T163 The Solidification Path of Magma—Information from Igneous Rocks, Eruptions, and Experimental Petrology: In Honor of the Mineralogical Society of America Awardee for 2019, Olivier Namur

Tuesday, 24 September 2019
8:00 AM - 12:00 PM
Room 126ABC, North Building

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

Paper number: T163-9
Program: Technical Sessions
Time: 10:10 AM – 10:25 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-336830

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