Collaborative Research: Documenting the Fine-Structure of Metamorphic Events and Ages in a Complex Polymetamorphic Orogen

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Hames9903077In this collaborative project we will focus our research effort on the Taconic (Ordovician-Silurian) and Acadian (Devonian) metamorphic episodes in a transect from Westchester County, New York, to the western margin of the Bronson Hill terrane in Connecticut. In the western area (NY), the Taconic rocks appear to be undisturbed by any Acadian thermal effects, whereas the Acadian is the only thermal event in the east (CT), with little or no evidence of any pre-Acadian thermal history. In between, highly complex textures and fabrics suggest there may be a variably intense Acadian overprint on earlier-metamorphosed Taconic rocks. Alternatively, the rocks in the apparent overlap zone may contain no evidence for an earlier (Taconic) event, and in fact may not have been present at all in this location when the Taconic metamorphism occurred. The two orogenic events in this region cover a range in age of about 100 Ma, from earliest documented Taconic activity at about 475 Ma to a peak at about 450 Ma, to an Acadian peak at 390 Ma followed by rapid post-Acadian decompression and unroofing at 375 - 355 Ma. We propose to use petrologic, geochronologic and thermochronometric techniques to test the hypothesis that there is in fact an overlap zone with Taconic metamorphic rocks of middle amphibolite grade overprinted by another (Acadian) metamorphic event of similar grade about 60 Ma later. This project will involve thorough integration of petrologic, thermobarometric, U-Pb geochronologic and 40Ar/39Ar thermochronometric analyses to deduce (1) the "fine structure" of the Taconic and Acadian metamorphisms, (2) the presence, nature and extent of Acadian overprint on the earlier Taconic orogen and (3) the ultimate cooling and unroofing history of the terrane. In work primarily done at Virginia Tech (supervision of Tracy), detailed metamorphic reaction histories will be extracted from key samples and quantitative P-T estimates will be made with the maximum accuracy possible. Zoned minerals will be compositionally imaged for major and trace elements for use both in reconstruction of the metamorphic reaction history and also as a guide to selection of appropriate material to sample for radiometric dating. U-Pb dating of carefully chosen metamorphic porphyroblast minerals at SUNY-Stony Brook (supervision of Hanson) will provide key constraints on the timing of growth of these minerals. 40Ar/39Ar geo- and thermochronology will be done by W.E. Hames using various laser-extraction Argon laboratories (primarily that at MIT) to constrain the cooling histories for the entire orogenic belt and primary mineral growth ages for lower-grade (sub-500 degrees C) portions.