Geochemistry of the Eocene limestones of the Jaisalmer basin,Rajasthan, India: Implications on depositional conditions and sources of rare earth elements |
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| Authors: | A Patra B P Singh |
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| Institution: | 1.Centre of Advanced Study in Geology,Banaras Hindu University,Varanasi,India |
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| Abstract: | Major, trace and rare earth elements (REE) concentration of the Eocene limestones, Jaisalmer Basin, Rajasthan, India are analysed to reconstruct the depositional conditions and to identify sources of REEs. Among the major oxides, CaO is the dominant oxide followed by SiO2 in the studied limestones. Trace element Ba dominates over the other trace elements and it shows negative correlation with CaO. The Sr, occurring in small concentration, shows positive correlation with CaO. Other trace elements such as V, Zr, Sc, Y, Rb, Ni, Pb Co, Cu, U occur in small concentrations. The studied limestones show a positive correlation of ΣREE with Fe2O3, Ni, Th, Sc, and Y. These limestones possess sea-water like shale-normalized REE + Y pattern with light REE depletion, slight Gd enrichment, slightly positive La anomaly, positive Y anomaly, positive Eu anomaly, negative Ce anomaly and superchondritic Y/Ho ratio from 23.12 to 28.57. The dominance of CaO and low percentage of MgO suggest that mineral phase is calcite and there is absence of dolomitization. The occurrence of SiO2 and Al2O3 in appreciable percentages may be because of the siliciclastic input during the limestone precipiatetion. The low concentration of Uranium (0.4-3.7) and authigenic Uranium (Average Total U-Th/3 value = 0.74) indicate that the studied limestones were precipitated in oxic condition from seawater. The depletion of LREE suggests that the limestones were precipitated from the seawater. The positive correlation of ΣREE with Al2O3 Fe2O3, Ni, Th, Sc, and Y and negative correlation with CaO suggest an input of siliciclastic sediments from the land during limestone precipitation. The negative Ce anomaly, slightly positive La anomaly, slight Gd enrichment, positive Y anomaly, and positive Eu anomaly also suggest that the limestone was precipitated from the seawater with some siliciclastic input from continent. The low values of the Y/Ho ratio (23.12 to 28.57) in the studied limestones suggest some modification of the seawater by the input of freshwater in a coastal environment. The REEs of the studied limestones are correlable with the shallow sea water REEs with exception of a few elements. We envisage a coastal/shallow marine depositional environment where mixing of the continental material in sea water appears feasible. |
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