Mineralogy and Petrology (2007) 90: 223–248DOI 10.1007/s00710-006-0177-xPrinted in The Netherlands

New evidence from a calcite-dolomitecarbonatite dyke for the magmatic originof the massive Bayan Obo ore-bearingdolomite marble, Inner Mongolia, China

M. J. Le Bas1;�, Y. Xueming2, R. N. Taylor3, B. Spiro4,J. A. Milton3, and Z. Peishan5

1 Department of Geology, University of Leicester, Leicester, UK2 INCO Exploration, Copper Cliff, Ontario, Canada3 National Oceanography Centre, University of Southampton, Southampton, UK4 Department of Mineralogy, Natural History Museum, London, UK5 Institute of Geology, Chinese Academy of Sciences, Beijing, China

Received March 23, 2006; accepted October 2, 2006Published online January 23, 2007; # Springer-Verlag 2007Editorial handling: L. G. Gwalani

Summary

New data on Sr and Nd isotope composition and major and trace element distribution indolomite-calcite carbonatite dykes at Bayan Obo are provided, and a Mid-Proterozoicage is deduced. The dykes and the neighbouring massive dolomite (H8) body havesimilar geochemical characteristics, interpreted to indicate a carbonatitic magmaticorigin. The occurrence of riebeckite-bearing fenitized quartzites marginal to both dykesand H8 dolomite body, and the presence of xenoliths in the latter, supports thisconclusion. Taken together with previously published stable isotope data, these dataconfirm a mantle-derived origin for the H8 body.

The oxygen isotope composition of the dolomite and magnetite in the dykes is lowerthan that in the fine-grained dolomite. Oxygen data from samples of the coarse-graineddolomite host are either similar to the dykes or to the fine-grained type in agreementwith their other geochemical characteristics. The carbonate-magnetite thermometricpairs of the fine-grained dolomite indicate a range of 350–540 �C, which is probablylower than that of the original main magmatic emplacement. This supports the dis-tinction made between the original coarse-grained dolomite marble and dyke com-position from the later fine-grained dolomite.

� Currently at National Oceanography Centre, University of Southampton, Southampton, UK

Thus the large H8 dolomite is interpreted as a carbonatite intrusion that containswall-rock xenoliths and caused fenitization of the hanging wall, foot wall and thexenoliths, and that the coarse-grained portions of the H8 marble are those portions that,in the Late Proterozoic to Palaeozoic, escaped recrystallization to fine-grained dolomiteand subsequent REE-Fe mineralization.

Introduction

The origin of the massive H8 dolomite marble host to the giant Fe-REE ore depositat Bayan Obo, Inner Mongolia remains controversial. Chao et al. (1997), Campbell(1999) and Artz et al. (2005) provide evidence for a sedimentary origin, whereasLe Bas et al. (1992, 1997), Rankin et al. (2003) and Yuan et al. (1992, 2003) havepresented evidence for an igneous carbonatitic origin. Other contributions thatreview the origin of the Bayan Obo rocks are by Wu et al. (1996) and Campbelland Henderson (1997). This contribution focuses on the geochemistry of a dolo-mite-calcite dyke cutting quartzites, on the fenites formed, on the relationship toother dykes, on the adjacent controversial H8 dolomite marble and the possiblecommon genesis, but not on the origin of the ore deposit, a later event. For thisreason, samples were collected from as far away from the ore bodies as possible,usually a kilometre or more.

The evidence for a sedimentary origin of the H8 marble is: (a) that the massivedolomite rock occurs within the Mid-Proterozoic Bayan Obo Group of sediments,between shales on top and quartzites, shales and limestones below; (b) that it isapparently stratiform, being >10 km along strike, 1–2 km thick; (c) that it containsclusters of apatite and magnetite crystals that are often elongate and parallel to theapparent stratification; and (d) that the cathodoluminescent properties of the apatitehave been interpreted to indicate a sedimentary origin.

In the Le Bas et al. (1992) paper, it was argued that the bulk chemical similarityof the H8 dolomite marble to the carbonatite dykes nearby, indicated a carbonatiticorigin of the marble, but as that paper uncritically accepted local knowledge pre-sented to us in 1988 that H8 was a component of the sedimentary sequence of thearea, the only conceivable conclusion was that it must, although unlikely, be amassive carbonatite tuff deposit. Subsequent field work, together with O-C-Sr iso-tope and trace element geochemistry, indicated to Le Bas et al. (1997) that the H8dolomite marble was a large intrusion of apatite dolomite carbonatite caught upand deformed within the thrust pile of Bayan Obo sediments, and that the apparentstratification was the product of tectonically induced plastic flow well-recognizedin many marbles and carbonatites (Garson, 1955). Furthermore, the presence ofabundant clusters of apatite and magnetite is atypical of sedimentary carbonaterocks although exceptions are known (Le Bas et al., 2002), but they are a char-acteristic of carbonatites (Beccaluva et al., 1992). Also, the amphibole-rich fenitesdeveloped at the contact of the H8 dolomite with the host quartzites (Liu, 1985;Yang and Le Bas, 2004) indicate a carbonatitic intrusive character for the coarse-grained H8 dolomite marble.

This contribution examines new data of the Zhang dyke, so named after ZhangPeishan who discovered it, also the fenites and the element and Nd-Sr-O isotopegeochemistry of the coarse and finely crystalline portions of the H8 dolomite

224 M. J. Le Bas et al.

marble away from mineralized area, and discusses the relationship of the dykes tothe H8 dolomite marble.

The term ‘dolomite’ can be ambiguous being both a mineral and a sedimentaryrock. Leaving out the limestones and dolomites north of the Kuangou Fault (Fig. 1)that are sedimentary in origin, the H8 dolomite marble south of the Fault was alsooriginally interpreted as a sediment that had been dolomitized and recrystallized to,petrographically, a coarse-grained dolomite marble (Philpotts et al., 1991; Chaoet al., 1997). This coarse-grained H8 dolomite marble is now shown petrogeneti-cally to be an igneous body in common with other dolomite carbonatites, e.g.Newania (Viladkar et al., 1986) confirming previous investigations of Le Baset al. (1997).

Geology

The Zhang dyke lies at N 41� 48.50; E 109� 57.60 (located by GPS) approximatelyone kilometre NW of the Main Ore Body at Bayan Obo and about 100 m north ofthe contact with the coarse-grained H8 dolomite marble at locality 1 (Fig. 1). It is

Fig. 1. Map of the Bayan Obo area, Inner Mongolia, modified after Drew et al. (1990)showing the long outcrop of the H8 dolomite marble, the localities 1 and 2 detailed in thetext and the location of the borehole (BH). Some limestones and dolomites are includedamong the Hu sediments, which were correlated by Drew et al. (1990) with the H8 rocks

Calcite-dolomite carbonatite dyke at Bayan Obo 225

exposed for sixteen metres in length and one metre width with slight pinching andswelling along the length, and dips N at 70�. It cuts H4 quartzite that strikes E–Wsub-parallel to H8 contact further down the mountainside. Initially it was consid-ered that the dyke was an offshoot from the adjacent main H8 dolomite marble(now better interpreted as a dolomite carbonatite) but it may be just another mem-ber of the local carbonatite dyke swarm (Le Bas et al., 1992). A vertical 1 m widecarbonated hornblende trachyte dyke striking NE–SW cuts the Zhang dyke. Asecond area of the coarse-grained H8 dolomite marble lies at locality 2 (Fig. 1).

In both areas, each some hundreds of metres wide, the dolomites are coarselycrystalline right up to the sharp contacts with H4 quartzite. To the south, outcrop islost beneath alluvium and the next outcrop is the fine-crystalline H8 dolomitemarble described by Chao et al. (1997).

Adjacent to the Zhang dyke, and to the H8 dolomite marbles at both localities,the H4 quartzites are fenitized with the strong development of blue amphibolefibres.

Petrography

The four samples collected across the Zhang dyke (Fig. 2a) are all texturallysimilar in thin section and, apart from the Fe-oxides, show few signs of alteration.On the basis of the mineralogy, they are identified as carbonatites. No fine-grainedmargins were observed.

Dyke sample P3.45 taken 5 cm from the N margin of the dyke is coarse-grainedand slightly coarser than the rest of the dyke, and is composed almost wholly oftwinned calcite plates with triple junctions. A few altered relicts of Fe-oxidesoccur.

Dyke samples P3.47 and 48 are texturally similar except that P3.47 includesirregular bands of subhedral apatite grains. P3.48 is wholly composed of twinnedferroan dolomite crystals with smooth contacts and triple junctions, and clusters ofsecondary Fe-oxides. The ferroan dolomite grains in P3.47 are similar, but adjacentto and within the apatite bands are small Sr-calcite crystals revealed by microprobeanalysis. Cutting the apatite bands are dark-rimmed veins of strained quartz withsome calcite.

Dyke sample P3.52, taken from the S margin of the dyke, is coarse- to medium-grained. Dolomite occurs as individual and clusters of crystals with ragged marginsentrained in calcite crystals, and the texture suggests a calciocarbonatite magmahaving intruded and broken up an earlier dolomite carbonatite. The calcite showstwinning, sutured crystal margins and poorly developed triple junctions. Smallpyrochlore and apatite crystals occur, as do rare red-brown Fe-oxide pseudomorphsafter magnetite octahedra. In most respects, the calcitic portion of P3.52 appearstexturally similar to calcite carbonatite P3.45.

The rock hosting the dyke is a feldspathic quartzite, the H4 member of theBayan Obo sedimentary sequence (Fig. 1). The quartzite P3.51 collected 122 cmfrom the dyke and some 80 m from the main H8 dolomite marble outcrops to thesouth, is arkosic with abundant, minute, rounded, quartzite pebbles and plagioclaseclasts (? oligoclase) set in a recrystallized quartzite matrix. The quartz crystalsshow undulose extinction. The pebbles and clasts are surrounded by clusters of

226 M. J. Le Bas et al.

blue-grey sodic amphibole prisms. These prisms together with apatite grains alsooccur within the quartzite matrix. Much interstitial fluorite is present. The quartzitehas evidently suffered metasomatic alteration, recognizably fenitization.

The quartzite, P3.46, collected only 6 cm from the dyke shows greater altera-tion. Many quartz grains are elongated with 3:1–4:1 ratios, are recrystallized andrarely show undulose extinction. The few plagioclase grains interlock with thequartz grains. Sodic amphibole abounds in trails of blue-grey prisms and fibresbetween the quartz grains together with iron-stained calcite, stumpy prisms ofapatite and interstitial fluorite.

The hillside outcrops, 100 m south of the dyke (Fig. 2b), of the massive H8dolomite marble in contact with the H4 quartzite do not constitute a ‘‘sill-likebody’’, as mistakenly recorded by Campbell (1999), but form the northern margin

Fig. 2. (a) Sketch of the Zhang carbonatite dyke and adjacent fenitized H4 quartzite atlocality 1, showing the locations of the samples analysed; (b) sketch map of the H4=H8contact area at locality 2, showing location of samples studied and strike=dip of theintrusive contact and the H4 bedding

Calcite-dolomite carbonatite dyke at Bayan Obo 227

(locality 1 in Fig. 1) of the extensive H8 dolomite marble body that extends severalkilometers ENE–WSW (Chao et al., 1997; Le Bas et al., 1997). On this hillside,the H8 dolomite marble makes intrusive and penetrating contact into the H4 quartz-ite, typical of an intrusive igneous contact (Fig. 2b). It is coarse-grained, but on thenext hill to the south, and across a few hundred metres of unexposed terrane, therock out-cropping is the fine-grained H8 dolomite marble that constitutes the bulkof H8 (Fig. 1). Similar intrusive relationships were also reported by Yang andLe Bas (2004). These relationships are not new observations, having been pre-viously recorded by Liu (1985) with detailed field observation that H8 makes anintrusive contact with the quartzite, includes xenoliths of the quartzite and pro-duces a sodic amphibole reaction margin with both. Liu (1985) goes further todescribe the intrusive contact of the H8 dolomite marble into H9 slate, with achilled margin 10–20 cm wide and sodic amphibole produced in the reaction rimin the slate. The Wu dyke, named and described in Yang et al. (2003), similarly hasan amphibole-rich fenitic reaction rim at its contact with slate.

B239, B240 and B241 are all coarse-grained H8 dolomite marbles comprisingmainly dolomite with triple junctions, some sub-prismatic apatite crystals, magne-tite and occasional pyrochlore grains. In the samples closer to the fenitized quart-zite contact, there are increasing quantities of scattered blue sodic amphiboleneedles evidently derived from the fenite (Yang and Le Bas, 2004).

Collected one metre and ten metres to the north of the contact are fenitesamples B242 and B243 (Fig. 2b). They are petrographically similar to the quart-zite P3.46 described above, with B242 showing strong development of the bluesodic amphibole replacing the quartz grains, and B243 lesser amounts. B245 wascollected from an angular xenolith at least 15�10 m2 in the H8 dolomite marble30 m south of the H8–H4 contact (Fig. 2b). It showed an even stronger develop-ment of blue amphibole replacing quartz.

The H8 dolomite marble (88=150) next to the quartzite contact at locality 2(Fig. 1), described and analysed by Le Bas et al. (1992), is formed of coarse-grained triple-junctioned dolomite crystals with abundant Ti-poor magnetite(preliminary analysis showed <1.5 wt% TiO2) and apatite crystals, plus minorpyrochlore, fluorite, sodic amphibole and occasional veinlets with monazite-(Ce)and barite.

The fine-grained H8 dolomite marble is composed of deformed and stretchedferroan dolomite caused by mylonitization and recrystallization of the coarse H8dolomite marble, sometimes producing alternating thick and thin bands simulatingsedimentary strata (Chao et al., 1997). Apatite is less common than in the coarseH8 dolomite marble, lensoid clusters of iron oxides are common and interstitialREE minerals, mostly monazite, commonly abound. The localities and analyses ofthese samples are given in Le Bas et al. (1997).

Analytical techniques

Carbonate and apatite chemical compositions were analyzed on a Cameca SX50wavelength-dispersive electron microprobe at the Natural History Museum,London at 15 kV and 10 nA. Amphibole compositions were determined at 15 kVand 1 nA on a Hitachi S2500 SEM equipped with a Link AN10=55S energy-

228 M. J. Le Bas et al.

dispersive analysis system also at the Natural History Museum, London, as detailedby Yang et al. (2000).

For XRF analysis, pressed pellets made at 15 tonnes per square inch wereprepared and subsequently analysed with the Phillips PW1400 X-ray fluorescencespectrometer at the University of Leicester using Rh and W tubes. The trace ele-ments Rb, Sr, Zr, Y, Nb, Ta, Hf, Pb, Th, U and REE were measured on a VGPlasmaquad 2þ ICP-MS at the National Oceanography Centre at SouthamptonUniversity. Precision on concentrations is better than �6% 2 s.d. and better than�2% 2 s.d. on element ratios.

Sr and Nd isotope ratios were measured on a VG Sector 54 mass spectrometerat the National Oceanography Centre, Southampton using>100 multidynamic ratios.Errors quoted on isotope ratios are within run 2 s.e. on the final quoted figures. Aver-age values of the SRM 987 Sr standard around the run period are 0.710245� 14(2 s.d. n¼ 12; relative to 86Sr=88Sr¼ 0.1194) and for the JNdi Nd standard are0.512106� 9 (2 s.d. n¼ 11; relative to 146Nd=144Nd¼ 0.7219). Values presented inTable 6 are uncorrected results. Details of the analytical methods for the isotopes aregiven in Le Bas et al. (2004), Yang et al. (2003) and Taylor and Nesbitt (1998).

Carbonate samples were prepared for oxygen stable isotope analysis accordingto the method of McCrea (1950) and magnetite separates according to Clayton andMayeda (1963). Measurements were carried out in a VG Optima dual inlet massspectrometer. Results are reported with respect to VSMOW with an overall analy-tical precision of 0.1ø.

Mineralogy

Carbonates

Microprobe analyses of the ferroan dolomite in the dolomite carbonatites (Table 1)show the CaO=MgO ratio of the dolomite in P3.47 and P3.52 remains unchangedacross the dyke and the same as that in the coarse-grained H8 dolomite marble nearby.However, the dolomite and calcite in the calcite carbonatite at the margin of thedyke do show a decrease in FeO and MnO. No zoning is evident from microprobetraverses of the calcite, nor from the cathodoluminescence that appears orange-redin common with the calcite in other Bayan Obo carbonatite dykes (Le Bas et al.,1992), and in carbonatites worldwide. Calcite in carbonatite usually does show zon-ing unless the original texture has been modified by recrystallization (Barker, 1993).

The high SrO content of the carbonate minerals, shown in Table 1, is typical ofcarbonatites, and that SrO is greater than MnO in the calcites but less than MnO inthe dolomites is indicative of relatively little fractionation (Clarke et al., 1994;Yang and Le Bas, 2004). The calcite of P3.45 includes small blebs of dolomitein optical continuity and some minute grains of Sr-bearing barite. On the inter-pretation that the dolomite is exsolved from the calcite, the geothermometer ofPowell et al. (1984) indicates a temperature of <300 �C.

Analyses of the dolomite in two samples, B241 and 88=150, of the coarse-grained H8 dolomite marble are included in Table 1. The ferroan dolomite in thedyke is identical, within error margins, to the dolomite in the H8 samples takenfrom localities 1 and 2 (Fig. 1).

Calcite-dolomite carbonatite dyke at Bayan Obo 229

Apatite

Three fluorapatite crystals were analysed in P3.47, two unzoned and one zoned(Table 2). The differences are slight. All are, relative to apatite in common igneousand metamorphic rocks, rich in SrO, REE and Na2O, as is characteristic of carbon-atites (B€uuhn et al., 2001; Hogarth, 1989). Under cathodoluminescence (CL), theapatites in P3.47 show blue, unlike apatite from common igneous and metamorphicrocks. Blue is characteristic of carbonatites (Hogarth, 1989), and the apatites in thecoarse-grained H8 dolomite marble also luminesce blue (Le Bas et al., 1992; andanalysis B241 in Table 2). In the zoned apatite (Table 2), the central area lumi-nesces blue and is surrounded by a zone slightly enriched in Na2O and REE thatshows green luminescence, and that zone is rimmed by grey-blue luminescentapatite deficient in Na2O and REE (Table 2). The type 1 of the four CL-charac-terized types of apatites in the mineralized H8 dolomite marbles at Bayan Obostudied by Campbell and Henderson (1997) also luminesces green, but the grey-blue rims observed here find no correspondence with the other CL identified byCampbell and Henderson (1997), unless the slight intermediate rise in Na2O andREE is correlated with the similar but stronger enrichment in their type 2 apatite.

Amphiboles

Two quartzites near the Zhang dyke and three near the contact with the H8 dolo-mite marble were microprobe-analyzed to see if the blue, zoned amphiboles show

Table 1. Comparative individual analyses and average compositions of dolomite and calcite

Dolomite P3.47 av. n¼ 8 st. dev. P3.52 av. n¼ 4 st. dev. P3.45 B241av 26�

88=150av 19�

MgO 17.80 17.91 0.5 18.06 17.88 0.52 19.92 18.34 17.66CaO 29.77 29.48 0.25 29.71 28.95 1.13 30.68 28.86 28.68MnO 0.67 0.73 0.06 0.78 0.77 0.03 0.25 0.67 0.68FeO 3.73 3.86 0.14 3.80 3.63 0.20 1.18 3.28 3.59SrO 0.61 0.68 0.10 0.73 0.65 0.26 0.12 0.49 0.29BaO 0.13 0.10 0.05 bd bd bd 0.10 0.13

Total 52.71 52.76 53.08 51.88 52.15 51.74 51.03

Calcite P3.47 av. n¼ 9 st. dev. P3.52 av. n¼ 7 st. dev. P3.45 Other dykes�

MgO bd bd bd bd bd 0.39CaO 54.39 53.7 1.11 54.05 52.49 1.37 54.69 54.55MnO 0.31 0.24 0.15 0.55 0.82 0.34 0.22 1.41FeO 0.22 0.39 0.42 0.36 0.37 0.16 0.16 1.24SrO 0.91 0.61 0.54 1.28 1.71 0.45 0.81 0.53BaO bd 0.06 0.05 bd 0.13 0.09 0.12 0.11

Total 55.83 55.00 56.24 55.52 56.00 58.23

bd Below detectionP3.45 is dolomite bleb and host calcite. La2O3 below detection in all carbonates� Data from Yang and Le Bas (2004)

230 M. J. Le Bas et al.

variable composition with distance from the contact (Table 3). The pairs of anal-yses shown in Table 3 indicate the range of composition found in each sample.The two samples P3.46 and P3.51 showed complete overlap of the twelve compo-sitions analyzed in each except for two Fe-rich compositions found in P3.46. Over-

Table 2. Analyses of fluorapatites in Zhang dyke and H8 dolomites

P3.47 5 4 6 3 7 2 8 1zoning centre centre mid mid mid=edge edge rim rim

CL blue blue blue blue green green grey-blue grey-blueNa2O 0.15 0.23 0.13 0.20 0.28 0.26 0.05 0.05SiO2 bd bd bd bd bd bd bd bdP2O5 41.44 41.80 41.19 40.89 42.22 42.43 42.32 42.43CaO 54.28 54.12 54.02 54.24 53.75 55.17 54.55 55.17MnO bd bd bd bd bd 0.05 0.05 0.05FeO bd 0.10 bd 0.01 0.06 0.01 0.04 0.01SrO 1.31 1.22 1.26 1.21 1.31 1.19 1.49 1.19Y2O3 bd bd bd 0.05 0.07 0.09 0.09 0.09BaO 0.10 bd 0.07 bd bd bd bd bdLa2O3 0.20 0.11 0.14 0.10 0.32 bd bd bdCe2O3 0.42 0.44 0.43 0.29 0.62 0.05 0.14 0.05Nd2O3 0.15 0.06 0.13 0.08 0.23 bd bd bdF 3.28 3.41 3.36 3.29 3.42 3.50 3.36 3.50

Total 100.06 100.10 99.37 99.02 100.85 101.13 100.71 101.13

Less F¼O 1.36 1.43 1.41 1.39 1.44 1.47 1.41 1.47Total 98.67 98.67 97.67 97.63 99.42 99.66 99.3 99.66

P3.47 H8c B241 H8f C&H ’97 1988 dykeunzoned CL blue yellow green blue

av. n¼ 5 st. dev. type 1 av. n¼ 24Na2O 0.18 0.02 0.12 0.09 0.17SiO2 0.02 0.01 bd 0.06 <0.05P2O5 42.10 0.18 41.18 41.68 41.20CaO 54.06 0.20 54.40 56.97 54.10MnO 0.03 0.01 0.09 0.04FeO 0.05 0.02 <0.06 0.05SrO 1.36 0.01 1.18 0.19 1.00Y2O3 0.04 0.02 0.06 0.27BaO 0.04 0.03 0.10 0.07 naLa2O3 0.09 0.04 0.10 0.08Ce2O3 0.40 0.02 0.18 0.17 0.28Nd2O3 0.13 0.03 <0.11 0.24F na 2.87 3.26 3.00

Total 98.51 100.28 102.49 100.43

Less F¼O 1.20 1.37 1.25Total 99.08 101.12 99.18

CL Cathodoluminescence; na not analysedH8c H8 dolomite coarse-grained; H8f H8 dolomite fine-grained

Calcite-dolomite carbonatite dyke at Bayan Obo 231

all, the deep blue to pale blue zoning observed in most crystals analysed corre-sponded to the progressive increase in the Mg=Fe ratio shown in Fig. 3. TheNaþK content remains constant. The three samples B242, B243 and B245 adja-cent to H8 similarly show overall increase in Mg=Fe, with complete overlap of

Table 3. Pairs of sodic amphibole compositions in fenites and distance from contact

Location P3.46-4 P3.46-7 P3.51-14 P3.51-2 B242-14 B242-1 B243-78 B243-88 B245-40 B245-42Distance 0.06 m 1.22 m c. 2 m c. 10 m c. 0.5 m

SiO2 54.00 55.37 55.00 55.57 54.26 54.06 55.09 55.21 56.36 56.51TiO2 0.09 0.18 0.10 0.21 0.12 0.18 0.27 0.25 0.04 0.22Al2O3 0.58 0.51 0.53 0.48 0.60 0.43 0.57 0.22 0.69 0.54FeO 26.27 19.92 22.98 19.34 17.46 15.97 20.74 19.98 16.88 14.56MnO 0.17 0.19 0.44 0.44 0.42 0.79 0.37 0.40 0.14 0.16MgO 7.06 11.30 9.35 11.40 11.94 12.55 9.16 11.34 14.02 15.49CaO 0.30 1.22 0.32 0.68 0.94 1.13 0.82 0.91 0.86 1.90Na2O 7.22 6.88 7.16 7.28 7.08 7.15 7.43 7.26 8.09 7.66K2O 0.33 1.52 0.63 1.17 1.19 1.20 1.11 1.89 0.93 1.14F na na na na 0.63 0.71 0.65 0.69 1.01 1.05LessF¼O

– – – – �0.27 �0.30 �0.27 �0.29 �0.43 �0.44

Total 96.02 97.09 96.51 96.57 94.37 93.87 95.94 97.86 98.59 98.79

Ions on basis of 23(O)Si 8.012 8.017 8.013 8.004 8.033 8.036 8.206 8.009 7.934 7.924Ti 0.010 0.020 0.011 0.023 0.013 0.020 0.030 0.027 0.004 0.023Al 0.101 0.087 0.091 0.081 0.105 0.075 0.100 0.038 0.114 0.089Fe(3) 1.619 1.249 1.621 1.406 1.233 1.155 0.784 1.208 1.362 1.175Fe(2) 1.674 1.165 1.179 0.983 0.928 0.832 1.799 1.216 0.625 0.531Mn 0.021 0.023 0.054 0.054 0.053 0.100 0.047 0.049 0.017 0.019Mg 1.562 2.439 2.031 2.448 2.635 2.783 2.034 2.452 2.942 3.238Ca 0.048 0.189 0.050 0.105 0.149 0.180 0.131 0.141 0.130 0.285Na 2.077 1.931 2.023 2.033 2.032 2.062 2.146 2.042 2.208 2.053K 0.062 0.281 0.117 0.215 0.225 0.228 0.211 0.350 0.167 0.204

Name riebeckite mg-rieb mg-rieb mg-rieb mg-rieb mg-rieb mg-rieb mg-arf mg-arf mg-arf

mg-rieb Magnesio-rebeckite; mg-arf magnesio-arfvedsonite

Fig. 3. Central part of (NaþK)–Fe–Mgtriangular plot showing progressive zon-ing with increase of Fe in the riebeckiticand arfvedsonitic amphiboles developedin the fenitized H4 quartzites, with minordiagram showing position within com-plete triangle

232 M. J. Le Bas et al.

B242 and 243 amphiboles and B245 extending to higher Mg=Fe ratios. They alsoshow slightly higher NaþK contents (Fig. 3). Table 3 shows that as MgOincreases, so does the F content, a feature noted in other carbonatite dykes at BayanObo (Le Bas et al., 1992).

Optically identical blue amphiboles also occur as occasional trains of smallprisms in the coarse H8 dolomite adjacent to the contact with the fenitized quart-zite, as observed by Liu (1985).

Figure 4 shows that the zoned compositions converge to magnesio-arfvedso-nite, similar to those as also found for the fenitic sodic amphiboles near Inver-ness, Scotland (Garson et al., 1984), at Newania, Rajasthan, India (Viladkar andWimmenauer, 1986), at Loe Shilman, NW Pakistan (Mian and Le Bas, 1986), atAln€oo, Sweden (Morogan and Woolley, 1988), and at the 1988 dyke of Bayan Obo(Le Bas et al., 1992). This direction of convergence is consistently found only ingranitic and other silica-rich rocks – each of the above examples. In more Mg-richfenitized source rocks such as at Chipman Lake, Ontario (Platt and Woolley, 1990),magnesio-arfvedsonite is again the ultimate composition but with decreasingMg=Fe ratio (Le Bas and Srivastava, 1989). These observations are independentof whether the carbonatite causing the fenitization is calcitic or dolomitic.

The cause of the two trends A and B shown in Fig. 4 is not clear. Both are in H4quartzite and it might be the response to compositional change within the quartziteor variation in the composition of the fenitizing fluids. It is thought more likely thatthe difference may be the result of differing temperatures attained during fenitiza-tion and the duration of the fenitizing process. Both the temperature and durationof fenitization adjacent to a dyke would be lower and shorter than that obtainedalongside a major igneous intrusion, the latter leading to more intense fenitization.A similar process was described by Yang et al. (1999) in their study of apatites incoarse-grained dolomite marble, and in carbonatites dykes where it was noted thatthe former were more intensely affected by fluids bearing fluorine and REEs thanthe latter.

Fig. 4. Differing trends of zoning shown by the Strong and Taylor (1984) plot for amphi-boles showing differing mainly alkali contents along trends A and B that converge onmagnesio-arfvedsonite

Calcite-dolomite carbonatite dyke at Bayan Obo 233

Magnetite

The iron oxides in the samples were not specifically micro-probe analysed but itwas noted in one case that TiO2 content was low (2 wt% or less).

Geochemistry

Major and minor elements

The four Zhang dyke samples analysed vary from calcio-carbonatite with <1%MgO to magnesiocarbonatite with >14% MgO (Table 4). Apart from an increasein Fe2O3 in the more MgO-rich rocks and the change in the CaO=MgO ratio acrossthe dyke, there is little change in other major oxides. The trace element distributionshown in Fig. 5a is closely similar to that established for average C1 carbonatite(Le Bas, 1999) and therefore may be considered the normal pattern of carbonatiteswith the exception that in some cases the REE, Nb and P contents are lower thanusual. The scatter of Nb and P shown in Fig. 5a is taken to reflect the differingpyrochlore and apatite contents (the ‘nugget effect’) between the carbonatite sam-ples, a feature that has been described and seen in carbonatites elsewhere e.g. atBailundo, Angola, Chilwa, Malawi, and Chishanya, Zimbabwe (Woolley, 2001).The trace element distributions for the two samples from the H8 dolomite marble(88=150 and B241), although separated by 3 km, are almost identical and confirmthe uniformity of the coarse-grained H8 dolomite marble previously demonstrated(Le Bas et al., 1992), and the traces differ little from the Zhang dyke average(Fig. 5b) apart from peaks in Nb and P.

Also plotted on Fig. 5b are two samples of mildly fenitized quartzite, one of H4(B243 in Fig. 2b) and one of H1 quartzite (90=51) taken near the Wu carbonatitedyke described and analyzed in Yang et al. (2003). The relatively higher contents ofNa, Ba, Th, Nb, REE, higher than those normally encountered in quartzites, pro-vide evidence for the composition of the fenitizing fluids, which were notablydeficient in Sr. The high Na2O content of B243 (Table 4) indicates the fenitizationis a Na-metasomatism made evident by the presence of blue sodic amphiboles.This is distinct from the K-metasomatism that has occurred in the H9 slate whichcaps and roofs H8 and both ore bodies. That cap-rock comprises a fenitic rockwholly composed of nearly pure K-feldspar; here microcline, with up to 16.6 wt%K2O, i.e. Or98. The K-feldspar cap-rock above the East ore body is also penetratedlocally by a two metre wide ‘enclave’ of albitite (mainly composed of albite withup to 11.8 wt% Na2O, i.e. Ab99, plus a scatter of magnesio-arfvedsonite needles,analysis of which is close to that in Table 3.

Rare earth elements (REE)

The overall REE content in the dyke increases from magnesiocarbonatite to cal-ciocarbonatite (Fig. 6), and the REE pattern for H8 dolomite marble (88=150) isclose to the average for the dyke. This increase could indicate that the magnesio-carbonatite was formed before the calcio-carbonatite, assuming that REEs increasewith fractionation, but the accompanying decrease in La=YbN ratio could be inter-

234 M. J. Le Bas et al.: Calcite-dolomite carbonatite dyke at Bayan Obo

Table 4. Compositions of carbonatites across the Zhang dyke, H8 dolomite and fenite

Sample P3.45 P3.47 P3.48 P3.52 H8c B241 H4 B243locality N margin 32 cm from S 10 cm from S S margin qtzite

SiO2 [wt%] 0.30 1.60 0.48 2.02 2.72 78.13TiO2 bd bd bd bd 0.01 0.15Al2O3 2.72 5.53 2.89 3.75 0.21 5.35Fe2O3 0.52 4.33 3.67 2.26 5.78 2.53MnO 0.38 0.69 0.77 0.47 0.78 0.48MgO 0.84 13.75 14.89 7.03 14.76 1.30CaO 45.45 27.17 28.25 38.17 26.25 3.83Na2O 0.3 0.18 0.11 0.26 bd 3.58K2O 0.41 0.02 bd 0.03 0.07 0.31P2O5 bd 0.61 0.05 0.05 2.38 0.07

Total 50.52 53.48 51.11 54.04 52.96 95.73

S [ppm] 84 47 38 76 na naF 1000 789 436 1004 na naCl 16 bd 12 29 na naSc 30 33 27 34 18.2 3V 19 20 14 19 8.7 16Cr bd bd bd 9 7.9 33Co 9 bd 1 19 20.1 4Ni 2 bd bd 7 0.2 3Cu 26 20 8 28 na naZn 11 32 29 26 30.1 55Rb 6 2 3 3 0.1 3Sr 10037 3857 4392 5646 3988.8 49Y 57 5 5 36 20.2 8Zr 1 0.6 bd 0.5 1.0 56Nb 19 7.7 0.4 39 472.8 14Ba 1492 63 99 569 116.7 322

La 189 29 31 71 99.5 21Ce 402 58 67 165 217.8 44Pr 39 5.5 6.8 17.7 na 3.6Nd 143 19.3 24.6 69 109.6 13.1Sm 20 2.7 3.3 12.6 na 2.7Eu 5.3 0.64 0.8 3.6 na 0.83Gd 14.2 1.8 2.09 9.4 na 2.6Tb 1.8 0.2 0.24 1.3 na naDy 9.4 0.96 0.96 6.5 na 2.3Ho 1.76 0.16 0.16 1.17 na naEr 4.3 0.48 0.38 2.85 na naTm 0.55 0.05 0.03 0.36 na naYb 3.58 0.29 0.34 2.2 na 0.94Lu 0.55 0.07 0.06 0.33 na 0.15Th 13.8 0.8 1.5 1.9 10.5 1.5Pb 9.3 2 1.7 8.9 9.3 11U 0.6 0.2 0.07 0.7 0.5 0.2Hf 0.1 bd 0.03 0.1 na naTa bd bd 0.04 0.06 na na

La=YbN 35.3 66.9 60.9 21.6 – 14.8Eu=Eu� 0.97 0.89 0.93 1.02 – 0.96

preted as providing contrary evidence. The total REE content and La=YbN ratio ofboth dyke and H8 are unusually low for carbonatites but close to the acceptedrange (Woolley and Kempe, 1989). They are comparable, for example, to thosein the carbonatites of Spitskop, South Africa (Ionov and Harmer, 2002). Althoughlow for carbonatites, the REE content is considerably greater than in sedimentarycarbonate rocks (McLennan, 1989) but addition of REEs during metasomatism is apossibility, especially near the ore bodies.

Surprisingly, the light REE content of the fenitized quartzites is little differentfrom that in the two REE-poor members of the Zhang dyke (Fig. 6) apart fromH1.90=51 still showing the negative Eu anomaly common in sediments. The flatterREE pattern, La=YbN is only 14.8 (Table 4), reflecting the original flat REE pattern

Fig. 5. (a) Trace elements distribution normalized to ‘pyrolite’ of McDonough and Sun(1995), showing general increase in trace element contents from the dolomite carbonatite tocalcite carbonatite in the Zhang dyke, apart from Nb and P; (b) comparison of averageZhang dyke with H8 and the fenites

236 M. J. Le Bas et al.

of the quartzite and the lesser mobility of the heavy REEs. This provides evidencefor the greater mobility of the lighter REEs under fenitizing conditions.

Oxygen isotopes

Table 5 presents the oxygen isotope data of H8 dolomite and magnetite fromoutcrops and the ZK88 borehole and from carbonatite dykes. The �18OVSMOW

values of dolomite range from 9.6 to 15.0ø, with the fine-grained H8 marblehaving values higher than some of the coarse-grained H8 marble, whereas the�13CVPDB values remain similar (Le Bas et al., 1997).

The �18O values of magnetite of some of the east and Wu dykes (described inYang et al., 2003) are �2.7 to �2.4ø and are lower than those of magnetite in thefinely crystalline dolomite (�1.8 to 1.9ø); also two of the �18O values of the dykedolomite are lower than those of the dolomite in the fine and coarsely crystallinerocks. Overall, the dolomites from the ZK88 borehole have lower �18Odol valuesthan the fine dolomite samples whilst the �18O values of magnetite are higher thanthose of the magnetite in the fine dolomite. The carbon isotopic compositions ofcarbonatite dykes have values no different from those of the H8 dolomite marble(Le Bas et al., 1992, 1997; Yang et al., 2003).

Figure 7 presents �18O of the calcite=dolomite and magnetite vs the difference�18Ocalcite=dolomite-magnetite which, for equilibrium fractionation, is a measure oftemperature (calculated after Chiba et al., 1989, see Table 5). Regression linesfor the calcite=dolomite and magnetite from the fine and ZK88 fine-grained dolo-mites intersect at 6ø at �18Odol-mt¼ 0ø. This would be the value of the originalwater in equilibrium with both minerals at very high temperatures, which lies

Fig. 6. Rare earth element distribution pattern normalized to C1-chondrite values(Rollinson, 1993) showing: (a) the increasing total REE content from dolomite-rich tocalcite-rich members of the Zhang dyke; (b) the similarity of the REE contents of thefenitized quartzites to the earlier members of the Zhang dyke dolomite carbonatite (apartfrom Dy and Yb determinations in H4 243); (c) the negative Eu anomaly, characteristic ofmany sediments, still evident in fenitized quartzite H1 90=51

Calcite-dolomite carbonatite dyke at Bayan Obo 237

within the primary igneous composition field (Reid and Cooper, 1992). The regres-sion lines are interpreted as representing equilibrium fractionation as there is asystematic relationship between the actual values for the carbonates and the mag-netite and the difference between mineral pairs. Several samples of this study notbelonging to the finely crystalline dolomite deviate from these lines. The coarselycrystalline 88=152 and 90=62 magnetite and dolomite have lower �18O values thanthe finely crystalline dolomite for the same �18Ocalcite=dolomite-magnetite valueswhereas samples B222 and B241 are similar to the fine grained dolomite. Themagnetite and carbonate samples from the dykes 88=141e, 88=142e and90=45w, deviate from the regression lines while the carbonate and magnetite valuefor 90=39 fall within the array of the fine-grained dolomite marble. The magnetiteof the dykes has a narrow range of �2.7 to �1.6ø whilst their respective carbo-nates vary greatly 2.6 to 14.0ø. Regression lines through these two arrays intersect

Table 5. Oxygen isotope data (ø) for carbonates and magnetite in H8 and dykes

Sample cc=dol mt T (�C)

B222 c 12.2 1.4 467B241 c 11.6 1.7 50188=152 c 9.6 �1.2 46790=62 c 10.5 �2.6 39888=117 f 13.0 �1.2 40288=147A f 12.1 0.0 42688=147B f 12.1 �0.1 42688=147X f 12.1 0.1 42688=148 f 13.8 �1.1 38288=157 f 14.7 �1.7 36090=58 f 13.1 �1.6 39990=64 f 15.0 �1.8 35588=140 e – �1.8 –88=141 e 8.8 �2.4 54688=142 e 2.6 �2.4 124390=39 w 14.0 �1.6 37790=45 w 11.9 �2.7 432

Data: Inst. Geochem. Chinese Acad. 1988ZK88-1 11.2 �0.3 453ZK88-2 10.7 0.1 470ZK88-3 11.0 �0.5 460ZK88-3B 11.2 0.4 453ZK88-7 12.1 1.0 426ZK88-8 11.4 1.2 447ZK88-10 11.9 1.7 432ZK88-11 11.4 1.3 447ZK88-13 11.1 1.9 457ZK88-17 11.0 0.9 460

Sample locations in Le Bas et al. (1997)c coarse-grained H8; f fine-grained H8; e east dyke; w Wu dykeTemperatures calculated from Chiba et al. (1989)

238 M. J. Le Bas et al.

at �2.2ø at �18Odol-mt¼ 0ø. It seems therefore that these samples have a dif-ferent origin from the fine-grained dolomites and their respective magnetites. Asthere is, at present, no suitable O isotope geothermometer for the dolomite-mag-netite pair, and because the oxygen fractionation between carbonates is only slight(Bottinga and Javoy, 1973), we can use the calibration of Chiba et al. (1989) forcalcite-magnetite pair to approximate the equilibrium temperatures of carbonateminerals and magnetite. The calculated temperatures (Table 5) are 340–501 �C forthe samples from the H8 dolomite marble, and 377–546 �C for the carbonatitedykes. One exception is for sample 88=142 from a calcite carbonatite dyke with�18O carbonate of 2.8ø, which yielded unreasonable high temperature up to1243 �C. This sample differs slightly in its 87Sr=86Sr ratio (Le Bas et al., 1997).

Fig. 7. Oxygen isotope composition of carbonate and magnetite samples plotted againstthe difference between respective values of the carbonate-magnetite mineral pairs.Regression lines through the values for the fine-grained dolomites including the ZK 88series intersect at a value of 6ø. The mineral pairs from the dyke form different arraysthat intersect at �2ø. The coarse-grained samples are similar either to the fine-graineddolomites (B222, B241) or to the dykes (88=152, 90=62), while the regression line for thefine-grained dolomite seems to represent equilibrium fractionation and hence temperature(see Table 5). The temperatures calculated from the mineral pairs of the dykes areplausible, however the reason for the constancy of the respective magnetite values is notfully understood

Table 6. Nd and Sr isotope data for Zhang dyke

P3.45 P3.47 P3.48 P3.52

87Sr=86Sr (measured) 0.702867� 9 0.702906 � 8 0.702975� 8 0.703845� 8143Nd=144Nd (measured) 0.511627� 4 0.511570 � 5 0.511590� 18 0.511797� 4

Rb 6 2 3 3Sr 10037 3857 4392 5646Nd 143.0 19.3 24.6 69.0Sm 20.0 2.7 3.3 12.6

Calcite-dolomite carbonatite dyke at Bayan Obo 239

Compared with the values of unaltered carbonatites (6.5–9.5ø) deduced by Deines(1989), the �18OSMOW values of these samples are higher. Furthermore the calcite-magnetite geothermometer gave temperatures for various carbonatites, higher than

Fig. 8. (a) 87Sr=86Sri vs "Nd(t¼ 1280) for the Zhang dyke. Error bars shown. (b) 147Sm=144Nd vs 143Nd=144Nd for Bayan Obo dykes. An isochron for the ore body samples fromZhang et al. (1994) is shown for comparison. Errors bars are smaller than the symbol size

240 M. J. Le Bas et al.

the temperatures from Bayan Obo where dolomite predominates, for example Oka(745–770 �C), Magnet Cove (700–760 �C) and Jacupiranga (700 �C) (Haynes et al.,2003).

Sr and Nd isotopes

The Sr and Nd isotopes (Table 6) indicate that the Zhang dyke was mantle-sourced(87Sr=86Sr¼ 0.70287–0.70298). P3.52 (0.70384) taken from the south margin ofthe dyke differs from the rest (Fig. 8a), the difference possibly being the result ofcontamination by feldspar from the wall rock and incorporation of radiogenic Sr.The 147Sm=144Nd–143Nd=144Nd systematics of the Zhang dyke are fairly concor-dant with those for the Wu dyke (Zhang et al., 1994), as might be expected sincethey are considered to be in the same dyke swarm (Fig. 8b). They are also close tothose of the H8 dolomite in the bore-hole near the western ore bodies (Zhang et al.,1994), and have broadly similar "Ndt¼ 1280 (�1.7 to �2.8) suggesting that all thecarbonatite intrusions were derived from a similar mantle source.

The Zhang dyke measurements are insufficient to constrain the age, especially assample P3.52 appears to be contaminated, but if the data are combined on the Zhanget al. (1994) isochron with the data for the Wu dyke, a date of 1157� 160 Maemerges (Fig. 8b), and together they lie at a slightly lower angle to the H8 bodyisochron (1290� 96 Ma). Ren et al. (1994) also give a ca. 1300 Ma date for the orebody, while Philpotts et al. (1991) and Chao et al. (1997) give dates that range1400� 200 Ma for the Bayan Obo H8 dolomite marble and initial mineralization.

The imprecise nature and overlap of these data make it impossible to decidewhether the dykes followed or preceded the emplacement of H8 dolomite marbleintrusion. They must be considered as broadly contemporaneous. More certain isthat both were succeeded by further phases of late Proterozoic and early Palaeozoicmineralization, ca. 800 and 400 Ma, as outlined by Chao et al. (1997).

Discussion

The distinction of Pre-Cambrian carbonatites from marbles of sedimentary originis a constant problem and source of controversy. The H8 dolomite marble at BayanObo does have the sedimentary character of occurring as a layer within a stratiformsequence of quartzites, shales and limestones, as outlined above. The igneouscharacter of the H8 dolomite marble is however indicated by its intrusive contactrelationships shown above, its mineral content described above, and its coarse-grained texture that is typical of carbonatites. The intrusive relationships couldhowever, relate only to some sill components within it, and it could be argued thatthe remainder of the H8 body was sedimentary but that recrystallization hadobscured evidence of original contacts – an unlikely proposition.

The high Sr content of ca. 1% in the calcite and ca. 0.5% in the ferroandolomite (Yang and Le Bas, 2004) in the Zhang dyke, together with the high Srcontent and blue CL of the apatite (Table 2), leave little doubt that the dyke is acarbonatite and not some rheomorphosed dolomite marble, despite the lack ofappreciable apatite, magnetite and pyrochlore in the dyke; these minerals are nor-mally present in carbonatite and are abundant in the H8 dolomite marble. The

Calcite-dolomite carbonatite dyke at Bayan Obo 241

coarse-grained H8 dolomite marble similarly has high Sr content in its dolomiteand apatite, and blue CL in the latter. The common factor of sodic metasomatism,producing amphibole-rich fenites at the quartzite contact zones of both the Zhangdyke and H8 dolomite nearby, as well as adjacent to the other members of thecarbonatite dyke swarm at Bayan Obo, again points to an igneous carbonatitesource, and the oxygen, carbon and strontium isotope data for the coarse-grainedH8 dolomite marble provided by Le Bas et al. (1997) further confirm the igneousorigin.

Rankin et al. (2003) give also evidence for its possible magmatic source basedon fluid inclusion geochemistry (SEM-EDS, Laser Raman, etc.) within the H8dolomite. They found that in some rocks the fluid (¼melt) compositions wereenriched in REE which they related to hydrothermal activity, but in other rocksthe melt compositions were poor in REE and were best related to carbonatiticmagma compositions. With all the features described here for H8, it can no longerbe interpreted as a sedimentary rock and it is better referred to as the H8 dolomitecarbonatite.

To the east and west of the main ore bodies, Le Bas et al. (1997) recordedfurther minor areas of coarse-grained H8 dolomite carbonatite (not shown onFig. 1) that appear in the field as groups of outcrops within the vast area dominatedby outcrops of the fine-grained H8 marble described by Chao et al. (1997). Someof the areas of coarse-grained H8 dolomite carbonatite show tectonic deformation,with the clusters of apatite and of magnetite being drawn out into cataclasticlensoid shapes resembling flow, a process long recognised (Garson, 1955). Thereis little doubt that the fine-grained H8 dolomite is the result of recrystallization ofcoarse-grained H8 dolomite as deduced by Chao et al. (1997). The result of thisinterpretation is that the original extent of coarse-grained H8 dolomite carbonatitemust have been vast, making it by far the largest carbonatite in the world, some15 km long and a kilometre wide or thick. This is difficult to believe with mostcarbonatites rarely exceeding 1 km diameter, but Bongo in Angola is one thatexceeds 3 km diameter (Lapido-Loureiro, 1973). Instead of a single intrusive mas-sive body, Wang et al. (2002) proposed intrusion of several sub-horizontal sills,later changed into a synform by tectonic deformation, but no internal contacts havebeen reported.

As shown above, some of the H8–H4 contacts are intrusive but the H8 mass asa whole is sandwiched within the Bayan Obo Group of Proterozoic sediments,comprising low grade metamorphosed limestones, quartzites and shales stackedinto a pile of thrust sheets, and including a minor ophiolite. The thrusting wasthe result of the Solonker orogeny along the Solonker suture on the northern edgeof the North China craton that closed in the end-Permian, although some thrustingdid continue into the Triassic-Jurassic (Xiao and Windley, 2003).

The question remains of whether the H8 dolomite carbonatite and the carbo-natite of the dykes belong to a single magmatic event or two separate events.General contemporaneity must be concluded, with the majority of dates givenfor the H8 rocks being greater than ca. 1300 Ma whilst the few dates relating tothe dyke rocks mostly lie in the 1200–1300 Ma bracket (Chao et al., 1997). How-ever, a recent contribution by Fan et al. (2002) on the U–Pb dating of a zircontaken from a carbonatite of the dyke swarm, gave an upper intercept date of

242 M. J. Le Bas et al.

2070� 33 Ma, but it is argued that the zircon in the carbonatite dyke is a con-taminant from the adjacent Wutai gneiss (Le Bas, 2006).

Comparison of the bulk geochemistry of the H8 and dyke rocks does showmuch in common between the two. The trace element distribution patterns aresimilar (Fig. 5b) apart from Nb and P noted above, and the REE content of H8falls close to the average for the dyke (Fig. 6), but the chondrite-normalizedLa=YbN ratio for H8 is 81 which is greater that of the dyke (average 46). If it istaken that the La=YbN ratio normally increases in any inter-relationship betweenmelts, then the dyke should precede the H8 intrusion, but the continuity of thecoarse-grained texture right up to the margins to the dyke could suggest that thedyke was intruded into county rocks warmed by the intrusion of H8. Generalcontemporaneity must again be concluded.

The fewer apatite and pyrochlore in the dyke relative to the abundance in theH8 dolomite carbonatite may indicate a difference in the F=OH ratios of the twomelts. Mitchell and Kjarsgaard (2004) have shown that carbonate melts with highF=OH ratios favour the precipitation of pyrochlore, whereas F deficient melts donot. Purple fluorite is visibly present (Le Bas et al., 1992) in some H8 outcropsindicative of a high F=OH ratio, but whether the F is primary or associated with thewidespread Bayan Obo mineralization is not evident. The dyke shows only ca.1000 ppm F (Table 4).

The presence of calcite carbonatite in the dyke as well as dolomite carbonatitepresents problems. The textural data for the Zhang dyke indicate that intrusionbegan with emplacement of magnesiocarbonatite magma into warm country rocks(no chilled margins are evident), and that the final phase of emplacement wasmarked by formation of calciocarbonatite. No internal boundaries are evidentwithin the dyke but they are not always visible unless there is a change in grainsize, which was not evident here. This sequence of emplacement finds support inthe increasing REE contents but not in the decreasing La=YbN ratio.

Whether or not magnesiocarbonatite magma can fractionate to calcio-carbona-tite, or the reverse, is briefly discussed below. Replacement of dolomite is anotherpossibility. Fieldwork has indicated that dolomitic and sideritic carbonatites wereusually intruded or emplaced later than calcite carbonatites (e.g. Garson, 1965; LeBas, 1977; Harmer, 1999) but, with dolomitic rocks, the distinction between intru-sion and replacement can be difficult to make, as pointed out by Barker (1993).

Lee and Wyllie (1998a) investigated experimentally the possibility of fractiona-tion and came to the conclusion that ‘‘phase relationships in CaCO3–MgCO3 areconsistent with generation of crustal magnesiocarbonatite magmas by fractionationof calciocarbonatite parents’’. But in the presence of water, their experimentsshowed that hydrous dolomitic carbonatite magma could precipitate cumulates€oovite (Lee and Wyllie, 1998b), and so produce a calcio-carbonatite rock but notcalciocarbonatite melt. However, Keller (1989) has described chilled calcio-carbo-natites thought to represent the composition of such a melt.

Harmer and Gittins (1997) argue cogently that fractionation of magnesiocar-bonatite magma escaping from the mantle could produce calcitic carbonatites ascumulates or crystal mushes that, on solidification, would become s€oovites. Wyllieand Lee (1998) likewise deduce from experimental work that at depths between 70and 40 km, carbonate magma can differentiate from magnesiocarbonatite to cal-

Calcite-dolomite carbonatite dyke at Bayan Obo 243

ciocarbonatite. This would find broad agreement with the observed field and tex-tural relationships that H8 came first together with magnesiocarbonatite in thedykes, with a later phase of calciocarbonatite magma in the dykes. The oxygenisotope composition of the carbonatites supports the trace element geochemistryand Sr and Nd isotopes indication of the magmatic origin of the H8 dolomite.Furthermore they agree with the distinction between the dykes and coarse-graineddolomite not affected by mineralization and the fine-grained dolomite which isaffected by the process that takes place at lower temperature as is also indicatedby the fluid inclusion microthermometry (Smith and Henderson, 2000) and theREE composition of the fluid inclusions (Fan et al., 2004). The difference in theoxygen isotope systematics between dykes and fine-grained dolomite host is re-flected in lower �18O values of carbonate and magnetite for the same �18Odol-mt

values. The parent fluid for the dykes would have a lower �18O value than the H8dolomite, which may be due to the lowering of �18O of the residual fluid a result ofthe crystallization of the H8 carbonates.

Conclusions

1. The Zhang dyke comprises magnesio- and calciocarbonatites that contain fewof the minor minerals apatite, magnetite and pyrochlore that characterize othercarbonatites at Bayan Obo, but have the typical Na-amphibole fenites developedat the contacts.

2. The magnesiocarbonatites of the Zhang dyke have REE contents well towardsthe lower end of the range common for carbonatites.

3. The dyke and the intrusion of the coarse-grained H8 dolomite carbonatite arebroadly contemporaneous.

4. The overall mineralogical and geochemical similarity of the Zhang dyke andcoarse-grained H8 dolomite carbonatite indicate a common mantle source.

5. The fine-grained H8 dolomite may have a similar source, but evidence forthat is obscured by the subsequent recrystallization and overprinting REE-Fe mineralization.

6. The oxygen carbonate-magnetite geothermometer indicates formation of the H8dolomite carbonatite in the range 340–501 �C from a magmatic fluid, whereasthe East and Wu dykes have a slightly different oxygen isotope systematicspossibly reflecting subsequent formation.

7. Sodic amphibole-bearing fenites occur in the quartzite at the contact of the H8dolomite carbonatite and in quartzite xenoliths in the H8 dolomite carbonatite,as well as in the contacts with the Zhang dyke.

8. Zonation in the amphiboles from riebeckite to magnesio-riebeckite to magne-sio-arfvedsonite is attributed to degrees of fenitization by carbonatite magma.

Acknowledgements

The authors thank Dr. L. S. Campbell and Prof P. Henderson for collecting the samples of theZhang dyke, Dr. F. Wall and John Spratt (Natural History Museum, London) for the electronmicroprobe analysis of the minerals, P. Boella (Southampton University) for the preparationof samples for ICP-MS and Sr and Nd isotope analysis, Dr. N. Marsh (Leicester University)

244 M. J. Le Bas et al.

for XRF analysis and Dr. A. R. Woolley, Dr. F. Wall and Prof P. Henderson for usefuldiscussions that materially improved this contribution, and to the referees Prof K. Bell andDr. XiaoYong Yang for valuable criticisms.

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Authors’ addresses: M. J. Le Bas (corresponding author; e-mail: m.lebas@btinternet.com),Rex N. Taylor, J. A. Milton, National Oceanography Centre, School of Ocean and EarthScience, University of Southampton, Southampton SO14 3ZH, UK; Yang Xueming, INCOExploration, Highway 17 West, Copper Cliff, Ontario, POM 1NO Canada; B. Spiro, Miner-alogy Department, Natural History Museum, SW7 5BD London, UK; Zhang Peishan, Insti-tute of Geology, Academia Sinica, PO Box 9825, Beijing 100029, China.

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