AN INTEGRATED PETROLOGICAL, GEOCHEMICAL, AND GEOCHRONOLOGICAL STUDY OF MEGACRYSTIC K-FELDSPARS FROM CRETACEOUS-AGED INTRUSIONS, CENTRAL NEVADA: INSIGHTS INTO EARTH’S MAGMATIC RECORD KEEPERS

Remnants of the Mesozoic subduction zone which once characterized the western North American continental margin are preserved as complex intrusive suites from present-day northern Idaho to southern California. To date, study of this fossilized arc system has primarily focussed on the extensive outcrops associated with the Idaho Batholith and the Sierra Nevadas with limited investigation of the central arc complex, largely due to its obscuration by recent Basin and Range-related extensional tectonics in present-day Nevada. In this work we aim to contribute insights to Mesozoic-arc activity and related intrusive magmatic activity through study of the Shoshone Granite in central-Nevada. The Shoshone Granite is located in Nye County, Nevada, and consists of two granitic lobes: the Round Mountain and Belmont Lobe. Here we focus on the Belmont Lobe, and specifically a suite of cm-scale megacrystic K-feldspars. Several thin sections were prepared perpendicular to known crystal faces. The megacrysts display visible, physical zoning with mineral inclusions (e.g plagioclase) that are euhedral in shape and oriented such that their long axes are parallel to crystal faces. In summary, the mineral inclusion population is as follows: plagioclase feldspar (45%), quartz (35%), muscovite (5-15%), titanite (5-10%), biotite (5-10%), oxides (3%), zircons (~1%), and apatite (<1%; notably up to ~1.2 mm in length). Study via SEM-EDS confirmed the presence of these minerals, and revealed complex zonation patterns within studied titanite inclusions. Subsequently, sixteen in-situ U-Pb analyses of select titanites via LA-ICP-MS were discordant but yielded lower intercept ages of 83.58 Ma (+/- 4.42Ma). Repeat analyses of titanite standard MKED-1 during the analytical run yielded an age of 1057.26 Ma (+/- 4.86Ma; n=16). The Belmont Lobe is therefore interpreted as Late Cretaceous in age. Future work will focus on zircon U-Pb geochronology, mineral inclusion geochemistry via EPMA, and 3D analysis of megacrystic growth zones via computed tomography (CT).