Pronounced and rapid exhumation of the Connecticut Valley Trough revealed through quartz in garnet Raman barometry and diffusion modelling of garnet dissolution–reprecipitation reactions

Abstract

AbstractMetamorphic rocks from the Connecticut Valley Trough (CVT), Vermont, and Massachusetts, have been examined using quartz‐in‐garnet (QuiG) and conventional thermobarometry, thermodynamic reaction modelling, diffusion modelling, and 40Ar/39Ar thermochronology to constrain their P–T–t paths during Acadian metamorphism and subsequent exhumation. Numerous samples, collected in the vicinity of the Acadian domes, contain garnet porphyroblasts that display cloudy zones characterized by numerous fluid inclusions and modified garnet compositions associated with the replacement of the original garnet by biotite±muscovite±plagioclase±quartz±low Xgrs/enriched Xsps. QuiG and conventional thermobarometry constrain both the conditions of garnet nucleation and peak P–T conditions to have occurred at ~0.85–1.05 GPa, ~550–600°C. Most notably, QuiG barometry was performed on inclusions adjacent to these reaction zones in conjunction with Gibbs method reaction modelling to reveal that these dissolution–reprecipitation reactions occurred during nearly isothermal decompression from the peak P–T conditions to around ~0.3 GPa, 550°C. Diffusion modelling reveals that the Mn zoning profiles created during garnet resorption that accompanied decompression formed in less than c. 3 Ma, which constrains the tectonic exhumation to have occurred at 8–10 mm/year. Subsequent cooling to 500°C occurred rapidly at a rate of 100°C/Ma, followed by slower cooling reaching 1.7°C /Ma by the mid Carboniferous. This is the first reported example of QuiG barometry revealing a multi‐stage metamorphic history and highlights the utility of this method for unravelling complex metamorphic terranes.