B. Krishna Rao

Whole-rock PbPb isotopic ages of Late Archaean limestones, Karnataka, India

Application of the PbPb isotopic dating technique to metamorphosed limestones from Late Archaean low- and high-temperature greenschist-amphibolite-facies terrains in Karnataka, southern India, has revealed distinct differences in the evolution of each terrain and in their model μ1 values. The western low-temperature terrain is characterised by tracts of Late Archaean volcanic and sedimentary rocks (Dharwar Supergroup), their basement of orthogneisses and granodiorites (3400-2900 Ma) and enclaves of older supracrustal rocks. Intrusions of Late Archaean granite (∼ 2600 Ma) occur sporadically in the low-temperature terrain. In contrast, the eastern high-temperature terrain is dominated by Late Archaean polyphase, syn- and post-tectonic granites (∼ 2500 Ma) which intruded tracts of supracrustal rocks that are similar to the Dharwar Supergroup in the low-temperature terrain. The low- and high-temperature terrains are separated by a N to NNW trending steep belt of mylonites which is over 400 km in length. The PbPb data show that metamorphic recrystallisation of the limestones in the low-temperature terrain took place 2639 ± 32 Ma ago, whereas limestones in the high-temperature terrain recrystallised 2475 ± 65 Ma ago. This finding is consistent with previously published ages of Late Archaean granites in the low- and high-temperature terrains. Limestones from the low-temperature terrain have a μ1 value of 7.79, whereas those from the high-temperature terrain have a μ1 value of 8.50. These values suggest that the U and Pb that was incorporated in the limestones of the low-temperature terrain came from a less evolved source than the U and Pb in the limestones in the high-temperature terrain and indicate a further contrast between these two divisions of the Archaean craton in Karnataka. The nature of the sources is enigmatic, but they may reflect differences in composition of the lithospheric plates that were involved in the Dharwar collision system. Data from the high-temperature terrain indicate disequilibrium between the measured Pb isotopic ratios of carbonate and residue components of the limestones. Pb isotopic compositions of HCI-insoluble residues are consistently more radiogenic than those of associated carbonate minerals and they define a younger, rotated palaeoisochron that has no age significance. This is the first documentation of carbonate-residue disequilibrium on the scale of a single hand specimen. It highlights the need for a thorough evaluation of Pb isotope systematics and the potential effects of acid leaching for multi-component limestones.