A PETROGRAPHIC STUDY OF THE SPRING POINT FORMATION, CASCO BAY, MAINE

A systematic petrographic study of the Spring Point formation, an Ordovician-Devonian metavolcanic (Lawrence, this volume), mapped from South Portland to Orr's Island Maine, shows a progression from greenschist to amphibolite facies metamorphism. Greenschist facies Spring Point rocks, from the South Portland area, show a mineral assemblage of Chl + Qtz + Ms ± Pl ± Gt ± Ep. The Spring Point reaches amphibolite facies 50km to the northeast, with an assemblage of Hbl + Qtz + Alm ±- Ms + Pl ± Bt ± Ilm ± Cal ± Zircon. In the amphibolites, garnet porphyroblasts have inclusions of plagioclase and biotite, and some hornblendes are mantled by plagioclase. Based on the minerals observed, a potential reaction for the transition from greenschist to amphibolite facies in the Spring Point formation is the dehydration reaction of chlorite + epidote + quartz=amphibole + plagioclase + H2O.

The Spring Point Formation, part of the Casco Bay Group, includes pelitic, calcareous, and volcanic protoliths. Regional metamorphism, folding, intrusion and faulting events are observed throughout the Casco Bay Group, and are interpreted to be part of the Acadian orogenic event (Hussey 1989). Previous mapping (Hussey 1987) of the pelitic Casco Bay Group formations shows a regional Buchan progression from South Portland to Harpswell.

Based on mineral chemistry and Fe-Mg exchange equilibria, the Spring Point amphibolite from the Cribstone Bridge area, yields metamorphic temperatures of 550-600°C. Sillimanite in nearby pelitic rocks constrains the metamorphic pressure to approximately 4 kbar. The low to intermediate metamorphic pressures and temperatures most likely reflect the influence of heating from plutonism in the Coastal Maine Magmatic Province.