DISCOVERY OF HIGH PRESSURE AND TEMPERATURE GRANULITES IN GOSHEN DOME, WESTERN MASSACHUSETTS

A field study was initiated of structural and petrological relationships between gneissic rocks of the Goshen Dome Collinsville formation (Oco) faulted against Goshen schists (Dg) in the Goshen Quadrangle, western Massachusetts. Metapelite schists at the boundary of the dome are nominally mapped as Cobble Mountain formation (Ocb) described as a quartz-muscovite-feldspar-garnet schist and quartz-feldspar-biotite gneiss. However, detailed study of these metapelitic rocks on the western edge of the dome indicates a peak assemblage of garnet + kyanite + biotite + K-feldspar + rutile + gedrite. This provides well bounded implications for pressure and temperature. The assemblage is bounded on the low-T side by the lack of muscovite and on the high-T side by the lack of clinopyroxene. This presents a temperature range from 700-850 degC. The minimum pressure is established by the presence of first, kyanite and second, ilmenite and rutile allowing the use of the ilmenite + kyanite = almandine + rutile (GRAIL) equation. These two indicators imply formation conditions in the range of 13-16 Kbar. The lack of plagioclase indicates much higher pressures. No high pressure boundaries have been identified; however lack of evidence of coesite in garnet restricts pressure range of less than 25-28 Kbar based on our temperature range. This peak metamorphic assemblage indicates a scenario of high pressure and temperature (HP) granulites. These are rocks that were deeply subducted and later reduced in pressure through decompressional uplift as shown by kyanite retrograding to cordierite, chlorite breaking down biotite and garnet, and ilmenite rimming rutile. No such HP granulites have been mapped in this area previously.