TRACE ELEMENT STUDY ON THE MIXED CARBONATE-PYROCLASTIC SEDIMENTS (LOWER MIOCENE, CENTRAL BASIN, IRAN)

Geochemical investigations have permitted the refinement of the environmental interpretation of the f-Member of the Qum formation, Central Basin, Iran, which has been established through detailed microfacies (MF) analysis. The analysis revealed that the f-Member carbonates consist of eight distinct microfacies designated by the letters A through H. On the basis of petrographic characteristics and environmental significance, the studied MF have been interpreted in terms of depositional environments as follows: MF-A (basin); MF-B (lower slope); MF-C (reef flank); MFD (proximal talus), MF-E (organic reef); MF-F (lagoon); MF-G (restricted marine shoal), MF-H (restricted shelf). The concentrations of Ti, V, Fe, Co, Ni, Cu, Sr, Zr, U and Rb have been measured in limestones from outcrop samples of this formation. The analysis revealed a wide range of concentration both within and between different microfacies. A volcanic eruption in the nearby area located in the northeastern part of the sedimentary basin exerted an influence on trace element composition by supplying pyroclastic materials to carbonate sediments. In general, the basin microfacies is characterized by having the highest concentrations of Ti and Zr. The lower slope microfacies has the highest values of V, Fe, Ni, Cu, Sr, U and ratios of Ni /Co, Cu/Co and Ni/V, whereas the reef flank microfacies have only elevated concentrations of Co. All of these traces and elemental ratios are depressed in both proximal talus and reef rnicrofacies. The restricted marine shoal microfacies possesses elevated Rb values and the restricted shelf microfacies is characterized by a high concentration of Sr and elevated Cu , U and Rb values and show a slight increase in the Ti. Fe, V, Co, Ni, Zr values.