Tracking Precambrian Crustal Sources through Heavy Mineral and Bulk Geochemistry: Cauvery River Sediments, Dharwar Craton–Southern Granulite Terrain, India

Abstract

Provenance and chemical weathering studies of river sediments usually work at one scale, either the bulk sample or a single mineral, and the two rarely meet. Here we combine bulk geochemistry and detrital ilmenite chemistry for bed sediments of the Cauvery (Kaveri) River, which drains the Dharwar craton and the Southern Granulite Terrain of peninsular India. We analysed 15 bed-load samples from Mettur dam to the river mouth by XRF, and detrital ilmenite from eight sites along the Tamil Nadu reach by electron probe. We then compared the results with published data for sand, clay and suspended sediment from major Indian and world rivers. The bulk sands are silica-rich (SiO2 64.9–84.6 wt%) and their chemical index of alteration (CIA 41.7–61.4; mean 53.7) is close to that of the upper continental crust. CIA is controlled almost entirely by the amount of Ca-bearing plagioclase and mafic minerals (CIA–CaO, r = −0.89), not by weathering intensity. Seen at grain scale, the same bed load is equally fresh. Ilmenite is 84–97 mol% FeTiO3, no grain exceeds the stoichiometric TiO2 of ilmenite, and its Fe3+ is held in primary ilmenite–hematite solid solution (TiO2–Fe2O3, r = −0.99) rather than produced by alteration to pseudorutile. Both scales point to a mixed source. Bulk chemistry identifies Ca–Mg-rich sands from charnockite and gneiss in the upper reach, quartz-rich felsic sands in the middle reach and a heavy-mineral concentrate at the mouth. Ilmenite separates Mg-bearing grains from mafic granulites and Mn- and Fe3+-rich grains from felsic or metamorphic rocks, which are concentrated in the middle reach. Compared with other rivers, the Cauvery bed load resembles bed sediments of large Tibetan–Himalayan rivers (CIA 46.5–69.6). It is about 28 CIA units less weathered than the river's own clay (81.6) and far below the world mean for suspended sediment (72.1). Cauvery ilmenite is also fresher and lower in TiO2 than altered beach-placer ilmenite on the Indian coast. We argue that bulk indices from bed load describe sorting and source mineralogy rather than climate, and that single-grain chemistry, checked first for alteration, is needed to read provenance reliably. We close with a practical framework for future river-sediment studies.