Abstract
Jersovec chert deposit is a part of the transitional zone between the internal and the external Dinarides. Tethyan cherts are often interlaid with fine-grained material, as is also the case in Jersovec, where chert beds interchange with several thin, up to 10 cm thick layers of fine-grained sedimentary rocks. The source of this material is often questionable and interpreted to be terrigenous, volcanogenic or even combination of both. In order to determine origin and depositional environment of fine-grained material, detailed mineralogical and geochemical analyses were performed. Fine-grained sedimentary rocks were characterised by X-ray diffraction (XRD), scanning electronmicroscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and inductively coupled plasma emission spectroscopy (ICPES) and mass spectroscopy (ICP-MS). All samples are mineralogically very similar but differ in mineral ratios. They contain quartz and clay minerals, predominantly illite/muscovite and chlorite group minerals. SEM/EDS analysis additionally revealed zircon, monazite, Ti-oxide (probably rutile) and iron oxides/hydroxides in all samples, whereas chromite, ilmenite, xenotime, apatite and baryte were found only in some of them. The average sizes of accessory minerals range from 3-20 µm. Two samples differ from the others by their brown colour and higher Fe2O3 and lower SiO2 contents. All other samples are green. Chemical analysis showed that they consist mostly of SiO2 (46.3-69.3 %), Al2O3 (15-24.1 %) and minor contents of K2O (2.8-3.4 %), Fe2O3 (2.1-8.4 %), MgO (1.6-2.3 %), TiO2 (0.5-0.7 %), CaO (0.3-1.4 %), Na2O (0.1 %) and P2O5 (up to 0.5 %). Position of all samples in the Zr/TiO2 vs. (V+Ni+Cr)/ Al2O3 diagram points to terrigenous origin and Sedimentation on the Continental margin. According to the chemical weathering indices (CIA and CIW) material has been subjected to the intense and long weathering, supported also by the index of compositional variability (ICV), which corresponds to the compositionally mature material, and by rounded zircon and ilmenite grains.
Highlights
Sedimentary rocks with cherts are very common Mesozoic pelagic sediments of the entire Tethys (Baumgartner, 2013)
Very similar clastic material of Jurassic age has been found and studied in the abandoned quarry near Izvir village in Krško depression (Skobe et al, 2013), in Italy, where the interbedded material is of Jurassic age (Barrett, 1981), and in Croatia, where the clastics are of Triassic and Jurassic age (Peh & Halamić, 2010)
scanning electron microscopy/ energy dispersive X-ray spectroscopy (SEM/EDS) analysis supports the results of X-ray diffraction (XRD) analysis and reveals the presence of some accessory minerals (Table 1)
Summary
Sedimentary rocks with cherts are very common Mesozoic pelagic sediments of the entire Tethys (Baumgartner, 2013). The origin of the fine-grained material could be either terrigenous and/or volcanogenic; it reflects the depositional environment, ranging from a shelf to a mid-ocean ridge or continental margin (Boström, 1973; Murray, 1994; Girty et al, 1996; Andreozzi et al, 1997; Di Leo et al, 2002a). It helps to interpret the palaeoenvironmental and palaeogeographic position of the area in Tethys realm. Very similar clastic material of Jurassic age has been found and studied in the abandoned quarry near Izvir village in Krško depression (Skobe et al, 2013), in Italy, where the interbedded material is of Jurassic age (Barrett, 1981), and in Croatia, where the clastics are of Triassic and Jurassic age (Peh & Halamić, 2010)
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