Determining the amount of overstepping required to nucleate garnet during Barrovian regional metamorphism, Connecticut Valley Synclinorium

Abstract

AbstractThe novel method of inclusion barometry coupled with the calculation of the required affinity for garnet nucleation is applied to three samples from the previously well‐characterized Connecticut Valley Synclinorium in central Vermont. Raman shifts for quartz inclusions record a range of maximum peak shifts of the quartz 464 cm−1 peak from 2.4 to 3.0 cm−1. Temperature of garnet nucleation was constrained by calculating mineral assemblage diagrams in the MnNCKFMASHT system and plotting the intersection of quartz inclusion in garnet barometry (QuiG, quartz‐in‐garnet) with Zr‐in‐rutile thermometry. Utilizing the intersection of Zr‐in‐rutile thermometry with QuiG barometry, garnet nucleation is inferred to have occurred within a P–T range of ~8.6–9.5 kbar and ~560–575°C. These P–T conditions for garnet nucleation are significantly higher than calculated equilibrium garnet‐in isograds for the three samples. Affinities for garnet nucleation were calculated as the difference between the free energy of a fictive garnet composition based on the matrix assemblage and the free energy of the nucleated garnet. The calculated nucleation affinity varied from 300 to 600 kJ/mol O for St–Ky grade samples. These results suggest that the assumption that metamorphism proceeds as a sequence of near‐equilibrium conditions cannot, in general, be made for regional metamorphic terranes. This body of work agrees with numerous recent studies showing that garnet‐producing reactions must be overstepped in order to for garnet to nucleate.