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A PETROGRAPHIC STUDY OF THE SPRING POINT FORMATION, CASCO BAY, MAINE MEYER, Edward Eric, Geology, Bowdoin College, 786 Smith Union, Brunswick, ME 04011, [email protected] and BEANE, Rachel, Geology, Bowdoin College, 6400 College Station, Brunswick, ME 04011 A systematic petrographic study of the Spring Point formation, an OrdovicianDevonian 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 550600°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. Northeastern Section - 36th Annual Meeting (March 12-14, 2001) © Copyright 2001 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions. Meyer, E. E.* and Beane, R. J., 2001. A petrographic study of the Spring Point Formation, Casco Bay, Maine. Abstracts with Programs, Geological Society of America, v. 33, p. A-10.
