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Following the Middle Devonian Acadian deformation an extensive belt of high grade metamorphism was formed in New England. In south-western Maine, at the northern end of this belt, there occurs a transition along the strike from regional low-pressure/high-temperature metamorphism to contact metamorphism in low-grade rocks. Petrological studies indicate that this transition occurs along a surface plunging to the northeast at about 3.5". with respect to the Middle-to-Late Devonian erosion surface. In addition, detailed petrological mapping has defined a history of temporally separate, localized metamorphic events associated with plutonism and occurring at increasingly deeper levels to the south-west. Geochronological studies constrain ambient temperatures in the transition zone at the time of metamorphism to be less than u ) ( p C in the north-east and between 350" C and 500" C in the south-west.They also establish a pattern of diachronous cooling due to differential uplift and erosion, with cooling occurring later and most rapidly to the south-west. Geophysical evidence suggests that along with this spatial variation in metamorphic style the shapes of the plutons in Maine undergo a transition from laterally extensive sheet-like bodies in the high grade terrane to more equant-shaped bodies in the low-grade terrane. Using the results of these petrological, geochronological and geophysical studies, as well as those of stratigraphical and structural studies we construct a thermal model for the transition zone. The model suggests that the Acadian metamorphism in south-westem Maine is a result of deeplevel contact metamorphism near laterally extensive granitic sills dipping to the northeast with respect to the present erosion surface. The plutons themselves are interpreted to be a result of lower crustal melting in response to crustal thickening in the presence of normal or slightly augmented mantle heat flux. A thermodynamic study of the pseudobinary join muscovite-paragonite in the system KAISi,0,-AIz0,-Si0z-H20, American Mineralogist, 60, 985-993. Chatterjee, N. D. & Johannes, W. 1974. Thermal stability and standard thermodynamic properties of synthetic 2M muscovite, KAI,(AISi,O,,(OH)z. Confributions to Mineralogy and Petrology, 48, 89-114. 66, 151-165. 381-400. 278,769-797.

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