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 granitoids, comprising peraluminous granites and subordinate metaluminous dioritic rocks (redwitzites) from the Fichtelgebirge (FG) and Northern Oberpfalz (NOP), NE Bavaria. The data, combined with a number of earlier geochromological studies on these granitoids, place severe constraints on the crustal evolution of this region during the Carboniferous and pre-Carboniferous epoch. zites range in ϵNd(T) from −4 to 0 (inclusive data from P.K. Holl and coworkers). The ϵNd(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 ϵNd(T)-values of > −4 whilst the younger granites (315-305 and ∼290 Ma) have ϵNd(T)-values of < −4. The diversity observed in Nd isotopic characteristic is interpreted in terms of different source material. The Nd isotopic compositions of the redwitzites exhibit source heterogeneity suggesting mixing between mantle magma and crust. The Nd isotopic features of the older granites are consistent with the magmatic precursors having been generated by partial melting of pre-existing mature crust variably contaminated by mantle material or, alternatively, by melting of chemically less evolved crust resembling paragneisses of teh ZEV structural unit. The origin of the younger granites can be confidently linked to anatexis of common Moldanubian and Saxothuringian metasediments. Two-stage Nd model ages (TDM) of the granitoids are in the range 1.1–1.7 Ga. These ages provide further support of substantial involvement of pre-Phanerozoic crust in the generation of the granitoids. uchtenberg granite and the G2 and G3 granites yield late Variscan SmNd isochron ages which are concordant with previous RbSr data on these granites. By contrast, the Nd isotopic data for Bärnau and Flossenbürg define pseudo-isochrons suggestive of mixing between contrasting felsic melts.