SmNd, PbPb and RbSr geochronology and petrogenesis of the mafic dyke swarm of Mahbubnagar, South India: implications for Paleoproterozoic crustal evolution of the Eastern Dharwar Craton

Abstract

The SmNd, RbSr and PbPb isotopic systematics of whole rock and separated mineral fractions from four tholeiitic mafic dykes of the Mahbubnagar swarm have been investigated. This dyke swarm intrudes the granitic basement of the Cuddapah basin in the Eastern Dharwar Craton (EDC). The SmNd data on whole rock samples of four gabbros and one dolerite and 15 mineral fractions yielded a linear array with an isochron age of 2173 ± 64 Ma and an initial ϵ Nd value of 0.4 ± 0.3. The SmNd data of a metapyroxenite and its three mineral fractions plot parallel to this array yielding an isochron age of 2189 ± 123 Ma with a negative initial ϵ Nd value of − 1.8 ± 1.4. RbSr and PbPb internal isochron ages of three gabbros and a PbPb internal isochron age of a metapyroxenite dyke range between 2031 and 2284 Ma and, within the limits of experimental error, broadly conform to the SmNd isochron age. By contrast, RbSr internal isochron ages of three other samples range between 1474 and 1952 Ma, and one sample yields a PbPb internal isochron age of 3446 Ma, showing the effects of partial isotopic resetting, possibly due to hydrothermal alteration. These data suggest that mafic dykes of the Mahbubnagar swarm were emplaced at c. 2170 Ma, marking an episode of the crustal extension and fracturing which ultimately resulted in the formation of the Cuddapah basin. Different petrological and geochemical variants of mafic dykes seen in the area most probably represent generation and/or crystallization of the progressively evolved batches of magma during a relatively short time interval. The initial ϵ Nd values calculated at 2170 Ma range from 0.6 to − 2.0 and can be explained as reflecting contamination of depleted mantle derived magma by crustal rocks of EDC. The initial 87Sr 86Sr ratios (0.7027–0.7064) and model μ 1 values (8.3–8.5) of apparently undisturbed samples are also consistent with this suggestion.