THE GRANITES OF DAMARISCOVE ISLAND, MAINE: A MICROSCALE CHARACTERIZATION

Much research has been done concerning the coastal geology of Maine, particularly the metamorphic terranes resulting from the Taconic and Acadian orogenies. However, very little work has been done on the granitic intrusions spanning the Casco Bay region. Damariscove Island, located off the southwestern coast of Maine (43.45°N, 69.37°W), sits on the southern edge of the transpressional Norumbega Fault Zone. The surrounding Cape Elizabeth amphibolite is host to numerous granitic intrusions which have been correlated with the late Devonian Waldoboro Pluton (368 ± 2 ma) (Hussey and Berry, 2002). Field evidence suggests two separate intrusive phases: an earlier coarse-grained series of intrusions followed by a more prevalent, fine-grained phase possessing a weak magmatic foliation. However, rare instances of coarse-grained granite cross-cutting the fine-grained granite are also observed. A thin-section characterization of samples taken across the island lends additional insight to the processes and relative timing of magma emplacement episodes.

In contrast to field observations, a microscopic view reveals that both coarse-grained and fine-grained granite samples have a seriate texture, with a higher concentration of biotite mica defining the foliation in the fine-grained phase. Grain shape ranges from anhedral to subhedral, with micas comprising the vast majority of the subhedral grains. There is little difference in the amount of deformation exhibited by each phase, suggesting similar timing for the emplacement of two different phases of the same magmatic event. Observed deformation structures include undulose extinction and development of subgrains in quartz, and mechanical twinning in plagioclase feldspar. Along phase contacts, coarse-grained samples also exhibit intercrystalline fracture in feldspars as well as both intra- and intercrystalline fracture in quartz. Based on thin-section evidence, one can infer that the fine-grained granite succeeded the coarse-grained granite, thus substantiating most of the cross-cutting relations observed in the field.