POSTCRYSTALLIZATION DISTURBANCE IN THE NEODYMIUM AND LEAD-ISOTOPE SYSTEMS OF METABASALTS FROM THE RAMAGIRI SCHIST BELT, SOUTHERN INDIA

Archean metabasaltic rocks from three separate blocks that constitute the north-south trending Ramagiri schist belt of the Eastern Dharwar C raton have large variations in epsilon(Nd)(2750). Even samples of the same rock type from one outcrop that are less than 50 cm apart from ea ch other show a range of up to 5.5 epsilon(Nd)(2750) units. The trends of the fields defined by the three suites in f(Sm/Nd) VS. epsilon(Nd) (2750) space intersect at f(Sm/Nd) = +0.1 to +0.2 and epsilon(Nd)(2750 ) = -8 to -12, suggesting that these racks have lost LREEs and have in teracted with and exchanged at least Nd with a common extraneous phase . The isotopic composition of this phase suggest derivation of its neo dymium from sources which had a long-term history of LREE enrichment. This phase may have derived its isotopic composition from older contin ental crust. One of the unaltered dikes cutting the three blocks in th e belt and the surrounding gneisses gives a Sm-147/Nd-144-Nd-143/Nd-14 4 age of 2454 +/- 100 Ma, a minimum for the time of block assembly and alteration of REEs including Nd. While a set of samples from the cent ral block lie along an isochron with a Pb-Pb age of 2746 +/- 64 Ma, th e samples from the eastern and western blocks show large uncorrelated variations in Pb-207/Pb-204 with Pb-206/Pb-204. Residues after acid le aching of sample powders have lead isotope ratios that are distinct fo r samples from each of the block, and are compatible with their deriva tion from Archean mantle. The lead isotope composition of the acid lea ches suggest disturbance of the metabasalts by an isotopically extrane ous phase that had variable and high Pb-207/Pb-204 and Pb-208/Pb-204 r atios for a given Pb-206/Pb-204, relative to expected Archean mantle v alues. Both lead and neodymium isotope disturbances are best explained as a result of alteration of these rocks by an extraneous phase which had derived its isotopic composition from ca. 3.5 Ga continental crus t.