U-Pb SHRIMP and Nd isotopic data from the western Bohemian Massif (Bayerischer Wald, Germany): Implications for Upper Vendian and Lower Ordovician magmatism

Abstract

U-Pb SHRIMP dating of zircons of metamagmatites from the Bayerischer Wald (Germany) reveals a complex evolution of this section of the Moldanubian Zone exposed in the western Bohemian Massif of the central European Variscan belt. In the south-western part of the Bayerischer Wald Upper Vendian magmatism is constrained by pooled 206 Pb/ 238 U mean ages of 555€12, 549€7 and 549€6 Ma from metarhyolites and a metabasite. Inherited zircon cores were not observed. Zircon overgrowths, yielding pooled 206 Pb/ 238 U ages of 316€10 and 319€5 Ma, provide evidence for Variscan metamorphic zircon growth; cathodoluminescence imaging reveals a two-stage metamorphic overprint. In contrast, Lower Ordovician magmatism and anatexis are documented in the north-eastern parts of the Bayerischer Wald by metagranitoids (480€6, 486€7 Ma), an eclogitic metabasite (481€8 Ma) and a leucosome (491 to 457 Ma). Inherited zircon cores are found in Lower Ordovician metagranitoids and the leucosome, indicating a Palaeoproterozoic-Archaean (ca. 2.7, 2.0 Ga) source region, presumably of Gondwana affinity (West African craton), and documenting Cadomian magmatism (ca. 640 Ma). Post-Cadomian metamorphism is inferred from concordant ages of 433€4 and 431€7 Ma. Upper Vendian magmatism is assumed at an active continental margin with ensialic back-arc development ( Nd(t) -3.01 to +1.22); the lack of inherited zircon is due to either derivation from juvenile (?volcanic arc) material or complete isotopic resetting of pre-existing zircon. An active continental margin setting, possibly with some lateral variation (accretion/collision) is envisaged for the Lower Ordovician, producing granitoids, rhyolites and leucosomes ( Nd(t) -0.5 to -6.27); MORB-type metabasites may be related to ZEV or MariµnskØ Lµzně Complex metabasites. A tentative palaeogeographic reconstruction puts the "Bayerischer Wald" in close relationship with the Habach terrane (proto-Alps), as the "eastern" extension of terranes of the northern Gondwana margin.

Host rock compositional controls on zircon trace element signatures in metabasites from the Austroalpine basement

“…A continuous range of Th/U from high ''igneous'' towards lower ''metamorphic'' values has been reported from distinct overgrowths on zircon in leucocratic gneisses (Teipel et al, 2004). On the other hand it has been shown that metamorphic zircon may keep the Th/U systematics of its inherited magmatic population during high-grade granulite conditions and that metamorphic zircon of the same age population can have widely different trace element characteristics (Mö ller et al, 2002(Mö ller et al, , 2003.…”

The birth of the Rheic Ocean — Early Palaeozoic subsidence patterns and subsequent tectonic plate scenarios

“…Since our comprehensive discussion on Ordovician granitoids von Raumer et al, 2003) and their narrow relation to Ordovician subduction and subsequent crustal extension, new data are at disposal from different Alpine realms (Bertrand et al, 2000;Guillot et al, 2002;Schulz and Bombach, 2003), from the Carpathian mountains (Gaab et al, 2005), from Sardinia (Helbing 2003;Helbing and Tiepolo 2005;Giacomini et al, 2006), from Sicily (Trombetta et al, 2004), from the Pyrenees and from the Bohemian Massif (Bavarian Forest, Teipel et al, 2004).…”

P–T–t evolution of spinel–cordierite–garnet gneisses from the Sauwald Zone (Southern Bohemian Massif, Upper Austria): is there evidence for two independent late-Variscan low-P/high-T events in the Moldanubian Unit?

“…On the other hand, the ca. 320 Ma age of low-P/high-T metamorphism in the Sauwald Zone, as derived from monazite dating in this work, is independently strengthened by a recent SHRIMP study of Teipel et al (2004), which was carried out on metaigneous rocks east of Passau. Metamorphic zircon rims considered as having formed during the low-P/high-T metamorphic event gave ages between 314 ± 4 and 326 ± 6 Ma.…”

“…Thus, it can be speculated that the Sauwald Zone could have been situated immediately above a significant late-Variscan heat reservoir. Ar-Ar hornblende and mica cooling ages published in the literature (Friedl 1997;Scharbert et al 1997;Kalt et al 2000;Teipel et al 2004). For the prograde evolution of the Sauwald Zone, two possible T-t paths labeled 1 and 2 are discussed in the text…”