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MINERALOGICAL AND GEOCHEMICAL CHARACTERISTICS OF QUATERNARY PELITIC SEDIMENTS FROM GRMOŠČICA (ZAGREB) ABANDONED CLAY PIT

Two outcrops (GRMOŠČICA-I 17.5m and GRMOŠČICA- II 10.4m) were studied in the area of Zagreb during the mapping of the Basic Geological Map of the Republic of Croatia 1:50 000 twenty years ago. Sediments from outcrops are composed of clay, silt, sand and gravel. They belong to the Pontian and Quaternary, informal lithostratigraphic Formations Andraševac and Bistra (AVANIĆ, 1997, KOVAČIĆ, 1998). In this paper, five samples of sedimentary rocks, which belong to Bistra Formation, from sample storage of the Croatian Geological Survey, were investigated. According to AVANIĆ (1997), clay, sand and silt facies are present, which are typical for alluvial environments of floodplains. The aim of this research was to determine quantitative mineralogical composition and provenance of the analysed samples. Mineralogical analysis included X-ray powder diffraction (XRPD) analysis that was made on random mounts of bulk samples, and oriented mounts of <63µm and <2µm fraction. Oriented mounts were recorded after following treatments: a) air drying, b) saturation with K+ and Mg2+ , c) ethylene-glycol solvation, d) DMSO solvation, e) glycerol solvation, f) heating to 300°C, 400°C and 550°C. These bulk samples consist of quartz, illite/muscovite and swelling clays. Calcite is present in three samples from outcrop GRMOŠČICA-II. Quartz, illite/muscovite, smectite and kaolinite were identified within the oriented mounts of the <63µm and <2µm fraction, while chlorite and vermiculite were present in some samples only. Quartz is the most abundant mineral in <63µm fraction, while <2µm fraction is rich in clay minerals. The carbonate content determinated by Scheibler calcimeter in sample GRM 2-1, GRM 2-2 and GRM 2-3 from GRMOŠČICA-II outcrop was 2.5%, 6.1% and 20.2% respectively. The granulometric analysis was made by Laser Diffraction Particle Size Analyzer on <63µm fracton separated by wet sieving method. Silt fraction dominates all samples, but samples from GRMOŠČICA-I outcrop were determined as clayey sandy silt, while samples from GRMOŠČICA-II outcrop were described as sandy clayey silt. Geochemical analysis was made at ACME Laboratory (Canada) using ICP-ES method for main elements determination, and ICP-MS method for REE and trace elements determination. These geochemical data was used to determine provenance and CIA (Chemical Index of Alteration) and ICV (Index of Compositional Variability)(CULLERS & PODKOVYROV, 2002). Weight percentage (wt%) of Al2O3–(CaO*+Na2O)– K2O and La-Th-Sc were plotted in triangular diagrams and La/Sc-Th/Co to scatter plot in order to distinguish chemical weathering and source rock composition (CULLERS & PODKOVYROV, 2000). These results accompanied by CIA and ICV implied that the source rocks originate from different types of rocks, predominantly from acidic, quartz enriched rocks.

Keywords: Quaternary. clay minerals, X-ray diffraction, chemical composition, provenance

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