Variscan and post-Variscan processes in the Central-Sudetic Ophiolite: records from carbonate and silicate rocks.

The northeastern part of Bohemian Massif is composed of various lithotectonic domains interpreted as microplates, sedimentary basins, and fragments of ancient oceanic lithosphere, which have been amalgamated during the Late Devonian multistage collision. Fragments of the Variscan Central-Sudetic Ophiolite (CSO) preserve information on the nature of the mantle at the onset of Variscan orogeny. They are mainly composed of ultramafic-mafic rocks (UMR) dated at 400 Ma, but these are not the only components. Other lithologies include (a) carbonate veins crosscutting the UMR, (b) dolomite-rich domains associated with clinopyroxenites, and (c) silicic dyke of diorite composition also crosscutting the UMR. The origin of these lithologies may be contemporaneous with UMR or later (Variscan or Cenozoic) and obtaining the ages is the first step to understanding which events they record. Zircons from the diorite yield a concordia age of 378.0 ± 5.0 Ma (SHRIMP) consistent with the diorite representing an early Variscan magmatic episode. The obtained age of the intrusion suggests an affinity with a located nearby outcrop of ultrapotassic syenites (from 378.2 ± 2.4 to 354.7 ± 4.3 Ma). A striking relationship between the two rocks is evident; if certain elements are strongly enriched in one rock they are equally impoverished in the other. Such unusual chemical fractionation can be achieved during the formation of alkaline and carbonatite melts. Also, dolomite domains recently found in clinopyroxenites or puzzling anhydrite inclusions in Ca-amphiboles may support this hypothesis suggesting an enriched mantle as a common source of dioritic, syenitic, and dolomitic lithologies. On the other hand, carbonate veins record another episode. Recently, the U-Pb radiometric dating of calcite sampled from one of the CSO massifs yielded an isochrone age of 15.4 ± 19.7 Ma that generally suits Paleogene and Neogene tropical weathering events, moreover, some parts of CSO contain abundant carbonates mineralization accompanied by plenty of quartz zonal clusters. The co-occurrence of these phases may suggest hydrothermal origin and becomes a foothold for further studies on the carbonation of obducted oceanic lithosphere.
Altogether, it is important to bear in mind CSO’s 400 Ma-long evolution. It seems that Central-Sudetic Ophiolite and associated younger lithologies still have more to tell us about the orogenic and post-orogenic history of the northeastern Bohemian Massif.

Funding: The research is funded by NCN grant PRELUDIUM no. UMO-2022/45/N/ST10/00879 awarded to Błażej Cieślik.