Abstract

The Proterozoic rocks of Sausar Group, comprising a pack of metasedimentaries and biotite gneisses, are subject of debate from the beginning. Presence of similar rocks as enclaves within the basement gneisses, and also within the cover sediments of Sausar Group is mind boggling. In absence of undoubted sedimentary structures, erection of stratigraphic column of the Sausar metasedimentaries, leaves ample scope of reasonable doubts.

Selective association of the debatable conglomerates with mylonite zones, along with stretching, boudin aging and rotation of the hydrothermal emplacements along these mylonites are suggestive of the formation of autoclasts, deceiving for sedimentary conglomerates. The granitic/gneissic pebbles of these autoclastic conglomerates are the stretched and rotated boudins of the pink feldspar bearing quartzofelspathic emplacements. Biotite gneisses, including Tirodi biotite gneiss and the Junewani gneiss, are products of synkinematic emplacement of huge mass of white feldspar bearing granitic material within pre-existing carbonate dominated sediments. Enclaves of granulites occuring within the biotite gneiss represent ‘skarns’, derived metasomatically from the peripheral part of the Sausar metacarbonates. Both the varieties have identical mineralogy and chemical composition. Further granitization of these skarns/ granulites produce ‘biotite granulites’, ‘hornblende gneiss’ and ‘calc-silicate gneisses’, depending upon the degree of assimilation. The pink feldspar magmatism during late D1 deformation is manifested as widespread quartzo-felspathic activity, especially along the available weak zones, and occasionally form small stocks of pink foliated granite. Boron metasometism and greissening associated with the pink magmatism is responsible for replacement of feldspars by tourmaline and muscovitization/sericitization of the feldspars respectively, producing tourmaline bearing quartz-muscovite schist/gneiss along the periphery of such emplacements. The Sausar metamorphites and the gneisses of the area, therefore, are the products of granitic activity within the largely carbonate rich sediments. It is thought provoking, therefore, if these rocks are amenable to stratigraphic hierarchy.

Keywords: Autoclastic conglomerate; Metacarbonates; Metasedimentaries; Synkinematic emplacements; Skarns; Granulites; Calc-silicates; pink magmatism; Greissening