A Study of Garnet and Chromian Spinel Xenocrysts from the Atri South Ultramafic Intrusion, Bundelkhand Craton, India

Abstract

Abstract The Bundelkhand craton is one of five Archean cratons that make up the Indian shield, four of which have yielded diamond discoveries. The Bunder diamond project consists of seven ultramafic intrusions, with the Mesoproterozoic Atri pipe being the largest and most prospective in terms of its diamond content. Despite the Majhgawan diamond mine also being located on the Bundelkhand craton, little information exists regarding the section of subcontinental lithospheric mantle that these deep-seated intrusions have sampled. This paper reports major and trace element data for xenocrystic chromian spinels and garnets, allowing some new interpretations of the mantle section and geotherm to be made. All of the xenocrysts recovered are of peridotitic paragenesis, with no evidence of eclogitic material being observed in concentrate. Calculated equilibration temperatures for garnet and chromian spinel indicate sampling at a range of depths. The chromian spinel can be divided into three groups based on major and minor element characteristics, with each group being derived from different horizons within the subcontinental lithospheric mantle. The garnet data can be divided into five groups based on their Ca, Cr, and rare earth element (REE) contents. The REE profiles of the groups span from a near-primitive mantle signature to extremely sinusoidal, which can be accounted for by varying amounts of initial melt depletion and/or metasomatic reenrichment. Equilibration temperatures for garnet overlap with chromian spinel at temperatures from ~1,100° to 1,250°C, near the base of the sampling profile. Using calculated minimum garnet equilibration pressures, the data suggest a geotherm relating to a model heat flow of ~40 mW/m2, which is similar to that previously determined using xenoliths from numerous intrusions in the eastern Dharwar craton to the south. Despite the similarity in their geotherms, previously reported geophysical data have suggested differences in the thickness and composition of the subcontinental lithospheric mantle between the two cratons. While modification or destruction of the Dharwar cratonic root after the breakup of Gondwana has been documented, seismic evidence suggests the same event may not have affected the Bundelkhand craton where the root appears to be preserved. If the subcontinental lithospheric mantle beneath the Bundelkhand craton escaped significant heating and metasomatism in the Mesozoic, then Cretaceous-aged kimberlite or lamproite intrusions may have greater potential to be diamond bearing, whereas intrusions of this age in the Dharwar or Bastar cratons do not.