LATEST PRECAMBRIAN (CADOMIAN) ZIRCON AGES, ND ISOTOPIC SYSTEMATICS AND P-T EVOLUTION OF GRANITOID ORTHOGNEISSES OF THE ERZGEBIRGE, SAXONY AND CZECH-REPUBLIC

Single zircons from two orthogneiss com complexes, the 'Grey Gneiss' and 'Red Gneiss', the lower-most tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a Pb-207/Pb-206 emplacement age of 550 +/- 7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean Pb-207/Pb-206 age of 555 +/- 7 Ma as well as xenocrysts ranging in age between similar to 850 and similar to 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform Pb-207/Pb-206 ages between 550 +/- 9 and 554 +/- 10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. epsilon(Nd(t)) values for the grey gneisses are -7.5 and -6.0 respectively, (mean crustal residence ages of similar to 1.7-1.8 Ga). The red gneisses have a wider range in epsilon(Nd(t)) values from -7.7 to -2.8 (T-DM ages of 1.4-1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism (similar to 8 kbar/600-650 degrees C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the;range 1000-1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveco-norwegian domain in the Baltic Shield.