Open Session

Michael Hanel1

(fax: +49 761 2036407)

Michael Montenari2

(monte@perm.geolgie.uni-freiburg.de)

Hans-Joachim Lippolt3

(hans.lippolt@urz.uni-heidelberg.de)

1 Institut fuer Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universitaet Freiburg i. Br., D-79104 Freiburg i. Br., Germany

2 Geologisches Institut, Albert-Ludwigs-Universitaet Freiburg i. Br., D-79104 Freiburg i. Br., Germany

3 Laboratorium fuer Geochronologie, Ruprecht-Karls-Universitaet Heidelberg, D-69120 Heidelberg, Germany

High-grade paragneisses are widespread in the Central Schwarzwald Gneiss Complex (CSGC) which is a part of the central crystalline region of the Variscan fold belt (Moldanubian Zone). The protoliths of the paragneisses are generally believed to be Precambrian in age. This is however difficult to prove because the intense Variscan deformation and metamorphism usually obliterated the very early stages of the geological record. In amphibolite facies paragneisses from the nothern CSGC we recently detected mircofossils in thin sections and in maceration residues. The samples investigated are drill cores from the geothermal research drilling Buehl-1 which penetrated gneisses of the nothern CSGC to a final deph of about 2700 m after crossing the Rhinegraben master fault. The determinable microfossils are chitinozoans and minor acritarchs. Although flattened and altered to graphite the morphology of the chitinozoans remain distinct enough to allow an identification down to the level of genus (e.g. Conochitina sp.). The chitinozoan spectrum found in in the Buehl-1 gneisses points to an Ordovician sedimentation age of the gneiss protoliths and indicates the former existence of an Early Paleozoic marine basin of still unknown dimensions in the CSGC. On the other hand the recent descovery of acritarchs (e.g. Melanocyrillium sp.) in high-grade kinzigites from the central part of the CSGC (Montenari 1996) confirms also the existence of Late Proterozoic sedimentary units. Another feature of the Buehl-1 paragneisses is a late-stage mylonitization at greenschist facies P-T conditions. The tectonic overprint could be due to the Rhinegraben formation or alternativly being caused by a Variscan system of N 35o trending sinistral shear zones which are known to shift the Schwarzwald units relative to the Vosges about 30 to 40 km to the north (Edel & Weber, 1995). A 40Ar/39Ar age of 309+- 2 Ma (1-sigma) of newly formed muscovite from a mylonitic paragneiss is in line with an Upper Carboniferous (Westphalian) age of the mylonitization and dates presumably the sinistral movements in this part of the Schwarzwald.