REDEFINING AN IGNEOUS SYSTEM: MAGMATIC EVOLUTION OF THE RIVER MOUNTAINS VOLCANIC SUITE AND WILSON RIDGE PLUTON

A new model for the evolution of igneous systems is based on the linked River Mountains volcanic suite and Wilson Ridge pluton of Nevada and Arizona. Evidence for this link includes new field work, U/Pb zircon geochronology, major and trace element geochemistry, whole rock Sr and Nd isotopes, and in situ plagioclase chemistry. Faulting and uplift from Basin and Range extension expose both the volcanic and plutonic sections, allowing us to expand the definition of igneous systems and our understanding of how they evolve. The River Mountains volcanic suite - Wilson Ridge pluton igneous system is the result of continuous bimodal magmatism and formed in three stages. Early and late stages of the system preserve bimodal magmas as separate intrusions, lavas, and dikes. Early hypabyssal units (14.91 ± 0.37 to 13.55 ± 0.42 Ma) and late stage dikes and lavas (13.64 ± 0.43 to 12.95 ± 0.32 Ma) are basalt and rhyolite. During the middle stage, bimodal magmas coalesced to form a magma chamber of intermediate composition. In contrast, the main phase of the system (13.94 ± 0.38 to 13.42 ± 0.23 Ma) is the result of extensive magma mixing and comingling producing quartz monzonite in the pluton and dacite in the volcanic suite. Mafic enclaves in the main phase of the pluton are largest and most abundant near the base, where feeder dikes entered the magma chamber. Enclaves occur near the margins of the pluton, but become smaller and less abundant towards the interior, where mixing was more thorough. Magma mixing models reproduce trends in major, trace elements, and Sr and Nd isotopes. A large component of assimilation of crustal material also explains 87Sr/86Sr (0.7076 to 0.7107) and 143Nd/144Nd (0.51225 to 0.51206) values for the entire system. Major element EPMA transects across plagioclase grains demonstrate a difference between volcanic and plutonic magmatic records. Volcanic plagioclase grains preserve smaller increments of this history than plutonic grains. On average, An numbers in volcanic grains only vary up to 15 mol.%. In comparison, plutonic grains vary up to 30 mol.%. The truncated growth of volcanic grains at the time of eruption translates to less variation in major element chemistry across volcanic grains compared to plutonic grains. This new data give a more complete understanding of systems with incomplete preservation and exposure.