THERMOBAROMETRY IN KYANITE S-TECTONITE GNEISSES FROM THE GOSHEN DOME, CONNECTICUT VALLEY ZONE, WESTERN MASSACHUSETTS

Electron microprobe analyses and qualitative X-ray maps were collected for metapelitic rocks from the Goshen formation (Dg) in the Goshen Quadrangle, western Massachusetts. These metapelite schists are located at the margin of the Goshen Dome, which consists of schists and gneisses of the Cobble Mountain (Ocb) and Collinsville (Oco) formations. Mapping of isotherms of the Goshen formation in the surrounding quadrangle, shows a local prograde metamorphic trend to the southeast, with the highest grade rocks (ky + sta) found in close proximity to the Goshen Dome (Abbott, Jr. 1979). In order to understand the formation conditions of the Devonian s-tectonites of the Goshen schists in relation to the local prograde Acadian metamorphism, a detailed study was performed on the metapelitic rocks on the western and eastern edge of the dome. This study revealed a peak assemblage of garnet + kyanite + plagioclase + biotite + rutile + quartz. It was previously argued based on mineral assemblage alone that these rocks reached pressures and temperatures of 13-16 kbar and 700-850 degC, indicating high pressures and temperatures of upper granulite facies metamorphism. Garnets occur as inclusions in coarse-grained kyanite porphyroblasts, while others are located in the matrix. The garnets located in the matrix exhibit sigmoidal quartz inclusion trails. Both sets of garnets show zoning patterns in which there is strong Ca and weak Mn prograde growth zoning. Also, slight Mg zoning is apparent at garnet rims, along with local resorption, consistent with retrograde re-equilibration. Plagioclase displays reverse zoning at the rims, seen by a slight increase in Ca from the cores to rims. Microprobe analyses were obtained for matrix biotite and plagioclase, and selected areas of garnet just inside the retrograde rim. Use of GASP, GARB, GRIPS, and GRAIL geothermobarometery yields values of 11 ± 2 kbar and 720-740 ± 23 degC for the measured compositions. This peak metamorphic assemblage indicates a P-T range in the lower granulite facies. Calculated pressures and temperatures are slightly higher to those of previous workers in the Connecticut Valley Synclinorium from CT to VT (Ratcliffe et al. 1992; Moecher et al. 1996).