抄録
Bundelkhand Craton is one of the Archean cratons in the Indian shield. It is exposed over 29,000 sq. km and is bound by the Son Narmada lineament in the south, the Great Boundary Fault in the west, and probably the Lesser Himalaya in the north. The eastern, western, and southern margins of the Bundelkhand craton are enclosed by the Proterozoic Vindhyan Supergroup and the northern part is overlain by the Indo-Gangetic alluvium plain.
The Bundelkhand craton is composed of high-Al TTG gneisses (3.5-3.3 Ga), meta-supracrustals (basaltic pillow lava, ultramafics, calc silicates, quartzite, BIF, and chert), granite intrusions (2.5-2.1 Ga), NE-WS trending giant quartz veins (2.0-1.8Ga), and NW-SE trending tholeiitic mafic dykes (2.0-0.9 Ga). In present work, we studied mode of field occurrences of metamorphosed supracrustal sequences (Bundelkhand greenstone belt), petrological, geochemical characteristics, and Nd isotopic compositions of metamorphosed mafic and ultramafic rocks, exposed along the highly sheared east-west striking Bundelkhand tectonic zone along 25015`. Field disposition suggest that these supracrustal units predate the granite emplacements of 2.5-2.2 Ga. The metamorphosed mafic volcanics (amphibolites) occur within supracrustal sequences comprising BIF, quartzite, ultramafics, chert and tuff. About 80 represented rock samples were analysed for whole rock major and trace elements. The amphibolites are characterized by higher LIL elements (Ba, Sr) and lower HSF elements (Nb, Zr, Hf, Ta, and Ti). They exhibit nearly flat to LREE enriched chondrite normalized REE patterns. In the MORB normalized plots they exhibit marked Nb and Ta trough. The mode of field occurrences and geochemical characteristics collectively suggest that these amphibolites are metamorphosed basaltic volcanics. The geochemical characteristics are comparable to the modern-day basaltic rocks formed at the convergent plate margins.
Nd isotope data of the amphibolites show wide range of variation (143Nd/144Nd, 0.511509-0. 512626). Since the rocks have relatively high 147Sm/144Nd ratios, we are not able to obtain reasonable depleted mantle model age constraint for the mafic rocks. Nevertheless, the Nd evolution diagram indicates that the mantle beneath the Bundelkhand craton was depleted in Archean time. The present data collectively imply that basaltic rocks were formed during Archean from the depleted mantle source in the subduction related setting.
