Rubidium-strontium age determinations of muscovites and biotites from pegmatites of the Blue Ridge and Piedmont

Abstract

Eighteen mica specimens from pegmatites of the Blue Ridge and Piedmont provinces yield Rb/Sr apparent ages of 250 to 510 million years (m.y.). Muscovites from the Piedmont generally appear to be younger (avg. about 285 m.y.) than those from the Blue Ridge (avg. about 315 m.y.). Highly discordant ‘ages’ were found for coexisting biotite and muscovite from two Blue Ridge pegmatites; the biotite ‘ages’ are about 275 m.y. while the muscovites both yield ‘ages’ of 510 m.y. Loss of radiogenic Sr87 from the biotite seems the most likely explanation of the differences; some of this may have been gained by the muscovite, thereby raising its ‘age.’ Some possible causes of discordance seem excluded by this study; these include (a) different crystallization times for biotite and muscovite, (b) preferential loss of rubidium from the muscovite, (c) original Sr87 abundances in the pegmatitic fluids markedly higher than 7.02 per cent, and (d) assimilation by the crystallizing pegmatites of country rock with an anomalously high Sr87 abundance. In general, the study supports the previous suggestion by others that widespread metamorphism and pegmatite emplacement took place in the Blue Ridge in two periods, about 500 and 350 m.y. ago; however, the ‘ages’ found for the eight Piedmont samples range from 255 to 330 m.y. and do not support the idea of a single 250-m.y. event in this region.