Major and Trace Element Characteristics of the Average Indian Post-Archean Shale: Implications for Provenance, Weathering, and Depositional Environment
Esha Ray, Debajyoti Paul
Abstract
We estimate the average chemical composition of Indian post-Archean Shale (IPAS) and conclude that it is comparable to that of global shale standards: post-Archean Australian Shale (PAAS) and North American Shale Composite (NASC). These shales are widely considered as continent wide, average compositional representatives of the post-Archean upper continental crust (UCC). Our IPAS estimate includes compiled literature (n = 444) and newly obtained (23 samples from 16 shale litho-units) data. The average IPAS has a composition comparable to that of the average, fine-grained matrix of glacial diamictites (tillite). Compared to a global model of UCC, notable differences in some elements are observed. Our estimated IPAS composition is more comparable to PAAS than NASC, with the latter being distinctly different likely due to having been recently glaciated and significantly resurfaced by the last glaciation, resulting in new shale production starting only about 12,000 years ago. Comparisons between the post-Archean and Archean Indian shales finds the latter with lower ΣREE; smaller negative Eu anomaly; lower Th/Sc, Th/Cr, and Th/U ratios; and higher Sc, V, Cr, Cu, and Zn contents. These findings suggest a greater contribution from mafic sources to Archean shales. A higher chemical index of alteration (CIA = 80 ± 2) for Indian Archean shales, relative to the post-Archean shales (72 ± 3) and UCC (59–65), suggests more intense weathering in the pCO2-rich Archean time. Indian shales were mostly deposited in plate interiors or intracratonic basins along a passive margin based on tectonic discrimination diagrams. Our IPAS estimate reflects the average composition of Indian sedimentary rocks and serves as the first step toward estimating the chemical composition of Indian UCC.