Type of Document Dissertation Author Hames, Willis E. URN etd-08232007-113000 Title Multidisciplinary analysis of a polymetamorphic terrane, western New England Degree PhD Department Geology Advisory Committee
Advisor Name Title Tracy, Robert J. Committee Chair Bodnar, Robert J. Committee Member Ratcliffe, N. M. Committee Member Sinha, A. Krishna Committee Member Sutter, John F. Committee Member Keywords
Date of Defense 1990-04-05 Availability restricted AbstractA combined petrologic, structural, and geochronologic approach has been used to characterize a zone in southwestern New England affected by superimposed metamorphisms, and the unroofing history of an area that records only the youngest metamorphism. The area chosen for the study of polymetamorphism, northwestern Connecticut and adjacent New York and Massachusetts, records Taconian (Ordovician) peak metamorphic effects in the west and Acadian (Devonian) peak effects in the east. In between, a complicated zone of overlap contains predominantly Taconian mineral assemblages that have been variably affected by Acadian processes. 40Ar/39Ar data presented here indicate that the timing of the Taconian thermal maximum was approximately 445 Ma, and that of the Acadian was approximately 390 Ma. Cooling ages suggest that the highest-grade Taconian assemblages at the present erosion surface remained at elevated metamorphic conditions between the Taconian and Acadian thermal maxima.
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 500°C, the rim compositions of porphyroblasts yield P-T estimates that are much more consistent, and vary by only a maximum of ±50° C and 1 kb. These results are interpreted to indicate that the rims of preexisting Taconian porphyroblasts reequilibrated pervasively at approximately 500°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/°C, increasing eastward. In an area mainly affected by Acadian metamorphism, peak. metamorphic temperatures were 8.2 kb and 575°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 (≤ 0.3 mm/yr). This proposed unroofing history is consistent with the Paleozoic sedimentary and igneous record of western New England.
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