Evaluation of terrigenous input, diagenetic alteration and depositional conditions of Lower Carboniferous carbonates of Tethys Himalaya, India

Abstract

Geochemical and isotopic signatures from Lower Carboniferous Limestone of the Lipak Formation of Spiti region, Tethys Himalaya (for the first time) have been studied for palaeo-climatic reconstruction of the region and to evaluate the influence of terrigenous input into the Tethyan Ocean. The principal component analysis (PCA) suggests that CaO exhibits different modes of origin as compared to other elements. The low and relatively constant Al/Si ratio (0.19–0.46) of Lipak limestone suggests that Si has multiple sources besides clay minerals. Higher Al2O3 of the limestone than siliciclastic-contaminated carbonates (1.59%) as well as comparable Y/Ho ratio to terrestrial material suggests terrigenous input into the basin. The Er/Nd ratio ranges from 0.07 to 0.18 which further indicates the influence of detrital input (in normal seawater Er/Nd is 0.27). The Mn/Sr ratio suggests insignificant diagenetic alterations. The lack of correlation between Sr and LREE concentration in the limestone (r = 0.036) lends credence to this conclusion. Geochemical palaeo-redox parameters such as negative Mn and positive Ce anomalies (V/V + Ni) and Th/U ratios indicate that the limestone was deposited in anoxic environmental conditions. δ18O and δ13C isotope composition of whole-rock samples ranges from −10%o to −24%o and +3.34%o to −8.24%o respectively indicating significant climatic change during Carboniferous period. These observations are consistent and match with those of contemporaneous carbonates elsewhere in the world, which can be linked to the alternative occurrences of cold and warm paleoclimates in the Carboniferous. The onset of the Permo-Carboniferous glaciation would have resulted in more positive δ13C and less negative δ18O values for these carbonates. The isotopic values also indicate eustatic sea level changes due to the glacial and interglacial phases during the period. Thus the signatures of Gondwana glaciation events are well preserved in the Lipak limestone of Spiti region. Keywords: Carboniferous limestone, Geochemistry, Isotopes, Spiti region, Tethys Himalaya