Geochemistry and tectonic significance of high-Fe bearing tholeiitic dykes from the southern part of Bastar Craton, Central India: An evidence for continental arc magmatism.

Low-Mg and Fe-rich tholeiitic dykes of basaltic andesite to andesites are common in many Precambrian cratons and they develop under arc-related tectonics and also due to the melting of the pyrolitic mantle. Such equivalent rock types are well embedded in the older trondhjemite gneisses of Sukma–Bengpal group in the southern part of Bastar Craton, Central India. These are well demarcated around the Mesoproterozoic Indravati and Sukma basins of the craton, occurring in the form of thin dykes and veins, trending parallel as well as diagonally to the NW–SE and ENE–WSW striking brittle–ductile shear zones. Their petrography shows that they are dominant with ferro-hornblende and magnesio-hornblendes, along with plagioclase, chlorite and accessory minerals of magnetite and ilmenites resembling the older amphibolite variety. The mineral chemistry of amphiboles shows that they are sub-alkaline in nature and crystallised under high oxygen fugacity conditions. The hornblende-plagioclase thermo-barometry indicates that these were re-equilibrated at temperatures of 600–650°C within the low pressures of 3–5 kbar under greenschist to lower amphibolite facies of metamorphism. The whole-rock chemistry represents the andesitic nature of sub-alkaline Fe-rich tholeiitic type. The primitive mantle and MORB normalised spider diagrams show strong enrichment of LIL elements (K, Rb, Ba, Th) and depletion of HFSE (Zr, Hf, Ti, Yb, Sc) with negative Nb anomalies. The above geochemical data on various tectonic discrimination plots reveal that they originated under continental arc volcanism of subduction-related tectonics and were derived from the spinel-garnet lherzolite melting source as similar to various metabasalts and arc tholeiites described from several parts of the craton. [ABSTRACT FROM AUTHOR]