ALLEGHANIAN MAGMATISM AND METAMORPHISM IN THE SOUTHERN APPALACHIAN OROGEN: RECORD OF TRANSPRESSIONAL ASSEMBLY OF PANGEA IN SOUTHERN LAURENTIA

The final magmatic expression of the assembly of central Pangea is a suite of late syn- to post-tectonic granitoids emplaced in the core of the Alleghanian-Variscan Orogen. U-Pb ages from Mississippian to Permian granitoids in the southern Appalachian orogen (SAO) yield ages of c. 323-335 in the Inner Piedmont and c. 303-293 Ma along the boundary between the Inner and Central Piedmont and accreted Gondwanan terranes. Trace elemental and isotopic data of whole rocks along with ages and Hf-isotopic compositions of zircon xenocrysts indicate derivation from older continental crust with some mantle input. Thermochronologic data reveal peak metamorphic temperatures at or just before intrusion of granitoids, and relatively rapid cooling after the c. 303-293 Ma pulse of magmatism in most areas. The inner belt (c. 323-335 Ma) plutons cooled though intermediate temperatures during intrusion of the outer belt (303-290 Ma). The younger group of plutons cooled rapidly during transtension and initial rifting of the South Georgia basin in Mesozoic time. A similar age progression of 330 to 290 Ma magmatism continues into the Variscian Orogen after oroclinal bends are restored (Johnston et al). Intrusion of Alleghanian plutons followed orogenic pulses – ie., transpressional emplacement of Carolinia and Suwanee, and the Alleghanian megathrust. Crustal anatexis was likely associated with deeper crustal dehydration reactions during crustal thickening and incremental mantle delamination (delamination cycling of DeCelles et al). Systematic relationships with strike slip faults suggest that deformation contributed to upward migration of magma and shallow emplacement. The lack of subduction signatures suggests that the accretion of Gondwana terranes to the SOA occurred highly obliquely with Rheic Ocean subduction along terranes west of those now along the northern Gulf of Mexico.