- © 2013 The Mineralogical Society
A detailed mineralogical examination of representative material from the P2-West ‘kimberlite’ located in the Wajrakarur Kimberlite Field (India) demonstrates that significant differences exist between these rocks and archetypal hypabyssal kimberlite. The intrusion consists of an olivine-phyric facies which has been transected by, and includes clasts of, a consanguineous phlogopite-rich pegmatitic facies. The olivine-rich parts of P2-West are relatively fresh and consist of euhedral-to-subhedral microphenocrystal olivine set in a groundmass of fine-grained anhedral monticellite, amoeboid apatite, and subhedral-to-euhedral perovskite within a partially chloritized-to-fresh phlogopite-rich mesostasis. The rock lacks the abundant olivine macrocrysts characteristic of kimberlite. Monticellite crystals are commonly partially or completely replaced by pectolite and hydrogarnet. Similar material occurs as irregular aggregates randomly scattered throughout the groundmass. The groundmass, in contrast to that of hypabyssal kimberlites, is relatively poor in spinels. Atoll spinels are absent, with the majority of spinels occurring principally as mantles upon microphenocrystal olivine. Disaggregated cumulate-like assemblages of intergrown anhedral perovskite and spinel are common. Spinel compositions are unlike those of kimberlites and their evolutionary trend is similar to that of lamproite and lamprophyre spinels. The pegmatitic facies of the intrusion are highly and pervasively altered, and characterized by the presence of large clasts, veins, and irregular aggregates consisting of large (1–5 mm) crystals of pinkish-bronze Al-poor phlogopite intergrown with and/or including: apatite; pectolite-hydrogarnet pseudomorphs after an unidentified euhedral phase; chlorite laths; barytolamprophyllite; perovskite; tausonite; diverse Sr-Ba-carbonates; and baryte. The presence of barytolamprophyllite and tausonite are typical of potassic undersaturated alkaline rocks and have never been reported from kimberlite; however, neither feldspar nor feldspathoids are present in P2-West. Micas in fresh and altered rocks are Al2O3- and BaO-poor, and exhibit compositional evolutionary trends towards tetraferriphlogopite rather than kinoshitalite. On the basis of these mineralogical data it is suggested that P2-West represents an unusual lamproite-like intrusion which has undergone extensive hydrothermal deuteric alteration and should not be considered a bona fide kimberlite.
Diamonds originating from India were introduced to the world without regard to the primary sources of this exotic mineral. In ancient India diamond was collected as a placer mineral along the banks of the majority of rivers traversing the Singhbhum and Dharwar Cratons. Despite the plethora of discoveries of lamproite and kimberlite pipes by the Indian geological community, and by De Beers, and Rio Tinto during the last two decades, no potentially diamond-rich pipes have been discovered. Thus, the primary sources of large diamonds such as the Kohinoor, Hope, Regent, Shah, and Great Moghul remain elusive (Fareeduddin and Mitchell, 2012).
It is now widely accepted that kimberlites and lamproites are the two major primary host rocks of diamonds. These genetically different rock types, which occur in stable cratons, have distinctive mineralogical and geochemical characteristics (Mitchell, 1995, 2006). Classification of a particular potentially diamond-bearing rock as either kimberlite or lamproite requires a detailed mineralogical study to characterize the typomorphic major, minor and accessory mineral populations which reflect the characteristics of the parental magma (Mitchell, 1995, 2006). Both kimberlites and lamproites can contain varying amounts of crustal and mantle xenoliths, together with xenocrysts derived from the fragmentation of these materials. This work describes the mineralogy of the P2-West ‘kimberlite’ intrusion occurring in the Wajrakarur Kimberlite cluster, Andhra Pradesh, India, and compares these data with the mineralogy of bona fide kimberlites and olivine lamproites.
Kimberlites and lamproites discovered in southern India are intrusive into the Dharwar Craton (Rao and Phadtre, 1966; Swami Nath et al., 1976; Drury et al., 1984; Chadwick et al., 2000; Sravan Kumar et al., 2004; Paul et al., 2006; Mukherjee et al., 2007; Srinivas Choudary et al., 2007; Ravi et al., 2009; Fareeduddin and Mitchell, 2012). This craton is divided into the Eastern and Western Dharwar Cratons by the Chitradurga Boundary Fault (Fig. 1). The kimberlites and lamproites occur only in the Eastern Dharwar Craton; there are no reports of kimberlite or lamproite fields from the Western Dharwar Craton (Fig. 1). The Dharwar Craton, in general, consists of extensive greenstone belts, granites, gneisses, sedimentary basins, and mafic dykes. Geological and geophysical studies have shown that both the eastern and western cratons differ in their lithological assemblages, age of the Archaean basement (>3 Ga and 2.6–2.5 Ga for Western and Eastern Cratons, respectively), and crustal thickness, with the Eastern Craton being thinner, 33–39 km compared to the Western Craton 42–51 km (Gupta et al., 2003; Ramakrishnan and Vaidyanadhan, 2008; Fareeduddin and Mitchell, 2012; Chalapathi Rao et al., 2013).
Currently, four ‘kimberlite’ fields are recognized in the Eastern Dharwar Craton: Wajrakarur; Narayanpet; Raichur; and Tungabhadra. Three lamproite fields are found in this craton: Krishna; Nallamalai; and Ramadugu (inset of Fig. 1; Nayak and Kudari, 1999; Ravi et al., 1999; Neelkantam, 2001; Chalapathi Rao et al., 2004, 2013; Paul et al., 2006; Fareeduddin and Mitchell, 2012). All of the kimberlite and lamproite fields consist of clusters of pipes (Fig. 1).
The Wajrakarur cluster mainly hosts poorly diamondiferous kimberlites, whereas the Narayanpet and Raichur kimberlites are essentially non-diamondiferous with reports of a few microdiamonds from the latter (Lynn, 2005; Chalapathi Rao et al., 2013). The kimberlites are mostly of Middle-to-Late Proterozoic age, whereas the lamproites were emplaced between ~1225 and 1500 Ma and thus are older than the kimberlites (Chalapathi Rao et al., 1996, 1999, 2013; Anil Kumar et al., 1993, 2001, 2007; Osborne et al., 2011).
Wajrakarur Kimberlite Field
The Wajrakarur Kimberlite Field (~120 km× 60 km), located in the Anantapur district of Andhra Pradesh, contains 31 kimberlite intrusions distributed in five clusters (Fig. 1): (1) Wajrakarur – 7 pipes (P-1, P-2, P-6, P-10, P-11, P-12, P-15); (2) Lattavaram – 7 pipes (P-3, P-4, P-5, P-7, P-8, P-9, P-13); (3) Chigicherla – 5 pipes (CC-1 to CC-5); (4) Kalyandurg – 6 pipes (KL-1 to KL-6); and (5) Timmasamudram – 6 pipes (TK-1 to TK-6). Summaries of the mode of occurrence and characteristics of each pipe in the Wajrakarur Kimberlite Field are given by Neelkantam (2001) and Fareeduddin and Mitchell (2012). Recently, Gurmeet Kaur et al. (2013) reclassified the P-5 and P-13 ‘kimberlites’ as lamproites on the basis of the groundmass mineralogy of the rocks.
Geology of P2-West
Pipe P2 belongs to the Wajrakarur cluster and is located ~2.5 km east of Wajrakarur village (Fig. 1). The locality consists of two pipes P2-West and P2-East. The latter is poorly exposed and extensively altered; consequently little is known of the petrology. In contrast, P2-West crops out as fresh rock forming a compact crescent-shaped body (Neelkantam, 2001; Fareeduddin, 2008; Ravi et al., 2009; Fareeduddin and Mitchell, 2012). The periphery of the intrusion encloses fragments of country rock. The principal feature of P2-West is the presence of veins and clasts of pegmatitic phlogopite-rich rock set in a dark fine-grained matrix. Chalapathi Rao et al. (2004) noted that P2-West is poor in kimberlite indicator minerals compared to other Wajrakarur kimberlites. Diamonds have not been reported from this pipe (cf. Fareeduddin and Mitchell, 2012). Paton et al. (2009) did not identify perovskites from this occurrence. P2-West has previously been classified as a hypabyssal-facies monticellite kimberlite by Scott Smith (1989) and Chalapathi Rao et al. (2004). In contrast, Reddy (1987) classified P2-West as a lamproite on the basis of the presence of sanidine and richterite. Our study was unable to verify this observation. Mitchell (2010) and Fareeduddin and Mitchell (2012) have suggested P2-West is an unusual lamproite-like intrusion.
Polished thin sections of rocks representative of both facies were investigated by back-scattered electron (BSE) imagery and quantitative energy dispersive X-ray spectrometry using either a JEOL JSM-5900 or an Hitachi scanning electron microscopes at Lakehead University. Analytical protocols and standards used with the JEOL instrument and the Link ISIS 300 analytical system are given by Liferovich and Mitchell (2005). Data obtained with the Hitachi instrument employed the same standards as the Oxford Aztec analytical system using a beam current of 300 pA and an accelerating voltage of 20 kV.
Petrography of P2-West
P2-West consists of only two petrographically different, but consanguineous phases. An early-formed melanocratic olivine-phyric facies and a later, more evolved, phlogopite-rich pegmatitic facies. The latter occurs as clasts and thin veins cross-cutting the olivine-phyric rocks.
The olivine-phyric facies exhibits an inequigranular texture consisting of rounded-to-ovoid, fractured olivine macrocrysts enclosed in a fine-grained groundmass (Fig. 2a). The macrocrysts constitute <5 vol.% of the rock (this work; Chalapathi Rao et al., 2004). The olivine-phyric rock consists primarily of relatively fresh euhedral-to-subhedral phenocrystal and microphenocrystal olivine and anhedral spinel aggregates set in a groundmass of fine-grained anhedral monticellite, amoeboid apatite, spinel and subhedral-to-euhedral perovskite within a partially chloritized-to-fresh phlogopite-rich mesostasis (Fig. 2a,b,d). The margins of monticellite crystals are typically replaced by thin rims of pectolite and hydrogarnet (Fig. 3b,c). Similar material occurs as irregular patches randomly scattered throughout the groundmass (Figs 3c, 4b). The majority of the spinel population mantles olivine crystals (Figs 2d, 3a). Aggregates of anhedral spinel also occur in the groundmass (Fig. 3c). Disaggregated cumulate-like assemblages of intergrown anhedral perovskite and spinel are common (Fig. 3d). Phlogopites in the groundmass material are commonly chloritized along the cleavages (Fig. 3d). Large poikilitic phlogopite plates, compositionally and optically similar to those of the pegmatitic facies, are found in the groundmass of the olivine-phyric rock and include all previously formed minerals.
The pegmatitic veins and clasts are characterized by aggregates of large (1–5 mm) crystals of pinkish-bronze optically-zoned phlogopite (Fig. 2c) intergrown with and/or including apatite, pectolite-hydrogarnet pseudomorphs after an unidentified euhedral phase (?monticellite), chlorite laths, barytolamprophyllite, perovskite, tausonite, diverse Sr-Ba-carbonates and baryte (Figs. 4b–d, 5a,b). Baryte, strontianite and apatite are residual phases occurring throughout the altered parts of the rock (Figs. 4c,d; 5a,b). The phlogopite in thin section exhibits a yellow-orange pleochroism (Fig. 2c) similar to that seen in lamproite phlogopite (Mitchell and Bergman, 1991). The margins of the phlogopite prisms exhibit a red pleochroism characteristic of tetraferriphlogopite.
The P2-West olivine-phyric facies contains two texturally distinct varieties of olivine: anhedral macrocrysts and/or xenocrysts (Fig. 2a) and subhedral-to-euhedral phenocrysts and microphenocrysts (Figs 2b; 3a). P2-West lacks the abundant olivine macrocrysts (~25 vol.%) characteristic of kimberlite (Mitchell, 1986, 1995). The macrocrysts are rounded-to-ovoid, or rarely, sub-angular, and are typically completely pseudomorphed by chlorite and/or serpentine. Rare partially pseudomorphed examples are replaced along fractures by chlorite and serpentine. The outer margins are characteristically mantled by chlorite (Fig. 2a). Most of the macrocrysts are resorbed at their margins and commonly contain randomly orientated rutile needles. The macrocrysts (Fo90–85) are typically zoned with the cores higher in Fe and lower in Mg contents as compared to the rims, i.e. reverse zoning (Table 1). The phenocrysts and microphenocrysts of olivine (Fo91–85) are relatively fresh compared to the olivine macrocryst population (Figs 2b,d; 3a). The alteration of the olivine phenocrysts and microphenocrysts is mainly concentrated around the rim and along fractures traversing the crystals. The phenocrysts and microphenocrysts exhibit normal zoning with Mg-rich cores and Fe-rich rims, in contrast to the reverse zoning observed in the olivine macrocrysts (Table 1).
Both varieties of olivines are closely associated with spinels. The spinels typically decorate the margins of olivines as subhedral crystals and appear to have crystallized contemporaneously with the marginal olivine (Figs. 2d, 3a). All olivines, regardless of paragenesis, are forsteritic (Fo91–85 mol.%; Table 1). The olivine compositions of P2-West are similar to the olivine compositions reported from olivine lamproites and kimberlites (Mitchell, 1986; Mitchell and Bergman, 1991).The paucity of olivine macrocrysts, reverse zoning in macrocrysts and xenocrysts, normal zoning in phenocrysts, and bulk composition indicate the similarity of P2-West olivine-phyric rocks to some olivine lamproites (Jaques et al., 1986; Mitchell and Bergman, 1991).
Monticellite occurs as a groundmass mineral (Fig. 3b,c). Some crystals are replaced only at their margins, by pectolite and hydrogarnet (Fig. 3b). However, in most of the altered P2-West rocks, monticellite is completely pseudomorphed by a complex intergrowth of pectolite and hydrogarnet, with preservation of the original morphology (Fig. 3c). Pseudomorphs, interpreted to be after monticellite, also occur as inclusions in pegmatitic mica. Representative compositions of monticellite are given in Table 2. The monticellite data plotted in the forsterite–monticellite–kirschteinite ternary diagram (Fig. 6) straddle the kimberlite and alnöite fields and thus are not of unusual composition, and therefore not diagnostic of any particular petrological affinity.
Phlogopite occurs in two generations: mica-I, forming small anhedral plates in the olivine-phyric groundmass and mica-II as late-stage large poikilitic plates in the groundmass of the olivine-phyric rock and large prisms in the pegmatitic facies (Figs. 2c, 3d, 4a). Mica-I is primary phlogopite with high Al2O3 and BaO contents compared to mica-II, which is enriched in FeO(T) and TiO2 (Table 3). Representative compositions of mica-I and II are given in Table 3. Mica-II in fresh and altered rocks exhibits a compositional evolutionary trend towards tetraferriphlogopite rather than kinoshitalite. These phlogopites are characteristically low in Al2O3 (<12 wt.%) and BaO (<2 wt.%) with relatively large FeO(T) (>5 wt.%) and TiO2 (1.5–3.5 wt.%) contents. The fine-grained groundmass phlogopites are not zoned, and have high Al2O3 together with low FeO(T) and TiO2 contents relative to the poikilitic mica plates in the groundmass and the pegmatitic phlogopites. The large laths in the groundmass and pegmatitic veins and patches have low Al2O3 and are zoned, with the cores enriched in Al2O3 and rims enriched in FeO and TiO2. The compositional trend (Fig. 7a,b) is similar to that shown by zoned phlogopite found in lamproites (Table 3) and clearly demonstrates the affinity of P2-West phlogopite with that crystallizing from lamproitic magmas (Mitchell and Bergman, 1991).
Perovskite occurs in two generations: perovskite-I is an earlier anhedral phase (Fig. 3d), whereas perovskite-II is found as a later euhedral-to-subhedral phase (Fig. 3b). Anhedral perovskite and spinel are commonly intergrown (Fig. 3d). Euhedral-to-subhedral perovskite grains occur scattered throughout the groundmass (Fig. 3c,d). Perovskite-II crystals are enriched in light rare earth elements (Table 4) relative to the resorbed anhedral perovskite-I (Tables 4, 5). Both perovskite-I and II are poor in SrO but have significant BaO (>1 wt.%) and Nb2O5 (> 0.65 wt.%) contents (Tables 4, 5).
Spinels of two parageneses are present: spinel-I, euhedral-to-subhedral (zoned and zonation-free), and anhedral spinels in the fresh-to-altered groundmass of the olivine-phyric rocks; and spinel-II, in the pegmatitic facies. The majority of spinel-I occurs either as clusters at the serpentinized margins of olivine phenocrysts or as a transported assemblage of anhedral resorbed single crystals in the groundmass (Fig. 3a,c,d). Disaggregated cumulate-like assemblages of intergrown anhedral perovskite and spinel are common. Atoll spinels are absent. Euhedral spinels enclosed within olivines are typically Cr-rich relative to all other spinels, and are spinel-chromite-ulvöspinel-magnetite solid solutions. Other spinels consist of euhedral relict spinel-chromite-ulvöspinel-magnetite-cores overgrown by strongly continuously-zoned qandiliteulvöspinel-magnetite solid solutions. Spinels of diverse compositions can be found clustered around a single olivine. Resorbed spinels occurring as isolated crystals also consist of qandiliteulvöspinel-magnetite solid solutions. Spinel-II included within phlogopite in the pegmatitic areas are ulvöspinel-magnetites.
Representative compositions of spinels are given in Table 6 and 7. The spinels in all rocks are Al-poor (<7 wt.% Al2O3), and evolve from Cr-rich spinel with Fe2+/(Fe2++Mg) ratios ranging from 0.45 to 0.85 at near constant Ti/(Ti+Cr+Al) ratios (0.1 to 0.15), to Ti-Fe-rich spinels with increasing Ti/(Ti+Cr+Al) ratios (0.5–1.0) at near-constant Fe2+/(Fe2++Mg) ratios (0.85–0.95). The spinel data are projected onto the front face of the reduced iron spinel compositional prism in Fig. 8, from which it is clear that all spinels follow spinel compositional trend-T2 of Mitchell (1995) i.e. a lamproitic or lamprophyric trend.
Apatite occurs as primary euhedral and late crystallizing anhedral amoeboid crystals (Fig. 4a,b). Representative compositions are given in Table 8. The apatite in fresh rock has only traces of Sr (Table 8) and no detectable Ba in contrast to apatite occurring in many lamproitic rocks (Mitchell and Bergman, 1991). Anhedral apatite in the altered groundmass and the pegmatitic facies is fluorapatite, establishing the fluorine-enriched character of the late-stage fluids.
Barytolamprophyllite [(Na,K)3(Ba,Sr,Ca)2(Ti, Fe)3O2(Si2O7)2(O,OH,F)4] is a complex titanosilicate typically found in peralkaline potassic rocks (Chakhmouradian and Mitchell, 1999, 2002). To date, it has not been reported from either kimberlites or lamproites. In P2-West it occurs as a late-stage residual phase in the pegmatitic facies (Fig. 4c,d). Barytolamprophyllite is partially altered to pectolite, tausonite, baryte and hydrogarnet (Fig. 4c,d). Representative compositions of barytolamprophyllite are given in Table 9. Barytolamprophyllite is enriched in fluorine signifying the presence of fluorine in late-stage magmatic fluids. The compositional variation (atomic %) illustrated in the ternary system Sr–(Na + K + Ca)–Ba (Fig. 9; after Chakhmouradian and Mitchell, 2002), shows that this barytolamprophyllite is enriched in Ba and poor in Sr compared to other barytolamprophyllites from diverse potassic alkaline rocks. The absence of Sr is unexpected given that residual fluids in this facies crystallized Sr-rich minerals such as tausonite and strontianite. This occurrence represents the first recognition of barytolamprophyllite from India.
Tausonite, baryte, Sr-carbonates
Tausonite, baryte and strontium-bearing carbonates are common in altered groundmass and pegmatitic parts of P2-West. Baryte and strontianite occur as anhedral grains (Figs 4b,c,d; 5b), whereas tausonite occurs as subhedral-to-anhedral small crystals (Fig. 5a,b). Representative compositions of tausonite, baryte and strontianite are given in Table 10. Strontianite is enriched in barium with up to 15 wt.% BaO (Table 10). Baryte occurs as pure barium sulfate with up to 1.5 wt.% CaO. Tausonite is pure SrTiO3 and exhibits no solid solution towards loparite. Tausonite exhibiting very little solid solution towards loparite or other perovskites is very rare (Mitchell, 2002). Although some domains in tausonite–loparite solid solutions from the Little Murun complex (Mitchell and Vladykin, 1993) approximate those of SrTiO3 (87 mol.%), the only other “pure” tausonite (98 mol.%) recognized to date is from a jadeitite (Miyajima et al., 2002). The P2-West tausonite occurrence represents the first record of this mineral in India. All of these late-forming minerals are associated intimately with the phlogopite and barytolamprophyllite occurring in the pegmatitic clasts and veins (Figs 4c,d; 5a).
Hydrogarnet and pectolite
Hydrogarnet and pectolite are the products of deuteric alteration of pre-existing minerals and result from the circulation of residual fluids generated in the later stages of crystallization of the P2-West parental magma. In the altered groundmass of the olivine-phyric rocks, these minerals commonly occur as alteration rims on monticellite crystals (Fig. 3b). Alteration can be so extensive that the complete pseudomorphing of monticellite occurs (Fig. 3c). The phlogopites in the pegmatitic facies also show replacement along cleavage planes to pectolite and hydrogarnet (Fig. 4b). Representative compositions of hydrogarnet and pectolite are given in Table 11.
Discussion and conclusions
P2-West rocks exhibit the following petrographic and mineralogical features:
The intrusion consists of an early olivine-phyric phlogopite-poor facies and a later consanguineous phlogopite-rich pegmatitic facies. The olivine-phyric facies is poor in macrocrystal and/or xenocrystal olivine. Phenocrysts and microphenocrysts are set in a groundmass of monticellite, spinel, perovskite, apatite, hydrogarnet, pectolite, poikilitic phlogopite, chlorite and serpentine.
The pegmatitic facies consists of coarse phlogopite prisms and later-formed barytolamprophyllite, tausonite, strontianite, baryte, apatite and the secondary phases, pectolite and hydrogarnet.
The olivine habit and paragenesis is clearly unlike that of hypabyssal kimberlite in that subhedral microphenocrysts predominate over rounded macrocrystal olivines.
Micas in fresh (Al2O3 <12 wt.%; BaO <2 wt.%), altered and pegmatitic (Al2O3 <10 wt.%; BaO <3wt.%) rocks are Al2O3- and BaO-poor, and exhibit compositional evolutionary trends towards tetraferriphlogopite rather than kinoshitalite. This trend is typical of that of lamproitic micas (Fig.7a,b).
The spinels in all rocks are Al-poor (<7 wt.% Al2O3), and the diverse spinel compositions follow a lamproite (or lamprophyric) compositional trend (Fig. 8) of decreasing Fe2+/(Fe2++Mg) ratios (0.5–0.9) at near constant Ti/(Ti+Cr+Al) ratios (0.1–0.15), followed by increasing Ti/(Ti+Cr+Al) ratios (0.15–1.0) at near constant Fe2 +/(Fe2 ++Mg) ratios (0.85–0.95). The groundmass, in contrast to that of hypabyssal kimberlites, is relatively poor in spinels. Atoll spinels are absent.
Barytolamprophyllite and tausonite, typical minerals occurring in potassic undersaturated alkaline rocks, are present. These have never been reported from kimberlite. Pyroxenes, feldspars and/or feldspathoids are not present.
Late stage phlogopite, barytolamprophyllite and apatite are enriched in fluorine indicating the enrichment of late stage fluids in fluorine.
The sequence of crystallization for the olivine-phyric facies is: olivine, Cr-spinel-Ti-magnetite, monticellite, perovskite, apatite, mica-I (high Al-type).
The sequence of crystallization for the pegmatite veins is: mica-II (low Al-type), apatite, Ti-magnetite, barytolamprophyllite, tausonite, and pseudomorphing of earlier-formed phases by hydrogarnet and pectolite as a result of alteration by residual fluids.
The above textural and mineralogical data are at variance with the classification of P2-West as a hypabyssal-facies kimberlite by Scott Smith (1989) and Chalapathi Rao et al. (2004). Although P2-West contains monticellite, a common mineral in kimberlite, it lacks calcite and/or dolomite which are characteristic phases in kimberlites (Mitchell, 1986). Importantly, P2-West does not contain clinopyroxene, Ti-K-richterite, potassium feldspar, or leucite; important but not essential phases in lamproites. (Mitchell and Bergman, 1991; Mitchell, 1995).
In summary, P2-West has mineralogical characteristics more typical of olivine lamproites than archetypal kimberlites, although the presence of monticellite is atypical of lamproites. Commonly, monticellite is associated with melilite. We did not recognize any melilite in our samples of P2-West, but given the extensive alteration of the groundmass in the monticellite-bearing olivine-phyric rocks we do not rule out the possiblity that this mineral was originally present.
In our opinion, P2-West is not a ‘contaminated kimberlite’, although the presence of secondary phases such as pectolite and hydrogarnet might suggest contamination. The absence of clinopyroxene, a common product of contamination of kimberlite magmas by crustal material (Mitchell, 1986), coupled with the bulk-rock Sr and Nd isotopic signatures of P2-West (Paton et al., 2009, 2007) suggests that crustal contamination of the magma has not occurred. The Sr isotopic composition of perovskites in other Wajrakarur and Narayanpet kimberlites also indicates the absence of crustal contamination (Paton et al., 2007). Further, the Nd and Hf whole-rock isotopic compositions suggest an asthenospheric derivation for the Wajrakarur magma with no Sub-Continental Lithospheric Mantle (SCLM) contamination (Paton et al., 2009). We conclude that P2-West is not a contaminated kimberlite and that primary monticellite has been altered by deuteric Na- and Ca-bearing hydrothermal fluids.
Although the phlogopite and spinel evolutionary trends of P2-West are similar to those of lamproites, we have not observed Ti-K-richterite, potassium feldspar or leucite. Mineralogically, the P2-West olivine-phyric facies is similar to, but unlike, all other olivine lamproites, although pegmatitic facies are mineralogically similar to some phlogopite lamproites
Consequently, it is obvious that P2-West cannot be classified as either a bona fide kimberlite or lamproite, following the mineralogical classifications devised for kimberlites and lamproites (Scott Smith and Skinner, 1984a,b; Mitchell, 1995; Mitchell and Bergman, 1991; Woolley et al., 1995). The magma from which P2-West was formed is best regarded as a local manifestation of a particular variety of cratonic potassic magmatism (Mitchell, 2006). The presence of monticellite, barytolamprophyllite, tausonite and Sr-Ba residual carbonates suggests affinities with potassic magmatism such as found at Gordon Butte, Montana (Chakhmouradian and Mitchell, 2002) or Haystack Butte, Montana (Buie, 1941; Wendlandt, 1977), and some potassic complexes of the Aldan shield (Mitchell and Vladykin, 1993). P2-West has mineralogical affinities with rocks occurring at Haystack Butte, i.e. the presence of the assemblage of olivine, monticellite and phlogopite and the lack of melilite.
It is not the objective of this study to devise a petrogenetic scheme for the origin of the magma from which the P2-West rocks crystallized. However, it is apparent that the magma which formed this intrusion must be potassic and derived, in part, from metasomatized lithospheric mantle. Metasomatism is required to enrich the source of this magma in K, Ba, Ti, F and other incompatible elements (Sr, REE). Whether this enrichment results from crystallization of mantle-derived magma as veins, as proposed by Foley (1992), or subduction of sedimentary material (see review by Mitchell and Bergman, 1991), is as yet unknown. Regardless, this enrichment must have occurred near-contemporaneously with the tectonic processes which initiated partial melting of the mantle such that long term radiogenic isotopic enrichment of the sources of the P2-West magma did not occur.
On the basis of the above discussion we propose that the P2-West rocks represent an unusual lamproite-like intrusion consisting of relatively un-evolved olivine-phyric rocks and consanguineous phlogopite-rich pegmatites. All rocks have undergone extensive subsolidus hydrothermal deuteric alteration and should not be considered as bona fide kimberlite. As a result of the extensive alteration, bulk-rock compositions cannot be representative of those of the parental magmas. The P2-West parental magma cannot be regarded in any sense as being ‘transitional’ between kimberlite and lamproite, as suggested by Paul et al. (2006) as it represents a distinct local variant of potassic intracratonic magmatism.
This work was supported by the Natural Sciences and Engineering Research Council of Canada, Almaz Petrology and Lakehead University. Staff of the Geological Survey of India in Bangalore and Wajrakarur are thanked for assistance in the field. Gurmeet Kaur wishes to acknowledge Panjab University, Chandigarh, India for granting leave to pursue research on Indian Kimberlites at Lakehead University. Valerie Dennison is thanked for pre-production copy editing of the text.
- Manuscript received 4 October 2013.
- Manuscript Accepted for publication 5 November 2013.