Reconstruction of Gmstal Blocks 

Of California on the Basis of 

Initial Strontium Isotopic Compositions 

Of Mesozoic Granitic Rocks

GEOLOGICAL S U K V E Y P It O F E S S I ON A L P A P E
Reconstruction of Crustal Blocks 

Of California on the Basis of 

Initial Strontium Isotopic Compositions 

Of Mesozoic Granitic Rocks

By RONALD W. KISTLER and ZELL E. PETERMAN

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1071

A study of regional variation of initial 

strontium isotopic composition of 

Mesozoic granitic rocks in California

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

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Catalog-card Number 78-600103

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

Washington, D.C. 20402

Stock No. 024-001-03109-4
CONTENTS

Page

Abstract __________________________________ l

Introduction ------------__-__---------------- 1

Fault block reconstruction --------------------------- 4

Development of the continental margin --------------------- 7

Trace element variations ---------------------------- 10

Source materials for the granitic rocks ---------------------- 11

Summary and conclusions --------------------------- 13

Rock type and locality description of rocks investigated -------------- 15

References cited -----------____----__---------- 16

ILLUSTRATIONS

Page

FIGURE 1. Index map showing specimen localities of granitic rocks investigated ____________________________ 3

2. Map of California showing boundaries between different crustal types ___________________________ 4

3. Map showing distribution of rj of Mesozoic granitic rocks and mafic Cenozoic volcanic rock with late

Cenozoic lateral displacements removed from San Andreas and Garlock fault systems _______________ 6

4. Location and configurations of lines rj = 0.7040 and 0.7060 in the western United States ________________ 8

5. Position of Salinian-western Mojave terrane after 800 km of left-lateral displacement is removed along

the discontinuity in rj in the central Mojave Desert ___________________________________ 9

6. Diagram of model of continental rifting used to account for present configurations of lines rj = 0.7040

and rj = 0.7060 _________________________________________________________ 10

7. Plots of: Rb concentration against rj _______________________________________________ 11

8. Plots of: Sr concentration against rj _______________________________________________ 11

9. Rubidium concentration relative to strontium concentration in California granitic rocks with rj less

than 0.7060 ___________________________________________________________ 12

10. Plots of: Rb/Sr against rj _____________________________________________________ 13

TABLES

Page

TABLE 1. Strontium analytical data ____________________________________________________ 2

2. Chemical analyses of granitic rocks ________________________________________________ 2

3. Potassium-argon dates ______________________________________________________ 2

III

RECONSTRUCTION OF CRUSTAL BLOCKS OF CALIFORNIA

ON THE BASIS OF INITIAL STRONTIUM ISOTOPIC 

COMPOSITIONS OF MESOZOIC GRANITIC ROCKS

By RONALD W. KISTLER and ZELL E. PETERMAN

ABSTRACT

Initial ^Sr/^Sr was determined for samples of Mesozoic granitic 

rocks in the vicinity of the Garlock fault zone in California. These 

data along with similar data from the Sierra Nevada and along the 

San Andreas fault system permit a reconstruction of basement rocks 

offset by the Cenozoic lateral faulting along both the San Andreas 

and Garlock fault systems.

The location of the line of initial "Sr/^Sr = 0.7060 can be related 

to the edge of the Precambrian continental crust in the western 

United States. Our model explains the present configuration of the 

edge of Precambrian continental crust as the result of two stages of 

rifting that occurred about 1,250 to 800 m.y. ago, during Belt sedimentation, and about 600 to 350 m.y. ago, prior to and during the 

development of the Cordilleran geosyncline and to left-lateral translation along a locus of disturbance identified in the central Mojave 

Desert. The variations in Rb, Sr, and initial "'Sr/^Sr of the Mesozoic 

granitic rocks are interpreted as due to variations in composition 

and age of the source materials of the granitic rocks. The variations 

of Rb, Sr, and initial "'Sr/^Sr in Mesozoic granitic rocks, the sedimentation history during the late Precambrian and Paleozoic, and 

the geographic position of loci of Mesozoic magmatism in the western United States are related to the development of the continental 

margin and different types of lithosphere during rifting.

INTRODUCTION

The isotopic composition of strontium was determined for specimens of granitic rocks in the vicinity of 

the Garlock fault, in the southern Sierra Nevada, and 

in the northern Mojave Desert of California. Values 

for Rb, Sr, K/Sr, K/Rb, ^Sr/wSr, initial ^Sr/^Sr, and 

age for each specimen are given in table 1. Chemical 

analyses of some of these rocks are given in table 2, 

and K-Ar ages of some are listed in table 3. Locations 

of specimens investigated are shown in figure 1, and 

petrographic descriptions and sample locations are 

given in the Appendix, Analytical techniques for Rb, 

Sr, and 87Sr/86Sr are the same as those described in 

Kistler and Peterman (1973), and those for K-Ar 

dates are the same as those described in Kistler 

(1968).

Previously, we found (Kistler and Peterman, 1973) 

that initial 87Sr/86Sr (hereafter called ri) values in 

Mesozoic granitic rocks to the north of the Garlock 

fault in California show a systematic areal variation, 

independent of age, and that the ri of superjacent upper Cenozoic basalts and andesites in that area show 

the same areal variation. We suggested that the 

boundary between granitic rocks with i{ less than 

0.7060 and greater than 0.7060 was coincident with 

the seaward edge of marine miogeosynclinal sedimentation during the late Precambrian and Paleozoic in 

California, that is, the edge of the Paleozoic continental shelf. Armstrong, Taubeneck, and Hales (1977) established that the ri of Mesozoic granitic rocks and 

Cenozoic volcanic rocks had the same relation in 

Washington and Idaho to Precambrian and Paleozoic 

sedimentation.

One goal of the present study was to extend to the 

vicinity of the Garlock fault distinctive pattens of ri 

previously established in the Sierra Nevada north of 

the Garlock fault by us (Kistler and Peterman, 1973) 

and along the San Andreas fault system (Kistler and 

others, 1973). Offsets of the pattern of ri along these 

fault systems could then be used to help establish limits of Cenozoic displacements along them.

The pattern of variation of strontium isotopes of 

Mesozoic granitic rocks in California appears to indicate that the State is composed of four fundamentally 

different types of crust. The boundaries between 

these crustal types where presently known are indicated in figure 2. The first type is crust that has been 

intruded by Mesozoic granitic rocks that are principally granodiorite and quartz monzonite and that 

have ri greater than 0.7060. The second type is crust 

that has been intruded by Mesozoic granitic rocks 

that are principally tonalite and granodiorite and 

have ri greater than 0.7040 but less than 0.7060; this 

can be subdivided into two types on the basis of traceelements abundances in Mesozoic granitic rocks that 

intrude it. The third type is crust intruded by Mesozoic granitic rocks that are principally quartz diorite
[n.d.

INITIAL STRONTIUM ISOTOPIC COMPOSITIONS OF MESOZOIC GRANITIC ROCKS

Table 1.—Strontium analytical data

not determined K20 and Na20 determined by L.B. Schlocker. Rb and Sr determined by W.P.Doering "'Sr/^Sr determined by R.A. Hildreth and R.W. Kistler]

Specimen 

No.

C_ i nn

C- 9 70

S_ Q_7Q

C. R_ 70

C- Q 70

S r Q 7Q

Sr 1A_7Q

C. 1 1 70

Sr 19 7Q

C_ if* n o

CU 1 7 7O

C. 1ft_7Q

S r 1 Q 7O

C_ OA 7Q

pf\O 1A_O

COS 13-42-2 

AQ

AC

D15

D16

f1 OA1

1?1 A

Wt percent

Map No.

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

27 

28 

29 

26

K20

1.79 

1.76 

1.71 

3.01 

3.40 

3.38 

2.87 

2.39 

2.84 

4.92 

2.45 

2.00 

1.70 

3.83 

2.45 

3.29 

3.03 

4.76 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

3.63 

4.33 

1.58 

n.d.

Na20

3.16 

4.02 

3.45 

2.96 

2.81 

4.00 

3.82 

3.91 

3.70 

3.13 

4.56 

3.91 

3.68 

3.50 

4.33 

4.50 

3.32 

3.45 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

3.62 

3.17 

3.97 

n.d.

Rb 

(ppm)

77.2 

57.7 

44.0 

146. 

154. 

140. 

98. 

81.8 

89.6 

118. 

80.1 

55.0 

53.8 

98.1 

72.0 

108. 

105. 

132. 

80.1 

10.3 

169. 

57.7 

146. 

203. 

89. 

144. 

180. 

27.7 

139.

Sr 

(ppm)

360 

399 

514 

249 

238 

349 

737 

695 

634 

609 

612 

347 

341 

382 

848 

731 

350 

339 

485 

664 

261 

686 

489 

311 

592 

345 

219 

970 

171

Weight ratios

Rb/Sr

0.215 

.145 

.086 

.586 

.646 

.401 

.133 

.118 

.141 

.193 

.131 

.159 

.158 

.257 

.085 

.148 

.301 

.390 

.165 

.016 

.548 

.084 

.299 

.653 

.150 

.417 

.824 

.029 

.815

K/Rb

193 

253 

323

m

183 

201 

243 

242 

263 

346 

253 

302 

262 

324 

282 

253 

239 

299

225 

211 

473

K/Sr

41.4 

36.6 

27.6 

100.4 

118.5 

80.5 

32.3 

28.5 

37.2 

67.0 

33.2 

47.8 

41.3 

83.2 

23.9 

37.3 

71.7 

116.5

89.1 

174. 

13.5

Atom ratios

o'Sr/^Sr

0.7048 

.7050 

.7036 

.7089 

.7084 

.7073 

.7080 

.7084 

.7076 

.7084 

.7062 

.7055 

.7060 

.7062 

.7069 

.7092 

.7065 

.7089 

.7057 

.7062 

.7106 

.7047 

.7094 

.7130 

.7083 

.7182 

.7174 

.7063 

.7107

ri

0.7038 

.7043 

.7032 

.7061 

.7053 

.7058 

.7075 

.7080 

.7070 

7077 

.7058 

.7050 

.7055 

.7042 

.7066 

.7081 

.7041 

.7060 

.7041 

.7060 

.7064 

.7041 

.7070 

.7070 

.7070

.7063 

.7070

Age

120 

120 

120 

120 

120 

90 

90 

90 

90 

90 

81 

80 

81 

210 

90 

180 

200 

180 

200 

180 

184 

200 

210 

210 

210 

pC(?)

p (?)

13 

100

Table 2.—Chemical analyses of granitic rocks

[Analyst, H. Smith]

Spec. No.

Si02 ---

A1203 --

Fe203 -

FeO---

MgO --

CaO---

Na20 --

K20---

H20+--

H20~--

Ti02 --

PA --

MnO --

C02 ——

Sum

1-73

63.2

16.5

0.7

4.3

2.8

5.8

3.3

1.8

0.53

0.25

0.62

0.15

0.07

0.03

100

2-73

68.7

15.7

1.1

2.2

1.2

3.8

4.2

1.8

0.54

0.26

0.44

0.14

0.04

0.05

100

3-73

65.8

15.9

1.6

2.6

2.1

5.0

3.8

1.8

0.52

0.28

0.52

0.16

0.08

0.04

100

4-73

65.8

15.1

0.3

3.8

2.0

4.2

3.1

3.0

0.49

0.29

0.57

0.12

0.06

0.02

99

5-73

65.1

15.2

1.0

3.8

2.2

3.9

2.9

3.6

0.45

0.23

0.66

0.11

0.06

0.05

99

6-73

68.1

15.9

1.3

2.0

0.9

2.4

4.1

3.3

0.64

0.27

0.52

0.21

0.04

0.02

100

8-73

65.8

16.2

1.5

2.6

1.2

3.5

4.0

2.9

0.52

0.38

0.77

0.23

0.04

0.03

100

9-73

63.8

17.1

1.3

3.5

1.3

3.9

4.1

2.5

0.81

0.29

0.82

0.25

0.04

0.02

100

10-73

63.8

16.3

1.7

3.4

2.1

4.5

3.7

2.8

0.45

0.28

0.87

0.23

0.06

0.04

100

11-73

72.7

14.0

0.90

0.96

0.30

1.5

3.1

5.0

0.27

0.25

0.20

0.06

0.00

0.06

99

12-73

59.0

16.8

1.4

4.5

2.6

5.2

4.7

2.6

0.75

0.35

1.2

0.38

0.10

0.08

100

15-73

61.8

16.8

1.2

4.7

2.2

4.6

4.1

1.7

0.79

0.31

0.91

0.22

0.09

0.08

100

16-73

71.9

14.5

0.90

1.2

0.36

2.0

3.5

3.8

0.33

0.36

0.19

0.08

0.06

0.06

99

17-73

62.1

15.6

0.40

4.5

1.8

4.9

4.5

2.5

0.52

0.28

1.1

0.35

0.06

0.24

99

18-73

68.4

16.3

1.8

1.3

0.70

2.7

4.6

3.4

0.25

0.33

0.35

0.15

0.04

0.08

100

19-73

65.8

15:8

2.1

2.5

2.0

4.3

3.5

3.1

0.57

0.28

0.56

0.14

0.08

0.04

101

20-73

70.3

15.2

1.5

1.3

0.68

1.9

3.4

4.8

0.58

0.27

0.36

0.13

0.05

0.08

101

TABLE 3.—Potassium-argon dates

[K20 determined by L. B. Schlocker. Moles <°Arrad determined by R. W. Kistler ]

Map No.

7

10

15 — ___ — _ — __ — __ —

17

1 ft

27 - -

00

Mineral

-do-

— do—

—doHornblende 

Biotite

Hornblende 

Biotite

K20 

(wt percent)

0 0-1

Q QH

7.22

ft 77

1.62

Q (HA

.55

ft 1 1

c on

"Arrad 

(x 10~" moles/gm)

QQ 71

110.99

103.07

77.22

oq 07

18.12 

184 40

10.72 

95.48

1OQ QQ

QO QQ

"Arrad 

(percent)

77

94

70

71

50

44

Q1

28

ft7

7Q

92

Age (m.y.)

A7 1-4-17

7ft 7 -4- 9 fl

74.8 ± 1.9

71 1 -4- 1 ft

1ft 9 + (\ *L

74.2 ± 1.9

127.4 ± 8.0

74 ft -4- 1 Q

1197 -4- 9 Q

fto fl -+• 9 Q
INTRODUCTION

and trondjhemite and have rj less than 0.7040; this 

third type is also characterized by the principal exposures of ophiolites in California. The fourth type is 

Franciscan melange.

In our original study of rj in Mesozoic granitic rocks 

(Kistler and Peterman, 1973), we found not only that 

a simple pattern of variation in chemistry and ri of 

Mesozoic and Cenozoic igneous rocks exists in California, but that variations in chemistry and ri could not

be related to depths of magma generation along subduction zones or to the age of the igneous activity. 

However, the variation was dependent on geographic 

position of specimens investigated and was correlated 

with long-lived crustal features. Granitic rocks with ri 

greater than 0.7060 were intruded into regions with a 

crust as much as 50 km thick and in many areas with 

exposures of Precambrian crystalline rocks (Oliver, 

1977). Similar studies (Early and Silver, 1973; Kistler

119° 118° 117°

Cenozoic volcanic 

and sedimentary 

rocks and alluvium

.-"•' •-. -•••" ^PilltJr nMoiave 0

- I A " ^ - <\ —i-l

Contact

Fault-Dashed where 

inferred; dotted 

where concealed

Speciman locality — 

For analysis of 

numbered samples 

see tables 1-3

35° -

25 50 KILOMETERS

FIGURE 1.—Generalized geologic map of vicinity of Garlock fault zone, California (modified from Dibblee, 1960) showing specimen localities of granitic rocks investigated for present study. Unnumbered sample localities are from Kistler and Peterman (1973) and 

Kistler, Peterman, Ross, and Gottfried (1973).
INITIAL STRONTIUM ISOTOPIC COMPOSITIONS OF MESOZOIC GRANITIC ROCKS

and others, 1973; Kistler, 1974; Armstrong and others, 

1977; PetO and Armstrong, 1976; Le Conteur and 

Templeman-Kluit, 1976) have simply reinforced this 

observation. As a consequence, we conclude that areas 

in the western United States intruded by Mesozoic 

granitic rocks with rj greater than 0.7060 are underlain by ensialic crust.

Considering the line rj = 0.7060 as a reflection of 

limits of Precambrian continental crust and the edge 

of the paleozoic continental shelf gives further insight 

into the reasons for the location of loci of magmatism 

of different ages and into the nature of the source materials for the granitic rocks. This insight is especially 

clear after the disrupted boundaries of the isotopic 

pattern are restored along the San Andreas and 

Garlock fault systems and the resultant pattern is related both to a known Precambrian aulocogen in the 

Death Valley region and to the age and type of sediments deposited in the Paleozoic cordilleran 

geosyncline along the margin of the western United 

States.

123 119°

42°

38°

34°

EXPLANATION 

ii> 0.7060

0.7040< ri< 0.7060 

ri < 0.7040 

Franciscan rocks

SEALCOV 

SAN GREGORIO 

FAULT

RINCONADA' 

FAULT

GABRIEL

150 KILOMETERS

FIGURE 2.—Generalized geologic map of California showing 

boundaries between four different crustal types characterized 

by Mesozoic granitic rocks with distinctive rj.

FAULT BLOCK RECONSTRUCTION

In order to test suggested offsets of basement rocks 

to the west of the San Andreas fault, Kistler, 

Peterman, Ross, and Gottfried (1973) determined the 

ri of Mesozoic granitic rocks in the vicinity of the San 

Andreas fault zone from the Gualala area in the north 

into the southern California batholith in the south. 

Two stages of motion of basement terrane were utilized to bring the granitic rocks with distinctive ri to 

the west of the San Andreas fault zone into a reasonable prefault configuration adjacent to granitic rocks 

in the Mojave Desert east of the fault. For the first 

stage, which is required to restore about 320 km of 

post-lower Miocene offset, the entire length of the 

present-day San Andreas fault (fig. 2) and the San Gabriel fault zone was considered the break between 

lithosphere blocks (Anderson, 1971; Crowell, 1968). 

After this restoration, granitic rocks from Ben Lomond to Bodega Head remain in an apparently 

anomalous position to the west of the Great Valley. 

Stratigraphic studies indicate these rocks arrived in 

this position during Late Cretaceous and Paleocene 

time and provided debris to Eocene submarine fan deposits, some of which are now cut by the San Andreas 

fault (Nilsen and Clarke, 1975). The second stage, 

along a proto-San Andreas fault, utilized the San 

Andreas fault north of the Garlock fault, the southern 

part of the Sur-Nacimiento fault, and the RelizEspinosa-San Marcos-Rinconada fault zone to restore 

granitic rocks from Ben Lomond to Bodega Head to a 

possible pre-Late Cretaceous position to the south 

(Kistler and others, 1973).

The second-stage restoration of Kistler, Peterman, 

Ross, and Gottfried (1973) indicated about 150 km of 

pre-middle Eocene, post-Late Cretaceous motion 

along the Reliz-Espinosa-San Marcos-Rinconada 

fault zone in the interior of the Salinian block. However, only about 60 km of pre-Miocene lateral displacement is likely along the south end of this fault 

zone (Dibblee, 1972, 1976), and the new strontium 

isotopic data indicate that the Ben Lomond region is 

not in an anomalous position. An additional fault or 

process is necessary, however, to remove granitic rocks 

from Montara to Bodega Head from their anomalous 

position west of the Great Valley. Up to 100 km of late 

Cenozoic right-lateral displacement is suggested for 

the Seal Cove-San Gregorio fault zone, a western 

branch of the San Andreas fault system (Silver, 1975; 

Graham, 1975; Dibblee, 1976). If 100 km of right-lateral displacement is removed along the Seal Cove-San 

Gregorio fault zone, however, the granitic rocks from 

Montara to Bodega Head would no longer be in a position suitable to shed debris into lower Tertiary sub-
FAULT BLOCK RECONSTRUCTION

marine fans in the location shown by Nilsen and 

Clarke (1975). Early Cenozoic lateral displacement of 

up to 50 km in the interior of the Salinian terrane 

along the San Juan fault and the Rinconada fault zone 

of Dibblee (1976) is possible, but the configuration of 

the isotopic pattern does not require any pre-Miocene 

Cenozoic lateral displacement of continental basement rocks along this segment of the California borderlands.

Smith (1962), on the basis of apparent offsets of 

dike swarms in Mesozoic granitic rocks, suggested 

about 64 km of Cenozoic left-lateral displacement 

along the Garlock fault. Subsequent geologic studies 

in the vicinity of the fault have supported Smith's 

conclusion, as summarized by Davis and Burchfiel 

(1973). Troxel, Wright, and Jahns (1972), Davis and 

Burchfiel (1973), and Garfunkel (1974) concluded 

that the Garlock fault is a continental transform related to Cenozoic crustal extension in the Great Basin 

and that displacement along it is not everywhere the 

same. Offsets in the strontium isotopic pattern in the 

vicinity of the Garlock fault zone indicate a maximum 

of about 52 km of displacement along the fault.

In figure 3, major post-Miocene right-lateral displacements of continental rocks along the San 

Andreas and San Gabriel fault zones have been removed from the California borderlands (Kistler and 

others, 1973). In addition, 52 km of left-lateral displacement has been removed along the Garlock fault. 

Post-Late Cretaceous and Pre-middle Eocene displacement of 50 km could be removed from the RelizEspinosa-San Marcos-Rinconada fault zone of 

Dibblee (1972) and its extension, the King City fault 

zone of Ross and Brabb (1973), but this displacement 

provides no improvement in alinement of ri and is not 

shown. The sliver of basement bounded by the San 

Gabriel and San Juan fault zones may be shown in a 

too southerly position in figure 3, because geologic evidence indicates a right-lateral offset of only 40 km on 

the San Gabriel fault zone (Crowell, 1952; Dibblee, 

1968). If the geologic estimates of displacement along 

the San Gabriel fault zone are correct, the sliver 

bounded by the San Gabriel and San Juan fault zones 

would have to lie further to the north. If this were the 

case, considerable right-lateral slip along the San 

Juan fault zone would have to be removed to place the 

rest of the Salinian block in the position shown. As 

much as 37 km of post-lower Tertiary right-lateral slip 

has been suggested for this fault, and earlier movements with the same sense may also have occurred 

along it (Dibblee, 1976, p. 21). The strontium data do 

not uniquely define a position for this sliver in the interior of the Salinian block, and its position in figure 3

is established only if there is no internal deformation 

within the block, an unlikely possibility (Garfunkel, 

1974).

The continuity of ri across the San Andreas fault 

zone to the south of Ben Lomond and across the 

Garlock fault zone after the indicated restoration is 

remarkable. An 80-km restoration along the Seal 

Cove-San Gregorio-Palo Colorado fault zone would 

juxtapose granitic rocks with similar ri (0.7061 to 

0.7068) at Bodega Head, Point Reyes, Montara, and 

the Farallon Islands. These granitic rocks still appear 

to be in an anomalous position relative to sedimentary 

rocks of the Great Valley sequence of late Mesozoic to 

Tertiary age. However, moving them further south 

laterally along a fault does not produce a post-Late 

Cretaceous pre-middle Eocene juxtaposition of ri that 

is any better than that in the position shown. The position of these granitic rocks shown in figure 3 places 

them just west of the junction between Franciscan 

melange and the Great Valley sequence—the Coast 

Range thrust. The projection of this junction south 

lines up with a zone of ultramafic rocks in the western 

part of the northern Santa Lucia Range (Ross, 1976). 

We believe that the Coast Range thrust and the zone 

of Ultramafic rocks may be related. If so, the granitic 

rocks under discussion could have arrived from the 

west to their apparently anomalous position prior to 

the Paleocene and early Eocene and, in effect, could 

be part of the Franciscan melange. It should be noted 

that their ri values are derived on the assumption that 

these granitic rocks are about 110 m.y. old (Mattinson 

and others, 1972; Kistler and others, 1973). However, 

this age has never been established unequivocally, as 

zircon ages from these rocks are discordant and indicate a pre-Mesozoic component (Mattinson and others, 1972) in the zircon populations. Plotting the raw 

RbSr data from granitic rocks from Bodega Head, 

Point Reyes, and Farallon Islands (Kistler and others, 

1973) on a strontium evolution diagram yields an apparent age of about 200 m.y. and a common ri of 

0.7058. If the pluton at Montara is assumed to have 

the same ri, its age is about 320 m.y. These calculations are not meant to indicate real ages; they are 

meant to show that the granitic rocks are possibly 

older than Mesozoic and possibly have an ri less than 

0.7060.

The small exposures of granitic rocks from Montara 

to Bodega Head are the isotopically unusual rocks of 

the Salinian block. The oldest strata of the Gualala 

basin to the north of Bodega Head lie on spilitic volcanic rocks similar to those of the Franciscan Formation and with oceanic affinities (Wentworth, 1966). 

The other basins of early Tertiary sedimentation are
INITIAL STRONTIUM ISOTOPIC COMPOSITIONS OF MESOZOIC GRANITIC ROCKS

118°

35°

EXPLANATION V

ISSl^^

'i .2 I

•i i ^v

§ 3 0.7040^

o >

>0.7030 < 0.7040 A A 

>0.7040 < 0.7050 X X 

>0.7050 < 0.7060 V V 

>0.7060 < 0.7070 -+- -O- 

>0.7070 < 0.7080 • O 

>0.7080 < 0.7090 • D 

>0.7090 

\

- Limit of granitic exposure

-1- -1- -1- Boundary of aulacogen

- — ---- Contact - Dashed where approximate; '' 

dotted where inferred

- ... Fauit. Dashed where approximate; 

dotted where inferred

-*—*— Thrust fault - Dashed where approximate

50 100 KILOMETERS

FIGURE 3.—Distribution of rj of Mesozoic granitic rocks and mafic 

Cenozoic volcanic rocks with late Cenozoic lateral displacements removed from San Andreas and Garlock fault systems. 

Stippled area is where 0.704 <r\< 0.706. Locations of geographic features mentioned in text are: (1) Gualala area, (2) 

Bodega Head, (3) Point Reyes, (4) Farallon Islands, (5) Montara, (6) Ben Lomond, (7) Santa Lucia Mountains, (8) El Paso 

Mountains, (AA)—Amargosa aulacogen. Faults mentioned in text 

are: (A) Coast Range thrust, (B) Seal Cove-San Gregorio, (C) Rinconada, (D) San Andreas, (E) San Gabriel, (F) San Juan, (G) Garlock. Thin lines outline areas of Mesozoic granite rocks (see fig. 1). 

Reconstruction along faults is made relative to a fixed Mojave 

block.
DEVELOPMENT OF THE CONTINENTAL MARGIN

suggested to have formed by slicing and fragmentation of continental crust along transform boundaries, 

and some may be floored by oceanic crust (Nilsen and 

Clarke, 1975). With these facts in mind and because 

there is no compelling reason to shift to the south the 

granitic exposures from Montara to Bodega Head, we 

suggest these grantitic rocks may represent the deeply 

exposed rocks of a volcanic arc terrane that were 

emplaced from the west into their positions shown in 

figure 3 during the Late Cretaceous and Paleocene.

DEVELOPMENT OF THE CONTINENTAL MARGIN

Another noteworthy feature of the pattern of ri 

after these major fault displacements are removed is 

an area in the southern Sierra Nevada and central 

Mojave Desert characterized by Mesozoic granitic 

rocks and late Cenozoic volcanic rocks with ri between 

0.7040 and 0.7060 between terranes intruded by 

Mesozoic granitic rocks with ri greater than 0.7060. 

The ri of Mesozoic granitic rocks in this area indicates 

a discontinuity underlain by ensimatic crust between 

two areas underlain by ensialic crust. The northern 

(eastern) terrane characterized by Mesozoic granitic 

rocks with rj greater than 0.7060 we will call Sierran, 

and the southern (western) terrane characterized by 

Mesozoic granitic rocks with rj greater than 0.7060 we 

will call Salinian-western Mojave. The time of formation of the discontinuity between the Salinian-western Mojave and the Sierran ensialic terranes is only 

surmised from geologic features in its vicinity; these 

features are discussed below. The systematics of the 

isotopic data from granitic rocks in the discontuinuity 

are compatible with the timing inferred from the geologic considerations.

A trough that controlled the Pahrump Group and 

subsequent Precambrian sedimentation, named the 

Amargosa aulacogen (Wright and others, 1974), occurs within the Sierran terrane (fig. 3) immediately 

east of the discontinuity between the ensialic terranes. 

The basal sedimentary unit in the trough, the Crystal 

Spring Formation, is intruded by basaltic dikes and 

sills of probable 1,200 m.y. age and lies unconformably 

on crystalline basement of approximately 1,700 m.y. 

age. The proximity of the aulacogen to the discontinuity between ensialic terranes suggests that these features are related: the aulacogen would be the failed 

arm of a triple junction (Burke and Dewey, 1973; 

Hoffman and others, 1974) with the other arms represented by the ensimatic terrane in the southern Sierra 

Nevada and central Mojave Desert now characterized 

in part by Mesozoic granitic rocks and Cenozoic basalts with ri less than 0.7060. These relations suggest

the discontinuity could have formed as long as 1,200 

m.y. ago.

The time of the formation of the discontinuity between the Sierran and Salinian-western Mojave terranes is also limited by the oldest crystal rocks within 

the discontinuity; these old rocks unfortunately are 

known only poorly. The oldest of these known include 

metamorphosed Paleozoic sedimentary rocks and volcanic rocks. Ordovician oceanic sedimentary rocks occur in the El Paso Mountains (fig. 3), but because 

these strata have affinities to western-facies eugeosynclinal rocks of the Cordilleran geosyncline, they 

are considered by some workers to be allochthonous 

and emplaced from the west (Poole, 1974). Other metamorphosed carbonates and eugeosynclinal rocks of 

Paleozoic age lie in the discontinuity south of the 

Garlock fault in the central Mojave Desert (Jennings 

and others, 1962). These rocks are flanked on both the 

east and the west by outcrops of Precambrian crystalline basement of approximately 1,700 m.y. age and by 

miogeosynclinal rocks of Paleozoic age (Stewart and 

Poole, 1975).

The strontium isotopic systematics of Mesozoic 

granitic rocks in the discontinuity with rj less than 

0.7060, as discussed below, are compatible with the 

development of the discontinuity during Precambrian 

continental rifting at the time of formation of the aulacogen. However, geologic evidence indicates that 

parts of the discontinuity were reactivated or became 

active during renewed rifting of the North American 

continent immediately prior to sedimentation in the 

Cordilleran geosyncline. If the interpretations above 

are correct, the Salinian-western Mojave terrane was a 

continental mass that lay west of the site of Paleozoic 

eugeosynclinal sedimentation in the Cordilleran geosyncline. Its present position in southern California is 

the result of subsequent Mesozoic convergence along 

the western margin of North America; it may result in 

part from about 800 km of middle Mesozoic leftlateral displacement along the extension of the zone of 

disruption of Precambrian basement, as described by 

Silver and Anderson (1974).

Figure 4 shows the configuration, where presently 

known, of the lines ri = 0.7040 and T{ ~ 0.7060 on an 

outline map of the western United States after removal of lateral displacements along the San Andreas 

and Garlock fault systems. The configuration of these 

lines in Idaho and Washington is from Armstrong, 

Taubeneck, and Hales (1977). The line ri = 0.7060 

marks the boundaries of ensialic crust and the seaward edge of late Precambrian and Paleozoic marine 

miogeosynclinal sedimentation along the Sierran terrane. The north-south trend of the edge of ancient
8 INITIAL STRONTIUM ISOTOPIC COMPOSITIONS OF MESOZOIC GRANITIC ROCKS

continental crust in central Idaho and northern Nevada changes abruptly at about lat 38° N in central 

Nevada to westward, running into eastern California. 

At about long 120° W, the trend of the margin changes 

abruptly again to about north-south as far as the discontinuity between Sierran and Salinian-western 

Mojave terranes.

Lower Paleozoic strata in northern California, 

Oregon, and Idaho can be divided into three stratigraphic belts (Churkin, 1974). The western volcanic rock 

and graywacke belt occupies the region where the rj of 

Mesozoic granitic rocks are less than 0.7040. The eastern belt of carbonate rock and quartzite occupies the

120° 115°

45°

40°

35°

Be 

Lomo:

0 100 200 300 400 KILOMETERS SOUTHERN CALIFORNIA 

t-"-1——'————i————i————i BATHOLITH

FIGURE 4.—Location and configurations of lines rj = 0.7040 and 

0.7060 in western United States. Displaced basement rocks along 

San Andreas and Garlock fault systems have been restored to 

positions shown in figure 3.

region where rf is greater than 0.7060. The central belt 

of graptolite shale and chert occupies the region where 

ri of Mesozoic granitic rocks are between 0.7040 and 

0.7060.

In the Mojave Desert, the geologic history of basement rocks and the record of Paleozoic sedimentation 

is only poorly known. This lack of knowledge is principally because of poor exposures of basement rocks and 

the poor fossil control in those rocks that are exposed. 

However, the remarkable correspondence between 

strontium isotopic ratios in Mesozoic granitic rocks 

and late Cenozoic mafic volcanic rocks and lower 

Paleozoic stratigraphy to the north suggests to us the 

following: the discontinuity between the Salinianwestern Mojave and Sierran terranes defined by ri between 0.7040 and 0.7060 in its Mesozoic granitic rocks 

is an indication of a Paleozoic sedimentation history 

in this discontinuity like that in the two belts of eugeosynclinal rocks to the north. If this postulate is correct, a puzzling exception in plate-tectonic models of 

Mesozoic igneous activity in California, Nevada, and 

Arizona can be resolved.

A locus of Jurassic magmatic activity along a northwest-southeast trend extends from southern Arizona 

to northwestern California and is crossed in the central Sierra Nevada by a locus of Cretaceous magmatic 

activity with a more northerly trend (Kistler and others, 1971, fig. 2; Kistler, 1974, fig. 2). These loci place 

older Mesozoic igneous rocks to the west of younger 

ones in Northern California and Nevada and younger 

Mesozoic igneous rocks to the west of older ones in 

southern California and Arizona. Of the many existing 

plate-tectonic models for the Mesozoic magmatic activity in California (Hamilton, 1969), none has accounted for the inland locus of Jurassic magmatism, 

extending from southern Arizona across the eastern 

Mojave Desert and into the Inyo-White Mountains in 

eastern California. In fact, the Jurassic magmatic activity in Arizona cuts the Precambrian craton and was 

hundreds of kilometers inland from the present continental margin. This fact led Kistler, Evernden, and 

Shaw (1971) to account for it as simply a locus of magmatism manifesting a linear zone of high heat flow in 

the mantle that had characteristics like present-day 

oceanic rises.

Continued geologic, geochronologic, and isotopic 

tracer studies have now identified other features associated with the inland locus of Jurassic magmatism. A 

zone of disruption that extends S 50° E from the 

southern Inyo Mountains into the Sierra Madre Occidental of Sonora offsets Precambrian crystalline rocks 

1,725-1,800 m.y. old some 500 km in a left-lateral 

sense and lies along the locus of Jurassic magmatism; 

700-800 km of left-lateral offset of Paleozoic deposi-
DEVELOPMENT OF THE CONTINENTAL MARGIN

tional trends occurs near the same structure (Silver 

and Anderson, 1974). The discontinuity between the 

Sierran and Salinian-western Mojave terranes lies 

along the western margin of the Jurassic magmatic 

locus.

The apparent left-lateral shear zone identified by 

Silver and Anderson (1974) led these investigators to 

suggest that the locus of Jurassic magmatism marks 

the position of a former plate boundary. The coincidence of the discontinuity between the, continental 

Sierran and Salinian-western Mojave terranes with 

the western margin of the Jurassic intrusive locus 

strengthens this concept.

Figure 4 shows the relative positions of the ensialic 

Salinian-western Mojave and Sierran terranes after 

removal of lateral displacements along the San Andreas and Garlock fault systems. Stewart and Poole 

(1975) have correlated two miogeosynclinal Precambrian and Paleozoic sections in the Salinian-western 

Mojave terrane with two similar stratigraphic sections 

in the Sierran terrane. This correlation requires that 

these two terranes have been in the same relative positions since the late Precambrian (Stewart and Poole, 

1975). On the other hand, Silver and Anderson (1974) 

suggest a correlation of Precambrian and Paleozoic 

strata in the Sierran terrane with strata on the west 

side of the zone of disruption in the Precambrian 

basement about 800 km to the south. To us these differing interpretations indicate that individual stratigraphic sections do not uniquely define relative 

positions of deposition of sedimentary strata in the 

Cordilleran miogeosyncline. Apparent truncation and 

offset of ancient basement terrane, however, are provocative. Therefore, we tested where the Salinianwestern Mojave terrane would lie in the early Mesozoic if its position shown in figure 4 resulted from a leftlateral displacement during the middle Mesozoic 

along the extension of the zone of disruption of Precambrian basement described by Silver and Anderson 

(1974). The position is shown in figure 5. Similar 

shapes and juxtaposition make it possible to speculate 

that the Salinian-western Mojave terrane once occupied the wide region between the ri = 0.7040 and ri = 

0.7060 lines in northwestern Nevada.

We envision the configuration of the margin of continental crust, indicated by the line ri = 0.7060 in the 

western United States (fig. 4), as developing in the following way. During the time of development of 

intracontinental basins that received the Belt Supergroup sediments, about 1,200 to 850 m.y. ago (Obradovich and Peterman, 1968), a true continental separation occurred along a locus now marked by the line ri 

= 0.7040 (fig. 64). Burke and Dewey (1973) propose a 

continental separation at this time, but their locus of

separation is indicated to be well to the east of the line 

ri = 0.7040. The western continental plate moved 

away, and its present location is unknown. This rifting 

event died out probably about 850 m.y. ago. Just prior 

to the Early Cambrian, rifting began again. The initial 

locus of the new separation is marked by the line ri = 

0.7060. In Washington and Idaho, the new locus of 

rifting coincided with the earlier locus and the lines ri 

= 0.7040 and 0.7060 are essentially coincidental (Armstrong and others, 1977). In Nevada and California, 

the new locus of rifting extended into the continental 

terrane. As a consequence, the western plate consisted 

of both mafic lithosphere made 1,200-850 m.y. ago

120°

35° -

FIGURE 5.—Position of Salinian-western Mojave terrane after 800 

km of left-lateral displacement is removed along extension of 

zone of dislocation in Precambrian basement described by Silver 

and Anderson (1974).
10 INITIAL STRONTIUM ISOTOPIC COMPOSITIONS OF MESOZOIC GRANITIC ROCKS

and older continental fragments, including the Salinian-western Mojave terrane (fig. 6B). During Mesozoic convergence along the western margin of North 

America, the continental fragments in the western 

plate were returned close to their former positions.

Accepting the rifting history as indicated in figure 

6, material intruded into the rift zones would produce 

mafic (oceanic) lithosphere. We prefer to use the term 

mafic lithosphere because modern oceanic lithosphere 

is defined on the basis of seismic characteristics, and 

even though this ancient rift filling was probably oceanic crust, it is no longer identified as such seis0.7060

= 0.7040

~ 600 - 350 m.y.

FIGURE 6.—Diagram of model of continental rifting used to account for present configurations of lines q = 0.7040 and ri = 

0.7060. A, Continental separation 1,200-850 m.y. ago during development of Precambrian aulacogens of Belt age. Locus of separation is now marked by line rj = 0.7040. B, Continental 

separation during early Paleozoic beginning about 600 m.y. ago. 

Note that western rifting plate is made up of both continental 

and mafic lithosphere. Initial locus of rifting is now marked by 

line rj = 0.7060 (edge of Precambrian continental crust).

mically. This mantle-derived material would have different Rb, Sr, and ri than the Precambrian lower continental crust. In the discussion that follows, we will 

show that the strontium isotopic systematics are compatible with deriving Mesozoic magmas west of the 

line ri = 0.7060 from the lithosphere produced during 

the two rifting events in our model.

TRACE ELEMENT VARIATIONS

Two values of ri, 0.7040 and 0.7060, mark natural 

separations of Mesozoic granitic rocks in the Sierra 

Nevada into three types on K-Rb, K-Sr, and Rb/SrRb variation diagrams (Kistler and Peterman, 1973). 

Using the alkali-lime index of classification of siliceous plutonic rocks (Peacock, 1931), calcic plutonic rocks have ri less than 0.7060, and calc-alkalic 

granitic rocks have ri values greater than 0.7060 

(Kistler, 1974). Enough Rb, Sr, and ri data now exist 

for California granitic rocks to make some general 

statements about elemental abundances in them relative to ri.

Rubidium concentration and ri (fig. 7) correlate in 

almost all the samples. For samples with ri greater 

than 0.7040, Rb and SiC>2 are also positively correlated. Tie lines between some points join samples 

from mapped cogenetic granitic rock sequences. SiC>2 

does not fall below 60 weight percent in the rocks investigated that have rf greater than 0.7080 or below 55 

weight percent in the rocks investigated that have ri 

between 0.7040 and 0.7080. In granitic rocks with ri 

greater than 0.7040, rubidium reaches a maximum 

concentration of about 200 ppm and the maximum 

SiC>2 content is about 75 weight percent. Three samples that plot in an anomalous position relative to 

other specimens with ri less than 0.7040 are 

trondjhemites with SiC>2 that average 71 weight percent but contain lower Rb concentrations than any 

other specimens investigated.

Strontium concentration is plotted against ri in figure 8 for each granitic rock specimen. Granitic rocks 

with ri greater than 0.7060 have maximum values of 

about 800 ppm strontium in the most mafic specimens, and those with ri less than 0.07060 have maximum values of about 650 ppm Sr in the most mafic 

specimens. Concentration of strontium in the most 

felsic rocks is about 100 ppm regardless of ri.

When rubidium is plotted against strontium (fig. 9) 

for all granitic rocks investigated with ri less than 

0.7060 as well as for average oceanic basalts (Hart and 

others, 1970), points can be separated into three discrete groups. Points representing granitic rocks with 

ri between 0.7030 and 0.7040 from the western Sierra 

Nevada lie along the oceanic basalt line and trend into
SOURCE MATERIALS FOR THE GRANITIC ROCKS 11

0.7120

0.7100

0.7080

~ 0.7060

0.7040

0.7020

0.7000 100

RUBIDIUM CONTENT, IN PARTS PER MILLION

200

FIGURE 7.—Rb concentration plotted against initial mSr/S7Sr values for specimens of Mesozoic granitic rocks in California. Lines join

samples from mapped cogenetic granitic rock sequences.

the group of points representing Sierran granitic rocks 

with ri between 0.7040 aind 0.7060.Those specimens 

with Rb and Sr abundances equivalent to oceanic basalts are trondjhemites, and the others are tonalites.

Points representing other granitic rocks with ri less 

than 0.7060 define two fields in figure 6. Granitic rock 

specimens from the southern California batholith, two 

specimens from the southern Sierra Nevada near Tehachapi, and the single specimen from Ben Lomond in 

the Salinian block lie in a field that does not overlap 

the field for granitic rocks in the Sierra Nevada with 

comparable ri. The separation of granitic rocks into 

groups on the diagram that can have similar ri but 

grossly different strontium concentration for any given rubidium concentration suggests that source materials for the southern California batholith and several 

of the granitic rocks in the southern Sierra Nevada 

had trace-element compositions different from the 

source materials that yielded granitic rocks with similar ri in the Sierra Nevada. The difference could also 

reflect a different history for the source materials or 

simply result from a different depth of magma generation in a similar source. These granitic rocks lie at the 

south and north ends of the Salinian-western Mojave 

terrane.

Points representing granitic rocks with ri greater 

than 0.7060 form a field that overlaps the field of

granitic rocks in the Sierra Nevada with rj between 

0.7040 and 0.7060. These points are not shown in 

figure 9.

SOURCE MATERIALS FOR THE GRANITIC ROCKS

In order to account for the observed variation in ri 

of Mesozoic granitic rocks north of the Garlock fault 

in California, Kistler and Peterman (1973) adopted a 

model in which the melts that resulted in the granitic 

rocks with ri greater than 0.7060 were derived from a 

lower continental crust that acquired its chemical and 

isotopic characteristics about 1,700 m.y. ago and had a 

ri of 0.7020 and Rb/Sr values that regionally varied 

between 0.06 and 0.10. If the source for the granitic 

rocks with ri less than 0.7060 was assumed to have ac0.7100

,1- 0.7060

0.7020

• • ••••«• %•*•* "'£ V^VVh*'

•_• • _.

0 400 800

STRONTIUM CONTENT, IN PARTS PER MILLION

FIGURE 8.—Sr concentration plotted against rj values for specimens of Mesozoic granitic rocks in California.
12 INITIAL STRONTIUM ISOTOPIC COMPOSITIONS OF MESOZOIC GRANITIC ROCKS

quired its Rb-Sr characteristics at the same time, the 

ratios required to produce the presently observed ri 

variation range from 0.02 to 0.06. It was pointed out, 

however, that as the ri values of the Mesozoic granitic 

rocks approach 0.7030, it becomes increasingly more 

difficult to decipher a possible pre-Mesozoic history 

for the source material on the basis of ri data alone. 

We will refine our model for the pre-Mesozoic history 

of the source region for the Mesozoic granitic rocks 

with ri less than 0.7060 on the basis of our model for 

development of the configuration of the western margin of ensialic crust (fig. 6). 

We plotted Rb/Sr against ri for Mesozoic granitic

100

10

AB

EXPLANATION

Sierra Nevada and Central Mojave 

< 0.7040 

< 0.7050 

< 0.7060 

Southern Call brnia batholith

0.7030 < 

0.7040 <

< 0.7040 

< 0.7050

fOFB

Northern boundary Salinian —

W. Mojave terrane 

V 0.7050 < TJ < 0.7060 

• Average oceanic basalts 

(Hart and others, 1970) 

OFB = Ocean floor basalt 

LKT = Low potassium

tholeiitic

TB = Tholeiitic basalt 

AB = Alkali basalt

100 1000 

STRONTIUM CONTENT, IN PARTS PER MILLION

FIGURE 9.—Rubidium concentration relative to strontium concentration in California granitic rocks with rj less than 0.7060.

rocks in California (fig. 10). An isochron with ri equals 

0.7020 and 1,600-m.y. age (1,600 m.y. reference isochron used to account for the average 100 m.y. age of the 

granitic rocks investigated) is also shown on the diagram. With the data available only from the granitic 

rocks north of the Garlock fault (Kistler and Peterman, 1973), all points fell to the right of a 1,600-m.y. 

isochron on a similar diagram. This distribution was 

compatible with an increase in Rb/Sr in melts derived 

during partial melting and subsequent differentiation 

from a 1,600 m.y.-old source with ri = 0.7020 and 

Rb/Sr between 0.02 and 0.10. The new data also fit 

this distribution except for two specimens with ri 

greater than 0.7080. However, the most mafic specimens with ri less than 0.7060 have Rb/Sr about twice 

that implied by the 1,600 m.y.-old source, and the 

most mafic specimens with ri greater than 0.7060 have 

Rb/Sr only slightly higher than that implied by the 

source. We feel that these Rb/Sr differences are significant and bear on the history of the source materials for the granitic rocks.

If the source for all the Mesozoic granitic rocks in 

California formed 1,700 m.y. ago and had ri of 0.7020 

at that time, the relatively high values of Rb/Sr in the 

granitic rocks with rj less than 0.7060 could be due to a 

lesser degree of partial melting of these source materials than the degree of partial melting of similar source 

material for granitic rocks with rj greater than 0.7060. 

Rb/Sr is higher in the initial melt than in the granite 

source and would vary inversely with the degree of 

melting. However, in terms of major-element chemistry, granitic rocks with ri less than 0.7060 are calcic, 

and those with ri greater than 0.7060 are calc-alkalic 

(Kistler, 1974). This fact suggests that different abundances and ratios of large-ion lithophile elements in 

the groups of granitic rocks are due to processes other 

than degree of melting of similar sources, because lesser degrees of melting should, in terms of major elements, yield more alkalic rather than more calcic 

magmas. We will elaborate below.

Our model (fig. 6) indicates that the ensimatic lithosphere to the west of the Sierran terrane formed between about 600 and 350 m.y. ago. In figure 10, we 

have drawn a 500 m.y. isochron (500 m.y. isochron is 

used to account for the average 100 m.y. age of the 

granitic rocks investigated) with ri of 0.7030. The ri of 

0.7030 was selected because this is the lowest measured by us in any Mesozoic and younger igneous 

rocks to the west of the 0.7060 line. All points for granitic rocks with ri less than 0.7060 lie to the right of this 

isochron, a feature that is compatible with deriving 

the magmas represented by these granitic rocks from 

a source terrane about 600 m.y. old and characterized 

by Rb/Sr between 0.012 and 0.10. Granitic rocks with
SUMMARY AND CONCLUSIONS 13

0.7100

0.7080

0.7060

~ 0.7040

0.7020

0.7000

•

•

• •

•

EXPLANATION

Southern Catifornia batholith, 

Ben Lomond and Tehachapi Mountains 

samples

All other samples

0.1 0.2 0.3 0.4 0.5 

Rb/Sr

0.6 0.7 0.8 0.9 1.0

FIGURE 10.—Isochron plot for California granitic rocks. Rb/Sr plotted against rj the 1,600-m.y., 1,100-m.y., and 500-m.y. isochrons shown 

for reference with regard to possible age of source material of granitic rocks as discussed in text.

ri greater than 0.7060 could not be derived from a 

source terrane with these characteristics.

Southern California batholith specimens, located in 

an area extending south from the south edge of the 

Salinian-western Mojave terrane, and the four specimens with ri less than 0.7060 along the north edge of 

the Salinian-western Mojave terrane (fig. 9), have 

generally lower Sr for any given Rb abundance than 

other California granitic rocks with equivalent rj. This 

fact indicates that the source for these granitic rocks 

could have been depeleted in Sr relative to the source 

for the granitic rocks with similar ri in the northern 

and western Sierra and in the eastern part of the discontinuity. The indicated depletion is compatible 

with a different source history and composition than 

that for the source of Sierran granitic rocks with similar ri. The terrane into which these granitic rocks are 

intruded would be mafic lithosphere formed between 

1,200 and 850 m.y. ago that became part of the western plate during the second rifting stage beginning in 

the Cambrian (fig. 6B). The lithosphere source material for these granitic rocks is older than that for granitic rocks with similar ri in the Sierra Nevada, and it 

possibly formed with a ri of around 0.7010. A 1,000- 

m.y. isochron (1,000-m.y. isochron used as an average

age for the source) with this rj lies close to the left of 

those points (shown as black squares in fig. 10) representing these granitic rocks.

SUMMARY AND CONCLUSIONS

Kistler, Evernden, and Shaw (1971) and Armstrong, 

Taubeneck, and Hales (1977) interpreted values of ri 

greater than 0.7060 in Mesozoic granitic rocks in the 

western United States as being due to contamination 

of magmas with less radiogenic strontium by radiogenic strontium from the host rocks. We (Kistler and 

Peterman, 1973) tested the possibility of contamination by intruded wallrocks of granitic magmas for 

rocks with similar ri and rejected the possibility. With 

the additional data available, including those of Armstrong, Taubeneck, and Hales (1977), we still reject 

contamination as a dominant cause of isotopic variation and consider variation in ri as time dependent 

and reflecting Rb/Sr differences established much 

earlier in the source materials for the granitic magmas. Ages inferred for the source materials from the 

systematics of relation between Rb, Sr, and ri in the 

granitic rocks are consistent with ages inferred for the 

sources from geologic considerations. This interpreta-
14 INITIAL STRONTIUM ISOTOPIC COMPOSITIONS OF MESOZOIC GRANITIC ROCKS

tion implies that the pattern of ri of igneous rocks 

maps a lithosphere zone of melting, probably at a 

depth of 30 to 50 km, that intersects ancient crust of 

different ages and compositions.

An alternative to the constant depth zone of lithosphere melting to explain the variation in ri has to be 

considered. The granitic rocks with ri greater than 

0.7060 also have the highest strontium concentrations 

(fig. 8). Partial melting of ensialic crust is not necessarily a unique source for increased Sr relative to partial melting of ensimatic crust. Also, a mixing model 

(mixed source, not mixed magmas) with basaltic (oceanic crust) and granitic (continental crust) end members should show a negative correlation between ri 

and Sr. However, Hart and others (1970) showed a 

positive relation between depth to seismic zone and Sr 

concentration in overlying volcanic rocks in the major 

active subduction zones of the Pacific. Their model to 

account for this relationship utilized the concept of 

large-ion-lithophile "depleted" versus "undepleted" 

mantle as source materials for the eruptive magmas. 

The model could be modified to include "suboceanic" 

versus "subcontinental" mantle sources, and the 

depth aspect would not be ignored when the Sr contents of the samples most distant from the trench are 

so high. Even though the granitic rocks with high ri 

are not alkalic, this may not be a discriminating factor. Many Cenozoic mafic lavas with high ri are alkalic. With strontium concentration being a function of 

depth of melting, our major argument, the necessity of 

continental crust associated with the plutons having ri 

greater than 0.7060 is unaffected. The ri variation 

shows that the source yielding high-ri magmas must 

have older and (or) higher Rb/Sr material than that 

yielding low-ri magmas. Whether the ri = 0.7060 contour reflects an "edge" or simply an intermediate degree of melting of the two end members that 

accomplished the same thing does not much affect the 

plate-tectonic interpretation. The low-ri areas still reflect the contribution of the low-Rb/Sr end members 

and require some interpretation such as a rift that removed the high-Rb/Sr and (or) older parent. Both 

the constant-depth zone and variable-depth zone of 

lithosphere melting to account for the observed variation in ri and Sr abundance in the Mesozoic granitic 

rocks in California are within regions of granitic 

magma generation considered possible according to 

experimental evidence summarized by Wylie, Huang, 

Stern, and Maalee (1976).

The strontium isotope data support the concept of 

rifting of the western margin of North America during 

the time of Belt sedimentation (Burke and Dewey, 

1973) and again at the inception of sedimentation in 

the Cordilleran geosyncline (Stewart, 1972; Churkin,

1974). Churkin's model of the early Paleozoic sedimentation history of western North America involves 

migration of frontal arc systems away from the continent and creation of interarc basins and marginal 

ocean basins that were sites of deposition of the sediments in the graptolite shale-chert belt. Churkin's 

concept requires the derivation of thick quartzite sequences in the graptolite shale-chert belt from the 

craton far to the east. The petrography and age of 

some of the quartzites in this belt have led other workers (Ketner, 1966; Gilluly and Gates, 1965) to deny the 

possibility of an eastern source and to appeal to an unknown continental source to the west to provide the 

materials for these units. Westward rifting of a continental fragment, possibly the Salinian-western Mojave terrane, from the area between lines ri = 0.7040 

and 0.7060 in northwestern Nevada and northeastern 

California beginning in the Cambrian (fig. 6B) would 

support the latter view.

Strontium-isotope studies of granite plutons have 

proved to be useful in tectonic studies. Investigations 

of possible offsets along transcurrent faults in the past 

have relied on apparent displacements of sedimentary 

rocks, because igneous and metamorphic basement 

rocks seldom have distinctive physical characteristics 

that can be correlated unequivocally over long distances. Current methods of classification of igneous 

rocks restrict their use as indicators of transcurrent 

fault displacement—granodiorites around the world 

look pretty much alike. However, in granodiorites investigated so far in California, ri ranges from about 

0.7040 to 0.7095. Strontium isotope ratios, therefore, 

provide an easily determined parameter in basement 

terrane with enough variation to test transcurrent offsets suggested by apparent displacement of 

superjacent sedimentary strata or other geologic features.

Our test of the suggestion of Silver and Anderson 

(1974), that a minimum of 500 km of left-lateral displacement during the middle Mesozoic occurred along 

the trend of the Jurassic locus of magmatism extending from California to Sonora, supports but does not 

prove the concept. We do not know where the locus of 

dislocation could be to the west of the Sierra Nevada 

nor its exact location in southern California; any or all 

of the faults in the Foothills fault system (Clark, 1960; 

Duffield and Sharp, 1975) are possible candidates in 

the Sierra Nevada. Clark (1960), Baird (1962), and 

Cebull (1972) have presented evidence for possible 

strike-slip displacement along the components of the 

Foothills fault system.

The model of two-stage rifting of the western margin of North America during the Precambrian and 

early Paleozoic not only helps to resolve the problem
ROCK TYPE AND LOCALITY DESCRIPTION OF ROCKS INVESTIGATED 15

of source materials for quartzites in Paleozoic eugeosynclinal assemblages in northwestern Nevada and 

northern California but also helps to account for a sequence of lower Mesozoic quartzites, marbles, and pelitic schists exposed discontinuously in a belt of roof 

pendants in the western Sierra Nevada south of lat 

38° N. The position of these rocks has been considered anomalous because they are separated from strata of similar age and lithology to the east by a belt of 

volcanic and sedimentary strata of similar age (Stanley and others, 1971). Source materials for the quartzite-carbonate sequence in the roof pendants would 

have to lie to the west, where none now exist. The 

western plate that encroached on North America during the early Mesozoic, however, was made up of both 

oceanic and continental material (fig. 6£). An ophiolite-melange sequence of Permian and Triassic age occurs in a belt parallel to and west of the western belt of 

lower Mesozoic sedimentary rock (Saleeby, 1975). 

This situation places the lower Mesozoic sedimentary 

rocks in the western belt in a trench of early Mesozoic 

age (Schweikert and Cowan, 1975). Continental fragments in the plate encroaching from the west would 

provide a source for the quartz sands in this sequence. 

The proposed minimum of 500 km of left-lateral middle Mesozoic disruption along this plate boundary 

(Silver and Anderson, 1974) would place the continental rocks of the western plate in a position that is now 

far to the south of the site of deposition of western belt 

lower Mesozoic sedimentary rocks.

Thus, the early Mesozoic trench associated with the 

converging plates lay to the east of the Salinian-western Mojave terrane and along the western margin of 

the Sierran terrane. Magmatic activity associated 

with this stage of convergence occurred along the 

northwest-southeast locus extending at least from 

northern California to south-central Arizona (Kistler 

and others, 1971). When continental parts of the western converging plate reached the trench, subduction 

was no longer possible. Continued convergence had to 

be relieved in a new trench developed to the west of 

the Salinian-western Mojave terrane, a trench now represented by Franciscan rocks. The late Mesozoic and 

Tertiary Great Valley sequence was deposited in the 

shadow of the Salinian-western Mojave terrane. The 

granitic rocks exposed discontinuously from Montara 

to Bodega Head (fig. 3) could have been brought to 

their apparently anomalous position west of the Great 

Valley sequence on the plate subducting beneath the 

Franciscan melange.

Our model (fig. 6) requires two stages of rifting of 

the western margin of North America and molds into 

compatibility modern tectonic theory and geologically 

determined sedimentation history. The configuration

of the plate boundaries as indicated by the strontium 

isotopic pattern is a crucial factor in the development 

of the model. In addition, geochronologically determined magmatic history and isotope geochemistry 

permit a model to be developed for this same marginal 

configuration when it changed from accretionary to 

convergent. Reactivation of continental rift zones is 

apparently a common phenomenon (McConnell, 1972; 

Garson and Krs, 1976). A triple junction associated 

with continental rifting is inferred to be active now in 

southern Idaho (Prostka and Oriel, 1975), centered on 

the line ri equals 0.7060. A geophysically defined axis 

of symmetry for the Great Basin (Eaton, 1976) extends from about lat 42° N to 34° N. The northern 

part of this axis lies near the line ri = 0.7060, but the 

southern part is entirely in crust characterized by 

Mesozoic granitic rocks with ri greater than 0.7060. If 

the axis is the locus of spreading under the Great Basin (Eaton, 1976), it is a modern analog of the rifting 

that occurred along the line ri = 0.7060 prior to deposition in the Cordilleran geosyncline (fig.6B).

ROCK TYPE AND LOCALITY DESCRIPTION 

OF ROCKS INVESTIGATED

Sr 1-73 Hornblende quartz diorite, specimen locality 124

of Evernden and Kistler (1970). 

Sr 2-73 Hornblende-bearing biotite quartz monzonite,

specimen locality 125 of Evernden and Kistler

(1970). 

Sr 3-73 Foliated quartz diorite, 2.6 km east of Glennville

on California Hwy. 155. 

Sr 4-73 Sphene-bearing biotite-hornblende granodiorite,

specimen locality 129 of Evernden and Kistler

(1970).

Sr 5-73 Foliated biotite-hornblende granodiorite, specimen locality 130 of Evernden and Kistler

(1970). 

Sr 6-73 Porphyritic biotite quartz monzonite, 13.0 km

south from junction of California Hwy. 178 on

Kelso Valley Road.

Sr 8-73 Biotite quartz monzonite, 12.2 km south of locality Sr 6-73 on Kelso Valley Road. 

Sr 9-73 Biotite quartz monzonite, on Jawbone Canyon

Road, 5.5 km east of junction with Kelso Valley

Road. 

Sr 10-73 Biotite-hornblende granodiorite, on Jawbone

Canyon Road, 12.6 km east of junction with

Kelso Valley Road. 

Sr 11-73 Biotite quartz monzonite, on California Hwy. 58,

18.7 km east of Mojave Calif. In small quarry

on north side of road. 

Sr 12-73 Biotite-hornblende quartz diorite, 17.4 km north

of Willow Springs turnoff on Willow SpringsTehachapi Road. 

Sr 14-73 Biotite granodiorite, on road to Kern County

Park, 3.2 km from park entrance. 

Sr 15-73 Biotite-hornblende granodiorite, on California

Hwy. 58, 13.2 km west of Tehachapi Railway

Station.
16 INITIAL STRONTIUM ISOTOPIC COMPOSITIONS OF MESOZOIC GRANITIC ROCKS

Sr 16-73 Biotite quartz monzonite, first granitic rock 

outcrop on east side of Last Chance Canyon 

Road in El Paso Mountains. Triassic age for 

this granite (table 3) is from Armstrong and 

Suppe (1973).

Sr 17-73 Hornblende-biotite granodiorite, on county road, 

1.2 km south of Randsburg, Calif.

Sr 18-73 Biotite quartz monzonite, 19.2 km north of junction with U.S. Hwy. 395 of county road, 0.4 km 

south of Johannesburg, Calif, on county road 

that intersects with California Hwy. 178.

Sr 19-73 Hornblende-biotite granodiorite, on road 2.3 km 

north of location of specimen Sr 18-73.

Sr 20-73 Foliated porphyritic biotite quartz monzonite, 

outcrop on east side of U.S. Hwy. 395 at Little 

Lake turnoff on road to lower Little Lake 

Ranch.

Cos 10-2 Biotite granodiorite, outcrop at center of south 

edge of sec. 10 T. 21S., R. 38 E., Haiwee Reservoir, Calif, quadrangle.

Cos 13-42-2 Hornblende quartz diorite, outcrop in center of 

sec. 34, T. 21 S., R. 38 E., Haiwee Reservoir, 

Calif, quadrangle.

A3 Fine-grained porphyritic alaskite, outcrop at center of west edge of sec. 10, T. 30 S., R. 43 E., 

Cuddeback Lake, Calif, quadrangle.

A6 Sphene-bearing hornblende quartz diorite, 

outcrop on small hill, 0.4 km east of hill 3699 

near center of north edge of T. 30 S., R. 44 E., 

Cuddeback Lake, Calif, quadrangle.

D15 Biotite-hornblende granodiorite, outcrop 305 m 

east of BM 2745 on Randsburg Road, Quail 

Mountains, Calif, quadrangle.

D16 Medium-grained biotite quartz monzonite, 

outcrop at 792 m on northwest-trending ridge 

in northeast corner of sec. 16, T. 17 N., R. 2 E., 

Quail Mountains, Calif, quadrangle.

E10 Biotite quartz monzonite, outcrop at Two 

Springs, Camp Irwin Military Reservation, 

Leach Lake, Calif, quadrangle.

E12 Medium-grained biotite quartz monzonite, 

outcrop at Desert King Spring, Camp Irwin 

Military Reservation, Leach Lake, Calif, quadrangle.

C-203 Gray biotite granite, outcrop at Black Magic 

Mines, Owlshead Mountains, Leach Lake, Calif, quadrangle.

C-204 Gray biotite granite, outcrop on hill 3342, 

Owlshead Mountains, Leach Lake, Calif, quadrangle.

C-201 Trachyandesite, outcrop on south edge near west 

corner of sec. 32, T. 19 N., R. 3 E., Leach Lake, 

Calif, quadrangle.

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