Reconstructing the Physical and Chemical Development of a Pluton-Porphyry Complex in a Tectonically Reorganized Arc Crustal Section, Tioga Pass, Sierra Nevada

Abstract

In ancient or partially eroded arc sections, a protracted history of tectonism and deformation makes interpretation of local volcanic-plutonic relationships challenging. The fragmentary preservation of volcanic rocks relative to the extensive plutonic record in upper-crustal arc sections also suggests that a broader-scale approach that includes volcanic-hypabyssal-plutonic “fields” is useful. In this context, studies of hypabyssal intrusions emplaced at the intersection of volcanic and plutonic fields provide additional physical and chemical constraints on shallow-level magmatic processes. New mapping, U-Pb zircon geochronology, and geochemistry at Tioga Pass, in the central Sierra Nevada arc section, document the physical and chemical evolution of the Tioga Pass hypabyssal complex, a ca. 100 Ma system that includes an intrusive dacite-rhyolite porphyry unit and comagmatic Tioga Lake quartz monzodiorite. We interpret these units as a Cretaceous subvolcanic magma feeder system intruding a package of tectonically displaced Triassic and Jurassic volcanic and sedimentary rocks, rather than the previous interpretation of a Triassic caldera. The Tioga Pass magmatic system is a well-exposed example of a hypabyssal complex with meso-to micro-scale structures that are consistent with rapid cooling and emplacement between 0–6 km depth and compositions suggestive of extensive fractionation of largely mantle-derived magma. The Tioga Pass porphyry unit is one of many hypabyssal intrusions scattered along a ~50-kilometer-wide belt of the east-central Sierra Nevada that are spatially associated with coeval volcanic and plutonic rocks due to tectonic downward transfer of arc crust. They provide a valuable perspective of shallow magmatic processes that may be used to test upper-crustal plutonic-volcanic links in tectonically reorganized arc sections.