HIGH-GRADE EXTRUSION TO LOW-GRADE EXTENSION AS MECHANISMS OF LATE PALEOZOIC TERRANE MODIFICATION IN THE SOUTHERN BRONSON HILL TERRANE, NEW ENGLAND, U.S.A

Deformation and high-grade metamorphism in the Bronson Hill terrane (BHT) of CT and MA have long been attributed to Acadian (420-380) and Neo-Acadian (380-360 Ma) orogenesis based largely on correlations with lower-grade equivalents to the north in NH and ME. However, increasingly numerous, detailed, geochronologic studies within and around the BHT in MA and CT constrain high- to low-grade metamorphism, and deformation to 305-260 Ma. Reconnaissance fieldwork in the BHT from central CT to northern MA, petrography, and quartz CPO, along with published studies in south-central New England indicate northward, orogen-parallel extrusion of two orthogneisses. The Monson gneiss records flattening strains near Bolton, CT progressing to constriction to the north; high-strain zones are only recognized in surrounding cover units. The Bonemill/Conant Brook shear zone bounds the eastern contact of the Monson and suggests progressive deformation (sillimanite-grade) from west-side-up to dextral; near Bolton, sinistral shearing has been reported along the western contact. Bounding high-strain zones for the Glastonbury gneiss have not yet been identified, but quartz CPO of the orthogneiss and overlying cover units record constriction and plane strain shearing, respectively. Metamorphic grade increases from epidote-amphibolite facies in central CT to garnet-amphibolite facies in southern MA. In central MA near the Glastonbury-Belchertown complex contact, structural relationships, high-strain zones, and associated quartz CPO indicate underplating and envelopment of the Belchertown by the Glastonbury. This high-grade deformation has been overprinted by a distinct, lower-greenschist grade extensional event, and is recognized throughout the BHT. Within the same structural position on the western boundary of the Monson gneiss, low-grade shearing has been recognized locally in central MA and CT; it isn't clear if these are correlatives along a major zone of extension, or localized deformation. Observations throughout the BHT of both the Monson and Glastonbury gneisses are consistent with northward extrusion of the orthogneisses, and associated metamorphic mineral assemblages indicate that rocks were at sufficient temperatures for ductile flow; geochronology constrains deformation to the late Paleozoic.