Certain oceans, whose obliteration during Paleozoic time led to climactic deformation of the Appalachian-Caledonian orogenic belt, closed at different times along the belt. Irregularity of the continental margins that collided during Late Silurian, Early Devonian, and Middle Devonian times caused great variations in structural style and igneous and metamorphic associations. The impingement of projections caused intense deformation leading to cryptic sutures and, in places, basement reactivation. Wider zones of less intense deformation are believed to represent embayments in the colliding margins. Late granite and associated high-grade metamorphic aureole rocks, are more common in the narrower, more intensely deformed regions; we suggest that they represent partial melts from lower parts of a thickened continental crust in these areas. “Granite” in embayments, floored by deformed oceanic basement, is suggested to represent the partial melting of that basement and (or) the covering sediment. In both cases, the magmatism reflects depression of crustal material to a depth below a particular critical isotherm and does not necessitate any abnormal heat input from below the crust.

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