Early-Palaeozoic Acid Magmatism in the Fichtelgebirge,
NE Bavaria

W. Siebel GeoForschungsZentrum, 14 473 Potsdam, Germany


W. Irber GeoForschungsZentrum, 14 473 Potsdam, Germany

I. Wendt GeoForschungsZentrum, 14 473 Potsdam, Germany

H. Kreuzer Bundesanstalt für Geowissenschaften und Rohstoffe, 30 655 Hannover, Germany

H. Raschka Bundesanstalt für Geowissenschaften und Rohstoffe, 30 655 Hannover, Germany

A. Höhndorf Bundesanstalt für Geowissenschaften und Rohstoffe, 30 655 Hannover, Germany

K.-L. Lenz Glockenholzweg, 29 331 Lachendorf, Germany

Pre-Variscan felsic igneous rocks from the Fichtelgebirge are variously refered to as epigneisses or porphyroid schists. On the basis of textural evidence and local emplacement phenomena, the rocks can be subdivided into an orthogneiss unit and a metavolcanic unit. The orthogneiss precursors were emplaced as discordant bodies at shallow crustal levels into country rocks of Cambrian age. The metavolcanic formations occur in a series of sheets and intermittent outcrops which are concordantly intercalated with upper Cambrian and lower Ordovician strata.

The orthogneisses are dominated by chemically evolved (monzo)granites, the metavolcanic rocks comprise rock types ranging from rhyodacites to dacites. In contrast to
the orthogneisses, most of the metavolcanic rocks are strongly tectonized and show evidence of hydothermal alteration reactions causing depletion in Mn, Ca and Na and enrichment in silica relative to average rhyolite composition.

Sm-Nd data for samples of three investigated orthogneiss bodies yield a composite Sm-Nd whole-rock isochron corresponding to 560 ± 45 Ma (1s). This age estimate, however, is bracketed to less than about 560 Ma by detrital Cadomian zircons in the host rocks (Grauert et al., 1973). The orthogneiss body of Wunsiedel gives a Rb-Sr isochron of 480 ± 4 Ma (ISr = 0.7095 ± 0.0007). Rb-Sr results obtained from the metavolcanic rocks show that Sr isotopic equilibrium, if ever reached, was significantly modified during later events. K-Ar isotope analyses of minerals of the orthogneisses reveal ages ranging from 316-298 Ma for muscovites and from 306-280 Ma for biotites (Kreuzer et al., 1989). The radiometric ages reported here are related to the intrusion of the orthogneiss precursors during early Ordovician times, and - after a regional metamorphic reactivation around the Visean/Namurian boundary - to thermal influences caused by the early Namurian to early Permian pulses of granite intrusions.

The orthogneisses are characterized by a limited variation of eNd(500Ma) values (-2.9 to -4.0), irrespectively of the gneiss type which is consistent with the derivation from a similar source. The metavolcanic rocks, by contrast, show a larger range of eNd(500Ma)values (-3.8 to -6.4) reflecting more diverse source environments.

Two stage Nd model ages are in the range 1.4 -1.5 Ga for the orthogneisses and 1.5 -1.7 Ga for the metavolcanic rocks suggesting that most of the material involved had a long crustal pre-history. Within a larger geological framework, the Nd model ages of both rock types agree with Nd model ages of granitoids from the Lausitz (Kröner et al., 1994) and the Erzgebirge (Kröner et al., 1995) supporting the existence of a common pre-Cadomian basement for these Saxothuringian domains. Besides, Nd model ages likewise agree with those of late-Variscan granites from the Fichtelgebirge (Siebel et al., 1995). This suggests that the pre-metamorphic magmatic evolution generated protoliths similar in Nd composition to those from which the fertile late-Variscan granites were derived.


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