Pb and Nd isotope constraints on the origin of high Mg and tholeiitic amphibolites, Kolar Schist Belt, South India

Abstract

The late Archean, north-south trending Kolar Schist Belt in south India, 4 km wide by 80 km long, is thought to be a suture between two gneiss terranes (Krogstad et al. 1989). Within this volcanics-dominated belt are recognized both tholeiitic and high Mg (komatiitic and picritic) amphibolites, which make up some 70% and 5% respectively of the exposed outcrops. A massive tholeiitic amphibolite separates the belt into western and eastern parts. A Pb-Pb whole-rock age of 2732±155 Ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock. Samples of this rock have ε_Nd values at 2700 Ma that range between +3.8 and +6.8, μ₁ (initial ²³⁸U/²⁰⁴Pb) of 7.5 and κ₁ (initial ²³²Th/²³⁸U) of about 4. Two different types of high-Mg amphibolites are recognized from the western part of the belt: a picritic or P-type, and a komatiitic or K-type. The P-type have highly variable Ce/Al ratios all greater than chondritic, Nd/Yb ratios greater than chondritic, ε_Nd at 2700 Ma of +1.5 to +8, and Pb isotope compositions variable in ²⁰⁷Pb/²⁰⁴Pb with μ₁ of about 8.0 and κ₁ of about 4. The trace-element data suggest that the light-REE enrichment is a character of the mantle source and is not due to residual garnet. The K-type amphibolites have near chondritic Ce/Al and Nd/Yb ratios, ε_Nd at 2700 Ma of +1.5 to +8, and μ₁ of about 8 and k 1 of about 4. Although the P-type is light-REE enriched compared to the K-type, both types have similar Ce/Nd ratios as well as initial Pb and Nd isotopes. If the 2696±136 Ma age for the Sm-Nd isochron which includes both types of high-Mg amphibolite has any significance it dates the time of light-REE enrichment of the mantle source for the P-type komatiitic amphibolites. The high-Mg amphibolites in the eastern part of the belt are light-REE enriched, have Pb isotopic compositions that are variable in ²⁰⁷Pb/²⁰⁴Pb with a μ₁ about 8.5 and ε_Nd at 2700 Ma of +1.8 to +4.5. Hydrothermal fluids associated with metamorphism and shearing prior to about 2400 Ma ago were responsible for the introduction of gold-quartz-carbonate veins into the Kolar Schist Belt. The Pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous Pb from adjacent older granitoid gneisses into the amphibolites, which could be responsible for the variability in the ²⁰⁷Pb/²⁰⁴Pb ratios of the samples. This extraneous Pb probably is not responsible for the distinct Pb isotope character of each type of amphibolite.