Early precambrian mafic dyke swarms from the central Archaean Bastar Craton, India: geochemistry, petrogenesis and tectonic implications

Abstract

The Archaean Bastar craton is known for the presence of different generations of mafic dykes. Less studied many NW-SE trending mafic dykes (intruded into the Archaean supracrustal rocks), encountered from the central part of the Bastar craton, are studied for their petrological and geochemical characteristics. There are many geological evidences which suggest emplacement of these mafic dykes in an intracratonic setting. Two distinct types of mafic dykes are recognized. Petrographically it is difficult to discriminate these two types. Many samples show metamorphic textures and amphibolites facies mineral assemblage but a few samples preserved original igneous texture and mineralogy. Major oxides show sub-alkaline thoeliitic basalt/basaltic andesite nature. High-Fe and high-Mg contents classify them as high-iron and high-magnesium tholeiites. They fall in the gabbronorite field on R1-R2 plot. Geochemical characteristics, particularly high-field strength and rare-earth elements, clearly distinguished them into two types: Group 1 and Group 2 mafic dykes. When compared with well-studied mafic dyke swarms of the Southern Bastar craton, the Group 1 is recognized as Meso-Neoarchaean sub-alkaline mafic dykes (BD1-CBC) and the Group 2 as Neoarchaean-Palaeoproterozoic boninite-norite mafic dykes (BN-CBC). The boninite-norite nature of the second group is also corroborated through immobile trace element ratios. BD1-CBC dykes are characterized by a relatively higher concentration of HFSE and REE in comparison to BN-CBC dykes. Some extent of crustal contamination is observed in BN-CBC dykes but BD1-CBC dykes do not show any indication of crustal contamination. Trace element modelling suggest that BD1-CBC mafic dykes are derived from a melt originated through ~20% melting of a depleted mantle source, whereas BN-CBC mafic dykes are probably derived from a high-Mg magma generated through ~25% melting of a refractory mantle source. Both melts have undergone 30-40% olivine fractionation before the emplacement. Geochemistry also points out involvement of a plume in the genesis of these mafic dykes.