Regional Metamorphism in the Condrey 

Mountain Quadrangle^ North-Central 

Klamath Mountains, California
Regional Metamorphism in the Condrey 

Mountain Quadrangle, North-Central 

Klamath Mountains, California

Bv PRESTON E. HOTZ

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1086

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress catalog-card No. 79-600048

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

Washington, D.C. 20402

Stock Number 024-001-03160-4
CONTENTS

Page

Abstract _____________________________ 1

Introduction__________________________ 2

Greenschist facies ______________________ 2

Condrey Mountain Schist ____________________ 2

Quartz-muscovite schisL_________________ 6

Chemical composition _______________ 6

Actinolite-chlorite schist ______________ 6

Chemical composition ______________ 7

Metamorphosed granitic (?) rock___________ 7

Age and correlation ___________________ 8

Metavolcanic and metasedimentary rocks of the 

western Paleozoic and Triassic belt ____________ 9

Metavolcanic rocka__________________ 9

Chemical composition _____________10

Metasedimentary rocks.________________11

Age and correlation _________________ 11

Stuart Fork Formation___________________ 11

Page

Amphibolite facies__ — __ — __ —— _ ——————— — —— 12 

Foliated amphibolite ___ — _ — — _ ———— — — ——12

Nonfoliated amphibolite ___________ — — _—— 14

Chemical composition ___________ —— _——15

Metasedimentary rocka__ —— — _ —— __ — —— ——— 15

Age _________________________________ _ 17

Mafic and ultramafic rocks______—_ — ——— — —— -17

Serpentinite __________;____———— — _—17

Gabbro and pyroxenite_____—___ ———— _————19

Age __________________________________ 20 

Plutonic rocks______-l_______-__ —— —————— 20

Pyroxene quartz diorite and granodiorite — — __ — —— -20 

Granitic rocks _______ — _ — —— — _ —— _ —— -21

Age__________________________________-23 

Structural and metamorphic relations —_ ——————— — —— 23 

References cited _—— — — __ — — ___ — — __ ————— 24

ILLUSTRATIONS

Page

FIGURE 1. Generalized geologic map of Condrey Mountain quadrangle and vicinity_____________ — — ——— __ _______ 3

2. Geologic map of Condrey Mountain quadrangle and parts of Seiad Valley and Hornbrook quadrangles ————— — — ——— 4

3. Graphs showing Na2O and CaO relative to SiO2 for greenschist from Condrey Mountain Schist__ ——— — _-— ————— — 8

4. AFM diagram of metavolcanic rocks and amphibolite from Condrey Mountain and Hornbrook quadrangles

and greenschist from Condrey Mountain Schist ______________________—___ —— __ ——— _— 8

5. Graphs showing Na 2O and CaO relative to SiO 2 for metavolcanic rocks from Condrey Mountain and

Hornbrook quadrangles ______________________________— — ___ —— —— — —— — 11

6. Photomicrographs of amphibolite, foliated and nonfoliated ___________________ — __ ———— —— —— - 13

Diagram of amphiboles in metavolcanic rock and amphibolite from the Condrey Mountain 

and Hornbrook quadrangles.________________________________—— ——— — — —— 14

Graph showing change in cation content of amphibolite at contact with serpentinite__________ —— ___ —— — __ 19

TABLES

TABLE 1. Chemical

2. Chemical

3. Chemical

4. Chemical

5. Chemical

6. Chemical

7. Chemical

8. Chemical

9. Chemical 

10. Chemical

Page

analyses and CIPW norms of quartz-muscovite schist from the Condrey Mountain Schist.__ —— _____ — —— 6

analyses, CIPW norms, and modes of greenschist from the Condrey Mountain Schist——— —— _ —— __ — _— 7 

analysis, CIFW norm, and mode of albite-quartz-muscovite-epidote schist———__ — —————— ———— ____— 9

and spectrographic analyses and CIPW norms of metavolcanic rock __ —— ___ —— ———— __ ——— — — __10 

and spectrographic analyses, CIPW norms, and modes of amphibolite ________ — — —— _ ——— — — — — __16

and spectrographic analysis and CIPW norm of quartz-biotite phyllite—— — —— _ —— _ — _____ —— — —17

analyses of serpentinite and magnesian schist _________ — _____ _____________ — __ ——— — —18

analyses illustrating metasomatic reaction at serpentinite-amphibolite contact.——— ___ — __ — — _—-_ —18 

analyses, CIPW norms, and modes of gabbro, pyroxenite, and pyroxene quartz diorite__ —— — — ____ ———20

and spectrographic analyses, CIPW norms, and modes of rocks from the Vesa Bluffs and Ashland plutons^__ —— 22

III

REGIONAL METAMORPHISM IN THE

CONDREY MOUNTAIN QUADRANGLE,

NORTH-CENTRAL KLAMATH MOUNTAINS, CALIFORNIA

By PRESTON E. HOTZ

ABSTRACT

A subcircular area of about 650 km2 in northern California and 

southwestern Oregon is occupied by rocks of the greenschist 

metamorphic facies called the Condrey Mountain Schist. This 

greenschist terrane is bordered on the east and west by rocks belonging to the amphibolite metamorphic facies that structurally overlie 

and are thrust over the Condrey Mountain Schist. The amphibolite 

facies is succeeded upward by metavolcanic and metasedimentary 

rocks belonging to the greenschist metamorphic facies.

The Condrey Mountain Schist is composed predominantly of 

quartz-muscovite schist and lesser amounts of actinolite-chlorite 

schist formed by the metamorphism of graywacke and spilitic volcanic rocks that may have belonged to the Galice Formation of Late 

Jurassic age. Potassium-argon age determinations of 141 ±4 m.y. and 

155±5 m.y. obtained on these metamorphic rocks seem to be incompatible with the Late Jurassic age usually assigned the Galice.

The rocks that border the amphibolite facies are part of an extensive terrane of metavolcanic and metasedimentary rocks belonging 

to the western Paleozoic and Triassic belt. The metavolcanic rocks 

include some unmetamorphosed spilite but are mostly of the 

greenschist metamorphic facies composed of oligoclase (An15_20) and 

actinolite with subordinate amounts of chlorite and clinozoisiteepidote. The interbedded sedimentary rocks are predominantly argillite and slaty argillite, less commonly siliceous argillite and chert, 

and a few lenticular beds of marble. On the south, high-angle faults 

and a tabular granitic pluton separate the greenschist metavolcanic 

terrane from the amphibolite facies rocks; on the east, nonfoliated 

amphibolite is succeeded upward, apparently conformably, by 

metasedimentary rocks belonging to the greenschist metavolcanic 

terrane.

In the southern part of Condrey Mountain quadrangle, an outlier 

of a thrust plate composed of the Stuart Fork Formation overlies the 

metavolcanic and metasedimentary rocks. The Stuart Fork in this 

region is composed of siliceous phyllite and phyllitic quartzite and is 

believed to be the metamorphosed equivalent of rocks over which it is 

thrust. In the Yreka-Fort Jones area, potassium-argon determinations on mica from the blueschist facies in the Stuart Fork gave ages 

of approximately 220 m.y. (Late Triassic) for the age of metamorphism.

Rocks of the amphibolite facies structurally overlie the Condrey 

Mountain Schist along a moderate to steeply dipping thrust fault. 

The amphibolite terrane is composed of amphibolite and 

metasedimentary rocks in approximately equal amounts accompanied by many bodies of serpentinite and a number of gabbro and 

dioritic plutons. Most of the amphibolite is foliated, but some is nonfoliated; the nonfoliated amphibolite has an amphibolite mineralogy 

and commonly a relict volcanic rock texture. The nonfoliated amphibolite occurs on the southern and eastern borders of the amphibolite terrane between the areas of foliated amphibolite and the overlying metavolcanic and metasedimentary rocks. Hornblende and 

plagioclase (An30_35) are the characteristic minerals, indicating that 

the rocks are of the almandine-amphibolite metamorphic facies. The 

metasedimentary rocks interbedded with the amphibolites include 

siliceous schist and phyllite, minor quartzite, and subordinate 

amounts of marble. Potassium-argon age dates obtained on 

hornblende from foliated amphibolite yield ages of 146±4 and 148± 4 

m.y, suggesting a Late Jurassic metamorphic episode.

Mafic and ultramafic rocks are widespread in the amphibolite terrane but are almost entirely absent from the area of greenschist 

facies metavolcanic and metasedimentary rocks. The ultramafic 

rocks, predominantly serpentinite, occur as a few large bodies and 

many small tabular concordant bodies interleaved with the foliated 

rocks. The ultramafic rocks include harzburgite and dunite and their 

serpentinized equivalents. In the Condrey Mountain quadrangle, 

probably more than 90 percent of the ultramafic rocks is serpentinized. Most of the serpentinite is composed of antigorite formed by 

metamorphism of serpentinized ultramafic rocks; it is not known 

whether the metamorphism was of the same episode as that which 

produced the enclosing amphibolite facies. Some of the serpentinite 

has been converted to tremolite-, talc-tremolite-, and talc-carbonate 

rocks. Numerous small bodies of quartz-bearing hornblende gabbro 

and hornblende pyroxenite are closely associated with some of the 

serpentinite.

The age of the mafic and ultramafic rocks is uncertain, but they 

occur as pendants in granitic plutons of Late Jurassic age. The 

numerous discontinuous bodies of mafic and ultramafic rock scattered throughout the terrane of amphibolite facies rocks suggests a 

melange, and the association of serpentinite, gabbro, and pyroxenite 

with amphibolite and siliceous metasedimentary rocks suggests that 

this assemblage may be part of a dismembered ophiolite suite.

Two large granite plutons, the Ashland and the Vesa Bluffs, range 

in composition from hornblende gabbro and diorite to quartz diorite; 

they contain some granodiorite, and alaskitic rock in minor amounts. 

The Ashland pluton is mainly in Oregon; its southern part is in the 

northeastern Condrey Mountain and northwestern Hornbrook quadrangles of California. The Vesa Bluffs pluton, a south-dipping tabular body in its western part, occurs between the amphibolite facies on 

the footwall and a hanging wall of greenschist facies metavolcanic 

and metasedimentary rocks in the southern Condrey Mountain and 

southwestern Hornbrook quadrangles. Both plutons enclose roof 

pendants of serpentinite and pyroxenite or gabbro. Potassium-argon 

age determinations on hornblende and biotite from these plutons 

range from 166±5 to 144±4 m.y; biotite ages, with one exception, 

are younger, having a range of 144 ±4 to 147 ±4 m.y.; concordant ages 

of 146 ±4 and 147 ±4 m.y. from hornblende and biotite, respectively, 

were obtained from only one sample, rock from the Ashland pluton.

The contact between the Condrey Mountain Schist and the overly-
REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

ing amphibolite and associated metasedimentary rocks is clearly a 

fault, undoubtedly a thrust modified at many places by high-angle 

faults. The relation between the higher grade metamorphic rocks 

and the lower grade metavolcanic and metasedimentary rocks is less 

well denned. Northwest of the Condrey Mountain quadrangle, 

metavolcanic rocks overlie metasedimentary rocks belonging to the 

amphibolite facies, probably along a fault. In the southern Condrey 

Mountain quadrangle, high-angle faults, serpentinite bodies, and 

the southward-dipping tabular Vesa Bluffs pluton separate the two 

terranes. No well-defined contact has been recognized on the east; 

nonfoliated metavolcanic rocks of the amphibolite facies are succeeded upward conformably by fine-grained metasedimentary rocks 

that are overlain by metavolcanic rocks of the greenschist facies, 

apparently with a gradual decrease in intensity of metamorphism 

stratigraphically upward. A fault that occurs between the foliated 

and nonfoliated amphibolite may have been the original discontinuity between foliated amphibolite and the metavolcanic and 

metasedimentary rocks.

During a Triassic tectonic episode, the Stuart Fork Formation, 

which lies between metavolcanic and metasedimentary rocks of the 

western Paleozoic and Triassic belt and lower Paleozoic rocks of the 

eastern Klamath belt, was metamorphosed under lowtemperature-high-pressure conditions of the lawsonite-glaucophane 

(blueschist) metamorphic facies. Thrusting of the Stuart Fork Formation over the metavolcanic and metasedimentary rocks probably took 

place during the Triassic and may have continued or have been renewed in the Jurassic.

INTRODUCTION

Throughout most of the area occupied by the western 

Paleozoic and Triassic belt in the Klamath Mountains, 

California and Oregon, the rocks are regionally 

metamorphosed to the greenschist facies. In the 

north-central part of the Klamath Mountains, the belt 

contains rocks belonging to the almandine-amphibolite 

facies. Other areas of moderate to high-grade 

metamorphism are known elsewhere in the Klamath 

Mountains (Kays, 1968, 1970), but this area in the 

north-central part of the province west and northwest 

of Yreka is the most extensive (fig. 1). Parts of the area 

of higher grade metamorphism in California have been 

mapped in detail by Barrows (1969), Hotz (1967), and 

Medaris (1966) and in reconnaissance by Pratt (1964); 

in Oregon, the area is included in reconnaissance maps 

of the Grants Pass and Medford 30-minute quadrangles 

(Wells, and others 1940; Wells, 1956) and larger scale 

studies of small areas by Engelhardt (1966) and Heinrich (1966).

A circular area of approximately 650 km2 on the 

California-Oregon border, north of the Klamath River, 

is occupied by highly foliated low-grade schist. The 

schist is of two main types, quartz-muscovite schist, 

commonly graphitic, and actinolite-chlorite schist, 

herein named the Condrey Mountain Schist. Rocks of 

amphibolite metamorphic facies are in thrust-fault 

contact with the Condrey Mountain Schist on the east,

west, and south (figs. 1, 2). The area occupied by the 

higher grade metamorphic rocks contains many large 

and small bodies of ultramafic rock. The amphibolite 

facies rocks are succeeded outward, and apparently 

upward in most places, by weakly metamorphosed volcanic and sedimentary rocks. At some places, the 

change is abrupt as a result of faulting; at other places, 

the transition is gradual. In the area of lower grade 

rocks, ultramafic rocks are absent. The weakly 

metamorphosed volcanic and sedimentary rocks are 

part of the western Paleozoic and Triassic belt, a broad 

continuous north-south-trending belt that constitutes 

the most extensive lithologic subdivision in the 

Klamath Mountains province.

GREENSCHIST FACIES

Rocks of the greenschist facies include the Condrey 

Mountain Schist, metavolcanic and metasedimentary 

rocks of the western Paleozoic and Triassic belt, and 

the Stuart Fork Formation.

CONDREY MOUNTAIN SCHIST

The rocks that occupy the subcircular area mainly 

north of the Klamath River are here named the Condrey Mountain Schist for a peak of that name (sec. 11, 

T. 47 N., R. 10 W) in northwestern Condrey Mountain 

quadrangle. The type locality is designated the exposures on the south side of the Klamath River between 

Dona and McKinney Creeks (sees. 8 and 9, T. 46 N., R. 9 

W.) (fig. 2). The principal reference locality is designated the exposures along a ridge from Condrey 

Mountain to the saddle between West Fork of Beaver 

Creek and Wards Fork (NE1A sec. 29, T. 48 N., R. 9 W). 

Exposures in the valley of Elliot Creek (T. 48 N., R. 10 

and 11 W), northwest Condrey Mountain, and northern 

Seiad Valley quadrangles, are considered a supplementary reference locality. This schist is largely constituted of quartz-muscovite schist, commonly graphitic, 

and actinolite-chlorite schist. Of the two, quartzmuscovite schist is more abundant; greenschist occurs 

mainly on the borders of the schist outcrop area adjacent to the contact with amphibolite facies rocks.

The following mineral assemblages have been observed: quarts-muscovite-(albite-chlorite-graphite); 

quartz-albite-muscovite-epidote-(chlorite); quartzalbite-actinolite-chlorite-epidote; albite-actinolitechlorite- epidote-(quartz-muscovite). All are stable 

mineral assemblages of the quartz-albite-muscovitechlorite (chlorite zone) subfacies (Turner, in Fyfe, and 

others, 1958, p. 218) of the greenschist facies. An unusual rock with the mineral assemblage glaucophane-
Condrey Mountain 

Schist

*.

I

I

w a »• 3

x> -o 3

III.

i iilf

3 ^ ** •§ 13

<* " 3 & «

I |sl

B- DEVONIAN PALEOZOIC AND TRIASSIC (?)

o |

JURASSIC

b3 •;

"> S

t» &

u

S ?

Upper 

Cretaceous

|_______| 

I

CRETACEOUS

I'l

\ 

PERMIAN

AND 

TRIASSIC

JURASSIC SILURIAN OR 

OLDER
REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

123°00' 122°45'

42° 00'

41°45'

FIGURE 2.—Geologic map of Condrey Mountain quadrangle
GREENSCHIST FACIES

METAMORPHIC ROCKS 

Greenschist facies

EXPLANATION

Hornbrook Formation

42°00'

Condrey Mountain Schist

Quartz-muscovite schist and

actinolite-chlorite schist

Amphibolite facies

Metavolcanic and metasedimentary rocks of the western Paleozoic 

and Triassic belt

•'•J

Stuart Fork Formation

Siliceous phyllite and

phyllitic quartzite Siliceous schist, phyllite,

quartzite and minor

marble

Contact 

Dashed where inferred

Fault 

Dashed where inferred

Thrust fault 

Sawteeth on upper plate

\Q

o

.75 62

PLUTONIC ROCKS

Pyroxene quartz diorite 

and granodiorite

MAFIC AND ULTRAMAFIC ROCKS

Serpentinite and 

magnesian schist

Paradise Craggy

A Analyzed specimens of Condrey Mountain Schist;

Tables 1, 2, and 3 

v Metavolcanic rocks; Table 4 

• Foliated amphibolite; Table 5 

T Nonfoliated amphibolite; Table 5 

o Quartz-biotite phyllite; Table 6 

A Serpentinite and magnesian schist; 

x Amphibolite-serpentinite reaction; 

» Gabbro and pyroxenite; Table 9 

Q Granitic rocks; Table 10

Table 7 

Table 8

41°45*

and parts of Seiad Valley and Hornbrook quadrangles.
REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

epidote-quartz-albite-(chlorite-sphene), found locally, 

contains no lawsonite and hence also belongs to the 

greenschist facies despite the glaucophane content.

QUARTZ-MUSCOVITE SCHIST

Quartz-muscovite schist is the most common variety 

of the Condrey Mountain Schist. The common mineral 

assemblage is quartz-muscovite-(albite-chloritegraphite). This schist is a gray to brownish-gray, in 

some places bluish-gray, fine-grained rock with a 

well-developed schistosity along which the rock readily 

splits, revealing shiny cleavage surfaces coated with 

muscovite. Thin lenticular white microcrystalline 

siliceous laminae alternate with dark micaceous 

laminae. Brown iron-oxide pseudomorphs of pyrite as 

much as 5 mm in diameter are fairly common. The 

rocks are very uniform in appearance, grain size, and 

composition over wide areas. They have no obvious 

compositional layering except for podlike lenticular 

bodies of white quartz less than 2.5 cm to 5 to 7.5 cm 

thick and several centimeters long that occur irregularly parallel to the schistosity.

Quartz, the dominant mineral of the schist, occurs as 

lenticular laminae of elongate anhedral interlocking 

grains that show obvious strain shadows in crosspolarized light under the microscope.

Albite is generally sparse or absent; where present, 

it has a habit similar to the quartz except in finegrained mica-rich laminae in some specimens where 

it occurs as subhedral porphyroblasts with major diameters of 0.7 to more than 2.5 mm. In one specimen, 

the albite porphyroblasts contained parallel trains of 

carbonaceous dust in their central parts; rims are of 

clear albite in the pressure shadows of the metacrysts. 

The dust trains occur at an angle to the rock foliation, 

which has a slight tendency to wrap around the crystals; this tendency indicates that there was further 

movement in the direction of the schistosity after crystallization of the porphyroblasts.

Muscovite, the most abundant micaceous mineral, 

occurs in alternating laminae with the quartz and as 

thin folia between some of the quartz grains. Commonly a few flakes and irregular masses of chlorite 

accompany the muscovite. Chlorite may have a 

porphyroblastic habit in some specimens where it is 

oriented with its cleavages at a slight to moderate 

angle to the schistosity, along which slight movement 

has caused crinkling and development of an incipient 

secondary cleavage of the porphyroblasts.

The dark color of this schist, whose major mineral 

components are light colored, is imparted by a pervasive black dust, apparently carbon concentrated in the 

micaceous folia.

Many specimens of quartz-muscovite schist contain 

no other accessory minerals, but some contain a few 

prismatic grains of actinolite and granules of clinozoisite. Sphene, apatite, and brown tourmaline may be 

minor accessories in some.

CHEMICAL COMPOSITION

The quartz-muscovite schist is undoubtedly a 

metasedimentary rock. Although recrystallization has 

destroyed all vestiges of original structures and textures and no relict textures can be seen microscopically, 

the mineralogy, especially the occurrence of graphite, 

is compatible with derivation from a sedimentary rock, 

perhaps a fine-grained sandstone. Chemical analyses 

of two samples of quartz-muscovite schist are given in 

table 1.

ACTINOLITE-CHLORITE SCHIST

Actinolite-chlorite schist—believed to be the 

metamorphosed equivalent of basaltic tuffs and flows,

TABLE 1.—Chemical analyses and CIPW norms of quartz-muscovite 

schist from the Condrey Mountain Schist

[Chemical analyses by rapid methods; analysts: P. L. D. Elmore, S. D. Botts, Gillison Chloe, 

Lowell Artis, and Hezekiah Smith. Sample localities shown in fig. 2]

Sample No. 120-63 114-62

Chemical analyses (weight percent)

SiO2 - 

AkOs 

FezOs 

FeO _ 

MgO 

CaO _ 

NaaO 

K2O _ 

H2OH2O + 

TiCfe _

MnO 

CCfe

69.8

13.5

1.3

4.3

2.7

.45 

1.9 

1.6 

.16 

3.1 

.79 

.24 

.05 

< .05

73.6

11.9

.90

3.0

1.9 

.73

2.0

1.5 

.07

2.2 

.67 

.22 

.04 

.71

Total_. 

Sp. gr. (bulk)

100

2.73

99

2.74

CIPW norms (percent)

q

C_ —

or . 

ab . 

an . 

di . 

hv. 

ol . 

mt. 

il . 

ap . 

cc _ mg.

45.8

8.4

9.5

16.1

.66

12.4

1.9

1.5 

.57

52.5

7.0

8.9

17.0

7.7

1.3

1.3 

.52 

.79 

.70

NOTE. —120-63 Quartz-(albite)-muscovite-chlorite schist. East of Dog Fork, NEV4 NE'A sec.

25, T. 48 N., R.10 W. 

114-62 Quartz-muscovite schist, Center sec. 9, T.46 N., R.9 W.
GREENSCHIST FACIES

constitutes a distinctive variety of the Condrey 

Mountain Schist. It occurs in a wide continuous belt 

between quartz-muscovite schist and the amphibolite 

facies metamorphic rocks in the northern part of Condrey Mountain quadrangle (Hotz, 1967) and as narrow 

discontinuous bodies elsewhere, both along the boundary with the higher grade rocks and interbedded with 

the quartz-muscovite schist. It is fine grained, greenish 

gray to grayish green, and varies from very uniform to 

finely laminated with thin flattened lenticular laminae 

alternately rich in ferromagnesian minerals and albite. Textures are crystalloblastic schistose; the average grain size ranges from 0.05 mm to 1.0 mm. Original 

textures and structures have been mostly obliterated 

by recrystallization. Some have a faint compositional 

layering that suggests laminae of sedimentary origin, 

and a few contain ovoid twinned albite phenocrysts 

that may be pseudomorphs of plagioclase and clots of 

chlorite and actinolite that possibly represent original 

phenocrysts of mafic minerals.

The mineral assemblages quartz-albite-actinolitechlorite-epidote and albite-actinolite-chloriteepidote-(muscovite) are common. A fine-grained rock 

with a bluish tinge seen in one place has the assemblage glaucophane-epidote-quartz-albite-(chlorite), 

but this assemblage is unusual. Sphene is a ubiquitous 

minor accessory, occurring as tiny, droplike granules, 

typically enclosed in chlorite and the amphibole. Apatite is present in trace amounts. The quartz content is 

variable but generally small; quartz is absent from 

some samples. Quartz occurs with albite, commonly as 

blebs within the feldspar, but it is also intergrown with 

it in grains of similar habit; it may also occur in small 

segregated lenses. Some flakes of muscovite have been 

observed in nearly all specimens.

CHEMICAL COMPOSITION

Chemical analyses of three samples of greenschist, 

given in table 2, show them to be basaltic in composition. Owing to their relatively high Na2O and low 

CaO content, they plot in the spilite field on SiO2- 

Na2O and SiO2-CaO diagrams (fig. 3) Hamilton, 1963). 

On an AFM plot (fig. 4), they are similar to the 

metavolcanic rocks and amphibolites in the area that 

surrounds the window of schist but they have a slightly 

lower iron and higher magnesia content than most of 

the samples.

METAMORPHOSED GRANITIC (?) ROCK

A small indefinite body of albite-quartz-muscoviteepidote schist (not shown in fig. 2) occurs in the West 

Fork of Beaver Creek along the contact between 

greenschist and amphibolite. Megascopically, it is a

TABLE 2.—Chemical analyses, CIPW norms, and modes of greenschist 

from the Condrey Mountain Schist

[Chemical analyses by rapid methods, analysts: P. L. D. Elmore, S. D. Botts, Gillison Chloe, 

Lowell Artis, and Hezekia Smith. Sample localities shown in fig. 2]

Sample No.

SiCfe — ___

FfeOs

FeO ________

MgO ______

CaO________

KaO— ———

H20- ______

H2O+ ______

TiC-2 ______

MnO ______

CCfe— — __

Total

Sp.gr.

ab ________

fs __________

di ________

hy ________

fo_ —— —— ——

fa ________

il __________

cc

Albite __

Chlorite

Muscovite

Apatite

113-62

Chemical analyses (weight percent)

________________________ 49.6

_ __ ____ ___ ___ 15.3

________________________ 3.8

________________________ 6.3

________________________ 7.9

________________________ 8.1

________________________ 4.0

________________________ .22

____________________ .22

________________________ 3.1

___ _ __________________ 1.0

____ _ __ .__ ____ .10

______—_____—___„_- .21

.39

__________________100 100

(bulk)__'_ _ _ _ _ _ 2.96

CIPW norms (percent)

_———____——_——__— 1.3

33.8

23.1

__—____—— _——__——_ 5.8

_ _ _ _. __ ____ _ __ 13.0

_ _ ___ __ _ ____ _ _ 4.7

___________________ 11.2

________________________ 12.4

_———_——____——_—_ 4.7

________________________ 1.9

5.5

1.9

.24

.89

Modal analyses (volume percent)

121-63

50.3

16.4

4.1

5.4

6.8

8.5

4.2

.14

.10

2.5

1.1

.12

.15

<.05

100

2.91

.8

35.6

25.6

6.7

12.8

3.8

12.7

10.5

2.9

.94

6.0

2.1

.28

38

21

15

23

2

—

12-60

51.3

15.9

3.2

5.4

5.7

8.3

5.1

.39

.07

2.1

1.3

.12

.18

1.1

2.93

2.3

43.1

19.3

5.9

8.4

3.2

11.3

6.2

4.1

1.7

4.6

2.5

.28

2.5

32

35

11

15

4

3

.7

.1

NOTE—113-62 Greenschist, SEV4 sec. 14, T.46 N, R.10 W.

121-63 Greenschist, Elliot Creek, east edge SEV4, sec. 23, T.48 N., R.10 W. 

12-60 Greenschist, West Fork Beaver Creek, eastern part, sec. 1, T.47 N., R.9 W.

light-colored, medium-grained schistose rock with 

visible quartz, feldspar, and white mica. It has large 

irregular to augen-shaped feldspars and looks like a 

metamorphosed granitic rock.

Under the microscope, the rock is seen to be composed of anhedral, clear, untwinned albite and quartz. 

The albite is vaguely rectangular and has a slight preferred orientation; the quartz is in anhedral interstitial 

bodies, inclusions, and veinlets. Muscovite occurs in 

roughly parallel flakes and folia that give the rock its 

faint foliation. Minor quantities of chlorite, epidote, 

and sphene are present.

A chemical analysis of this rock, presented in table 3, 

is somewhat puzzling, for it is unlike that of most granitic rocks. SiO2 is very high; A12O3 is relatively low, 

MgO is somewhat high. Alkali content is rather low for
REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

Spilite

12-60. /

121-63 /

113-62. \_

— •— — — 

Basalt

/ Keratophyre /

-4-

.a.

45 50 55 60 

SiO, CONTENT, IN WEIGHT PERCENT

65

ai 

O

OC12 

01

£10

- 6 (-"

Z

oi 4 IZ

O 2 O

(0

Spilite

45 50 55 60

SiO 2 CONTENT,IN WEIGHT PERCENT

65

FIGURE 3.—Na2 O and CaO relative to SiO2 for 

greenschist from Condrey Mountain Schist (from data 

given in table 2). Classification from Hamilton 

(1963, p. 71).

a plutonic rock of this SiO2 content, and Na2O is more 

than twice as abundant as K2O. The rock may be trondhjemitic, although normative quartz and orthoclase 

seem too high.

Perhaps this unusual rock is an intrusive of the same 

general age as the other granitic rocks in the area 

K 140 m.y.) and locally has been metamorphosed contemporaneously with the Condrey Mountain Schist.

AGE AND CORRELATION

Potassium-argon age determinations on muscovite 

and whole-rock analyses gave dates of 141 m.y. (Lanphere and others, 1968) and 155 ±3 m.y. (Suppe and 

Armstrong, 1972), respectively, for the Condrey 

Mountain Schist. The age of metamorphism is therefore Late Jurassic, but the age and correlation of its 

protolith among known lithologic units of the region is 

somewhat speculative.

The overall lithology and composition of the Condrey 

Mountain Schist are indicative of the formation of 

these metamorphic rocks from sedimentary rocks and

2Fe FeO+ MnO

EXPLANATION

A Metavolcanic rocks 

D Nonfoliated amphibolite 

• Foliated amphibolite 

Greenschist

Na,O + K,O MgO

FIGURE 4.—AFM diagram of metavolcanic rocks and amphibolite 

from Condrey Mountain and Hornbrook quadrangles and 

greenschist from Condrey Mountain Schist.

subordinate amounts of spilitic volcanic rocks. The 

Stuart Fork Formation (Davis and Lipman, 1962) has 

been regarded as a possible correlative (Medaris, 1966), 

and the style of deformation in these two terranes is 

similar, but the metasedimentary rocks of the Stuart 

Fork are predominantly metachert. Furthermore, 

elsewhere the Stuart Fork is thrust over rocks of the 

western Paleozoic and Triassic belt, whereas here the 

schist underlies amphibolite that is beneath rocks of 

that belt. Suitable parental rocks might be found 

among some of the Mississippian, Triassic, and Jurassic rocks of the eastern Klamath belt, but the structural problems inherent in explaining their relative 

positions seem almost unresolvable.

Suppe and Armstrong (1972) thought that the Condrey Mountain Schist of this report resembled the 

South Fork Mountain Schist, and they concluded therefore that this schist was metamorphosed Franciscan 

rocks, noting, however, that the date of 155 m.y. they 

obtained is slightly older than any available Franciscan dates. If these are metamorphosed Franciscan 

rocks, a structural difficulty exists, for in the Klamath 

Mountains the Galice Formation lies structurally 

above Franciscan rocks and the South Fork Mountain 

Schist and structurally below the western Paleozoic 

and Triassic belt. It would be difficult to account for the 

occurrence of a window of Franciscan with no intervening Galice.

The Galice Formation is of suitable lithology and is 

structurally below the western Paleozoic and Triassic
GREENSCHIST FACIES

TABLE 3.—Chemical analysis, CIPW norm, and mode of 

albite-quartz-muscovite-epidote schist

[Chemical analyses by rapid methods; analysts: P. L. D. Elmore, S. D. Botts, Gillison Chloe, 

Lowell Artis, and Hezekiah Smith. Sample locality shown in fig. 2]

Sample No.____________________________________________113-63

_________Chemical analysis (weight percent)__________

Si02

AkOa

Fe-Oa

FeO__

MgO

CaO__

N320

K_0__ H2OH20+ 

Ti02-_

P2 05

MnO 

CCfe __

.74.2 

.13.3

- 1.2 

_ .88

- 1.2

- 2.1

- 3.9 

_ 1.6 

_ .09 

_ .91 

_ .20 

_ .03 

_ .08 

_ .32

Total 100

CIPW norm (percent)

q

c

ab __________________________

an

hy __________________________

mt

il ____________________________

cc

39.9

_____________________ _ __ 2.1

__ __ ______________________ 9.5

____________________________33.0

_____________________________8.2

____________________________ 3.4

1.7

____________________________ .38

.07

____________________________ .73

Modal analysis (volume percent)

Quartz _ _ 

Albite__ 

Muscovite 

Chlorite __ 

Epidote _- 

Sphene __ 

Garnet __

_31.6 

_41.7 

.16.9 

. 1.4 

. 6.6 

_ .7 

_ .1

NOTE—113-63 West Fork Beaver Creek, line between sees. 1 and 2, T.49 N, R. 9 W.

belt elsewhere. The area of Condrey Mountain Schist is 

only about 25 km east of a reentrant of the Galice at 

Happy Camp, where Klein (1976) found sedimentary 

and volcanic rocks of the Galice grading upward into a 

greenschist facies overridden by amphibolite-grade 

rock along a thrust fault. The Galice seems to be a 

suitable protolith structurally as well as lithologically, 

although its apparent youthfulness (Oxfordian and 

Kimmeridgian) is incompatible with the metamorphic 

age of Condrey Mountain Schist. As fossils are not 

abundant in the Galice and its stratigraphy is poorly 

known, there may conceivably be parts of the formation older than the metamorphic episode.

METAVOLCANIC AND METASEDIMENTARY ROCKS OF THE 

WESTERN PALEOZOIC AND TRIASSIC BELT

Metavolcanic and metasedimentary rocks are almost 

continuously exposed along the Klamath River from 

the overlap by rocks of the Cretaceous Hornbrook Formation 6 km northeast of the mouth of the Shasta 

River (fig. 2) to the confluence of Empire Creek with

the Klamath River, a distance of about 18 km. South of 

the Vesa Bluffs pluton, metavolcanic and 

metasedimentary rocks occur as metavolcanic rocks 

with interbedded chert, slate and phyllite with subordinate amounts of volcanic rock, and interbedded chert 

and argillite with minor greenstone (Hotz, 1967). 

Along the Klamath River, metavolcanic rocks crop out 

from the overlap of Cretaceous sedimentary rocks 

westward to Croy Gulch, 4 km west of the mouth of 

Shasta River (fig. 2). Between Croy Gulch and Deadman Gulch, 7 km to the west, rocks that crop out are 

mostly metasedimentary. From Deadman Gulch to the 

body of ultramafic rocks, rocks exposed are predominantly metavolcanic.

METAVOLCANIC ROCKS

The metavolcanic rocks are characteristically gray 

to greenish gray and fine grained to microcrystalline, 

but locally porphyritic. They are mostly massive and 

virtually structureless; in some places planar structures are made apparent by variations in composition 

or grain size. Some are clearly volcanic breccias; pillow 

structures are recognizable in some exposures but are 

not common.

The least metamorphosed metavolcanic rocks of the 

Klamath River section are to the east; they are represented by specimens of spilite from road cuts on the 

north side of Klamath River at the junction with 

Shasta River (table 4, sample no. 2-64) and fragmental 

basalt from exposures on Paradise Craggy about 4 km 

southeast of the junction (table 4, sample no. 20-66). 

These rocks are characterized by the coexistence of unaltered clinopyroxene, albite, and chlorite. There is no 

alteration of the pyroxene to amphibole, but minor 

amounts of chlorite occur interstitial to the 

groundmass albite and pyroxene. Veinlets of carbonate 

are commonly accompanied by prehnite, and the rock 

may contain a few small amygdules of calcite rimmed 

by pale-green chlorite.

Except for these occurrences of unmetamorphosed 

spilite, the metavolcanic rocks in nearby Shasta Canyon and westward through the Klamath River canyon 

to the contact with the ultramafic rocks are composed 

mainly of sodic plagioclase, actinolitic amphibole, 

minor amounts of chlorite, biotite, and clinozoisiteepidote, accessory sphene, and variable amounts of calcite; typically, opaque accessories are present in very 

small amounts or absent. To a large extent, the original textures and structures are preserved.

The plagioclase most commonly ranges from oligoclase (An15_20) to oligoclase-andesine (An30); an exceptional few specimens have feldspar as sodic as albite. 

Plagioclase commonly occurs as clear subhedral to
10 REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

TABLE 4.—Chemical and spectrograpic analyses and CIPW norms 

of metavolcanic rocks

[Chemical analyses by rapid methods; analysts: P. L. D. Elmore, S. D. Botts, Gillison Chloe, 

Lowell Artis, James Kelsey, and Hezekiah Smith. Semiquantitative spectrographic 

analyses by W B. Crandell, Chris Heropoulos and Carolyn Pickett. Sample localities 

shown in fig. 2]

Sample No. 75-62 20-66 16-61 2-64 132-64

Chemical analyses (weight percent)

SiO2 _____________

AbO. ___________

FezO. ___________

FeO—————

MgO ___ _—————

CaO— — — — -

NazO ___ ___ _____

fcO— _— ——— —

H 2 0- _—————

H 2O+ __ _ ————

TiCte _____________

P205 ____________

MnO ————— __

COz— — — ———

_____ 45.3

— — 14.2

. _ .7

10.3

.__ __ 4.3

______ 8.3

. ___ 4.2

—— __ .55

______ .07

______ 3.5

______ 2.4

______ .49

_ ____ .19

______ 6.0

45.7

13.0

5.0

6.4

12.2

7.2

2.3

1.1

.5

3.8

2.2

.35

.18

.08

48.1

15.0

1.2

9.0

5.7

9.3

4.9

1.4

.08

1.5

2.6

.39

.14

.85

50.6

15.0

.66

7.4

9.5

7.4

3.8

.91

.20

3.4

.87

.11

.15

.14

53.5

13.7

.55

6.5

10.9

9.5

3.1

.16

.07

1.1

.23

.04

.13

.06

Total _ 

Sp. gr. (bulk)_

100

2.82

99

2.99

100

2.99

100

2.95

99

2.71

Semiquantitative spectrographic analyses (ppm)

Ba _____________

Ce ____________

Co ————— ___

Cr ____________

Cu _____________

Ga ____________

La ____________

Nb ____________

Ni ____________

Pb ____________

Sc_— _ _______

Sn ____________

Sr— ———— __ __

V ______________

Y_——— ___ —— _

Yb ____________

Zr————— ——— _

______ 100

______ 150

_______ 50

200

_______ 70

______ 15

_______ 100

_______ 50

_ __ __ 200

______ 70

_______ 30

______ 0

1000

_______ 300

_______ 50

_______ 5

_____ 200

200

100

70

700

70

15

30

30

300

0

30

0

300

150

20

2

150

300

0

70

500

150

10

0

10

100

10

30

5

500

300

20

2

100

300

0

50

500

150

10

0

0

150

70

50

15

200

200

15

1.5

30

70

0

30

1500

500

7

0

0

300

0

50

0

200

200

7

0

7

CIPW norms

ab

di-wo

di-en

di-fshy-en

hy-fs

ol-fool-famt -_

il _,_

ap -_

5.7

5.7

3.2

35.5

.2

10.6

14.7

1.0

4.6

1.2

13.6

6.5

19.6

22.0

4.6

3.6

.5

15.2 

2.2 

8.2 

1.3 

7.3 

4.2 

.83 

.18

8.3

27.1

14.8

7.8

9.8

5.2

4.2

6.3 

5.6 

1.7 

4.9 

.92 

1.9

5.4

32.2

21.2

5.8 

3.6 

1.8 

3.8 

1.9 

11.4 

6.3 

.96 

1.6 

.26 

.32

.94

26.2

23.0

9.8 

6.4 

2.7 

16.8 

7.0 

2.8 

1.3

.8

.44

.1

.14

NOTE.—Name and location of specimens:

75-62 Variolitic spilite. Klamath River Road, SE%, sec. 16, T.46 N, R.7 W,

Hornbrook quadrangle.

20-66 Fragmental basalt. Paradise Craggy, Hornbrook quadrangle. 

16-61 Spilite. NWV4, sec. 5, T.45 N., R.8 W, Condrey Mtn. quadrangle. 

2-64 Spilite. Klamath River Road, SWVi, sec. 18, T.46 N., R.6 W, Hornbrook

quadrangle.

132-64 Fragmental basalt. Empire Creek, S. part NW1/., sec. 36, T.42 N., R.8 W, 

Hornbrook quadrangle.

anhedral relict phenocrysts that are clouded in some 

specimens. Faint twinning lamellae are preserved in 

some specimens, but in many the original phenocrysts 

have been partly to wholly recrystallized to a mosaic of 

anhedral untwinned grains with the outlines of the 

original phenocrysts preserved. Plagioclase in the 

groundmass of some specimens preserves the outlines 

of original lath-shaped crystals, but most of the 

groundmass feldspar has been recrystallized to anhedral granules intergrown with and penetrated by acicular actinolite. 

Pale-green actinolite, the most abundant mineral in

these rocks, occurs as matted needles in the 

groundmass, as anhedral matted porphyroblasts, and 

as euhedral to subhedral fibrous masses (uralite) that 

pseudomorph original pyroxene phenocrysts. Uncommonly, original pyroxene is preserved in the central 

part of actinolite phenocrysts. West of Cayuse Gulch, in 

rocks that were probably originally tuffaceous, actinolite has a subparallel orientation.

Chlorite is common but not abundant. It is intergrown with plagioclase and actinolite in the 

groundmass of the metavolcanic rocks. West of Cayuse 

Gulch, pale-brown biotite in small flakes occurs with 

actinolite and plagioclase. Chlorite partly replaces 

biotite and, to a lesser extent, actinolite, hence is probably a retrograde mineral.

Clinozoisite and epidote are not abundant in any of 

these rocks, and they are absent from some specimens. 

Clinozoisite tends to be more abundant. Both occur as 

small granules in the groundmass, as grains enclosed 

in feldspar, and in a few places, as veinlets.

Sphene, ubiquitous as a minor accessory, occurs as 

very small grains or clusters of grains and gray 

translucent masses. It may or may not be accompanied 

by fine opaque dust and grains.

Carbonate is fairly common as irregular and lenticular masses, some of which are clearly relict amygdules.

CHEMICAL COMPOSITION

Chemical analyses of five representative specimens 

of metavolcanic rocks from the Klamath River canyon 

and vicinity in the Condrey Mountain and Hornbrook 

quadrangles are presented in table 4; they are 

classified according to their plotted positions on diagrams of SiO2 relative to Na2O2 and CaO (fig. 5). They 

have the composition of spilite and basalt, except for 

no. 132-64, which, owing to its higher SiO2 content, is 

andesite. They have a moderate water content relative 

to terrestrial basaltic rocks, an expectable feature considering their low-grade metamorphic state. These 

rocks have a lower content of CaO than ocean floor 

basalts (Cann, 1971; Kay and others, 1970); Na2O is 

distinctly higher than in basalt in samples 16-61, 2-64, 

and 75-64, about the same in sample 20-66. K2O is 

higher than in basalt, except for sample 132-64 (table 

4). Total iron (as FeO) and A12 O3 are about the same. 

The higher Na2O content and lower CaO are possibly 

the result of spilitization of basaltic lavas. Amounts of 

TiO2 and P2 O5 are greater than in ocean floor basalts 

in samples 20-66 and 16-61, about the same in 2-64, and 

markedly lower in the andesite, sample 132-64.

Sample 75-62, a variolitic spilite, is somewhat unusual because of its high CO2 content. The rock has 

abundant carbonate in microscopic vesicles; the low
GREENSCHIST FACIES 11

Z 7

UJ

o 

a: 6

UJ

CD= .

z

~ 3

H 

Z

Spilite / Keratophyre /

/ / 

.16-61 / /

7S-62

BasaH 

20 - 66

132-64

45 50 55 60

SiO2 CONTENT, IN WEIGHT PERCENT

65

.75-62 

-•20-66

1

132-64

2-64\

45 50 55 60

SiO2 CONTENT,IN WEIGHT PERCENT

65

FIGURE 5.—Na 2O and CaO relative to SiO2 for metavolcanic 

rocks from Condrey Mountain and Hornbrook quadrangles (from data given in table 4). Classification from 

Hamilton (1963, p. 71).

Fe2O3 and high FeO content combined with relatively 

low CaO suggest that not all the carbonate is as calcite, 

and may include a considerable component of siderite.

METASEDIMENTARY ROCKS

The metasedimentary rocks are predominantly 

dark-gray to black argillite and slaty argillite; they 

contain a few thin interbeds of siliceous or cherty argillite and microcrystalline recrystallized chert and a 

very few thin lenticular beds of marble. Some of the 

argillaceous rocks are also calcareous.

East of approximately the longitude of Cayuse 

Gulch, the metasedimentary rocks are composed 

mainly of microcrystalline quartz, muscovite, and carbon. Very pale green to colorless chlorite is a fairly 

common accessory mineral, and hazy albitic feldspar 

and actinolite are locally present. Calcite is abundant 

in some layers.

West of Cayuse Gulch, the rocks are similar, but 

pale-brown biotite is a fairly common constituent instead of chlorite, which, where present, appears to be 

mainly a retrograde mineral that replaces biotite and 

actinolite; some layers are rich in calcite; others are 

dark and carbonaceous. Some layers that probably 

were tuffaceous are made up of a foliated matrix of 

actinolite, biotite, and granular plagioclase that wraps 

around relict plagioclase phenocrysts.

Near the mouth of Empire Creek, gray, massive to 

thick-bedded, sugary-grained microcrystalline quartzite crops out on both sides of the river. These rocks, 

probably recrystallized chert, are composed mainly of 

very finegrained to microcrystalline quartz; some 

specimens contain minor amounts of biotite; others 

contain parallel-oriented fibers of tremolite. Finegrained to microcrystalline quartz-biotite schist occurs 

as thin interbeds in some of the quartzite, probably 

representing originally argillaceous layers.

AGE AND CORRELATION

These metavolcanic and metasedimentary rocks are 

continuous with similar rocks in the central and southern Klamath Mountains that are part of the western 

Paleozoic and Triassic belt (Irwin, 1960, 1966).Similar 

rocks in southern Oregon were included in the Applegate Group of Late(?) Triassic age (Wells and others, 

1949; Wells, 1956; Wells and Peck, 1961). In the southern Klamath Mountains, the belt includes fossiliferous 

limestone lenses of Permian and Triassic age (Irwin, 

1972), and a limestone lense northeast of Yreka that 

has yielded Late Permian (Ochoan)fossils (Elliott, 

1971). Discovery that some of the cherts in this belt 

contain Triassic and Jurassic radiolarians has been 

used as evidence that at least some of the limestone 

bodies are blocks floating in a Jurassic melange (Irwin, 

1977; Irwin and others, 1977).

STUART FORK FORMATION

A small area of Stuart Fork Formation shown in the 

southern part of the region of figure 2 is an outlier of a 

thrust plate that occurs farther south in the Yreka-Fort 

Jones area (fig. 1) (Hotz, 1973). It is the north end of a 

small klippe that overlies metavolcanic and metasedimentary rocks of the western Paleozoic and Triassic belt.

The rocks are contorted siliceous phyllite and phyllitic quartzite composed of fine-grained recrystallized 

quartz and discontinuous plicated micaceous laminae 

composed of muscovite, pale biotite, and chlorite. Locally there are small amounts of actinolitic phyllite. 

Where these rocks occur to the south in Fort Jones and 

Yreka quadrangles, they commonly contain lawsonite 

and a blue sodic amphibole.
12 REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

The Stuart Fork has been correlated with rocks in 

the western Paleozoic and Triassic belt (Davis and 

Lipman, 1962; Davis and others, 1965; Davis, 1968). A 

Middle to Late Jurassic age has been postulated for 

metamorphism of the Stuart Fork on the basis of limited potassium-argon age determinations ranging from 

133 to 158 m.y. for samples collected in the southern 

Klamath Mountains (Lanphere and others, 1968). In 

the Yreka-Fort Jones area, age determinations of approximately 220 m.y. (Middle Triassic) were obtained 

on mica from three samples of blueschist (Hotz and 

others, 1977).

AMPHIBOLITE FACIES

The Condrey Mountain Schist is bordered on the 

east, north, and west by a terrane in which the rocks 

are of the amphibolite facies (figs. 1, 2). The higher 

grade metamorphic rocks structurally overlie the Condrey Mountain Schist along a moderate to steeply dipping fault. In contrast to the schist, which is devoid of 

plutonic rocks and has only a few small bodies of serpentinite, the terrane underlain by rocks of the amphibolite facies contains many bodies of ultramafic 

rock and some gabbroic and dioritic plutons. Approximately equal amounts of amphibolite and 

metasedimentary rocks occur in this terrane in the 

Condrey Mountain and western Hornbrook quadrangles. Rocks of the two types are to some extent interbedded, but extensive tracts are predominantly of one or 

the other (fig. 2).

Most of the amphibolite is foliated, but some of it is 

massive and resembles a metavolcanic rock (fig. 6 A), 

even to retaining a porphyritic texture (fig. 6B). Two 

kinds of amphibolite, foliated and nonfoliated, are described here and are shown separately on the geologic 

map (fig. 2).

FOLIATED AMPHIBOLITE

Megascopically, the foliated amphibolite ranges from 

very fine to medium grained and is commonly lineated. 

In many specimens, the foliation is made apparent by 

segregation of plagioclase and mafic minerals into parallel layers. In exposures that have no visible layering 

and no micas, the schistosity may not be visible; under 

the microscope, a preferred orientation of hornblende 

that defines a planar structure can usually be seen. 

Many amphibolites have a lineation made apparent by 

parallel orientation of hornblende prisms that lie in 

the plane of foliation.

The texture is crystalloblastic and the rocks are 

mostly fine grained (fig. 6C,D). Equant xenoblastic 

plagioclase is intergrown with subhedral to euhedral 

nematoblastic hornblende. Plagioclase grains range in

size from approximately 0.02 to 0.1 mm, most commonly around 0.06 mm; hornblende prisms are 0.05 to 

0.5mm long, most commonly about 0.1 with a lengthto-width ratio of 2.5:1 to 4:1. Commonly, the rocks have 

a more or less well-developed layering or streaking 

caused by alternating plagioclase and hornblende-rich 

layers. Most laminae are very narrow, generally a millimeter or less in width, discontinuous or lenticular. 

Recrystallization has essentially obliterated original 

textures and structures, but in many specimens relict 

phenocrysts of plagioclase and hornblende can be identified. The original plagioclase phenocrysts are completely recrystallized to a polyhedral mosaic with subrectangular boundaries. Some specimens are 

porphyroblastic. The metacrysts are most commonly 

coarser hronblende grains, but porphyroblasts of 

plagioclase occur.

Foliated amphibolite has a simple mineralogy. Its 

principal components are hornblende and plagioclase, 

characteristic of the almandine-amphibolite facies. 

Sphene and an opaque black metallic mineral are 

common minor accessories. Other minerals that may 

be present, usually in subordinate amounts, are epidote group minerals, clinopyroxane, mica (usually biotite), quartz, and calcite.

Plagioclase is somewhat more calcic than in the nonfoliated amphibolite. It ranges from approximately 

An25 to An55, in most specimens it is An30_ 35 . The 

composition in pyroxene-bearing amphibolites is more 

calcic, in the range An40_ 55. Most of the grains of 

plagioclase are clear and inclusion-free. Commonly 

they are untwinned; a few may show twinning lamellae under crossed-polarized light. Some may show simple progressive zoning; most do not.

Hornblende in thin section is almost always strongly 

colored and commonly has a blue-green pleochroic 

color in the Z direction; from specimen to specimen, it 

may range from green to brown. Hornblende that has a 

brown pleochroism commonly occurs in amphibolite 

specimens that contain pyroxene and a more calcic 

plagioclase; the pyroxene and more calcic plagioclase 

are indicative of a slightly higher metamorphic grade.

The pyroxene in some of the amphibolite is colorless 

to very pale green nonpleochroic diopside that occurs 

as anhedral granules intergrown with plagioclase and 

hornblende, generally occurring in laminae accompanied by plagioclase with subordinate hornblende. In 

a few specimens, pyroxene was observed to occur as 

anhedral porphyroblasts.

Very minor to trace amounts of biotite occur in some

FIGURE 6.— Photomicrographs of amphibolite. A, B, Nonfoliated 

variety showing randomly oriented hornblende and relict 

porphyritic texture. C, D, Foliated variety showing welldeveloped preferred orientation of hornblende.
AMPHIBOLITE FACIES 13
14 REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

of the amphibolite, but it is not usually present. 

Anhedral, droplike granules of sphene are common. 

Sphene may range from trace amounts to as much as 4 

or 5 percent volumetrically and commonly is as abundant as 1.5 to 2 percent; it is absent from some 

specimens. Grains of unidentified black opaque metallic grains, probably magnetite, are ubiquitous, ranging 

from trace amounts to as much as 1.5 percent. Quartz, 

calcite, and chlorite may be present in minor quantities in some specimens. Garnet is notably absent from 

the foliated amphibolite. Only one specimen, obtained 

from float, contained pink andraditic garnet.

Typically, either epidote or clinozoisite is absent from 

the foliated amphibolite. Some specimens contain trace 

to very minor amounts of epidote, and some contain 

clinozoisite that, in each sample examined, is retrograde from the alteration of plagioclase.

NONFOLIATED AMPHIBOLITE

Nonfoliated amphibolite occurs on the south and east 

borders of the amphibolite terrane, largely between the 

areas of foliated amphibolite and the terrane occupied 

by metavolcanic and metasedimentary rocks of the 

western Paleozoic and Triassic belt.

Adjacent to the large serpentinite body in the southeastern part of the amphibolite terrane and in small 

areas in the southern part, the mafic rocks have a 

mineralogy characteristic of the amphibolite facies yet 

they retain many original textural features of 

metavolcanic rocks. In general, the rocks are massive 

and unfoliated, though completely recrystallized, except locally where they may have a fine-grained compositional layering possibly inherited from originally 

tuffaceous rocks. Characteristically, there is no welldeveloped preferred orientation of the minerals. These 

rocks are characterized by the occurrence together of 

plagioclase and hornblende and the absence or occurrence in very minor amounts of epidote group minerals; hence they are of the almandine-amphibolite 

facies.

Plagioclase in these rocks ranges in composition 

from approximately An25 to An35 . It occurs as small 

anhedral crystalloblastic grains in the groundmass intergrown with hornblende, as granoblastic 

pseudomorphs of original phenocrysts, and as unrecrystallized subhedral to euhedral relict phenocrysts. 

The recrystallized grains are seldom twinned. Some of 

the relict phenocrysts are polysynthetically twinned. 

Composition of the relict phenocrysts is apparently the 

same as the recrystallized plagioclase in the 

groundmass.

Hornblende rather than actinolite is the amphibole 

of these rocks. In thin section the hornblende is light

colored but slightly darker than the actinolite of the 

greenschist facies rocks. Characteristically, its pleochroism is: (x), colorless to pale greenish yellow; (y), 

pale yellowish green; (z), pale green, commonly with a 

faint blue tinge to pale blue green. On a /8-2Va plot 

(fig. 7), these amphiboles lie in the hornblende field. 

Unlike actinolitic amphibole, they are not fibrous in 

habit, but rather are well-formed prismatic. The 

hornblende is randomly oriented and commonly matted; in some specimens, a slight tendency toward parallel alignment is apparent. Some of the hornblende is 

pseudomorphous after original pyroxene phenocrysts. 

Uncommonly, some of the hornblende pseudomorphs 

have a small central core of relict augite.

Most of these rocks contain no epidote-group minerals; a few specimens contain minor amounts of either 

clinozoisite or epidote, minerals that seem not to occur 

together in the rock. Epidote is possibly more common. 

Some of the clinozoisite occurs with chlorite in veinlets 

that cut the rock. The presence of epidote minerals may 

indicate that some of these rocks are transitional between the upper greenschist facies and the amphibolite 

facies. In some, it may indicate retrograde metamorphic reactions.

Sphene, apatite, and a black opaque mineral which 

is presumably ilmenite-magnetite are common minor 

accessories in these rocks. The sphene occurs as very 

small anhedral droplike granules. The black opaque 

minerals are fine grained and may occur as fine 

granules, irregular skeletal and spongy bodies, or fine 

rodlike grains. Apatite commonly occurs as long slender needles and as stubby subhedral crystals.

Pale-brown biotite occurs in minor to trace amounts

90'

80°

60°

n

EXPLANATION 

A Metavolcanic rocks 

n Nonfoliated amphibolite 

• Foliated amphibolite

1.64 1.65 1.66 1.67 

REFRACTIVE INDEX (beta)

1.68 1.69

FIGURE 7.—Diagram of amphiboles in metavolcanic rock and amphibolite from Condrey Mountain and Hornbrook quadrangles. 

Boundary line from Holdaway (1965).
AMPHIBOLITE FACIES 15

in some specimens but it is not a common accessory. 

Pale chlorite, which is present in a few specimens, is 

absent from most specimens. In the few samples examined, it could not be certainly determined whether it is 

a relict or a retrograde mineral. Calcite, which is very 

uncommon, occurs in a few specimens as veinlets and 

as ovoid bodies that probably are relict amygdules.

CHEMICAL COMPOSITION

It is apparent from analyses of six samples of foliated 

amphibolite and six of nonfoliated amphibolite (table 

5) that chemically they are practically the same, the 

main difference being the lower water content of the 

more strongly metamorphosed foliated amphibolite. 

The average content of the oxides is very similar; that 

of the total alkalis and TiO2 is identical. On an AFM 

plot (fig. 4), both amphibolites occupy essentially the 

same field; the foliated variety is somewhat more restricted. The close similarity in composition between 

the two varieties strongly suggests that rocks of the 

same type were involved and that the metamorphism 

was essentially isochemical except for the loss of H2O 

in the formation of the foliated amphibolite.

Relative to metabasalt and spilite of the less strongly 

metamorphosed volcanic rocks east and south of the 

amphibolite terrane (fig. 2; table 4), the SiO2 content of 

amphibolite (table 5) has a narrow range and H2O and 

CO2 are low. The amphibolites have a more constant 

total alkali content, the average K2O is lower, and the 

CaO is notably higher. A contrast in TiO2 content is 

apparent: in all the analyses of amphibolite, TiO2 is 

less than 2 percent; in three of the metavolcanic rock 

samples, TiO2 content is greater than 2 percent. The 

AFM plot (fig. 4) illustrates the somewhat narrower 

compositional range of the amphibolite relative to the 

metavolcanic rocks.

METASEDIMENTARY ROCKS

Metasedimentary rocks are interbedded with amphibolite in the Condrey Mountain quadrangle. They 

occur in a belt east of Beaver Creek and east of the 

north-northeast leg of the Klamath River between Little Humbug Creek and Beaver Creek. The belt extends 

northward from the northern boundary of the Vesa 

Bluffs pluton to the northern boundary of the quadrangle and beyond into Oregon, becoming dominant 

over amphibolite northward. These metasedimentary 

rocks underlie the large eastern body of serpentinite 

and largely overlie the foliated amphibolite, although 

there may be some intertonguing of the two units. The 

contact between the metasedimentary rocks and the 

underlying amphibolite is occupied at many places by a

thin tabular body of serpentinite. Another belt of 

metasedimentary rocks occurs east of the serpentinite 

belt, extending from Lumgrey Creek to the east fork of 

Beaver Creek, where it joins with the northern area of 

metasedimentary rocks. Some smaller lenticular 

bodies of metasedimentary rock are interbedded with 

the amphibolite.

The metasedimentary rocks are mainly siliceous 

schist and phyllite, minor quartzite, and subordinate 

amounts of marble in discontinuous thin beds and 

lenses.

The siliceous rocks are most commonly dark gray, 

less commonly light, fine grained, and finely foliated. 

Individual minerals cannot usually be recognized 

megascopically, although porphyroblastic grains of 

pink garnet are easily recognized where they occur. 

Where fine-grained biotite is abundant, it imparts a 

dusky red-purple coloration. In places where the rocks 

attain medium grain size, individual constituents are 

more easily seen.

Under the microscope, these rocks are seen to be 

crystalloblastic and finely foliated with a welldeveloped planar structure, both compositional and as 

produced by the parallel orientation of platy minerals. 

As the average grain size is commonly about 0.05, very 

fine, most of these rocks are more properly classed as 

phyllites. Medium-grained schistose varieties in which 

the grain size is 1-4 mm are much less common.

A common variety is quartz-biotite phyllite; quartzbiotite schist is less common. Some are quartzofeldspathic rocks, the feldspar generally being an untwinned sodic plagioclase; potassium feldspar may 

occur in some varieties. Other phases commonly present include muscovite, pink almandine garnet, and retrograde chlorite. An opaque mineral, probably 

magnetite, tourmaline, and apatite are ubiquitous 

minor accessories, and sphene is present in some. Almandine garnet commonly forms small porphyroblasts, conspicuous in hand specimens. Muscovite 

also forms porphyroblasts, commonly poikiloblastic, 

but typically recognizable only in thin section. A variety of quartzo-feldspathic phyllite observed in thin section but not recognized megascopically contains 

hornblende in addition to biotite.

Another common metasedimentary rock type is dark 

gray to black, commonly sooty appearing, very fine 

grained, and phyllitic. Lighter quartzose laminae and 

lenses are commonly visible. The microscope shows 

that these rocks are composed of fine polyhedral 

quartz, colorless mica, and abundant graphite. Biotite 

and fine colorless fibers of tremolite may be present in 

small amounts. Fine-grained pyrite was recognized in 

some specimens.

Fine-grained finely foliated quartzite is somewhat
16 REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF. 

TABLE 5.—Chemical and spectrographic analyses, CIPW norms, and modes of amphibolite

[Chemical analyses by rapid methods; analysts, P. L. D. Elmore, S. D. Botts, Gillison Chloe, Lowell Artis, James Kelsey, Hezekiah Smith, and J. L. Glenn. Semiquantitative spectrographic analyses: samples 3, 5, 6, 9, 12 by W B. Crandell; 2, 4 by Chris Heropoulos; 1, 7, 8, 11, 19 by Carolyn Pickett. Sample localities shown in fig. 2J

Sample

SiO2 -- —

Al2 Oa _____

Fe2O3 _____

FeO

MgO _____

CaO _____

Na2 0 __ -

K2 0 _____

H2 0- _____

H2 0+ _____

TiO2 _____

P2Os

MnO _____

C02 _______

Total--- _

Sp Gr (bulk)

Foliated amphibolite

106-62 32-63 22-63 91 63 3-64 106-64 61 61

Chemical analyses (weight percent)

.____ _____ 49.1 49.1 49.5 49.6 50.4 50.9 46.2

._ — __ 14.0 16.4 18.0 17.8 13.8 13.8 17.2

— __ 5.0 3.4 1.6 1.0 7.1 1.6 1.8

---— 8.8 8.0 6.6 8.5 9.7 8.5 8.1

__________ 5.0 4.9 4.0 5.5 6.8 7.5 6.8

_ - __ 11.3 11.3 15.2 10.7 9.7 9.7 11.3

- ___ 2.2 3.4 2.2 3.6 3.6 3.3 4.0

.70 .53 .45 1.2 .40 .63 .64

__________ .04 .05 .06 .02 .08 .09 .06

.__— ___ .93 .76 .56 .74 .92 1.2 1.4

.__ ___ - .20 1.6 .51 1.0 1.8 1.7 .99

_ ______ .11 .19 .13 .12 .16 .21 .13

_________ .24 .20 .15 .13 .20 .16 .20

_________ <.05 <05 .21 .06 .05 .05 1.3

__ _ - _ 100 100 99 100 100 99 100

________ 3.09 3.01 3.02 3.02 2.99 2.99

Nonfoliated amphibolite

74-61

48.4

20.1 

2.8 

7.6 

4.5 

10.0 

2.1 

.64 

.36 

2.3 

.63 

.21 

.17 

<.05

100 

2.97

65-62

49.2 

16.1 

1.0 

7.1 

7.7 

11.9 

2.7 

.80 

.10 

1.3 

1.3 

.16 

.12 

.16

100 

3.02

79-62

49.7

18.8 

2.4 . 

8.7 

4.6 

10.6 

2.4 

.44 

.21 

1.6 

.70 

.12 

.15 

<.05

100 

3.08

49-61

49.8 

14.8 

1.6 

9.2 

7.9 

8.7 

3.7 

.47 

.12 

1.4 

1.1 

1.0 

.18 

<.05

99

34-64

50.0 

14.8 

.71 

10.9 

6.7 

8.4 

4.2 

.14 

.04 

1.3 

1.9 

.22 

.17 

.11

100 

3.00

Semiquantitative spectrographic analyses (ppm)

B _________

Ba _________

Co— — —

Cr _________

Cu _________

Ga—— _____

Ni _________

Pb _________

Sc _________

Sn _________

Sr — — —

V _____ —

Y _________

Yb _________

Zr _________

q _--- — __

or

ab — — —

an

wo

fa ______ __

fo _________

fa _________

mt

il _________

on

CC

__ ____ 100 50 200 70 20 100 200

__ ____ 30 50 30 50 30 70 70

----- _ 200 300 50 500 200 500 700

_-- ___ 30 50 300 50 30 70 150

—— —— _ 20 15 10 15 10 10 15

— -- __ 70 100 30 100 70 20 300

__ _ 20 70 15

_________ 30 50 30 50 30 50 50

— — — — — — — 5 15 15

__ __ 300 200 500 500 100 150 1500

- — __ 300 200 200 200 200 200 300

_________ 50 50 10 30 30 30 30

_-- __ - 551333 3

-_ __ __ 150 100 15 50 50 50 70

CIPW norms (weight percent)

47 0 35

— — — 4.1 3.1 2.7 7.1 2.4 3.7 3.8

_________ 18.7 28.8 18.6 23.9 30.5 27.9 21.8

__ ____ 26.4 28.0 37.9 28.9 20.3 21.0 27.1

Q f^ f\ ^

_________ 11.6 11.3 14.7 9.6 11.0 10.6 8.3

------ 12.5 7.5 10.0 4.8 11.0 14.6 4.6

_________ 9.2 5.9 10.2 4.7 8.8 9.2 3.3

-- ______ 3.3 6.3 4.1 2.9 8.6

2.9 6.7 3.6 2.0 6.8

------- 7.3 4.9 2.3 1.5 3.0 2.3 2.6

________ 3.8 3.0 .97 1.9 3.4 3.3 1.9

— ___ __ .74 .45 .31 .28 .38 .50 .31

------- .48 .14 .11 .11 2.9

30 

300 

30 

30 

200 

15 

30 

500 

30 

30 

1000 

300 

15 

2 

50

2.2

3.8 

17.9 

43.8

2.0 

11.3 

11.0

4.1 

1.2 

.50

150 

50 

500 

50 

10 

100 

5 

30

150 

200 

15 

1.5 

50

4.7 

22.8 

29.5

11.5 

8.7 

4.7 

7.3 

4.3 

1.5 

2.5 

.38 

.36

30 

150 

30 

30 

150 

15 

30 

50 

30

1000 

300 

15 

1.5 

20

1.6

2.6 

20.3 

39.1

5.2 

11.4 

13.1

3.4 

1.3 

.28

70 

50 

500 

20 

15 

150 

15 

50

700 

300 

30 

3 

70

2.8 

31.6 

22.6

8.5 

7.4 

5.3 

8.8 

6.9 

2.3 

2.1 

.24

70 

30 

700 

50 

10 

100

30

100 

200 

20 

2 

50

.83 

35.5 

21.1

7.7 

6.0 

6.0 

7.5 

8.2 

1.0 

3.6 

.52 

.25

Modal analyses (volume percent)

Plagioclase 

Hornblende 

Clinopyroxen 

Biotite 

Sphene 

Apatite 

Opaque 

Epidote-clino 

Carbonate 

Chlorite 

Sericite

NOTE.— 106-62 

32-63 

22-63 

91-63 

3-64 

106-64 

61-61 

74-61 

65-62

ll(A4ti) 33(A37> 34(As5 ) (Ass) 35(A3s) 34(Ag8) KKAss)

QO C 1 07 CO £JA H Q

10 1 Q 91

2 1^7 ~ 9 O

tr 1 ^

1 Z. Q 9 O

izoisite 497 7 9

4 tr 4

tr 9 ^

tr °

36 

56

3

2 

tr

2

16(As2) 

60

.6

2 

20

.5

1

23

46

1 

25

4

23(An25 ) 

73

1 

<.5

3

1

38(An32 ) 

73

4 

.3

.3

South side Klamath River, NE'/i, sec. 6, T.46 N., R.8 W Condrey Mountain quadrangle. 

SE'ASW'/i, sec. 9, T. 47 N., R. 8 W Condrey Mountain quadrangle. 

North part NE'A, sec. 17, T. 47 N., R. 8 W Condrey Mountain quadrangle. 

South central part SW'/i, sec. 31, T. 48 N. R. 8 W Condrey Mountain quadrangle. 

McKinney Creek, SEV4, sec. 16, T. 46 N., R. 9 W. Condrey Mountain quadrangle. 

Beaver Creek, line between sec. 24, T. 47 N., R. 9 W, and sec. 19, T. 47 N., R. 8 W. Condrey Mtn. quadrangle. 

SEl/4, sec. 17, T. 46 N., R. 8 W. Condrey Mountain quadrangle. 

SE'4, sec. 24, T. 46 N., R. 9 W. Condrey Mountain quadrangle 

Lumgrey Creek, N part sec. 35, T. 47 N., R. 8 W Hornbrook quadrangle.

79-62 South side Klamath River, SE'/4 , sec. 1, T. 46 N., R. 8 W Hornbrook quadrangle.

49-61 SW'/iNW'/i, sec. 8, T. 46N., R. 8 W

34-64 South side Klamath River, SEV4 , sec. 4, T. 46 N., R. 8 W Condrey Mountain quadrangle.
MAFIC AND ULTRAMAFIC ROCKS 17

less common but nevertheless frequently observed. 

Fine (0.01-0.1 mm) polyhedral crystalloblastic quartz is 

the principal constituent, but colorless mica is ubiquitous and minor pale biotite may be present. Tourmaline, fine-grained opaque magnetite, and apatite are 

usually present. Stilpnomelane was observed in several thin sections of quartzite, where it commonly has a 

sheaflike radiating habit with the blades lying across 

the rock foliation. One specimen of quartzite contained 

a blue amphibole, probably crossite, and epidote in 

laminae with magnetite. An uncommon variety is calcareous quartzite composed of intergrown quartz and 

calcite accompanied by minor amounts of tremolite.

The thin beds and lenses of marble are composed 

predominantly of crystolloblastic calcite that ranges 

from fine to coarse grained. The marble is generally 

quite pure but it may contain some impurities, seldom 

more than about 10 percent. These include quartz, 

white mica, and some carbonaceous material. At one 

place an impure marble has been metamorphosed to a 

silica-carbonate rock composed of coarse diopside 

poikilitically enclosing calcite and a few blades of tremolite.

The only metasedimentary rock analyzed is a sample 

of garnetiferous quartz biotite phyllite; the analysis is 

presented in table 6.

AGE

Rocks herein assigned to the amphibolite facies in 

the Seiad Valley quadrangle have been considered 

pre-Mesozoic by Rynearson and Smith (1940); Medaris 

(1966) tentatively correlated them with the Salmon

TABLE 6.—Chemical and spectrographic analysis and CIPW norm of 

quartz-biotite phyllite

[Chemical analysis by rapid methods; analysts: P. L. D. Elmore, S.D. Botts, Gillison Chloe, 

Lowell Artis, and Hezekiah Smith. Spectrographic analysis by Carolyn Pickett. Sample 

locality shown in fig. 2]

Chemical analysis 

(weight percent)

SiO2 — _

AkOs __

FeaOs __

FeO ____

MgO-__

CaO ____

KaO ____

H2O-____

TiOa ---

MnO

CO> ___ .

Total____

Powder 

density

— 58.3

___ 15.8

____ 1.0

____ 2.0

____ 2.7

_ __ 2.4

.____ .10

_____ 1.2

____ 1.7

. — _ .93

.____ .14

.__ -100

__. _ 2.90

Semiquantitative 

spectrographic 

analysis (ppml

Ba

Be— —

Co— —

Cr ______

Cu__— _

La

Nb

Ni ______

Pb— —

Sc ______

Sr ______

V ______

Y ______

Yb— —

Zr ______

700

______ 1

— ___ 10

______ 20

^__ ___ 70

______ 20

_ —.100

______ 50

______ 30

______ 15

______ 20

100

______ 50

______ 5

CIPW 

norms 

(percent)

q ———— _

ab— ——

fs _______

il _______

16 6

__ __ 5.1

28 0

_ — — 7.3

_____ 5.0

_____ 1.5

._—— 3.2

______ 2.2

NOTE.—Quartz-plagioclase-biotite-almandine phyllite (CM-58-61), Road, south side 

Klamath River, SW'A, sec. 4, T. 46 N., R. 8 W.

and Abrams Formations of the south-central Klamath 

Mountains. Wells (1956) interpreted them to be 

metamorphic equivalents of the Upper Triassic Applegate Group, and Hotz (1967) considered them to be the 

metamorphic equivalent of the western Paleozoic and 

Triassic belt.

Isotope analysis of hornblende from two samples of 

amphibolite from the Condrey Mountain quadrangle 

yielded ages of 148 and 146 m.y (Lanphere and others, 

1968). These ages suggest metamorphism during the 

Jurassic rather than an older metamorphic episode 

represented by the Salmon and Abrams Formations.

MAFIC AND ULTRAMAFIC ROCKS

SERPENTINITE

In both the Condrey Mountain and Seiad Valley 

quadrangles, there is a direct relation between the distribution of ultramafic rocks and the occurrence of 

amphibolite-grade rocks (figs. 1,2). Bodies of ultramafic 

rocks occur widely in the areas of amphibolite but are 

absent from the areas of weakly metamorphosed volcanic and sedimentary rocks. A few small tabular 

bodies of serpentinite are infolded with the Condrey 

Mountain Schist. The association of ultramafic rocks 

with amphibolite-grade metamorphism continues 

south of Seiad Valley quadrangle in the Scott Bar 

quadrangle in the Marble Mountains (Pratt, 1964) and 

persists northward into Talent quadrangle, Oregon 

(Wells, 1956).

The amphibolite-grade rocks in the Condrey 

Mountain quadrangle are in general structurally 

below the main ultramafic body, but many small discontinuous tabular bodies of serpentinite occupy ridges 

and summits in the area. Tabular concordant bodies of 

serpentinite are interleaved with the metemorphic 

rocks and in places are found at the contact between 

metasedimentary rocks and amphibolite. In the Seiad 

Valley quadrangle, the ultramafic rocks apparently 

occur beneath as well as above the higher grade rocks 

(Medaris, 1966; Barrows, 1969). A characteristic structural feature of the ultramafic bodies throughout the 

area of high-grade metamorphism is their concordance 

with the large-scale planar structure of the metamorphic rocks. Discordant dikelike bodies are very rare. 

The typically curvilinear form of many of the serpentinite bodies, especially in the Condrey Mountain quadrangle, is produced by erosion of gently dipping thin 

concordant serpentinite bodies.

The ultramafic rocks include harzburgite and dunite 

and their serpentinized derivatives. Surbordinate 

amounts of wehrlite and Iherzolite occur in an ultramafic complex in the Tom Martin Creek area of
18 REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

southeastern Seiad Valley and northeastern Scott Bar 

quadrangles (Barrows, 1969). Harzburgite and its serpentinized equivalent are by far the most common ultramafic rocks. In the Condrey Mountain quadrangle, 

probably more than 90 percent of the ultramafic rocks 

are serpentinized, even the large body north of the 

Klamath River in the eastern part of the quadrangle. 

Much of the serpentinite is not sheared, even the 

smaller bodies, except locally along faults and some 

contacts. In many places, however, thin tabular bodies 

of serpentinite in the metamorphic terrane are converted to schistose talc-actinolite rock.

Serpentinite in the Condrey Mountain quadrangle 

has not been extensively studied petrographically; several specimens have been examined in thin section and 

some X-ray studies were made. Primary minerals of 

the ultramafic rock are seldom preserved; in all the 

specimens examined, only relict olivine was seen. 

Some of the serpentinite is composed of a web of 

chrysotile-lizardite mixture, minor brucite, and very 

minor amounts of olivine; some consists entirely of antigorite. Most commonly the serpentinite is composed 

of antigorite with minor amounts of chrysotilelizardite. Magnetite, formed by the release of iron from 

olivine during serpentinization, is a common secondary 

mineral in all the serpentinite. A carbonate mineral, 

probably magnesite, is present in small amounts in 

some specimens, and talc and tremolite may be present 

in minor amounts. Antigorite is the metamorphic 

equivalent of lizardite, chrysotile, and brucite (Page, 

1966; Trommsdorff and Evans, 1974). Presumably 

these antigoritic serpentinites were formed by the 

metamorphism of previously serpentinized ultramafic 

rocks. It is not certain if the metamorphic episode was 

the same as that which produced rocks of the amphibolite facies that enclose the serpentinite.

At many places in a narrow zone adjacent to the 

contact with the enclosing metamorphic rocks, the serpentinite has been converted to fine-grained tremolite 

rock and talc-tremolite schist. One of the largest of 

these bodies is south of the Klamath River east of Little Humbug Creek. In this body, the rock is schistose 

and very fine grained and is composed of variable 

amounts of talc, tremolite, and magnesite. Within it 

are fine-grained unsheared nodular masses of talc and 

magnesite. When seen in thin section, serpentinite 

that megascopically appears to be unaltered adjacent 

to talc-carbonate rock shows considerable replacement 

by talc and carbonate.

The alteration of serpentinite to talc-carbonate rock 

(table 8) is accomplished almost entirely by carbondioxide metasomatism accompanied by loss of water 

(Chidester, 1962). Talc-tremolite schist (table 7) is possibly formed from serpentinite by the addition of

TABLE 7.—Chemical analyses, in weight percent, of serpentinite and 

magnesian schist

[Rapid rock analyses by P. L. D. Elmore, Samuel Botts, and Lowell Artis. 

Localities shown in fig. 2]

Sample No. 39-61 83-64 82-64 76-64

SiO2 -

AbOa

FeaOs

FeO _ MgOCaO -

Na2O

K2O _ H20-

H2O+

TiO2 _

P2Q5 -

MnO_ 

CO2 -

37.5

.46

3.1

2.7

39.2

.12

.12

.00

.66

11.9

.05

.02

.22

3.5

38.5 

.40 

4.2

2.6 

35.3

2.7 

.12 

.00 

.72 

11.4 

.03 

.02 

.17

3.8

17.7

.40

.89

5.1

39.2

.22

.05

.00

.03

1.5

.03

.02

.22

34.8

56.2

2.8

.81

4.9

23.5

7.1

.30

.11

.14

3.7

.07

.04

.17

.09

Total_ 

Sp gr (bulk)

100

2.66

100

2.53

100

2.98

100

2.90

NOTE.—39-61 Serpentinite with minor magnesite, sec. 10, T. 46 N, R. 8 W

83-64 Serpentinite with relict olivine, magnetite, and minor talc, S. Edge sec. 8,

T. 46 N., R. 8 W

82-64 Unsheared talc-carbonate rock S. edge sec. 8, T. 46 N., R. 8 W 

76-64 Tremolite-talc schist, extreme NE'A sec. 7, T. 46 N., R. 8 W

TABLE 8.—Chemical analyses, in weight percent, illustrating 

metasomatic reaction at serpentinite-amphibolite contact

[Rapid rock analysis by P. L. D. Elmore, Samuel Botts, and Lowell Artis. 

Sample localities shown in fig. 2]

Clinozoisite 

Amphibolite metasomatite Serpentinite

Sample no.

124-64 125-64 126-64 122-64 121-64

SiC-2 . 

AkOs . 

Fe2O3 . 

FeO ... 

MgO . 

CaO. 

Na2O _

HzO- . 

H2O+. 

TiCb . 

P2O5 

MnO 

COa —

Total ___ 

Sp gr (bulk)

57.8 

13.4 

.63 

6.7 

6.0 

7.6 

5.6 

.35 

.09 

.81 

.64 

.07 

.15 

<.05

55.4

12.6

.86

8.4

6.9

7.6

4.5

.10

.18

1.7

1.1

.11

.21

.09

36.6

21.8

3.6

10.2

6.9

13.7

.25

.08

.11

4.5

1.3

.04

.21

.11

42.6

23.2

3.5

3.7

2.5

21.9

.25

.09

.09

1.4

.33

09

.19

.08

40.7

1.6

4.6

2.7

37.8

.09

.05

.00

.41

11.6

.02

.02

.11

.12

100

2.88

100

2.89

100

322

100

2.72

NOTE.—All specimens located in SW'4 sec. 19, T. 47 N., R. 8 W

124-64 Retrograded amphibolite 2m from contact (albite-hornblende-sphene).

125-64 Retrograded amphibolite-1.5 cm from contact (albite-chloriteactinolite-sphene).

126-64 Clinozoisite metasomatite at contact (clinozoisite-chlorite-tremolitesphene). 

122-64 Clinozoisite metasomatite at contact (clinozoisite-minor chloritetremolite-sphene-trace of garnet). 

121-64 Serpentinite 1m above contact.

alumina and silica from the adjacent schist or amphibolite accompanied by loss of magnesia and water 

from the ultramafic rock. The metamorphism that produced the talc-tremolite rocks probably postdates the 

earlier regional metamorphism. At many places it ap-
MAFIC AND ULTRAMAFIC ROCKS

pears to be related to faulting between the higher 

grade country rocks and serpentinite.

Contact effects of the ultramafic rocks are not apparent in most places. Where such effects have been observed, they are generally not extensive. Adjacent to 

an ultramafic complex in Seiad Valley quadrangle, 

amphibolite is recrystallized to pyroxene granulite and 

hornblende-pyroxene granulite; yet such contact 

metamorphic effects are absent from another peridotite 

body in the same area (Medaris, 1966). One very localized occurrence of pyroxene granulite less than a 

meter in width occurs adjacent to a thin serpentinite 

body in northern Condrey Mountain quadrangle. 

Barrows (1969) noted that greenschist facies rocks 

have been converted to amphibolite next to an ultramafic complex in southeastern Seiad Valley quadrangle.

Another kind of metamorphic phenomenon was observed in the Condrey Mountain quadrangle where 

amphibolite adjacent to a thin concordant serpentinite 

body has retrograded to a greenschist facies albitechlorite-actinolite rock as much as 2 m from the contact, and a clinozoisite-tremolite rock has been formed 

at the contact. This is a postmetamorphic metasomatic 

reaction involving movement of A12 O3 and CaO from 

the amphibolite toward the contact with concomitant 

loss of alkalis and some SiO2 in the same direction. The 

main contribution from the serpentinite side is a slight 

increase in H2O (table 8; fig. 8).

GABBRO AND PYROXENITE

Numerous small bodies of gabbro and pyroxenite are 

associated with serpentinite in an area of amphibolite 

facies rocks north of Klamath River and east of Beaver 

Creek. These mafic bodies have forms similar to the 

serpentinite bodies. They occur with the metasedimentary rocks and amphibolite and as apparently 

transgressive bodies in serpentinite, although they 

may be in part interlayered with the serpentinite. 

These bodies are composed of both gabbro and pyroxenite; gabbro is more abundant, the pyroxenite generally occuring as localized bodies with indefinite 

boundaries within the gabbro. More specifically, the 

rocks are quartz-bearing hornblende gabbro with subordinate pyroxene and hornblende clinopyroxenite.

The gabbro is a mesocratic to melanocratic (CI 

43-73) fine- to medium-grained rock composed predominantly of hornblende and plagioclase. 

Clinopyroxene may or may not be present as a minor 

constituent; most commonly, it occurs as relicts enclosed by hornblende. The plagioclase is labradorite 

(An60), and may be zoned as sodic as andesine (An40 ); in 

many specimens, the plagioclase is gray and sausAmphibolite

Clinozoisite 

Metasomatite Serpentinite

10.0

IZ

UJ Iz 

o 

u 

z 

o I- 

< 

o 1.0

- Na"— -

0.1

.05

Si .

V

v \ \

Al -

\

^\

Na,Ca x* 

•to 0.0 ,

124 - 64 125-64 126-64 122-64 121-64

SAMPLE NUMBER

FIGURE 8.—Change in cation content of amphibolite at contact with

serpentinite.

suritized; in some, the plagioclase may be mostly replaced by clinozoisite. Small quantities of biotite occur 

in some specimens. Some specimens contain as much 

as 8 percent quartz. Besides clinozoisite, late-stage alteration products include small amounts of chlorite 

and sericite. A somewhat unusual variety of gabbro in 

the Condrey Mountain quadrangle contains as much as 

15 percent clinopyroxene, relict hypersthene amounting to 0.5 percent, and 13 percent biotite in addition to 

hornblende, plagioclase, and a trace of quartz. Two 

analyses of quartz-bearing gabbro are presented in 

table 9, Nos. 52-63 and 59-63. One of them (59-63), a 

porphyritic gabbro intermediate between gabbro and 

hornblende pyroxenite, is nearly 60 percent amphibole, 

nearly 10 percent clinopyroxene, and nearly 20 percent
20 REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

TABLE 9.—Chemical analyses, CIPW norms, and modes of gabbro, 

pyroxenite, and pyroxene quartz diorite

[Chemical analyses by rapid methods; analysts: P. L. D. Elmore, S. D. Botts, Gillison Chloe, 

Lowell Artis, and Hezekiah Smith. Sample localities shown in fig. 2]

Chemical analyses (weight percent)

Sample No.

SiO2 —„-_-___

AkOs __ _ _ _

FezOs ____ _ .

RsO— - — — —

MgO _ __ .

CaO___ __ _______

NazO ____________

KzO— -— — — _

IfcO-— — — —

H2O+— — — —

Ti02 ____________

P20s ____________

MnO ____________

Total

27-63

.— ___ 49.5

— ____ 13.8

_______ 1.0

— — _ 9.8

_______ 8.6

. __ 11.4

. — ___ 1.6

_______ .44

_______ .12

. __ _ 1.2

_______ 1.4

_______ .60

______ .21

100

Sp. gr. (bulkl ___ 2.95

q ________________

ab ______________

di-fs______. _______

hy-fs ____________

ol-fb— — — — -

ol-fa-_— — — —

il ________________

CIPW

_______ 1.8

. _ __ 2.6

_______ 13.5

.-____ 29.2

_______ 9.8

5.5

______ 3.9

______ 15.9

. _ _ _ 11.3

.._____ 1.44

_______ 2.6

._ __ 1.4

52-63 59-63

50.9 52.1 

14.8 10.7 

2.5 1.2 

9.0 6.4 

6.6 11.7 

9.5 12.6 

2.4 1.7 

1.2 .82 

.23 .34 

1.3 1.1 

1.2 .78 

.35 .16 

.21 .15

100 100 

2.93 2.98

norms (percent)

1.3 —— 

7.1 4.8 

20.3 14.4 

26.1 19.1 

7.8 17.7 

4.2 12.2 

3.3 4.1 

12.2 16.4 

9.5 5.5 

__ 4

—— '.I 

3.6 1.7 

2.3 1.5 

.83 .38

53-63

53.6 

4.6 

.92 

5.7 

15.5 

15.5 

.73 

.58 

.20 

.76 

.30 

.16 

.15

99 

2.98

1.7 

3.4 

6.2 

7.6 

28.5 

20.8 

5.1 

17.8 

4.4

1.3 

.57 

.38

26-63

57.9 

14.9 

1.2 

6.2 

6.1 

7.9 

2.5 

1.2 

.16 

.52 

.84 

.34 

.13

100

11.6 

7.1 

21.2 

25.9 

4.6 

2.7 

1.8 

12.5 

8.2

1.7 

1.6 

.80

Modal analyses (volume percent)

Biotite _ _ .

Chlorite

._-_____ 0.3

________ 40

________ 48

_______ 4

.6

. ___ .7

. ___ 6

. _ tr

3 2 

31 19 

48 58 

— 9

3 .3 

.3 tr 

3 1 

10 7 

2 3

7 

60 

32

.2 

.4

12 

49 

4 

15 

8 

16

.1 

.1

NOTE.—27-63 Hornblende-pyroxene gabbro south part SW'/_, sec. 3, T47 N., R.8 W

52-63 Altered hornblende-quartz bearing gabbro west central part, SW/4 sec.

34, T. 48 N., R.8 W 

59-63 Porphyritic hornblende-pyroxene-quartz bearing gabbro SW part NEV.

sec. 28 T. 48 N. R. 8 W

53-63 Hornblende clinopyroxenite dike N part SE'4 sec. 33, T. 48 N., R. 8 W 

26-63 Biotite-hornblende-pyroxene quartz diorite SW1/. SEV4 sec. 3, T 47 N, R. 8 W

plagioclase. An analysis of hornblende gabbro with a 

trace of quartz and a small amount of clinopyroxene is 

given in table 9, No. 27-63.

Hornblende pyroxenite is commonly a grayish-green 

medium-grained hypidiomorphic to allotriomorphic 

granular rock, locally with a subporphyritic "spotted" 

appearance. It has a color index above 90 and is mainly 

composed of hornblende and clinopyroxene with less 

than 10 percent plagioclase. The plagioclase is consistently saussuritized. Hornblende commonly amounts 

to about 60 percent and encloses—poikilitically in 

some specimens—colorless clinopyroxene. Typically, 

the hornblende is pleochroic from pale brown to colorless; some is pale green. Sphene and apatite are present in amounts less than 1 percent. Chlorite, which 

amounts to less than 1 percent, partly replaces 

hornblende. Analysis of a sample of hornblende 

clinopyroxenite (53-63) is given in table 9.

AGE

There is no direct evidence for the age of the serpentinite except for its occurrence as pendants in the Ashland and the Vesa Bluffs granitic plutons; the pendants 

indicate that emplacement of the ultramafic rocks occurred before intrusion and crystallization of the 

granitic rocks in Late Jurassic time. The close association of serpentinite with rocks of the amphibolite facies 

suggests that both ultramafic and metemorphic rocks 

were involved in the episode of deformation and 

metamorphism. The Seiad ultramafic complex of 

Medaris (1966) and the Tom Martin ultramafic complex 

of Barrows (1969) were emplaced tectonically at high 

temperatures during a period of deformation and regional metamorphism. In the Condrey Mountain quadrangle, the presence of numerous small discontinuous 

bodies of serpentinite scattered throughout the terrane 

of amphibolite rock facies is suggestive of a melange. 

The association of serpentinite with amphibolite derived from basaltic rocks accompanied in places by 

quartzite that may represent original chert suggests 

that these rocks may be part of a dismembered ophiolite suite. This concept is strengthened by the occurrence in the same terrane of gabbro and pyroxenite 

bodies similar in form and habit to the serpentinite. A 

Permian and Triassic ophiolite has been recognized in 

the Preston Peak area (Snoke, 1977), approximately 65 

km west of the Condrey Mountain quadrangle. This 

ophiolite has been displaced and now rests in thrust 

fault contact on metasedimentary rocks of the Late 

Jurassic Galice Formation. Indirect evidence suggests 

that the ultramafic rocks in the Condrey Mountain 

quadrangle belong to an ophiolite of Permian and (or) 

Triassic age. In the Seiad Valley quadrangle, the ultramafic rocks may represent peridotite emplaced as 

hot diapirs.

PLUTONIC ROCKS

Several bodies of plutonic rock are intrusive into 

rocks of the amphibolite terrane and the lower grade 

metavolcanic and metasedimentary rocks. None intrude the Condrey Mountain Schist.

PYROXENE QUARTZ DIORITE AND GRANODIORITE

An irregular body of plutonic rock ranging from 

quartz diorite to granodiorite in which pyroxene is the 

most abundant characterizing accessory crops out in 

the northeast Condrey Mountain quadrangle southwest of the Ashland pluton. Its extent eastward in the 

Hornbrook quadrangle is unknown. A body of 

hornblende gabbro lies between it and the Ashland 

pluton, which intrudes the gabbro. Although the gab-
PLUTONIC ROCKS 21

bro and pyroxene quartz diorite are in contact, their 

relative ages are uncertain. Field relations suggest 

that the bodies are gradational into one another.

The rock is mesocratic (C.I. 30-40) fine grained with 

an hypidiomorphic granular texture. Unlike the gabbro and hornblende pyroxenite associated with ultramafic rocks and amphibolite, the plagioclase is clear, 

unaltered, and zoned normally with cores of labradorite (Ango) and rims of andesine (Ari4o). The rock contains about 12 percent quartz and may have as much as 

10-13 percent potassium feldspar, although the 

analyzed sample did not contain any. Subhedral 

pyroxene is the chief mafic mineral, amounting to 12 

percent to nearly 19 percent in the specimens examined 

microscopically. The pyroxene is colorless augite and 

hypersthene. Green hornblende (4 to approximately 12 

percent) forms reaction rims on the pyroxene; biotite 

(approximately 7-16 percent), commonly with a large 

central grain of magnetite, occurs as isolated 

irregular-shaped flakes or as partial replacements of 

hornblende. Apatite, sphene, and black opaques are 

common minor accessories. An analysis of a specimen 

of hornblende biotite pyroxene quartz diorite (No. 

26-63) is given in table 9.

A potassium-argon age of 152 m.y. has been obtained 

(M. A. Lanphere, oral commun., 1970) on biotite from a 

sample of the pyroxene quartz diorite represented by 

analysis 5, table 9. This date was obtained by rerunning a sample that earlier (Lanphere and others, 1968, 

table 4, No. 31) yielded an age of 133 m.y.

GRANITIC ROCKS

Granitic rocks in the Condrey Mountain and 

Hornbrook quadrangles include the Vesa Bluffs pluton 

in southern Condrey Mountain and southwestern 

Hornbrook quadrangles and the Ashland pluton in 

northeastern Condrey Mountain and the northwestern 

Hornbrook quadrangles, California and the greater 

part in Oregon. The Vesa Bluffs body is relatively 

small, 19 km2 , whereas the Ashland pluton—400 km2 , 

has batholithic proportions. The plutonic rocks include 

hornblende gabbro and diorite, quartz diorite, 

granodiorite, and minor quantities of alaskitic rock. 

Many plutons of similar size and composition occur 

elsewhere in the Klamath Mountains (Hotz, 1971).

The Vesa Bluffs pluton is a narrow elongate pluton 

oriented east-west in southern Condrey Mountain and 

southwestern Hornbrook quadrangles. In the Condrey 

Mountain quadrangle, the pluton is a southwarddipping tabular body with amphibolite facies rocks in 

the footwall and a roof of weakly metamorphosed volcanic and sedimentary rocks. In the Hornbrook quadrangle, it loses its apparently tabular form and becomes a steep-sided trangressive body. The western 

three-fourths of the pluton is composed predominantly 

of hornblende diorite and gabbro with local areas of 

quartz diorite. The eastern one-fourth of the pluton is 

quartz diorite. A small body of alaskite of quartz 

monzonite composition intrudes the quartz diorite on 

Craggy Mountain (fig. 2). A small roof pendant of serpentinite occurs in the western end of the pluton west 

of Dona Creek. The boundary between the predominantly dioritic western three-fourths of the pluton and 

the eastern quartz diorite cannot be determined, as exposures are poor; it may be either a gradational or an 

intrusive contact.

The Ashland pluton crops out mainly in Oregon. It is 

not well known in California and has been studied 

mainly in reconnaissance in Oregon (Wells, 1956). It is 

a steep-sided transgressive body that appears to be 

composed mainly of quartz diorite and granodiorite. In 

northeastern Condrey Mountain quadrangle, it intrudes hornblende gabbro and has small roof pendants 

of serpentinite and hornblende pyroxenite.

Gabbro and diorite in the Condrey Mountain quadrangle are mesocratic (C.I. 40-60) granitic rocks that 

contain plagioclase. The plagioclase is commonly so 

saussuritized that its composition is uncertain; therefore classification of the rock as gabbro or as diorite is 

doubtful. The tendency is to classify the darker rocks 

as gabbro.

The principal mafic mineral is green hornblende that 

may contain traces of relict clinopyroxene. Minor 

amounts of chlorite partly replace the hornblende, and 

some epidote-clinozoisite is generally present. Minor 

accessories include sphene, apatite, and an opaque 

oxide, probably ilmenite-magnetite. Quartz is not generally a constituent but it may be present in minor 

amounts, as much as 5-6 percent. Analyses of three 

samples of gabbro, diorite, and quartz-bearing gabbro 

from the Vesa Bluffs pluton (Nos. 20-62, 99-64,100-64) 

are given in table 10.

The quartz diorite is leucocratic to mesocratic (C.I. 

11-35) granite rock. In contrast to the gabbro and diorite, plagioclase is generally only slightly altered and 

shows normal compositional zoning from as calcic as 

labradorite in the central parts of crystals to rims of 

sodic andesine. Small amounts of potassium feldspar 

are recognizable in some specimens. Quartz is an essential constituent. Green hornblende is commonly 

the predominant mafic mineral; biotite generally occurs in lesser amounts. Many specimens, however, contain biotite and hornblende in nearly equal amounts, 

and in some, biotite is the more abundant. Other minerals present in minor amounts include apatite, chlorite, and a member of the epidote-clinozoisite group. 

Black opaque minerals constitute less than 0.5 percent;
22 REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

TABLE 10.—Chemical and spectrographic analyses, CIPW norms, and modes of rocks from the Vesa Bluffs and Ashland plutons

[Chemical analyses: No. 116-38, conventional analysis by F. S. Grimaldi; all others, rapid analyses by P. L. D. Elmore, S. D. Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith. 

Semiquantitative spectrographic analyses: samples 2, 3,10 by W B. Crandell; 4, 6, 7, 8, 9 by R. E. Mays; 11,11 by Carolyn Pickett; 8 by Chris Heropoulos. Sample localities shown in 

fig. 2]

Sample No. 20-62 99-64 100-64 28-60 116-38

Chemical analyses

SiCfe ________________

AkOs — _ ________

FeaOs _ _ _

FeO

MgO— — _ _________

CaO _____________ ___

NazO- __ __________

KaO ________________

HaO- ______________

H2O+ ___ __ _______

TiCfe ________________

p2 a ________________ MnO— _ __________

CO2 ________________

Total _ _ _.__

Sp. Gr. (bulk) __

46.5

18.0

2.9

8.5

6.2

10.9

2.7

.50

.07

2.2

.94

.36

.21

<.05

100

3.02

50.9

18.7

1.4

6.3

4.5

10.5

3.5

1.0

.19

1.6

.73

.18

.14

.19

100

2.94

53.9

16.2

2.4

5.7

5.4

10.0

2.7

1.1

.09

1.3

.65

.15

.16

.11

100

2.92

55.7

16.5

3.9

4.6

5.2

8.7

2.0

.89

.13

1.7

.62

.30

.16

<.05

100

2.93

57.85

17.57

1.98

5.22

3.42

7.06

3.27

1.29

.07

.33

1.04

.36

.12

.14

99.72 —

118-63 109-63

(weight percent)

60.0 61.1

17.2 17.2

1.7 2.6

4.5 3.4

2.9 2.3

6.8 7.5

2.8 3.2

1.4 1.1

.19 .10

1.7 .95

.39 .29

.23 .35

.15 .14

<.05 <.05

100 100

2.77 2.78

7-67

62.3

16.3

1.3

3.2

3.3

5.4

3.5

2.9

.05

.54

.66

.35

.13

<.05

100

2.77

77-63

63.3

15.8

1.3

3.0

3.4

6.1

3.9

1.4

.20

1.0

.48

.26

.10

.17

100

2.75

89-64

63.5

16.4

.79

4.1

2.0

4.6

3.9

2.5

.12

1.2

.49

.14

.15

.05

100

2.73

29-60

64.2

16.9

2.4

2.7

1.9

6.2

2.8

1.6

.11

1.0

.33

.17

.14

<.05

100

2.79

108-63

74.6

14.6

.53

.16

.26

.10

4.7

4.6

.12

.65

.04

.01

.03

<.05

100

2.56

Semiquantitative spectrographic analyses (ppm)

B

Ba __________________

Be __________________

Co __________________

Cr __________________

Cu _________________

Ga__________________

La __________________

Nb__________________

Ni _______ _ ________

Pb __________________

Sc __________________

Sr __________________

V __________________

Y

Yb __________________

Zr __________________

30

200 —

30

20

700

15 —

__

30

20

30

1000

300

30

3

30

__

300 —

30

70

15

15 —

_

50 —

30

700

200

7 —

30

_

500

_

20

30

100

10

_

_

50

_

30

500

200

10

1

10

_

500

_

20

30

200

15

_

_

15

_

20

1000

200

15

2

70

_

_

_

_

_

_

_

_

_

_

_

_

—

—

_

_

—

_ _

700 500

_ _

15 15

7 17

70 70

15 15

_ _

_ _

5 5

_ _

15 15

1500 1000

150 150

15 15

2 2

50 70

20

1000

_

20

70

50

15

50

10

50

20

20

1000

100

20

2

15

15

700 —

10

30

50

15 —

_

10

20

15

1500

100

20

2

100

_

1000

—

10

20

10

10 —

_

<30

—

10

500

100

7 —

70

<30

1000 —

15

20

30

15 —

—

—

30

15

1500

150

20

2

100

_

100

3 —

2

5

15 —

—

—

—

—

70 —

—

1

100

CIPW norms (percent)

q

or

ab __________________

an

di-wo

di-en _

di-fs ________________

hy-en

hy-fs

fo __________________

fa __________________

mt

hm _

il __________________

ap _____________ ____

cc _

3.0

22.8

35.5

6.8

3.7

2.9

1.9

1.5

6.9

5.9

4.2

_

1.8

.85 —

5.9

29.6

32.4

7.2

3.8

3.2

3.3

2.8

2.9

2.7

2.0

_

1.4

.43

.43

5.8

6.5

22.8

28.8

8.0

4.8

2.8

8.6

5.0

_

_

3.5

_

1.2

.36

.25

14.7

5.2

16.9

33.4

3.2

2.2

.8

10.7

3.7

_

_

5.6

_

1.2

.71 —

12.63

7.62

27.67

29.45

.98

.53

.41

7.98

6.05

_

_

2.87

_

1.98

.85

.32

17.6 19.2

8.3 6.5

23.7 27.1

30.2 29.3

.8 2.3

.4 1.3

.4 .9

6.7 4.4

6.1 3.0

_ _

_ —

2.5 3.8

_ _

.74 .55

.54 .83

— —

14.5

17.2

29.6

20.2

1.8

1.1

.55

7.1

3.4 —

—

1.9

_

1.2

.83 —

17.9

8.3

33.0

21.5

2.5

1.6

.73

6.8

3.1 —

—

1.9

_

.91

.62

.39

16.4

14.8

33.0

19.9

.7

.3

.4

4.7

5.9 —

—

1.1

_

.93

.33

.11

25.1

9.5

23.7

28.8

.35

.21

.12

4.5

2.6 —

—

3.5

_

.63

.40 —

29.1

1.7

27.2

39.8

.4 —

—

_

.6 —

—

—

.5

.2

.08

.02 —

sphene is sparse or absent, and typically zircon is present in traces. Analyses of representative samples of 

quartz diorite from the Vesa Bluffs and Ashland plutons are given in table 10.

Granodiorite is not so abundant as quartz diorite and 

in most places has not been mapped separately. In general, it is lighter than quartz diorite; its color index is

less than 10 although it may have as much as 25 percent mafic constituents. Potassium feldspar, including 

both orthoclase and microcline, amounts to about 

13-20 percent in the specimens studied. Hornblende, 

biotite, and to a minor extent muscovite, are the mafic 

minerals. Biotite may be predominant; in some 

specimens, it is the only mafic mineral except for minor
STRUCTURAL AND METAMORPHIC RELATIONS 23

TABLE 10.—Chemical and spectrographic analyses, CIPW norms, and modes of rocks from the Vesa Bluffs and Ashland plutons—Continued

Sample No.

Quartz

K-feldspar

Biotite _ __

Muscovite (sericite) __ 

Sphene

Zircon

Opaques

Chlorite __ __

Epidote-clinozoisite __

20-62

0.7 

441

46

.6 

.5

.6 

3.2

4.2

99-64

1

591 

37

.5 

.5

.1 

2.4 

.8

100-64

6.5

43 

43

.2

.3 

6.5

28-60 116-38

Modal analyses

12 14

32 56 

25 26 

— 4

.3 .5 

— tr 

.3 .1 

10 — 

21 —

118-63 109-63 7-67 77-63 89-64 29-60 108-63

(volume percent)

16 

1.3 

56 

14 

8

tr

tr 

3 

2.3

17

57 

9 

11 

1.3

.3

.8

13 

16 

48 

13 

9

.3 

.3 

tr 

tr 

.1

18 

tr 

54 

14 

10

.1 

tr 

tr 

tr 

.9 

1

22 

5 

52 

13

7

tr 

tr 

.1 

.3 

.6

24

1

51 

4 

13 

.5

.2 

.5

33 

24 

40

2.4

'Plagioclase completely saussuritized.

NOTE.— 20-62 Hornblende gabbro, Grouse Greek, center, west edge sec. 31, T.46 N., R.8 W, Condrey Mtn. quadrangle.

99-64 Hornblende diorite, Barkhouse Creek, eastern part sec. 26, T.46 N., R.9 W, Condrey Mtn. quadrangle.

100-64 Hornblende quartz-bearing gabbro, Barkhouse Creek, SEV4, sec. 6, T.46 N., R.8 W, Condrey Mtn. quadrangle.

28-60 Altered hornblende quartz diorite, Rider Gulch, eastern part sec. 34, T.46 N., R.8 W., Condrey Mtn. quandrangle. 

116-38 Biotite hornblende quartz diorite, West Branch Long John Creek, Talent quadrangle, Oregon (not shown in fig. 2). 

118-63 Biotite hornblende quartz diorite, Dona Creek, S. part sec. 20, T.46 N., R.9 W, Condrey Mtn. quadrangle. 

109-63 Hornblende biotite quartz diorite, The Craggies, NEV4, sec. 27, T.46 N., R.8 W, Condrey Mtn. quadrangle. 

7-67 Biotite hornblende granodiorite, center, SEV4, sec. 18, T.39 S., R.I E., Ashland quad., Oregon (not shown in fig. 2).

77-63 Biotite hornblende quartz diorite, Hungry Creek, NE'/i, sec. 33, T.48 N., R.8 W, Condrey Mtn. quadrangle.

89-64 Biotite hornblende quartz diorite, Line between sees. 5 and 8, T.45 N., R.9 W, Condrey Mtn. quadrangle.

29-60 Hornblende biotite quartz diorite, Vesa Bluffs, W part sec. 22, T.46 N., R.8 W, Condrey Mtn. quadrangle. 

108-63 Alaskite of quartz monzonite composition, E. part sec. 27, T.46 N., R.8 W, Condrey Mtn. quadrangle.

secondary chlorite. The rock contains apatite and 

opaque metallic minerals in minor amounts and traces 

of zircon. Analysis of a sample of granodiorite from the 

Ashland pluton is given in table 10, No. 7-67.

Alaskite of quartz monzonite composition on Craggy 

Mountain in the eastern part of Vesa Bluffs pluton has 

no hornblende or biotite. It is composed principally of 

plagioclase, quartz, and potassium feldspar with minor 

amounts of white mica. An analysis of a sample of the 

rock from Craggy Mountalin is given in table 10, No. 

108-63.

AGE

Potassium-argon ages have been determined for the 

Vesa Bluffs and Ashland plutons. For the Ashland pluton, concordant ages of 146 and 147 ± 4 m.y. were obtained on hornblende and biotite, respectively, from a 

sample in Condrey Mountain quadrangle (Lanphere 

and others, 1968); two determinations on hornblende 

from a sample of quartz diorite collected near Ashland, 

Ore., yielded ages of 160 ±5 m.y. and 166 ±5 m.y, 

whereas biotite from the same sample gave an age of 

144 ±4 m.y. (Hotz, 1971, fig. 10). Discordant ages of 

160±5 m.y. and 146±4 m.y. were obtained on 

hornblende and biotite, respectively, from a sample of 

quartz diorite from the Vesa Bluffs pluton (Lanphere 

and others, 1968). In all the mineral pairs except one 

from the Ashland pluton, hornblende,with a spread of 

13 m.y, yields a somewhat older age than biotite; biotite is younger and has a range of only 3 m.y. In view of

the narrower range of the biotite ages and the concordance with hornblende in one sample, the biotite ages 

of 144-147 m.y, which correspond to Late Jurassic 

time, are assumed to be the approximate age of crystallization of the pluton.

STRUCTURAL AND METAMORPHIC RELATIONS

The Condrey Mountain Schist is structurally beneath the higher grade amphibolite and associated 

metasedimentary rocks. The contact is undoubtedly a 

thrust fault, probably modified in many places by 

high-angle faulting. In some places a thin gently dipping tabular body of serpentinite occurs at the contact, 

and at a few places far out in the central part of the 

quartz-mica schist terrane, a few small thin sheets of 

serpentinite overlie and are infolded with the schist 

(fig. 1); these sheets are probably remnants of more 

continuous serpentinite that may have occurred between the schist and the overriding amphibolite terrane.

The structural and metamorphic history must explain the absence of ultramafic and plutonic rocks in 

the window of Condrey Mountain Schist and the concentration of these rocks in the amphibolite facies zone. 

The absence of serpentinite, except for the few remnants infolded with the schist, is strong evidence that 

the thrusting that juxtaposed these two terranes was 

later than the episode of ultramafic rock emplacement. 

These serpentinite remnants, possibly at or near the
24 REGIONAL METAMORPHISM, CONDREY MOUNTAIN QUADRANGLE, KLAMATH MOUNTAINS, CALIF.

contact with the underlying amphibolite, may have 

been emplaced cold at the time of thrusting.

In general, the change from rocks of the amphibolite 

facies to overlying metavolcanic and metasedimentary 

rocks of the greenschist facies to the east and west is 

less well defined. Westward, in the Seiad Valley quadrangle and the southern part of the Ruch quadrangle, 

Oregon (fig. 1), metasedimentary rocks belonging to 

the amphibolite facies are overlain by metavolcanic 

rocks, probably along a fault (N. J Page, written commun., 1976). In the southern Condrey Mountain quadrangle, faults and serpentinite bodies separate the two 

terranes, and the roughly tabular southward-dipping 

Vesa Bluffs pluton occurs between amphibolite facies 

rocks and greenschist facies metavolcanic and 

metasedimentary rocks. No well-defined contact between nonfoliated metavolcanic rocks of the amphibolite facies and those of the greenschist facies has been 

recognized east of the large eastern ultramafic body in 

eastern Condrey Mountain quadrangle. The nonfoliated amphibolite that occurs east of the serpentinite is succeeded upward, apparently conformably, by 

fine-grained greenschist facies metasedimentary and 

metavolcanic rocks of the western Paleozoic and Triassic belt. The biotite isograd in metasedimentary rock is 

approximately 3 km east of the main serpentinite body. 

There appears to be a gradual decrease in the intensity 

of metamorphism stratigraphically upward east of the 

easternmost ultramafic bodies.

The significant break in the Condrey Mountain 

quadrangle between the terranes of foliated and nonfoliated amphibolite is north of the Vesa Bluffs pluton, 

where a gently dipping contact interpreted as a fault 

separates them. In Little Humbug Creek, the fault is 

partly occupied by gently dipping tabular serpentinite 

bodies. The western part of the fault is intersected by 

the Vesa Bluffs pluton. The northern continuation of 

the fault may be the nearly east-west steeply dipping 

fault south of and parallel to the Klamath River east of 

the mouth of Beaver Creek. North of the Klamath 

River, the identity of the break becomes lost beyond the 

place where the east-west-trending fault is intersected 

by the large eastern serpentinite body. This break may 

have been the original discontinuity between foliated 

amphibolite and the metavolcanic-metasedimentary 

rock terrane.

The association of ultramafic rocks and amphibolite 

is fairly common worldwide. Some geologists have ascribed the amphibolite to contact metamorphism of 

basaltic rocks by the ultramafic intrusions (Challis, 

1965; Green, 1964). Williams (Williams and Smyth, 

1973) regards amphibolite associated with ultramafic 

rocks in Newfoundland as contact dynamothermal aureoles related to obduction and transport of ophiolite

slices. In the Condrey Mountain quadrangle, the occurrence of numerous discontinuous metamorphosed 

serpentinite bodies interlayered conformably with the 

foliated amphibolite is indicative of simultaneous deformation and metamorphism of disrupted ophiolitic 

rocks, possibly a melange. The potassium-argon age of 

the amphibolite is in general agreement with the age 

of the plutonic rocks; either the metamorphism and 

plutonism were simultaneous events or the 

metamorphism was earlier than the plutonism and the 

potassium-argon ages were modified by heat of the intrusions. As both samples of dated amphibolite were 

taken about 5 km from the Vesa Bluffs and Ashland 

plutons, it seems unlikely that they would be affected 

by later heating.

Clearly for the Condrey Mountain Schist and the 

amphibolite facies to be brought together, these rocks 

of contrasting lithologies have been juxtaposed by 

large-scale horizontal displacements. The ages of 

metamorphism of the amphibolite and schist are approximately the same, suggesting that at about the 

time in the Late Jurassic the metamorphism of the 

amphibolite took place, sedimentary and minor 

amounts of volcanic rock, probably of the Galice Formation, were overthrust by the amphibolite or dragged 

beneath it. Penetrative deformation and heating 

caused by deeper burial, possibly together with the 

higher temperature of the overthrust sheet, resulted in 

metamorphism of rocks below the thrust sheet to form 

the Condrey Mountain Schist.

An earlier tectonic-metamorphic episode under lowtemperature high-pressure conditions is represented 

by phyllite of the Stuart Fork Formation, which contains blueschist of Triassic age (220 m.y.) in the 

Yreka-Fort Jones area (Hotz and others, 1977). Here 

the Stuart Fork occurs adjacent to serpentinite of Ordovician (?) age, which is between lower Paleozoic 

rocks of the eastern Klamath belt and metavolcanic 

and metasedimentary rocks of the western Paleozoic 

and Triassic belt. Thrusting of the Stuart Fork over 

these rocks probably took place during the Triassic 

metamorphism and may have continued or may have 

been renewed in the Jurassic.

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