Generation, Segregation, Ascent, and Emplacement of Alleghanian Plutons in the Southern Appalachians

The Carboniferous-Permian (Alleghanian) orogeny in the southern Appalachians produced a distinctive 45 m.y.-long intrusive event that formed a NE-trending band of dispersed, composite plutons covering 10,500 km². Of these plutons, 75% are metaluminous biotite or amphibole + biotite granitoids with lower crustal sources, and 20% are peraluminous biotite + muscovite ± garnet or cordierite + biotite granitoids with more evolved crustal sources. The remaining 5% are gabbroids, intermediate rocks, and possible alkaline granitoids. The granitoid to gabbroid + intermediate rock area ratio is 180:1. The exposed intrusive rocks were produced at a relatively slow rate of 2 km³/ linear km/m.y. along the orogen, indicating an inability of the source to produce much melt, inefficient melting mechanisms, or difficulty of magma segregation and ascent. The absence of spatial trends in rock composition and age of emplacement, the small amount but relative abundance of the granitoids, and coincidence with contemporaneous strike-slip faults favor deformation-driven granitoid magma generation, segregation, and ascent. Melting occurred by juxtaposition of high-temperature rocks against lower-temperature melting rocks, decompression, fluxing, or a combination of these as a result of either transpressional arching or faulting. The magmas separated from their source by dilatancy pumping into active shear zones. Energy considerations favor magma ascent by conduit flow in the fault zones with separate magmas pulses channelled into and up the same preheated pathways, resulting in the common occurrence of composite plutons. Emplacement was confined to areas of extension or least compression in faults and folds. Plutons away from shear zones were probably emplaced in their final stages by ductile ballooning. Increasing viscosity with progressive crystallization stopped magma ascent at 8-20 km depths, though the heat of crystallization locally softened the wall rocks and allowed final ductile magma ascent and emplacement in the brittle crust. Segregation, ascent, emplacement, crystallization, and cooling for each magma center was rapid.