CONSTRAINTS ON TECTONIC BURIAL FROM QUIG BAROMETRY, CONNECTICUT VALLEY SYNCLINORIUM, CENTRAL VERMONT

QuiG (quartz-in-garnet) barometry, traditional thermobarometry, and pseudosection analysis have been applied to garnet and staurolite-kyanite zone metapelites of the Gile Mountain, Waits River, and the Northfield formations from the Connecticut Valley Synclinorium (CVS), central Vermont. Over fifteen samples were examined in detail along an east-west transect from the Strafford Dome to Bethel, Vermont, encompassing both the garnet and staurolite-kyanite zones.

QuiG barometry was used to determine the pressure at which garnet overgrew quartz grains. Quartz inclusions are plentiful in garnet and inclusions on the order of five microns were selected for Raman spectroscopy. Overall, QuiG barometry indicated garnet nucleated at conditions around 9-13 kbars throughout the CVS. Compared to garnet-plagioclase barometry, QuiG barometry revealed pressures consistent with or up to 3 kbars higher than GASP and GPMB barometers. In addition, pressures determined from QuiG barometry on inclusions from the garnet core to the rim showed no systematic variation, which is interpreted as a consequence of near isobaric and isothermal garnet growth. Garnet-biotite thermometry consistently resulted in temperatures of 450-550°C. This temperature range is believed to occur during peak burial, and does not necessarily reflect peak temperature. Several samples recorded temperatures up to 700°C reflecting a possible maximum temperature.

Garnet is zoned with decreasing Mn from core to rim, with rims enriched in Mn where resorption occurred. Mn diffusion models reveal the formation of these rims to have occurred on a time scale of around 1Ma. Such a short time scale is consistent with the near isobaric and isothermal growth of garnet inferred from QuiG barometry. Furthermore, comparison of QuiG and traditional barometry with equilibrium pseudosection analysis requires considerable overstepping of the equilibrium garnet-in reaction curve.

The combined QuiG, thermobarometry, diffusion, and pseudosection analysis is consistent with rapid porphyroblast formation following considerable overstepping of the garnet isograd at nearly isobaric and isothermal conditions. This could be accomplished by rapid burial, metamorphism, and exhumation of the CVS.