U-PB GEOCHRONOLOGY OF GRENVILLE ROCKS IN THE EASTERN U.S.: DECIPHERING COMPLEX MULTI-AGE ZIRCONS

The Grenville province in the Appalachians of the eastern United States is composed of upper amphibolite to granulite grade Mesoproterozoic rocks (ca. 1.0-1.4 Ga). Orthogneisses of both volcanic and plutonic origin generally contain abundant zircon and frequently also contain monazite or sphene. These U-bearing minerals can be used to develop a temporal framework for igneous and metamorphic events in conjunction with detailed geologic maps. Unfortunately, the high grade of metamorphism(s) and the refractory character of the minerals often results in the occurrence of multi-age components in individual grains. The development of the sensitive high resolution ion microprobe (SHRIMP) enables in situ analysis of small (~25 micron) areas on individual grains. Cathodoluminescence (CL) and backscatter imaging of mounted half-grains, together with reflected and transmitted light photomicrography, aid in identification of homogeneous zones for SHRIMP analysis. Although relatively large SHRIMP errors sometimes make it difficult to distinguish distinct events that are separated by only a few million years, SHRIMP data provide insights for complex grains that are may not be obtainable through conventional analyses. Case histories from high grade Grenville rocks highlight the problems presented by zircons which contain multi-age components. Interpretations of ages of protoliths for these gneisses are determined utilizing external morphology, characterization of internal features such as cracks and inclusions, CL zoning patterns, identification of inherited components, identification of metamorphic components, and statistical analysis of ages from different zones. Independently derived geologic relationships (lithology, structure, metamorphic grade) gained from detailed geologic mapping and geochemistry are used to develop internally consistent geochronologic interpretations for broad regions. Once complex age relations within individual grains are deciphered, then petrologic and tectonic models can be developed, perhaps for the entire Grenville province.