Petrotectonic evolution of Ledge Mountain migmatites, Adirondack Highlands, New York

Abstract

Ledge Mountain 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. This study provides a multifaceted methodology involving detailed petrographic analysis and extensive thermodynamic modeling using Perple_X to reveal a metamorphic history with significantly higher peak P-T conditions than those determined by classical thermobarometry from elsewhere in the region. Ultrahigh-temperature granulite-facies peak metamorphic P-T conditions of 11-18 kbar and ≥960°C are determined using the intersection of peritectic garnet compositions from five samples and garnet isopleth models. Water content is determined to be ~0.12 wt. % based on hydrous mineral volume and T-MH2O modeling. Thermodynamic models test possible protolith compositions and water content to determine whether lower temperature conditions would be produced; all model configurations generate UHT metamorphic conditions. Modeling reveals that 10% to 44% melt may have been produced and subsequently lost during the prograde history. To account for melt loss, prograde compositions are estimated by reintegrating a modeled melt composition back into the bulk composition. A melt-reintegrated pseudosection reveals stable mineral assemblages along the prograde path that are consistent with observed microtextures and melt inclusions within peritectic garnet. From peak conditions, the retrograde path follows a near isothermal decompression path followed by near isobaric cooling to retrograde conditions of 6.5-8 kbar and ~750°-850°C. These retrograde metamorphic conditions align with those reported in other studies as peak metamorphic conditions. This modeling methodology may reveal a hidden P-T history not captured elsewhere in the Adirondack Highlands, and may be applicable more widely to the Grenville province or other orogens containing felsic migmatites.