PETROGENESIS OF LATE-STAGE QUARTZ-RICH, APLITIC, AND PEGMATITIC VEINS IN THE SHOSHONE GRANITE, CENTRAL NEVADA: INSIGHTS INTO MESOZOIC ARC MAGMATISM

During the Mesozoic the western margin of the North American continent was characterized by a magmatic arc. To investigate the spatial and temporal evolution of this active margin many studies have focused on the granitic batholiths throughout western Idaho and eastern California. This is likely due to their extensive exposure and the relative paucity of spatiotemporal equivalent lithologies in Nevada which are otherwise obscured by Oligocene arc-related magmatism and more recent Basin and Range extensional activity. To contribute to our understanding of Mesozoic arc magmatism this study focuses on the Shoshone Granite of central Nevada. Specifically, we focus on a suite of late-stage, SiO₂-rich veins.

The Shoshone Granite is located within the Big Smokey Valley, Nye County, and outcrops at the base of the Toquima Mountain Range. Field work in March 2022 included sampling the Shoshone Granite (Stitle et al., this meeting) and its cross-cutting veins of which 3 types were documented: quartz-rich, aplitic, and pegmatitic. The host Shoshone Granite units are coarse grained, equigranular, and characterized by variable proportions of muscovite and biotite (+ quartz, microcline, and plagioclase as major phases).

Vein samples are phaneritic, holocrystalline, and characterized by anhedral-subhedral grains. The aplite vein is equigranular and contains oxides (5%), microcline (40-50%), quartz (40-50%), plagioclase (10%), and trace amounts of muscovite (1%). The quartz vein displays a seriate texture and contains oxides (5-10%), muscovite (20%), microcline (30-40%), quartz (50-60%), and plagioclase (5%). To date, no accessory phases have been observed in the aplitic or quartz veins. The sampled pegmatite also displays a seriate texture and contains muscovite (10-15%), quartz (25-35%), plagioclase (20%), microcline (25-30%), minor oxides (1%), and trace zircon.

Current and future work will involve detailed scanning electron microscopy (SEM), coupled with energy dispersive X-ray spectroscopy (EDS), to further investigate trace mineral assemblages. In addition, the identification of zircon permits the application of the U-Pb chronometer which would constrain the timing of late-stage vein activity and help place the petrogenesis of the Shoshone Granite within the context of Mesozoic arc magmatism.