Rare earth element geochemistry of the Permo-Carboniferous clastic sedimentary rocks from the Spiti Region, Tethys Himalaya: significance of Eu and Ce anomalies

Abstract

Siliciclastic sedimentary rocks, including sandstones and associated shales, from the Permo-Carboniferous Kanawar Group of NW Tethys Himalaya, Spiti Region, India were examined geochemically to monitor the evolutionary changes in the upper continental crust in the Himalaya. The rocks are characterized by consistent rare earth element (REE) patterns with light REE enrichment (LaN/YbN = 5.3–28.2) and flat heavy REE patterns. The ∑REE values are high (up to 281 ppm) with large negative Eu anomalies (avg. Eu/Eu* = 0.57). The REE characteristics of the sediments are similar to those of post-Archean Australian shales and North American shale composite. La/Th values (avg. 2.34) correspond to a relatively felsic composition of the terrestrial igneous rocks standard (La/Th of G-1 = 2.3). The evolved felsic composition of the sediments probably relates to widespread acidic activity in the source. The REE patterns and Th/U values seem to have been affected by the sedimentary environment as well as by the provenance. The presence of positive Ce anomalies in some sediments may be the result of post-depositional processes. Moreover, the Permo-Carboniferous sediments indicate that hydraulic sorting, even over short transport distances, is capable of concentrating enough accessory phases to influence REE composition and to develop negative Eu anomalies. High ∑REE, La/Yb, and Th/U contents and large negative Eu anomalies reveal that the sediments were deposited in an oxidizing environment, suggesting the surficial environment became oxidizing around the Carboniferous-Permian boundary in the Indian craton.