GORE MOUNTAIN GARNET AMPHIBOLITE RECORDS UHT CONDITIONS: IMPLICATIONS FOR THE RHEOLOGY OF THE LOWER CONTINENTAL CRUST DURING OROGENESIS

The Gore Mountain Garnet Amphibolite (GMGA), part of the Mesoproterozoic Grenville province in the Adirondack Highlands, NY, USA, is an iconic rock known for the world’s largest garnets (up to 1 m diameter). Here we present a detailed petrographic study of these rocks. Field relations, whole rock, and mineral major and trace element chemistry suggest these rocks formed via a prograde hydration reaction of a metagabbro during an increase in pressure and temperature. U-Pb geochronology applied to metamorphic turtleshell-textured zircon date this metamorphic reaction to 1054.6±7.1Ma (2σ; MSWD = 0.94), during the expected peak metamorphic conditions of the Ottawan Orogeny (1090-1020 Ma). Our results on peak metamorphic P-T conditions based on thermobarometry, diffusion models, and thermodynamic modelling indicate that these rocks formed at ultra-high temperature (UHT, >900˚C) conditions (P = 8.5–10 kbar, T = 950±40˚C), significantly hotter than previously estimated. Diffusion models pinned by nearby cooling ages require the GMGA to initially cool quickly (9.1 ˚C Myr^-1), followed by slower cooling (2.6 ˚C Myr^-1). This cooling history suggests that the region was hot enough to undergo topography-driven lower crustal flow similar to hypothesized for modern Tibet for 0–25 Myr depending on the deviatoric stress. Including estimated effects of melt increase the potential duration to 5–35 Myr. The two-stage cooling history could reflect initial advection-dominated cooling followed by conduction-dominated cooling once flow ceases.