Description: 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 18 O shift of ~1.5 between igneous (~5.5) and recrystallized (~7) domains. These results, taken with previous zircon and whole-rock 18 O 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.

Description: New mapping in the Lake Kaweah pendant of the southwestern Sierra Nevada batholith reveals a previously unrecognized nonmarine sequence of metamorphosed sedimentary and volcanic strata, defined herein as the Goldstein Peak Formation. The nonmarine origin distinguishes the Goldstein Peak Formation from all other Sierra Nevada metasedimentary pendants and from virtually all other coeval deposits associated with the Sierra Nevada arc. Basic structural relations, supplemented by new U-Pb zircon ages, suggest an Early Cretaceous depositional age, a time that is poorly represented within the stratigraphic record of California. This unusual age makes the Goldstein Peak Formation the youngest sedimentary deposit preserved within the metamorphic framework of the exhumed batholith, one that was deposited concurrently with some of the earliest deposits in the Great Valley forearc basin and just preceding the mid-Cretaceous Sierra Nevada arc surge. Preserved sedimentary and volcanic structures, along with whole-rock geochemistry, are consistent with deposition of Goldstein Peak conglomerates and sandstones within fluvial and alluvial fan environments, deposition of mud-rich sediments and air-fall tuffs within a lacustrine(?) environment, and subaqueous to subaerial extrusion of basaltic to dacitic arc volcanic rocks. This volcano-sedimentary section was intruded soon after deposition, with peak hornblende hornfels to low-pressure amphibolite facies metamorphism ultimately driven by intrusion of the surrounding Early Cretaceous Stokes Mountain ring dike complexes. Deposition of the nonmarine Goldstein Peak Formation within a fault-bounded, possibly transtensional, intra-arc basin during the transition from the low-standing, moderately extensional, Late Jurassic fringing arc to the high-standing, compressional mid- to Late Cretaceous arc indicates that at least one section of the Sierra Nevada arc was a fully emergent continental margin arc by the Early Cretaceous Epoch.