Lead isotopes in feldspars from selected granitic rocks associated with regional metamorphism

Abstract

Paleozoic regional metamorphism of Precambrian gneiss near Baltimore, Maryland, altered the lead-isotope ratios in Precambrian feldspars but did not affect the Rb-Sr ages. This fact indicates that the alteration took place without solution and redeposition or melting of the feldspars. The metamorphism of amphibolite facies (kyanite zone) undoubtedly caused the observed differences in the lead-isotope ratios. The Paleozoic metamorphism was of lower grade to the west and had little effect on the isotopic composition of lead in feldspars of the pyroxene-bearing granitic rocks underlying Shenandoah National Park, Virginia. The feldspars of these rocks contain lead of uniform isotopic composition that is similar to those of younger granites of nearly equivalent age (about 1000 m.y.) near Balmat, New York. The isotopic composition of lead in the Paleozoic granitic rocks and pegmatites near Baltimore, Maryland, is less uniform than in the 1000-m.y.-old rocks at Shenandoah National Park. Age data and geologic relations suggest thermal effects sufficient to lower biotite ages by 50 to 150-m.y. This heating plus the fact that much older rocks (Precambrian) underlie the region, may account for the lack of a unique ‘Appalachian lead’ through natural contamination. Isotopic uniformity of leads in the feldspars from the Vermilion granite, Minnesota, is comparable to that of the 1000-m.y.-old granitic rocks at Balmat and Shenandoah National Park. Model lead ages are about 2900 m.y., or approximately 350 m.y. greater than biotite K-Ar ages. This relationship is in contrast to the Paleozoic and younger Precambrian granitic rocks of Maryland, Virginia, and New York, for which the model lead ages are less than the biotite K-Ar ages.