Petrography of the archive thin sections of the Palaeozoic fine-grained siliciclastic rocks of borehole Horváthertelend–1 (W Mecsek)

Abstract

The Horváthertelend Unit, NW foreland of the Mecsek Mountains (Hungary), is known as a pre-Alpine nappe remnant. In this study petrographic examination of the archive thin section collection from the borehole Horváthertelend–1 was carried out. The goal of this research is to characterize the mineralogical, petrographic and microstructural features of the black or dark grey fine-grained metasedimentary rocks in the depth interval of 720–790 m. This core section of the borehole Horváthertelend–1 was previously correlated with the Silurian Szalatnak Slate Formation.
The main lithology is the dark metasandstone–slate with quartz (~70%), volcanic rock fragments with intermediate composition (~20%) and detrital plagioclase (~10%) grains. The sandstone is classified as a quartz-rich greywacke with large amount of matrix (~30–40%). In this lithology there are red arkose lenses composed of clay mineral pseudomorphs after detrital feldspar grains (~60%), detrital quartz (~20%) and opacitic biotite (~15%) grains. Idiomorphic, colourless–bluish green, pleochroic epigenic tourmaline with a size of 40–100 µm in the dark rock type and vein-filling prehnite suggests the effect of hydrothermal fluids.
The sigmoidal shape of the lenses and the weakly or moderately developed pressure solution cleavage and the folding of the sedimentary bedding suggest tectonic deformation of the rock body.