Rare earth element geochemistry of Post- to Neo-archean shales from Singhbhum mobile belt, Eastern India: implications for tectonic setting and paleo-oxidation conditions

Abstract

In the present study, trace element (including rare earth element) chemical data has been interpreted with the aim to decipher the tectonic setting and paleo-redox conditions of the Post- to Neo-archean shales from the Singhbhum Mobile Belt, eastern India. The data show moderate enrichment of compatible elements [such as Cu (avg. 59.28 ppm), Ni (avg. 59.49 ppm), V (avg. 234.24 ppm) and Cr (avg. 181.23 ppm)] relative to the Post-Archean Australian Shale. Their chondrite normalized light rare earth elements are moderately fractionated [(La/Sm)N ranges from 2.21 to 5.78], whereas heavy rare earth elements show a nearly flat pattern [(Gd/Lu)N ranges from 0.74 to 1.68]; this indicates that the rare earth element (REE) concentrations, rather than being severely affected by the diagenesis and weathering processes, decreased gradually from Gd to Lu. The Post-Archean Australian Shale normalized multi-element diagram shows the slight enrichment of Cr, V, Zr, Y, U and Sc, whereas Sr, Pb, Hf and Th are depleted. The notable negative anomaly of Sr indicates the least accumulation of plagioclase, which is also supported by the negative Eu-anomaly in these rocks. The La–Th–Sc and Th–Zr–Sc tectonic setting diagrams indicate their continental arc setting. The geochemical parameters, such as U/Th, V/Cr, Ni/Co, and Cu/Zn, indicate that these shales were deposited under oxic to anoxic environmental conditions.