DECIPHERING P-T HISTORIES FROM MULTIPLE GARNET GROWTH GENERATIONS WITH QUARTZ INCLUSION RAMAN BAROMETRY

Multiple generations of garnet (either growth or dissolution-reprecipitation) may occur during metamorphism as a response to changing pressure-temperature (P-T) conditions. Correlation of each growth event to specific P-T conditions may be difficult due to disequilibrium processes, unconstrained equilibrium length scales, or absence of assemblages required for thermobarometry. Quartz inclusion Raman (QuiG) barometry provides a means of estimating potential pressure conditions of each event in the absence of any other reliable P-T information. Several examples of varying core to rim variations in measured inclusion Raman shifts highlight how the garnet-quartz host-inclusion pairs may survive changes in P-T and be used to constrain possible P-T paths. Examples from the Connecticut Valley Trough in western New England reveal that core inclusions within garnet preserve peak P-T conditions of ~575 C and ~8.5 kb, whereas rim quartz inclusions record a low-pressure garnet dissolution-reprecipitation event that occurred after decompression to ~3-4 kb. QuiG barometry combined with Zr-in-Rutile thermometry, Ti-in-Qtz thermometry, and conventional thermobarometry for several different samples further constrain a clockwise P-T path with a period of near isothermal decompression. A second example from the Fall Mountain nappe in western New Hampshire shows how quartz inclusions reflect different P-T conditions from a sequence of five different generations of garnet along a counter clockwise heating then loading P-T path. This result is entirely consistent with prior equilibrium thermobarometry work and further shows how QuiG may be integrated with previous P-T results. A third example from a low-pressure contact metamorphic terrane from the Perry Mountain Fm. in southeastern New Hampshire contains quartz inclusions that record two distinct inclusion pressures: a moderate-pressure event (ca. 10 kb) and a low-pressure event consistent with the ~550 ˚C and 4 kb regional P-T conditions. As there is no evidence of a clockwise Barrovian style P-T path for this terrane, this raises the questions as to whether there was a yet unknown early loading event that formed early garnet cores followed by the observed low P-T andalusite zone metamorphism recorded in the surrounding terrane.