Post-Acadian metamorphism in the Appalachians

Abstract

Summary Well-documented post-Acadian metamorphism occurs at three locales along the eastern flank of the Appalachians, where it has affected Avalonian basement and/or Carboniferous cover. Carboniferous rocks of coastal New Brunswick display a greenschist-facies metamorphism of Pennsylvanian to Permian age which largely post-dates associated Alleghanian ductile deformation. In southeastern New England sedimentary rocks of late Pennsylvanian age have undergone an amphibolite-facies Barrovian metamorphic event (275–290Ma) that is syn- to post-ductile deformation and is truncated by peraluminous granite. Considerations of patterns of metamorphism in surrounding crystalline rocks implies that at least two distinct late Palaeozoic tectonothermal events have affected New England. In the Piedmont of Virginia and the Carolinas amphibolite-facies metamorphism (roughly 260–310 Ma) is synchronous with the emplacement of syn-kinematic granitoids. In all three areas isogradic patterns, where determinable, are complex, reflecting the effects of later faulting and fault-controlled retrograde metamorphism. In the USA at least, Alleghanian metamorphism also affected areas well beyond the boundaries of Avalonian terranes sensu stricto into the polydeformed crystalline terrane to the W (Inner Piedmont, Merrimack synclinorium). Within these crystalline rocks the Permian thermal overprint appears to be focussed along shear zones. Late Palaeozoic regional metamorphism in the Appalachians probably reflects crustal thickening related to continent-continent or continent-microcontinent collisions characterized by heat transfer along fundamental tectonic boundaries, and not simply proximity to late Palaeozoic igneous activity or subduction-zone tectonics.