The Southwest Harbor granite : a field, petrographic and geochemical characterization of the forgotten unit on Mount Desert Island, Maine

Plummer, Benjamin

'The Southwest Harbor Granite (SWHG), is a light pink, fine-grained, two feldspar calc-alkaline muscovite granite of Silurian age. The SWHG lies between the Cadillac Mountain Intrusive Complex (CMIC) and the Cranberry Island Series (CIS) volcanics on Mount Desert Island (MDI) Maine (Gilman et al. 1988). Its field relations indicate that it was emplaced along the same unconformity that controlled the intrusion of the CMIC (Wiebe 1993). Other field evidence suggests that the SWHG originated through multiple pulses of silicic material at two different times. Vugs, granophyre, and the existence of two feldspars offers evidence that the SWHG was formed in a low pressure, high water content environment. In thin section, the SWHG has highly variable textures and mineralogical characteristics. Myrmekite, quartz crystals showing undulatory extinction, and plagioclase zoning have definite aerial distributions that describe how the SWHG has been affected since its initial emplacement. Geochemical data shows a direct correlation among the SWHG, the Somesville Granite (SG), and the Cadillac Mountain Granite (CMG). Models taken from Pearce, Harris, and Tindle, (1984), characterize the SWHG as a calc-alkaline granite associated with the formation of a volcanic island arc.' /STUDY LOCATIONS:/ /MOUNT DESERT ISLAND