A DETAILED ISOTOPIC AND PETROLOGICAL STUDY OF A SINGLE GARNET FROM THE GASSETTS SCHIST, VERMONT

Garnets, up to 1.2 cm across, from a metapelite in the Acadian metamor phic terrain of eastern Vermont have been analysed for major elements and segregated into different fractions for isotopic analysis. The gar nets preserve abundant inclusions of minerals present during garnet gr owth which allow a nearly complete reaction history to be established. The Sm-Nd and U-Pb isotopic analyses yield concordant ages of approxi mately 380 Ma for the rim of one garnet and this is interpreted as the formation age. The changing mineral assemblages through garnet growth , their evolving compositions and the thermodynamic dataset of Holland and Powell (1990) are used to put constraints on the P-T evolution du ring growth. These imply growth during heating from 540 to 635-degrees -C and decompression from 9.7 to 7.2 kbar, representing a temperature increase of 95-degrees-C and an uplift of 7 km during growth of the ga rnet. While growth during heating and decompression is consistent with both field evidence and analysis of garnet microstructures and is pre dicted by theoretical models of regional metamorphism, the extent of t he temperature increase requires either very slow uplift (less-than-or -equal-to 0. 1 5 mm a - 1) or an additional magmatic heat input. Slow uplift is precluded by existing constraints on both the duration of th e uplift event and that of garnet growth and it is concluded that an e xternal magmatic heat input is required. Comparison with published dat a on the timing of metamorphism in other parts of the terrain suggests that the peak occurred earlier in lower grade regions, a conclusion t hat is again supported by theoretical studies. Following the peak, coo ling and uplift occurred at a modest rate consistent with simple isost atic recovery.