DETERMINING THE PETROTECTONIC EVOLUTION OF LEDGE MOUNTAIN MIGMATITES WITH PHASE EQUILIBRIA MODELING AND MELT REINTEGRATION: ADIRONDACK HIGHLANDS, NEW YORK

Migmatites exposed at Ledge Mountain in the Adirondack Highlands are part of a classic granulite terrane and provide an analogue for deep crustal processes in modern orogens where direct study or sampling would be impossible. Petrographic analysis and phase equilibria modeling with Perple_X reveal significantly higher peak metamorphic conditions than those indicated by classical thermobarometry from elsewhere in the region. We determine ultrahigh-temperature (UHT) conditions for peak metamorphism (13-18 kbar, >1000°C) and granulite-facies retrograde metamorphism (6.5-8 kbar, ~750°C-850°C) - like results from classical thermobarometry. Preliminary modeling using MELTS-rhyolite reveals lower TiO₂ activity values (0.45-0.5) that increase Ti-in-zircon temperature estimates of 750-800°C by 65°C-85°C, but probably record cooling during buoyancy-induced exhumation. Biotite volume limits H₂O content to ~0.12 wt% and indicates these rocks were quite dry; overestimating H₂O content up to 2.0 wt% in models provides a minimum temperature of 950°C which is still within UHT conditions. Leucocratic channels may have helped move melt out of the residuum; Perple_X modeling determines at least 25% melt may have been generated and transported during peak metamorphism. Melt volumes are consistent with petrographically observed peak mineral assemblage of Qz + Pl + Ksp + Ilm + Opq + Grt + Sil ± Rt, garnet mineral chemistry, and microtextures that indicate large melt volumes. Prograde conditions and protolith composition are estimated by reintegrating determined volumes of modeled (and measured) melt chemistries back into melt-depleted compositions, completing the P-T-t path. We reintegrated melt and show that even incorporating 40% melt does not shift our peak assemblage field more than 500 bar and 20-25°C lower. Our modeling has important implications for late- to post orogenic crustal dynamics during extensional collapse of the southern Grenville Orogen. Wider application of thermodynamic modeling in the Grenville Province, in combination with geo/thermochronology may reveal more extensive UHT granulite-facies metamorphism in the Grenville.