CONSTRAINING PRESSURE-TEMPERATURE CONDITIONS OF GARNET DISSOLUTION-REPRECIPITATION REACTIONS IN THE GOSHEN FORMATION OF WESTERN MASSACHUSETTS WITH QUARTZ INCLUSION BAROMETRY

Quartz inclusion barometry, garnet-xenotime thermometry, and garnet-biotite thermometry were applied to a staurolite zone sample from Goshen Formation in western Massachusetts to determine the pressure-temperature (P–T) conditions where garnet experienced dissolution-reprecipitation reactions. BSE imaging and compositional X-ray maps of garnet within the matrix show distinct low grossular, porous rims surrounding reaction zones where garnet was replaced by muscovite and biotite (±quartz). By contrast, garnet partially overgrown by staurolite does not exhibit any mica replacement textures. Contained within these reaction zones are inclusion suites containing quartz, rutile, chlorite, xenotime, and apatite. Application of quartz-in-garnet (QuiG) Raman barometry to this sample revealed that quartz inclusions preserve two distinct populations of inclusion pressures (Pinc) and hence two distinct entrapment P–T conditions. Within unaltered garnet cores are quartz inclusions that preserve Raman shifts of the 464 cm^-1 Raman band of ~2.7 cm^-1, whereas inclusions near porous alteration textures record Raman shifts of -2.0 cm^-1. When combined with garnet-xenotime thermometry this indicates that a lower bound on garnet nucleation was ~480 °C and ~8 kbar, and rim reequilibration occurred at ~580 °C and ~3 kbar. The peak temperature of 580 °C is nearly the same as the maximum garnet-biotite temperature of 575 °C measured in this sample. It is unknown however, if the garnet core temperature is underestimated due to recent reevaluation of the P–T conditions reached by several calibration samples of the empirical Grt-Xen thermometer.

These observations suggest that fluid infiltration at near peak temperatures occurred after staurolite formation and during exhumation to drive garnet dissolution-reprecipitation reactions and create several domains of localized equilibrium within this sample. Garnet partially overgrown by staurolite appears to have been unaffected by dissolution reactions whereas garnet within the matrix has been substantially altered by fluid infiltration. Texturally this sample provides evidence that staurolite-forming reactions (e.g. Grt + Chl + Ms = St + Bt + Qtz + H₂O) did not substantially contribute to garnet breakdown, and instead grew from matrix chlorite and muscovite.