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

The Sm-Nd, Rb-Sr and Pb-Pb isotopic systematics of whole rock and sepa rated mineral fractions from four tholeiitic mafic dykes of the Mahbub nagar swarm have been investigated. This dyke swarm intrudes the grani tic basement of the Cuddapah basin in the Eastern Dharwar Craton (EDC) . The Sm-Nd data on whole rock samples of four gabbros and one dolerit e and 15 mineral fractions yielded a linear array with an isochron age of 2173+/-64 Ma and an initial epsilon(Nd) value of 0.4+/-0.3. The Sm -Nd data of a metapyroxenite and its three mineral fractions plot para llel to this array yielding an isochron age of 2189+/-123 Ma with a ne gative initial epsilon(Nd) value of -1.8+/-1.4. Rb-Sr and Pb-Pb intern al 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 ag e. By contrast, Rb-Sr internal isochron ages of three other samples ra nge between 1474 and 1952 Ma, and one sample yields a Pb-Pb internal i sochron age of 3446 Ma, showing the effects of partial isotopic resett ing, possibly due to hydrothermal alteration. These data suggest that mafic dykes of the Mahbubnagar swarm were emplaced at c. 2170 Ma, mark ing an episode of the crustal extension and fracturing which ultimatel y resulted in the formation of the Cuddapah basin. Different petrologi cal and geochemical variants of mafic dykes seen in the area most prob ably represent generation and/or crystallization of the progressively evolved batches of magma during a relatively short time interval. The initial epsilon(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 Sr-87/Sr-86 ratios (0.7027-0.7064) and model mu(1) values (8.3-8.5) of apparently undistu rbed samples are also consistent with this suggestion. (C) 1997 Elsevi er Science B.V.