Pronounced and rapid exhumation of the Connecticut Valley Trough revealed through quartz in garnet Raman barometry and diffusion modelling of garnet dissolution–reprecipitation reactions. (16th April 2021)

• Record Type: Journal Article
• Authors: Wolfe, Oliver M.; Spear, Frank S.; Harrison, T. Mark
• Is Part Of: Journal of metamorphic geology. Volume 39:Number 8(2021)
• Page Start: 1045
• Page End: 1069
• Publication Date: 2021-04-16
• Subjects: Connecticut Valley Trough -- fluid alteration -- garnet diffusion -- P–T–t paths -- QuiG barometry; Metamorphism (Geology) -- Periodicals; 552.4
• DOI: 10.1111/jmg.12601
• Languages: English
• ISSNs: 0263-4929

Abstract

Abstract: Metamorphic 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 40 Ar/ 39 Ar 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 X grs /enriched X sps . 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.