Geochemical study on Upper Bhuban shale in Aizawl district of Mizoram, India: an implication of chemical weathering, geochemical classification, tectonic setting and provenance


Shales from the Upper Bhuban formation in the Muthi, Aizawl district, was analysed for major, trace and rare earth elements. These elements are competent alternative for determining lithological composition, geochemical classification, provenance and tectonic setting of the basin. It shows high content of SiO2 varies from 62.85 wt% to 73.22 wt% and a small variation in Al2O3 (14.22–19.61 wt%). The average of K2O/Al2O3 ratio vary from 0.15 to 0.22 with the average value of 0.18 which are close to the upper limit of the clay mineral range indicated that the illite is the dominant clay mineral in these shales. The value of chemical index alteration in the Muthi shale is high (75.10 to 81.77) indicating high intensity of chemical weathering in the source areas. The Muthi shales are classified as litharenite. Th/Sc, Th/Co, Th/Cr, Cr/Th, and La/Sc ratios of shales from this study are compared with those of sediments derived from felsic and basic rocks (fine fraction) as well as to upper continental crust (UCC) and PAAS values. This comparison suggested that these ratios are within the range of felsic rocks. Furthermore, it shows slightly LREE enriched and flat HREE patterns with negative Eu anomalies 0.65 (average). The (Gd/Yb)N ratio 2.47 (average) of Muthi shale is higher than 2 which suggested that these shales were derived from the large HREE depleted source rocks.

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