The Eo-Variscan domain in the Variscan belt: New data from the Sudetes, southern Poland

Abstract

The Variscan belt in Europe includes Eo-Variscan domains whose tectono-metamorphic evolution ended before the Carboniferous. In the Bohemian Massif, these are the Teplá-Barrandian terrain and the Góry Sowie Massif – Kłodzko Metamorphic Massif terrain, jointly referred to as the Bohemian Terrane, which is partially concealed beneath the North Bohemian Cretaceous Basin. Terrains on either side of this basin differ in several aspects. Northern Bohemia is accompanied by dismembered ophiolite (c. 400 Ma) and preserves fragments of late Devonian overstep succession – features missing in the Teplá-Barrandian terrain. New zircon UPb and O isotope analyses indicate that the Góry Sowie Massif is composed primarily of metasedimentary rocks with a maximum depositional age of 540 Ma, along with minor metaigneous rocks with protolith ages of c. 500–490 Ma. Oxygen isotope study suggests crustal derivation of felsic rocks (δ¹⁸O_Zrn values of 8.14–11.29 ‰) and local pulses of mantle melting for minor basites (δ¹⁸O_Zrn values of 5.10–8.36 ‰). Neither Cadomian basement nor Neoproterozoic magmatic arc, characteristic for the Teplá-Barrandian domain, is in evidence in the Góry Sowie Massif. Detrital zircon age spectra for the Góry Sowie Massif reveal three clusters: 2.8–2.4 Ga, 2.1–1.9 Ga, and 650–540 Ma also known from the adjacent Saxothuringian units. However, the Góry Sowie Massif spectra contain more Neoarchaean and Mesoproterozoic components, and fewer Neoproterozoic components, suggesting different positions for the Góry Sowie Massif and Saxothuringia in the peri-Gondwana realm during the Cambrian. The two Eo-Variscan parts of the Bohemian terrane have probably constituted the composite terrane of Bohemia since the Early/Middle Devonian and further acted as a single microcontinent. In the Góry Sowie Massif, metamorphic zircon grains dated to ca. 390–380 Ma preserve evidence of the mid-Devonian collision between the western margin of the Góry Sowie Massif–Kłodzko Metamorphic Massif block and the Saxothuringia-related sub-terrane. In the Kłodzko Metamorphic Massif, slightly metamorphosed mudstones (Frasnian depositional age), below the nonconformity, contain detrital zircons of metamorphic origin dated at c. 390–370 Ma, sourced from the Góry Sowie area. The Eo-Variscan domain was finally brought to the surface in the Frasnian/Famennian as constrained by previous biostratigraphical data from a nonconformably overstepping sedimentary succession on its flanks. The more distant Famennian pelagic deposits in the Bardo Structure acquired c. 390–365 Ma euhedral zircons of volcanogenic provenance, prior to the Variscan collision between the Saxothuringian and Bohemian terranes.