Mineralogy and Geochemistry of Kimberlites Nk-2 and Kk-6, Narayanpet Kimberlite Field, Eastern Dharwar Craton, Southern India: Evidence for a Transitional Kimberlite Signature

Abstract

We have carried out a mineralogical and geochemical study on two previously uncharacterized kimberlites of coherent facies, NK-2 and KK-6, from the Narayanpet kimberlite field (NKF), Eastern Dharwar craton (EDC), southern India. Both these kimberlites have a distinct inequigranular texture resulting from two generations of variously sized olivine macrocrysts and groundmass microphenocrysts. Other groundmass phases include clinopyroxene (diopside), serpentine, phlogopite, calcite, spinel, perovskite and apatite. Electron-microprobe analyses of various groundmass minerals support their kimberlitic affinity. Primitive Mg-numbers, Ni and Cr contents and high enrichment in incompatible trace elements are hallmarks of these kimberlites. From major-and trace-element geochemistry, crustal contamination seems to be minimal. The LILE and HFSE display excellent correlations amongst themselves and also with each other, thereby implying limited effects of post-emplacement hydrothermal alteration. Ratios of highly mobile and immobile incompatible trace elements (e.g., La, Ce, Nb, Ba, Pb and Th) strongly resemble those of transitional kimberlites of southern Africa and are confined to the arrays of mixing lines between Group-I and Group-II kimberlites. The REE abundances of NK-2 and KK-6 kimberlites suggest variable degrees of partial melting. Petrogenetic modeling depicts them to be products of a greater degree of melting. A mixed Group-I and Group-II "southern African" kimberlite source is inferred for the generation of NK-2 and KK-6 magmas, as well as for several other EDC kimberlites. This can explain several overlapping petrological and geochemical characters of Group-I and Group-II kimberlites displayed by the kimberlites of southern India. Lower whole-rock Ti and Fe contents in the NK-2 and KK-6, compared to those in other non-diamondiferous kimberlites from the NKF, suggests that they are promising diamond prospects; we recommend their bulk processing.

Origin and diamond prospectivity of Mesoproterozoic kimberlites from the Narayanpet field, Eastern Dharwar Craton, southern India: insights from groundmass mineralogy, bulk‐chemistry and perovskite oxybarometry

“…Olivine: Microphenocrystal olivine grains show a range in composition (Table 1) in Fo 83-91 and NiO (0.15-0.41 wt%), and NiO varies with Fo content (Figure 3). The composition of the olivine of this study is indistinguishable from data ( Figure 3) reported for other NKF pipes with Fo 86-92 and 0.10-0.40 wt% NiO; Chalapathi Rao et al, 2004;Chalapathi Rao and Dongre, 2009), Wajrakarur kimberlites (Fo 83-93 and 0.9-0.42 wt% NiO; Chalapathi Rao et al, 2004;Chalapathi Rao and Srivastava, 2009), Behradih orangeite, Bastar Craton (Fo 84-91 and 0.23-0.38 wt% NiO; Chalapathi Rao et al, 2011) and world-wide kimberlites (Fo 81.7 to 91.5 and up to 0.42 wt% NiO; Arndt et al, 2010). BSE imaging reveals little compositional variation (zoning) in olivine of this study ( Figure 2I).…”

“…Clinopyroxene: Clinopyroxene occurs only as diopside and displays a tight chemical range (Table 3). The composition of diopside from kimberlites of this study is indistinguishable in Mg-Fe-Ca space from that previously reported from other NKF (Chalapathi Rao et al, 2004;Chalapathi Rao and Dongre, 2009) and WKF (Chalapathi Rao et al, 2004) pipes ( Figure 4A). However, their Ti (atomic) and Al (atomic) contents ( Figure 4B) have a restricted composition compared to that in other NKF pipes Behradih orangeite (Chalapathi Rao et al, 2011) and Krishna lamproites (at the NE margin of the Cuddapah Basin, Eastern Dharwar Craton; Chalapathi Rao et al, 2010a).…”

“…SrO contents show a much smaller range of 0.17 to 1.28 wt% with an average of 0.31 wt%, distinctly different from southern African orangeites ( Figure 7B Mitchell, 1995). WKF and NKF field is from Chalapathi Rao et al (2004) and Chalapathi Rao and Dongre (2009). (B) Al total versus Ti total expressed as atoms per formula unit for clinopyroxene from the kimberlites of this study (black circles).…”

“…In this paper, we present comprehensive mineral chemistry data for these occurrences, as well as additional whole-rock geochemical data. We evaluate the relative contributions of the underlying subcontinental lithospheric and asthenospheric (convecting) mantle in light of the recent petrogenetic models envisaged for the Wajrakarur and Raichur kimberlite fields from Eastern Dharwar Craton and Mainpur orangeite field, Bastar Craton, Central India (e.g., Paul et al, 2006;Chalapathi Rao and Srivastava, 2009;Chalapathi Rao and Dongre, 2009;Chalapathi Rao et al, 2010a, b;. We also estimate the oxygen fugacity ( fO 2 ) of the NKF magmas by applying Fe-Nb oxybarometry to their perovskite chemistry and thereby attempt to assess the role of oxidation state in their diamond prospectivity.…”

Mineralogy of the P-12 K-Ti-richterite diopside olivine lamproite from Wajrakarur, Andhra Pradesh, India: implications for subduction-related magmatism in eastern India

“…The depth (160-220 km) of this feature also coincides with the diamond stability field. Clearly such subducted material, if metasomatized, could provide a source for lamproitic magmatism, and it is important to note that several geochemical studies of eastern Indian para-kimberlites and lamproites have suggested a role for subducted components in their genesis, together with formation in the subcontinental lithospheric mantle, rather than convecting asthenospheric mantle (Chalapathi Rao and Dongre 2009;Chalapathi Rao et al 2010).…”