Abstract

Rb-Sr whole-rock radiometric age determinations were made on Precambrian basement rocks from the Blue Ridge province of northern North Carolina and adjacent areas of Tennessee and Virginia. Parts of the Cranberry Gneiss in North Carolina and the Grayson Gneiss in Virginia have ages of 1,252 ± 45 m.y. and 1,174 ± 14 m.y., respectively. Thus far, these are the oldest rocks found in the southern Appalachians, and their ages suggest that they are coeval with much of the basement of the stable U.S. interior. The Wilson Creek Gneiss of North Carolina yields model ages averaging 1,135 m.y. The Blowing Rock Gneiss of North Carolina and the Cranberry Gneiss in North Carolina and Tennessee have ages of 1,027 ± 36 m.y. and 1,063 ± 41 m.y., respectively. A third Cranberry Gneiss location, in Tennessee, may be only 871 ± 17 m.y. old.

A radiogenic Sr87 growth curve for these gneisses is compatible with a model in which the Blue Ridge crust of the area studied has behaved as a closed Rb-Sr system for much of its Precambrian history. The 1,300- to 1,200-m.y.-old terrane was largely remobilized by a major orogenic episode about 1,050 m.y. ago, which suggests synchrony with the Grenville orogeny. The approximately 1,050-m.y.-old gneisses are products of anatexis. There is no evidence for the addition of new material to the Blue Ridge crust between 1,300 and 700 m.y. ago.

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