FLUID INCLUSION EVIDENCE FOR AN ANTICLOCKWISE METAMORPHIC P-T PATH IN CENTRAL MASSACHUSETTS

Acadian (Late Silurian to Early Devonian) metamorphism in the Central Maine Terrane (CMT) in central Massachusetts is characterized by an early low-P, high-T (Buchan-type) metamorphism followed by thickening at high temperature (>650-degrees-C) and then by cooling to 100-200-degrees-C below peak recorded temperatures before eventual unroofing. Mineralogical and textural evidence for this path includes sillimanite pseudomorphs after early andalusite, abundant cordierite in pelitic lithologies, replacement of low-P cordierite-bearing assemblages by high-P garnet-bearing assemblages, and recrystallization of mylonites associated with late shear zones to form lower-T and higher-P assemblages. Peak conditions in the highest grade rocks were 685-780-degrees-C and 5-6 kbar; the cooling path passed through 550-degrees-C at about 6.5 kbar. The well-constrained P-T path documented from geological and mineralogical evidence for the CMT offers an unusual opportunity to examine characteristics of fluid inclusions that have experienced a long-lived metamorphic event spanning a broad range of P-T conditions. Fluid inclusion data from the CMT document a range of fluid compositions (CO2-rich, mixed CO2-N2-rich, N2-rich and H2O-rich) and densities during metamorphism. Densities of CO2 fluid inclusions range from 0.20 to 1.03 g cm-3. Medium-density CO2 fluid inclusions are contained in quartz inclusions within garnets in partial melt leucosomes, and in quartz grains within migmatites. Fluid inclusions within the quartz inclusions indicate trapping conditions of 650-700-degrees-C at pressures below 5 kbar. Other CO2 fluid inclusions from matrix quartz yield isochores which pass through 700-degrees-C and 5.2 kbar. The highest density inclusions associated with rocks containing the late high-P assemblages have isochores which pass below the estimated P-T conditions for recrystallization of the mylonite. Fluid inclusion evidence suggests an early low-P heating event, followed by thickening at high temperature, and then by nearly isobaric cooling to about 500-degrees-C with later decompression. This interpretation is also consistent with previously published petrological models and supports an anticlockwise P-T path for the CMT of south-central Massachusetts.