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- Generated on 2014-10-29 19:49 GMT / http://hdl.handle.net/2027/mdp.39015010076944 Public Domain, Google-digitized / http://www.hathitrust.org/access_use#pd-google PRELIMINARY REPORT ON INITIAL LEAD AND STRONTIUM ISOTOPES FROM OPHIOLITIC AND BATHOLITHIC ROCKS SOUTHWESTERN FOOTHILLS SIERRA NEVADA CALI FORNI A By Jason Saleeby 1 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 posttecton- ic suite of voluminous olivine-hornblende gab- bro s, 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 al ce1, u,,:, 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 and from 15.45 to 15.52 for 207 Pb/ The Jurassic suite yield s a more radio- genic lead of 18.47 for 7 D 6 Pb/ 7 0 4 Pb and 15.60 for 2 0 1 Pb; : o 4 pb. The Cretaceous olivine gab- bros yield ratios similar to the ophiolitic gab- bros. In a 20'Pb/2'l' 1 Pb - 2° 7 Pb/70 4 Pb diagra111. the ophiolitic and Cretaceous gabbros plot with- in the ocean ridge basalt field. Tile Jurassic gabbro lie s within the field of Sierran granit- oids (Chen and Tilton, 197 8). Initial HlS r/ 8 :S r value on a gabbro fro m the ophiolitic s uite is 0.7026. Initial 8 GSr values determined on t he Cretaceous suite ranqe from 0. 7031 to 0. 7039, and for the Jurc1 s- si c s uite a value of 0.7032 vldS det er mined. These data are in agreement with the values mea- sured on other batholithic rocks of the we s tern Sierra by Kistler and Peterman (1973). Lead and strontium isotopic studies on other fractio ns of the Kings-Kaweah mafic-ultramafic belt are in progress. Several significant conc lusion s can be drawn from this study. l. T he ophiolitic and Cretaceous s uite s bear some of the lea st radio genic leads on the Digitized by Go gle west coast. They are less radiogenic than the oceanic-affinity volcanic rocks of the Francis- can 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 (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 sui te petrogenesis was in the oceanic realm far removed from the Sierran region, whereas t he C retaceou s suite was generated be- neath 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; C hen and Tilton, 1978), these data support th e notion that ophiolitic rocks of the Kiny s- r'., rneah be lt mark a fossil suture be- twe en oceanic and continental lithospheric plates (Saleeby, 1977). The fundamental nature of this su tu re has been 111asked by the Sierra Ne- vada ba tho lit h. Chen, J. H., and Tilton, G. R., 1978 , Lead and strn ntit w1 i so topi c studies of the sout hern Sierra Nevada batholith, California: Geo- logical Soc iety of Am erica Abs. with Pro- rirarns, 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 S r, Rb, K, Na and initial Sr8 7 / i 11 Mc sozo i c granitic rock s and i ntrud- ed wallrotks in Ce ntr al California: Geo- l oc1ic a l Soc iet y of An.e rica Bulletin, v. 84, p. Sa l eeby , J . G., Fracture zone tectonics, conti- 11e11tal r »a rgin frag1 :1entc1tion, 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., North f\ 1 ·1e ri can Ophiolite Volume: Port- land, (Jr·f·ijon Departl:'ent of Geology and Mi111 ·r ·c11 !11du s trie s Gulletin, v. 95, p. 141 - ll.0. 375 Origilkll from UNIVERSITY OF MICHIGAN
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Open-file report / U.S. Geological Survey · the ophiolite. The third suite is a posttecton-ic suite of voluminous olivine-hornblende gab-bros, which are intimately related to norites,
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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
3?6
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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