The San Emigdio Schist of Southern California permits examination of partial melting and devolatilization processes along a Late Cretaceous shallow subduction zone. Detrital and recrystallized zircon of the structurally highest portions of the schist bracket the depositional age to between ca. 102 and 98 Ma. Zircon oxygen isotope data from both lower-plate schist and upper-plate assemblages of the Sierra Nevada batholith (SNB) reveal a δ18O shift of ∼1.5‰ between igneous (∼5.5‰) and recrystallized (∼7‰) domains. These results, taken with previous zircon and whole-rock δ18O measurements, provide evidence for devolatilization and/or partial melting of the schist and fluid ascent through overlying southwestern SNB upper-plate assemblages. Furthermore, the timing of mobile phase–rock interaction in the southwestern SNB is coincident with voluminous S-type magmatism in the southeastern SNB. We posit that during flattening of the Farallon slab, the schist was emplaced into the root zone of the southeastern SNB, where ensuing partial melting triggered a magmatic flare-up. Shallow subduction of the Cocos plate beneath central Mexico represents a close modern analog to this model.