Contrib Mineral Petrol (1996) 122: 368–386 C Springer-Verlag 1996

Jingfeng Guo ? Suzanne Y. O’ReillyWilliam L. Griffin

Corundum from basaltic terrains:a mineral inclusion approach to the enigma

Received: 15 March 1995y Accepted: 30 June 1995

Abstract This paper investigates the origin of corun-dum megacrysts that occur in many basaltic terrains, andwhich are considered to be eroded from basaltic rocks.Geochemical data for over 80 primary mineral inclu-sions within corundum megacrysts are used to gain a newinsight into the petrogenetic history of the corundummegacrysts. A wide spectrum of minerals is present asinclusions in the corundum; the most common areNb2Ta oxides (such as titaniferous columbite and uran-pyrochlore), alkali feldspar, low-Ca plagioclase (albite-oligoclase) and zircon. Rare inclusions include Fe,Cu-sulphide (low in Ni), cobalt-rich spinel, Th,Ce-richphosphate and uraninite. The similar chemistry of someinclusion minerals from corundum occurring in widelyseparated areas suggests that the corundum megacrystsin basalts have a similar petrogenesis. Geochemicalcharacteristics of the inclusions indicate a bimodalgrouping, which is best explained by a mixing-hybridisa-tion process. This study indicates that the corundummegacrysts are not cogenetic with their basaltic hostsbut are crustal fragments accidentally incorporated intothe erupting magma. It is suggested that interactionsbetween a silicic component and an intruding carbon-atitic or similar Si-poor magma is responsible for Al-oversaturation, resulting in locally distributed lensesof corundum-bearing rock of mixed paragenesis (“ hy-brid rock hypothesis ”). Feldspar exsolution texturesprovide strong evidence that this hybridisation occurredat mid-crustal levels. Subsequent volcanic eruptionsbrought the corundum-bearing rocks (later disintegratedin the magma) up to the Earth’s surface. This petrogenet-

ic model for corundum megacrysts is experimentallytestable.

Introduction

Most of the world’s current sapphire production is takenfrom recent or palaeo-alluvial deposits in basaltic ter-rains sparsely distributed in eastern Australia, Thailandand eastern China. The lack of understanding of corun-dum genesis and the geology ofcorundum-bearing par-ent rocks in basaltic terrains has greatly limited thesearch for new economic deposits.

There is an empirical relationship between the occur-rence of alluvial sapphire deposits and the presence ofbasaltic rocks in certain volcanic provinces worldwide,and basaltic rocks have been considered the source of thecorundum. Examples include eastern Australia (e.g.MacNevin 1972; Stephenson 1976; Coldham 1985;Krosch and Cooper 1990), eastern China (Keller andWang 1986; Guo et al. 1992), the Indochina region ofsoutheastern Asia (e.g. Vichit et al. 1978; Barr and Mac-Donald 1978), Nigeria (Kiefert and Schmetzer 1987),and Scotland (Upton et al. 1983; Aspen et al. 1990).Weathering and erosion of basaltic lavas, pyroclastics,plugs and diatremes, and subsequent alluvial processesresulted in economic deposits of the gem-quality varietysapphire in eastern Australia, eastern China, the In-dochina region of southeastern Asia (Thailand, Vietnamand Cambodia) and northern Africa (Kenya and Nige-ria). Notwithstanding these observations, petrologicalevidence suggests that corundum could not crystallisefrom basaltic magmas (e.g. Green et al. 1978).

However, apart from the empirical observation andexperimental evidence above, few studies have been car-ried out to elucidate the origin of the alluvial corundumas well as the corundum megacrysts found in basalticrocks. Many aspects of the “ sapphire enigma ” are stilluntouched although corundum clearly forms part of thewell-documented megacryst suite in alkalic basalticrocks. A knowledge of corundum genesis may improve

J-F. Guo (✉) ? S.Y. O’ReillyKey Centre for Geochemical Evolution and Metallogenyof Continents (GEMOC),School of Earth Sciences, Macquarie University,Sydney, NSW 2109, Australia

W.L. GriffinCSIRO Division of Exploration and Mining,PO Box 136, North Ryde, NSW 2113, Australia

Editorial responsibility: J. Touret

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our understanding of the whole megacryst suite. Thisforms the subject of the present investigation.

Occurrence

Kopecky et al. (1967) were among the fist to reportcorundum occurring as discrete crystals in diatremes ofkimberlitic and carbonatitic affinity; they were consid-ered to be xenocrysts. Reports of in situ occurrence ofcorundum in basaltic rocks (and volcanic rocks in gener-al) have been rare. MacNevin (1972) documented rare insitu occurrences of corundum megacrysts in basalt fromthe Inverell-Glen Innes region, northern New SouthWales, Australia. These corundum megacrysts showmany similarities to the corundum recovered from thenearby alluvial deposits. Stephenson (1976) reported theoccurrence of corundum-anorthoclase compositemegacrysts and a number of euhedral corundum crystalsin basalts of the Hoy Volcanic Province in centralQueensland, Australia.

At two localities in Scotland, Loch Roag in the westand Ruddon’s Point of Fife in the east, corundum-bear-ing xenoliths are found in alkali olivine basalt host rocks(Upton et al. 1983; Aspen et al. 1990). The Scottishcorundum forms part of a cogenetic assemblage domi-nated by alkali feldspar; other phases include zircon,magnetite, ilmenorutile, yttro-niobate, and probably bi-otite and apatite. Many large euhedral corundum crystalshave been extracted directly from basaltic lava flows atChangle, China (Guo et al. 1992). At this locality, corun-dum is concentrated in massive alkali basalt flows thatlocally contain abundant ultramafic xenoliths (formingup to 30–40% by volume), and there appears to be apositive correlation between the abundance of xenolithsand of corundum (Fig. 1 a). The gem-quality corundum

recovered from the surrounding alluvial materials clear-ly is derived from the breakdown of the corundum-bear-ing basalts (Guo et al. 1992). Recently, a unique, largecorundum aggregate (5 cm32 cm) was found in situ inbasaltic scoriaceous lava matrix in the Chudleigh Vol-canic Province, northern Queensland (M. Zhang and O.Gaul, personal communication 1994); the host basaltcontains numerous peridotite xenoliths.

In several basaltic terrains, corundum crystals arefound in the weathered residuals or soil profiles lyingdirectly above the basaltic plugs, lavas or pyroclasticrocks; these include localities of the Inverell-Glen Innesregion, NSW, Australia (MacNevin 1972), Lava Plains,northern Queensland, Australia (Krosch and Cooper1990), Bo Ploi, Thailand (Bunopas and Bunjitradulya1975), Wenchang, Hainan, China (Wang 1988; Guo1993) and Luhe, Jiangsu, China (Guo 1993). Similarcorundum occurrences were reported at the Pailin rubyysapphire deposit, Cambodia (Jobbins and Berrange´1978).

Alluvial corundum is equivalent to the corundummegacrysts found in situ in basalts in many aspects (Guo1993). This confirms the empirical observation of acorundum-basalt association. The similarities are:1. They are all coloured stones ranging from very dark

blue (nearly black), through blue, green to less com-mon yellow.

2. They have similar trace element fingerprints, whichare distinctly different from those of corundum fromnon-basaltic terrains (Guo et al. in preparation).

3. Their crystal habits and surface features are directlycomparable. The corundum megacrysts are typicallyof barrel-shaped habit (Fig. 1 a). The alluvial corun-dum commonly occurs as broken fragments, but someof these preserve original crystal faces and shapesidentical to those of the megacrysts. Both themegacrysts and the alluvial corundum exhibit surfacefeatures typical of high temperature corrosion(Fig. 1 b).

Indirect evidence also supports the megacryst origin ofthe alluvial sapphires, e.g.i. Some alluvial corundum exhibits only minor me-

chanical abrasion along the crystal edges and theoriginal surface features are well preserved, which

Fig. 1 a A deep blue corundum megacryst in the matrix of alkalibasalt from Changle, Shandong Province, China; the yellow frag-ments adjacent to the megacryst are altered peridotite xenoliths.b SEM image of the surface of a corundum from Lava Plains,Queensland, Australia, showing irregular corrosion patterns re-sulting from the corundum-basalt interaction

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Table 1 List of corundumsamples examined in thisstudy

Country Stateyprovince Area of sampling Longitude Latitude

Australia New South Wales Grabben Gullen 149825’E 34832’SNew South Wales Mittagong 150827’E 34826’SNew South Wales Kings Plains, Inverell-Glen Innes 151805’E 29847’SQueensland Anakie-Rubyvale region 147844’E 23835’SQueensland Lava Plains 144848’E 18829’S

China Hainan Wenchang 110820’E 19850’NFujian Mingxi 117815’E 26825’NJiangsu Luhe 118845’E 32820’NShandong Changle 118850’E 36840’N

Thailand Kanchanaburi Bo Ploi 90830’E 14810’N

USA Arizona San Carlos 110830’W 33825’N

Kenya Mangari (Uncertain)

demonstrates that the alluvial corundum has not beentransported far from the source rock.

ii. In the Inverell-Glen Innes region, it is observed thatrivers draining from small basaltic bodies carryheavy minerals of corundum, zircon, spinel and gar-net whereas those cutting across granite bodies andyor metamorphic sequences transport dominantlyquartz and zircon with very little (if any) corundum.

iii. A geological and geophysical study on the Tertiaryvolcanics and sapphires in the New England districtof New South Wales revealed that sapphire-bearingbasaltic pyroclastics are widespread at the base of theTertiary volcanic piles in the Inverell-Glen Innes re-gion, and are considered to represent a period of in-tense maar-type eruption marking the onset of Ter-tiary basaltic volcanism in this area (Lishmund 1987;Pecover 1987).

Corundum megacrysts in basalts are apparently not inequilibrium with the host rock and commonly show reac-tions with the magma, resulting in etched and roundedsurfaces, sometimes with a reaction rim of black spinel.Such disequilibrium has been confirmed by petrologicalexperiments (Green et al. 1978), which suggest thatcorundum could not crystallise from basaltic magmasunder realistic geological conditions. The genetic rela-tionship between corundum and basalt has thereforebeen enigmatic.

Methodology

The chemistry of corundum is very simple and in isolation pro-vides very limited information on petrogenesis. Rarely, corundumhas been found intergrown with other minerals (e.g. Stephenson1976; Upton et al. 1983). However, many corundum crystals con-tain inclusions which are a potential source of genetic informationabout the parent rock.

Inclusions in a mineral may preserve valuable informationabout the chemical environment, physical conditions and the geo-logical processes that affected the host mineral at the time of itsformation. This type of investigation has been employed in dia-mond studies and has generated important genetic informationabout diamonds (e.g.Meyer and Boyd1972;Meyer1987; Gurney1989). The presence of mineral inclusions within corundum there-fore allows the use of a similar approach to examine the genesis of

corundum. This investigation is based on the identification andquantitative analysis of the mineral phases that coexisted with andwere enclosed in corundum during its crystallisation. More than1000 corundum crystals, grains and fragments were examined forthis study; more than 80 inclusions were identified. The mineralparagenesis and the chronological relationship between individualminerals in principle can be used to identify the source character-istics and the processes involved in the generation of corundum.

Samples and preparation

Corundum grains used in this study are mainly from five areasfrom Australia, four areas from China and one area in Thailand(Table 1). Other samples from Mangari, Kenya and San Carlos,USA also were examined. Most of these corundum samples arealluvial; a few were recovered directly from basaltic rocks.

Corundum samples were first examined under a binocular mi-croscope and those with inclusions were mounted in epoxy, fol-lowed by careful cutting using a fine diamond wheel. When theinclusions appeared close to the cut surface, the samples wererepeatedly ground on 15-micron corundum powder until the min-eral inclusions were exposed at the surface. This lengthy processensures the largest inclusion exposure and minimal loss of therelatively soft inclusions. The impregnated corundum sampleswere then cut into cubes and fixed in one or more epoxy discs forefficient micro-beam analysis. Subsequent polishing utilised 6-mi-cron, 3-micron and 1-micron diamond pastes, producing a goodquality polish on both the corundum hosts and the much softermineral inclusions.

Analytical methods

ETEC electron microprobe

An ETEC electron microprobe equipped with a LINK energy dis-persive spectrometer system (EDS) at Macquarie University wasused to identify and quantitatively analyse the inclusions. The ac-celerating voltage was set at 15 kV and the electron beam on thespecimen was focused down to a 4–5mm diameter spot. The beamcurrent was calibrated on pure cobalt metal with a specimen cur-rent of 50 nA being used for routine quantitative analysis. Eachspectrum was collected during a 100-second counting time. Anybeam drift was corrected by remeasuring the beam current for 5seconds immediately after the spectrum acquisition. The built-inZAF correction programme of the LINK EDS system was used forquantitative calculation of element concentrations.

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der of formation relative to the host gemstones. The pro-togenetic inclusions are those originating before the for-mation of the enclosing crystal, the syngenetic inclusionsare those formed simultaneously with the crystallisationof the host crystal, and the epigenetic inclusions arethose developed after the formation of the host crystal. Itis the syngenetic and protogenetic inclusions that bearmost of genetic information about the host mineral. Pro-togenetic and syngenetic inclusions are sometimes diffi-cult to distinguish from each other but both are “ prima-ry ” relative to the host mineral. The descriptions in thefollowing text include only the primary inclusions.

Mineral inclusions

Earlier descriptive studies of gem corundum (sapphireand ruby) from basalt fields worldwide have identified awide spectrum of mineral inclusions, such as zircon, ap-atite, albite, spinel, Zn-rich spinel, pyrrhotite, pyrite,calcite, dolomite, phlogopite, muscovite, biotite, tour-maline, graphite, columbite, pyrochlore and almandinegarnet (Gübelin 1983; Gübelin and Koivula 1986). In thepresent study, more than 80 mineral inclusions wereidentified; they are mainly oxides, followed by silicate,phosphate (minor) and sulphide (minor) minerals. Thetypes of mineral inclusions in each sample suite aregiven in Table 2 and summarised in Table 3 and Fig. 2.

Cameca CAMEBAX autoprobe

Selected mineral inclusions were analysed using the CamecaCAMEBAX automated electron microprobe at the CSIRO Divi-sion of Exploration and Mining; the probe is equipped with fourwavelength dispersive spectrometers, which enables four elementsto be analysed simultaneously. Two of these spectrometers areequipped with PET and LIF crystals, a third with PET and TAP, andthe fourth is equipped with TAP and ODPB. A high voltage of15 kV was used with a stable beam current of 20 nA (measured byFaraday cup at the sample position, and regulated to better than+0.1%). In order to analyse elements at lower levels, the countingtime for background was extended to the same value as that usedon the peak. Prior to quantitative analysis, the background in thevicinity of the analytical peak was investigated to examine thepresence of any potential interferences from other elements. Thiswas carried out for each element of interest by step-wise scanningof the spectrometers from20.1 sinu to 10.1 sinu of the analyticalpeak position. Most scans were made with an accelerating voltageof 20 kV and a beam current of 50 nA to ensure efficient X-rayexcitation. By this method, many elements in a silicate matrix canbe determined at concentration levels as low as 100 ppm. Detaileddescriptions of analytical and calibration procedures are given byRamsden and French (1990). The standards for each element wereas follows: F (CaF2), Na (jadeite), Mg (olivine), Al (kyanite), Si(wollastonite), P (apatite), Ca (wollastonite), Ti (rutile), Mn (Mnmetal), Fe (hamatite), Y (Y metal), Zr (zirconymetal), Nb (Nbmetal), Sn (cassiterite), Hf (Hf metal), Ta (Ta metal), Th (ThO2),and U (UO2). For REE-bearing phases, rare earth elements (REE)were included in the analytical matrix: La (LaB6), Ce (CeO2), Pr(PrSi), and Nd (NdSi), Sm (Sm metal).

HIAF proton microprobe

The recently developed proton microprobe in the CSIRO HeavyIon Analytical Facility (HIAF) laboratory is used to analyse quan-titatively minor and trace element contents in the inclusions. Thebeam spot on the sample surface is regulated to approximately20–30mm in diameter. The X-rays produced by the proton beambombardment were screened by Al- andyor Be-filters and thencollected by a Si(Li) energy dispersive detector. The use of anAl-filter combined with an additional Be-filter attenuates thecontinuum background dramatically and enables a precise analysisof elements with Z.(Cl). Details of the facility and data reduc-tion methods are given in Griffin et al. (1988) and Guo et al.(1994).

Inclusion categorisation

For descriptive purposes, inclusions are generally cate-gorised as solid, liquid, vapour inclusions, and single-,double-, triple- as well as multi-phase inclusions withina mineral. This categorisation has been widely used inthe study of fluid inclusions to characterise theP-T-Xrelationship of the geochemical system concerned.

In this paper, we consider only the mineral inclusionswithin the corundum samples. Mineral inclusions may beclassified on their genetic links with the host mineral:primary, pseudo-secondary and secondary inclusions,though the genetic information needed to apply this clas-sification is frequently difficult to obtain as pointed outby Roedder (1984). In the study of inclusions within avariety of gemstones, Gübelin (1983) employed termssuch as “ protogenetic ”, “ syngenetic ” and “ epigenetic ”to group inclusions according to their chronological or-

Table 2 Identified mineral inclusions

Area Nb2Ta oxides Silicates Others

Kings Plains, Columbite Zircon SulphideNSW Uranpyrochlore Albite Uraninite

K-feldspar BrockiteQuartza

Mt Leura Columbite Feldspar(Anakie), IlmenorutileQueensland Uranpyrochlore

BetafiteSamarskite

Lava Plains, Columbite ZirconQueensland Ilmenorutile Feldspar

Bo Ploi, Albite Co-rich spinelKanchanaburi

Wenchang, Columbite ZirconHainan Ilmenorutile Albite

Muscovite(?)

Mingxi, Columbite K-feldspar SpinelFujian Ilmenorutile

FersmiteSamarskite

Luhe, Nb2Ta melt (?) Feldspar SpinelJiangsu

Changle, Columbite ZirconShandong Oligoclaseb

a Quartz identified only from decomposed zirconsb Taken from Ding (1991)

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Table 3 Summary of mineralinclusions in corundum frombasaltic terrains

Group Mineral species Comment

Oxide Columbite, ilmenorutile, pyrochlore, 1) Nb-Ta oxides dominant.fersmite, samarskite, uraninite, Co- 2) Fe-Ti oxide and hematite asrich spinel, Fe-Ti oxide, hematite, exsolved products, i.e. non-baddeleyite, quartza primary

3) Baddeleyite from zircondecomposition, non-primary

Silicate Zircon, albite, K-feldspar, 4) Mullite and sillimanite frommuscovite(?), mullite, sillimanite, zircon decomposition, non-

primary

Sulphide Pyrrhotite

Phosphate Brockite

a see note of Table 2

Columbite is the most common mineral in the groupand occurs in corundum from all Australian and Chineselocalities except Grabben Gullen and Mittagong. Due tothe limited number of corundum samples from areasother than the Australian and Chinese localities, fewinclusions were found in corundum from those areas.Electron microprobe analyses show that the columbiteinclusions from widely separated areas have a narrowrange of composition, suggesting a common petrogenet-ic process (Table 4). Their compositions are very distinc-tive: high TiO2, (up to 17% TiO2) low Ta2O5 (generallyless than 4%), low but constant MgO (0.3–1.2%). Thesecompositions are similar to those of columbites fromcarbonatite-related intrusions, and readily distinguish-able from columbites occuring in granites and graniticpegmatites (low TiO2, variable Ta2O5 and lower MgO;Fig. 4).

Uranpyrochlore, though not as abundant as colum-bite, is a distinctive phase: cubic, reddish to dark brownand commonly surrounded by radial cracks and brown“ haloes ” (Fig. 3 b). The uranpyrochlore inclusions arerich in U, Nb and Ti (Table 4), and are typical of py-rochlores from carbonatites and related alkaline rocks(e.g.Hogarth1989; Hodgson and Le Bas 1992).

Ilmenorutile has the same physical appearance ascolumbite but elongated and is only reported to occur ingranites and granitic pegmatites (e.g. Cerny et al. 1989).Compared with those in granitic rocks, the ilmenorutileinclusions are characteristically rich in Nb (Nb2O5 up to53%), Fe (FeO up to 20%), Al (Al2O3 up to 2.5%) and Zr(ZrO2 up to 4%). They are sometimes intergrown withtitaniferous columbites, forming composite inclusionswithin corundum.

Zircon

Zircon is another common inclusion in corundum. Thezircon inclusions are generally small, from a few to tensof microns but can reach 1 mm across. They generallyhave a short prismatic habit with the {110} prism moredeveloped than the {100} prism (Fig. 3 c, d), indicating

Fig. 2 The relative abundances of the identified 82 primary min-eral inclusions in corundum

Glass inclusions, often associated with megacrysts inbasaltic rocks, are the result of basaltic melt infiltrationduring transport and are not regarded as primary. Thecorundum megacrysts studied appear to be lacking glassinclusions, which may be due to their extraordinaryphysical properties in contrast to silicate minerals suchas pyroxene and amphibole.

Nb2Ta oxides

This is a broad category including all oxides containingNb and Ta as major constituents. The Nb2Ta oxides arethe most abundant mineral inclusions in the corundumgrains studied. They are typically black, opaque, andhave a sub-metallic lustre similar to that of ilmenite(Fig. 3). They range in size from 1–2 microns up to sev-eral millimetres. The recognised phases include colum-bite, pyrochlore, betafite, ilmenorutile, fersmite and sa-marskite. A brief summary of the three most importantNb2Ta oxide phases is given here, and a comprehensiveaccount of the Nb2Ta oxide inclusions will appear in aseparate publication.

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crystallisation from alkaline-peralkaline environments(Kostov 1973). Some zircon decomposition reactions re-sult in pseudomorphs composed of ZrO21mullite1sil-limanite, suggesting the possible presence of quartz inthe primary corundum paragenesis (Guo et al., submit-ted). The zircon inclusions studied here are composition-ally similar to those occurring in the Scottish corundum-bearing xenoliths (e.g. Aspen et al. 1990; Upton et al.1983). These corundum-related zircons form a distinctgenetic group and are identifiable by their geochemicalcharacteristics: high Hf, U, Th, Y and REEs (Table 4).The unusually high contents of these elements are con-

Fig. 3a–f Common mineral inclusions within corundum.a Pris-matic columbite in a Lava Plains blue sapphire.b Two small, cubicpyrochlore grains in a Kings Plains light blue sapphire; the brownhalo and radial cracks are distinctive features.c Colourless zircongrains in a Kings Plains light blue sapphire; note the characteristicradial cracks around zircon.d Several small zircon grains in a LavaPlains sapphire with typical crystal morphology. e Several albitecrystals with contrasting sizes in a Kings Plains yellow sapphire.f Two large albite grains in a Kings Plains sapphire, with thecross-cut fractures typical of feldspar inclusions. All photographsare taken under a binocular microscope with transmitted lightillumination

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Table 4 Representative microprobe analyses of Nb2Ta oxides andzircon inclusions. All samples analysed by the CSIRO CAMEBAXautoprobe (WDS), with all iron recorded as FeO except zircon(Fe2O3). (– below detection limit,NA not analysed)

Area Wenchang Lava Plains Anakie Kings PlainsCorundum HN13 LP32 LRA4 KP17Inclusion Columbite Columbite Pyrochlore Zircon

SiO2 NA 0.01 NA 32.12TiO2 6.62 3.57 11.48 –Al2O3 NA 0.07 – 0.02FeO 15.89 16.59 1.51 0.2a

MnO 4.32 2.99 NA NAMgO 0.34 1.18 NA –CaO 0.01 0.03 5.58 0.02Na2O NA NA 4.27 NANb2O5 66.50 71.98 32.99 –Ta2O5 4.10 2.98 6.58 NAThO2 – 0.01 8.56 0.49UO2 0.17 0.12 21.80 0.84ZrO2 2.51 0.40 0.20 63.78HfO2 NA NA NA 2.88P2O5 NA NA NA 0.34Y2O3 0.08 0.01 0.66 0.34La2O3 NA NA 0.01 NACe2O3 NA NA 0.48 NANd2O3 0.04 NA 0.54 NASm2O3 0.17 NA 0.24 NA

Total 100.77 99.94 94.9 101.04

Calculated cations assuming stoichiometryO 6 6 7 4Si – 0.001 – 0.989Al – 0.005 – 0.001Ti 0.276 0.150 0.708 –Fe 0.738 0.774 0.104 0.046a

Mn 0.203 0.141 – –Mg 0.028 0.098 – –Ca 0.001 0.002 0.490 0.001Na – – 0.679 –Nb 1.669 1.816 1.222 –Ta 0.062 0.045 0.147 –Th – – 0.160 0.003U 0.002 0.002 0.398 0.006Zr 0.068 0.011 0.008 0.957Hf – – – 0.025P – – – 0.009Y 0.002 – 0.029 0.006La – – – –Ce – – 0.014 –Nd 0.001 – 0.016 –Sm 0.003 – 0.007 –

Sum 3.054 3.044 3.980 2.001

a Fe2O3 or Fe31

Fig. 4 The compositional ranges of columbite-tantalite groupminerals occurring in granitic pegmatites compared with thecolumbite inclusions within corundum megacrysts. The pegmatitecolumbite data and the columbite-tapiolite gap are taken fromCerny and Ercit (1985). Thesolid diamondsare two titaniancolumbites from carbonatite-syenite alkaline complexes (Svesh-nikova et al. 1965)

Fig. 5a, b X-ray mapping sketches of a feldspar inclusion in aKings Plains corundum (KP19). The compositional data for thealbite and K-feldspar domains are given in Table 4;b is the sameinclusion ground several microns deeper

transparent grains (Fig. 3 e, f). Most feldspars have thetypical albite crystal habit except for a probable perthiticintergrowth of albite and K-feldspar in a Kings Plainscorundum, which bears important implications for thelast equilibration temperature of the parent rock (Fig. 5).When the exsolved feldspar is rehomogenised (by calcu-lation), an alkali feldspar of composition in the range ofanorthoclase is obtained. Microfractures in the hostcorundum are common around feldspar inclusions, andminute fluid inclusions (pseudo-secondary) are spreadalong the fractures. In some samples, the enclosed indi-vidual feldspar grains tend to be arranged in a plane par-allel to a particular orientation of the host, which is in-

sistent with the crystallisation of zircon from highlyevolved silicic melts, which have undergone extensivefractional crystallisation (Guo et al., submitted).

Feldspar

Feldspar is a common inclusion in corundum in almostall areas studied. The feldspar inclusions are easily iden-tified as most occur as single discrete, euhedral and

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Table 5 Representative mi-croprobe analyses of feldsparinclusions in corundum.Analysed using the LINK(EDS) analyser attached to theETEC probe Macquarie Uni-versity (– below detection lim-it)

Area King Plains Wenchang Changle

Corundum KP19 KP26 KP36 KP61 HN32 A1–2b

Species K-feldsa Albite a Albite Albite Albite Albite Oligoclase

Major elements (wt%)SiO2 64.81 67.43 61.34 65.79 65.72 65.52 65.16Al2O3 18.56 20.45 26.86 21.76 21.94 21.85 22.50CaO – 0.73 1.84 1.42 1.60 0.73 2.79Na2O 0.73 10.97 9.26 10.07 10.09 9.53 9.16K2O 15.50 – 0.45 0.68 0.68 0.58 0.37

Total 99.60 99.58 99.75 99.72 100.03 98.21 99.98

Principal cations calculated on the basis of 8 oxygensSi 2.997 2.957 2.705 2.896 2.886 2.911 2.861Al 1.011 1.057 1.396 1.129 1.136 1.144 1.164Ca – 0.034 0.087 0.067 0.075 0.035 0.131Na 0.065 0.933 0.792 0.859 0.859 0.821 0.780K 0.914 – 0.025 0.038 0.038 0.033 0.021

Sum 4.988 4.981 5.005 4.989 4.995 4.944 4.957

An 0.0 3.5 9.6 6.9 7.7 3.9 14.1Ab 6.7 96.5 87.6 89.1 88.3 92.4 83.7Or 93.3 0.0 2.8 4.0 3.9 3.7 2.2

a Perthitic intergrowth comprising albite and K-feldsparb Feldspar inclusions taken from Ding (1991)

ferred to represent the corundum-melt interface at thetime of crystallisation of the feldspar. The formation offeldspar may be due to either local nucleation on thegrowing corundum surface or to the crystallisation ofmeltyfluid trapped during corundum growth.

Representative microprobe analyses of feldspar inclu-sions show compositions of potassium-rich (K-feldspar)and sodium-rich feldspars (albite-oligoclase) (Table 5).The presence of low-Ca feldspar inclusions appears to bedistinctive. The K-feldspar is Ca-free and has a composi-tion of Ab6.7Or93.3and the plagioclase has a narrow com-positional range An3.5–10.9Ab87–96.5Or0–4.5. Ding (1991)also identified oligoclase inclusions in Changle corun-dum (Table 5). Gübelin and Koivula (1986) documentedplagioclase inclusions in some sapphires and rubies butconsidered the inclusions to be related to basaltic volcan-ism (e.g. Bo Ploi in Thailand, Anakie in Australia andPailin in Cambodia). Although no chemical data are pro-vided by Gübelin and Koivula (1986), the crystal habitsand comparison with the feldspar inclusions recognisedin this study suggest their feldspars are sodic plagioclas-es.

Sulphide

Sulphides were found in only two corundum samples,from Kings Plains (KP1) and Lava Plains (LP19). Sam-ple KP1 is a large and very pale blue stone with colourzonation; the blue colour bands arenarrow, concentricand parallel to the crystal faces, representing progressivegrowth of corundum (Fig. 6 c). Sulphide inclusions in

KP1 are primary small droplets, generally less than10mm across, arranged parallel to the colourygrowthbandings of the host corundum. Some smaller sulphidedroplets are associated with healed fractures para-con-centric to the larger droplets, the “ fingerprint ” texturereferred to by fluid inclusion researchers. Electron mi-croprobe analysis shows that these sulphide droplets areFe,Cu-bearing with detectable Zn (Table 6). The Ni con-tent is below the analytical detection limit in contrast tothe Ni-rich sulphides commonly occurring in basalticrocks. The detected Al2O3 is probably due to contamina-tion from the enclosing corundum during the analysis.The sulphide droplet inclusions in LP19 are up to 1 mmacross and have been thoroughly oxidised, resulting inspongy “ blebs ” (Fig. 6 a, b). Electron microprobe analy-sis reveals that these blebs contain the elements Fe, S, Si,Al, Na, Cu and Zn in descending order of characteristicX-ray intensity, and are probably a mixture of predomi-nantly hematite (Fe2O3), FexS and other components.

The presence of sulphides as inclusions in corundummegacrysts indicates S-saturation of the parent melt inthe course of crystallisation. The spatial arrangement ofthe sulphide droplets in the host corundum KP1 is astrong indication that the parent melt achieved local S-saturation at the crystal-melt interface as corundum con-tinually grew, similar to sulphide melt inclusions withinclinopyroxene megacrysts (e.g. Andersen et al. 1987).The difference between the sulphide inclusions in corun-dum and pyroxene megacrysts lies in their chemistry.The former suggest a Ni-poor, Fe,Cu-dominant system(Fe,Cu,Zn2S) with Cu up to 2.6 wt% whereas the lattersuggests a Cu,Zn-poor, Fe,Ni-dominant one (Fe,Ni2S)

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trace element composition is important with high con-tents of the transition metals and detectable Ga, Nb, Zr,Sn, As and Sr; the Nb content is as high as 1252 ppm atthe rim while it is very low (24 ppm) at the core of thelarge spinel grain. Because of the small number of sam-ples available for this study, only cobalt-rich spinel andalbite were found as inclusions in the Bo Ploi corundum(Table 2). The inclusion mineral suite in the Bo Ploicorundum (including those identified by Gunawardeneand Chawla 1984) is: Co-rich spinel1albite1K-feldspar1Na2Mg2Al amphibole(?)1pyrrhotite. It isanticipated that Nb2Ta oxides, common to most of stud-ied basalt-related corundum, may also be found as inclu-sion phases in other Bo Ploi corundum grains. In fact, thehigh Nb concentration along the margin of the largecobalt-rich spinel grain indicates a Nb-rich environ-ment.

Phosphate

Phosphate inclusions have been observed in a Mingxi(MX4) and in a Kings Plains corundum (KP37). Chemi-cal data for the phosphates are given in Table 6.

Sample MX4 is a light blue corundum, with two typesof mineral inclusions, ilmenorutile and phosphate. Bothare euhedral and the phosphate is intergrown with one ofthe ilmenorutile grains (Fig. 6 d). The two phosphate

Fig. 6a–d Sulphide and brockite inclusions in corundum.a Twosulphide blebs found exposed (and oxidised) on a fracture plane ofa Lava Plains blue corundum, LP19 (reflected light illumination).b An SEM image of the LP19 oxidised sulphide grains illustratingtheir porous surfaces.c Sketch of a colour-banded Kings Plainscorundum (KP1) with sulphide inclusions. The concentric hexag-onal rings depict the colour zonation that is parallel to the growthbanding of the corundum. Small sulphide droplets are distributedexclusively along the outer colour bands.Sometimes healed frac-tures (" fingerprints ") are associated with sulphide inclusions.d Abackscattered electron image of an intergrowth of a large, homo-geneous columbite (grey area) and Th,Ce-rich phosphate phases(white areas), identified as brockite, in a Mingxi sapphire

with Ni up to around 58 wt%. The corundum-hosted sul-phides also show higher contents of sulphur (S,43.5 wt%) compared with the pyroxene-hosted sul-phides (S,,40 wt%; Andersen et al. 1987). Such differ-ences must be related to the chemistry of melts that pro-duced corundum and clinopyroxene megacrysts as dis-cussed below.

Cobalt-rich spinel

The cobalt-rich spinel (Guo et al. 1994) was found onlyin a Bo Ploi corundum, a greenish blue stone. The inclu-sion is an elongated multi-phase assemblage (compositeinclusion), with the largest spinel grain being approxi-mately 250mm3150mm and chemically zoned. The

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Table 6 Electron microprobe analyses of some less common min-eral inclusions in corundum (brockite, uraninite, sulphide). Alliron is recorded as FeO except for sulphide (Fe); phosphates anduraninite were analysed by the CSIRO Cameca CAMEBAX auto-

probe (WDS); sulphide was analysed using the LINK (EDS)analyser attached to the ETEC probe at Macquarie University. (–below detection limit,NA not analysed)

Host corundum MX4 KP37 KP1

Inclusion Brockite Brockite or ningyoite (?) Uraninite Sulphide

Label MX4by1.1 MX4by2.1 KP37bylight KP37bydark KP37uy1.1 KP37uy1.2 KP1sulf

SiO2 3.12 3.02 0.78 0.92 – – Ti –TiO2 – – 0.02 – 0.08 0.04 Al 0.43Al2O3 – – 0.72 0.70 – – Fe 52.72FeO 0.09 0.11 0.19 0.19 0.03 0.17 Mn –MnO – – – – – – Cu 2.61MgO – – 0.01 0.03 – – Zn 0.78CaO 4.20 4.49 1.49 2.04 0.08 0.06 S 43.53Na2O – – – – 0.01 –Nb2O5 – – 0.04 – 0.12 0.14 Total 100.07Ta2O5 – – – – – –ThO2 30.35 30.68 48.21 50.19 25.41 27.11UO2 0.63 0.24 24.96 16.37 65.06 61.32ZrO2 – – – – 0.01 –P2O5 24.90 24.84 10.97 13.31 – –Y2O3 1.82 1.66 0.17 0.05 0.24 0.72La2O3 4.80 4.94 NA NA 0.06 0.13Ce2O3 14.54 13.39 NA NA 0.13 0.19Pr2O3 1.82 2.05 NA NA – –Nd2O3 5.80 5.59 NA NA – 0.33Sm2O3 1.79 2.03 NA NA 0.23 0.10F – – – – – –

Total 93.86 93.04 87.56 83.80 91.46 90.31

*REE1Y oxides 30.57 29.66 0.66 1.47*Th1U oxides 30.98 30.92 73.17 66.56 90.47 88.43

grains are relatively large compared with many otherinclusions (25350mm and 25380mm). Electron mi-croprobe analyses show that they are compositionallyidentical, predominantly P2O5, ThO2 and REE2O3

(Table 6). Other significant oxides are CaO and SiO2.This composition is most similar to that of brockite, ahydrous Ca,Th,REE-phosphate (Fisher and Meyrowitz1962). The abundance of volatiles (H2O and possiblyCO2) indicated by low totals in the phosphate inclusionsis approximately 7 wt%, comparable to the level of H2Opresent in brockite (¥8 wt%).

Brockite is a Ca,Th-dominant hydrous phosphate,isostructural with rhabdophane, a hexagonal REE-domi-nant hydrous phosphate. An ideal brockite may be ex-pressed by the formula of (Ca,Th)PO4? H2O, withCa:Th51: 1 (Fisher and Meyrowitz 1962), whereas rhab-dophane is (Ce,La)PO4? H2O. The major difference be-tween the phosphate inclusions of the present study andthat reported in the literature is the high abundance ofREE, the dominance of Ce among the rare earth ele-ments and the presence of significant Si in the inclusions.The brockite inclusions have REEs 50%, Th 30% and Ca20% (REE1Th1Ca normalised to 100 atoms), i.e. ap-proximately equal proportions of brockite and rhab-dophane components. Cerium forms nearly half of thetotal REE atoms. Such chemical characteristics, and the

departure of the phosphate inclusion chemistry fromideal brockite and rhabdophane, suggest a rhabdophanestructure more tolerant with regard to Ca,Th- and REE-substitutions, as previous data suggest. The incorpora-tion of REEs into the brockite structure, replacing Ca,may be coupled with the substitution of P by Si, a com-mon mechanism operating in apatite and cheralite([Ca,Ce,Th][P,Si]O4) (Bowie and Horne 1957;Hogarth1989).

Sample KP37 is also a light blue corundum. The phos-phate phase forms part of a uranpyrochlore-uraninite-phosphate aggregate intergrown with corundum (Fig. 7).Uranpyrochlore is zoned and forms the main body of theaggregate, with subsidiary uraninite. The phosphatephase is small (30mm360mm) and interlocked betweenuranpyrochlore and uraninite. Electron microprobeanalyses show that the KP37 phosphate is extremely en-riched in Th and U in agreement with the coexistinguraninite. The chemistry of this phosphate is unique anddoes not resemble any of the knownnatural phosphateminerals. Considering the presence of a significantquantity of Ca and the hydrous nature of this phosphatephase, it may be tentatively identified as either a brock-ite-type phase or a mineral similar to ningyoite([U,Ca]2[PO4]2? 1–2H2O; Muto et al. 1959).

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Fig. 7 a A backscattered elec-tron image of a complex inter-growth of corundum (dark atlower-left), uranpyrochlore(grey in the middle), uraninite(brilliant white) and Th-richphosphate (dark grey at thelower-right corner); scalebar5100mm. b Close-up ofpart of the multi-phase inter-growth, showing the mineralsfrom left to right: uraninite,phosphate and uranpyrochlore;scale bar510mm. c PKaX-rayimage ofb. d UMaX-ray imageof b. e NbLaX-ray image ofb.f ThMaX-ray image ofb

Uraninite

Uraninite was found only as part of the uranpyrochlore-uraninite-phosphate aggregate, intergrown with a bluecorundum from Kings Plains (KP37; Fig. 7). The pho-tograph of the multi-phase aggregate shows thaturaninites occur as irregular patches within a zoneduranpyrochlore matrix. The uraninite appears to replacethe uranpyrochlore. The electron microprobe analyses ofthe uraninites are given in Table 6.

Uraninite is isostructural with fluorite CaF2, and isideally expressed as the dioxide of uranium, UO2. How-ever, all natural uraninite samples are more or less oxi-dised with partial conversion of U41 to U61 so that achemical formula (U41

1-xU61x)O21x (x,1) more accu-

rately describes the composition. The upper limit of theoxidation effects is undefined (e.g. Frondel 1958).Uraninite and thorianite form a complete solid solutionseries (U,Th)O2 with 50 mol% of U or Th serving as adivision for the two terms. The most distinctive featuresof the uraninite in the present study are its high concen-

tration of Th (ThO2¥25–27 wt%), with a Thy(U1Th)ratio ¥0.30, and a significant content of Nb(Nb2O5¥0.13 wt%). The rare earth elements account foronly about 1 wt% of the total oxides. A significantamount of H2O is thus inferred to be present in theuraninite. The high concentration of Th in this uraniniteis likely to reflect a magmatic crystallisation rather thana hydrothermal origin (Frondel 1958), as discussed be-low.

Discussion

Bimodal source paragenesis: evidence from inclusions

Inclusions are common in the corundum fragments (overone thousand) examined and most are euhedral andmonomineralic. More than one inclusion phase may oc-cur in a single host corundum. They may be discrete andseparated from other inclusions or occur as bi- or poly-minerallic intergrowths. In addition to the inclusions,

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some minerals were found to form intergrowths withcorundum. The inclusion mineral suite and the chemistryof the phases typify the environment of corundum crys-tallisation.

Aspen et al. (1990) studied the feldspar and com-posite megacrysts in basaltic rocks from several areas atFife, Scotland. At two localities, Ruddon’s Point andLoch Roag, they found the following intergrowths:Rud-don’s Point, anorthoclase1corundum1zircon1REE,Y-rich niobate; Loch Roag, alkali feldspar1oligoclase1corundum1zircon1ilmenorutile1Fe-biotite1Fe-sali-te1apatite1samarskite. The Loch Roag alkali feldsparcomprises anorthoclase and sanidine. These assem-blages are very similar to those derived from inclusionsin corundum megacrysts in the present study. Aspen et al.(1990) interpreted the Scottish alkali feldspar-dominat-ed assemblages as debris from a deep-seated syenite.

Columbite is commonly found in various graniticrocks and their pegmatitic equivalents, but the composi-tion of the titaniferous columbite inclusions in corundumdoes not resemble most ofthem in terms of NbyTa andFeyMn ratios (e.g. Cerny and Ercit 1985). The columbiteinclusions are, however, comparable with those occur-ring in alkaline rocks of carbonatite complexes (e.g.Sveshnikova et al. 1965), indicating a source composi-tion with carbonatitic characters (Fig. 4). The presenceof uranium-rich pyrochlores as inclusions in corundumalso indicates the involvement of a carbonatitic composi-tion in the source region of the corundum because uran-pyrochlore is known to be characteristic of carbonatiteand related alkaline rocks.

The morphology of thezircon inclusions is character-istically dominated by the {110} prism over the {100}prism. Kostov (1973) showed that U,Th, REEs, alkalisand H2O in the melt tend to favour the growth of {110}-dominated zircon. Pupin (1980) regarded the {110}-dominated zircons as the result of low temperature crys-tallisation in a peralkaline and hyperaluminous melt. Theimplication of the above studies is that the source ofcorundum megacrysts must be alkaline to peralkaline andrich in U, Th and REEs,consistent with the chemical char-acteristics of zircon inclusions (Guo et al., submitted).

The feldspar inclusions found in this study are mainlysodium plagioclase (albite and oligoclase) with a minoramount of alkali feldspar. The low Ca content of thefeldspars suggests further that the source rock of thecorundum megacrysts was evolved and felsic.

Phosphates constitute a small part of the mineral in-clusion suite in corundum but their unique chemical sig-nature has important implications for the nature of thecorundum source. The Th,U,REE-rich phosphates arerare minerals. Brockite occurrences so far reported areunique to granitic pegmatite veins and rare granitic rocks(Fisher and Meyrowitz 1962).

Uraninite is mainly found in granites and granite peg-matites and moderate temperature hydrothermal veinsassociated with granite intrusions. The former includenormal granite pegmatite comprising microcline,quartz, muscovite and zircon, andmineralised peg-

matiteswith either Nb2Ta2REE enrichment in the formof Nb2Ta2REE oxides such as columbite-tantalite, fer-gusonite, samarskite, silicates and phosphates, or Li-en-richment characterised by lepidolite, columbite-tantaliteand spodumene. The major differences between the peg-matite-type and hydrothermal vein-type uraninites arethe concentrations of Th, Y and REE especially Ce. Assummarised by Frondel (1958), these elements are oftenpresent in relatively large amounts in pegmatite-typeuraninites but absent from the hydrothermal vein-typeuraninites. If such a generalisation is valid, the uraninitesassociated with the corundum of the present study areunlikely to have originated from hydrothermal activitybut rather from magmatic crystallisation because of theirenrichment in Th (ThO2 up to 27 wt%), higher than anyof the uraninite analyses compiled by Frondel (1958).The maximum ThO2 value given in Frondel (1958) is14 wt%.

The recorded occurrence of ilmenorutile is limited togranitic rocks and their pegmatitic equivalents, suggest-ing that ilmenorutile belongs to a felsic mineralogicalassociation.

From this information, it is reasonable to divide themineral inclusions in corundum into two groups: anevolved alkaline felsic suite (alkaline granite or syenite)and a carbonatitic suite. The former is exemplified byfeldspar, zircon, uraninite, ilmenorutile and Fe,Cu-sul-phide; the latter by titaniferous columbite, uranpyro-chlore and fersmite. (Although apatite is considered themost important phosphate in carbonatite, no apatite in-clusion was found in the present study). This bimodalcharacteristic of corundum paragenesis suggests that asingle rock type is not adequate to interpret the complexmineralogy, and thus the system that produces corundummay not be a simple one. It is likely to be a mixed sourceinvolving a mixingyreaction process or processes. The in-tergrowths between members of the two inclusion groupswithin a single corundum sample, e.g. columbite-zircon,columbite-ilmenorutile and uranpyrochlore-uraninite,reinforce such a inference (Guo et al., submitted).

Evaluation of hypotheses

There have been two contrast views about the origin ofcorundum megacrysts: (1) The corundum megacrystsform part of the basaltic magma system, representingdirect crystallisation from basaltic magmas at high pres-sure (mantle); (2) The corundum megacrysts do not formpart of the basaltic magma at any stage; they representaccidental fragments from some depth in the Earth, in-corporated into the magmas only at the time of basalticeruptions.

Phenocryst hypothesis

The basis for this hypothesis has been that, firstly, corun-dum megacrysts are commonly found as euhedral anddiscrete crystals and, secondly, they often occur in asso-

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ciation with other aluminous Mg2Fe silicate megacrystssuch as aluminous clinopyroxene, aluminous orthopy-roxene and aluminous amphibole. The latter megacrystsmay have originated from near-liquidus crystallisation ofa range of basalt-like compositions at upper mantle con-ditions; evidence for this is derived mainly from highpressure experimental studies (e.g. Green and Hibberson1970; Knutson and Green 1975). However, experimentalstudies and the results of the present inclusion studyprovide strong evidence against such a “ phenocryst hy-pothesis ” for corundum.

This study has revealed that a wide range of mineralsis intimately associated with the formation of corundummegacrysts. The most petrologically significant are ti-taniferous columbite, pyrochlore, REE,Nb,Ta-oxidessuch as samarskite, uraninite, Hf-rich zircon, sodiumplagioclase and alkali feldspar. As discussed above, thismineral suite is typical of an evolved silicic and carbon-atitic system rather than a primitive basaltic one.

Liu and Presnall (1990) investigated the liquidus rela-tionships on the join anorthite-forsterite-quartz at20 kbar and showed the presence of a stable corundumfield (Fig. 8). Compared with experimental results atlower pressure, the expansion of the corundum field athigh pressure is mainly at the expense of plagioclase.Although information derived from the anorthite-forster-ite-quartz system is a close analogue to the evolution of anatural mafic maga, it is not possible for corundum to bestable in a basaltic magma even at elevated pressure. Theprojection of a basaltic composition lies within the lower

half of the CaAl2SiO82Mg2SiO42SiO2 (“ anorthite-forsterite-quartz ”) join (Fig. 8), and at pressures in therange of 10–20 kbar olivine, orthopyroxene, spinel, sap-phirine and plagioclase can appear as liquidus phasesfrom a basaltic magma, but not corundum. At such highpressures, fractional crystallisation of a basaltic magmawill drive the residual liquid towards Al-enrichment un-til a point where spinel precipitates followed by sap-phirine and plagioclase. The final eutectic assemblagewould be plagioclase1pyroxene1quartz (no corundum).The spinel-sapphirine-plagioclase assemblage serves asan aluminium “ sink ” preventing the onset of Al-oversat-uration and the crystallisation of corundum. The persis-tent presence of such a barrier in the mafic system for anyreasonable geologicalP2T conditions means that a nor-mal fractional crystallisation process cannot result in aperaluminous composition, and thus in the crystallisa-tion of corundum. This is consistent with the result ofGreen et al. (1978).

Xenocryst hypothesis

The mineralogical and chemical evidence against the“ phenocryst hypothesis ”, discussed above, providesstrong support for its alternative, the “ xenocryst hypoth-esis ”. The presence of corrosion patterns of some corun-dum samples are consistent with this hypothesis(Fig. 1 b). The hypothetical corundum-bearing rock ormagma, the ultimate source of corundum megacrysts inbasalts, should include phases such as columbite, uran-pyrochlore, zircon and albite. The corundum megacrystsare regarded as the fragments of such a rock, entrained inan ascending basaltic magma. The possible conditionsand mechanism involved in the formation of such acorundum-bearing rock are discussed below.

Space, time and conditions

Space

Corundum megacrysts are known only in basalts eruptedthrough continental crust, and appear to be associatedwith alkalic basaltic provinces in continental regionswhich have undergone extensional rifting at some time.According to Knutson et al. (1989), basaltic lava fieldvolcanism in eastern Australia, Southeast Asia and mostof eastern China occurs in regions associated with exten-sional processes. There also is evidence that basalt un-derplating was associated with the lava field volcanismin eastern Australia (Griffin and O’Reilly 1986, 1987;O’Reilly and Griffin 1994). The magmas generated inthese rifting environments rise through trans-lithospherefractures and produce dominantly alkalic basaltprovinces with some subalkalic and tholeiitic basalts.The spatial relationship between basalts containingcorundum and their tectonic settings seems valid at leastfor the Australian and Chinese situations. It is thereforeinferred that the process(es) responsible for generating

Fig. 8 The experimentally determined liquidus phase boundarieson the join anorthite-forsterite-quartz. The 20 kbar lines are takenfrom Liu and Presnall (1990) and the 10 kbar lines are from Senand Presnall (1984) but adjusted later by Liu and Presnall (1990).(fo forsterite,en enstatite,q quartz,sp spinel,sa sapphirine,ananorthite,cor corundum) Thearrows indicate the descending lineof temperature. The final eutectic points are indicated bylargecircles

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corundum-bearing rocks take place in rifting environ-ments.

Time

The timing of the corundum-forming process is poten-tially given by the crystallisation age of the zircon inclu-sions within corundum megacrysts. As discussed earlier,neither zircon nor corundum can crystallise out of abasaltic composition under any geologically reasonableconditions, and both are thus interpreted to be the prod-ucts of a process occurring prior to the basaltic magma-tism. U2Pb dating of zircon inclusions has shown thatthe apparent ages of the zircons are almost identical tothe age of the basalt from which corundum was recov-ered (Coenraads et al. 1990; Guo et al., in preparation).However, as demonstrated by Guo et al., (in preparation),at the high temperature of basaltic magmas, the zirconinclusion U2Pb chronometer can be reset within thetime of a magma’s eruption-cooling cycle due to Pb dif-fusion. As a result, the real timing of zircon crystallisa-tion and thus the timing of corundum formation, couldhave been significantly before the volcanism representedby the host basalts.

P2T conditions

While the present data do not allow a precise evaluationof the temperature and pressure conditions of corundumformation, chemical characteristics and textures of cer-tain inclusions provide useful information in constrain-ing these conditions. The feldspar inclusion in a KingsPlains corundum (KP19) is an intimate intergrowth ofK-feldspar and albite and probably represents a coars-ened and well-equilibrated alkaline perthite (Fig. 5).Such an intergrowth relationship suggests low tempera-ture K- and Na-diffusion in the feldspar has taken placeover an extended period of time. Applying the strain-freesolvus in the alkalic feldspar series determined byThompson and Waldbaum (1969), we estimate that theperthitic feldspars were equilibrated at a temperaturearound 4008 C or lower. The implication of this tempera-ture estimate for its host corundum is that the corundum-inclusion association must have been in a “ cold ” envi-ronment at the time of corundum entrainment intobasaltic magmas; this is strong evidence supporting the“ xenocryst hypothesis ”. Temperature information canalso be derived from the cobalt-rich spinel inclusion in aBo Ploi corundum; the crystal chemistry of the CoAl2O4-based spinel tends to suggest a temperature of less than7008 C based on the cation distribution in spinel struc-ture (Guo et al. 1994). Therefore, the temperature for thecorundum at the time of entrainment in the basalt may beestimated conservatively to be within a large range of300–6008 C. Although the temperature range is large, itplaces the “ corundum-bearing rock ” at mid-crust levels(10–20 km depth) by reference to the southeastern Aus-

tralian geotherm of O’Reilly and Griffin (1985). Thisgeotherm is believed to reflect the ambientT2P relation-ship during each volcanic episode.

Stephenson (1990) measured the densities of CO2 flu-id inclusions in a corundum fragment from Mt Leura, theAnakie-Rubyvale region, central Queensland. The ob-tained density information suggests a minimum trappingpressure of 10 kbar for an assumed temperature of10008 C. However, as discussed above, the “ corundum-bearing rock ” probably formed at mid-crustal levels(¥5 kbar,¥4008 C). Under these conditions, the calcu-lated CO2 fluid density corresponds well with the densi-ty data derived from Stephenson (1990) using experi-mental CO2 density isochors. Therefore, Stephenson’sassumption of a 10008 C trapping temperature, and thederived depth of ca 30 km for corundum megacrysts maybe invalid. This warrants a further geobarometric studyof the fluid inclusions in corundum.

Genetic model

Information derived from mineral inclusions providesthe most direct constraints on genetic models for thebasalt-associated corundum megacrysts. The complexcorundum paragenesis, based on the mineral inclusionsrecognised, suggests that a single source is not adequateto explain all the inclusion data. At least two componentsmust be involved in the source of corundum to explain thebimodal mineral inclusion suites, which indicate both anevolved alkaline-peralkaline felsic composition and an-other of carbonatitic affinity. This suggests that mixingandyor interaction between an alkaline granite or syenitepegmatite composition (magma or rock) and a carbon-atitic magma may be responsible for the formation ofcorundum. Such a hybridisation process is likely to berestricted to the intracontinental rifting regimes wheremost carbonatitic magmas occur, and is also constrainedto take place at mid-crustal level according to the inclu-sion mineral-based temperature estimate.

Granitic and syenitic rocks occur in almost all conti-nental tectonic environments. Their pegmatitic equiva-lents result from crystallisation of late-stage magmas ofcrustal origin, except for a few cases where the parentmagma may originate in the mantle.

Carbonatite magmas are considered to have a mantleorigin (e.g. Gittins 1989) and are generally associatedwith major faults defining rift valleys or other grabenstructures, which correspond to structural domes atdepth; this indicates a close genetic connection betweenmagmatism and faulting. Carbonatitic magmas may beproduced either, (1) through direct melting of carbonate-bearing mantle peridotite, i.e. “ primary ” (e.g. Wyllie1977; Wallace and Green 1988; Gittins 1989), or (2)through low pressure separation of immiscible liquidfrom carbonated silicate magmas derived from themantle, i.e. “ derivative ” (e.g. Koster van Groos and Wyl-lie 1973; Kjarsgaard and Hamilton 1988, 1989; Bakerand Wyllie 1990).

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localised hybrid zones. Such interactions have a tenden-cy to drive the initial composition towards the silicatelimb of the two-liquid field. The bulk compositions ofthe hybrid rocks will lie on the mixing trends (I) or (II) asshown in Fig. 9.

Wyllie and Watkinson (1970) evaluated the crysytalli-sation process of Si-oversaturated magma during the ad-dition of CO2 on the basis of petrological experiments.They pointed out that addition of CO2 to an H2O-under-saturated magma at constantP-T conditions could forcethe liquid to crystallise. They further suggested that suchsituations could occur when a granitic magma is em-placed into a limestone sequence, resulting in dehydra-tion andrapid crystallisation of the magma. The intru-sion of carbonatite into granitoid systems suggested inour model can be regarded as an analogue to the lime-stone assimilation process of Wyllie and Watkinson(1970). Because the crystallisation of magma in the pres-ence of CO2 can be completed at almost constant temper-ature and pressure, the interaction process (hybridisa-tion) has to be short-lived. In other words, the hybridisa-tion zones formed as the result of arapid interactionbetween carbonatite and pegmatite.

In an experimental study, Green et al. (1978) notedthat the presence of CO2 in the silicate liquid couldmarkedly reduce the solubility of Al2O3 in a magma. It isspeculated that corundum may crystallise in the rapidlydeveloping hybridisation zone due to the rapid achieve-ment of Al-oversaturation. In fact, Koster van Groos andWyllie (1973) conducted a series of 1 kbar experimentson the 6-component system NaAlSi3O82CaAl2Si2O8

2Na2CO32H2O, in which the phase relationships weredetermined along the joins Albite-Na2CO3, Ab80An20

2Na2CO3 and Ab50An502Na2CO3 (Fig. 10). Atthe feldspar ends of the joins, a stable wollastonite

Fig. 9 The experimentally determined carbonate-silicate liquidimmiscibility relationship in the system (Na2O1K2O)2(SiO2

1Al2O31TiO2)2(CaO1MgO1FeO) at 12508 C. The 5 kbar im-miscibility curves are taken from Kjarsgaard and Hamilton (1989)and tielines between the silicate and the carbonate limbs indicatedifferent equilibrium carbonate-silicate pairs. The 25 kbar immis-cibility curveand the primitive alkalic basaltf ield are taken fromBaker and Wyllie (1990). Thef ield showing granitic pegmatites isbased on data of Cerny (1982). Some natural rocks are plotted asreferences; they are a cogenetic alkalic suite within the EastAfrican Rift (Price et al. 1985) (Nos. 1–4) and several Si-undersat-urated rocks associated with carbonatite (Le Bas 1977) (Nos. 5–9except for the Oldoinyo Lengai suite). The compositions of theOldoinyo Lengai suite are taken from Dawson (1989). (1 basanite,2 benmoreite,3 trachyte,4 phonolite,5 phonolite,6 nephelinite,7 phonolite (Oldoinyo Lengai),8 nephelinite (Oldoinyo Lengai),9 melilitite)

Figure 9 shows the experimentally determined car-bonate-silicate liquid immiscibility gaps at high temper-ature (12508 C). The 5 kbar data of Kjarsgaard andHamilton (1989) are highlighted as they approximate themid-crustal conditions required in our model for corun-dum genesis. The carbonate liquid lies on the(Na2O1K2O)-(CaO1MgO1FeO) side, the silicate liq-uid at the corner of (SiO21Al2O31TiO2), and the two-liquid field occupies the middle of the diagram. Thecompositions of silicate liquids in equilibrium with a“ typical ” carbonatite (alkali-poor carbonatite) and a na-trocarbonatite (alkali-rich carbonatite) are indicated bythe tielines between the carbonate and the silicate limbs.It is obvious that granitoid compositions are not chemi-cally in equilibrium with either of the two types of car-bonatites. However, if a carbonatitic magma were to in-trude into a partly solidified pegmatitic system (late-stage magma) at mid-crustal level, it is predictable thatthe two components will mingle and interact, resulting in

383

Fig. 10 Simplified phase rela-tions on the joins feldspar-Na2CO3 with 10 wt% H2O at1 kbar pressure, based on theexperiments of Koster vanGroos and Wyllie (1973)

field appears with the introduction of CaO and thewollastonite field expands considerably with increasingCa content of the feldspar. The solidus temperaturesof these silicate-carbonate mixing experiments arearound 7008 C. Considering that wollastonite is an Al-free phase (CaSiO3), rapid crystallisation and removal ofsuch phase would cause rapid increase of the AlySi ratioof the crystallising media, which leads to a sudden Al-oversaturation relative to Si within the hybridisationzone. If significant amounts of MgO andyor FeO arepresent in the intruding carbonatite magmas, phasessuch as olivine, diopside, monticellite and akermanitealso may be stable in the hybrid rocks (Wyllie 1966).These Al-free phases are probably additional contribut-ing factors to changes in the AlySi ratio of the crystallis-ing media.

Because of the presence of the two-liquid field serv-ing as a thermal barrier in the silicate-carbonate mixingexperiments, once the carbonate content in the silicateliquid reaches a critical level, further addition of carbon-ate material in the mix prevents the silicate liquid frombecoming more carbonaceous but it remains Si-under-saturated (Fig. 10). The crystallisation of wollastonitefrom such undersaturated media is consistent with the 1atmosphere experimental results of Schairer and Yoder(1964), in which melilite, nepheline, plagioclase anddiopside crystallise in accompaniment with wollastonite.However, Schairer and Yoder’s experiments were on aCO2-free system. The addition of CO2 probably changesthe stabilities of some of these phases and the paths ofcrystallisation.

So far, there are no experimental data on the silicate-carbonate system at pressures equivalent to mid-crustaldepths (¥5 kbar), but the simple phase relations revealedfrom experiments at near-surface pressure conditions(e.g. Koster van Groos and Wyllie 1973) probably do notdeviate dramatically from those at mid-crust pressures.At detailed study on the phase relations of felsic compo-sitions under carbon-saturated conditions, which is most re-levant to the hybridisation zone conceived of in the presentstudy, may be a test for our model of corundum genesis.

Figure 11 depicts the geological processes that may beresponsible for the genesis and distribution of corundumin basaltic terrains. Tectonically, the corundum-bearingbasalts are confined to crustal extension zones, inboardfrom the continental margins. Two stages of magmatismare involved. In an early melting event due to the up-welling of asthenospheric mantle, mafic magmas form atthe thermal head, followed by volcanic eruptions and in-trusions into the lower crust and upper mantle regions ofthe lithosphere. The injected mafic magmas then cool aslayered intrusions near the crustymantle boundary, andprovide the heat source to produce granitoid magmas.Some of these magmas may crystallise pegmatitically, toform the pyroxene+amphibole+garnet+spinel mega-cryst suite.

When carbonate phases are present, partial melting ofcarbonate-bearing peridotite could result in either pri-mary carbonatite magmas or carbonated silicate mag-mas which may evolve subsequently at higher levels toform immiscible carbonatite magmas. These carbonatitemagmas intrude into mid-crustal levels,¥10–20 kmdepth. The extremely reactive carbonatite magmas atmid-crust hybridise with local granitoid systems (mag-ma+rocks) resulting in rapid crystallisation of corun-dum in the hybridisation zone. This process may producelenses of corundum-bearing rocks characterised by com-plex mineralogy, locally distributed at mid-crustal levels(“ hybrid rock hypothesis ”).

A later mantle melting event triggered a majorepisode of basaltic magmatism. The mafic magmas, es-pecially those rich in alkalis and volatiles, ascendedrapidly to the surface entraining fragments of wall rockson their way. The corundum-bearing lenses may be en-countered and their fragments entrained into the risingmagmas, in a similar way to the deeper, mantle-derivedpyroxene megacrysts (Irving 1974). This explains theobserved association of corundum with mantle-derivedxenoliths and megacrysts: the host magma simply acts asan elevator, entraining fragments from different levelsand carrying them to the surface because of their rapidascent (e.g. O’Reilly 1989; O’Reilly et al. 1989).

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Fig. 11 A schematic illustra-tion accounting for the originof corundum megacrysts basedon a crustal growth model foreastern Australia (Griffin andO’Reilly 1987). (see text fordiscussion)

Conclusions

This study presents unique mineralogical and geochemi-cal data relevant to the origin of the corundummegacrysts that occur in basaltic rocks.1. The data from mineral inclusions in the corundum

grains studied demonstrate that a wide range of miner-al phases crystallised together with corundummegacrysts; they are dominantly Nb2Ta oxides, low-Ca feldspars and zircon along with Fe,Cu-sulphideand Ca,Th-rich phosphates. The unique chemicalcharacteristics of these inclusions indicate that at leasttwo components, graniteysyenite pegmatite and car-bonatite, are involved in the formation of corundummegacrysts. The inclusion assemblage is not compat-ible with mafic compositions.

2. The corundum formed prior to entrainment in the hostmagma by interaction of carbonatitic (or similar Si-poor) magmas with very evolved felsic systems; thisinteraction changes the path of crystallisation towardenrichment of Al in the crystallising magma. Suchan interaction-hybridisation process is likely to berapid, resulting in corundum-bearing lenses locally

distributed in the crust, probably at depths of 10–20 km.

3. This inclusion-based study has clarified the long-standing confusion about the origin of corundummegacrysts, i.e. “ phenocryst? ” or “ xenocryst? ”. Itprovides strong evidence that corundum megacrystshave not crystallised from basaltic magmas at highpressure (upper mantle) but are xenocrysts from mid-crustal levels.

4. The model presented here provides a new “ hybridrock hypothesis ” for corundum megacrysts. Some ele-ments predicted by the model are absent in the presentinclusion data sets. However, this model is testable byexperimental studies coupled with further inclusionstudies.

5. This is the first detailed study of inclusions in corun-dum and is thus at an analogous stage to the knowledgeof diamond inlcusions more than 20 years ago. It isanticipated that more inclusion species will be foundwhen more samples are examined as has been the casewith studies of diamond genesis.

385

Acknowledgements We wish to thank G and J Gems Pty Ltd.(Sydney) for generously providing valuable sapphire samples andsome funding for this investigation. Thanks are also due to McEl-roy Brian Geological Services Pty Ltd for their assistance in theNSW part of the fieldwork. Dr Ian Ridley is acknowledged forproviding the San Carlos corundum samples. J.G. would like tothank Prof. Wang Fuquan of the Geological Museum of China forhis support and assistance while conducting a sapphire field surveyin China. Many persons have helped on various aspects at variousstages of the project, and we would like to thank Richard Flood,David French, Trevor Green, Carol Lawson, Norm Pearson, TonyRamsden, Chris Ryan and Tin Tin Win. Comments by RichardFlood on an early draft helped to improve greatly the quality of themanuscript. Constructive reviews by Tom Andersen and JacquesTouret have also contributed to the improvement of the final paper.This study is partly supported by funding to S.Y.O’R. from Aus-tralian Research Council (ARC) and internal Macquarie sources.The study was carried out while J.G. was the recipient of a Mac-quarie University Postgraduate Research Award. Publication num-ber 9 from the Key Centre for Geochemical Evolution and Metal-logeny of Continents (GEMOC).

References

Andersen T, Griffin WL, O’Reilly SY (1987) Primary sulphidemelt inclusions in mantle-derived megacrysts and pyroxenites.Lithos 20: 279–294

Aspen P, Upton BGJ, Dickin AP (1990) Anorthoclase, sanidineand associated megacrysts in Scottish alkali basalts: high-pres-sure syenitic debris from upper mantle sources? Eur J Mineral2: 503–517

Baker MB, Wyllie PJ (1990) Liquid immiscibility in a nephelinite-carbonate system at 25 kbar and implications for carbonatiteorigin. Nature 346: 168–170

Barr SM, MacDonald AS (1978) Geochemistry and petrogenesisof late Cenozoic alkaline basalts of Thailand. Geol Soc MalaysBull 10: 25–52

Bowie SHU, Horne JET (1957) Cheralite, a new mineral of themonazite group. Mineral Mag 30: 93–99

Bunopas S, Bunjitadulya S (1975) Geology of Amphoe Bo Phloi,North Kanchanaburi with special notes on the “ KanchanaburiSeries ”. J Geol Soc Thai 1: 51–67

Cerny P (1982) Petrogenesis of granitic pegmatites. In: Cerny P(ed) Granitic pegmatites in science and industry. MineralAssoc Canada, (MAC Short Course Handb 8) pp 405–461

Cerny P, Ercit TS (1985) Some recent advances in the mineralogyand geochemistry of Nb and Ta in rare-element granitic peg-matites. Bull Mineral 108: 499–532

Cerny P, Chapman R, Göd R, Niedermayr G, Wise MA (1989)Exsolution intergrowths of titanian ferrocolumbite and niobianrutile from the Weinebene spodumene pegmatites, Carinthia,Austria. Mineral Petrol 40: 197–206

Coenraads RR, Sutherland FL, Kinny P (1990) The origin of sap-phires: U2Pb dating of zircon inclusions sheds new light.Mineral Mag 54: 113–122

Coldham T (1985) Sapphires from Australia. Gems Gemol21: 130–146

Dawson JB (1989) Sodium carbonatite extrusions from OldoinyoLengai, Tanzania: implications for carbonatite complex gene-sis. In: Bell K (ed) Carbonatites. Unwin Hyman, London, pp255–277

Ding Z (1991) Mineralogical studies on Shandong sapphires (un-published). MSc thesis, Academia Sinica Inst Geochem,Guiyang, China

Fisher FG, Meyrowitz R (1962) Brockite, a new calcium thoriumphosphate from the Wet Mountains, Colorado. Am Mineral47: 1346–1355

Frondel C (1958) Systematic mineralogy of uranium and thorium.US Geol Surv Bull 1064, Washington

Gittins J (1989) The origin and evolution of carbonatite magmas.In: Bell K (ed) Carbonatite. Unwin Hyman, London, pp 580–600

Green DH, Hibberson WO (1970) Experimental duplication ofconditions of precipitation of high-pressure phenocrysts in abasaltic magma. Phys Earth Planet Inter 3: 247–254

Green TH, Wass SY, Ferguson J (1978) Experimental study ofcorundum stability in basalts (abstract). Abstr Programme 3rdAust Geol Conv, Townsville, pp 34

Griffin WL, O’Reilly SY (1986) The lower crust in eastern Aus-tralia: xenolith evidence. In: Dawson JB, Carswell DA, Hall J,Wedepohl KH (eds) The nature of the lower continental crust.Geol Soc Spec Publ 24, pp 363–374

Griffin WL, O’Reilly SY (1987) Is the continental Moho the crust-mantle boundary? Geology 15:241–244

Griffin WL, Jaques AL, Sie SH, Ryan CG, Cousens DR, Suter GF(1988) Conditions of diamond growth: a proton microprobestudy of inclusions in West Australian diamonds. Contrib Min-eral Petrol 99: 143–158

Gübelin EJ (1983) Internal world of gemstones. ABC Edition,Zurich

Gübelin EJ, Koivula JI (1986) Photoatlas of inclusions in gem-stones. ABC Edition, Zurich

Gunawardene M, Chawla SS (1984) Sapphires from KanchanaburiProvince, Thailand. J Gemmol 19: 228–239

Guo J (1993) Origin and distribution of corundum from basalticterrains (unpublished). PhD thesis, Macquarie Univ, Sydney

Guo J, Wang F, Yakoumelos G (1992) Sapphires from Changle inShandong Province, China. Gems Gemol 28: 255–260

Guo J, O’Reilly SY, Griffin WL (1994) A cobalt-rich spinel inclu-sion in a sapphire from Bo Ploi, Thailand. Mineral Mag 58:247–258

Gurney JJ (1989) Diamonds. In: Kimberlites and related rocks,vol. 2. Geol Soc Aust Spec Publ 14, Blackwell, Singapore, pp935–965

Hodgson NA, Le Bas MJ (1992) The geochemistry and crypticzonation of pyrochlore from San Vicente, Cape Verde Islands.Mineral Mag 56: 201–214

Hogarth DD(1989) Pyrochlore, apatite and amphibole: distinctiveminerals in carbonatite. In: Bell K (ed) Carbonatites. UnwinHyman, London, pp 105–148

Irving AJ (1974) Megacrysts from the Newer Basalts and otherbasaltic rocks of Southeastern Australia. Geol Soc Am Bull85: 1503–1514

Jobbins EA, Berrange´ JP (1981) The Pailin ruby and sapphiregemfield, Cambodia. J Gemmol 17: 555–567

Keller PC, Wang FQ (1986) A survey of the gemstone deposits ofChina. Gems Gemol 22: 3–13

Kiefert L, Schmetzer K (1987) Blue and yellow sapphire fromKaduna Province, Nigeria. J Gemmol 20: 427–442

Kjarsgaard BA, Hamilton DL (1988) Liquid immiscibility and theorigin of alkali-poor carbonatites. Mineral Mag 52: 43–55

Kjarsgaard BA, Hamilton DL (1989) The genesis of carbonatitesby immiscibility. In: Bell K (ed) Carbonatites. Unwin Hyman,London, pp 388–404

Knutson J, Green TH (1975) Experimental duplication of a high-pressure megacrystycumulate assemblage in a near-saturatedhawaiite. Contrib Mineral Petrol 52: 121–132

Knutson J, Sun SS, Ewart A (1989) Comparison with other in-traplate volcanic areas. In: Johnson RW (ed) Intraplate volcan-ism in Eastern Australia and New Zealand. Cambridge Univer-sity Press, Singapore, pp 313–320

Kopecky L, Pisova´ J, Pokorny L (1967) Pyrope-bearing diatremesof the Ceske´ Stredohori Mountains. Sb Geol Ved GIZ: 81–130

Koster van Groos AF, Wyllie PJ (1973) Liquid immiscibility in thejoin NaAlSi3O82CaAl2Si2O82Na2CO32H2O. Am J Sci 273:465–487

Kostov I (1973) Tetrahedral and octahedral coordination in theclassification of silicates. Sov Phys-Crystallogr 16: 1069–1073

Krosch NJ, Cooper W (1990) Queensland mineral commodityreport – sapphire. Queensl Gov Min J July Issue: 299–306

386

Le Bas MJ (1977) Carbonatite-nephelinite volcanism. John Wileyand Sons, New York

Lishmund SR (1987) Regional distribution of sapphire, diamondand volcanoclastic rocks. In: Extended abstracts from seminaron Tertiary Volcanics and Sapphires in the New England Dis-trict. NSW Geol Surv Rep 1987y058, pp 9–12

Liu T-C, Presnall DC (1990) Liquidus phase relationships on thejoin anorthite-forsterite-quartz at 20 kbar with applications tobasalt petrogenesis and igneous sapphirine. Contrib MineralPetrol 104: 735–742

MacNevin AA (1972) Sapphires in the New England district, NewSouth Wales. Rec Geol Surv NSW 14: 19–35

Meyer HOA (1987) Inclusions in diamond. In: Nixon PH (ed)Mantle xenoliths. John Wiley and Sons, Chichester, pp501–522

Meyer HOA, Boyd FR(1972) Composition and origin of crys-talline inclusions in natural diamonds. Geochim CosmochimActa 36: 1255–1273

Muto T, Meyrowitz R, Pommer AM, Murano T (1959) Ningyoite,a new uranous phosphate from Japan. Am Mineral 44: 633–650

O’Reilly SY (1989) Xenolith types, distribution, and transport. In:Johnson RW (ed) Intraplate volcanism in Eastern Australia andNew Zealand. Cambridge University Press, Singapore, pp249–253

O’Reilly SY, Griffin WL (1958) A xenolith-derived geotherm forsoutheastern Australia and its geophysical implications. Tec-tonophysics 111: 41–63

O’Reilly SY, Griffin WL (1994) Moho and petrologic crust-mantleboundary coincide under southeastern Australia: comment.Geology 22:666–667

O’Reilly SY, Nicholls IA, Griffin WL (1989) Xenoliths andmegacrysts of mantle origin. In: Johnson RW (ed) Intraplatevolcanism in Eastern Australia and New Zealand. CambridgeUniversity Press, Singapore, pp 254–274

Pecover SR (1987) Tertiary maar volcanism and the origin of sap-phires in northeastern New South Wales. In: Extended ab-stracts from seminar on Tertiary Volcanics and Sapphires inthe New England District. NSW Geol Surv Rep 1987y058, pp13–21

Price RC, Johnson RW, Grey CM, Frey FA (1985) Geochemistryof phonolites and trachytes from the summit region of MtKenya. Contrib Mineral Petrol 89: 394–409

Pupin JP (1980) Zircon and granite petrology. Contrib MineralPetrol 73: 207–220

Ramsden AR, French DH (1990) Routine trace-element capabi-lities of electron-microprobe analysis in mineralogical in-

vestigation: an empirical evaluation of performance usingspectrochemical standard glasses. Can Mineral 28: 171–180

Roedder E (1984) Fluid inclusions. (Reviews in mineralogy, vol12) Mineral Soc Am, Washington, DC

Schairer JF, Yoder HS Jr (1964) Crystal and liquid trends in sim-plified alkali basalts. Carnegie Inst Washington Yearb 63: 65–74

Sen G, Presnall DC (1984) Liquidus relationships on the join anor-thite-forsterite-quartz at 10 kbar with applications to basaltpetrogenesis. Contrib Mineral Petrol 85: 404–408

Stephenson PJ (1976) Sapphire and zircon in some basaltic rocksfrom Queensland, Australia. Abstr 25th Int Geol Congr, Syd-ney, 2: 602–603

Stephenson PJ (1990) The geological context of sapphire occur-rences in the Anakie region, Central Queensland. Abstr 10thAust Geol Conv, Hobart, 232–233

Sveshnikova EV, Zhabin AG, Yakovlev-Skaya TA, Alexandrov VB(1965) On the titanium-bearing columbite from alkaline mas-sives (in Russian). Trudy Mineral Muz Acad Sci USSR16: 266–270

Thompson JB Jr, Waldbaum DR (1969) Mixing properties ofsanidine crystalline solutions. III. calculations based on two-phase data. Am Mineral 54: 811–838

Upton BGJ, Aspen P, Chapman NA (1983) The upper mantle anddeep crust beneath the British Isles: evidence from inclusionsin volcanic rocks. J Geol Soc London 140: 105–121

Vichit P, Vudhichativanich S, Hansawek R (1978) The distributionand some characteristics of corundum-bearing basalts in Thai-land. J Geol Soc Thai 3:M4.1–M4.38

Wallace MT, Green DH (1988) An experimental determination ofprimary carbonatite magma composition. Nature 335: 343–345

Wang F (1988) The sapphires of Penglai, Hainan Island, China.Gems Gemol 24: 155–160

Wyllie PJ (1966) Experimental studies of carbonatite problems:the origin and differentiation of carbonatitic magmas. In: Tut-tle OF, Gittins J (ed) The carbonatites. John Wiley and Sons,New York, pp 311–352

Wyllie PJ (1977) Mantle fluid compositions buffered by carbon-ates in peridotite-CO22H2O. J Geol 85: 187–207

Wyllie PJ, Watkinson DH (1970) Phase equilibrium studies bear-ing on genetic links between alkaline and subalkaline magmas,with special reference to the limestone assimilation hypothe-sis. Can Mineral 10: 362–374