LINKING METAMORPHIC PETROLOGY AND GEOCHEMISTRY THROUGH GEOTHERMOBAROMETRY AND TECTONIC DISCRIMINATION OF SUBDUCTION ZONE ROCKS: ROWE-HAWLEY ZONE, VERMONT
Original spatial distribution of source region between the two types of basalts influenced subsequent metamorphism along P-T-t subduction and exhumation paths. Bulk chemistry and amphibole compositional data integrated in Perple_X (Connolly, 2005) through pseudosection and isopleth calculations indicate that Taconian subduction metamorphism of an ultramafic-mafic-pelitic unit in Stockbridge reached 535-540°C and exceeded 8.8 kbar. Under these types of conditions, chlorite-zoisite-phlogopite-rutile and quartz are stable with two amphiboles indicating miscibility along the subduction zone at about 26 km depth. Exhumation of these mafic rocks and polymetamorphism at lower pressures and temperatures is predicted theoretically by decreasing NaM4SiCa-1Al-1 and AlVIAlIVMg-1Si-1 substitution from core to rim in amphiboles preserving three distinct chemical zones. Projections of electron microprobe data using Na and Al compositional isopleths calculated in Perple_X indicate metamorphism for one amphibole equilibria at 445-475°C and 6.3-7 kbar, corresponding to 20-21 km depth. A final phase of exhumation is marked by actinolite growth at ~10 km depth during greenschist facies metamorphism at 400°C and 3 kbar, which may correspond to Acadian intraplate deformation.