A vestige of an Early Devonian active continental margin in the East Sudetes (SW Poland) -- evidence from geochemistry of the Jegłowa Beds, Strzelin Massif

Abstract

The Early Devonian metasandstones of the Jegłowa Beds (Strzelin Massif, NE Bohemian Massif) are low- to medium-grade metamorphosed siliciclastic deposits showing differences in modal composition, especially in the amount of micas and feldspars. Despite the similarity in relative concentrations of trace elements, three chemical groups can be distinguished among the metasandstones that differ in the total amounts of major and trace elements. The negative Ta and Nb anomalies visible on a spider plot normalised to the average upper continental crust and specific ratios of e.g. Th, Zr, Hf, La/Th and Ti/Sc characterize the majority of the metasandstones, indicating the provenance of their sedimentary precursors from a subduction-related tectonic setting. However, few of the samples analysed show strongly differentiated geochemical characteristics with high Zr and Hf contents. This suggests input of relatively strongly reworked material. Thus, it seems that a back-arc setting can account for the mixed nature of the inferred source areas with old basement and arc-related detritus respectively as the end-members of the mixtures. The composition of the metasandstones indicates deposition of the siliciclastic material near an Early to Middle Devonian volcanic arc that was superimposed on rocks originally forming a part of the pre-Devonian continental margin of the Brunovistulicum microplate. A plausible tectonic scenario involves formation of the Devonian arc due to east-directed subduction of an oceanic domain between the Brunovistulian microcontinent in the east and the Central Sudetic terrain located further west.