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915 The Canadian Mineralogist Vol. 46, pp. 915-932 (2008) DOI : 10.3749/canmin.46.4.915 FENITES ASSOCIATED WITH CARBONATITES Michael J. LE BAS§ 9 Park Lands, Blandford Forum, Dorset DT11 7BA, UK Abstract A review of occurrences of potassic and sodic fenitization around carbonatites is presented, including several new examples and oceanic ones. Taken in conjunction with the many examples quoted in the literature, conclusions are drawn that the K and Na contents of carbonatitic melts that induce fenitization are primary constituent parts of both calciocarbonatitic and magnesiocarbonatitic melts, which have a source in the mantle, and that the K and Na are not derived from the crust during passage by these melts. Potassic fenitization can lead to the production of pseudotrachytes with up to 16 wt% K2O; these are genetically unrelated to common magmatic trachytes, which have only 5–10 wt% K2O. Keywords: fenite, carbonatite, sodic metasomatism, potassic metasomatism, K-feldspar, pseudotrachyte. Sommaire Le but de ce travail est de passer en revue plusieurs exemples de fénitisation potassique et de fénitisation sodique développées autour des carbonatites, y inclus de nouveaux exemples, dont certains dans un contexte océanique. Ces exemples, ainsi que de nombreux cas cités dans la littérature, mènent à la conclusion que les teneurs en K et Na des magmas carbonatitiques qui provoquent cette fénitisation sont primaires, à la fois dans les magmas calciocarbonatitiques et magnésiocarbonatitiques, qui ont leur origine dans le manteau. C’est donc dire que le K et le Na ne seraient pas dérivés de la croûte lors du passage de ces liquides carbonatés. La fénitisation potassique peut mener à la production de pseudotrachytes ayant jusqu’à 16% de K2O (poids). Ces roches n’ont aucune filiation génétique avec les trachytes magmatiques cummunes, qui ne contiennent que 5–10% de K2O. (Traduit par la Rédaction) Mots-clés: fénite, carbonatite, métasomatose sodique, métasomatose potassique, feldspath potassique, pseudotrachyte. Introduction Fenitization is defined as the process of alkali metasomatism associated with igneous activity, usually alkaline igneous activity. Carbonatites are not usually recognized as alkaline, as most contain negligible alkalis. However, it is argued here that many carbonatitic magmas were originally alkaline, and that the alkalinity was lost during the process of fenitization. Thus “alkaline igneous activity” may be understood to be “alkali silicate and associated carbonatite igneous activity”. Although a side issue from the purpose of this contribution, the use above of the word “associated” requires explanation in light of the 2003 paper by Gittins & Harmer. They asserted that usually “there is no liquid line of descent relationship between silicate magma and carbonatite magma”, and therefore the word § E-mail address: [email protected] “accompanying” is more appropriate than “associated”. However, the fact that ca. 80% of carbonatites occur together with alkali silicate rocks in time and space (Woolley 2003) is a strong argument that they are associated in the genetic sense, and hence “associated” is used here. The intimate proximity of silicate magma and carbonatitic magma erupting up the same volcanic stem is well illustrated by the three Tanzanian volcanic edifices Mosonik, Oldoinyo Lengai and Kerimasi that occur within 20 km of each other; each has coexisting carbonatite and nephelinite. In this contribution, I consider what the alkalis in fenites tell us about “carbonatitic” processes, and build on the many recent contributions by Andersen (1989), Kresten (1991), Cooper & Reid (2000), Sindern & Kramm (2000), Drüppel et al. (2005) and others. I concentrate on those fenites specifically associated with carbonatites. Therefore, fenites around ijolites, e.g., 916 the canadian mineralogist at the type locality of ijolite at Iivaara, northeastern Finland (Lehijarvi 1960), and those around syenites, e.g., at the Mlanje Mountains in Malawi (Platt & Woolley 1986), are not considered here. Background Information Problems arise in the cases where fenites occur around both alkali silicate rocks and carbonatites, and whether the fenites produced are the result of the intrusion of the alkali silicate or the carbonatitic magmas. Examples where they cannot be distinguished are numerous and include some classic cases, e.g., at Kovdor in the Kola Peninsula, northern Russia, and also at Alnö, Sweden (von Eckermann 1948, Morogan & Woolley 1988), Bingo, Democratic Republic of Congo (Woolley et al. 1995), Chilwa Island, Malawi (Woolley 1969) and Fen, Norway (Brøgger 1921). As a result of this uncertainty, these occurrences are excluded from this analysis. Such exclusion greatly limits the number of examples available for consideration, but sufficient carbonatite-associated fenites remain to make this study viable. Fenites around carbonatites come in many varieties. Heinrich (1985) identified three principal types: potassic, sodic–potassic and sodic. All are syenitic in appearance, and some are all-too-easily mistaken for igneous syenites unless close attention is paid to the mineralogy, chemical composition and field relationships. As the purpose of this contribution is to examine the role of the alkalis K and Na, only the more extreme K and Na types that best illustrate their properties are considered. They can be characterized as: 1) potassic, and dominated by the presence of K-rich feldspar, and 2) sodic, and dominated by the presence of alkali feldspar with an alkali amphibole and sodic pyroxene. The sodic–potassic fenites are intermediates between the two extremes; therefore, data about them are not specific to either potassic fenitization or to sodic fenitization. The tables of analytical data given with this contribution are only representative data. The complete data files of all tables may be obtained from the Depository of Unpublished Data on the MAC website [document Fenites cm46_915]. The majority of the samples are stored in the Le Bas Collection at the Natural History Museum, London. The Term “Fenite” The terms fenite and fenitization were coined by Brøgger in 1921 for certain rocks of the intrusive complex at Fen in southern Norway. He described fenite as any rock, whether felsic or basic, produced by in situ metasomatism of older country-rock in contact with the igneous rocks of the Fen complex; however, he did not apply the term to rocks produced by remobilization or rheomorphism of the metasomatic rock, even if fenitic in origin. In this contribution, the term fenite is extended to include rheomorphic and intrusive rocks because, as will be shown, they are intrinsically part of the fenitization process, producing, for example, the intrusive rocks described as “feldspathic breccia” and “rheomorphic leucotrachytes” by Garson (1965), Sutherland (1965a), Woolley (1969), Baldock (1973) and as “ochreous breccias and tuffs” by Flegg et al. (1977) and described below. Heinrich (1966, Chapter 3) gave a valuable account of occurrences of fenitization around carbonatites, pointing out that intensive fenitization can ultimately produce the singular composition, a “feldspar rock”, no matter what the original chemical composition of the protolith may have been. This process of “convergent” fenitization is common for the potassic fenites, but Vartiainen & Woolley (1976) have shown that it is less common for sodic fenites. Many of the examples used in this contribution were unknown to Heinrich and result from more recent decades of seeing and collecting fenites adjacent to carbonatite complexes all over the world, both continental and oceanic. Potassic Fenites Field relationships show that potassic fenites are developed mainly around sövitic carbonatites, some fine-grained calcite carbonatites and, less commonly, around other carbonatites of the same intrusive complex (Dixey et al. 1955, Brown 1964, Garson 1965, von Eckermann 1960, Woodward & Hölttä 2005). Furthermore, the evidence in the field shows that the formation of fenite is usually caused by the metasomatic action of the earliest intrusive sövite, the later phases of intrusion commonly appearing only to brecciate the earlier-formed fenite without metasomatism (Garson 1965, McKie 1966, Le Bas 1977). The feldspar rocks produced by fenitization of quite different protoliths are commonly termed, if coarsegrained, “orthoclasite” being composed largely of Orca. 90 (Sutherland 1965a, Le Bas 1981). The term is not always appropriate because the K-feldspar of the rock may be microcline, as pointed out by Heinrich & Moore (1970). The more general term “feldspathic fenite” is more appropriate. However “orthoclasite” is used in this contribution where that name is already employed in published occurrences, so that there is no uncertainty about the rock being considered. The term “syenitic fenite” is avoided, despite the common syenitic appearance in the field, because of the possible subsequent misinterpretation that it may be an igneous syenite largely composed of alkali feldspar. Nor do these rocks have evidence of a melt with solidus feldspar composi- fenites associated with carbonatites tions, lying as they do in the leucite solidus field of the system quartz – nepheline – kalsilite. Pseudotrachyte In the fenites surrounding the carbonatites of the Tundulu complex, Malawi, Garson (1962) traced in detail, with supporting illustrations, the transition from recrystallization of coarse-grained feldspathic rock to feldspathic breccia to a mobilized product described as trachyte that intruded the host rocks as small bodies and dykes. Since they are not true trachytes, being neither truly magmatic nor of the chemical composition of common trachyte, the term “pseudotrachyte” is more suitable, although the term “potassic trachyte” is used in some cases to highlight the K-rich nature (Sutherland 1965b, Baldock 1973, Cooper & Reid 2000). These rocks are petrographically composed of small fragments of feldspar recrystallized to aligned laths of water-clear K-rich feldspar identified as sanidine by Garson (1962), in a microcrystalline matrix. X-ray powder diffraction of similar rocks elsewhere has shown that the feldspar is not sanidine but is structurally maximum microcline 917 (Heinrich & Moore 1970, Heinrich 1985, Mian & Le Bas 1988). Some pseudotrachytes in eastern Uganda carry large fragments of feldspar crystals that can be mistaken for phenocrysts, falsely giving the rock the appearance of common trachyte (Sutherland 1965b). Cooper & Reid (2000) described the process of formation of potassic trachyte dykes as the melting of a cupola of high-grade fenite in response to the advective flux of heat from rising carbonatitic magma or fluid which, with continued fluid fluxing, produces a melt with or without xenocrysts. This melt then could be emplaced as dykes, sills and plugs into the fenitized collar around the carbonatite intrusion. This development of extreme K-feldspathization associated with carbonatite complexes is not unusual and, for example, is seen in dykes and plugs in the Toror Hills (Sutherland 1965b) and at Bukusu (Baldock 1973), both in eastern Uganda, and in the dykes at Seladinha in San Vicente, Cape Verde Islands, described below. This process may also have given rise to the potassic glassy extrusive lapilli described by Rosatelli et al. (2003) at the Rangwa caldera complex in western Kenya. Also, well exposed at Landslide Gully on the western cliffs of the Homa Mountain carbonatite complex in western Kenya is a further example of a “pseudotrachyte” or “ochreous breccia”, an intrusive sill-like sheet up to 4 m thick between carbonatite and shattered, fenitized Archean Nyanzian country rocks (Le Bas 1977, Chapter 18). The sheet is a tuff composed of minute fragments aligned in cross-cutting flow patterns. A CIPW norm of the XRF-derived composition of the tuff (Table 1) gives the calculated composition: orthoclase 44%, quartz 14%, calcite 25% and Fe–Ti oxides 17%. Potassic fenites of Ruri Hills, western Kenya The main host rocks to the Ruri carbonatites are chlorite-grade Nyanzian (Archean) metabasalts, mostly bluish green and fine grained, but with some blackish variolitic-textured pillow lavas. Within a few meters of the contact with the sövites of the South Ruri Hills, the metabasalts change in color to dark green with conspicuous whitish patches. Thin sections of this rock show the matrix feldspar as indeterminate cloudy plagioclase enclosing opaque material, formerly intergranular pyroxene. The white patches are prismatic plates of euhedral clear K-feldspar, in some cases rimmed by albite. Toward the contact, this clear feldspar is seen in places to take on a trachytic texture parallel to the contact, indicative of some remobilization, in a matrix of fine-grained, indeterminate opaque material and white mica. Xenoliths of this trachytic rock are caught up in the adjacent sövite. Analysis of the grains of clear feldspar indicated compositions varying from Or89 to Or92. For several meters from the contact, the feldspar-rich rock is commonly brecciated. Strongly fenitized rocks identified near the carbonatite are in some cases also seen away from the contact, but on 918 the canadian mineralogist inspection, are visibly adjacent to both sides of a fracture or joint surface along which fenitizing fluids evidently migrated. The bulk composition of these rocks (Table 1) shows progressive enrichment in K, corresponding with the appearance and increase in K-feldspar and the increases in Ba and Rb contents (Fig. 1). The fenites analyzed and plotted in Figure 2 come from the twin carbonatite complexes of South Ruri (RS) and North Ruri (RN). Figure 2 shows that the path of fenitization (a) is initially one of increasing Na, followed by an increase in K (path b), producing the euhedral plates of clear K-feldspar and ultimately the feldspar rock. The path is one of falling temperature. products after hornblende, all cut by quartz veins (sample U 1071). Relict alkali feldspar from the host rock is in some cases preserved in the cores of the feldspars analyzed (Table 2). Some outcrops show a much finer-grained rock of flinty and foliated appearance, and composed of fine-grained granular feldspars with some quartz and chlorite, but with no apparent signs of fenitization (U 1069). At other outcrops, thin sections reveal fragments of the granular feldspar rock, “Orthoclasites” of Wasaki Peninsula, western Kenya Surrounding the large (2 3 2 km) Wasaki body of sövite west of Homa Bay is a zone 200 to 300 m wide of feldspar rock and feldspathic breccias in which the dominant feldspar is Or85–95, with some albite and aegirine-augite (Table 2). The Wasaki sövite is locally brecciated, and was described by Le Bas (1977, Fig. 10.1). It outcrops from Ugongo in the north to Sokolo southward, where it extends underground toward Kiumba and is penetrated by the Nyamaji phonolitic vent that brings to the surface fragments of granitic, sövitic and feldspar rocks, amongst others, providing a better image of this large carbonatite intrusion. A selection of fenite compositions from least fenitized to feldspar rock is presented in Table 3. The basement rock is a hornblende-bearing porphyritic granodiorite described by McCall (1958). Even the least fenitized sample collected is strongly fractured and is, especially in the south, reduced to minute fragments of dusty albite-twinned plagioclase and quartz set in a matrix of Fe-stained and chloritized secondary Fig. 1. Concentrations of K2O versus Ba and Rb for K-fenites developed next to sövites at Ruri, western Kenya. Fig. 2. Normative feldspar plot for fenites from Ruri, western Kenya (filled circles, labeled SR and NR for South and North Ruri) and for fenites from Seladhina, San Vicente, Cape Verde Island (filled squares). The Ruri feldspars initially show an albite-enrichment trend (arrow a), then an orthoclase-enrichment trend (b) with increase in fenitization of the metabasalt protolith. The trend c for the fenitization of an essexite protolith (E) at Seladhina, also begins with albite enrichment and then joins trend b of orthoclase enrichment, culminating in a pure K-feldspar rock. fenites associated with carbonatites each rimmed by water-clear K-feldspar plates. Filling the fractures is a mixture of Fe oxides, green aegirine– augite prisms, Fe-stained feldspars and some carbonate. Quartz veins are still evident. These rocks grade into feldspathic breccias. 919 At higher grades of fenitization, the plates of clear feldspar are larger and more abundant, with fractures containing a turbid and Fe-stained matrix in which abundant small laths of clear K-feldspar occur (U965). Other outcrops to the north and south of the Wasaki sövite show K-feldspar rocks veined and penetrated by needles of green sodic pyroxene and some analcime (SK34). There is only a general increase in proportion of the K-feldspar and size of crystals on approaching the sövite; the actual distribution is variable, and probably related to the extent of brecciation accompanying the fenitization. The coarser the grain size of the feldspars, the closer the composition approaches pure K-feldspar, and the greater the Ba content in most cases (Table 2). Figure 3 demonstrates the path of the fenitization, with the early stage (a) of Si loss and K gain, then increasing K (stage b) leading to the feldspar-rock compositions near pure K-feldspar. It follows the same path as that shown by Baldock (1973) for fenitization around the Bukusu and other carbonatites in eastern Uganda (Sutherland 1965b). In the area labeled Kfs in Figure 3 are the mineral compositions of the K-rich feldspar in the feldspar rocks, mostly Or87–97. Individual grains show zoning, with K and Ba increasing toward the rim. Of the trace elements listed in Table 3, Ba shows an appreciable increase with progressive fenitization around the Wasaki sövite, Rb less so (Fig. 4). Total Fe and Nb both show a poor correlation with progressive fenitization, as measured by increasing K2O. The Nb content instead shows a positive correlation to total Fe. Fig. 3. Normative felsic components of the Wasaki whole-rock fenite compositions plotted on the quartz – kalsilite – nepheline diagram (wt%) showing the progressive increase of the orthoclase (Or) component as fenitization increases from G (the local granodiorite) along trend arrows a and b. The points plotted in area labeled Kfs pertain to electron-microprobe data on feldspar in the fenites and orthoclasites. 920 the canadian mineralogist Fenites on Brava, Cape Verde Islands On the barren hillsides of Minhoto on the eastern part of the island of Brava, coarse-grained sövite is exposed from near the coast, across the Ribeira Grande inland for a kilometer to Cachaço. The carbonatite intrudes ijolite, and in the contact area there are “orthoclasites” (K-fenite in Fig. 5) that are usually brecciated. The exposures in some cases are whitish, but more commonly reddish and brecciated, the red color coming from the Fe-rich matrix to the breccia. Right up to the sövite, the ijolite in most places remains coarse grained with unaltered nepheline, aegirine-augite, apatite and titanite. In some cases, there is clear K-feldspar between the nepheline crystals. In other cases, the nepheline is replaced by cancrinite, and some prisms of pyroxene are rimmed and replaced by biotite and brown to opaque Fe-rich material. Thin section petrography reveals that the “orthoclasite”, where not altered, is composed of fresh, Carlsbadtwinned feldspar crystals 5 mm long. In general, the feldspar is fractured, in some cases comminuted. The feldspar has not been analyzed, but optically, it has the same clear, subhedral and Carlsbad-twinned habit of K-rich feldspar seen in other “orthoclasites”. The database (Table 4) is limited but reveals increases in Ba and Rb similar to those noted above. Strontium, Nb and the REE, normally in abundance in carbonatites, show no increase with fenitization. Instead here they decrease. The fenitizing fluids here, although emanating from carbonatite, evidently did not transport these elements. Fenites on San Vicente, Cape Verde Islands On the hillside north of the road at Seladinha da Cal (GR 163663), midway between Mindelo and Ponta do Calhau, is a brownish white northeasterly trending dyke-like body 2 m wide that cuts extensively brecciated essexitic gabbro. In the middle of the body is a Fig. 4. Concentration of K2O versus Ba and Rb for K-fenites around the Wasaki sövite, western Kenya. Fig. 5. Normative felsic components of potassic fenites plotted on the quartz – kalsilite – nepheline diagram (wt%) showing the progressive increase of the orthoclase component as fenitization increases in the nepheline syenites (ne sy) at Koga, northern Pakistan and in the ijolites (ij) at Brava, Cape Verde Islands. fenites associated with carbonatites pale brown carbonatite dyke ca. 40 cm wide showing dolomitization (MgO varies from 5 to 18 wt%). The data in Table 5 are taken from samples across the dyke body into the country rocks at Seladinha, with an additional set of samples prefixed HV and sample 82LV5 from similar occurrences 150 m to the northwest of Seladinha, where the carbonatite dykes are better seen. The whole area hereabouts exposes similar rocks. The data plotted as filled squares on Figure 2 show the same initial reduction in SiO2 (trend c) comparable to the Ruri example, followed by a similarly marked increase in K and ending with rocks of almost pure K-feldspar composition. In thin section, the original albite-twinned plagioclase of the essexite is seen to be replaced progressively by almost clear K-feldspar, and the original ophitic mafic minerals are altered to opaque material with a matrix of carbonate. These country rocks are considerably brecciated. Some fragments were formerly ankaramite rather than essexite. In less brecciated areas nearby, ankaramite dykes are seen cutting the essexitic gabbros. Feldspar rock at Koga, Swat district, northern Pakistan, 72°50’E, 36°20’N Forming the top of the hill of Naranji Kandao is an enormous body of feldspar rock capping a plug of white carbonatite that intrudes and fenitizes nepheline syenite and ijolite. The ca. 300 Ma Koga nepheline syenite is an oval body of about 40 km2 and is the eastern member of the Ambela granitic complex (Chaudhry et al. 1982, Le Bas et al. 1987). Naranji Kandao hill dominates the southern margin of the nepheline syenite. That syenite is coarse grained with perthitic orthoclase and albite, cloudy euhedral nepheline, minor sodic amphibole and pyroxene, biotite, titanite and, in some cases, brown garnet. Toward the carbonatite at Naranji Kandao, the grey syenite becomes whiter, and the proportion of K-feldspar increases and becomes notable as large white crystals. The carbonatite exposed in places on the Naranji Kandao hillsides is a coarse-grained sövite with triplejunctioned calcite, minor prisms of apatite, and rounded crystals of perthitic albite. Feldspar-rich syenite occurs higher up the hillsides, where it is invaded by sövite and K-feldspar rock. The hilltop is capped by almost pure feldspar rock that includes occasional elongate patches of the feldspathized nepheline syenite a meter across, some cut by veins of feldspar rock and sövite. The feldspar rock covers an area 1 km across surrounded by a further 0.5 km collar of feldspar-rich rocks. It is very coarse-grained and composed of cloudy, twinned plates of microcline up to 2 cm across, commonly rimmed by albite. Rare large prisms of green-brown aegirine occur randomly. Jabeen & Mian (1992) provided detailed descriptions of the rocks and compositions of the feldspars (Or94–98 and Ab89–98). Figure 5 shows the 921 transition from nepheline syenite to K-feldspar rock (analytical data in Table 6). As in other K-dominant fenites, Ba increases as K2O increases. The REE show no gain and instead decrease because the Koga sövite, which contained ca. 400 ppm La and ca. 800 ppm Ce, contributed nothing to the nepheline syenite initially being fenitized. The sövite compositions are given in Le Bas (1999). 922 the canadian mineralogist Mian & Jabeen (1990) subsequently found evidence for a second and minor carbonatite that produced Na-fenitized rocks bearing magnesio-arfvedsonite. K-fenites on Silai Patti, Northwest Frontier Provinces, Pakistan On the southern edge of the Himalayan Mountains in Malakand Agency of northern Pakistan, near the Afghanistan border, is a carbonatite sheet that occupies a southerly dipping thrust plane (the Main Mantle Thrust) between amphibolites to the north and granite gneisses and schists to the south. The sheet is 20 to 50 m thick and extends east–west for 12 km before disappearing into inaccessible tribal areas. It was first reported by Ashraf & Chaudhry in 1977, was studied in detail by Mian (1987), and dated at 29 ± 5 Ma by Le Bas et al. (1987). It comprises a white sheet of biotite– apatite sövite and a later brown and more extensive sheet of amphibole–apatite sövite. The sövites are coarse grained, with triple-junctioned plates of calcite, and include rounded prismatic crystals of apatite and biotite flakes. Outcrops of mildly fenitized rocks away from the carbonatites are few. The biotite sövite, rarely more than 20 m thick, is best developed in the east, where it fenitizes the overlying granitic gneiss. The gneiss next to the sövite contact is medium to coarse grained, well foliated (marked by muscovite and Fe oxides), but weathered and friable. The onset of fenitization is marked by the disappearance of the muscovite and quartz, the foliation being marked by alignment of feldspar, biotite and Fe oxides. Toward the contact, the grain size increases, with K-feldspar crystals (Or88–96), some grid-twinned, reaching up to 2 3 3 cm, usually surrounded by clusters of small granules of albite (Ab99), some albite-twinned, Fig. 6. Whole-rock fenite compositions, each with two feldspars, adjacent to carbonatites at Silai Patti, northern Pakistan plotted on the quartz – kalsilite – nepheline diagram, showing the trends for the two successive stages of fenitization of the Malakand granitic gneiss protolith, each commencing with a phase of desilication. giving an augen texture appearance. Red-brown biotite flakes around the Fe oxides and the prisms of aegirine are present in some cases (Table 7). The change in whole-rock compositions (Table 8) is plotted in Figure 6, which shows the same patterns as those shown by Woolley (1969) for the fenites of Alnö and other carbonatitic complexes. The Na-fenites associated with the amphibole sövite are discussed below. Similar K- and Na-fenites occur adjacent to the sheets of sövitic carbonatite at Loe Shilman on the Pakistan–Afghanistan border northwest of Peshawar (Mian & Le Bas 1986, 1987, 1988). K-fenites at Bayan Obo, Inner Mongolia, China Above the massive sheet of H8 dolomite carbonatite emplaced in Middle Proterozoic Bayan Obo sediments, large bodies of pure feldspar rock are developed in the overlying H9 slates. Despite the many contributions describing the Bayan Obo rocks, these feldspar-rich rocks have not received much recognition; they are briefly mentioned by Le Bas et al. (1992) and were declared by Yuan et al. (2004) not to be slate but metasomatized trachytic volcaniclastic rock or volcanic tuff. The feldspar rocks are best displayed in the quarry faces above the enormous Main and East pits mined for REE, Fe and Nb, and in the hanging wall at several places along the length of the sheet (>10 km). Figure 7 shows the two-stage fenitization seen above (Figs. 2, 3, 6); the preliminary decrease in Si is followed by an increase in K. Woolley (1969) and Vartiainen & Woolley (1976) also demonstrated similar paths of fenitization for the Chilwa, Alnö and other carbonatite complexes. The feldspar-rich rocks are in sharp contact with the mineralized H8 dolomite carbonatite body and extend into the Proterozoic slates for 2–4 meters. They are very fine grained, grey to white, and penetrated by pure white veins 10 to 50 mm wide. Preliminary Fig. 7. Fenitized H9 slate compositions at Bayan Obo, Inner Mongolia, plotted on the quartz – kalsilite – nepheline diagram, showing the progressive increase of the orthoclase component as fenitization increases, beginning with desilication and ending with pure K-feldspar rock. fenites associated with carbonatites field interpretation of these flinty rocks was that they are albitites cut by calcite veins, but XRF analysis reveals that they are K-feldspar rocks cut by albite 923 veins (Table 9), with some later indistinct carbonate veins. In thin section, the feldspar rocks are seen to be microcrystalline, and electron-microprobe analyses show that the bulk of the microcrystalline material has the composition Or95–98 with some Ab99 and occasional prisms of magnesio-arfvedsonite, especially toward the outer, darker (graphitic) and less fenitized slates. Similar rocks have also been identified in dykes nearby as potassic trachytic tuffs with K2O > 15 wt% (Yuan et al. 2000), and described as the product of metasomatic fluids emanating from the H8 dolomite carbonatite (Yuan et al. 2004). Feldspathization of carbonatite tuff at Kruidfontein, South Africa The caldera-filling carbonatitic tuffs of the Proterozoic carbonatite complex at Kruidfontein, in South Africa, provide evidence for both K and Na fenitization of the tuff. As well as clasts of fresh barian K-feldspar (Or85–93) in the sövitic tuffs and clasts of K-feldspar (Or96–100) in the ankeritic tuffs presumably derived from earlier fenites, pervasive and microcrystalline K-feldspar (Or97–100) occupies the matrix of the tuffs (Clarke et al. 1991). In addition, veins of both K-feldspar (Or98) and albite (Ab99) cut the mildly ankeritized tuffs. 924 the canadian mineralogist Sodic Fenites (Including some Sodic–Potassic Types) Sodic fenites are characterized by Na-rich amphibole and an alkali feldspar or albite, with K-feldspar and Na-rich pyroxene playing a lesser role. An extreme example is the occurrence of albitite around the Kirumba carbonatite in Kivu Province, Democratic Republic of Congo (Denaeyer 1966). Among the Tertiary carbonatite complexes of East Africa, evidence of sodic fenitization is rare. In the volcanic vent of Nyamaji (Le Bas 1977), fragments of albitite occur that might result from sodium fenitization. On Homa Mountain, the central updomed mass of Nyanzian metabasalts, andesites and rhyolites is mildly fenitized to Na-amphibole-bearing fenites of uncertain origin, but they are likely to be related to the emplacement of the swarms of carbonatite cone-sheets centered under the mountain. The fenites are characterized by widespread shattering and are cut by veinlets of minute sheaves of aegirine and magnesio-arfvedsonite together with crystals of albite projecting from the walls. An apatite-group mineral and calcite also occur, in some cases in conspicuous amounts. Cutting these veins and along their edges are numerous veinlets composed of fine-grained K-feldspar, which in places appear to develop into patches composed of K-feldspar, aegirine and Na-amphibole. These fenites were described and analyzed by Sutherland (1969) and Le Bas (1977). The Na-fenites at Silai Patti, North West Frontier Province, Pakistan Near the contact between granitic gneiss countryrock and the brown amphibole sövite sheet at Silai Patti, veins of sövite cutting the K-fenitized gneiss are bordered by aegirine prisms, indicating the passage of later sodic fluids. The gneiss near the sövite sheet is also recrystallized to a syenitic composition. The early stages of fenitization are seen unfortunately only in deeply weathered exposures in the field, where Na-pyroxene and some Na-amphibole progressively replace the quartz, and albite becomes more prominent. Nearer to the sövite contact, the fenites are feldsparrich, with aligned prisms and aggregates of dark green aegirine-augite, in some cases rimmed by blue to grey pleochroic magnesio-arfvedsonite (Table 10). The albite and K-feldspar occur in the proportions of 2:1 in the coarser-grained fenites, with albite increasing in the finer-grained fenites to 3:1. These variable proportions show no relationship to distance from the contact, but the field outcrops do suggest a relationship of distance to former joints in the gneiss, along which fenitizing fluids may be presumed to have traveled. Table 11 provides representative analytical data for the local granite fenitized to increasing degrees. Plots of these data on a Qtz–Ks–Ne normative diagram (Fig. 6) shows the contrasting paths between the K and Na fenitization processes. The Na-fenites at Bayan Obo, Inner Mongolia, China As well as the K-fenites at Bayan Obo described above, Na-fenites occur adjacent to sövitic carbonatite dykes, and also at the contact with the massive H8 dolomite carbonatite intrusion (Le Bas et al. 1992, 2007), and bordering the north–south-striking Wu sövitic carbonatite dyke a kilometer to the north of the H8 carbonatite described by Yang et al. (2000, 2003). At the western contact of the Wu dyke, the adjacent H1 arkosic quartzite is strongly fenitized to a brilliant blue rock for the first 15 to 40 cm. The bedding is fenites associated with carbonatites almost perpendicular to the contact, which is strongly dislocated by shear. The bulk of the blue rock is a meshwork of amphibole fibers varying in composition from ferrowinchite to magnesioriebeckite to glaucophane (Table 12) that surround patches (the remains of pebbles in the quartzite) of granular, fresh, twinned albite crystals and occasional Fe oxide phases, with some cloudy areas of microcline. Further from the contact and marked by a sharp change in color from blue to white, evidently indicating the limit achieved by the metasomatic front, the white fenitized quartzite is penetrated by an anastomosing network of dark amphibole-bearing veinlets. The veining appears to mark the path of migrating carbonate-free fenitizing fluids along annealed fractures and joints. Former quartz pebbles are recognizable, but are now represented by granular aggregates of albite, small and fractured grains of quartz, and some microcline, all with strained extinction and sutured margins. Between the aggregates are veinlets of pale mica flakes (Table 13), with blue-grey pleochroic amphibole prisms varying in composition from magnesioriebeckite to magnesio-arfvedsonite (Table 12) and some apatite prisms. Further from the carbonatite dyke, the proportion of amphibole diminishes and disappears some 10 meters from the contact, where low-Al phlogopite is present. At 34 m from the contact, no mafic minerals are present, only fresh albite (Ab95–97) and cloudy microcline (Or97–99) grains around areas of strained quartz (Table 14). Results of XRF and ICP–MS analyses of these fenitized quartzites show (Table 15) an increase in Na2O (sample 90/48 is recalculated to 3.15 wt% Na2O without calcite and barite) and loss of SiO2 toward the carbonatite. There are also increases in Sr, Ba, Sc, V, Zn, Nb and REE (Figs. 8a, b). The strong enrichment of REE in the fenite alongside the fractured and sheared western margin of the carbonatite dyke is unique among all the other carbonatites dykes nearby and is interpreted 925 as a later REE-rich hydrothermal event, most probably related to the formation of the massive REE–Nb–Fe orebodies for which Bayan Obo is famous. This leaves Sr, Ba and the alkalis as the main elements of the fenitizing process. Full analytical data for the minerals in H1 fenites 90/48, 90/47, 90/49, 90/50 and 90/51 and H3 fenites 90/40, 90/55 and 90/56 are given in Tables 15a–h, obtainable from the Depository of Unpublished Data on the MAC website [document Bayan Obo CM46_915]. On its eastern contact, the dyke cuts bedded H3 shale of the Bayan Obo sedimentary sequence with a sharp and undeformed contact perpendicular to the bedding. Instead of the blue fenitic zone to the west, the fenite to the east is striped dark and light according to the psammitic content of the beds. Next to the contact, sample 90/40 is unusually potassic (K2O = 11.94 wt%, Table 15), with the dark bands composed mainly of low-Al phlogopite flakes with minor prisms of sodic amphibole, aegirine and granules of feldspar (Table 15f). The lighter bands differ only in having a higher proportion of feldspar. Ten centimeters from the 926 the canadian mineralogist contact (90/55), the dark bands are composed dominantly of wispy sodic amphibole (magnesioriebeckite to magnesio-arfvedsonite), euhedra of titanite, rare brown pleochroic aegirine crystals and phlogopite flakes, and much feldspar as clear grains of albite Ab98–99 and K-feldspar Or94–95 (Table 15g). A meter from the contact (90/56), the bands of bedding are still evident, and the mineralogy is little different apart from more biotite and Fe oxides (Table 15h). Geochemical changes across this fenite zone are not as marked as those in the western contact zone: Na, Nb and REE show some increase and SiO2 decreases (Table 15). The narrow potassic zone next to the contact is chemically no different from the potassic fenites described earlier. Its position here is unusual, but it may mark a later phase of K-fenitization following Na-fenitization, similar to events described by Garson (1965) and Woolley (1969) at Kangankunde, Malawi, and by Platt & Woolley (1990) for the carbonatites and fenites of Chipman Lake, Ontario. Discussion Heinrich (1985) rightly pointed out that there are “infinite variations on a fenite theme”. Therefore, fruitful discussion on the process of fenitization associated with carbonatites is focused on those occurrences where the metasomatic fluids or emanations that induce the fenitic effect can be traced to a carbonatite source. This limits, as mentioned earlier, the examples available for study because it rules out the fenitization at the numerous centers where carbonatites coexist with alkali silicate rocks and at which the source cannot be distinguished, e.g., at Alnö, Chilwa, Fen, Kola and Meimecha–Kotui in Siberia. In these and many other localities, it is not entirely clear whether the fenitization, usually sodic, is related to the intrusion of carbonatitic magma or of alkali silicate magma (generally syenitic or ijolitic). In some cases, too, there is uncertainty concerning the genesis of the syenite associated with a carbonatite, for example at Xiluvo in Mozambique, where a syenite, with K-rich feldspar (Or96–97) as the most abundant phase together with low-Al phlogopite or biotite, is reported (Melluso et al. 2004), supposedly as an igneous rock but seemingly comparable with the fenites considered above. The examples of fenitization considered above and many more in the literature (e.g., McKie 1966, Woolley 1969, 1987) have led to the general observation, noted by several authors including Garson (1965), Fig. 8. Plot against distance in H1 quartzite from the Wu carbonatite dyke north of Bayan Obo, Inner Mongolia, China showing: (a) increase of Ba and Rb as the contact is approached; (b) increase of Sc, Zn, V and Nb toward the contact. fenites associated with carbonatites Baldock (1973) and Heinrich (1985), that feldspathic fenitization tends to occur at the upper and subvolcanic levels of carbonatite intrusion. These levels are also usually the sites of the younger carbonatite occurrences, where relatively little erosion has taken place. In contrast, sodic fenites are more commonly found around the older complexes such as Oldoinyo Dili, Tanzania (McKie 1966), where the deeper levels have become exposed as the result of prolonged erosion. At these deeper levels, a higher-temperature part of the thermal gradient (> ca. 600°C) would have prevailed, favoring the formation of sodic fenites, as deduced by Rubie & Gunter (1983). This temperature relationship accords with the earlier preferential loss of Na over K during the magmatic evolution of a carbonatite, as noted by Woolley (1982). One exception to the depth–time relationship is seen at the >2000 Ma old Kruidfontein carbonatite in South Africa (see above), where K-rich fenites in the caldera-filling sediments somehow escaped much erosion. Another is seen with the trachytic fenites around the “Paleozoic Limestone” [interpreted by Woolley (1987) as carbonatite] at Brent Crater in Ontario (Currie 1971). It is also a fact that extensive and intensive brecciation commonly occurs among the K-feldspar fenites and, in cases of extreme brecciation, results in the emplacement of pseudotrachyte tuffs or, if higher fluid pressures exist, of intrusive sheets of the comminuted material explosively reaching the surface to become extrusive tuffs, as experimentally demonstrated by Lorenz et al. (1991). Brecciation does not characteristically occur among the sodic fenites. It would seem evident from this that a “head” of volatiles or fluids must migrate to the topmost levels of a chamber of carbonatitic magma. Most likely, the fluid is H2O in part expelled from crystallizing carbonatitic magma in which up to 5% H2O at 1 kbar can be dissolved, as recently confirmed by Keppler (2003). The H2O may be an intrinsic part of the carbonatitic magma or, more likely, may have been derived from the country rocks traversed by the carbonatitic magma. Explosive release of this fluid within the subvolcanic complex could produce dense clouds of dust leading to the formation of the tuffs described above or, if droplets formed, to lapilli tuffs, as observed in the caldera of the Kruidfontein carbonatite complex in the Transvaal and in the Tertiary Rangwa carbonatite caldera of western Kenya. The potassic glasses found at Rangwa, recently described by Rosatelli et al. (2003), and the newly discovered trachytic glasses intermingled with dolomite carbonatite globules in the Massif Central of France (Chazot et al. 2003), may have a similar explosive origin. It has also been observed that where there is more than one phase of intrusion of a sövite, the earlier sövite produces the feldspathic fenites, but the later intrusions of sövite may not, only cutting the earlier sövite and fenite, and apparently without reaction. Garson (1965) 927 commented on this effect in describing the occurrence of fresh xenoliths of feldspar-rich fenites in some sövites evidently produced by earlier sövites. One feature common to many fenites is the absence or very limited presence of calcite or other carbonate mineral. Morogan (1994) did propose that for fenitization associated with carbonatite, the fluid phase would, as could be expected, be dominated by CO2 over H2O, and with abundant calcite formed. Were that the case, one might expect to see in thin section in between the fenitic minerals, calcite penetrating from the adjacent carbonatite that induced the fenitization. But this is not commonly the case, and it would seem as if there is some impermeable membrane not permitting the passage of CO2-bearing fluids from the crystallizing carbonatitic magma. The answer to the question of the nature of the fluid that carries the fenitizing agents may lie in mineralogy. The presence of amphiboles, micas and apatite, particularly in the sodic fenites, suggests that the fluid included hydroxyl and fluorine ions, a conclusion drawn by Gittins et al. (1990) as a result of experimental work. However, they cautioned that the role of H2O in the evolution of a carbonatitic magma was probably minor rather than major. Jago & Gittins (1991) provided further evidence for significant F in carbonatitic magma. In a later discussion with Treiman & Essene (1992), Gittins et al. (1992) reiterated more strongly the evidence for a dominance of F over H2O in carbonatitic magma. Treiman & Essene (1992) did not dispute that a significant proportion of F may be present in carbonatite-related fluids, but maintained that in the case of the Husereau dyke at Oka, Canada, it “was fluxed by H2O rather than by HF or F2”. Fluorine clearly is a dominant factor, but whether it is in the vapor phase or in the melt remains open to argument. The near-absence of H2O (and F) in the K-feldspar fenites indicates that these fenite-producing fluids are different, notwithstanding the fact that explosive action, presumably fluid-driven, is associated with many K-feldspar fenites. Phlogopite reaction rims are, however, commonly seen between sövite and its hosts, e.g., on the west coast of Fuerteventura, Canary Islands, and may indicate (OH), K and F activity at relatively higher temperatures (Bailey 1966). This may also have been the case at the phlogopite-rich K-rich fenite margin at the eastern margin of the Wu dyke described above. However, variability in the ratio F/(OH) is to be expected, as demonstrated by Williams-Jones & Palmer (2002) for the potassic fenites around the Amba Dongar carbonatite in India, where F clearly is a major component of the fenitizing fluids. The significant presence of F in carbonatitic magma is borne out by fluid-inclusion studies recently summarized by Veksler & Lentz (2006) and by Rankin (2005). The trapped primary daughter minerals include halides and the trapped fluids are halogen-rich, indicating that F was present at an early stage. 928 the canadian mineralogist It is significant that calciocarbonatites and magnesiocarbonatites each can induce both potassic and sodic fenitization at their margins, and that the potassic fenites produced by calciocarbonatites and the potassic fenites produced by magnesiocarbonatites cannot be distinguished unless calcite or dolomite is present. Similarly, sodic fenites produced by calciocarbonatites and by magnesiocarbonatites cannot be distinguished. Fenites around calciocarbonatites have been discussed above; examples around magnesiocarbonatites are less common but nevertheless well reported in the literature, for example at Kangankunde, Malawi, where zones of potassic feldspathization form collars around the magnesiocarbonatite vents (Garson 1965). At Newania in Rajasthan, India, eckermannite-rich fenites surround a three-km-long dolomite carbonatite sheet (Viladkar 1980) and are also associated with the Swartbooisdrif ferrocarbonatite in Namibia (Drüppel et al. 2005). Veksler & Lentz (2006) also reviewed the significance of melt inclusions in carbonatites, distinguishing between inclusions of primary melts and inclusions of late-stage fluids. Here it is the melt inclusions in early liquidus minerals in carbonatitic magma, such as apatite, that provide the telling evidence on the primary composition of those magmas. After reviewing the data derived from carbonatites in East and South Africa, Europe, North America, and in Russia from East to West, they concluded that in general the inclusions are alkali-rich. Costanzo et al. (2006) and Rankin (2005) came to broadly similar conclusions, based for the most part on other carbonatite complexes, that REE as well as Sr, Na and K are significant minor components. Rankin in particular stated that evidence from inclusions trapped in carbonatitic minerals showed that Na and K are, besides Ca and Fe, the main components of orthomagmatic carbonatitic fluids, commonly with percentage levels of Sr, Ba and REE. Mitchell (2005) also arrived at the conclusion in his wide-ranging study that carbonatite-forming magma must contain Na and K. Despite the evident appreciable contents of Na and K in primitive carbonatitic magma, the Na and K invariably does not remain in the crystallized product of the magma, but is apparently lost via late-stage fluids to induce fenitization. Fluid-inclusion evidence coming from Xie et al. (2006) in a new study of a Chinese carbonatite lends further support to the hypothesis that these fluids have high contents of K, Na, Ba, Sr and REE and that “the melt–fluid inclusions should represent the original carbonatite fluid” (Xie, pers. commun., 2007). As observed long ago in 1937 by Dixey et al. (1955) and commented on by Kresten (1991), the element most commonly added during fenitization, besides Na and K, is Ba, much of it going into barian K-feldspar (see Table 14), particularly near the contact. Iron also is usually introduced, appearing as finely disseminated oxides, as are Sc, V, Zn, Sr and Rb, but not the REE in most instances. Drüppel et al. (2005) listed Na, Sr, Ba, Nb and LREE as the elements released by the carbonatite into the fenitizing fluids at Swartbooisdrif in Namibia. Strontium and Ba are common trace elements in carbonatites, either in discrete mineral phases or in solid solution, variously in the carbonates, phosphates and phlogopite. There are noteworthy contents of Ba in the core of phlogopite that are possibly mantle-sourced (Gaspar & Wyllie 1982, McCormick & Le Bas 1996). The common presence of biotite or phlogopite is an indicator of K in the primary carbonatite, but the usual almost total absence of Na from carbonatites is problematic. The eruption of Na–K carbonate lavas at Oldoinyo Lengai in northern Tanzania proves that Na–K-rich carbonatitic magmas can exist, but such compositions are not found in any of the other >500 carbonatite complexes in the world, and this has relegated Oldoinyo Lengai to its status as an exceptional occurrence and an unlikely parental composition. However, the work of Wallace & Green (1988) shows that sodic dolomitic carbonatitic melts can be generated from an amphibole-bearing mantle, which under suitable geothermal conditions could rise through the crust and be a potential source for Na-metasomatism around a dolomitic carbonatite intrusion. The recent observations by Bailey et al. (2006) give further evidence for a mantle source of dolomite carbonatite melts. The presence of the rare minerals burbankite and carbocernaite in some carbonatites is a clear indication that alkali-bearing carbonatitic magmas can exist, as at Oldoinyo Lengai. Burbankite, (Na,Ca)3(Sr,Ca,Ce)3(CO3)5, is reported by Kapustin (1980) in several carbonatites, both as primary crystals and at a late stage. Primary burbankite (also sylvite) is recorded by Costanzo et al. (2006) trapped in fluid inclusions in apatite of the Jacupiranga carbonatite. Platt & Woolley (1990) reported burbankite in the dolomitic carbonatite at Chipman Lake, Bühn et al. (1999) similarly in the carbonatite at Kalkfield in Namibia, and Sindern et al. (2004) also in the calcite carbonatite at Khibina, Kola Peninsula, Russia. But burbankite is unstable; upon its breakdown, the alkalis are likely to be lost, dissolved in fluids during the later-stage processes of carbonatite crystallization (Zaitsev et al. 2002). The fact that burbankite is unstable and that it includes many of the elements involved in fenitization may be taken as a pointer to the possible source of the components of fenitization. Carbocernaite, (Ca,Na)(Sr,Ce,Ba)(CO3)2 is even more rare; it is reported by Kapustin (1980) in a few carbonatites, by Drüppel et al. (2005) in the Swart booisdrif ferrocarbonatite in Namibia, and by Wall et al. (1993) in a carbonatite dyke in Rajasthan, India. It is a stable mineral, in some cases primary, but is also formed by the breakdown of burbankite. On the basis of isotope evidence, Zaitsev et al. (2002) deduced that the burbankite-bearing carbonatites at Vuoriyarvi, and probably at Khibina, in the Kola Peninsula of Russia, have a mantle source, and that fenites associated with carbonatites these Kola carbonatites lack crustal contamination. For other carbonatitic magmas passing through continental crust, contamination with K and Na from the crust remains a possibility. However, the mineralogical and geochemical evidence presented above shows that the Na and K fenitization associated with the carbonatites in the Cape Verde and Canary Islands is no different from fenitization associated with carbonatites in continental areas. But the Cape Verde Islands and arguably the Canary Islands are on oceanic crust, and therefore the carbonatitic magmas of the islands during passage through oceanic crust would fail to receive significant K and Na contamination and more likely receive none. Therefore, the source of the alkalis must be in the mantle. In the case of Na, the source most likely is amphibole-bearing mantle as Wallace & Green (1988) have shown possible. A phlogopite-bearing mantle might similarly be the source for the K, and the K:Na ratio in the mantle-derived carbonatitic magma would then depend on the sources sampled during its passage through the mantle. Experimental evidence on the process of metasomatism, in particular fenitization, is required, but the uncertain rates of metasomatic chemical reactions and intra-lattice diffusion processes make the process difficult to be reproduced in the laboratory. More detailed petrography and element-distribution maps are needed to gain greater insight into the process of fenitization. Summary Significant features, some new, about alkalis and carbonatites are: 1. Primary mantle-derived carbonatite melts carry appreciable Na and K in widely varying proportions that can be subsequently lost to fenitization. 2. Calciocarbonatites and magnesiocarbonatites of any age can both produce potassic fenites that are indistinguishable. Likewise, both carbonatites can produce sodic fenites that are indistinguishable. 3. Potassic fenites are characterized by a high proportion of K-rich orthoclase or microcline, in the composition range Orca. 90, or in some cases by concentrations of low-Al phlogopite or biotite. 4. Sodic fenitization is characterized by an abundance of sodic amphiboles and some pyroxenes, accompanied by K- and Na-feldspars. 5. The fenitizing fluids carrying the Na and K are halide-rich, principally F; CO2 is commonly absent. The H2O content varies with locality and may depend on the country rocks. 6. Potassic fenites are formed mostly in the upper levels of an intrusive body of calciocarbonatite and magnesiocarbonatite, whereas sodic fenites more commonly are formed earlier and at deeper levels and at relatively higher temperature. 7. The first marked effect of fenitization is loss of silica, followed by increase in the alkalis. 929 8. The ultimate product of potassic fenitization can be pseudotrachyte with 15% K 2 O, which by remobilization (manner remains uncertain) may be intruded as dykes or, at high fluid pressures, can grade into feldspathic breccias, ochreous breccias and tuffs emplaced as near-surface sheets or, in extreme cases, as extrusive tuffs. 9. Barium is characteristically enriched in potassic and sodic fenites, in some cases also Fe, Sr, Sc, V, Zn and Rb. Acknowledgements I have many thanks to make for the enjoyable years spent cogitating on carbonatites, primarily to Basil King, Tom Deans and Diana Sutherland, who introduced me to them in East Africa and provided subsequent encouragement, and to John Gittins for illuminating reflections beginning in our research student days. I am also indebted my many research students, including John Dixon, Ihsanullah Mian, Neil Hodgson, Yang Xueming and Anatoly Zaitsev, for permitting extensive use of data from their theses and for collaborative work, and to the other many colleagues, especially Alan Woolley, with whom I most profitably shared discussions on and visits to carbonatite complexes worldwide. This contributed also greatly benefitted from profound, critical and helpful comments from editors Robert F. Martin and David R. Lentz, and from reviewers Don Hogarth and Yang Xueming on many major and minor points. References Andersen, T. (1989): Carbonatite-related contact metasomatism in the Fen complex, S.E. Norway: effects and petrogenetic implications. Mineral. Mag. 53, 395-414. Ashraf, M. & Chaudhry, M.N. (1977): A discovery of carbonatite from Malakand. Geol. Bull. Punjab Univ. Lahore 14, 91-94. Bailey, D.K. (1966): Potash feldspar and phlogopite as indices of temperature and partial pressure of CO2 in carbonatite and kimberlite. Mineral. Soc. India, IMA Vol., 5-8. Bailey, K., Kearns, S., Mergoil, J., Mergoil Daniel, J. & Paterson, B. (2006): Extensive dolomitic volcanism through the Limagne Basin, central France: a new form of carbonatite activity. Mineral. Mag. 70, 231-236. Baldock, J.W. (1973): Potassic fenitization, trachytes and agglomerates at the Bukusu carbonatite complex, Uganda. Overseas Geol. Mineral Resources 42, 1-24. Brøgger, W.C. (1921): Die Eruptivgesteine des Kristiania gebietes. IV. Das Fengebiet in Telemark, Norwegen. Videns. Skrift. I. Mat-Naturv. Klasse 91. Brown, P.E. (1964): The Songwe Scarp carbonatite and associated feldspathization in the Mbeya Range, Tanganyika. Quart. J. Geol. Soc. London 120, 223-240. 930 the canadian mineralogist B ühn , B., R ankin , A.H., R adtke , M., H aller , M. & Knöchel, A. (1999): Burbankite, a (Sr,REE,Na,Ca)-carbonate in fluid inclusions from carbonatite-derived fluids: identification and characterization using Laser Raman spectroscopy, SEM-EDX and synchrotron micro-XRF analysis. Am. Mineral. 84, 1117-1125. Chaudhry, M.N., Ashraf, M. & Hussain, S.S. (1982): Petrology of Koga nepheline syenites and pegmatites of Swat District. Geol. Bull. Punjab Univ. 16, 1-14. Gittins, J., Beckett, M.F. & Jago, B.C. (1990): Composition of the fluid phase accompanying carbonatite magma: a critical examination. Am. Mineral. 75, 1106-1109. Gittins, J., Beckett, M.F. & Jago, B.C. (1992): Composition of the fluid phase accompanying carbonatite magma: a critical examination – reply. Am. Mineral. 77, 666-667. Gittins, J. & Harmer, R.E. (2003): Myth and reality in the carbonatite–silicate rock “association”. Per. Mineral. 72, 19-26. Chazot, G., Bertrand, H., Mergoil, J. & Sheppard, S.M.F. (2003): Mingling of immiscible dolomitic carbonatite and trachyte in tuffs from the Massif Central, France. J. Petrol. 44, 1917-1936. Heinrich, E.W. (1966): The Geology of Carbonatites. Rand McNally & Co., Chicago, Illinois. Clarke, L.B., Le Bas, M.J. & Spiro, B. (1991): Rare earth, trace element and stable isotope fractionation of carbonatites at Kruidfontein, Transvaal, S. Africa. Proc. 5th Int. Kimberlite Conf. 1. Kimberlites, Related Rocks and Mantle Xenoliths (O.H. Leonardos & H.O.A. Meyer, eds.). Companhia de Pesquisa de Recursos Minerals (CRPM), Brasilia, Brazil (236-251). Heinrich, E.W. & Moore, D.G. (1970): Metasomatic potash feldspar rocks associated with igneous alkalic complexes. Can. Mineral. 10, 571-584. Cooper, A.F. & Reid, D.L. (2000): The association of potassic trachytes and carbonatites at the Dicker Willem Complex, southwest Namibia: coexisting, immiscible, but not cogenetic magmas. Contrib. Mineral. Petrol. 139, 570-583. Costanzo, A., Moore, K.R., Wall, F. & Feely, M. (2006): Fluid inclusions in apatite from Jacupiranga calcite carbonatites: evidence for a fluid-stratified carbonatite magma chamber. Lithos 91, 208-228. Currie, K.L. (1971): A study of potash metasomatism around the Brent Crater, Ontario, a Paleozoic alkaline complex. Can. J. Earth Sci. 8, 481-497. Denaeyer, M.E. (1966): Sur la présence d’une carbonatite ankéritique (rauhaugite) en bordure alcalin de Kirumba (Kivu). C.R. Acad. Sci. Paris 263, 9-12. Dixey, F., Campbell Smith, W. & Bisset, C.B. (1955): The Chilwa Series of Southern Nyasaland. Geol. Surv. Dep. Nyasaland, Bull. 5 (revised 1937 edition). Drüppel, K., Hoefs, J. & Okrusch, M. (2005): Fenitizing processes induced by ferrocarbonatite at Swartbooisdrif, NW Namibia. J. Petrol. 46, 377-406. Flegg, A.M., Clarke, M.C.G., Sutherland, D.S. & Le Bas, M.J. (1977): Homa Mountain II: the main carbonatite centre. In Carbonatite–Nephelinite Volcanism (M.J. Le Bas, ed.). John Wiley & Sons, London, U.K. (222-232). Garson, M.S. (1962): The Tundulu carbonatite ring-complex in southern Nyasaland. Geol. Surv. Nyasaland, Mem. 2. Garson, M.S. (1965): Carbonatites in southern Malawi. Geol. Surv. Dep., Bull. 15. Gaspar, J.C. & Wyllie, P.J. (1982): Barium phlogopite from the Jacupiranga carbonatite, Brazil. Am. Mineral. 67, 997-1000. Heinrich, E.W. (1985): Infinite variations on a fenite theme. Indian Mineral., Sukheswala Vol., 151-162. Jabeen, N. & Mian, I. (1992): Alkali-feldspar from Koga syenites, Ambella granitic complex, NW Pakistan. Geol. Bull. Univ. Peshawar 25, 77-84. Jago, B.C. & Gittins, J. (1991): The role of fluorine in carbonatite magma evolution. Nature 349, 56-58. Kapustin, Yu.L. (1980): Mineralogy of Carbonatites. Amerind Publ. Co. PVT, Ltd., New Delhi, India. Keppler, H. (2003): Water solubility in carbonatite melts. Am. Mineral. 88, 1822-1824. Kresten, P. (1991): Chemistry of fenitization at Fen, Norway and Alnö, Sweden. Proc. 5th Int. Kimberlite Conf. 1. Kimberlites, Related Rocks and Mantle Xenoliths (O.H. Leonardos & H.O.A. Meyer, eds.). Companhia de Pesquisa de Recursos Minerals (CRPM), Brasilia, Brazil (252-269). Kretz, R. (1983): Symbols for rock-forming minerals. Am. Mineral. 68, 277-279. Le Bas, M.J. (1977): Carbonatite–Nephelinite Volcanism. John Wiley and Sons, London, U.K. Le Bas, M.J. (1981): Carbonatite magmas. Mineral. Mag. 44, 133-140. Le Bas, M.J. (1999): Sövite and alvikite: two chemically distinct calciocarbonatites C1 and C2. S. Afr. J. Geol. 102, 109-121. Le Bas, M.J., Keller, J., Tao Kejie, Wall, F., Williams, C.T. & Zhang Peishan (1992): Carbonatite dykes at Bayan Obo, Inner Mongolia, China. Mineral. Petrol. 46, 195-228. Le Bas, M.J., Mian, I. & Rex, D.C. (1987): Age and nature of carbonatite emplacement in north Pakistan. Geol. Rundschau 76, 317-323. fenites associated with carbonatites Le Bas, M.J., Yang Xueming, Taylor, R.N., Spiro, B., Milton, J.A. & Zhang Peishan (2007): New evidence from a calcite–dolomite carbonatite dyke for the magmatic origin of the massive Bayan Obo ore-bearing dolomite marble, Inner Mongolia, China. Mineral. Petrol. 90, 223-248. L ehijarvi , M. (1960): The alkaline district of Iivaara, Kuusamo, Finland. Bull. Comm. Géol. Finlande 185. Lorenz, V., Zimanowski, B. & Fröhlich, G. (1991): Experiments on explosive basic and ultrabasic, ultramafic and carbonatitic volcanism. Proc. 5th Int. Kimberlite Conf. 1. Kimberlites, Related Rocks and Mantle Xenoliths (O.H. Leonardos & H.O.A. Meyer, eds.). Companhia de Pesquisa de Recursos Minerals (CRPM), Brasilia, Brazil (270-282). McCall, G.J.H. (1958): Geology of the Gwasi area. Geol. Surv. Kenya, Rep. 45. McCormick, G.R. & Le Bas, M.J. (1996): Phlogopite crystallization in carbonatite magmas from Uganda. Can. Mineral. 34, 469-478. McKie, D. (1966): Fenitization. In Carbonatites (O.F. Tuttle & J. Gittins, eds.). Interscience, New York, N.Y. (261-294). Melluso, L., Censi, P., Perini, G., Vasconcelos, L., Morra, V., Guerreiro, F. & Bennio, L. (2004): Chemical and isotopic (C, O, Sr, Nd) characteristics of the Xiluvo carbonatite (central-western Mozambique). Mineral. Petrol. 80, 201-213. 931 Platt, R.G. & Woolley, A.R. (1986): The mafic mineralogy of the peralkaline syenites and granites of the Mlanje complex, Malawi. Mineral. Mag. 50, 85-99. Platt, R.G. & Woolley, A.R. (1990): The carbonatites and fenites of Chipman Lake, Ontario. Can. Mineral. 28, 241-250. Rankin, A.H. (2005): Carbonatite-associated rare metal deposits: composition and evolution of ore-forming fluids – the fluid inclusion evidence. In Rare-Element Geochemistry and Mineral Deposits (R.L. Linnen & I.M. Samson, eds.). Geol. Assoc. Can., Short Course Notes 17, 299-314. Rosatelli, G., Wall, F. & Le Bas, M.J. (2003): Potassic glass and calcite carbonatite in lapilli from extrusive carbonatites at Rangwa Caldera Complex, Kenya. Mineral. Mag. 67, 931-955. Rubie, D.C. & Gunter, W.D. (1983): The role of speciation in alkaline igneous fluids during fenite metasomatism. Contrib. Mineral. Petrol. 82, 165-175. Sindern, S. & Kramm, U. (2000): Volume characteristics and element transfer of fenite aureoles: a case study from Iivaara alkaline complex, Finland. Lithos 51, 75-93. Sindern, S., Zaitsev, A., Demény, A., Bell, K., Chakmouradian, A.R., Kramm, U., Moutte, J. & Ruskhlov, A.S. (2004): Mineralogy and geochemistry of silicate dyke rocks associated with carbonatites from the Khibina complex (Kola, Russia) – isotope constraints on genesis and small scale mantle sources. Mineral. Petrol. 80, 215-239. Mian, I. (1987): The Mineralogy and Geochemistry of the Carbonatites, Syenites and Fenites of North West Frontier Province, Pakistan. Ph.D. thesis Univ. of Leicester, Leicester, U.K. Sutherland, D.S. (1965a): Nomenclature of the potassicfeldspathic rocks associated with carbonatites. Geol. Soc. Am., Bull. 76, 1409-1412. Mian, I. & Jabeen, N. (1990): Sodic pyroxenes and amphiboles from Koga syenites of Ambela granitic complex, N.W.F.P., Pakistan. Geol. Bull. Univ. Peshawar 23, 67-85. Sutherland, D.S. (1965b): Potash trachytes and ultra-potassic rocks associated with the carbonatite complex of the Toror Hills, Uganda. Mineral. Mag. 35, 363-378. Mian, I. & Le Bas, M.J. (1986): Sodic amphiboles in fenites from the Loe Shilman carbonatite complex, NW Pakistan. Mineral. Mag. 50, 187-197. Sutherland, D.S. (1969): Sodic amphiboles and pyroxenes from fenites in East Africa. Contrib. Mineral. Petrol. 24, 114-135. Mian, I. & Le Bas, M.J. (1987): The biotite–phlogopite series in fenites from the Loe Shilman carbonatite complex, NW Pakistan. Mineral. Mag. 51, 397-408. Treiman, A.H. & Essene, E.J. (1992): Composition of the fluid phase accompanying carbonatite magma: a critical examination – discussion. Am. Mineral. 77, 663-665. Mian, I. & Le Bas, M.J. (1988): Feldspar solid solution series in fenites from Loe Shilman carbonatite complex, NW Pakistan. Geol. Bull. Univ. Peshawar 21, 71-83. Vartiainen, H. & Woolley, A.R. (1976): The petrography, mineralogy and chemistry of the fenites of the Sokli carbonatite intrusion, Finland. Geol. Surv. Finland, Bull. 280. Mitchell, R.H. (2005): Carbonatites and carbonatites and carbonatites. Can. Mineral. 43, 2049-2068. Morogan, V. (1994): Ijolite versus carbonatite as sources of fenitization. Terra Nova 6, 166-176. Morogan, V. & Woolley, A.R. (1988): Fenitization at Alnö carbonatite complex, Sweden; distribution, mineralogy and genesis. Contrib. Mineral. Petrol. 100, 169-182. Veksler, I.V. & Lentz, D. (2006): Parental magmas of plutonic carbonatites, carbonate–silicate immiscibility and decarbonation reactions: evidence from melt and fluid inclusions. In Melt Inclusions in Plutonic Rocks (J.D. Webster, ed.). Mineral. Assoc. Can., Short Course Vol. 36, 123-150. Viladkar, S.G. (1980): The fenitized aureole of the Newania carbonatite, Rajasthan. Geol. Mag. 117, 285-292. 932 the canadian mineralogist Von Eckermann, H. (1948): The alkaline district of Alnö Island. Sveriges Geol. Unders., Ser. Ca 36. Von Eckermann, H. (1960): Borengite. A new ultrapotassic rock from Alnö Island. Arkiv Mineral. Geol. 2(39), 519-528. Wall, F., Le Bas, M.J. & Srivastava, R.K. (1993): Calcite and carbocernaite exsolution and cotectic textures in a Sr,REE-rich carbonatite dyke from Rajasthan, India. Mineral. Mag. 57, 495-513. Wallace, M.E. & Green, D.H. (1988): An experimental determination of primary carbonatite magma composition. Nature 335, 343-346. Williams-Jones, A.E. & Palmer, D.A.S. (2002): The evolution of aqueous-carbonic fluids in the Amba Dongar carbonatite, India: implications for fenitization. Chem. Geol. 185, 283-301. Woodward, J. & Hölttä, P. (2005): The Naantali alvikite vein-dykes: a new carbonatite in southwestern Finland. Geol. Surv. Finland, Spec. Pap. 38, 5-10. Woolley, A.R. (1969): Some aspects of fenitization with particular reference to Chilwa Island and Kangankunde, Malawi. Brit. Mus. (Nat. Hist.) Mineral. Bull. 2(4) 191220. Woolley, A.R. (1982): A discussion of carbonatite evolution and nomenclature, and the generation of sodic and potassic fenites. Mineral. Mag. 46, 13-17. Woolley, A.R., Williams, C.T., Wall, F., Garcia, D. & Moute, J. (1995): The Bingo carbonatite – ijolite – nepheline syenite complex, Zaire: geology, petrography, mineralogy and petrochemistry. J. Afr. Earth Sci. 21, 329-348. Xie, Y-L., Yin, S-P., Xu, J-H., Chen, W. & Yi, L-S. (2006): A study on the fluids in carbonatite of the Mianning–Dechang REE metallogenic belt. Bull. Mineral. Petrol. Geochem. 25, 66-74 (in Chinese with English abstr.). Yang, X.-M., Yang, X.-Y., Zheng, Y.-F. & Le Bas, M.J. (2003): A rare earth element-rich carbonatite dyke at Bayan Obo, Inner Mongolia, North China. Mineral. Petrol. 78, 93-110. Yang, X.-M., Zheng, Y.-F., Yang, X.-Y., Zhang, P.-S. & Le Bas, M.J. (2000): A geochemical study of a REE-rich carbonatite dyke at Bayan Obo in Inner Mongolia, Northern China. Acta Geol. Sinica 74, 605-612. Yuan, Z., Bai, G. & Zhang, Z. (2000): Trachytic rock and associated fenitization in the Bayan Obo ore deposit, Inner Mongolia, China. Acta Geol. Sinica 74, 148-153. Yuan, Z., Bai, G. & Zhang, Z. (2004): Autometasomatic phenomena of host rocks in the Bayan Obo ore deposit and their significance. Acta Petrol. Mineral. 23, 1-7. Zaitsev, A.N., Demény, A., Sindern, S. & Wall, F. (2002): Burbankite group minerals and their alteration in rare earth carbonatites – source of elements and fluids (evidence from C–O and Sr–Nd isotopic data). Lithos 62, 15-33. Woolley, A.R. (1987): Alkaline Rocks and Carbonatites of the World. 1. North and South America. Brit. Mus. (Nat. Hist.) London, U.K. Woolley, A.R. (2003): Igneous silicate rocks associated with carbonatites: their diversity, relative abundances and implications for carbonatite genesis. Per. Mineral. 72, 9-17. Received November 27, 2006, revised manuscript accepted October 1, 2007.