CONSTRAINING A SHRIMP U-PB AGE - MICROSCALE CHARACTERIZATION OF ZIRCONS FROM SAXONIAN ROTLIEGEND RHYOLITES

We present results of a detailed investigation of zircons from two rhy olites from St. Egidien and Chemnitz, Saxony, using a combination of m icroprobe techniques (SHRIMP ion probe, Raman microprobe, SEM: SE, BSE , and CL imaging). These rhyolites belong to the so-called ''lower vol canics'', which is the older of two series of Late Variscan volcanic r ocks occurring in the Saxonian Sub-Erzgebirge basin (Germany). The pur pose of the present contribution is to demonstrate that detailed chara cterization of zircons, as provided by the different micro-techniques, facilitates soundest interpretation of geochronological data. The zir cons (at most 40 to 80 mu m in size) show oscillatory growth zoning, w ith reversely correlated CL and BSE signal intensities. These zircons are interpreted to have grown during crystallization of the rhyolite b ecause, apart from some cracking, they do not appear to have experienc ed any alteration since the time of their growth: The shapes of the zi rcons and their internal structures revealed by CL and BSE imaging app ear to be magmatic, and neither annealing of the accumulated alpha-dec ay damage nor disturbance of the U-Pb system is observed. The SHRIMP i on probe measurements on the zircons gave a Permian (206)pb/U-238 age of 278 +/- 5 Ma (95% confidence). The concordance of this age is suppo rted by the correlation between the low degrees of metamictization (es timated from Raman parameters) and the accumulated alpha fluxes (calcu lated from SHRIMP data). The 278 Ma zircon age is interpreted to repre sent the age of the ''lower rhyolites'' series and, with that, the age of postkinematic Late Variscan volcanism in the Sub-Erzgebirge basin, which has been related to anorogenic extension and uplift as a result of intracontinental rifting. Because of genetic association of rhyoli tes in the Sub-Erzgebirge basin and Li-F granites and lamprophyres in the neighbouring Erzgebirge, the rhyolite age also indirectly contribu tes to the understanding of the geological history of the Erzgebirge. The 278 Ma age is the first absolute dating result for rhyolites from the Saxonian Sub-Erzgebirge basin.