Geochemical Constraints on the Petrogenesis of the Metasedimentary Rocks Forming the Basement of the Shillong Plateau, Northeast India

Abstract

Granite gneisses, cordierite bearing granulitic gneisses (pelitic gneisses) and quartz-sillimanite schists (pelitic schists) comprise the Precambrian metamorphic basement complex of the Shillong plateau. Pelitic gneisses and pelitic schists were geochemically studied to determine the paleoweathering conditions and provenance characteristics of their precursor sediments. The chemical indices of alteration and weathering (CIA and CIW) values of the pelitic gneisses vary from 62 to 84 and from 74 to 96 respectively, which indicate a moderate to extremely weathered precursor that was formed probably in a warm and humid climate. The pelitic schists are characterized by CIA and CIW values that range from 99.2 to 99.6 indicating the schists probably had extremely weathered precursor(s).Chondrite normalized rare earth elements (REE) patterns of the rock suites display highly fractionated REE patterns along with prominent negative Eu anomaly [pelitic gneisses: LaN/YbN{}={}19.7 − 81.7, Eu/Eu*{}={}0.17 − 0.68; pelitic schist: LaN/YbN{}={}27.4 − 43.7, Eu/Eu*{}={}0.15 − 0.32]. Post-Archean Australian Shale (PAAS) normalized multi-element diagram for the pelitic gneisses exhibits enrichment of Nb, Th, Ce, La, Ba, K, Rb with strong depletion of Zr and Sr., suggesting precursor sediments were rich in clay fractions. The pelitic schists however exhibit a highly fractionated trend with enrichment of Zr, Th, Ce and La and depletion of P, Nb, Ti, Y, Sr, Ba, K and Rb relative to PAAS. The geochemical characteristics of both the rock types thus suggest that the precursor sediments were derived from felsic sources; however, those for the schists were derived from a recycled felsic source.