Postcrystallization disturbance in the neodymium and lead isotope systems of metabasalts from the Ramagiri schist belt, southern India

Abstract

Archean metabasaltic rocks from three separate blocks that constitute the north-south trending Ramagiri schist belt of the Eastern Dharwar Craton have large variations in ε_Nd²⁷⁵⁰. Even samples of the same rock type from one outcrop that are less than 50 cm apart from each other show a range of up to 5.5 ε_Nd²⁷⁵⁰ units. The trends of the fields defined by the three suites in f^Sm/Nd vs. ε_Nd²⁷⁵⁰ space intersect at f^Sm/Nd=+0.1 to +0.2 and ε_Nd²⁷⁵⁰ -8 to -12, suggesting that these rocks have lost LREEs and have interacted with and exchanged at least Nd with a common extraneous phase. The isotopic composition of this phase suggest derivation of its neodymium from sources which had a long-term history of LREE enrichment. This phase may have derived its isotopic composition from older continental crust. One of the unaltered dikes cutting the three blocks in the belt and the surrounding gneisses gives a ¹⁴⁷Sm/¹⁴⁴Nd- ¹⁴³Nd/¹⁴⁴Nd 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 central block lie along an isochron with a Pb-Pb age of 2746±64 Ma, the samples from the eastern and western blocks show large uncorrelated variations in ²⁰⁷Pb/²⁰⁴Pb with ²⁰⁶Pb/²⁰⁴Pb. Residues after acid leaching of sample powders have lead isotope ratios that are distinct for samples from each of the block, and are compatible with their derivation from Archean mantle. The lead isotope composition of the acid leaches suggest disturbance of the metabasalts by an isotopically extraneous phase that had variable and high ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios for a given ²⁰⁶Ph/²⁰⁴Pb, relative to expected Archean mantle values. 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 crust.