PRELIMINARY REPORT ON INITIAL LEAD AND STRONTIUM

ISOTOPES FROM OPHIOLITIC AND BATHOLITHIC ROCKS

SOUTHWESTERN FOOTHILLS SIERRA NEVADA

CALIFORNIA

By Jason Saleeby1 and James H. Chen-

department of Geology and Geophysics

University of California

Berkeley, California 94720

department of Geological Sciences

University of California

Santa Barbara, California 93106

Initial lead and strontium isotopic ratios

have been determined for three suites of gabbros

from the mafic-ultramafic belt of the Kings and

Kaweah Rivers in the southwest Sierra Nevada

foothills. Geochronological control is derived

from zircon studies on the same suites (Saleeby

and Sharp, 1978). The oldest suite of gabbro is

from the metamorphosed Kings-Kaweah ophiolite

belt whose petrogenetic age is 250 to 300 m.y.

The second suite is a 169 m.y. (Middle Jurassic)

syntectonic gabbrodiorite complex which intrudes

the ophiolite. The third suite is a posttecton-

ic suite of voluminous olivine-hornblende gab-

bros, which are intimately related to norites,

pyroxene diorites, and quartz diorites, all

yielding ages from 115 to 125 m.y. (Early Creta-

ceous). The Jurassic and Cretaceous gabbroscon-

stitute the mafic members of the composite

Sierra Nevada batholith. The ophiolitic gabbros

are part of an allocthonoub Lcnm;:, which forms

part of the western metamorphic wall of the

batholith.

The isotopic compositions of lead from the

ophiolitic suite range from 17.90 to 17.95 for

20GPb/20^Pb7 and from 15.45 t0 15_52 for 207Pb/

z01*Pb. The Jurassic suite yields a more radio-

genic lead of 18.47 for 206Pb/201*Pb and 15.60

for 207Pb/''0'*Pb. The Cretaceous olivine gab-

bros yield ratios similar to the ophiolitic gab-

bros. In a 206Pb/201*Pb - 207Pb/20l4Pb diagram,

the ophiolitic and Cretaceous gabbros plot with-

in the ocean ridge basalt field. The Jurassic

gabbro lies within the field of Sierran granit-

oids (Chen and Tilton, 1978).

Initial 87Sr/8GSr value on a gabbro from

the ophiolitic suite is 0.7026. Initial 87Sr/

86Sr values determined on the Cretaceous suite

range from 0.7031 to 0.7039, and for the Juras-

sic suite a value of 0.7032 was determined.

These data are in agreement with the values mea-

sured on other batholithic rocks of the western

Sierra by Kistler and Peterman (1973). Lead and

strontium isotopic studies on other fractions of

the Kings-Kaweah mafic-ultramafic belt are in

progress.

Several significant conclusions can be

drawn from this study.

1. The ophiolitic and Cretaceous suites

bear some of the least radiogenic leads on the

west coast. They are less radiogenic than the

oceanic-affinity volcanic rocks of the Francis-

can assemblage to the west (Sinha and Davis,

1971), and they are comparable to leads measured

on the eastern Taiwan ophiolite (Chen, unpub.

data) and on Mid-Atlantic ridge basalts (Tatsu-

moto, 1978).

2. The ophiolitic and Cretaceous suites

were derived from a source that had the charac-

teristics of oceanic upper mantle. Geologic re-

lations indicate that in the case of the ophio-

litic suite petrogenesis was in the oceanic

realm far removed from the Sierran region,

whereas the Cretaceous suite was generated be-

neath its present position.

3. When compared with the more radiogenic

initial lead and strontium values measured on

batholithic rocks to the east (Kistler and

Peterman, 1973; Chen and Tilton, 1978), these

data support the notion that ophiolitic rocks of

the Kings-Kaweah belt mark a fossil suture be-

tween oceanic and continental lithospheric

plates (Saleeby, 1977). The fundamental nature

of this suture has been masked by the Sierra Ne-

vada batholith.

Chen, J. H., and

strontium i

Sierra Neva

logical Soc

grams, v. 1

Kistler, R. W. ,

ations in S

Sr86 in Mes

ed wal1 rock

logical Soc

p. 3489-351

Saleeby, J. B.

nental marg

ment of the

southwest S

Coleman, R.

North Ameri

land, Orego

Mineral Ind

160.

Tilton, G. R., 1978, Lead and

sotopic studies of the southern

da batholith, California: Geo-

iety of America Abs. with Pro-

0, p. 99-100.

and Peterman, Z. E., 1973, Vari-

r, Rb, K, Na and initial Sr87/

ozoic granitic rocks and intrud-

s in Central California: Geo-

iety of America Bulletin, v. 84,

2.

Fracture zone tectonics, conti-

in fragmentation, and emplace-

Kings-Kaweah ophiolite belt,

ierra Nevada, California, i_n,

G. , and Irwin, W. P. , eds.,

can Ophiolite Volume: Port-

n Department of Geology and

ustries Bulletin, v. 95, p. 141-

3/5

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PRELIMINARY REPORT ON INITIAL LEAD AND STRONTIUM ISOTOPES FROM OPHIOLITIC AND BATHOLITHIC ROCKS

SOUTHWESTERN FOOTHILLS SIERRA NEVADA CALI FORNI A

By Jason Saleeby1 and James H. Chen: 1 Department of Geology and Geophysics

University of California Berkeley, California 94720

2 Department of Geological Sciences University of California

Santa Barbara, California 93106

Initial lead and strontium isotopic ratios have been determined for three suites of gabbros from the mafic-ultramafic belt of the Kings and Kaweah Rivers in the southwest Sierra Nevada foothills. Geochronological control is derived from zircon studies on the same suites (Saleeby and Sharp, 1978). The oldest suite of gabbro is from the metamorphosed Kings-Kaweah ophiolite belt whose petrogenetic age is 250 to 300 m.y. The second suite is a 169 m.y . (Middle Jurassic) syntectonic gabbrodiorite complex which intrudes the ophiolite. The third suite is a posttecton­ic suite of voluminous olivine-hornblende gab­bros, which are intimately related to norites, pyroxene diorites, and quartz diorites, all yielding ages from 115 to 125 m.y. (Early Creta­ceous). The Jurassic and Cretaceous gabbroscon­stitute the mafic members of the composite Sierra Nevada batholith. The ophiolitic gabbros are part of an al locthonou~ ce1, u,,:, '~hi ch forms part of the western metamorphic wall of the ba tho l it h.

Tile isotopic compositions of lead from t he ophiolitic suite range from 17.90 to 17.95 for 2 o~Pb/~ ~ 4 Pb, and from 15.45 to 15.52 for 207 Pb/ ~0 4 Pb. The Jurassic suite yields a more radio­genic lead of 18.47 for 7 D6 Pb/ 7 04 Pb and 15.60 for 2 0 1 Pb; : o4 pb. The Cretaceous olivine gab­bros yield ratios similar to the ophiolitic gab­bros. In a 20'Pb/2'l'1 Pb - 2° 7 Pb/70 4 Pb diagra111. the ophiolitic and Cretaceous gabbros plot with­in the ocean ridge basalt field. Tile Jurassic gabbro lies within the field of Sierran granit­oids (Chen and Tilton, 1978).

Initial HlS r/ 8 :S r value on a gabbro fro m the ophiolitic suite is 0.7026. Initial ~~Sr/ 8 GSr values determined on t he Cretaceous suite ranqe from 0. 7031 to 0. 7039, and for the Jurc1 s ­s i c suite a value of 0.7032 vldS determined. These data are in agreement with the values mea­sured on other batholithic rocks of the wes tern Sierra by Kistler and Peterman (1973). Lead and strontium isotopic studies on other fractions of the Kings-Kaweah mafic-ultramafic belt are in progress.

Several significant conc lusions can be drawn from this study.

l. The ophiolitic and Cretaceous suites bear some of the lea s t radiogenic l eads on the

Digitized by Go gle

west coast. They are less radiogenic than the oceanic-affinity volcanic rocks of the Francis­can assemblage to the v1est (Sinha and Davis, 1971), and they are comparable to leads measured on the eastern Taiwan ophiolite (Chen, unpub. data) and on Mid-Atlantic ridge basalts (Tatsu­moto, 1978).

2. The ophiolitic and Cretaceous suites were derived from a source that had the charac­teristics of oceanic upper mantle. Geologic re­lations indicate that in the case of the ophio­litic sui te petrogenesis was in the oceanic realm far removed from the Sierran region, whereas t he Cretaceous suite was generated be­neath its present position.

3. When compared with the more radiogenic initial lead and strontium values measured on batholithi c rocks to the east (Kistler and Peterman, 1973; Chen and Tilton, 1978), these data support the notion that ophiolitic rocks of the Kinys- r'.,rneah belt mark a fossil suture be­tween oceanic and continental lithospheric plates (Saleeby, 1977). The fundamental nature of this su tu re has been 111asked by the Sierra Ne­vada ba tho lit h.

Chen, J. H., and Tilton, G. R., 1978 , Lead and strnntitw1 i so topi c studies of the southern Sierra Nevada batholith, California: Geo­logical Soc iety of America Abs. with Pro­rirarns, v. 10, p. 99-100.

f'. i s t l c 1· , R . \~ . , and Pete rma n , Z . E . , l 9 7 3 , Va r i -ations in Sr, Rb, K, Na and initial Sr8 7 /

Sr'~ i: i 11 Mc sozo i c granitic rock s and i ntrud­ed wallrotks in Central California: Geo­l oc1ical Soc iety of An.e rica Bulletin, v. 84, p. 3'1~:9 -3 5 1 2 .

Sa l eeby , J . G., Fracture zone tectonics, conti-11e11tal r»a rgin frag1:1entc1tio n, and emplace-11:c> nt of tlH' r'. i ngs-K.rneah ophiol i te belt, soutl l\·1est Sie rra flevada, California, in. Cole111<1n . R. G., and In1in, W. P., eds~ North f\1·1e ri can Ophiolite Volume: Port­land, (Jr·f·ijon Departl:'ent of Geology and Mi111 ·r·c11 !11dus tries Gulletin, v. 95, p. 141 -ll.0.

375

Origilkll from

UNIVERSITY OF MICHIGAN

Saleeby, J. B., and Sharp, W. D. , 1978, Prelimi-

nary report on the behavior of U-Pb zircon

and K-Ar systems in polymetamorphic ophiol-

itic rocks and batholithic rocks, south-

western Sierra Nevada foothills, Califor-

nia: (this conference).

Sinha, A. K., and Davis, T. E., 1971, Geochemis-

try of Franciscan volcanic and sedimentary

rocks from California: Carnegie Institu-

tion of Washington Year Book 69, 1969-1970,

p. 394-400.

Tatsumoto, M., 1978, Isotopic composition of

lead in oceanic basalt and its implication

to mantle evolution: Earth and Planetary

Science Letters, v. 38, p. 63-87.

PRELIMINARY REPORT ON THE BEHAVIOR OF U-Pb

ZIRCON AND K-Ar SYSTEMS IN POLYMETAM0RPHOSED

OPHIOLITIC ROCKS AND BATHOLITHIC ROCKS,

SOUTHWESTERN SIERRA NEVADA FOOTHILLS,

CALIFORNIA

By Jason Saleeby and Warren Sharp

Department of Geology and Geophysics

University of California

Berkeley, California 94720

Ophiolitic rocks of the southwest Sierra

Nevada foothills form part of the western meta-

morphic wall of the Sierra Nevada batholith

(fig. 1). Geochronological studies were under-

taken to determine: (1) The igneous petrogenet-

ic age of the ophiolite; (2) The age of a pre-

batholith dynamic greenschist to amphibolite fa-

cies metamorphic event experienced by the ophi-

olite; and (3) The emplacement ages of the

batholithic rocks. Geochronological work was

interfaced with detailed structural and petro-

logic studies. A summary of the geochronologi-

cal data is given in figure 2.

Zircon from four widely spaced leucocratic

dikes within the ophiolite yield suites of in-

ternally consistent discordant U-Pb ages. Each

zircon sample was split into size and magnetic

fractions, which were analyzed separately. Each

sample showed the same pattern of greater dis-

cordance with increase in U concentration and

decrease in grain size. Zircon discordance is

attributed to regional thermal metamorphism re-

lated to emplacement of the Sierra Nevada batho-

lith. Metamorphism affected the hornblende

hornfels facies, but one of the main factors

which is believed to have affected the zircon is

the intense metasomatic system, which was set up

between the batholithic rocks and serpentinite

that enclosed the ophiolitic blocks containing

the sampled dikes.

Numerous concordant zircon ages measured on

the cross-cutting batholithic rocks of gabbroic

to granodioritic composition range from 100 to

125 m.y. Mafic metaigneous rocks of the ophiol-

ite, which enclose the leucocratic-dike rocks,

were sampled for K-Ar age determinations. The

K-Ar ages were reset to ages which fall within

the range of concordant batholithic-zircon ages.

Concordia plots made for each of the discordant

ophiolitic zircon populations yielded lower in-

tercept ages, which are in agreement with the

concordant batholithic-zircon ages and the reset

ophiolitic K-Ar ages.

The intercept age ranges for each ophiolit-

ic-zircon population are derived by fitting a

family of lines through the error brackets of

the discordant U-Pb points on a concordia plot,

and noting the ranges over which these lines in-

tercept concordia. The upper intercept falls

around 300 m.y. and may represent an initial

crystallization age for the ophiolitic-dike

rocks. The oldest 206Pb/238U age obtained from

the discordant populations is 247 m.y. 20GPb/

207Pb ages for the discordant populations range

back to 275 m.y. These data together suggest

that the igneous ages of the ophiolitic-dike

rocks are between 250 and 300 m.y. The dike

rocks are an integral part of the ophiolite as-

semblage and thus this petrogenetic age range is

assigned to the entire igneous assemblage of the

ophiolite. The petrogenetic age is considered

an oceanic spreading center age.

Mafic metamorphic tectonites of amphibolite

facies were sampled from the ophiolite in do-

mains of lowest textural and mineralogic contact

metamorphic grade. K-Ar ages on amphibole re-

present a minimum age on the dynamic metamorphic

event that was related to tectonic disruption of

the ophiolite. The minimum ages range from 179

m.y. to 190 m.y. Where sampled adjacent to the

batholith a similar mafic tectonite sample (not

shown on fig. 2) had its K-Ar system reset to

the batholithic-zircon age. Geological rela-

tions suggest that the dynamic metamorphic age

should be close to the petrogenetic age of the

ophiolite, and that this metamorphism occurred

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_use

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oogle

Saleeby, J. B., and Sharp, W. D., 1978, Prelimi­nary report on the behavior of U-Pb zircon and K-Ar systems in polymetamorphic ophiol­itic rocks and batholithic rocks, south­western Sierra Nevada foothills, Califor­nia: (this conference).

Sinha, A. K., and Davis, T. E., 1971, Geochemis­try of Franciscan volcanic and sedimentary

rocks from California: Carnegie Institu­tion of Washington Year Book 69, 1969-1970, p. 394-400.

Tatsumoto, M., 1978, Isotopic composition of lead in oceanic basalt and its implication to mantle evolution: Earth and Planetary Science Letters, v. 38, p. 63-87.

PRELIMINARY REPORT ON THE BEHAVIOR OF U-Pb ZIRCON AND K-Ar SYSTEMS IN POLYMETAMORPHOSED

OPHIOLITIC ROCKS AND BATHOLITHIC ROCKS, SOUTHWESTERN SIERRA NEVADA FOOTHILLS,

CALIFORNIA

By Jason Saleeby and Warren Sharp Department of Geology and Geophysics

University of California Berkeley, California 94720

Ophiolitic rocks of the southwest Sierra Nevada foothills form part of the western meta­morphic wall of the Sierra Nevada batholith (fig. 1). Geochronological studies were under­taken to determine: (1) The igneous petrogenet­ic age of the ophiolite; (2) The age of a pre­batholith dynamic greenschist to amphibolite fa­cies metamorphic event experienced by the ophi­olite; and (3) The emplacement ages of the batholithic rocks. Geochronological work was interfaced with detailed structural and petro­logic studies. A surrmary of the geochronologi­cal data is given in figure 2.

Zircon from four widely spaced leucocratic dikes within the ophiolite yield suites of in­ternally consistent discordant U-Pb ages. Each zircon sample was split into size and magnetic fractions, which were analyzed separately. Each sample showed the same pattern of greater dis­cordance with increase in U concentration and decrease in grain size. Zircon discordance is attributed to regional thermal metamorphism re­lated to emplacement of the Sierra Nevada batho­lith. Metamorphism affected the hornblende hornfels facies, but one of the main factors which is believed to have affected the zircon is the intense metasomatic system, which was set up between the batholithic rocks and serpentinite that enclosed the ophiolitic blocks containing the sampled dikes.

Numerous concordant zircon ages measured on the cross-cutting batholithic rocks of gabbroic to granodioritic composition range from 100 to 125 m.y. Mafic metaigneous rocks of the ophiol­ite, which enclose the leucocratic-dike rocks, were sampled for K-Ar age determinations. The K-Ar ages were reset to ages which fall within the range of concordant batholithic-zircon ages.

Digitized by Go gle

Concordia plots made for each of the discordant ophiolitic zircon populations yielded lower in­tercept ages, which are in agreement with the concordant batholithic-zircon ages and the reset ophiolitic K-Ar ages.

The intercept age ranges for each ophiolit­ic-zircon population are derived by fitting a family of lines through the error brackets of the discordant U-Pb points on a concordia plot, and noting the ranges over which these lines in­tercept concordia. The upper intercept falls around 300 m.y. and may represent an initial crystallization age for the ophiolitic-dike rocks. The oldest 206Pb/2 3Bu age obtained from the discordant populations is 247 m.y. 206pb/ 207pb ages for the discordant populations range back to 275 m.y. These data together suggest that the igneous ages of the ophiolitic-dike rocks are between 250 and 300 m.y. The dike rocks are an integral part of the ophiolite as­semblage and thus this petrogenetic age range is assigned to the entire igneous assemblage of the ophiolite. The petrogenetic age is considered an oceanic spreading center age.

Mafic metamorphic tectonites of amphibolite facies were sampled from the ophiolite in do­mains of lowest textural and mineralogic contact metamorphic grade. K-Ar ages on amphibole re­present a minimum age on the dynamic metamorphic event that was related to tectonic disruption of the ophiolite. The minimum ages range from 179 m.y. to 190 m.y. Where sampled adjacent to the batholith a similar mafic tectonite sample (not shown on fig. 2) had its K-Ar system reset to the batholithic-zircon age. Geological rela­tions suggest that the dynamic metamorphic age should be close to the petrogenetic age of the ophiolite, and that this metamorphism occurred

376

Origilkll from

UNIVERSITY OF MICHIGAN