PETROLOGIC, STRUCTURAL, AND GEOCHRONOLOGICAL CHARACTERISTICS OF THE ACADIAN METAMORPHIC OVERPRINT ON THE TACONIDE ZONE IN PART OF SOUTHWESTERN NEW-ENGLAND

A combined petrologic, structural, and geochronologic approach has been used to characterize the polymetamorphic area of northwestern Connecticut and adjacent New York and Massachusetts. This area records a Taconian (Ordovician) peak metamorphic event in the west and Acadian (Devonian) peak effects in the east. Ar-40/Ar-39 data presented here indicate that the timing of the Taconian thermal maximum in the staurolite zone was about 445 Ma and that of the Acadian thermal maximum was about 390 to 400 Ma. A complicated zone of overlap between these areas contains predominantly Taconian mineral assemblages thermally and mechanically affected by Acadian metamorphism. Within the Taconian zone, the rim compositions of the highest-grade porphyroblasts yield P-T estimates that are inconsistent for different subareas within a given thin section. Samples that experienced superimposed Acadian metamorphic conditions sufficient to produce fibrolitic sillimanite (about 500-degrees-C at about 4 kb) have porphyroblasts with rim compositions that provide more consistent P-T estimates, varying by only a maximum of 50-degrees-C and 1 kb, despite significant textural complexity within a thin section. These results are interpreted to indicate that the rims of preexisting Taconian porphyroblasts were in equilibrium during Acadian metamorphism at temperatures of about 500-degrees-C and above, whereas reequilibration was incomplete at lower overprinting temperatures. The thermobarometric results and porphyroblast textures demarcate a zone where Acadian overprinting was in excess of 500-degrees-C.