VOLCANIC RECORDS IN THE IRON MOUNTAIN PENDANT, SIERRA NEVADA, CA: INSIGHTS INTO JURASSIC-CRETACEOUS MAGMATIC EVOLUTION THROUGH FIELD RELATIONSHIPS AND GEOCHRONOLOGY

Downward flow during episodic arc magmatism preserved fragments of the volcanic and sedimentary rock record within roof pendants that represent rare, often incomplete, windows into the surface evolution of the Sierra Nevada arc system. The Iron Mountain pendant (IMP), located in the west-central Sierra Nevada, preserves a metavolcanic record coeval with Late Jurassic to Early Cretaceous plutonism. Identified units provide insight into a relatively understudied period in the region’s surficial history that is missing from the neighboring Mineral King, Ritter Range, and Oak Creek pendants, due to faulting and unconformable contact relationships.

The volcanic sequence of the IMP consists of three distinct volcanic packages of moderately dipping (30-55°) rhyolite and andesite deposits that were regionally metamorphosed to greenschist facies. The lowermost package is underlain by a meter-thick debris flow and is dominated by massive andesitic lava flows, with a weighted mean age of 155.6 ± 3.58 Ma. These fine-grained flows contain mm-sized plagioclase and/ or hornblende and are capped by a lahar deposit.

An unconformable coherent package, ranging from 124-119 Ma, is predominantly rhyolite, with minor andesitic components. Centimeter-scale rhyolite flow banding is observed at the basal contact with overlying fine-grained massive lava flow units. Volcaniclastic units and a mafic enclave bearing unit are interbedded in the lava flows. Andesite-rhyolite sheets, alternating between lava flows with foliated groundmass to boulder-sized volcaniclastic deposits form the uppermost unit.

An overlying andesite crosscuts multiple units of the rhyolite package, containing primary volcanic lavas and reworked volcaniclastic material, similar to the basal andesite package. This sequence is capped by a highly altered rhyolite unit. Geochronological analysis will test if the contact is depositional, reflecting eruptive patterns, or a cryptic fault. The IMP volcanic sequence records a transition from effusive andesitic lava flows in the Jurassic arc, with reworking of volcanic arc materials, to primarily explosive and effusive rhyolite flows by the Early Cretaceous. The preserved sequence highlights the shift in composition and volcanic process throughout the evolution of arc magmatism.