Applications of single-grain zircon evaporation analyses to detrital grain studies and age discrimination in igneous suites

Abstract

207Pb 206Pb ages determined by evaporation of single zircon grains from rocks of the New England Appalachians in the USA permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenesis. Tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age, whose U-Pb upper intercept zircon ages obtained through isotope dilution methods show profound post-crystallization discordance, demonstrate that the evaporation method is successful in recovering protolith ages. Uncertainties are ±10–15 My for rocks of Paleozoic age or older. Preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics. 207Pb 206Pb ages of detrital zircon grains from two metasedimentary units from opposite sides of the New England Appalachians reveal the distinct provenances of these rocks. The Missisquoi Formation in southeastern Vermont, USA, is comprised, in part, of detritus derived from the 1.0–1.2 Ga Grenvillian province of Laurentia; whereas this component is conspicuously absent from the Plainfield Formation in easternmost Connecticut. The detrital zircon age data suggest that the Missisquoi Formation accumulated peripheral to the Laurentian craton during the early Paleozoic, whereas the Plainfield Formation detritus was supplied by non-Laurentian, ossibly West African, sources during the Late Proterozoic or early Paleozoic. In southern Vermont, the evaporation method successfully distinguishes between two groups of augen gneisses with ages of ~960 Ma and ~ 1120 Ma, which were first recognized through isotope dilution methods. Collectively, these results indicate that the dating of single zircon grains by the evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed rocks.