T201 From the Micro to Macro in Metamorphic Geology: Constraining Tectono-Metamorphic Processes with High-Resolution Approaches

Thursday, 29 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

TECTONIC SWITCHING, EXTREME HEAT FLOW, AND CRUSTAL MELTING IN PALEOPROTEROZOIC MIGMATITES WHERE THE YAVAPAI AND MOJAVE DOMAINS MEET: MCCULLOUGH RANGE, NEVADA

Paper number: T201-15
Program: Technical Sessions
Time: 4:40 PM – 4:50 PM

Authors

Abstract

The McCullough mountains, in the southern tip of Nevada, expose c. 1.7 Ga low-pressure metasedimentary gneisses with spectacular evidence for partial melting, melt extraction, and melt migration. This presentation consists of petrography and phase-equilibrium calculations of residual garnet–cordierite gneisses to understand the tectonic environment in which they formed. To determine near-peak metamorphic conditions, phase-equilibrium calculations were compared using multiple thermodynamic datasets and bulk compositions determined by both XRF and quantified EPMA maps of mm-scale domains in thin section. Conditions of prograde metamorphism, under which melting occurred, were determined by petrographic assessment of mineral inclusions and coronae, coupled with additional phase-equilibrium calculations in which the bulk composition was iteratively adjusted to account for the progressive loss of melt that was likely to have occurred. As much as 30 vol% partial melting occurred during isobaric heating to ~800°C at ~0.35 GPa. A small increase in pressure (~+0.1 GPa) at peak temperature indicates an overall counter-clockwise shape to the P–T path. This low-pressure counter-clockwise evolution contrasts with broad clockwise P–T paths recorded in contemporaneous migmatites of NW Arizona, indicating a highly dynamic and spatially variable orogenic environment where the Yavapai and Mojave domains meet, characterized by tectonic switching and substantial advective crustal heating.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-353467

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