Volume 38: January - December 1997

Issue 2: February 1997

Abstract

Early palaeozoic acid magmatism in the saxothuringian belt: new insights form a geochemical and isotopic study of orthogneisses and metavolcanic rocks from the Fichtelgebirge, SE Germany

W. Siebel, H. Raschka², W. Irber¹, H. Kreuzer³, K-L. Lenz⁴, A. Hohndorf² and I. Wendt¹ ¹Geoforschungszentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany, ²Bundesanstalt fur Geowissenschaften und Rohstoffe, Stilleweg 2, 30 655 Hannover, Germany, ³Hamsunstrasse 19, 30 655 Hannover, Germany, ⁴Glockenholzweg, 29 331 Lachendorf, Germany and Corresponding author Email: siebel@gfz-potsdam.de
ABSTRACT
Orthogneisses and acid rocks from the Fichtelgebirge, NE Bavaria, Germany, are predominantly chemically evolved (monzo)granites and rhyodacites-dacites, respectively. The metavolcanic rocks are variably tectonized and include samples with anomalously low CaO, Na2O and MnO (<0.01 wt%) and high SiO2 (>80 wt%) and K2O (>6 wt%) concentration, implying selective element mobility during post-eruptive events. Sm-Nd isotope data for samples from three main orthogneiss units (Wunsiedel, Selb, Waldershof) yield a composite Sm-Nd whole-rock isochron corresponding to an age of 560 +-45 Ma. This age estimate is constrained, however, to be less than 560 Ma by the presence of detrital Cadomian zircons in the country rocks. Fifteen specimens from the Wunsiedel orthogneiss give an Rb-Sr whole-rock isochron of 480 +- 4 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7095 +- 0.0007 (MSWD = 2.7). Rb-Sr isotope data from the Waldershof orthogneiss and the metavolcanic rocks suggest, however, that, in general, Sr isotopic equilibrium, if ever reached, was significantly modified during later events. Taking recent geochronological 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 successions during the Late Ordovician. K-Ar ages of the investigated rocks range from 316 to 298 Ma for muscovites and from 306 to 280 Ma for biotites, demonstrating thermal influences form Late Visean to Late Stephanian (325-290 Ma) granite intrusions. The involvement of dominantly crustal-derived melts is considered to account for the peraluminous character (A/CNK >1.08), high initial ⁸⁷Sr/⁸⁶Sr (>0.709) and negative [epsilon]Nd(500 Ma) (-2.9 to 6.4) of the intrusive and volcanic rocks. The generation of the melts is assumed to have taken place within an overall extensional geodynamic setting. The wider range in [epsilon]Nd(500 Ma) for the metavolcanic rocks (-3.8 to -6.4; orthogneisses: -2.9 to -4.0) suggests that differential admixture of a pelagic sedimentary component during emplacement may have occurred. Nd model ages range form 1.5 to 1.7 Ga and are consistent with those of granitoids form the Erzgebirge and the Lausitz but completely different from those of metavolcanic rocks from the Thuringer Wald. In addition, Nd model ages of the Early Palaeozoic granitoids in the Fichtelgebirge are consistent with those of the Late Carboniferous granitoids from the same area. This suggests similar source material for the pre-Variscan and the alte Variscan Fichtelgebirge granitoids.

Keywords: Fichtelgebirge; geochemistry; orthogneisses; metavolcanic rocks; radiogenic isotopes; Saxothuringian belt

Pages: 203 - 230

Part of the OUP Journal of Petrology WWW service

General Information

Click here to register with OUP.

This page is maintained by OUP admin

Last updated 06 Mar 97

Part of the OUP Journals World Wide Web service.