5..
Pliocene Volcanic Rocks of the 

Coso Range, Inyo County, 

California

By STEVEN W. NOVAK and CHARLES R. BACON

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1383

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1986
DEPARTMENT OF THE INTERIOR

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging-in-Publication Data

Novak, Steven W.

Pliocene volcanic rocks of the Coso Range, Inyo County, California.

U.S. Geological Survey Professional Paper 1383

Bibliography: p. 40-42

Supt. of Docs. No.: I 19.16:1383

1. Geology, Stratigraphic Pliocene. 2. Volcanic ash, tuff, etc. California Coso Range. 3. Geology California Coso

Range. I. Bacon, Charles R. II. Title. III. Series: Geological Survey Professional Paper 1383. 

QE695.N67 1986 552'.2'0979487 86-600084

For sale by the Books and Open-File Reports Section, 

U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225
CONTENTS

Page 

Abstract _________ __________________ 1

Introduction ______________________________ 1

Petrography _____________________________ 3

Basalt _______________________________ 3

Olivine ____________________________ 6

Chrome spinel ______________________ 6

Plagioclase ________________________ 7

Clinopyroxene _______________________ 11

Xenocrysts _________________________ 13

Intermediate-composition rocks ________________ 13

Olivine _________________________ 14

Plagioclase ________________________ 14

Pyroxenes ___________________________ 14 

Hornblende _ __ _ _ _ ____ _ _ ____ _ _ ____ 15

Biotite _____________________________ 15 

Commingled basalt-dacite bomb _______________ 16 

Rhyodacite ___________________________ 17

17

18

18

19

19

Plagioclase 

Alkali feldspar 

Amphibole ____

Biotite ________

Fe-Ti oxides __

ILLUSTRATIONS

FIGURE 1. Simplified geologic map of volcanic rocks and vents of the Coso volcanic field

2. Sample locality map _____________________

3. Photomicrographs of representative Pliocene volcani

4. Nickel vs. forsterite content of olivine _____________

5. Cr/Cr + Al vs. Fe/Fe + Mg in chrome spinel _________

6. Anorthite, albite, and orthoclase contents of plagioclai

7. Wollastonite, enstatite, and ferrosilite contents of pyroxene

8. Ca, Mg, and Fe contents of amphibole _________

9. Mg, AT^ + Ti, and Fe + Mn contents of biotite __

10. Photomicrographs of commingled basalt-dacite bomb

11. Fe/Fe + Mg in amphibole vs. anorthite content of coexisting plagiocl

12. CaO, Na2O + K2O, and FeO*/FeO* + MgO vs. SiO2

13. Major14. MgO vs. P2O5 for Pliocene volcanic rocks

15. AFM diagram for Pliocene volcanic rocks

16. Th, Co, Sc, Ba, and Cr vs. SiO2

17. Chondrite-normalized rare-earth-element plots

18. Th vs. Cr content for Pliocene volcanic rocks _.

19. Maps showing development of Coso volcanic field

-element oxide variation diagrams for Pliocene volcanic rocks

content for Pliocene volcanic rocks

Petrography Continued 

Rhyodacite Continued

Accessory phases _________ __ 19

Rhyolite _______________________________ 19

Compositional variation______________________ 20

Major elements ________________ __ 20

Trace elements ____________ 24

Transition metals (Sc, Cr, Co, Zn) ____________ 24

Alkali metals and alkaline earths (Rb, Cs, Ba) ____ 25

Rare earths ________________ 25

Highly charged cations (Zr, Hf, Ta, Th, U) ________ 27

Discussion ___ _____ ________ 28

Petrogenesis ___________________________ 29

Basalt _____________________________ 29

Intermediate-composition rocks 31

Rhyodacite ____________ 33

Rhyolite ____________________________ 34

Age and duration of volcanism ________ 35

Relation of volcanism to tectonic processes 36

Conclusions __________________ 40

References cited _____________________ ____ 40

Page

._______________________ 2

.________________________________________ 5 

rocks of the Coso Range __________________________ 10 

.________________________________________ 11 

.________________________________________ 11

36 ________________________________________________________ 12

._______________________ 13

._____________-____________ 15

.________________________ 16 

________________________ 17 

ase________________________________ 19

for Pliocene volcanic rocks ____________________________ 25

______________________ 26

._______________________ 27 

._______________________ 28 

________________________ 29

for Pliocene volcanic rocks ______________________________ 30

.________________________________________ 31

_________________________________________ 38

TABLES

Page

TABLE 1. Phenocryst contents, K-Ar ages, and map units of analyzed Coso Range Pliocene volcanic rocks, arranged in order of

increasing SiO2* _____ _________________________________________________________ 4

2. Microprobe analyses and structural formulas of oliviae from Coso Range Pliocene volcanic rocks _______________ 6

3. Microprobe analyses and structural formulas of chrome spinel from Coso Range Pliocene volcanic rocks ___________ 6

4. Microprobe analyses and structural formulas of pyroxene from Coso Range Pliocene volcanic rocks ______________ 7

5. Microprobe analyses and structural formulas of amp libole from Coso Range Pliocene volcanic rocks

6. Microprobe analyses and structural formulas of biotite from

7. Chemical analyses of Coso Range Pliocene volcanic rocks _ 

Chemical analyses of some basalts and andesites fro:n

Coso Range Pliocene volcanic rocks ________________ 9

_____________ 21

the western United States ____________ ____________ 24

III
IV CONTENTS

TABLE 9. Least squares solutions to magmatic differentiation and combined differentiation-assimilation models using pairs of

analyzed samples of Coso Range Pliocene volcanic rocks as assumed parent and derivative compositions ____ 32 

10. Sample localities for Coso Range Pliocene volcanic rocks _________________________________ 44
PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, 

INYO COUNTY, CALIFORNIA

By STEVEN W. NovAi^ora/ CHARLES R. BACON

ABSTRACT

The Coso Range is east of the Sierra Nevada and immediately south 

of Owens Valley, California. Major volcanic episodes occurred in the 

Coso Range at about 6 m.y, 4-2.5 m.y, and later than 1 m.y. The 

Pliocene episode (4-2.5 m.y. ago) was by far the most voluminous, and 

during it more than 30 km3 of basalt to high-silica rhyolite magma was 

erupted. Basalt originated from monogenetic vents in the eastern part 

of the range, andesite and dacite generally from polygenetic centers 

within the-basalt field, and rhyodacite and high-silica rhyolite from a 

large silicic center in the northwest.

Petrographic features, such as quartz xenocrysts in basalt, sieved 

plagioclase phenocrysts, broad compositional ranges of phenocrysts 

within single samples, and occurrence of commingled bombs and lava 

flows, indicate that many of the intermediate-composition rocks 

formed by mixing of basaltic magma with silicic material. Sieved 

plagioclase, common in the intermediate silica rocks, reflects sudden 

thermal and compositional changes of magma brought about by mixing; in general, finely sieved crystals have been partially resorbed and 

coarsely sieved crystals have experienced rapid growth. Major- and 

trace-element contents of the intermediate-composition rocks also are 

compatible with an origin involving mixing; mass-balance calculations show that fractionation of olivine + plagioclase + clinopyroxene, combined with assimilation of silicic magma, accurately models 

the composition of andesite and dacite. This mixed magma was generated in small volume, thermally and compositionally zoned magmatic 

systems underlying polygenetic volcanoes.

A larger volume, highly silicic center near Haiwee Ridge evolved 

3.1-2.5 m.y. ago. This system erupted high-silica rhyolite air-fall and 

ash-flow tuff and in its later stages rhyodacite lava flows. The Haiwee 

Ridge system probably consisted of well-mixed, converting rhyodacite 

capped by high-silica rhyolite during its early stages.

The evolution of the Coso volcanic field, from early basalt to polygenetic intermediate-composition volcanoes to a comparatively largevolume silicic center, is thought to reflect systematic changes in the 

least principal stress (S3) and the related tectonic extension rate. 

Beginning about 4 m.y. ago, basalt was erupted onto a surface of little 

relief, ponding in a shallow north-trending basin. Average residence 

time of magma in the crust evidently was short and the eruption/ 

intrusion ratio was high. At this time the magnitude of S3 was greatest 

and the extension rate probably the least.

Intermediate-composition magma was produced mainly between 

3.5 and 3.3 m.y, at a time when S3 may have decreased and the 

extention rate increased. Residence time is inferred to have been low 

to medium, and the eruption/intrusion ratio also medium. By the time 

relatively voluminous rhyodacitic and rhyolitic magma was erupted 

from the Haiwee Ridge center, a series of north-south-trending grabens had developed, indicating that the extension rate probably was 

high and S3 had decreased to a minimum value. Absence of basalt or 

intermediate-composition lava in these late stages suggests long residence time and a low eruption/intrusion ratio. Thus, Pliocene volcanism progressed through a sequence of compositions and eruptive 

patterns that reflect the transition from tectonic stability to extension.

'Present address: Charles Evans and Associates, 301 Chesapeake Dr., Redwood City, 

CA 94063.

INTRODUCTION

The Coso volcanic field (Duffield and others, 1980; 

Duffield and Bacon, 1981), located east of the Sierra 

Nevada between Owens Valley and the Garlock fault, is 

one of many Neogene volcanic centers in the Basin and 

Range province. Three periods of volcanic activity 

occurred within the Coso Range during the last 6 million years. The oldest episode of mainly basaltic volcanism with subordinate intermediate-composition 

lava, and minor high-silica rhyolite occurred on the 

north flank of the range in the late Miocene, about 6 m.y. 

ago (Bacon and others, 1982). Pleistocene high-silica 

rhyolite and associated basaltic rocks occur in the southwest part of the range (Bacon, 1982; Bacon and others, 

1981,1984; Bacon and Metz, 1984). The Pliocene volcanic 

episode, the subject of this paper, was by far the most 

voluminous of the three and its products were the most 

varied. This episode was centered in the east and central 

parts of the range and took place between about 4.0 and 

2.5 m.y. ago (fig. 1). Pliocene volcanic rocks include 

basalt in the north-central and eastern areas, andesite 

and dacite at localized polygenetic centers generally 

within the area of basalt, and rhyodacite and high-silica 

rhyolite in a silicic eruptive center that makes up the 

northwestern part of the Coso Range. Granitic rocks of 

the Sierra Nevada batholith and associated metamorphic rocks form the basement of the volcanic field.

Normal faults cut all of the Pliocene rocks of the Coso 

Range. Right-lateral strike-slip faulting within the field 

is suggested by topography, the left-stepping sense of a 

series of echelon grabens that cut Pliocene rocks, and 

current seismicity (Walter and Weaver, 1980). The overall pattern of faulting indicates extension in a westnorthwest east-southeast direction. Flow directions 

and thickness variations of Pliocene lava flows indicate 

that little fault offset occurred in the area before about 3 

m.y. ago. Increased displacement of older lavas relative 

to younger flows demonstrates that extension has continued throughout the history of the Coso volcanic field. 

Thus, the Pliocene volcanic rocks were erupted while 

the area was in transition from comparative stability to 

rapid extension.

This paper describes the petrography and chemical 

composition of the Pliocene volcanic rocks, discusses

l
PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

118°00'

36° 15

36° 00' -

117°30'

FIGURE 1. Simplified geologic map of the Coso volcanic field (modified from Duffield and Bacon, 1981). Faults omitted for clarity.
PETROGRAPHY

their petrogenesis, and attempts to relate the compositional evolution of the volcanic field to tectonic processes. Electron-microprobe mineral analyses are 

interpreted to show that many of the intermediate rocks 

contain mixed populations of phenocrysts derived from 

basaltic and silicic magmas. These analyses are also 

used in conjunction with major-element whole-rock 

chemical analyses to model the petrogenesis of the. 

lavas. Models based on major-element data are tested 

for compatibility with trace-element abundances determined by instrumental neutron activation analysis 

(INAA). Finally, we relate the composition and volume of 

volcanic rocks to the rate of tectonic extension through 

the inferred relative magnitudes of regional principal 

compressive stresses.

Rock names used in this paper are based on SiO2 

weight percent expressed on an anhydrous basis with 

the analysis recalculated to total 100 percent. Because 

many samples have suffered posteruption oxidation, 

analyses were recalculated with a uniform atomic Fe2 +/ 

Fe2+ +Fe3+ratio. A value of 0.86 was chosen for this ratio 

on the basis of a plot of Fe2+ versus Fe2++Fe3+, but we 

recognize that this probably misrepresents the true 

oxidation state of some samples. In this classification, 

basalt has <53 percent SiO2 , andesite 53-63 percent, 

dacite 63-68 percent, rhyodacite 68-71 percent, and 

rhyolite >71 percent. Breaks between basalt and 

andesite and between andesite and dacite are arbitrary 

because there is both mineralogical and chemical continuity among these rock types. Rhyodacite forms a

EXPLANATION

VOLCANIC ROCKS

Basalt (0.04-0.4 m.y.)

Rhyolite (0.06-1.0 m.y.)

Basalt (1.1 my.)

Basalt and andesite (1.8-2.1 m.y.)

Rhyodacite (2.5-3.0 m.y.)

Dacite (-3.3 m.y.)

Andesite (- 3.3 m.y.)

Basalt (3.0->3.6 m.y.) 

VENTS

Rhyodacite

Dacite

Andesite

Basalt and andesite

FIGURE 1. Continued

readily separable group confined to the Haiwee Ridge 

eruptive center. The break between rhyodacite and 

rhyolite is placed so as to minimize the terminology 

necessary to describe the silicic rocks of the Haiwee 

Ridge eruptive center. This classification is convenient 

in that names assigned on the basis of phenocryst mineralogy and field criteria correspond well to names 

based on anhydrous SiO2 content. It differs slightly 

from the scheme used by Duffield and others (1980), but 

results in little change in relative volumes of rock types.

PETROGRAPHY

We believe that much of the compositional variation 

among Pliocene volcanic rocks of the Coso field resulted 

from mixing of basaltic and silicic magmas, or from 

contamination of basaltic melts by crustal material. 

This section describes textural and mineralogical features of the volcanic rocks and presents petrographic 

evidence in support of the mixing and contamination 

hypotheses. A summary of K-Ar ages and estimated 

modes for all samples examined is given in table 1, along 

with their respective map units from Duffield and Bacon 

(1981). Petrographic descriptions and mineral compositions determined by microprobe are presented together 

for a given rock type so the two can be easily related. 

Minerals in thin sections of seven representative rocks 

were analyzed by electron microprobe using techniques 

described by Bacon and Duffield (1981) (see fig. 2 and 

table 10 for sample localities). Photomicrographs of representative rocks are shown in figure 3. Diagrams summarizing the compositional variation of minerals are 

shown in figures 4 to 9 and representative analyses are 

given in tables 2 to 6. Listed mineral analyses are averages of at least five spot analyses, and core and rim 

compositions were determined for most crystals.

BASALT

Rocks classified as basalt (<53 weight percent SiO2) 

are holocrystalline diktytaxitic lavas containing phenocrysts of olivine and plagioclase, and in some cases 

clinopyroxene, in a groundmass of finer plagioclase, 

olivine, clinopyroxene, and Fe-Ti oxides. Phenocrysts 

tend to form glomeroporphyritic clots. Two groups can 

be distinguished within the basalts. Samples with <51 

percent SiO2 have euhedral phenocrysts and only rarely 

contain quartz and sieved plagioclase xenocrysts; these 

appear to be the least contaminated basalts. In basalts 

with >51 percent SiO2, sieved plagioclase is common,
TABLE 1. Phenocryst contents, K-Ar ages, and map units of analyzed Coso Range volcanic rocks

[Samples arranged in order of increasing SiO2 * (SiO2 * = SiC>2 recalculated with analysis normalized to total 100 percent volatile free and with atomic Fe2+ /Fe2+ Fe3+ = 0.86; see text); minera

abundances by visual estimation; K-Ar dates from Dufneld and others (1980); map units from Dufneld and Bacon (1981)]

Symbols:

A - abundant (>5 percent) 

C - common (1-5 percent) 

R - rare (<1 percent)

a - included in olivine

b - resorbedc - included in sieved plagioclase

d - included in clinopyroxene

e - included in plagioclase

f - rims or replaces hornblende

g - included in hornblende 

h - included in biotite 

i - overgrowth on orthopyroxene j - included in orthopyroxene

K-Ar age (m.y.) 

Plagioclase sieved R 

Plagioclase clear- A

Potassium feldsparPlagioclase sieved C 

Plagioclase clear- A 

Clinopyroxene C,b 

Orthopyroxene R,bPotassium feldspar

3.64 3.64 - 3.56 - -----C RCRRRC 

AA---C CCCARR

ACACRCAR AC RCC A 

CRR-ARCR,bCC - R - 

R R,b C - C,e R - R - R R R R R 

-R--R-C RR RRR R

° _--___ e

76

52.7

R 

A-C 

A

53.5 53.5 53.6 54.5 54.5 54.5 55.1 55.2 55.3

ARAA- R-AR 

CAC.dCA R - R - AACCAA R - 

CRARCR AC

R,c - - R R,b R.b

R,b - - - - C

HH

O

o

A

S8GO

§

!_gffi M

8CO

Os

s

LIFORN]

t>
PETROGRAPHY

118°00'

36° 15' -

FIGURE 2. Sample localities for rocks described in this paper. Outlines of rock units from figure 1.
6 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

TABLE 2. Microprobe analyses and structural formulas ofolivine phenocrysts from Coso Range Pliocene volcanic rocks

[SiO2* = whole-rock SiO2 recalculated with analysis normalized to total 100 percent volatile free and with atomic Fe+ 2/Fe+ 2 + Fe+ 3 = 0.86; oxides in weight percent; formulas based on 4

oxygens; total Fe reported as FeO]

sio2 * 

FeOMgO 

MnO 

Total 

Fe 

Mn 

Fo-(percent)

6f

50.

Core

39.8

15.9

43.7

.31

.25

.16

100. 12

1.004

.334

1.641

.008

.005

.003 

82.3

0

Rim

39.6

17.7

42.2

.37

.16

100.31

1.004

.376

1.595

.010

.006

.003 

80.2

45

50.!

Core

14.6

44.6

 Z o

.24

.14

100.00

1.006

.306

1.667

.008

.005

.003 

83.8

3

Rim

39.1

21.3

Jo . J

.46

.41

.07

99.61

1.015

1.483

.013

.009

.001 

75.3

84

55.

Core

40.3

14.2

.29

.20

.21

100.50

1.003

.295

1 .682

.008

.004

.004 

84.3

2

Rim

39.1

20.3

39.8

.33

.32

.15

lOu.Gu

1.007

.438

1.529

.009

.007

.003 

77.0

9

59

Core

40. 1

11.9

47.7

34

.16

.27

100*. 4 7

.991

1.756

.009 

.003

.005 

86.9

2

.7

Rim

38.8

39.8

.28

.29

.28

100.85

.997

.460

1.527

.008 

.006

.006

76.1

107

Core

40.1

14.7

44.8

.27

.23

.11

100.21

1.004

. JUo

1.670

.007 

.005

.002

83.8

Rim

39.3

20.4

39.1

.31

.37

.13

99.64

1.016

1.509

.009 

.008

.003

76.6

99

66.0

40.5

7.48

50.5

.26

.22

.37

99.33

.993 

.153

1.843

.007

.005

.007

91.5

TABLE 3. Microprobe analyses and structural formulas of chrome 

spinel from Coso Range Pliocene volcanic rocks

[SiO2 * = whole-rock SiO2 recalculated with analysis normalized to total 100 percent volatile 

free and with atomic Fe +2/Fe+ 2 + Fe+ 3 = 0.86; oxides in weight percent; formulas based 

on 32 oxygens; total Fe reported as FeO; calculated FeO and Fe2C>3 derived from charge 

balance, a, euhedral inclusion in olivine; b, intergrown with plagioclase in inclusion in 

olivine]

O J-V/_

FeO 

MgO 

MnO 

Cr 2 03 - 

Total 

Calculated

6 2 3 

Total 

Fe +3 ______

Fe+2 

Mg 

Ti 

a

48.2 

14.6 

18.3 

.03

.45 

14.2

95.73

10.8

4.27

96.21

12.64 

.72 

2.00 

6.07 

.01 

.08 

2.50

65 

50.0

b

30.0 

28.3 

12.6 

.18 

2.02 

24.2

97.34

18.5 

10.9

98.39

8.61 

2.00 

3.76 

4.57 

.04 

.37 

4.66

45 

50.8

a

26.4 

27.1 

13.3 

.17 

1.22 

30.5

98.64

16.6 

11.7

99.86

7.56 

2.14 

3.37 

4.82 

.04 

.22 

5.86

84 

55.2

a

35.4 

27.5 

12.9 

.13 

1.03 

21.0

98.08

18.2 

10.3

98.99

9.87 

1.83 

3.61 

4.55 

.03 

.18 

3.93

a

38.6 

23.2 

15.0 

.07 

.75 

19.5

97.01

15.0 

9.07

98.03

10.57 

1.59 

2.92 

5.20 

.01 

.13 

3.58

orthopyroxene rimmed by clinopyroxene may be present, and quartz xenocrysts with granular clinopyroxene 

armor are frequently encountered. Quartz xenocrysts 

are present, however, even in the most mafic basalts.

OLIVINE

Olivine occurs as euhedral 1-5 mm phenocrysts that 

make up 5-10 percent of the basalts. Most crystals have a 

thin orange brown rim of iddingsite, and many contain 

small chrome spinel inclusions. Olivine phenocrysts are 

commonly intergrown with plagioclase in glomeroporphyritic clots. Compositions of olivine phenocrysts 

from two basalts are given in table 2 and plotted in 

figure 4. Cores of crystals with spinel inclusions are 

forsterite-rich (Fo86.87) and show little variation in forsterite content, whereas cores without spinels show 

greater variation in forsterite content (for example, 

sample 45: Fo74.82). The Fo content of olivine in the 

basalt is not as high as in some other Pliocene Coso 

Range volcanic rocks. Nearly all crystals are zoned to 

more Fe-rich rims which have the same or lower Ni 

content and higher Mn content than cores.

CHROME SPINEL

Many olivine phenocrysts in the basalts contain small 

(<0.1 mm) inclusions of chrome spinel (fig. 3A).
PETROGRAPHY 7 

TABLE 4. Microprobe analyses and structural formulas of pyroxene from Coso Range Pliocene volcanic rocks

[SiO2* = whole-rock SiO2 recalculated with analysis normalized to total 100 percent volatile free and with atomic Fe +2/Fe +2 + Fe+ 3 = 0.86; oxides in weight percent; formulas based on 6

oxygens; total Fe reported as FeO]

SampleSic^* 

Clinopyroxene

45

50.8

Rim Core

84

55.2

Rim Core

92

59.7

Rim Core

99

66.0

Core

Orthopyroxene

84

55.2

Core

92

59.7

Rim Core

SiC>2 

A1 2 03 

FeO 

MgO 

\JCi\S

Na 20 

MnO 

Cr203 

c A -7

5.06

n n -7

.32

.11

.84

.45

51.4

4.46

15.7

// . o

 Zo

.12

.79

.27

49.5

6.11

15.3

.56

.12

 oo

.91

JU. O

3.89

 o /

15.7

20.9

.37

.17

.95

.27

. D 1

4.91

10. 8

20.5

/ O

.13

.40

.37

1.91

7.01

16.7

19.4

.JO

.22

.33

. Uo

50.5

2.57

4.91

20.5

.47

.13

.84

.30

52.3

25.0

lo y

.07

.71

.19

.02

54.5

2.03

10.2

30.7

1.61

.05

.20

.19

.14

54.6

. JO

7.96

1.43

.06

.15

.16

. JO

Total----- 100.27 100.58 !.92 99.86 99.28 97.04 99.86 99.65 100.11

Si 1.851

.149

1.871

.129

1.828

.172

1.869

.131

1.949

.051

1.964

.036

1.894

.106

1.988

.012

1.920

.080

1.896

.104

ZTet- 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000

AlVIFe 

Mg 

Mn 

Ti 

Cr 

Ca 

Na 

ZOCT-

.069

.148

.831

.003

.023

.013

.890

.023

.063

.145

.851

.004

.022

.008

.889

.020

.094

.161

.843

.004

.024

.027

.806

.040

.037

.212

.862

.005

.026

.008

.823

.026

.062

.151

.916

.004

.011

.011

.807

.031

.047

.217

.922

.007

.009

.002

.767

.026

.008

.154

.942

.004

.024

.009

.825

.034

.000

.796

1.070

.023

.005

.001

.101

.005

.005

.302

1.614

.006

.005

.004

.061

.003

.002

.232

1.685

.004

.004

.016

.053

.004

2.000 2.000 2.000 2.000 1.993 1.997 2.000 2.001 2.000 2.000

Although most of the spinel inclusions have similar 

Fe/Fe + Mg and Cr/Cr + Al ratios, a range of compositions occurs in basalt sample 65 (fig. 5 and table 3). 

Chrome spinel is present in two forms in this sample: as 

euhedral octahedra, and as spinel-silicate symplectic 

intergrowths. The two appear identical in transmitted 

light, and there is no systematic relationship between 

spinel morphology and composition. The silicate phase 

in the symplectic intergrowths is rich in Ca and Al and 

is probably anorthite-rich plagioclase.

PLAGIOCLASE

Plagioclase in 3-7 mm phenocrysts forms 1-10 percent 

of the basalt and commonly is intergrown with olivine in 

glomeroporphyritic clots (fig. 3A). The groundmass of

the basalt is dominated by minute plagioclase laths with 

intergranular olivine, pyroxene, and Fe-Ti oxides.

Basalts with 51-53 percent SiO2 contain, in addition 

to a small proportion of clear euhedral grains, large 

sieved plagioclase phenocrysts. The sieve texture consists of fine-grained irregular inclusions in the cores or 

outer zones of crystals. Most sieved phenocrysts have 

clear plagioclase overgrowths of varying thickness 

which commonly give the crystal a euhedral outline. 

These phenocrysts appear blocky in contrast to the 

lathlike habit of nonsieved crystals (fig. 3B), although 

this may be an artifact of the orientation of crystals 

relative to the thin section, since the number examined 

is small. Inclusions are interconnected and are filled 

with the same phases which make up the groundmass of 

the rock; they are inferred to represent trapped silicate
PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

melt that later crystallized. Sieved plagioclase crystals 

in andesites and dacites described later contain brown 

glass inclusions which were obviously trapped melt.

Melt inclusions in phenocrysts can form either by 

partial dissolution of a phenocryst during reaction with 

surrounding liquid or by entrapment during rapid 

growth of the host crystal. Partial dissolution requires

rapid diffusion of plagioclase components outward, and 

magmatic liquid components into small channelways 

within the sieved crystal. Sieve texture has been duplicated experimentally in this manner (partial dissolution) by introducing plagioclase crystals that are more 

sodic than equilibrium phenocryst compositions into 

basaltic melts (Lofgren and Norris, 1981). Similar texTABLE 5. Microprobe analyses and structural formulas ofamphibole from Coso Range Pliocene volcanic

rocks

[SiO2* = whole rock SiO2 recalculated with analysis normalized to total 100 percent volatile free and with atomic Fe + 2/Fe + 2 + Fe + 3 = 0.86; 

oxides in weight percent; formulas based on 24 anions; total Fe reported as FeO]

Sample -- 

Si0 2*-

Core

99

66.0

Rim Core Rim Core

100

67.5

Rim Core Rim

sio2 

A 1 r\

i? -, f\

MgO 

r n n

Na 20 

K

f\

'i i n

MnOLess 0 = F, Cl

40.8 42.1 41.1 41.6 41.1 42.8 40.4 40.4

11.5 9.74 11.3 11.0 14.8 11.6 15.3 15.2

15.8 18.4 15.1 16.1 9.78 15.2 11.8 12.8

11.3 10.0 12.2 11.4 15.0 12.5 13.6 12.6

10.8 10.9 9.74 9.89 10.6 10.2 10.7 10.3

2.00 1.66 2.54 2.38 2.39 1.70 2.33 2.29

.94 1.24 .81 .92 .62 .75 .76 .75

2.46 1.43 3.06 2.70 2.88 1.71 3.17 3.04

.51 .52 .31 .31 .14 .36 .16 .17

.14 .16 .19 .17 .15 .17 .14 .17

.02 .05 .02 .02 .01 .03 .01 .01 

.07 .08 .09 .08 .06 .08 .06 .07

Total 96.20 96.12 96.28 96.41 97.41 96.94 98.31 97.66

SiAlIV

>:TetTi 

Mg 

Fe

2M1-M3- 

Fe 

Mn - 

CaNa -

ZM4-

CaNaK 

F-- 

Cl6.236

1.746

8.000 

.308

.283

2.575

1.834

5.000 

.186

.066

1.748

~

2.000 

.021

.593

.183

.797 

.068

.005

6.513

1.487

8.000 

.289

.166

2.306

2.239

5.000 

.141

.068

1.791

-

2.000 

.016

.498

.245

.759 

.078

.013

6.236

1.764

8.000 

.257

.349

2.760

1.634

5.000 

.282

.040

1.583

.095

2.000

.652

.156

.808 

.091

.005

6.328

1.672

8.000 

.300

.309

2.585

1.806

5.000 

.242

.040

1.612

.106

2.000

.596

.179

.775 

.082

.005

5.995

2.005

8.000 

.539

.316

3.262

.883

5.000 

.310

.017

1.657

.016

2.000

.660

.115

.775 

.069

.002

6.406

1.594

8.000 

.452

.192

2.789

1.567

5.000 

.335

.046

1.619

2.000 

.017

.493

.143

.653 

.080

.007

5.906

2.094

8.000 

.542

.348

2.964

1.146

5.000 

.297

.020

1.675

.008

2.000

.652

.142

.794 

.065

.002

5.961

2.039

8.000 

.604

.337

2.771

1.288

5.000 

.291

.021

1.628

.060

2.000

.595

.141

.736 

.079

.003
PETROGRAPHY

tural effects have been observed in plagioclase that has 

been partly melted during laboratory experiments 

(Tsuchiyama and Takahashi, 1984). Sigurdsson (1971) 

suggested that sodic plagioclase in a hybrid rock from 

Iceland formed a sieve zone by partial fusion following 

mixing of silicic and mafic magmas. Sieved plagioclase

in many ocean floor basalts (Dungan and Rhodes, 1978) 

and in andesitic rocks of the Medicine Lake volcano 

(Gerlach and Grove, 1982) is believed to have formed by 

partial dissolution.

Skeletal growth of plagioclase crystals has been produced experimentally by undercooling the melt from

TABLE 6. Microprobe analyses and structural formulas ofbiotite

[SiO2* = whole-rock SiO2 recalculated with analysis normalized to total 100 percent volatile free and with atomic Fe +2/Fe +2 + Fe+3 = 0.86; total Fe reported

as FeO]

SampleSi °2*~

99

66.0

100

67.5

Si02 - 

A1 2°3 -

FeO 

MgO

Na 20 

TL02 

MnO 

Cl 

Less 0 = F,C1

36.9

14.2

21.2

10.7

.03

.53

9.10

4.10

.32

.22

.04 

.10

Q /: c

13.7

19.5

11.3

.11

9.00

.35

.04 

.16

o /- ri

14.0

21.1

10.6

.06

.52

9.10

.31

.25

.04 

.12

JJ. O

20.0

1(J« O

.09

.55

8.90

.29

.05 

.17

37.4

14.5

46.9

13.2

.06

. oo

8.40

4.65

.16

.29

.04 

.13

37.2

14.4

16.6

 to /

.05

.92

8.43

4.60

.21

.37

.04 

.17

11 Q

14.1

20.5

11.3

.04

.37

9.03

4.14

.25

.04 

.12

37.9

ID D

.03

.70

9.08

3.99

.22 

.30

.03

.14

Total 97.14 95.27 95.95 94.92 96.22 95.89 97.62 97.38

Si 

Al IV 2.849

1.151

2.865

1.135

2.820

1.180

2.823

1.177

2.848

1.152

2.841

1.159

2.884

r.ii6

2.883

1.117

STet- 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000

Ti 

Mg - 

Fe - 

Mn -

lOct-

.141

.238

1.231

1.369

.021

.132

.248

1.322

1.280

.017

.112

.247

1.238

1.382

.021

.143

.249

1.270

1.319

.019

.149

.266

1.498

1.076

.010

.137

.264

1.525

1.060

.014

.151

.238

1.285

1.308

.018

.156

.228

1.406

1.196

.014

3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000

CaNaK 

.002

.079

.896

.009

.096

.901

.005

.079

.909

.008

.084

.895

.005

.130

.816

.004

.136

.821

.003

.055

.879

.002

.103

.881

.977 1.006 ,993 .987 .951 .961 .937 .986

F 

Cl-

.054

.005

.087

.005

.062

.005

.095

.007

.070

.005

.089

.005

.060

.005

.072

.004
10 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

which they were crystallizing (Lofgren, 1974), forming 

coarse sieve texture. Undercooling might be caused by 

mixing of cooler differentiated magma with basaltic 

magma. Several natural examples of sieve texture have 

been attributed to rapid crystal growth following a mixing or contamination event that undercooled the host 

magma (Hibbard, 1981; Kuo and Kirkpatrick, 1982). Disequilibrium textures of sieved phenocrysts are therefore 

thought to have formed during mixing of magmas of 

different composition, either by dissolution or by rapid 

growth. The interconnected, irregular shape of melt 

inclusions, the common coincidence of polysynthetic 

twin planes with inclusion boundaries, and the rounded 

or amoeboid cores of crystals suggest that sieved pla-
PETROGRAPHY 11

0.010

10.005 

oc

o

65 70 75 80 85 

Fo, IN MOLE PERCENT

EXPLANATION 

SPINEL SPINEL 

INCLUSIONS INCLUSIONS 

PRESENT ABSENT

90 95

Dacite 99

Andesite 92

Andesite 84

Basalt 45

Basalt 65

Basalt portion of 107

FIGURE 4. Nickel vs. forsterite content of olivine. Symbols show core 

composition; lines extend to rim composition.

FIGURE 3. Photomicrographs of representative Pliocene volcanic 

rocks of the Coso Range. A, Basalt (sample 66) showing 

glomeroporphyritic clot of olivine (Ol) and plagioclase (PI) phenocrysts. The olivine contains a spinel (Sp) inclusion. Crossed 

nicols; width of field 18 mm. B, Basalt (sample 75) showing phenocrysts of euhedral olivine (Ol) and sieved plagioclase (PI). 

Quartz (Q) xenocryst with clinopyroxene rim at lower left. Planepolarized light; width of field 18 mm. C, Andesite (sample 87) 

showing differing types of sieved plagioclase phenocrysts. 

Resorbed oval phenocryst on far left (PI) contains coarse, 

devitrified melt inclusions. Most inclusions are parallel to or 

bounded by polysynthetic twin planes. Euhedral grain at upper 

center (Pz) has a zone of fine interconnected devitrified melt 

inclusions around a clear core. A thin continuous overgrowth of 

clear plagioclase surrounds the entire grain. Textures of both 

these grains are interpreted as resulting from incomplete resorption. Crossed nicols; width of field 9 mm. D, Andesite (sample 84) 

showing a zoned and partially resorbed plagioclase phenocryst. 

Numbers on figure indicate anorthite content of the grain as 

determined by microprobe analyses. Crossed nicols; width of view 

7 mm. E, Rhyodacite (sample 100) showing phenocrysts of 

hornblende (Hb), biotite (Bt), and coarsely sieved plagioclase (PI) 

in glassy groundmass. Plane-polarized light; width of view 18 mm. 

F, Detail of coarsely sieved plagioclase phenocryst in rhyodacite 

(sample 100). Angular blebs are inclusions of highly silicic glass 

(G). One inclusion near the center of the figure contains a biotite 

(Bt) crystal. A small plagioclase (PI) is also present. This texture 

apparently resulted from rapid growth of plagioclase. Planepolarized light; width of view 2.6 mm.

MgCr2C>4 

0.6 

FeCr2 C>4

t

0.4

0.2

0

MgAl204

0.2 0.4 0.6

Fe/Fe+Mg

0.8 1.0

FeAl2O4

FIGURE 5. Cr/Cr+Al vs. Fe/Fe + Mg (atomic basis) in chrome spinel. 

Fields enclosing compositions of crystals from individual samples 

indicated by sample numbers.

gioclase phenocrysts in the Pliocene basalts of the Coso 

volcanic field developed sieve texture during partial dissolution and reaction with liquid, rather than by rapid 

growth. The dissolution origin also is suggested by the 

composition of plagioclase phenocrysts.

Compositions of plagioclase phenocrysts and microphenocrysts in basalt are plotted in figure 6. Plagioclase 

phenocrysts in basalt that lacks sieved plagioclase are 

very calcic (An60_70). Most phenocrysts are zoned to 

more sodic rims, but the compositional range is not 

large. Rim compositions match those of groundmass 

feldspars, suggesting the rims formed during the last 

stages of crystallization of the basalt. In basalt with 

sieved plagioclase, cores of phenocrysts are more sodic 

and show a greater range of anorthite content. The 

sieved phenocrysts are either normally or reversely 

zoned; rim compositions match those of microphenocrysts. Plagioclase compositions within sieve 

zones are highly variable. Sieve zones are clearly more 

sodic than cores of nonsieved crystals or than phenocryst rims. These findings are consistent with the 

dissolution hypothesis in which sieve zones form when 

relatively sodic plagioclase is introduced into basaltic 

magma during a mixing event, later to be overgrown 

with clear rims of a composition in equilibrium with the 

hybrid liquid.

CLINOPYROXENE

Euhedral phenocrysts of clinopyroxene compose up to 

3 percent of a few basalt samples, and interstitial 

clinopyroxene grains are a common groundmass constituent of all the mafic lavas. In a few generally diktytaxitic basalts, clinopyroxene also forms larger ophitic 

crystals in the groundmass. Clinopyroxene phenocrysts
12 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

in some higher silica, contaminated basalts have fine 

sieve texture similar to that of plagioclase. We consider 

this to reflect partial dissolution of clinopyroxene phenocrysts, which had grown in more silicic magma, as a 

result of magma mixing. 

Clinopyroxene compositions (fig. 7) have been determined in sample 45, one of a few basalts with euhedral 

clinopyroxene phenocrysts. The compositions cluster 

tightly on the pyroxene quadrilateral and appear to be 

appropriate for equilibrium with basaltic magma. This 

sample is representative of the basalt of Coso Peak, the 

only known basaltic unit in the Coso Range that conAn An

Basalt " / Basalt

(45) f\ (65)

SiO2 *=50.8/ SiO2 *=50.

Basalt / Basalt / Basalt / Basalt 

(72) A" (77) /\ (73) /V (75)

SiO2*=53.5/ SiO2*=52.1/ SiO2*=52.6

FIGURE 6. Anorthite, albite, and orthoclase contents (mole percent) of plagioclase (sample number given in parentheses). Each plot shows 

crystal compositions for a single sample, solid circles are core composition; lines extend to rim composition. Plots for samples 99 and 100 also 

shown enlarged for clarity.
PETROGRAPHY 13

tains euhedral olivine and clinopyroxene phenocrysts. 

Most of the Coso Peak flows also lack plagioclase phenocrysts.

XENOCRYSTS

Several of the higher silica basalt samples contain 

xenocrysts that provide evidence of contamination. 

Sieved plagioclase and clinopyroxene evidently are 

xenocrystic, having been derived from admixed more 

silicic magma. In addition, resorbed quartz rimmed by 

fine-grained clinopyroxene occurs in three analyzed 

samples and many other specimens. One sample contains a large aggregate of oxide grains that appears to be 

a completely reacted amphibole crystal. Another has a 

small metamorphic xenolith that consists of finegrained polygonal plagioclase enclosing oval aggregates 

of green spinel.

The presence of xenocrysts suggests that the more 

silicic basalt has been contaminated with crustal material or with more silicic magma. Even the most primitive 

basalt may contain minor quartz xenocrysts. The basalt 

of Rose Valley is notable among the Pliocene basalts for 

containing the largest amount of xenocrystic material. 

This is among the most silicic of all the basaltic lavas, 

and many samples fall in the andesite compositional 

range. Large resorbed clinopyroxene crystals and gabbroic and metamorphic microxenoliths are common in 

some of these rocks, in addition to more familiar 

xenocrystic plagioclase and quartz. Apatite needles are

common in xenocrysts and zircon has been noted. The 

most silicic (sample 81) of the three analyzed samples 

from the basalt of Rose Valley (Duffield and Bacon, 1981) 

contains only euhedral olivine, clinopyroxene, and plagioclase phenocrysts, suggesting that all xenocrysts had 

been completely dissolved.

INTERMEDIATE-COMPOSITION ROCKS

Andesite and dacite are considered together here 

because the intermediate-composition volcanic rocks 

form a continuum ranging from highly contaminated 

basalt to dacite. Most of these rocks are coarsely porphyritic, but nearly aphyric silicic andesite also is present. Rock compositions within specific map units may be 

highly variable. For instance, the andesite northwest of 

Petroglyph Canyon (Duffield and Bacon, 1981) probably 

varies from highly contaminated basalt at the margins 

of the flow to low-silica dacite near the vent. Similar 

compositional variation exists in other units. Some 

rocks called dacite on the basis of field characteristics 

may actually fall in the andesite range according to their 

anhydrous SiO2 content.

Evidence for mixing or contamination in these rocks 

is widespread. Phenocryst assemblages are mixed, containing crystals of both basaltic and silicic origins. Plagioclase phenocrysts are euhedral to rounded and 

almost always sieved or resorbed. Resorbed olivine phenocrysts may be present, although less commonly than 

in basalt. Both orthopyroxene and clinopyroxene are

Wo

_____ ___________ t

A A/ A TV

 ^ »

FIGURE 7. Wollastonite, enstatite, and ferrosilite contents (mole percent) of pyroxene. Sample numbers in parentheses. Solid circles are

core composition; lines extend to rim composition.
14 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

found in many samples. Resorbed quartz xenocrysts are 

ubiquitous and commonly are rimmed by fine-grained 

clinopyroxene. Pseudomorphs after amphibole occur 

rarely in the andesite, but resorbed amphibole and biotite rimmed or replaced by fine-grained oxides characterize the dacite. The following sections describe the 

petrographic and compositional characteristics of phenocryst minerals throughout the broad range of rock 

compositions present.

OLIVINE

Olivine phenocrysts =^3 mm make up less than 3 

percent of the andesite and are common only in rocks 

with less than 60 percent SiO2 . The phenocrysts typically are slightly resorbed and rimmed by iddingsite, 

and they may contain chrome spinel inclusions. As in 

the basalt, olivine cores with spinel inclusions vary little 

in Fo content (fig. 4). All crystals are zoned to rims richer 

in Fe and Mn; the Ni content of rims can be higher or 

lower than the cores (table 2). Resorbed microphenocrysts (<1 mm) of strongly iddingsitized olivine 

make up about 1 percent of the analyzed dacite (sample 

99). These crystals have the highest forsterite content 

(Fo915) of any analyzed olivine, yet occur in the most 

silicic rock that contains olivine. The presence of forsterite-rich olivine, particularly when it is intergrown 

with anorthite-rich plagioclase, as described below, is 

further evidence for contamination of silicic magma by 

basaltic material to form the dacite.

PLAGIOCLASE

Plagioclase phenocrysts dominate the appearance of 

the andesites. Euhedral blocky 1-5 mm phenocrysts 

make up 5-20 percent of the rocks and commonly are 

sieved. The sieve zones are usually much finer textured 

than those of plagioclase in the basalt. They may occur 

as oval zones around a clear core and are commonly 

overgrown by clear euhedral rims (fig. 3C). These features, plus the fact that in at least one crystal the sieve 

zone cuts across compositional zoning in the core, suggest that sieving of plagioclase in intermediate-composition rocks was formed by preferential melting of the 

outer zones of crystals.

A particularly instructive example of sieved plagioclase in an andesite (sample 84) is illustrated in 

figure 3D. This crystal consists of an oscillatory-zoned 

clear core surrounded by a finely sieved zone that cuts 

across the compositional zoning. The sieved zone is in 

turn overgrown by a euhedral clear rim. Microprobe 

analyses show that the clear core is normally zoned 

from An32 to An25 , compositions that are substantially

more sodic than expected equilibrium phenocrysts in 

this rock. The outer euhedral rim, however, is An38 (fig. 

6). We infer that the core of this crystal grew in relatively 

sodic silicic magma and that the sieve zone represents 

resorption of the most sodic zones of the crystal when it 

was incorporated into hotter, more mafic magma. A 

euhedral rim of more calcic plagioclase later grew over 

the sieved zone. The crystal was apparently broken after 

the overgrowth formed, and nonsieved sodic plagioclase 

in the core is now in direct contact with the groundmass 

along part of the break.

Plagioclase in the dacites commonly is subhedral and 

has rounded outlines with a finely sieved zone near the 

rim. A clear rim may occur around the sieved zone as in 

the andesites. The cores of these crystals are clear or 

have coarse silicic melt inclusions, and many show 

oscillatory zoning.

Plagioclase composition can vary widely within a 

given rock. In the andesite (sample 84) containing the 

sieved crystal described earlier, core compositions range 

from An29 to An58 . Most of these crystals have euhedral 

rims of An60 to An67, and one euhedral microphenocryst 

has a composition of An71 (fig. 6). Euhedral rims have 

compositions similar to groundmass plagioclase crystals and probably crystallized at the same time. In the 

other analyzed andesite (sample 92), plagioclase cores 

vary from An30 to An65 and show both normal and 

reverse zoning to rims of An35.42. In this sample, two 

crystals of An35.40 are intergrown with a completely 

oxidized amphibole pseudomorph, suggesting they originally crystallized in a more silicic and more hydrous 

melt. Also present in this sample is plagioclase of An69 

intergrown with olivine. Both andesites thus contain a 

wide range of plagioclase core compositions, suggesting 

that some phenocrysts originated from basaltic magma 

while others originated from rhyodacitic or even 

rhyolitic magma.

Plagioclase phenocrysts in the analyzed dacite (sample 99) show a much narrower range of composition. 

This sample is somewhat atypical of the dacite in that it 

is the most silicic, it contains little basaltic material, 

and the plagioclase phenocrysts do not exhibit the sieving that is common in the other intermediate rocks. 

Phenocryst cores range from An38 to An28 and are both 

reversely and normally zoned to rims of An35 to An29 

(fig. 6). An An55 plagioclase inclusion in forsteritic 

olivine provides evidence of the basaltic component in 

this rock.

PYROXENES

Clinopyroxene and orthopyroxene phenocrysts of 2 to 

7 mm make up from 1 to 5 percent of many andesite 

samples (table 1). Relative proportions vary, but
PETROGRAPHY 15

clinopyroxene is usually more common than 

orthopyroxene. Some clinopyroxene phenocrysts are 

sieved. Both pyroxenes may occur as euhedral to subhedral crystals in the groundmass. In dacite, 

orthopyroxene is present as small laths that have V-- 

shaped or swallowtail terminations and skeletal growth 

forms. In another dacite, clinopyroxene forms rims on 

resorbed orthopyroxene cores.

Compositions of analyzed pyroxenes are plotted in 

figure 7 and representative analyses are given in table 

4. Clinopyroxene in andesite has lower Ca than that in 

basalt but is otherwise similar. Most crystals are zoned 

to more Fe-rich rims, but some orthopyroxene is reversely zoned. The pyroxene projection of Lindsley (1983) 

shows that many of the Coso Range pyroxene phenocrysts contain more than 10 percent "other" components and thus are not suitable for geothermometry. 

Tieline orientation shows that the pyroxene in sample 

92 represents equilibrium conditions; two pairs yield 

temperatures near 1,030 °C. The single, comparatively 

Fe-rich, Cr-poor orthopyroxene in sample 84 definitely 

was not in equilibrium with Mg- and Cr-rich clinopyroxene in the same rock. This orthopyroxene occurs within 

a sieved plagioclase phenocryst and apparently was 

derived from magma more silicic than that from which 

the clinopyroxene crystallized.

HORNBLENDE

Hornblende crystals as large as 5 mm characterize 

the dacite and elongate or lathlike aggregates of very

Ca50 M950

fine-grained oxides, which are apparently 

pseudomorphs after amphibole, are present in some 

andesite. Amphibole crystals in dacite generally are 

ovoid and have rims of fine-grained oxides. Sample 99 is 

an exception and was chosen for microprobe study 

because its amphibole and biotite phenocrysts are not 

highly altered. The amphibole is magnesian hastingsite 

and magnesio-hastingsitic hornblende according to the 

classification of Leake (1978). All crystals are slightly 

zoned to rims with higher Fe/Fe + Mg ratios and higher 

fluorine contents. Representative analyses are presented in table 5; figure 8 compares the composition of 

Coso amphibole with that of amphibole from some other 

calc-alkalic volcanic suites.

BIOTITE

Biotite phenocrysts are common constituents of the 

dacite, forming up to 5 percent of the rock. Biotite occurs 

as books up to 1 mm thick that commonly contain minute apatite needles. In the single dacite studied with the 

microprobe (sample 99), the biotite is quite similar to 

that of the high-silica rhyolitic Bishop Tuff (Hildreth, 

1979), although containing slightly more Al and Ti (fig. 

9, table 6). Biotite in the dacite is richer in Fe than that 

in Haiwee Ridge rhyodacite, despite the former's occurrence in a less silicic rock. The Fe/Fe + Mg ratio in biotite 

is strongly affected by the temperature and oxygen 

fugacity of the magma from which it crystallized; these 

biotite compositions suggest either a lower temperature 

or more reducing conditions for the dacite than for the 

rhyodacite. Alternatively, the slightly rounded biotite

FIGURE 8. Fields of Ca, Mg, and Fe proportions (atomic basis) of amphibole from samples 99 and 100. Plotted for comparison are 

compositions of amphibole from Crater Lake rhyodacites (Ritchey, 1979), Lassen dacites (Carmichael, 1967), and Rio Grande Rift 

andesites (Zimmerman and Kudo, 1979).
16 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

phenocrysts may have been derived from a rhyolitic end 

member of the demonstrably mixed dacite and have 

been out of equilibrium with the hybrid liquid.

COMMINGLED BASALT-DACITE BOMB

A bomb composed of commingled basalt and dacite 

(sample 107) is described separately because it contains 

discrete areas of differing bulk composition and illustrates the process of magma mixing (fig. 10). This specimen is from a polygenetic center located 2 km south of 

Petroglyph Canyon and 4 km west of Volcano Butte (fig. 

1). The 10-cm bomb consists of light-brown dacite and 

dark olivine basalt interfingered on a scale of centimeters. The groundmass of the basaltic part is composed of randomly oriented plagioclase laths, finely 

granular pyroxenes and Fe-Ti oxides, and very small 

patches of brown interstitial glass. Phenocrysts consist 

of about 5 percent subhedral to euhedral plagioclase, 

some crystals of which are finely sieved, and about 2 

percent microphenocrysts of olivine. Most olivine is in 

small bipyramidal crystals with square or elongate central voids that represent sections of hopper-shaped crystals. Some larger olivine crystals with dendritic 

terminations are also present.

The dacitic part of sample 107 consists dominantly of 

greenish-brown glass containing about 15 percent plagioclase phenocrysts. Plagioclase phenocrysts are both 

larger and more abundant in the dacite than in the 

basalt. A few crystals have finely sieved zones around

their margins, but most are clear. Approximately 5 percent of the dacite consists of aggregates of fine-grained 

oxides in lathlike or oval patches set in plagioclase(?). 

The oxides are arranged in linear arrays parallel to the 

long axes of the patches. These aggregates are probably 

pseudomorphs after amphibole, although no relict 

amphibole remains in them. Vesicles in the glass are 

associated with the pseudomorphs, but are rare 

elsewhere in the rock. Fine-grained granular 

clinopyroxene forms less than 1 percent of the dacite, 

and a single resorbed quartz crystal was observed.

The contact between the two rock types is sharp but 

crenulate, the basalt convex toward the dacite. This 

form of contact is typical of mafic inclusions in silicic 

volcanic rocks (see Bacon and Metz, 1984), including 

many Pliocene dacites of the Coso volcanic field, and 

probably results from differences in surface tension and 

viscosity of the two magmas. The basalt of the bomb 

could be an erupted mafic inclusion. Mafic inclusions 

are believed usually to crystallize at depth after incorporation into cooler silicic magma, and their 

groundmass textures generally reflect crystallization at 

substantial undercooling. Because such textures are 

lacking in the bomb, we believe it represents concurrent 

venting of basaltic and dacitic magmas.

Between the two rock types in the bomb is a zone 

about 0.25 cm wide that appears to be a zone of reaction 

or diffusive mixing. This zone consists of an interlocking 

mass of minute plagioclase laths with scattered 

clinopyroxene crystals and rare olivine. It appears texFe+Mn

FIGURE 9. Mg, AT^1 + Ti, and Fe + Mn proportions (atomic basis) in biotite of dacite (sample 99) and rhyodacite (sample 100). Solid points are 

core composition; lines extend to rim composition. Plotted for comparison are compositions of biotite in Lassen dacites (Carmichael, 1967), 

the Bishop Tuff (Hildreth, 1977), and granitic biotite (Foster, 1960).
PETROGRAPHY 17

turally similar to the basalt groundmass, but is lighter 

colored and lacks fine-grained oxides.

Rounded phenocrysts from the dacite occur in the 

basalt singly or in clusters or trains linked with dacitic 

glass. One elongate group of quartz and plagioclase 

phenocrysts is surrounded by a lighter colored aureole 

similar to the reaction or mixing zone separating the 

rock types; small patches of clear dacitic glass remain 

between the crystals. Rounded and embayed quartz 

phenocrysts with fine-grained clinopyroxene rims are 

common in the basalt.

FIGURE 10. Photomicrographs of commingled basalt-dacite bomb. A, 

Commingled sample 107 showing two rock types in contact. Light 

area on left is dacite containing phenocrysts of plagioclase and 

opaque aggregates, which probably are pseudomorphs after 

amphibole. Note vesicle (clear) between two largest aggregates. 

Dark area on right is basalt containing phenocrysts of plagioclase. 

Olivine and clinopyroxene phenocrysts also are present in basalt, 

but are not visible here. Plane-polarized light; width of view 7 

mm. B, Detail of reaction zone between two rock types in figure 

10A. Dacite on left, basalt on right. Phenocrysts in dacite are 

plagioclase and small granular olivine. Phenocrysts in basalt are 

olivine and plagioclase. Note central void in bipyramidal olivine 

at extreme right. Plane-polarized light; width of view 2.6 mm.

A few crystals in the dacite have been derived from the 

basalt. The dacite contains a plagioclase phenocryst 

that has inclusions of deep-red-brown glass similar to 

that of the basalt. In addition, this crystal has patches of 

dark glass on its margin that appear to be basaltic 

groundmass trapped in embayrnents of the crystal rim. 

Near this crystal several small olivine grains with 

clinopyroxene rims form a trail back to the basaltic part 

of the sample.

Interchange of phenocrysts is also shown by mineral 

compositions (figs. 4 and 6). Cores of plagioclase crystals 

within the sample range from An65 to An28. The many 

crystals near An30 are mainly in the dacite, but some 

are in the basalt. The most calcic plagioclase analyzed is 

a microphenocryst intergrown with olivine that occurs 

in the dacite. Most of the sodic plagioclase phenocrysts 

incorporated in the basalt are not sieved.

This bomb speciman documents the textures formed 

by commingling of differing magmas erupted at the 

same time. It shows that crystals from different magmas may be intermixed without complete homogenization of the two liquids. The texture of the bomb probably 

illustrates syneruptive magma mixing, but similar processes may have operated within magma reservoirs and 

conduits during the formation of many of the intermediate-composition magmas erupted in the Coso volcanic 

field.

RHYODACITE

Rocks classified as rhyodacite occur in a large complex 

of flows and domes near Haiwee Ridge and as scattered 

deposits of air-fall pumice throughout the Coso area 

(Duffield and others, 1980). The lavas are highly porphyritic, some containing more than 20 percent phenocrysts (figure 3E and table 1). Unlike the less silicic 

rocks, the rhyodacite consistently has a glassy 

groundmass. Plagioclase phenocrysts are very abundant and generally coarsely sieved, the sieve pockets 

containing silicic glass. Rounded quartz phenocrysts 

are present in the more silicic samples of rhyodacite. 

Mafic phenocrysts consist of euhedral hornblende and 

biotite, which may contain oxide, zircon, or apatite 

inclusions. The glassy groundmass is generally crowded 

with microlites of most of the above minerals.

PLAGIOCLASE

Plagioclase phenocrysts of 1 to 5 mm make up 10 to 20 

percent of the rhyodacites. Most plagioclase phenocrysts are euhedral and coarsely sieved. A few relatively small phenocrysts lack sieve texture and are 

rounded.

The composition of plagioclase phenocrysts in the 

rhyodacite studied with the microprobe (sample 100)
18 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

varies widely, cores ranging from An28 to An62 (fig. 6). 

Both normally and reversely zoned crystals are present. 

The most calcic and the most sodic crystals are normally 

zoned, but zoning of intermediate-composition phenocrysts can be either normal or reverse. Some of the 

phenocrysts show oscillatory zoning. Many of the intermediate-composition crystals have rim compositions 

from An39 to An42, while others, along with the most 

calcic crystals, have rim compositions from An51 to An56. 

A sharply euhedral microphenocryst is An39. The intermediate-composition phenocrysts are generally 

euhedral and sieved. Sodic phenocrysts with rims near 

An30 and crystals with cores more calcic than about 

An52 are nonsieved and tend to be rounded as though 

partially resorbed. Hornblende is intergrown with or 

included in crystals spanning the entire compositional 

range, and more magnesian hornblende is associated 

with more calcic plagioclase.

The texture of the coarsely sieved plagioclase phenocrysts is unique to the rhyodacite. Most of the sieve 

pockets, which can be as large as 0.5 mm, are filled with 

glass, commonly appearing similar to the light 

brownish glass of the groundmass (fig. 3F). Some of the 

larger plagioclase phenocrysts have biotite, hornblende, 

Fe-Ti oxide, apatite, or zircon microphenocrysts trapped 

within large glass inclusions. Boundaries of the inclusions may be straight and show sides that parallel faces 

of the host crystal. A few biotite inclusions have step like 

outlines that mimic the shape of nearby glass inclusions; some groups of such biotite inclusions are in 

optical continuity, indicating they form part of a single 

crystal intimately intergrown with the surrounding plagioclase. Some coarsely sieved phenocrysts are composed of a few adhering skeletal plagioclase crystals 

with an overall euhedral shape. These may enclose 

euhedral earlier-formed plagioclase or glass inclusions 

containing ferromagnesian phases. Some of these plagioclase phenocrysts have a crude hopper shape, and 

appropriate sections show a core of glass surrounded by 

inward-projecting euhedrally terminated tablets within 

a rectangular euhedral ring of plagioclase. These textural features suggest that the glass inclusions in coarsely sieved plagioclase are primary and thus represent a 

growth feature (Hibbard, 1981; Kuo and Kirkpatrick, 

1982) rather than resorption. Similar textures in granitic rocks Hibbard (1981) attributed to skeletal growth 

following mixing of magmas.

Partial microprobe analyses indicate that glass inclusions within large coarsely sieved plagioclase phenocrysts have compositions that are nearly identical to 

those of groundmass glass immediately adjacent to the 

phenocrysts. They are severely depleted in CaO and are 

richer in K2O and SiO2 than would be expected of glass 

produced by partial melting of the plagioclase or influx

of reacting liquid. The highly silicic glass in the sieve 

pockets probably formed, in the same manner as that in 

the groundmass, from local domains of residual liquid 

that was depleted in plagioclase components, particularly anorthite, owing to comparatively slow diffusion of Al in the melt during rapid phenocryst growth. 

The wide range of plagioclase phenocryst core compositions in the rhyodacite argues for contamination or 

mixing of different magmas. Crystals that were in equilibrium with a given melt should be zoned to a similar 

rim composition, and the large number of rims of An51 to 

An55 suggests that these rims crystallized at the same 

time from the same melt (fig. 6). Note that both normally 

and reversely zoned crystals are included. A number of 

crystals also have compositions that converge to a more 

sodic rim composition near An40. There may have been 

two (or more) mixing episodes causing rims to crystallize toward different compositions at different times. 

The absence of relict plutonic microxenoliths and lack of 

evidence for deformation in crystals argues against a 

wallrock xenocrystic origin for any of the plagioclase. 

Most likely the more calcic plagioclase phenocrysts originated in relatively mafic (dacitic?) magma that underlay the more silicic magma. Convective mixing could 

have introduced these comparatively refractory crystals 

into the silicic melt, causing them to partially dissolve 

and resulting in the observed rounded forms.

ALKALI FELDSPAR

A single large crystal of alkali feldspar was found in 

sample 102. Resorbed and optically slightly zoned, the 

crystal shows exsolution lamellae that might suggest a 

wallrock xenocrystic origin. We infer that this xenocryst 

was derived from rhyolitic magma that mixed with the 

rhyodacitic magma.

AMPHIBOLE

Large green-brown pleochroic, euhedral 1 to 3 mm 

phenocrysts of amphibole form up to 3 percent of the 

rhyodacite. Smaller crystals are present throughout the 

glassy groundmass and commonly are included in plagioclase phenocrysts. Representative amphibole analyses are given in table 5. Amphibole s in the Coso 

rhyodacite is classified as magnesian hastingsite in the 

scheme of Leake (1978). Figure 8 compares the Coso 

amphibole to those of Lassen dacites (Carmichael, 1967) 

and of andesites of Mount Mazama (Ritchey, 1979) and 

the Rio Grande Rift (Zimmerman and Kudo, 1979). Coso 

amphibole is notably lower in Ca than those from the 

other localities. The Fe/Fe + Mg ratio for amphibole in 

the rhyodacite (sample 100) is highly variable but generally lower than amphibole in the dacite (sample 99). 

This is opposite to the Fe/Fe+ Mg relations of the rocks
PETROGRAPHY 19

and is consistent with the inference that the amphibole 

in the dacite originated in more silicic magma.

A relationship exists between the compositions of 

amphibole and coexisting plagioclase in the dacite and 

rhyodacite studied. Figure 11 shows compositions of 

pairs of crystals in contact with each other. The plot 

indicates that Mg-rich amphibole coexists with calcic 

plagioclase and Fe-rich amphibole with sodic plagioclase. Note that crystals in the rhyodacite (sample 

100) span a wide compositional range. This suggests a 

protracted crystallization history for this phenocrystrich lava, with the various mineral pairs forming at 

different times as the magma evolved to a more silicic 

composition. Alternatively, the different mineral pairs 

could have formed in different parts of a compositionally 

zoned magma reservoir and subsequently been brought 

together by convective mixing. The relationship 

between amphibole and plagioclase compositions also 

suggests that the spread in phenocryst composition is 

not due to contamination by wallrock materials. The fact 

that plagioclase phenocrysts of different compositions 

are separate entities, not joined together or overgrown 

in compositional sequence, argues against their origin 

in a protracted crystallization history and favors the 

convective mixing hypothesis.

BIOTITE

Biotite phenocrysts occur as euhedral brown books up 

to 1 mm thick that make up 1 to 5 percent of the rock. 

Most are strongly pleochroic from chocolate brown to a 

lighter reddish brown, and some are sieved, showing a 

texture similar to the clinopyroxene in the intermediate 

rocks. Many biotite phenocrysts contain minute crystals of zircon and apatite. Biotite commonly is present

0.9

0.8

0.6

0.5

EXPLANATION

Phenocrysts in rhyodacite (100) 

Phenocrysts in dacite (99)

20 30 40 50 60 

An CONTENT OF PLAGIOCLASE, IN MOLE PERCENT

70

FIGURE 11. Mg/Mg + Fe in amphibole (atomic basis) vs. anorthite 

content of coexisting plagioclase in dacite and rhyodacite.

also as small inclusions in plagioclase and as microlites 

scattered throughout the glassy groundmass of the 

rhyodacite. Compositions of biotite from the rhyodacite 

are given in table 6 and plotted in figure 7, where they 

are compared to those of the Coso dacites and to biotite 

in some other volcanic suites. Biotite in the rhyodacite is 

quite similar to that in the dacite, having only slightly 

higher TiO2 and Na2O contents. The crystals are 

slightly zoned to rims that are lower in total Fe and 

higher in F. The Coso biotite is lower in F and Cl than 

biotite from the Bishop Tuff (Hildreth, 1977) and has a 

more restricted compositional range.

FE-Ti OXIDES

Both ilmenite and magnetite are present in the 

rhyodacite but make up less than 1 percent of the rock. 

They occur as minute (less than 0.01 mm) euhedral 

titanomagnetite and subhedral ilmenite microphenocrysts scattered throughout the groundmass and 

as inclusions in other phenocryst phases. Microprobe 

analyses of these crystals yielded low oxide totals and 

are not presented here. The low totals may be due to 

submicroscopic hematite intergrowths, formed during 

oxidation of the oxide grains, although the rock is 

unaltered apart from hydration of the glass. Oxide composition is much less varied than in the dacite, suggesting that the crystals may have been in equilibrium at 

one time and variations are now mainly due to differences in alteration. Minor elements vary somewhat 

both in ilmenite and in titanomagnetite.

ACCESSORY PHASES

The rhyodacite contains tiny needles of apatite as 

inclusions in most, if not all, phenocryst phases. Zircon 

is a less abundant companion that also can be found in 

the groundmass. Very rare allanite has been identified 

intergrown with hornblende in sample 106, the most 

differentiated rhyodacite.

RHYOLITE

Rocks classified as rhyolite occur locally as ash-flow 

and air-fall tuffs on Haiwee Ridge (Duffield and others, 

1980). White cellular pumice in these rocks contains 

about 5 percent euhedral phenocrysts of quartz, plagioclase, and sanidine and less than 1 percent each of 

biotite, titanomagnetite, and ilmenite. Compositions of 

plagioclase (An18 5 Ab79 4 Or2^) and sanidine (An0 6 

Ab26.5 Or?2.9) have been determined by Bacon and others (1982). Microprobe analyses of ilmenite and 

titanomagnetite crystals in a mineral concentrate from 

the pumice yielded low oxide totals similar to those of 

Fe-Ti oxides in the previously described rhyodacite samples. The sole analyzed ilmenite crystal has much more
20 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

MnO and less MgO and A12O3 than ilmenites in the 

rhyodacite. Titanomagnetite also has relatively high 

MnO and low MgO, A12O3, and TiO2 contents. These 

compositional differences are consistent with the oxide 

minerals in the rhyolite having crystallized in a relatively low-temperature, differentiated rhyolitic melt.

COMPOSITIONAL VARIATION

A broad spectrum of magmas ranging from basaltic to 

rhyolitic was erupted during the Pliocene volcanic episode in the Coso Range. In the following sections we 

describe the compositional variations of the Coso suite 

as a whole, but it should be kept in mind that these rocks 

represent magmas erupted over a long time interval and 

across a broad area. Major- and trace-element analyses 

are given in table 7 (see fig. 2 and table 10 for sample 

localities). Compositionally similar volcanic rocks have 

been described from several other areas in the western 

United States. Analyses of some of those rocks are presented in table 8 for comparison. Particularly pertinent 

to the present discussion is the study of Doe and others

(1969) on basalt of the southern Rocky Mountains. 

These authors document contamination of basalt by 

crustal material and show that this type of contamination can produce quartz-bearing intermediatecomposition lavas quite similar to those of the Coso 

volcanic field. Chemical analyses of volcanic rocks from 

areas near the Coso Range appear in papers by Ross

(1970) and Moore and Dodge (1980).

MAJOR ELEMENTS

A plot of total alkalis and CaO against SiO2 (fig. 12) 

shows that the Pliocene Coso volcanic suite is calcalkalic, with an alkali-lime index (Peacock, 1931) of 60. 

This plot also demonstrates that according to the 

alkalic-subalkalic division of Miyashiro (1978), the Coso 

basalts are alkalic to transitional. Most of the basalt is 

hypersthene normative, but some of the most mafic 

samples contain small amounts of normative nepheline 

(<2 percent). All Coso rocks with more than 54 percent 

SiO2 are quartz normative. Small amounts of corundum 

(<2 percent) appear in the rhyodacite and rhyolite 

norms, probably because alkalis were selectively 

depleted during hydration of these glassy rocks, leaving 

excess A12O3.

Oxide variation diagrams for the Coso suite (fig. 13) 

show roughly linear trends with much scatter in basalt 

and andesite. This is partially due to the fact that SiO2 

makes up an increasing percentage of the silicic rocks, 

allowing less scatter in the other oxides. However, we 

suggest that much of the scatter is due to mixing of 

variably fractionated basaltic magma with silicic

magma. Several diagrams (CaO, Na2O, P2O5) have 

slight inflections below about 53 percent SiO2 , suggesting that crystal fractionation produced chemical variation in the basalt. Both MgO and A12O3 are extremely 

variable in the basalt and andesite; variation in MgO, 

particularly in andesite may be attributed to the combined effects of olivine fractionation in a basaltic end 

member and addition of an MgO-poor silicic contaminant. Na2O varies about a subhorizontal line but 

decreases, probably because of Na loss during hydration 

of glass, in rhyodacite and rhyolite.

Crystal fractionation from a single parent composition should produce well-defined curved paths on variation diagrams because the oxides are removed at 

different rates at different stages depending on which 

phases are crystallizing. On the other hand, mixing of 

two different magma compositions should produce linear trends, the position of a point depending on the 

proportions of the end members. The well-defined linear 

trends on most of the plots of Coso data (for example, 

K2O, CaO) strongly suggest mixing was important in 

producing compositional variations. Curvature and 

scatter in other diagrams (for example, MgO, A12O3 ) 

suggest that multiple end members, variable amounts 

of contamination by crustal melts, and some degree of 

crystal fractionation all contributed to the overall variations in the Coso suite.

The plot of P2O5 against MgO (fig. 14) is useful for 

demonstrating mixing versus fractionation processes. 

Varying degrees of partial melting or fractional crystallization should produce a systematic increase of P2O5 

with decreasing MgO, followed by decreasing P2O5 in 

fractionated magmas once melts become saturated with 

apatite; mixing of silicic and basaltic magmas would 

tend to produce a covariance. The wide range of P2O5 

contents in Coso rocks with more than 5 percent MgO 

argues that neither crystal fractionation nor varying 

degrees of partial melting was the only process responsible for variation in the basalt and mafic andesite. The 

low-P2O5, intermediate-MgO rocks probably formed by 

addition of silicic material, and the high-P2O5 basalts 

can be explained by partial melting or crystal fractionation. The very high-P2O5, intermediate-MgO rocks 

(basalt of Rose Valley (Duffield and Bacon, 

1981) = andesites with >1.2 percent P2O5) probably 

cannot be explained by extreme fractionation, but may 

call for either unusual parent magmas or contamination 

with unusual materials. The decrease in P2O5 with 

decreasing MgO in rhyodacite reflects crystal fractionation of apatite, which is common as inclusions in phenocryst phases (table 1).

An important feature of the variation diagrams (figs. 

13 and 14) is the compositional gap between the
TABLE 7. Chemical analyses of Coso Range Pliocene volcanic rocks

[Major elements determined at Reston Rapid Rock Analysis Laboratory of the U.S. Geological Survey by methods described under "single solution" by Shapiro (1975); analysts: K-Coate

N. Skinner, and H. Smith. Trace elements determined by instrumental neutron activation analysis at the U.S. Geological Survey, Reston, Virginia; analyst: P.A. Baedecker

recalculated with analysis normalized to total 100 percent volatile free and with atomic Fe+ 2/Fe + 2 + Fe +3 = 0.86]

Sample No. 64 

49.5

65 66 

50.0 50.1

67 68 

50.3 50.4

45 

50.8

69 56 

50.8 51.0

70 

51.0

Major elements, in weight

Si02 - 

A1 2 03 

Fe2°3 

MgO 

CaC\- 

Na 20 

v n K oTI f)+ _ ____ _

TI n ___ __

P n P 2°5 

C0 2 

Total 

48.6 

17.0 

4.6 

4.8 

7.0 

9.3 

3.5 

1.3 

.40 

.21 

1.8 

.47 

.12 

.09

99.19

49.1 50.2 

16.8 16.8 

3.6 1.5 

6.1 7.5 

7.5 8.9 

9.6 9.5 

3.2 3.3 

.62 .71 

.33 .28 

.17 .23 

1.4 1.3 

.29 .29 

.12 .12 

.23 .07

99.06 100.70

49.5 50.0 

16.9 18.1 

2.5 2.6 

6.2 5.3 

7.4 5.9 

10.2 10.5 

3.5 3.5 

.67 1.5 

.25 .23 

.21 .17 

1.3 1.4 

.30 .41 

.12 .11 

.04 .04

99.05 99.76

50.6 

16.5 

3.5 

5.0 

7.8 

9.9 

3.3 

1.4 

.36 

.08 

1.3 

.43 

.12 

.02

100.31

50.0 50.5 

15.5 17.0 

6.9 3.2 

3.9 5.2 

5.4 6.9 

10.2 9.8 

3.8 3.7 

.82 1.2 

.46 .21 

.27 .23 

1.9 1.2 

.38 .33 

.16 .13 

.03 .02

99.72 99.62

Trace elements, in

Sc -

Cr -

Co 

f7

DU _________ _

Cs 

Ba 

La -

Nd 

f*A

Tm 

Lu 

 af __ ____

Ta 

-

30.5 30.5 

207.1 300.2 

40.5 42.4 

114 118 

60 50 

600 500 

3 3 

284 495 

12 14 

26 32 

14 16 

4.1 4.4 

1.23 1.26 

3.6 3.7 

.34 .33 

2.9 2.7 

.38 .39 

3.0 2.8 

.48 2 

.6 1.3

Q 0

27.0

50.4 

29.3 

110 

50 

160 

2 

952

54 

29 

5.7 

1.52 

4.8 

.33 

2.1 

.33 

4.1 

.51 

2.3

28.8 

222.4 

35.5 

111 

70 

500 

3 

1020 

25 

52 

29 

5.6 

1.47 

3.8 

.24 

2.1 

.29 

3.9 

.38 

2.4

26.3 

138.2 

32.3

110 

50 

500 

2 

724 

20 

39 

25 

1.7 

1.23 

3.5 

.33 

2.4 

.34 

3.5 

.64 

2.2

49.8 

18.2 

5.6 

2.8 

5.2 

8.8 

4.1 

1.3 

.93 

.55 

1.8 

.47 

.11 

.09

99.75

71 

51.5

percent

50.5 

16.5 

4.9 

3.7 

5.9 

10.2 

3.8 

.91 

.33 

.32 

1.4 

.44 

.13 

.09

99.12

72 

51.7

50.9 

16.5 

2.5 

6.0 

6.8 

9.3 

3.6 

1.0 

.34 

.18 

1.4 

.35 

.13 

.02

99.02

63 

51.8

51.6 

17.2 

2.0 

6.0 

6.7 

9.5 

3.8 

.88 

.28 

.12 

1.5 

.39 

.11 

.02

100.10

73 74 

52.1 52.5

51.6 52.0 

18.5 16.0 

3.0 3.3 

5.0 4.0 

5.3 7.5 

8.1 9.9 

4.3 3.3 

1.4 1.5 

.28 .47 

.38 .34 

1.6 1.2 

.36 .50 

.11 .11 

.05 .06

99.98 100.18

parts per million

21.0 

58.4 

28.6 

102 

26 

190 

2 

685 

30 

62 

32 

6.3 

1.81 

5.2 

4.2 

2.2 

.32 

4.5

2.4

-

-

-

25.0 

153.6 

30.2 

115 

40 

290 

2 

539 

20 

41 

25 

5.0 

1.44 

4.2 

.33 

2.2 

.31 

3.5

1.2

25.5 

264.2 

30.4 

112 

50 

500 

2 

968 

32 

62 

30 

5.9 

1.33 

3.1 

.25 

1.9 

.27 

3.3

2.7 

i i
TABLE 7. Chemical analyses of Pliocene volcanic rocks of the Coso Range Continued

Sample No. 

Sl °2

Si02 

A1 2 03 

FeO 

MgO 

Na 20 

Kn _ __

H 2 0~ -----

Ti02 

P 2°5

MnO - 

C02 

Total -----

Cr 

Co 

Zn 

DU 

JucL

Nd 

Eu - 

Lu 

TK _ - ___

IT ->

77 

53.5

10 Q

2.7 

 » a

5.0

4.0

1.7

.40

.19 

1.2

.32

.09

.10

99.20

78 

53.5

53.3

19.5

 O

4.1

4.9

1.6

.42

.30 

1.2

.27

.09

.01

100.49

17.6

27.0

95 

23

190

.5

Sf.t

16

32

1ft

4.0

1.15

4.1

.30

.25

3.0

.47

1.3 

9 n

79 80 

53.6 54.5

52.9 53.4

16.3 16.7

5.4 2.0

4.0 5.6

4.8 3.6

7.4 6.8

1.9 2.5

.40 .39

.42 .52

2.0 1.7

 Jo 1. J

.15 .12

.03 1.7

99.98 100.73

81

54.5

53.6

17.3

. 6

5.0

3.5

4.1

2.5

.73

.51

1.6

1.3

.12

99.66

14.1

21.3

105

oU

250

3

711

JO

67

6.3

1.48

4.5

.32

. 6

.34

4.6

1.63

4.4 

i a

82 83

54.5 55.1

Major elements

54.1 54.0

18.7 17.6

3.3 3.5

3.2 3.3

5.0 5.2

7.4 7.3

4.2 4.3

2.0 1.6

.66 .71

.34 .74

1.3 1.1

. ^O ./ /

.11 .09

.03 .06

100.62 99.77

Trace elements,

lo.j iy . j

40.6 43.1

26.0 27.4

92 97

24 50

210 500

2.0 2.0

846 849

lo 1 /

19 50

3.9 4.1

1.12 1.22

3.1 3.0

.29 .26

1.7 1.6

.24 .24

.51 .56

» o 1 . / 

n A

84

55.2

, in weight

55.1

16.4

3.2

4.6

7.1

3.6

1.9

* J O

.44

1.4

.50

.11

.Uo

101.01

in parts pe

19.0

197.1

29.2

103

21

270

2.0

yy j

32

63

29

6.5

1.55

4.0

.32

2.0

.31

4.0

1.07

2.9

i i

85 40

55.3 56.5

percent

55.1 56.9

16.7 17.4

3.6 1.9

5.6 5.5

3.6 3.9

i y i . o

.60 .26

.33 .32

1.0 1.1

.JO .^O

. uy . uo

.02 .02

100.80 101.34

r million

86

56.7

57.0

1. o

4.5

5.4

 £.

3.6

1.9

.42

.24

.98

.34

.(Jo

101.26

/U .O

ol j

// *o

97

25

lou

0.6

1130

31

60

32

5.6

1 .43

4.6

 /o

1.7

.23

3.3

.46

3.3

n Q

87 88

56.7 56.9

56.7 56.8

16.7 16.3

1.7 1.4

7.7 7.0

3.9 3.7

1.9 2.0

.48 .14

.23 .22

1.2 .89

.29 .20

.08 .07

.09 .02

100.87 100.24
TABLE 7. Chemical analyses of Pliocene volcanic rocks of the Coso Range Continued

Sample No. -- 

SiO * 

2

91 

58.8

92 

59.7

93 

61.3

94 95 

61.4 62.1

96 

63.4

97 98 

63.7 64.0

99 

66.0

Major elements, in weight

SiOo 

M f\ _ _

Feo3 -

FeO 

MgO 

Na 20 

Kn

H

rt +

H 2 0~ 

Ti02 

MnOTotal 

Cr 

Co 

Rb 

Ba 

Po _ 

Sm 

Eu 

r* A

Yb 

TT f

U 

57.0 

16.2 

1.6 

4.0 

3.2 

8.0 

3.7 

1.9 

1.1 

.48 

1.0 

.31 

.08 

.90

99.47

-

59.0 

15.3 

2.1 

3.0 

6.5 

6.0 

3.7 

2.3 

.53 

.50 

.79 

.21 

.07 

.02

100.02

14.1 

262.2 

26.8 

81 

44 

120 

0.7 

1400 

21 

38 

19 

3.7 

.96 

3.4 

.24 

1.6 

.23 

3.1 

.43? 

2.5 

.7

60.5 

16.8 

1.3 

3.4 

3.6 

5.2 

4.0 

2.8 

.91 

.33 

.87 

.21 

.06 

.03

100.01

11.8 

50.5 

17.2 

70 

53 

600 

2 

1230 

24 

45 

17 

4.0 

1.01 

2.6 

.22 

1.3 

.19 

3.8 

.81 

3.6 

1.4

60.9 61.6 

16.8 16.5 

1.5 1.1 

3.2 3.1 

3.5 4.3 

6.1 5.0 

3.7 4.0 

2.6 2.6 

1.1 .63 

.25 .12 

.72 .74 

.25 .22 

.06 .05 

.01 .06

100.69 100.02

13.2 10.4 

56.4 132.6 

14.7 17.8 

82 69 

24 52 

400 170 

1.0 1.1 

1270 1180 

26 21 

49 38 

21 17 

4.2 3.2 

1.01 .88 

2.6 2.8 

.18 .23 

1.3 1.3 

.19 .17 

3.4 3.5 

.57 .66 

3.3 3.3 

1.0 1.1

63.0 

16.2 

1.6 

2.6 

3.5 

4.8 

4.1 

2.7 

.72 

.52 

.73 

20 

.05

100.72

Trace

9.9 

95.7 

lb.3 

74 

63 

230 

1.0 

1330 

21 

41 

16 

3.6 

.90 

2.4 

.15 

1.1 

.17 

3.5 

.67 

3.6 

1.4

63.0 63.6 

16.3 16.5 

1.4 1.2 

2.6 2.6 

3.2 3.2 

4.5 4.3 

3.7 4.1 

3.3 3.0 

1.1 1.4

.35 

.68 .68 

.20 .19 

.05 .04 

.11 .01

100.49 100.82

elements, in

9.03

65.7 

14.1 

72

64

400

1.4

1220

49 

50 

3.6 

.94 

2.4

.22

1.3 

.19

4.5

 /o

4.5

62.6 

15.0 

2.9 

.12 

2.4 

4.5 

3.6 

3.2 

2.0 

1.1 

.50 

.18 

.03 

.80

98.93

100 

67.5

percent

66.4 

15.9 

1.3 

1.8 

1.5 

3.4 

3.9 

3.4 

1.8 

.15 

.58 

.20 

.06 

.03

98.47

101 

68.3

65.7 

15.5 

1.5 

1.6 

1.3 

3.5 

3.4 

3.1 

3.7 

.53 

.54 

.15 

.04 

.01

100.04

102 103 

68.7 68.9

67.3 66.1 

15.8 15.5 

1.4 1.2 

1.4 1.6 

1.3 1.2 

3.0 3.0 

3.8 3.3 

3.4 3.4 

2.0 3.2 

.24 .47 

.46 .50 

.03 .16 

.04 .04 

.05 .02

98.08 99.2

104 

68.9

67.0 

15.8 

1.0 

1.5 

1.1 

2.9 

3.9 

3.5 

2.2 

.21 

.44 

.15 

.03

97.32

parts per million

5.12 

15.5 

6.4 

42 

64 

200 

1.3 

1260 

28 

50 

21 

3.4 

.79 

2.7 

.21 

1.2 

.18 

3.5 

.81 

5.6 

1.7

5.18 

13.1 

6.2 

58 

71 

190 

1.8 

1190 

25 

48 

20 

3.8 

.87 

2.6 

.2 

1.3 

.18 

3.5 

.93 

5.4 

2.2

5.00 

10.1 

5.7 

56 

74 

300 

1.8 

1310 

23 

44 

19 

3.5 

.74 

2.5 

.20 

1.1 

.15 

3.6 

.84 

5.4

4.52

9.5

5.1

5.1 

76 

240 

2.0 

1150 

23 

41

19 

3.1 

.69 

2.0 

.14

1.0 

.16

3.7 

.74 

5.6 

2.2

4.16 

10.0 

4.9 

4.9 

80 

200 

1.9 

1260 

25 

47 

19 

3.0 

.76 

2.6 

.20 

1.0 

.16 

3.8 

.77 

5.2 

1.8
24 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

rhyodacite and most of the dacite. The single dacite 

pumice (sample 99) plots at the mafic end of the 

rhyodacite trend, presumably because it is unique 

among the analyzed dacite samples in containing a very 

small basaltic component. The compositional gap may 

indicate that hybrid magmas with less than some minimum component of basaltic material are unlikely to 

form in zoned systems for mechanical reasons. Compositional gaps in the dacite range are very common in 

volcanic systems that produce relatively silicic rocks 

(Smith, 1979; Hildreth, 1981).

An AFM plot (fig. 15) shows a broad band not unlike 

trends of other calc-alkaline suites, although the Coso 

rocks are somewhat lower in Fe than continental margin 

suites (Gill, 1981). Rhyodacite and rhyolite form a linear 

array, dacite a tight cluster (excepting the pumiceous 

sample 99, which has suffered oxidation and Na loss), 

and basalt and andesite overlapping fields. The scatter 

in andesite is notable and is mainly due to variation of 

the Fe/Fe + Mg ratio at constant alkali content. The 

marked dispersion of the andesite data on figure 15 is 

probably due to competing effects of crystal fractionation and mixing, superimposed on original variation in 

basaltic end-member composition. If the dacite was 

derived in part by mixing, the silicic end member apparently had a lower Fe/Fe-l-Mg ratio than the erupted

rhyodacite.

The FeO*/FeO* + MgO ratio plotted against SiO2 for 

the Coso Pliocene volcanic rocks shows a broad diffuse 

pattern that trends subhorizontally from basalt to 

dacite, but the rhyolite and rhyodacite cluster at comparatively high levels (fig. 12). The lowest FeO*/ 

FeO* -I- MgO ratios occur in andesite and dacite. Such 

low values are hard to reconcile with fractionation of 

observed phenocrysts in reasonable proportions but can 

be explained by addition of a silicic component to relatively primitive basaltic magma. Conversely, andesite 

and silicic basalt with high FeO*/FeO* + MgO ratios 

might be derived by crystal fractionation. Most of the 

andesitic rocks, however, would be most readily formed 

by mixing of moderately differentiated basalt and a 

silicic component.

TRACE ELEMENTS

Trace-element abundances have been determined by 

instrumental neutron activation analysis (INAA) for 

selected samples (table 7).

TRANSITION METALS (Sc, CR, Co, ZN)

The transition metals Sc, Cr, Co, and Zn tend to be 

partitioned into early formed crystals during the crysTABLE 8. Chemical analyses of basalts and andesites from the western United States

[Oxides in weight percent]

Sample 

Si0 2 48.4 56.3 50.54 54.25 51.3 56.8

Al.,03 17.4 17.0 15.97 16.40 17.1 14.3

Fe 2°3 3 ' 4 5 ' L 2 ' 81 2 ' 63 2 ' 9 3>0

FeO 5.9 1.8 5.02 4.24 6.1 4.9

MgO 2.5 4.1 8.96 6.24 6.6 5-6

CaO 9.1 6.8 9.20 8.04 7.6 6.6

Na 2 0 5.1 4.0 3.50 3.72 4.2 3.5

K2 0 1.6 2.7 1.51 1.50 1.7 2.4

H 2 0+ 2.7 .66 .13 .68 .25 .43

H 2 CT .17 .12 .14 .28 .10 .47

Ti02 1.9 1.1 1.36 1.09 1.6 1.3

P 9 05 .80 .41 .44 .38 .45 .39

MnO .12 .12 .14 .12 .14 .12

C0 2 .14 .26 .38 .25 .05 .21

Total 99.23 100.47 100.10 99.82 100.04 100.02

1. Trachyandesite caprock, Saline Range, CA (Ross, 1970). 

2. Trachyandesite caprock, Saline Range, CA (Ross, 1970). 

3. Alkali olivine basalt, Kern volcanic field, CA (Moore and Dodge, 

4. Andesite, Kern volcanic field, CA (Moore and Dodge, 1980). 

5. "Primitive" basalt of the Hinsdale Formation, CO (Doe and others 

6. Very contaminated basalt of the Hinsdale Formation, CO (Doe and < 

7. Hawaiite, AZ (Best and Brimhall, 1974). 

8. Quartz-bearing basaltic andesite, AZ (Best and Brimhall, 1974).

15.7

'10.5

7.6

1.4

1.7

98.4

1980).

, 1969) 

others ,

CO £

15.4

1 Q Q

6.6

7.b

1.6

98. y

1969) .

- Total Fe expressed as FeO.
COMPOSITIONAL VARIATION 25

tallization of magmas; consequently, abundances of 

these elements tend to correlate with one another and to 

decrease with increasing SiO2 . This trend is generally 

shown by the Coso rocks (fig. 16). Chromium varies 

considerably in the basalt because of widely differing 

proportions of chrome spinel inclusions in olivine phenocrysts (table 1); the most magnesian basalt has Cr 

contents several times that of less magnesian basalt of 

the same silica content. The Cr-rich primitive basalt (for 

example samples 65, 66) also has the lowest abundance 

of incompatible elements such as K, Ba, and Th.

Rhyodacite has inonotonically lower contents of Sc, 

Cr, Co, and Zn with increasing differentiation. The 

rhyolite has the lowest abundance of these compatible 

elements, which are greatly depleted relative to even the 

most differentiated rhyodacite. The single dacite 

pumice (sample 99) extends the rhyodacite trend to 

slightly higher values (fig. 16). Andesite and less differentiated dacite have highly variable compatible-element abundances, and this is reflected particularly well

10

o 8 

oc

- 4

W) 111 a 

x

O 2

0 

1.0

0.8

0.6

0.4

0.2

50 60 70

Basalt Andesite

RhyoDacite dacite Rhyolite

50 60 70 

SiO2 , IN WEIGHT PERCENT

FIGURE 12. CaO (circles), Na2O + K2O (dots), and FeO7 

FeO* + MgO (dots) vs. SiO2 for Pliocene Coso Range volcanic 

rocks. Alkalic-subalkalic fields (separated by dashed line) 

from Miyashiro (1978). Crossing point of CaO and Na2O + K2O 

trend lines gives alkali-lime index of 60, indicating that the 

Coso suite is calc-alkalic (Peacock, 1931).

by Cr. Because crystal fractionation should result in 

rather similar Cr contents for a given degree of differentiation, the only conceivable mechanism for producing 

this great variation is mixing of silicic magma and relatively primitive Cr-rich basaltic magma.

ALKALI METALS AND ALKALINE EARTHS (RB, Cs, BA)

Concentrations of the alkali metals Rb and Cs are 

high enough to be detected by neutron activation (INAA) 

only in silicic andesite and more differentiated rocks. 

Both these elements are present at nearly uniform levels in the rhyodacite and are highly enriched in the 

rhyolite. Dacite and silicic andesite have lower and variable Rb contents. The same appears to be true of Cs, but 

the abundance of Cs commonly is below the detection 

limit.

Barium is the only alkaline-earth trace element 

determined by INAA. Of the common phenocryst species, only alkali feldspar and biotite have a strong 

affinity for Ba. Thus Ba would be expected to rise 

rapidly with differentiation of basaltic magma and drop 

in silicic melts from which biotite or alkali feldspar was 

being removed. The plot of Ba versus SiO2 (fig. 16) confirms this general model, but several complications are 

worth noting. Some of the most Ba-rich basalt samples 

also have very high Cr and other compatible-element 

contents, characteristic of primitive basalt; these lavas 

also have unusual rare-earth-element patterns. Mixing 

with silicic magma will not readily explain these chemical characteristics. Barium contents of silicic andesite, 

dacite, and the less-differentiated rhyodacite are similar, possibly reflecting the opposing effects on silicic 

magma of separation of biotite and addition of basaltic 

magma. Like most trace elements in the andesite, Ba 

shows wide variation at a given SiO2 content. This is 

readily explained by the effects of crystal fractionation 

and of mixing with variably differentiated or contaminated basaltic magma, as will be discussed later 

with reference to all of the analytical data. Barium 

content decreases with increasing SiO2 in the rhyolite 

and the more differentiated rhyodacite.

RARE EARTHS

The rare-earth elements (REE) La, Ce, Nd, Sm, Eu, 

Yb, and Lu were determined with acceptable precision 

in all samples; Gd and Tm also were determined, but 

results are believed to be less accurate because of inconsistency with those for other REE. In this discussion, 

light rare earth elements (LREE) are those with atomic 

numbers less than Eu, and heavy rare earth elements 

(HREE) are those with atomic numbers greater than 

Sm. Chondrite-normalized REE abundances are plotted
26 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

for different rock types in figure 17. The two most primitive basalt samples (65 and 66) have moderately fractionated REE patterns. They are similar to Pleistocene 

basalt erupted in the Coso Range (Bacon and Metz, 

1984), but have slightly lower LREE and higher HREE 

contents. Other Pliocene basalt samples that are 

unfractionated based on high compatible-element contents have higher LREE contents than samples 65 and 

66. These LREE-enriched rocks also have higher K2O, 

P2O5 , and Ba contents than the most primitive basalt

(for example, samples 68, 74). More extreme enrichments of these elements occur in the ultrapotassic 

basaltic lavas of the Sierra Nevada studied by Van 

Kooten (1980).

Dacite and rhyodacite have rather similar REE abundance patterns. Relative to the primitive basalt, they are 

enriched in LREE and strongly depleted in HREE. The 

patterns are similar in overall shape to those of many 

Sierra Nevada granitoids (Dodge and others, 1982), but 

abundances of all REE are notably lower in the volcanic

18

16 

14

10 

8

6

4 

2

0 

10

8

6

4

2 

n

1

 * A12 O3 

- * - 

 * 

* 

i i i

 ' ' ' 

V.' FcO*

 f

0 

 * \

1 1 1

1 1 1 

- , MgO

7*' .

* » .

~ *" % * ".

. ~

i i i 

10

8 

6

4

2

5 

4 

3

4

2 

1

0

2

1

0

0.75 

0.50 

0.25

n

I ' I ' I ' 

CaO _

- 

"\ *

"I , I i I , "

I ' I ' I 

Na2 O

** / «** * *** 

 % ,** »

_" '

I I I i I i

_l ' I ' I ' ?_ 

 K2 O

 . ».4 *

I i i 1

jiftiW..1 ,. T0^

1 i 1 i * *"*! i 

_l ' 1 ' 1 ' _

_ _

50 60 70 50

Si02 , IN WEIGHT PERCENT

60 70

FIGURE 13. Major-element oxide variation diagrams for Pliocene Coso Range volcanic rocks. Prior to plotting, analyses were

recalculated to 100 percent as described in text.
COMPOSITIONAL VARIATION 27

rocks. Patterns for andesite generally are displaced 

from those of primitive basalt in the same sense as for 

the silicic rocks, rotating clockwise about a point 

between Nd and Sm, a feature consistent with an origin 

involving magma mixing. Absence of a Eu anomaly in 

all of the lavas except the rhyolite argues against significant plagioclase fractionation.

One mafic andesite (sample 81) is unusual in that it 

has the highest REE (except Eu) concentrations of any 

of the analyzed rocks. Its REE pattern is similar in 

shape to those of some LREE-rich basalt samples, but is 

displaced upward. The lava flow from which this sample 

was obtained issued from a vent near the western margin of the volcanic field, in the vicinity of the Sierra 

Nevada fault zone. Another sample from the same unit 

(basalt of Rose Valley; Duffield and Bacon, 1981) shows 

petrographic evidence of contamination (table 1; sample 

80), and both these samples have very high KgO and, 

especially, P2O5 contents. Bacon and Metz (1984) suggested that Pleistocene lavas erupted in this same area 

which have unusual trace-element contents were contaminated by crustal material. Alternatively, an 

enriched mantle source could be postulated, but this 

appears inconsistent with the petrographic data.

The high-silica rhyolite (sample 44) is depleted in all 

REE relative to basalt and it is the only analyzed rock 

with a significant Eu anomaly. Although the abundances are low, the LREE show a steep fractionated 

pattern and the HREE have a positive slope. Note that 

although mixing of basaltic and silicic magma is pro1.6

1.4

1.2

oc 1.0

5 0.8

0.4

0.2

0

EXPLANATION

T Rhyolite

O Rhyodacite

 Dacite 

A Andesite

 Basalt

"

987654321 0 

MgO, IN WEIGHT PERCENT

FIGURE 14. MgO vs. P205 for Pliocene Coso Range volcanic rocks. 

Andesites with very high P20s contents (two samples) are from 

the basalt of Rose Valley (Duffield and Bacon, 1981).

posed to explain variations in REE patterns of intermediate-composition rocks, the silicic end member is 

not represented by the analyzed high-silica rhyolite. 

None of the Pleistocene high-silica rhyolites of the Coso 

Range have REE patterns similar to that of the Pliocene 

rhyolite (Bacon and others, 1981). A small Pleistocene 

high-silica rhyolite dome in the southern Sierra Nevada 

has an REE pattern somewhat similar to that of the 

Coso Pliocene rhyolite, but with higher abundances, 

especially of HREE (Bacon and Duffield, 1981).

HIGHLY CHARGED CATIONS (ZR, HF, TA, TH, U)

Concentrations of Hf, Ta, and Th are believed to be 

accurately determined in virtually all samples. The contents of U and Zr are less well known, particularly Zr in 

basalt and andesite, and they will not be described in 

detail. The behavior of Hf is similar to that of many 

incompatible elements levels are relatively constant 

in rhyodacite and dacite and variable in basalt and 

andesite. Rhyodacite and rhyolite do not have the highest Hf contents. The may be because zircon, which is a 

common accessory in these rocks, preferentially incorporates Hf and was probably a fractionating phase during their generation.

Tantalum abundance is erratic. The rhyolite has the 

highest concentration of Ta, but basalt and andesite 

show the widest range among the other rocks. Dacite 

and rhyodacite have moderate Ta content, and the combined effects of mixing and fractionation may explain 

the variable concentration of this strongly incompatible 

element in the basalt and andesite.

Thorium behaves in a manner similar to many other 

incompatible elements in that abundances generally 

increase with SiO2, but scatter in a manner that is 

consistent with combined crystallization differentiation 

and magma mixing. Concentrations are high and relatively constant in the most silicic dacite and all but the 

most differentiated rhyodacite, which has an elevated 

Th content (fig. 16). The rhyolite contains nearly three 

times the Th of most of the rhyodacite. Primitive basalt 

has the lowest Th levels. The mafic andesite of sample 81 

is an exception to this pattern, possessing unusually 

high Th, as well as high U and Ta. Uranium levels in all 

rocks are roughly one-third those of Th, except in the 

rhyolite, where U is approximately one-half Th. A low 

Th/U ratio is characteristic of high-silica rhyolite.

A plot of Th versus Cr (fig. 18) is instructive in showing 

possible effects of mixing and crystal fractionation. Cr is 

a highly compatible trace element that is depleted by 

fractionation of early-formed spinel and pyroxene. 

Therefore, its concentration drops rapidly to low levels 

in early stages of crystallization. Th is a highly incompatible element, so its concentration remains relatively
28 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

low until a significant amount of crystallization has 

taken place. Crystal fractionation will therefore produce curved paths on the Th versus Cr plot; two calculated possible paths are shown in figure 18. Mixing, on 

the other hand, will produce compositions that plot 

along straight lines within the concavity of the fractionation curves. Such possible mixing trends are schematically shown by dashed lines on figure 18. The Coso 

data scatter widely on the diagram, basalt varying in 

both Cr and Th content and most rhyodacite and differentiated dacite forming a tight cluster. Intermediatecomposition rocks commonly have Th concentration in 

excess of what might be expected for a given Cr content. 

We suggest that the variation in Cr content of the basalt 

is due to varying amounts of crystal fractionation and 

that most of the spread in the andesite values is due to

mixing of basaltic magma with silicic melts much like 

the rhyodacite. Note that some low-Cr andesite also has 

quite low Th content, however, and may have been 

derived from relatively Th-rich basalt mainly by crystal 

fractionation.

DISCUSSION

In this section we discuss models for petrogenesis of 

the Pliocene volcanic rocks, based on the data presented 

earlier. These models include specific fractionation and 

mixing schemes, which are quantitatively evaluated, 

and inferred processes that operated to form the compositional spectrum of volcanic rocks. We also attempt to 

relate the dominant processes to changes in the tectonic 

regime of the Coso Range.

FeO*

50 50

Na2O+K2 O

EXPLANATION

COSO RANGE SAMPLES 

Rhyolite 

Rhyodacite 

Dacite 

Andesite 

Basalt

MgO

FIGURE 15. Ternary AFM diagram for Pliocene volcanic rocks. The field labeled Volcano Butte shows variation within a single polygenetic 

Coso Range volcano. Field for Basin and Range basalts from Leeman and Rogers (1970); field for Pliocene andesite of the Lava Mountains, 

Calif, from Smith (1964); Cascade Range trend from Smith and Carmichael (1968).
DISCUSSION 29

Quantitative mass-balance calculations referred to in 

this section (table 9) were performed using the program 

of Stormer and Nicholls (1978). Input for the calculations consisted of major-element analyses of the lavas 

and microprobe analyses of phenocrysts. In an attempt 

to provide additional constraints, most of the modelling 

was done on lavas erupted from the Volcano Butte center 

(fig. 1), all of which appear to be nearly contemporaneous.

PETROGENESIS

BASALT

Few basaltic lavas of the Coso volcanic field have 

chemical characteristics that would be considered primitive (for example, high MgO and Cr, low incompatibleelement contents). Those that do generally form extensive, thin diktytaxitic flows that erupted from monogenetic vents. These rocks are the closest to unmodified 

partial melts of the mantle, but even they may have 

undergone some fractionation prior to eruption. The 

samples of primitive basalt have gently sloping LREEenriched chondrite-normalized REE patterns that suggest derivation from a garnet-bearing mantle source. 

Compositional variation among these samples may be 

ascribed to small heterogeneities in source composition 

and to differing degrees of partial melting, with superimposed fractionation of olivine+chrome spinel. All 

other basaltic lavas are believed to have been significantly affected by fractional crystallization, commonly 

accompanied by assimilation of crustal material.

Some basalt samples (45 and 74) have high MgO and 

Cr contents and also high contents of I^O, P2O5 , Ba, 

and LREE. The primitive characteristics of these rocks 

contrast with their high contents of many elements that 

are believed to behave as incompatible elements in

15

10

o

40

Th

 : .: *

20

0

30

20

10

Co

 ** :

 

Sc

1500

1000

500

Ba

 

 

300

200

100

Cr

50 60 70 50

SiO2, IN WEIGHT PERCENT

60 70

FIGURE 16. Th, Co, Sc, Ba, and Cr vs. SiO2 content for Pliocene Coso Range volcanic rocks.
30 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

basaltic magmas. They have SiO2 contents 1 to 2 percent 

higher than other "primitive" lavas erupted at approximately the same time (Duffield and Bacon, 1981). Van 

Kooten (1980) described ultrapotassic mafic lavas from

the Sierra Nevada which show extreme development of 

many of these chemical characteristics. The Sierran 

lavas, however, have relatively low SiO2 contents and 

much higher trace-element abundances. Van Kooten

100

50

10

I 1

"Primitive" basalts 

<51 percent SiO2

100

50

10

"Primitive" basalts

La Ce Nd SmEuGd Dy TmYbLu La Ce Nd SmEuGd Dy TmYb Lu

100

50

10

"Primitive" basalts^

5 

100

50

10

Dacites

"Primitive" basalts

5 

La Ce Nd SmEuGd Dy TmYbLu La Ce Nd SmEuGd Dy TmYbLu

100

50

10

Primitive" basalts

La Ce Nd Sm Eu Gd Dy Tm Yb Lu

FIGURE 17. Chondrite-normalized rare earth element plots for Pliocene Coso Range volcanic rocks. Analyses are normalized to values for 

the Leedey Chondrite (Masuda and others, 1973) divided by 1.2 and plotted as a function of ionic radius (Whittaker and Muntus, 1970).
DISCUSSION 31

(1980) appealed to a phlogopite-bearing mantle source 

to explain the composition of the Sierran lavas, whereas 

Dodge and Moore (1981) called upon a source containing 

amphibole. Some Pleistocene lavas of the Coso volcanic 

field are comparable to the LREE-rich Pliocene rocks, 

and Bacon and Metz (1984) suggested that contamination with deep crustal material caused at least 

some trace-element enrichment in those Pleistocene 

rocks. Watson (1982) showed experimentally that certain elements, notably K, are preferentially incorporated into basaltic magma interacting with partially 

melted crustal rocks. Selective contamination provides 

an attractive mechanism for increasing some incompatible-element concentrations in basaltic magma erupted 

through continental crust. Some crystallization would 

be expected to accompany assimilation of crustal material because of the loss of heat necessary to partially fuse 

country rocks. MgO and Cr contents of the LREE-rich 

Coso basalts are high, however, implying that extensive 

crystal fractionation could not have occurred. Heterogeneity of the mantle source remains a possible explanation for the chemical characteristics of these basalt 

samples.

Basalts with MgO contents below 7 percent are 

believed to have undergone some fractional crystalliza6 

EXPLANATION

T Rhyohte

 Rhyodacite

 Dacite 

A Andesite

 Basalt

fa A x\ Fractional crystallization 

Fractional crystallization

100 200 

Cr, IN PARTS PER MILLION

300

FIGURE 18. Th vs. Cr content for Pliocene Coso Range volcanic rocks. 

Open circles represent hypothetical parent magmas for the calculated fractionation curves shown. Dashed lines show possible 

compositions formed by mixing variably fractionated basaltic 

magma with rhyodacitic magma. Arrow indicates that rhyolite 

sample is off scale at a Th content of 14.9 ppm.

tion and will be referred to as differentiated. Massbalance calculations employing microprobe analyses of 

representative phenocrysts suggest that separation of 

reasonable amounts of olivine + plagioclase ± 

clinopyroxene will account for most of the major-element variation in differentiated basalt (table 9, model 1). 

Residuals for K2O typically are greater than for other 

oxides and suggest that additional processes, such as 

selective contamination (Watson, 1982), must have been 

operative. The assimilation of some crustal material 

seems inevitable for mantle-derived magmas intruded 

into, and perhaps stored in, the deeper part of the crust. 

The most sensitive indicator of crustal interaction 

would be expected to be K. Indeed, Doe and others (1969) 

cited elevated K content as one of the criteria for recognizing contamination in basaltic lavas from the Rocky 

Mountain region which showed isotopic evidence of a 

crustal component. Pleistocene differentiated and contaminated basaltic lavas of the Coso volcanic field have 

relatively radiogenic Pb and Sr isotopic compositions 

consistent with the addition of crustal material (Bacon 

and others, 1984). Simple two-component mixing of 

primitive basaltic magma and small amounts of silicic 

magma would have recognizable effects on the majorelement composition, and perhaps phenocryst mineralogy, of the basalt and cannot readily produce the composition of the differentiated basalt. Least-squares 

fractionation calculations, however, show that separation of olivine + plagioclase ± clinopyroxene combined with assimilation of small amounts of crustal 

material (table 9, model 4) can account for that composition.

A significant difference between Pliocene and 

Pleistocene basalt of the Coso volcanic field is the commonly high TiO2 content of the younger rocks (Bacon 

and Metz, 1984). None of the Pleistocene lavas has such 

low KgO, P2O5 , or TiO2 content as some of the Pliocene 

basalt. This may be because prolonged igneous activity 

affects the extent to which basaltic magma differentiates and interacts with crustal rocks. In the later history 

of the volcanic field, cumulative advection of heat to the 

deep crust may have promoted partial melting and 

caused increased modification of basaltic magma on its 

trip from the mantle to the surface.

INTERMEDIATE-COMPOSITION ROCKS

Andesite and most dacite samples contain mixed populations of phenocrysts and have textures that reflect 

sudden thermal disturbances brought about by mixing 

of magmas of contrasting composition and temperature 

(see Hibbard, 1981; Lofgren and Norris, 1981). Many 

previously noted geochemical features also suggest the 

derivation of intermediate-composition rocks by mixing 

of magmas. Mass-balance calculations indicate that the
32 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

Coso andesites cannot be derived from primitive basalt 

by crystal fractionation unless unrealistic amounts of 

hornblende are removed (table 9, model 2). Because 

hornblende is not present as a stable phase in rocks less 

silicic than dacite, its participation in the genesis of 

andesite is considered unlikely. Acceptable mass-balance solutions were obtained on the assumption that the 

andesite was generated from basalt by crystal fractionation combined with assimilation of silicic material (table 

9, model 5; DePaolo, 1981). Because the calculations 

indicate a very large fraction of silicic material must be 

added, mixing with silicic magma is favored over assimilation of crustal rocks to avoid excessive cooling. Since 

the very common xenocrysts in the intermediate rocks 

neither appear deformed nor show subsolidus reequilibration effects, it is more likely that they originated as phenocrysts in silicic magma than as 

disaggregated plutonic or metamorphic rocks.

Fractional crystallization may have produced some 

variation within the intermediate rocks, for example 

from andesite to dacite, if small amounts of hornblende 

were present in the fractionating assemblage (table 9, 

model 3). In addition, the more silicic intermediate magmas could be derived from less silicic magmas by separation of plagioclase + clinopyroxene combined with 

assimilation of silicic magma (table 9, model 6). As in the

previous assimilation-fractionation model, the amount 

of assimilated material is much greater than the 

amount of crystals removed.

Trace-element plots and ratio-ratio plots also can be 

used to quantify mixing relationships (Langmuir and 

others, 1978). The Th versus Cr plot (fig. 18) already has 

been discussed in this connection. A number of ratioratio plots were constructed in an attempt to further 

quantify the amount of mixing needed to derive the Coso 

intermediate-composition rocks. Some plots show regular relationships, but none meet the criteria for 

rigorously demonstrating two-end member mixing. 

This probably is a result of our inadequate knowledge of 

specific end members, since the Coso suite represents 

rocks of many ages erupted from widely distributed 

centers.

Trace-element contents of intermediate-composition 

rocks are most readily explained by mixing of variably 

differentiated and (or) contaminated basaltic magma 

with silicic magma. Chondrite-normalized REE patterns for most andesite samples fall between those of 

basalt and silicic rocks. Absence of marked Eu anomalies virtually rules out significant plagioclase fractionation. The rotation of patterns about a point between Nd 

and Sm, such that LREE become enriched and HREE 

depleted, is inconsistent with fractionation of observed

TABLE 9. Least squares solutions to magmatic differentiation models

[Each model used pairs of analyzed Coso Range Pliocene rocks and the listed phenocrysts or rock compositions. Granodiorite BP-2 is from 

Bateman and others (1984). SiO2 * = SiO2 in weight percent recalculated with analysis normalized to total 100 percent volatile free and 

with atomic Fe +2/Fe+ 2 + Fe+3 = 0.86]

Modelc .1 r\ st co i

Derivative: (sample) 56

n r\ -jf CIO

Mineral Phases: 

(wt. percent) 

Dacite (99) --- - 

Olivine »-6.1

Whole Rock analyses: 

(wt. percent) 

Dacite (99) 

Granodiorite (BP-2) 

66 

50.1 

83 

55.1

'-8.1 

3 -18.9 

6 -3.7 

7 -20.8 

a -.7

92 

59.7 

95 

62.1

'-4.4 

"-4.8 

6 -5.1 

7 -2.9

a-.21

66 66 

50.1 50.1 

56 89 

51.0 57.2

'-5.2 2 -3.9 

"-2.0 3 -1.9 

6-4.5

+68.4 

+3.1

88 

56.9 

92 

59.7

s -10.5 

6 -1.6

+39.0

Eresid 2 - .34 .17 .02 .38 .11 .33

Compositions of mineral phases used in fractionation calculations are given in tables 

2-5. Mineral phases used in the calculations are phenocrysts that occur in the parent 

or derivative, or are in compositionally similar rocks.

1 olivine in sample 65 s plagioclase in sample 92

2 olivine in sample 45 6 clinopyroxene in sample 45

3 plagioclase in sample 65 7 hornblende in sample 99 

* plagioclase in sample 99 a ilmenite in sample 65
DISCUSSION 33

phenocryst minerals, a process which should not 

deplete differentiates in HREE. Mafic andesite (sample 

81), which has elevated REE but rather low compatibleelement abundances, could have been derived by fractionation of LREE-rich basaltic magma, although its 

high A12O3 content and lack of a Eu anomaly suggests 

plagioclase was not involved. Abundant mineralogic evidence for contamination in this lava is not consistent 

with simple crystallization differentiation.

Continental interior andesite compositionally similar 

to andesite of the Coso Range has been described from 

the San Juan volcanic field in southwestern Colorado. 

The San Juan andesite has REE patterns nearly identical to those of the Coso rocks (Zielinski and Lipman, 

1976; Lipman, unpub. data 1982). Zielinski and Lipman 

(1976) surmised that the REE patterns of the San Juan 

rocks suggest they were ultimately formed as mafic 

melts from a garnet-bearing source; however, isotopic 

evidence indicates extensive contamination with lower 

crustal material (Lipman, unpub. data, 1982). Such an 

origin is consistent with that inferred for the Coso 

andesites, in which the lower crustal component is 

thought to have been silicic magma derived by partial 

fusion of garnet-bearing rocks.

The first intermediate-composition magmas were 

erupted several hundred thousand years after the 

inception of basaltic volcanism in the Coso Range. Most 

of the andesite and dacite was erupted about 3.6 to 3.3 

m.y. ago from polygenetic centers where volcanism was 

focused for relatively long periods; intermediate-composition lavas from monogenetic vents were only 

erupted later in the history of the field. Clear evidence 

for the coexistence of more than one magma composition 

in specific centers is present in the form of commingled 

bombs and of lava flows with magmatic inclusions. Separation of liquids from crystals, by whatever process, was 

accompanied by episodes of mixing. Magma mixing is 

easily imagined to take place in igneous systems, particularly those of comparatively small volume, which 

are zoned from relatively differentiated magma downward into basaltic roots. Such a model will be more fully 

developed later, after the silicic rocks have been treated.

RHYODACITE

Major-element compositions of rhyodacite and silicic 

dacite may be consistent with derivation of these lavas 

by fractional crystallization of less-differentiated intermediate-composition magmas. Mass-balance calculations suggest that this is a feasible origin for the less 

evolved silicic rocks if hornblende is a fractionating 

phase (table 9, model 3). Trace-element abundances in 

the rhyodacite, however, do not appear to be consistent 

with such an origin. Rather, partial melting of deep

crustal rocks is suggested. The LREE-enriched, HREEdepleted patterns imply the presence of garnet in the 

residual assemblage. Fractionation of hornblende may 

indeed have played a role in the genesis of the 

rhyodacite, but this probably took place after generation 

of a silicic melt. In any fractionation or partial melting 

scheme, the abundance of plagioclase in both Coso lavas 

and prospective lower crustal rocks suggests that it 

would have to be an important participant. This is 

inconsistent with REE data which do not show the 

expected negative Eu anomalies. Partition coefficients 

for Eu between many crystalline phases, especially apatite, and silicic melts are very small relative to those for 

other REE (Arth, 1976; Watson and Green, 1981; Mahood 

and Hildreth, 1983). Perhaps absence of Eu anomalies in 

the silicic rocks was brought about by a tendency for the 

effects of other phases to balance those of feldspar in a 

fractionating or residual assemblage.

Although trends in elemental abundances indicate 

that the silicic rocks have experienced some fractional 

crystallization, the high Ba content of the rhyodacite 

and silicic dacite is best explained by the parental silicic 

liquids originating through partial melting of deep 

crustal rocks, during which all K-feldspar and biotite 

are consumed. Wyllie (1979) summarized a large 

amount of experimental data on fusion of typical crustal 

rocks, including several from the Sierra Nevada. These 

studies show that quartz and alkali feldspar are the first 

phases to melt during partial fusion of granodioritic to 

tonalitic rocks under H2O-saturated conditions. 

Because the majority of the Ba in such rocks resides in 

alkali feldspar, liquids created by small to moderate 

degrees of melting, in which alkali feldspar is consumed, will be enriched in Ba. More extensive melting 

increasingly involves other phases and would be 

expected to cause the Ba content of liquids to decrease 

toward that of the parent material. A similar process 

may contribute to Ba enrichment of basaltic magmas 

during interaction with crustal material.

Mafic inclusions in dacite, commingled bombs, and 

basaltic xenocrysts in andesite and dacite indicate that 

the dacite of the 3.6- to 3.0-m.y.-old part of the volcanic 

field evolved in compositionally zoned igneous systems. 

The most silicic magmas in these relatively small reservoirs may have been derived in part by fractionation of 

more mafic intermediate-composition magmas and in 

part by partial melting of country rocks, probably deep 

in the system. Signatures of these processes are blurred 

by the effects of magma mixing. The most differentiated 

erupted dacite (sample 99) itself contains forsteritic 

olivine xenocrysts derived from mixing, so the true silicic end member has not been sampled. This end member very likely would be similar in composition to the 

rhyodacite of Haiwee Ridge.
34 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

The Haiwee Ridge silicic center, which produced 

rhyodacite 3.0 to 2.5 m.y. ago, has no associated basaltic 

or intermediate-composition lavas (Duffield and Bacon, 

1981). The rhyodacite is nearly uniform in composition 

and crystal content, is relatively volumious, and was 

erupted from vents spread over a comparatively large 

area. This magma probably came from a reservoir substantially larger than those of the earlier, polygenetic 

centers to the east. The wide range of plagioclase composition in individual samples and resorption and 

rapid-growth textures of the phenocrysts in the 

rhyodacite suggest that the fairly uniform composition 

of this rock may be related to convection and mixing of 

silicic magma that previously had been zoned. Explosive 

eruption of highly differentiated rhyolite at 3.1 m.y, 

followed at 3.0 m.y. by explosive eruption of rhyodacite, 

and final emplacement of thick lava flows as late as 2.5 

m.y. ago (Duffield and others, 1980) suggests degassing 

and gradual death of a compositionally zoned system. 

High-silica rhyolitic magma erupted from the upper 

part of the system, which subsequently extruded lessdifferentiated, higher-temperature magmas; the earliest rhyodacite originally may have underlain this 

rhyolitic cap. The longevity of the Haiwee Ridge center 

necessitates a basaltic heat source, unless the uniform 

rhyodacites represent independent igneous events. A 

magma reservoir of several cubic kilometers or more is 

indicated by the erupted volume and the 0.6-m.y.-long 

eruptive history. The greater extent of differentiation 

and far larger volume of silicic eruptives than found at 

earlier Coso volcanoes is consistent with this hypothesis.

Depletion of Ba in the most differentiated rhyodacite 

suggests fractionation of either biotite or alkali feldspar. 

Gradual increase in Rb and Cs throughout the 

rhyodacite series suggests that biotite was not a major 

fractionating phase because it has partition coefficients 

for these elements of the same order as that for Ba, 

whereas alkali feldspar concentrates Ba relative to Rb 

and Cs. Alkali feldspar has not been recognized in the 

rhyodacite, but K-rich sanidine is present in the 

rhyolite. If the rhyodacite contains a component derived 

from high-silica (Ba-poor) rhyolitic liquid that was 

mixed with less-silicic differentiated magma, the 

decreased Ba in the most evolved rhyodacite (sample 

106) might be reconciled with the lack of evidence for 

saturation with alkali feldspar. In fact, textural evidence for mixing supports this suggestion.

RHYOLITE

The rhyolite is a special case because of its highly 

differentiated composition. Extreme depletion and 

enrichment of trace elements in high-silica rhyolite

have been attributed to various processes, notably fractionation of accessory minerals (for example, Miller and 

Mittlefehldt, 1982) and liquid-state differentiation 

(Hildreth, 1979,1981). Because rhyolite is spatially and 

temporally associated with the rhyodacite and because 

many trace-element abundances seem to fall on projections of trends defined by the rhyodacite, the two rock 

types are probably genetically related. Mahood and 

Hildreth (1983) showed that partition coefficients for 

most trace elements between phenocrysts and highsilica rhyolitic liquids are substantially larger than for 

less silicic magmas because it is difficult for highly polymerized melts to accept trace constituents. For this reason, we believe that the trace-element abundances in 

the rhyolite largely reflect extreme fractional crystallization of silicic magma.

Fractionation of feldspars and quartz tends to enrich 

differentiates in most trace elements and deplete them 

in Ba, Sr, and Eu. Conversely, fractionation of mafic 

silicates, Fe-Ti oxides, and accessory minerals depletes 

high-silica liquids in most trace elements except 

alkaline earths (apart from micas) and alkali metals. 

Accessory phases, such as sphene, allanite, zircon, and 

apatite have characteristic patterns of partition coefficients, as shown well by the REE. The concave REE 

pattern for the rhyolite may be attributed to combined 

fractionation of apatite, which has an affinity for the 

middle REE (Watson and Green, 1981), and of allanite, 

which has very large partition coefficients for LREE 

(Mahood and Hildreth, 1983). Apatite inclusions are 

ubiquituous in phenocrysts of the rhyodacite and a single crystal of allanite intergrown with hornblende has 

been found in the most differentiated rhyodacite (sample 106). Alternatively, sphene might be the phase 

responsible for depleting the differentiated rhyolite liquid in the middle REE because of high partition coefficents for these relative to other REE (Simmons and 

Hedge, 1978). The Eu anomaly is presumably caused by 

feldspar fractionation. The REE pattern is not similar to 

that of high-silica rhyolite postulated to have been 

affected by thermogravitational diffusion (Hildreth, 

1981), although diffusion-controlled processes may have 

been important. Recent experimental studies (Harrison 

and Watson, 1983) suggest that the common occurrence 

of apatite inclusions in phenocrysts in silicic volcanic 

rocks (see Bacon and Duffield, 1981) is brought about by 

very slow diffusion of P in the liquid, causing local saturation in apatite near growing crystals of other phases. 

Apatite, and possibly other accessory minerals, 

included in unexpectedly great amounts in larger phenocrysts by such a process could produce enhanced fractionation effects. In addition the Soret effect, in which 

species migrate under the influence of a thermal gradient, may produce enhanced mobility of many ele-
DISCUSSION 35

ments toward cooling surfaces (Lesher and Walker, 

1983). Equilibrium fractionation thus may not be such a 

common process as fractionation enhanced by diffusion 

and thermal gradient effects.

Various studies, particularly the exhaustive analysis 

of the Bishop Tuff by Hildreth (1979), have shown that 

crystal settling is not a viable mechanism for differentiation of silicic magmas. More likely, crystallization 

causes bouyant rise of low-density differentiated liquid 

near the margins of a magma reservoir (McBirney, 

1980); this liquid may be mixed into a convecting chamber or lodge near its roof. Steep thermal gradients and 

liquid-state diffusion processes may enhance the ability 

of such boundary-layer differentiation to strongly 

deplete liquids in some components and enrich them in 

others. The magmatic system is kept active by repeated 

intrusion of basaltic magma below the silicic volume, 

and thermal gradients are maintained by conduction of 

heat into country rocks, generally greatly aided by 

hydrothermal convection. The Haiwee Ridge volcanic 

center must have been fed by such a system, whose 

highly differentiated portion was eventually mixed into 

the main volume. Silicic magmas at other Pliocene centers in the Coso volcanic field may have experienced the 

same processes, but the igneous systems beneath these 

centers were smaller in volume and shorter-lived. As a 

result, magma erupted from them was universally contaminated with more mafic magma from deeper in the 

systems.

AGE AND DURATION OF VOLCANISM

Volcanism in the Coso Range was episodic, consisting 

of discrete eruptive episodes separated by relatively 

lengthy periods of inactivity. The earliest episode, which 

produced basalt, high-silica rhyolite, and minor 

amounts of intermediate-composition lava occurred 

between about 6.0 and 5.3 m.y ago in the northeastern 

Coso Range (Bacon and others, 1982). These late 

Miocene rocks are not discussed in this paper because 

they are geographically separated from the main 

Pliocene volcanic field. Extensive K-Ar dating (Duffield 

and others, 1980) has shown that a second volcanic episode took place in the eastern part of the Coso Range 

around 4.0 to 3.0 m.y. ago. Basalt was erupted from 

monogenetic vents throughout this period, a time when 

basaltic volcanism was commonplace near the present 

western margin of the Basin and Range province and 

within the adjacent Sierra Nevada. Most of the polygenetic centers that produced dacitic lava in the eastern 

part of the Coso Range were active approximately 3.5 to 

3.3 m.y. ago; local dacitic flows were erupted as late as 

about 3.0 m.y. The volume of volcanic products 

increased to a maximum during the period of activity at 

polygenetic centers; at the same time, the proportion of

intermediate-composition products was greatest 

(Duffield and others, 1980). This pattern probably 

reflects about one million years of continuity in the flux 

of mantle-derived basaltic magma intruded into and, in 

many cases, flowing through the crust beneath this 

region. Approximately 0.5 m.y. after the onset of basaltic 

volcanism, the local crust had become sufficiently 

heated and tectonic conditions (discussed in the next 

section) were appropriate to allow differentiation and 

assimilation processes to generate evolved magmas. 

These silicic magmas mixed to varying degrees with 

basaltic magma beneath them to form the intermediatecomposition volcanics erupted from the comparatively 

long-lived polygenetic centers.

Before the main stage of volcanism in the eastern 

Coso Range had ended, silicic eruptions began approximately 10 km to the west, near Haiwee Ridge. Highsilica rhyolitic explosive eruptions took place at 3.1 m.y, 

followed about 0.1 m.y. later by more voluminous 

explosive eruption of rhyodacite (Duffield and others, 

1980). Rhyodacitic lava was erupted from several widespread vents at least as late as 2.5 m.y. ago. More than 9 

km3 of silicic magma erupted from the Haiwee Ridge 

center, a volume at least twice as great as that of dacite 

from the eastern center. Moreover, no basaltic or intermediate-composition rocks have been recognized that 

could have erupted from within the area of silicic vents. 

The closest intermediate to mafic eruption was that of 

the andesite of Cactus Flat (Duffield and Bacon, 1981) 

from a small polygenetic center about 5 km northeast of 

the nearest rhyodacite vent, probably before the end of 

rhyodacitic volcanism at Haiwee Ridge. These observations, coupled with the compositional homogeneity of 

the rhyodacite and other petrologic information presented earlier, argue for a relatively large crustal 

magma reservoir that may have been active for at least 

0.6 m.y. This reservoir presumably was created and 

sustained by intrusion of subjacent basaltic magma. 

Tectonic conditions in this part of the volcanic field 

evidently allowed this chamber to develop such that it 

prevented any basaltic or intermediate magma from 

reaching the surface.

Other than the rhyodacite of Haiwee Ridge, no volcanic rocks are known in the Coso Range that have ages 

between about 3.0 and 2.1 m.y. A few basaltic to 

andesitic monogenetic vents were active in Rose Valley 

2.1 m.y. ago, and several were active east of Volcano 

Butte, one of which has been dated at 1.8 m.y. (Duffield 

and others, 1980). Both sets of vents are considered as 

one eruptive episode here because they are separated by 

0.7 m.y. of inactivity from the next younger volcanic 

episode (Duffield and others, 1980; Bacon and Metz, 

1984). The duration of activity at these clusters of monogenetic cones is unknown, but is thought to have been
36 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

comparatively brief. The highly porphyritic basalt of 

Rose Valley (samples 79, 80, and 81) (Duffield and 

Bacon, 1981) is compositionally unusual in having elevated KjO, P2O5 , Rb, and Th contents (table 7). It is 

similar in this respect to younger lavas erupted nearby 

along the Sierra Nevada fault zone (for example, the 

basalt of Red Hill and basalt southeast of Little Lake; 

Duffield and Bacon, 1981; Bacon and Metz, 1984). Less is 

known about the composition of rocks east of Volcano 

Butte, but they appear to be typical basaltic (sample 70) 

and andesitic lavas, compositionally and texturally similar to those of the main Pliocene volcanic field nearby.

RELATION OF VOLCANISM TO TECTONIC PROCESSES

Neogene extension of the western Basin and Range 

province, including the Coso area, is well established 

(see Stewart, 1978). Volcanism has been associated with 

this tectonic activity in many places, some of which have 

developed into volcanic fields with longevities of several 

million years (Smith and Luedke, 1984). The Coso volcanic field is no exception: Evidence abounds for faulting 

during the lifetime of the field. Results of our petrologic 

study suggest that the variation in composition of 

erupted products is tied to the extent of interaction of 

mantle-derived magma and crustal rocks, or, in other 

words, to the crustal residence time of magma. In the 

Coso Range there is sufficient control on the timing of 

volcanism and tectonism to formulate some general 

relations between residence time and long-term measures of deformation.

There are intriguing correlations between times and 

styles of volcanism in entire provinces that suggest that 

these qualities are functions of regional tectonic processes. In the Sierra Nevada 40 km northwest of the 

Coso volcanic field, three rhyolite domes were emplaced 

2.4 m.y. ago, and a high-silica rhyolite dome 0.2 m.y. ago 

(Bacon and Duffield, 1981). Monogenetic basaltic vents 

3.6 m.y. old (Dalrymple, 1963) and a small field of 

Quaternary basaltic vents, one of which is approximately 0.2 m.y. old, (Moore and Lanphere, 1983) are 

present about 20 km southwest and 10 km northwest, 

respectively, of the rhyolite domes. Approximate synchroneity for both basaltic and silicic lavas, of volcanism 

in the Sierra Nevada and the Coso Range thus suggests 

tectonic influence on the timing and nature of igneous 

activity (Bacon and Duffield, 1981).

Because the tectonic-magmatic pattern alluded to 

above is regional, some tectonic factor that affects large 

areas and that ultimately can be quantified is needed to 

relate activity in one area to that in another and to relate 

tectonic processes to igneous products. A useful concept 

is that of the regional stress regime, defined in terms of 

magnitude and orientation of principal compressive

stresses; these stresses are believed to be virtually constant within large provinces (Zoback and Zoback, 1980) 

which have narrow transistion zones between them 

(McGarr, 1982). At present, there are few areas in which 

magnitudes of stresses are known, but it is possible to 

express tectonic styles in terms of the orientations and 

relative magnitudes of the principal stresses (see 

Nakamura and Uyeda, 1980). We have deduced 

qualitatively the principal stresses that existed within 

the Coso Range at various times during the last 4 m.y. 

The results allows us to speculate on the relation 

between the stress field and the composition and eruptive style of igneous products. We confine our analysis to 

the area south of 36°15'N.; this is the approximate 

northern limit of Pliocene volcanic rocks, and the transition to the tectonically different Owens Valley deformational province lies north of this latitude.

Some assumptions and principles upon which our 

analysis is based are: (1) orientations of principal stress 

axes (S^Sg, and S3) and relative magnitudes of tectonic 

stresses can be inferred from geologic observations; (2) 

magma typically migrates in tabular conduits oriented 

perpendicular to the least principal stress, S3 ; (3) driving pressure of magma is proportional to Pmagma - S3 

such that a decrease in S3 may cause intrusion by extensional failure; (4) for large crustal domains, the magnitude of S3 is inversely related to the externally induced 

extension rate; and (5) the residence time of magma in 

the crust at any particular depth depends upon the 

magnitude of S3 at that depth. From these it follows that 

the volume ratio of eruption to intrusion depends upon 

the magnitude of S3 . Decreasing S3 , which might be 

brought about by an increase in extension rate, would 

lead to increasing residence time and decreasing eruption/intrusion ratio, allowing greater differentiation 

and interaction with crustal rocks.

Application of this model to the Pliocene history of the 

Coso Range involves some speculation. Although the 

volcanic history is well documented, long-term rates of 

extension can only be inferred from limited information 

on the timing of basin formation and normal faulting. 

Thus, the relative magnitudes of principal stresses can 

only be estimated; orientations of stress axes, however, 

can be specified with some confidence. For example, 

Bacon and others (1980) synthesized data on azimuths 

of faults and alignments of volcanic vents with earthquake focal mechanisms (Weaver and Hill, 1978/79; Walter and Weaver, 1980) to define the orientation of stress 

axes for the Coso Range during the Quaternary: S3 is 

west-northwest, Sj^ is vertical, and S2 is north-northeast; because both strike-slip and normal faulting take 

place from time to time, Sj^ and S2 are thought to be 

similar in magnitude such that locally, during periods of 

strike-slip faulting, Sj^ may be oriented north-northeast
DISCUSSION 37

and S2 vertically. Pliocene vent alignments and fault 

traces are consistent with these orientations of stresses, 

but the geologic history suggests their magnitudes were 

slightly different as the region underwent the change 

from relative stability to extension.

Prior to about 4 m.y. ago the Coso Range evidently did 

not exist and the landscape was one of subdued topography developed on intrusive rocks of the Sierra Nevada 

batholith. Remnants of this erosion surface are visible 

where volcanic cover is sparse. To the north, clastic 

sediments were accumulating and volcanoes were active 

at least as early as 6 m.y. ago in the basin that became 

Owens Valley (Bacon and others, 1982). Accumulation of 

basalt flows in a shallow basin was the first indication of 

tectonism within the present Coso Range (south of 

37°15'N.) (fig. 19A). These flows erupted from monogenetic vents, begining around 4 m.y. ago, and flowed 

southwest or west-southwest into a generally northtrending elongate depression. Normal faults with detectable offset of this age are unknown, yet the presence of 

a shallow basin suggests that extensional deformation 

had begun. This activity continued for 0.5 m.y, during 

which time at least 5 km3 of fluid basalt were erupted. 

The abundance of K-Ar ages of about 3.6 m.y. for these 

diktytaxitic basalt flows (Duffield and others, 1980) 

implies that this was a time of particularly intense 

tectonic activity. In any case, the long-term eruption 

rate was high, and the composition reflects rapid transport of mantle-derived magma to the surface. This 

would have been a period of high eruption/intrusion 

ratio, one in which the magnitude of S3 perhaps was the 

largest for any time during evolution of the volcanic 

field.

By about 3.5 m.y. ago intermediate-composition magmas had begun to be erupted in substantial volumes 

from polygenetic centers within the basalt field (fig. 

19B). This type of activity reached a maximum between 

3.5 and 3.3 m.y. ago, and declined thereafter until only 

local, monogenetic centers erupted dacite as late as 3.0 

m.y. ago. At the close of this period, lavas were still 

flowing toward the center of the same shallow elongate, 

north-south basin, as evidenced by intermediate-composition flows erupted near both east and west margins 

of the field (Duffield and Bacon, 1981). Basalt continued 

to be erupted from monogenetic vents throughout this 

interval. The overall eruption rate was probably comparable to the earlier rate but the eruption/intrusion 

ratio was smaller (mean residence time was greater). If 

the inferred high extension rate of the late Quaternary 

was achieved by an approximately monotonic increase, 

then the rate was somewhat higher 3.5-3.0 m.y. ago than 

it was 4.0-3.5 m.y. ago. We can assume that Sx remained 

constant, since it reflected the weight of overburden, 

and S2 would have had no particular reason to change,

being perpendicular to the direction of extension. By 

inference, therefore, S3 had decreased below its value in 

the previous 0.5 m.y.

The calc-alkaline trend shown by the Pliocene Coso 

lavas may reflect the same kind of tectonic environment 

in which arc volcanism occurs. In arcs, the volcanic front 

probably forms in the zone where S3 changes from vertical to horizontal as one moves onland from the trench. 

The magnitude of S3 would be relatively large there, 

allowing only short to medium residence time for 

magma in the crust and opportunity for mixing. Early 

in the history of extension in the Coso Range, the relative magnitudes of principal stresses may have been 

similar to those in the typical arc case. Only when 

extension was well underway would S3 have decreased 

to a level such that magmas could have long crustal 

residence times and large volumes of silicic magma 

could form as a result.

Eruption of high-silica rhyolitic pumice from the 

Haiwee Ridge silicic center took place 3.1 m.y. ago, 

before the last intermediate-composition lava had 

erupted in the eastern part of the volcanic field. Voluminous explosive eruptions of rhyodacitic pumice 

occurred approximately 0.1 m.y later from the same 

general part of the field. Air-fall deposits of the 

rhyodacitic pumice are interbedded with coarse alluvial 

fan deposits that overlie the intermediate-composition 

lavas to the east (fig. 19C). The fans developed as normal 

faulting began and granitic debris was shed into a 

north-south-trending left-stepping series of echelon 

grabens that bisects the Coso Range. We surmise from 

this that extension was then proceeding at an increased 

rate in comparison to the previous 1 m.y. and that S3 had 

decreased further. The surface manifestation of this 

extension was normal faulting along the western margin 

of the earlier basin. The absence of volcanic products 

less silicic than rhyodacite in the Haiwee Ridge area and 

the moderate long-term eruption rate of silicic magma 

indicate a decreased eruption/intrusion ratio. This episode evidently lasted until at least 2.5 m.y. ago, by which 

time several thick rhyodacitic lava flows had been 

emplaced, some of which flowed down canyons cut into 

earlier pyroclastic and lacustrine deposits (Duffield and 

others, 1980).

Extension continued from 2.5 to 0.6 m.y. ago, as evidenced by larger offsets on normal faults that cut older 

rocks than those which cut younger rocks, and was 

punctuated by brief local eruptions of monogenetic 

basaltic cones (fig. 19D). A single high-silica rhyolite 

dome was extruded 1.04 m.y. ago, a few tens of thousands 

of years after several basaltic lava flows. Most of this 

period of nearly 2 m.y, however, left little record of 

volcanism. We have no evidence of how the extension 

rate may have varied during this period between the
38

118°00'

PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

117°30' 118°00' 117°30'

36° 

15'

A. 3.6 m.y. ago

36°

o- u> ^ * 

5 10 

» Iz

B. 3.0-3.6 m.y. ago

Lava flows into 

deepening basin

Polygenetic

andesite-dacite

volcano

118° 00' 11 f3tf 118° 00' 117°30'
DISCUSSION 39

118° 00' 117°30'

EXPLANATION

VOLCANIC ROCKS

Basalt (0.04-0.4 m.y.)

Rhyolitc (0.06-1.0 my.) [-Pleistocene

Basalt (1.1 my.)

Basalt and andesite (1.8-2.1 my.)

Rhyodacite (2.5-3.0 m.y.)

Dacite(~3.3m.y.)

Andesite (-3.3 my.)

Basalt (3.0-> 3.6 m.y.)

VENTS

Rhyodacite

Dacite 

Andesite

Basalt and andesite 

 CONTACT

Pleistocene 

and Pliocene

r Pliocene

QUATERNARY

QUATERNARY 

AND TERTIARY

- TERTIARY

FAULT - Bar and ball on

downthrown side 

DIRECTION OF FLOW OF

LAVA OR STREAMS

Pliocene tectonic regime, which was characterized by 

the inception and gradual increase of extension and the 

onset of normal faulting, and the Quaternary, in which 

extension may have reached a steady state and both 

normal and strike-slip faulting acted concurrently.

Extrusion of numerous high-silica rhyolite domes and 

eruption of peripheral basaltic lava flows in the southwest part of the Coso volcanic field appear to have taken 

place at approximately constant long-term rates (Bacon, 

1982) for about the past 0.3 m.y. (fig. 19E\ These rates 

are low compared to those of the Pliocene volcanic episode. One high-silica rhyolite dome was emplaced 0.6 

m.y. ago in the center of the area of later rhyolitic extrusions. The highly differentiated composition of the 

rhyolite implies a very low eruption/intrusion ratio, and 

anomalously high heat flow in the central part of the 

rhyolite field (Combs, 1980) is consistent with the continuance of the underlying magmatic system. Holocene 

fault scarps immediately east of the rhyolite field and 

historical seismicity show that extension is continuing. 

Focal mechanisms demonstrate that strike-slip faulting 

alternates with normal faulting, indicating that Sx and 

S2 are similar in magnitude. We suggest that during the 

last 0.3, or even 0.6 m.y. S3 has been less than at any 

other time in the history of the volcanic field and that it 

may have reached a steady-state minimum value.

FIGURE 19. Maps showing development of Coso volcanic field (modified from Duffield and Bacon, 1981). Areas of rock units not 

restored to pre-extension configuration. Most faults dominantly 

normal but some, particularly those that trend northwest, have a 

strike-slip component of motion. Vents indicated only where location is certain. A, Approximate minimum area of volcanic rocks 

3.6 m.y. ago. Basement surface beneath volcanic rocks was characterized by low relief. Inferred north-northwest-trending shallow 

basin may have resulted from incipient west-northwest eastsoutheast extension. Age of two patches of basaltic rocks near east 

margin of map inferred. B, Approximate minimum area of volcanic rocks 3.6-3.0 m.y. ago. Flow of lava towards axis of shallow 

basin suggests continuing basin subsidence. Differentiated 

magma evolved at polygenetic volcanoes within the area of earlier 

basaltic volcanism. C, Approximate minimum area of volcanic 

rocks 3.0-2.5 m.y. ago. Rhyodacitic volcanic center, from which 

minor rhyolite was first erupted, developed at northwest corner of 

volcanic field. Onset of faulting and deposition of coarse alluvial 

fan deposits from western source about 3.0 m.y. ago. D, Approximate minimum area of volcanic rocks 2.5-0.6 m.y. ago. Basalt and 

andesite (contaminated basalt) erupted in southern part of volcanic field; high-silica rhyolite dome emplaced 1.04 m.y. ago near 

center of field. Faulting continued. E, Minimum area of volcanic 

rocks 0.6 m.y. ago to present. Continued basaltic volcanism near 

southwestern corner of volcanic field. High-silica rhyolite formed 

extensive field of domes and short thick flows near west-central 

part of field (vents not shown). Faulting continued.
40 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

CONCLUSIONS

Pliocene volcanism in the Coso Range progressed 

through a sequence of compositions and eruptive patterns at the same time as the area was making the 

tectonic transition from stability to extension. Early 

lavas were of comparatively primitive basalt, erupted 

from monogenetic vents during a period of shallow basin 

formation 4.0 to 3.5 m.y ago. Subsequent eruptions in 

this area consisted of concurrent intermediate-composition magmas from polygenetic centers and basaltic 

magmas from outlying monogenetic vents. Together 

with the onset of normal faulting and alluvial fan formation about 3 m.y. ago, major silicic eruptions began in 

the northwest part of the field near Haiwee Ridge. The 

silicic activity ceased about 2.5 m.y. ago. The rest of the 

Pliocene and the early Pleistocene were characterized 

by local eruptions from short-lived monogenetic, generally basaltic centers.

The compositional variety of eruption products can be 

related to the upward movement of basaltic magma, 

ultimately derived from the mantle, into and through 

the crust. Differentiation of basaltic magma took place 

by separation of phenocrysts, a process commonly 

accompanied by assimilation of crustal material. With 

time, crystallization of basalt in the deep crust locally 

produced sufficient heating to cause partial melting of 

the crust and formation of relatively silicic magma. 

Small-volume zoned magmatic systems developed, in 

which silicic magma derived from the crust overlay 

basaltic magma. Eruptions from these systems formed 

polygenetic volcanic centers. Because of the relatively 

small volume of silicic magma in these short-lived reservoirs, there was abundant opportunity for the basaltic 

and silicic material to mix and hybridize, producing the 

nearly continuous spectrum of intermediate rocks found 

in the Coso Range. Toward the end of the Pliocene magmatic episode, when volcanism had shifted to the 

Haiwee Ridge center, the volume of silicic magma was 

sufficient that intermediate magma never erupted. Mixing in the Haiwee Ridge chamber probably took place 

between a dominant volume of rhyodacitic magma and 

more differentiated high-silica rhyolitic magma.

Sparse data on fault displacements and on the deformational history of the Coso Range suggest a gradual 

change during the late Cenozoic from stability to 

tectonic extension. Extension may have reached a 

steady state by the late Pleistocene. The development of 

the volcanic field seems to reflect this change in tectonic 

environment. We have attempted to relate this change 

to decrease in the least principal compressive stress, S3. 

By encouraging longer residence of magma in the crust, 

decrease in S3 may lead to increased differentiation and 

the tendency to form large volumes of silicic magma. We

conclude that in the evolution of the calc-alkaline Coso 

volcanic field, magma mixing was an important 

petrogenetic process that occurred during a time when 

S3 was intermediate between a value characteristic of 

tectonic stability and one typical of rapid extension.

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origin of continental-interior andesite: Geological Society of 

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Zimmerman, C., and Kudo, A. M., 1979, Geochemistry of andesites 

and related rocks, Rio Grande Rift, New Mexico, in Rieker, R. E., 

ed., Rio Grande Rift: Tectonics and magmatism: Washington, 

American Geophysical Union, p. 355-381.

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p. 6113-6156.
TABLE 10
44 PLIOCENE VOLCANIC ROCKS OF THE COSO RANGE, CALIFORNIA

TABLE 10. Sample localities for Coso Range Pliocene volcanic rocks

Locality

Field

Latitude N. Longitude W.

40 

44 

45 

56 

/ -3

64 

65 

66 

67 

, o

69 

70 

71 

72 

73 

74 

75 

76 

77 

79 

81 

O n

83 

84 

85 

Q £

87 

o o

89 

90 

91 

92 

93 -

94 

95 - 

96 

97 -

o o

99 - 

100 

1 AO

103 - 

104 

1U J

106 

107 -

 9-85-2 

. 13-108-1

 1-103-3

 8-191-1

. 9-12-3

 13-114-2

 1-103-1

 8-194-8

 9-7-2

 8-195-1

 8-194-6

 9-10-3

 8-196-5

8

1 O O Q

 1-106-6

 1-106-7

 1-106-3

 1-103-2

 8-194-4

 13-111-2

 13-111-7

 13-113-21

 1-101-12

 1-103-4

 13-47-3

O/ £.

- 1-101-1

- 1-101-13

 13.111-4

 1-101-10

 1-101-4

 13-113-19

- 1-103-7

 13-45-1

- 13-42-4

- 13-42-5

 13-43-13

- 13-45-5

 9-86-4

- 1-103-5

36°07.85' 

36°59.29' 

36°11.88' 

35°59.80' 

35°56.63' 

36°14.01' 

36°13.54' 

36°01.71' 

36°00.03' 

36°02.47' 

36°09.41' 

36°02.47' 

35°56.11' 

36°02.18' 

36°10.61' 

36°08.60' 

35°09.35' 

36°09.89' 

36°05.86' 

35°53.02' 

36°03.40' 

35°59.74' 

36°00.04' 

36°00.26' 

36°00.37' 

36°00.42' 

36°09.33' 

36°06.93' 

36°07.87'

36°02-73' 

36°00.19' 

36°00.09' 

36°12.63' 

36°13.92' 

35°57.94' 

36°00.24' 

36°07.89' 

35°59.19 

35°59.03' 

36°03.85' 

36°03.12' 

35°58.93' 

36°08.17' 

36°03.93' 

36°06.75' 

36°03.93' 

36°06.48' 

36°07.73' 

36°11.14' 

35°59.29'

117°45.61' 

117°41.78' 

117°44.30' 

117°42.52' 

117°36.00' 

117°45.20' 

117°48.65' 

117°40.46' 

117°42.44' 

117°39.97' 

117°44.75' 

117°39.97' 

117°39.62' 

117°46.91' 

117°45.86' 

117°44.68' 

i!7°47.26' 

117°49.18' 

117°45.06' 

117°35.30' 

117°38.24' 

117°53.01' 

117°52.74' 

117°54.23' 

117°41.74' 

117°41.24' 

117°47.16' 

117°42.91' 

117°43.02' 

117°42/42' 

117°38.75' 

117°42.08'

117°50.95' 

117°54.82'

117°38.58' 

117°38.69' 

117°43.29' 

117°38.47' 

117°38.84' 

117°44.66' 

117°43.94' 

117°41.52' 

117°52.37' 

117°50.33' 

117°53.14' 

117°50.33' 

117°53.53' 

117°53.36' 

117°53.63' 

117°41.78'

687-049/4501 7