Mineralogical Magazine, February 2005, Vol. 69(1), pp. 21±38 Basalt petrology, zircon ages and sapphire genesis from Dak Nong, southern Vietnam V. GARNIER1, D. OHNENSTETTER1, G. GIULIANI2,1, A. E. FALLICK3, T. PHAN TRONG4, V. HOAÁNG QUANG4, L. PHAM VAN5 AND D. SCHWARZ6 1 CRPG/CNRS, UPR 2300, BP 20, 15 rue Notre-Dame des Pauvres, 54501 Vandoúuvre-leÁs-Nancy Cedex, France 2 IRD, UR154, LMTG, 14 avenue Edouard Belin, 34100 Toulouse, France 3 Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride G75 0QF, Scotland Institute of Geological Sciences, CNST, Nghia DoÃ, CaÃu GiaÃy, Hanoi, Vietnam Vietnam National Gem and Gold Corporation, 91 Dinh Tien Hoang Street, Hanoi, Vietnam GuÈbelin Gemmological Laboratory, 102 Maihofstrasse, CH-6000 Lucerne 9, Switzerland 4 5 6 ABS TR AC T Basalts associated with sapphire deposits are widespread in Europe, Africa, Asia and Australia. In Vietnam, blue, green and yellow sapphires are recovered from eluvial and alluvial placers hosted in basaltic fields of the Dak Lak Province. Two distinct basalt suites are recognized in this field: a tholeiitic suite without any xenocrysts and an alkaline suite with mantle and lower crustal xenocrysts. The sapphires are enriched in Fe (0.43 to 1.26 wt.%), with moderate contents of Cr (33ÿ1582 ppm), Ti (35ÿ1080 ppm), Ga (149ÿ308 ppm) and V (28ÿ438 ppm) and they are poor in Zn and Mg. Their Oisotope18compositions range from 6.0 to 6.9% and are not in equilibrium with the alkali basalts which have d O values between 5.0 and 5.7%. The U-Pb dating of zircons recovered from the basaltic placers provides evidence of two eruptional events: one at ~6.5 Ma followed by another one at ~1 Ma. The petrography of the basalts and the oxygen isotopic composition of the sapphires suggest that the sapphires are xenocrysts and that they crystallized in a deep magma chamber, at the lower continental lithosphere and the upper mantle boundary, in evolved melts issued from the fractionation of alkali basaltic magmas contaminated with lower crustal fluids. K EY WORDS : Vietnam. sapphire, basalt, petrography, geochemistry, zircon, U-Pb dating by SIMS, oxygen isotopes, THE majority of gem corundums on the world market today, particularly blue and so-called fancy sapphires, come from secondary erosional deposits in southeast Asia, Australia and Madagascar. The basalt ®elds yielding corundums are widespread throughout six continental regions, within 15 countries and >40 main basalt ®elds (Sutherland and Schwarz, 2001). Sapphires and sometimes rubies are recovered from secondary alluvial and eluvial deposits. Recent * E-mail: virginie_garnier@inrs-ete.uquebec.ca DOI: 10.1180/0026461056910233 { Present address: INRS-ETE, 490 rue de la Couronne, QueÂbec, Canada G1K 9A9 # 2005 The Mineralogical Society studies (Sutherland ., 1998 ; Sutherland and Schwarz, 2001) reveal that corundums associated with the lavas fall into two main suites: (1) the `basaltic' suite with blue, green, yellow and colour-zoned sapphires; these sapphires have high Fe (0.4 < Fe2O3 < 1.8 wt.%) and Ti (0.05 < TiO2 < 0.2 wt.%), slightly high Ga (0.015 < Ga2O3 < 0.04 wt.%) and very low Cr (Cr2O3 <0.005 wt.%) contents; (2) `the metamorphic' suite with variously pinkish to purple, pastel blue sapphires and ruby; their mineral inclusions are similar to metamorphic assemblages as they include sapphirine and pyroxene; they have lower Fe and Ga and higher Cr contents than magmatic corundums. Besides the coexistence of these et al b V. GARNIER ET AL. Geological setting `basaltic' and `metamorphic' suites in basalthosted corundum (2002) proposed deposits, that some Sutherland deposits et al. Neogene±Quaternary intraplate basaltic volcanism present is widespread in east and southeast Asia (Fig. 1), `multi-modal' gem suites. The aim of this study is to decipher the origin forming plateaux associated with extensional rifts of sapphires hosted in the basalts of the Dak Nong (Barr and McDonald, 1981; Whitford-Stark, 1987). province the Volcanic activity post-dates the Early Tertiary petrographic, geochemical and O-isotopic study India-Asia collision and may be related to astheno- of the sapphires and their host rocks. New oxygen spheric isotopic data on sapphire and ruby originating processes (Tapponnier from different kinds of protolith possibly derived plateaux in southern and central Vietnam often from permit exceed 100 km in diameter, are up to several discussion of the possible origin of corundum hundred metres thick, and cover a total area of xenocrysts and zircon found in the alkali basaltic ~23,000 km suite. the eruptive centres are associated with pull-apart the from southern mantle or the Vietnam lower from crust and 2 lithospheric et al., tectonic extrusion 1982, 1986). Basalt (Hoang and Flower, 1998). Most of FIG. 1. Distribution of Tertiary and Quaternary basalts and related sapphire deposits in southeast Asia with K-Ar and Ar-Ar ages of the basaltic ¯ows (modi®ed from Jobbins and Berrange Â, 1981; ages in Thailand, from Barr and MacDonald, 1979, 1981; in Vietnam, from Hoang and Flower, 1998; in Dak Nong: this study). The thick dashed lines are the political borders. 22 BASALTS AND SAPPHIRES, S VIETNAM structures bounded comprising by short strike-slip extensional faults analyses were: 26 ppm for Cr, 37 ppm for Fe, rifts et al., (Rangin 39 ppm for Ga, 22 ppm for Mg, Ti and V and 1995). The basalts comprise large volume tholeiite 54 ppm plateaux with lesser amounts of alkali basalts, sapphire-bearing usually marking major intersections of extensional observed by cold cathodoluminescence (CL) on ®ssures and their conjugate strike-slip faults (Hoang a et al., produced quartz and Cambridge Zircons alluvial Image were sampled deposits. Technology from They Ltd were CLmk4 France). involved at least two eruptive episodes. Early usually Zn. apparatus at the GR2 lab. (University of Nancy, 1996). The volcanic centres appear to have episodes for Oxygen olivine isotope were analyses performed at of basalt the SUERC and tholeiite ¯ows, with rare alkali basalt, whereas sapphires later episodes erupted olivine tholeiite, alkali basalt, Scotland, following a modi®cation of the laser- in basanite and rarely nephelinite. This bi-episodal ¯uorination technique described by Sharp (1990). pattern is recognized at the Dalat, Phuoc Long, The method involves complete reaction of ~1 mg Pleiku, Buon Ma Thuot, Xuan Loc and Re Island of powdered sapphire, heated by a CO2 laser, with volcanic centres. At Buon Ma Thuot this composi- ClF3 as the ¯uorine reagent. The released oxygen tional trend is reversed (Hoang and Flower, 1998). is passed through an in-line Hg-diffusion pump It is estimated from known stratigraphic thickness, before conversion to CO2 on platinized graphite. surface exposures and preliminary age dates that The yield is measured by capacitance manometer 3 ~8000 km of basalts were erupted mostly between and the gas-handling vacuum line is connected to 8 and 2 Ma, implying comparatively large eruption the inlet system of a dedicated VG PRISM 3 dual rates for continental rift-related volcanism (Hoang inlet isotope ratio mass spectrometer. Precision et al., 1996). and accuracy on quartz standards are Ô0.1 % (1s) Sapphires have been found in basaltic alluvial and duplicate analyses of sapphire give a similar deposits in four provinces of southern Vietnam: degree of uncertainty. Data are reported in the Binh Thuan, Lam Dong, Dong Nai and Dak Lak conventional delta notation relative to V-SMOW. (Smith et al., as U-Pb dating was performed on a Cameca IMS pyroxene, olivine, plagioclase, garnet and zircon 1270 ion probe following the analytical procedure occur as 1995). Corundums megacrysts in as well undersaturated lavas detailed by Deloule et al. (2002). Corrections for 204 (Hoang and Flower, 1998). All these minerals common lead were made using the are found in recent and palaeo-alluvial deposits of and the Stacey and Kramers (1975) lead evolution Dak model. Nong. The Dak Nong sapphire-mining Pb content district lies in the Dak Lak Province near the Petrography Cambodian border, in basaltic plateaux. Sapphires are recovered from Quaternary and Upper- Pleistocene alluvial formations. The sapphires Two groups of basalts are distinguished in the are black, blue, green to yellow and colour-zoned. Dak There is no ruby but some ``trapiche-like'' blue tholeiitic Nong area. suite A stratigraphically characterized by lower porphyritic sapphires were found: they show radiating blue (V74b, V74c, V75a, V75c', V75d) and glomero- and white coloured zones but no skeletal arms as porphyritic basalt (V74a) with pilotaxitic texture in true ``trapiche'' minerals (Garnier et al., (Fig. 2a). They are sometimes vesicular (V75a) 2002). with Analytical techniques amygdales ®lled with smectites. Glomerocrysts (up to 2 mm) of orthopyroxene (opx) are rounded and slightly altered to This study was performed on ten rocks sampled serpentine. from basaltic ¯ows from the mining district of pyroxene (cpx) and opx (up to 3 mm) show Dak Nong. Chemical analyses for the samples plagioclase (pl) lath inclusions in subophitic to were performed by ICP-ES Jobin Yvon 70 for nesophitic major elements and ICP-MS Perkin Elmer Elan arrangement was formed in a shallow magmatic Intratelluric arrangement phenocrysts suggesting of clino- that this 5000 for trace elements at the CRPG-CNRS chamber (Fig. 2b). Phenocrysts (2 mm), micro- (Nancy, France). Thin sections were studied using phenocrysts and microlites of plagioclase with an Hitachi 2500 SEM and a Camebax SX100 swallow tails and belt buckle textures are also electron microprobe at the University of Nancy, found as well as small rounded microlites of cpx, France; see Garnier (2003) for the analytical rare titanomagnetite (tmt) and scarce skeletal conditions. olivine (ol) microlites (V75a). Sometimes cpx Detection limits for corundum 23 V. GARNIER ET AL. 24 BASALTS AND SAPPHIRES, S VIETNAM .e57V elpmas ;ssamd nuorg )lg( yssalg a ni )tmt( etitengamonatit dna enexoryponilc ,esalcoigalp fo setilorcim dna )xpo .tsed( enexorypohtro fo tsyrconex dezilibatsed ,)xpc( enexoryponilc dna )lo( enivilo fo stsyrconehporciM )f( .1g57V elpmas ;ssamdnuorg yssalg a ni enexoryponilc dna esalcoigalp fo setilorcim ,enexoryponilc dna enivilo fo stsyrconehporciM .tlasab citiryhprop enivilo na ni )xpo( enexorypohtro fo stsyrconex dezilibatseD )e( . c57V elpmas ;stsyrconehp )lp( esalcoigalp no depoleved )xpc( enexoryponilc hcneuQ )d( . c57V elpmas ;sliat wollaws htiw esalcoigalp dna tibah elkcub tleb ,ertnec wolloh htiw )lp( esalcoigalp latelekS )c( . c57V elpmas ;erutxet citihpobus ni htworgretni )xpo( enexorypohtro dna )lp( esalcoigalP )b( . c57V elpmas ;ssamduorg yssalg a ni setilorcim enexoryponilc tnadnuba dna setilorcim )lo( enivilo ecracs ;esalcoigalp fo setilorcim dna stsyrconehporcim ,)xpo( enexorypohtro fo tsyrconehP )a( .erutxet kcor fo egami ESB )egap gnicaf( .2 .GIF shows a quench texture, and smectite is sometimes developed in the groundmass (V74a,c). Both pl and cpx also demonstrate quench textures (V75c ), pl with swallow tails and belt buckle habits and cpx with swallow tails, feather and cervicone habits (Fig. 2c,d). The second basaltic suite is composed of alkali basalts with porphyritic olivine (ol) basalt (V75c, V75e, V75g1) with hyalopilitic texture (Fig. 2e,f). They are sometimes vesicular (V75b, V75f, V75h1) with amygdales ®lled with smectite and calcite. Xenoliths of protogranular spinel-lherzolite (up to 6 mm) are observed (V75b, V75c). Xenocrysts of opx are highly resorbed with rounded edges and embayments. Xenocrysts of feldspar (1 mm) have a sieve texture (V75g1, V75h1). Xenocrysts of ol (1.5 mm) show kink band boundaries with a small corona of cpx and sometimes show iddingsitic alteration. Xenocrysts of opx are destabilized showing at least two corona of secondary cpx and ol. Brown xenocrysts of Cr-spinel (sp) present a dark secondary sp rim, up to 1 mm. Xenocrysts of quartz (up to 0.7 mm) present corona of secondary cpx with radial arrangement (V75f, V75h1). Phenocrysts of olivine are common, whereas, plagioclase phenocrysts, when present, are highly zoned and have resorbtion features; within the embayments dendritic cpx overgrows the pl phenocrysts (V75e). Olivine microphenocrysts are associated with microlites of pl, cpx and tmt; smectites occur in the glassy groundmass. Plagioclase microlites are sometimes in intersertal arrangement. Patches of yellow smectites develop as alteration of the previous glass. The fractionation trends of the two suites are different. In the tholeiitic series the sequence is ol+sp-opx-pl-cpx whereas in the alkaline one the crystallization order is ol+sp-cpx-pl according their textural relationships. ' ' ' ' Mineral chemistry ' The tholeiitic and alkali basaltic suites show different mineralogical trends. Feldspar microlites (Fig. 3) evolve from labradorite to andesine (An65.0ÿ36.0) in the tholeiitic suite and are characterized by low Or content (up to 5.3%). The range of An content in the feldspar microlites of the alkali basaltic suite is similar (An59.0ÿ37.3) but they differ by their higher Or content (up to 7.4%). Plagioclase phenocrysts (Fig. 3) show a restricted labradorite composition (An64.2ÿ51.1) with low Or content (up to 1.4%) in the tholeiites, 25 V. GARNIER ET AL. FIG. 3. Composition of plagioclases from the two basaltic suites from Dak Nong. cpx microlites of both suites are also different (Fig. 4a). The cpx microlites of the tholeiitic suite occupy mainly the augite and ferroaugite ®elds and few analyses lie in the subcalcic ferroaugite ®eld de®ned by Poldervaart and Hess (1951). whereas in the alkaline suite, phenocrysts and xenocrysts of pl are scarce and evolve from labradorite to andesine (An59.6ÿ24.7) and are also characterized by higher Or content (up to 12.1% for the most sodic members). The composition of FIG. 4. Composition of pyroxene and olivine from the two basaltic suites from Dak Nong. (a) Microlites; open squares = tholeiitic suite, black circles = alkaline suite; (b) pyroxene phenocrysts of the tholeiitic suite, open squares = phenocrysts without plagioclase; black squares = phenocryst in sub- to nesophitic arrangement with plagioclase; (c) pyroxene and olivine composition in the alkaline suite: open circles = in xenoliths; grey circles = coronitic pyroxenes; full circles = coronitic and secondary olivines. 26 BASALTS AND SAPPHIRES, S VIETNAM contrast, the cpx microlites from the alkaline suite have a restricted compositional range and plot in the diopside and salite ®eld of Poldervaart and Hess (1951) and in the diopside ®eld of Morimoto et al. (1988). The diopsides are enriched in Ca (Wo up to 54.6%) and often lie above the diopside±hedenbergite join. The pyroxene pheno- The pyroxenes of the alkaline suite are more magnesian and more calcic than those from the tholeiitic suite. In the Morimoto et al. (1988) classi®cation, the cpx analyses are mostly restricted to the augite ®eld, few are pigeonite. The wide range in composition of the cpx for the tholeiitic suite is explained by quenching. In TABLE 1. Chemical analyses of the basalts studied. Alkali basalt suite Sample V75b V75f V75g V75h V75e SiO2 Al2O3 Fe2O3* MnO MgO CaO Na2O K2O TiO2 P2O5 LOI Total Ba Co Cr Nb Ni Rb Sr Ta Zr Ce Dy Er Eu Gd Ho La Lu Nd Pr Sm Tb Tm Y Yb 45.10 13.41 13.02 0.18 10.68 7.54 2.02 2.24 2.15 0.67 3.03 100.04 685 56 438 56.1 338 60.2 879 3.9 241 82.6 4.65 1.76 2.29 6.10 0.72 41.6 0.19 36.8 9.06 7.34 0.86 0.23 21.4 1.31 45.23 13.43 12.87 0.17 10.76 7.49 2.36 2.17 2.14 0.62 2.78 100.02 674 57 451 56.6 349 65.6 897 4.0 244 81.3 4.68 1.79 2.54 6.29 0.75 41.2 0.21 36.5 9.12 7.36 0.86 0.24 22.0 1.40 45.42 13.46 12.59 0.16 10.87 7.52 2.46 2.12 2.14 0.62 2.63 99.99 676 58 384 54.4 361 65.0 885 4.0 236 80.1 4.51 1.74 2.50 6.50 0.72 40.5 0.22 38.0 9.35 7.69 0.88 0.22 21.0 1.41 45.09 13.20 13.22 0.19 10.35 7.63 2.27 2.04 2.12 0.61 3.29 100.01 663 55 419 53.3 336 62.0 863 3.7 228 81.6 4.31 1.70 2.46 5.95 0.75 40.9 0.20 36.7 9.04 7.57 0.86 0.22 20.7 1.26 44.88 13.49 12.86 0.17 10.63 7.40 2.65 2.23 2.15 0.62 2.61 99.69 669 54 418 46.1 316 52.5 880 3.9 232 83.5 4.66 1.79 2.42 6.40 0.77 37.6 0.18 37.2 9.43 7.71 0.89 0.24 21.3 1.40 Tholeiitic basalt suite Subgroup I Subgroup II V74a V74c V74b V75a V75d 46.17 16.89 12.69 0.13 7.05 6.06 1.65 0.13 1.77 0.18 7.12 99.84 111 63 258 7.9 172 1.8 159 0.7 90 31.3 7.65 3.46 2.68 8.59 1.38 12.8 0.41 23.3 4.68 6.99 1.32 0.46 37.0 2.75 46.72 17.75 10.00 0.13 5.60 6.84 1.88 0.20 2.04 0.2 8.49 99.85 104 42 194 9.3 102 2.9 150 0.8 110 29.3 6.61 3.41 2.07 7.74 1.30 14.6 0.43 20.6 4.13 5.93 1.14 0.45 56.5 2.71 * Total Fe LOI: loss on ignition. Trace elements in ppm and major elements in wt.% 27 52.25 15.67 10.64 0.18 5.05 8.97 2.94 0.41 1.58 0.2 1.94 99.83 98 65 138 7.4 185 8.7 237 0.7 87 19.0 4.40 2.25 1.50 4.84 0.86 8.8 0.30 13.0 2.71 4.00 0.76 0.31 28.5 1.90 52.70 15.20 11.06 0.13 5.44 8.54 2.79 0.64 1.63 0.17 1.56 99.86 105 48 164 7.7 147 13.1 226 0.7 90 18.5 4.09 1.89 1.44 4.54 0.75 8.1 0.25 13.0 2.67 3.84 0.70 0.28 21.2 1.68 53.49 15.31 10.64 0.15 5.94 8.72 2.78 0.70 1.59 0.22 0.92 100.46 121 51 206 9.5 143 14.5 277 0.7 110 23.2 4.22 2.05 1.51 5.10 0.85 11.5 0.27 14.0 2.93 4.09 0.73 0.27 29.7 1.77 V. GARNIER ET AL. alkaline suite with compositions between XCr0.48ÿ0.08 and XMg0.83ÿ0.12; the rim is tmt. crysts of the tholeiitic suite (Fig. 4b) are mostly bronzite, augite and rarely sub-calcic augite. In the alkaline suite (Fig. 4c), xenocrysts of both pyroxenes show complex reequilibration trends due to the disequilibrium between xenocryst composition and the melt. Most of the cpx are diopside-endiopside salite and few fall in the augite ®eld, the opx xenocryst composition lies close to the enstatite-bronzite boundary (En91.2ÿ85.9). Olivines are ubiquitous in the alkaline suite and scarce in the tholeiitic, their compositions are also different in both series (Fig. 4a). They are more magnesian in the alkaline suite (Fo89.6ÿ65.1) and more evolved in the tholeiitic (Fo74.0ÿ64.4). Olivine never occurs as a phenocryst in the tholeiitic suite; the ol xenocryst (Fo90.1ÿ77.0) cores are more magnesian than the microlites and the rim and the secondary ol (Fo up to 88.5%) in the corona are close in composition to the composition of the microlites. Cr-spinels occur only in the Whole-rock chemistry All the whole-rock analyses are given in Table 1. The suites are distinguished by their chemistry. The tholeiitic suite plots in the quartz tholeiite ®eld and the alkalic suite is Ol and sometimes Ne normative (Fig. 5a). The tholeiitic suite can be divided into two sub-groups; the ®rst lies in the ®eld de®ned by Hoang et al. (1996), the second lies along the QzÿHy join due to their high Al2O3 content and normative plagioclase. The trace elements also separate both series in a 2Nb-Zr/ 4-Y triangular diagram (Fig. 5b) of Meschede (1986). The alkaline suite plots in the within-plate alkaline basaltic ®eld, whereas the tholeiitic suite crosses the within-plate tholeiitic, the volcanic arc and the MORB ®elds. Compared to MORB, the FIG. 5. Chemical variation diagrams of the two basaltic suites from Dak Nong. (a) Normative diagram, the grey ®eld corresponds to Vietnamese basalts studied by Hoang and Flower (1998); (b) 2Nb-Zr/4-Y triangular plot. The ®elds are de®ned as follows: AI ± within-plate alkali basalts; AII ± within-plate alkali basalts and within-plate tholeiites; B ± E-type MORB; C ± within-plate tholeiitic and volcanic-arc basalts; D ± N-type MORB and volcanic-arc basalts (from Meschede, 1986); (c) REE patterns (normalizing data from Evensen et al., 1978); (d) spider diagram (normalizing data from Hofmann, 1988). For all diagrams: open squares = tholeiitic suite; full circles = alkaline suite. 28 BASALTS AND SAPPHIRES, S VIETNAM 96.001ÿ75.001 )2 = n( neerg-wolley 59.99ÿ29.99 )5 = n( wolley 87.001ÿ88.99 61.001ÿ95.89 )2 = n( neerg-eulb 93.99ÿ81.99 )41 = n( eulb krad 13.001ÿ65.79 )8 = n( 'ekil-ehcipart` eulb 40.001ÿ45.89 Trace element chemistry of corundum )mpp( nZ )mpp( gM )mpp( V )mpp( aG )mpp( eF )mpp( iT )mpp( rC )%.tw( 3O2lA ruoloC Sapphires from Dak Nong deposit fall into eight types according to colour and texture. Characteristic chemical analyses of each type of sapphire are given in Table 2. The corundums are Zn- and Mg-poor (often below detection limit), they have low to medium Cr, Ti, Ga and V contents relative to corundums from Vietnamese marbles (Garnier, 2003) and are Fe-rich reaching 1.8 wt.% Fe2O3 in the black, blue and green sapphires. The green sapphires are the richest in Cr. This is consistent with the known chromophorous nature of blue, green and yellow sapphires: blue and yellow colours are linked to the substitution of Al by Fe and Ti (Schmetzer and Bank, 1981; Schmetzer, 1987); the presence of Cr3+ in octahedral sites is necessary to give the yellow colour (Schmetzer and Bank, 1981). According to Sutherland et al. (1998a) and Sutherland and Schwarz (2001), high Ga2O3 contents (up to 0.04 wt.%) as well as Cr2O3/ Ga2O3 ratios below 1 are typical of the sapphires from the `basaltic' suite. Dak Nong sapphires are plotted on Cr2O3/Ga2O3 vs. Fe2O3/TiO2 and TiO2/ Ga2O3 vs. Fe2O3/Cr2O3 diagrams in Fig. 6. For comparison, the ®elds corresponding to the `basaltic' and `metamorphic' suites of the basaltic deposits from Pailin in Cambodia and from eastern Australia have been added (data from Sutherland et al., 1998a), as well as rubies from marble-hosted deposits of northern Vietnam (Garnier, 2003; Pham Van et al., 2004). The sapphires from Dak Nong fall mainly in the characteristic ®elds of the `basaltic' suite. Some grains are more Cr-rich, the green-coloured desylana selpmas fo rebmun = n ;timil noitcetced woleb = ldb ;eulav naem = *)84( 78.89ÿ15.79 )11 = n( neerg ldb ldb ldb ldb )52( 821±ldb )8( 56±ldb 091ÿ131 )661( 834ÿldb )021( 091ÿ82 752ÿ452 )712( 172ÿ941 )532( 552ÿ312 89021ÿ00111 )1749( 07421ÿ0825 )06201( 93521ÿ1077 902ÿ861 )952( 0801ÿ53 )522( 073ÿ551 33 )65( 013±ldb *)84( 49±ldb )3 = n( kcalb ldb ldb ldb ldb ldb ldb ldb ldb )14( 28±ldb )01( 25±ldb )84( 94ÿ84 )251( 502ÿ89 )05( 891±ldb )391( 892ÿ46 )431( 303ÿ96 )752( 852ÿ652 )902( 422ÿ491 )772( 503ÿ242 )532( 752ÿ412 )122( 462ÿ781 )1084( 2135ÿ0924 )5717( 7727ÿ2707 )8175( 8188ÿ5054 )41511( 00621ÿ28401 )0338( 46911ÿ4594 )39( 921ÿ65 )471( 002ÿ741 )89( 351ÿ25 )455( 857ÿ744 )811( 091ÿ06 )701( 241ÿ37 )52( 05±ldb )88( 171±ldb )431( 961ÿ86 )774( 2851±ldb )3 = n( sselruoloc .gnoN kaD morf serihppas tnereffid eht fo egnar lanoitisopmoC .2 ELBAT tholeiitic suite is enriched in light rare-earth elements (LREE) (Fig. 5c) and is comparable to within-plate tholeiites from Vogelsberg (Bogaard and WoÈrner, 2003) or Ethiopian ¯ood basalts (Kiefer et al., 2004). The alkaline suite shows a very steep LREE enrichment and is comparable to continental alkaline intraplate basalts from Vogelsberg (Bogaard and WoÈ rner, 2003), Mongolia (Barry et al., 2003) or Ethiopian alkali basalts (Kieffer et al., 2004). The two sub-groups of the tholeiitic suite are distinguished by their REE patterns. On the spider diagram (Fig. 5c,d), the sub-group II is enriched in REE compared to the sub-group I which presents negative Rb, K and Sr anomalies. The alkaline suite is homogeneous and comparable to other Vietnamese alkaline rocks described by Hoang et al. (1996). 29 V. GARNIER ET AL. FIG. 6. Chemical variation diagrams showing oxide ratio plots of trace-element contents of green, blue and yellow sapphires from Dak Nong in southern Vietnam (open circles), from marble-hosted deposits in northern Vietnam (open triangles, unpublished data); for comparison, the ®elds corresponding to the `metamorphic' and `basaltic' corundum types of Pailin (Cambodia) and from Eastern Australia, de®ned by Sutherland et al. (1998a), have been added. is very poor and the errors are large. Thus, only sapphires, and these fall away from the `basaltic' ®eld. The sapphires from Dak Nong the are 206 Pb- 238 U ages Ô1 s completely different to the rubies from marble- Cathodoluminescence hosted deposits of northern Vietnam which are zircon are given in Fig. 7. Cr-richer and Fe- and Ga-poorer. Zircon from the will be considered. images U-Pb ages the analysed corundum-bearing formation of Dak Nong yield between of 1.05Ô0.05 Ma and 206 Pb- alluvial 238 U ages 7.13Ô0.88 Ma (Fig. 7). Sample `a' is an oscillatory zoned grain The U-Pb isotopic data of analysed zircon are with superimposed recrystallized patchy zoning in summarized in Table 3. Due to their young ages, its centre and at its rim. Both textures have been these zircons accuracy of 207 are Pb- 235 207 the dated and yield ages overlapping within uncer- U ages of young minerals tainty. Sample `b' shows clear oscillatory zoning depleted in Pb, so 30 BASALTS AND SAPPHIRES, S VIETNAM TABLE 3. U-Pb isotopic data of zircons analysed. Contents (ppm) 206 204 Pb Calibrated and corrected 206 238 206 238 U 62 0.2 209 643 3.08 0.000997Ô0.000042 0.161360Ô0.161397 23 0.1 85 153 1.80 0.010516Ô0.000109 0.126326Ô0.596716 6.78Ô0.70 Sample b 18 0.1 68 45 0.66 0.001107Ô0.000135 0.119176Ô0.822229 7.13Ô0.88 Sample e 59 0.1 737 726 0.99 0.000162Ô0.000007 0.129343Ô0.224884 1.05Ô0.05 238 Pb Pb- Th/U Pb- Pb/ 206 Th 206 U 207 U FIG. 7. Cathodoluminescence images, with Pb- Calibrated and corrected Pb Sample a Pb- ages (Ma) 6.42Ô0.27 U ages, of zircons recovered from the Dak Nong placer. 31 V. GARNIER ET AL. range of common in zircons (Hoffman and Long, 1984; (4.8 Ohnenstetter 6.4Ô1.1 % et al., 1991; Corfu et al., 2003). This d d sample yielded an age of 7.13Ô0.88 Ma. This age surrounded Sample by `e' has oscillatory a unzoned growth core zones. O values which of continental have % an and a median of 6.1 basalts average of (Harmon and % Hoefs 1995). The sapphires from Dak Nong have is in statistical agreement with those measured in `a'. 18 ÿ11.4 % ) 2000) as well as hour-glass sector zoning often sample d for a tholeiite. These isotopic values fall in the patterns, typical of magmatic zircons (Hoskin, 18 The core yielded an age of 1.05Ô0.05 Ma, and this d %n Ocorundum values ranging from 6.0 to 6.9 (average 18 18 Ocorundum = 6.6Ô0.4 O corundum O-isotopic values are equilibrium too with , = 5). These high the to be local in d alkali 18 age is probably a crystallization age and not a olivine basalts and tholeiites. The invariant reset age (see Fig. 7, sample e). These U-Pb ages of mantle olivine and the restricted range of suggest two events of zircon formation: the ®rst at mineral fractionation with cpx and opx (Mattey ~6.5 Ma followed by a second one at ~1 Ma. As d et al., 1994; Chazot et al., 1997), imply that the bulk zircon and corundum are xenocrysts, the age 18 % range of eruption, which is younger. 6.9 Barr and MacDonald (1981) reviewed K-Ar d Ocorundum composition has to be in the same obtained on zircon pre-date the age of basalt 18 O values. The higher values such as for the sapphires support the hypothesis that sapphires are xenocrysts in these basalts. This age data of southeast Asian Cenozoic basalts and hypothesis was also noted by Yui concluded the that basalt activity in O Indochina Denchai area. Based on et al. (2003) in the theoretical appeared by at least 12 Ma, after the end of the modi®ed South China Sea opening, and peaked in the last 3 oxygen million years. Sutherland and Schwarz (2001), expressed the corundum-water isotope fractiona- reviewing all the published geochronological data tion by the equation: of basalt-hosted sapphire deposits, emphasized 3 10 ln that the southeast Asian deposits are Neogene. Hoang and Flower (1998) suggest that the a increment isotope = 2.24 where a&d equilibrium the 10 ln Phuoc intervals: Long Cambodia, Dalat (straddling <8 ÿ3.4 Ma), (17.6 the ÿ7.9 Ma), border Buon Ma 3 with eruptive episodes. Coenraads U-Pb age of 1 ÿ2 sapphire-bearing Thuot sapphire 18 d Zheng 3 a 18 d d ÿ OH 18 O 2 18 OH O. 2 + 4.28 factor and In the case % in equilibrium with O corundum % between 1 (1991) is the corundum-water fractionation Ocorundum (average calculating ÿ 13.71610 Tÿ between 10.6 and 11.7 et al. (1995) found a of Tÿ2 isotope temperatures 1000 and = ) at 1200ëC, 6.6 is . It is apparent that the Dak Nong sapphires were in isotopic equilibrium (Ô<1) Ma for a zircon in a xenolith 6 of Dak Nong, the ÿ1.67 Ma), Pleiku (4.3ÿ0.8 Ma) and Xuan Loc (0.83ÿ0.44 Ma), con®rming the bi-episodal (5.8 10 of fractionations, T is in Kelvin and Vietnamese volcanic centres were active over following 6 method with Chanthaburi-Trat an oxygen reservoir d characterized by a crustal component as a lower-crust alumina-rich % and K-Ar ages of 3.0 to 0.44 Ma for the alkalic granulite which has volcanism in the Chanthaburi province. Sapphire/ 5 to 14 ruby-bearing basalts from Thailand yield K-Ar 18 O composition in the range (Hoefs, 1997; Yui et al., 2003). Discussion ages between 5.64Ô0.28 and 0.44Ô0.11 Ma (Barr and MacDonald, 1979, 1981). Thus, the U-Pb ages of alluvial Vietnamese zircons related to The oxygen isotopes can be used as a tracer for sapphire the formation process of the corundums (Giuliani deposits are in agreement with the periods of activity of the volcanic centres in et al., southern Vietnam and highlight the bi-episodal associated with alkali basalts. Three main models eruptive events as already proven in the Buon Ma have been proposed for the formation of sapphires Thuot area (Hoang and Flower, 1998). and rubies found in basaltic terrains: (1) crystal- 2004), especially for corundum xenocrysts lization of corundum from fractionated syenitic 1986; Aspen et al ., 1990; et al., 1990, 1995; Guo et al., 1996; Sutherland et al., 1998b; Upton et al., 1999); Oxygen isotopic composition of lavas and corundums melts The oxygen isotopic compositions of basalts and d (2) crystallization in the upper mantle, from a sapphires are listed in Table 4 and shown in Fig. 8. The 18 % % ma®c melt (Sutthirat morphic O values of the two alkali olivine basalts are 5.0 and 5.2 , and 5.7 (Irving, Coenraads (Levinson is obtained 32 et al., crystallization and Cook, of 2001); (3) metaaluminous 1994; rocks Sutherland and BASALTS AND SAPPHIRES, S VIETNAM 18 % TABLE 4. d O ( vs. V-SMOW) of lavas (whole rock) and variously coloured sapphires from basalt-hosted deposits and from different metamorphic types of ruby and sapphire deposits; n = number of analyses per sample. Country Mine Corundum Host rock Sample number ( Reference %, V-SMOW) 5.7Ô0.1 ( n = 3) 5.0Ô0.2 (n = 3) 5.2Ô0.1 (n = 3) 6.0 6.4 6.7 6.9 Vietnam Dak Nong basalt basalt basalt dark blue dark blue blue greenyellowish yellow tholeiitic basalt alkali basalt alkali basalt placer placer placer placer placer V76d 6.9 " Sri Lanka Neluyaya Kaltota pinkish gneiss (granulite) KCl-3 7.1 this work France Velay Beaune sur Arzon light blue V1 8.7 this work lilac colourless placer in a basanite (granulitic complex) " V2 V3 9.2 8.0 " " brown to light blue colluvium and alluvium from a basanite ± 5.9Ô1.1 Gaillou (2003) blue to green placer in volcanic ®elds ± 6.0Ô0.2 Gaillou (2003) Cantal Menoyre light blue anorthoclasite xenolith in trachyte ± 5.9Ô0.6 Gaillou (2003) India Mysore red gneiss (granulite) " MYHM96 3.8 this work RNDUD002 3.5 " Russia Karelia pink gneiss (granulite) " Karelia 1 3.0 this work Karelia 2 2.9 " Madagascar Soamiakatra red pyroxenite xenolith in alkali basalt RNANTA 4.7 this work Kenya Garba Tula yellowgreen syenite GT-1 5.2 this work Thailand Denchai dark blue placer in alkali basalt " " BK-MS 4.7 5.5 (n = 7) Mont Coupet Mont Dore Sioulot red pink blue blue-greenyellow V75d V75b V75f V76a DN-1 V76b V76c d18O this work " " " " " " ÿ Yui 4.9 8.4 (n = 5) 5.1 5.9 (n = 4) ÿ ÿ " " et al. (2003) et al. Canada Grenville Province corundum " " syenite " " S1 S2 S3 7.6 7.8 7.6 Kerrich (1987) " Scotland Loch Roag Colourless to brown " " " trachyte xenoliths in alkali basalts " " " LR core 4.8 LR rim LR-250 LR-299 5.25 4.65 4.8 Upton et (1999) " " " 33 al. V. GARNIER ET AL. FIG. 8. Oxygen isotopic composition of corundum from various protoliths. Source of data: see Table 4. Coenraads, 1996). As mantle and crustal rocks (2) In ma®c trapped melts in sub-continental have distinct oxygen isotope compositions, the peridotites, corundum occurs: in garnet clinopyr- genetic process for the formation of corundum oxenites associated with sapphirine as in Beni Bousera composition of ruby and sapphires of different aluminous geological origins: sapphirine, corundum, clinopyroxene, plagioclase, % have d18Ocorundum values ÿ5.25%) of 5.2 ma®c layers 1990); containing spinel in the Ronda massif (Morishita (1) In syenites: sapphires from Garba Tula (Kenya) (Kornprobst et al., will be discussed in the light of new O-isotopic in thin garnet, et al., 2001); in ma®c rocks and in garnet pyroxenite in the (Table 4) which are within the range of values Cabo Ortegal massif (Girardeau and Ibarguchi, found 1991); for sapphires (4.65 associated in ma®c layers composed of olivine, with trachyte xenoliths in Scottish alkali basalts plagioclase, margarite, spinel and amphibole in (Upton an the Horoman peridotite complex (Morishita and anorthoclasite xenolith in the trachyte of the Arai, 2001) and in a gabbroic boulder probably Do à me et al ., of 1999). Menoyre Blue % (Cantal, sapphire France) in a derived from the same massif (Morishita and 2003). Kodera, 1998), in a sapphire-ruby-garnet pyricla- has d18Ocorundum value of Kerrich et al . (1987) 5.9 between 7.6 and 7.8 for syenitic corundums % (Gaillou, obtained d18Ocorundum site and a sapphire-ruby-garnet clinopyroxenite xenoliths in alkali basalts from Denchai (Sutthirat et al., from the Grenville Province. 34 2001); and in ruby-garnet pyroxenite from BASALTS AND SAPPHIRES, S VIETNAM Soamiakatra, 2003). In d18Ocorundum et al., Madagascar Denchai, from an evolved syenitic melt contaminated by (Rakotosamizanany, % placer corundums values between 4.7 and 5.9 % lower crustal ¯uids. have (Yui Acknowledgements 2003) and in Soamiakatra, the ruby has a d18Ocorundum value of 4.7 (Table 4). (3) In granulites, corundums occurs either in This study was supported by IRD, CNRS (CRPG), ma®c granulites as in Mysore (India) and Karelia the PICS (CNRS-INSU and CNST) program. We (Russia), ranging with from aluminous Neluyaya d18Ocorundum low 2.9 to 3.8 % or in of Henri Poincare Â, Nancy) for SEM images and granulites as in microprobe analyses, and M. Champenois and % higher D. Mangin (CRPG) for their help in using the Velay ion probe. We are grateful for the assistance of the charnockitic Kaltota d18Ocorundum (Sri values granulites (France) 7.1 % with between 8.0 and 9.2 (Table 4); Lanka), of a thank S. Barda, F. Diot and A. Kohler (University compositions ; with and in d18Ocorundum Vietnamese range basalts d18Ocorundum have between 2.9 and 9.2 sources are granulites 18 Program on the % compositions gave the sapphires from Garba Tula, P. Francis (Fig. 8). For ruby, the from Kelaniya (Sri Lanka) for the sapphires of pyroxenites (d Research thank J.M. Saul, Director of ORYX company, who In summary, the xenocrysts of corundums in alkali Basic Geodynamics of the Red River fault zone. We . and Ocorundum = ÿ4.7 % ), Al-rich 2.9 Neluyaya Kaltota, F.H. Forestier (University of ma®c Nantes) as for the corundums from Beaune-sur- proposed for alluvial ruby in New South Wales Arzon and E. Gavrilenko (University of Madrid) (Australia) for the ruby from Karelia, S. Rakotosamizanany which contains inclusions (Sutherland et al., Al-rich diopside 2003). Sutthirat et and al. (2001) also proposed a garnet granulite origin for the ruby-bearing clinopyroxene Toulouse) xenocryst and for Madagascar. from Eastern Thailand. Regarding the source of % % M. Rakotondrazafy Antananarivo) B. access We (University Moine to thank (University their data of of on C.M.B. Henderson sapphires, two origins are possible (Fig. 8): a (University of Manchester) and P.W.O. Hoskin syenitic origin (4.8 < (University of Freiburg) for their critical reviews d18Ocorundum < 7.8 18 < d Ocorundum < 9.2 granulitic one (7.1 d18Ocorundum % of an earlier version of the manuscript. ). 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