Metamorphic History of the Blue Ridge Province of Tennessee and North Carolina

Abstract

A study of metamorphic index minerals in 354 panned alluvial samples has enabled delineation of metamorphic isograds in the Blue Ridge province of Tennessee and North Carolina. A well-defined axis of maximum metamorphic intensity (sillimanite and kyanite grades) is flanked to the southeast and northwest by lower metamorphic zones. Based on radiometric age determinations, metamorphic isograds are interpreted to have formed during the 320 to 375 m.y. event. In northeastern Tennessee and northwestern North Carolina, the isograds form a retrograde sequence in rocks previously metamorphosed during the Granville orogeny (≅1000 m.y.). In the southeastern Blue Ridge, a classical prograde sequence of the Barrovian type has developed in the Late Precambrian Ocoee Series. Mid-Paleozoic granites and pegmatites are essentially confined to the kyanite and sillimanite zones. The granites are interpreted to have formed by anatexis. Pegmatites may have formed by differentiation of granitic magmas in some areas, and by local anatexis or metamorphic differentiation in others. Ultramafic rocks are also confined mainly to the kyanite and sillimanite zones. These rocks were probably implaced during an early stage of regional metamorphism. The relationships between metamorphic isograds and faults indicate that the major period of movement of the Blue Ridge thrust sheet took place after mid-Paleozoic metamorphism.