TEMPORAL AND SPATIAL GEOCHEMICAL EVOLUTION OF MESOZOIC MAGMATISM IN THE CENTRAL SIERRA ARC, CALIFORNIA

Combined geochronology, geochemistry data sets from The Western North American Volcanic and Intrusive Rock Database (NAVDAT) and our studies across the central Sierra Nevada continental arc help to constrain the temporal and spatial evolution of the Mesozoic magmatism in the central Sierra Nevada batholith.

535 chemical analyses of both volcanic and plutonic rocks are included in this study. In the Soldier Lake area on the eastern margin of Tuolumne Batholith (TB), the Green Lake pluton (165 Ma), Soldier Lake granodiorite (97 Ma) and Cathedral Peak granodiorite (86 Ma) are intruded into Triassic (218-229 Ma) metavolcanic and metavolcaniclastic units. In the Cinko Lake area to the northwest of TB, the Cretaceous Harriet Lake (102 Ma) and Freemont Lake granodiorite (<95 Ma) are intruded into slightly older (102-107 Ma) metavolcanic rocks. In the Iron Mountain area to the southwest of TB, Cretaceous plutonic rocks are intruded into Cretaceous metavolcanic units (98-106 Ma). The 97 Ma Jackass Lake pluton to the southwest of TB is intruded into Cretaceous metavolcanic units (98-103 Ma).

From east to west across the main axis of the arc, magmatism becomes progressively younger, although the volumes of Triassic plutons and Jurassic volcanics are not large in the above four areas. From Triassic to Cretaceous and from east to west of the main arc axis, plutonic rocks became more evolved in terms of rock types while volcanic rocks evolve from more tholeiitic to calc-alkaline. All rocks are predominantly calc-alkaline, but intermediate and more mafic plutonic rocks show tholeiitic iron enrichment . Multiple trace element lineages, such as that for Zr and Ti for Jurassic and Cretaceous plutonic rocks, indicate that magma mixing is needed, in conjunction with the effects of fractionational crystallization, to account for chemical trends during magma evolution. Relative depletion of Ta, Nb and Ti to K, Ba in Sierran plutonic and volcanic rocks and existing Nd-Sr isotopic data indicate the important role of a continental crust component in the origin of Sierran magmas. The magnitude of a crustal component contributing to origin of Sierran magmas was increased for younger magmatic stages, including the late Cretaceous magmatic surge at and after 90 Ma.