Th [sbnd]Pb versus U [sbnd]Pb isotope systematics in allanite from co-genetic rhyolite and granodiorite: Implications for geochronology

Abstract

U Th Pb isotopic analyses were performed on single allanite crystals from a rhyolite of the Atesina volcanic complex (AVC) and a granodiorite of the Cima d'Asta pluton (CAP) which form the largest calc-alkaline volcano-plutonic association of Late Hercynian age in the South Alpine domain (northern Italy). The best approximation to the time of emplacement of the AVC and CAP is given by essentially coeval 232Th 208Pb ages of 276.3 ± 2.2 Ma (95% confidence limit; n = 2) and 275.5 ± 1.5 Ma ( n = 4) for AVC rhyolite and CAP granodiorite, respectively. Although the AVC/CAP magmatites have been overprinted by a Triassic hydrothermal event, the Th Pb isotopic system in allanite remained virtually undisturbed, whereas the U Pb systems display variable degrees of open-system behaviour. The allanite Th Pb ages are neither affected by initial radioactive disequilibrium nor by the inheritance of radiogenic Pb components, which underlines the importance of the Th Pb isotopic system for accurate dating of Th-enriched minerals. The isotopic results demonstrate that even hydrothermally altered magmatites can successfully be dated by this technique.