ASSESSING METAMORPHIC PRESSURE-TEMPERATURE CONDITIONS FOR LEDGE MOUNTAIN GRANULITES, ADIRONDACK HIGHLANDS, NY

This study combines petrography, mineral and whole-rock geochemistry, phase equilibrium modeling, and statistical analysis to investigate felsic migmatites exposed on Ledge Mountain in the central Adirondack Highlands (New York, USA), part of a migmatitic gneiss dome in the Mesoproterozoic Grenville orogen. Recent phase equilibrium modeling of Ledge Mountain suggests ultrahigh-temperature (UHT) metamorphic conditions (960-1025 ºC; 11-12.5 kbar), much higher than previously determined. New samples collected from the eastern side of Ledge Mountain will give insight into the pressure-temperature (P-T) conditions of metamorphism and help to develop the overall tectonic model. Ledge Mountain is a ridge-forming unit with the best exposures found on its steep south-facing cliff. Rocks are commonly migmatitic, tan to pink, fine- to medium-grained, sillimanite- and garnet-bearing UHT granulite-facies quartzofeldspathic gneisses. Migmatites are composed of melanosome, melanosome with interlayered leucosome, and pegmatitic leucosome with occasional amphibolite blocks (retrogressed gabbros?). These amphibolite blocks are not seen in the western part; they contain Amp + Pl + Opx + Bt + Fe-Ti oxides + Qz ± Rt. Preliminary pseudosection modeling suggests that the eastern side of Ledge Mountain experienced similar P-T conditions for metamorphism as the UHT conditions documented along its western half. We ask whether Ledge Mountain formed from a sedimentary protolith (based on high Al₂O₃ content) or an igneous protolith (based in part on similar sillimanite-quartz nodules). Ledge Mountain samples were compared to published geochemistry of igneous and sedimentary rocks using a P₂O₅/TiO₂ vs MgO/CaO discrimination diagram to determine the statistical probability of an igneous or sedimentary protolith. A better understanding of the possible protolith will let us refine our models for Ledge Mountain migmatites, and help us to interpret microtextures that record the petrologic history. Preliminary results suggest Ledge Mountain formed from an igneous protolith which supports the migmatite gneiss dome model. We will refine the P-T conditions of our pseudosections using electron microprobe data to model mineral composition isopleths.