EVIDENCE FROM ZIRCON DATING FOR EXISTENCE OF APPROXIMATELY 2.1 GA OLDCRYSTALLINE BASEMENT IN SOUTHERN BOHEMIA, CZECH REPUBLIC

Zircon ages are reported for three Moldanubian amphibolite grade ortho gneisses from the southern Bohemian Massif obtained by conventional U/ Pb analyses. For two of these orthogneisses, conventional U/Pb data ar e supported by ion microprobe single zircon ages or single grain evapo ration data. The amphibolite grade orthogneisses, occurring in three s mall tectonic lenses within the Varied Group close to the South Bohemi an Main Thrust, are of tonalitic, granodioritic or quartz dioritic com position. Conventional bulk size fraction and ion microprobe analyses of nearly euhedral zircons from a metatonalite, erroneously interprete d as a metagreywacke in a previous study, yielded an upper Concordia i ntercept age of 2048 +/- 12 Ma. The well preserved euhedral grain shap es of the zircons suggest crystallization from a magmatic phase, and t he upper Concordia intercept age is now interpreted as reflecting a ma gmatic event at that time. The age of this rock is compatible with the conventional zircon data and the (Pb-207/Pb-206) single grain evapor ation result from two further orthogneisses providing intrusion ages o f 2060 +/- 12, 2104 +/- 1 and 2061 +/- 6 Ma, respectively. For one sam ple a concordant U/Pb age for sphene of 355 +/- 2 Ma defines the age o f amphibolite facies metamorphism. The upper Concordia intercept ages of three orthogneisses constitute the first direct evidence for the pr esence of early Proterozoic crust under the supracrustal cover in the southern part of the Bohemian Massif.