I 

Potassium-Argon Ages for Plutons in the 

Eastern and Southern Sierra Nevada 

Batholith, California 

U.S. GEOLOGICAL SURVEY BULLETIN 1799 

Potassium-Argon Ages for Plutons in the 

Eastern and Southern Sierra Nevada 

Batholith, California 

By EDWARD A. du BRAY and DAVID A. DELLINGER 

A study of the ages of 26 plutons in the 

eastern and southern Sierra Nevada 

U.S. GEOLOGICAL SURVEY BULLETIN 1799 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas l. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1988 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Du Bray, E. A. 

Potassium argon ages for plutons in the eastern and southern 

Sierra Nevada batholith, California. 

{U.S. Geological Survey bulletin ; 1799) 

Supt. of Docs. no.: I 19.3:1799 

1. Potassium-argon dating. 2. Intrusions {Geology)-Sierra 

Nevada Mountains {Calif. and Nev.). I. Dellinger, D. A. II. Title. Ill. Series. 

QE75.B9 no. 1799 557.3 s 87-600266 

[QE508] [552' .3' 097944] 
CONTENTS 

Abstract 1 

Introduction 1 

Methods 1 

Analytical precision 1 

Geology 4 

Results and discussion 4 

Summary 9 

References 10 

FIGURE 

1. Map showing sample localities and generalized outcrop patterns of the dated 

plutons and the Independence dike swarm of the eastern and southern Sierra 

Nevada batholith 2 

TABLES 

1. Analytic data for samples with multiple K20 analyses 4 

2. Petrographic data for the dated plutons 5 

3. K-Ar ages of and analytical data for intrusive rocks, eastern and southern Sierra 

Nevada batholith, California 6 

Contents Ill 

Potassium-Argon Ages for Plutons in the 

Eastern and Southern Sierra Nevada 

Batholith, California 

By Edward A. du Bray and David A. Dellinger 

Abstract 

The ages of biotite and/or hornblende separates from 26 

plutons in the eastern and southern Sierra Nevada were 

determined by the K-Ar method. Both biotite and hornblende 

age determinations were made for 14 of these samples. Six of 

the biotite-hornblende pairs give concordant ages; the rest of 

the biotites give younger ages. Two samples of hornblende give 

ages considered to be too young, which may be a consequence 

of post-magmatic potassium metasomatic alteration of 

hornblende. Most of the dated plutons were emplaced in Late 

Cretaceous time at the culmination of magmatic activity in the 

Sierra Nevada. Samples of two plutons not cut by the 

Independence dike swarm have ages of around 123 Ma; the 

dike swarm must have been emplaced prior to this time. 

INTRODUCTION 

Age determinations have been made for many 

plutons of the Sierra Nevada batholith. Evernden and 

Kistler (1970) dated plutons in the accessible parts of the 

range, and ages were determined for plutons in the 

southern Sierra Nevada by Chen (1977) and Chen and 

Moore (1979, 1982). Ages for a number of plutons in the 

central Sierra Nevada were determined by Stern and 

others (1981). The ages of plutons in the eastern and 

southern Sierra Nevada, however, are less well known and 

are the subject of this study. 

METHODS 

Rock samples of 2-5 kg were collected at locations 

(fig. 1) as far from contacts as was practical to minimize 

the effects of argon loss due to reheating during 

subsequent magmatic events. Samples were crushed and 

sieved; biotite and hornblende separates, at least 99 

percent pure, were made from -60 + 80 mesh size fraction 

using standard laboratory techniques based on density, 

magnetic properties, and shape differences. Aliquots of 

biotite and hornblende were analyzed by the Analytical 

Labs of the U.S. Geological Survey in Menlo Park for 

K20 by a lithium metaborate flux fusion-flame 

photometry technique; lithium acts as an internal 

standard (Ingamells, 1970). 

All argon determinations were made in the U.S. 

Geological Survey's isotope geology laboratory in Menlo 

Park, California, using standard isotope-dilution techniques described by Dalrymple and Lanphere (1969). 

Samples of clean argon gas were analyzed using a 60 ° 

sector, 15.2 em-radius, Neir-type mass spectrometer, 

operated in the static mode. Ages were calculated using 

the constants of the subcommission on geochronolgy 

(Steiger and Jager, 1977). 

ANALYTICAL PRECISION 

Analytical precision (error), which is expressed at 

the one sigma level, assigned to the determined ages was 

computed using a modified version of the equation 

developed by Cox and Dalrymple (1967). In addition to 

the factors included in their equation (errors in 

determination of K2

0, of 40Ar/38Ar and 38Ar/36Ar, of 

38 Ar in the spike, and the percentage of 40 Ar in the 

sample) we have included spectrometer discrimination 

errors and errors in the measurement of isotopic ratios 

in the spike. The contributions to total error by these 

factors are small in most cases but are important 

components of total error in others. The total error 

computed by our equation is consistent with the 

empirically predicted error of Tabor and others (1985). 

The largest source of uncertainty in most of the age 

determinations was in the K20 analyses (table 1). 

Several replicate K20 determinations of split mineral 

separates were made. Relative standard deviations for 

replicate analyses of K2

0 in biotites (mean, 3.0 percent) 

ranged from 1.6 to 4. 7 percent (not including a chloritized sample with a standard deviation of 7. 7 percent) and 

from 0.5 to 10.3 percent in hornblendes. We believe the 

Analytical Precision 
D 

EXPLANATION 

Cenozoic roc k s and surficial deposits 

~ d·kes Independence ' 

k -Linewor k indicates I C :::) I Granitic roc s . of dated plutons known margms 

r::::::::::::::::::::::::::j Pre-Cenozoic strata 

howing sam- Figure 1 .. . Ma~n~ generalized 

le localities f the dated 

P tterns 0 d e outcrop p~ the lndepen enc 

plutons an f the eastern 

dike swarm oSierra Nevada and southern 

batholith 

37"00' 

36"00' 

>LONG 

VALLEY 

~ 

' 

. 

. 

. . 

( 

., 

.... 

118"00' 

~ 

"' .) 

( 

.. ., 

~ .) 

( 

> 

... L ., 

.) 

A ~ 

.. ~ L 

., 395 (. 

8 

L ( 
Table 1. Analytic data for samples with multiple K2

0 

analyses 

Field Hap Mineral 

no. no. 

8-133 D biotite 

8-133 D do. 

8-127 J do. 

8-127 J do. 

8-48 p do. 

8-48 p do. 

8-47 Q do. 

8-47 Q do. 

8-232 s hornblende 

8-232 s do. 

8-232 s do. 

8-59 T biotite 

8-59 T do. 

9-7 u hornblende 

9-7 u do. 

9-77 v do. 

9-77 v do. 

KzU .t-lean K2o Standard dewt. % wt. % viation wt. % 

2.31 2.28 0.05 

2.24 

9.01 8.79 0.31 

8.57 

7.84 7.44 0.57 

7.03 

2.98 2.98 

2.98 

1.475 1.410 4. 72 

1.342 

1.412 

8.89 8.79 1.61 

8.69 

1.147 1.07 10.25 

0.992 

1.271 1.276 0.55 

1.281 

10.3 percent error computed for one hornblende to be 

anomalous with respect to the pooled coefficient of variation determined for the laboratory by many replicate 

analyses of other samples; the mean error for other hornblende samples is 2.6 percent. These values are larger than 

those cited by Cox and Dalrymple (1967) and may be due 

to sample inhomogeneity rather than analytical error; 

neither cause can be ruled out. Errors of 2 percent 

(biotite) and 3 percent (hornblende) were assigned to 

samples for which only a single K20 determination was 

made. 

GEOLOGY 

The Sierra Nevada batholith is a composite intrusive complex composed of hundreds of individual 

Mesozoic-age plutons; the majority of its constituent 

plutons are Cretaceous in age (Stern and others, 1981; 

Chen and Moore, 1982). The batholith intruded older 

volcanic and sedimentary rocks that have been metamorphosed at greenschist to amphibolite facies conditions. 

These rocks also account for about 10 percent of the area 

within the batholith, where they occur as roof pendants. 

The intrusive rocks of the Sierra Nevada batholith range 

in composition from hornblende gabbro to alaskitic 

granite. The granitoid rocks are typically medium grained 

and homogeneous with respect to texture and composition within a given pluton, although some display welldeveloped concentric zoning. The granitoids contain 

quartz, potassium feldspar, plagioclase, biotite, and hornblende as principal constituents; minor constituents include iron-titanium oxides, zircon, apatite, and sphene. 

The individual characteristics of the 26 dated plutons are 

4 Potassium-argon ages, Sierra Nevada batholith, California 

presented in table 2, and their geographic distribution is 

shown in figure 1. 

RESULTS AND DISCUSSION 

Analytical data for the samples are summarized in 

table 3. 

The K20 contents of biotite separated from 

samples Q and Dare very low relative to K20 contents 

from other plutons of the Sierra Nevada batholith, which 

range from 8.28 to 9.98 weight percent (Dodge and 

others, 1969). Petrographic examination of these biotite 

separates indicates that much of the biotite in these 

samples is chloritized, yet the separates give geologically 

reasonable ages. Dalrymple and Lanphere (1969) indicate 

that chloritized biotites give appropriate ages in many 

cases, regardless of when chloritization occurred. 

Presumably, potassium loss during chloritization is accompanied by a matching argon loss. The 40Ar/39Ar 

spectra for recently analyzed chloritzed biotites, however, 

are wildly discordant (L. W. Snee, written commun., 

1986) and imply that care is needed in the interpretation 

of conventional K-Ar ages for chloritized biotite. 

The age of biotite separated from sample Q was 

determined twice; the determined ages for the two splits 

fall outside the range predicted for the other separates 

using our error analysis. This is the only case among our 

determinations for which replicate analyses did not result 

in overlapping errot bars. The ages calculated for these 

chloritized biotites, though reasonable, are neither as accurate nor as precise as those determined for other 

samples. 

The K2

0 contents of some of the hornblende 

separates are high compared to those for hornblende from 

other plutons of the Sierra Nevada batholith, which range 

from 0.37 to 0.95 weight percent (Dodge and others, 

1968). In most cases, contamination of hornblende by 

small inclusions of biotite is indicated. Contamination 

of this sort should not adversely affect the precision of 

the age determinations, provided that the splits used for 

K20 analysis and argon extraction contained the same 

proportion of impure grains. The accuracy of the hornblende age could have been adversely effected if there was 

significant contamination of the hornblende separate by 

biotite because of the variable argon retention temperatures for the two minerals. 

Ages for biotite-hornblende pairs are considered 

concordant when the difference between the ages determined for the two minerals is less than the critical value 

(CV), defined by Dalrymple and Lanphere (1969, p. 120). 

Using this criterion, 8 of our 14 biotite-hornblende pairs 

yield discordant ages; the observed discordancy has 

several possible explanations. The argon retention 

temperature for biotite is 280 ° ± 40 °C (Harrison and 
Table 2. Petrographic data for the dated plutons 

[--, leaders indicate no data available] 

Pluton Average mode Reference 

Map Plagi- Color Specific 

Quartz K-spar oclase index gravity Texture 1 

Grain 

size2 no. 

A ~uartz monzonite of Hilton 

Creek 

31.5 33.8 32.5 2.5 A, F x Rninehart and Ross (1964). 

ti Nonporphyritic phase of the 25.7 

quartz monzonite of Nono 

Recesses 

c 

l) 

E 

F 

H 

I 

J 

K 

L 

H 

N 

0 

p 

~ 

K 

s 

T 

u 

v 

w 

X 

y 

z 

Granodiorite of Chickenfoot 

Lake 

Fine grained facies of quartz 

monzonite of Bear Dome 

~uartz monzonite of Bear Dome 

~uartz monzonite of Turret 

Peak 

Granodiorite of Mount 

Reinstein 

~uartz monzonite of Finger 

Peak 

Tinemaha Granodiorite 

Granodiorite of Tombstone 

Creek 

~uartz monzonite of Brush 

Canyon 

Taboose pluton 

Goodale pluton 

S1. berian pluton 

Mule Lake pluton--north mass 

Spook pluton--light facies 

Spook pluton--dark facies 

Nc !Joogle pluton 

Mule Lake pluton--south mass 

lkGann pluton 

Granite of Carroll Creek 

Granodiorite of Quinn Peak 

Granite of Window Cliffs 

Granite of White Mountain 

Granodiorite of Doe Meadow 

Granodiorite of Redrock 

Head ow 

21.3 

24.6 

22.6 

25.1 

21.5 

31.6 

21.3 

21.6 

32.2 

30.9 

21.9 

29.5 

24.4 

24.5 

12.4 

17.2 

24.5 

23.5 

19.7 

24.9 

15.7 

18.() 

30.0 

25.0 

36.2 

24.() 

17.8 

11.6 

27.8 

23.6 

11.4 

25.3 

38.4 

32.1 

29.6 

19.3 

1~.3 

17.5 

35.5 

31.9 

12.2 

22.6 

24.9 

10.7 

20.1 

1Texture codes: A. Hypidiomorphic granular. 

B. Equigranular. 

39.6 

39.4 

36.b 

43.() 

413.8 

48.3 

36.5 

39.0 

36.3 

28.5 

34.2 

36.3 

47.9 

46.7 

46.6 

35.9 

36.6 

50.1 

46.4 

43.2 

52.0 

50.5 

4.7 

14.4 

2.6 

7.9 

8.3 

18.6 

4.1 

16.1 

18.6 

6.2 

2 .1 

1 L .8 

4.6 

8.4 

1U.6 

23.6 

11.5 

7 .1 

14.2 

11.3 

5.4 

21.6 

10.6 

2.63 

2. 70 

2.62 

2.67 

2.70 

2. 71 

2.63 

2.72 

2.74 

2.64 

2.60 

2.65 

2.60 

2.62 

2.6t> 

2.74 

2.64 

2.63 

2.72 

2.68 

2.62 

2.75 

2.67 

A, B 

A, C, G 

F 

F, G 

A, C 

A, B, I 

A, B 

A, C, I 

A, E, I 

A, B 

A, B 

A, ti 

A, B 

A,. C, G 

A, C 

A, C 

A, B 

A, C, G 

A, B 

A, B 

A, E 

A, C, G 

A, D 

A, B, H 

A, C 

y 

X 

y 

y 

y 

y 

y 

y 

y 

y 

y 

y 

y 

y 

y 

y 

y 

y 

z 

y 

y 

y-z 

y 

y 

Lockwood and Lydon (1975). 

Lockwood and Lydon (1975). 

Do. 

Lockwood and Lydon (1975). 

Do. 

Bateman (1965b). 

Do. 

Bateman (1965a). 

Do. 

Moore and Marks (1972). 

!Jo. 

Moore (1%3). 

Do. 

Do. 

Do. 

l)o. 

Do. 

LJo. 

Do. 

du Bray and Uellinger 

(1981). 

Do. 

Do. 

Do. 

Do. 

Do. 

F. Aplitic. 

r;. Sheared. 

c. Moderately porphyritic with respect to K-feldspar. H. Lineated. 

D. Strongly porphyritic with respect to K-feldspar/megacryst bearing. 

~. Porphyritic with respect to plagioclase or hornblende. 

I. Strongly foliated. 

2Grain size codes: x. Fine grained (<1 mm). 

y. Medium grained (1 to 5 mm). 

z. Coarse grained ()5 mm). 

McDougall, 1980; Snee, 1982) and 525 ° ± 25 OC for hornblende (Harrison, 1981). Both of these temperatures are 

significantly lower than granitoid magma solidus 

temperatures (Tuttle and Bowen, 1958), which implies 

that magma solidification occurs well before the onset 

of argon retention and that ages determined for slowly 

cooled plutons by the K-Ar method will be younger than 

true crystallization ages. In particular, biotite gives ages 

that in many instances are much too young because 

radiogenic argon does not begin to accumulate until the 

cooling pluton attains a very low temperature. It is also 

well established that the diffusion rate of argon from 

biotite exceeds that for hornblende (Hart, 1964; Hanson 

and Gast, 1967) and the difference leads to differential 

argon retention. The hornblende age of discordant 

biotite-hornblende pairs is considered a minimum age 

since the radiometric clock in both minerals can be at least 

partially reset during reheating. 

Results and Discussion 5 
0'1 

""C:C 

Q 

S' 

Ill 

Ill 

cr 

3 

~ 

al 

Q 

:I 

AI 

~ Table 3. K-Ar ages of and analytical data for intrusive rocks, eastern and southern Sierra Nevada batholith, California ~ 

!II 

;;· Map Field N. lat. W. long. Mineral Experiment Age, Ma 1 %40Ar 40Ar K2 ~ o Estimated C/D3 AI 0 -11 rad 

age, Ma2 

z no. no. deg min sec deg min sec no. (x10 mol/g) wt. % 

~ 

Cl. 

AI A 8-10 37 31 40 118 45 30 biotite 91221 83.7+1.9 90.54 104.774 8.50 AI B 8-4 37 28 54 118 45 23 biotite 91365 79.7+2.1 88.06 79.7209 6.80 r:r 

~ c 8-21 37 23 03 118 45 32 biotite 01116 81.4+"1.8 82.52 81.7138 6.82 79.8+2.3 ::r c 8-21 37 23 03 118 45 32 biotite 01133 78.2+1.8 71.74 72.468 6.82 ~ 

?- c 8-21 37 23 03 118 45 32 hornblende 91139 88.9+"2.9 73.77 27.7326 2.114 D 

~ 

~ D 8-133 37 18 08 118 53 46 biotite 91379 82.1+2.1 72.44 27.5220 2.28 

~ E 8-132 37 17 19 118 54 16 biotite 91366 81.7+"1.8 89.01 103.315 8.59 :I F 8-138 37 15 06 118 49 33 biotite 01213 76.5""+1.9 48.52 81.1123 7.21 76.3+1.7 £;' 

F 8-138 37 15 06 118 49 33 biotite 01329 76.0+"1.8 54.84 80.5507 7 .21 

F 8-138 37 15 06 118 49 33 biotite 01427 76.3""+1.8 56.77 80.9223 7.21 

F 8-138 37 15 06 118 49 33 hornblende 91114 81.6+"2.2 50.57 9.78249 0.814 D 

G 8-109 37 06 04 118 45 45 biotite 01118 81. 8+1. 9 77.84 111.527 9.26 80.9+1.0 

c; 8-109 37 06 04 118 45 45 biotite 01623 80 .0+"1.8 84.34 109.103 9.26 

G 8-109 37 06 04 118 45 45 biotite 01644 80.8+"1.8 84.47 110.145 9.26 

G 8-109 37 06 04 118 45 45 hornblende 91128 92.7+3.1 59.24 12.0796 0.882 95.1+3.3 

G 8-109 37 06 04 118 45 45 hornblende 01685 97.4+"3.1 60.83 12.7009 0.882 D 

H 8-110 37 02 36 118 44 37 biotite 91177 83.4+1.8 92.59 98.6526 8.03 82.6+1.1 

H 8-110 37 02 36 118 44 37 biotite 91657 81.8+"1.8 69.94 96.7111 8.03 

I 8-28 37 05 01 118 27 03 biotite OI211 69 .5+1.7 58.25 75.0628 7.36 69.2+1.4 

I 8-28 37 05 01 118 27 03 biotite OI619 68.9+"1.6 55.31 74.3693 7.36 

I 8-28 37 05 01 118 27 03 hornblende 91130 126.9"+4.3 58.91 14.5776 0.784 125.7+3.3 

I 8-28 37 05 01 118 27 03 hornblende 9I372 124.4+"4.6 43.33 14.2738 0.784 D 

J 8-127 36 54 45 118 51 45 biotite 91612 83.4+3.1 85.17 108.037 8.79 82.7+2.4 

J 8-127 36 54 45 118 51 45 biotite 01332 82 .o+3 .o 72.34 106.105 8. 79 

J 8-127 36 54 45 118 51 45 hornblende 91526 84 .0+3 .5 28.00 13.0270 1.053 c 

K 8-125b 36 54 33 118 52 25 biotite 91210 85.7+"1.9 86.78 115.413 9.14 

L 8-42 36 59 53 118 21 45 biotite 91220 83.1""+2.0 79.41 77 .5005 6.33 

M 8-40 36 59 11 118 22 56 biotite 91453 87.9+2.0 86.91 96.6100 7.45 

M 8-40 36 59 11 118 22 56 hornblende 91371 78.8"+8.3 9.43 12.2955 1.06 c 

N 8-224 36 54 45 118 22 45 biotite 91223 77 .6+2 .1 . 92.03 95.3255 8.35 

0 8-49 36 56 11 118 21 45 biotite 91375 77 .2+1.7 87.63 90.9322 8.01 

p 8-48 36 54 33 118 20 27 biotite 01624 72.5+"5.4 68.89 79.1234 7.44 
Q 8-47 36 53 52 118 21 22 biotite 91454 77.1+2.0 65.33 33.1H37 2.98 80.5+4.8 

Q 8-47 36 53 52 118 21 22 biotite 01660 83.0"+1.9 72.95 36.8424 2.98 

Q 8-47 36 53 52 118 21 22 hornblende 91373 75.4"+2.7 46.76 8.50243 0.767 c 

R 8-231 36 53 19 118 20 44 biotite 9Il62 80.9"+1.8 87.59 106.951 8.98 80.0+1.3 

R 8-231 36 53 19 118 20 44 biotite 01642 79.0"+1.8 70.01 104.426 8.98 

1.{ 8-231 36 53 19 118 20 44 hornblende 91374 86.0"+4.9 18.25 37.8118 2.98 c 

s 8-232 36 53 28 118 20 28 biotite 91457 79.8+1.8 81.47 76.7070 6.53 

s 8-232 36 53 28 118 20 28 hornblende 91378 85. 7"+4 .3 49.63 17.8254 1.410 c 

T 8-59 36 50 36 118 19 13 biotite OllOO 73.5"+1.4 68.46 94.8866 8.79 73.0+0.9 

T 8-59 36 50 36 118 19 13 biotite 01429 72.5"+1.4 66.64 93.5436 8.79 

T 8-59 36 50 36 118 19 13 hornblende 9Il40 121.9"+0.8 97.24 35.4797 1.954 D 

u 9-7 36 23 28 118 04 33 biotite 01068 78.4+1.9 80.99 89.6996 7.78 77.0+2.0 

u 9-7 36 23 28 118 04 33 biotite 01330 75.6+"2.1 34.87 86.4779 7.78 

u 9-7 36 23 28 118 04 33 hornblende 01067 94.5+"9.8 80.12 14 0 9289 1.070 c 

v 9-77 36 21 17 118 34 23 biotite Oll20 89.7+2.0 82.00 100.656 7.60 89 .1+1.5 

v 9-77 36 21 17 118 34 23 biotite 01643 88 .5+1. 9 78.84 99.2515 7.60 

v 9-77 36 21 17 118 34 23 hornblende 91523 94.0"+2.5 33.73 17.7164 1.276 95.1+1.6 

v 9-77 36 21 17 118 34 23 hornblende 01620 96.2+"1.8 65.85 18.1456 1.276 D 

w 9-17 36 17 27 118 20 03 biotite 01069 7 5 .3+1.8 59.01 78.0221 7.05 

w 9-17 36 17 27 118 20 03 hornblende 91609 102.1+3 .6 46.24 23.6360 1.555 104.3+2.4 

~-~ 9-17 36 17 27 118 20 03 hornblende 01331 106.0"+3.6 45.82 24.4388 1.555 D 

X 9-31 36 12 21 118 27 29 biotite 01098 80.3+"1.8 73.71 96.9623 8.20 80.5+1.6 

X 9-31 36 12 21 118 27 29 biotite 01335 80.7+"1.8 79.70 97.4634 8.20 

y 9-38 36 10 34 118 22 15 biotite 91611 85.1+1.9 80.40 96.7451 7. 71 

y 9-38 36 10 34 118 22 15 hornblende 91599 125.8"+4.0 66.37 19.4715 1.038 122.6+4.5 

y 9-38 36 10 34 118 22 15 hornblende 01659 119 .4+3 .8 67.61 18.438 1.038 u 

z 9-69 36 12 49 118 15 51 biotite 01212 81.1+"1.7 87.06 108.853 9.12 81.3+1.5 

z 9-69 36 12 49 118 15 51 biotite 01334 80.3+"1.8 75.45 107.769 9.12 

z 9-69 36 12 49 118 15 51 biotite 01686 83.9"+3.5 74.31 85.8294 9.12 

1 Uncertainty on ages is one sigma. 2 Best estimate of pluton ages based on multiple determinations and overlapping error. 3 Concordant (C) or discordant (D) biotite-hornblende pair; as determined using the critical value (C.V.) of Dalrymple 

and Lanphere (1969). 

;1:1 

~ 

"' i-

"' 

~ 

Q. = 

0 iii. 

n 

c 

"' s· "' 

= 

...... 
Chen and Moore (1979) determined that the Independence dike swarm was emplaced between 149 and 

103 Ma. The oldest pluton that they dated that is not cut 

by the dikes crystallized about 103 Ma. Hornblende ages 

for the McGann pluton (sample T) and granodiorite of 

Doe Meadow (sample Y) are 122 Ma and 123 Ma, respectively, and because neither of these plutons is cut by the 

dikes the age determinations for these plutons further 

restrict the emplacement time for the Independence dike 

swarm to between 123 and 149 Ma. The ages of these 

plutons indicate magmatism in the southern Sierra 

Nevada at a time coincident with the onset of Cretaceous 

magmatic activity elsewhere in the range (Evernden and 

Kistler, 1970; Chen, 1977; Chen and Moore, 1979, 1982; 

Stern and others, 1981). 

Samples 0 and S were dated to test Moore's (1963) 

correlation of two isolated masses of the Mule Lake 

pluton. Both masses are small and surrounded by younger 

plutons. Ages for sample S biotite-hornblende pair are 

concordant and imply crystallization between 80 and 86 

Ma; the age determined for biotite from sample 0 is 

about 77 Ma. Both masses are cut by numerous Independence dikes, however, which indicates that the 

radiometric ages are too young. The Mule Lake pluton 

probably crystallized between 149 and 123 Ma ago and 

was subsequently reheated about 80 Ma, which caused 

diffusive loss of previously accumulated argon from both 

biotite and hornblende. Simultaneous potassium metasomatism may have doubled the K2

0 content of sample 

S hornblende, yielding a value twice that characteristic 

of Sierra Nevada hornblende (Dodge and others, 1968). 

Our K-Ar determinations provide no definitive information concerning the correlation or intrusive age of the two 

intrusives that form the Mule Lake pluton. 

A determination on hornblende from sample W, 

which was collected from the sheared and altered granite 

of Window Cliffs (du Bray and Dellinger, 1981), indicates 

an age about 104 Ma, which i~ indistinguishable from the 

minimum age of the dike swarm. The pluton is cut by 

Independence dikes, and a U-Pb zircon age (Chen and 

Moore, 1982) suggests that the pluton crystallized at least 

160 Ma. The biotite and hornblende ages are discordant, 

and the K20 content of the hornblende is twice that 

characteristic of Sierra Nevada hornblende. This hornblende, like the hornblende in sample S, may have 

undergone potassium metasomatism. Shearing and concomitant alteration may have altered the composition of 

this hornblende and emplacement of younger plutons 

nearby reset its radiometric clock. 

The age determination for sample L, collected well 

within the Taboose pluton, indicates crystallization about 

83 Ma, but mapping by Moore (1963) shows that the 

pluton's west edge is cut by Independence dikes. The 

absence of mafic dikes elsewhere in the Taboose pluton, 

the uncertain nature of the contact, and the compositional 

8 Potassium-argon ages, Sierra Nevada batholith, California 

similarity between the adjacent alaskite of Red Mountain and the Taboose pluton suggest that the location of 

the contact as mapped by Moore (1963) is at least 1 km 

west of its actual location. Thus, the mapped mafic dikes 

cut the older alaskite of Red Mountain but not the 

Taboose pluton. The age determined for the Taboose 

pluton may be a crystallization age, rather than a reset 

age. 

Ages for hornblende (about 126 Ma) and biotite 

(about 69 Ma) from the Lower Jurassic Tinemaha 

Granodiorite (sample I), are discordant. Other K-Ar age 

determinations for the Tinemaha Granodiorite (Kistler 

and others, 1965), however, indicate that the unit may 

have crystallized about 180 Ma and probable correlative 

intrusives have U-Pb ages of about 165 Ma (Chen and 

Moore, 1982). The cause for the discrepancy between the 

various age determinations for the Tinemaha Granodiorite are unknown although a thermal event that may 

have accompanied emplacement of the Cretaceous Inconsolable Granodiorite about 98 Ma (Kistler and others, 

1965) may have caused argon loss from the Tinemaha 

Granodiorite. 

Sample J was collected from a granodiorite correlated with the granodiorite of Tombstone Creek (Moore 

and Marks, 1972). U-Pb zircon age determinations for 

two samples from the southern end of the pluton, collected 1 km apart, indicate crystallization of 115 and 99 

Ma, respectively (Chen and Moore, 1982). Lead isotopic 

data for the 115 Ma-old sample age are internally discordant; Chen and Moore (1982) suggested that the discordancy indicates inherited zircon. Data for the 99 Ma-old 

sample are fairly concordant. Hornblende from sample 

J gives an age of about 84 Ma, and biotite gives an age 

of about 82 Ma. The biotite-hornblende age concordance 

suggests that the K-Ar determinations give the pluton's 

crystallization age. However, the radiometric clocks in 

both biotite and hornblende may have been reset when 

younger plutons were emplaced around the granodiorite 

of Tombstone Creek. The discrepancy between the U-Pb 

zircon ages and the K-Ar results may be accounted for 

in a variety of ways: The correlation of the dated sample 

with the granodiorite of Tombstone Creek may be incorrect; or the granodiorite may have had a prolonged cooling history between 99 and 84 Ma, which could have 

inhibited argon retention in both biotite and hornblende; 

or the biotite-hornblende data indicate crystallization age 

and the zircon sample that gave the 99 Ma age, like the 

other zircon sample, could contain older, inherited zircon. However, the near concordance of the lead isotopic 

data suggests that this sample probably does not contain 

significant inherited zircon. 

The lithologic similarity and proximity of the quartz 

monzonite of Mono Recesses and the quartz monzonite 

of Turret Peak suggest that they may be connected at 

depth (Lockwood and Lydon, 1975). The two units crop 
out several kilometers apart and are separated by the Upper Cretaceous Lamarck Granodiorite. Ages for biotite 

(about 76 Ma) and hornblende (about 82 Ma) from the 

quartz monzonite of Turret Peak, sample F, are in agreement with those of Evernden and Kistler (1970) for the 

quartz monzonite of Mono Recesses. Thus, the quartz 

monzonite of Turret Peak may be an apophysis of the 

Mono Recesses pluton. If so, this is one of the few places 

in the Sierra Nevada batholith where part of a pluton is 

exposed as an apophysis, separate at the current level of 

exposure, from the main mass of the pluton. 

Ages for biotite (about 81 Ma) and hornblende 

(about 95 Ma) from the granodiorite of Mount Reinstein 

(sample G) and for biotite (about 83 Ma) from the quartz 

monzonite of Finger Peak (sample H), respectively, were 

determined to evaluate the hypothesis that these plutons 

constitute a comagmatic, concentrically zoned intrusive 

suite. According to this hypothesis, the granodiorite of 

Mount Reinstein crystallized from early, relatively 

primitive magma; whereas, the quartz monzonite of 

Finger Peak crystallized from more evolved, younger 

magma. Age spans of 5-10 Ma determined by Stern and 

others (1981) for comagmatic suites of the central Sierra 

Nevada suggest that the 14 Ma age span between 

crystallization of the quartz monzonite of Finger Peak 

and the granodiorite of Mount Reinstein is too great and 

precludes a comagmatic relationship. Comparison of 

sample H biotite age with sample G hornblende age might 

be inappropriate if the biotite age was reset. However, 

the plutons adjacent to the quartz monzonite of Finger 

Peak, including the Upper Cretaceous Mount Givens 

Granodiorite and the plutons of the Goddard pendant, 

are all older than the quartz monzonite of Finger Peak 

(Stern and others, 1981; Chen and Moore, 1979) and thus 

could not have caused reheating and argon loss from sample H biotite. Furthermore, the granodiorite of Mount 

Reinstein (sample G) biotite-hornblende age discordance 

may be attributable to reheating of the Mount Reinstein 

biotite during emplacement of the quartz monzonite of 

Finger Peak. We conclude that the granodiorite of Mount 

Reinstein and the quartz monzonite of Finger Peak are 

not members of a comagmatic suite. 

Age determinations for samples Q and P suggest 

that the Spook pluton crystallized about 76 Ma and was, 

therefore, one of the last plutons to be emplaced in the 

Sierra Nevada batholith. The pluton is located on the 

eastern edge of the batholith near some of the oldest 

plutons. This instance of temporal juxtaposition contrasts 

with age-belt patterns described by Evernden and Kistler 

(1970), Chen (1977), and Moore and others (1979). 

Whether anomalous bodies, such as the Spook pluton, 

exist within an otherwise orderly age-belt pattern or 

whether the Sierra Nevada batholith evolved in a timespace random pattern will be determined only by additional geochronologic investigations. 

Among the biotite-hornblende pairs that are discordant and for which there is no specific geologic 

evidence that might account for the discordance are 

samples C, F, G, R, T, V, and Y. Biotite from these 

samples may have lost argon with or without the influence 

of thermal events that occurred subsequent to cooling 

through the argon retention temperature for biotite. 

Determinations based on hornblende indicate minimum 

crystallization ages because the hornblende may have 

undergone some argon loss as well. Intrusive age relationships and radiometric ages of nearby plutons provide 

permissive corroboration of the hornblende ages determined for these samples. 

SUMMARY 

On the basis of evidence from geologic relations and 

geochronologic studies by Evernden and Kistler (1970), 

Chen (1977), Chen and Moore (1979, 1982) and Stern and 

others (1981), it is probable that a number of the biotite 

ages determined in this study are too young. Pluton ages 

based on biotite separates alone, that lack substantiation 

either in field relations or other geochronologic evidence, 

are usually minimum ages. Concordant biotitehornblende pairs, however, suggest that a close approximation to the crystallization age may have been achieved 

(Dalrymple and Lanphere, 1969). 

Many of the samples dated in this study and many 

previously reported ages for plutons of the Sierra Nevada 

batholith are of Late Cretaceous age. It is noteworthy 

that between about 80 and 90 Ma a major thermal event 

reset the K-Ar clock of many of the batholith's older 

plutons. The thermal event may be associated with the 

emplacement elsewhere within the batholith of the 

Cathedral Peak-type granitoids including the Sonora Pass 

pluton (Slemmons, 1953), the Upper Cretaceous 

Tuolumne Intrusive Suite (including the Cathedral Peak 

Granodiotite) (Stern and others, 1981), the Mono 

Recesses pluton (Evernden and Kistler, 1970), the 

Whitney Granodiorite (Evernden and Kistler, 1970), the 

granite of White Mountain (du Bray and Dellinger, 1981) 

(sample X, table 3), and the Isabella Granodiorite (Bergquist and Nitkiewicz, 1982). These plutons, which are 

among the largest in the range, extend almost continuously from the north end to the south end of the batholith, 

and were emplaced about 80 Ma. The heat that accompanied their emplacement probably had a profound effect throughout the eastern part of the batholith. 

The tectonic events that triggered generation and 

emplacement of the Cathedral Peak-type magmas, all the 

same age, modal composition, texture, and chemistry, 

are not well known. Subduction dynamics, including 

shallowing of dip along the :,3enioff zone, at the edge of 

the Cretaceous North American plate may be partially 

Summary 9 
responsible for the great pulse of Late Cretaceous 

magmatism in the Sierra Nevada. The onset of lowangle subduction may also be responsible for the abrupt 

termination of magma generation in the Sierra Nevada 

region that followed production of the Cathedral Peaktype granitoids. Generation of these distinctive plutons, 

followed by an abrupt end to magma generation in 

the region suggests that the conditions that produced 

these plutons are characteristic of the Sierra Nevada 

batholith and its tectonic regime during Late Cretaceous 

time. 

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