MID-LATE DEVONIAN PLUTONIC ACTIVITY IN THE BOHEMIAN MASSIF - U-PB ZIRCON ISOTOPIC EVIDENCE FROM THE STARE SEDLO AND MIROTICE GNEISS COMPLEXES, CZECH REPUBLIC

A crystallization age of the plutonic protolith of the gneisses of the Stare Sedlo and Mirotice complexes in the range 380-365 Ma (mid-late Devonian) is indicated by an upper intercept U-Pb zircon age of 373 +/ - 5 Ma for the former and upper concordia intercept and Pb-206/Pb-204 vs. Pb-207/Pb-204 isochron ages of 369 +/- 4 and 375 +/- 5 Ma, respect ively, for zircon populations from both complexes. Oscillatory zoning in many zircons is consistent with igneous crystallization while prima ry crystallized cores and late overgrowths present in a small proporti on of grains are unlikely to have had a significant influence on the U -Pb isotopic system. The data points from both zircon populations grou p within the upper 16% of their chords and at least some of the small Pb-loss is related to fracturing and leaching along cracks, mainly dur ing the ductile deformation of the gneiss protolith.