Contents lists available at ScienceDirect Geochemistry journal homepage: www.elsevier.com/locate/chemer Evidences of felsic volcanism and hydrothermal activities from clays associated with the Palaeoproterozoic Porcellanite Formation of the Vindhyan Supergroup, Central India Mageswarii G. a , Sucharita Pal a , Meenal Mishra b , J.P. Shrivastava a, ⁎ a Department of Geology, University of Delhi, Delhi, 110007, India b School of Sciences, Indira Gandhi National Open University, New Delhi, 110068, India ARTICLE INFO Handling Editor: P.D. Roy Keywords: Porcellanite Vindhyan Supergroup Weathering Palaeoenvironment Clay minerals Hydrothermal alteration ABSTRACT Chemico-mineralogical attributes of authigenic clays associated with the altered volcanic tuffs that occur in the Palaeoproterozoic Porcellanite Formation contain evidences of hydrothermal alteration and diagenetic processes in a marine environment. Previous sedimentological and geochemical studies on Porcellanite Formation were restricted to the Chopan area, but, the details related to provenance, nature and source of volcanism archived in these clays have not been ascertained. In order to understand these aspects, present study on these authigenic clays were carried out. Clay minerals represent dominance of illite with subordinate amount of montmorillonite. Moreover, low abundance of kaolinite is also noticed. The illite fibers and plates associated with the kaolinite indicate illitization. The kaolinite to illite transformation is favoured by incorporation of K + ions, derived from the K-feldspar dissolution and its overgrowth. Major oxide contents of these clays and their ratios when plotted over diagrams marked with standard illite, kaolinite, smectite and chlorite compositional fields show clustering within or close to the illite field. Thermodynamic components calculated for these clays when plotted over AR 2 3+ Si 3 O 10 (OH) 2 -R 2 3+ SiO 4 O 10 (OH) 2 -A 3 AlSi 4 O 10 (OH) 2 ternary diagram, data plots lie within the illite, mixed layer I/S and smectite fields. Binary major oxide data plots between bulk rock and authigenic clay compositions showed felsic affinity. Montmorillonite and illite predominated in the eastern and western mar- ginal areas of the Vindhyan Basin, respectively. However, former resulted from the hydrothermal alteration of volcanic glass associated with the ferruginous breccia and altered tuffs and remnants of the volcanic vents, whereas, later is associated with the tuffaceous beds. Owing to the adsorption, Ba, Rb and Sr is enriched in clays comparing to the bulk rock composition. Low (< 15 ppm) Sc values suggested major contribution from the felsic component. Also, low Rb/Sr and Th/U values revealed moderate insitu weathering. The dominance of K-feldspar alteration and insitu weathering is also evident from clustering of clay data plots in the A-CN-K ternary diagram. Pronounced negative Eu anomaly together with higher LREE/HREE values associated with these clay minerals implied proximity to source and their possible derivation from the silicified felsic tuffs available in the prove- nance. 1. Introduction Volcanic eruptions yielded pyroclastics, including ash (≤ 2 mm), lapilli (2–64 mm), and bombs (≥ 64 mm). Moreover, volcanic glass (70–80%), phenocrysts (20–25 %) and negligible amount of amorphous clays (˜5%) constitute major component of the volcanic ash (Kawano and Tomita, 2001). As a result of compaction and lithification followed by deposition (Dehio, 1998), finer pyroclastics of the volcanic ash and dust settled down and form un-welded thinly bedded tuffs and tuffites (Mishra et al., 2017a). These tuffs transformed into bentonites through devitrification along with the chemical and mineralogical modifications (Huff and Türkmenoğlu, 1981; Fortey et al., 1996). Alteration process is largely dependent upon environmental conditions and pore-water chemistry of the depositional environments (Ddani et al., 2005; Cuadros et al., 2013; Fang et al., 2017). Devitrification of volcanic glass to authigenic clay minerals is perhaps causative for the significant changes in the bulk geochemistry of the ash or tuffs, especially with respect to K, Mg, and other mobile elemental contents (Zielinski, 1985; Mitchell et al., 2004; Huff, 2016). Process of devitrification yielded dissimilar authigenic clay minerals in diverse facies such as smectite https://doi.org/10.1016/j.geoch.2019.02.002 Received 10 July 2018; Received in revised form 16 January 2019; Accepted 3 February 2019 ⁎ Corresponding author. E-mail address: jpshrivastava.du@gmail.com (J.P. Shrivastava). Geochemistry xxx (xxxx) xxx–xxx 0009-2819/ © 2019 Elsevier GmbH. All rights reserved. Please cite this article as: Mageswarii G., et al., Geochemistry, https://doi.org/10.1016/j.geoch.2019.02.002