207/206Pb Single Zircon Evaporation Ages from Ordovician Sections of the Saxo-Thuringian Zone (Germany)

Michael Gehmlich TU Bergakademie Freiberg, Mineralog. Inst., Brennhausgasse 14, Freiberg, D-09596, Germany

Marion Tichomirowa TU Bergakademie Freiberg, Mineralog. Inst., Brennhausgasse 14, Freiberg, D-09596, Germany

Ulf Linnemann Museum für Mineralogie und Geologie Dresden, Augustusstr. 2, Dresden, D-01067, Germany

Harald Lützner Friedrich-Schiller-Universität Jena, Inst. f. Geowissenschaften, Burgweg 11, Jena, D-07749, Germany

The general problem of the lowermost parts of the Ordovician siliciclastic sections within the Saxo-Thuringian zone is the rarity of biostratigraphic indicators. Sponsored by a project of the Deutsche Forschungsgemeinschaft we use new 207/206Pb single zircon evaporation ages (after Kober, 1986, 1987) of one granite and acid subvolcanic to volcanic rocks to establish a better dating in the Early Palaeozoic rocks of the Schwarzburg anticline (Thüringisches Schiefergebirge) and the Elbe zone (Sachsen). Both sections can be correlated by use of sequence stratigraphy (Fig. 1). In the profile a number of sea level fluctuations is recorded. The top of both sections is the glaciomarine diamictite of the Ashgillian Sahara glaciation (Lederschiefer) (Fig. 1). Key fossils are only known from the upper part of the Ordovician in the Schwarzburg anticline. In this area as well as in the Elbe zone within the lower part of the profiles acid to intermediate rocks with hercynian overprint are distributed nearly in the same stratigraphic level (Fig. 1). The 207/206Pb single zircon evaporation ages of these rocks show the following ages:

Schwarzburg anticline: rhyolite at the base of the "Konglomeratische Arkose", i.e. the first Palaeozoic sediment: 480±15 My (n = 6 zircons); "Bärentiegel-Porphyroid" = rhyolithe: 471±9 My (n = 5 zircons).

Elbe zone: tourmaline granite: 471±13 My (n = 5 zircons); "Sericitgneis" = meta-rhyolite: 481 ± 4 (n = 4 zircons). We conclude, that during the Arenig (490-470 My) a magmatic event took place. The sedimentary rocks intruded by the acid igneous rocks were deposited during the Earliest Ordovician (Tremadocian), because in these sediments only two sequences (transgressive systems tract, highstand systems tract, regressive systems tract) are preserved, before the first Arenigian key fossil occurs. Very probable the hole Cambrian in the Schwarzburg anticline is not developed, because below the rhyolites (with exeption of the rhyolite at the base of the "Konglomeratische Arkose"), only two complete sequences overlap the Neoproterozoic basement. This is not enought to fill the 35 million years, that are required for the Cambrian.

Fig. 1.