U-PB ZIRCON AGES FOR HIGH-SILICA GRANITES IN THE CENTRAL SIERRA NEVADA BATHOLITH: IMPLICATIONS FOR CRUSTAL GENERATION IN CONTINENTAL ARCS

Recent work in the Sierra Nevada batholith suggests that plutonism is episodic, with large volumes of granodiorite emplaced in relatively short time periods (~10-15 m.y.). Two major magmatic episodes are now recognized in the eastern Sierra and White/Inyo Mountains - one in the Jurassic (~180-165 Ma) and one in the Cretaceous (the Sierra crest magmatic episode ~98-88 Ma). The later event is responsible for growth of a large part of the high Sierra and comes toward the end of a general eastward sweep of magmatism. We present new U-Pb zircon ages for four high-silica granites exposed across the central Sierra Nevada batholith that demonstrate the importance of these high silica rocks in the overall pattern of magmatism. In the west-central Sierra, a new U-Pb zircon age for the granite of Grant Grove is ~98 Ma, and for the granite of Lodgepole is ~115 Ma. To the east, the Evolution Basin Alaskite yields a preliminary age of 92-94 Ma and a garnetiferous phase of the granite mapped as the leucogranite of Rawson Creek gives an age of ~165 Ma. This last age distinguishes the garnetiferous phase from the leucogranite of Rawson Creek proper, which has been dated at about ~95 Ma (Stern et al., 1981).

These new ages bear on the question of the generation and modification of crust in continental arcs. Several studies suggest that the voluminous granodiorites in the Sierran arc are the products of mixing between coexisting dioritic and granitic magmas. The U-Pb dates presented here demonstrate that the history of high-silica magmatism in the arc mirrors that of diorites and granodiorites exposed nearby. Consequently, the geochronology permits the coexistence of the three magma types. This combined with field evidence for mixing, the bulk geochemistry, and the isotope geochemistry of rocks across the range of composition strongly supports the hypothesis that magma mixing is an important process in generation of the intermediate composition rocks. Isotope geochemistry also suggests very little involvement of old crustal material, even in the most siliceous lithologies, consistent with the idea that the Sierra crest magmatic event was responsible for growth of a large volume of new crust in the arc.