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

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.