WHOLE-ROCK PB PB ISOTOPIC AGES OF LATE ARCHEAN LIMESTONES, KARNATAKA,INDIA/

Application of the Pb/Pb isotopic dating technique to metamorphosed li mestones from Late Archaean low- and high-temperature greenschist-amph ibolite-facies terrains in Karnataka, southern India, has revealed dis tinct differences in the evolution of each terrain and in their model mu(1) values. The western low-temperature terrain is characterised by tracts of Late Archaean volcanic and sedimentary rocks (Dharwar Superg roup), their basement of orthogneisses and granodiorites (3400-2900 Ma ) and enclaves of older supracrustal rocks. Intrusions of Late Archaea n granite (similar to 2600 Ma) occur sporadically in the low-temperatu re terrain. In contrast, the eastern high-temperature terrain is domin ated by Late Archaean polyphase, syn- and post-tectonic granites (simi lar to 2500 Ma) which intruded tracts of supracrustal rocks that are s imilar to the Dharwar Supergroup in the low-temperature terrain. The l ow- and high-temperature terrains are separated by a N to NNW trending steep belt of mylonites which is over 400 km in length. The Pb/Pb dat a 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. Th is finding is consistent with previously published ages of Late Archae an granites in the low- and high-temperature terrains. Limestones from the low-temperature terrain have a mu(1) value of 7.79, whereas those from the high-temperature terrain have a mu(1) value of 8.50. These v alues suggest that the U and Pb that was incorporated in the limestone s 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 ind icate a further contrast between these two divisions of the Archaean c raton in Karnataka. The nature of the sources is enigmatic, but they m ay reflect differences in composition of the lithospheric plates that were involved in the Dharwar collision system. Data from the high-temp erature terrain indicate disequilibrium between the measured Pb isotop ic ratios of carbonate and residue components of the limestones. Pb is otopic compositions of HCl-insoluble residues are consistently more ra diogenic 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 sc ale of a single hand specimen. It highlights the need for a thorough e valuation of Pb isotope systematics and the potential effects of acid leaching for multi-component limestones.