Abstract
Abstract Sea-level rise after the Hirnantian glaciation resulted in the global inundation of continental shelf areas and the widespread formation of early Silurian black shales. Black shales that were deposited on shelves receiving drainage from earlier glaciated areas have high uranium (U) contents because large-scale glacial erosion brought rocks with leachable U to the surface. In contrast, black shales receiving drainage from non-glaciated areas that had lost leachable U earlier have low U contents. Early Silurian U-rich shales formed only on shelf areas that had not been separated from earlier-glaciated mainland Gondwana by oceanic lithosphere. Therefore, early Silurian U-rich black shales within the Variscan orogen provide direct evidence that these areas had not been separated from mainland Gondwana, but were part of the same, contiguous shelf. This implies that the Rheic Ocean was the only pre-Silurian ocean that opened during the early Paleozoic extension of the peri-Gondwana shelf.
Highlights
The assembly of western Pangea included the collision between Laurentia and the East Europ ean Craton to form Laurussia (Caledonian orogeny) followed by the collision of Gondwana with Laurussia in Europe (Variscan orogeny) and the final closure of the remaining Rheic Ocean between western Africa and North America (Alleghanian orogeny)
This implies that the Rheic Ocean was the only pre-Silurian ocean that opened during the early Paleozoic extension of the peri-Gondwana shelf
Black shale deposited on the Gondwana shelf is U-rich, whereas black shale deposited on the shelves of other continents has low U contents (Fig. 2)
Summary
The assembly of western Pangea included the collision between Laurentia and the East Europ ean Craton to form Laurussia (Caledonian orogeny) followed by the collision of Gondwana with Laurussia in Europe (Variscan orogeny) and the final closure of the remaining Rheic Ocean between western Africa and North America (Alleghanian orogeny). The second group of models includes two plates (Gondwana and Laurussia) (e.g., Robardet, 2003) and one single divergent plate boundary, i.e., the mid-ocean ridge of the Rheic Ocean In these models, post-Silurian collision resulted in long-lasting subduction-accretion processes of the segmented shelf of both plates, culminating in intraplate continental subduction during the late stages of the Variscan collision (Kroner and Romer, 2010, 2013). Paleomagnetic data were widely believed to provide key evidence for viewing the Variscan massifs between mainland Gondwana and mainland Laurussia as independently moving microplates that were separated by wide oceans (e.g., Tait et al, 1997). The latter point has seen major modifications: Franke et al
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