MULTIPLE EPISODES OF MAGMATISM, DEFORMATION, AND METAMORPHISM IN THE AVALON TERRANE OF EASTERN MASSACHUSETTS

Crystalline rocks at Black Rock Beach, Cohasset are part of the Avalon terrane of eastern Massachusetts and record a polyphase geologic history. Field and petrographic analysis reveal evidence of two phases of metamorphism and magmatic activity and three episodes of deformation. Lithologies present in the study area include; highly deformed mafic gneiss, Dedham granodiorite, and basaltic intrusions. The earliest phase of metamorphism and deformation are recorded by mafic gneiss which occur as large xenoliths within the more extensive 622 Ma Dedham granodiorite. The mafic gneiss is folded and preserves a mylonitic texture that may be remnants of the Burlington Mylonite Zone. Following crystallization the Dedham granodiorite experienced an episode of plastic deformation. This event resulted in the development of a weak foliation defined by aligned feldspar porphyroclasts. Quartz and feldspar display undulatory extinction and sutured grain boundaries, which suggest deformation occurred at temperatures that exceeded 450°C. A second phase of magmatic activity affected this area and resulted in the intrusion of several 1-2 meter wide, NE and E-W trending, porphyritic basalt dikes. These dikes cut across both the xenoliths and the foliation in the granodiorite and were emplaced during a time of crustal extension and brittle deformation and later experienced hydrothermal alteration. The different orientations of the dikes indicate two episodes of magmatism within a changing stress regime. During the time of emplacement of the dikes, the Dedham granodiorite also experienced brittle deformation and was highly fractured and brecciated. The absolute timing of the events mentioned above is difficult to determine unequivocally due to the lack of precise geochronologic constraints for this area. The crystallization age of the Dedham granodiorite has previously been reported as 622 Ma (Zartman et al., 1984). Thus, the earliest phase of metamorphism indicated by the presence of gneiss xenoliths must pre-date 622 Ma, whereas the plastic deformation preserved in the Dedham granodiorite and the emplacement of the basaltic dikes and brittle deformation must post-date this age.