CHEMOSTRATIGRAPHIC EVIDENCE FOR DIFFERENTIATION WITHIN THE MIOCENE SEARCHLIGHT PLUTON, NEVADA, USA

The origin of high-SiO₂ rhyolite magmas remains a persistent petrologic question. The migration of the haplogranite minimum towards higher SiO₂ content with decreasing pressure suggests that high-SiO₂ melts equilibrated in the upper crust. A prevailing model to explain this observation is that these melts are extracted from partially crystalline bodies at low pressures via compaction or crystal settling. Such an origin requires that complementary felsic-to-intermediate cumulates solidify within the upper crust as plutons. To test if a pluton, or part of a pluton, represents such a cumulate we expect that: a) it contained melt simultaneously, and b) it contains evidence for extraction of high-SiO₂ melt by gravity-driven processes. To date, very few plutons meet these criteria. Here, we present new U-Pb ID-TIMS zircon geochronology and whole rock geochemical measurements from the well exposed and ~90°-tilted Miocene Searchlight pluton in southern Nevada. Overlapping zircon dates from four samples of the middle Searchlight pluton suggest that this unit represents a sill that simultaneously contained melt over ~30 kyr and that it is distinct in age relative to the lower and upper Searchlight pluton. Lithological differences within the Middle Searchlight pluton include a cap of high-SiO₂ leucogranite underlain by granite. To assess whether these zones represent extracted melt and a felsic cumulate, respectively, we collected whole rock geochemical data along a single transect parallel to paleo-vertical. To obtain a high density of measurements (100m spacing over ~4 km) we used both traditional laboratory-based methods and a portable-XRF for field-based measurements. Results show vertical gradients in incompatible and compatible elements that are consistent with gravity-driven differentiation. We compare these vertical gradients to those expected for crystal-settling and compaction and suggest that crystal settling combined with compaction via mechanical grain rotation played an important role in generating the high-SiO₂ leucogranite cap in the middle Searchlight pluton. Given these geochemical trends, we propose that similar chemostratigraphic data will help better identify fossil magma reservoirs in otherwise homogenous granitoid complexes.