29-2 Petrologic Constraints on the High-Temperature Metamorphism of Garnet Metapelites in the Western Adirondacks

Session: Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories (Posters)

Presenting Author:

Authors:

Abstract:

Metapelitic rocks in the western Adirondacks preserve a record of granulite facies metamorphism and partial melting during the Grenville Orogeny. Previous studies have used mineral equilibria and U-Pb geochronology on zircon and monazite to constrain the metamorphic conditions of both these rocks and a nelsonite dike interpreted to have formed in association with an anorthosite body. Petrochronologic constraints from the metapelitic gneisses suggest an early metamorphic event (~1150 Ma) at 0.4 – 0.65 GPa and 735 – 780 °C with a later metamorphic event (1040 – 960 Ma) that reached temperatures >700 °C and is potentially associated with the timing of the nelsonite intrusion. Here, we further characterize the metamorphic history of these metapelitic gneisses using quantitative electron microprobe X-ray mapping and phase equilibrium modeling. X-ray maps and bulk rock compositions were obtained for four samples in the same unit; one metapelitic gneiss located near the contact with the nelsonite from the Black River area, and three metapelites that show compositional and textural (i.e., metamorphic vs partial melt) variations in the Moose River area. Deformed almandine-rich garnets in metamorphic samples from both areas do not show significant major element zoning. The Black River metapelite contains small hercynite crystals which suggest early metamorphism at ~850 °C and <0.5 GPa. These conditions are consistent with peak metamorphic temperatures in the Moose River samples. However, biotite is not stable in the Black River sample at these temperatures. Instead, biotite XMg isopleths suggest crystallization at ~780 °C. Garnet is predicted to be stable at peak metamorphic temperatures but isopleth intersections at ~0.66 GPa and 750 °C suggest that garnet compositions were modified via diffusion during high-temperature metamorphism. Liquid is also present in all modeled mineral assemblages at these P-T conditions. This, combined with the fact that these retrograde P-T conditions overlap with the stability field of the observed mineral assemblage in a garnet neoblast bearing metapelite, reaffirms that peritectic garnet metapelites were derived from the partial melting of the metapelitic gneisses. These results combined with previous geochronologic constraints imply that the metapelitic gneisses were metamorphosed along a counter-clockwise P-T path that involved partial melting associated with nelsonite intrusion.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.