INSIGHTS INTO MAFIC MAGMA CONTRIBUTIONS THROUGH STUDY OF AMPHIBOLE-RICH MAFIC ENCLAVES IN MESOZOIC GRANITE INTRUSIONS, NEVADA

The western margin of the continental United States was dominated by arc-related magmatism during the Mesozoic This resulted in the production of extensive intrusive suites within the Earth’s upper crust (at the time) At present, evidence of this once extensive system is largely preserved by the Sierra Nevada and Idaho Batholiths. Insights into arc evolution is however, obscured along strike in present-day Nevada owing to relatively recent Basin and Range extensional tectonics and associated volcanism and sedimentation. This study focuses on the study of a suite of Magmatic Mafic Enclaves (MMEs) within the granites sampled in Tonopah, Nye County, Nevada. Prior study of MMEs in other geologic settings have proposed various origins including: chemically modified xenoliths, disturbance of preexisting mafic dikes, residual magmatic material, cumulates derived from a host magma, and magma mixing and mingling processes. The objective of this study is to assess the role and origin of the granite-hosted Nevada MMEs within the context of MME formation, and Mesozoic arc evolution through detailed petrological and geochemical analyses. Petrography and SEM-EDS data reveal that the MMEs are dominated by amphibole (35%) and plagioclase (40%) with minor and accessory clinopyroxene (10%), biotite (7%) quartz (5%), oxides (2%), titanite (< 1%), and apatite (< 1%). Bulk rock major element analysis of 4 MMEs classify them as gabbros or monzogabbros: ~49 wt. % SiO₂ at ~5 wt. % Na₂O + K₂O. wt. % Total wt. % Al₂O₃, FeO*, CaO, and MgO are similarly consistent between samples at ~17%, ~11%, 9%, and ~5% respectively. Bulk Sr ranges from 573ppm to 601ppm, Rb from 97ppm to 103ppm, Ba from 170ppm to 193ppm, and Th from 10ppm to 12ppm. Primitive mantle-normalized spidergrams lack significant LILE enrichment and Nb-Ta anomalies while chondrite-normalized REE patterns are generally LREE enriched (La_N/Sm_N from 2.9-3.3) and exhibit minor Eu anomalies: 0.76 to 0.81 where Eu/Eu*= Eu_N/[(Sm_N/Gd_N)^^0.5]. Future work will focus on in-situ elemental analysis of amphibole and plagioclase via EPMA and LA-ICP-MS in order to further evaluate the magmatic origin of these granite-hosted MMEs.