Determining the Petrotectonic Evolution of Ledge Mountain Migmatites with Phase Equilibria Modeling and Melt Reintegration: Adirondack Highlands, New York.

Abstract:

Migmatites exposed in the Adirondack Highlands, an outlier of the Grenville province, are a part of a classic granulite terrane and represent deep crustal rocks that form modern orogenic roots. Analogue rocks in modern orogens exist at depths where direct study or sampling would be impossible. Ledge Mountain in the central Adirondack Highlands contains migmatitic granulites that record peak metamorphic conditions that are significantly higher temperature than other P-T conditions determined by classical thermobarometry from elsewhere in the region. Petrographic analysis and phase equilibria modeling with Perple_X suggests ultrahigh-temperature conditions for peak metamorphism (13-18 kbar, >1000°C), and granulite-facies conditions during retrograde metamorphism (6.5-8 kbar, ~750°C – 850°C) similar to results from classical thermobarometry. Preliminary modeling using MELTS-rhyolite reveals low TiO2 activity values (0.45-0.5) that increase Ti-in-zircon temperature estimate of 750-800°C by 65°C-85°C. Zircon temperature estimates probably record cooling during buoyancy induced isothermal decompression. Water content was determined to be ~0.12% based on hydrous mineral volume, however, increasing water content to 2% lowered peak assemblage field to ~950°C, which is still within ultrahigh-temperature granulite facies. Ledge Mountain migmatites host leucocratic channels that may have helped move a high volume of 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 microstructures 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.