Recurrent early Paleozoic rifting in the Northern Gondwana margin: Evidence from correlation of metabasalts across the Saxothuringian Zone of the Bohemian Massif

Geodynamic causes of the Rhenohercynian basin opening and Devonian–early Carboniferous rifting within the Saxothuringian Zone of northern peri-Gondwana and the Variscan basement of the Alps remain unresolved. Interpretations with varying numbers of oceanic tracts formed during Paleozoic fragmentation of Peri-Gondwana exist, heretofore without a comprehensive survey linking Rheic rift-related segments. A review of existing data, combined with new evidence from three separate Rheic/Saxothuringian rift system segments in the Bohemian Massif, informs a new three-stage model of their Paleozoic evolution: (1) Late Cambrian–early Ordovician (~505–470 Ma) rifting accompanying back-arc extension along the northern Gondwana margin, was coeval with Rheic Ocean opening between Avalonia and Saxothuringia. Initial bimodal felsic and mafic OIB-like magmas transitioned through T-MORB to MORB-like basalts, common in NE Saxothuringia. (2) Subordinate Silurian–early Devonian (~430–410 Ma) extension of both Saxothuringian and Teplá-Barrandian basement was possibly linked to closure of Iapetus Ocean and accretion of microcontinental fragments to Laurentia. Proximal to the Münchberg Massif, this manifests as strongly alkaline OIB-type volcanism, whereas more tholeiitic magmas prevail in the adjacent Teplá-Barrandian Unit. (3) Middle-Devonian–early Carboniferous (390–340 Ma) extension produced the most voluminous magmatism, similarly progressing from early bimodal alkaline rhyolite and OIB-type basalts to MORB-like compositions proximal to the Saxothuringian ocean tract. This rift-related volcanism overlaps in chemistry and time with alkaline volcanism of the SE edge of Avalonia, grading into of the Rheic/Rhenohercynian oceanic basalts. Late Devonian/early Carboniferous closure of this ocean is linked to cessation of extension, its shift back to the Gondwana margin and opening of the Paleotethys Ocean.