Polymetamorphism in medium- to high-grade pelitic metamorphic rocks, west-central Maine
A regional petrologic study of pelitic metamorphic rocks has been undertaken, with emphasis on the area bounded by the cities of Augusta, Lewiston, Norway, Rangeley, Kingfield, and Madison. This report involves a regional isograd map, interpretation of the nature of the various reactions, and discussion of petrologic problems of polymetamorphism, isograd distribution, and widespread occurrence of muscovite in the K-feldspar–sillimanite zone. A future contribution will deal more specifically with mineral chemistry, geotherrnometry, and geobarometry of much of the area. After a low-grade metamorphism, M 1 , the region suffered a higher-grade early metamorphism, M 2 , which in many places produced andalusite and cordierite as well as staurolite. This was followed by a somewhat higher-pressure metamorphism, M 3 , involving staurolite-chlorite and sillimanite. In some areas, M 3 has produced extensive retrograde effects. With increasing grade in MS, the major mineral reactions are (1) formation of staurolite from reaction of garnet, chlorite, and muscovite; (2) formation of sillimanite from staurolite, chlorite, and muscovite; (3) reaction of staurolite to form sillimanite, biotite, and in many rocks, garnet; and (4) formation of K-feldspar from the partial reaction of muscovite, quartz, and plagioclase. A possible fourth metamorphism, M 4 , produced narrow aureoles with andalusite and staurolite around the three latest plutons. Analysis of the isograd pattern demonstrates that the pressure was rather uniform throughout the area at a given time, but increased about 0.5 kbar or more between M 2 and M 3 metamorphism. The more regional distribution of isograds to the south is consistent with higher initial temperatures and more closely spaced magmatic heat sources, but not necessarily with increasing pressure to the south. The pressure increase between M 2 and M 3 might have been produced by intrusion of magma above the present level and/or extrusion of rhyolitic volcanics.