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
 |
ABSTRACT |
|---|
 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 |
|---|
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
|
INTRODUCTION |
|---|
|
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 |
|---|
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.
|
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.
|
|
GEOLOGICAL SETTING OF THE CUDDAPAH BASIN AND THE EASTERN DHARWAR CRATON |
|---|
|
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 |
|---|
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).
|
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 |
|---|
|
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 |
|---|
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.
|
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.
|
GEOLOGY AND PETROLOGY OF THE LAMPROITES FROM THE CUDDAPAH BASIN AND ITS NE MARGIN |
|---|
|
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 |
|---|
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.
|
AGE OF MAGMATISM |
|---|
|
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 |
|---|
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.
|
ANALYTICAL TECHNIQUES |
|---|
|
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 |
|---|
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).
|
MINERAL CHEMISTRY |
|---|
|
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 |
|---|
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.
|
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).
|
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.
|
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).
|
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).
|
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 %).
|
WHOLE-ROCK GEOCHEMISTRY |
|---|
|
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 |
|---|
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.
|
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:
 |
; 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).
|
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:
 |
|
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.
|
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, but not richterite, is likely to be a residual phase during melting of metasomatized peridotite (Van der Laan & Foley, 1994). Separation of phlogopite macrocrysts may also produce negative troughs at K on normalized multi-element plots but we have no conclusive evidence of this process occurring in the kimberlites and lamproites of southern India (e.g. see vector for phlogopite in Fig. 7).
|
The positive Ti spikes of the Mahbubnagar and some non-diamondiferous Anantapur kimberlites correlate with relatively high modal amounts of perovskite. The relatively high concentrations of Ti, Nb, Fe and Hf in the non-diamondiferous kimberlites may reflect melting and/or entrainment of ilmenite from their mantle source regions. Ilmenite has a high intrinsic oxygen fugacity, which is well above the stability field of solid carbon. Negative Ti anomalies are not considered characteristic of kimberlites (Mitchell, 1995
) but they are present in the Tres Ranchos (Brazil), Premier and Bellsbank (South Africa) kimberlites (see Fig. 10) and in the diamondiferous Anantapur Pipes 1, 7 and 9. Although alteration may account for Ti depletion in Pipes 1 and 9, the presence of a residual Ti-bearing phase in the source for the Pipe 7 kimberlite seems a more plausible explanation for its Ti depletion. The C.I. of this pipe (1·20–1·30, Table 4) is the lowest of the Anantapur pipes. It also has a low Ilm.I. (<0·52) suggesting little or no entrainment of ilmenite megacrysts. Negative spikes at P in some of the rocks may reflect residual apatite in their source. Pipe 7 is the most highly diamondiferous of the southern Indian kimberlites and shows considerable differences from the other pipes in its overall geochemistry.
Rare earth elements
Concentrations of REE are relatively unaffected by secondary processes, such as leaching or weathering, and their relatively high abundances mean that they are not significantly affected by contamination with crustal material. Perovskite and apatite are the main REE-bearing phases in kimberlites and lamproites (Mitchell & Bergman, 1991). The kimberlites and lamproites of southern India are strongly enriched in LREE with La abundances of 300–1100 times chondrite (Fig. 10). Pipes 7 and 9 from Anantapur, however, are more enriched in their LREE (700 times chondrite) than the other kimberlites and have similar concentrations to the lamproites (700– 1100 times chondrite). Within-pipe REE variations are in general minor and occur only in Anantapur Pipes 1 and 2. Heavy REE (HREE) abundances are low, and range from 5 to 10 times chondrite. Consequently, La/Yb ratios are high and show a wide range (from 70 to 176). All samples are enriched in LREE relative to MREE, with La/Sm ratios of 7–12 (Fig. 11).
|
It is well established that melts with high La/Yb ratios (60–180) can be produced by very small (<1%) degrees of partial melting of a phlogopite–garnet lherzolite (e.g. Mitchell & Brunfelt, 1975
; Mitchell & Bergman, 1991; Tainton & McKenzie, 1994). Furthermore, to generate such melts with high REE abundances the mantle source must have been previously enriched (e.g. Menzies & Wass, 1983; Vollmer & Norry, 1983). Evidence for metasomatism in the sub-Dharwar lithosphere comes from the REE data for the garnet lherzolite xenolith from Pipe 3 (Lattavaram), which are similar (particularly in terms of its LREE enrichment) to those for the metasomatized garnet lherzolites from southern Africa (see Nehru & Reddy, 1989).
Radiogenic isotopes
Bulk-rock Sr- and Nd-isotope determinations were undertaken on eight kimberlites and four lamproites (Table 5). These samples were selected on the basis of petrographic evidence (of minimal crustal contamination) and, where possible, low LOI (indicating restricted alteration) and high Rb contents. The measured isotopic ratios of the Anantapur kimberlites have been corrected to an emplacement age of 1090 Ma, whereas those for Mahbubnagar have been corrected to 1400 Ma. All lamproites were corrected to an emplacement age of 1418 Ma.
|
The diamondiferous Anantapur kimberlites have initial 87Sr/86Sr ratios (87Sr/86Sri) that range from 0·7021 (Pipe 2, Wajrakarur) to 0·7073 (Pipe 7, Venkatampalle) and correspond to
Sri values of –1·6 to 6 (Fig. 12). These 87Sr/86Sri ratios agree with the few ratios previously published for the Anantapur kimberlites (Paul, 1979; Anil Kumar et al., 1993). The Mahbubnagar kimberlites show a similar range from 0·7027 (Kotakonda) to 0·7070 (Maddur). The kimberlite suites have 143Nd/144Ndi ratios ranging from 0·51105 (Kotakonda) to 0·51133 (Maddur). These correspond to Ndi values that range from +4·3 to +4·6 for the non-diamondiferous Mahbubnagar pipes and from +0·5 to +2 for the diamondiferous Anantapur kimberlites (Fig. 12). These values, in conjunction with the 143Nd/144Ndi ratios from Basu & Tatsumoto (1979), show that all of the Eastern Dharwar Craton kimberlites have initial Nd isotopic signatures higher than Bulk Earth.
|
The Cuddapah lamproites have relatively high 87Sr/86Sri (from 0·7059, Ramannapeta, to 0·7218, Zangamarajupalle) that correspond to
Sri of 3–52. Two samples of the Chelima lamproites have very high 87Sr/86Sri ratios (0·7168 and 0·7390); these may reflect the presence of abundant secondary carbonate. Crawford & Compston (1973) also recorded high 87Sr/86Srm (0·7286) for the Chelima lamproite. The high 87Sr/86Sri (0·7218) for the Zangamarajupalle lamproite may similarly be due to secondary alteration. We are therefore cautious about interpreting the petrogenetic significance of these ratios. The lamproites have 143Nd/144Ndi ratios significantly lower than those of the kimberlites, with Ndi ranging from –7·3 to –8·3 (Table 5). The lamproites appear to be predominantly derived from melt source regions with lower time-integrated Sm/Nd ratios than Bulk Earth. The 87Sr/86Sri ratios do not show a systematic variation with 143Nd/144Ndi, suggesting that these two isotope systems are decoupled. This is almost certainly caused by the mobilization of Sr during hydrothermal alteration. |
COMPARISON WITH WORLDWIDE OCCURRENCES |
|---|
|
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 |
|---|
The Dharwar kimberlites are mica-rich and petrographically resemble the phlogopite-bearing kimberlites from the USA. The high TiO2 and low K2O contents, low La/Nb (0·6–0·9) and Ce/Y ratios (7–20) of both the diamondiferous Anantapur and non-diamondiferous Mahbubnagar kimberlites are similar to those of Group IA ‘on-craton’ kimberlites from southern Africa (Fig. 13). They are less rich in LREE than Group II South African kimberlites but have higher incompatible element abundances than the Proterozoic Premier kimberlite (Group I) of South Africa (McDonald et al., 1995
). The Ndi values of the Anantapur and Mahbubnagar kimberlites are similar to those of Group I kimberlites from southern Africa (Smith, 1983), North America (Alibert & Albarède, 1988) and Greenland (Nelson, 1989).
|
The Maddur kimberlite from the Mahbubnagar cluster and Pipe 11 from the Anantapur cluster show a small Sr dip on normalized multi-element plots (Fig. 10), a feature that is common in kimberlites and lamproites worldwide. This can be attributed either to the presence of residual clinopyroxene (Smith et al., 1985
) or residual phosphate (Mitchell, 1995) in the mantle source, or to depletion of the mantle source in Sr during a previous phase of melt extraction (Tainton & McKenzie 1994). The REE plots do not show any apparent depletion in MREE (Eu to Ho) and lack the downward concave shape that is characteristic of some other Gondwanaland kimberlites, e.g. the Aries kimberlite of Western Australia (Edwards et al., 1992) and the Koidu kimberlite of West Africa (Taylor et al., 1994).
The Cuddapah Basin lamproites appear to be less rich in K2O than other olivine lamproites (West Kimberley and Smoky Butte) but have contents comparable with those of lamproites from the Leucite Hills and Prairie Creek (Fig. 13). They have relatively high La/Nb (1–1·5) and variable Ce/Y ratios (10–27) that are similar to lamproites worldwide (e.g. Western Australia; western USA; Sover North, South Africa). On normalized multi-element plots (Fig. 10), the Cuddapah lamproites lack the peaks at Zr and Hf that characterize some other worldwide lamproites (e.g. those from Western Australia and the western USA). The Cuddapah lamproites have slightly higher Nd isotopic ratios than lamproites from Western Australia (McCulloch et al., 1983; Jaques et al., 1989) and Prairie Creek (Arkansas, USA; Fraser et al., 1985) but similar ratios to mafic potassic– ultrapotassic rocks from the Alto Paranaíba Province, Brazil (Gibson et al., 1995) and Italy (Hawkesworth & Vollmer, 1979).
|
PETROGENESIS |
|---|
|
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 |
|---|
The compositions of mafic potassic–ultrapotassic magmas can be used to investigate the relative contributions of asthenosphere- and lithosphere-derived melts and to probe compositional variations in the sub-continental lithospheric mantle (Gibson et al., 1995
, 1996). Studies over the last decade have suggested that Group I and II southern African kimberlites and lamproites are genetically distinct and that their parental melts were derived from different source regions (e.g. Mitchell & Bergman, 1991; Mitchell, 1995). The close spatial and temporal association of kimberlites and lamproites in southern India provides an opportunity to test the extent to which these models and observations hold in other portions of the Gondwana supercontinent.
Role of crustal contamination
Southern Indian kimberlites and lamproites have much higher abundances of incompatible trace elements such as Sr (550–1526 ppm), Nb (80–250 ppm) and Zr (180–600 ppm) than the continental crust. All of the studied rocks have Mg/(Mg + Fe) >0·69 and high Ni contents (>500 ppm), which are indicative of the ‘primitive’ nature of the magmas. Furthermore, the major oxide compositions of these rocks show low abundances of Al2O3 (<10 wt %) and Na2O (<0·5 wt %) that cannot be accounted for by crustal contamination. The granitic rocks through which the kimberlites of the Eastern Dharwar Craton have been emplaced have relatively low LREE/HREE ratios (La/Yb = 30; Divakara Rao et al., 1994). The absence of positive Eu anomalies and the low HREE and Y contents of the samples provide further additional evidence against significant crustal contamination. Thus, it can be concluded that variation in the Nd-isotopic signature of the kimberlites and lamproites is not simply due to crustal contamination, but reflects that of their contributing mantle source regions.
Source enrichment and mantle heterogeneity
Mantle metasomatism is typically attributed to either (1) fluids or melts generated by subduction processes (e.g. Peacock, 1990; Maury et al., 1992; Murphy et al., 2002), or (2) volatile- and K-rich, low-viscosity melts that leak continuously to semi-continuously from the asthenosphere and accumulate in the overlying lithosphere (e.g. McKenzie, 1989; Gibson et al., 1995; Wilson et al., 1995). The nature of the metasomatizing agent may vary widely from silicate to carbonate melts rich in CO2 and H2O (e.g. Wyllie, 1987; Wallace & Green, 1988), to Na metasomatism derived by peridotite slab–melt interaction (e.g. Takahashi, 1988; Kepezhinskas et al., 1995). The composition of these metasomatic melts is believed to change continuously as they percolate through the mantle from their source regions (Navon & Stolper, 1987). Kimberlite melt may also form an important metasomatizing agent (Kinny & Dawson, 1992), and Tainton & McKenzie (1994) have suggested that phlogopite and K-richterite-bearing peridotites may have been generated by infiltration of a metasomatic melt with a composition similar to that of kimberlites.
Normalized multi-element plots (Fig. 10) for kimberlites and lamproites of southern India do not show any subduction-related characteristics, such as large negative anomalies at Ta, Nb and Ti, and we therefore attribute the source enrichment to metasomatizing melts derived from the convecting mantle. McKenzie (1989) proposed that small-melt fractions—enriched in K2O, incompatible elements and volatiles—have the ability to migrate continuously or semi-continuously from the asthenosphere and invade the sub-continental lithospheric mantle. He suggested that these melts would freeze in the lithosphere where the ambient temperature was below their solidus and they would therefore be concentrated in thin zones over a wide range of depths. McKenzie (1989) pointed out that this process could continue over a long period of time, resulting in substantial volumes of frozen melt accumulating in the mechanical boundary layer as veins, dykes or sills. Their compositions would depend on the contents of CO2 and H2O but strongly resemble those of kimberlites, lamproites and carbonatites. Remelting of these modified layers by heating (mantle plume) or by extension (decompression melting) can lead to the generation of potassic–ultrapotassic rock types, such as kimberlites and lamproites, and long-term storage of these areas result in enriched mantle.
Experimental studies on the liquidus compositions of lamproites and other ultrapotassic rocks suggest that their enriched source regions melted under H2O-rich, reducing conditions (e.g. Foley, 1988, 1992, 1993; Mitchell & Bergman, 1991) in contrast to the CO2-rich nature of the kimberlite source regions (e.g. Canil & Scarfe, 1990; Edgar & Charbonneau, 1993; Girnis et al., 1995). Hence, their observed petrological and compositional differences are attributed to variations in the content and nature of volatiles in their source regions. The presence of accessory phases in vein assemblages, rich in incompatible elements, was suggested to be essential for the generation of lamproites and other ultrapotassic rocks (Becker et al., 1999). A lamproite-like trace element pattern can result from melting of a veined assemblage if both apatite and a titanate mineral are present in addition to diopside, phlogopite and K-richterite; however, the concentrations of these phases must be small (Edgar & Mitchell, 1997).
There is considerable debate as to the role of the convecting mantle in kimberlite genesis. The presence of syngenetic inclusions of majoritic garnets within diamonds (Moore et al., 1991) and ultra-deep (>400 km) xenoliths in some southern African kimberlites with ocean-island basalt (OIB)-like isotopic signatures, i.e. Group I kimberlites, led some workers to suggest that they are convecting mantle melts derived from a ‘transition zone’ source (Ringwood et al., 1992), or even from the core–mantle boundary (Haggerty, 1994). Invoking similar criteria, some of the kimberlites with Group I type isotopic signatures from the Eastern Dharwar Craton have been suggested to be products derived from partial melting of sources in the ‘transition zone’ (Murthy et al., 1997). In contrast, Tainton & McKenzie (1994) suggested that Group I and II kimberlites and lamproites are all predominantly derived from lithospheric mantle sources and that the difference in isotopic compositions requires that the source enrichment occurred at different times but not necessarily by different processes.
The isotopic systematics of southern Indian kimberlites and lamproites demonstrate the existence of variably enriched source regions beneath the intracratonic Cuddapah Basin and Eastern Dharwar Craton. The 143Nd/144Nd ratios of the Cuddapah lamproites require that their predominant contributing mantle source region(s) was isolated from the convecting mantle for a significant period of time, prior to their emplacement. Even though the Ramannapeta lamproite differs from the Chelima and Zangamarajupalle lamproites in terms of mineralogy and geochemistry, it has a similar Nd isotopic composition (Ndi of –6·4 to –8·29). Although we cannot rule out a small melt contribution from the convecting mantle, the predominant melt source of the lamproites appears to have resided within the mechanical boundary layer of the lithospheric mantle (see below). The positive Ndi values (4) of the non-diamondiferous Mahbubnagar pipes would have been only slightly less than the depleted upper mantle at 1400 Ma (Nd = 6; e.g. McCulloch & Bennett, 1994). This, combined with the high La/Sm ratios (6·9–8·6) of the kimberlites, suggests that their mantle source regions were enriched by small-fraction melts, derived from the depleted upper mantle, shortly prior to final melt generation. The slightly lower Ndi values (1·15–0·4) of the diamondiferous Anantapur kimberlites (and also the non-diamondiferous Padiripahad kimberlite from the Mahbubnagar cluster) correspond to the high macrocryst/phenocryst ratio (Table 2) and high Ni contents (Fig. 8) in these samples. We suggest that these kimberlites contain a greater proportion of entrained sub-continental lithospheric mantle peridotite than the Mahbubnagar kimberlites.
REE inversion modelling
We have used the REE inversion program (INVMEL) developed by McKenzie & O'Nions (1991) to model the melt distribution with depth that may have been responsible for the observed REE patterns in the Dharwar kimberlites and Cuddapah lamproites. The inversion technique uses the REE abundances and the slope (La/Yb) of the normalized pattern to constrain the melt distribution with depth, given an assumed source composition.
Xenolith data from the Dharwar Craton kimberlites are sparse, but some are similar to the ‘depleted’ garnet peridotite xenoliths entrained by southern African kimberlites (Ganguly & Bhattacharya, 1987; Nehru & Reddy, 1989). Therefore, we adopted as a source composition a garnet peridotite with 70% olivine, 22% orthopyroxene, 3% clinopyroxene and 5% garnet, phlogopite and apatite, corresponding to the composition of depleted garnet phlogopite peridotite xenoliths from the Kaapvaal craton (Erlank et al., 1987). In addition, chrome-spinel was added to the lherzolite source, and a correction was made to Ni to remove the effects of olivine xenocrysts (see Tainton & McKenzie, 1994).
Modelled REE concentrations are sensitive to the amount of melting and the depth at which it takes place. The amount of melting is important as it determines the extent to which phases (e.g. garnet, clinopyroxene) remain in the residue, and the depth of melting also influences the source mineralogy. Tainton & McKenzie (1994) investigated inversion models for Group I and II kimberlites from the Kaapvaal craton and the Argyle lamproite of Western Australia, and suggested that the observed REE patterns required three stages of melting: (1) depletion of the peridotite source by extraction of 20% melt in the garnet stability field; (2) metasomatic enrichment with a MORB type melt; (3) small-fraction melting of this enriched harzburgite source. The extensive initial melting event is required to produce the observed low concentrations of HREE in kimberlites and lamproites. Subsequent metasomatic enrichment is required to produce the observed LREE abundances.
We inverted the REE concentrations of the Dharwar kimberlites and Cuddapah lamproites using similar parameters to test whether the Tainton & McKenzie model holds good for these samples. We used the entire dataset for hypabyssal-facies kimberlites and lamproites from southern India (Table 4) in our inversion models.
Because most kimberlites are hybrid rocks, containing a significant proportion of entrained lithospheric material, interpretation of the primary magma compositions from bulk-rock analyses must be treated with caution. This is especially the case for the Anantapur kimberlites that have a high olivine macrocryst/phenocryst ratio. However, as the bulk-rock LREE/MREE ratios of the kimberlites are comparable with those in perovskites, which occur as an abundant euhedral early-crystallizing groundmass phase (Fig. 11), we suggest that these ratios are representative of those in the primary magma. This suggestion is consistent with previous studies of the REE contents of perovskite that have demonstrated that the whole-rock REE abundances in kimberlites and lamproites correspond primarily to the modal amounts of this mineral rather than any other phase (e.g. Jones & Wyllie, 1984; Mitchell & Reed, 1988; Mitchell & Steele, 1992).
The fractionated HREE ratios (e.g. [Ho/Lu]n = 2·5–5) of the Dharwar Craton kimberlites and Cuddapah Basin lamproites imply that partial melting occurred in the presence of garnet. As melting takes place entirely in the garnet stability region, the depth to the top of the melting column cannot be predicted. However, as some of the kimberlites contain diamond we have placed the top of the melting column at 150 km. This is the depth of intersection of the diamond–graphite transition with the cratonic geotherm (see below). Anand et al. (2003) estimated that the thickness of the lithosphere beneath the intracratonic Cuddapah Basin was 145 km prior to extension and we have used this as a maximum depth in our lamproite models.
The calculated REE abundances fit the observed values (Fig. 14) and the predicted amounts of mantle source depletion and enrichment are almost indistinguishable. These values, together with the calculated major element composition of the source, are summarized in Table 6. The difference between observed and calculated REE concentrations is less than twice the SD in all cases and hence is not significant. The amount of source depletion for the kimberlites and lamproites ranges from 18 to 28% over a depth range of 68–89 km. The proportion of metasomatic melt enrichment varies from 2 to 4% for the majority of Anantapur and Mahbubnagar kimberlites, and from 6 to 7% for highly diamondiferous Pipe 7 (Venkatampalle) and Pipe 9 (Lattavaram). Metasomatic melt enrichment of the lamproite mantle source region ranges from 4% (Ramannapeta) to 8% (Chelima and Zangamarajupalle) and is indistinguishable from that of the kimberlites. Increasing the degree of enrichment by metasomatic melt addition results in a poorer fit to the HREE data, whereas decreasing enrichment worsens the fit for LREE. The amounts of initial melt depletion and subsequent enrichment estimated for the mantle source regions of the southern Indian kimberlites and lamproites are similar to those for southern African kimberlites (Table 6).
|
|
The model that best fits the REE data does not, however, fully account for the observed concentrations of some trace elements (Fig. 14). This may be partly due to limitations in our assumptions regarding partition coefficient data. Nevertheless, some deductions made from these plots may provide additional information about the nature of the source. With the exception of non-diamondiferous Pipe 5 (Muligiripalle), the observed concentrations of K are low and suggest either the presence of residual phlogopite in the source, or hydrothermal alteration. Observed concentrations of Zr and Hf are lower than the predicted values for highly diamondiferous Pipes 7 and 9; this may be due to the presence of residual zircon in the source region. Concentrations of high field strength elements (Nb, Ta and Ti) in the kimberlites are generally underestimated by the model and this together with the high Ilm.I. (see above) suggests that a titanate phase may have been present in their melt source regions. Sr concentrations are overestimated and the lower Sr in all of the Dharwar kimberlite pipes may be attributed to: (1) the involvement of a carbonate phase; (2) errors in the assumed partition coefficient (Tainton & McKenzie, 1994
); or (3) extensive depletion of a calcium-bearing phase in the melt source region. Alteration by secondary carbonation (as in the case of lamproites) may lead to elevated Sr concentrations in the calculated compositions.
We consider that minor discrepancies between predicted and observed models, as discussed above, are not due to the assumptions in our modelling but are source related. For example, the low observed K values compared with the melting models can be attributed to phlogopite; the REE partition coefficients for both phlogopite (and also K-richterite) are considerably smaller than those for diopside in the nodules and therefore the influence of residual phlogopite on the REE concentrations in the melt can be neglected (Erlank et al., 1987). Likewise, the possibility that the melt that metasomatized the source could have been carbonatitic, rather than silicic, in nature can also be ruled out as (1) kimberlite magmas and metasomatic melts are dominantly silicic, and carbonate would have only a modest effect on bulk-partition coefficients (see Harte, 1983), and (2) extensive depletion of the source, followed by enrichment, is still required to model the HREE concentrations in kimberlites or lamproites even if the partition coefficients for pure carbonatite are used (Tainton & McKenzie, 1994).
Extensive melting (20%) of the source to form a harzburgite is a prerequisite for the generation of kimberlites and lamproites of the Eastern Dharwar Craton. Evidence for at least one episode of extensive melting beneath the Eastern Dharwar Craton during the Archaean and Early Proterozoic is provided by the presence of komatiites and metavolcanics in the greenstone belts (Rajamani et al., 1985, 1989; Mallikharjuna Rao et al., 1995). Tainton & McKenzie (1994) showed that the melt distribution curve obtained from the REE inversion of a komatiite, but not a tholeiite, can satisfy the extraction of 20% melt from a region in which garnet is stable. They also calculated that komatiitic melting results in the thickening of the lithosphere by a factor of two, thereby placing the depleted source well within the graphite– diamond transition (i.e. 150 km). Experimental studies have also highlighted the importance of a harzburgite mantle source for lamproite genesis (e.g. Foley et al., 1987; Jaques et al., 1989; Mitchell & Bergman, 1991).
|
GEODYNAMIC MODEL FOR THE PROTEROZOIC DHARWAR KIMBERLITES AND CUDDAPAH LAMPROITES |
|---|
|
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 |
|---|
Mantle plumes have been widely invoked in the genesis of kimberlites and lamproites, especially for those of Mesozoic age (e.g. Crough et al., 1980
; England & Houseman, 1984). Heat conducted from the mantle plumes, or advected by plume-derived small-fraction melts, is believed to have caused melting of the overlying metasomatized lithospheric mantle and generated the kimberlites and lamproites. The role of mantle plumes in the genesis of Proterozoic kimberlites is poorly constrained. There is, however, considerable evidence for the role of mantle plumes in the genesis of Proterozoic basalts (e.g. LeCheminant & Heaman, 1989) and Archaean komatiites (Abbott, 1996; Tomlinson et al., 1998; Gibson, 2002) but each of these examples involves relatively large-degree melting of the convecting mantle. To further understand the petrogenesis of the Dharwar kimberlites and the Cuddapah lamproites we have attempted to reconstruct the Proterozoic geothermal gradient (Fig. 15) using the following information.
|
(1) The cooling Earth models of Turcotte & Schubert (1982)
and Richter (1988) predict that the potential temperature of the convecting mantle at the time of genesis of the Dharwar kimberlites and Cuddapah lamproites would have been 1400°C. We have assumed an adiabatic gradient of 0·6°C/kbar (McKenzie & Bickle, 1988).
(2) Previous studies of garnet lherzolite and harzburgite xenoliths from the Anantapur kimberlites (Pipe 3) have reported the compositions of equilibrium phases (Ganguly & Bhattacharya, 1987; Nehru & Reddy, 1989). We have recalculated equilibration temperatures for these xenoliths using the geothermometer of Harley (1984) for the garnet harzburgites and those of Bertrand & Mercier (1985) and Brey & Kohler (1990) for the garnet lherzolites. These equations were solved simultaneously with the geobarometers of Macgregor (1974), Nickel & Green (1985) and Brey & Kohler (1990), and the results are shown in Fig. 15.
(3) The Anantapur kimberlites are known to be diamondiferous. The Proterozoic geothermal gradient below the Dharwar Craton must therefore pass through the diamond stability field. We have located this in Fig. 15a using the experimental results of Kennedy & Kennedy (1976).
(4) A recent study of Early Proterozoic tholeiitic basalts from the intracratonic Cuddapah Basin has suggested that the mechanical boundary layer (MBL) was 145 km thick, prior to lithospheric extension (Anand et al., 2003). This is the only information that we have to constrain the depth of melting involved in the genesis of the Cuddapah lamproites (Fig. 14b).
We have combined the above information with the results of experimental studies on CO2- and H2O-rich peridotite (Canil & Scarfe, 1990), anhydrous peridotite (Hirschmann, 2000), phlogopite (Sato et al., 1997) and K-richterite (Van der Laan & Foley, 1994; Konzett et al., 1997), to constrain the source regions of the primary kimberlite and lamproite melts. If our Mesoproterozoic geothermal gradient for the Dharwar Craton is correct, then Fig. 15a suggests that the primary melts of the diamondiferous Anantapur kimberlites could have been generated near the base of the MBL (at depths of 160 km) from a CO2-rich peridotite source by minor amounts of lithospheric extension. Phlogopite would also be stable under these conditions and this would explain the K anomalies that we have observed on normalized multi-element plots. Our geochemical data suggest that the non-diamondiferous kimberlites (i.e. Pipes 2, 5 and 10 from Anantapur, and all of those from Mahbubnagar) have passed through lithospheric mantle containing a significant amount of ilmenite. This may represent material from partially melted veined lithospheric mantle; ilmenite would, however, be residual only at very small degrees of partial melting (Foley, 1992). We do not have any evidence for a convecting mantle source contribution to the kimberlites but we believe that a significant melt contribution came from the sub-continental lithospheric mantle.
Our Nd-isotopic study of the lamproites suggests that they were generated from ancient enriched lithospheric mantle sources beneath the intracratonic Cuddapah Basin. Figure 15b suggests that the lamproites may have been generated from a metasomatized harzburgite near the base of the MBL, i.e. at depths of 145 km. K-richterite (OH) and phlogopite would be only marginally stable under these conditions, which is consistent with the relatively potassic nature of these rocks. The regional metasomatic events, which have influenced the source regions of the kimberlites and lamproites, might also be responsible for the enrichment of the source regions of of the Mid-Proterozoic alkaline plutons in the Eastern Ghat mobile belt. These are poorly dated but seem to be broadly contemporaneous with, or younger than, the lamproites. The contemporaneous generation of different types of small-volume igneous rocks has been noted in other igneous provinces where there are variations in lithospheric thickness (e.g. Gibson et al., 1995).
None of the scenarios that we have outlined above require elevated Proterozoic mantle potential temperatures or invoke significant melting of the convecting mantle. The 300 Myr interval of mid-Proterozoic kimberlite and lamproite emplacement in the Eastern Dharwar Craton and Cuddapah Basin is more consistent with a model of lithospheric mantle melting by extension rather than linked with a mantle plume. Indeed, the 1·9 Ga initiation and subsequent evolution of the Cuddapah Basin can be largely explained by lithospheric extension (Anand et al., 2003). The age of the Chelima lamproite (1417 ± 8·2 Ma; Chalapathi Rao et al., 1999), which intrudes the youngest unit of the Cuddapah Supergroup of rocks, is considered to be the minimum age for the termination of extension in the Cuddapah Basin. The thick sedimentary succession and genesis of tholeiitic lavas and sills does not require the involvement of a mantle plume but can be explained by the high ambient potential temperature (1500°C) of the underlying convecting mantle during the Early Proterozoic (Anand et al., 2003). Mafic dyke swarms in the Dharwar Craton may also have been generated by lithospheric extension. These are, however, poorly dated and little is known about their petrogenesis.
|
CONCLUSIONS |
|---|
|
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 |
|---|
(1) Detailed study of the petrology and mineral chemistry of Proterozoic mafic potassic igneous rocks from southern India suggests that those occurring within the Cuddapah Basin and at its NE margin are lamproites and those within the Dharwar Craton are kimberlites. The kimberlites are non-diamondiferous in the
1400 Ma Mahbubnagar cluster and predominantly diamondiferous in the 1090 Ma Anantapur cluster. Many are remarkably fresh, given their Proterozoic age. (2) The non-diamondiferous kimberlites from Mahbubnagar and Anantapur are characterized by higher contents of Fe, Ti, Zr, Hf and Sc and lower La/Sm ratios and Ni contents than the diamondiferous Anantapur kimberlites.
(3) The absence of Ta and Nb negative anomalies in normalized multi-element patterns suggests that metasomatic enrichment of the sub-continental lithospheric mantle was caused by the migration of low-temperature, small-fraction melts from the convecting mantle (McKenzie, 1989) rather than by subduction-derived melts (Murphy et al., 2002).
(4) The positive Ndi values and high La/Sm ratios of the Dharwar kimberlites suggest that their melt source regions were enriched by small-melt fractions derived from the depleted upper mantle shortly prior to kimberlite genesis. The low Ndi values of the diamondiferous Anantapur kimberlites relative to most of the Mahbubnagar kimberlites suggest that the former have entrained a greater amount of material from the sub-continental lithospheric mantle. This is consistent with the higher proportion of olivine macrocrysts in the Anantapur kimberlites.
(5) REE ratios of both the kimberlites and lamproites suggest that they are the products of low-degree partial melting of a garnet peridotite mantle source. Inversion modelling of REE concentrations suggests that the source regions of the Dharwar Craton kimberlites and lamproites have been extensively depleted (20%) prior to metasomatic enrichment and subsequent partial melting. These observations are consistent with those for Group I and II kimberlites and lamproites from Kaapvaal craton of southern Africa (Tainton & McKenzie, 1994). The diamondiferous Anantapur kimberlites appear to have been derived from more LREE-enriched mantle sources than the non-diamondiferous Mahbubnagar kimberlites. The observed petrological and geochemical differences between the kimberlites and lamproites reflect: (a) differences in the volatile contents of their respective source regions; (b) final depth of melting; (c) subsequent low-pressure crystallization of these magmas.
(6) The occurrence of kimberlites in the Dharwar Craton, lamproites in the Cuddapah Basin and other alkaline rocks within the Eastern Ghat Mobile belt is due to extension of lithosphere of variable thickness during the Mid-Proterozoic.
|
SUPPLEMENTARY DATA |
|---|
|
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 |
|---|
Supplementary data from this paper are available on Journal of Petrology online.
|
ACKNOWLEDGEMENTS |
|---|
We are grateful to S. V. Satyanarayana, K. R. P. Rao, Mohd Burhanuddin, K. Shivaji, B. K. Nagaraja Rao, M. G. Rao and K. S. Bhaskar Rao of the Geological Survey of India (Southern Region, Hyderabad) for their help during N.V.C.R.'s fieldwork; to D. James, K. Jarvis and B. Gibson for assistance with analytical work; and to D. McKenzie for providing the inversion code. A Cambridge University Nehru Trust Scholarship, Cambridge Philosophical Society grant and a bursary from the Wolfson College to N.V.C.R. made this research possible. Thorough reviews of an earlier draft of this manuscript by K. Bell, J. B. Dawson, P. Janney and D. G. Pearson, and the editorial comments of M. Wilson, are gratefully acknowledged. This is Department of Earth Sciences Contribution ES.7522.
|
FOOTNOTES |
|---|
* Corresponding author. E-mail: sally{at}esc.cam.ac.uk
Present address: Mineralogy Laboratory, Ore Dressing Division, Indian Bureau of Mines, Hingna Road, Nagpur 440016, India. 
|
REFERENCES |
|---|
|
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 |
|---|
Abbott, D. H. (1996). Plumes and hotspots as sources of greenstone belts. Lithos 37, 113–127.[CrossRef][Web of Science]
Akella, J., Rao, P. S., McCallister, R. H., Boyd, F. R. & Meyer, H. O. A. (1979). Mineralogical studies on the diamondiferous kimberlites of Wajrajkarur area, south India. In: Boyd, F. R. & Meyer, H. O. A. (eds.) Kimberlites, Diatremes and Diamonds: their Geology, Petrology and Geochemistry, Volume 1. Washington, DC: American Geophysical Union, pp. 172–177.
Alibert, C. & Albarède, F. (1988). Relationships between mineralogical, chemical and isotope properties of some North American kimberlites. Journal of Geophysical Research 93, 7643–7691.
Allen, P., Condie, K. C. & Narayana, B. L. (1985). The geochemistry of prograde and retrograde charnockite–gneiss reactions in southern India. Geochimica et Cosmochimica Acta 49, 323–336.[CrossRef][Web of Science]
Allsopp, H. L., Bristow, J. W., Smith, C. B., Brown, R., Gleadow, A. J. W., Kramers, J. D. & Garvie, O. G. (1989). A summary of radiometric dating methods applicable to kimberlites and related rocks. Geological Society of Australia Special Publication 14, 343–357.
Anand, M., Gibson, S. A., Subbarao, K. V., Kelley, S. P. & Dickin, A. P. (2003). Early Proterozoic melt generation processes beneath the intra-cratonic Cuddapah Basin, Southern India. Journal of Petrology 44, 2139–2171.
Anil Kumar, Padmakumari, V. M., Dayal, A. M., Murthy, D. S. N. & Gopalan, K. (1993). Rb–Sr ages of Proterozoic kimberlites of India: evidence for contemporaneous emplacement. Precambrian Research 62, 227–232.[CrossRef][Web of Science]
Anil Kumar, Gopalan, K., Rao, K. R. P. & Nayak, S. S. (2001). Rb–Sr ages of kimberlites and lamproites from Eastern Dharwar Craton, South India. Journal of the Geological Society of India 58, 135–142.
Babu, T. M. (1998). Diamonds in India. Geological Society of India, Bangalore, Economic Geology Series Publication, 332 pp.
Basu, A. R. & Tatsumoto, M. (1979). Sm–Nd systematics in kimberlites and in the minerals of garnet-lherzolite inclusions. Science 205, 398–401.
Beard, A. D., Downes, H., Hegner, E., Sablukov, S. M., Vetrin, V. R. & Balogh, K. (1998). Mineralogy and geochemistry of Devonian ultramafic minor intrusions of the southern Kola peninsula, Russia: implication for the petrogenesis of kimberlites and melilites. Contributions to Mineralogy and Petrology 130, 288–303.[CrossRef][Web of Science]
Beard, A. D., Downes, H., Hegner, E. & Sablukov, S. M. (2000). Geochemistry and mineralogy of kimberlites from the Arkhangelsk region, NW Russia: evidence for transitional kimberlite magma types. Lithos 51, 47–73.[CrossRef][Web of Science]
Becker, H., Wenzel, T. & Volker, F. (1999). Geochemistry of glimmerite veins in peridotites from Lower Austria—implications for the origin of K-rich magmas in collision zones. Journal of Petrology 40, 315–338.[CrossRef][Web of Science]
Bergman, S. C. (1987). Lamproites and other K-rich igneous rocks: review of their occurrence, mineralogy and geochemistry. In: Fitton, J. G. & Upton, B. G. J. (eds) Alkaline Igneous Rocks. Geological Society, London, Special Publications 30, 103–190.
Bertrand, P. & Mercier, J. C. C. (1985). The mutual solubility of coexisting ortho- and clinopyroxene; toward an absolute geothermometer for the natural system? Earth and Planetary Science Letters 76, 109–122.[CrossRef][Web of Science]
Bhaskara Rao, B. (1976). A note on the micaceous kimberlite in the Cumbum Formations near Zangamarajupalle, Cuddapah district, Andhra Pradesh. Indian Minerals 30, 55–58.
Bhattacharji, S. & Singh, R. N. (1984). Thermomechanical structure of the southern part of the Indian shield and its relevance to Precambrian basin evolution. Tectonophysics 105, 103–120.[CrossRef][Web of Science]
Boyd, F. R. (1974). Olivine megacrysts from the kimberlites of Monastery and Frank Smith Mines, South Africa. Carnegie Institution of Washington Yearbook 73, 282–285.
Brey, G. P. & Kohler, T. (1990). Geothermobarometry in four-phase lherzolites. Part II: New thermobarometers and practical assessment of existing thermobarometers. Journal of Petrology 31, 1353–1378.
Brey, G. P., Kohler, T. & Nickel, K. G. (1990). Geothermobarometry in four-phase lherzolites. Part I: Experimental results from 10 to 60 kb. Journal of Petrology 31, 1313–1352.
Canil, D. & Scarfe, C. M. (1990). Phase relationships in peridotite + CO2 system up to 12 GPa: implications for the origin of kimberlites and carbonate stability in the Earth's upper mantle. Journal of Geophysical Research 95, 15805–15816.[CrossRef][Web of Science]
Chalapathi Rao, N. V. (1997). Petrogenesis of Proterozoic kimberlites and lamproites from the Cuddapah Basin and Dharwar Craton, southern India. Ph.D. thesis, University of Cambridge.
Chalapathi Rao, N. V., Miller, J. A., Pyle, D. M. & Madhavan, V. (1996a). New Proterozoic K–Ar ages for some kimberlites and lamproites from the Cuddapah Basin and Dharwar Craton, south India: evidence for non-contemporaneous emplacement. Precambrian Research 79, 363–369.[CrossRef][Web of Science]
Chalapathi Rao, N. V., Reed, S. J. B., Pyle, D. M. & Beattie, P. D. (1996b). Larnitic kirschsteinite from the Kotakonda kimberlite, Andhra Pradesh, India. Mineralogical Magazine 60, 513–516.[Abstract]
Chalapathi Rao, N. V., Gibson, S. A., Pyle, D. M. & Dickin, A. P. (1998). Contrasting isotopic mantle sources for Proterozoic lamproites and kimberlites from the Cuddapah Basin and eastern Dharwar Craton: implication for Proterozoic mantle heterogeneity beneath southern India. Journal of the Geological Society of India 52, 683–694.
Chalapathi Rao, N. V., Miller, J. A., Gibson, S. A., Pyle, D. M. & Madhavan, V. (1999). Precise 40Ar/39Ar dating of Kotakonda kimberlite and Chelima lamproite, India: implication to the timing of mafic dyke swarm activity in the Eastern Dharwar Craton. Journal of the Geological Society of India 53, 425–433.
Chatterjee, N. & Bhattacharji, S. (1998). Formation of Proterozoic tholeiite intrusives in and around Cuddapah Basin, south India and their Gondwana counterparts in East Antarctica, and compositional variation in their mantle sources. Neues Jahrbuch für Mineralogie, Abhandlungen 174, 79–102.[Web of Science]
Chatterjee, N. & Bhattacharji, S. (2001). Petrology, geochemistry and tectonic setting of the mafic dykes and sills associated with the evolution of the Proterozoic Cuddapah Basin of South India. Proceedings of the Indian Academy of Sciences (Earth & Planetary Sciences) 110, 433–453.
Chetty, T. R. K. (1995). A correlation of Proterozoic shear zones between Eastern Ghats, India and Enderby Land, Eastern Antarctica. In: Yoshida, S. & Santosh, M. (eds) India and Antarctica during the Precambrian. Geological Society of India Memoir 34, 205–220.
Clement, C. R. (1982). A comparative geological study of some major kimberlite pipes in northern Cape and Orange Free State. Ph.D. thesis, University of Cape Town.
Crawford, A. R. & Compston, W. (1973). The age of the Cuddapah and Kurnool systems, southern India. Journal of the Geological Society of Australia 19, 453–464.
Crough, S. T. (1981). Mesozoic hot spot epeirogeneity in Eastern North America. Geology 9, 2–6.
Crough, S. T., Morgan, W. J. & Hargraves, R. B. (1980). Kimberlites: their relation to mantle hot spots. Earth and Planetary Science Letters 50, 260–274.[CrossRef][Web of Science]
Cullers, R. L., Dorais, M. J., Berendsen, P. & Chaudhuri, S. (1996). Mineralogy and petrology of Cretaceous sub-surface lamproite sills, south-eastern Kansas, USA. Lithos 38, 185–206.[CrossRef][Web of Science]
Dalziel, I. W. D. (1997). Overview: Neoproterozoic–Paleozoic geography and tectonics: review, hypothesis, environmental speculations. Geological Society of America Bulletin 109, 16–42.
Dawson, J. B. & Smith, J. V. (1975). Chemistry and origin of phlogopite megacrysts in kimberlite. Nature 253, 336–338.
Dawson, J. B. & Smith, J. V. (1977). The MARID (mica– amphibole–rutile–ilmenite–diopside) suite of xenoliths in kimberlite. Geochimica et Cosmochimica Acta 41, 309–323.[CrossRef][Web of Science]
Divakara Rao, V., Subba Rao, M. V., Rama Rao, P. & Pantulu, G. V. C. (1994). Geochemistry, age and origin of Perur granite, Chittoor district, Andhra Pradesh. Journal of the Geological Society of India 44, 637–644.
Edgar, A. D. & Charbonneau, H. E. (1993). Melting experiments on a SiO2-poor CaO-rich aphanitic kimberlite from 5–10 GPa and their bearing on the source of kimberlite magmas. American Mineralogist 78, 132–142.[Abstract]
Edgar, A. D. & Mitchell, R. H. (1997). Ultra high pressure– temperature melting experiments on an SiO2-rich lamproite from Smoky Butte, Montana: derivation of siliceous lamproite magmas from enriched sources deep in the continental mantle. Journal of Petrology 38, 457–477.[CrossRef][Web of Science]
Edwards, D., Rock, N. M. S., Taylor, W. R., Griffin, W. J. & Ramsay, R. R. (1992). Mineralogy and petrology of the Aries diamondiferous kimberlite pipe, central Kimberley block, Western Australia. Journal of Petrology 33, 1157–1191.
Eggler, D. H. (1977). The principle of the zone of invariant vapor composition; an example in the system CaO–MgO–SiO2–CO2–H2O and implications for the mantle solidus. Carnegie Institution of Washington Yearbook 76, 428–435.
Eggler, D. H. (1989). Kimberlites: how do they form? Geological Society of Australia Special Publication 14, 489–504.
Emeleus, C. H. & Andrews, J. R. (1975). Mineralogy and petrology of kimberlite dyke and sheet intrusions and included peridotite xenoliths from SW Greenland. Physics and Chemistry of the Earth 9, 179–198.[CrossRef]
England, P. & Houseman, G. (1984). On the geodynamic setting of kimberlite genesis. Earth and Planetary Science Letters 67, 109–122.[CrossRef][Web of Science]
Erlank, A. J., Waters, F. G., Hawkesworth, C. J., Haggerty, S. E., Allsopp, H. L., Rickard, R. S. & Menzies, M. (1987). Evidence for mantle metasomatism in peridotite nodules from the Kimberley pipes, South Africa. In: Menzies, M. A. & Hawkesworth, C. J. (eds) Mantle Metasomatism. London: Academic Press, pp. 221–311.
Farmer, G. L. & Boettcher, A. L. (1981). Petrological and crystal chemical significance of some deep-seated phlogopites. American Mineralogist 66, 1154–1163.[Abstract]
Foley, S. F. (1988). The genesis of continental basic alkaline magmas—an interpretation in terms of redox melting. Journal of Petrology, Special Lithosphere Issue, 138–161.
Foley, S. F. (1992). Petrological characterisation of the source composition of K-magmas: geochemical and experimental constraints. Lithos 28, 187–204.[CrossRef][Web of Science]
Foley, S. F. (1993). An experimental study of olivine lamproite: first results from the diamond stability field. Geochimica et Cosmochimica Acta 57, 483–489.[CrossRef][Web of Science]
Foley, S. F., Venturelli, G., Green, D. H. & Toscani, L. (1987). The ultra-potassic rocks: characteristics, classification and constraints for petrogenetic models. Earth-Science Reviews 24, 81–134.
Fraser, K. J. (1987). Petrogenesis of kimberlites from South Africa and lamproites from Western Australia and North America. Ph.D. thesis, The Open University, Milton Keynes, 270 pp.
Fraser, K. J., Hawkesworth, C. J., Erlank, A. J., Mitchell, R. H. & Scott-Smith, B. H. (1985). Sr, Nd and Pb isotope and minor element geochemistry of lamproites and kimberlites. Earth and Planetary Science Letters 76, 57–70.[CrossRef][Web of Science]
Ganguly, J. & Bhattacharya, P. K. (1987). Xenoliths in Proterozoic kimberlites from southern India: petrology and geophysical implications. In: Nixon, P. H. (ed.) Mantle Xenoliths. New York: John Wiley, pp. 249–266.
Gibson, S. A. (2002) Major element heterogeneity in Archean to Recent mantle plume starting-heads. Earth and Planetary Science Letters 195, 59–74.[CrossRef][Web of Science]
Gibson, S. A., Thompson, R. N., Leonardos, O. H., Dickin, A. P. & Mitchell, J. G. (1995). The Late Cretaceous impact of the Trindade mantle plume: evidence from large-volume, mafic, potassic magmatism in SE Brazil. Journal of Petrology 36, 189–229.
Gibson, S. A., Thompson, R. N., Dickin, A. P. & Leonardos, O. H. (1996). High Ti- and low-Ti mafic potassic magmas: key to plume–lithosphere interactions and continental flood basalt genesis. Earth and Planetary Science Letters 141, 325–341.[CrossRef][Web of Science]
Girnis, A. P., Brey, G. P. & Ryabchikov, I. D. (1995). Origin of Group IA kimberlites: fluid-saturated melting experiments at 45–55 kbar. Earth and Planetary Science Letters 134, 283–296.[CrossRef][Web of Science]
Grasty, R. L. & Leelanandam, C. (1965). Isotopic ages of basic charnockite and khondalite from Kondapalle, Andhra Pradesh, India. Mineralogical Magazine 35, 529–535.
Greenwood, J. C., Gibson, S. A., Thompson, R. N., Weska, R. K. & Dickin, A. P. (1999). Cretaceous kimberlites from the Paranatinga– Batovi region, Central Brazil: geochemical evidence for subcratonic lithosphere mantle heterogeneity. In: Gurney, J. J., Gurney, J. L., Pascoe, M. D. & Richardson, S. H. (eds) Proceedings of the Seventh International Kimberlite Conference, 1. Cape Town: Red Roof Design, pp. 291–298.
Gupta, M. L. (1993). Is Indian shield hotter than other Gondwana shields? Earth and Planetary Science Letters 115, 275–285.[CrossRef][Web of Science]
Gupta, M. L., Sundar, A. & Sharma, S. R. (1991). Heat flow and heat generation in the Archaean Dharwar Cratons and implications for the southern Indian shield geotherm and lithospheric thickness. Tectonophysics 194, 107–122.[CrossRef][Web of Science]
Gurney, J. J. & Ebrahim, S. (1973). Chemical composition of Lesotho kimberlites. In: Nixon, P. H. (ed.) Lesotho Kimberlites. Maseru: Lesotho National Development Corporation, pp. 280–284.
Gurney, J. J., Moore, R. O., Otter, M. L., Kirkley, M. B., Hops, J. J. & McCandless, T. E. (1991). Southern African kimberlites and their xenoliths. In: Kampunju, A. B. & Lubala, R. T. (eds) Magmatism in Extensional Tectonic Settings: the Phanerozoic African Plate. New York: John Wiley, pp. 495–536.
Haggerty, S. E. (1982). Kimberlites in Western Liberia: an overview of the geological setting in a plate tectonic framework. Journal of Geophysical Research 87, 811–826.[CrossRef]
Haggerty, S. E. (1994). Super kimberlites: a geodynamic window to the Earth's core. Earth and Planetary Science Letters 122, 57–69.[CrossRef][Web of Science]
Harley, S. L. (1984). Comparison of the garnet–orthopyroxene geobarometer with recent experimental studies, and applications to natural assemblages. Journal of Petrology 25, 697–712.
Harte, B. (1983). Mantle peridotites and processes—the kimberlite sample. In: Hawkesworth, C. J. & Norry, M. J. (eds) Continental Basalts and Mantle Xenoliths. Nantwich: Shiva, pp. 46–91.
Hawkesworth, C. J. & Vollmer, R. (1979). Crustal contamination versus mantle enrichment, 143Nd/144Nd and 87Sr/86Sr evidence from Italian volcanics. Contributions to Mineralogy and Petrology 69, 151–165.[CrossRef][Web of Science]
Hirschmann, M. M. (2000). Mantle solidus: experimental constraints and the effects of peridotite composition. Geochemistry, Geophysics, Geosystems 2000GC000070.
Hwang, P., Taylor, W. R., Rock, N. M. S. & Ramsay, R. R. (1994). Mineralogy, geochemistry and petrogenesis of the Metters Bore No. 1 lamproite pipe, Calwynyardah Field, West Kimberley Province, Western Australia. Mineralogy and Petrology 51, 195–226.[CrossRef][Web of Science]
Janney, P. E., le Roex, A. P., Carlson, R. W. & Viljoen, K. S. (2002). A chemical and multi-isotope study of the Western Cape Olivine Melilitite Province, South Africa: implications for the sources of kimberlites and the origin of the HIMU signature in Africa. Journal of Petrology 43, 2339–2370.
Janse, A. J. A. & Sheahan, P. A. (1995). Catalogue of worldwide diamond and kimberlite occurrences: a selective and annotative approach. Journal of Geochemical Exploration 53, 73–111.[CrossRef][Web of Science]
Jaques, A. L., Lewis, J. D., Smith, C. B., Gregory, G. P., Ferguson, J., Chappell, B. W. & McCulloch, M. T. (1984). The diamond-bearing ultrapotassic (lamproitic) rocks of the west Kimberley region, western Australia. In: Kornprobst, J. (ed.) Kimberlites I. Kimberlites and Related Rocks. Amsterdam: Elsevier, pp. 225–254.
Jaques, A. L., Sun, S.-S. & Chappell, B. W. (1989). Geochemistry of Argyle (AK1) lamproite pipe, Western Australia. Geological Society of Australia Special Publication 14, 170–188.
Jarvis, K. E. (1990). A critical evaluation of two sample preparation techniques for low-level determination of some geologically incompatible elements by ICP-MS. Chemical Geology 83, 89–103.[CrossRef][Web of Science]
Jones, A. P. & Wyllie, P. J. (1984). Minor elements in perovskites from kimberlite and distribution of rare earth elements: an electron probe study. Earth and Planetary Science Letters 69, 128–140.[CrossRef][Web of Science]
Kaila, K. L., Roychowdhury, K., Reddy, P. R., Krishna, V. G., Harinarain, Subbotin, S. I., Sollogub, V. B., Chekunov, A. V., Kharetchko, G. E., Lazarenko, M. A. & Illchenko, T. V. (1979). Crustal structure along the Kavali–Udipi profile in the Indian peninsular shield from deep seismic soundings. Journal of the Geological Society of India 20, 307–333.
Kennedy, C. S. & Kennedy, G. C. (1976). The equilibrium boundary between graphite and diamond. Journal of Geophysical Research 81, 2467–2470.
Kepezhinskas, P. K., Defant, M. J. & Drummond, M. S. (1995). Na metasomatism in the island arc mantle by slab–melt peridotite interaction: evidence from mantle xenoliths in the Northern Kamchatka Arc. Journal of Petrology 36, 1505–1527.
Kinny, P. D. & Dawson, J. B. (1992). A mantle metasomatic injection event linked to Late Cretaceous kimberlite magmatism. Nature 360, 726–728.[CrossRef]
Klemme, S. & O'Neill, H. St. C. (2000). The near-solidus transition from garnet lherzolite to spinel lherzolite. Contributions to Mineralogy and Petrology 138, 237–248.[CrossRef][Web of Science]
Konzett, J., Sweeney, R. J., Thompson, A. B. & Ulmer, P. (1997). Potassium amphibole stability in the upper mantle: an experimental study in a peralkaline KNCMASH system to 8·5 GPa. Journal of Petrology 38, 537–568.[CrossRef][Web of Science]
Kramers, J. D. & Smith, C. B. (1983). A feasibility study of U–Pb and Pb–Pb dating of kimberlites using groundmass mineral fractions and wholerock samples. Chemical Geology 1, 23–38.
Larsen, L. M. & Rex, D. C. (1992). A review of the 2500 Ma span of alkaline–ultramafic–potassic and carbonatitic magmatism in West Greenland. Lithos 28, 367–402.[CrossRef][Web of Science]
LeCheminant, A. N. & Heaman, L. M. (1989). Mackenzie igneous events, Canada: Middle Proterozoic hot spot magmatism associated with ocean opening. Earth and Planetary Science Letters 96, 38–48.[CrossRef][Web of Science]
Macgregor, I. H. (1974). The system MgO–Al2O3–SiO2; solubility of Al2O3 in enstatite for spinel and garnet peridotite compositions. American Mineralogist 59, 110–119.[Web of Science]
Mahadevan, T. M. (1995). Deep Continental Structure of India: a Review. Geological Society of India Memoir 28, 562 pp.
Mahotkin, I. L., Gibson, S. A., Thompson, R. N., Zhuravlev, D. Z. & Zherdev, P.U. (2000). Late Devonian diamondiferous kimberlite and alkaline picrite (proto-kimberlite?) magmatism in the Arkhangelsk region, NW Russia. Journal of Petrology 41, 201–227.
Mallikharjuna Rao, J., Bhattacharjee, S., Rao, M. N. & Horne, O. (1995). 40Ar/39Ar ages and geochemistry of dyke swarms around the Cuddapah Basin. Geological Society of India Memoir 33, 307–328.
Matson, D. W., Mucnow, D. W. & Garcia, M.O. (1986). Volatile contents of phlogopite micas from South African kimberlites. Contributions to Mineralogy and Petrology 93, 399–408.[CrossRef][Web of Science]
Maury, R., Defant, C. & Joron, M. J. (1992). Metasomatism of the sub-arc mantle inferred from trace elements in Philippines xenoliths. Nature 360, 660–661.
McCulloch, M. T. & Bennett, V. C. (1994). Progressive growth of the Earth's continental crust and depleted mantle: geochemical constraints. Geochimica et Cosmochimica Acta 58, 4717–4738.[CrossRef][Web of Science]
McCulloch, M. T., Jaques, A. L., Nelson, D. R. & Lewis, J. D. (1983). Nd and Sr isotopes in kimberlites and lamproites from western Australia: an enriched mantle origin. Nature 302, 400–403.[CrossRef]
McDonald, I., De Wit, M. J., Smith, C. B., Bizzi, L. A. & Viljoen, K. S. (1995). The geochemistry of the Platinum Group of elements in Brazilian and southern African kimberlites. Geochimica et Cosmochimica Acta 59, 2882–2903.
McKenzie, D. (1989). Some remarks on the movement of small melt fractions in the mantle. Earth and Planetary Science Letters 95, 53–72.[CrossRef][Web of Science]
McKenzie, D. P. & Bickle, M. J. (1988). The volume and composition of melt generated by extension of the lithosphere. Journal of Petrology 29, 625–679.
McKenzie, D. & O'Nions, R. K. (1991). Partial melt distributions from the inversion of rare earth element concentrations. Journal of Petrology 32, 1021–1091.
Menzies, M. A. & Wass, S.Y. (1983). CO2 and LREE-rich mantle below eastern Australia: a REE and isotopic study of alkaline magmas and apatite-rich mantle xenoliths from the Southern Highlands Province, Australia. Earth and Planetary Science Letters 65, 287–302.[CrossRef][Web of Science]
Mitchell, R. H. (1985). A review of the mineralogy of lamproites. Transactions of the Geological Society of South Africa 88, 411–437.
Mitchell, R. H. (1986). Kimberlites: Mineralogy, Geochemistry and Petrology. New York: Plenum, 442 pp.
Mitchell, R. H. (1995). Kimberlites, Orangeites and Related Rocks. New York: Plenum, 406 pp.
Mitchell, R. H. & Bergman, S. C. (1991). Petrology of Lamproites. New York: Plenum, 447 pp.
Mitchell, R. H. & Brunfelt, A.O. (1975). Rare earth geochemistry of kimberlite. Physics and Chemistry of the Earth 9, 671–686.[CrossRef]
Mitchell, R. H. & Reed, S. J. B. (1988). Ion microprobe determination of rare earth elements in perovskites from kimberlites and alnoites. Mineralogical Magazine 52, 331–339.
Mitchell, R. H. & Steele, I. M. (1992). Potassium, zirconium and strontian cerian perovskite in lamproites from the Leucite Hills, Wyoming. Canadian Mineralogist 30, 1153–1159.[Web of Science]
Mitchell, R. H., Platt, R. G. & Downey, M. (1987). Petrology of lamproites from Smoky Butte, Montana. Journal of Petrology 28, 645–677.
Moore, A. E. (1988). Olivine: a monitor of magma evolutionary paths in kimberlites and olivine melilitites. Contributions to Mineralogy and Petrology 99, 238–248.[CrossRef][Web of Science]
Moore, R.O., Gurney, J. J., Griffin, W. L. & Shimizu, N. (1991). Ultra-high pressure garnet inclusions in Monastery diamonds: trace element abundance patterns and conditions of origin. European Journal of Mineralogy 3, 213–230.[Web of Science]
Murphy, D. T., Collerson, K. D. & Kamber, B. S. (2002). Lamproites from Gaussberg, Antarctica: possible transition zone melts of Archaean subducted sediments. Journal of Petrology 43, 981–1001.
Murthy, D. S. N., Dayal, A. M. & Natarajan, R. (1994). Mineralogy and geochemistry of Chigicherla kimberlite and its xenoliths, Anantapur district, South India. Journal of the Geological Society of India 43, 329–341.
Murthy, D. S. N., Dayal, A. M., Natarajan, R., Balaram, V. & Govil, P. K. (1997). Petrology and geochemistry of Chigicherla (pipe-2) kimberlite, Anantapur district, A. P. Journal of the Geological Society of India 49, 123–132.
Murthy, Y. G. K., Babu Rao, V., Guptasarma, D., Rao, J. M., Rao, M. N. & Bhattacharjee, S. (1987). Tectonic, petrochemical and geophysical studies of mafic dyke swarms around the Proterozoic Cuddapah Basin, South India. Geological Association of Canada Special Paper 34, 303–316.
Nagaraja Rao, B. K., Rajurkar, S. T., Ramalingaswamy, G. & Ravindra Babu, B. (1987). Stratigraphy, structure and evolution of the Cuddapah Basin. Geological Society of India Memoir 6, 33–86.
Naqvi, S. M. & Rogers, J. J. W. (1987). Precambrian Geology of India. Oxford Monographs on Geology and Geophysics 6, 223 pp.
Naqvi, S. M., Divakara Rao, V. & Harinarain (1974). The Protocontinental growth of the Indian shield and the antiquity of its rift valleys. Precambrian Research 1, 345–389.[CrossRef][Web of Science]
Navon, O. & Stolper, E. (1987). Geochemical consequences of melt percolation: the upper mantle as a chromatographic column. Journal of Geology 95, 435–453.
Nayak, S. S., Kasiviswanathan, C. V., Reddy, T. A. K. & Nagaraja Rao, B. K. (1988). New find of kimberlitic rocks in Andhra Pradesh near Maddur, Mahabubnagar district. Journal of the Geological Society of India 31, 343–346.
Nayak, S. S., Rao, K. R. P., Kudari, S. A. K. & Ravi, S. (2001). Geology and tectonic setting of kimberlites and lamproites of southern India. Geological Survey of India Special Publication 58, 603–613.
Nehru, C. E. & Reddy, T. A. K. (1989). Ultramafic xenoliths from Vajrakarur kimberlites, India. Geological Society of Australia Special Publication 14, 745–758.
Nelson, D. R. (1989). Isotopic characteristics and petrogenesis of lamproites and kimberlites of central west Greenland. Lithos 22, 265–275.[CrossRef][Web of Science]
Nelson, D. R., Black, L. P. & McCulloch, M. T. (1989). Nd–Pb isotopic characteristics of the Mordor complex, Northern Territory: Mid-Proterozoic potassic magmatism from an enriched mantle. Australian Journal of Earth Sciences 36, 541–551.[Web of Science]
Nickel, K. G. & Green, D. H. (1985). Empirical geothermobarometry of garnet peridotites and implications for nature of the lithosphere, kimberlites and diamonds. Earth and Planetary Science Letters 73, 157–170.[CrossRef]
Nixon, P. H., Davies, G. R., Rex, D. C. & Gray, A. (1992). Venezuela kimberlites. Journal of Volcanology and Geothermal Research 50, 101–115.[CrossRef][Web of Science]
Norrish, M. & Hutton, J. T. (1969). An accurate X-ray spectrographic method for the analyses of a wide range of geological samples. Geochimica et Cosmochimica Acta 33, 431–453.[CrossRef][Web of Science]
Padmakumari, V. M. & Dayal, A. M. (1987). Geochronological studies of some mafic dykes around Cuddapah Basin, south India. Geological Society of India Memoir 6, 369–379.
Paul, D. K. (1979). Isotopic composition of strontium in Indian kimberlites. Geochimica et Cosmochimica Acta 43, 389–394.[CrossRef][Web of Science]
Paul, D. K. & Ray Barman, T. (1988). Isotopic and geochemical evolution of granulites of the Eastern Ghats belt. Workshop on ‘Proterozoic Rocks of India’ IGCP-217. Calcutta: Geological Society of India (unpaginated).
Paul, D. K., Rex, D. C. & Harris, P. G. (1975). Chemical characteristics and K–Ar ages of Indian kimberlites. Geological Society of America Bulletin 86, 364–366.
Peacock, S. M. (1990). Fluid processes in subduction zones. Science 248, 329–337.
Pidgeon, R. T., Smith, C. B. & Fanning, C. M. (1989). Kimberlite and lamproite emplacement ages in Western Australia. Geological Society of Australia Special Publication 14, 369–381.
Radhakrishna, T. & Piper, J. D. A. (eds) (1999). The Indian Subcontinent and Gondwana: a Palaeomagnetic and Rock Magnetic Perspective. Geological Society of India Memoir 44, 270 pp.
Rajamani, V., Shiv Kumar, K., Hanson, G. N. & Shirey, S. B. (1985). Geochemistry and petrogenesis of amphibolites from Kolar schist belt, south India: evidence for komatiitic magma derived by low percentages of melting of the mantle. Journal of Petrology 96, 92–123.
Rajamani, V., Shirey, S. B. & Hanson, G. N. (1989). Fe-rich Archaean tholeiites derived from melt-enriched mantle sources: evidence from Kolar schist belt, south India. Journal of Geology 32, 487–501.
Rajaraman, S. & Deshpande, M. L. (1978). Banganapalle diamondiferous conglomerate in Kurnool district, Andhra Pradesh. Indian Minerals 32, 33–43.
Rao, K. R. P., Rao, K. S. B. & Satyanarayana, S. V. (1991). Assessment of diamond resources in kimberlites of Venkatampalle– Lattavaram areas, Anantapur district, Andhra Pradesh. Records of the Geological Survey of India 124, 33–39.
Rao, P. V. & Puffer, J. H. (1996). Geochemistry, petrogenesis and tectonic setting of Proterozoic mafic dyke swarms, Eastern Dharwar Craton, India. Journal of the Geological Society of India 47, 165–174.
Reddy, N. S. (1988). Pillowed spilitic lavas from the Tadpatri Formation of the Cuddapah Basin. Journal of the Geological Society of India 32, 65–67.
Reddy, T. A. K. (1987). Kimberlite and lamproitic rocks of Wajrakarur area, Andhra Pradesh. Journal of the Geological Society of India 30, 1–12.
Reddy, T. A. K., Ravi, S., Chakravarthi, V. & Neelakantam, S. (2000). Discovery of lamproite field in Krishna and Nalgonda districts in Andhra Pradesh, South India. Proceedings of a workshop on the status, complexities and challenges of diamond exploration in India held at Raipur, pp. 93–104.
Richter, F. M. (1988). A major change in the thermal state of the Earth at the Archean–Proterozoic boundary: consequences for the nature and preservation of continental lithosphere. Journal of Petrology, Special Lithosphere Volume, 39–52.
Ringwood, A. R., Kesson, S. E., Hibberson, W. & Ware, N. (1992). Origin of kimberlites and their related magmas. Earth and Planetary Science Letters 113, 521–538.[CrossRef][Web of Science]
Rogers, J. J. W., Unrug, R. & Sultan, M. (1995). Tectonic assembly of Gondwana. Journal of Geodynamics 19, 1–34.[CrossRef]
Rollinson, H. R., Windley, W. F. & Ramakrishnan, M. (1981). Contrasting high and intermediate pressures of metamorphism in the Archaean Sargur belts of South India. Contributions to Mineralogy and Petrology 76, 420–429.[CrossRef][Web of Science]
Sato, K., Katsura, T. & Ito, E. (1997). Phase relations of natural phlogopite with and without enstatite up to 8 GPa; implication for mantle metasomatism. Earth and Planetary Science Letters 146, 511–526.[CrossRef][Web of Science]
Satyanarayana, S. V. (2002). Diamond Provinces of India: an overview. In: Proceedings of the International Conference on Diamond & Gemstones organized by South Asian Association of Economic Geologists (SAAEG) at Raipur, Chattisgarh, India, 9–15 February, Extended Abstracts, pp. 44–45.
Satyanarayana, S. V., Rao, K. R. P., Shivaji, K., Nayak, S. S., Rao, M. G. & Viswanathan, T. V. (1992). Geological setting of the diamondiferous primary and secondary rocks in Andhra Pradesh. Proceedings of the International Round Table Conference on Diamond Prospecting and Exploration (NMDC and UNESCO sponsored), New Delhi (unpaginated).
Scott, B. H. (1979). Petrogenesis of kimberlites and associated potassic lamprophyres from central west Greenland. In: Boyd, F. R. & Meyer, H. O. A. (eds) Kimberlites, Diatremes and Diamonds: their Geology, Petrology and Geochemistry, Volume 1. Washington, DC: American Geophysical Union, pp. 190–225.
Scott, B. H. (1981). Kimberlite and lamproite dykes from Holsteinsborg, West Greenland. Meddelelser om Grønland, Geoscience 3, 13–24.
Scott-Smith, B. H. (1989). Lamproites and kimberlites in India. Neues Jahrbuch für Mineralogie, Abhandlungen 86, 193–225.
Scott-Smith, B. H. & Skinner, E. M. W. (1984). A new look at the Prairie Creek, Arkansas. In: Kornprobst, J. (ed.) Kimberlites I. Kimberlites and Related Rocks. Amsterdam: Elsevier, pp. 255–284.
Scott-Smith, B. H., Danchin, R. V., Harris, J. W. & Stracke, K. J. (1984). Kimberlites near Orroroo, South Australia. In: Kornprobst, J. (ed.) Kimberlites I. Kimberlites and Related Rocks. Amsterdam: Elsevier, pp. 121–142.
Scott-Smith, B. H., Skinner, E. M. W. & Loney, P. E. (1989). The Kapamba lamproites of the Luangwa valley, eastern Zambia. Geological Society of Australia Special Publication 14, 189–205.
Shee, S. R., Bristow, J. W., Bell, D. R., Smith, C. B., Allsopp, A. L. & Shee, P. B. (1989). The petrology of kimberlites, related rocks and associated mantle xenoliths from the Kuruman province, South Africa. Geological Society of Australia Special Publication 14, 60–82.
Skinner, E. M. W., Smith, C. B., Bristow, J. W., Scott-Smith, B. H. & Dawson, J. B. (1985). Proterozoic kimberlites and lamproites and a preliminary age for the Argyle pipe, Western Australia. Transactions of the Geological Society of South Africa 88, 335–340.
Smith, A. G. & Hallam, A. (1970). The fit of the southern continents. Nature 225, 139–144.[CrossRef][Web of Science]
Smith, C. B. (1983). Pb, Sr and Nd isotopic evidences for sources of southern African Cretaceous kimberlites. Nature 304, 51–54.[CrossRef]
Smith, C. B., Allsopp, H. L., Kramers, J. D., Hutchinson, G. & Roddick, J. C. (1985). Emplacement ages of Jurassic–Cretaceous South African kimberlites by the Rb–Sr method on phlogopite and whole-rock samples. Transactions of the Geological Society of South Africa 88, 249–266.
Smith, J. V., Breunscholtz, R. & Dawson, J. B. (1978). Chemistry of micas from kimberlites and xenoliths—I micaceous kimberlites. Geochimica et Cosmochimica Acta 42, 959–971.[CrossRef][Web of Science]
Smith, J. V., Delaney, J. S., Hervig, R. L. & Dawson, J. B. (1981). Storage of F and Cl in the upper mantle: geochemical implications. Lithos 14, 133–147.[CrossRef][Web of Science]
Spriggs, A. J. (1988). An isotopic and geochemical study of kimberlites and associated alkaline rocks from Namibia. Ph.D. thesis, University of Leeds.
Sreeramachandra Rao, K. (1988). Geological setting of the kimberlite dykes in the Zangamarajupalle Lead Zinc prospect, Cuddapah district, Andhra Pradesh. Records of the Geological Survey of India 116(3–8), 146–155.
Tainton, K. M. (1992). The petrogenesis of Group-2 kimberlites and lamproites from the Northern Cape Province, South Africa. Ph.D. thesis, University of Cambridge.
Tainton, K. M. & McKenzie, D. (1994). The generation of kimberlites, lamproites and their source rocks. Journal of Petrology 35, 787–817.
Takahashi, M. (1988). On the Na2O content of convergent zone high-alumina basalts. Chemical Geology 68, 17–29.[CrossRef][Web of Science]
Taylor, W. R., Tompkins, L. A. & Haggerty, S. E. (1994). Comparative geochemistry of West African kimberlites: evidence for a micaceous kimberlite end member of sub-lithospheric origin. Geochimica et Cosmochimica Acta 58, 4017–4037.[CrossRef][Web of Science]
Thompson, R. N. & Gibson, S. A. (2000). Transient high temperatures in mantle plume heads inferred from magnesian olivines in Phanerozoic picrites. Nature 407, 502–505.[CrossRef][Medline]
Thompson, R. N., Morrison, M. A., Hendry, G. L. & Parry, S. J. (1984). An assessment of the relative roles of crust and mantle in magma genesis: an elemental approach. Philosophical Transactions of the Royal Society of London, Series A 310, 549–590.[CrossRef]
Tomlinson, K. Y., Stevenson, R. K., Hughes, D. J., Hall, R. P., Thurston, P. C. & Hall, R. P. (1998). The Red Lake greenstone belt, Superior Province: evidence of plume-related magmatism at 3 Ga and evidence of an older enriched source. Precambrian Research 89, 59–76.[CrossRef][Web of Science]
Turcotte, D. L. & Schubert, G. (1982). Geodynamics: Applications of Continuum Physics to Geological Problems. New York: John Wiley, pp. 279–285.
Van der Laan, S. R. & Foley, S. F. (1994). MARIDs and mantle metasomatism. Mineralogical Magazine 58A, 505–506.
Vollmer, R. & Norry, M. J. (1983). Possible origin of K-rich volcanic rocks from Virunga, East Africa, by metasomatism of continental crustal material: Pb, Nd and Sr isotope evidence. Earth and Planetary Science Letters 64, 374–386.[CrossRef][Web of Science]
Wagner, C., Deloule, E. & Mokhtari, A. (1996). Richterite-bearing peridotites and MARID-type inclusions in lavas from NE Morocco: mineralogy and O/H ratios. Contributions to Mineralogy and Petrology 124, 406–421.[CrossRef][Web of Science]
Wallace, M. E. & Green, D. H. (1988). Mantle metasomatism by ephemeral carbonate melts. Nature 336, 459–462.[CrossRef]
Wilson, M., Rosenbaum, J. M. & Dunworth, A. D. (1995). Melilitites: partial melts of the thermal boundary layer? Contributions to Mineralogy and Petrology 119, 181–196.[Web of Science]
Woolley, A. R., Bergman, S. C., Edgar, A. D., Le Bas, M. J., Mitchell, R. H., Rock, N. M. S. & Scott-Smith, B. H. (1996). Classification of lamprophyres, lamproites, kimberlites and the kalsilitic, melilitic and leucitic rocks. Canadian Mineralogist 34, 175–186.[Web of Science]
Wyllie, P. J. (1987). Discussion of recent papers on carbonated peridotites bearing on mantle metasomatism and magmatism. Earth and Planetary Science Letters 82, 391–397.[CrossRef][Web of Science]
Yoshida, M. (1995). Assembly of East Gondwanaland during the Mesoproterozoic and its rejuvenation during the Pan-African period. Geological Society of India Memoir 34, 24–45.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  T. C. Birkett FIRST-ROW TRANSITION ELEMENTS, Y AND Ga IN KIMBERLITE AND LAMPROITE: APPLICATIONS TO DIAMOND PROSPECTIVITY AND PETROGENESIS Can Mineral, October 1, 2008; 46(5): 1269 - 1282. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. Paton, J. M. Hergt, D. Phillips, J. D. Woodhead, and S. R. Shee New insights into the genesis of Indian kimberlites from the Dharwar Craton via in situ Sr isotope analysis of groundmass perovskite Geology, November 1, 2007; 35(11): 1011 - 1014. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. BECKER and A. P. L. ROEX Geochemistry of South African On- and Off-craton, Group I and Group II Kimberlites: Petrogenesis and Source Region Evolution J. Petrology, April 1, 2006; 47(4): 673 - 703. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. Maas, M. B. Kamenetsky, A. V. Sobolev, V. S. Kamenetsky, and N. V. Sobolev Sr, Nd, and Pb isotope evidence for a mantle origin of alkali chlorides and carbonates in the Udachnaya kimberlite, Siberia Geology, July 1, 2005; 33(7): 549 - 552. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. KUMAR and K. GOPALAN Comments on: 'Petrogenesis of Proterozoic Lamproites and Kimberlites from the Cuddapah Basin and Dharwar Craton, Southern India' J. Petrology, June 1, 2005; 46(6): 1077 - 1079. [Full Text] [PDF]
|
|
|
|
|
|
N. V. CHALAPATHI RAO, S. A. GIBSON, D. M. PYLE, A. P. DICKIN, and J. DAY Petrogenesis of Proterozoic Lamproites and Kimberlites from the Cuddapah Basin and Dharwar Craton, Southern India: a Reply J. Petrology, June 1, 2005; 46(6): 1081 - 1084. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||