Abstract

Hornblende within the Helen and Tallulah Falls nappes of the eastern Blue Ridge and portions of the western Piedmont (higher grade sequences of the Chauga- Walhalla complex and eastern portions of the Six-Mile nappe) record 322-333-m.y. 40Ar/39Ar plateau ages which date post metamorphic cooling through temperatures required for intracrystalline retention of argon. Hornblende within lower-grade portions of the Chauga-Walhalla complex and western portions of the Six-Mile nappe (Alto allochthon) records older plateau ages (322-362 m.y.). None of these release spectra suggests a super-posed metamorphic history, and the plateau ages are interpreted to date regionally diachronous cooling of the allochthon following a Late Devonian or earlier high-grade metamorphism. Muscovite within all structural units of the eastern Blue Ridge and western Piedmont records 302-318-m.y.4OAr/39Ar plateau ages which date post meta-morphic cooling through lower closure temperatures (∼350 °C?). Biotite from these structural units yields 312-348-m.y. 40Ar/39Ar plateau ages. The geologic significance of the biotite ages is uncertain because of variable contamination with extraneous argon.

The Alto allochthon appears to have originated within the western Piedmont following a Late Devonian or earlier high-grade metamorphism which may have developed during initial terrane accretion to North America. Diachronous postmetamorphic cooling of the allochthon likely occurred during its movement to higher crustal levels associated with emplacement over lower-grade rocks of the Chauga-Walhalla complex. Emplacement occurred prior to local development of retrogressive mylonitic fabrics related to tectonic activity within the Brevard fault zone and prior to regional cooling through muscovite closure temperatures.

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