SLAB FLATTENING TRIGGER FOR ISOTOPIC DISTURBANCE AND MAGMATIC FLARE-UP IN THE SOUTHERNMOST SIERRA NEVADA BATHOLITH, CALIFORNIA

The San Emigdio Schist of the southwestern Sierra Nevada batholith (SNB) permits examination of partial melting and devolatilization processes along a Late Cretaceous shallow subduction zone. Detrital and metamorphic zircon of the structurally highest and earliest subducted portions of the San Emigdio Schist constrain the depositional age to between ca. 102 and 98 Ma. Zircon oxygen isotope data from both lower plate schist and upper plate batholithic assemblages reveal a δ¹⁸O shift of ~ 1.5‰ between igneous (~ 5.5‰) and metamorphic (~ 7‰) domains. These results, taken with previous zircon and whole-rock δ¹⁸O measurements, provide evidence for massive devolatilization of the San Emigdio Schist and fluid traversal of upper plate batholithic assemblages, thereby altering the isotopic composition of overlying material. Furthermore, the timing of fluid-rock interaction in the southwestern SNB is coincident with eastward arc migration and an associated pulse of voluminous magmatism. We posit that during flattening of the Farallon slab the schist was rapidly emplaced beneath the magmatic source, where ensuing devolatilization triggered a magmatic flare-up in the southeastern SNB. This short-lived (< 15 Myr) high-flux event was followed by the termination of arc magmatism as the shallow subduction zone approached thermal equilibrium.