A. Dongre

Origin of Ti-rich garnets in the groundmass of Wajrakarur field kimberlites, southern India: insights from EPMA and Raman spectroscopy

Although Ti-rich garnets are commonly encountered in the groundmass of many alkaline igneous rocks, they are comparatively rare in kimberlites. Here we report on the occurrence of Ti-rich garnets in the groundmass of the P-15 and KL-3 kimberlites from the diamondiferous Wajrakarur field in the Eastern Dharwar craton of southern India. These garnets contain considerable Ti (11.7–23.9 wt.% TiO2), Ca (31.3–35.8 wt.% CaO), Fe (6.8–15.5 wt.% FeOT) and Cr (0.04–9.7 wt.% Cr2O3), but have low Al (0.2–5.7 wt.% Al2O3). In the case of the P-15 kimberlite they display a range in compositions from andradite to schorlomite, with a low proportion of grossular (andradite(17.7–49.9)schorlomite(34.6–49.5)-grossular(3.7–22.8)-pyrope(1.9–10.4)). A few grains also contain significant chromium and represent a solid solution between schorlomite and uvarovite. The Ti-rich garnets in the KL-3 kimberlite, in contrast, are mostly schorlomitic (54.9─90.9 mol %) in composition. The Ti-rich garnets in the groundmass of these two kimberlites are intimately associated with chromian spinels, perhaps suggesting that the garnet formed through the replacement of spinel. From the textural evidence, it appears unlikely that the garnets could have originated through secondary alteration, but rather seem to have formed through a process in which early magmatic spinels have reacted with late circulating, residual fluids in the final stages of crystallization of the kimberlite magma. Raman spectroscopy provides evidence for low crystallinity in the spinels which is likely to be a result of their partial transformation into andradite during their reaction with a late-stage magmatic (kimberlitic) fluid. The close chemical association of these Ti-rich garnets in TiO2-FeO-CaO space with those reported from ultramafic lamprophyres (UML) is also consistent with results predicted by experimental studies, and possibly implies a genetic link between kimberlite and UML magmas. The occurrence of Ti-rich garnets of similar composition in the Swartruggens orangeite on the Kaapvaal craton in South Africa, as well as in other kimberlites with an orangeitic affinity (e.g. the P-15 kimberlite on the Eastern Dharwar craton in southern India), is inferred to be a reflection of the high Ca- and high Ti-, and the low Al-nature, of the parent magma (i.e. Group II kimberlites).

The Pipe-15 kimberlite: A new addition to the Wajrakarur cluster of the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India

This paper serves to report, for the first time, on the chemical composition of primary groundmass minerals occurring in a newly discovered kimberlite located 150m NE of the well known Pipe 2 kimberlite in the Wajrakarur cluster of the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India. These mineral compositions are also compared with newly acquired analyses of groundmass minerals in the Pipe 2 kimberlite. The kimberlite in this newly discovered intrusion (here referred to as Pipe 15) is characterized by a magmatic, microcrystic rock texture, consisting of microcrysts of serpentinised olivine in a finer groundmass rich in phlogopite, clinopyroxene, spinel, perovskite and apatite (in a base of serpentine). The rock is intensely altered. Phlogopite in the Pipe 15 kimberlite has comparatively low Al and higher Fe than phlogopite in Pipe 2 and (on this basis) is markedly different to phlogopite from Pipe 2. Spinels in Pipe 15 are also compositionally markedly different to that in Pipe 2 in that lower Ti/(Ti+Cr+Al) and Fe/ (Fe+Mg) are seen in spinels from Pipe 15, when compared to spinels from Pipe 2. Furthermore, Fe-poor, primary (groundmass) clinopyroxenes are abundant in Pipe 15, while it is almost totally absent in Pipe 2. In view of these compositional differences, it is here considered likely that Pipe 15 represents a new, separate intrusion and that it is not simply a satellite of Pipe 2, in the Wajrakarur kimberlite cluster. The compositions of the phlogopites, spinels and clinopyroxenes from the Pipe 15 kimberlite, as analyzed in the present study, are strikingly similar to that of the groundmass phases (phlogopite, spinel and clinopyroxene) in Group 2 kimberlites (orangeites) from the Bastar craton in Central India, as well as for typical orangeites from the Kaapvaal craton in South Africa. A genetic relationship to Group 2 kimberlites (orangeites) can therefore be inferred in view of modern mineralogical genetic classification. The recent discovery of Late-Cretaceous orangeite in the nearby Timmasamudram kimberlite cluster is also indicative of Group 2 kimberlite (orangeite) magmatism in this region of kimberlite intrusions, and serves to support the classification proposed here for Pipe 15.

Petrochemistry of crater facies Tokapal kimberlite pipe, Bastar craton, central India and its orangeitic affinities

Tokapal kimberlite is the only well preserved crater facies kimberlite intruded within sedimentary sequence of Indravati basin in Bastar craton of central India. We present detailed petrographical and whole rock geochemical studies, carried out on ten samples collected from different locations from Tokapal kimberlite to constrain its genesis and also the mantle processes involved in the origin of this earlier characterized Group I kimberlite. Geochemical studies show that only SiO2 content and the mobile trace elements Ba, Sr, and K vary in the crater facies while rest others show restricted range and can be successfully used in evaluating the petrogenetic processes. Very low abundances of Rb (<2 ppm), Sr (<28 ppm), Ba (<52 ppm) and Cs (0.02–3 ppm) are observed which show possible effects of late-stage alteration rather than significant crustal contamination. The LREE enriched REE pattern, absence of positive Eu anomalies and HREE depletion also provide further additional evidence against crustal contamination considerably modifying magma composition. We infer the presence of less enriched (metasomatised) mantle source regions and comparatively greater degrees of partial melting responsible for the genesis of Tokapal kimberlite. Present study also suggests that crater facies Tokapal kimberlite intruding the Indravati basin, Bastar craton has a Group II kimberlite (orangeite) affinity. This finding is important in light of recent identification of Mainpur kimberlites of Bastar craton as orangeites.

An alternate perspective on the opening and closing of the intracratonic Purana basins in peninsular India

absolutely unaffected and found as ‘fresh limestones’ as is directly evident from their micritic nature with clayey intercalations under microscope and the laminations/bandings visible even on a handspecimen scale besides their geochemical attributes (lack of influence of kimberlite bulk-geochemistry). Such limestone xenoliths must have been incorporated into the kimberlite magma at a much shallower depths (close to the surface), thereby necessitating their survival and non-reactiveness, and certainly not ‘exhumed’ being entrapped in the kimberlite as expressed by Prof Kale. We also wish to point out that the presence of ‘fresh limestones’ in a kimberlite is not a ‘remarkable’, aspect but in fact has been well-documented and quite familiar to those involved in research on kimberlites. For example, ‘fresh limestone xenoliths (some of them with profuse conodont fossils) have been recovered from the kimberlites occuring in the Slave (Cookenboo et al. 1998; Geology, v.26, n.5, pp. 391-394) and Saskatchewan (Lefebvre and Kurszlaukis, 2008; Journal of Volcanology and Geothermal Research v.174, pp. 171-185) cratons of Canada and constitute the only solid evidence that sea water inundated, in the geological past, in the now exposed cratons. Likewise, fossiliferous limestone xenoliths derived from the kimberlites of the Baffin Island, Nunavut, Canada (Zhand and Pell, 2014; Canadian Journal of Earth Sciences v.51, pp. 142-155) confirm the Paleozoic cover in this domain. We do agree with Prof. Kale that despite more than a century of study many apects of the Purana basins still remain open to debate. We also deeply appreciate the remarkable efforts made in recent years by Prof Abhijit Basu and his co-workers for reviving interest in these fascinating basins.