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The Sierra Nevada Batholith is a record of copious magmatism caused by subduction of the Farallon oceanic plate under the western margin of North America during much of the Mesozoic Era, between 256 and 80 Ma. The diversity of rocks produced during these sub-surface interactions depends on several variables, including fluid availability, melt source, and mantle partial melt emplacement geometry (Ducea et al., 2015). The analysis of zircon is particularly appealing because zircon is a robust mineral that endures periods weathering and erosion and commonly lingers as detrital crystals in the rock record. It thus has the potential to add value as a lens into global magmatism and planetary evolution given its use as a thermometer (Watson and Ferry, 2007), and measure of magma source composition (Davies et al. 2021). Several researchers suggest that zircon can be a useful tool for constraining depth of crystallization (Tang et al. 2020). Building on thesis work on the utility of europium anomalies in zircon to model depths and, by proxy, crustal thickness for batholithic granitoids, this project provides additional data and insight to understand spatially and temporally varied trends of the arc’s plutonic record. Magma emplacement occurs in pulses and typically exhibits an eastward younging trend during the Mesozoic (Chen and Moore, 1982). Chinen (2022) found that the arc’s Western Margin exhibits both younging and thickening trends towards the east. Recent research exposed the issues associated with traditional cerium anomaly calculation because of a reliance on lanthanum, a poorly analyzed element (Loader et al., 2022). We incorporate these new methods to calculate zircon metrics for our data; this project further constrains the precision of interpretations about geochemical trends using laboratory analysis and zircon because it draws on a large and prolific database of plutonic trace element geochemistry. Because multiple magmatic and environmental processes affect zircon crystallization compositions, we use broad suites of zircon (e.g. rare earth elements, oxygen isotopes) and whole rock (XRF, trace elements, isotopes, additional minerals) geochemical analyses to elucidate aspects of previous research (Brady and Lackey, 2022; Chinen, 2022) and to build upon noted trends of the plutonic Cordilleran record. 

The Sierra Nevada Batholith (SNB) records copious Mesozoic magmatism and is an important touchstone for understanding crustal growth at continental convergent margins. Recent research in the SNB has focused on defining magmatic cyclicity and arc “flare ups” based on the ages, magma production rates, and radiogenic isotope heterogeneities of the plutonic and volcanic rocks found throughout the batholith. Two main intervals at ca. 170–148 Ma and ca. 125–85 Ma delivered >95% of the magmas in the exposed plutonic bulk in the SNB and suggest elevated emplacement rates and hotter-than-usual magmas, though the Cretaceous is by far the most productive era and the most promising for understanding the factors modulating magmatic flux. The mid-Cretaceous of the Sierra (ca. 105–98 Ma) saw the appearance of conspicuous, high-silica (>65 wt.% SiO2; average ~71%) granitic plutons of similar chemical nature that span a large geographic area, breaking the well-established west-to-east “younging” trend found in the more common rocks of intermediate compositions. This study focuses on thirteen of these high-silica granites: the Bullfrog, Independence, McGann, Rawson Creek, and Spook Plutons of the eastern Sierra; and the Shaver Intrusive Suite, Grant Grove, Case Mountain, Coyote Pass, Dennison Peak, and Frys Point Plutons of the western/central Sierra. Whole rock geochemistry, zircon trace elements, and radiogenic isotope ratios (Sr and Nd) in these high-silica granites show some transitional patterns with other contemporaneous and geographically related plutons of intermediate compositions, suggesting fractionation trajectories; however, some distinct dissimilarities are observed, including: 1) elevated, but highly varied initial 87Sr/86Sr ratios, 2) elevated fluorine in granites, and 3) hotter apparent zircon saturation conditions. These geochemical data, hotter conditions, and higher flux suggest that mantle conditions favored more crustal melting and crustal source input than at any other time in the Cretaceous. We conclude that the granitic outburst of the mid-Cretaceous was a flare up like no other.