Chronology of major metamorphic events in the southeastern United States [Appalachians]

Abstract

K-Ar and Rb-Sr age measurements have been made on a variety of granites, pegmatites, gneisses, and schists which comprise the plutonic-metamorphic complex of the Piedmont and Blue Ridge of the southeastern United States. Large portions of the area appear to have been metamorphosed initially approximately at the same time as the Grenville province, i. e., about 900 to 1,100 million years ago. Superimposed on this older metamorphic province was a major orogenic event culminating at about 350 million years with widespread recrystallization of existing rocks and intrusion of pegmatites in the Spruce Pine, Franklin-Sylva and Bryson City districts, and granites in the Virginia and North Carolina piedmont. There is strong evidence of an additional metamorphic epoch between 350 and 1,000 million years, but its effects have been largely obliterated by the 350 million years event. In western North Carolina a transition of apparent ages from 355 to 890 million years occurs in the same rock unit (Cranberry gneiss) over a distance of about 10 mi. across the strike of the border of the 350 million years event. In the southeastern Piedmont of Georgia and South Carolina, a younger metamorphic event or events can be detected producing rocks of apparent age ranging from 230 to 310 million years. The time of these orogenies is compared with those in the central and northern Appalachians. Evidence is accumulating that the Holmes' time scale will have to be considerably lengthened.