The Rb-Sr isotope geochemistry of the metamorphosed bimodal volcanic association of the Rychory Mts crystalline complex, West Sudetes, Bohemian Massif.

Bendl Jiři, Patočka František


The Rb-Sr isotope data on porphyroid and greenschist samples of the Ponikla Group (the Rychory Mts crystalline complex, West Sudetes) indicate that the protolith of these rocks can be a bimodal volcanic association (dominated by acid components) related to continental rifting. The possible age of the magmatic origin of the protolith, based on the Rb-Sr isochron, is 501 ±8 Ma (Early Ordovician). A large-scale fragmentation of the Gondwana plate northern margin during the Cambrian and Ordovician, deduced from the abundance of mafic-felsic volcanic suites in the Lower Paleozoic of the European Variscides, is supported also by the interpretation of the Rb-Sr isotope data on the Ponikla Group metavolcanics.
The 350 and 365 Ma intersects of the hypothetical evolution curve of Sr isotope composition in the Rychory Mts marble, and the curve of Sr isotope ratio fluctuations in the Paleozoic marine carbonates, as well as the relics of oolitic texture, often observed in the marbles, may probably point to Famennian to Tournaisian age of the marble sedimentary precursor.
The Rb-Sr isotope data on the Rychory Mts metavolcanics and marble suggest that the Ponikla Group is a metamorphosed volcano-sedimentary sequence formed between the Early Ordovician and the Late Devonian/Early Carboniferous. As a place of its origin is suggested a rifted off passive continental margin of some of the Gondwana-derived fragments. 

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