ORIGINS OF WIDESPREAD, CONTEMPORANEOUS GRANITES IN THE SIERRA NEVADA BATHOLITH

The Sierra Nevada Batholith (SNB) records copious Mezosoic magmatism and is a touchstone for understanding crustal growth at continental convergent margins. Recent research in the SNB has focused on defining magmatic cyclicity or “flare ups” based on the ages, magma production rates, and radiogenic isotope heterogeneities of the plutonic and volcanic rocks found throughout the batholith. In the Cretaceous, there are widely recognized pulses of magmatism at 115-110; 105–98; and 95–87 Ma. The spatial relationships of these pulses within the broadly defined arc and host rocks provide an opportunity to evaluate transitions in magma productivity. The mid-Cretaceous (ca. 105–98 Ma) pulse hosts high-silica (>65 wt.% SiO2; average ~71%) granites that are more common than in the earlier and later pulses, and that do not follow the well-established west-to-east “younging” trend found in the more common granodiorites but are instead distributed across the batholith. This study considers geochronological (U-Pb zircon) and geochemical (whole rock geochemistry, zircon trace elements, zircon oxygen isotopes, whole rock radiogenic isotope ratios (Sr and Nd)) data on nine high-silica granites in the Central Sierra aged 97.5–102 Ma. Whole rock geochemical patterns are transitional with contemporaneous, nearby granodiorites, suggesting fractionation trajectories; however, zircon saturation temperatures are hotter in many of the granites. Some distinct zircon trace element (e.g., Ta, Sc) data suggest unusual fractionation conditions or sources in which these elements are elevated. Isotope ratios differ between the western and eastern Sierra, with higher δ¹⁸O Zircon in the west (avg. = 7.25±0.25‰, VSMOW) than east (avg. = 5.83±0.53‰); initial ⁸⁷Sr/⁸⁶Sr ratios that are more uniform in the west (0.705–0.707) than east (0.704–0.707); and eNd values in nearly all granites falling in a relatively uniform range of –2.5 to –4.5. These radiogenic and stable isotope ratios are like those in adjacent granodiorites indicating similar melt sources. Overall, the sources of the granites do not appear to relate to an unusual addition of a novel melt source (e.g., retro-arc insertion of crust), but rather a hotter arc thermal state during the mid-Cretaceous magmatic pulse, potentially driving unusual melting of oxide- or amphibole-bearing sources.