The Origin and Tectonic Setting of Precambrian Greywacke of Ribandar‐Chimbel, Goa, India: Petrological and Geochemical Evidence

Abstract

The Precambrian greywacke of Ribandar-Chimbel belonging to the Sanvordem Formation of the Goa Group, India, has been studied for petrography and analyzed for major trace elements. The greywacke is characterized by angular to sub-round grains of quartz, feldspar, biotite, chlorite and clay minerals. The abundance of clay in the matrix seems to have influenced the Al 2 O 3 content and the K 2 O/Al 2 O 3 ratio. The variation diagrams indicate a decreasing trend of TiO 2 , Al 2 O 3 , Fe 2 O 3 and MgO; whereas Na 2 O and CaO exhibit a scatter which could be a result of the variable presence of feldspar within the sediments. The immobile elements, vanadium (25 to 144 ppm), nickel (up to 107 ppm) and chromium (up to 184 ppm), reflect abundance of clay minerals. The greywacke shows strongly fractionated REE patterns with La N /Yb N = 8 to 26 and with higher total REE abundances (up to 245 ppm). The low REE enrichment and depletion in heavier REE with prominent negative Eu anomaly (Eu/Eu*= 0.54 to 0.79) suggest a derivation of the greywacke from an old upper continental crust composed chiefly of felsic components. Petrological evidence and geochemical data suggest that the deposition of the greywacke largely took place in a deep to shallow basin that progressively changed from that of a continental island arc to an active continental setting.saprolite with Fe-rich mottling and/or segregations, Fe-Al segregations and nodular concretions, laterite with fretworks of Fe-rich segregations, semi-indurated Fe-rich tubular textured laterite and indurated laterite cap. Below the fretworks, a palaeo-water-table occurs. It is to be noted that the Merces quarry, of which only the laterite was studied in detail by Widdowson (2009), is the same as the Chimbel quarry described in this paper.