Geochemistry and Sm–Nd isotope systematics of mafic-ultramafic rocks from the Babina and Mauranipur greenstone belts, Bundelkhand Craton, India: Implications for tectonic setting and Paleoarchean mantle evolution

Abstract

The Paleoarchean geodynamical evolution of the Bundelkhand Craton is discussed based on the geochemistry (major, trace and rare earth elements) and Sm–Nd isotope systematic of mafic-ultramafic rocks from volcano-sedimentary succession of the Babina and Mauranipur greenstone belts. The petrography and mineral assemblages of these mafic-ultramafic rocks imply that they were gone through greenschist to amphibolite facies metamorphism. Geochemical analyses indicate that mafic rocks from the Babina are characterized by SiO 2 = 43.9–51.2 wt%, MgO = 5.4–11.0 wt%, TiO 2 = 0.3–1.14 wt% and Mg# = 44–77, whereas the Mauranipur are characterized by higher silica (51.8–55.6 wt%), MgO = 6.9–9.5 wt% and Mg# = 59–70. The ultramafic rocks from the Babina and Mauranipur contain SiO 2 = 46.9–50.3 wt%, MgO = 20.2–21.1 wt%, TiO 2 = 0.33–0.46 wt% and Mg# = 77–82. Six samples of mafic-ultramafic rocks from the Babina greenstone belt yield 147 Sm– 143 Nd whole rock isochron age of ca. 3.4 Ga. Sm–Nd isotopic studies of mafic rocks from the Babina and Mauranipur reveal that they were derived from a depleted mantle source. Bivariate diagrams such as Cr vs. Ni and V suggest that these magmas might have evolved through fractionation of clinopyroxene and olivine. The mafic rocks from the Babina displayed almost a flat REE and HFSE profile [(La/Yb) PM = 0.87–1.40] with negative Nb (Nb/Nb* = 0.13–0.77) and positive Pb anomalies that could be attributed to metasomatic agents derived from subducting crustal rocks. The Mauranipur mafic rocks show slightly enriched REE [(La/Yb) PM = 1.1–1.7; (La/Sm) PM = 1.1–2.0] with negative Nb anomalies (Nb/Nb* = 0.13–0.77), which are most likely the effects of crustal contamination. The ultramafic rocks from both areas and ultramafic cumulate rock from the Mauranipur display a progressive enrichment from Th, La to Yb, with negative Nb anomalies (Nb/Nb* = 0.40–0.73) with lack of Zr anomalies (Zr/Zr* = 0.10–0.30) that could be attributed to their derivation from shallower mantle. These data collectively suggest that the mafic-ultramafic rocks have been interpreted to be derived from oceanic crust in a subduction-related setting with depleted mantle composition.