CONSTRUCTION OF THE BEAR VALLEY INTRUSIVE SUITE, A TRANS-CRUSTAL MAGMA SYSTEM IN THE SIERRA NEVADA, CALIFORNIA (Invited Presentation)

The Bear Valley Intrusive Suite (BVIS) in the Southernmost Sierra Nevada Batholith, California, exposes a trans-crustal magma system that spans emplacement pressures from 3-10 kbars. We will present new CA-ID-TIMS U/Pb zircon geochronology that show the bulk of the BVIS crystallized at all crustal levels between 100-101.4 Ma, and was constructed with ultra-high mantle-derived magma fluxes. Further, given the short duration of crystallization, the BVIS represents an unparalleled snapshot of magmatic processes within continental arc crust. Leveraging this fact, we present new field observations combined with whole rock geochemistry that show a fundamental dichotomy within the BVIS. The lower crust of the BVIS is dominantly composed of mafic cumulates that preserve originally shallow to horizontal magmatic fabrics, while the middle and upper crust is dominantly composed of voluminous homogenous tonalites with steeply dipping fabrics. Using a stochastic thermodynamic phase equilibria model of melt fractionation and extraction, we show that these observations strongly constrain the P-T paths along which BVIS magmas must have been emplaced: to create the observed abrupt transition from mafic lower crust to felsic middle and upper crust, evolving magmas must initially cool near isobarically in the lower crust before melt extraction and subsequent rapid emplacement in the upper crust following near-isothermal paths.

These results additionally constrain the ambient thermal state of the arc crust during BVIS emplacement to generally warm conditions compatible with conductive geothermal gradients with surface heat flows greater than 60mW/m², but less than 90 mW/m². Finally, we observe that the Sierra Nevada Batholith is typically characterized by felsic crust with low seismic velocities between 6.0-6.5 km/s to at least 30-35 km depth. This is significantly deeper than the transition at ~28 km depth in the BVIS from felsic crust to mafic cumulates with seismic velocities >7.0 km/s. Given this observation, we conclude that the bulk of the Sierra Nevada Batholith was emplaced into crust with a cooler background geotherm than was present during BVIS construction, and speculate that the BVIS high flux magmatism produced the elevated thermal structure and drove the shallowing of the mafic to felsic crust boundary.