Obervendischer und unterordovizischer Magmatismus im Bayerischen Wald

U-Pb-SHRIMP dating of zircons, zircon morphology, geochemistry and
isotopic data of metamagmatites from the Bayerischer Wald (Germany)
reveal 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 SHRIMP 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 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) and a leucosome (491 to 457 Ma). Inherited zircon
cores are found in Lower Ordovician metamagmatites indicating a
Palaeoproterozoic-Archaean (2.70, 2.02 Ga) source region,
presumably of Gondwana affinity (West African craton), and
documenting Cadomian magmatism (>615 to 560 Ma). Upper Vendian
magmatism is assumed at an active continental margin with ensialic
back-arc development (εNd(t) –4.58 to +1.22). An active continental
margin setting, possibly with some lateral variation
(accretion/collision) is envisaged for the Lower Ordovician
producing granitoids, rhyolites, leucosomes (εNd(t) -0.5 to -6.27)
and andesites. A tentative palaeogeographic reconstruction puts the
"Bayerischer Wald" in a close relationship with the Habach terrane
(proto-Alps), as the "eastern" extension of terranes of the
northern Gondwana margin.