N. V. Chalapathi Rao

A petrological and geochemical reappraisal of the Mesoproterozoic diamondiferous Majhgawan pipe of central India: evidence for transitional kimberlite – orangeite (group II kimberlite) – lamproite rock type

SummaryThe Mesoproterozoic diamondiferous Majhgawan pipe of central India is re-examined in the light of new and recently published petrological, geochemical and isotope data. This investigation reveals that its tectonic setting is similar to that of lamproites and orangeites (Group II kimberlite of southern Africa) and not that of a typical kimberlite. The petrography and mineralogy are comparable to lamproite and to some extent to orangeite, whereas the major element geochemistry is more akin to that of kimberlite. Trace element geochemistry is closer to that of lamproite but Nd isotope systematics are atypical of lamproite or orangeite. The inferred petrogenesis of the Majhgawan pipe is also similar to that of other such potassic ‘metasomatised mantle magmas’ without any strong affinity to a particular clan/group.It is demonstrated in this study that the Majhgawan pipe shares the petrological, geochemical and isotope characteristics of all three rock types. It is therefore suggested to constitute a transitional kimberlite–orangeite (Group II kimberlite)–lamproite rock. The existence of such transitional magmas in space and time in other cratons, outside India, is also highlighted. The name majhgawanite is proposed for this rock – keeping in mind the antiquity of the Majhgawan pipe, its intriguing petrological and geochemical characteristics and also on the basis of India’s legacy for introducing diamond to the world – to designate such mafic potassic-ultrapotassic transitional rock types so as to distinguish them from the classical kimberlite, lamproite or orangeite.It is concluded that the correlations between kimberlite petrography, geochemistry and isotopic types (viz., Group I and II), as established for kimberlites in southern Africa, need not be necessarily valid elsewhere. Hence, the recommendations of I.U.G.S. on classification of kimberlite, orangeite and lamproite are clearly inadequate when dealing with the transitional mafic potassic ultrapotassic rocks. It is further stressed that mineralogical, geochemical and isotopic aspects of mafic potassic-ultrapotassic rocks need to be considered in unison before assigning any name as the nomenclature of such exotic and rare alkaline rock types invariably implies economic and tectono-magmatic (regional) significance.

Mesoproterozoic emplacement and enriched mantle derivation of the Racherla alkali syenite, Palaeo-Mesoproterozoic Cuddapah Basin, southern India: insights from in situ Sr–Nd isotopic analysis on apatite

Abstract We report an in situ Sm–Nd isochron age of 1326±73 Ma, determined by LA-MC-ICP-MS (laser ablation-multiple collector-inductively coupled plasma-mass spectrometry), on crystalline apatite grains from the Racherla alkali syenite occurring in the Palaeo-Mesoproterozoic Cuddapah Basin, southern India. The obtained Mesoproterozoic age necessitates the Racherla syenite to be an intrusive into the sedimentary rocks of the Cuddapah Basin contrary to some previous suggestion that it represents an inlier of the basement pluton. Age of the dated syenite, within its error limits, is also coeval with that of the nearby Chelima lamproite (1354±52 Ma) and other syenite complexes (c. 1352 Ma) located elsewhere within the Eastern Ghats Mobile Belt, southern India, thereby implying a common geotectonic control in all of their generation. Incompatible trace element signatures (high La/Nb, Zr/Nb and La/Yb), 87Sr/86Sr (0.70432±10) and the lower Nd isotopic ratio (ϵNd(t) −8.9) of the apatite suggest derivation of the Racherla syenite parent magma from an enriched mantle source. Our findings provide compelling evidence for the existence of ancient (c. 2.6 Ga) metasomatized lithospheric mantle, at the Archaean–Palaeoproterozoic boundary, beneath the Cuddapah Basin. This enriched mantle appears to have had played a dominant role in the generation of co-spatial and coeval lamproites within and around the Cuddapah Basin and a plethora of alkaline and sub-alkaline syenites within the Eastern Ghats Mobile Belt. Implications of our study in the context of Indo-Antarctic collisional tectonics are also explored.

A new find of boninite dyke from the Palaeoproterozoic Dongargarh Super group: inference for a fossil subduction zone in the Archaean of the Bastar craton, Central India

The Dongargarh Supergroup (DSG), a bimodal Large igneous province (LIP), is one of the Palaeoproterozoic greenschist facies-metamorphosed volcano-sedimentary belts in the Bastar craton of the Central Indian shield. Two contrasting models are in vogue for the generation of the mafic volcanics from the DSG - a continental rifting model and an arc related model. In this paper, we report the occurrence of a boninite dyke from the Bijli rhyolite Formation, which is the lower volcanic horizon in the Nandgaon Group of the DSG. The boninite dyke is characterised by high magnesium (MgO : 18.32-18.80 wt.%), primitive Mgnumber (Mg# > 80), abundance of silica (SiO 2 : 51.63-51.95 wt.%), high Ni (~369 ppm), Cr (~2703 ppm), extremely low titania (TiO 2 : 0.04 wt.%), enrichment of LREE over MREE and HFSE and pronounced negative anomalies in Nb, Ti and Zr on primitive mantle normalized multi-element plots. The Dongargarh boninite dyke is inferred to have been derived from a primary magma and shares geochemical characteristics of modern- as well as Archaean-boninites. It comes under the high-Ca boninite category and displays distinct geochemical traits compared to the so far reported boninites from the Bastar craton. Its petrogenesis necessitates a two stage-model involving a refractory mantle as well as fluids derived from subducted sediments. Crustal assimilation (contamination) or a direct plume-derived melt cannot account for its observed geochemical characters. Even though we cannot constrain the generation of the mafic volcanics of DSG vis-a-vis rifting vs convergence with the available data, the occurrence and geochemistry of the boninite dyke indeed demonstrates that this domain represents a fossil subduction zone.

K-rich titanate from the Jharia ultrapotassic rock, Gondwana coal fields, eastern India, and its petrological significance

We report a rare accessory groundmass mineral of K-rich titanate, having a composition close to that of potassium triskaidecatitanate (K2Ti13O27), from an underground drill-core sample of ultrapotassic rock from southwestern part of the Jharia coal field in the Damodar valley, at the northern margin of the Singhbhum craton, Eastern India. Potassium triskaidecatitanate is regarded as a typomorphic mineral of orangeites (Group II kimberlites) of Kaapvaal craton, southern Africa, and its occurrence in the Jharia ultrapotassic rock is significant since ultrapotassic suite of rocks elsewhere from the Damodar valley have been recently suggested to be peralkaline lamproites based on mineral-genetic classification. The important role played by a unique geodynamic setting (involving a thinned metasomatised lithospheric mantle and inheritance of an Archaean subduction component) at the northern margin of the Singhbhum craton in deciding the petrological diversity of the early Cretaceous ultrapotassic intrusives from the Damodar valley is highlighted in this study.

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).

LARNITIC KIRSCHSTEINITE FROM THE KOTAKONDA KIMBERLITE, ANDHRA PRADESH, INDIA

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Tokapal tuff-facies kimberlite, Bastar craton, Central India: A nickel prospect?

We report the occurrence of garnierite (a general term referring to Ni-Mg bearing hydrous silicates in laterites) from the crater-facies Tokapal kimberlite of the Bastar craton, Central India. Garnierite occurs as discrete ovoid or amoeboid segregations (up to 200 mm) or as veinlets with up to 18.1 wt% NiO and high iron contents (up to 36.2 wt% FeOT). Chemical composition of the garnierite implies its derivation from a magnesium-rich protolith. Extensive lateritisation of the large crater-facies (∼2.5 km diameter) saucer-shaped kimberlite under tropical weathering conditions, aided by suitable topography, drainage and favourable structural set-up, are the factors inferred to be responsible for the formation of garnierite in the Tokapal system. As lateritic nickel ores constitute significant resources for nickel exploration, the perspective of the Tokapal kimberlite as a nickel prospect needs to be investigated.

Petrology, Bulk-Rock Geochemistry, Indicator Mineral Composition and Zircon U–Pb Geochronology of the End-Cretaceous Diamondiferous Mainpur Orangeites, Bastar Craton, Central India

The end-Cretaceous diamondiferous Mainpur orangeite field comprises six pipes (Behradih, Kodomali, Payalikhand, Jangara, Kosambura and Bajaghati) located at the NE margin of the Bastar craton, central India. The preservation of both diatreme (Behradih) and hypabyssal facies (Kodomali) in this domain implies differential erosion. The Behradih samples are pelletal and tuffisitic in their textural habit, whereas those of the Kodomali pipe have inequigranular texture and comprise aggregates of two generations of relatively fresh olivines. The Kosambura pipe displays high degrees of alteration and contamination with silicified macrocrysts and carbonated groundmass. Olivine, spinel and clinopyroxene in the Behradih and the Kodomali pipes share overlapping compositions, whereas the groundmass phlogopite and perovskite show conspicuous compositional differences. The bulk-rock geochemistry of both the Behradih and Kodomali pipes has a more fractionated nature compared to southern African orangeites. Incompatible trace elements and their ratios readily distinguish them from the Mesoproterozoic Wajrakarur (WKF) and the Narayanpet kimberlites (NKF) from the eastern Dharwar craton, southern India, and bring out their similarity in petrogenesis to southern African orangeites. The pyrope population in the Mainpur orangeites is dominated by the calcic-lherzolitic variety, with sub-calcic harzburgitic and eclogitic garnets in far lesser proportion. Garnet REE distribution patterns from the Behradih and Payalikhand pipes display “smooth” as well as “sinusoidal” chondrite-normalised patterns. They provide evidence for the presence of a compositionally layered end-Cretaceous sub-Bastar craton mantle, similar to that reported from many other cratons worldwide. The high logfO2 of the Mainpur orangeite magma (ΔNNO (nickel-nickel oxide) of +0.48 to +4.46 indicates that the redox state of the lithospheric mantle cannot be of first-order control for diamond potential and highlights the dominant role of other factors such as rapid magma transport. The highly diamondiferous nature, the abundance of calcic-lherzolitic garnets and highly oxidising conditions prevailing at the time of eruption make the Mainpur orangeites clearly “anomalous” compared to several other kimberlite pipes worldwide. U–Pb dating of zircon xenocrysts from the Behradih pipe yielded distinct Palaeoproterozoic ages with a predominant age around 2,450 Ma. The lack of Archean-aged zircons, in spite of the fact that the Bastar craton is the oldest continental nuclei in the Indian shield with an Eoarchaean crust of 3.5–3.6 Ga, could either be a reflection of the sampling process or of the modification of the sub-Bastar lithosphere by the invading Deccan plume-derived melts during the Late Cretaceous.

Kimberlites, lamproites, carbonatites and associated alkaline rocks: a tribute to the work of Rex T. Prider

This special issue is a tribute to the work of the late Rex T. Prider, who undertook pioneering work on the West Kimberley leucite lamproites and predicted a link to diamonds. The special issue incorporates both authoritative research and significant new contributions to the understanding of the mineralogy and petrology of carbonatites, kimberlites, lamproites and related alkaline rocks. Apart from their scientific interest, these rocks are of major and growing economic importance. They are significant repositories of certain metals and other mineral commodities, indeed the only source of some of them, including diamonds, niobium, the rare-earth elements, copper, phosphate, vermiculite, and raw materials for the manufacture of ceramics. Mining exploration efforts result in the discovery of new localities that add to our understanding of these rocks and their mantle source regions. Not only do such rocks provide a unique sample of the upper

Glimmeritic enclave in a lamprophyre from the Settupalle alkaline pluton, Eastern Ghats mobile belt

A rare occurrence of glimmeritic (mica-rich) enclave — composed of abundant modal biotite, subordinate proportions of clinopyroxene and apatite, minor amounts of feldspar, carbonate and sphene — is reported from the lamprophyre of Settupalle alkaline pluton, Eastern Ghats mobile belt (EGMB), India. The enclave displays very coarse grained equigranular texture (mica laths up to 5 mm and clinopyroxene grains up to 4 mm). In comparison, the host lamprophyre exhibits a marked porphyritic-panidiomorphic texture comprising phenocrysts of clinopyroxene; other phases such as biotite and potash- and plagioclase-feldspar are restricted to the groundmass. A tight closeness in mineral chemistry of the glimmerite and lamprophyre imply a possible genetic relationship between their parent magmas. Glimmeritic enclave is construed to be an autolith of the proto-lamprophyre magma, which failed to reach the surface, and lined the wall-rock along the conduit of the lamprophyric intrusion. Glimmerite enclave provides a direct evidence for the multi-stage modification of the lithospheric mantle due to the infiltration of the potassium-rich hydrous melts such as lamprophyres. Mineralogy of the glimmeritic enclave is also similar to that of a vein component of the hydrous, mafic and potassic-ultrapotassic veined lithosphere in the EGMB.