Early Palaeozoic Acid Magmatism in the Saxothuringian Belt: New Insights from a Geochemical and Isotopic Study of Orthogneisses and Metavolcanic Rocks from the Fichtelgebirge, SE Germany

Abstract

rocks range from 316 to 298 Ma for muscovites and from 306 to Orthogneisses and acid metavolcanic rocks from the Fichtelgebirge, 280 Ma for biotites, demonstrating thermal influences from Late NE Bavaria, Germany, are predominantly chemically evolved (monVisean to Late Stephanian (325–290 Ma) granite intrusions. The zo)granites and rhyodacites–dacites, respectively. The metavolcanic involvement of dominantly crustal-derived melts is considered to rocks are variably tectonized and include samples with anomalously account for the peraluminous character (A/CNK > 1·08), high low CaO, Na2O and MnO ( 80 initial Sr/Sr (> 0·709) and negative eNd(500 Ma) (–2·9 to –6·4) wt %) and K2O (>6 wt %) concentrations, implying selective of the intrusive and volcanic rocks. The generation of the melts is element mobility during post-eruptive events. Sm–Nd isotope data assumed to have taken place within an overall extensional geodynamic for samples from three main orthogneiss units (Wunsiedel, Selb, setting. The wider range in eNd(500 Ma) for the metavolcanic rocks Waldershof) yield a composite Sm–Nd whole-rock isochron cor(–3·8 to –6·4; orthogneisses: –2·9 to –4·0) suggests that differential responding to an age of 560 ± 45 Ma. This age estimate is admixture of a pelagic sedimentary component during emplacement constrained, however, to be less than ~560 Ma by the presence of may have occurred. Nd model ages range from 1·5 to 1·7 Ga and detrital Cadomian zircons in the country rocks. Fifteen specimens are consistent with those of granitoids from the Erzgebirge and the from the Wunsiedel orthogneiss give an Rb–Sr whole-rock isochron Lausitz but completely different from those of metavolcanic rocks of 480 ± 4 Ma with an initial Sr/Sr ratio of 0·7095 ± from the Thuringer Wald. In addition, Nd model ages of the Early 0·0007 (MSWD = 2·7). Rb–Sr isotope data from the WalPalaeozoic granitoids in the Fichtelgebirge are consistent with those dershof orthogneiss and the metavolcanic rocks suggest, however, of the Late Carboniferous granitoids from the same area. This that, in general, Sr isotopic equilibrium, if ever reached, was suggests similar source material for the pre-Variscan and the late significantly modified during later events. Taking recent geoVariscan Fichtelgebirge granitoids. chronological literature data into account, it now appears that Early Palaeozoic acid magmatism in the Fichtelgebirge commenced with the intrusion of the orthogneiss precursors during the Early and Mid Ordovician and ended with the eruption of the volcanic