Neoproterozoic rocks in the Saxo-Thuringian part of Armorica formed in an active margin setting and were overprinted during Cadomian

Saxo-Thuringia is classified as a tectonostratigraphic terrane belonging to the Armorican Terrane Collage (Cadomia). As a former part of the Avalonian Cadomian Orogenic Belt, it became (after Cadomian orogenic events, rift-related Cambro-Ordovician geodynamic processes and a northward drift within Late Ordovician to Early Silurian times), during Late Devonian to Early Carboniferous continent-continent collision, a part of the Central European Variscides. By making use of single zircon geochronology, geochemistry and basin analysis, geological processes were reconstructed from latest Neoproterozoic to Ordovician time: (1) 660 540 Ma: subduction, back-arc sedimentation and tectonomagmatic activity in a Cadomian continental island-arc setting marginal to Gondwana; (2) 540 Ma: obduction and deformation of the island arc and marginal basins; (3) 540-530 Ma: widespread plutonism related to the obduction-related Cadomian heating event and crustal extension; (4) 530-500 Ma: transform margin regime connected with strike-slip generated formation of Early to Mid-Cambrian pull-apart basins; (5) 500-490 Ma: Late Cambrian uplift and formation of a chemical weathering crust; (6) 490 470 Ma: Ordovician rift setting with related sedimentation regime and intense igneous activity; (7) 440-435 Ma: division from Gondwana and start of northward drift. The West African and the Amazonian Cratons of Gondwana, as well as parts of Brittany, were singled out by a study of inherited and detrital zircons as potential source areas in the hinterland of Saxo-Thuringia. the reconstructed Avalonian-Cadomian Orogenic Belt includes Western and Eastern Avalonia, Florida, Iberia, Carolina and Armorica. The last of these was named 'Cadomia' by Nance & Murphy (1994). As differential drift paths and individual block From." FRANKE, W., HAAK, V., ONCKEN, 0. r162 TANNER, D. (eds).

The Teplµ-Barrandian unit (TBU) of the Bohemian Massif was a part of the Avalonian-Cadomian belt at the northern margin of Gondwana during Neoproterozoic and Early Cambrian times. New detrital zircon ages and geochemical compositions of Late Neoproterozoic siliciclastic sediments confirm a deposition of the volcano-sedimentary successions of the TBU in a back-arc basin. A change in the geotectonic regime from convergence to transtension was completed by the time of the Precambrian-Cambrian boundary. The accumulation of around 2,500 m Lower Cambrian continental siliciclastics in a Basin-and-Range-type setting was accompanied by magmatism, which shows within-plate features in a few cases, but is predominantly derived from anatectic melts displaying the inherited island arc signature of their Cadomian source rocks. The geochemistry of clastic sediments suggests a deposition in a rift or strike-sliprelated basin, respectively. A marine transgression during Middle Cambrian times indicates markedly thinned crust after the Cadomian orogeny. Upper Cambrian magmatism is represented by 1,500 m of subaerial andesites and rhyolites demonstrating several geochemical characteristics of an intra-plate setting. Zircons from a rhyolite give a U-Pb-SHRIMP age of 499€4 Ma. The Cambrian sedimentary and magmatic succession of the TBU records the beginning of an important rifting event at the northern margin of Gondwana.

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