Abstract
The Paleoproterozoic Aravalli Supregroup of rocks, hosted in Aravalli Craton of NW shield, is deposited in shallow and deep water conditions. The major lithologies are phyllites and quartzites with significant components of greywacks and dolomite. Geochemical indices in particular, CIA (Chemical Index of Alteration) values (avg. phyllites: 51.6 - 81.5, avg. quartzites: 57.4 - 95.5) calculated from the data of clastic rocks of ASG suggest minimum to highly intense weathering in the source region. Other indices including PIA, CIW and ICV along with plot patterns on the A-CN-K diagram also nearly endorse CIA based interpretation. These rocks possess relatively high Th/U ratios compared to that found in fresh igneous rocks or their high grade metamorphic equivalents. This high Th/U ratio is neither a source inheritance nor a result of oxidation state rather a manifestation of Th hosting mineral accumulation through sorting. Viewed in the context of present stratigraphic succession, the weathering history adduced from geochemistry does not seem compatible but matches well with earlier classification scheme wherein the evolution of Aravalli Supergroup was considered episodic.
Highlights
Sedimentary rocks are of great interest since quite long time as they have been effectively used in unfolding the history of diverse geological settings
The Plagioclase Index of Alteration (PIA) values of Aravalli clastics suggests that initially the source rock(s) undergone relatively small degree of chemical weathering which became quite intense with the passage of time which significantly converted plagioclase into clay minerals
Sediments/sedimentary rocks are the final products of a chain of dynamic phenomena including weathering, ablation, transport, deposition and diagenesis
Summary
Sedimentary rocks are of great interest since quite long time as they have been effectively used in unfolding the history of diverse geological settings. S. Khan by many factors including, transportation mechanism, composition of the source rocks, environmental parameters, duration and intensity of weathering, depositional environment and post depositional makeup (e.g., diagenesis and metamorphism) [1]. Khan by many factors including, transportation mechanism, composition of the source rocks, environmental parameters, duration and intensity of weathering, depositional environment and post depositional makeup (e.g., diagenesis and metamorphism) [1] Among all these aspects, the main controlling features are source rock composition [2] [3] and the tectonic setting of the sedimentary basin [4]. Aravalli craton of NW Indian shield preserves thickest and extensively developed Proterozoic sedimentary sequence [18]. The present study is the maiden attempt wherein a comprehensive account of weathering history is presented on the basis of major and trace elements of entire clastic litho-spectrum of Aravalli fold belt
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