engleski

Clay minerals assemblage in the Upper Albian carbonate succession (Tinjanska Draga, Istria, Croatia)

Several clay-containing layers within the Upper Albian carbonate sequence (BARUDŽIJA et al.., 2004) from Tinjanska Draga area in Istria (Croatia) were analyzed. The aim of the study was to connect their mineral assemblage with the contemporaneous diagenetic silicification of carbonates appearing in the same sequence (BARUDŽIJA, 2003 ; DURN et al., 2003). Therefore, 12 samples from the four different horizons were analyzed by X-ray powder diffraction (XRD). Results were compared with petrographic analysis of heavy and light mineral assemblage from the same samples. Bulk samples were analyzed by XRD. Samples were treated with NaAc/HAc and diffraction patterns of obtained air dried and differently treated (with glycerine ; with ethylene glycol ; 2 hours heated on 600&#186; C ; dissolved for 24 hours in warm 18% HCl) insoluble residues were also recorded. Clay fractions (<2&#956; m) from insoluble residues were taken and diffraction patterns of non-oriented and oriented samples were made. In order to determine clay minerals, samples were saturated with Mg2+ (oriented air dried samples, ethylene glycol treated samples and samples heated on 300&#186; C/2h and 550&#186; C/2h were prepared) and K+ (oriented air dried samples, ethylene glycol and DMSO treated samples, 2 hours heated samples on 300&#186; C and 550&#186; C were prepared) and diffraction patterns were taken. Samples have significant percentage of carbonates (35-95wt% of calcite in 10 samples, and 2 samples with almost 95wt% of dolomite and less than 5wt% of calcite). Insoluble residues contain from 10wt% to 90wt% of quartz, as well as up to 5wt% of feldspars and variable content of phyllosilicates mainly present in clay fraction. Depending of sample clay fractions mostly contain illite and/or mixed-layer illite-smectite dominated by illite layers, illite-smectites with higher amount of smectite layers than previous and randomly interstratified minerals. Some of other phyllosilicates, probably smectites, kaolinite and chlorites, are also present. In the total heavy mineral sample there are transparent grains (up to 50% - predominatly epidote-zoisite-clinozoisite minerals, followed by pyroxenes (Cpx+Opx), garnets, hornblende and feroactinolite and some zircons, tourmalines and glaucophanes), muscovite (up to 40%), chlorite (up to 10%) and opaque minerals and grains with limonitic cover as well. In the light mineral assemblage quartz and feldspar (plagioclase) grains are the dominant components. Clay minerals assemblage in the Upper Albian carbonate succession from Tinjanska Draga is closely connected with silicification developed within limestones and dolomites from that section. Determined mixture of illite and highly illite-smectite clays indicate diagenesis of clays within marine conditions. Mineral association present within heavy and light mineral fraction shows a significant composition and can indicate volcanic origin of source material for development of determined clay minerals assemblage and source of silica for silicification as well.

clay minerals; Tinjanska Draga; carbonates; silicification

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