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Fluid inclusion evidence for basement decompression during Permo‐Triassic extension, SE New England, USA
Author(s) -
O'HARA K.
Publication year - 1991
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.1525-1314.1991.tb00549.x
Subject(s) - geology , metamorphism , geochemistry , basement , precambrian , gneiss , fluid inclusions , plateau (mathematics) , orogeny , inclusion (mineral) , petrology , metamorphic rock , geomorphology , paleontology , mineralogy , structural basin , quartz , mathematical analysis , civil engineering , mathematics , engineering
CO 2 ‐bearing fluid inclusions in strongly lineated but weakly foliated late Precambrian gneisses within the Hope Valley Shear zone of Connecticut and Rhode Island are of mixed composition ( X co2 ± 0.1; 7 wt% NaCl equivalent) and variable density (0.59–0.86 g/ml) and occur mainly as isolated inclusions. Also present are dilute (3 wt% NaCl equivalent) aqueous inclusions which occur on healed fractures related to greenschist facies retrograde metamorphism. Isochores for dense isolated CO 2 ‐bearing inclusions indicate pressures of 7.5–9 kbar at 500–600° C, the estimated temperature conditions of peak metamorphism. Published 40 Ar/ 39 Ar hornblende plateau age spectra indicate cooling through about 500° C at 265 ± 5 Ma. Isochores for low‐density CO 2 ‐bearing inclusions and aqueous inclusions intersect at the conditions of retrograde metamorphism (325–400° C) and indicate pressures of 3–4 kbar. Published 40 Ar/ 39 Ar biotite plateau ages indicate cooling through about 300° C at 250 ± 5 Ma. These data define a P–T uplift curve for the region which is convex towards the temperature axis and indicate uplift rates between 0.4 and 3.3 mm/year in Permian time. Exhumation of basement gneisses was coeval with normal (west‐down) motion along the regional basement–cover contact (Honey Hill–Lake Char–Willimantic fault system), and is interpreted as due to post‐orogenic extensional collapse of the Alleghanian orogeny.