Pages: 267 - 280
Abstract: Metamorphosed clastic sediments from three lithostratigraphic groups (the low- to medium-grade Velka Upa, and the low-grade Radcice and Ponikla Groups), exposed in a para-autochthonous to allochthonous unit of the Krkonose-Jizera Terrane (KJT, West Sudetes), were studied in order to discover whether they are chemically discriminable. According to chemical features of the metasediments (e.g. lower Th/Sc and CeN/YbN ratios), the Velka Upa Group is interpreted to be younger than the Neoproterozoic Machnin Group (of the KJT autochthonous unit) related to a Cadomian active continental margin setting. Metamorphosed clastics of the Ponikla and Radcice Groups proved to be chemically indistinguishable. As a result, these groups are herein combined within a single Vrchlabi Group of Cambrian-Ordovician to Silurian (±Devonian?) age. Precursors of the Velka Upa and Vrchlabi Groups were apparently deposited on a passive margin of the peri-Gondwanan Saxothuringian microplate. The passive margin originated in Cambrian and Ordovician times during extension and rifting of the former (Cadomian) active continental margin of NW Gondwana. Comparison of the metasediment geochemistry, together with the stratigraphic and structural affinities within the KJT, allows the Velka Upa and Vrchlabi Groups to be interpreted as broadly coeval: Cambrian to Silurian (±Devonian?) in age. Deposition of the Vrchlabi Group was more distal and sourced by sediment derived from a more dissected area of the Cadomian basement (fossil active margin) as demonstrated by an enrichment in Hf, depletion in Sc and Cr, somewhat higher CeN/YbN values and slightly more pronounced negative anomalies of Eu. The geochemistry of the metasediments reflects the complex paleotectonic evolution of the West Sudetes that started with Cambrian-Ordovician intracontinental rifting and subsequent sea-floor spreading during marginal fragmentation of Gondwana.
Keywords: Bohemian Massif, West Sudetes, Early Paleozoic, paleotectonic setting, geochemistry of clastic metasedimentsDownload PDF document Show Text Version