QUARTZ INCLUSION IN GARNET RAMAN BAROMETRY AT MT. MOOSILAUKE, NEW HAMPSHIRE: REVISITING CLASSIC GEOTHERMOBAROMETRIC CALIBRATIONS

Recent advances in the application of Raman band spectroscopy to mineral inclusions in metamorphic rocks (thermobaRamantry) have called into question the efficacy of traditional geothermobarometric methods. The mathematics of geothermobarometry require the assumption of chemical equilibrium between the mineral phases involved in metamorphic reactions. The equilibrium conditions, however, must be overstepped to provide a driving force for the reaction. Typically, the amount of overstepping is assumed to be negligible. ThermobaRamantry, however, only requires the assumption of mechanical equilibrium, as it relies entirely on empirical equations of state and thermoelastic models of the minerals. The results of thermobaRamantry have shown that overstepping can occur to a much greater degree than assumed. To test this, quartz inclusions in garnet (QuiG) barometry has been used on garnet-sillimanite-staurolite metapelites from Mt. Moosilauke, New Hampshire. These samples are particularly pertinent to this study because they were used in the past for calibrating popular geothermometers and geobarometers, and their P-T histories are thought to be well understood. The metapelites were metamorphosed during the Acadian orogeny, and are thought to have experienced peak temperatures and pressures at or around the aluminosilicate triple point, at 0.38 GPa, and 500 ˚C. Results from our recent microprobe analyses are in line with the previously published data. Preliminary QuiG work, however, shows that these samples may have been at pressures ranging from 0.6 GPa-0.8 GPa when garnet began to crystallize. QuiG results and garnet major element maps are also not homogenous across the Moosilauke septum. The garnet maps show complex growth zoning. This work could substantially change our understanding of the metamorphic and tectonic evolution of the Moosilauke septum. To this effect, the results of QuiG are compared to the results of the geothermobarometry and pseudosection calculations in order to further pin down the role of overstepping in metamorphic reactions and its tectonic implications.