The acid metavolcanic rocks of the Orlica-Śnieżnik Dome (Sudetes): their origin and tectono-metamorphic evolution

Abstract

The Orlica-Śnieżnik Dome (OSD) belongs to the mosaic of geological units forming the Sudetic part (NE margin of the Bohemian Massif) of the Variscan belt. Acid metavolcanic rocks referred to as leptites appear in the OSD as intercalations in the mica schists of the Stronie Formation - the supracrustal cover of the gneiss core of the unit. Two types of acid meta-volcanogenic rocks have been distinguished: massive leptites (ML) and biotite-bearing leptites (BBL); together they are referred the leptite series. The ML originate from acid volcanic rocks that could have contained only small amounts of sedimentary material. The BBL are metamorphosed tuffites with a variable share of sedimentary material. All the analysed leptites from the entire OSD area reveal similar geochemical characteristics. The high concentration of strongly incompatible elements such as REE, as well as the enrichment in large-ion-lithophile elements indicates the continental crust affinities of the leptites' protolith. On the other hand, ascertained the high Th/Ta ratios, negative Nb and Ta anomalies, and low Ti, Zr and Hf content can be interpreted as indicators of magmatism connected with a suprasubduction environment. Such ambiguous characteristics may indicate that an ensialic rift environment, considered as the place of origin for the magmatic rocks dated at c. 500 Ma and occurring in the OSD, developed due to extension behind a collisional zone of Lower Palaeozoic age. An extensional environment created during this event could be the location for the postulated early deformation and metamorphism of the Stronie Formation. The Śnieżnik metagranites would have been emplaced into such, already deformed, supracrustal cover. The signature of an active margin found in these continental crust rocks that were stretched and melted due to the increased heat transfer resulted from an earlier (Neoproterozoic?) tectono-thermal episode. The reconstructed path of the tectono-metamorphic evolution of the leptites entirely results from a Variscan collisional episode occurring in the borderland of the West and the East Sudetes. The sequence of the deformations established for the studied rocks is interpreted as an effect of the continuous convergence of crustal units in an E-W direction. Structural and phase-equilibria lines of evidence show that the tectono-metamorphic evolution of the Stronie Formation was that of early burial (D1) to a depth not exceeding upper amphibolite facies conditions, and subsequent uplift connected with subvertical shortening and flattening strain (D2). As a result of the continuous convergence, thrusting took place along the edges of the colliding units. The final adaptation of the colliding crustal slices led to zonal lateral oblique displacements (D3). This latter event is treated as the cause for disturbance of the isograd and isotherm pattern observed within the rocks of the Stronie Formation. It is also responsible for zonal shearing, generally taking place in a N-S direction. During the collision that occurred along the eastern margin of the OSD, the rocks of the Stronie Formation reached the highest P-T conditions of metamorphism at c. 9 kbar and 650°C. The presented data shows a considerable decrease in the peak-metamorphic conditions from the east to the west of the OSD.