ORIGIN OF LATE VARISCAN GRANITOIDS FROM NE BAVARIA, GERMANY, EXEMPLIFIED BY REE AND ND ISOTOPE SYSTEMATICS

Sm-Nd isotopic results are presented for late-and post-tectonic granit oids, comprising peraluminous granites and subordinate metaluminous di oritic rocks (redwitzites) from the Fichtelgebirge (FG) and Northern O berpfalz (NOP), NE Bavaria. The data, combined with a number of earlie r, geochronological studies on these granitoids, place severe constrai nts on the crustal evolution of this region during the Carboniferous a nd pre-Carboniferous epoch. Redwitzites range in epsilon(Nd(T)) from - 4 to 0 (inclusive data from P.K. Holl and coworkers). The epsilon(Nd(T ))-values of the granites s.s. are restricted to overlapping ranges of -8 to -3 (FG) and -8 to -2 (NOP). In both domains, the older granites (330-325 Ma) are characterized by epsilon(Nd(T))-values of > -4 whils t the younger granites (315-305 and similar to 290 Ma) have epsilon(Nd (T))-values of < -4. The diversity observed in Nd isotopic characteris tics is interpreted in terms of different source material. The Nd isot opic compositions of the redwitzites exhibit source heterogeneity sugg esting mixing between mantle magma and crust. The Nd isotopic features of the older granites are consistent with the magmatic precursors hav ing been generated by partial melting of pre-existing mature crust var iably contaminated by mantle material or, alternatively, by melting of chemically less evolved crust resembling paragneisses of the ZEV stru ctural unit. The origin of the younger granites can be confidently lin ked to anatexis of common Moldanubian and Saxothuringian metasediments . Two-stage Nd model ages (T-DM) of the granitoids are in the range 1. 1-1.7 Ga. These ages provide further support of substantial involvemen t of pre-Phanerozoic crust in the generation of the granitoids. The Le uchtenberg granite and the G2 and G3 granites yield late Variscan Sm-N d isochron ages which are concordant with previous Rb-Sr data on these granites. By contrast, the Nd isotopic data for Barnau and Flossenbur g define pseudo-isochrons suggestive of mixing between contrasting fel sic melts.