Plutons offer an opportunity to study the extended history of magmas at depth. Fully exploiting this record requires the ability to track changes in magmatic plumbing systems as magma intrudes, crystallizes, and/or mixes through time. This task has been difficult in granitoid plutons because of low sampling density, poorly preserved or cryptic intrusive relationships, and the difficulty of identifying plutonic volumes that record the contemporaneous presence of melt. In particular, the difficulty in delineating fossil magma reservoirs has limited our ability to directly test whether or not high-SiO2 rhyolite is the result of crystal-melt segregation. We present new high-precision U-Pb zircon geochronologic and geochemical data that characterize the Miocene Searchlight pluton in southern Nevada, USA. The data indicate that the pluton was built incrementally over ~1.5 m.y. with some volumes of magma completely crystallizing before subsequent volumes arrived. The largest increment is an ~2.7-km-thick granitic sill that records contemporaneous zircon crystallization, which we interpret to represent a fossil silicic magma reservoir within the greater Searchlight pluton. Whole-rock geochemical data demonstrate that this unit is stratified relative to paleo-vertical, consistent with gravitationally driven separation of high-SiO2 melt from early-formed crystals at moderate crystallinity. Zircon trace-element compositions suggest that our geochronologic data from this unit record most of the relevant crystallization interval for differentiation and that this process occurred in <150 k.y.
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