Geochemical Characteristics of Metabasites in Different Tectonic Units of the Northeast Bavarian Crystalline Basement

Abstract

Comprehensive geochemical investigations of rnetabasites yielded constraints for a correlation of, or discrimination between the different tectonic units within the northeast Bavarian crystalline basement.The Münchberg nappe pile consists of at least five large tectonic units which exhibit differences in lithology, in part also in metamorphie grade and in metamorphie history. The metabasites in each of these nappes show their own, significant geochemical characteristics. The lowermost tectonic unit, the Bavarian lithofacies, includes the anchimetamorphie Ordovician Randschieferserie which contains alkaline basalts. In their geochemistry, they are sirnilar to the metabasites of the Fichtelgebirge crystalline complex in the autochthonous Saxo-thuringian. The next higher tectonic unit of the Münchberg nappe pile, the Prasinit-Phyllit-Serie contains metabasites which can be derived from subalkaline basalts with a clear calc-alkaline tendency. There is a striking geochemical resemblance to the metabasites of the Erbendorf Greenschist Zone (EGZ) underscorinq the similar lithology of both allochthonous units which appear to be in a similar tectonic position. The Randamphibolit-Serie higher up in the Münchberg nappe pile consists of metabasites with tholeiitic characteristics and a pronounced differentiation trend. The next higher tectonic unit, the Liegendserie of the Münchberg gneiss cornplex s. str., contains metagabbros to metagabbronorites with a high-Al basaltic composition. The amphibolites and banded hornblende gneisses of the overlying Hangendserie are of subalkaline basaltic character with calc-alkaline affinity.The Zone Erbendorf-Vohenstrauss (ZEV) is currently regarded as an allochthonous unit equivalent to the higher crystalline nappes of the Münchberg pile. However, the geochemical character of the metabasites do not encourage such a correlation. Neither the schistose and striped amphibolites nor the flaseramphibolites of the ZEV with their N-KORB and E-MORB character respectively, find convincing counterparts in the crystalline nappes of the Münchberg pile. However, an interestingly close resemblance exists between the schistose and striped amphibolites in the ZEV, on the one hand, and in the autochthonous Zone Tirschenreuth-Mähring (ZTM) and the adjacent Moldanubian sensu strictu, on the other.Owing to the absence of age criteria, our results cannot be used, so far, to reconstruct the paleogeographical position of the individual tectonic units, based on the geochemical characteristics of their respective metabasites.

Monazite in a Variscan mylonitic paragneiss from the Münchberg Metamorphic Complex (NE Bavaria) records Cadomian protolith ages

“…This nappe pile is subdivided from the bottom to the top into the following units: Prasinit-Phyllit, Rand-Amphibolit, Liegendserie, and Hangendserie (e.g. Stettner, 1960a;Kreuzer et al, 1989;Okrusch et al, 1989). The Prasinit-Phyllit unit mainly consists of greenschist facies metavolcanic and metasedimentary rocks.…”

The fundamental Variscan problem: high-temperature metamorphism at different depths and high-pressure metamorphism at different temperatures

“…Okrusch et al 1989). Protolith ages for the eclogite, gabbro and augen-gneiss are around 500 Ma and K-Ar cooling ages from the amphibolite and greenschist facies overprint fall between 380 and 360 Ma (Gebauer & Grfinenfelder 1979;S611ner et al 1981;Kreuzer et al 1989).…”

Early Palaeozoic rift‐related magmatism in Variscan Europe: fragmentation of the Armorican Terrane Assemblage

“…Details of analytical techniques, accuracy and precision can be found in Floyd and Castillo (1992) and Turner et al (1999). This database is also supplemented by previously published analyses of Bohemian Massif metabasites (Okrusch et al, 1989;Patzak et al, 1991;PatocÏ ka et al, 1997). The comparative study is facilitated by use of geochemical data from the Massif Central Pin and Marini, 1993;Briand et al, 1995) and NW Iberian Massif (Peucat et al, 1990;Ribeiro, 1987).…”