Ion microprobe U-Pb dating of hydrothermal xenotime from an episyenite: evidence for rift-related reactivation

Hydrothermally intergrown xenotime-monazite from an episyenite in the Bohus granite, southwest Sweden, has been analyzed in situ by ion microprobe. Al though the analysed xenotime crystals are extremely small, sometimes less t han the ion beam diameter (similar to 17 mum), they retain relatively undis turbed U-Pb systems, which yielded a Pb-206/U-238 age of 252 +/- 8 Ma. The evolution of this episyenite involved an initial stage of quartz dissolutio n accompanied by albitization, followed by a second stage of vug-infilling and replacement of igneous Fe-Ti phases. The obtained age shows that the se cond alteration stage, during which xenotime was precipitated, was coeval w ith the thermal anomaly associated with Permo-Carboniferous crustal thinnin g and the development of the nearby Oslo Rift. Moreover, the development of the permeable episyenite structure must be ascribed to a preceding high-te mperature event, most likely the post-magmatic cooling of the host granite at 920 Ma. This study shows that U-Pb dating of young xenotimes may address geochronological problems, though further work is required to characterize possible matrix effects which presently limit the accuracy of the method. (C) 2001 Elsevier Science B.V. All rights reserved.