ICP-MS AND TIMS ZIRCON DATING IN THE SIERRA NEVADA: INITIAL IMPLICATIONS FOR TECTONIC AND MAGMATIC PROCESSES

LA-ICPMS geochronology of zircon populations in 54 new samples including 38 metasedimentary samples, 14 volcanic samples, and two plutonic samples from the central and southern Sierra Nevada are combined with 25 CA-TIMS zircon ages from several plutons intruding the above. We are using these data to (1) determine the age and origin of metasedimentary and metavolcanic packages in pendants in the Sierra Nevada batholith and terranes of the Western Metamorphic belt; (2) evaluate proposed tectonic scenarios for the Sierra; and (3) investigate magmatic processes and the degree of host rock assimilation by granitoids.

Our preliminary results confirm the following: (1) Golconda and younger sedimentary packages occur in the Saddlebag Lake pendant; (2) miogeoclinal strata are preserved in pendants from just north of the Tuolumne Batholith to the Lake Isabella area, southern Sierra, (3) Jurassic marine sequence(s) are recognized in pendants throughout the Sierra and may form an overlap assemblage on both of the above; (4) a belt of Cretaceous volcanic rocks is preserved in pendants along the central axis of the Sierra, and overlie the marine Jurassic rocks; and (5) clastic metasedimentary units in the Calaveras Complex are no older than Jurassic.

We further examined Death Valley and Inyo Facies units as potential source regions for miogeoclinal rocks in Sierran pendants. Our preliminary comparison of zircon populations from the Eureka quartzite in the Talc City Hills area with those in the May Lake pendant show considerable overlap. Thus zircon populations from the Inyo and Death Valley facies strata are equally permissive as source regions. Since the Eureka Quartzite is missing in the Mojave Desert, a ≤200 km amount of displacement along the Mojave-Snow Lake fault may be a more likely scenario.

Our new ages also indicate that Precambian zircon xenocrysts are largely absent in the dated plutons although common in some nearby host rocks, whereas Mesozoic xenocrystic zircons do locally occur in these plutons. Finally, even though aureole T’s range up to ~700 ^oC, LA-ICPMS zircon ages are not reset in samples from these aureoles nor do they fall below the TIMS ages of the intruding plutons.