Contrasting Precambrian Mafic Dykes of the Bastar Craton, Central India: Petrological and Geochemical Characteristics

Numerous mafic dykes occur within the Archacan-granite-gneisses of the Bastar (Bhandara) craton of central India. Width of these dykes varies from about 20 to 200 metres and are 1 to 20 kilometers long. The dykes trend mainly NW-SE, following the foliation of the granite gneiss and major regional structural trend. They mainly comprise two different types, one, amphibolitic and the other doleritic. The amphibolite dykes have a granoblastic texture whereas the unaltered dolerite dykes are ophitic or sub-ophitic. All of the dykes are sub-alkaline Fe-rich tholeiites. However, variations in the incompatible elements such as Nb, P, Ti, Zr, Y and the light rare earth elements (LREE), suggest that the Bastar dykes infact comprise two different sets. The amphibolite dykes were derived from an earlier relatively high-Mg, low-Ti olivine tholeiite magma. These dykes have high field strength element (HFSE) values only around six times higher than those of primordial mantle. In contrast, the doleritic dykes have fractionated HFSE levels 10 to 30 times higher than primitive mantle values; they are also characterised by a marked negative Sr anomaly not present in the older dykes. One subset of amphibolitic dykes corresponds geochemically to the predominantly doleritic swarm. These latcr dykes are derived from a low-Mg, high-Ti quartz-tholeiitic magma. The two different Bastar dyke sets are recognized here as the older, BD1 amphibolitic dykes and younger, BD2 doleritic dykes. Age constraints for these dykes are set by the ages of the enveloping host gneisses and discordant granitoids and hence they are placed at or near the end-Archaean.