Unraveling Magmatic Histories at Multiple Scales: Insight from the Searchlight Pluton - Highland Range Magmatic System

Abstract

We summarize herein three decades of previous research from the Searchlight pluton (SLP) and coeval Highland Range volcanic sequence (HRV), which have been suggested to represent intruded and erupted counterparts. We integrate these data with new observations to further constrain the SLP-HRV connection. Our results suggest that the crystal-poor high-silica rhyolite volcanic material was sourced from the middle SLP leucogranite and the crystal-rich rhyolite porphyry (RP) volcanic material was sourced from the middle SLP low-silica granite. Trace element and geobarometric results suggest the crystal-rich material represents a crystal mush from which the crystal-poor high-silica rhyolite material was extracted, consistent with the exposure within the middle SLP. The textures and whole-rock and mineral geochemistry of the crystal-rich RP (~730 ºC) volcanic deposits suggest that it was derived en masse following the intrusion of hotter more mafic magma (~1000 ºC). The intrusion of hotter more mafic magma into the middle SLP mush is directly recorded by a composite lava flow in the HRV. We use this unique exposure to gain insight into mineral scale record of mobilization and mush rejuvenation. Our data from reaction rims on olivine in the mingled vitrophyre suggest mobilization of the composite lava occurred within days to weeks following the intrusion of mafic magma. The disequilibrium textures in the mingled vitrophyre directly reflect the consequences of thermally perturbing a silicic mush and starkly contrast with the near euhedral textures in the RP zone. The internal textures of minerals in the RP clearly document an earlier thermal event. Our observations suggest that mush rejuvenation is a multi-step process; (1) recharge, (2) mush dissolution, (3) equilibration with elevated thermal conditions, (4) cooling and crystallization, (5) eruption or solidification. We use Rhyolite-MELTS to model the effects of mafic intrusion into a crystal-rich silicic mush (e.g. RP) by conserving enthalpy. Our models suggest that recharging a silicic mush with small amounts (as low as 10% mafic recharge) of mafic magma can reduce the crystal content of the mush, bringing it into a more eruptible state. Mafic recharge may be central to the production and maintenance of eruptible magmas within the upper crust.