Multidisciplinary analysis of a polymetamorphic terrane, western New England
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Rim compositions of the highest-grade porphyroblasts in the Taconian zone generally yield P-T estimates that are inconsistent for subareas within a given thin section. With the onset of Acadian metamorphic overprinting conditions of approximately 500Â°C, the rim compositions of porphyroblasts yield P-T estimates that are much more consistent, and vary by only a maximum of Â±50Â° C and 1 kb. These results are interpreted to indicate that the rims of preexisting Taconian porphyroblasts reequilibrated pervasively at approximately 500Â°C and above, whereas reequilibration was incomplete at lower Acadian overprinting temperatures. Garnet developed texturally and chemically distinct rims which appear to reflect prograde Acadian metamorphism in higher-grade parts of the Acadian metamorphic zone.
Previous studies of garnet zoning have emphasized volume and surface diffusion as the primary means of changing mineral composition. However, in this polymetamorphic zone changes in the composition of preexisting garnet occurred by metasomatic dissolution and reprecipitation along porphyroblast surfaces in the presence of a metamorphic fluid. Diffusion seems significant for changing the garnet composition during the initial overprint only as an agent to move material in an intergranular fluid.
The pressures and temperatures of the Acadian metamorphism a consistent metamorphic field gradient of 16 bar/Â°C, increasing eastward. In an area mainly affected by Acadian metamorphism, peak. metamorphic temperatures were 8.2 kb and 575Â°C. Nonlinear unroofing rates from ~10 to less than 1 mm/yr following the high-pressure Acadian metamorphism have been documented using combined petrologic, fluid inclusion, thermochronometric, and thermal modeling techniques. The calculated unroofing path is initially nearly isothermal and is followed by more isobaric cooling. Comparison of the inferred path to those predicted by thermal models suggests unroofing was characterized by initial rapid upward en bloc velocity (~ 1 cm/yr) of brief duration, followed by much slower unroofmg rates (â ¤ 0.3 mm/yr). This proposed unroofing history is consistent with the Paleozoic sedimentary and igneous record of western New England.
- Doctoral Dissertations