Crystalline Rocks of the Strawberry Lake Area, 

Front Range, Colorado 

A. Petrography and Geochemistry 

B. Petrology 

U.S. GEOLOGICAL SURVEY BULLETIN 1937 

Crystalline Rocks of the Strawberry Lake Area, 

Front Range, Colorado 

A. Petrography and Geochemistry 

B. Petrology 

By EDWARD J. YOUNG 

A petrographic and geochemical study of the 

bedrock and a petrologic discussion based on 

felsic-mafic and silica-saturation ratios 

This volume is published as chapters A and B. 

These chapters are not available separately 

U.S. GEOLOGICAL SURVEY BULLETIN 1937 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names In this publication Is for descriptive 

purposes only and does not imply endorsement by the U.S. Government. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1990 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box25425 

Denver, CO 80225 

Ubrary of Congress Cataloging-In-Publication Data 

Young, Edward J. (Edward Joseph), 1923-

Crystalline rocks of the Strawberry Lake area, Front Range, Colorado I by 

Edward J. Young. 

p. em. -(U.S. Geological Survey bulletin; 1937) 

"A petrographic and geochemical study of the bedrock and a petrologic 

discussion based on felsic-mafic and silica-saturation ratios." Includes 

bibliographical references (p. 

Supt. of Docs. no.: I 19.3:1937A-B 

1. Rocks, Crystalline-Colorado-Strawberry Lake Region (Grand 

County). 2. Rocks, Igneous-Colorado-Strawberry Lake Region (Grand 

County) 3. Geology-Colorado-Strawberry Lake Region (Grand 

County) 4. Petrology-Colorado-Strawberry Lake Region (Grand 

County) 5. Analytic geochemistry-Colorado-Strawberry Lake Region 

(Grand County) I. Title. II. Series. 

QE75.B9 no. 1937 

[QE445.C64) 

557.3 s-dc20 90-139564 

[552' .3'0978865) CIP 
CONTENTS 

[Letters designate the chapters] 

(A) Petrography and Geochemistry 

(B) Petrology 

Volume eontents Ill 

Chapter A 

Petrography and Geochemistry 

By EDWARD J. YOUNG 

A study of the petrography and 

geochemistry of the bedrock 

U.S. GEOLOGICAL SURVEY BULLETIN 1937 

CRYSTALLINE ROCKS OF THE STRAWBERRY LAKE AREA, 

FRONT RANGE, COLORADO 

CONTENTS 

Abstract A1 

Introduction A1 

Geologic setting A2 

Petrography of crystalline rocks A3 

Boulder Creek Granodiorite A3 

Granitic enclaves in Boulder Creek Granodiorite AS 

Silver Plume Granite AS 

Granitic enclaves in Silver Plume Granite A13 

Biotite gneiss A17 

Granitic enclaves in biotite gneiss A26 

Biotite-lamprophyre enclaves in Boulder Creek Granodiorite A27 

Small mafic enclaves in Boulder Creek Granodiorite A27 

Andesite porphyry dike A27 

Chemical, physical, and petrographic comparison of rock units A34 

Geochemistry A34 

Conclusions A38 

References cited A40 

PLATE 

[Plate is in pocket) 

1. Generalized geologic map of the Strawberry Lake area showing sample 

localities. 

FIGURES 

1. Index map of the Strawberry Lake area A2 

2-4. Photographs showing: 

2. Boulder Creek Granodiorite in contact with a small, fine-grained 

granitic enclave A12 

3. Small mafic enclave in Boulder Creek Granodiorite A12 

4. Pink porphyroblasts in Boulder Creek Granodiorite near contact 

with biotite gneiss A12 

5. Quartz-alkali feldspar-plagioclase (QAP) diagram showing Boulder Creek 

Granodiorite and Silver Plume Granite A13 

6. Quartz-alkali feldspar-plagioclase (QAP) diagram showing granitic 

enclaves and mafic rocks A17 

7. Photomicrograph showing monazite in biotite with intense, dark, radiationdamage halo A23 

8. Photomicrograph showing monazite in plagioclase with typical alteration 

ring of opaque material A23 

9-12. Photographs showing: 

9. Small enclave of biotite gneiss in Silver Plume Granite A23 

10. Thin, parallel biotite schlieren in Silver Plume Granite A23 

11. Crystalloblastic texture of granitic enclave in Silver Plume 

Granite A23 

Contents VII 
12. Block of biotite lamprophyre showing irregular veining by aligned 

pink potassium feldspar A34 

13. Photomicrographs showing biotite lamprophyre A34 

14-17. Graphs showing: 

TABLES 

14. Uranium and thorium contents of Boulder Creek Granodiorite and 

Silver Plume Granite A38 

15. Uranium and thorium contents of rock units in the Strawberry Lake 

area A39 

16. Coefficient of trace element accumulation, R, in rock units in the 

Strawberry Lake area A40 

17. Significant elemental enrichment in rock units in the Strawberry 

Lake area A42 

1. Parameters used in description of rocks in the Strawberry Lake area and 

number of analyses or measurements for each rock unit A4 

2. Uranium, thorium, chemical, spectrographic, and modal analyses and 

descriptive parameters of the Boulder Creek Granodiorite A6 

3. Uranium, thorium, spectrographic, and modal analyses and descriptive 

parameters of the Boulder Creek Granodiorite AS 

4. Modal analyses and descriptive parameters of the Boulder Creek 

Granodiorite AlO 

5. Uranium, thorium, chemical, spectrographic, and modal analyses and 

descriptive parameters of granitic enclaves in the Boulder Creek 

Granodiorite A14 

6. Modal analyses and descriptive parameters of granitic enclaves in the Boulder 

Creek Granodiorite A16 

7. Uranium, thorium, chemical, spectrographic, and modal analyses and 

descriptive parameters of the Silver Plume Granite A18 

8. Uranium, thorium, spectrographic, and modal analyses and descriptive 

parameters of the Silver Plume Granite A20 

9. Modal analyses and descriptive parameters of the Silver Plume 

Granite A22 

10. Uranium, thorium, chemical, spectrographic, and modal analyses and 

descriptive parameters of granitic enclaves in the Silver Plume 

Granite A24 

11. Uranium, thorium, chemical, spectrographic, and modal analyses and 

descriptive parameters of the biotite gneiss A25 

12. Uranium, thorium, spectrographic, and modal analyses and descriptive 

parameters of the biotite gneiss A26 

13. Uranium, thorium, chemical, spectrographic, and modal analyses and 

descriptive parameters of a granitic enclave in the biotite gneiss A28 

14. Uranium, thorium, spectrographic, and modal analyses and descriptive 

parameters of granitic enclaves in the biotite gneiss A30 

15. Uranium, thorium, chemical, spectrographic, and modal analyses and 

descriptive parameters of the biotite lamprophyre enclaves, a small mafic 

enclave in the Boulder Creek Granodiorite, and an andesite porphyry 

dike A32 

16. Rubidium:strontium ratios of rocks in the Strawberry Lake area A35 

17. Averages and ranges of chemical, physical, and petrographic parameters of 

rock units in the Strawberry Lake area A36 

VIII Contents 
Petrography and Geochemistry 

By Edward J. Young 

ABSTRACT 

The Strawberry lake area lies between the Continental 

Divide and Granby, Colorado, just north of Tabernash. It is 

underlain by Proterozoic rocks composed of biotite gneiss 

and two plutons-Boulder Creek Granodiorite of the Routt 

Plutonic Suite and Silver Plume Granite of the Berthoud 

Plutonic Suite. Relict enclaves of biotite gneiss are not 

uncommon in the Boulder Creek Granodiorite, in the Silver 

Plume Granite, and in the granitic enclaves in the biotite 

gneiss. Granitic and mafic enclaves in the Boulder Creek 

Granodiorite, granitic enclaves in the Silver Plume Granite 

and in the biotite gneiss, and a Tertiary andesite porphyry 

dike complete the rock types. 

I used chemical, physical, and petrographic data to 

delineate properties of the rock units. The Boulder Creek 

Granodiorite ranges from metaluminous to peraluminous 

and from granodiorite to tonalite, whereas the Silver Plume 

Granite is peraluminous, monazite bearing, and potassium 

rich. The Boulder Creek Granodiorite contains Jess uranium 

and thorium and the Silver Plume Granite contains more 

uranium and thorium in the Strawberry lake area than the 

same rocks, on average, in the entire Front Range. This 

finding correlates with a more mafic than average Boulder 

Creek Granodiorite and a more felsic than average Silver 

Plume Granite in the Strawberry lake area. 

Geochemical and petrologic problems are posed by 

the enclaves. The granitic enclaves in the two plutons and in 

the biotite gneiss exhibit greater enrichment in thorium and 

light rare-earth elements than their hosts. Small mafic 

enclaves in the Boulder Creek Granodiorite are enriched in 

mafic elements, whereas lamprophyres in the Boulder Creek 

Granodiorite are enriched in both felsic and mafic elements. 

INTRODUCTION 

The geology of the crystalline rocks of the 

Strawberry Lake area (northwestern flank of the central 

Front Range) in Colorado was not well known before 

Manuscript approved for publication, March 9, 1990. 

completion of David A. Schroeder's {1984) thesis. 

Concurrent with Schroeder's investigation, and 

supplementary to it, my independent investigation 

(Young, 1984) on the chemical, petrographic, and 

physical properties of the crystalline bedrock resulted in 

additional information, which must be considered for a 

more complete understanding of the geology. 

The geologic map of Colorado, compiled by Odgen 

Tweto (1979) at a scale of 1:500,000, shows that most of 

the area of this report is underlain by 1,700-m.y. granitic 

rocks. Mapping by Schroeder {1984) and myself suggests 

that a part of this area is underlain by the Middle 

Proterozoic Silver Plume Granite. A rubidium-strontium 

(Rb-Sr) age of 1,503 ± 100 m.y. (C.E. Hedge, written 

commun., 1982) confirmed this suggestion. I collected 

rock samples from the Strawberry Lake quadrangle and 

small parts of the surrounding quadrangles (fig. 1) in 

order to study carefully the rocks and minerals; I paid 

special attention to uranium and thorium, because the 

Front Range long has been called a "uranium and 

thorium province" (Phair and Gottfried, 1963). In 

addition, I determined foliations and other structural 

features in order to study the relationship between 

enclaves and their hosts. 

Major rock units of the Strawberry Lake area 

include an older (Early Proterozoic) biotite-gneiss 

complex, Boulder Creek Granodiorite of the Routt Plutonic Suite (Tweto, 1987), and younger (Middle Proterozoic) Silver Plume Granite of the Berthoud Plutonic 

Suite (Tweto, 1987). All major rock units contain granitic 

enclaves1 of various sizes. In addition, relict enclaves of 

biotite gneiss occur in the two plutons and in the granitic 

enclaves of the biotite gneiss. Small mafic enclaves, 

lamprophyres, and a Tertiary andesite porphyry dike are 

confined to the pluton of Boulder Creek Granodiorite. 

1

Enclave is a nongenetic term, which is useful for naming a 

body of rock enclosed within a host rock (Didier, 1973, p. 1). 

Petrography and Geochemistry A 1 
1 05°52'30" 1 05°45' 1 05°37'30" ~--------------~~----------------------------,---------------~----------, 

TRAIL 

MOUNTAIN 

0 

SHADOW 

MOUNTAIN 

10 

ISOLATION 

PEAK 

15 KILOMETERS 

Figure 1. Index map of the Strawberry Lake area (?1!2-minute quadrangles are underlined). 

Chapter A of this bulletin focuses on the 

petrography and geochemistry of the rock units. Chapter 

B addresses the question of origin (petrology) of the 

plutonic rocks and their enclaves. Chapter A has 

benefited from the reviews of Robert C. Pearson, 

Dolores J. Gable, and Alan R. Wallace. All the 

observations and views expressed, however, are mine. 

GEOLOGIC SETTING 

The 160-km2 Strawberry Lake area lies between 

the Continental Divide and Granby, Colo., just north of 

Tabernash. Fifty percent of the area is underlain by a 

pluton of Boulder Creek Granodiorite of the Routt 

Plutonic Suite (Tweto, 1987). The Boulder Creek Granodiorite in this area has not been dated, but it is similar in 

A2 Crystalline Rocks of the Strawberry Lake Area, Colorado 

all regards to Boulder Creek Granodiorite in the central 

Front Range, where it has been dated as 1,710 ± 70 m.y. 

(Hedge and others, 1967). A smaller pluton of Silver 

Plume Granite of the Berthoud Plutonic Suite (Tweto, 

1987) underlies about 20 percent of the area and has 

been dated at 1,503 ± 100 m.y. (C.E. Hedge, written 

commun., 1982). Elsewhere in the Front Range Silver 

Plume Granite has been dated at 1,390-1,450 m.y. 

(Peterman and others, 1968). A biotite-gneiss complex 

(country rock) underlies about 20 percent of the area; it 

surrounds the two plutons and also occurs as enclaves, 

generally small, within the two plutons and within the 

granitic enclaves of the biotite gneiss. Radiometric age 

determinations of rocks in the biotite-gneiss complex 

indicate that regional metamorphism peaked between 

1,700 and 1,775 m.y. ago in Colorado (Hedge and others, 

1967). Numerous enclaves of fine-grained granitic rock 
occur in the Boulder Creek Granodiorite and in the 

biotite gneiss, but fewer occur in the Silver Plume 

Granite. These granitic enclaves aggregate about 7 

percent of the total area. Small discoid, mafic enclaves, 

lamprophyric enclaves, and a Tertiary andesite porphyry 

dike occur only within the Boulder Creek Granodiorite. 

These last three rock units make up less than 1 percent of 

the map area. 

Plate 1 is a generalized geologic map of the 

Strawberry Lake area showing sampling localities. Most 

of the map area is in the Strawberry Lake quadrangle, 

which was mapped by Schroeder (1984). Small parts of 

the map area are found in the Trail Mountain quadrangle 

(mapped by Izett, 1974), in the Granby quadrangle 

(mapped by Schroeder, 1984), and in the Bottle Pass 

quadrangle (mapped by Taylor, 1975). Portions of the 

map area that are in the Shadow Mountain and Monarch 

Lake quadrangles were mapped by me. 

Regional foliations in the Boulder Creek Granodiorite and in surrounding biotite gneiss generally strike 

northeast and dip steeply. Foliations in biotite-gneiss 

enclaves within the Boulder Creek Granodiorite conform 

to foliations of the host. In the northeast corner of the 

map area, the northeast-striking regional foliation occurs 

in both biotite gneiss and biotite-gneiss enclaves within 

the Silver Plume Granite. Divergence from this 

northeast-striking regional foliation to a northweststriking foliation, generally of steep dip, is chiefly found 

in biotite gneiss near its contact with Silver Plume 

Granite and in biotite-gneiss enclaves Within Silver 

Plume Granite near its contact with biotite gneiss. This 

northwest-striking foliation extends from the center to 

the northwest corner of the map area. The zone of 

northwest-striking foliation near the contact of the biotite 

gneiss and the Silver Plume Granite indicates a structural 

relationship between these two rock units, but exposure 

is neither good enough nor continuous enough to reveal 

the structure. Although a fault between these two rock 

units would seem to be a possible inference, neither D.A. 

Schroeder nor I found any direct evidence for such a 

fault. 

A small north-trending fault is fo~nd in the westcentral part of the Strawberry Lake quadrangle, where 

normal faulting has preserved a small inlier of Mesozoic 

sedimentary rocks within the pluton of Boulder Creek 

Granodiorite. Two small faults between Mesozoic 

sedimentary rocks and Boulder Creek Granodiorite 

occur in the southwest corner of the area, and two small 

faults between Boulder Creek Granodiorite and biotite 

gneiss occur in the southeast part of the area. An inferred 

bifurcating fault was mapped by Izett (1974) in the 

southeast corner of the Trail Mountain quadrangle 

(northwest corner of plate 1). 

PETROGRAPHY OF 

CRYSTALUNE ROCKS 

Eight of the nine crystalline rock types described 

here are Proterozoic; the ninth is a Tertiary andesite 

porphyry dike. In general, the rocks are described in 

order of their volumetric importance in tables 2-17. 

Table 1 gives an overview of the type and number of 

analyses or measurements made on these rocks. 

Boulder Creek Granodiorite 

The Boulder Creek Granodiorite (tables 2-4) is a 

massive to foliated, medium-grained, light- to dark-gray, 

hypautomorphic granular, granitoid rock of granodioritic 

to tonalitic composition, similar to the Boulder Creek 

Granodiorite west of Boulder, Colo. (Gable, 1980). 

Biotite and hornblende commonly cluster, which makes 

the rock appear to be coarse grained. Essential minerals 

are quartz, plagioclase (generally andesine, locally labradorite), and biotite. The plagioclase is commonly 

myrmekitic and antiperthitic but is not zoned. Microcline, hornblende, muscovite/sericite, chlorite, epidote, 

clinozoisite, and allanite occur locally. Apatite and 

sphene, magnetite, hematite, and zircon are common 

accessory minerals and are rarely absent in a sample. 

Black opaque minerals in thin section present a problem 

to the modal analyst. Oblique reflected light aids 

discrimination between pyrite and the iron and titanium 

minerals. A strong hand magnet is useful in 

discriminating between magnetite-bearing and hematitebearing rocks. Rocks designated as nonmagnetic may 

contain hematite or ilmenite although determinations by 

Isabelle Brownfield on black opaque minerals from such 

rocks yielded only hematite. 

Because these accessory minerals are relatively 

abundant, they provide an opportunity for closer scrutiny 

of their properties and mode of occurrence. Some 

colorless apatite grains, which commonly occur as 

crystals, show radiation-damage halos in biotite (not as 

intense as the halos that surround zircon), but other 

apatite grains within the same rock specimen do not show 

halos. Sphene commonly surrounds biotite flakes as 

small noncontiguous grains and also riddles the interior 

of biotite flakes with trains of smaller grains. Sphene also 

occurs at the contact of biotite and magnetite or hematite. Radiation-damage halos do not surround sphene in 

biotite. 

The zircons are small (less than 0.14 mm). Some 

zircons show syntaxial overgrowths, and all zircons 

invariably show radiation-damage halos in biotite. 

Allanite grains tend to be relatively large (some 

more than 3 mm). They are yellow brown to red brown 

and commonly isotropic ( metamict ), and they show 

Petrography and Geochemistry A3 
Table 1. Parameters used in description of rocks in the Strawberry Lake area and number of analyses or measurements for 

each rock unit 

[ <, less than; < <, much less than] 

Rock unit Boulder Silver Biotite Granitic Granitic Biotite Granitic Small mafic Andesite 

Creek Plrne gneiss enclaves in enclaves in lampro- enclaves in enclaves in pot Granodio- Granite biotite Boulder Creek phyre Silver Plrne Boulder Creek 

rite gneiss Granodiorite Granite Granodiorite 

Cllemical paramaters 

ChEmical analysis ••••••• 30 35 15 5 20 10 10 5 5 

Ktl:Na20, 

A:CNKl, il, s3. 

spectrographic ana10sis 60 125 25 35 25 15 10 5 5 

La:Ni, U, Th, Th: • 

Rb:Sr ••••••••••••••••••• 3 3 3 3 4 2 

Physical paramaters 

Radioactivity4, in. ••••• 26 33 8 8 12 4 2 0 

COl.Ults per second. 

Magnetic propertyS •••••• 26 34 9 8 12 5 2 

Specific gravity6 ••••••• 25 31 7 7 12 5 2 

Petrographic pat'al"leters 

M:>dal. analysis, color ••• 

index7• 

48 64 16 16 24 10 4 2 2 

Average ffain size, ••••• 26 34 9 8 12 5 2 

in rid.l i.nlaters. 

Total nunber of analy- 244 359 92 90 121 55 34 16 17 ses for ~ramaters for each unit. 

Areal percentage of •••• so 20 20 5 aboot 1 <1 «1 1? <1 

rock unit in nap area. 

1 AlPJ 

A:CNK = alund.na saturation=----- (in roolecular proportions). 

Ca<*Na2C»Kil 

(oxides in weight percent). 

Si~ 

3s=---------------------- (oxides in weight percent). 

A12DJ+Na2C»K~FeO+-FePJ 

~asured on the ootcrop with a M:lunt Sopris scintillareter, M:xlel SC-132. 

Sm, rock reacts postively to a large hard negnet; nm, oo reaction. 

6netermined with an air pyct10111:!ter, described by r-tlntyre and others (1965). An average of three determinations for each rock specinlm 

yields a specific gravity with :1: 0.008 gran per cubic centimeter maxinun deviation. 

7 Color index is the SliD of the colored (nelanocratic) minerals expressed in percent. O>lored minerals are: biotite, anphibole, pyroxene, chlorite, scapolite, all opaque minerals, garnet, allanite, zircon, sphene, tounnaline, epidote, and monazite. Light (leucocratic) 

minerals are: quartz, feldspar, lll.lscovite, cordierite, calcite, fluorite, apatite, sillimanite, and clinozoisite. 

A4 Crystalline Rocke of the Strawberry Lake Area, Colorado 
radiation-damage halos in biotite. Locally, allanite is 

surrounded by yellow-green epidote. Clinozoisite 

commonly occurs as microcrystalline aggregates along 

biotite cleavages and appears almost isotropic. Table 17, 

which gives average volume percentages of accessory 

minerals in the Boulder Creek Granodiorite and other 

rocks, shows that sphene, apatite, and epidote each 

average more than 0.5 percent. 

Much of the Boulder Creek Granodiorite appears 

homogeneous, but it contains enclaves of rock that range 

from felsic to mafic. The largest of these enclaves are 

granitic and range from less than 2 m to more than 1 km 

in greatest dimension. Figure 2 shows typical Boulder 

Creek Granodiorite in contact with a small granitic 

enclave. Some granitic enclaves locally contain enclaves 

(roughly equant) of granodiorite about 1m in size. This 

"enclave within enclave" pattern indicates the complexity 

involved in the process of formation or transformation of 

the Boulder Creek Granodiorite. Wisps of biotite gneiss 

and larger enclaves of biotite gneiss are not uncommon. 

At least three enigmatic biotite-lamprophyre enclaves 

occur in the granodiorite, and small (1-30 em) mafic 

(dioritic) enclaves are common locally (fig. 3). Large 

pink porphyroblasts, as much as 10 em long, are common 

near contacts with the biotite-gneiss unit (fig. 4). 

The Streckeisen (1976) classification is used for all 

rock names. Quartz-alkali feldspar-plagioclase (QAP) 

ratios are plotted on a QAP diagram in figure 5 for all 

modally analyzed rocks (24 samples) of the Boulder 

Creek Granodiorite. Almost one-half of these specimens 

are tonalite, thus placing the average composition near 

the granodiorite-tonalite dividing line. It is unusual for a 

rock of such mafic intermediate composition to be almost 

peraluminous. Average alumina composition 

(A:CNK=Al2 0 3/(Ca0+Na20+K20) in molecular 

proportions, as defined by Clarke (1981)] is 0.98. The 

Boulder Creek Granodiorite in the Strawberry Lake area 

is decidedly more mafic than the batholithic Boulder 

Creek Granodiorite west of Boulder, Colo. (Gable, 

1980), which plots as felsic granodiorite (close to the 

dividing line with monzogranite). 

Granitic Enclaves in 

Boulder Creek Granodiorite 

Granitic enclaves in the Boulder Creek Granodiorite (tables 5 and 6) range from 1m to more than 1 

km in greatest dimension. The larger ones are shown on 

plate 1, but many are too small to be shown. Contacts 

between the granite and the host granodiorite are 

generally sharp and abrupt. The smaller enclaves are 

generally fine grained, and the larger enclaves are only 

slightly coarser. All are gray to buff and have hypautomorphic granular texture. 

Essential minerals are quartz, microcline, oligoclase (rarely albite), and biotite. The plagioclase is locally 

antiperthitic and commonly myrmekitic. Microcline 

locally shows vermicular quartz and quartz blebs. 

Muscovite or sericite and magnetite are generally 

present. Hematite does not occur in the smaller enclaves 

and is subordinate to magnetite in the larger enclaves. 

Apatite and zircon occur in all the smaller enclaves and 

in most of the larger enclaves. The largest apatite grain is 

0.15 by 0.12 mm; it contained an inclusion of zircon that, 

in turn, contained a core of older zircon. Many of the 

zircon grains show zircon cores. Chlorite, epidote, 

allanite, monazite, and sphene occur locally. The monazite grains are very small-the largest one is 0.02 by 0.025 

mm. Some monazite grains are pale yellow and gemmy in 

appearance. Monazite grains included in biotite produce 

dark, intense radiation-damage halos. 

Twelve samples of these granitic enclaves are 

plotted on a QAP diagram in figure 6. For comparison, 

eight granitic enclaves in biotite gneiss and two granitic 

enclaves in Silver Plume Granite are also plotted. All the 

granitic enclaves, regardless of host rock, contain similar 

amounts of quartz, but the granitic enclaves in the 

Boulder Creek Granodiorite are the richest in alkali 

feldspar. All the granitic enclaves are peraluminous. 

Silver Plume Granite 

The Silver Plume Granite (tables 7-9) is a 

generally massive, pale-gray to buff, hypautomorphic 

granular, medium-grained, peraluminous granite, which 

forms an almost homogeneous mass. Essential minerals 

are quartz, microcline (locally perthitic ), oligoclase 

(unzoned, locally myrmekitic or containing quartz blebs), 

biotite, and muscovite. Monazite is present in every 

sample. Zircon, apatite, and hematite or magnetite are 

present in most samples; hematite is more common. 

Chlorite and sillimanite occur locally, and epidote and 

allanite are rare. Trace amounts of pyrite and fluorite, 

which fill fractures, occur in only one sample. 

Microcline locally forms laths or is anhedral; it 

invariably has the largest grain size of the major minerals. 

Plagioclase, which is commonly sericitized, and quartz 

are anhedral. Biotite and muscovite occur as much 

smaller grains than the feldspar and quartz and locally 

form inclusions in the feldspars. Biotite:muscovite ratios 

are not at all consistent; they vary from almost 30 to less 

than 1. 

Monazite is usually found as single, small, anhedral 

(less than 0.1 mm) grains, but also locally in clusters of 

several grains. The largest monazite grain is 0.36 by 0.2 

mm. Monazite is hosted by all the major silicate minerals 

but is also intimately associated with magnetite or hematite. If hosted by biotite it produces an intensely dark 

radiation-damage halo, which is invariably 0.03 mm thick 

Petrography and Geochemistry AS 
Table 2. Uranium, thorium, chemical, spectrographic, and modal analyses and 

descriptive parameters of the Boulder Creek Granodiorite 

[Delayed neutron activation analyses for U and Th by H.T. Millard, Jr., M. Solt, M. Coughlin, B. 

Vaughn, M. Schneider, W. Stang, R. Bies, B. Keaten, and S. Lasater. Rapid rock chemical analyses 

of sample S~35 by K. Coates, and sample S~51 by N. Skinner and D. Kopilis. Chemical (X-ray 

spectrographic) analyses of samples S~57, 66, 67, and 70 by J.S. Wahlberg, J. Taggart, and J. 

Baker. Semiquantitative spectrographic analyses of sample S~35 by G. Kaczanowski; sample 

S~51 by T. Fries; and samples S~57, 66, 67, and 70 by N.M. Conklin. •, significant trace elements 

(2 x crustal abundance, or more) and diagnostic accessory minerals. n.d., not determined; tr, trace; 

-,not found; <,less than] 

Sample No. 

O:mtx>sition SL-35 SL-51 SL-57 SL-66 SL-67 

Uraniun and thoriun analyses, in parts per million 

u ••••••••••••••••••• 

Th •••••••••••••••••• 

Th:U •••••••••••••••• 

1.67 

3.7 

2.2 

0.81 

5.5 

6.8 

2.32 

8.8 

3.8 

0.84 

7.7 

9.2 

<llanical analyses, in ~ight percent 

Sio2 •••••••••••••••• 

Al20J· •••••••••••••• 

Fe2o3 ••••••••••••••• 

FeO ••••••••••••••••• 

~ ................ . oo ................ . Na2o •••••••••••••••• 

K20 ••••••••••••••••• 

H2or •••••••••••••••• 

H2o-•••••••••••••••• 

TiOz •••••••••••••••• 

P2os· ••••••••••••••• 

MnO ••••••••••••••••• 

COz· •••••••••••••••• 

60.6 

16.1 

2.4 

3.8 

3.5 

4.9 

3.2 

3.4 

.73 

.33 

.78 

.34 

.08 

• 01 

Total............. 100 

~O/Na o •••••••••••• 1.1 

A:CNK3 •••••••••••••• .90 

~- ................ . 4.60 

sS •••••••••••••••••• 1.62 

58.1 

16.5 

2.0 

4.3 

3.7 

5.3 

3.2 

3.2 

1.0 

99 

.21 

.84 

.41 

.11 

.01 

1.0 

.90 

4.22 

1.52 

60.3 

16.9 

1

6.03 

n.d. 

3.1 

5.16 

3.5 

2.32 

} 2.78 

.94 

.40 

.07 

n.d. 

100 

.66 

.% 

4.63 

1.63 

61.6 

16.7 

1

5.78 

n.d. 

2.7 

4.91 

3.3 

2.61 

2.70 

.73 

.3) 

.06 

n.d • 

99 

• 79 

.97 

5.04 

1.71 

Spectrographic analyses, in parts per millioo 

B. • • • • • • • • • • • • • • • • • • :sa. •••••••••••••••••• 

:Be •••••••••••••••••• 

c:e •••••••••••••••••• 

Co •••••••••••••••••• 

Cr •••••••••••••••••• 

Cll •••••••••••••••••• 

Ga. ••••••••••••••••• La. ••••••••••••••••• 

Nb •••••••••••••••••• 

Nd •••••••••••••••••• 

Ni •••••••••••••••••• 

Pb •••••••••••••••••• 

Sc •••••••••••••••••• 

Sr •••••••••••••••••• 

v ••••••••••••••••••• 

Y. • • • • • • • • • • • • • • • • • • Yb •••••••••••••••••• 

Zn. • • • • • • • • • • • • • • • • • Zr •••••••••••••••••• 

I.a:Ni ••••••••••••••• 

20 

690 

3.3 

<100 

22 

78 

28 

24 

41 

<25 

n.d. 

56 

<10 

2()* 

480 

140 

33 

n.d. 

87 

120 

.73 

(10 

1000 

3.3 

(100 

24 

98 

39 

24 

43 

<25 

n.d. 

60 

<10 

22* 

72()k 

150 

3) 

n.d. 

64 

150 

.72 

A& Crystalline Rocks of the Strawberry Lake Area, Colorado 

<10 

300 

1.5 

(100 15 

70 

70 

15 

3) 

(10 

<70 

50 

<10 

7 

70CJk 

70 

20 

3 

<300 

150 

.60 

<10 

700 

1.5 

(100 

15 

70 

15 

15 

50 <10 

70 

3) 

10 

2()* 

70CJk 

70 

3) 

3 

<300 

200 

1.7 

0.87 

10.4 

12.0 

64.7 

15.5 

1

6.05 

n.d. 

2.3 

3.53 

2.8 

3.10 

2.80 

.74 

.20 

.07 

n.d. 

100 

1.1 

1.08 

5.94 

1.94 

<10 

700 

1.0 

(100 15 

3) 

15 

15 

3) 

<10 

<70 

15 

10 

15 

500 

70 

15 

1.5 

<300 

150 

2.0 

SL-70 

1.24 

16.8 

13.5 

66.1 

15.3 

1s.66 

n.d. 

2.0 

3.49 

3.0 

2.83 

2.60 

.70 

.20 

.05 

n.d. 

100 

.94 

1.07 

6.45 

2.05 

<10 

500 

1.0 

100 

15 

3) 

15 

15 

7()* 

(10 

<70 

15 

10 

7 

500 

70 

15 

1.5 

(3)0 

150 

4.7 
Table 2. Uranium, thorium, chemical, spectrogrphic, and modal analyses and 

descriptive parameters of the Boulder Creek Granodiorite-Continued 

Sample No. 

Conposition SL-35 SL-51 SL-57 SL-66 SL~7 SL-70 

t1ldal. analyses, in volwe percent 

c·············· 16.9 20.7 19.0 21.9 25.4 31.3 

ocli.rle. •••••••••• 10.8 9.4 2.6 3.3 5.5 

Plagioclase6 •••••••• 42.5 37.5 59.0 55.7 52.4 37.1 

Anorthite ••••••••• 35 43 38 36 32 32 

Biotite ••••••••••••• 16.9 11.3 11.7 12.6 16.6 23.3 

MUscovite/sericite •• 4.0 .3 

OOorite •••••••••••• .1 .3 

Hornblende •••••••••• 9.4 13.7 7.5 5.4 .5 ~tite ••••••••••• .2 .1 .1 .5 .9 

Hematite •••••••••••• .4 

~tite* •••••••••••• .7 .5 .8 .7 .8 1.0 

Sphene* ••••••••••••• 2.4 1.2 1.3 .9 .2 .3 

Zircon. ••••••••••••• tr tr tr tr .1 tr 

!tidote ••••••••••••• .2 1.6 .3 tr 

lanite •••••••••••• tr tr .2 tr 

Total ••••••••••••• TIX) TIX) Tii) TIX) ]X) 1iX) 

Points counted •••• 801 826 710 684 610 690 

Color index7 •••••••• 29.1 27.9 21.2 19.1 18.1 24.8 

Average flain size, 2.5 2.5 3 3 2.5 2.5 

in mil i.rreters. 

Magnetic, m, or ••••• m m IDl m m m 

nonmagnetic, m 8• 

Radioactivity ,9 in.. 110 150 125 105 120 165 

counts per second. 

Specific gravity10 •• 2.770 2.830 2.814 2.795 2.745 2.750 

Rc>ck. name. • • • • • • • • • • ~e. ft~e. tonalite tonalite tonalite 

(Streckeisen, 1976). ~~e. 

1rotal iron reported as Fe20:3. 

2wr at 900 °C. 

3A:CNK = alumina saturation = Al2~ (in nnlecular proportions). 

CaO+NazO+K2o 

4F = Sio2+Na20+Kz0 

(oxides in ~ight percent). 

Fe0+-Fe 0)~ 

5s = SiOz 

(oxides in ~ight percent). 

A12o3+Na20+K20+Mg(}+Q!O+-FeO+-Fez0:3 

&plagioclase determinations ~re nade by a combination of maxim..nn extinction angles 

to the 010 trace, optic sign, am refractive index conparisons against quartz. 

7 Color index is the sun of the colored (nelanocratic) mi.nerals expressed in 

percent. Colored minerals are: biotite, 811l>hibole, pyroxene, chlorite, scapolite, all 

opaque mi.nerals, garnet, allanite, zircon, sphene, tounnaline, epidote, and monazite. 

Light (leucocratic) minerals are: quartz, feldspar, nuscovite, cordierite, calcite, 

fluorite, apatite, sillimanite, and clinozoisite. 

8m, rock reacts postively to a large han:! magnet; nrn, no reaction. 

~asured on the outcrop with a M:>tmt Sopris scintill011Eter, M:>del SC-132. 

1

0oetermined with an air pycnotreter, described by ~Intyre and others (1965). An 

average of three determinations for each rock speci.nen yields a specific gravity with 

2:: 0.008 gram per cubic centi.neter maxirrun deviation. 

Petrography and Geochemistry A7 
Table 3. Uranium, thorium, spectrographic, and modal analyses and descriptive parameters of 

the Boulder Creek Granodiorite 

[Delayed neutron activation analyses for U and Th by H.T. Millard, Jr., M. Solt, M. Coughlin, B. Vaughn, M. 

Schneider, W. Stang, R Bies, B. Keaten, and S. Lasater. Semiquantitative spectrographic analyses of sample 

SL--8 by L. Castillo; samples SL-50 and 52 by T. Fries; and samples SL-53, 54, and 61 by N.M. Conklin. •, 

significant trace elements (2x crustal abundance, or more) and diagnOstic accessory minerals. tr, trace;-, not 

found; n.d., not determined; n.a., not applicable) 

Sanple No. 

<hnposition SL-8 SL-50 SL-52 SL-53 SL-.54 SL-61 

Uraniun and thotiun analyses, in parts per million 

u ••••••••••••••••••• 

Th •••••••••••••••••• 

Th:u •••••••••••••••• 

B ••••••••••••••••••• 

'Ba.. ••••••••••••••••• :Be •••••••••••••••••• 

c.e •••••••••••••••••• 

Cl> •••••••••••••••••• 

Cr •••••••••••••••••• 

ili •••••••••••••••••• 

Ga. ••••••••••••••••• ~ ................. . Li •••••••••••••••••• 

Ml •••••••••••••••••• 

Nb •••••••••••••••••• 

r.«:t •••••••••••••••••• 

Ni •••••••••••••••••• P ••••••••••••••••••• 

Pb •••••••••••••••••• 

Sc •••••••••••••••••• 

Sr •••••••••••••••••• 

Ti •••••••••••••••••• v ••••••••••••••••••• 

Y ••••••••••••••••••• 

'Yb •••••••••••••••••• 

.z-n. •••••••••••••••••• 

Zr •••••••••••••••••• 

I,a.:Ni ••••••••••••••• 

Quartz •••••••••••••• Microcline •••••••••• 

1 Plagioclase •••••••• Anorthite ••••••••• Biotite ••••••••••••• 

MUscovite/sericite •• Chlorite •••••••••••• Hornblende •••••••••• Magnetite ••••••••••• Hematite •••••••••••• 

Apatite* •.••••••••••• 

Spllene*. • • • • • • • • • • • • Zircon. ••••••••••••• Epidote ••••••••••••• Clinozoisite •••••••• Allanite •••••••••••• calcite ••••••••••••• 

Total ••••••••••••• Points counted •••• 

1.83 

6.78 

3.7 

1.38 

14.0 

10.1 

1.18 

8.64 

7.3 

1.62 

18.5 

11.4 

Spectrographic analyses, in parts per million 

(10 

840 

3.1 

110 

23 

55 

21 

27 

68* 

96* 

790 

<25 

n.d. 

47 

1500 

3.7 

(10 

550 

6300 

150 

23 

n.d. 

52 

100 

1.4 

(10 

890 

4.2 

(100 

23 

97 

56 

25 

56 

<SO 

1100 

<25 

n.d. 

57 

2900* 

<10 

23* 

610 

5400 

150 

36 

n.d. 

71 

140 

1.0 

(10 

1300* 

2.7 

(100 15 

58 

9.4 

19 

36 

<SO 

710 

(25 

n.d. 

40 

1500 

<10 

13 

550 

4200 

92 

21 

n.d. 

55 

67 

.9 

<10 

700 

1 

200* 

15 

70 

30 

15 

150* 

(100 

700 

(10 

100* 

30 

<2000 

15 

7 

700* 

3000 

70 

20 

2 

<300 

200 

5.0 

M:.x:lal analyses, in voh.JIE percent 

16.8 

10.2 

32.6 

30 

32.2 

.1 

2.4 

2.0 

1.4 

2.3 

100.0 

651 

no 

thin 

section 

available 

16.6 

27.6 

29.5 

31 

15.3 

3.8 

.3 

3.9 

1.0 

.7 

.7 

.1 

.1 

.4 

100.0 

767 

25.3 

11.3 

39.3 

38 

22.4 

.6 

.1 

.1 

.5 

.4 

tr 

tr 

100.0 

835 

AS Crystalline Rocks of the Strawberry Lake Area, Colorado 

2.52 

11.6 

4.6 

<10 

1000 

1 

100 

15 

70 

15 

15 

]()k 

(100 

700 

(10 

100* 

30 

<2000 

10 

15 

700* 

3000 

70 

30 

3 

<300 

150 

2.3 

23.4 

1.24 

9.74 

7.9 

(10 

700 

1 

100 

15 

70 

30 

15 

]()k 

(100 

500 

(10 

70 

30 

<2000 

10 

7 

700* 

3000 

70 

10 

1.5 

<300 

150 

2.3 

5.3 no 

36.3 

35 

20.9 

thin 

11.6 

.1 section 

.5 

1.1 

tr 

.4 available 

.4 

100.0 

786 
Table 3. Uranium, thorium, spectrographic, and modal analyses and descriptive parameters of 

the Boulder Creek Granodiorite-Continued 

Cbnpositicn 

Cblor index2 •••••••• 

Average .grain size, 

in mi.ll:in:Eters. 

M:!gnetic, m, or.·y· 

non:nagnetic, rm • 

Radioactivity4, in.. 

coonts per second. 

Specific gravityS ••• 

SL-8 

38.4 

3.5 

m 

165 

2.720 

Rock. name. • • • • • • • • • • grano- (Streckeisen, 1976). diorite. 

SL-50 

n.a. 

about 3 

m 

150 

2.812 

n.a. 

Sanple No. 

SL-52 SL-53 SL-54 SL-61 

21.4 23.0 34.5 n.a. 

3 2 2.5 about 3 

rm m rm m 

170 120 130 140 

2.759 2.807 2.789 2.850 

granite grano- grano- n. a. diorite. diorite. 

1Plagioclase detenni.nations ~re mde by a combinaticn of maxim.m extinction angles 

to the 010 trace, optic sign, and refractive index conpa.ri.sons against quartz. 

2Color index is the sun of the colored (tlElanocratic) minerals expressed in 

percent. Cblored minerals are: biotite, anphibola, pyroxene, chlorite, scapolite, all 

opaque minerals, garnet, allanite, zircon, sphene, tounnaline, epidote, and m:>nazite. 

Light (leucocratic) minerals are: quartz, feldspar, llllSCOvite, cordierite, calcite, 

fluorite, apatite, sillimanite, and clinozoisite. 

~' rock reacts positively to large harrl magnet; rm, no reaction. 

4r.teasured on outcrop with t1>lm.t Sopris scintillooeter, M:xie1 SC-132. 

Snetermined with an air pyCilOllEter, described by 1-tlntyre and others (1965). An 

average of three detenni.nations for each rock spec:in:En yields a specific gravity with 

:1: 0.008 gram per cubic centi.nEter maxinun deviation. 

Petrography and Geochemistry A9 
Table 4. Modal analyses and descriptive parameters of the Boulder Creek Granodiorite 

[•, diagnostic accessory minerals. tr, trace; -, not found) 

Sample No. 

Cbllp)sition SL-84 SL-86 SL-93 SL-94 SL-96 SL-97 SL-98 

Mxlal analyses, in vol"l.lre percent 

Quartz •••••••••••••• 29.4 21.3 18.8 20.8 28.9 30.7 27.1 

Microcline. 1 •••••••• 18.1 .5 2.8 12.4 17.8 .4 

Plagioclase •••••••• 35.1 41.1 36.7 44.4 53.5 38.1 42.2 

Anorthite ••••••••• 32 32 28 32 32 32 30 

Biotite ••••••••••••• 11.9 29.4 31.8 14.7 17.0 11.5 27.7 

MUscovite/sericite •• .6 .4 tr 

Chlorite •••••••••••• tr tr 1.0 

Hornblende •••••••••• 1.4 1.3 5.8 6.3 

Magnetite ••••••••••• - .9 

Hematite •••••••••••• tr .3 tr .1 tr 

Apatite* •••••••••••• .3 .8 .4 .6 .5 .2 .3 

Sphene*••••••••••••• .3 1.8 1.0 .6 .7 .9 Zirco~ ••••••••••••• tr tr tr tr tr tr 

Epidote ••••••••••••• 2.8 2.4 2.3 .1 tr .4 

Allanite •••••••••••• tr tr .1 

Calcite ••••••••••••• .1 

Total ••••••••••••• 100 100 100 100 100 100 100 

Points counted •••• 725 615 702 712 647 583 669 

Color indexZ ........ 16.4 36.3 40.9 21.8 17.0 13.2 30.0 

Av~r~ J.flain size, 2.5 2.5 2.5 2.5 3 2.5 2.5 

1n nu l.Ileters. 

Magnetic, m, or ••••• rnn rnn rnn rnn rnn rm m . 3 nonma.gnet1c, IJII • 

Radioactivity, 4 in.. 130 110 120 100 160 175 125 

counts per second. 

Specific gravityS ••• 2.727 2.812 2.786 2.803 2.756 2.685 

Rock name. • • • • • • • • • • grano- tonalite tonalite grano- tonalite grano- tonalite 

(Streckeisen, 1976). diorite. diorite. diorite. 

A 10 Crystalline Rocks of the Strawberry Lake Area, Colorado 
Table 4. Modal analyses and descriptive parameters of the Boulder Creek Granodiorite-Continued 

Q:mp>sitian 

()uartz •••••••••••••• 

Microcline •••••••••• 

Plagioclase ••••••••• 

Anorthite ••••••••• 

Biotite ••••••••••••• 

MUscovite/sericite •• 

Chlorite •••••••••••• 

Hornblende •••••••••• 

Magnetite ••••••••••• 

Hematite •••••••••••• 

Apatite* •••••••••••• 

Sphene* ••••••••••••• Zirco~ ••••••••••••• 

Epidote ••••••••••••• 

Clinozoisite •••••••• 

Allanite •••••••••••• 

Total ••••••••••••• 

Points counted •••• 

Color indexZ •••••••• 

Aw;ragf7 . ~ain size, 1n nn.ll:i.Ireters. 

Magnetic, m! or •• "t•. 

nornnagnet1c, mr'. 

Rad . . . 4 . 1oact1V1ty, 1n •• 

counts per second. 

S .f. . 5 peC1 1C graV1tY • • • 

SL-109A 

28.0 

12.8 

35.7 

33 

18.4 

4.1 

.5 

.5 

tr 

tr 

tr 

100 

592 

18.9 

1.5 

m 

120 

2.762 

SL-110 

18.7 

13.8 

43.7 

35 

20.6 

.1 

.6 

.6 

.8 

tr 

tr 

.8 

.3 

tr 

100 

647 

22.6 

2.5 

m 

120 

2.774 

Sanple No. 

SL-lllA SL-112 

20.1 

2.0 

59.8 

35 

17.4 

tr 

.2 

tr 

tr 

.5 

tr 

tr 

100 

5% 

17.9 

2.5 

rnn 

100 

2.770 

16.5 

12.7 

48.6 

35 

15.8 

4.9 

.2 

.5 

.5 

tr 

.3 

tr 

100 

612 

21.7 

2.5 

rnn 

100 

2.767 

SL-113 

16.8 

9.0 

46.1 

32 

13.1 

.2 

.3 

8.6 

.2 

.7 

.7 

tr 

1.6 

1.4 

1.3 

100 

579 

25.8 

2 

rnn 

100 

2.773 

SL-114 

27.6 

44.2 

35 

23.6 

.7 

2.5 

.8 

.5 

tr 

.1 

tr 

100 

601 

26.7 

2 

m 

90 

2.811 

SL-117 

20.3 

1.6 

47.3 

32 

25.7 

.2 

2.7 

.6 

.6 

tr 

tr 

1.0 

100 

630 

30.0 

2 

m 

90 

2.787 

Rock name. • • • • • • • • • • grano- (Streckeisen, 1976). diorite. grano- tonalite grano- grano- tonalite tonalite 

diorite. diorite. diorite. 

1

Plagioclase detenninations Y.ere nade by a combination of nmdrrum extinction angles to the 

010 trace, optic sign, and refactive index conparisons against quartz. 

2Color index is the sun of the colored (relanocratic) minerals expressed in percent. Colored 

minerals are: biotite, amphibole, pyroxene, chlorite, scapolite, all opaque minerals, garnet, 

allanite, zircon, sphene, tounnaline, epidote, and nnnazite. Light (leucocratic) minerals are: 

quartz, feldspar, m.JScovite, cordierite, calcite, fluorite, apatite, sillimanite, and clinozoisite. 

\t, rock reacts positively to a large hand nagnet; nm, no reactio~ 

~ured on outcrop with M:>unt Sopris scintill011Eter, Model SC-132. 

Snetennined with an air pycnooeter, described by Mcintyre and others ( 1965). An average of 

three determinations for each rock speci.nEn yields a specific gravity with a= 0.008 gram per cubic 

cent:i.Ireter nmdrrum deviation. 

Petrography and Geochemistry A11 
Figure 2. Boulder Creek Granodiorite (on right) in contact 

with a small fine-grained granitic enclave. Note atypical weak 

zoning in granitic rock parallel to contact. Specimen label is 

2 centimeters long. 

Figure 3. Small mafic enclave in Boulder Creek Granodiorite. Toothpick on right is 6 centimeters long. 

(fig. 7). Locally, very small monazite grains (less than 

0.02 mm) in biotite leave telltale, dark radiation-damage 

halos. Commonly, the peripheral contact of monazite 

with a feldspar or quartz host is marked by a diagnostic, 

dark reaction(?) or alteration ring of opaque material 

(fig. 8). Apatite grains are generally larger than monazite 

grains (the largest apatite grain is 1.8 by 0.5 mm) and are 

hosted by all the major silicate minerals. Radiationdamage halos caused by apatite do not occur in biotite. 

Most zircons are single small grains, but some have older 

zircon cores and syntaxial overgrowths, which are 

commonly zoned. Some zircons produce weak radiationdamage halos in biotite. Sillimanite occurs as single, 

needlelike grains or sheafs of needlelike grains in 

muscovite or quartz. 

Foliation in the Silver Plume Granite (pl. 1) was 

read on enclaves of biotite gneiss or schist within the 

A 12 Crystalline Rocks of the Strawberry Lake Area, Colorado 

Figure 4. Pink porphyroblasts in Boulder Creek Granodiorite near contact with biotite gneiss. Poptop is 4.5 

centimeters long. 

granite (fig. 9), on alignment of tabular microcline laths, 

or on biotite schlieren. The Silver Plume Granite is 

generally featureless in the Strawberry Lake area and 

does not show any mineral orientation, except for local 

biotite schlieren (fig. 10) or relicts of biotite gneiss in all 

stages of replacement or assimilation. 

The foliation in the biotite-gneiss enclaves in the 

Silver Plume Granite generally conforms to the foliation 

in surrounding biotite gneiss. The planar orientation of 

the tabular microcline laths in the Silver Plume Granite, 

where it infrequently occurs, conforms reasonably well 

with the foliation in nearby enclaves of biotite gneiss. The 

feldspar alignment is generally attributed to magmatic 

flow structure (Pearson and U.S. Bureau of Mines, 1980, 

p. 16), but such alignment may be due to recrystallization 

(feldspathization) in conformity with regional foliation in 

this area. 

Pegmatites in the Silver Plume Granite are 

uncommon and small. Several pegmatites, which are 

located along Lake Granby and are too small to show on 

plate 1, are notable for small amounts of tourmaline 

(black schorl). Granitic enclaves in the Silver Plume 

Granite that are more radioactive than their host are less 

than 100 m in greatest dimension and occur on the south 

shore of Lake Granby. 

Whole-rock sample SM-4 was dated by C.E. Hedge 

(written commun., 1982) using the rubidium-strontium 

method. He assumed an initial 87Srl6Sr of0.7036, which 

was determined previously from suites of Silver Plume 

Granite, and calculated an apparent age of 1,503 ± 100 

m.y. The substantial uncertainty (± 100 m.y.) results 

from the use of a single sample. Nevertheless, 1,503 ± 

100 m.y. is closer to the 1,390-1,450-m.y. grouping of the 

Silver Plume Granite than to the 1,700-1,800-m.y. 
/::,. ... SYENOGRANITE ... /::,. 

• 

+ 

• • • + • 

• 

EXPlANATION 

• Boulder Creek Granodiorite 

+ Boulder Creek Granodiorite: 

chemically analyzed samples 

[J Boulder Creek Granodiorite: 

average of chemically analyzed 

samples 

/::,. Silver Plume Granite 

-'. Silver Plume Granite: chemically 

analyzed samples 

0 Silver Plume Granite: average of 

chemically analyzed samples 

QUARTZ MONZONITE 

ALKALI 

FELDSPAR .__ __ .....lL.. __ ___;~t...-__ ___lL _ _,___lol,.._ __ _y_ ___ lL...___:L___y_ ___ y__ __ ___.Y. __ __l PLAGIOCLASE 

20 30 40 50 60 70 80 90 100 

Figure 5. Quartz-alkali feldspar-plagioclase (QAP) diagram showing Boulder Creek Granodiorite and Silver Plume Granite. 

grouping of the Boulder Creek Granodiorite (Peterman 

and others, 1968). 

Quartz-alkali feldspar-plagioclase ratios are plotted on a QAP diagram in figure 5 for all modally analyzed 

rocks (32 samples) of the Silver Plume Granite. In the 

Streckeisen {1976) classification most of the rocks are 

monzogranites, a sizable number are syenogranites, one 

is a granodiorite, and one is a quartz monzonite. 

Potassium predominates over sodium; the average 

~O:N~O ratio of seven samples is 1.9. Recent analyses 

of Silver Plume Granite by Anderson and Thomas {1985) 

also yield an average K2 0:Na20 ratio of 1.9 {10 

samples). Another indication of how felsic the Silver 

Plume Granite is, in contrast to the Boulder Creek 

Granodiorite, is the fact that even the ranges of values for 

the two rock types do not overlap for color index, specific 

gravity, radioactivity, thorium, and ~O:N~O (table 17). 

Granitic Enclaves in Silver Plume Granite 

Granitic enclaves in the Silver Plume Granite are 

mappable only where exposure is very good because they 

are small and their megascopic appearance is similar (at 

first glance) to that of the Silver Plume Granite. Three 

sizable enclaves and two smaller ones occur in the same 

general area on the south shore of Lake Granby (pl. 1 ). 

The three largest bodies are about 75 m by 12 m, 45 m by 

23 m, and 30 m by 8 m. The latter two are close to water's 

edge, and they are susceptible to seasonal flooding. The 

longest enclave, which is confined to a small ravine, is 

high enough above lake level for exposure preservation 

(pl. 1). 

The rock that composes these enclaves is 

approximately twice as radioactive, more melanocratic, 

Petrography and Geochemistry A 13 
A14 

Table 5. Uranium, thorium, chemical, spectrographic, and modal analyses and 

descriptive parameters of granitic enclaves in the Boulder Creek Granodiorite. 

[Delayed neutron activation analyses for U and Th by H.T. Millard, Jr., M. Solt, M. Coughlin, B. 

Vaughn, R. Bies, B. Keaten, S. Lasater, J. Storey, and S. Danahey. Rapid rock chemical analyses 

of sample SL-11 by V. Smith and J. Reid, and sample SL-20 by K. Coates. Chemical (X-ray 

spectrographic) analyses of sample SL-147 by A. Bartel; and sample SL-58 by J.S. Wahlberg, J. 

Taggart, and J. Baker. Semiquantitative spectrographic analyses of sample SL-58 by N.M. 

Conklin; samples SL-3, 11, and 20 by G. Kaczanowski; and sample SL-147 by L. Bradley. 

*, significant elements (2 x crustal abundance, or more) and diagnostic accessory minerals. n.d., 

not determined; tr, trace;-, not found; <,less than; >,greater than] 

Size of enclave 

Snall: 1-S m Large: 0. S km or 100re 

Sanple No. 

Conposition SL-11 SL-20 SL-.58 SL-59 SL-147 

Uraniun and thoriun analyses, in parts per million 

u •••••••••••••••••••••• 1.96 1.77 l.SS 2.08 1.87 

Th. •••••••••••••••••••• 86.2* 33.9* SS.l* 23.7 8.65 

Th:U ••••••••••••••••••• 44.0 19.2 35.5 11.4 4.63 

Olemi.cal analyses, in weight percent 

SiOz· •• ••••• ••••••••••• 69.3 71.2 71.8 n.d. 63.8 

Al20:3· ••••••••••••••••• 14.7 13.4 15.4 n.d. 14.7 

Fe20:3 •••••••••••••••••• 2.0 1.9 11.86 n.d. 16.94 

FeO. ••••••• •••••• •••••• 1.0 .96 n.d. n.d. n.d. 

~-··················· .64 • 78 .68 n.d • 1.09 

eao •••••••••••••••••••• .98 1.1 2.19 n.d. 2.51 

Na2o ••••••••••••••••••• 2.4 1.9 3.4 n.d. 3.20 

K2~..;: •••••••••••••••••• 7.5* 7.4* 3.75 n.d. 4.41 

H20 ••••••••••••••••••• .42 .64 

} 2.70 2 n.d. .51 

H20-••••••••••••• •••••• .21 .32 

Ti.Oz ••••••••••••••••••• . ss .51 .28 n.d • 1.02 

P20s· • • • ••••• •••• ••• •• • .23 .09 (.1 n.d. .SO* 

fu() •••••••••••••••••••• .02 .04 (.02 n.d. .07 

COz·· •••••••••••••••••• • 03 .02 n.d. n.d. n.d • 

Total •••••••••••••••• 100 100 100 n.d. 99 

K20:Naz0· •••••••••••••• 3.13 3.90 1.10 n.d. 1.38 

A:CNK3 •••• ••••••••••••• 1.16 1.05 1.02 n.d. 1.13 

~ ..................... 17.1 17.0 16.7 n.d. 6.78 

ss •......•.••.•........ 2.37 2.59 2.63 n.d. 1.94 

Spectrographic analyses, in parts per million 

B. • • • • •• • • • • • •• • • • • • • • • <10 <10 (10 <10 (10 

Ba. •••••••••••••••••••• 2400* 1500* 700 700 3000* 

Be ••••••••••••••••••••• 1.1 1.1 <I (1 1.5 

Ce ••••••••••••••••••••• 420* <IOO ISO* ISO* 300* 

Co ••••••••••••••••••••• 5.3 6.0 3 3 7 

Cr ••••••••••••••••••••• (10 (10 7 15 s 

Cu. •••••••••••••••••••• 7 12 I 2 10 

Ga. •••••••••••••••••••• 20 14 15 15 30 

La. •••••••••••••••••••• 230* 67* ISO* 100* ISO* 

Nb. •••••••••••••••••••• <25 <25 (10 (10 IS 

Nd ••••••••••••••••••••• n.d. n.d. 70 70 ISO* 

Ni ••••••••••••••••••••• 5.3 8.4 3 3 <10 

Pb ••••••••••••••••••••• 38* 38* 30 30 30 

Sc. • • • • ••• •• • • • • • • • • • • • (10 <10 7 7 15 

Sr ••••••••••••••••••••• 270 240 700* 700* 1000* 

v •••••••••••••••••••••• 42 33 IS 30 70 

Y. • • • •• • • • • • • •• • • • • • • • • 15 12 10 IS 50 

Yb ••••••••••••••••••••• n.d. n.d. 1 7 3 

Zn. •••••••••••••••••••• 57 <SO <300 <300 <300 

Zr ••••••••••••••••••••• 340* 380* 150 150 200 

La:Ni •••••••••••• •••• •• 43 8.0 50 33 )15 

Crystalline Rocks of the Strawberry Lake Area, Colorado 
Table 5. Uranium, thorium, chemical, spectrographic, and modal analyses and 

descriptive parameters of granitic enclaves in the Boulder Creek Granodiorite- · 

Continued 

Size of enclave 

Small: 1-5m Large: 0. 5 km or rore 

Sample No. 

Colq>osition SL-11 SL-20 SL-58 

MxJal analyses, in volure percent 

~z ••••••••••••••••• 23.2 

Microcline ••••••••••••• 54.3 

Plagioclase6 ••••••••••• ll.6 

Anorthite •••••••••••• 14 

Biotite. ••••••••••••••• 8.5 

M.Jscovite •••••• ·• ••••••• .s 

Sericite ••••••••••••••• OOorite ••••••••••••••• 

Magnetite. ••••••••••••• Hematite ••••••••••••••• 1.0 

Apatite. ••••••••••••••• MJnazite ••• •••••••••••• .7* 

Sphene. •••••••••••••••• Zircon. •••••••••••••••• .2 =e •••••••••••••••• 

te ••••••••••••••• 

Total •••••••••••••••• 100 

Points counted ••••••• 801 

O:>lor index7 ••••••••••• 9.7 

Average ~ain size, •••• <1 

in mi. iaeters. 

Magnetic, m, or •• 8 ..•.• m 

I10llliEgl1etic, nm • 

Radioactivity9, in. •••• 490 counts per second. 

S peel. T 1c graVl.t . ylO ••••• 2.650 

Rock name ••••••••••••• fsanite (Streckeisen, 1976). yeno- granite). 

1Total iron reported as Fe2

o3• 

2

101 at 900 °C. 

27.9 32.9 

41.8 30.5 

17.6 29.1 

OS 12 

8.4 3.4 

2.7 

.2 tr .s .3 

2.1 

.9 

.3* trk 

tr .8 tr .3 tr tr .2 

.1 

100 100 

1803 584 

12.2 4.8 

<1 1.5 

m nm 

525 220 

2.630 2.677 

fsanite rmanite 

yeno- IOOilZogranite). granite). 

SL-59 

28.5 

13.3 

50.7 

15 

5.0 

2.1 

.1 

.3 

trk 

tr 

100 

701 

5.4 

2.5 

nn 

200 

2.689 

ID:~e 

Alz03 

3A:CN< = alumina saturation = ----- (in rolecular proportions). 

4F = Si0z+Na20li<.z0 

FeO+Fe 0J~ 

Ca0+Nazo+K20 

(oxides in ~ht percent). 

Si02 

Ss = ------------ (oxides in weight percent). 

Al2~+NazO!i<.2~FeO+Fe2~ 

SL-147 

22.6 

27.8 

35.3 

24 

8.4 

2.5 

tr 

1.9 

1.4* 

.1 

tr 

100 

806 

10.4 

<1 

m 

150 

2.758 

~anite IOOilZogranite). 

6plagioclase detenninations were nede by a combination of umdnum extinction angles 

to the 010 trace, optic sign, and refractive index conparisons against quartz. 

7 Color index is the sun of the colored (aelanocratic) minerals expressed in 

percent. O:>lored minerals are: biotite, anphibole, pyroxene, chlorite, scapolite, all 

opaque minerals, garnet, allanite, zircon, sphene, tounnaline, epidote, and lll)nazite. 

light (leucocratic) minerals are: quartz, feldspar, llllScovite, cordierite, calcite, 

fluorite, apatite, sillimanite, and clinozoisite. 

But, rock reacts postively to a large hand magnet; nm, no reaction. 

~asured on the outcrop with a M::>unt Sopris scintilloaeter, M::>del SC-132. 

1

Doetermi.ned with an air pycooaeter, described by M:Intyre and others (1%5). An 

average of three detenninations for each rock speciaen yields a specific gravity with 

:t: 0.008 gram per cubic centiaeter maxi.nun deviation. 

Petrography and Geochemistry A 15 
Table 8. Modal analyses and descriptive parameters of granitic enclaves in the Boulder Creek Granodiorite 

[*,diagnostic accessory minerals. tr, trace;-, not found; <,less than) 

Size of enclave 

SnBll: 1-5m Large: loo-400 m 

J..e No. 

Couposition SL-95 SL-109B SL-lllB 81~5 SL-99 SL-115 SL-116 

M:ldal. analyses, in voJ..me percent 

~z·••••••••••••••••• 31.8 33.0 35.5 33.0 25.3 24.3 25.2 

crocline. ••••••••••••• 41.8 32.6 36.2 55.8 45.4 48.8 44.2 

Plagioclase1 •••••••••••• 19.5 21.6 18.8 7.4 20.2 15.1 18.9 

Anorthite ••••••••••••• 28 20 26 13 13 27 25 

Biotite. •••••••••••••••• 5.0 5.7 6.4 1.6 5.1 7.1 4.6 

Muscovite ••••••••••••••• 6.1 tr 2.1 2.0 2.1 4.1 

Ollorite. ••••••••••••••• 0.5 tr 0.7 tr 0.1 0.8 

Magnetite ••••••••••••••• 0.5 1.0 1.2 1.0 1.8 1.0 

H:ma.tite ••• ••••••••••••• - Apatite ••••••••••••••••• tr'* tr'* 0.5* 0.1* 0.5* 0.3* 0.5* 

M:mazite •••••••••••••••• tr tr tr Sphene •••••••••••••••••• 0.2 tr 0.5 

Zircon. ••••••••••••••••• tr"< tr"< tr'* tr 0.4 0.1 ~e ••••••••••••••••• 0.7 tr 0.7 0.3 te •••••••••••••••• tr tr 0.3 

Total. ••••••••••••••••• 100" ""i1X) ""i1X) RO RO RO 100"" 

Points counted •••••••• 559 592 563 578 605 708 608 

Color index2 •••••••••••• 6.9 6.7 9.0 1.6 6.6 9.4 7.4 

Average J.ft;ain size, ••••• <1 1.5 <1 <1 

in mi. ~ters. 

Magnetic, m, or •• 3 •••••• m m m Ill1 m m m 

nonmagnetic, nm • 

Radioactivity4 , in. ••••• COWltS per second. 

180 480 365 165 270 470 400 

Specific gravityS ••••••• 2.666 2.666 2.665 2.625 2.670 2.674 2.675 

Rock name............... granite 

(Streckeisen, 1976). (syenogranite). granite granite 

(nvnzogranite). (syenogranite). 

granite granite granite granite 

(syenogranite). (syenogranite). (syenogranite). (syenogranite). 

1

Plagioclase determinations \~Jere tmde by a ccrnbination of rraxim..mt extinction angles to the 010 trace, optic sign, and refractive index 

corrpa.risoos against quartz. 

Zcolor index is the sun of the colored (rel.anocratic) minerals expressed in percent. Colored minerals are: biotite, arphibole, pyroxene, 

chlorite, scapolite, all opaque minerals, garnet, allanite, zircon, sphene, tounnaline, epidote, and JOOllaZite. Light (leucocratic) minerals are: 

quartz, feldspar, lll.ISCOvite, cordierite, calcite, fluorite, apatite, sillimanite, and clinozoisite. 

3

m, rock reacts postively to a large harxl. magnet; rnn, m reaction. 

~ured on the ootcrop with a M:>unt Sopris scintillOIIEter, M:>del SC-132. 

Snetermined with an air pyCl'lbi!Eter, described by M:Intyre and others (1965). kJ. average of three determinations for each rock specimen 

yields a specific gravity with~ 0.008 gram per cubic centineter maxinun deviation. 

A 16 Crystalline Rocks of the Strawberry Lake Area, Colorado 
• 

• 

EXPlANATION 

• Granitic enclaves in the biotite gneiss 

+ Granitic enclaves in the biotite gneiss: 

chemically analyzed samples 

A Granitic enclaves in the Boulder Creek 

Granodiorite 

A Granitic enclaves in the Boulder Creek 

Granodiorite: chemically analyzed 

samples 

0 Granitic enclaves in the Silver Plume 

Granite: chemically analyzed samples 

X Small mafic enclave (SL-101B) in the 

Boulder Creek Granodiorite 

Andesite porphyry dike (SL-179) 

A 

MONZOGRANITE 

GRANODIORITE 

QUARTZ MONZONITE QUARTZ 

MONZODIORITE 

ALKALI 

FELDSPAR .__ __ __,._ ___ ....__ __ ___.,____,__--"----..lL.---__.ljt:,__--l.--¥...---.x._--__,.::l---~ PLAGIOCLASE 

10 20 30 40 50 60 70 80 90 100 

Figure 6. Quartz-alkali feldspar-plagioclase (QAP) diagram showing granitic enclaves and mafic rocks. 

and slightly finer grained than the Silver Plume Granite, 

and it shows a subtle crystalloblastic texture (fig. 11 ). 

Contact relations between the granitic enclaves and the 

Silver Plume Granite are generally sharp, but there is no 

semblance of a chilled border in the enclaves. 

Quartz, microcline (locally perthitic ), oligoclase 

(locally antiperthitic), biotite, and muscovite are essential 

minerals. Chlorite is secondary after biotite, and 

magnetite, monazite, apatite, zircon, and sillimanite 

(occurring as elongate prisms) are common accessory 

minerals (table 10). Monazite is definitely more 

abundant than zircon or apatite. Monazite occurs as 

small inclusions in all the essential minerals; the largest 

monazite grain is 0.09 by 0.06 mm. Dark, intense 

radiation-damage halos in biotite that were produced by 

monazite are invariably 0.03 mm thick. Halos that were 

produced by zircon are less intense and no more than 

0.01 mm thick. 

The granitic enclaves are plotted on a QAP 

diagram in figure 6, where they appear as monzogranites. 

In contraSt to the host Silver Plume Granite, the granitic 

enclaves are more peraluminous (tables 10 and 17), show 

higher 0:N~O ratios, higher color indices and specific 

gravities (primarily due to more biotite), and contain one 

and one-half times as much radioactivity and twice as 

much thorium. 

Biotite Gneiss 

The biotite-gneiss unit is predominantly a fine- to 

medium-grained biotite gneiss (tables 11 and 12) but 

includes minor granite gneiss, amphibolite, quartzite, 

pegmatite, and cordierite-garnet-sillimanite-biotite 

gneiss and schist. The biotite-gneiss unit is host to 

fine-grained granitic enclaves of variable size as 

described below. 

Petrography and Geochemistry A17 
A18 

Table 7. Uranium, thorium, chemical, spectrographic, and modal analyses and descriptive parameters of the 

Silver Plume Granite 

[Delayed neutron activation analyses for U and Th by H.T. Millard, Jr., M. Solt, M. Coughlin, B. Vaughn, M. Schneider, W. Stang, 

R. Bies, B. Keaton, S. Lasater, J. Storey, and S. Danahey. Rapid rock chemical analyses of samples SM-3 and SL-34 by K. Coates, 

and sample SL-40 by N. Skinner and D. Kobilis. Chemical (X.;.ray spectrographic) analyses of samples SM-4 and 43A by AJ. Bartel; 

and samples SL-64 and 65 by J.S. Wahlberg, J. Taggart, and J. Baker. Semiquantitative spectrographic analyses of samples SM-3 

and SL-34 by G. Kaczanowski; sample SM-4 by L. Bradley; sample SL-40 by T. Fries; and sample SM-43A and SL-64 and 65 by 

N.M. Conklin. •, significant elements (2 x crustal abundance, or more) and diagnostic accessory minerals. n.d., not determined; tr, 

trace; -,not found; <,less than) 

Sa.nple No. 

O:>npositioo. SM-3 SM-4 SM-43A SL-34 SL-40 SL-64 SL-65 

Uranit.m an:l thorit.m analyses, in parts per nrl.llion 

u .•••..•...•...•..•. 8.79* 3.65 6.8* 6.77* 7.44* 6.57* 8.27* 

1h. ••••••••••••••••• 37.0* 30.0* 41.0* 48.0* 27.1* 38.2* 52.8* 

Th:U •••••••••••••••• 4.2 8.5 6.0 7.1 3.6 5.8 6.4 

<han:i.cal analyses, in weight percent 

Si02 •••••••••••••••• 72.8 71.0 71.0 71.4 72.0 73.0 71.8 

Ali:>:3·· ••••••••••••• 14.5 14.7 14.8 14.5 14.3 14.7 14.7 

Fe:t>J ••••••••••••••• 1.2 1

2.36 11.82 1.4 0.83 11.38 11.37 

FeO ••••••••••••••••• 0.68 n.d. n.d. 1.0 .68 n.d. n.d. ~- ................ .36 0.67 0.48 0.65 .34 0.40 0.40 

eao ••••••••••••••••• 1.0 .99 1.27 1.80 .96 .92 1.05 

Na2o •••••••••••••••• 3.0 2.74 2.82 2.80 3.30 2.80 3.1 

K~. • • • • • • • • • • • • • • •• 6.0* 5.49 5.63 6.00* 5.00 5.66 5.49 

Hzo+ •••••••••••••••• .59 }2.94 .51 .53 } 2.71 2.76 2.58 

H2o-•••••••••••••••• .24 .26 .23 

Ti~······ .......... .27 .32 .28 .33 .17 .19 .23 

P20s• • • • • • • • • • • • • • • • .13 .15 .14 .18 .14 (.10 .10 

~ ................. .02 <.02 <.02 .04 .04 <.02 (.02 

C0:2· •••••••••••••••• .03 n.d. n.d. .01 • 01 n.d • n.d. 

Total ••••••••••••• ~ ~ 99 ~ ~ ~ 9<J 

I<:lO:NazO· ••• • • • •• • •• 2.0 2.0 2.0 2.1 1.5 2.0 1.8 

A:rnK3 •••••••••••••• 1.09 1.20 1.13 1.01 1.14 1.19 1.13 

~- ................. 25.2 19.7 22.3 16.5 28.6 30.2 24.2 

s5 •••••••••••••••••• 2.72 2.63 2.65 2.54 2.83 2.82 2.75 

Spectrographic analyses, in parts per nrl.llioo. 

B ••••••••••••••••••• (10 (10 (20 <10 (10 (10 (10 

Ba. ••••••••••••••••• 850 500 700 610 430 D) 500 

Be •••••••••••••••••• 1.6 <1.5 (1 1.3 1.7 (1.0 (1 

Ce •••••••••••••••••• (100 (100 150* 170* (100 150* 200* 

Co •••••••••••••••••• 2.4 <S (3 1.5 1.4 (3 (3 

Cr •••••••••••••••••• (10 5 3 (10 (10 1.5 7 

01. ••••••••••••••••• 6.2 15 30 23 12 7 30 

Ga. ••••••••••••••••• 17 30 30 20 21 15 15 

La. ••••••••••••••••• 54 70* 70* 100* 31 70* 150* 

Nb •••••••••••••••••• <25 10 15 <25 <25 (10 (10 

Nd •••••••••••••••••• n.d. 150* n.d. n.d. n.d. 70 ISO* 

Ni •••••••••••••••••• 4.9 <S (3 4.4 2.7 <2 2 

Pb •••••••••••••••••• 53* 50* 30 36* 21 30 30 

Sc •••••••••••••••••• (10 (10 5 (10 (10 (10 (10 Sr •••••••••••••••••• 200 200 200 120 81 150 150 

v ................... 17 30 30 21 11 15 30 

Y. • • • • •• • • • • • • • • • • • • 13 (10 10 18 (10 10 15 

Yb. ••••••••••••••••• n.d. (1 n.d. n.d. n.d. 1 1 

L.rl. ••••••••••••••••• (5() <300 <300 61 (5() (5() <SO 

Zr •••••••••••••••••• 160 70 150 120 62 70 150 

La:Ni ••••••••••••••• 11 )14 )23 23 12 )35 75 

CrystaiUne Rocks of the Strawberry Lake Area, Colorado 
Table 7. Uranium, thorium, chemical, spectrographic, and modal analyses and descriptive parameters of the 

Silver Plume Granite-Continued 

Sanple No. 

<bnposition SM-3 SM-4 SM-43A SL-34 

M:xia.1 analyses, in volme percent 

~:::::::::: ~:~ Plagiocl.ase6••• • • • • • 20.7 

Anorthite. • • • • • • • • 12 

Biotite. • • • • • • • • • • • • 7.1 

Muscovite. • • • • • • • • • • 3. 6 

Qllorite. ••••••••••• Magnetite. •••••••••• Hematite............ 1.5 

Apatite............. .2 

r-tmazite*. • • • • • • • • • • • 2 

Zircon*............. tr Sphene •••••••••••••• 

E(>idote ••••••••••••• Sillinenite ••••••••• 

Allanite •••••••••••• Fluorite. ••••••••••• Pyrite •••••••••••••• calcite ••••••••••••• 

Total ••••••••••••• 100 

Points counted •••• 813 

Color index7 •••••••• 

Av~r~1 ff!n size, m ters. 

Magnetic, m, or ••••• 

normagnetic, rJ3. 

Radioactivity, 9 in.. counts per second. 

Specific gravityl-0 •• 

Rock naJ:JJ: 

( Streckeisen, 

1976). 

8.8 

1.5 

rnn 

350 

2.630 

granite 

(syenogranite). 

1

Total iron reported as Fez03• 

2

LC>I at 900 °C. 

28.6 

39.0 

22.8 

15 

.9 

3.7 

3.1 

.9 

.6 

tr 

tr 

.3 

tr 

tr 

tr 

.1 

ill) 

703 

5.2 

3.0 

m 

460 

2.668 

= granite). 

Al20J 

3A:<N< = alunina saturation = -----

Ca01-Na2o+Kz0 

28.1 

43.2 

20.7 

15 

3.3 

3.5 

.6 

.1 

.4 

tr 

tr 

.1 

ill) 

691 

4.0 

2.5 

m 

335 

2.662 

granite). 

r~ 

23.5 

43.0 

23.5 

25 

4.5 

3.1 

.8 

.9 

-

.3 

.4 

tr 

tr 

ill) 

800 

6.6 

2.5 

m 

450 

2.660 

granite 

{nnnzogranite). 

(in mlecular proportions). 

Si.Oz+NazO+K2o 

~=----- (oxides in weight percent). 

SiOz 

5S=--------------- (oxides in weight percent). 

A12~+NazO+K2~FeOtFe20:J 

SL-40 

35.1 

31.3 

26.5 

12 

2.8 

3.2 

.3 

tr 

tr 

tr 

tr 

.8 

ill) 

713 

3.1 

1.5 

rnn 

390 

2.643 

granite 

{nnnzogranite). 

SL-64 

32.3 

28.1 

33.3 

13 

4.1 

1.3 

.1 

tr 

tr 

.8 

tr 

ill) 

716 

4.2 

2.5 

rm 

320 

2.647 

= granite). 

SL~5 

24.7 

28.3 

40.0 

15 

4.9 

1.3 

tr 

.6 

.1 

tr 

tr 

.1 

5.5 

3.5 

360 

2.664 

granite 

{nnnzogranite). 

&plagioclase detenninations were made by a combination of nmdm.nn extinction angles to the 010 trace, optic sign, 

and refractive index conparisons against quartz. 

7 Color index is the sun of the colored (uelanocratic) minerals expressed in percent. Colored minerals are: 

biotite, anphibole, pyroxene, chlorite, scapolite, all opaque minerals, garnet, allanite, zircon, sphene, tourmaline, 

epidote, and n:onazite. Light (leucocratic) minerals are: quartz, feldspar, llllSCOvite, cordierite, calcite, fluorite, 

apatite, sillimanite, and clinozoisite. 

f\n, rock reacts postively to a large hand negnet; run, oo reaction. 

~ on the outcrop with a t-b.m.t Sopris scintil.l.o!reter, M:xiel SC-132. 

1

0netennined with an air pyCilOileter, described by l-tintyre and others (1965). kt average of three detenninations 

for each rock spec:f.mm yields a specific gravity with ::1: 0.008 gran per cubic cent:im:!ter maxim.m deviation. 

Petrography and Geochemistry A19 
t 

0 

(') 

~ Table 8. Uranium, thorium, spectrographic, and modal analyses and descriptive parameters of the Silver Plume Granite 

i [Delayed neutron activation analyses for U and 1b by H.T. Millard, Jr., M. Solt, M. Coughlin, B. Vaughn, M. Schneider, W. Stang, R. Bies, B. Keaten, and S. Lasater. Semiquantitative S' spectrographic analyses of samples ML-1, 2, and 4, and samples SL-29, 30, and 31 by G. Kaczanowski; samples SL-37, 39, 41, 42, 44, 45, 46, 47, 48, and 49 by T. Fries; and samples SL-62 and • ::zJ 63 by N.M. Conklin. •, significant trace elements (2 x crustal abundance, or more), and diagnostic accessocy minerals, tr, trace; -, not found; n.d., not determined; n.a., not applicable; <, less 

g than] 

=- 0 

- Sanple No. !f • (bnpositioo. ML-1 ML-2 ML-4 SL-29 SL-:r> SL-31 SL-37 SL-39 SL-41 SL-42 SL-44 SL-45 SL-46 SL-47 SL-48 SL-49 SL~2 SL~3 

w • i Uraniun and thoriun analyses, in parts per ml.llion 

.. 

~ u ••••••••••••••••••••• 2.23 2.78 5.80* 3.33 5.00* 4.42 4.39 6.08* 5.59* 9.03* 3.36 5.83* 3.04 5.42* 5.91* 7~35* 5.60* 5.76* 

r- Th. ••••••••••••••••••• 71.5* 71.2* 45.2* 70.7* 33.8* 38.7* 21.6 33.6* 31.9* 61.7* 41.1* 60.0* 32.8* 64.0* 50.1* 71.2* 31.9* 26.4* 

• r Th:U •••••••••••••••••• 32.1 25.3 7.8 21.2 6.8 8.8 4.9 5.5 5.8 6.8 12.2 10.3 10.8 11.8 8.5 9.7 5.7 4.6 

:!: 

J Spectrographic analyses, in parts per ml.llion 

~ B ••••••••••••••••••••• (10 (10 (10 (10 (10 (10 (10 (10 (10 (10 (10 (10 <10 (10 (10 (10 (10 (10 

Ba. ••••••••••••••••••• 1300* 1500* 700 720 470 590 470 620 580 720 890 1000 1200 830 810 730 500 300 .. Be •••••••••••••••••••• 1.2 1.4 2.4 (1 2.1 2 3.6 2.9 1.8 1.8 1.6 2.2 1.9 1.6 1.4 (1 <I (1 • a. Oe •••••••••••••••••••• 200* 280* 200* 270* (100 140* 140* 150* 150* 190* 140* 190* 130 230* 200* 280* 100 200* 

0 <h •••••••••••••••••••• 3.4 3.9 3.0 4.7 1.1 2.8 2.3 2.3 2.2 4.1 2.6 6.0 4.9 3.1 3.1 4.6 (3 (3 

Cr •••••••••••••••••••• 11 11 (10 (10 (10 (10 (10 (10 (10 (10 11 (15 13 (10 (10 11 3 1 

Cu. ••••••••••••••••••• 24 19 14 22 5.8 18 11 25 6.6 51 17 9.9 5.1 24 19 9.6 7 15 Ga. ••••••••••••••••••• 21 22 26 20 20 23 23 24 18 22 20 24 20 16 22 19 15 15 

La •••••••••••••••••••• 85* 170* 120* 150* 45 80* 60* 75* 69* 100* 72* 130* 58* 140* 120* 160* 70* 150* ~ ................... (200 <200 2:r> 290 (200 260 230 <200 (200 <230 <200 270 250 (200 210 (200 100 70 

Nb •••••••••••••••••••• <25 <25 <25 <25 <25 <25 <25 <25 <25 <25 <25 <25 (25 (25 <25 <25 (10 (10 

Nd •••••••••••••••••••• n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 70 150* 

Ni •••••••••••••••••••• 7.6 7.0 5.5 8.5 2.5 3.7 3.2 3.5 3.0 5.1 4.8 7.7 6.6 4.1 4.0 5.9 2 (2 

P ••••••••••••••••••••• 900 100) 900 900 1100 900 1600 1500 100) 2100* 1400 1600 1400 1100 1500 900 <2000 (2000 

Pb•••••••••••••••••••• 31 61* 46* 46* 34* 35* 23 26 20 49* 52* 25 25 41* 31 32* .D .D 

Sc. ••••••••••••••••••• (10 (10 (10 (10 (10 (10 (10 (10 (10 (10 (10 (10 (10 (10 (10 (10 <7 (7 

Sr •••••• •••••••••••••• 160 210 130 100 82 120 100 120 140 120 170 100 210 160 160 150 150 150 

Ti •••••••••••••••••••• 2200 2300 1300 2100 1200 1700 1200 1400 100) 1900 1800 3000 2100 2400 1900 2100 1500 700 

v ••••••••••••••••••••• 39 41 38 23 (10 23 14 19 16 .D 21 48 37 27 26 32 15 15 

Y •••••• ••••••••• •••••• 16 21 14 33 12 21 16 13 12 21 11 12 16 15 15 24 10 15 

Yb •••••••••••••••••••• n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 1 1 

Zn. ••••••••••••••••••• 68 89 57 (50 (50 (50 (50 (50 (50 84 61 86 50 (54 66 63 <300 (300 

Zr •••••••••••••••••••• 150 130 170 230 64 91 33 96 98 210 170 200 170 1.D llO 160 100 70 

La:Ni. •••••••••••••••• ll 24 22 18 18 22 19 21 23 20 15 17 9 34 .D 27 35 )75 
j 

; 

., 

::r 

~ 

• :II 

Q. 

Ci) 

I 

:1 

J 

t 

.... 

O:>tq:losition 

()Jartz •••••••••••• •••• 

Microcline. ••••••••••• 

Plagioclase1 •••••••••• Anorthite ••••••••••• Biotite ••••••••••••••• 

Muscovite ••••••••••••• <hlorite •••••••••••••• Magnetite ••••••••••••• Henatite •••••••••••••• Apatite ••••••••••••••• 

M::mazite* ••••••••••••• Zircon* ••••••••••••••• Epidote. •••••••••••••• Sillimanite. •••••••••• Allanite •••••••••••••• Calcite ••••••••••••••• 

ML-1 

23.7 

35.1 

31.8 

15 

3.8 

4.7 

tr 

.7 

tr 

tr 

tr 

.2 

tr 

ML-2 

31.7 

38.0 

19.7 

15 

4.1 

4.1 

1.5 

.3 

.3 

.2 

tr 

.1 

tr 

ML-4 

46.6 

17.7 

32.5 

15 

.2 

1.7 

.6 

.6 

tr 

tr 

.1 

tr 

tr 

SL-29 

33.7 

41.1 

17.7 

12 

2.7 

2.3 

1.0 

1.4 

.1 

tr 

SL-3:> 

29.6 

42.8 

18.1 

14 

2.8 

6.2 

.5 

tr 

tr 

SL-31 

26.2 

26.7 

35.7 

15 

.9 

6.3 

3.4 

.3 

.5 

tr 

tr 

Semple No. 

SL-37 SL-39 SL-41 SL-42 

M:xlal analyses, in volwe percent 

35.7 

34.8 

22.9 

13 

3.2 

3.4 

tr 

tr 

tr 

tr 

tr 

29.1 

31.3 

28.0 

23 

4.1 

6.6 

.1 

.1 

tr 

tr 

• 7 

33.3 

33.1 

27.0 

25 

4.2 

1.1 

I:"8 

tr 

tr 

.5 

19.8 

37.0 

31.6 

18 

4.0 

4.4 

.5 

12."3 

tr 

.2 

tr 

.2 

SL-44 

34.0 

33.4 

24.3 

20 

3.7 

1.1 

13.4 

.1 

tr 

tr 

tr 

SL-45 

21.0 

20.5 

46.1 

17 

10.0 

1.3 

.2 

.6 

.3 

tr 

tr 

SL-46 

17.2 

38.8 

34.0 

15 

.8 

1.8 

6.3 

1.1 

tr 

tr 

tr 

tr 

SL-47 

22.4 

47.8 

20.4 

15 

4.1 

3.9 

1.2 

tr 

.2 

tr 

SL-48 SL-49 

18.6 

48.3 No 

23.0 

17 

4.6 

2.5 thin 

1.2 

1.2 

tr section 

tr 

tr 

.6 

available 

Total............... 100 

tr 

100 

604 

100 

661 

100 

824 

TcX) 

794 

TcX) TcX) "'iij) 100 TcX) TcX) 100 100 TcX) 100 

Points coonted...... 553 

O:>lor inc!ex3 •••••••••• 

Avi~~lf~e~e,. • • 

Magnetic, m, or •• 4 •••• 

Il0011llgiletic, nn • 

4.7 

1.5 

nn 

6.2 1.5 

1.5 5 

m nm 

587 682 

5.2 3.2 4.6 3.2 

(1 1.5 3 1.5 

nm rm nm nn 

740 644 1210 618 629 652 608 679 

4.2 5.0 7.0 7.1 10.8 8.2 5.5 7.0 n.a. 

3 2.5 3 2 1.5 1.5 2 2.5 2 

nm nm nn rm nn m nn nn nn 

Radioactivity>, in. ••• coonts per second. 420 410 200 240 300 33:> 255 350 390 505 300 390 360 350 43) 300 

Specific gravit1' ••••• 

Rock naue 

(Streckeisen, 1976). 

2.660 2.650 2.63:> 2.640 2.63:> 2.670 2.655 2.651 2.659 2.655 2.683 2.641 

granite granite granite granite granite granite granite granite granite granite granite grano- quartz 

{lDJDZ()- {syeno- {Ill)[lZo- {syeno- {syeno- {Ill)[lZo- {m:mzo- {Ill)[lZo- {lDJDZ()- {m:mzo- {m:mzo- diorite m:mzo- granite). granite). granite). granite). granite). granite). granite). granite). granite). granite). granite). nite. 

1About one-half of tre opaque mineral is secondary in grain interstices • 

2.679 2.664 

granite granite {syeno- {syenogrimite). granite). 

2

Plagioclase determinations -were nade by a corrbination of naxim..Dn extinction angles to the 010 trace, optic sign, and refractive index COI\l)8riSOOS against quartz. 

n.a. 

SL-62 SL-63 

22.4 

36.2 No 

33.7 

12 

2.9 

3.8 thin 

.6 

.3 section 

tr 

tr available 

.1 

TcX) 

662 

3.5 

3 

nn 

33:> 

2.660 

= granite). 

n.a. 

3 

nm 

3)5 

n.a. 

30:>lor index is the sun of the colored (nel.anocratic) minerals expressed in percent. O:>lored minerals are: biotite, anphibole, pyroxene, chlorite, scapolite, all opaque minerals, garnet, 

allanite, zircon, sphene, toonnaline, epidote, and mmazite. Light (leucocratic) minerals are: quartz, feldspar, Ill.IScovite, cordierite, calcite, fluorite, apatite, sillimanite, and clinozoisite. 

4m, rock reacts postively to a large ham uegnet; nm, m reaction. 

~ured on the ootcrop with a l-t>unt Sopris scintillooeter, Mxlel SC-132. 

flnetermined with an air pycnooeter, described by M:!Intyre and others (1965). An average of three determinations for each rock spec:ilren yields a specific gravity with± 0.008 gram per cubic 

centineter maxinun deviation. 
Table 9. Modal analyses and descriptive parameters of the Silver Plume Granite 

(*, diagnostic accessory minerals. tr, trace; -, not found] 

Sample No. 

Corrposition SL-75 SL-81 SL-91 SL-92 SM-35 SM-52 

H:xia.l analyses, in volme percent 

Quartz ••••••••••••••••• Microclirle. ••••••••••••• 

Plagioclase! ••••••••••• Anorthite •••••••••••• Biotite •••••••••••••••• 

MUscovite •••••••••••••• <lll.orite ••••••••••••••• Magnetite •••••••••••••• Hematite. •••••••••••••• Apatite •••••••••••••••• 

~ite* •••••••••••••• ZirCOil* •••••••••••••••• ~idote •••••••••••••••• S1llimanite •••••••••••• Allanite. •••••••••••••• ~te ••••••••••••••••• calcite •••••••••••••••• 

Total •••••••••••••••• 

Points counted ••••••• 

Color ind~ ••••••••••• 

Average grain size, •••• in mill:ineters. 

Magnetic, m, or •••••••• 

nonmagnetic, ~. 

Radioactivity, 4 in. •••• counts per second. 

Specific gravityS •••••• 

36.1 

30.1 

23.5 

18 

3.2 

5.7 

.5 

.5 

.2 

tr 

tr 

100 

631 

.2 

4.2 

2.5 

m 

450 

2.660 

29.8 

37.8 

29.0 

17 

2.2 

.9 

.3 

tr 

tr 

tr 

100 

1273 

2.5 

3 

run 

400 

2.642 

36.7 

40.9 

12.7 

25 

8.6 

.3 

.3 

tr 

100 

640 

.5 

9.4 

3 

IID 

370 

2.635 

36.0 

36.2 

20.7 

15 

4.7 

1.9 

.5 

tr 

tr 

tr 

tr 

100 

580 

5.2 

1 

l'lll 

260 

2.643 

27.3 

30.7 

30.4 

15 

5.2 

2.4 

2.0 

1.5 

.4 

tr 

tr 

tr 

tr 

100 

735 

8.8 

2 

m 

2.668 

28.4 

30.6 

32.7 

15 

6.1 

1.2 

.6 

.4 

tr 

tr 

100 

668 

6.7 

1.5 

m 

200 

2.668 

SM-55 

34.0 

39.6 

21.8 

13 

1.8 

2.7 

.1 

tr 

tr 

tr 

100 

670 

1.9 

2 

run 

300 

2.652 

SM-72 

33.7 

27.6 

24.6 

12 

2.0 

11.2 

.1 

.1 

tr 

tr 

100 

802 

.7 

2.1 

2.5 

IID. 

2.654 

SM-73 

32.2 

33.1 

24.2 

13 

4.8 

3.8 

.6 

.3 

100 

715 

.1 

.1 

.1 

.7 

5.9 

2.5 

m 

300 

2.662 

RoCk~ •••••••••••• 

(Streckeisen, 1976). granite granite granite granite granite granite granite granite granite 

(IOOilZo- {100I1Zo- {syeno- (mmzo- {100I1Zo- {IOOnZo- {100I1Zo- {roonzo- {100I1Zogranite). granite). granite). granite). granite). granite). granite). granite). granite). 

1

Plagioclase determinations were made by a combination of maxinun extinction angles to the 010 trace, optic sign, 

and refractive index cooparisons against quartz. 

2

Color index is the sun of the colored (m=lamcratic) minerals expressed in percent. Colored minerals are: 

biotite, anphibole, pyroxene, chlorite, scapolite, all opaque minerals, garnet, allanite, zircon, sphene, tounnaline, 

epidote, and IIDnazite. Light (leucocratic) minerals are: quartz, feldspar, nuscovite, cordierite, calcite, fluorite, 

apatite, sillimanite, and clinozoisite. 

~' rock reacts postively to a large hand Ill'3gnet; run, no reaction. 

~ed on the outcrop with a M:>unt Sopris scintillOIIEter, Mxiel SC-132. 

Snetenni.ned with an air pyCI100Eter, described by ~Intyre and others (1965). An average of three detenni.nations 

for each rock spec:inen yields a specific gravity with :1:: 0.008 gran per cubic cent:ineter maxinun deviation. 

A22 Crystalline Rocks of the Strawberry Lake Area, Colorado 
Figure 7. Monazite in biotite shows intense, dark, radiationdamage halo. Although thin section is from biotite gneiss, 

similar monazite and halos are common in biotite of the Silver 

Plume Granite. Note garnet in upper right, quartz in lower 

right, vermicular quartz in biotite at bottom, and three small 

radiation-damage halos interpreted to be the result of thin 

section cutting above or below a monazite grain, thereby 

slicing through the halo only. Plane-polarized light. 

Figure 8. Monazite in plagioclase with typical alteration ring 

of opaque material. Dark grain to right of monazite is 

radiation-darkened biotite. Plane-polarized light. 

Grain size of the biotite gneiss is variable, from less 

than 1 mm to 10 mm, but much, if not most, of the unit 

is fine grained. Foliation is ubiquitous, and the rock 

becomes schistose as biotite and muscovite increase. 

Essential minerals are quartz and biotite (locally altered 

to chlorite). Muscovite (both primary and secondary), 

microcline, and plagioclase (oligoclase, andesine, and 

rarely labradorite) are generally present; locally the 

plagioclase is antiperthitic. Hornblende, garnet, siiiimanite, and cordierite occur locally. Commonly, garnet 

has biotite inclusions, and brown biotite fills fractures in 

garnet. Cordierite has poikilitic inclusions of sillimanite 

Figure 9. Small enclave of biotite gneiss in Silver Plume 

Granite. Rock hammer is 33 centimeters long. 

Figure 10. Thin, parallel biotite schlieren shown here are 

probably relicts of biotite gneiss in Silver Plume Granite. 

Hammer handle is 3 centimeters in diameter. 

Figure 11. Crystalloblastic texture of granitic enclave in 

Silver Plume Granite. Coin is 19 millimeters in diameter. 

Petrography and Geochemistry A23 
Table 10. Uranium, thorium, chemical, spectrographic, and modal analyses and descriptive parameters of granitic enclaves 

in the Silver Plume Granite 

[Delayed neutron activation analyses for U and Th by H.T. Millard, Jr., M. Solt, M. Coughlin, B. Vaughn, R. Bies, B. Keaten, S. Lasater, J. Storey, 

and S. Danahey. Chemical (X-ray spectrographic) analyses of samples SM-43B and SM-48 by A. Bartel. Semiquantiative spectrographic analyses 

of samples SM-43B and SM-48 by N.M. Conklin. •, significant elements (2 x crustal abundance, or more) and diagnostic accessory minerals. 

n.d., not determined; tr, trace;-, not found; <,less than] 

Size of enclave 

Snall: <100 m 

Semple No. 

O>rqx>sition 

Uraniun and thoriun analyses, in parts per million 

u •••••••••••••••••••••• 

Th .................... . 

Th:U ••••••••••••••••••• 

3.4 

75.9* 

22.3 

5.0* 

123.0* 

24.6 

Otemi.cal analyses, in ~ight percent 

SiOz••••••••••••••••••• 70.2 70.7 

A12~·················· 14.6 14.4 

Fe2o3 •••••••••••••••••• 1

3.24 12.84 

FeO .................... n.d. n.d. 

~ .................... • 86 .69 

eao .................... 1.14 .85 

Nazo ••••••••••••••••••• 2.33 2.23 

K20.t.••••••••••••••••••• 5.33 5.83 

H2o ................... 

} 2.9! 2 .79 

H20-................... 

Tio2··················· .44 .41 

P2o5··················· .12 .16 

lliO. ••••••••••••••••••• <.02 <.02 COz···················· n.d. n.d. 

Total ................ 99 99 

K20:Na2o ••••••••••••••• 2.29 2.61 

A:CNK3 ••••••••••••••••• 1.25 1.25 

~ ..................... 14.9 18.0 

s5 ..................... 2.55 2.63 

Spectrographic analyses, in parts per million 

B •••••••••••••••••••••• <20 <20 

Ba ..................... 1000 700 Be ..................... <1 <1 

Ce ••••••••••••••••••••• 200* 300* 

Co ..................... <3 7 

Cr ..................... 15 15 

OJ ••••••••••••••••••••• 30 7 

Ga. •••••••••••••••••••• 30 30 

La. •••••••••••••••••••• 150* 150* 

Nb ••••••••••••••••••••• 10 10 

Nd ••••••••••••••••••••• 150* 150* 

Ni ••••••••••••••••••••• 7 7 

Ph ••••••••••••••••••••• 30 30 

& ••••••••••••••••••••• 7 5 

Sr ••••••••••••••••••••• 300 150 

v •••••••••••••••••••••• 30 30 

Y •••••••••••••••••••••• 15 15 

Yb ••••••••••••••••••••• 1.5 1.5 

Zn. •••••••••••••••••••• <300 <300 

Zr ••••••••••••••••••••• 150 150 

La:Ni •••••••••••••••••• 21 21 

A24 Crystalline Rocks of the Strawberry Lake Area, Colorado 

Size of enclave 

Small: (100 m 

Sanple No. 

<blqx>Sition SM-43B 

!otxlal analyses, in volrne percent 

~:~::::::::::: 

Plagioclase .......... . Anorthite ........... . 

29.9 29.7 29.8 34.3 

25.2 24.2 27 27 Biotite ............... . 9.6 5.9 

l'hlscovite ............. . 3.0 4.3 Sericite ............. .. Ollorite .............. . 1.0 .1 11lgnetite. ............ . Apatite ............... . 11:>nazite ............. .. 

1.1 .9 tr"' tr"' .1* .1* 

~=:::::::::::::::: tr"' tr* Epidote ............... . 

Allanite .............. . Sillimanite ........... . .3 .5 

Total .............. .. 100 100 Points counted ••••••• 1769 1036 

Color index7 .......... . 11.8 7.0 

1.5 1.5 

M3gnetic, m, or ........ 

nonmagnetic, J. 

Radioactivity9, in..... 500 550 counts per second. 

Specific gravity1o..... 2.673 2.675 

1

Total iron reported as Fez03. 

2toss on ignition at 900 •c. 

3A:<H< = alumina saturation = __ Al_z_DJ_ 

ca>+Na2o+K2o 

(in molecular proportions). 

4F = Si0z-+Na2o+K2o 

FeO+FezO~ 

(oxides in weight 

perecent). 

5s _______ s_i~-----

Al:zOJ+Nazo+K2~FeO+Fez03 

(oxides in weight percent). 

6plagioclase determinations were trade ~ a 

combination of maxinun extinction angles to the 010 

trace, optic sign, and refractive index OOlparisons 

against quartz. 

7 Color index is the sun of the colored 

(nclanocratic) minerals expressed in percent. 

Colored minerals are: biotite, mphibole, pyroxene, 

chlorite, scapolite, all opaque minerals, garnet, 

allanite, zircon, sphene, tounnaline, epidote, and 

monazite. Light (leucocratic) minerals are: 

quartz, feldspar, nuscovite, cordierite, calcite, 

fluorite, apatite, sillimanite, and clinozoisite. 

~. rock reacts postively to a large hand 

magnet; nm, no reaction. 

~asured on the outcrop with a M:mnt Sopris 

scintiUOI!Eter, M:xlel SC-132. 

1

Dnetermined with an air pycOOI!Eter, described 

~ M::Intyre and others (1965). An average of three 

determinations for each rock specimen yields a 

specific gravity with ± 0.008 gran per cubic 

centimeter nax1m.nn deviation. 
Table 11. Uranium, thorium, chemical, spectrographic, and modal analyses and descriptive parameters of the biotite gneiss 

[Delayed neutron activation analyses for U and Th by H.T. Millard, Jr., B.A. Keaten, P.M. Luman, J. Storey, S. Danahey, B. Vaughn, and M. 

Coughlin. Chemical (X-ray spectrographic) analysis of sample SM-152 by A. Bartel, and samples SL-69 and 71 by J.S. Wahlberg, J. Taggart, and 

J. Baker. Semiquantitative spectrographic analyses of samples SL-69 and 71 by H.M. Conklin, and sample SM-152 by L. Bradley. •, significant 

elements (2x crustal abundance, or more) and diagnostic accessory minerals. n.d., not determined; tr, trace; -,not found; n.a., not applicable; 

<, less than) 

Sanple No. 

Cbupositioo. SL-69 SL-71 SM-152 

Uraniun aid thoriun analyses, in parts per million 

u •••••••••••••••••••••• 2.02 2.69 5.33* 

!t4 •••••••••••••••••••• 24.7 23.4 22.2 

Th:U ••••••••••••••••••• 12.2 8.7 4.2 

Chanica! analyses, in ~t percent 

Si02 ••••••••••••••••••• 52.4 66.5 62.0 

Al20:3· 0 0 0 0 0 0 0 0 0. 0 0 0. 0 0 0 26.5 17.0 21.5 

Fe20:3 •••••••••••••••••• 112.4 17.15 1

8.89 

Fe<>. • • • • • • • • • • • • • • • • • • • n.d. n.d. n.d. 

Jt.ga •••••••••••••••••••• 2.6 2.6 2.23 

oo .................... .27 .82 .38 ~o ••••••••••••••••••• .6 1.0 .58 

Kz~+· •••••••••••••••••• 3.84 2.47 2.59 

~0 0 0 0 0 000 •• 0 •• 0 00 0 0 0 } 2.94 21.20 2.45 

H2o+ •••••••••••••••••• 

Ti~. 0 0 0 •• 0 0 0 •• 0 0 •• 0 0. 0 1.07 .76 1.09 

PA··· ................ <.1 <.1 .08 

~- ................... .1 .08 .11 

~- ................... n.d. n.d. n.d. 

Total •••••••••••••••• 101 100 100 

K20:Na2o ••••••••••••••• 6.40 2.47 4.47 

A:CNK3 ••••••••••••••••• 4.64 2.95 4.81 

~ .........•...•....... 3.72 6.62 5.67 

ss •••••••••.•.•.•.•.••• 1.13 2.14 1.71 

Spectrographic analyses, in parts per million 

B •••••••••••••••••••••• <10 <10 (10 ~ .................•.. 700 500 300 

:Be, ••••••••••••••••••••• 1 <1 <1 

l.e: ••••••••••••••••••••• 100 100 (100 

Co. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 30 15 20 

Cr. 0 0 0 0 0 0 0 0. 0 0 0 0 0 0 0 0 0 0 0 150 70 100 

lli. •••••••••••••••••••• 3 70 10 

Ga. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 30 15 30 ~ .................... 70* 70* 50 

Nb. •••••••••••••••••••• (10 10 10 

Nd. ••••••••••••••••••••• 70 70 70 

Ni ••••••••••••••••••••• 70 20 30 

Pb ••••••••••••••••••••• 15 10 15 

Sc ••••••••••••••••••••• 30* 15 20* 

Sr ••••••••••••••••••••• 150 150 50 

v ••••••••••••••••••••.• 70 70 150 

Y. • • • • • • • • • • • • • • • • • • • • • 50 70* 15 

Yb. 0 0 0 0 0. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 7* 7* n.d. 

Zrl. •••••••••••••••••••• <300 <300 <300 

Zr ••••••••••••••••••••• 150 300 150 

I.a.:Ni •••••••••••••••••• 1.0 3.5 1.7 

Sanple No. 

Cbiq!OSition SL-69 SL-71 SM-152 

!1xial analyses, in voli.IIB percent 

~Z··••••••••••••••• 12.0 31.1 · crocline. 6 ••••••••••• 7.5 4.4 

Plagioclase ••••••••••• 16.5 No Anorthite •••••••••••• 35 

Biotite. ••••• •••••••••• 21.5 16.5 

allorite ••••••••••••••• 2.5 1.8 Garnet ••••••••••••••••• 42.1 6.2 thin Cbrdierite ••••••••••••• 6.8 12.8 Sillilmnite •••••••••••• 6.2 10.1 Hematite ••••••••••••••• 1.3 .6 

Apatite ••••••••• ••••••• section 

Sphene. • • • • • • • • • • • • • • • • ~ite ••••••••••••••• .1* tr* Zircon. •••••••••••••••• tr tr ifidote •••••••••••••••• .1 available lanite ••••••••••••••• 

Total •••••••••••••••• 100 Points (bunted ••••••• 601 

Cblor inc!ex7 ••••••••••• 67.4 

Av~~ff~re~!~e, •••• 3 

Magnetic, m, or •••••••• rm 

nonmagnetic, J. 

Radioactivity9, in. •••• 210 counts per second. 

Specific gravitylO ••••• 2.920 

Rode name. ••••••••••••• n.a. (Streckeisen, 1976). 

1rotal iron reported as Fez~· 

2Loss on ignition at 900 °C. 

100 

662 

25.1 

2.5 

rm 

155 

2.872 

n.a. 

Alz~ 3A:CN< = ali.Ulli.na saturation = ----

OO+NazO+KzO 

(in rolecular proportions). 

n.a. 

2 

m 

n.a. 

4F = _s_i~_r+-_Naz_o+K_z_o_ (oxides in ~ight percent). 

FeO+-Fe ~ 

SiOz 5s = (oxides in 

Alz~+NazO+Kz~FeO+-Fez03 IOeight percent). 

6plagioclase determinations ~re !IBde by a combination 

of maxinun extinction angles to the 010 trace, optic sign, 

and refractive index comparisons against quartz. 

7 Cblor index is the sun of the colored (nelanocratic) 

minerals expressed in percent. Cblored minerals are: 

biotite, anphibole, pyroxene, chlorite, scapolite, all 

opaque minerals, garnet, allanite, zircon, sphene, 

tourmaline, epidote, and roonazite. Light (leucocratic) 

minerals are: quartz, feldspar, lll.lScovite, cordierite, 

calcite, fluorite, apatite, sillimanite, and clinozoisite. 

1\n, rock reacts postively to a large hand mggnet; nm, 

no reaction. 

~ured on the outcrop with a ~t Sopris 

scintillOOEter,. l>bdel SC-132. 

10oetermined with an air pyCOOlEter, described by 

M::Intyre and others (1965). An average of three 

determinations for each rock specinEn yields a specific 

gravity with :1: 0.008 gram per cubic centi.treter maxinun 

deviation. 

Petrography and Geochemistry A25 
Table 12. Uranium, thorium, spectrographic, and modal analyses and descriptive parameters of the biotite gneiss 

[Delayed neutron activation analyses for U and Th by H.T. Millard, Jr., M. Solt, M. Coughlin, B. Vaughn, M. Schneider, and W. Stang. 

Semiquantitative spectrographic analyses of sample SL-1 by L. Castillo, and sample SL-36 by T. Fries. *, significant elements (2x crustal 

abundance, or more) and diagnostic accessory minerals. tr, trace;-, not found; n.d., not determined; n.a., not applicable; <,less than] 

Sanple No. 

(bnposition SL-1 SL-36 SL-76 SL-79 SL-aJ SL-89 

Uraniun ani tlnriun in parts per million 

u ••••••••••••••••••••• 3.04 2.65 n.d. n.d. n.d. n.d. Th. ••••••••••••••••••• 12.6 7.6 n.d. n.d. n.d. n.d. 

Th:U •••••••••••••••••• 3.7 2.9 n.d. n.d. n.d. n.d. 

Spectrographic analyses, in parts per million 

B ••••••••••••••••••••• 15 15 n.d. n.d. n.d. n.d. Ba. ••••••••••••••••••• 81}) 1300 n.d. n.d. n.d. n.d. Be •••••••••••••••••••• 3 3.1 n.d. n.d. n.d. n.d. ee. ••••••••••••••••••• <IOO <IOO n.d. n.d. n.d. n.d. Co •••••••••••••••••••• 16 9.9 n.d. n.d. n.d. n.d. 

Cr •••••••••••••••••••• 84 <10 n.d. n.d. n.d. n.d. Cu. ••••••••••••••••••• 8.9 4.5 n.d. n.d. n.d. n.d. Ga. ••••••••••••••••••• 24 22 n.d. n.d. n.d. n.d. La. ••••••••••••••••••• 54 39 n.d. n.d. n.d. n.d. Li •••••••••••••••••••• 1701< <SO n.d n.d. n.d. n.d. 

z.n. ••••••••••••••••••• 600 1200 n.d. n.d. n.d. n.d. Nb. ••••••••••••••••••• <25 <25 n.d. n.d. n.d. n.d. Nd •••••••••••••••••••• n.d. n.d. n.d. n.d. n.d. n.d. Ni •••••••••••••••••••• 34 6.5 n.d. n.d. n.d. n.d. P ••••••••••••••••••••• 600 2()()')* n.d n.d. n.d. n.d. 

Pb. ••••••••••••••••••• <10 <10 n.d. n.d. n.d. n.d. &. ••••••••••••••••••• 17 21 n.d. n.d. n.d. n.d. Sr •••••••••••••••••••• I30 180 n.d. n.d. n.d. n.d. Ti •••••••••••••••••••• 5200 4100 n.d. n.d. n.d. n.d. v ••••••••••••••••••••• 92 52 n.d. n.d. n.d. n.d. 

¥ ••••••••••••••••••••• 43 59* n.d. n.d. n.d. n.d. Yb. ••••••••••••••••••• n.d. n.d. n.d. n.d. n.d. n.d. Zn. ••••••••••••••••••• 91 90 n.d. n.d. n.d. n.d. Zr •••••••••••••••••••• 190 240 n.d. n.d. n.d. n.d. 

La:Ni. •••••••••••••••• 1.6 6.0 n.d. n.d. n.d. n.d. 

M:xlal analyses, in volune percent 

QJartz •••••••••••••••• 51.9 41.6 58.8 47.3 33.3 25.5 

:~~~:::::::::: .6 4.2 .3 13.2 4.5 

13.3 23.2 24.6 32.2 36.9 20.0 28 68 15 17 28 28 Biotite ••••••••••••••• 16.4 27.8 9.9 18.9 I4.7 36.9 

M.Jscovite ••••••••••••• I7.9 3.I 1.7 .s .3 .8 Ollorite •••••••••••••• .I .I .2 Hornblende. ••••••••••• .9 Garnet •••••••••••••••• .2 I.8 Sillinrmite. •••••••••• .I .I .2 10.3 

=it~:'::::::::::::: - .6 - .7 .5 .5 .2 Apatite. •••••••••••••• .4 .5 tr Sphene •••••••••••••••• tr tr M:mazite. ••••••••••••• tr* tr* tr* .1* tr* 

Zircon. ••••••••••••••• tr tr tr tr 

~~~:::::::::::::: 2.2 tr tr 

Total •••••••••••••• IOO IOO 100 100 100 100 Points coomted. ••••• 801 68I 710 640 1314 5I4 

A26 Crystalline Rocks of the Strawberry Lake Area, Colorado 

Sample No. 

(bnposition SL-1 SL-36 SL-76 SL-79 SL-aJ SL-89 

Color ~.......... I6.9 3I.O 10.6 19.7 I5.6 38.9 

<I <I 

JIBgnetic, m, or ••••••• 

rtO!Ill3glletic, rm'l. 

Radioacti vit!, in. • • • 180 counts per second. 170 230 240 185 165 

Specific gravityS ••••• 2.760 2.807 2.719 2.718 2.749 

Rock ruure ••••••••••••• (Streckeisen, 1976). n.a. 

1 Plagioclase determinations were nede ~ a rombination of nmcl.nun extinction angles to the 

010 trace, optic sign, ani refractive index conp3Iisons against quartz. 

2eolor index is the sun of the colored ( nel.anocratic) minerals expressed in percent. 

Colored minerals are: biotite, .mphibole, pyroxene, chlorite, scapolite, all opaque minerals, 

garnet, allanite, zircon, sphene, tournaline, epidote, and 111)[18Zite. Light (leucocratic) 

minerals are: quartz, feldspar, JDJscovite, cordierite, calcite, fluorite, apatite, sillinmrl.te, 

and clinozoisite. 

~. rock reacts postively to a large haOO mgnet; nm, no reaction. 

~on the oUtiCrop with a l'bunt Sopris scintillorooter, l-txlel 80-132. 

Soetermined with an air pycnooeter, described ey ltlntyre and others (l%5). An average of 

three determinations for each rock specinEn yields a specific gravity with± 0.008 gran per cubic 

centim=ter nmcim.nn deviation. 

and quartz and displays twinning that is similar to, but 

diagnostically different from, that of plagioclase. Monazite and zircon are common; apatite is less so. Monazite 

produces intense, dark, radiation-damage halos in biotite 

(fig. 7) and lighter ones in cordierite. The largest monazite grain is 0.14 by 0.08 mm. Zircons commonly contain 

older zircon cores. Hematite or magnetite are normally 

present, epidote and allanite are uncommon, and sphene 

is rare. 

Three chemical analyses (table 11) of biotite gneiss 

indicate that it is very peraluminous, has high K20:Na2 0 

ratios, and is poor in calcium. 

Granitic Enclaves in Biotite Gneiss 

Granitic enclaves in the biotite gneiss range from 1 

m to about 7 km in greatest dimension. The long, narrow 

enclave in the northwest part of the area was mapped as 

Silver Plume Granite by Schroeder (1984) and as quartz 

monzonite by Izett (1974). My interpretation of this 

elongate body as being different from the Silver Plume 

Granite is based on its finer grain size and lower uranium 

content. Moreover, the area bounded by this elongate 

body is poorly exposed and contains considerable 

amounts of biotite gneiss. These generally fine grained 

granitic rocks are gray to buff and have hypautomorphic 

granular texture. 
Essential minerals are quartz, microcline, oligoclase (rarely andesine), and biotite (locally altered to 

chlorite). Muscovite is generally present, zircon is very 

common, and apatite and monazite are less common 

(tables 13 and 14). Monazite is conspicuous by its 

production of intense, dark, radiation-damage halos in 

biotite. The largest monazite grain is 0.04 by 0.08 mm. 

Monazite commonly shows an alteration ring and locally 

produces small fractures in quartz. The largest apatite 

grain is 0.5 by 1.2 mm. Hematite is generally present 

(magnetite occurs in only one sample); garnet, epidote, 

allanite, sillimanite, and tourmaline are rarely present. 

The tourmaline is dichroic from almost colorless to dark 

gray blue. 

One chemical analysis of a typical sample (sample 

SL-3, table 13) from a large granitic enclave in the biotite 

gneiss indicates that the rock is a peraluminous and very 

potassic monzogranite. Eight samples (including the 

chemically analyzed one) of the granitic enclaves in the 

biotite gneiss are plotted on a QAP diagram in figure 6; 

six are monzogranites, one is a syenogranite, and one is a 

granodiorite. On figure 6 the trends of all the granitic 

enclaves (those in Boulder Creek Granodiorite, those in 

the Silver Plume Granite, and those in biotite gneiss) are 

similar; in other words, plagioclase content increases with 

increasing quartz content, or conversely, alkali feldspar 

content increases with decreasing quartz content. 

Furthermore, granitic enclaves in the biotite gneiss 

contain considerably more plagioclase than do granitic 

enclaves in the Boulder Creek Granodiorite. Granitic 

enclaves in the Silver Plume Granite contain more 

plagioclase than do granitic enclaves in the Boulder 

Creek Granodiorite but less than granitic enclaves in the 

biotite gneiss. 

Biotite Lamprophyre Enclaves in 

Boulder Creek Granodiorite 

Enclaves of biotite lamprophyre are tens of meters 

in length and cluster in the south-central portion of the 

area (pl. 1, table 15). Individual pods (broken blocks of a 

dike?) show sharp contact with their host. A single 

enclave, which is 30 m in length, is 3.5 km to the 

west-southwest of the cluster. Another enclave was 

mapped 3 km to the north-northeast of the cluster by 

Schroeder (1984). The biotite lamprophyre is dark gray, 

fine grained, and holocrystalline; some parts of the 

lamprophyre show irregular, delicate veining by pink 

feldspar (fig. 12). Most specimens are massive, but 

sample SL-10 (table 15) showed a pronounced foliation. 

I use the name "enclaves," rather than "dikes," in order 

to emphasize that their origin is open to question. 

Biotite and hornblende are essential minerals, 

which are set in a matrix of microcline and microcline 

microperthite that invariably is heterogeneously light 

brown (fig. 13). Biotite is commonly intergrown with 

hornblende, which implies simultaneous crystallization. 

Monoclinic pyroxene occurs in sample SL-142 (table 15) 

as very small phenocrysts. Quartz occurs in sample 

SL-82 (table 15). Sphene and apatite are very abundant. 

Hematite, epidote, and triangular inclusions of rutile in 

biotite occur in all samples; the hematite is locally 

bordered with granular sphene. Zircon and allanite occur 

locally. Allanite, where included in biotite, produces 

dark, intense radiation-damage halos. 

The narrow range of specific gravities for these five 

samples (2.903; maximum deviation does not exceed 

± 0.029) (table 15) indicates the relative homogeneity of 

the rock. According to the Streckeisen (1979) 

classification for lamprophyres, all five samples are 

minette-vogesites, and they are metaluminous. 

Small Mafic Enclaves in 

Boulder Creek Granodiorite 

Small (generally 1-20 em), fine-grained, generally 

discoid enclaves in the Boulder Creek Granodiorite are 

abundant locally (fig. 3). I would not call them xenoliths 

because no country rock fits their description. One could 

assume that they come from a source at depth, but that 

may be an unwarranted assumption. 

Essential minerals (table 15) are andesine, biotite, 

and hornblende. Small amounts of quartz and muscovite 

(secondary?) are present in some samples. Accessory 

minerals are epidote, sphene, apatite, magnetite, zircon, 

and allanite. Some apatite grains show weak radiationdamage halos in biotite; others do not. Zircons, some 

with older zircon cores, show moderate radiationdamage halos in biotite, whereas sphene does not 

produce halos. A few, very small, unidentified nonopaque grains show dark, intense radiation-damage halos 

in biotite; these grains may be monazite. 

Sample SL-101B, which is a metaluminous 

representative of this rock type, is plotted on a QAP 

diagram in figure 6. It is classified as a quartz diorite 

according to the Streckeisen (1976) system. The mafic 

composition (rich in Fe, Mg, Ca, Cr, V, and Sc) of the 

small mafic enclaves (table 15) matches that of the biotite 

lamprophyres, although the latter are also enriched in K 

and other felsic trace elements. 

Andesite Porphyry Dike 

The Tertiary(?) andesite porphyry dike (table 15) 

in the southwest part of the area is about 9 m wide and at 

least 150m long, and it intrudes Boulder Creek Granodiorite. The andesite porphyry is dark and fine grained. 

Petrography and Geochemistry A27 
Table 13. Uranium, thorium, chemical, spectrographic, and modal analyses and descriptive parameters of a granitic enclave 

in the biotite gneiss 

[Delayed neutron activation analyses for U and Th by H.T. Millard, Jr., M. Solt, M. Coughlin, B. Vaughn, R. Bies, B. Keaten, S. Lasater, J. Storey, 

and S. Danahey. Rapid rock chemical analysis by V. Smith and J. Reid. Semiquantitative spectrographic analysis by G. Kaczanowski. *, significant 

elements (2x crustal abundance, or more) and diagnostic accessory minerals. n.d., not determined; tr, trace;-, not found; <,less than] 

Size of enclave 

Large: 0. 5 km or TIDre 

Semple No. 

Conp>Sition SL-3 

Uraniun and thoriun analyses, in parts per million 

u •••••••••••••••••••••• 

Th ••••••••••••••••••••• 

Th:U ••••••••••••••••••• 

4.50 

53.1* 

11.8 

<llemical analyses, in ~ight percent 

Sio2 ••••• •••• •••••••••• 

Al20J· ••••••••••••••••• 

Fe20J •••••••••••••••••• 

FeO •••••••••••••••••••• 

~ ................... . eao •••••••••••••••••••• 

Na2o ••••••••••••••••••• 

KzO+••••• •• ••• ••• •••••• H2o ••••••••••••••••••• 

HzO-•••••• ••••••••••••• 

Ti.Oz ••••••••••••••••••• 

P20s· • • • • • • •• • • • • • • • • • • MnO •••••••••••••••• ••. • COz· ••••••••••••••••••• 

Total ••••••••••••••• 

KzO:NazO ••••••••••••••• 

A:<NK2. • • • • • • •• • • •• • •• • 

~- ................... . s4 •••.••••.•••••••••••• 

72.4 

14.5 

1.6 

.52 

.29 

.66 

2.7 

6.3* 

} 1.03 

100 

.29 

.12 

.oo 

.08 

2.33 

1.16 

26.5 

2.72 

Spectrographic analyses, in parts per million 

B. • • • • • • • • • • • • • • • • • • • • • :sa. ••••••••••••••••••••• 

13e. ••••••••••••••••••••• 

(;e: ••••••••••••••••••••• 

():) .................... . Cr ••••••••••••••••••••• 

Ol ••••••••••••••••••••• 

Ga. •••••••••••••••••••• I..a.. •••••••••••••••••••• Nb ••••••••••••••••••••• 

-m ••••••••••••••••••••• 

Ni ••••••••••••••••••••• 

Pb ••••••••••••••••••••• 

Sc. ••••••••••••••••••••• 

Sr ••••••••••••••••••••• 

v •••••••••••••••••••••• 

Y. • • • • • • • • •. • • • • • • • • • • • Yb. •••••••••••••••••••• Zn. •••••••••••••••••••• Zr ••••••••••••••••••••• 

la:Ni •••••••••••••••••• 

<10 

1100 

2.4 

210* 

1.8 

<10 

27 

19 

130* 

<25 

n.d. 

4 

23 

<10 

120 

14 

17 

n.d. 

<SO 

170 

33 

A28 Crystalline Rocks of the Strawberry Lake Area, Colorado 

Size of enclave 

Large: 0. 5 km or mre 

Semple No. 

Conposition SL-3 

M:xlal analyses, in volt.~~e percent 

~~::::::::::::: 

Plagioclase5 ••••••••••• Anorthite •••••••••••• Biotite •••••••••••••••• 

Muscovite •••••••••••••• Ollorite ............. .. Hematite .............. . Apatite. ............. .. ~nazite ••••••••••••••• 

Zircon. .............. .. Epidote .............. .. Allanite ............. .. Sillimanite •••••••••••• 

Total ............... . Points counted ••••••• 

Color indelt> .......... . 

M:tgnetic, m, or ••••• r. or nollllBglletic, rm • 

Radioactivitfl, in ••••• counts per second. 

Specific gravity9 ...... 

Rock name ........... . (Streckeisen, 1976). 

32.7 

27.6 

27.2 

14 

4.8 

6.5 

.6 

.1* 

.5* 

t:r* 

100 

805 

5.9 

1.5 

345 

2.650 

granite 

(100!1Zogranite). 

1

I.css on ignition at 900 °C, 

Al2~ 2

A:CNK = ah.mi.na saturation = ----

CaO+Na20+Kz0 

(in mlecul.ar proportions). 

3F = Si0z+Na20+K20 

FeOtFe ~ 

(oxides in ~.eight 

percent). 

sw2 4s=--------------- Al20J+NazO+K2~FeOtFez03 

(oxides in weight percent). 

5Plagioclase detemrl.nations ~.ere nade by a 

combination of maxi11lJ11 extinction angles to the 010 

trace, optic sign, and refractive in:lex crnparisons 

against quartz. 

6Color index is the sun of the colored 

(melanocratic) minerals expressed in percent. 

Colored minerals are: biotite, aq>hibole, pyroxene, 

chlorite, scapolite, all opaque minerals, garnet, 

allanite, zircon, sphene, tourmaline, epidote, and 

100118Zite. Light (leucocratic) minerals are: 

quartz, feldspar, Rllscovite, cordierite, calcite, 

fluorite, apatite, sillinBnite, and clinozoisite. 

7

m, rock reacts postively to a large hand 

DBgnet; run, no reaction. 

%asured on the outcrop with a ~t Sopris 

scintillooeter, l'b:lel SC-132. 

9netemrl.ned with an air pycnooeter, described 

by M::Intyre and others (1965). An average of three 

detenni.nations for each rock spec:i.nen yields a 

specific gravity with± 0.008 gram per cubic 

cent:i.neter nmd.num deviation. 
Essential minerals are plagioclase {An30), biotite, 

hornblende, monoclinic pyroxene, potassium feldspar, 

and minor quartz. Most of the plagioclase occurs as 

micro lites; some of the larger plagioclase phenocrysts are 

zoned. Potassium feldspar and quartz are largely 

interstitial. Magnetite, apatite, sphene, and epidote are 

accessory minerals. An unusual mineral in the dike is 

scapolite, which occurs as phenocrysts as much as 2.5 by 

3.5 mm in size and aggregates about 2 percent of the 

dike. This is the first reported occurrence of meionite 

(calcium scapolite) from Colorado (Eckel, 1961). All the 

scapolite phenocrysts have a light -colored reaction rim, 

which may be microcrystalline epidote. The scapolite 

phenocrysts have a lustrous, black, glassy appearance in 

hand specimen, break conchoidally, and show no 

cleavage. Under the polarizing microscope the scapolite 

phenocrysts are light brown, slightly dichroic, and uniaxial negative. Cleavage is not evident, but many fractures 

(some curved) are filled with an unidentified mineral that 

is probably identical to that of the light-colored reaction 

rim. Refractive indices of the scapolite are w = 1.600 and 

e = 1.565. 

Data shown below are from the studies of the 

scapolite by Isabelle Brownfield utilizing an X-ray 

diffraction-powder camera and a microprobe. 

X-ray diffraction-powder camera study 

d, in 28-Cu:Ka, in Intensity! hkP 

angstroms degrees 

9.509 9.3 ms 

3.867 23.0 s 

3.493 25.5 vs 112 

3.100 28.8 vs 400 

2.814 31.8 w 

2.739 32.7 vs 

2.344 38.4 s 

2.181 41.4 w 

2.113 42.8 s 

2.045 44.3 s 

1.949 46.6 vs 

1.903 47.8 vw 

1.787 51.1 ww 

1.755 52.1 vw 

1.598 57.7 w 

1.5465 59.8 vw 

1.5033 61.7 w 

1.4568 63.9 w 

1.4041 66.6 s 

1.3507 69.6 vw 

1vs, very strong; s, strong; ms, medium strong; w, weak; vw, very 

weak; ww, very very weak. 

2--, not determined. 

Microprobe study 

Average of eight Average of three 

Composition1 determinations on determinations on 

first phenocryst second phenocryst 

Si02 42.65 43.27 

Al20$ 26.30 26.72 

FeO .76 .80 

MgO .33 .38 

CaO 16.44 16.00 

Na20 3.68 3.85 

~0 .26 .29 

Ti02 .09 .11 

MnO .03 .02 

F .02 .03 

a .08 .08 

Total 90.64 91.55 

CaO:Na20 ratios range from 4.1 to 4.7 and suggest 

that this scapolite could be considered meionite (Me) 

[Ca4Al60 24{C03,S04 )], which is the Ca-rich end 

member of the marialite-meionite series (Fleischer, 

1983). However, comparison of the 2,813 gram-atoms of 

Ca of this scapolite with the gram-atoms of Ca of other 

analyzed scapolites (Shaw, 1960) indicates that this 

scapolite is about 73 percent Me. Therefore, on the basis 

of CaO:N~O ratios, this scapolite is a mizzonite 

(Meso-Meso)· 

If the refractive indices are used as a gauge of 

composition, the use of Shaw's (1960) equation 

e ~ w = 1.5346 + 0.000507 (Me percent) yields 94.5 

percent 2 Me, which indicates this scapolite is a meionite. 

I also utilized the X-ray method of Burley and 

others (1961), which uses the difference in 28-CuKa 

spacings between the (112) and ( 400) hkl planes; it was 

necessary to extrapolate their curve. The result obtained 

for the scapolite of this report was about 87 percent Me. 

The average of the three methods is 85 percent Me, 

which would justify the name meionite. 

Sample SL-139, which is a metaluminous 

representative of the andesite porphyry dike, is plotted on 

a QAP diagram in figure 6, which indicates that it is a 

monzodiorite in the Streckeisen (1976) classification. Of 

all the rock units, this rock alone can unequivocally be 

called igneous, because of its dikelike structure and two 

types of plagioclase phenocrysts: larger zoned ones and 

microlites. 

Petrography and Geochemistry A29 
Table 14. Uranium, thorium, spectrographic, and modal analyses and descriptive parameters of 

granitic enclaves in the biotite gneiss 

[Delayed neutron activation analyses for U and Th by H.T. Millard, Jr., M. Solt, M. Coughlin, B. Vaughn, M. Schneider, 

W. Stang, B. Keaten, and F. Luman. Semiquantitative spectrographic analyses of samples SL-4 and SL-17 by L. Castillo, 

samples TM-2 and SL-25 by G. Kaczanowski, and samples SL-38 and SL-43 by T. Fries. •, significant elements (2 x 

crustal abundance, or more) and diagnostic accessocy minerals. tr, trace; -, not found; n.d., not determined <, less than; 

>, greater than] 

Size of enclave 

Small: 1-5m Small: <SO m Large: 0.5 km or mre 

Sanple No. 

Conposition SL-103 SL-25 SL-38 SL-43 SL-4 

Uranium and thorium ana.lses, in parts per million 

u. 0 ..... 0 ....... 0....... n.d. Th...................... n.d 

Th:U.................... n.d. 

6.97* 

26.4* 

3.8 

2.73 

29.9k 

11.0 

1.36 

30.4* 

22.0 

2.78 

75.2* 

27.1 

Spectrographic analyses, in parts per mi.llioo 

B....................... n.d. 

&. • • • • • • • • • • • • • • • • • • • • • Il.d. Be....................... n. d. Ce...................... Il.d. 0:>...................... n.d. 

Cr...................... n.d. OJ....................... n.d. Ga....................... tl. d. 14...................... n. d. u...................... Il.d. 

fu..................... n.d. Nb...................... Il.d. Nd...................... n.d. Ni...................... Il.d. P....................... n.d. 

Pb...................... n.d. Sc...................... n.d. Sr...................... 11.d. Ti...................... n.d. v..... .. . . . . . . . . . . . . . . . . Il.d. 

Y........... . . . . . . . . . . . . n. d. Yb...................... n.d. Zrl...................... n.d. Zr...................... 11.d. 

I..a.:Ni................... n.d. 

<10 

340 

1.9 

(100 

1.1 

(10 

<I 

18 

44 

<SO 

<200 

<25 

n.d. 

2.4 

600 

51* 

<10 

170 

900 

(10 

(10 

n.d. 

<SO 

<20 

18 

(10 

220 

2.8 

(100 

1.5 

(10 

8.7 

18 

<20 

<SO 

260 

<25 

n.d. 

4.3 

1000 

14 

(10 

73 

900 

(10 

17 

n.d. 

<SO 

43 

(5 

17 

1700* 

3.6 

310* 

10 

13 

l') 

25 

190* 

<SO 

860 

<25 

n.d. 

11 

)4500* 

(10 

11 

290 

6300 

77 

43 

n.d. 

170* 

420* 

17 

M:xlal analyses, in voltJie percent 

Q.Ja.rtz.................. 32.2 

Microcline. • • • • • • • • • • • • • 11. 7 

Plagioclase1•••••••••••• 36.2 

Anorthite. • • • • • • • • • • • • 33 

Biotite................. 15.1 

Muscovite. • • • • • • • • • • • • • • 3. 6 

011.orite •••••••••••••••• Magnetite ••••••••••••••• Hematite................ 0.3 Apatite................. . 7* 

::r1Jnazite. • • • • • • • • • • • • • • • trk 

Zircon. •••••••••• -....... tr* 

.Allanite. • • • • • • • • • • • • • • • tr 

Sillimanite ••••••••••••• Tourmaline. ••••••••••••• Epidote................. tr (;a.n}et.................. .2 

Total ••••••••••••••••• 100 

Points counted •••••••• 583 

31.1 

34.1 

32.8 

13 

2.0 

tr 

tr 

0.1 

tr* 

tr 

100 

549 

28.3 

38.6 

29.1 

12 

3.2 

0.8 

tr 

tr 

tr* 

tr 

100 

622 

A30 Crystalline Rocks of the Strawberry Lake Area, Colorado 

28.7 

20.8 

32.3 

32 

13.8 

1.1 

0.1 

2.5 

.6 

.1* 

tr 

100 

832 

<10 

2000* 

1.4 

390* 

4.3 

(10 

11 

25 

230* 

<SO 

<200 

<25 

n.d. 

6.2 

700 

43* 

(10 

190 

3100 

41 

17 

n.d. 

54 

280 

37 

29.2 

30.8 

27.0 

13 

7.3 

2.8 

0.6 

1.5 

.4* 

.4* 

tr* 

100 

800 

SL-17 

3.89 

75.6* 

19.4 

<10 

1100 

1.5 

320* 

4.2 

<10 

6.5 

21 

160* 

<SO 

230 

<25 

n.d. 

4.7 

500 

50* 

<10 

150 

2200 

27 

16 

n.d. 

55 

220 

34 

30.1 

39.5 

20.8 

12 

7.1 

2.5 

tr 

tr* 

tr* 

tr* 

tr 

100 

552 

'IM-2 

3.99 

26.2* 

6.6 

<10 

370 

3.2 

110 

1.8 

(10 

12 

22 

54 

55 

<200 

<25 

n.d. 

2.3 

900 

31 

<10 

61 

1300 

(10 

20 

n.d. 

<SO 

130 

23 

35.3 

23.9 

29.1 

16 

1.7 

8.8 

0.8 

.4* 

tr* 

tr* 

100 

809 
Table 14. Uranium, thorium, spectrographic, and modal analyses and descriptive parameters of 

granitic enclaves in the biotite gneiss-Continued 

Size of enclave 

Small: 1-5m Small: <SOm Large: 0. 5 km or mre 

Sample No. 

Cooposition SL-I03 SL-25 SL-38 SL-43 SL-4 SL-I7 '11+-2 

Color ind~ •••••••••••• I5.6 2.I 3.2 I6.5 9.8 7.I 2.5 

Av~rag~ Ji:ain size, ••••• <I I.S 2 <I <1 I 

J.n tm. l.IIEters. 

Magnetic, m, or ••••••••• nm nm rm m rJil rJil nn 

ru:>IJnagnetic, ~. 

RadioactivitY., in. ••••• 50 330 I65 325 385 325 280 

counts per second. 

Specific gr.avityS ••••••• 2.620 2.649 2.776 2.630 2.640 2.640 

Rock nane grano-

= fmanite = fmanite ~anite =te (Streckeisen, I976). diorite. IOOnZo- IOOnZo- syeno- o- granite). granite). granite). granite). granite). granite). 

1

Plagioclase detenninations ~re nade by a combination of maxi.nun extinction angles to the 010 

trace, optic sign, and refractive index coopari.sons against quartz. 

20Jlor index is the sun of the colored (nElanocratic) minerals expressed in percent. OJlored 

minerals are: biotite, amphibole, pyroxene, chlorite, scapolite, all opaque minerals, garnet, allanite, 

zircon, sphene, tourmaline, epidote, arrl nx>nazite. IJ.ght (leucocratic) minerals are: quartz, feldspar, 

IDJScovite, cordierite, calcite, fluorite, apatite, sillimanite, and clinozoisite. 

~' rock reacts postively to a large harrl magnet; nm, no reaction. 

~asured on the ootcrop with a M>unt Sopris scintill()I"(Eter, M>del SC-132. 

Soetermi.ned with an air pyCllOilEter, described by M:Intyre arrl others (1965). M. average of three 

detenninations for each rock spec.:inen yields a specific gravity with :i: 0.008 gram per wbic cent:im=ter 

maxi.nun deviation. 

Petrography and Geochemistry A31 
Table 15. Uranium, thorium, chemical, spectrographic, and modal analyses and descriptive 

parameters of the biotite lamprophyre enclaves, a small mafic enclave in the Boulder Creek 

Granodiorite, and an andesite porphyry dike 

[Delayed neutron activation analyses for U and Th by H.T. Millard, Jr., B.A. Keaten, F.M. Luman, J. Storey, S. 

Danahey, B. Vaughn, and M. Coughlin. Chemical (X-ray spectrographic) analyses of samples SL-88, 101B, 139, and 

142 by A. Bartel. Semiquantitative spectrographic analyses of sample SL-10 by L. Castillo; and samples SL-88, 101B, 

139, and 142 by L. Bradley. *, significant elements (2 x crustal abundance, or more) and diagnostic accessory minerals. 

n.d., not determined; tr, trace;-, not found; <,less than) 

Con:position 

u •••••••••••••••••••••• 

nt. •••••••••••••••••••• 

Th.:U. • • • • • • • • • • • • • • • • • • 

Si<>z ••••••••••••••••••• 

A12~···· •••••••••••••• 

Fe20:3 •••••••••••••••••• 

FeO •••••••••••••••••••• 

~ ................... . eao •••••••••••••••••••• 

Na2o ••••••••••••••••••• 

K2~.;.········ .......... . H20 ••••••••••••••••••• 

~o-•••••••••.••••••••• 

Ti.o2 ••••••••••••••••••• 

P2o5 ••••••••••••••••••• 

t-tl.O •••••••••••••••••••• 

~- .................. . Total •••••••••••••••• 

KzO:NazO. ••• ••••••••••• 

A:CNK3 ••••••••••••••••• 

~- ................... . 

s5 ••••••••••••••••••••• 

B. • • • • • • • • • • • • • • • • • • • •• Ba. •••••••••••••••••••• 

Be ••••••••••••••••••••• 

Ce ••••••••••••••••••••• 

Co ••••••••••••••••••••• 

Cr ••••••••••••••••••••• 

Cll. •••••••••••••••••••• 

Ga. •••••••••••••••••••• La. •••••••••••••••••••• Nb ••••••••••••••••••••• 

Nd. •••••••••••••••••••• Ni ••••••••••••••••••••• 

Pb. • • • • • • • • • • • • • • • • • • • • Sc ••••••••••••••••••••• 

Sr ••••••••••••••••••••• 

v •••••••••••••••••••••• 

Y. • • • • • • • • • • • • • • • • • • • • .- Yb ••••••••••••••••••••• 

Zn. •••••••••••••••••••• 

Zr ••••••••••••••••••••• 

La:Ni •••••••••••••••••• 

Biotite l.aiq>rophyre enclaves Snall Andesite 

mafic enclave prophyry dike 

Semple No. 

SL-88 SI..-142 SL-10 SL-82 SL-83 SL-101B SL-139 

Uraniun and thoritm analyses, in parts per million 

8.31* 

51.fik 

6.2 

50.0 

11.2 

17.77 

n.d. 

9.20* 

7.01 

.71 

7.67* 

} 2.57 

1.39 

1.53* 

• 13 

n.d. 

97 

10.8 

.50 

2.44 

1.15 

9.52* 

62.fik 

6.6 

47.7 

11.7 

17.84 

n.d. 

9.04* 

7.43 

.78 

7.84* 

2• 75 

1.12 

1.79* 

.11 

n.d. 

96 

10.1 

.50 

2.32 

1.07 

7.68* 

43.9* 

5.7 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d • 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

Spectrographic analyses, in parts per million 

<10 

5000* 

3 

.500* 

20 

.500* 

15 

20 

300* 

(10 

300* 

1.50* 

30 

30* 

1500* 

200* 

50 

3 

<300 

150 

2.0 

<10 

15<XX)* 

5 

700* 

20 

300* 

50 

20 

300* 

(10 

700* 

1.50* 

50* 

20* 

3000* 

150 

70* 

3 

<300 

150 

2.0 

<10 

4100* 

11* 

440* 

38k 

420* 

36 

25 

230* 

<25 

n.d. 

220* 

<10 

29* 

1700* 

240* 

64* 

n.d 

<SO 

510* 

1.0 

n.d. 

n.d. 

n.d • n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

2.15 

<2..7 

(1.3 

46.8 

16.5 

113.4* 

n.d. 

6.07 

8.54 

2.76 

2.38 

21.21 

1.25 

.42* 

n.d. 

n.d. 

100 

.86 

.73 

1.85 

0.94 

(10 

300* 

2 

(100 

20 

300* 

30 

30 

<SO 

<10 

100* 

30 

15 

50* 

500 

200* 

70* 

5 

<300 

70 

(1.7 

3.38 

22.9 

6.8 

52.2 

15.0 

18.51 

n.d. 

6.01 

6.41 

2.80 

3.51 

21.29 

1.02 

1.06* 

.12 

n.d. 

98 

1.25 

.75 

2.80 

1.24 

(10 

5000* 

1.5 

300* 

20 

300* 

30 

20 

200* 

(10 

200* 

70 

50* 

15 

3000* 

150 

30 

3 

<300 

100 

2.9 

A32 Crystalline Rocks of the Strawberry Lake Area, Colorado 
Table 15. Uranium, thorium, chemical, spectrographic, and modal analyses and descriptive 

parameters of the biotite lamprophyre enclaves, a small mafic enclave in the Boulder Creek 

Granodiorite, and an andesite porphyry dike-Continued 

Biotite l.arrprophyre enclaves Srre11 Andesite 

mafic enclave prophyry dike 

Conposition 

Q.lartz ••••••••••••••••• Mi.crocline. , ••••••••••• 

Plagioclase6 ••••••••••• Anorthite •••••••••••• Biotite ••• •••••••••• ••• 

Hornblende ••••••••••••• M:mocline pyroxene ••••• 

Scapolite (neionite) ••• Magnetite •••••••••••••• Hematite ••••••••••••••• Apatite •••••••••••••••• 

Sphene ............... .. 

Zircon. •• •••••••••••••• Epidote •••••••••••••••• Allanite ••••••••••••••• Rutile ••••••••••••••••• 

Calcite •••• •••••••••• •• 

Total •••••••••••••••• Points eotmted ••••••• 

O>lor index7 ••••••••••• 

Average grain size, •••• in millilreters. 

Magnetic, m, or •••••••• 

nornnagnetic, nm8• 

Radioactivity, 9, in. ... COlmts per second. 

Specific gravitylO ••••• 

SL-88 

45.8 

1.6 

5 

23.6 

20.8 

1.3* 

2.3* 

2.6* 

2.0* 

tr 

tr* 

100 

696 

50.3 

<1 

nm 

295 

2.899 

Sample No. 

SL-142 SL-10 SL-82 SL-83 

r.txlal analyses, in volume percent 

0.4 

24.9 37.6 37.4 37.7 

0.5 12.3 2.8 

8 20 8 

38.1 20.7 12.1 20.5 

15.6 34.2 30.5 29.3 

5.7 

.5* .5* 1.9* .1* 

4.2* 2.1* .8* 3.7* 

4.8* 4.3* 4.4* 5.8* 

.1 

5.9* .1* .2* tr* 

.3 tr trk trk trk tl"'< tr 

100 100 100 100 

598 007 571 711 

70.9 59.8 49.1 55.8 

<1 <1 <1 

!'Ill !'Ill !'Ill !'Ill 

250 300 210 

2.932 2.890 2.896 2.899 

Rock name ............ .. (Streckeisen, 1976). 

minette- minette- minette- minette- minettevogesite. vogesite. vogesite. vogesite. vogesite. 

1

rotal iron reported as Fez<>]• 

2

Loss on ignition at 900 °C. 

Alz03 

3A:<:NK = alunina saturation=-----

CaO+Na2o+K20 

(in molecular proportions). 

SiOz+NazO+KzO 

4F=------- (oxides in ~ight percent). 

sw2 

(oxides in ~ight percent). 

Al +Na 0+K ~FeOt-Fe 0:3 

SL-101B 

5.7 

tr 

37.9 

32 

28.0 

20.7 

.9 

1.0 

1.0 

tr 

4.8 

tr 

100 

690 

55.4 

<1 

m 

SL-139 

1.9 

10.4 

39.1 

30 

20.7 

17.3 

2.9 

2.1 

2.7 

1.9 

100 

701 

.3 

.7 

46.7 

<1 

m 

160 

2.852 

llXXlZO"" 

gabbro. 

6plagioclase determinations ~re I!Ede by a combination of m:ocinurn extinction angles to the 010 trace, 

optic sign, and refractive index conparisons against quartz. 

7 Color index is the sun of the colored (nelanocratic) minerals expressed in percent. O>lored nd.nerals 

are: biotite, anphibole, pyroxene, chlorite, scapolite, all opaque nd.nerals, garnet, allanite, zircon, 

sphene, tounnaline, epidote, and monazite. Light (leucocratic) minerals are: quartz, feldspar, Ill.lScovite, 

cordierite, calcite, fluorite, apatite, sillimanite, and clinozoisite. 

8

m, rock reacts postively to a large hand nagnet; nm, no reaction. 

~ured on the outcrop with a M::>tm.t Sopris scintilloneter' M:xlel sc-132. 

1

0netermi.ned with an air pycnoneter, described by ~Intyre and others (1965). An average of three 

determinations for each rock spec:im:m yields a specific gravity with: 0.008 gran per cubic centineter 

maxinum deviation. 

Petrography and Geochemistry A33 
Figure 12. Block of biotite lamprophyre (minette-vogesite) 

showing irregular veining by aligned pink potassium feldspar 

(raised, lighter veinlets near coin). Coin is 19 millimeters in 

diameter. 

CHEMICAL, PHYSICAL, AND 

P.ETROGRAPHIC COMPARISON OF 

ROCK UNITS 

Table 16 gives Rb:Sr ratios for each of the rock 

units. Although limited data indicate variability within a 

rock unit, the Rb:Sr averages are in direct proportion 

(loosely) to the felsic nature of the rock. 

Table 17 summarizes significant data for each of 

the rock units by listing ranges and averages of chemical, 

physical, and petrographic properties; enrichment factors 

of significant elements; and average contents of accessory 

minerals. Table 17 also clearly points out how different 

the Boulder Creek Granodiorite is from the Silver Plume 

Granite by showing that their ranges of values for 

~O:N~O, Th, La:Ni, radioactivity, specific gravity, and 

color index are mutually exclusive. A glance at the 

significantly enriched elements of all the rock types shows 

that the three mafic rocks (biotite lamprophyre, small 

mafic enclaves in the Boulder Creek Granodiorite, and 

the andesite porphyry dike) are the most enriched 

(discussed in detail in the geochemistry section of this 

report). Table 17 also shows significant differences in 

accessory-mineral composition of the rock types; for 

example, the zircon content of the biotite gneiss is about 

one-half that of either the Boulder Creek Granodiorite 

or the Silver Plume Granite. 

GEOCHEMISTRY 

Uranium and thorium analyses of the Boulder 

Creek Granodiorite and the Silver Plume Granite of the 

Strawberry Lake area, which are summarized in figure 

A34 Crystalline Rocks of the Strawberry Lake Area, Colorado 

Figure 13. Photomicrograph of biotite lamprophyre 

(minette-vogesite); microcline shows inhomogeneous brown 

color under the microscope. A, Plane-polarized light; B, 

Crossed nicols. Minerals: apat, apatite; biot, biotite; hb, 

hornblende; mic, microcline. 

14, differ but slightly from previous analyses of the same 

rock types by Phair and Gottfried (1963) and Hills and 

Dickinson (1982). In the Strawberry Lake area these two 

plutonic rocks show slightly less uranium and thorium for 

the Boulder Creek Granodiorite and slightly more 

uranium and thorium for the Silver Plume Granite than 

these rocks do elsewhere in the Front Range. This 

difference is probably explained by the fact that the 

Boulder Creek Granodiorite in the Strawberry Lake area 

is more mafic than Boulder Creek Granodiorite in the 

central and eastern parts of the Front Range and that the 

Silver Plume Granite in the Strawberry Lake area is 

more felsic than Silver Plume Granite in the eastern part 

of the Front Range. The following averages for silica 

illustrate this point: 
Samples Silica, in percent 

Average of 35 samples of Boulder 65.4 

Creek Granodiorite from the 

Boulder Creek batholith (Gable, 

1980, p. 61). 

Average of 6 samples of Boulder 61.9 

Creek Granodiorite from the 

Strawberry Lake area (this report, 

table 2). 

Average of 10 samples of Silver 69.9 

Plume Granite from the Silver 

Plume batholith (Anderson and 

Thomas, 1985). 

Average of 7 samples of Silver Plume 71.9 

Granite from the Strawberry Lake 

area (this report, table 7). 

Uranium and ·thorium contents of the rock units of 

the Strawberry Lake area are compared in figure 15. 

Notable are the high thorium content of the small 

granitic enclaves in the Silver Plume Granite and the very 

low thorium content of the small mafic enclaves in the 

Boulder Creek Granodiorite. More importantly, 

however, the thorium content in 9 of the 11 rock units is 

above crustal abundance. The rock unit richest in 

uranium is the biotite lamprophyre, and the uranium 

content in 7 of the 11 rock units is above crustal 

abundance (fig. 15). Altogether, the rock units of the 

Strawberry Lake area support the appellation "uranium 

and thorium province" that Phair and Gottfried (1963) 

gave to rocks of the Front Range. 

I used the method of Shaw (1961) to compare the 

trace-element contents of different rocks. Shaw proposed 

the term "coefficient of trace element accumulation" 

• 

(R ), which is defined as R = lin 2: k0c; where ki equals i- 1 

the concentration of element i in a rock or mineral, Ki 

equals the crustal abundance of element i, and n equals 

the number of trace elements determined. In figure 16, R 

is shown for each rock unit in the Strawberry Lake area 

for 21 elements: Ba, Be, Ce, Co, Cr, Cu, Ga, La, Mn, Nd, 

Ni, P, Ph, Sc, Sr, Th, Ti, U, V, Y, and Zr. For those few 

spectrographic analyses near a detection limit that are 

reported as a "less than" value, a conservative estimate of 

one-half that value was chosen for calculation. The 

coefficient of accumulation for the crust, or lithosphere, 

is 1.00, by definition. 

All the rock units accumulated more trace 

elements than crustal abundance. As figure 16 shows, 

two of the rock units, the biotite lamprophyre and the 

andesite porphyry dike, are accumulators of trace 

elements far in excess of crustal abundance. Most of the 

granitic enclaves and the small mafic enclaves have 

accumulated slightly more trace elements than did their 

much larger hosts, the Boulder Creek Granodiorite, the 

Silver Plume Granite, and the biotite gneiss. 

M 

0 

0 

00 

,....., 

0 

8 

0 

00 

,....., 

0 

$ 

0 

,....., 

N 

0 

P~trography and Geochemistry A35 
~ 

0 

1 

S' 

CD 

:a 

g 

ri 

2. 

g. 

CD 

~ 

I 

~ 

" rf 

~ 

.; 

~ 0 

Dl 

a. 

0 

Table 17. Averages and ranges of chemical, physical, and petrographic parameters of rock units in the Strawberry Lake area 

[Key to system of averages and ranges: 25 ---Number of samples contributing to average. 

7 ------Average. 

(3 to 10) ----Range. 

Key to system of significant elements: 618 (1.8) --Enrichment factor = Average/crustal abundance (Average). 

-,no data; <,less than; n.d., no determination; -,approximately. Accessory minerals: computed as averages (volume percentage), trace counted as 0.01 percent] 

Granitic Granitic Granitic enclaves Granitic enclaves Granitic enclaves Mafic enclaves Boulder Silver enclaves in enclaves in in Boulder Creek in Boulder Creek in Silver in Boulder Creek Creek Pltm! Biotite ~n~~i:s· f!*~~~ Granodiorite. Granodiorite. Large: Biotite Pltm! Granite. Granodiorite. Granodiorite Granite gneiss Small: 1-5m 100m to >0.5 km lamprophyre Small: <80 m Small 

lwerages and ranges 

6 7 3 1 K'fJ/Na'/) 0.93 1.9 4.45 - 2.33 3.52 1.24 10.5 2.45 0.86 (0.66 to 1.10) (1.5 to 2.1) (2.47 to 6.40) (3.13 to 3. 90) (1.10 to 1.38) (10.1 to 10.8) (2.29 to 2.61) 

6 7 3 1 2 2 2 A:CNC 0.98 1.13 4.13 - 1.16 1.11 1.08 0.50 1.25 0.73 (0.90 to 1.08) (1.01 to 1. 20) (2. 95 to 4. 81) (1.05 to 1.16) (1.02 to 1.13) (0.5 to 0.5) (1.25 to 1.25) 

6 7 3 2 2 2 2 F 5.15 23.8 5.34 - 2.65 17.1 11.7 2.38 16.5 1.85 (4.22 to 6.45) (16.5 to 30.2) (3. 72 to 6.62) (17.0 to 17.1) (6. 78 to 16. 7) (2.32 to 2.44) (14. 9 to 18.0) 

6 7 3 1 2 2 2 1.75 2.71 1.66 - 2.72 2.48 2.29 1.11 2.59 0.94 (1.52 to 2.05) (2.54 to 2.83) (1.13 to 2.14) (2. 37 to 2. 59) (1.94 to 2.63) (1.07 to 1.15) (2.55 to 2.63) 

12 25 5 3 4 2 3 3 

U, ~:urc:s per 1.46 5.57 3.15 3.89 3.79 1.87 1.83 8.50 4.2 2.15 (0.81 to 2.52) (2.23 to 9.03) (2.02 to 5.33) (1.36 to 6.97) (2.78 to 4.50) (1. 77 to 1. 96) (1.55 to 2.08) (7.68 to 9.52) (3.4 to 5.0) 

12 25 5 3 4 2 3 3 Th, in parts per 10.2 45.3 18.1 28.9 57.5 60.1 29.2 52.7 99.5 <2.7 million. (3.7 to 18.5) (21.6 to 70.5) (7 .6 to 24. 7) (26.4 to 30.4) (26.2 to 75.6) (33.9 to 86.2) (8.65 to 55.1) (43.9 to 62.6) (75. 9 to 123) 

12 25 5 3 4 2 3 3 Th:U 7.0 8.13 5.75 7.4 15.2 32.1 16.0 6.2 23.7 (1.3 (2.2 to 12.0) (3.6 to 32.1) (2.9 to 12.2) (3.8 to 22.0) (6.6 to 27.1) (19.2 to 44.0) (4.6 to 35.5) (5.7 to 6.6) (22.3 to 24.6) 

12 25 5 3 4 2 3 3 La:Ni 1.9 25 2.8 13 32 26 >33 1.7 21 (1.7 (0.6 to 5.0) (9 to >75) (1 to 6) (<S to 18) (23 to 37) (8 to 43) (>15 to SO) (1 to 2) (21 to 21) 

26 33 8 4 4 5 7 4 Radioactivity, in 127 355 192 218 334 408 268 284 525 - counts per Second. (90 to 175) (240 to 505) (155 to 240) (50 to 330) (280 to 385) (180 to 525) (150 to 470) (250 to 310) (500 to 550) 

~dc~rmn. 26 34 9 4 4 

(14m, 12mn) (9m, 2Snm) (3m, 6nm) (lm, 3m) (<b, 4nn) (Sm, Onm) (4m, 3rnn) (<b, Snm) (2m, Onm) (2m, Onm) 

25 31 7 3 4 5 7 5 2 Specific gravity 2.778 2.654 2.792 2.682 2.640 2.655 2.681 2.903 2.674 - (2.685 to 2.850) (2.630 to 2.683) (2.718 to 2.920) (2.620 to 2. 776) (2.630 to 2.650) (2.630 to 2.666) (2.625 to 2. 758) (2.890 to 2.932) (2.673 to 2.674) 

Andesite 

~ 

1.25 

0.75 

2.80 

1.24 

3.38 

22.9 

6.8 

2.9 

160 

(lm) 

1 2.852 
24 32 Color index 24.9 5.0 28.2 9.4 6.3 8.9 6.5 57.2 9.4 55.4 46.7 (13.2 to 40.9) (1.5 to 10.8) (10.6 to 67.4) (2.1 to 16.5) (2.5 to 9.8) (6. 7 to 12.2) (1.6 to 10.4) (49.1 to 70.9) (7 .0 to 11.8) 

Avi~~&:!!~;~· 26 34 9 4 4 5 7 5 2 2 2.5 2.3 <1.5 -1.2 1 <1 1 <1 1.5 <1 <1 (1.5 to 3.0) (<1 to 5.0) ((1 to 3) (<1 to 2) (<l to 1.5) (<1 to 1.5) (<1 to 2.5) (<1 to l) (1.5 to 1.5) 

Crustal Significant elerrent [average, in -weight percent ( enrichle1t factor)] abundance 

---

Fe 

!'""l - - 6.64 (1.4) - - - - - Ti 0.45 - - - - - - - 9.38 r) 

'·"rl - 0.75 1.7~ 0.61 ~1.4~ ~ 1.87 - - - - - - - 5.47 2.9 - 3.64 1.9 3.61 1.9 2.96 - - - - - 5.16 1.7 - 6.10 2.1 4.58 1.5 K 2.5) - 4.66 (1.9) - 5.23 (2.1) 6.18 (2.5) - 6.44 2.6 4.63 (1.9) p 0.093) - - - -0.22 (2.4) - - - 0.72 7.7 0.18 (1.9) 0.46 (4.9) 

Crustal Significant elerrent (in parts per million) abundance 

---

~ !Bl" -

~57 !'"'l 3.89 (1.6) 3.79 r-5~ - -

'·~r·l ~, r·'l -

'·~rl - 45.3 3.5 18.1 ~1.4) 28.9 ~2.2~ 57.5 4.4 60.1 t6~ 29.2 (2.2) 52.7 4.1 99.5 7.7 - 22.9 1.8 La 29 60 (2.1) 96 3.3 57 2) - 80 2.8 144 5) 149 5.1 133 t6~ 2n 9.6 150 5.2 - 200 6.9 Ce 70 166 2.4 -150 2.1 258 3.7) 245 3.5 200 2.9 547 7.8 250 3.6 - 300 4.3 w 37 - 118 3.2 70 p-9~ n.d. n.d. n.d. 97 2.6 500 (13.5 150 4.1 100 ~2.7~ 200 5.4 y 29 - - 47 1.6 - - - - 61 (2.1 - 70 2.4 

Yb (3~ - - 7 (2.3) n.d. n.d. n.d. - 5 (1.7) Ba 650 - - - 1143 (1.8) 1950 (3) 1467 F-3~ ""' (12.4l - - 5000 

~dm 618 (1.8) - - - - - 800 2.4 F-7~ 2067 r-1 - 500 (1.5) )J()() 8.8 - - - - - 360 (2.1) - 270 1.6 - - - Be (3.8) - - - - - 6.3 1.7 - - - Pb (16) - 35 (2.2) - - 37 (2.3) 38 (2.4) 30 (1.9) 28 1.8 30 (1.9) - 50 (3.1) 

&!"l - - 21 (2.1) - - - - 26 !'"'l - 50 B~6> - Cr 83 - - - - - - - 407 4.9 - 300 300 (3.6) 

~ ~ - - - - - - - 173 3) - - - - - - - - - 197 2.2~ - 200 (2.2) 150 (1.7) Co 18 - - - - - - - 26 1.4 

Al::cessory mineral (average, in voliJIE percent) 

Magnetite 0.48 0.19 0.16 0.63 0 1.16 0.81 0 1.0 0.9 2.7 Henatite 0.10 0.52 0.39 0.08 0.73 0 0.17 0.86 0 0 0 Pyrite 0 0.0006 0 0 0 0 0 0 0 0 0 Sphene 0.73 0.009 0.0025 0 0 0.20 0.09 4.4 0 1.0 0.3 Apatite 0.64 0.14 0.11 0.33 0.23 0.30 0.40 2.6 Mxlazite 0.01 1.0 1.9 0 0.067 0.03 0.008 0.23 0.002 o.ro; 0 0.1 0 0 j Zircon 0.017 0.015 0.0075 0.033 0.01 O.ll 0.08 0.02 0.01 0.01 0 Rutile 0 0 0 0 0 0 0 0.01 0 0 0 

~~isite 0.58 0.01 0.29 0.003 0 0.28 0.07 1.6 0 4.8 0.7 0.09 0 0 0 0 0 0 0 0 0 0 • Allanite 0.21 0.0016 0.0025 0.005 0 0.02 0.04 0.06 0 0.01 0 ., Fluorite 0 0.0003 0 0 0 0 0 0 ::r 0 0 0 '< Toutmaline 0 0 0 0.003 0 0 0 0 0 0 0 Garnet 0 0 6.0 0.05 0 0 0 0 0 0 0 • Cordierite 0 0 2.5 0 0 0 0 0 0 0 0 ::::. Sillimanite 0 0.17 3.4 0.003 0.003 0 0 0 a. 0.4 0 0 

Ci) 

§ 

::r 

CD 

3 

j 

t 

...., 
BOULDER CREEK 

GRANODIORITE 

SILVER PLUME 

GRANITE 

100~---------------,,----------------, 

45.3 

13* 

o~~~~~~-=~~~~~-===~==~~ 

10~--------------~~--------------~ 

a: 

UJ 

0.. 

CJ)Z 

tc§ 

~:::! 

~~ 

::) 

5.57 5.2 5.2 

2.5* 

o~~~~~a_~~~--~~_m~L_~~~ 

10~--------------~----------------~ 

8.1 

7.0 

A B c D E F 

EXPlANATION 

A Averages of 12 samples of Boulder Creek Granodiorite, 

Strawberry Lake area, this report. 

B Averages weighted for areal abundance in Boulder Creek-type 

intrusives, Front Range, Colorado (Phair and Gottfried, 1963). 

C Averages of 69 samples of Boulder Creek Granodiorite from the 

Tallahassee Creek area, Colorado (Hills and Dickinson, 1982). 

D Averages of 25 samples of Silver Plume Granite, Strawberry 

Lake area, this report. 

E Averages weighted for areal abundance in Silver Plume-type 

intrusives, Front Range, Colorado (Phair and Gottfried, 1 %3). 

F Averages of 50 samples of Silver Plume Granite from the 

Tallahassee Creek area, Colorado (Hills and Dickinson, 

1962). 

5.2* 

Figure 14. Uranium and thorium in the Boulder Creek 

Granodiorite and in Silver Plume Granite in the Front Range, 

Colorado. Crustal abundances(*) from Vinogradov (1962). 

The ratio La:Ni is useful to distinguish felsic rocks 

from mafic and intermediate rocks. As shown in table 17 

La:Ni averages about 25 for the felsic rocks and about 2 

for the mafic to intermediate rocks in the Strawberry 

Lake area. 

Figure 17 shows the significant elemental 

enrichment (1.4 times crustal abundance or greater) in 

the rock units of the Strawberry Lake area. From this 

figure, several conclusions can be drawn: 

1. The biotite lamprophyre shows the greatest 

number of enriched elements (22) and the greatest 

enrichment above crustal abundance of the 11 rock 

units, primarily due to the enrichment of K and Mg, 

A38 Crystalline Rocks of the Strawberry Lake Area, Colorado 

which are incompatible major elements. Elements 

associated with K enrichment, the felsic suite, are: 

Nd, Ba, La, Ce, Th, U, Ph, Be, and Zr. Elements 

associated with Mg enrichment, the mafic suite, are: 

P, Sr, Cr, Ni, Sc, V, Y, Ti, Ca, Co, and Mn. 

2. The andesite porphyry dike is the rock unit next 

most enriched in elements. Because of the mafic 

composition of the andesite porphyry, Sr, P, Cr, Mg, 

V, Sc, Ca, and Ti are enriched elements, but in the 

process of intrusion through relatively felsic country 

rock, the dike also may have become enriched with a 

felsic suite of Ba, La, Nd, Ce, Pb, Th, and U. 

3. The most consistent enrichment of any element 

or coherent element group in all the rock units is 

that of the light rare-earth elements (LREE). 

Enrichment in heavy rare-earth elements (HREE), 

which is represented by Y and Yb, occurs only in the 

small mafic enclaves in the Boulder Creek Granodiorite, although there is some enrichment of Y in 

the biotite gneiss and the biotite lamprophyre. 

4. Thorium is the next most enriched element in all 

the rock units, except in the Boulder Creek Granodiorite and in the small mafic enclaves in the 

Boulder Creek Granodiorite. 

5. All the granitic enclaves in the major host rocks 

consistently show enrichment in LREE, Th, and Ph. 

CONCLUSIONS 

The Boulder Creek Granodiorite in the Strawberry 

Lake area contains slightly less uranium and thorium 

than the average of the Boulder Creek Granodiorite in 

the Front Range, whereas the Silver Plume Granite in the 

Strawberry Lake area contains slightly more uranium 

and thorium than the average of the Silver Plume Granite 

in the Front Range. Such divergence from the mean 

correlates with the more mafic nature of the Boulder 

Creek Granodiorite and the more felsic nature of the 

Silver Plume Granite in the Strawberry Lake area. 

The granitic enclaves in the Boulder Creek Granodiorite, in the Silver Plume granite, and in the biotite 

gneiss are notable for their relatively high thorium and 

LREE contents compared to their hosts. The biotite 

lamprophyres are equally enriched in felsic elements (K, 

U, Th, La, Ce, Nd, Ba, Zr, Be, and Ph) and mafic 

elements (Mg, Ca, Ti, P, Y, Sr, Sc, Cr, Ni, V, and Co), 

whereas the small mafic enclaves in the same host are 

only enriched in mafic elements. The Tertiary(?) andesite dike is enriched in mafic elements, LREE, and 

thorium. The selective enrichment that occurred in all 

the enclaves and the dike is related to their origin and 

that of their hosts; this topic is discussed in chapter B of 

this bulletin. 

Three problems that stand out involve the enclaves. 

Petrographically, their constitution is known, but 
a: 8.5 

i ~Or--- ------t---a=[1[i1[~~:_1-+--Ii~-t-~rti-ii-~l~!i- 2.S* 

40r-----+-----r-----+----~------+-----~------~----~------~------~----~ 

32.1 

20 11--+--------t-------+--------+----£ 23.71---+-----+------l-------+-------1-------l 

~ 0~~--1--it--+---~-.... ~-.. :~-. -+--£:

8

.

1 

15.2 

f----+--::=::::l~i +------til:-1~.~11! ---"j;,;,;,;,;,;l!';ll"lr\...--.1 5.

2* 

Boulder Small Large Small Silver 

Creek granitic granitic mafic Plume 

Granodiorite enclaves enclaves enclaves Granite (12) in Boulder in Boulder in Boulder (25) 

Small 

granitic 

enclaves 

in Silver 

Biotite 

gneiss 

(5) 

Small 

granitic 

enclaves 

in biotite 

Large Biotite Andesite 

granitic lamprophyre porphyry 

enclaves (3) dike 

in biotite (1) 

Creek Creek Creek 

Granodiorite Granodiorite Granodiorite 

(2) (3) (1) 

Plume 

Granite 

(2) 

gneiss gneiss 

(3) (4) 

Figure 15. Uranium and thorium contents of rock units in the Strawberry Lake area. Number in parentheses represents 

number of samples that contributed to average. Crustal abundances(*) from Vinogradov {1962). 

petrologically they are enigmatic. The granitic enclaves in 

the Boulder Creek Granodiorite and in the Silver Plume 

Granite are notable for their relatively high thorium and 

high rare-earth-element contents compared to their 

hosts. How and why is such enrichment achieved? An 

inquiry into the process of formation of these enclaves is 

necessary to an understanding of this geochemical 

behavior. 

A more complex problem is the multielemental 

enrichment of the biotite lamprophyre (minettevogesite), which profits from the best of two worlds: the 

felsic world (enrichment inK, Nd, Ba, La, Ce, Th, U, Pb, 

Be, and Zr) and the mafic world (enrichment in Mg, P, 

Sr, Cr, Ni, Sc, V, Y, Ti, Ca, Co, and Mn). A conventional 

explanation of lamprophyres invokes metasomatic 

alteration of diabase dikes, but this is far from being 

proven. 

Constrained in size, the small mafic enclaves in the 

Boulder Creek Granodiorite may figure prominently in 

the genesis of the host, but how? Thus, knowledge of the 

origin of these felsic and mafic enclaves is mandatory to 

a full understanding of the formation of their host 

plutons. 

A discussion of the origin of the enclaves and their 

host rocks is deferred to chapter B of this bulletin, which 

covers the petrology of the crystalline rocks of the 

Strawberry Lake area. Because the isolated enclaves 

have no connection with an igneous source they cannot 

be considered intrusive. The following options for 

formation of these enclaves might be considered: 

1. As the plutons were being emplaced they 

transported their enclaves as accompanying 

different magmas. 

Petrography and Geochemistry A39 
5.------.-------.-------.------~------~------.-------.------.-------,-------.------. 

4.32 

4r------1-------+------~------~------4-------+-------~----~-------+-4: 

3.07 

3r-----_,-------+------~------4-------4-------+-------~----~-------+-;: 

2r-----_,-------+------~------~------4-------+-------~----~-------+-4: 

1.41 1.50 

I I 1.30 

1.02 -~ 1.15 

Boulder Small Large Small Silver 

Creek granitic granitic mafic Plume 

Granodiorite enclaves enclaves enclaves Granite 

(12) in Boulder in Boulder in Boulder (25) 

Creek Creek Creek 

Granodiorite Granodiorite Granodiorite 

(2) (3) (1) 

1.54 

I 

Small 

granitic 

enclaves 

in Silver 

Plume 

Granite 

(2) 

1.14 

~ 

Biotite 

gneiss 

(5) 

1.07 

Small 

granitic 

enclaves 

in biotite 

gneiss 

(3) 

1.36 

I 

Large Biotite Andesite 

granitic lamprophyre porphyry 

enclaves (3) dike 

in biotite (1) 

gneiss 

(4) 

Figure 16. Coefficient of trace element accumulation, R, in rock units in the Strawberry Lake area. Number in 

parentheses represents number of samples that contributed to average. 

2. The enclaves are xenoliths (country rock not 

completely assimilated by magma). 

3. The host plutons represent largely granitized 

or ultrametamorphic rock (does not exclude local 

melting), and the enclaves are relicts of country 

rock that resisted granitization or ultrametamorphism. 

4. The process of enclave formation (metamorphic differentiation) is an outgrowth of the 

metamorphic-metasomatic processes that were 

involved in formation of the host. Local melting is 

not excluded. 

Options 1 and 2 would represent a relatively short 

time of formation (10 million years or less), whereas 

A40 Crystalline Rocks of the Stra~rry Lake Area, Colorado 

options 3 and 4 would entail a long time of formation 

(100 million years or more). Whichever option is more 

likely, it must account for the relatively high LREE and 

Th contents of the granitic enclaves, the HREE 

enrichment in the small mafic enclaves, and the felsic and 

mafic elemental enrichment of the biotite lamprophyre. 

REFERENCES CITED 

Anderson, J.L., and Thomas, W.M., 1985, Proterozoic 

anorogenic two-mica granites-Silver Plume and St. 

Vrain batholiths of Colorado: Geology, v. 13, p. 177-180. 

Burley, B.J., Freeman, E.B., and Shaw, D.M., (1961), Studies 

on scapolite: Canadian Mineralogist, v. 6, pt. 5, 

p. 670-679. 
aarke, D.B., 1981, The numeralogy of peraluminous granites-A review: Canadian Mineralogist, v. 19, p. 3-17. 

Didier, J., 1973, Granites and their enclaves: New York, 

Elsevier Scientific Publishing, 393 p. 

Eckel, E.B., 1961, Minerals of Colorado-A 100-year record: 

U.S. Geological Survey Bulletin 1114, 399 p. 

Fleischer, Michael, 1983, Glossary of mineral species: Tucson, 

Ariz., Mineralogical Record, Inc., 202 p. 

Gable, D.J., 1980, The Boulder Creek Batholith, Front Range, 

Colorado: U.S. Geological Survey Professional Paper 

1101, 88 p. 

Hedge, C.E., Peterman, Z.E., and Braddock, W.A., 1967, Age 

of the major Precambrian regional metamorphism in the 

northern Front Range, Colorado: Geological Society of 

America Bulletin, v. 78, no. 4, p. 551-558. 

Hills, F.A., and Dickinson, K.A., 1982, Silver Plume GranitePossible source of uranium in sandstone uranium 

deposits, Tallahassee Creek and High Park areas, 

Fremont and Teller Counties, Colorado: U.S. Geological 

Survey Open-File Report 82-404, 23 p. 

lzett, G.A., 1974, Geologic map of the Trail Mountain 

quadrangle, Grand County, Colorado: U.S. Geological 

Survey Map GQ-1156, scale 1:24,000. 

Mcintyre, D.B., Welday, E.E., and Baird, A.K., 1965, Geologic 

application of the air pycnometer-A study of the 

precision of measurement: Geological Society of 

America Bulletin, v. 76, no. 9, p. 1055-1060. 

Pearson, R.C., and U.S. Bureau of Mines, 1980, Mineral 

resources of the Indian Peaks Study Area, Boulder and 

Grand Counties, Colorado: U.S. Geological Survey 

Bulletin 1463, 109 p. 

Peterman, Z.E., Hedge, C.E., and Braddock, W.A., 1968, Age 

of Precambrian events in the northeastern Front Range, 

Colorado: Journal of Geophysical Research, v. 73, no. 6, 

p. 2277-2296. 

Phair, George, and Gottfried, David, 1963, The Colorado Front 

Range, Colorado, U.S.A., as a uranium and thorium 

province, in Adams, J.A.S., and Lowder, W.M., eds., The 

National Radiation Environment: Houston, Tex., Rice 

University, p. 7-38. 

Schroeder, D.A., 1984, Geology of the Granby and Strawberry 

Lake 7lh' quadrangles, Grand County, Colorado: 

Boulder, University of Colorado, Ph. D. thesis, 340 p. 

Shaw, D.M., 1960, The geochemistry of scapolite; Pt. I, 

Previous work and general mineralogy, and Pt. II, Trace 

elements, petrology, and general geochemistry: Journal 

of Petrology, v. 1, pts. 2 and 3, p. 218-285. 

___ 1961, Coefficient of accumulation-A measure of the 

total trace element content of a mineral or a rock: 

Canadian Mineralogist, v. 6, pt. 5, p. 694-697. 

Streckeisen, A.L., 1976, To each plutonic rock its proper name: 

Earth Science Reviews, v. 12, p. 1-33. 

___ 1979, Classification and nomenclature of volcanic 

rocks, lamprophyres, carbonatites, and melilitic rocksRecommendations and suggestions of the lUGS 

subcommission on the systematics of igneous rocks: 

Geology, v. 7., p. 331-335. 

Taylor, R.B., 1975, Geologic map of the Bottle Pass 

quadrangle, Grand County, Colorado: U.S. Geological 

Survey Map GQ-1224, scale 1:24,000. 

Taylor, S.R., 1964, The abundance of chemical elements in the 

continental crust-A new table: Geochimica et Cosmochimica Acta, v. 28, p. 1273-1285. 

Tweto, Ogden, 1979, Geologic map of Colorado: U.S. 

Geological Survey, prepared in cooperation with the 

Geological Survey of Colorado, scale 1:500,000. 

___ 1987, Rock units of the Precambrian basement in 

Colorado: U.S. Geological Survey Professional Paper 

1321-A, 54 p. 

Vinogradov, A.P., 1962, Average contents of chemical elements 

in the principal types of igneous rocks of the earth's crust: 

Geokhimiya 1962, no. 7, p. 555-571 [in Russian], 

translation in Geochemistry, 1962, no. 7, p. 641-664. 

Young, E.J., 1984, Analytical data on the crystalline rocks of 

the Strawberry Lake area, Grand County, Colorado: U.S. 

Geological Survey Open-File Report 84-568, 30 p. 

Petrography and Geochemistry A41 
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Small granitic 

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Boulder Creek 

Granodiorite 

(2) 

~ 

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1: 

·lllll~ La Ce Nd Sr Ba Th Pb Sc Cr Nd Y V Ca Fe P Mg Mn Ti Yb Ga Sr Th La Nd Ce U Pb K Th La Nd Ce K Pb U Ga Yb Sc La Nd Y Th Fe 

Large granitic 

enclaves in 

Boulder Creek 

Granodiorite 

(3) 

Small mafic 

enclaves in 

Boulder Creek 

Granodiorite 

(1) 

Silver Plume 

Granite 

(25) 

Small granitic 

enclaves in 

Silver Plume 

Granite 

(2) 

Biotite 

gneiss 

(5) 
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Small granitic 

enclaves in 

biotite gneiss 

La Th Ce Nd' Pb K Ba U Nd Ba La Ce P Sr Cr Th U Ni Mg Sc K V Y Pb Ti Ca Be Zr Co Mn Sr Ba La Nd P Ce Cr Pb MgTh V Sc Ca Ti U 

(3) 

Large granitic 

enclaves in 

biotite gneiss 

(4) 

Biotite 

lamprophyre 

(3) 

Andesite 

porphyry 

dike 

(1) 

Figure 17 (above and facing page). Significant elemental enrichment (1.4 x crustal abundance or greater) in rock units of the Strawberry Lake area. 

Number in parentheses represents number of samples that contributed to average. Number at top of distribution bar is average in parts per million. 

Crustal abundances from Vinogradov (1962), except for Yb (Taylor, 1964). Nd1 identifies samples where Nd content was estimated from 

Nd/Ce+La = -0.32. 

Chapter B 

Petrology 

By EDWARD J. YOUNG 

A discussion of the petrology and 

felsic-mafic and silica-saturation ratios 

U.S. GEOlOGICAl SURVEY BUllETIN 1937 

CRYSTALLINE ROCKS OF THE STRAWBERRY LAKE AREA, 

FRONT RANCE, COLORADO 

CONTENTS 

Abstract Bl 

Introduction Bl 

The F-S Method B2 

Petrology B3 

Boulder Creek Granodiorite B3 

Granitic enclaves in Boulder Creek Granodiorite BS 

Silver Plume Granite BS 

Granitic enclaves in Silver Plume Granite B9 

Biotite gneiss Bll 

Granitic enclaves in biotite gneiss B12 

Biotite lamprophyre enclaves in Boulder Creek Granodiorite B14 

Small mafic enclaves in Boulder Creek Granodiorite B15 

Andesite porphyry dike B17 

Conclusions B18 

References Cited B18 

FIGURES 

1. F-S diagram of crystalline rocks B4 

2. F-S diagram of Boulder Creek Granodiorite and its granitic enclaves B6 

3. IA12 0 3 + Na20 + K2 0 versus Si02 diagram of Boulder Creek Granodiorite 

and its granitic enclaves B7 

4. F-S diagram of Silver Plume Granite and its granitic enclaves BlO 

5. IA12 0 3 + N~O + K20 versus Si02 diagram of Silver Plume Granite and its 

granitic enclaves Bll 

6. F-S diagram of biotite gneiss and a granitic enclave B12 

7. IA12 0 3 + Na2 0 + K2 0 versus Si02 diagram of biotite gneiss and a granitic 

enclave B13 

8. F-S diagram of biotite lamprophyres and other mafic rocks B14 

9. IA12 0 3 + Na2 0 + K20 versus Si02 diagram of biotite lamprophyres and other 

mafic rocks B17 

TABLES 

1-5. Classification of rock types of the: 

1. Boulder Creek Granodiorite, Strawberry Lake area BS 

2. Granitic enclaves in the Boulder Creek Granodiorite, Strawberry Lake 

area BS 

3. Silver Plume Granite, Strawberry Lake area B9 

4. Granitic enclaves in the biotite gneiss, Strawberry Lake area B13 

5. Biotite lamprophyre, Strawberry Lake area B15 

6. Comparative chemical analyses of lamprophyres B16 

Contents Ill 

Petrology 

By Edward J. Young 

Abstract 

Felsic-mafic ratios and silica-saturation ratios (F-8 

method) lead to inferences concerning the origin of 

crystalline rocks. Corroboration of the validity of the 

inferences is supported by two different methods, which 

involve biotite analysis and the IAI2 0 3 + Na2 0 + ~0 in the 

rock. 

The Boulder Creek Granodiorite, which is tonalitic in 

composition, in the Strawberry Lake area of northern 

Colorado is part of a large Early Proterozoic plutonic 

complex. Evidence from the F-8 method, from the 

IAI203 + Na2 0 + K20 method, and from a few biotite 

analyses favors a metamorphic-metasomatic origin for most 

of the Boulder Creek Granodiorite in the study area. Other 

evidence for a metamorphic-metasomatic, rather than a 

totally intrusive, origin is: 

1. Enclaves of biotite gneiss within the Boulder Creek 

Granodiorite show foliation attitudes similar to foliation 

attitudes in surrounding country rock of biotite gneiss. 

2. The average composition of the Boulder Creek 

Granodiorite is approximately the same as that of the 

biotite-gneiss country rock. 

3. Many, if not most, zircons in the Boulder Creek 

Granodiorite were inherited from the biotite gneiss as 

evidenced by zircon overgrowths on older zircon 

cores. 

4. Dissimilarities in uranium content of apatites favor 

a metamorphic-metasomatic origin. 

Granitic enclaves within the Boulder Creek Granodiorite 

probably represent an outgrowth of metamorphicmetasomatic processes during formation of the host. 

Attributes of the granitic enclaves compared to their host are: 

increased ~O:Na 0, Ba, Th:U, and light rare-earth 

elements; a tendency toward lower silica saturation; and 

accessory apatite and sphene, which represent a reflection of 

the same chemistry that produced apatite and sphene in the 

host. 

The Middle Proterozoic Silver Plume Granite from the 

Strawberry Lake area is more felsic than Silver Plume Granite 

from the Front Range as a whole. Evidence from the F-8 

method, from the IAI2 0 3 + Na2 0 + K2 0 method, and from a 

few biotite analyses generally favors a metamorphicmetasomatic origin for the Silver Plume Granite. Local partial 

melting is possible. Foliation of biotite-gneiss enclaves within 

the Silver Plume Granite and planar orientation of feldspar 

laths in the Silver Plume Granite generally conform to foliation 

in surrounding biotite gneiss. Biotite:muscovite ratios are 

highly variable. Some zircon was inherited from older 

precursor rock. 

Granitic enclaves in the Silver Plume Granite may 

represent zones of partial melting. The enclaves are more 

mafic, more radioactive, and finer grained than their host. 

Granitic enclaves in the biotite-gneiss country rock are 

compositionally and mineralogically similar to Silver Plume 

Granite, but they are finer grained. They may be Silver Plume 

Granite precursor rock that has been subjected to the same 

metamorphic-metasomatic conditions that produced the 

Silver Plume Granite in this area. 

Textural evidence and low IAI20 3 + Na20 + ~0 values 

suggest that the biotite lamprophyres had attained a 

magmatic state. The small mafic enclaves in the Boulder 

Creek Granodiorite are interpreted to be restites, which are 

geochemically immobile segregations. The Tertiary(?) andesite porphyry dike is unequivocally igneous as shown by 

structure, two types of plagioclase phenocrysts and low 

IA120 3 + Na2 0 + K2 0 values. 

INTRODUCTION 

This report uses an entirely new method of 

classifying crystalline rocks, the F-S method, which is 

based on felsic-mafic and silica-saturation ratios (Young, 

1989). The petrologic inferences that are suggested by 

the F-S method have been corroborated by two other, 

completely different methods, which use biotite analysis 

and the IA120 3 + Na20 + K20 in the rock (Young, 

1989). 

In particular, my work strongly suggests that most, 

if not all, of the Proterozoic plutonic rocks in the 

Strawberry Lake area are metamorphic-metasomatic, 

rather than magmatic, in origin. These inferences may be 

disturbing to many geologists. If the inferences are 

invalid, no change need be made in the opinion of most 

"magmatists." However if the inferences are valid, or 

Petrology 81 
even partially valid, further inquiry along similar lines 

needs to be undertaken by other curious investigators. It 

is appropriate to keep in mind the following words of 

Roddick (1982): 

We can conclude safely that the logic applied to 

granitic rocks has not been rigorous. In spite of the 

inconclusiveness of the evidence, the view that most 

plutonic bodies were emplaced in a magmatic state is 

held by the overwhelming majority of geologists. Why 

this should be is almost as complex as the granite 

problem itself. Geologists, by and large, have no corn.er 

on perverse logic, and if at times they lean too heav1ly 

on the collective wisdom of individual ignorance, it is 

probably because the demands of their specialites force 

them, in peripheral fields, to rely more on the judgment 

of others than on the evidence. For some specialties, 

however, the origin of granite is not merely a peripheral 

aspect but a square-one assumption that must be 

correct if the subsequent edifice is not to collapse. 

Theories of ore deposition, geochronology and some 

geochemical arguments are peculiarly sensitive to any 

erosion of this assumption. 

Until the origin of granite is truly understood a large 

part of geological science has no reliable foundation. 

If the ~esults of this study were the only suggestion 

that the plutonic rocks of this area are largely 

metamorphic-metasomatic, rather than magmatic, 

perhaps the evidence would not be strong enough to 

justify a reappraisal of the origin of these rocks. 

However, I believe that many other geologic and 

petrographic observations of these rocks are more 

compatible with a metamorphic-metasomatic, rather 

than a magmatic, origin, and consequently that this 

reappraisal is not only justified but necessary. 

The terms "metamorphic-metasomatic" and 

"magmatic" are from Gokhale (1968). "Magmatic" 

means the involvement of fully molten rock with no or 

variable amounts of phenocrysts. "Metamorphicmetasomatic" describes processes that range from solidstate reactions to in-situ partial melting (anatexis), in 

short, granitization. As Krauskopf (1967) pointed out, 

The important geological question, however, is not 

whether partial melting is possible-no one would 

seriously deny this-but on what sort of a scale the 

process operates in nature. Is partial melting a rare and 

local phenomenon to be found occasionally in areas of 

high-grade metamorphism, or is it a general process 

responsible for the origin of most granite plutons? Field 

observations should be the best approach to an answer, 

but unfortunately their interpretation is not wholly 

unambiguous. Relations at some granite contacts 

demonstrate beyond question that high-grade metamorphic rocks are locally transitional into igneousappearing materials, but the great bulk of most large 

plutons shows no structures that give conclusive 

evidence as to origin. From field evidence alone most 

plutons may apparently have moved into their present 

positions as molten material from far below, or they may 

82 Crystalline Rocks of the Strawberry Lake Area, Colorado 

consist chiefly of material already present and made 

over into granitic rock by extreme metamorphism, or 

they may contain materials of both kinds. The question 

as to which process is the more important-the old 

argument between the 'granitizers' a~d the 'm.agmatists' -must wait for an ultimate solut1on on ev1dence 

other than field criteria. 

The current status of this controversy we have 

reviewed in an earlier discussion (Sec. 14-6). Here we 

need only emphasize that, from a purely chemical 

standpoint, a metamorphic origin of all granite out .of 

crustal materials seems entirely possible. Some gran1te 

bodies may have remained practically static during the 

process of extreme metamorphism, while others may 

have moved as a liquid or partly liquid mass from deep 

levels into cooler parts of the crust and hence may show 

all the familiar phenomena of intrusion. Chemical 

support for the orthodox magmatic hypothesis, 

envisioning differentiation of granitic liquid out of 

primordial subcrustal material, is currently more difficult 

to muster because of experimentally demonstrated 

changes in the differentiation process at high pressures. 

But evidence is by no means all on the one side, so that 

characterization of granites in general as a product of 

extreme metamorphism is hardly warranted. 

Metamorphic-metasomatic processes include ultrametamorphism and metamorphic differentiation. 

Metamorphic-metasomatic processes require relatively 

long periods of time and elevated temp~rature~ and 

pressures; such conditions can not be duplicated m ~he 

laboratory. By contrast, magmatic processes, whtch 

usually produce textures that are unambiguous, are 

relatively short lived. 

Chapter B has benefited from the reviews of 

Robert C. Pearson, Thomas P. Frost, and James E. 

Elliott. All the observations and views expressed, 

however, are mine. 

THE F-S METHOD 

The F-S method of classifying crystalline rocks is 

based on a felsic-mafic ratio, 

F 

FeO + Fe2 0 3 + MgO + CaO 

and a silica-saturation ratio, 
A full explanation of the rationale, validity, and use of the 

F-S method is contained in Young (1989). Because the 

F-S method is used extensively in this report, I will briefly 

describe its main features here. Figure 1 illustrates how 

the method is applied. If F and S of any crystalline rock 

are known, a compositional-equivalent name may be 

obtained as well as information about the degree of silica 

saturation of the rock. The compositional-equivalent 

names are derived from comparison with Nockolds' 

(1954) average rocks (Young, 1989). 

From consideration of a test set of five 

metamorphic-metasomatic and seven magmatic 

(igneous) rocks (Young, 1989), I found that 

metamorphic-metasomatic rocks tend to show lower 

silica saturation( s) than magmatic rocks; in other words, 

metamorphic-metasomatic rocks plot left of the oversaturation trendline, whereas magmatic rocks plot right of 

the trendline. Therefore, inferences about the origin of 

quartz-bearing crystalline rocks should be obtainable 

with the F-S method. Several examples presented in 

Young (1989) demonstrate that the petrologic inferences 

are consistent with field data. 

Two independent methods support and 

corroborate the F-S method (Young, 1989). One 

method involves biotite in the rock; Gokhale (1968) has 

shown that biotite from magmatic (igneous) rocks tends 

to contain less Mg than biotite from metamorphicmetasomatic rocks. The other method demonstrates that 

the IA120 3 + N~O + ~0 tends to be less than 22 

percent in granitic magmatic rocks and greater than 23 

percent in granitic metamorphic-metasomatic rocks; 

values between 22 and 23 percent are generally 

equivocal. 

PETROLOGY 

The petrology of the crystalline rocks of the 

Strawberry Lake area is discussed in the same order as in 

Chapter A of this bulletin, which described the 

petrography and geochemistry of these rocks. First, 

comparisons of the results of classifying these rocks using 

the Johannsen (1939) and Streckeisen (1976, 1979) 

classifications and the F-S method are made. Then, 

inferences about the origin of these rocks based on the 

F-S method and IAI20 3 + Na20 + K20 are discussed. 

Finally, geologic and petrographic observations bearing 

on the origin are covered. 

Boulder Creek Granodiorite 

Table 1 compares the classification of the Boulder 

Creek Granodiorite in the Strawberry Lake area using 

the Johannsen (1939) and Streckeisen (1976) 

classifications and the F-S method. The F-S method 

suggests that the Boulder Creek Granodiorite in the 

Strawberry Lake area would be more accurately 

classified as a tonalite than a granodiorite. 

The six chemically analyzed samples of Boulder 

Creek Granodiorite are plotted on an F-S diagram in 

figure 2. For comparison, the average of 42 chemically 

analyzed samples of Boulder Creek Granodiorite in the 

Boulder Creek batholith (Gable, 1980) is also plotted. 

This comparison shows that Boulder Creek Granodiorite 

in the Strawberry Lake area is considerably more mafic 

than Boulder Creek Granodiorite in the batholith-even 

the most felsic sample of Boulder Creek Granodiorite in 

the Strawberry Lake area is not as felsic as the average 

Boulder Creek Granodiorite in the batholith. The two 

most felsic samples from the Strawberry Lake ~.rea (fig. 

2) plot on the· high-S side of the oversaturation trendline-this position is indicative of magmatic conditions. 

More support for this inference is shown by figure 

3, where Boulder Creek Granodiorite is plotted on the 

IA120 3 + Na20 + K20 versus Si02 diagram. Both figures 

2 and 3 suggest that more magmatic conditions were 

attained by more felsic portions of the precursor Boulder 

Creek Granodiorite. This is a reasonable suggestion 

because the felsic part of the precursor (biotite gneiss) 

would achieve partial melting before the mafic part, 

when and if higher pressure-temperature conditions 

were attained. The two most felsic (highest Si02) 

samples plot on the magmatic side of figure 3. These two 

samples are the only samples of peraluminous Boulder 

Creek Granodiorite. The more mafic samples and the 

average of 35 samples of Boulder Creek Granodiorite in 

the batholith fall in the equivocal part of the diagram 

(22-23 percent). 

Biotite in the Boulder Creek Granodiorite may be 

helpful as an indicator of the mode of origin of the host 

rock. Gable (1980) presented seven chemical analyses of 

biotites from the Boulder Creek batholith and chemical 

analyses of the rocks from which the biotite was 

separated. Using the method of Gokhale (1968), all 

seven biotites plot in the metamorphic-metasomatic 

field. Furthermore, all seven rocks from which the 

biotites were removed plot as metamorphic-metasomatic 

in both the F-S method and the IAI20 3 + Na20 + K20 

method. 

Where the Boulder Creek Granodiorite is foliated 

(pl. 1 of Chapter A of this bulletin), the foliation attitude 

conforms to that of the surrounding biotite-gneiss 

country rock. Even more indicative of metamorphicmetasomatic conditions are the relict enclaves of biotite 

gneiss within the pluton of Boulder Creek Granodiorite. 

The foliations in the these enclaves are conformable with 

that of host Boulder Creek Granodiorite or nearby 

biotite-gneiss country rock. If the Boulder Creek Granodiorite had been emplaced as a gross intrusion, it would 

be difficult to account for not only the foliation pattern 

but also the necessary space. 

Petrology 83 
50 

40 

30 

20 

~10 

O~g 

~0 + Ol 

<&~ 8 Ill ('I') 

zo 

+ N NQ) 

Q~7 

1/)Q 

~ 6 II 

u. 

5 

4 

3 

2 

00 

4 

Ultrafelsic CD 

~I 

{I 

~I 33.7 

I 

I 

Felsic granite I 

I 

23.6 

Granite 

15.2 

Adamellite 

Granodiorite 

6.80, ___________ _.. 

Tonalite 

4.52 

Monzonite 

2.98 

Diorite 

2.14 

Gabbro 

1.51 

Ultramafic 

EXPLANATION 

00 00 

CD. 4 e Nockolds' (1954) average rocks 

· X Speci&c rocks: Nepheline syenite .!I sl ~I ~I (average of 80 samples)-

Normative percent nepheline is 23.3, 

51 I olivine Is 0.2 

I I Alkali syenite (average of 25 

I I samples)-Normatlve percent quartz 

I I Is 1.7 

I I Calc-alkali syenite (leucocratic) 

(average of 18 samples)-

Normative percent quartz Is 2.0 

Calc-alkali syenite (melanocratic) 

(average of 6 samples)-Normative 

percent olivine Is 11.8 

Urtite (average of 6 samples)-

Normative percent nepheline is 64.5 

ljolite (average of 11 samples)-

Normative percent nepheline is 43.7 

Normal tholeiitic basalt (and dolerite) (average of 137 samples)-

Normative percent quartz Is 3.5 

1.0 2.0 3.0 

Figure 1. F-8 diagram of crystalline rocks. F, felsic-mafic ratio; S, silica-saturation ratio. All oxide values are in weight 

percent. Data from Nockolds (1954) for average rocks (dots) and other rocks (described below). 

84 Crystalline Rocks of the Strawberry Lake Area, Colorado 
Table 1. Classification of rock types of the Boulder Creek Granodiorite, Strawberry Lake area 

[=,equals) 

Johannsen (1939) Streckeisen (1976) 

method method 

F-S method1 

(Combined F's from specific 

gravity and modal analysis) 

F-S method! 

(Chemically analyzed samples) 

15 granodiorite 13 granodiorite 

8 tonalite 10 tonalite 

15 tonalite 

6 monzonite 

5 tonalite 

1 monzonite 

1 adamellite 1 monzogranite 2 granodiorite 

1 adamellite 

Average F = 5.75 

(tonalite). 

Average F = 5.15, 

average S = 1.75 

(tonalite). 

1Rock names are compositional-equivalent names. 

Additional compelling evidence for a 

metamorphic-metasomatic origin for the Boulder Creek 

Granodiorite is the similarity in composition of the 

biotite gneiss (F=5.34) and the Boulder Creek Granodiorite (F = 5.15), both of which are tonalitic in 

composition (Chapter A of this bulletin). The F-S 

classification points explicitly to this similarity. The 

composition of an intruding magma would probably not 

match that of the host rock unless the intruding magma 

originated by the melting of identical host rock. 

Petrographic evidence favors a metamorphicmetasomatic origin for the Boulder Creek Granodiorite 

in the Strawberry Lake area. Many, if not most, zircons in 

the Boulder Creek Granodiorite contain older zircon 

cores that are surrounded by recrystallized zoned or 

unzoned overgrowths of zircon. Stern and others (1971) 

called these zircons noneuhedral and considered them to 

have been inherited by assimilation, in contrast with 

euhedral, magmatic zircons. Both types of zircon are 

found in the Boulder Creek Granodiorite of the 

Strawberry Lake area. Thus, zircon morphology is 

consistent with a metamorphic-metasomatic history of 

the Boulder Creek Granodiorite; that is, many zircons 

were inherited from the country rock (biotite gneiss), and 

some crystallized as a result of partial melting. 

Some apatite in the Boulder Creek Granodiorite 

shows weak radiation-damage halos in biotite, but other 

grains of apatite do not; the difference is evidently due to 

variation in uranium content among the apatite 

population. This is not a likely circumstance for a 

magma, but instead, suggests the heterogeneous 

environment of a metamorphic-metasomatic origin. 

Biotite in the Boulder Creek Granodiorite locally 

contains many small grains of sphene, and some biotite 

shows borders of small grains of sphene. These 

occurrences of sphene suggest that temperatures were 

not high enough for biotite to keep Ti in solid solution 

during crystallization. 

Thus, geologic relations, similar composition of 

host rock (biotite gneiss) and the Boulder Creek Granodiorite, petrographic observations, and, most of all, 

evidence from included biotite, the F-S method, and the 

method ofiA12 0 3 + N~O + ~0 in the rock suggest that 

a metamorphic-metasomatic origin for the Boulder 

Creek Granodiorite in the Strawberry Lake area is 

reasonable. 

Granitic Enclaves in 

Boulder Creek Granodiorite 

Granitic enclaves in the Boulder Creek Granodiorite range from 1 m to greater than 1 km in greatest 

dimension. The range in rock type of these enclaves is 

shown in table 2, which describes them according to three 

classifications. For rocks of this composition (granite to 

adamellite), the three methods are in reasonable 

agreement. These rocks fit the description of the Twin 

Spruce Quartz Monzonite of Gable (1980) in their fine 

grain size, chemical composition, and petrography. Gable 

(1980, p. 27) noted that the Twin Spruce Quartz Monzonite forms not only mappable bodies but also local thin 

lenses that are commonly intermixed with the granodiorite or are gradational into it. Gable (1980, p. 27) also 

stated that the Twin Spruce Quartz Monzonite is in part 

the same age as upper Boulder Creek Granodiorite and 

in part younger than Boulder Creek Granodiorite. From 

these two observations of Gable (1980), it is clear that the 

Boulder Creek Granodiorite and the Twin Spruce 

Quartz Monzonite demonstrate an intimate and genetic 

interdependency. 

Gable (1980) presented chemical analyses of nine 

biotites from the Twin Spruce Quartz Monzonite. Again, 

using the method of Gokhale (1968), every biotite plots 

in the metamorphic-metasomatic field. Complications 

arise, however, because six of the biotites come from 

Petrology B5 
50,-------------------------~----~--~----~----~--~ EXPlANATION 

Ultrafelsic 

Felsic granite 

Granite 

Adamellite 

Granodiorite 

Tonalite 

Monzonite 

Diorite 

Gabbro 

Ultramafic 

1.0 2.0 3.0 

e Nockolds' (1954) average rocks 

• Boulder Creek Granodiorite-Strawberry 

Lake area 

• Boulder Creek Granodiorite-Average of 

42 samples from Gable (1980, p. 61) 

£. Granitic enclaves in Boulder Creek 

Granodiorite-Strawberry Lake area 

S = SiOz 

AI20 3+Na20+Kz0+Mg0+Ca0+Fe0+Fe20 3 

Figure 2. F-8 diagram of Boulder Creek Granodiorite and its granitic enclaves. F, felsic-mafic ratio; S, silica-saturation 

ratio. 

Twin Spruce Quartz Monzonite (average 39.5 percent 

modal quartz) enclaves in metasedimentary rock and 

three of the biotites come from Twin Spruce Quartz 

Monzonite (average 21.1 percent modal quartz) enclaves 

in Boulder Creek Granodiorite. The reason for bringing 

modal quartz into the picture is that four of the six 

samples of the Twin Spruce Quartz Monzonite enclaves 

86 Crystalline Rocks of the Strawberry Lake Area, Colorado 

in metasedimentary rock plot as igneous in both the F-S 

and IA120 3 + Na20 + ~0 methods, which, I believe, is 

due to the excess quartz. On the other hand, all three 

samples of the Twin Spruce Quartz Monzonite enclaves 

in Boulder Creek Granodiorite plot as metamorphicmetasomatic using the F-S and IAI20 3 + N~O + ~0 

methods. 
80 

!--Equivocal zoneSL-58.._ .6. SL-20 

70 

60 

50 

20 

MAGMATIC 

.sL-70 

.sL-67 

21 

.... SL-147 

22 

I .a.SL-11 

i 

METAMpRPHICMETASPMATIC 

SL-66 

• ~L-35 

SL-57 

SL-51• 

23 24 25 

EXPlANATION 

• Boulder Creek Granodiorite-Strawberry Lake area 

• Boulder Creek Granodiorite-Average of 42 samples from Gable 

(1980, p. 61) 

.6. Granitic enclaves in Boulder Creek Granodiorite-Strawberry Lake area 

Figure 3. IAI2 0 3 + Na2 0 + K.zO versus Si02 diagram of Boulder Creek Granodiorite and its granitic 

enclaves. 

The four chemically analyzed samples in this report 

are plotted on an F-S diagram in figure 2, which shows 

that three are very similar in composition (granite) and 

the fourth is close to the line between tonalite and 

granodiorite. All four samples are peraluminous. Sample 

SL-11 contains 54 percent modal microcline and shows 

the least oversaturation of any sample on the diagram. 

Sample SL-11 shows the highest IA120 3 + N~O + ~0 

in figure 3 and demonstrates a tendency toward 

saturation, which is indicative of a process similar to the 

formation of small pegmatites by replacement (notable 

enrichment in alkali feldspar). The other three samples 

are more equivocal. 

The granitic enclaves in the Boulder Creek Granodiorite appear to be completely surrounded by their host. 

Of course their contacts with their host at depth cannot 

be seen, but many enclaves are so small that pipelike 

extensions at depth can be discounted, which means that 

they are enclaves and not intrusions into the Boulder 

Creek Granodiorite. Some possible explanations for the 

presence of these enclaves are the following: 

1. The enclaves were carried along as immiscible 

magmas within the host magma of Boulder Creek 

Granodiorite as the pluton of Boulder Creek Granodiorite was being intruded into the country rock of 

biotite gneiss: 

(a) I have not seen any evidence of wholesale 

intrusion of Boulder Creek Granodiorite into 

the biotite gneiss. 

(b) Other data, which I discussed previously, 

do not support a full magmatic history for the 

Boulder Creek Granodiorite. 

(c) The physical process implied by this 

explanation should attenuate and distort the 

"carried along" immiscible magmas. 

(d) For the granitic enclaves to be magmatic 

differentiates requires a fully magmatic system, 

which necessitates high temperatures for which 

there is no evidence. 

2. The enclaves are xenoliths (country rock not 

completely assimilated by magma): 

(a) Objections (a) and (b) for explanation 1 

apply here also. 

(b) No evidence for any assimilation of these 

granitic enclaves has been found. 

Petrology 87 
Table 2. Classification of rock types of the granitic enclaves in the Boulder Creek Granodiorite, Strawberry Lake area 

( =, equals; >, greater than) 

Johannsen (1939) Streckeisen (1976) F-S method 1 

(From modal analysis) 

F-S method1 

(Chemically analysed samples) 

6 

2 

2 

1 

1 

method method 

granite 7 

adamellite 3 

syenogranite 

monzogranite 

5 adamellite 

3 granite 

3 granite 

tonalite 

leucoadamellite 1 alkali feldspar 3 granodiorite 

kaligranite granite. 1 ultrafelsic granite 

granodiorite granodiorite 

Average F > 14.7 

(adamellite or 

more felsic). 

Average F = 14.4, 

average S = 2.38 

(adamellite). 

1Rocks names are compositional-equivalent names. 

(c) No country rock matches the granitic 

enclaves. 

3. The host pluton represents largely granitized or 

ultrametamorphic rock (does not exclude local 

melting), and the enclaves are relicts of country rock 

that resisted granitization or ultrametamorphism. 

(a) Objections (b) and (c) of explanation 2 

apply here also. 

4. The process of enclave formation (metamorphic 

differentiation?) is an outgrowth of the 

metamorphic-metasomatic process~s during 

formation of the host. Local or partial melting is not 

excluded. 

(a) The enclaves are genetically related to 

their host by the following: 

{1) The enclaves have relatively high 

apatite and sphene contents as does their 

host (table 17 of Chapter A of this 

bulletin). 

(2) Rb:Sr ratios of the enclaves are 

more similar to Rb:Sr ratios of the host 

than to Rb:Sr ratios of other granitic rocks 

in the area (table 16 of Chapter A of this 

bulletin). 

(b) Other than by metamorphic differentiation, it is difficult to explain why the 

enclaves 

(1) show high K20:Na20 ratios and 

enrichment in Ba and 

(2) show high Th:U ratios and 

enrichment in LREE (light rare-earth 

elements). 

Explanation 4 fits the data the best. 

Silver Plume Granite 

The Silver Plume Granite in the Strawberry Lake 

area is an acid or felsic type granite as is made clear in 

table 3. To further illustrate how felsic the Silver Plume 

Granite in the Strawberry Lake area is, the seven 

chemically analyzed samples are plotted on figure 4. For 

comparison, data on Silver Plume Granite from other 

areas are also plotted on the same diagram: GSP-1, 

Flanagan (1976); average of five samples, Lovering and 

Tweto (1953); and average of ten samples, Anderson and 

Thomas (1985). The Silver Plume Granite in the 

Strawberry Lake area is the most felsic of all. 

Without exception all plots of the Silver Plume 

Granite fall on the low-S side of the oversaturation 

trendline, which may be an indication of metamorphicmetasomatic origin. Further support for that inference 

may be seen in figure 5, which shows that all the Silver 

Plume Granite data points have high or equivocal 

IA12 0 3 + Na20 + ~0 (an indication of metamorphicmetasomatic conditions or derivation from pelitic 

metasediments or both). The three data points from 

chemically analyzed biotite from Silver Plume Granite in 

Young (1989) all fall in the metamorphic-metasomatic 

field also. Thus, three different chemical methods are in 

agreement on the metamorphic-metasomatic origin of 

the Silver Plume Granite, small though the number of 

samples may be. 

Certain structural features in the Silver Plume 

Granite are more simply explained by a metamorphicmetasomatic history than by a magmatic history. For 

example, the foliation in the biotite-gneiss enclaves in the 

Silver Plume Granite generally conforms to the foliation 

in surrounding biotite gneiss (pl. 1 of Chapter A of this 

bulletin). These relicts of biotite gneiss would not show 

foliation conformity if they were xenoliths that had been 

carried along in an invading magma. Similarly, planar 

B8 Crystalline Rocks of the Strawberry Lake Area, Colorado 
Table 3. Classification of rock types of the Silver Plume Granite, Strawberry Lake area 

[ =, equals; >, greater than] 

F-S method1 F-S method1 Johannsen (1939) 

method 

Streckeisen (1976) 

method (From modal analysis) (Chemically analyzed samples) 

12 leucoadamellite 

11 adamellite 

22 monzogranite 

8 syenogranite 

12 adamellite 4 felsic granite 

10 ultrafelsic granite 3 granite 

6 granite 1 quartz monzonite 

1 granodiorite 

9 granite 

2 leucogranite 

1 granodiorite 

1 granodiorite 

Average F > 20.8 Average F = 23.8, 

average S = 2.71 

(felsic granite). 

(granite or more felsic). 

1Rocks names are compositional-equivalent names. 

orientation of tabular microcline laths in the Silver 

Plume Granite (pl. 1 of Chapter A of this bulletin) 

conforms reasonably well with foliation in nearby 

enclaves of biotite gneiss. I think that this orientation is 

probably due to feldspathization in conformity with 

regional foliation rather than to flow structure in a 

magma. 

Mineralogical features in the Silver Plume Granite 

favor a metamorphic-metasomatic origin rather than a 

magmatic origin. For example, biotite:muscovite ratios 

are highly variable from much greater than 1 to much less 

than 1. Such variability is more reasonably explained by 

heterogeneity of metasedimentary source material than 

by exotic magmatic (more or less homogeneous) 

intrusion. Zircons are of two types in the Silver Plume 

Granite; most are single, small euhedral crystals, but 

some show zoned or unzoned, recrystallized overgrowths 

on older zircon cores. The single, euhedral zircons may 

represent crystallization from an ultrametamorphic 

condition; the recrystallized zircons obviously inherited 

cores from older, precursor material. Zircon content of 

the Silver Plume Granite is about twice that of the 

surrounding biotite gneiss (table 17 of Chapter A of this 

bulletin; Schroeder, 1984). This fact is consistent with 

new crystallization and recrystallization of zircon in the 

Silver Plume Granite, which added zircon mass to the 

inherited portion. 

Koschmann (1960) was one of the first geologists 

to point out that the Silver Plume Granite in the Tenmile 

Range, Colo., and in the Montezuma area of the Front 

Range, gave evidence of having originated from local 

centers of fusion or recrystallization. This perception 

especially applied to small, ductless, isolated bodies that 

are surrounded by metamorphic rocks. 

Much later, Anderson and Thomas (1985) 

concluded that the Silver Plume and St. Vrain batholiths 

were derived from largely metasedimentary crust. Their 

scenario, however, involved generation of a magma from 

an oxidized, peraluminous quartzofeldspathic source at a 

depth of 36 km or more (pressure of at least 10 kbar) and 

emplacement at a depth as shallow as 8-9 km (pressure 

of2.3 kbar) at a P810 of 487-560 bar and a temperature 

of 740-760 °C. I do not accept Anderson and Thomas' 

(1985) hypothesis of emplacement or intrusion of a body 

of magma of batholithic size with all its attendant space 

problems, especially in an area of compressed m.etamorphic rocks. Moreover, such a hypothesis does nothing 

to explain the small isolated bodies of Silver Plume 

Granite mentioned by Koschmann (1960). 

DePaolo (1981) concluded from a study of Nd-Sm 

characteristics that the Silver Plume-type magmas of the 

Colorado Front Range were crustally derived. Fountain 

and others (1983) also suggested a crustal source for the 

Silver Plume and Sherman Granites and the granite of 

the Log Cabin batholith. 

Thus there seems to be general agreement on the 

source rocks of the Silver Plume Granite. The point of 

contention, however, is whether the Silver Plume Granite 

represents emplacement (intrusion) of a magma or 

whether most, if not all, of the granite is actually 

transformed pelitic metasediments with a small metavolcanic component as a result of metamorphicmetasomatic processes. Partial or minor melting is not 

excluded from these metamorphic-metasomatic processes. 

Granitic Enclaves in Silver Plume Granite 

The five granitic enclaves in the Silver Plume 

Granite are small (largest dimension is about 75 m) and 

generally elongate (pl. 1 of Chapter A of this bulletin). 

Compared to their host they are twice as thoriferous, 

more melanocratic, and slightly finer grained. The 

enclaves show a subtle crystalloblastic texture. According 

to the Johannsen (1939) classification they are adamellites, and according to the Streckeisen (1976) 

Petrology 89 
50.-------------------------~----~----~----~----~-~ EXPlANATION 

40 

30 

20 

0 

Ill 

ou 10 

N+ 

~ 0 + Ol 9 0~ N + Ill c 8 ~ N C'.l(l) 

ou.. ii)b 7 

tl 6 II 

L.L. 

5 

4 

3 

2 

Ultrafelsic 

33.7------------------+---+----I-----S+-L---64--+-1-----I 

~L-40~ 

15.2 

6.80 

Felsic granite SM-3 

SL~65~ ---------------+--~--~M-43A~~+---~ 

, .... SM-4'e 

Granite 

Adamellite 

Granodiorite 

Tonalite 

Monzonite 

Diorite 

Gabbro 

1.51--------------~--~--+-----~--4---4 

Ultramafic 

1.0 2.0 3.0 

e Nockolds' (1954) average rocks 

• Silver Plume Granite-Strawberry Lake 

area 

+ Silver Plume Granite-GSP-1 from 

Flanagan (1976) 

A. Silver Plume Granite-Average of 5 

samples from Lovering and Tweto (1953) 

C Silver Plume Granite-Average of 10 

samples from Anderson and Thomas 

(1985) 

.A Granitic enclaves in Silver Plume GraniteStrawberry Lake area 

Figure 4. F-8 diagram of Silver Plume Granite and its granitic enclaves. F, felsic-mafic ratio; S, silica-saturation ratio. 

classification they are monzogranites. According to the 

F-S method one i1i a granite and one is an adamellite 

(close to granite). On figure 4 they appear somewhat 

more mafic than their host. 

Sample SM-43B, the adamellite, plots on the high-S 

side of the oversaturated trendline in figure 4; sample 

SM-48, the granite, plots slightly on the low-S side of the 

oversaturated trendline. From this tendency toward 

810 Crystalline Rocks of the Strawberry Lake Area, Colorado 

higher S, more so than any of the samples of Silver Plume 

Granite, one might infer that these granitic enclaves 

represent sites of partial melting. Indeed, in figure 5, 

which shows IA12 0 3 + N~O + ~0, the position of SM43B appears possibly more magmatic than that of SM-48. 

Of course, both positions are in the equivocal zone of 

22-23 percent, but it is significant that their plots are to 

the left of the Silver Plume Granite plots. 
80~----------~-----------.~----------,.-----------,------------, 

,._Equivocal zone_, 

SL-64 SM-3 

SL~40 SM-4 e • SL-34 

S 8 '- I • ...--5M-43A C'l MAGMATIC M!r A-SM- 48 •sL-SS _c ~ 70~----------~----------~~~-=~~~~~~~~----~------------~ 

+ 

METAMORPHICMETASOMATIC 

60L------------L----------~L-----------~----------~L-----------~ 

20 21 22 23 24 25 

EXPlANATION 

• Silver Plume Granite-Strawberry Lake area 

+ Silver Plume Granite-GSP-1 from Flanagan (1976) 

A Silver Plume Granite-Average of 5 samples from Lovering 

and Tweto (1953) 

C Silver Plume Granite-Average of 10 samples from Anderson 

and Thomas (1985) 

.6. Granitic enclaves in Silver Plume-Strawberry Lake area 

Figure 5. IA120 3 + Na20 + K.zO versus Si02 diagram of Silver Plume Granite and its granitic 

enclaves. 

There are more similarities than differences 

between the granitic enclaves in the Silver Plume Granite 

and the granitic enclaves in the Boulder Creek Granodiorite. The similarities common to both enclaves, 

compared to their hosts, are higher Th:U and K20:Na20 

ratios and enrichment in Th and LREE. The main 

difference is that the granitic enclaves in the Silver Plume 

Granite are more mafic than their host and the granitic 

enclaves in the Boulder Creek Granodiorite are more 

felsic than their host. 

Biotite Gneiss 

It is not surprising that a highly variable rock unit 

such as biotite gneiss should show such diversity on the 

F-S diagram (fig. 6). The three chemically analyzed 

samples range from the compositional equivalent of 

monzonite (between saturated and undersaturated) to 

very oversaturated tonalite (close to granodiorite). This 

range in composition includes all the compositional 

varieties of Boulder Creek Granodiorite. Similar 

compositions were reported for an average of ten 

samples of sillimanitic biotite gneiss and schist associated 

with the Boulder Creek Granodiorite batholith (Gable, 

1980, p. 61 ). This average is also plotted on figure 6 along 

with the average of six samples of Boulder Creek Granodiorite from the Strawberry Lake area to show the 

prevalence of tonalitic composition. Therefore, the 

derivation of the Boulder Creek Granodiorite from the 

biotite gneiss is not only plausible, it is almost mandatory. 

It is highly unlikely that the Boulder Creek Granodiorite 

in the Strawberry Lake area intruded its way, as a single 

pluton, into its present position-the space problem is a 

formidable barrier. Transformation by metamorphicmetasomatic processes of biotite gneiss into Boulder 

Creek Granodiorite with local, partial melting solves the 

space problem, and the compositional similarity of the 

two rock types obviates the need to add other material. 

Plotting the points of the three analyzed samples of 

biotite gneiss on the IA120 3 + Na20 + K20 versus Si02 

diagram (fig. 7) also shows the heterogeneous nature of 

the raw materials from which the Boulder Creek Granodiorite may have been produced. In fact, one sample 

(SL-71) of the biotite gneiss plots on the magmatic side 

of figure 7, whereas averages of this rock type, both from 

the Strawberry Lake area and the Boulder Creek batholith, plot on the metamorphic-metasomatic side. Figure 7 

may indicate that the source of the biotite gneiss varied 

from metamorphosed, oversaturated igneous materials 

(volcanic ash, tuft) to metamorphosed, undersaturated 

pelitic materials. 

Petrology 811 
50 EXPlANATION 

40 Ultrafelsic • Noc:kolds' (1954) average rocks 

• Biotite gneiss-strawberry Lake area 

0 Sillimanitic: biotite gneiss and schist30 Average of 10 samples from Gable (1980, 

p. 61) 

Felsic granite 

+ Boulder Creek Granodiorite-Strawberry 

Lake area; average of 6 samples 

A Granitic: enclave in biotite gneiss20 Strawberry Lake area 

Granite 

Adamellite 

Granodiorite 

Tonalite 

5 

4 

Monzonite 

3 

Diorite 

2 

Gabbro 

1.51-----------+----11----+---t---+------t 

Ultramafic 

1.0 2.0 3.0 

Figure 6. F-8 diagram of biotite gneiss and a granitic enclave. F, felsic-mafic ratio; S, silica-saturation ratio. 

Granitic Enclaves In Biotite Gneiss 

Granitic enclaves in the biotite gneiss vary in size 

from small (about 1m) to large (several kilometers). The 

larger granitic enclaves themselves contain enclaves or 

bodies of biotite gneiss, hence they are not homogeneous 

812 Crystalline Rocks of the Strawberry Lake Area, Colorado 

bodies of granite. The range in rock type of the granitic 

enclaves is shown in table 4. The plot of the one 

chemically analyzed sample of a granitic enclave in 

biotite gneiss on the F-S diagram (fig. 6) and on the 

IA120 3 + Na20 + ~0 versus Si02 diagram (fig. 7) 

suggests that it is metamorphic-metasomatic in origin. 

What is more revealing is that its position on both 
Table 4. Classification of rock types of the granitic enclaves in the biotite gneiss, Strawberry Lake area 

[=,equals;>, greater than] 

F-S method1 F-S method1 Johannsen (1939) 

method 

Streckeisen (1976) 

method (From modal analysis) (Chemically analyzed sample) 

3 

3 

1 

1 

leucoadamellite 6 

adamellite 1 

monzogranite 

syenogranite 

3 ultrafelsic 

2 adamellite 

F = 26.5, S = 2.72 

(felsic granite). 

granite 1 granodiorite 

granodiorite 

2 granodiorite 

1 tonalite 

Average F ) 18.6 

(granite or more felsic). 

1Rocks names are compositional-equivalent names. 

80 

70 

60 

50 

20 

SL-71 

• 

MAGMATIC 

-Equivocal 1--

22 

zone 

+ 

A 

SL-3 

0 

24 

METAMORPHICMETASOMATIC 

• 

SM-152 

26 

I,A1203+Na20+K20 

EXPLANATION 

• Biotite gneiss-Strawberry Lake area 

28 

0 SUiirnanitic biotite gneiss and schist-Average of 10 samples from Gable 

(1980, p. 61) 

+ Boulder Creek Granodiorite-Strawberry Lake area; average of 6 samples 

A Granitic enclave in biotite gneiss-Strawberry Lake area 

Figure 7. IAI20 3 + Na20 + ~0 versus Si02 diagram of biotite gneiss and a granitic enclave. 

.sL-69 

30 32 

diagrams is very close to that of Silver Plume Granite. In 

addition, both composition and mineralogy of the granitic 

enclaves are similar to those of the Silver Plume Granite. 

A notable difference, however, is that the granitic 

enclaves are finer grained. 

The granitic enclaves may be precursor Silver 

Plume Granite that has been subjected to the same 

metamorphic-metasomatic conditions that produced the 

pluton of Silver Plume Granite, but the smaller size of 

the enclaves limited grain-size development. 

Petrology 813 
EXPlANATION 

e Nockolds' (1954) average rocks 

33.7--------+-----t---+---+------1--J. Lamprophyres 

30 

Felsic granite 

20 

Granite 

+ Strawbeny Lake area 

• Other areas (see Table 6) 

• Small mafic enclave in Boulder Creek 

Granodiorite 

A Andesite porphyry dike-Strawberry 

Lake area 

15.2-------~~-~--+--~--~~--~ 

II 

u. 

5 

4 

2 

Adamellite 

Granodiorite 

Tonalite 

Monzonite 

Diorite 

Gabbro 

1.51---------'---------'---_.J._-_J.__ _____ ___.J 

1.0 2.0 

S = Si02 

AI203+Na20+K20+MgO+CaO+FeO+Fe203 

Figure 8. F-8 diagram of biotite lamprophyres and other mafic rocks. F, felsic-mafic ratio; S, silica-saturation ratio. 

Biotite Lamprophyre Enclaves in 

Boulder Creek Granodiorite 

Enclaves of biotite lamprophyre, which are tens of 

meters in length, occur only in the Boulder Creek Granodiorite, hence it is reasonable to suspect a genetic 

connection between the two rock types. The biotite 

lamprophyre is compared by using three classifications in 

table 5. The plot of the two chemically analyzed samples 

of biotite lamprophyre on an F-S diagram (fig. 8) shows 

that both are saturated and compositionally equivalent to 

diorite; sample SL-142 has a tendency toward undersat814 Crystalline Rocks of the Strawberry Lake Area, Colorado 

uration. Several other lamprophyres from Colorado and 

elsewhere are plotted on figure 8; the ones from 

Colorado cluster fairly closely. Chemical analyses are 

given in table 6. The samples (SL-88 and SL-142) from 

the Strawberry Lake area are the most enriched in ~0, 

P20 5 , and BaO. 

Figure 9 shows that all the lamprophyres plot on 

the magmatic side of the IA120 3 + N~O + ~0 versus 

Si02 diagram except the vaugnerite from France. The 

position of the vaugnerite, however, does not mean that it 

is not magmatic, but rather that it has a strong tendency 

toward undersaturation (evidenced by the lowest 
Table 5. Classification of rock types of the biotite lamprophyre, Strawberry Lake area 

[=,equals] 

Johannsen (1939) 

method 

Streckeisen (1979) 

method 

F-S method1 

(From modal analysis) 

F-S method1 

(Chemically analyzed samples) 

3 mela-kalisyenite 

2 mela-sodaclase 

syenite. 

5 minette-vogesite 4 diorite 

1 gabbro 

2 diorite 

Average F = 2.14 

(between diorite 

and gabbro). 

Average F = 2.38, 

average S = 1.11 

(diorite). 

1Rocks names are compositional-equivalent names. 

Si02and highest Al20 3 values of these lamprophyres). 

Figure 8 shows that the vaugnerite has the strongest 

degree of undersaturation of the compared lamprophyres. 

The interstice-filling, allotriomorphic microcline 

and microcline microperthite indicates that the lamprophyres of the Strawberry Lake area attained a magmatic 

state. Grain size remained small (generally less than 1 

mm), perhaps due to relatively rapid crystallization. 

It is not necessary to call upon an intrusion of 

foreign magma to form the dikelike bodies of biotite 

lamprophyre in the Boulder Creek Granodiorite. All 

components of the biotite lamprophyres could have been 

obtained from its host. This is convincingly shown by the 

similarity in accessory minerals between the biotite 

lamprophyre and its host (table 17 of Chapter A of this 

bulletin), except for the greater enrichment of sphene, 

apatite, and epidote in the lamprophyre. The most 

outstanding characteristic of the biotite lamprophyre, at 

'least in the Strawberry Lake area, is its propensity for 

unusual enrichment of many elements. A quantitative 

measure of this propensity, called coefficient of trace 

element accumulation (Shaw, 1961), indicates that the 

biotite lamprophyre is enriched in 21 elements four times 

more than its host, Boulder Creek Granodiorite (see 

Geochemistry section in Chapter A of this bulletin). 

Potassium enrichment in the biotite lamprophyre may 

have been in two stages: most of the enrichment occurred 

as anhedral microcline and microcline microperthite, and 

the remainder of the enrichment occurred as delicate 

veins of the same minerals throughout some parts of the 

lamprophyre. 

Potassium enrichment mayrepresent end stage ultrametamorphism in the Boulder Creek Granodiorite. The 

granitic enclaves in the Boulder Creek Granodiorite also 

show pronounced potassium enrichment. 

Small Mafic Enclaves in 

Boulder Creek Granodiorite 

The small (generally 1-20 em) mafic enclaves in 

the Boulder Creek Granodiorite are roundish to discoid, 

fine grained, and notably similar in appearance. They are 

abundant locally but are not omnipresent. Sample 

SL-101B, which is a chemically analyzed representative 

of the small mafic enclaves, is classified as a melatonolite 

or vaugnerite in the Johannsen (1939) classification and 

as a quartz diorite in the Streckeisen (1976) 

classification. The thin section of vaugnerite that 

Johannsen (1939) had at his disposal contained only 1-2 

percent of orthoclase, whereas the vaugnerite described 

by Sabatier (1980) contained 5-10 percent perthitic 

orthoclase (table 6). The biotite lamprophyres (minettevogesite) of this report contain 25-46 percent potash 

feldspar. In the F-S classification sample SL-101B 

appears as a compositional equivalent of gabbro, which is 

somewhat undersaturated (F = 1.85, S = 0.94; fig. 8). 

Sample SL-101B plots on the magmatic side of the 

IA120 3 + Na20 + K2 0 versus Si02 diagram (fig. 9). 

The small mafic enclaves are too homogenous to 

be xenoliths in the Boulder Creek Granodiorite, and 

moreover, no rocks that fit their description occur in the 

country rock (biotite gneiss). This last point was also 

made by Vernon (1984), who, in discussing the origin of 

these small mafic enclaves in intermediate plutonic rocks, 

believed them to be mainly trapped globules of hybrid 

magma. 

The following facts and interpretations (in 

parentheses) indicate to me that the small mafic enclaves 

in the Boulder Creek Granodiorite are restites, that is, 

they were geochemically immobile parts of a rock during 

partial mobilization (anatexis) of rock components: 

Petrology 815 
Table 6. Comparative chemical analyses of lamprophyres 

Sample 

Component A B c D E F G 

Si02 50.0 47.7 50.41 48.8 50.7 52.26 46.73 

Al2o3 11.2 11.7 12.27 12.5 13.3 10.63 17.38 

Fe2o3 17.77 17.84 5.71 2.7 3.0 2.47 0.90 

FeO n.d. n.d. 3.06 5.2 5.7 5.45 6.36 

MgO 9.20 9.04 8.69 8.4 7.9 9.32 9.25 

CaO 7 .o 1 7.43 7.08 9.6 7.8 5.62 8.42 

Na2o 0.71 0.78 0.97 1.5 1.7 1.60 1.28 

K2o 7.67 7.84 7.53 5.9 6.6 5.99 4.51 

H2o+ 2.57 2.75 1.80 1.3 0.40 1.97 n.d. 

H2o - n.d. n.d. .46 .14 .15 0.98 n.d. 

Ti02 1.39 1.12 1.47 1.7 1. 3 1.92 1.74 

P2os 1.53 1.79 .46 1.2 1.2 .98 .94 

MnO .13 .11 .15 .04 .16 .12 .10 

co2 n.d. n.d. n.d. <.OS <.OS .75 n.d. 

F n.d. n. d. n.d. • 62 • 59 n.d • n.d • 

Cl n.d. n.d. n.d. .04 • 04 n.d • n.d. 

v2o3 3.03 3.02 • 03 3.02 3.02 n.d • n.d. 

NiO 3.02 3.o2 • 04 3.04 3.03 n.d. n.d • 

BaO 3.56 31.67 • 23 3.11 3.11 n.d. n.d • 

SrO 3.18 3.35 • 06 3.24 3.24 n.d. n.d • 

Total 98 98 100.42 100 101 100.06 98 

K2o/Na2o 10.8 10.1 7.76 3.93 3.88 3.74 3.52 

F4 2.44 2.32 2.40 2.17 2.42 2.62 2.11 

sS 1.15 1.07 1.11 1.07 1.10 1.27 0.97 

1rotal iron reported as Fe2o3• 

2Loss on ignition at 900 °C. 

3oxide value calculated from semiquantitative spectrographic analysis. 

4F = felsic-mafic ratio Si02 + Na2o + K20 

(in weight percent) FeO + Fe2o3 + MgO + CaO 

Ss = silica saturation ratio = Si02 

(in weight percent) Al2o3 + Na2o + K2o + MgO + CaO + FeO + Fe2o3 

816 Crystalline Rocks of the Strawberry Lake Area, Colorado 
60 

40 

18 

MAINE 

• 

19 

MAGMATIC 

Two Buttes, 

Colo. 

SL-88 '-

387, Colo~ +SL-142 

20 

!"-Equivocal zone- METAMORPHICMETASOMATIC 

SL-139 • 

388, Colo.e 

• • SL-1018 Massif Central, 

FRANCE 

22 23 24 

EXPLANATION 

Lamprophyres 

+ Strawberry Lake area 

• Other areas (see Table 6) 

• Small mafic enclave in Boulder Creek Granodiorite 

+ Andesite porphyty dike-Strawberry Lake area 

Figure 9. IA120 3 + Na20 + ~0 versus Si02 diagram of biotite lamprophyres and other mafic rocks. 

(1) Small mafic enclaves are abundant locally but 

are not omnipresent. (Some portions of the biotitegneiss precursor contain more mafic components 

than others-such local volumes produce restites as 

more felsic parts of the biotite gneiss are mobilized 

to form Boulder Creek Granodiorite.) 

(2) About nine-tenths of the small mafic enclaves 

consist of andesine, biotite, and hornblende. (This is 

the mafic residue of excess immobile components.) 

(3) Thorium is severely depleted in the small mafic 

enclaves with respect to their host. (Thorium was 

mobilized away from restite to its more felsic host 

under ultrametamorphic conditions.) 

Speculation on the possible genetic relationship 

between the small mafic enclaves and the biotite lamprophyres is supported by the dual classification of the small 

mafic enclaves as melatonalites or vaugnerites in the 

Johannsen (1939) classification. Figure 8 shows that the 

small mafic enclaves and the biotite lamprophyres are 

not very far apart in composition, which would be 

brought even closer with an infusion of potassium. Table 

17 of Chapter A of this bulletin shows that the accessory 

minerals of the two rock types are virtually identical. 

Locally, swarms of the small mafic enclaves are abundant 

enough to provide the mafic component of the biotite 

lamprophyres. Thus, local or partial melting and (or) 

metamorphic differentiation processes could produce the 

biotite lamprophyres from in-situ sources. 

Andesite Porphyry Dike 

The Tertiary(?) andesite porphyry dike is about 9 

m wide and at least 150 m long; it intrudes Boulder 

Creek Granodiorite. Sample SL-139, which is 

representative of the andesite porphyry dike, is classified 

as a syenodiorite porphyry in the Johannsen (1939) 

classification and as a monzodiorite in the Streckeisen 

(1976) classification. In the F-S classification it is the 

compositional equivalent of a felsic andesite (F = 2.80, 

S = 1.24; fig. 8), which is saturated in silica. Sample 

SL-139 plots on the magmatic side of 

IA120 3 + N~O + ~0 versus Si02 diagram (fig. 9). 

Of all the rock units in the Strawberry Lake area, 

this one alone can unequivocally be called igneous, 

because of its dikelike structure and two types of plagioclase phenocrysts: large zoned crystals and small microlites. The source of the magma that formed the andesite 

porphyry dike probably was the same that produced the 

biotite lamprophyres-namely, crustal material-but the 

degree and length of time of heating may have been 

different. A general similarity in composition and 

accessory minerals supports this supposition. 

Petrology 817 
CONCLUSIONS 

Chemical, structural, and mineralogic evidence 

favors a metamorphic-metasomatic origin for the 

Boulder Creek Granodiorite and the Silver Plume 

Granite in the Strawberry Lake area. Partial melting 

occurred, but no evidence has been found for gross 

intrusion. Granitic enclaves in the Boulder Creek Granodiorite and in the Silver Plume Granite probably 

represent end phases of metamorphic-metasomatic processes in these two plutons, but the granitic enclaves in the 

Silver Plume Granite may have undergone partial 

melting. Granitic enclaves in the biotite-gneiss country 

rock may be synchronous with the Silver Plume Granite. 

The small mafic enclaves in the Boulder Creek 

Granodiorite are restites, and the biotite lamprophyres in 

the same host probably represent end-stage ultrametamorphism. 

The Tertiary(?) andesite porphyry dike is 

unequivocally igneous. 

REFERENCES CITED 

Anderson, J.L., and Thomas, W.M., 1985, Proterozoic 

anorogenic two-mica granites-Silver Plume and St. 

Vrain batholiths of Colorado: Geology, v.13,p. 177-180. 

Bastin, B.S., 1906, Some unusual rocks from Maine: Journal of 

Geology, v. 14, no. 3, p. 173-187. 

Cross, Whitman, 1906, Prowersose (syenitic lamprophyre) from 

Two Buttes, Colorado: Journal of Geology, v. 14, no. 3, 

p. 165-172. 

DePaolo, D.J., 1981, Neodymium isotopes in the Colorado 

Front Range and crust-mantle evolution in the Proterozoic: Nature, v. 21, p. 193-196. 

Flanagan, F.J., compiler and ed., 1976, Descriptions and 

analyses of eight new USGS rock standards: U.S. 

Geological SuiVey Professional Paper 840, 192 p. 

Fountain, J.C., Hodge, D., and Waddell, C., 1983, Petrogenesis 

of the Silver Plume, Log Cabin, and Sherman granites, 

Colorado and Wyoming: Geological Society of America 

Abstracts with Programs, v. 15, p. 575. 

Gable, D.J., 1980, The Boulder Creek Batholith, Front Range, 

Colorado: U.S. Geological SuiVey Professional Paper 

1101, 88 p. 

818 Crystalline Rocks of the Strawberry Lake Area, Colorado 

Gokhale, N.W., 1968, Chemical composition of biotites as a 

guide to ascertain the origin of granites: Geological 

Society of Finland Bulletin, v. 40, p. 107-111. 

Johannsen, Albert, 1939, A descriptive petrography of the 

igneous rocks: University of Chicago Press, 4 v., 1,629 p. 

Koschmann, A.H., 1960, Mineral paragenesis of Precambrian 

rocks in the Tenmile Range, Colorado: Geological 

Society of America Bulletin, v. 71, p. 1357-1570. 

Krauskopf, Konrad, 1967, Introduction to geochemistry: 

McGraw-Hill Co., p. 566-567. 

Lovering, T.S., and Tweto, Ogden, 1953, Geology and ore 

deposits of the Boulder County Tungsten District, 

Colorado: U.S. Geological SuiVey Professional Paper 

245, 199 p. 

Nockolds, S.R., 1954, Average chemical compositions of some 

igneous rocks: Geological Society of America Bulletin, v. 

65, p. 1007-1032. 

Roddick, J.A., 1982, On granite logic, in Drescher-Kaden, F.K., 

and Augustithis, S.S., eds., Transformists petrology: 

Athens, Theophrastus Publications, S.A., p. 87-104. 

Sabatier, Henri, 1980, Vaugne'rites et granites: une association 

de roche grenues acides et basique: Bulletin, v. 103, p. 

507-522 [translated by R.E. Wilcox]. 

Schroeder, D.A., 1984, Geology of the Granby and Strawberry 

Lake 7 lh' quadrangles, Grand County, Colorado: 

Boulder, University of Colorado, Ph. D. thesis, 340 p. 

Shaw, D.M., 1961, Coefficient of accumulation-A measure of 

the total trace element content of a mineral or a rock: 

Canadian Mineralogist, v. 6, pt. 5, p. 694-697. 

Stern, T.W., Phair, George, and Newell, M.F., 1971, Boulder 

Creek batholith, Colorado, Part 11-Isotopic age of 

emplacement and morphology of zircon: Geological 

Society of America Bulletin, v. 82, p. 1615-1634. 

Streckeisen, A.L., 1976, To each plutonic rock its proper name: 

Earth Science Reviews, v. 12, p. 1-33. 

___ 1979, Classification and nomenclature of volcanic 

rocks, lamprophyres, carbonatites, and melilitic rocksRecommendations and suggestions of the lUGS 

subcommission on the systematics of igneous rocks: 

Geology, v. 7, p. 331-335. 

Vernon, R.H., 1984, Microgranitoid enclaves in granitesGlobules of hybrid magma quenched in a plutonic 

environment: Nature, v. 309, p. 438-439. 

Young, E.J., 1989, Felsic-mafic ratios and silica saturation 

ratios-Their rationale and use as petrographic and 

petrologic indicators: U.S. Geological SuiVey Open-File 

Report 89-651,32 p. 

*U.S. GOVERNMENT PRINTING OFFICE: 1991-573-049/26019