DETAILED FIELD, GEOCHEMICAL AND ISOTOPIC CONSTRAINTS ON THE ASSEMBLY OF THE SENTINEL GRANODIORITE, SIERRA NEVADA BATHOLITH, USA

Granitoid plutons in continental arcs bear testimony to magmatic processes that drive the growth and evolution of arc crust. We present new field, geochemical, and isotopic data (including in situ zircon trace element and Hf isotope data) from the 95 Ma Sentinel granodiorite (Sierra Nevada Batholith, USA), that help to better evaluate rates of mass addition and mechanisms of magma transfer, storage, and evolution in arc crust.

This high spatial resolution data set demonstrates that multiple processes operated at different temporal and spatial scales. A combination of modest granite source heterogeneity, intra-pluton magma mixing, and hornblende-biotite-plagioclase-titanite fractionation accounts for the observed trace element and whole rock Sr-Nd-Hf isotopic variation. Magma mixing between diorite and granodiorite magmas is most clearly evident in enclave-bearing granodiorite bodies, whereas more evolved granite and leucogranite sheets are derived by fractional crystallization of the host granodiorite as clearly shown by trace element data. Zircon trace element and Hf isotope data reveal highly heterogeneous zircon populations that point to remobilization and local mixing in the Sentinel magmatic system. These results are likely relevant for other Sierran arc intrusions.

Pluton assembly involved multiple, slightly isotopically and chemically heterogeneous magma pulses that successively intruded a crystal mush, allowing for incomplete mixing and modest degrees of fractional crystallization to occur. Isotopic data indicate that the source of the Sentinel pluton (95 Ma) is more similar to the slightly older Yosemite Valley Intrusive Suite than to the younger Tuolumne Intrusive Suite, with which the Sentinel is usually grouped. More high-resolution U-Pb zircon geochronology is now required to examine the aforementioned processes as a function of time.