Petrogenesis of ultramafic rocks with abyssal peridotite affinity from the Central Bundelkhand Craton, India

The Bundelkhand craton in India preserves important records of archean geological evolution, where several ultramafic rocks belonging to the Babina Greenstone Belt (BGB) occur as isolated and oval shaped bodies. These rocks are composed of olivine, orthopyroxene, amphiboles, and serpentine along with accessory mineral phases like chromian spinel and ilmenite. Here we present the major and trace element geochemistry of these ultramafic rocks that are characterised by low SiO₂ (45.16–49.00 wt%), high MgO (24.41–29.15 wt%) and moderate Fe₂O₃ (5.82–9.95 wt%) with high Ni (1164–1674 ppm), Cr (1532–3477 ppm) and Cu (14.7–39.5 ppm) suggesting primary magmatic nature. The rocks show low rare earth element (REE) content (ΣREE 2.1–3.5 ppm) with depleted LREE pattern and flat to slightly fractionated HREE pattern similar to abyssal peridotite signature. The Nb/Yb ratio ranges between 0.01 to 0.20 (average = 0.03), similar to that of N-MORB, suggesting magma derivation from a depleted mantle source, further substantiated by the Th/Yb vs. Ta/Yb plot. Trace elements like Ta and Pb show positive spikes, whereas La, Nb, Pr and Ce show depleted nature. The rocks generally have low platinum group elements (PGE) content (<150 ppb) except one sample where it goes up to 388 ppb. The ΣPPGE concentration is higher than ΣIPGE for all the samples and the high Pd/Ir ratio (7.55–20.98) indicating the derivation of these ultramafic rocks from low degree of partial melting. Our data suggest that the ultramafic rocks were derived from a depleted mantle source at a shallow depth with affinity towards abyssal peridotite. These rocks might represent residue after extraction of low degree melt (∼2–10%) in a mid-oceanic ridge (MOR) setting, which were captured and brought to shallow levels and subsequently exposed on the surface.