Geochemistry of Rohtas Limestone from Vindhyan Supergroup, Central India: Evidences of detrital input from felsic source

Abstract

The Rohtas Limestone of Semri Group is conformably overlain by Lower Kaimur Group of Vindhyan Supergroup in the Son Valley, Central India. Lower Kaimur Group comprising Sasaram Sandstone, Ghurma Shale and Markundi Sandstone are dominantly quartz arenite in composition. While, Rohtas limestones are micritic carbonates with considerable input of detrital grains/rock fragments. Major oxide data shows that Lower Kaimur sandstone have SiO2 = 92 wt % while limestone exhibit SiO2 = 9.52 wt %, CaO = 47 wt %. Limestones exhibit Sr = 352 ppm, Zr = 20 ppm, while Lower Kaimur sandstones are enriched in Zr = 400 ppm, due to the presence of heavy minerals like zircon. The average limestone and sandstone shows a fractionated LREE pattern and a nearly flat HREE pattern with a small negative Eu anomaly. REE patterns and abundance of limestone are quite similar and comparable to sandstones from Lower Kaimur than to the pure micritic limestone. This indicates that the REE pattern and abundance of Rohtas limestone have been controlled by detrital components possibly derived from the similar source as that for Lower Kaimur sandstones. Apart from the similarity in the REE patterns of limestone and sandstone, the trace element ratios such as Ti/Zr, La/Sc, Th/Sc, Th/Co, Th/U, Zr/Sc, Ba/Sr, La/Th, La/Th, Zr/Hf, [La/Yb]N, [Ce/Yb]N, [Gd/Yb]N, [Sm/Nd]N, [La/Lu]N and Eu/Eu* strongly support the above inference. The ubiquitously comparable REE patterns and incompatible elemental ratios of Rohtas limestones and Lower Kaimur sandstones indicate their derivation from a similar felsic source. The geochemical signatures suggest that the Bundelkhand Gneissic Complex had significantly contributed sediments to the Vindhyan basin prior and during the deposition of Lower Kaimur Group.