The Acadian thermal history of western Maine

Abstract

ABSTRACT 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 north‐east 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 300° 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‐western Maine is a result of deep‐level contact metamorphism near laterally extensive granitic sills dipping to the north‐east 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.