ROOTING AROUND BENEATH AN ARC: ZIRCON U-PB GEOCHRONOLOGIC AND HF ISOTOPIC CONSTRAINTS ON THE EVOLUTION OF THE BASE OF THE SIERRA NEVADA BATHOLITH

The bulk of the Sierra Nevada batholith of California formed during a Cretaceous episode of high-flux magmatism. Xenoliths hosted in Miocene volcanic centers in the central Sierra Nevada provide a glimpse into processes occurring in the deep crust and upper mantle while the arc was producing large volumes of magma. We present new zircon U-Pb geochronologic and Hf isotopic data to better understand the formation and evolution of the mantle under the central Sierra Nevada. We sampled gabbroic, granulitic, and eclogitic xenoliths from three localities (Chinese Peak, Big Creek, and Pick and Shovel Mine). The majority (~80%) of calculated ages range from ca. 120-80 Ma, precisely overlapping the timing of Late Cretaceous “flare-up” magmatism responsible for constructing the bulk of the Sierra Nevada batholith. This range of U-Pb zircon ages overlaps existing Lu-Hf and Sm-Nd mineral ages from the same xenolith suites, requiring rapid cooling following construction of the sub-batholithic root. One analyzed sample of pyroxenite yielded a weighted mean age of ca. 11 Ma and an εHf values ranging from +2.24 to +4.86, suggesting that asthenospheric material flowed into the void left following Late Miocene to Pliocene delamination of the Late Cretaceous root. A minor proportion (~3%) of analyzed grains yield Proterozoic to Paleozoic ages, most likely derived from pre-batholithic metamorphic framework rocks. We observe no clear relationship between rock type and age or Hf isotopic value. However, we note a strong correlation between U-Pb age and Hf isotope composition, with Hf isotope values declining from +4 to -15 from ca. 120 to 80 Ma. These Hf values are significantly lower than those reported from the Fine Gold Intrusive Suite and Dinkey Creek granodiorite, in which the sampled volcanic centers occur. This relationship suggests that supracrustal material was continuously transferred to the deep crust-upper mantle during batholith construction, which likely had the effect of “stoking” arc magmatism. In aggregate, this work provides evidence for rapid construction of sub-batholithic root material, an increasing metasedimentary component in the root zone with time, and abrupt shutoff of the engine of the Sierran arc at ca. 80 Ma.