Structural, geochemical and temporal relations in the southern Maine migmatite-granite complex, USA

In southern Maine, USA, coexisting granite and migmatite are demonstrated to reflect separate, distinct, episodes of crustal melting. The crystallization of a complex of migmatite and granite at c . 376 Ma contrasts with younger granite plutonism at c . 300 Ma that featured the intrusion of the two-mica granite of the Sebago pluton. Migmatites in this area preserve evidence of both widespread granitic melt loss and local retention of fractionated melt. These rocks record deformation coincident with crystallization and, later at subsolidus conditions in the east, much of which is associated with the Norumbega shear zone system. The Sebago pluton is shown to be far smaller than is indicated on existing maps. Among widespread granite in the complex that surrounds the pluton, some are Permian (Sebago-age, undeformed) and others formed contemporaneously with migmatite, c . 100 Ma earlier (and later deformed). Although geochemically distinct, the differing granites are largely indistinguishable in the field. Radiogenic isotopes point to primary source components from circum-Gondwanan (i.e., Gander) crustal sources and leave little room for essential mantle source involvement for any of the granitic rocks.