UNDER THE SURFACE: UNRAVELING THE MAGMATIC HISTORY OF EOCENE INTRUSIONS AT SWALES MOUNTAIN, NV

Many modern volcanic systems change over the course of their lifetime in their eruptive behavior and products. How these changes are recorded in subsurface magmatic records remains poorly understood. By studying magmatic systems that expose the subvolcanic and plutonic levels of old volcanic systems we can reconstruct the spatial and temporal assembly of small to intermediate-sized systems. Nevada has been affected by the westward convergent plate margin for over 150 million years causing volcanism and magma intrusions in the Jurassic, Cretaceous, and Eocene through Miocene. The associated subvolcanic and plutonic plumbing systems have been exhumed and tilted as a result of Basin and Range extension enabling the study of their assembly in time and space. We present emerging results from geologic mapping combined with geochemical analyses from Swales Mountain, NV, which hosts a shallow to mid-crustal Eocene magmatic system. The peraluminous, calc-alkaline intrusions range from 56 to 76 wt% SiO₂ and mainly include monzonite, quartz monzodiorite, rhyolite, and trachyandesite units. Two texturally and compositionally-distinct plutons each approximately 2 km in width are exposed in different parts of the plumbing system. In between, various dikes, sills, and breccias interact with Paleozoic sedimentary units. Field observations exhibit the trachyandesite cross-cutting the rhyolite. Mixing textures with mafic material have been observed in the plutonic units, indicating a contemporaneous heterogeneity in the Swales Mountain system. Units exhibit weak to moderate argillic alteration with minor copper oxides. Swales Mountain is less than 20 km from the Carlin Trend causing the intrusions to be a potential heat source leading to Carlin-type gold mineralization. Results indicate that the oldest preserved episodes of magmatism at Swales Mountain were evolved and felsic, potentially related to more crustally-derived magmas that then transitioned to less evolved, more mafic, mantle-derived compositions. We speculate that this transition to more mafic compositions is related to more direct magma transport from the mantle as slab roll back progressed. Small systems, like Swales Mountain, may potentially be used to explore how initial crustal magmatism integrates mantle material over time.