Heavy minerals in the Carboniferous sediments of the Intra-Sudetic Basin as palaeogeographic indicators

Abstract

The Intra-Sudetic Basin, a Variscan intramontane trough in the central Sudetes (NE part of the Bohemian Massif), is filled with Carboniferous, Permian, Triassic, Upper Cretaceous and Cenozoic deposits. The Carboniferous sediments display considerable lateral and vertical facies variation, which reflects intense synsedimentary, tectonic and volcanic activities. The sedimentary material was transported from various directions, and both the pebble lithology and the heavy minerals present are good palaeogeographic indicators of the alimentary areas. This paper presents the results of heavy mineral analyses of the Carboniferous deposits of the Intra-Sudetic Basin and discusses the possible usage of the heavy mineral spectra as palaeogeographic and stratigraphic indicators. Samples representing all the Carboniferous formations were taken along two profiles: between Ciechanowice and Kamienna Góra (profile W), and between Sady Górne and Głuszyca (profile E). The heavy mineral analyses show the variation of the heavy fraction, both in the stratigraphic column and laterally. The Upper Tournaisian sediments display a characteristic heavy mineral assemblage: epidote, biotite, and chlorite - pointing to the Kaczawa Mts as the source area. In the Visean sediments, a clear regional distinction can be seen: in profile W there are no significant differences compared to the older sediments; in profile E - garnet appears as a major component, pointing to possible transport from the Góry Sowie Block. In the Upper Visean, the regional differences disappear, probably as a result of redeposition of the older material. There is an important change in the mineralogy of the heavy fraction in the middle part of the Biały Kamień Formation. A considerable decrease in the garnet and ZRT (zircon+rutile+tourmaline) -mineral content is observed, and minerals formed during diagenesis (e.g. haematite) and/or hydrothermal processes (barite and siderite), become more abundant. The chemical composition of some minerals (especially garnet and chlorite) appears to be very useful for establishing the alimentary areas. The garnets of the Ciechanowice and Lubomin Formations are rich in Grs and resemble the garnets from the contact-metamorphic basic rocks of the Rudawy Janowickie (garnets rich in Sps, typical of HP rocks of the eastern cover of the Karkonosze Pluton, were not found). In the Sady Górne Formation, garnets similar to those from the gneisses and migmatites of the Góry Sowie Block are the most common (a few grains rich in Sps, typical of low-grade metamorphic rocks are also found). The younger deposits display a wide variation of garnet formulae, which indicates various source areas and an important role of resedimentation. The heavy mineral analyses confirm that the alimentary areas for the oldest sediments of the Intra-Sudetic Basin were the Kaczawa Mts and the eastern cover of the Karkonosze granite, whereas for the younger deposits, most probably, there was not a single source area; rather the materials derived from different sources and were redeposited. The distinct change in the heavy mineral spectra in the middle part of the Biały Kamień Formation suggests an important change in the factors controlling sedimentation in the basin. The heavy mineral analyses, in general, confirm previous interpretations based on sedimentological research; however, the direct transport from the Góry Sowie Block area is questionable (apart from garnets, other minerals typical of the rocks of that area do not occur).