LEAD ISOTOPES AS A PROVENANCE TOOL FOR QUARTZ - EXAMPLES FROM PLUTONS AND QUARTZITE, NORTHEASTERN MINNESOTA, USA

The U-Pb isotope system in plutonic quartz samples approximates the crystallization age of the host granitoid, and thus provides a useful provenance tool for detrital quartz grains. We present lead isotopic data and U and Pb concentrations of quartz from three Late Archean granitoids and from the Early Proterozoic Pokegama Quartzite, all in northeastern Minnesota. Lead in granitic quartz is considerably more radiogenic than in its associated whole rock host, but it is similar or only slightly more thorogenic than its host. Pb-Pb ages obtained from quartz whole-rock-leached feldspar from the Late Archean granitic plutons are consistent with existing geochronology, suggesting that the U-Pb system in quartz remained closed since crystallization. U-Pb isotopic data for detrital clear quartz populations in the Pokegama Quartzite record Late Archean ages (2550-2650), consistent with a source from the Superior Province. Lead isotopic compositions of detrital K-feldspar populations and lead and neodymium isotopic compositions of whole rocks samples from the Pokegama Quartzite also support a Superior Province source. This study highlights the potential of the lead isotope system in detrital quartz, K-feldspar, and whole rocks in provenance studies of sandstones.