40Ar/39Ar age spectra of biotite from Grenville basement gneisses in northwest Georgia

Abstract

Biotite from two samples of Grenville gneisses exposed near Cartersville, Georgia (Corbin and Salem Church units), record undisturbed 40 Ar/ 39 Ar release spectra with total-gas ages of 735 and 702 m.y. (both ± 15 m.y.). The ages are discordant with >1 b.y. Rb-Sr whole-rock and U-Pb zircon ages from the same units. It is suggested that the biotite ages date the time of cooling below temperatures required for argon retention following Grenville metamorphism. Release spectra of biotite from samples along the southeastern border of the gneissic terrain are variably disturbed, reflecting the overprinting of a Paleozoic metamorphic event. The regional distribution of disturbed release spectra permits a more definitive evaluation of the extent of Paleozoic thermal effects in this area. The data also help to resolve late Precambrian–early Paleozoic stratigraphic relations.