears a few percent 

microcline. The various rock types have gradational 

contacts with one another. The complete range of rock 

types extends from olivine gabbro to biotite-augite 

granodiorite. Gabbro with augite and median or sodic 

labradorite as major constituents and with minor hornblende, biotite, ilmenite, and magnetite is perhaps the 

most abundant single petrographic variety in the intrusive. These rocks are all massive, medium to coarse 

grained, of various brown to dark-gray shades, and, 

with one exception, relatively fresh and free from secondary sericite and other fine-grained minerals. That

exception is in the easternmost area of this intrusive, in 

the canyon of the Animas River, where plagioclase has 

been extensively altered to sericite, clay, and epidote, 

and biotite and augite are moderately to wholly chloritized. These effects may be due to effects of the Eolus 

Granite, which lies about one-eighth mile east of the 

gabbro. Actually, the relative order of intrusion of the 

Electra Lake Gabbro and the Eolus Granite is not 

known, for the isotopic age methods do not have the 

necessary resolution. Plagioclase has crystallized mostly 

as subhedral laths, some of which enclose small grains 

of augite. Augite and hypersthene are stubby subhedral 

to anhedral prisms. Hornblende rims on augite are common, but biotite forms discrete grains of very irregular 

shape. Microcline shows excellent grid twinning and 

occurs as blocky subhedra and anhedra that have grown 

between plagioclase and biotite grains. Quartz is interstitial to all other minerals.

The dike of mixed fine- to medium-grained alaskite 

and granophyre that crops out on the west shor^- of 

Electra Lake is 10-12 feet thick, strikes about N. 10° E., 

and dips vertically. This rock consists of about 55 percent microcline, 30 percent quartz, 15 percent albite, and 

traces of muscovite, magnetite, and chlorite. A chemical 

analysis of this rock (sample LS-E1, table 2) also shows 

it to be of granophyric composition, although perhaps 

more potassic than average, containing 5.83 percent 

K2O.

TBIMBLE GBANITE

Cross and Howe (Cross, Howe, Irving, and Emmons, 

1905, p. 8) named the Trimble Granite from its exposures at Trimble Pass. This granite underlies approximately 5 square miles of Vallecito Basin, East Silver 

Mesa, Missouri Gulch, and the area from Florida Mountain to Castelleia Lake. Its contacts are well definec1 except where many dikes extend outward into the Eolus 

Granite.

The Trimble Granite is a pale-pink to light-brick-red 

massive homogeneous fine- to medium-grained granite. 

It commonly is porphyritic and contains 1-5 percent 

blocky to tabular %- to 1-inch-long microcline ph?,nocrysts. In thin section the rock is seen to consist largely 

of fresh blocky anhedral microcline, ^-3 mm in size 

and showing superb grid twinning and rare Carlrbad 

twinning, and anhedral irregular quartz grains, mostly 

l%-3 mm in maximum dimension. The larger microcline grains are poikilitic with randomly oriented subhedral grains of plagioclase %o--/4 mm in diameter. 

Calcic oligoclase to sodic aiidesine forms 10-15 percent 

of the rock, is blocky subhedral to equant anhedral, 

shows excellent albite twinning except where altered, 

and commonly is slightly to intensively altered to clay 

and sericite. Biotite is present as thick tablets, is pleo-
A28 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

chroic golden light brown to dark olive green, forms 

6-10 percent of the granite, and in some specimens is 

entirely altered to green chlorite, white clay, and sericite. Trace amounts of anliedral interstitial muscovite 

are present. Original accessory magnetite and ilmenite 

were altered to hematite and leucoxene, respectively. 

The ubiquitous nature of the alteration of the plagioclase, biotite, and opaque oxides suggests that this alteration is deuteric type. A chemical analysis of a specimen 

from Trimble Pass, BSJ-133, is given in table 2. As it 

contains 71 percent SiO2 , 5.60 percent K2O, and 3.19 

percent Na2O, this rock is not greatly different from the 

BSJ-158 type of Eolus Granite (table 2).

The Trimble Granite has intruded the Eolus Granite 

and therefore is younger. A rubidium-strontium wholerock age determination by Bickford, Barker, Wetherill, 

and Lee-Hu (1968) gives approximately 1,350±50 m.y.

MINOR INTRUSIVE ROCKS

Several types of intrusive rocks noted during this 

investigation occur in bodies too small to be shown in 

figure 2. For completeness of presentation, these varied 

rock types are briefly described.

METADACITE

The Vallecito Conglomerate is cut by a dike of metadacite in the southernmost area of its exposure on the 

west side of the Vallecito valley. The dike is fine grained, 

massive, and homogeneous and consists of sericitized 

plagioclase, quartz, biotite, pale-blue-green hornblende, 

epidote, and magnetite. The dike is mostly 3-7 feet 

thick, and it sharply crosscuts the bedding of the Vallecito at about right angles. It can be traced for about 500 

feet.

RHYOLLTE POEPHYEY DIKES

Dikes of rhyolite porphyry about 1-15 feet thick cut 

the Bakers Bridge Granite (Cross and Hole, 1910, p. 4) 

north of the latitude of Bakers Bridge and cut the Irving Formation along Vallecito Creek south of Quartz 

Mill bridges.

A 10- to 15-foot-thick dike in the Bakers Bridge 

Granite that was selected as typical of its kind is about 

two-thirds phenocrysts and one-third groundmass that 

is deep buff pink, massive, homogeneous, and fresh. The 

phenocrysts are approximately 45 percent quartz, 40 

percent microcline microperthite, 10 percent oligoclase, 

and 5 percent pseudoinorphs of chlorite after biotite (?). 

The quartz, which is bipyramidal, shows resorbed crystal faces and is 2-3 mm in maximum dimension. The 

microcline microperthite is fresh, subhedral to euhedral, 

and 2-8 mm in size, commonly shows carlsbad twinning, 

and contains 10-20 percent albite in visible streaks. The

oligoclase is subhedral, 1-3 mm in maximum dimension, 

and faintly zoned; it shows mild to moderate sericitization and mostly albite twinning and rarely carlsbad 

twinning. The groundmass consists of a mosaic fabric of 

%Q- to i/io-nim grains of microcline and quartz, with 

minor, scattered sericite, chlorite, opaque minerals, zircon, and rutile. A chemical analysis of a specimen of 

this rock, CO-199, is given in table 2, and is notable for 

its relatively low content of ALO3 , 12.9 percent, and its 

high content of FeO, 2.2 percent. Both values set it 

apart from the rhyolite porphyry dike of Vallecito 

Creek. This analysis, however, is similar to that of 

Bakers Bridge Granite (BSJ-28, -53, and -210). The 

rhyolite dikes in the Bakers Bridge Granite, though, 

apparently are much younger than the granite, for they 

yield a rubidium-strontium whole-rock age of 1,314±38 

m.y. (Bickford and others, 1968).

The dike of rhyolite porphyry that cuts the Irving 

Formation in Vallecito Creek is about 15 feet thick 

and is similar both in gross appearance and in thin section to the dike, specimen CO-199, in the Bakers Bridge 

Granite. This rhyolite porphyry in Vallecito Creek has 

about 60 percent phenocrysts, of which 40 percent is 

microcline, 35 percent is quartz, 20 percent is median 

albite, and 5 percent is a mixture of chlorite, muscovite, 

and limonite pseudomorphous after biotite. Kagged 

muscovite grains are disseminated througl the rock, 

and some of these cut both phenocrysts and groundmass 

and, thus, may be of secondary origin. The chemical 

analysis of this rock, BSJ-218, is given in table 2. The 

analysis is similar to that of Eolus Granite (BSJ-158) 

except for its relatively low content of K2O 4.0 percent. The low content, however, may be th°> result of 

loss of K2O during the formation of the secondary 

muscovite. Better evidence for believing that this rhyolite porphyry is genetically related to the Eolus batholith is that uranium and lead isotopes of t^vo samples 

of its zircon plot on a remarkably linear 1,460-m.y. 

chord (L. T. Silver, written conimun., Nov. 1967) on a 

concordia diagram. A rubidium-strontium whole-rock 

determination of this dike, however, by Bicl'ford, Barker, Wetherill, and Lee-Hu (1968) lies on the same 

1,314-m.y. isochron as the rhyolite porphyry dikes in the 

Bakers Bridge Granite. The problem of discordance of 

these rubidium-strontium ages is not yet resolved.

APLITE AND PEGMATITE

Dikes of aplite and pegmatite are scattered throughout the various granitic rocks and the Irving Formation. 

They are rare to nonexistent, however, in large tracts 

of the Vallecito Conglomerate and Uncomp^hgre Formation, perhaps because these formations are much
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A29

stiffer and were resistant to igneous intrusion. These 

dikes were intruded at various times: some are of Twilight age, others are of Tenmile and Eolus ages, and still 

others are apparently only about 1,314 m.y. old (M. E. 

Bickf ord, G. W. Wetherill, Fred Barker, and Chin-Nan 

Lee-Hu, unpub. data), the same age that Bickf ord, 

Barker, Wetherill and Lee-Hu (1968) obtained for the 

two rhyolite porphyry dikes. They were not studied 

in detail but are mentioned separately because of their 

areally wide range.

MAFIC QUARTZ SYENITE OF TJTE CREEK

Biotite-hornblende quartz melasyenite is exposed in 

the valley of Ute Creek at and around Black Lake, as 

mentioned by Larsen and Cross (1956, p. 29). This 

syenite is northeast of the Needle Mountains and so is 

mentioned here only briefly. The syenite body is about 

1 to 2 miles in size and is bounded on three sides by 

Tertiary volcanic rocks, which unconformably overlie 

it. On the fourth, northeast, side it appears to have an 

intrusive contact into massive gray quartzite. This 

quartzite is lithologically similar to extensive outcrops 

of quartzite of the Uncompahgre Formation that lie 4 

to 5 miles to the south and west. The melasyenite is a 

dark-greenish-gray massive medium- to coarse-grained 

rock and consists of 40-50 percent biotite, hornblende, 

and augite and 50-60 percent microcline, microperthite, 

plagioelase, and quartz. Isotopic age determinations indicate that this intrusive is of Eolus or immediate postEolus age (K. F. Marvin, written commun., Feb. 1968).

BIOTITE QUARTZ MONZONITE NEAR LAKE SILEX

Biotite quartz monzonite has intruded slate and 

quartzite of the Uncompahgre Formation at the 12,800- 

foot saddle that lies 0.4 mile west of Lake Silex. Exposures are poor to good; the eastern parts of the dike 

are covered with talus. This body is lenticular, has contacts mostly parallel to the slaty cleavage of the enclosing slate, and is several hundred feet long and about 

60 feet thick. Randomly oriented inclusions of both 

slate and quartzite are common in this intrusive, and 

the quartzite inclusions have rims of biotite 1-2 mm 

thick. The quartz monzonite has an unusual fabric in 

which 2- to 4-mm grains of blocky andesine and aggregates of ragged %-mm biotite grains contain finer interstitial microcline, quartz, micropegmatite, biotite, and 

epidote. The plagioclase is heavily sericitized. Both the 

fabric and the association of andesine with micropegmatite in such a salic rock are unusual. The aiidesinemicropegmatite association, in particular, may be the 

result of assimilation of slate. The absolute age of this 

intrusive is not known; all .that is known is that it postdates folding and metamorphism of the Uncompal gre 

Formation.

LAMPROPHYRE DIKES

Cross raid Ho we (Cross, Howe, Irviiig, and Einmons, 

1905, p. 8) stated:

Dikes of mica-syenite or minette are found in considerable 

abundance in the northern third of the [Needle Mountains] 

quadrangle, cutting the schists, granites, and quartzites, the 

greater number being found in the Uncompahgre formation. A 

few occur as far south as Emerson Mountain and the Qvartz 

Mill. They are commonly found intruding the softer shales and 

slates of the Uncompahgre, and in their field occurrence are 

mainly remarkable for their variability in thickness, which in 

few cases exceeds 10 feet, and for their lack of continuity. The 

occurrences in the region of the Continental Divide and ir the 

neighborhood of Elk Park are the most noteworthy.

These dikes are not known to cut Upper Cambrian or 

younger rocks, and they thus may be of Middle or Early 

Cambrian age, or of Precambrian age. A thin section 

from one of these dikes consists of almost equal amounts 

of microcline, green biotite, and clear amphibole that 

shows lamellar twinning and secondary calcite, serpentine, and chlorite.

DIABASE DIKES

Dikes of diabase, mostly a few feet to about 15 feet 

thick, occur in Precambrian rocks of the Needle Mountains, as described by Cross and Howe (Cross, Howe, 

and Bansome, 1905; Cross, Howe, Irving, and Emmons, 

1905). The most noteworthy occurrence is in the Irdng 

Formation from the Animas Canyon near Deer Park 

Creek and Whitehead Gulch eastward to the headwaters 

of Deep Creek. These dikes were not studied in detail by 

me. They may be of Cambrian age, for Hansen and 

Peterman (1968, p. C87) found that the diabase dikes in 

Precambrian rocks of the Black Canyon of the Gunnison River are about 510 m.y. old.

STRUCTURAL GEOLOGY

In this reconnaissance investigation observations of 

geologic structures were neither detailed nor systematic. 

However, some major aspects of the structure are discussed here.

FOLDS OF VALLECITO CONGLOMERATE AND IRVING 

FORMATION

The sharp lithologic contrast between the Vallecito 

Conglomerate and the Irving Formation and the crossstratification in the conglomerate that indicates tops 

and bottoms enable the noses of two folds to be deHneated. One of these folds is immediately west of Emerald 

Lake, and the other is about 2.2 miles west of that lake 

and just northeast of Table Mountain. These folds originally were anticlines but the western one, at taast,
A30 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

probably now is an inverted anticline. The VallecitoIrving contact is not sufficiently well exposed around 

the nose of this fold for determination of the attitude 

of its axis but axes of minor folds in the conglomerate 

core of this fold plunge about 45° at S. 10° W. Thus, if 

the axes of the minor folds parallel the axis of the major fold, this structure is an inverted anticline. The 

eastern fold has a post-Mississippian fault along its 

western flank, and the small part of the nose lying 

between this fault and the lake to the east was not examined. These folds were formed during amphibolite-facies metamorphism of the Irving Formation and were 

later rotated during the Uncompahgran disturbance. 

The eastern fold was rotated perhaps as much as 90°, 

and the western one by an unknown amount, for the unconformity between the Irving and Uncompahgre Formations east of Emerald Lake was rotated about 90°. 

The Irving Formation adjacent to these two folds shows 

only its prominent north-south foliation.

TRENDS IN THE IRVING FORMATION

Trends of foliation and compositional banding in the 

Irving Formation are shown in figure 2. In the southeastern area of exposure these trends are from north to 

about N. 20° E., except northwest of Emerald Lake, 

where they are roughly east and dips are variable. This 

eastern trend apparently was formed during the Uncompahgran disturbance and is superimposed on the 

older, north trends.

The southwestern mass of the Irving Formation has 

northeast trends, which were not displaced by intrusion 

of the Eolus batholith. North of the Tenmile Granite 

body these trends swing parallel to the Uncompahgre 

unconformity. The northern area of Irving rocks has 

east to N". 70° E. trends, and much or all of this change 

in trend may have formed in the Uncompahgran 

disturbance.

Determining the amount of rotation of the Irving 

rocks, and of the Twilight Gneiss and Tenmile Granite 

as well, during the Uncompahgran disturbance remains 

a problem. One would expect the rocks immediately 

under the unconformity to have been rotated as much as 

the unconformity itself, but this may not be true 'because 

of shearing and attendant strain. The amount of rotation in the Irving and the two granites at distances of 1 

mile or more from the unconformity is even more problematical, especially from Lime Creek to Vallecito Creek, 

where the contact between the Irving and Uncompahgre 

Formations has variable dips. Exposures of the southern unconformity in the canyon of the Animas Eiver 

show it to be approximately vertical; yet to the east and 

west it is smoothly folded, and its dip decreases upward and southward to about zero. In spite of this variation in rotation of the unconformity, from 0° to 90°, 

the attitudes of foliation and compositional banding 

in the underlying rocks are uniform. This problem deserves detailed study.

THE UNCOMPAHGRAN DISTURBANCE

The intense folding, with attendant low- to high-rank 

metamorphism, that affected the Uncompahgre Formation is named the Uncompahgran disturbance, as mentioned on page All. The Uncompahgre Formation was 

deposited on a surface that truncates l,7fO-m.y.-old 

granite, was then folded and metamorphosed, and later 

was intruded by the posttectonic, l,460-m.y.-old Eolus 

batholith. Those two dates thus bracket deposition of the 

Uncompahgre Formation as well as the Uncompahgran 

disturbance. To my knowledge no metamorohic rocks 

or minerals of the Uncompahgre Formation have been 

dated by radioactive methods.

FOLDS OF THE UNCOMPAHGRE FORMATION

The Uncompahgre Formation is intensely folded, in 

isoclinal fashion, as shown by Cross and Ho we (Cross, 

Howe, Irving, and Emmons, 1905, map of areal geology) and in figure 2. The quartzites and irterlayered 

slates, phyllites, and schists responded very differently 

to the tectonic environment; the folds of individual 

quartzite layers are of parallel type, approaching the 

pure flexure folds of Ramsay (1962, p. 316), whereas 

those in the interlayerecl pelites are of similar type. The 

folds probably are cylindroidal, but further work is 

needed here. Axial surfaces of the folds are not planar 

but gently curved; these generally are steep but locally 

are gently dipping. In most outcrops of the pelites 

bedding is not parallel to cleavage or schis*osity, but 

parallelism of the two is not uncommon. Minor folds of 

irregular cross section and of many sizes occur in most 

of the pelite; the fold axes, for the most pr.rt, plunge 

less than 30° in easterly and westerly directions west of 

Vallecito Creek and in southeasterly and northwesterly 

directions east of Vallecito Creek. The axes of these 

folds may well parallel those of the major foHs but this 

has not been demonstrated. However, a few minor folds 

with very steep to vertically plunging axes were seen. 

Gently warped axes of minor folds were noted in the 

Animas Canyon; this may be a consequence of local 

flattening of parallel folds (Ramsay, 1962, p. 313). 

Minor folds are common in the quartzite layers in the 

Elk Creek drainage and westward but are rare to the 

east. Slip cleavage and crinkles with amplitude as large 

as one-Jialf inch are prominent in much of the pelite, 

especially along Vallecito Creek and the several forks 

of Ute Creek.
PRECAMBRIAN GEOLOGY OF THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A31

FAULTS

No major faults were found in this investigation, and 

these rocks are not known to contain any notable shear 

zones similar to those found in Precambrian rocks of 

Arizona, New Mexico, and other parts of Colorado. 

Figure 2 shows some of the faults discovered by Cross 

and his associates (Larsen and Cross, 1956, p. 1) : some 

minor ones along the southern margin of the Precambrian uplift; several in the Uncompahgre Formation; 

and two that cut the Irving Formation, Vallecito Conglomerate, and Paleozoic rocks west of Emerald Lake. 

Several of the faults portrayed by Cross and others 

(1905, 1905, 1907) in the geologic folios and by Larsen 

and Cross, however, were found not to exist; these include the intrusive contact between the Pine River 

batholith of Eolus Granite and the Uncompahgre Formation to the west, the intrusive contact between Eolus 

Granite and the Irving Formation, most of the depositional contact between the Vallecito Conglomerate and 

the Irving Formation, and the unconfomiable depositional contact between the Uncompahgre Formation and 

older rocks. The latter contact, which apparently has 

phyllonite developed along a part of it, is a zone of 

shearing and relative movement of unknown magnitude 

and certainly is a fault if my interpretation of the 

phyllonite is correct. Some doubt remains, nonetheless, 

and so this feature is not classed as a fault in figure 2. 

The relative straightness of the contact between the 

Eolus Granite and the Vallecito Conglomerate southeast 

of Emerald Lake may be due to intrusion along a fault.

METAMORPHISM

Metamorphism of the Irving Formation was to the 

amphibolite f acies, and later retrograde metamorphism 

affected much of it. These rocks were not studied in 

enough detail, however, to warrant separate discussion 

here. Metamorphism of the pelitic rocks of the Uncompahgre Formation was studied more intensively in 

regard to the assemblages formed, even though individual mineral phases were not analyzed, and metamorphism of those rocks is treated separately.

METAMORPHISM OP THE UNCOMPAHGRE PELITES

The pelitic rocks of the Uncompahgre Formation 

were regionally metamorphosed to slate and phyllite. 

Later, a higher temperature thermal type of metamorphism was superimposed on these rocks from Vallecito Creek east and southeast to their contact with the 

eastern batholith of Eolus Granite.

REGIONAL METAMOHPHISM

The Uncompahgre pelitic rocks from their westward exposure at Lime Creek eastward to the western

and northern headwaters of Vallecito Creek are slates, 

typically of the assemblage quartz-sericite-chlorite. 

They commonly contain carbon or graphite and ilmenite 

and pyrite. Poikiloblastic andalusite with hematite was 

found in this assemblage in one layer in the canyon of 

the Animas River, and biotite and oligoclase also were 

observed, with the quartz-sericite-chlorite assemblage in 

a pelitic layer near the Animas River. These relatively 

simple assemblages do not violate the phase rule and 

probably are equilibrium assemblages. They consist of 

the components A12O3 , C, FeO, H2O, K2O, MgO, S, 

SiO2 , and TiO2 .

THERMAL METAMORPHISM

The Uncompahgre pelites east and south of upr)er 

Vallecito Creek, east of Trinity Lake, and south of 

Vallecito Lake and Nebo Creek have progressively developed biotite, chloritoid and garnet, chiastolite, and 

in a small area, staurolite. These rocks, here loosely 

termed "schists," of which the muscovite-biotitechiastolite type is most abundant, occur in an arer, of 

about 25 square miles. Sillimanite- and microcline-b^aring schists, of granoblastic fabric, occur to the east 

within l/^-l mile of the Eolus Granite (p. AM). These 

latter schists have a direct spatial relationship to the 

Eolus Granite and thus are of contact metamorphic 

origin. The muscovite-biotite-chiastolite and other 

types of schist in the upper Vallecito area also probrMy 

are of thermal metamorphic type, presumably caused 

by heat from the underlying Eolus Granite. Reasons for 

suggesting this are:

1. Transitions from sericite-quartz-chlorite slate? to 

the biotite-chiastolite-garnet-muscovite-staurolite 

schist and associated schists are relatively abript. 

The rocks change within 0.5 mile southward from 

the sericite-quartz-chlorite slates containing randomly oriented postkinematic biotite of Nebo 

Creek to sericite-quartiz-biotke-chiastolite phyllite, and within 1.2 miles \vest-southwestwarcltc the 

chiastolite-garnet-staurolite locality. Simils.rly, 

from the quartz-sericite-graphite slate just northwest of Trinity Lake it is only 0.8 mile southe astward to quartz-muscovite-chlorite phyllite rath 

chiastolite and chloritoid, 0.5 mile southeastward 

to muscovite-quartz-chloritoid-garnet phyllite, 

and 1.9 miles farther southeastward to the chiastolite-garnet-staurolite locality. Thus within 2 to 3 

miles there is a transition from sericite-chlorite 

slates to chiastolite-garnet-staurolite schist.

2. The fabric changes from slates with well-developed 

slaty cleavage and microfolds and phyllites rath 

clearly apparent schistosity cut by randomly 

oriented biotite to granoblastic porphyroblrstic
A32 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

fabrics in which neither muscovite nor biotite shows 

preferred orientation.

3. The chiastolite-bearing phyllites and schists and the 

associated rocks are similar to those of other contact metamorphic aureoles. (See, for example, 

Pitcher and Read, 1963; Compton, 1960.)

4. An offshoot of the Eolus batholith lies in quartzite 

1.9 miles west-southwest of the chiastolite-garnetstaurolite locality; thus, this abrupt metamorphic 

gradient lies close to exposed granitic rocks. This 

granite is postkinematic and would be expected to 

cause the formation of hornfels with granoblastic 

fabrics, as are found in the sillimanite-bearing 

hornf els of Flint Fork.

The alternate possibility that the metamorphism 

here is of Buchan type (Read. 1952) cannot, though, 

be ruled out.

Assemblages of the chiastolite-bearing schists that lie 

0.4 mile north of the junction of Rock and Vallecito 

Creeks contain more phases, in apparent textural equilibrium, than can be plotted in a Thompson diagram 

(Thompson, 1957). Whether this is due to ferric iron 

in chloritoid, to lime in the garnet, to H2O as a determining inert component, or to other reasons was not 

determined.

Chlorite grains similar in size and habit to biotite 

grains in schists of Rock Creek, Flint Fork, and Ute 

Creek apparently are stable with chiastolite, staurolite, 

and garnet and very nearly are stable to the sillimanite 

zone. Chlorite, furthermore, locally has formed with 

magnetite and in place of biotite. I infer that activity 

of H2O was relatively high and locally variable, as 

would be expected from nearby bodies of granite.

Cordierite, with the possible exception of a pinitebearing rock east of Ute Lake, is not present in any of 

the chiastolite- or sillimanite-bearing rocks of the Uncompahgre Formation. Cordierite, of course, is a common constituent of pelitic hornfels and schists of 

Buchan type metamorphism. Its absence in the Needle 

Mountains must be due to the relatively ferrous nature 

of these pelites, which have regularly developed garnet, 

chloritoid, and staurolite. Cordierite probably would 

have formed had any of the pelites been more 

magnesian.

OXIDATION OF DETRITAL MAGNETITE TO 

HEMATITE

The dark laminae in the clastic rocks of the Uncompahgre Formation and the Vallecito Conglomerate are 

of special interest. These laminae are largely hematite, 

commonly with accessory ilmenite and rutile. The hematite not uncommonly shows octahedral forms, which

indicate derivation from magnetite by oxidation. The 

cause of such a widespread oxidation, in which the 

interlayered pelites and metabasalts were not affected, 

apparently lies in the dissociation of interst'tial water. 

Fugacities of oxygen in pure H2O were calculated from 

227°C to 927° C and at 1 bar and 1,200 bars water pressure, using fugacities of water. Two important conclusions reached in this investigation (Barker, 1969b) are: 

(1) The oxygen fugacities obtained for temperatures of 

low- to medium-rank metamorphism are 8 to T.O orders of 

magnitude greater than those of the magnetite-hematite 

univariant curve, meaning that pure water will oxidize 

stoichiometric magnetite (FeoO3 ) under tHse conditions; and (2) the oxygen fugacities for 1 bar and for 

1,200 bars converge with decreasing temperature, at 

about 150°C, so that this effect is almost independent 

of pressure below 400° C. Thus, a relatively pure interstitial water in these clastic rocks would be expected to 

oxidize detrital magnetite during metamorphism. The 

hydrogen generated in the oxidation of magnetite may 

be responsible for the hydrolysis of detrital feldspar 

to sericite in these rocks.

REFERENCES CITED

Alien, J. R. L., 1963, The classification of cross-stratified units, 

with notes on their origin: Sedimentology, v. 2. no. 2, p. 93- 

114.

Barker, Fred, 1969a, Gold investigations in Precairbrian clastic 

and pelitic rocks, southwestern Colorado and northern New 

Mexico: U.S. Geol. Survey Bull. 1272-F, 22 p.

 1969b, Occurrence and genesis of hematite in Precambrian clastic rocks in southwestern Colorado and northern 

New Mexico, in Abstracts for 1968 : Geol. Soc. A merica Spec. 

Paper 121.

Barker, Fred, and Friedman, Irving, 1969, Carbon isotopes in 

pelites of the Precambrian Unconipahgre Formation, 

Needle Mountains, Colorado: Geol. Soe. America Bull., v. 80, 

no. 7, p. 1403-1408.

Biekford, M. E., Barker, Fred, Wetherill, G. W., and Lee-Hu, 

Chin-Nan, 1968, Precambrian chronology in the Needle 

Mountains, southwestern Colorado Pt. 2, Rv~Sr results, 

in Abstracts for 1967: Geol. Soc. America Spe~. Paper 115, 

p. 14.

Burbank, W. S., and Luedke, R. G., 1964, Geology of the Ironton 

quadrangle, Colorado: U.S. Geol. Survey Geo). Quad. Map 

GQ-291.

Compton, R. R., 1960, Contact metamorphism in Santa Rosa 

Range, Nevada: Geol. Soc. America Bull., v. 71, no. 9, p. 

1383-1416.

Comstoek, T. B., 1883, Notes on the geology and mineralogy 

of San Juan County, Colorado: Am. Inst. Mining Eng. 

Trans., v. 11, p. 165-191.

Cross, Whitman, and Hole, A. D., 1910, Description of the 

Engineer Mountain quadrangle, Colorado: U.S. Geol. Survey 

Geol. Atlas, Folio 171,14 p.

Cross, Whitman, Howe, Ernest, and Irving, J. D., 1907, Description of the Ouray quadrangle, Colorado: U.S. Geol. 

Survey Geol. Atlas, Folio 153, 20 p.
PRECAMBRIAN GEOLOGY OP THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO A33

Cross, Whitman, and Larsen, E. S., Jr., 1935, A brief review of 

the geology of the San Juan region of southwestern Colorado : U.S. Geol. Survey Bull. 843,138 p.

Cross, Whitman, Howe, Ernest, and Ransome, F. L., 1905, Description of the Silverton quadrangle, Colorado: U.S. Geol. 

Survey Geol. Atlas, Folio 120, 34 p.

Cross, Whitman, Howe, Ernest, Irving, J. D., and Emmons, 

W. H., 1905, Description of the Needle Mountains quadrangle, Colorado: U.S. Geol. Survey Geol. Atlas, Folio 131, 

14p.

Emmons, S. F., 1890, Orographic movements in the Rocky Mountains : Geol. Soc. America Bull., v. 1, p. 245-286.

Endlich, F. M., 1876, Report on the San Juan district, Colorado: 

U.S. Geol. Geog. Survey Terr. (Hayden), 8th Ann. Rept, p. 

181-240.

Hansen, W. R., and Peterman, Z. E., 1968, Basement-rock 

geochronology of the Black Canyon of the Gunnison, Colorado, in Geological Survey research 1968: U.S. Geol. Survey 

Prof. Paper 600-C, p. C80-C90.

Hem, J. D., 1960, Some chemical relationships among sulfur 

species and dissolved ferrous iron: U.S. Geol. Survey 

Water-Supply Paper 1459-C, p. 57-73.

Hinds, N. E. A., 1936, Uncoinpahgran and Beltian deposits in 

(western North America: Carnegie Inst. Washington Pub. 

463, p. 53-136.

Kelley, V. C., 1946, Geology, ore deposits, and mines of the 

Mineral Point, Poughkeepsie, and Upper Uncompahgre districts, Ouray, San Juan, Hinsdale Counties, Colorado: 

Colorado Sci. Soc. Proc., v. 14, no. 7, p. 287-466.

Kleeman, A. W., 1965, The origin of granitic magmas: Geol. 

Soc. Australia Jour., v. 12, pt. 1, p. 35-52.

Lakes, Arthur, 1889, Geology of Colorado coal deposits: Colorado 

School Mines Ann. Rept. Field Work, 264 p.

Larsen, E. S., Jr., and Cross, Whitman, 1956, Geology and

petrology of the San Juan region, southwestern, Colorr <lo:

U.S. Geol. Survey Prof. Paper 258,303 p. 

Luedke, R. G., and Burbank, W. S., 1962, Geology of the Orray

quadrangle, Colorado: U.S. Geol. Survey Geol. Quad. Map

GQ-152, with text.

McKee, E. D., and Weir, G. W., 1953, Terminology for stratification and cross^stratification in sedimentary rocks: Geol. Soc.

America Bull., v. 64, no. 4, p. 381-389. 

Pitcher, W. S., and Read, H. H., 1963, Contact metamorpl '^m

in relation to manner of emplacement of the granite'* of

Donegal, Ireland: Jour. Geology, v. 71, no. 3, p. 261-296. 

Ramsay, J. G., 1962, The geometry and mechanics of formation

of "similar" type folds: Jour. Geology, v. 70, no. ?, p.

309-327. 

Read, H. H, 1952, Metamorphism and migmatisation in the

Ythan Valley, Aberdeenshire: Edinburgh Geol. Soc. Trvinis.,

v. 15, p. 265-279. 

Silver, L. T., and Barker, Fred, 1968, Geochronology of Precambrian rocks of the Needle Mountains, southwestern

Colorado Pt. 1, U-Pb zircon results [abs.] : Geol. Soc.

America Spec. Paper 115, p. 204-205. 

Thompson, J. B., Jr., 1957, The graphical analysis of mireral

assemblages in pelitic schists: Am. Mineralogist, v. 42, nos.

11-12, p. 842-858. 

Van Hise, C. R., 1892, Correlation papers; Archean and Alsronkian : U.S. Geol. Survey Bull. 86, 549 p. 

Varnes, D. J., 1963, Geology and ore deposits of the South

Silverton mining area, San Juan County, Colorado: U.S.

Geol. Survey Prof. Paper 378-A, p. A1-A56. 

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geophysical laboratory: Carnegie Inst. Washington Year

Book 56, p. 206-214.

INDEX

Page

Accessibility of NeedleMountains____.. A2 

Acknowledgments.____.__.._..... 2

Age, Bakers Bridge Granite..... ..'.___ 18,24

ElectraLake Gabbro.__.___._.... 18,27

Eolus Granite............................ 18,26

rhyolite porphyry dikes.........___.. 18,28

Tenmfle Granite......................... 18,22

Trimble Granite_...................... 18,28

Twilight Gneiss..-..-......-.....-.---... 18,19

Whitehead Granite__..-.-.......-.--.- 18,22

Analyses, amphibolite from Irving Formation. 7 

Bakers Bridge Granite_................ 20

Electra Lake Gabbro (alaskite dike)___ 21 

Eolus Granite............................ 21

Ground water ___. _____..... __. 14

quartz diorite of Pine River.. _...._.. 20

rhyolite prophyry dikes..___._..._ 21 

Tenmile Granite_...................... 20

Trimble Granite......................... 21

Twilight Gneiss.......................... 20

Whitehead Granite....................... 20

Animas River, exposures of Irving Formation

along.___..._.____..__ 7,8 

pelitic rocks..___________...._ 12

Uncompahgre Formation along.__.... 11,12

Aplite and pegmatite............___._.. 28

Artis, Lowell, analyst.....__...._........ 21

Bakers Bridge Granite............__...... 20,25

intrusion...____.____._....__ 3

Balsam Lake, estimated section of Uncompahgre Formation__...____ 15 

Biot'te quartz monzonite__________. 29 

Botts, S. D., analyst.__.................... 21

Budinsky, J., analyst___.._......__ 21

Chloe, Gilson, analyst..._________. _ 20 

Compositional layering, Irving Formation _ 5,

8,10,30 

Uncompahgre Formation_------------- 13

Contact relations, Eolus Granite... __... _ 26 

Irving Formation.............. 6,15,16,20,21,26

rocks of Weminuche Creek._...__._ 10 

Twilight Gneiss..--.......--..-.--.--- 15,20,21

Uncompahere Formation_____.... 11,15,26

Vallecito Conglomerate___.--....--.- 5,26,31

Whitman Cross' interpretation_... __ 2 

Cross,Whitman, quoted...___....____ 2,29

Cross-stratification, Uncompahgre Formation.............................. 11

Vallecito Conglomerate___ _. __... 4,29

Detrital magnetite, oxidation to hematite_.. 32

Dikes, diabase_______......__.... 29

in Irving Formation..__.___.____ 7

lamprophyre_...___.._...____ 29

[Italic page numbers indicate major references]

Page 

ElectraLake Gabbro._................ A18.21.S7

intrusion.._.____..._____.. .. 4 

Elmore, P. L. D., analyst................... 21

Engleman, Edythe, analyst..._____..... 21

Eolus Granite............................. 18,21,85

intrusion........__.._._.__ . 4

Erosion, places hi geologic sequence hi area... 3,4

Faults ......... . 31

Folding...................................... 3

Irving Formation__.........._..... 29

Uncompahgre Formation........_..-.- 30

Foliation, Irving Formation... _... _ 5,7,8,10,30

Geologic sequence.....__._..___....... 2

Glenn, J., analyst______..______... 21

Gray, E., analyst.....__....__..-.-.-..- 21

Grenadier Range, quartzite.......__..... 11

Ground water, Uncompahgre Formation..... 14

HighlandMary Lakes, Irving Formation..... 8 

Howe, Ernest, quoted_............__.._ 2,29

Intrusive rocks, minor......._....-.-.---... 28

Iron-formation, banded_.__...____... 5 

Irving Formation......... 5,15,16,20,21,22,26,29,30

deposition___..................._.... 2

northern Needle Mountains.___.. __ 8

southeastern Needle Mountains......--... 5

western Needle Mountains_....._.... 7

Kelsey, J., analyst. 20

Lime Creek, pelitic rocks exposed............ 12

Lineations, in Irving Formation_____.. 7,8,10 

Location of NeedleMountains. ___ _...... 2

Mafic quartz syenite__..._.___....__ 29 

Metadacfte_............................... 28

Metamorphism....__.__._____.__ SI

contact, Uncompahgre schist............. 14,31

directional increase hi grade...-.-.-----.- 12

effects, Vallecito Conglomerate_... __ 4 

regional.._......_._.__._...... 31

retrograde, Irving Formation... _.. 5,7,8,9,22 

thermal................._.............. 31

Midgett,M. R., analyst...................... 14

Origin, Twilight Gneiss......

Uncompahgre Formation. 

Vallecito Conglomerate...

Parker, C. L., analyst...................... 7,20,21

Pelitic rocks, Uncompahgre Formation_._ 111,31 

Phyllonite .............................. IS

origin____..__.__...._____... 16

Page

Physical features of NeedleMountains. .... A2 

Pillow structure_______. ____. ___ 9 

Pine River, superb Vallecito Conglomerate

exposures along..._______ 4 

Previous investigations___..___...... 2

Quartz diorite of Pine River_. ____ __ &4

Quartzite, Uncompahgre Formation.... .. 11

Vallecito Conglomerate.......... 4

References cited..... .................... S&

Rhyolite porphyry dikes. . If, 21,28 

Rock Lake, pelitic rocks..................... 12,13

San Juan Primitive Area..__...__. 2

Scarbro, G. F., analyst ___ 14 

Schists, Uncompahgre Formation...... 13,14,31,32

Sericite, in quartzite, possible sources ...... 12

Sills, in Irving Formation___....... .. 7

Slates, Uncompahgre Formation...... 12

Smith, H., analyst..... . 20

Structural geology.. . 29

Sub-Uncompahgre unconformity. .. 15

Tectonic events........... . ...... 2

Tenmile Granite. . .......... 18,20,22

intrusion -... ... 3 

Trimble Granite..-.-..... -- ---- lf,2l,«7

intrusion.....................--.-..------ 4

Trinity Lake, pelitic rocks..--.------ . 12

Twilight Gneiss IS

original rocks...-.---___-- . .-. 3

Uncompahgran disturbance. ---------. 3,30

Uncompahgre Formation........ 10,26.30

basal conglomerate...... . 11

deposition- __ . . 3

pelitic rocks..___.._--------- 1&

metamorphism- ... .... SI

quartzite _....- 11

thickness-.......-----..-.---..----------- 15

Uncompahgre sy nclinorium ------. -... 11

Vallecito Conglomerate._........ . 4

deposition..-._ .. ... -- 2

folding-__.-.-........----.------.-.---. 29

origin_ - 15

source terrane.-..._....... .-.- -. 4,5

Vallecito Creek, Uncompahgre Formation

 . 13

Weminuche Creek, rocks at head ....... 10

White Dome, pelitic rocks..... ..... 12

Whitehead Granite. .. . M,20,22

A35

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