Journal of Petrology | Volume 45 | Number 5 | Pages 907-948 | 2004
Journal of Petrology 45(5) © Oxford University Press 2004; all rights reserved.
Petrogenesis of Proterozoic Lamproites and Kimberlites from the Cuddapah Basin and Dharwar Craton, Southern India
1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF CAMBRIDGE, DOWNING STREET, CAMBRIDGE CB2 3EQ, UK
2 DEPARTMENT OF GEOLOGY, McMASTER UNIVERSITY, HAMILTON L8S 4M1, ONT., CANADA
RECEIVED JANUARY 2, 2003; ACCEPTED OCTOBER 16, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTING OF THE...GEOLOGY AND PETROLOGY OF...GEOLOGY AND PETROLOGY OF...AGE OF MAGMATISMANALYTICAL TECHNIQUESMINERAL CHEMISTRYWHOLE-ROCK GEOCHEMISTRYCOMPARISON WITH WORLDWIDE...PETROGENESISGEODYNAMIC MODEL FOR THE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Proterozoic mafic potassic and ultrapotassic igneous rocks emplaced in the Cuddapah Basin and Dharwar Craton of the southern Indian shield are among the earliest recorded on Earth. Lamproites intrude the basin and its NE margin, whereas kimberlites intrude the craton to the west of the basin. Kimberlites occur in two spatially separate groups: the non-diamondiferous Mahbubnagar cluster that was emplaced at 1400 Ma and is of a similar age to the Cuddapah lamproites, and the predominantly diamondiferous Anantapur cluster, emplaced at
1100 Ma. Despite their Proterozoic ages, some of the kimberlites are petrographically fresh. Distinct variations are evident in the major and trace element concentrations of the diamondiferous and non-diamondiferous kimberlites. The latter have higher concentrations of Fe, Ti, Zr, Hf and Sc, and lower Ni contents and La/Sm ratios. All of the kimberlites have high La/Yb ratios (65–180) and positive 
Ndi values (0·5–4·5), which suggests that their source regions were metasomatized by small-fraction melts derived from the depleted upper mantle, shortly prior to kimberlite genesis. Cuddapah Basin lamproites have similar La/Yb ratios but much lower Ndi values (–6 to –7) and appear to have been derived from ancient metasomatized sub-continental lithospheric mantle. The Proterozoic ambient mantle is believed to have had a higher potential temperature than at the present day such that small amounts of lithospheric extension may account for the genesis of the kimberlites and lamproites of southern India without the need for a mantle plume. KEY WORDS: Proterozoic; diamonds; India; kimberlites; lamproites
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...GEOLOGY AND PETROLOGY OF...GEOLOGY AND PETROLOGY OF...AGE OF MAGMATISMANALYTICAL TECHNIQUESMINERAL CHEMISTRYWHOLE-ROCK GEOCHEMISTRYCOMPARISON WITH WORLDWIDE...PETROGENESISGEODYNAMIC MODEL FOR THE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Global kimberlite and lamproite occurrences are known from the Early Proterozoic to the Quaternary (Janse & Sheahan, 1995
). However, certain periods seem to have been the focus for kimberlite and lamproite emplacement. During the Cretaceous many kimberlites and lamproites were emplaced in Africa, South America and North America. This includes the majority of the Group I kimberlites (85–100 Ma) and almost all of the Group II kimberlites (200–100 Ma) of South Africa (Gurney et al., 1991). Skinner et al. (1985) recognized the period between 1100 and 1200 Ma as an important global ‘mantle event’ for kimberlite and lamproite emplacement. Evidence for this event is widespread across the Gondwana supercontinent and also in Greenland. Proterozoic kimberlites and lamproites are relatively rare (Table 1) compared with their Phanerozoic counterparts (Nelson et al., 1989), and petrogenetic studies of the former are few in number.
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There is overwhelming evidence that many of the present-day continents of the southern hemisphere formed the Gondwana ‘supercontinent’ at the end of the Palaeozoic (e.g. Smith & Hallam, 1970
). The existence of a preceding Mesoproterozoic supercontinent (Rodinia) at 1300–1000 Ma is less certain, but has been proposed on the basis of palaeomagnetic data and tectonic reconstructions (e.g. Dalziel, 1997). The reorganization of Rodinia is thought to have been responsible for the formation of Gondwana (e.g. Rogers et al., 1995), and the widespread Pan African radiometric ages (as young as 500 Ma) within the Gondwana fragments are thought to represent a thermal event post-dating the final suturing of this Proterozoic supercontinent. It is in this context that the locations of Proterozoic kimberlites and lamproites in the pre-Cretaceous Gondwana fit assume significance.
A number of kimberlites and lamproites were emplaced during the Proterozoic in the Eastern Dharwar Craton and Cuddapah Basin of the southern Indian shield (Fig. 1). Known kimberlite occurrences in the Dharwar Craton are confined to an area immediately west of the Cuddapah Basin (Fig. 1). They occur as two clusters, in the Anantapur and Mahbubnagar districts of Andhra Pradesh, that are separated from each other by >150 km. In contrast, lamproites are confined to the Cuddapah Basin (Chelima and Zangamarajupalle) and its NE margin (Ramannapeta and Jaggayyapeta; Reddy et al., 2000). The kimberlites and lamproites of the Indian sub-continent offer a rare opportunity to investigate Proterozoic tectono-magmatic processes operating when southern India formed a part of the Gondwana supercontinent. Current understanding of the petrogenesis of kimberlites and lamproites has been strongly influenced by studies of examples from South Africa (e.g. Smith, 1983; Tainton & McKenzie, 1994; Mitchell, 1995; Janney et al., 2002) Australia (e.g. Jaques et al., 1984; Fraser et al., 1985), South America (e.g. Gibson et al., 1995) and Russia (e.g. Beard et al., 1998, 2000; Mahotkin et al., 2000). This work complements these recent studies and sheds further insight into the genesis of, and genetic relationship between, kimberlites and lamproites.
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GEOLOGICAL SETTING OF THE CUDDAPAH BASIN AND THE EASTERN DHARWAR CRATON |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...GEOLOGY AND PETROLOGY OF...GEOLOGY AND PETROLOGY OF...AGE OF MAGMATISMANALYTICAL TECHNIQUESMINERAL CHEMISTRYWHOLE-ROCK GEOCHEMISTRYCOMPARISON WITH WORLDWIDE...PETROGENESISGEODYNAMIC MODEL FOR THE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Southern India comprises an extensive supracrustal sequence of Proterozoic rocks, believed to lie above a number of Archaean cratons. The evolution of the Indian shield was initially suggested to have started around Archaean nuclei that developed into three cratons (Dharwar, Singhbhum and Aravalli; Naqvi et al., 1974
). Subsequent studies revealed additional tectonic sutures, and the Indian shield is now subdivided into seven cratons (e.g. Naqvi & Rogers, 1987). Although the nature of the cratonic boundaries remains unresolved (see Mahadevan, 1995), there is a consensus that the Indian shield has been a single lithospheric unit since mid-Proterozoic times (e.g. Yoshida, 1995; Radhakrishna & Piper, 1999).
The Dharwar Craton mainly comprises schists and gneisses that have undergone greenschist- to granulite-facies metamorphism (Rollinson et al., 1981; Allen et al., 1985). A NW–SE-trending suite of Early Proterozoic granitic bodies (Closepet granite) divides the Dharwar Craton into western and eastern parts (Naqvi & Rogers, 1987). The Eastern Ghat orogeny affected the Eastern Dharwar Craton, resulting in a narrow, highly deformed granulite-facies belt extending from Madras in the south to near Calcutta in the north. This belt may extend into Eastern Antarctica (e.g. Chetty, 1995). The timing of the Eastern Ghat orogeny is poorly constrained, with age estimates for the granulite-facies metamorphism ranging from 2900 Ma (Sm–Nd whole rock; Paul & Ray Barman, 1988) to 500 Ma (Grasty & Leelanandam, 1965).
The Cuddapah Basin lies in the Eastern Dharwar Craton (Fig. 1) and includes igneous and sedimentary rocks of Middle to Late Proterozoic age. The basin is crescent shaped, covers an area of 44 000 km2 and shows increasing metamorphic grade from west to east, attributed to the Eastern Ghat orogeny. The Cuddapah Basin includes two sub-basins, the Kurnool and Palnad (see Fig. 1), in which the Upper Proterozoic Kurnool Group sediments were deposited. A marked angular unconformity separates sediments of the older Cuddapah Supergroup from the Kurnool Group. Intrusive and extrusive rocks are present throughout much of the Cuddapah Supergroup, and extensive lava flows, ash beds and sills occur along the SW fringes of the Cuddapah Basin (Reddy, 1988; Anand et al., 2003). Additional intrusives include lamproites (at Chelima and Zangamarajupalle), rare alkali syenites and granitoid domes.
The crust beneath the Cuddapah Basin is 40–50 km thick (Kaila et al., 1979), whereas the lithospheric thickness beneath the Dharwar Craton exceeds 200 km (Bhattacharji & Singh, 1984; Gupta et al., 1991). The thermal structure of the Dharwar lithosphere is considered identical to cratons in Africa, Australia and South America (Gupta, 1993). Evidence for the existence of a thickened Proterozoic Indian lithosphere comes from studies of mantle-derived xenoliths from a kimberlite from the Dharwar Craton [Pipe 3 from Lattavaram in the Anantapur cluster (Fig. 2; Ganguly & Bhattacharya, 1987; Nehru & Reddy, 1989)]. Thermobarometry of garnet peridotites from this pipe suggests that the mid-Proterozoic lithosphere was at least 170 km thick (see below).
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The Archaean basement surrounding the basin is intruded by an extensive swarm of dolerite dykes, which trend ENE–WSW but do not intrude the basin sediments. Radiometric age determinations are of variable precision but suggest that at least three major episodes of dyke emplacement occurred at 1900–1700 Ma, 1400–1300 Ma and 1200–1000 Ma together with a minor younger event at 650 Ma (Murthy et al., 1987
; Padmakumari & Dayal, 1987; Mallikharjuna Rao et al., 1995; Chatterjee & Bhattacharji, 2001). The oldest dykes trend east–west and NW–SE and are tholeiitic, whereas the main dyke activity (1300–1400 Ma) comprises tholeiitic to mildly alkaline basalts (Chatterjee & Bhattacharji, 1998). Some north–south-trending mafic dykes, at the northern end of the Cuddapah Basin, are of basaltic komatiite affinity and were emplaced between 1400 and 1300 Ma (Mallikharjuna Rao et al., 1995). Two broad compositional groups of dykes have been recognized: (1) a depleted type similar to mid-ocean ridge basalts (MORB); (2) an enriched type similar to continental flood basalts (Rao & Puffer, 1996). |
GEOLOGY AND PETROLOGY OF THE DHARWAR KIMBERLITES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...GEOLOGY AND PETROLOGY OF...GEOLOGY AND PETROLOGY OF...AGE OF MAGMATISMANALYTICAL TECHNIQUESMINERAL CHEMISTRYWHOLE-ROCK GEOCHEMISTRYCOMPARISON WITH WORLDWIDE...PETROGENESISGEODYNAMIC MODEL FOR THE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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We have used petrography, mineral chemistry and geochemistry to classify the kimberlites and lamproites of southern India following the IUGS scheme proposed by Woolley et al. (1996)
. The freshest possible surface samples, augmented by drill-core samples, were used in this study. Details of sample locations, sample descriptions and photomicrographs are available in Electronic Appendices, which can be downloaded from the Journal of Petrology website (http://www.petrology.oupjournals.org), and only the salient aspects are discussed below. The major petrographic features of the studied rocks are summarized in Table 2.
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Anantapur kimberlites
Twenty-one kimberlite pipes intrude Archaean granites and gneisses in the Anantapur district (Babu, 1998
; Nayak et al., 2001). The pipes were numbered in order of discovery by the Geological Survey of India (GSI) and this terminology is retained here. Pipes 1, 2 and 6 are located near Wajrakarur; Pipes 3, 4, 8 and 9 occur near Lattavaram; Pipes 5 and 7 are located near Muligiripalle and Venkatampalle, respectively, and Pipes 10 and 11 are located close to Chigicherla (Fig. 2).
Crustal xenoliths (chiefly granitic or gneissic rocks) are present in all of the kimberlite pipes, and amphibolite xenoliths also occur in Pipe 7. Mantle xenoliths are reported only from Pipe 3 (lherzolite, harzburgite, dunite and eclogite; Akella et al., 1979; Ganguly & Bhattacharya, 1987; Nehru & Reddy, 1989) and Pipe 10 (spinel harzburgite; Murthy et al., 1994). Typical kimberlite indicator minerals (Cr-diopside, picro-ilmenite and pyrope-garnet) are abundant in all pipes, except 2 and 5, where they are rare. Diamonds have been recovered from all pipes, apart from 2, 5 and 10 (Rao et al., 1991; Satyanarayana et al., 1992). Pipe 1, which has abundant crustal xenoliths and a ‘brecciated’ appearance, is classified as a diatreme-facies tuffisitic kimberlite (Table 2). All of the others represent hypabyssal-facies kimberlite.
The hypabyssal kimberlites show a characteristic inequigranular porphyritic texture with: (1) subrounded to rounded megacrysts; (2) corroded megacrysts (>5 mm); (3) macrocrysts (0·5–5 mm); (4) subhedral to euhedral phenocrysts (<0·5 mm) of olivine. Modal olivine content and degree of serpentinization varies from pipe to pipe. Olivine is thoroughly serpentinized in Pipes 1, 6, 8 and 9, but fresh in the remaining pipes (Table 2). Some olivines exhibit ragged and embayed margins suggesting resorption prior to serpentinization. Rutile inclusions are noted in some of these olivines. In Pipe 8, olivine pseudomorphs commonly contain opaque cores mantled by carbonate minerals.
Monticellite is an important groundmass phase in Pipes 2, 5 and 6. Its presence is consistent with these rocks being classified as kimberlite, and not lamproite (e.g. Woolley et al., 1996). Phlogopite constitutes an important groundmass phase in all pipes, and is particularly abundant in Pipes 2 and 5, where it occurs either as coarse poikilitic plates or as interstitial grains. Phlogopite macrocrysts (2 mm) are occasionally present. Some phlogopite shows alteration to serpentine, calcite and/or chlorite. Subhedral to euhedral crystals of perovskite (usually <0·04 mm) occur throughout the groundmass of these pipes, and are particularly abundant in Pipes 2 and 5 (see Table 2). Perovskite may also occur as ‘necklaces’ around olivine grains, and as inclusions within phlogopite. Various spinels (chromite, titano-magnetite, titano-chromite and aluminous spinel) also constitute important groundmass phases (Table 2). Randomly oriented lath-shaped clinopyroxene crystals occur in some pipes, suggesting late-stage crystallization. Light green laths and plates of clinopyroxene, probably of secondary origin, are reported from Pipe 3 for the first time. Other groundmass phases in the Anantapur kimberlites include calcite, chlorite, apatite, serpentine and clay minerals. Rare minerals such as hazelwoodite (Ni3S2), pectolite and melilite have been reported from some of the pipes (Akella et al., 1979; Murthy et al., 1994). Reddy (1987) reported richterite and sanidine in Pipes 2 and 5. However, we observed no amphibole or sanidine in any of the studied samples and suggest that the phases noted by Reddy (1987) may have been derived from granitoid country rocks.
Mahbubnagar kimberlites
Hypabyssal-facies kimberlites, emplaced into migmatites and igneous intrusives, occur near Maddur, Kotakonda, Narayanpet and Padiripahad, and constitute the Mahbubnagar cluster (Fig. 1). Numerous new kimberlite dykes or diatremes have been discovered in this cluster in recent years (Chalapathi Rao et al., 1998). In contrast to the Anantapur kimberlites, preliminary studies suggest that the Mahbubnagar pipes are non-diamondiferous (Satyanarayana, 2002) despite the presence of alluvial diamonds in the vicinity. Chalapathi Rao et al. (1998) gave a preliminary account of the indicator minerals and mantle xenoliths in some of these rocks.
Pipes in the Maddur–Narayanpet areas were previously suggested to be olivine lamproites, similar to those in Western Australia (e.g. Nayak et al. 1988). However, petrographic, geochemical and isotopic studies demonstrate that they are true kimberlites. Olivine forms euhedral to subhedral phenocrysts and rare, larger (3–5 mm) sub-rounded crystals are also present. Fresh olivine is present in both the Maddur and Narayanpet hypabyssal-facies kimberlites (Table 2). Strongly pleochroic phlogopite forms 0·2–8 mm long euhedral grains in the groundmass, and monticellite inclusions are seen in some phlogopites in the Kotakonda kimberlite. Colourless clinopyroxene is present in the Padiripahad and Narayanpet kimberlites. Perovskite forms an abundant groundmass phase (5–8 modal %) occurring as discrete euhedral grains and as ‘garlands’ around olivine. The rest of the groundmass predominantly consists of calcite, serpentine, apatite and spinel. The Kotakonda kimberlite also has groundmass rutile, monticellite and kirschsteinite.
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GEOLOGY AND PETROLOGY OF THE LAMPROITES FROM THE CUDDAPAH BASIN AND ITS NE MARGIN |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...GEOLOGY AND PETROLOGY OF...GEOLOGY AND PETROLOGY OF...AGE OF MAGMATISMANALYTICAL TECHNIQUESMINERAL CHEMISTRYWHOLE-ROCK GEOCHEMISTRYCOMPARISON WITH WORLDWIDE...PETROGENESISGEODYNAMIC MODEL FOR THE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The Chelima and Zangamarajupalle lamproite dykes intrude slates and phyllites of the Nallamalai Group, within the Cuddapah Basin (Bhaskar Rao, 1976
; Sreeramachandra Rao, 1988). Neither dyke has yet been tested for diamond by modern exploration techniques. The Ramannapeta lamproite intrudes granitoid country rocks at the NE margin of the Cuddapah Basin and lies close to the alluvial workings on the banks of the Krishna River where many famous Indian diamonds, including the Koh-I-Noor, have been recovered. The primary source of these diamonds is uncertain. Earlier workers proposed the Anantapur kimberlites and Chelima lamproite as the source rock for these diamonds (e.g. Rajaraman & Deshpande, 1978). Based on the close proximity (<10 km) of the Ramannapeta lamproite to these workings and the >200 km distance of the nearest known kimberlites, we suggest that the Ramannapeta body may be the primary source for the diamonds of the Krishna valley. In the Chelima and Zangamarajupalle hypabyssal-facies lamproites, olivine is completely altered and replaced by ferroan dolomite and rarer serpentine. Two olivine generations can be distinguished from pseudomorph textures. Olivine pseudomorphs are rare in the Ramannapeta lamproite. Highly pleochroic, strongly zoned titaniferous phlogopite phenocrysts (0·5–0·7 mm) are abundant in all these rocks (see Table 2). Talc and chlorite occur as marginal alteration products in the phlogopites of the Chelima lamproite.
Poikilitic laths of a pale pink pleochroic titanian potassic richterite are scattered through the groundmass of the Ramannapeta lamproite. The groundmass also contains abundant pale green or colourless clinopyroxene that lacks the quench textures reported from other lamproites (Mitchell, 1985), and rutile, apatite, serpentine, secondary carbonate and opaque oxides. The abundance of ferroan dolomite and lack of other well-preserved phases (apart from phlogopite) in the Chelima and Zangamarajupalle lamproites may be due to secondary alteration. Therefore, much of the petrological information in these rocks has to be extracted from phlogopite.
The Chelima dyke has been variously classified as minette, lamprophyre, carbonatite–kimberlite and lamproite (e.g. Bergman, 1987; Scott-Smith, 1989). The absence of clinopyroxene precludes this rock from being a minette and the mineral chemistry of the phlogopite is consistent with its classification as a lamproite (see below). The striking petrographic similarity of the Zangamarajupalle lamproite to that at Chelima was noticed by earlier workers, who suggested that both were kimberlites (Bhaskara Rao, 1976; Sreeramachandra Rao, 1988). The composition and zoning trends of the micas and whole-rock geochemistry presented here, however, suggest that both are lamproites.
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AGE OF MAGMATISM |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...GEOLOGY AND PETROLOGY OF...GEOLOGY AND PETROLOGY OF...AGE OF MAGMATISMANALYTICAL TECHNIQUESMINERAL CHEMISTRYWHOLE-ROCK GEOCHEMISTRYCOMPARISON WITH WORLDWIDE...PETROGENESISGEODYNAMIC MODEL FOR THE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The precise emplacement ages of the kimberlites and lamproites are important in constraining models of their origin, whether by lithospheric extension, or the drift of the Indian continent over a mantle plume (e.g. Crough, 1981
). Anil Kumar et al. (1993) reported phlogopite Rb–Sr ages of 1090 Ma in some Anantapur kimberlites (Pipes 1, 2, 5 and 7), and the Majhgawan lamproite of central India, and suggested that all southern Indian kimberlites and central Indian lamproites were emplaced around 1090 ± 30 Ma. This contrasts with the much wider age range (850–1350 Ma) reported in earlier work (Paul et al., 1975; Paul, 1979). Preliminary K–Ar dating of phlogopite separates from four Dharwar Craton kimberlites (Kotakonda and Pipe 5 at Muligiripalle) and two Cuddapah Basin lamproites (Chelima and Ramannapeta) yielded Mesoproterozoic ages of 1100– 1380 Ma (Chalapathi Rao et al., 1996a). These ages, however, have variable precision (±10–50 Ma).
40Ar/39Ar plateau ages of groundmass phlogopite separates from the Kotakonda kimberlite and Chelima lamproites are 1401 ± 5 Ma and 1418 ± 8 Ma, respectively (Chalapathi Rao et al., 1999). These ages are consistent (within error) with conventional K–Ar dates on phlogopite separates from the same samples (Chalapathi Rao et al., 1996a). The emplacement of the Kotakonda kimberlite and Chelima lamproite could have been contemporaneous; both are older than the Anantapur kimberlites (1090 Ma; Anil Kumar et al., 1993). Anil Kumar et al. (2001) reported younger phlogopite Rb–Sr ages of 1085 ± 14 and 1099 ± 12 Ma for the Kotakonda and Mudalabad kimberlites of the Mahbubnagar cluster. Those workers also reported a range of Rb–Sr ages for the southern Indian lamproites: Ramannapeta, 1224 ± 14 Ma; Chelima, 1354 ± 17 Ma; Zangamarajupalle, 1070 ± 22 Ma. It is evident that, although further work remains to be done to improve the constraints on their emplacement ages, the Proterozoic kimberlites and lamproites of southern India were non-contemporaneous and emplaced over a protracted period of time.
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ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...GEOLOGY AND PETROLOGY OF...GEOLOGY AND PETROLOGY OF...AGE OF MAGMATISMANALYTICAL TECHNIQUESMINERAL CHEMISTRYWHOLE-ROCK GEOCHEMISTRYCOMPARISON WITH WORLDWIDE...PETROGENESISGEODYNAMIC MODEL FOR THE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Electron microprobe analyses were conducted using a CAMECA CAMEBAX SX50 in the Department of Earth Sciences, University of Cambridge, using both energy- and wavelength-dispersive spectrometry (EDS and WDS). Routine EDS analyses were obtained on carbon-coated polished thin sections with an accelerating voltage of 20 kV, 10 nA beam current, 3 µm beam diameter and live counting time of 50 s. Both natural and synthetic standards were used for calibration. On-line peak stripping and corrections were performed using LINK Analytical ZAF4 software. WDS was conducted by simultaneous counting on three spectrometers equipped with TAP, LIF and PET crystals with an accelerating voltage of 20 kV, 15 nA beam current and a counting time of 15–20 s for each element. The accuracy is not better than ±1%. Rare earth element (REE) contents of perovskite were determined by WDS with an accelerating voltage of 20 kV and a beam current of 50 nA, using the LiF crystal for REE L lines and Fe K
; the PET crystal for Ca K, Ti K and Nb L, and the TAP crystal for Sr L. The Lß lines of Pr and Eu were used to minimize interference effects. Standards used were LaB6, CeAl2, PrAl2, NdAl2, SmF3, EuVO4, pure Gd, wollastonite (for Ca), TiO2, pure Nb, pure Fe and Sr-celestine (for Sr).
Whole-rock X-ray fluorescence (XRF) analyses for major and some trace elements (Ba, Cr, Cu, Nb, Ni, Rb, Sc, Sr, V, Y, Zn and Zr) were carried out at the Grant Institute of Geology and Geophysics, University of Edinburgh, on a Phillips PW 1480 automated XRF spectrometer using standard procedures (Norrish & Hutton, 1969). Major elements were analysed on fused discs and trace elements on pressed powder pellets. Data quality was assessed by repeat analysis and analysis of internal standards. Typical uncertainties are <5% for major oxides and <10% for trace elements.
REE and trace elements (Cs, Hf, Ta, Pb, Th and U) were analysed using a VG Plasma Quad PQ2 STE instrument at the NERC ICP-MS facility, Silwood Park, Surrey. Samples were prepared by open digestion following Jarvis (1990). The instrument was calibrated using three multi-element standard solutions in dilute HNO3 containing 10 ppb, 20 ppb and 30 ppb of each element. These were analysed in triplicate. Standards BCR-1 and BHVO-1 were run with the samples. The difference in individual light REE (LREE) and middle REE (MREE) concentrations in replicate samples is <10% (Chalapathi Rao, 1997).
Sample dissolutions for Sr- and Nd-isotope ratios were carried out at McMaster University, Hamilton, Canada. All of the samples were leached with warm 6 M HCl and rinsed before dissolution. Sr and Nd isotopic ratios were measured on a VG354 mass spectrometer at McMaster by dynamic multicollection. 87Sr/86Sr ratios were normalized for within-run fractionation to 86Sr/88Sr = 0·1194, and are quoted relative to a value of 0·71024 for the NBS 987 standard. 143Nd/144Nd ratios were normalized for within-run fractionation to 146Nd/144Nd = 0·7219, and are quoted relative to a value of 0·51185 for the La Jolla standard. Within-run precision of Sr and Nd isotope ratios averages 0·00003 and 0·000015 (2
), respectively, and the accuracy of calculated initial ratios is estimated to be about 10% of the magnitude of the age corrections. Details of procedures adopted, including run conditions and leaching experiments to back-correct Rb and Sr values, have been given by Gibson et al. (1995) and Chalapathi Rao et al. (1999).
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MINERAL CHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...GEOLOGY AND PETROLOGY OF...GEOLOGY AND PETROLOGY OF...AGE OF MAGMATISMANALYTICAL TECHNIQUESMINERAL CHEMISTRYWHOLE-ROCK GEOCHEMISTRYCOMPARISON WITH WORLDWIDE...PETROGENESISGEODYNAMIC MODEL FOR THE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The compositions of mineral phases in 70 mafic potassic rocks from the Cuddapah Basin and Eastern Dharwar Craton were determined by electron microprobe analysis; representative analyses are presented in Table 3.
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Olivine
Fresh olivine is present in Pipes 3, 4, 5, 7, 10 and 11 from Anantapur, and the Maddur and Narayanpet kimberlites of the Mahbubnagar cluster. Olivine phenocrysts in the kimberlites range from Fo87 to Fo92. Most olivine macrocrysts have similar compositions (Fo84–92) although serpentinized olivine from Pipes 2 and 6 (Wajrakarur) range up to Fo95, possibly reflecting Mg enrichment as a result of magnetite precipitation during serpentinization. Some macrocrysts are relatively iron rich (Fo84–86), similar to those in some southern African kimberlites (e.g. Boyd, 1974
). Because of the considerable compositional overlap, no distinction can be made between olivine phenocrysts and xenocrysts on the basis of their Fo contents. However, olivines from disaggregated mantle xenoliths and phenocrysts from mantle-derived melts may be distinguished on the basis of their CaO contents (Brey et al., 1990; Thompson & Gibson, 2000). A significant proportion of the olivine macrocrysts in the Dharwar kimberlites have CaO <0·18 wt % and are probably xenocrysts (Fig. 3).
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Compositional zoning of olivine to iron-rich rims is present in most of the pipes. Ni contents also decrease subtly from core to rim in the phenocrysts. However, reverse zoning is also apparent, especially in Pipe 3 from the Anantapur cluster. Such reverse zoning has been noted in many Group I kimberlites of southern Africa (Moore, 1988
). Minor compositional differences between the phenocrysts may result from olivine phenocrysts crystallizing from differing batches of magma. In the Cuddapah lamproites, olivine is either very rare or completely pseudomorphed by calcite, and its composition could not be determined.
Monticellite
Monticellite is an important groundmass phase in Pipes 2, 5 and 6 from Anantapur, and in the Kotakonda kimberlite. Molar Mg/(Mg + Fe) ranges from 0·80 to 0·87 (Table 3) and is slightly lower than in coexisting olivine phenocrysts. Although most monticellites are relatively Mg rich, a few from the Kotakonda kimberlite show appreciable solid solution towards kirschsteinite (CaFeSiO4; Chalapathi Rao et al., 1996b). Monticellite crystallizes at low pressures and temperatures after spinel and perovskite, but before late-stage calcite and serpentine (Eggler, 1989). It is characteristically absent from lamproites and Group II kimberlites (Woolley et al., 1996).
Spinel
Spinel is not observed in any of the Cuddapah Basin lamproites. This is a feature characteristic of most lamproites (Mitchell & Bergman, 1991). Mitchell (1986) delineated two trends amongst kimberlite groundmass spinels: a magnesian ulvöspinel trend (Trend 1) containing Ti-rich spinels with substantial amounts of magnesian ulvöspinel molecule and a Ti-magnetite trend (Trend 2) containing a range in composition from aluminous– magnesian chromites to titanian–magnesian chromites, titanian chromites and members of the ulvöspinel magnetite series. Most of the spinels from the Dharwar Craton kimberlites follow Trend 2 (Fig. 4a). The Maddur kimberlite is characterized by abundant titanian magnetite whereas Pipe 8 (Lattavaram), the Kotakonda kimberlite and the Padiripahad kimberlite contain abundant chromite. The rest of the pipes contain different proportions of titanian–magnesian chromite, chromite and Ti-magnetite. Extreme variations in spinel compositions may reflect variation in the bulk composition of their parental magmas. Spinels from the Dharwar Craton kimberlites are relatively poor in aluminium and magnesium compared with those from southern Africa (Fig. 4b); those from Pipes 1 and 3 (Anantapur) have the highest Al2O3 and MgO contents.
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Clinopyroxene
Clinopyroxene occurs as a groundmass phase in the Anantapur kimberlites (Pipes 3, 4, 5, 8, 10, 11) and in the Maddur, Narayanpet and Padiripahad pipes from the Mahbubnagar cluster. Most clinopyroxenes are diopsides and those in Pipe 10 are the most Fe rich. The TiO2 contents of diopside in the Maddur kimberlite and Pipes 4, 5 and 11 are high (up to 4 wt %) compared with the other pipes. With the exception of Pipe 10 (Chigicherla), the Al2O3 contents of kimberlite diopside are <1·0 wt %. In Pipe 10, Al2O3 ranges from 1 to 2·5 wt %, suggesting that the clinopyroxene may be xenocrystal (see Mitchell, 1986
). Crust-derived xenocrystic augite occurs in Pipe 9. Clinopyroxene is a rare groundmass phase in Group I kimberlites (see Mitchell, 1995), but has been reported from some archetypal kimberlites from the Dharwar Craton (Scott-Smith, 1989) and Greenland (Scott, 1981). Its occurrence may be due to contamination of the magma by crustal xenoliths.
In the lamproites, clinopyroxene occurs only in the samples from Ramannapeta, where it is a pure diopside and shows little compositional variation. This is considered characteristic of lamproite diopsides and is attributed to either rapid quenching of the magma after the onset of pyroxene crystallization (Mitchell, 1985) or its early replacement as a liquidus phase by amphibole (Mitchell & Bergman, 1991). The TiO2 content of clinopyroxene in the Ramannapeta lamproite ranges from 1·37 to 2·2 wt % and Cr2O3 and Al2O3 contents are 0·06–0·48 wt % and 0·2–0·53 wt %, respectively. Such diopsides are considered to be typical of lamproites (Bergman, 1987).
Phlogopite
Phlogopite, along with olivine, is the most characteristic ferromagnesian mineral in kimberlites and lamproites, where it occurs as megacrysts, macrocrysts and microphenocrysts. Megacrysts (>1 cm) are xenocrystal and may be derived from mantle peridotites (Dawson & Smith, 1975), but it is not always possible to distinguish them from phenocrysts (Mitchell, 1995). All of the kimberlite phlogopites have TiO2 <4·5 wt % (Fig. 5) and zoning trends that indicate enrichment in Al2O3 and depletion in TiO2. Minor variations in zoning patterns shown by some micas may reflect changes in the magma composition on a small scale or mixing processes. Some Pipe 4 and Pipe 6 micas have relatively low Al2O3 (<4 wt %) but substantial tetrahedrally coordinated Fe3+ and are therefore tetraferriphlogopites. They strongly resemble phlogopites from the Orroroo kimberlites, Australia (Scott-Smith et al., 1984), and others from West Greenland (Emeleus & Andrews, 1975).
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Lamproite micas are more strongly zoned than those in the kimberlites (Fig. 5). They are all titaniferous (>4 wt %), with Zangamarajupalle micas containing up to 10·5 wt % TiO2. Ramannapeta and Zangamarajupalle micas show core–rim zoning trends of increasing TiO2 and FeOT and falling MgO and Al2O3. Although most of the Chelima micas also show marked Ti enrichment, a few show slight Ti depletion that may reflect mixing processes. In the lamproite phlogopites, the cation sum (Al + Si) is typically less than 8 atoms per 23 oxygens, so some Fe3+ probably occupies the tetrahedral site (e.g. Farmer & Boettcher, 1981
). The high TiO2 content of the Chelima, Ramannapeta and Zangamarajupalle micas is consistent with the classification of the host rocks as lamproites.
Cr2O3 contents of the kimberlite micas are mostly below the levels of detection, but Pipe 7 contains chromian micas with 0·5–0·7 wt % Cr2O3. Lamproite micas collectively show relatively high Cr2O3 values (0·2–0·35 wt %). Fluorine contents of kimberlite and lamproite micas worldwide are very poorly documented. The Cuddapah lamproite phlogopites contain 0·49–1·57 wt % F, with the highest values in the Chelima micas. In contrast, the Dharwar kimberlite micas have F contents ranging up to 3·1 wt %, whereas the Kotakonda micas consistently contain around 2 wt % F. The fluorine concentrations obtained here are consistent with the compositions of primary phlogopites (Smith et al., 1981), and are higher than in phlogopites from South African kimberlite peridotite nodules (Matson et al., 1986).
Amphibole
Pleochroic amphibole (richterite) is found only in the Ramannapeta lamproite (Table 3). It is rich in TiO2 (1·2–6·1 wt %) and K2O (3·8–4·5 wt %), but poor in Al2O3 (0·4 wt %) and CaO (<6·4 wt %). Bergman (1987) noted that richterites of such composition are ‘unique’ to lamproites. The Ramannapeta richterite TiO2 contents are more varied than in most other lamproites, where TiO2 is characteristically >3 wt % (Mitchell & Bergman, 1991), although richterites with 0·38–1·37 wt % TiO2 are found in the groundmass of some Kansas lamproites (Cullers et al., 1996). The Ramannapeta richterite has appreciably higher TiO2 than richterites in MARID suite nodules (e.g. Dawson & Smith, 1977, fig. 6) and richterite-bearing peridotites (Wagner et al., 1996). FeOT and Na2O contents show little variation. The Ramannapeta richterite appears to have a tetrahedral site deficiency (Si + Al <8), so either Fe3+ or Ti4+ may enter the tetrahedral sites (e.g. Mitchell et al., 1987; Hwang et al., 1994). Richterites in the Ramannapeta lamproites are broadly similar in composition to those from Barkley West (South Africa) and Western Australia (Fig. 6).
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Perovskite
Perovskite is an important groundmass phase in the Dharwar kimberlites but is present only in minor amounts in the Cuddapah lamproites. Perovskites have CaO (
38 wt %) and TiO2 (58 wt %) contents near that of the stoichiometric end-member, indicating little substitution by other elements. FeO* (as total iron) contents of perovskites range from 0·79 to 1·02 wt % in the non-diamondiferous Mahbubnagar kimberlites and from 0·92 to 2·22 wt % in the diamondiferous Anantapur kimberlites. These are similar to the Fe contents of kimberlite perovskites from elsewhere (Mitchell, 1986), indicating little Fe substitution in the Ti site. Nb2O5 contents are markedly different in the Anantapur (0·34–2·33 wt %) and Mahbubnagar (0·22–0·32 wt %) kimberlites, and low compared with Nb contents in perovskites from kimberlites elsewhere. This is consistent with the bulk-rock Nb contents (see below). LREE oxide contents range from 0·88 to 1·97 wt % in the Mahbubnagar perovskites and from 2·57 to 5·37 wt % in Anantapur perovskites. La/Sm ratios in the perovskites range from 6·5 to 8·5 and from 8 to 12·5 in the Mahbubnagar and Anantapur kimberlites, respectively. These are identical to the La/Sm ratios of the host kimberlites (see below) and confirm the significant role of perovskite in controlling the LREE contents of kimberlites.
Apatite
Apatite is a ubiquitous phase in southern Indian kimberlites and lamproites, and shows no significant variation in composition.
Carbonate
Calcite is an abundant primary and secondary phase in kimberlites, whereas lamproites are virtually devoid of primary calcite, and any carbonate is of secondary origin (Mitchell & Bergman, 1991). Calcite occurs in Pipes 7 and 9 (Anantapur). Ferroan dolomite is present in the Chelima and Zangamarajupalle lamproites, where its texture suggests that it is a secondary phase.
Rutile
Rutile is a common accessory phase in the Dharwar kimberlites, where it chiefly occurs as needle-shaped crystals in olivine. Although it is considered an uncommon mineral in lamproites (Mitchell & Bergman, 1991), rutile is a fairly abundant groundmass phase in the Chelima and Zangamarajupalle lamproites. It is relatively poor in Cr2O3 (<0·3 wt %) compared with rutile from the Maddur kimberlite (1·43 wt %).
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WHOLE-ROCK GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING OF THE...GEOLOGY AND PETROLOGY OF...GEOLOGY AND PETROLOGY OF...AGE OF MAGMATISMANALYTICAL TECHNIQUESMINERAL CHEMISTRYWHOLE-ROCK GEOCHEMISTRYCOMPARISON WITH WORLDWIDE...PETROGENESISGEODYNAMIC MODEL FOR THE...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Whole-rock chemical analyses of 69 representative samples of southern Indian Proterozoic kimberlites and lamproites are presented in Table 4. Kimberlites and lamproites incorporate varying proportions of crustal and mantle xenoliths on ascent from the mantle to the Earth's surface, and they are highly susceptible to hydrothermal alteration. It is therefore necessary to evaluate the extent of contamination and alteration in these samples before interpreting their bulk-rock compositions.
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Alteration and contamination indices
Loss on ignition (LOI) values of the southern Indian kimberlites and lamproites typically range from 3·9 to 12·3 wt %; Pipe 1 (22·1 wt %), Pipe 9 (27·2 wt %) and the Chelima lamproite (14·2 wt %) have relatively high LOI as a result of the presence of secondary carbonate minerals and talc. As mentioned above, some samples are petrographically fresh (Anantapur Pipes 3, 4, 5, 7, 10 and 11, and the Maddur and Narayanpet pipes from the Mahbubnagar cluster) and these tend to have lower LOI (3·7–10 wt %). Analyses of these latter rocks provide important constraints on the effects of hydrothermal alteration, especially on highly mobile elements such as Rb and K.
We have used Clement's (1982) contamination index (C.I.) to assess the extent of crustal contamination of the samples:
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; Taylor et al., 1994; Beard et al., 2000). Kimberlites with C.I. <1·4 are generally regarded as uncontaminated. The contamination index is influenced by alteration, but also by olivine accumulation. The relatively low Ni contents of the Dharwar kimberlites, however, suggest that their contamination indices have not been significantly influenced by olivine accumulation. Anantapur Pipe 1 has relatively high SiO2 (59·1 wt %), Al2O3 (7·1 wt %) and Na2O (0·9 wt %) contents and hence a high contamination index (5·7), which is consistent with the presence of abundant crustal xenoliths in this diatreme-facies kimberlite. The remaining Anantapur hypabyssal-facies kimberlites have C.I. values of 0·87–2·4 (Table 4). The more micaceous and non-diamondiferous Anantapur kimberlites have moderately elevated values, i.e. Pipe 2 (1·7–2·9) and Pipe 5 (1·5–1·9), whereas the rest have C.I. 1·5. Contamination indices for the non-diamondiferous Mahbubnagar kimberlites vary from 1·3 to 1·9. The Maddur kimberlite and the Cuddapah lamproites have high contamination indices (2·5–3·5) reflecting their high modal phlogopite contents (Fig. 7).
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Major elements
MgO contents range from 21 to 25 wt % in the Mahbubnagar kimberlites and from 18 to 29 wt % in the Anantapur kimberlites (excluding altered rocks from Pipes 1 and 9). All of the Dharwar kimberlites show a well-defined increase in CaO and Al2O3 with decreasing MgO, and all have low SiO2 contents (28–40 wt %). The Fe2O3* (Total Fe) range of the diamondiferous Anantapur kimberlites (6·9–13·4 wt %) is lower than that of non-diamondiferous kimberlites from Mahbubnagar (13·5–14·8 wt %). The latter also have higher TiO2 contents (3·8–7·2 wt %) than those from Anantapur (0·7–4·7 wt %; Fig. 8). We have used the ‘Ilmenite Index’ of Taylor et al. (1994)
to identify kimberlites and lamproites that may have accumulated and/or assimilated ilmenite megacrysts and xenocrysts (Fig. 7). This index (Ilm.I.) is defined thus:
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Cuddapah Basin lamproites have relatively low MgO contents (<15 wt %) and show limited major elemental compositional variation at a given MgO value. They do not lie on the extension of linear arrays defined by the kimberlites, i.e. they have a distinct chemistry (Fig. 8). A single lamproite sample (Chelima: C5 sample) plots amongst the Anantapur kimberlites in Fig. 8. This has an unusually high LOI (15·92 wt %, Table 4) and appears to have undergone extreme secondary carbonation that has resulted in a low SiO2 (35·4 wt %) and high CaO (17·21 wt %) content. The remaining Cuddapah Basin lamproites have higher SiO2 (42–46 wt %) than the kimberlites and are similar to other lamproite occurrences worldwide (Fig. 9). The relatively high SiO2 (46·3 wt %), CaO (9·2 wt %) and Na2O (0·43 wt %) contents of the Ramannapeta lamproite are consistent with its high modal proportion of groundmass phlogopite (Table 2). The lamproites have Fe2O3* contents that range from 7·5 to 11·4 wt % and the TiO2 enrichment (3·1–4·8 wt %) corresponds to the Ti-rich nature of their phlogopite and, in the case of Chelima and Zangamarajupalle, to rutile abundance. They have relatively high K2O (
2–4·8 wt %) and hence high K2O/Na2O ratios (>6). The low K2O content (1–2 wt %) of the Chelima lamproite (C1-C) is probably due to its highly altered nature. All of these rocks may therefore be classified as potassic–ultrapotassic (Foley et al., 1987). P2O5 contents of the Cuddapah Basin lamproites range from 0·64 to 1·5 wt %, with Chelima and Zangamarajupalle having the highest abundances (up to 1·1 wt % and 1·4 wt %), consistent with their relatively high modal apatite contents.
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Trace elements
The variability of compatible element abundances in kimberlites is due to widely varying macrocryst/ phenocryst–matrix ratios and does not necessarily reflect the composition of the liquids from which their parental melts were generated (Mitchell, 1986
). Both Ni and Cr show a good positive correlation with MgO (Fig. 8) in the kimberlites. The Mahbubnagar kimberlites show lesser variation and lower abundances of Ni (772–932 ppm) and Cr (888–1000 ppm) than the Anantapur kimberlites (491–1367 ppm Ni, 765–1491 ppm Cr). Sc contents of kimberlites from Pipes 2, 5 and 6 are slightly higher (13–27 ppm) than those of the other Anantapur kimberlites (<15 ppm), consistent with their higher modal phlogopite contents (Table 2). Non-diamondiferous Mahbubnagar kimberlites show uniformly high Sc contents (20 ppm). Vanadium in kimberlites is hosted primarily by phlogopite and spinel. The Mahbubnagar kimberlites collectively show higher V abundances (144–253 ppm) than most of the Anantapur kimberlites. The diamondiferous Anantapur kimberlites have Zr <200 ppm and Hf between 2 and 5 ppm (Table 4). Non-diamondiferous Pipe 5 (Muligiripalle) has relatively high Zr (400–478 ppm) and Hf (>10 ppm). The non-diamondiferous Mahbubnagar kimberlites have moderate Zr (>200 ppm) and Hf (5·5–7·5 ppm) contents. The Rb contents of the Anantapur kimberlites range from 73 to 203 ppm whereas those of Mahbubnagar range from 67 to 89 ppm (Table 4). The high Nb contents of the non-diamondiferous Mahbubnagar kimberlites and non-diamondiferous Pipes 2 and 5 from the Anantapur cluster are consistent with their high modal percent of perovskite. Highly diamondiferous Pipe 7 has the highest concentrations of Nb (250 ppm) for a given TiO2 content.
In the Cuddapah lamproites, Ni (487–718 ppm) and Cr (667–909 ppm) concentrations are low relative to the Dharwar kimberlites but Sc contents are high (20 ppm). Relatively high values of Zn in the Chelima (300 ppm) and Ramannapeta (120 ppm) lamproites may reflect secondary alteration. The lamproites have higher Zr (400–1149 ppm), Hf (10–30 ppm) and Rb contents (55–316 ppm) and higher K/Rb (12–50) than the kimberlites (2·6–10·4). The extremely high K/Rb (50) of the Ramannapeta lamproite is primarily due to its low Rb abundance (60 ppm). This lamproite has low Nb (100 ppm) and low modal perovskite compared with the other kimberlites and lamproites.
On normalized multi-element plots the majority of the southern Indian kimberlites and lamproites exhibit troughs at K and rarely at Rb (Fig. 10). Such negative anomalies may reflect either hydrothermal alteration or the presence of residual phases in contributing melt source regions. The altered nature of Pipes 1 and 9 might be responsible for their relatively low concentrations of both K and Rb (Fig. 10), which is supported by their high LOI and high contamination indices (Table 4). Negative Rb and K anomalies were recorded in the Group I and II kimberlites of southern Africa (e.g. Smith et al, 1985; Mitchell, 1995) and a trough at K is seen in many mafic–potassic rocks (e.g. Gibson et al., 1995). The negative Rb anomaly of the Ramannapeta lamproite could be due to either hydrothermal alteration or residual amphibole (K-richterite) in its mantle source, whereas a relative depletion in K and the lack of a corresponding negative Rb anomaly in most of the southern Indian kimberlites and lamproites suggests that the residual mantle phase may be phlogopite. Our interpretations are consistent with experimental data suggesting that phlogopite, bu