The Acadian thermal history of western Maine

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 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.

Ammonium loss and nitrogen isotopic fractionation in biotite as a function of metamorphic grade in metapelites from western Maine, USA

“…The metamorphic history of these metapelites includes initial greenschist facies M1 metamorphism of Ordovician to lower Devonian basinal sediments at slightly before 400 Ma followed by D1 passive flow folding which resulted in northeast-trending tight folds and the development of a penetrative slaty-cleavage over a broad region of Maine (Guidotti, 1970(Guidotti, , 1974(Guidotti, , 1985(Guidotti, , 1989Guidotti et al, 1975;Holdaway et al, 1982Holdaway et al, , 1988DeYoreo et al, 1989). M2 metamorphism was initiated by the intrusion at $400 Ma of granitoid plutons at a pressure of 300 MPa, which outlasted contemporaneous D2 late-slip cleavage and resulted in the formation of characteristic andalusite-staurolite-biotite assemblages.…”

Paleozoic orogens in New England, USA

“…Not very much, nor have we learned much about plate interactions based on the detailed study of shear sense within the zone in Massachusetts. In Maine, where the Late Pennsylvanian effects are mainly thermal with apparently little accompanying deformation, thermal models (De Yoreo et al 1989a, 1989b can account for the advection of magmas in the Pennsylvanian as a consequence of conductive heating of the lower crust following Acadian crustal thickening. But with a growing body of data indicating that other areas of New England were geologically active at the same time, it is tempting to think of these as being early effects of the collision with Africa as discussed by Mosher et al (1993) and Cogswell & Mosher (1994).…”

Guidebook for Field Trips in Western Maine and Northern New Hampshire

“…This model may also explain the variations in metamorphic P-T paths observed along strike in the Central Maine Terrane. In Maine, P-T paths show isobaric heating DeYoreo et al, 1989), while in central New Hampshire, P-T paths show increasing pressure during heating, and in Massachusetts, P-T paths are "counterclockwise" loops showing continued increasing pressure during cooling (Tracy & Robinson, 1980;Robinson et al, 1989). These P-T paths can be explained by the transfer of heat and mass by pluton migration upward through the crust during metamorphism.…”