Fluid inclusions: evidence for a counterclockwise P-T trajectory in the Central Maine terrane of South-Central Massachusetts

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Virginia Tech


Acadian (Late Silurian-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 compression and nearly isobaric cooling to 100-200°C below peak recorded temperatures before eventual unroofing. Evidence for this path includes sillimanite pseudomorphs after early andalusite, abundant cordierite in pelitic lithologies, late replacement of low-P cordierite-bearing assemblages by high-P garnet-bearing assemblages, recrystallization of mylonites associated with late shear zones to form lower-T and higher-P assemblages, and development of high density CO₂ inclusions in the host rock of the mylonite. Peak conditions in the highest grade rocks were 685 - 780°C and 5-6 kbar; the cooling path passed through 550°C at pressures of 6.5-8 kbar.

Fluid inclusion data show a range of fluid composition and density during metamorphism. Densities of CO₂ fluid inclusions range from 0.25 g cm⁻³ to 1.04 g cm⁻³. High density fluid inclusions are contained within quartz inclusions in garnets found in partial melt leucosomes, and in quartz grains within migmatites. Fluid inclusions within the quartz inclusions indicate trapping conditions of 650-700°C at pressures below 5 kbar. Other CO₂ fluid inclusions from matrix quartz yield isochores which pass through conditions of 700°C and 5.2 kbar. The higher density inclusions associated with rocks containing the late high-P assemblages have isochores which pass through the estimated P-T conditions for the recrystallization of the mylonite.

Fluid inclusions recording late stage conditions include CO₂-rich, N₂-rich and aqueous fluid inclusions. Cathodoluminescence maps of quartz along healed microfractures are used to constrain the late stages of the uplift path.

Fluid inclusion evidence suggests an early isobaric heating event followed by cooling and compression with a later isothermal decompression. This evidence is in accordance with previously obtained petrologic evidence and supports a counterclockwise P-T model for the CMT of south-central Massachusetts.