TH-PB VERSUS U-PB ISOTOPE SYSTEMATICS IN ALLANITE - FROM CO-GENETIC RHYOLITE AND GRANODIORITE - IMPLICATIONS FOR GEOCHRONOLOGY

U-Th-Pb isotopic analyses were performed on single allanite crystals f rom a rhyolite of the Atesina volcanic complex (AVC) and a granodiorit e of the Cima d'Asta pluton (CAP) which form the largest calc-alkaline volcano-plutonic association of Late Hercynian age in the South Alpin e domain (northern Italy). The best approximation to the time of empla cement of the AVC and CAP is given by essentially coeval Th-232-Pb-208 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. Al though the AVC/CAP magmatites have been overprinted by a Triassic hydr othermal event, the Th-Pb isotopic system in allanite remained virtual ly undisturbed, whereas the U-Pb systems display variable degrees of o pen-system behaviour. The allanite Th-Pb ages are neither affected by initial radioactive disequilibrium nor by the inheritance of radiogeni c Pb components, which underlines the importance of the Th-Pb isotopic system for accurate dating of Th-enriched minerals. The isotopic resu lts demonstrate that even hydrothermally altered magmatites can succes sfully be dated by this technique.