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' a nd '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 Lauen stein contains idiomorphic zircons with a mean Pb-207/Pb-206 age of 55 5 +/- 7 Ma as well as xenocrysts ranging in age between similar to 850 and similar to 1910 Ma. Red gneisses from the central Erzgebirge cont ain complex zircon populations, including numerous xenocrysts up to 24 64 Ma in age. The youngest, idiomorphic, zircons in all samples yielde d uniform Pb-207/Pb-206 ages between 550 +/- 9 and 554 +/- 10 Ma. Nd i sotopic data support the interpretation of crustal anatexis for the or igin 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)) valu es from -7.7 to -2.8 (T-DM ages of 1.4-1.8 Ga). The zircon ages docume nt a distinct late Proterozoic phase of granitoid magmatism, similar i n age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism (simil ar to 8 kbar/600-650 degrees C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and re d gneisses were overthrust by units with abundant high-pressure relict s 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 L usatia and the West-Sudetes, and document a pre-Cadomian basement in p arts of east-central Europe that, chronologically, has similarities wi th the Sveco-norwegian domain in the Baltic Shield.