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Fluid inclusion evidence for an anticlockwise metamorphic P‐T path in central Massachusetts
Author(s) -
WINSLOW D. M.,
BODNAR R. J.,
TRACY R. J.
Publication year - 1994
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1994.tb00029.x
Subject(s) - geology , metamorphism , metamorphic rock , fluid inclusions , geochemistry , mylonite , andalusite , quartz , greenschist , cordierite , sillimanite , shear zone , petrology , tectonics , paleontology , biochemistry , chemistry , biotite , catalysis
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 (>650d̀ C) and then by cooling to 100‐200d̀ 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 685‐780d̀ C and 5‐6 kbar; the cooling path passed through 550d̀ 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 (CO 2 ‐rich, mixed CO 2 ‐N 2 ‐rich, N 2 ‐rich and H 2 O‐rich) and densities during metamorphism. Densities of CO 2 fluid inclusions range from 0.20 to 1.03 g cm ‐3 . Medium‐density CO 2 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 650‐700d̀ C at pressures below 5 kbar. Other CO 2 fluid inclusions from matrix quartz yield isochores which pass through 700d̀ 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 500d̀ 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.