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 (>650d? C) and then by cooling to 100-200d? 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 685-780d? C and 5-6 kbar; the cooling path passed through 550d? 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 650-700d? C at pressures below 5 kbar. Other CO₂ fluid inclusions from matrix quartz yield isochores which pass through 700d? 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 500d? 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.