petrogenesis of cenozoic basalts from vietnam - journal of petrology

JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 PAGES 369–395 1998

Petrogenesis of Cenozoic Basalts fromVietnam: Implication for Origins of a‘Diffuse Igneous Province’

NGUYEN HOANG∗ AND MARTIN FLOWER†DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF ILLINOIS AT CHICAGO (M/C 186), 845 W. TAYLOR STREET,

CHICAGO, IL 60607-7059, USA

RECEIVED JULY 8, 1996; REVISED TYPESCRIPT ACCEPTED AUGUST 28, 1997

low-viscosity thermal boundary layer (TBL), and caused upwardBasalt magmatism occurred throughout east and southeast Asiapenetration and polybaric melting of TBL–asthenosphere columns.after the early Tertiary India–Asia collision. This activity does

not conform to the ‘Large Igneous Province’ model in view of

lower eruption and melt production rates, wide dispersal of

centres and the apparent absence of deep mantle upwelling. Age

data for Vietnamese plateau basalts reflect spatial–temporalKEY WORDS: Vietnam; basalt; Cenozoic; geochemistry

patterns consistent with a rotating stress field rather than supra-

hotspot lithosphere migration. For most of the volcanic centres

there are two eruptive episodes: an early series formed by high-

SiO2, low-FeO∗ quartz and olivine tholeiites—large melt fractions

of refractory (lithosphere-like) mantle—and a later series made INTRODUCTIONup of low-SiO2, high-FeO∗ olivine tholeiites, alkali basalts and Neogene–Quaternary intraplate volcanism is widespreadbasanites—smaller melt fractions of more fertile (asthenosphere- in east and southeast Asia (Fig. 1a) forming basalt plateauxlike) mantle. Comparison of Mg-15 normalized basalt com- associated with pull-apart, extensional rifts (Barr &positions with parameterized anhydrous and hydrous experimental McDonald, 1981; Whitford-Stark, 1987). Althoughmelt compositions allowed calculation of melt segregation pressures widely dispersed the activity shares common source iso-and temperatures. Computed for anhydrous conditions these range topic and lithosphere structural character with intraplatefrom <4 GPa and ~1470°C (for alkali basalts) to <0·5 GPa and back-arc volcanism in the western Pacific and hasand ~1400°C (quartz tholeiites), and for H2O-undersaturated been referred to as a ‘diffuse’ igneous province (Hoangconditions, from <3·5 GPa and ~1450°C to ~1·5 GPa and et al., 1996). The activity post-dates the early Tertiary1350–1400°C, respectively. Hydrous conditions are more realistic India–Asia collision and may be related to asthenosphericin view of high measured basalt H2O

+ contents, pressure and lithospheric tectonic extrusion processes (Tapponnierestimates consistent with melting below a thinned mechanical et al., 1982, 1986). The province is bounded to the eastboundary layer (MBL) and interpolated mantle adiabats of and southeast by active subduction at the Izu–Bonin,2–3°C/km (compared with <1°C/km for anhydrous conditions), Mariana and Indonesian archipelagos, and to the north-consistent with fluid dynamic constraints and a 1440°C potential west by the collision-thickened Tibet plateau (Flower ettemperature. After collision-induced ‘extrusion’ of east and al., 1998a). The Indochina and China plates appear tosoutheast Asia, the lithosphere was probably thinned during have been tectonically extruded along regional strike-slip

faults, with concomitant opening of the South Chinaheating and transtension; this converted refractory MBL into a

∗Present address: University of Tokyo, Ocean Research Institute,1-15-1 Minamidai, Nakano, Tokyo 164, Japan.†Corresponding author.Extended data set can be found at: http://www.oup.co.uk/jnls/list/petroj Oxford University Press 1998

JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998

Fig. 1.

Sea, Japan Sea and Andaman Sea basins (Tapponnier as the ‘southeast Asia DUPAL anomaly’ (e.g. Hickey-Vargas et al., 1995; Castillo, 1996). This has been dis-et al., 1982, 1986; Briais et al., 1993).

Although widely dispersed, east Asian Cenozoic basalt cussed by Hoang et al. (1996) with reference to Vietnamesebasalts, and by Flower et al. (1998a) with respect to eastmagmatism does not conform to the ‘Large Igneous

Province’ model (Coffin & Eldholm, 1994) in view of the Asia in general.The causes of dispersed, relatively sudden mantle melt-lower rates of eruption and melt production, dispersal of

eruptive centres and apparent absence of deep mantle ing events have not been extensively discussed in theliterature although there has been progress in under-upwelling (Su et al., 1994). In common with other ‘diffuse’

provinces (e.g. Ormerod et al., 1988; Hoernle et al., 1995) standing the constraints of pressure, temperature, P(H2O)and mantle fertility on melt composition and volumethe east Asian activity reflects contemporaneous, rapid

appearance of dispersed basalt centres, a transtensional (e.g. McKenzie & Bickle, 1988; Latin & White, 1990;Wilson, 1993). Here, we advance the idea that ‘diffuse’setting, and proximity to major continent–continent plate

collisions. In addition, east Asian continental and western volcanic provinces may reflect the combined effectsof lithosphere transtension and asthenospherePacific back-arc volcanics appear to share a common

asthenosphere isotopic signature, sometimes referred to decompression concomitant with collision-induced

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HOANG AND FLOWER CENOZOIC BASALTS FROM VIETNAM

Fig. 1. (a) Map of the Indochina region showing fault systems (fine lines), Cenozoic volcanic centres (shaded), national boundaries (fine dashedlines) and lithospheric sectors (bold dashed lines) (Tung & Tri, 1992). I, Northern accretionary belt (Precambrian to Palaeozoic); II,Central—Kontum Massif (Archaean, Proterozoic, Palaeozoic); III, Southwest—Khorat Plateau (Precambrian) and surrounding Palaeozoic andMesozoic belts; IV, Southeast—undifferentiated Precambrian overlain by Mesozoic. (b). Map of south–central Vietnam (insert from a). Basaltcentres shaded, drill site locations numbered, and surface or dredge sample site numbers italicized. Lithospheric sector boundaries shown bydashed lines, lithospheric sub-sector boundaries by dotted lines (IIa–d, Archaean, Proterozoic, Cambrian and Permo-Triassic, respectively).

extrusion of thermally anomalous mantle. Assuming that Quoc & Giao, 1980; Hoang & Han, 1990; Thi, 1991;the volumes, supply rates and compositions of mantle and references given by Whitford-Stark (1987)]. Most ofmelts are simple functions of mantle composition, thermal these centres are associated with pull-apart structuresstructure and the extent of lithosphere stretching [fol- comprising short extensional rifts bounded by strike-sliplowing McKenzie & Bickle (1988) and Latin & White faults (Rangin et al., 1995a). The centres appear to have(1990)], we develop a petrogenetic model for the Viet- involved at least two eruptive episodes, referred to herenamese basalts as a basis for understanding possible as ‘early’ and ‘late’ eruptive series, thick palaeosols mark-asthenospheric thermal responses during the closure of ing the intervening quiescent periods (Hoang, 1996).eastern Tethys. Early episodes usually produced quartz and olivine thole-

iite flows, with rare alkali basalt, whereas later episodeserupted olivine tholeiite, alkali basalt, basanite and (rarely)nephelinite. This bi-episodal pattern is recognized at the

CENOZOIC MAGMATISM IN Dalat, Phuoc Long, Pleiku, Buon Ma Thuot, Xuan LocVIETNAM and Re Island centres, and probably other offshoreAges and eruption rates of basalts localities, although at Buon Ma Thuot the compositional

trend is inverted (see below). Tholeiitic eruptives are theBasalt plateaux in southern and central Vietnam oftenmost voluminous, with flows up to 30 m thick eruptedexceed 100 km in diameter, are up to several hundredfrom axial rifts. Alkali basalts, basanites, and rare ne-metres thick, and cover a total area of ~23 000 km2

(Fig. 1a) (Lacroix, 1933; Carbonnel & Saurin, 1975; phelinites formed thinner, more sporadic flows erupted

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mostly from small central volcanoes aligned on conjugate rates of up to ~2700 km3/my, assuming a total of atstrike-slip faults (Quoc & Giao, 1980; Hoang & Han, least 8000 km3 basalt were erupted at the major centres.1990; Thi, 1991; Hoang, 1996). However, such estimates underplay the effects of frac-

Barr & Macdonald (1981) reviewed new and published tional crystallization and also ignore the presence ofK–Ar age data for southeast Asian Cenozoic basalts and trapped melt consolidated at depth (see Latin et al., 1993).concluded that basalt activity in Indochina appeared byat least 12 Ma, after the cessation of South China Seaopening, and peaked in the last 3 my with an equally

Structural and dynamic controls onrapid abatement. More recent K–Ar data (Novikov et al.,volcanism1989; Arva-Sos et al., 1990; Rangin et al., 1995a) and 24The Indochina peninsula comprises fragments of Gond-high-precision Ar–Ar age dates for our stratigraphicallywana, made up of Precambrian, Palaeozoic and Mesozoicselected core samples (Lee et al., 1998) suggest the Viet-crust, that migrated northwards during the Palaeozoicnamese centres were active over the following intervals:and accreted to pre-Tethyan Eurasia (e.g. Gatinski et al.,Dalat (17·6–7·9 Ma), Phuoc Long (straddling the border1984; Hutchison, 1989; Tung & Tri, 1992). These arewith Cambodia) (<8–3·4 Ma), Buon Ma Thuot (5·8–1·67preserved as distinct lithospheric sectors separated byMa), Pleiku (4·3–0·8 Ma), Xuan Loc (0·83–0·44 Ma) andtectonic sutures of known age (Fig. 1a and b) (e.g. Tungthe Ile des Cendres (0·8–0 Ma), and confirm the bi-& Tri, 1992). A northern sector (I) comprises an accretedepisodal eruptive pattern (Fig. 1b).Archaean, Proterozoic and Palaeozoic complex in north-Although the bulk of volcanism in Indochina post-ern Vietnam and Laos, and southern China throughdates South China Sea spreading, Paleogene activity haswhich Cenozoic basalts were erupted at Dien Bien Phu,been recorded by drilling on the southern Chinese andPhu Quy, Con Co island and Khe Sanh (Fig. 1a),Vietnamese continental shelves (e.g. Zhang Qi Ming,and also localities in Hainan, Yunnan and GuangdongNanhai West Co., personal communication, 1989) andprovinces (South China), and the South China Sea (e.g.appears in Thailand, NW Vietnam, Yunnan and SichuanFlower et al., 1992; Tu et al., 1992). A central sector (II)close to the Ailao Shan–Red River (ASRR) and othercomprises the Kontum Massif, a quasi-cratonic blockstrike-slip shear zones (Flower et al., 1998b). Historicwith a 2·8 Ga core (Archaean) (Tung & Tri, 1992)activity in Vietnam is confined to the offshore Con Sonsurrounded by concentric ‘sub-sectors’ separated by Prot-swell (Ile des Cendres, Fig. 1a), sporadic ash eruptionserozoic, Cambrian and Permo-Triassic sutures (Fig. 1b).in the central highlands (e.g. Pleiku, April, 1993), andCenozoic basalts were erupted within this sector at Songsubmarine activity along the eastern seaboard (KoloskovCau (Archaean), Buon Ma Thuot, Pleiku and Konget al., 1986). Late series undersaturated lavas carry mantlePlong (Proterozoic), and at smaller centres in Quangxenoliths, including garnet lherzolites, spinel lherzolitesNgai and Re Island, and offshore north of latitude 15°Nand harzburgite, and megacrysts of pyroxene, olivine,(Cambrian) (Fig. 1a and b). A southwestern sector (III)plagioclase, garnet, zircon and corundum (e.g. Sa-includes the eastern part of the Khorat Plateau, whichpozhinkov et al., 1979; Han & Hoang, 1985; Hoang &probably has a Precambrian core (in Thailand) enclosedHan, 1990). The latter characterize centres in southernby accretionary Palaeozoic and Mesozoic belts (in Cam-Vietnam (especially Pleiku, Xuan Loc, Buon Ma Thuotbodia and southwest Vietnam) (Hutchison, 1989). Thisand Ile des Cendres), Cambodia and Thailand (Lacombe,sector contains the largest single basalt complex in Indo-1967; Barr & MacDonald, 1981), and Hainan Island,china (Phuoc Long; 200 km across and up to 500 mMingxi and other southern Chinese localities [Flower etthick) which straddles the Cambodia–Vietnam border.al., 1992; compare Sutherland (1983) and Irving & FreySmaller centres (as yet unstudied) occur in central and(1984)].western Cambodia and Thailand (Barr & MacDonald,Three factors allow fairly precise estimates of magmatic1981; Mukasa & Zhou, 1994; Intasopa et al., 1995;volumes erupted in Vietnam: first, the large number ofMukasa et al., 1996) (Fig. 1a). A southeastern sector (IV)cored hydrologic sections drilled through the basalts toprobably includes most of southern Vietnam and consistsbasement, second, the well-documented areal extent ofof accreted Proterozoic, Palaeozoic and Mesozoic base-the eruptives, and third, their essentially basaltic char-ment (Tung & Tri, 1992). With estimated crustal thick-acter. Provisional estimates of magmatic volume for thenesses of ~30 km (Tien, personal communication, 1993)principal centres are 1500 km3 for Dalat, 2200 km3 forthis sector includes basalts of both the oldest (Dalat) andPhuoc Long, 1500 km3 for Buon Ma Thuot, 2000 km3

youngest (Xuan Loc) onshore complexes, and the activefor Pleiku and 500 km3 for Xuan Loc, and for smalleroffshore Ile des Cendres and Katwit centres (Fig. 1acentres to the north are, for example, 180 km3 for Songand b).Cau and 160 km3 for Kong Plong (Fig. 1a and b). Despite

Tapponnier et al. (1982, 1986) proposed that the Indo-a lack of age data for some smaller centres, the knownage and volume relationships suggest magma production china plate was extruded southeastwards and rotated

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anticlockwise after the India–Asia collision. Developed moderately phyric (<10% phenocrysts) with plagioclase(An83–72), olivine (Fo78–74) and augite (Wo47–49En40–36Fs13–14).on the basis of scaled experiments, the extrusion model

has dominated post-Mesozoic plate tectonic re- Most are petrographically similar to mid-ocean ridgebasalt (MORB), with plagioclase preceding clino-constructions in the region and provides a simple ex-

planation for the opening of contiguous marginal basins. pyroxene at the low-pressure liquidus. Quartz tholeiitesfrom early Phuoc Long and Dalat series and Dien BienAlthough the extent of extrusion has been questioned in

the light of additional experiments (e.g. Jolivet et al., Phu, however, may contain small, unreacted phenocrysts1990), and palaeomagnetic (McCabe et al., 1993), gravity of orthopyroxene (~En82–78), in which respect they re-(Harder et al., 1993) and seismic data (Rangin et al., semble volcanic arc rather than MORB tholeiite (e.g.1995b), metamorphic thermochronology (Leloup et al., Kushiro, 1990). Orthopyroxene-phyric tholeiites are1995) and igneous age dates (Chung et al., 1998) associated highly unusual in intraplate tectonic settings and theirwith the ASRR shear zone confirm that 500–600 km petrogenesis may reflect high P(H2O) in the mantle sourceleft-lateral motion occurred between ~30 and 17 Ma, (see below). Olivine tholeiites are aphyric to sparselyfollowed by a few tens of kilometres of right-lateral phyric and are dominant at most other centres, usuallymotion. Whether South China Sea opening was wholly interlayered with quartz tholeiite and lesser amounts ofa response to extrusion (Tapponnier et al., 1986; Briais alkali basalt. Phenocrysts rarely exceed 10–15% by vol-et al., 1993) or represents Pacific plate-induced back-arc ume and are mostly olivine (Fo82–78) and augite (Wo44–

spreading (Taylor & Hayes, 1983) is unresolved (Rangin 43En42–40Fs16–19), with lesser amounts of plagioclaseet al., 1995b). In either case, the migration of magmatism (An85–68). Alkali basalts and basanites are common in latefrom the South China Sea spreading axis to Indochina series of Xuan Loc and Re Island centres, whereas alkalimarks a major mid-Miocene shift in the locus of litho- basalts are also prominent in the Pleiku late series andsphere extension (Le Pichon et al., 1995). Buon Ma Thuot early series. These are moderately

On the basis of palaeostress measurements in Vietnam, phyric, with 7–15% olivine phenocrysts (Fo89–70), severalRangin et al. (1995a) argued that faulting during the generations of which may be distinguished on the basisPaleogene and Neogene was determined by two super- of morphology and composition, together with lesserimposed stress systems. An older system compatible with amounts of augite.an east–west maximum compressional axis predominates Mantle xenoliths include garnet lherzolite, spinel lher-in northern and central Vietnam and produced NW–SE zolite and harzburgite, along with eclogite of unknownleft-lateral strike-slip faults (parallel to the ASRR shear provenance and cumulate xenoliths comprising wehrlite,zone) with conjugate SW–NE right-lateral faults (Fig. 1a). websterite and pyroxenite. Megacrysts include olivine,A younger system compatible with a north–south max- Al-rich clinopyroxene, orthopyroxene, Ti-amphibole,imum compressional axis produced dominantly NNW– anorthoclase, phlogopite, sapphire and zircon (see FlowerSSE to north–south right-lateral faults. This pattern of et al., 1992). Spinel lherzolites are abundant at Phu Quystress redistribution is confirmed by our basalt Ar–Ar and Con Co island (sector I), Quang Ngai, Re Island,ages (Lee et al., 1998), which record a clockwise rotation Buon Ma Thuot and Pleiku (sector II), Dalat, Xuan Locof transtensional fractures, an initial NE–SW trend (Dalat, and Ile des Cendres (sector IV) along with cumulates18–8 Ma) superceded by NW–SE (Buon Ma Thuot and and megacrysts. Eclogites were found in Pleiku basaltsPleiku, 8–2 Ma) and north–south (Pleiku, Xuan Loc and comprise idiomorphic garnet (40%) and euhedral toand offshore, 4–0 Ma) trends. Palaeomagnetic data for subhedral clinopyroxene (60%), with or without or-Vietnamese Mesozoic to Quaternary eruptives (McCabe thopyroxene (see Halton & Gurney, 1987). Feldsparet al., 1993; Chi et al., 1998) suggest little or no tectonic megacrysts were also found in Pleiku and range fromrotation of Indochina occurred since the India–Asia sanidine to anorthoclase in composition. Clinopyroxene,collision, apparently at variance with predictions of the orthopyroxene and Ti-amphibole megacrysts were en-extrusion model (Tapponnier et al., 1986). However, this countered mostly in Pleiku and Dalat centres, often withcan be reconciled with extrusion if Indochina behaved amphibole-free lherzolites, and are common in Ile desas a non-rigid plate (Rangin et al., 1995a, 1995b) in which Cendres basalts.case basalt magmatism may reflect stretching associatedwith the change from left- to right-lateral ASRR shearzone motion (Leloup et al., 1995).

GEOCHEMISTRY OF BASALTS ANDXENOLITHS

PETROGRAPHY OF BASALTS AND Geochemical data for basalts and xenoliths are used toXENOLITHS define compositions and stratigraphic relations of the

erupted magmas, identify compositions of their un-Quartz tholeiites are dominant in Dalat and PhuocLong and are generally aphyric (<3% phenocrysts) to fractionated (mantle-equilibrated) parent melts, and from

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this basis estimate melt segregation conditions and ther- dominated by quartz tholeiite with subsidiary olivinetholeiite, basalts from Pleiku and Buon Ma Thuot includemal state of the asthenosphere. Assuming the entrained

lherzolites and harzburgites represent lithospheric mantle quartz and olivine tholeiites and alkali basalts in ap-proximately equal amounts, and Xuan Loc and offshorefragments and that high-pressure undersaturated melts

are probably generated in the asthenosphere, estimated seamounts comprise olivine tholeiite and alkali basalt(Fig. 2).melt segregation conditions help define the ‘thermal

boundary layer’ (TBL) between convecting as- ‘Chemical types’ were established as a basis for stra-tigraphic correlation between drill sites (Hoang, 1996)thenosphere and a rigid ‘mechanical boundary layer’

(MBL). Sr, Nd and Pb isotopic data for representative and for comparing primitive magma types within andbasalts—summarized below and discussed in detail by between centres (this work). Where possible, chemicalHoang et al. (1996)—confirm the presence of anomalous types were taken to include stratigraphic intervals of(DUPAL-like) asthenosphere beneath Indochina and re- similar composition—cooling unit batches identified fromcord evidence for the interaction of asthenospheric melts drill core stratigraphy, or coherent groupings identifiedwith enriched lithospheric mantle and continental crust. from surface sampling of mapped flows—whose internalThermobarometric estimates of xenolith equilibration, a variation is consistent with crystal–liquid redistributionbasis for estimating conductive geotherms in the li- or mixing. Distinctions were based on major elementthospheric mantle, are being published separately along oxides and CIPW normative character, although surfacewith chronologic interpretations of xenolith Re–Os and sample groups were further constrained by element ratiosSm–Nd isotopic decay systematics (Hoang et al., 1998). such as K/Na, Rb/Sr and Ba/Zr. This approach is

exclusive rather than inclusive such that chemical typesidentified at different sites may represent different partsof the same erupted magma batch (e.g. types XL-B andSampling and analysis-D at Xuan Loc; types DT-A, -B and -C at Dalat)

Most samples analysed were selected from fresh, un- whereas a single type is unlikely to include compositionsweathered drill-core from the Dalat, Phuoc Long, Buon from distinct eruptive episodes. Type averages are givenMa Thuot, Pleiku and Xuan Loc plateaux, surface out- in Table 1, with representative incompatible elementcrops in these and other basalt plateaux, and dredge ratios, ages (where known) and melt segregation pressureshauls from the South China Sea. Stratigraphic sections and temperatures—the last computed from simulatedwere developed from hydrologic drill records of the primitive magma compositions (see below).Vietnamese Geological Survey and are accessible with Figure 3a–f shows chemical type variation in plots ofgeochemical data at the Journal of Petrology website (http:// SiO2 and TiO2 vs MgO (wt %) annotated for age wherewww.oup.co.uk/jnls/list/petroj). Dredge samples were

possible, with early and late series outlined by continuouscollected by Vietnamese and Soviet scientists duringand dashed lines, respectively. Dalat centre basalts rangecruises of the R.V. Vulkanolog between 1981 and 1987between 6 and 9 wt % MgO and are exclusively tholeiitic(Koloskov et al., 1986) (Fig. 1a and b).(Fig. 3a), including four chemical types from drill sitesRepresentative major and trace element analyses were711, 736 and 756. Early series eruptives (10·5–14·0 Ma)published with isotopic data by Hoang et al. (1996)include low-Ti olivine and quartz tholeiite types DT-Atogether with descriptions, precisions and accuracies ofand -B, whereas late series (1·8–2·6 and < ~1·8 Ma)the analytical techniques employed. Whole-rock majorcomprise low- and high-Ti olivine tholeiite types DT-Cand trace elements were determined using X-ray fluor-and -D. Basalts from Phuoc Long section 804 also rangeescence spectrometry (Michigan State University) andbetween 6 and 9 wt % MgO and show a quartz tholeiiteinstrumental neutron activation analysis (University oftype (PH-A) and two olivine tholeiite types (PH-B andMichigan), H2O+ and CO2 using a CHN analyser (Ar--C), types PH-A and -B forming a 15–15·1 Ma earlygonne National Laboratory), and phenocryst com-series and PH-C a late series of ~5 Ma (Fig. 3a).positions using a scanning electron microscope with

Pleiku basalts are more variable, ranging between 5energy-dispersive attachment (UIC).and 12 wt % MgO and showing a broad range of SiO2

and TiO2 contents (Fig. 3b). Early series eruptives (4·8–2·6Ma) include quartz tholeiite (chemical types PL-A and

Major elements -B), olivine tholeiite (PL-D and -E) and low-Ti alkalibasalt (PL-F), whereas the late series type PL-C (< ~2·5CIPW normative variation defines a range of quartzMa), a high-Ti alkali basalt, succeeds type A at site 90.tholeiite (QT) (qz-normative), olivine tholeiite (OT) (olThe early series types appear to comprise two groups,+ hy-), alkali basalt (AB) (ne- up to 5%) and basanitewith 5–8 wt % MgO (types PL-A and -B) and 8–12 wt(BA) (ne-normative >5%) at most centres, reflecting an% MgO (PL-C, -D and -F), types PL-F and -B, and PL-overall increase in undersaturation from older to younger

centres (Fig. 2). The Phuoc Long and Dalat plateaux are D and -A representing possible cogenetic pairs. Type

374


Fig. 2. CIPW normative variation for analysed basalts from early and late eruptive series of Phuoc Long (SW sector), Pleiku, Buon Ma Thuot,Song Cau, Quang Ngai and Re Island (central sector), Xuan Loc, Dalat and Ile des Cendres (SE sector), and Dien Bien Phu, Khe Sanh andCon Co island (northern centres) (Appendices A and B: http://www.oup.co.uk/jnls/list/petroj) (computed for Fe3+ = 0·15Fe2+ total), showingthe range between quartz tholeiite, olivine tholeiite, alkali basalt and basanite.

PL-C, the youngest of the early series (2·6 Ma), resembles and late alkali basalt series (type RE-B) (1·2–0·4 Ma) arerecognized on Re Island, and analogous distinctions existthe late series type PL-E in SiO2 but not TiO2 content

and appears to be unique. Buon Ma Thuot shows a for Ile des Cendres (IC-A and -B) and Katwit Islandseamounts. Thus, with the important exception of Buonsimilar compositional range, including quartz tholeiite

(type BMT-B), olivine tholeiite (BMT-A and -C) and Ma Thuot, early eruptive series comprise quartz andolivine tholeiite with only rare alkali basalt (e.g. type PL-alkali basalt (BMT-D, -E and -F) (Fig. 3b). However,

in contrast to the other centres, SiO2-undersaturated D in Pleiku), whereas late series are generally formed byolivine tholeiite, alkali basalt and basanite. Reasons for theeruptives are confined to the early BMT series (4·6–3·2

Ma) with late series made up exclusively of quartz tholeiite ‘inverted’ Buon Ma Thuot pattern are unclear, although itis possible these eruptives represent late and early re-(1·9–0·3 Ma). Although Buon Ma Thuot chemical types

were identified from several sections, this ‘inverted’ spective phases of distinct magmatic cycles.In general, the alkali basalts show higher contents ofsequence is best seen at site 45, where quartz tholeiite

(type BMT-B) overlies olivine tholeiite (BMT-A) and TiO2 and FeO∗ than quartz and olivine tholeiites atequivalent MgO contents, such that a rough inversealkali basalt (BMT-D).

Xuan Loc early and late series are distinguished at site correlation exists between SiO2 undersaturation and TiO2

or FeO∗. Stratigraphic successions from low- to high-507, where olivine and quartz tholeiites (types XL-Aand -C) (2·42–2·2 Ma) precede basanites (types XL-B TiO2 basalts are common in intraplate flood basalts and

have been interpreted to reflect variable source ratherand -D) (1·1–0·4 Ma) (Fig. 3c). Offshore islands andseamounts show similar associations of tholeiite and alkali than fractional crystallization or contamination effects

(e.g. Hawkesworth et al., 1988). However, as noted, thisbasalt although stratigraphic control is poor or lacking(Fig. 3c). An early tholeiite series (type RE-A) (12·0 Ma) pattern is not ubiquitous in Vietnam, as both Pleiku and

375


Tab

le1:

Che

mic

alT

ype

aver

ages

for

Vie

tnam

ese

basa

lts

Mag

ma

seri

es:

DT-

ID

T-I

DT-

ID

T-II

DT-

IID

T-II

PH

-IP

H-I

PH

-II

PL-

IP

L-I

Ch

em.

typ

e:D

T-A

/711

DT-

B/7

36D

T-C

/756

DT-

D/7

36D

T-D

/756

DT-

D/7

11P

H-A

/804

PH

-B/8

04P

H-C

/804

PL-

A/1

21P

L-B

/90

Mel

tty

pe:

QT

QT

QT

OT

OT

OT

QT

OT

OT

QT

QT

SiO

252

·05

52·5

553

·85

48·0

552

·01

51·4

252

·72

50·7

048

·84

53·9

953

·12

TiO

21·

861·

882·

042·

482·

042·

331·

802·

162·

361·

591·

78

Al 2

O3

14·2

414

·93

14·8

915

·29

14·3

815

·65

15·5

715

·28

14·2

214

·55

14·5

6

FeO∗

11·1

410

·87

9·69

11·7

010

·04

10·0

210

·85

11·0

611

·09

10·0

210

·22

Mn

O0·

180·

160·

160·

190·

140·

230·

180·

160·

150·

150·

16

Mg

O7·

927·

046·

178·

198·

016·

456·

287·

3510

·26

6·72

6·89

CaO

9·12

8·40

9·08

8·72

8·57

7·49

9·07

8·64

8·47

8·71

8·79

Na 2

O2·

653·

203·

042·

703·

043·

492·

873·

152·

563·

002·

96

K2O

0·59

0·67

0·80

2·14

1·40

2·47

0·43

1·11

1·50

0·83

1·24

P2O

50·

230·

300·

270·

530·

360·

450·

240·

400·

540·

280·

27

K2O

/Na 2

O0·

227

0·21

0·26

30·

793

0·46

10·

708

0·15

0·35

20·

585

0·27

70·

419

Zr/

Y4·

26·

13·

74·

75

5·6

56·

87·

84·

76·

7

Zr/

Nb

185·

119

3·2

138·

610

199

·127

2·2

502·

131

7·1

233·

216

8·8

221·

2

Rb

/Sr

0·03

0·04

0·05

0·11

0·12

0·06

0·03

0·05

0·06

0·06

0·03

Ba/

Zr

1·91

1·82

2·31

3·3

3·46

2·74

0·61

1·32

2·3

1·71

2·65

n4

156

31

28

64

63

Ag

e(M

a)>1

0·5

10·5

–14·

018

–26

<1·8

<1·8

<1·8

15·1

15·5

7·0

3·9

2·6

T°C

(dry

)14

2714

1913

7814

4713

8613

9014

1814

2514

4214

0114

21

T°C

(H2O

)14

1714

0413

4514

4613

5713

6314

0314

1414

3913

7914

08

P(d

ry)

11·5

9·8

6·3

23·9

11·5

14·5

10·5

15·7

19·6

7·9

15·6

P(H

2O)

17·9

16·9

14·8

25·2

17·9

19·7

17·3

20·4

22·7

15·8

20·3

376


Mag

ma

seri

es:

PL-

IP

L-I

PL-

IP

L-I

PL-

IIB

MT-

IB

MT-

IB

MT-

IIB

MT-

IIB

MT-

IIB

MT-

I

Ch

em.

typ

e:P

L-C

/90

PL-

D/9

0P

L-E

/90

PL-

F/12

1P

L-G

/121

BM

T-A

/63

BM

T-A

/45

BM

T-B

/911

BM

T-B

/45

BM

T-B

/858

BM

T-C

/45

Mel

tty

pe:

QT

AB

OT

OT

AB

OT

OT

QT

QT

QT

OT

SiO

252

·04

45·6

149

·75

49·0

048

·69

48·3

348

·82

52·3

852

·22

51·7

548

·77

TiO

21·

932·

071·

962·

582·

442·

182·

061·

651·

581·

621·

88

Al 2

O3

15·1

913

·36

13·9

914

·55

13·5

214

·04

14·3

515

·03

15·2

215

·38

13·9

9

FeO∗

10·3

413

·16

11·4

911

·75

11·3

311

·51

11·2

910

·04

10·1

510

·12

11·6

4

Mn

O0·

160·

230·

180·

170·

170·

130·

160·

150·

140·

160·

12

Mg

O6·

4410

·21

10·0

07·

769·

7610

·05

8·68

7·09

6·99

7·46

9·89

CaO

9·11

10·1

38·

198·

458·

649·

189·

829·

429·

228·

729·

24

Na 2

O3·

143·

012·

742·

992·

752·

632·

832·

973·

083·

152·

54

K2O

1·33

1·40

1·28

2·00

2·04

1·45

1·43

0·95

1·09

1·28

1·56

P2O

50·

270·

740·

430·

590·

930·

500·

550·

310·

310·

370·

35

K2O

/Na 2

O0·

424

0·46

50·

467

0·66

90·

741

0·55

10·

505

0·32

00·

354

0·40

60·

614

Zr/

Y6·

56·

58·

77·

79

7·1

6·2

4·2

4·8

55·

1

Zr/

Nb

170·

911

8·4

260·

223

726

8·5

153

137·

210

1·8

118·

610

7·1

121

Rb

/Sr

0·02

0·07

0·06

0·07

0·05

0·06

0·06

0·06

0·05

0·07

0·06

Ba/

Zr

2·74

4·45

2·44

2·59

3·98

3·24

5·4

3·54

3·46

3·85

3·8

n5

47

78

13

74

Ag

e(M

a)?

3·4

?<2

·54·

84·

024·

021·

9–0·

31·

9–0·

31·

9–0·

33·

2

T°C

(dry

)14

0214

3514

1914

3914

4414

3614

3113

8913

9213

9114

42

T°C

(H2O

)13

8014

2814

0514

3414

4114

3014

2213

6113

6613

6314

39

P(d

ry)

11·1

20·1

17·2

2022

·920

·420

·510

·411

·212

·420

·4

P(H

2O)

17·7

2321

·324

·621

·823

·123

·217

·317

·718

·523

·1

377


Tab

le1:

cont

inue

d

Mag

ma

seri

es:

BM

T-I

BM

T-I

BM

T-I

BM

T-I

BM

T-1

BM

T-I

XL-

IX

L-I

XL-

IIX

L-I

XL-

I

Ch

em.

typ

e:B

MT-

C/6

3B

MT-

D/4

5B

MT-

D/6

3B

MT

D/8

58B

MT-

E/9

60B

MT-

F/63

XL-

A/5

07X

L-A

/508

XL-

B/5

11X

L-C

/507

XL-

C/5

16

Mel

tty

pe:

OT

AB

AB

AB

AB

AB

OT

OT

BA

QT

OT

SiO

250

·73

45·9

047

·44

45·6

148

·43

49·8

850

·31

50·0

743

·40

49·5

449

·32

TiO

21·

801·

992·

192·

002·

402·

692·

192·

173·

042·

272·

22

Al 2

O3

15·0

214

·52

13·6

414

·54

15·4

015

·08

13·9

914

·01

12·9

313

·77

13·4

5

FeO∗

10·9

411

·81

12·4

911

·40

11·9

310

·69

11·2

911

·06

12·6

411

·43

10·8

3

Mn

O0·

210·

190·

260·

180·

180·

160·

210·

160·

240·

240·

17

Mg

O7·

579·

749·

069·

986·

927·

388·

949·

1411

·05

9·34

9·95

CaO

9·31

10·3

19·

9310

·03

8·54

8·37

8·51

8·57

12·2

89·

129·

30

Na 2

O2·

942·

812·

572·

892·

972·

942·

842·

842·

452·

872·

77

K2O

1·19

2·12

1·76

2·57

2·31

2·18

1·46

1·54

0·71

0·97

1·54

P2O

50·

300·

610·

650·

800·

910·

640·

250·

431·

250·

450·

44

K2O

/Na 2

O0·

405

0·75

40·

626

0·88

90·

786

0·74

10·

514

0·54

20·

230

0·33

80·

556

Zr/

Y5·

35·

75·

86·

29·

411

·37·

96·

99·

49·

26·

2

Zr/

Nb

132·

915

3·6

160

168

424·

423

7·8

204·

119

1·5

166·

725

516

7·6

Rb

/Sr

0·06

0·07

0·06

0·06

0·08

0·07

0·1

0·07

0·1

0·11

0·05

Ba/

Zr

3·28

4·74

4·71

5·4

2·0

2·9

2·11

2·29

5·82

2·35

3·25

n6

44

23

22

24

32

Ag

e(M

a)3·

24·

64·

64·

64·

5>4

·6>2

·4>2

·4>1

·12·

2–2·

422·

2–2·

42

T°C

(dry

)14

1814

4514

4914

3114

5614

3214

3114

1814

8014

3314

28

T°C

(H2O

)14

0414

4314

4914

2314

5914

2414

2214

0314

9514

2614

19

P(d

ry)

15·9

25·7

30·9

26·6

23·8

18·8

15·3

1638

·818

·218

·2

P(H

2O)

20·5

26·3

29·3

26·8

25·2

22·2

20·6

3433

·821

·820

·1

378


Mag

ma

seri

es:

XL-

IIX

L-II

I.C

end

res

I.C

end

res

Kat

wit

Is.

Re

Is.-

IIR

eIs

.-I

QN

Co

nC

oK

he

San

hD

BP

Ch

em.

typ

e:X

L-D

/511

XL-

D/5

07IC

-AIC

-BR

E-A

RE

-B

Mel

tty

pe:

BA

AB

AB

OT

OT

OT

QT

AB

QT

OT

QT

SiO

243

·26

45·0

149

·50

50·4

049

·44

49·8

752

·99

44·7

752

·67

48·4

855

·26

TiO

22·

762·

932·

341·

992·

072·

291·

572·

851·

782·

682·

04

Al 2

O3

12·2

813

·84

13·5

113

·91

13·8

715

·32

15·0

114

·17

16·5

014

·61

16·0

0

FeO∗

12·1

512

·78

11·6

711

·52

11·4

810

·93

9·43

11·5

110

·22

10·8

39·

66

Mn

O0·

200·

150·

160·

150·

160·

140·

140·

220·

120·

150·

16

Mg

O12

·96

9·22

7·86

8·18

8·50

7·10

7·46

11·2

14·

178·

625·

36

CaO

10·9

99·

869·

008·

879·

138·

488·

9010

·12

5·67

9·45

6·15

Na 2

O2·

872·

763·

273·

123·

133·

113·

283·

224·

923·

093·

00

K2O

1·32

2·48

2·27

1·49

1·83

2·19

0·92

0·94

3·22

1·54

2·04

P2O

50·

940·

960·

420·

360·

380·

570·

291·

000·

720·

540·

32

K2O

/Na 2

O0·

460·

899

0·69

40·

478

0·58

50·

704

0·28

0·29

10·

654

0·49

80·

68

Zr/

Y10

·910

7·1

5·5

7·8

5·3

1111

6·2

9·1

Zr/

Nb

236·

626

1·8

262·

315

2·9

164

169

263·

24·

0620

0·1

5·98

Rb

/Sr

0·17

0·08

0·06

0·05

0·09

0·05

0·07

0·08

0·04

0·22

Ba/

Zr

3·64

3·29

2·39

3·12

3·25

2·24

3·9

3·35

3·93

1·75

n2

36

38

43

21

11

Ag

e(M

a)0·

4–1·

10·

4–1·

11·

27–0

??

0·4–

1·2

12·0

7·1

0·35

??

T°C

(dry

)14

6914

7414

4614

3914

3814

0913

6914

3714

1514

14—

T°C

(H2O

)14

7814

8614

4514

3414

3213

9113

3214

3114

0113

27—

P(d

ry)

38·8

33·7

19·3

15·9

19·2

19·2

7·7

33·9

9·7

22·5

—

P(H

2O)

3431

22·5

20·5

22·5

22·4

15·4

31·1

16·7

24·2

—

Ave

rag

eso

fd

ry-w

eig

ht

no

rmal

ized

anal

yses

,an

no

tate

das

follo

ws:

QT,

qu

artz

tho

leiit

e;O

T,o

livin

eth

ole

iite;

AB

,alk

alib

asal

t;B

A,b

asan

ite.

I,‘e

arly

seri

es’;

II,‘la

tese

ries

’eru

pti

on

epis

od

es.N

ost

rati

gra

ph

icd

isti

nct

ion

iso

bse

rved

for

Iled

esC

end

res

seam

ou

nts

,alt

ho

ug

hlo

w-a

nd

hig

h-T

igro

up

sar

ere

cog

niz

ed.P

–Tca

lcu

lati

on

sar

efo

rM

g-1

5n

orm

aliz

edch

emic

alty

pe

aver

ages

,ex

pla

ined

inte

xt.

Bas

alt

pla

teau

x:D

T,D

alat

;P

G,

Ph

uo

cLo

ng

;B

MT,

Bu

on

Ma

Th

uo

;P

L,P

leik

u;

XL,

Xu

anLo

c;Q

N,

Qu

ang

Ng

ai;

DB

P,D

ien

Bie

nP

hu

.40

Ar–

39A

rag

ed

ata

on

thes

esa

mp

les

fro

mLe

eet

al.

(199

8).

Ch

emic

alty

pes

nam

edal

ph

abet

ical

lyw

ith

resp

ect

tod

rill

site

nu

mb

er.

FeO∗,

tota

lFe

oxi

de.

379


Fig. 3.

Buon Ma Thuot high-Ti alkali basalts precede lower-Ti range 90·1–95·3. These data suggest that Indochineselithospheric mantle is variably refractory and similar tovariants in the sequence. The existence of both coupled

and decoupled variation between TiO2 and SiO2 may Phanerozoic subcontinental mantle elsewhere (e.g. Song& Frey, 1989; Hawkesworth et al., 1990).result from the interplay of variable melt fraction, source

fertility and P(H2O) in generating primitive melts. Wesuggest therefore that parental magmas reflect a spectrumbetween large melt fractions, low pressures and a (low-Ti, Fe) refractory source (on the one hand), and small

Trace elements and isotopesmelt fractions, high pressures and a (high-Ti, Fe), fertilesource (on the other). Isotopic and trace element data for representative lava

Clinopyroxene separates from representative spinel samples (Hoang et al., 1996) are reviewed here as a basislherzolites and harzburgites are in the range En46·6–49·5 for evaluating mantle boundary layer models derivedFs3·6–5·1Wo45·4–47·5, with mg-numbers [100×Mg/ from the major element variation (see below). As-(Mg+ Fe2+)] (80–95), Al2O3 (5·6–7·9 wt % ) and CaO similation–fractional crystallization appears to have(19·7–22·2 wt %) overlapping with those of peridotites affected several flows in Pleiku (olivine tholeiite, type PL-from mid-ocean ridge (e.g. McDonough & Frey, 1989; A), Dalat (quartz tholeiite, DT-A), Buon Ma ThuotJohnson et al., 1990) and continental settings (e.g. Cao (quartz tholeiite, BMT-B), Xuan Loc (tholeiitic andesite,& Zhou, 1987; Hawkesworth et al., 1990; Qi et al., 1995). XL-A) and Dien Bien Phu (orthopyroxene-bearing quartzCr2O3 contents range from 1 to 0·7 wt %, with cr- tholeiite), which show negative correlations of MgO/numbers [Cr2O3× 100/(Cr2O3+Al2O3)] in the range FeO∗ and 87Sr/86Sr and strong enrichment in EM2

[Hoang et al., 1996; compare DePaolo (1981) and Arndt1·2–10·38. The mg-number shows a positive covariancewith SiO2 and Cr2O3, and a negative covariance with et al. (1993)]. Involvement of crustal components is also

indicated by positive Rb and Ba anomalies in mantle-Al2O3, consistent with control by partial melting (Fig. 4),whereas Fo contents of coexisting olivines are in the normalized incompatible element distributions (Fig. 5)

380


Fig. 3.

(e.g. in Pleiku, Buon Ma Thuot and Xuan Loc patterns) lie mostly between two and four. Whereas variation isslight to negligible in Dalat and Phuoc Long, BMT-Iand in plots of Sr/Zr, Rb/Zr and Ba/Zr vs Ti/Zr (Fig. 6).

However, covariance of MgO/FeO∗ and 87Sr/86Sr (Fig. 7) and -II and, to a lesser extent, PL-I and XL-II seriesshow marked increases in Sr/Zr consistent with crustalsuggests some of these reflect partial melting of an en-

riched source, generated by sediment-derived me- input.Crust addition, whether resulting from wallrock re-tasomatism, rather than wallrock contamination of

ascending melt. action or mantle metasomatism, is thus significant atBuon Ma Thuot and Xuan Loc, and is further supportedTi/Zr typically decreases from early to late series

eruptions (except in Buon Ma Thuot) and is generally by the presence of isotopic EM2 (Hoang et al., 1996).However, two points should be noted. The presencematched by increasing ratios of more- to less-incompatible

elements (e.g. Fig. 6a–c). In addition to source hetero- of systematic isotopic differences between basalts fromdistinct lithospheric sectors suggests that the effects ofgeneities these arrays reflect a decrease in melt fraction

from tholeiite to alkali basalt (indicated by dashed parallel enriched lithospheric mantle predominate over wallrockreaction. Second, at most centres there is a secularlines with decreasing Ti/Zr) and contrast with divergent

trends towards higher Ti/Zr, reflecting the addition of change from EM2-rich early series to EM1-rich lateseries compositions. Hoang et al. (1996) proposed that,crust components (CC). For example, negative co-

variation bands of Rb/Zr and Ba/Zr vs Ti/Zr include irrespective of whether EM2 is incorporated by risingmelt or derives from lithospheric mantle metasomes,members of both early and late series at each centre

(Fig. 6a and b), whereas sharp increases in Rb/Zr and EM1-like components must have been present in theconvecting asthenosphere and were perhaps delaminatedBa/Zr with increasing or near-constant Ti/Zr (shown

by DT-I, BMT-I, BMT-II and XL-II series) (Fig. 6a and from the Sino-Korean craton.As samples of sub-Indochina lithospheric mantle, theb) are consistent with the addition of crustal material. In

contrast, again with the exception of Buon Ma Thuot, spinel lherzolite xenoliths show low Rb/Sr (0·001–0·008)and high Sm/Nd (0·3–0·65) values, with 87Sr/86Sr andSr/Zr ratios show little overall variation with Ti/Zr and

381


Fig. 3. Plots of SiO2 and TiO2 vs MgO (wt %) showing ‘chemical types’ defined in the text for cored sections of Vietnamese basalt plateaux(average compositions in Table 1). These are labelled alphabetically with drill site numbers in parentheses (shown in Fig. 1b) and age rangesbased where possible on Ar–Ar data (Lee et al., 1998). ‘Early series’ types are shown by continuous outlines, ‘late series’ by dashed outlines.Tholeiites are shown by fine lines and alkali basalts and basanites by bold lines.

143Nd/144Nd ratios corresponding to N-MORB ± EM2 averages of chemical types based on samples with >7·5(Hoang et al., 1998). Although these differ from the MgO wt %. The rationale for this procedure (followingEM1-rich host alkali basalts, the (decoupled) negative Scarrow & Cox, 1995) was that: (1) olivine (Fo89–83) iscovariance of Sm/Nd and mg-number suggests cryptic the only significant phenocryst phase in basalts withEM2-rich metasomatism of the refractory lithospheric >7·5 MgO wt % which shows negligible effects ofmantle (Fig. 8) (see Frey & Green, 1974). clinopyroxene and plagioclase fractionation, (2) parent

magmas are almost certainly more magnesian than themost Mg-rich erupted lavas, (3) liquids with >13 wt %MgO are likely to have equilibrated with Fo85–90PRIMITIVE MELT COMPOSITIONS(Roeder & Emslie, 1970), and (4) experimental peridotite

To interpret basaltic melt segregation conditions, we melts fall in the range 12–17 wt % MgO. Thesehave considered the compositions of primitive, mantle- criteria apply to both anhydrous and hydrous conditionsequilibrated, melts to be a simple function of lithospheric (Kushiro, 1990; Hirose & Kushiro, 1993; Hirose &stretching (b) and asthenosphere potential temperature Kawamoto, 1995). The effect of adding olivine is(T p), in turn reflecting melt segregation pressure, melt relatively trivial for most oxides but provides a basisfraction and bulk source composition (after McKenzie &

for establishing the variation of primitive melts, andBickle, 1988; Latin & White, 1990).by comparison with published experimental results,estimating melt segregation pressures and temperatures.

Variation of Mg-15 normalized eruptive Olivine (Fo89) was added to the eruptive (i.e. chemicalcompositions type) averages until MgOp = 15 wt %, assuming

K Mg/Fe"ol/liq = 0·30 (Roeder & Emslie, 1970), accordingPrimitive melt compositions with assumed 15 wt %

MgO were simulated by adding forsteritic olivine to to the equations

382


Fig. 4. Plots of mg-number [Mg× 100/(Mg+ Fe2+)] vs oxides wt % for clinopyroxenes separated from spinel lherzolites [data published byHoang et al. (1998)]. Η: northern and central sectors; Α: Dalat; open crosses: Ile des Cendres.

log(FeO/Al2O3) = K log(MgO/Al2O3)+ log(FeO/ basalt centres. Second, several of these range from rel-atively SiO2-rich (quartz or olivine tholeiite) to SiO2-poorMgO)p·(Al2O3/MgO)pK – 1 (1)(olivine tholeiite, alkali basalt or basanite) end-members,

(Al2O3/FeO)i = K (MgO/FeO)p· (Al2O3/MgO)i+ replicating the overall pattern of regional variation. Third,(Al2O3/FeO)p·(1 – K ) (2) at several centres (e.g. Dalat, Phuoc Long, Pleiku, Xuan

Loc and Re Island) the low- and high-FeO∗ suites cor-where p denotes the primary melt composition, i denotes respond, respectively, to early and late eruptive series.the initial (i.e. erupted) liquid composition, and K is the Whereas the covariation of Si-15 with Fe-15 is negative,distribution coefficient K Mg/Fe"

ol/liq (from Pearce, 1978). that of Ti-15 and P-15 is mostly positive, and K-15Figure 9a–d shows Mg-15-normalized variation from appears to vary independently of Fe-15 (Fig. 9a–d).

which three distinctive features are evident. First, discrete Covariance of Si-15 and K-15 (not shown) is mostly‘suites’ of chemical type averages are recognized in terms negative within magmatic suites, except for the Xuan

Loc and Pleiku upper series basalts, which show a positiveof FeO∗, SiO2, TiO2, K2O and P2O5 at most of the

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Fig. 5. Primitive mantle-normalized incompatible element distributions for representative basalts from: (a) Phuoc Long (SW sector), (b) Pleiku,(c) Buon Ma Thuot, (d) Quang Ngai and Re Island (central sector), (e) Xuan Loc and Dalat (SE sector), and (f ) Dien Bien Phu and Con Coisland (northern centres), and Ile des Cendres (SE sector) (Appendix B). Normalizing data are from Hofmann (1988). (Note the negative Baanomaly in Phuoc Long, high Rb anomaly in Xuan Loc, and positive Sr anomaly in Buon Ma Thuot basalts.)

deflection consistent with isotopic and trace element in mantle fertility as reflected by the bulk source mg-number, and contents of CaO, Al2O3 and Na2O. Theseindications for wallrock reaction (Hoang et al., 1996).contrasting vectors are analogous to those recognized inA simple model was developed as a basis for petro-oceanic (Wilkinson, 1991; Nicolson & Latin, 1992) andgenetic interpretation in terms of relevant publishedcontinental (Turner & Hawkesworth, 1995) intraplateexperimental data (in this paper), and Sr, Nd and Pbbasalts.isotopic variation (Hoang et al., 1996). The within-suite

variation of SiO2 saturation and incompatible elementcontents represents a spectrum of melts generated froma discrete volume of decompressing mantle, reflecting

Source fertility and melt segregationminor isotopic and trace element source heterogeneities,conditionsand the combined effects of variable melt fraction, se-

gregation pressure and temperature (Langmuir et al., Establishing pressure and temperature conditions of prim-1992; Scarrow & Cox, 1995). In contrast, the between- itive melt segregation, although problematic, can be

approached in at least two ways. The first involvessuite differences in Fe-15 were attributed to differences

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(e.g. Albarede, 1992; Scarrow & Cox, 1995; Turner &Hawkesworth, 1995). A complementary approachinvolves mathematical inversion of erupted meltcompositions with respect to an assumed mantle sourceand known subsolidus assemblages determined ex-perimentally as a function of pressure and temperature,assuming fractional melting of a polybaric melt column

Fig. 6.

comparing natural melt compositions with those pro-duced experimentally at known pressures, temperaturesand P(H2O), from a source of known composition, as-suming experimental melt compositions are realistic, thatbatch equilibrium melting is a valid analogue for mantlemelting (even if fractional melting is more realistic) andthat an independent measure of P(H2O) is available

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Hirose & Kushiro, 1993; Baker & Stolper, 1994) whosecompositions correlate with equilibration pressures andtemperatures. Melts produced under H2O-under-saturated conditions show similar compositional variationwith respect to pressure and temperature, although theiroxide covariation slopes differ significantly and SiO2

contents are consistently higher than those of the an-hydrous melts (Kushiro, 1990; Hirose & Kawamoto,1995).

Pressure and temperature calibrationsCovariation of FeO∗ in melts with both temperature andpressure has been recognized in experimental studies(e.g. Jaques & Green, 1980; Falloon et al., 1988) althoughsimple temperature-dependent functions have been elu-sive as a result of problems such as Fe loss from ex-perimental capsules. For a given bulk composition thetemperature dependence of Fe is pronounced at lowpressures (0·5–1 GPa) ( Jaques & Green, 1980) but lessso at higher pressures where garnet appears at the sub-solidus. Scarrow & Cox (1995) discussed this relationshipwith respect to Hirose & Kushiro’s (1993) data andprojected isopleths for Fe and Mg in pressure–temperature space for melts equilibrated with fertilelherzolite HK-66. For melts of restricted MgO range(e.g. 14–16 wt %) over a pressure range of 1–3 GPa(implying decreasing melt fraction with increasing pres-sure) the Fe–temperature relationship is quasi-linear forboth fertile and refractory bulk compositions (Fig. 11a;see below). Equivalent melt fractions formed in hydrous‘sandwich’ and ‘diamond aggregate’ experiments alsoshow this relationship over the pressure range 1·2–2·5GPa, although temperatures are 50–100°C less for meltsof equivalent FeO∗ content (Fig. 11a). The relationshipbetween SiO2 and pressure in experimental melts hasbeen quantified by Albarede (1992) and Scarrow & Cox(1995), and is shown in Fig. 11b (see below) for anhydrousand hydrous conditions. Compositions of experimentalmelts from fertile peridotite (HK-66) were therefore cal-ibrated for pressure and temperature according to thereported experimental conditions of Kushiro (1990) and

Fig. 6. Plots of (a) Rb/Zr, (b) Ba/Zr and (c) Sr/Zr vs Ti/Zr ratios for Hirose & Kushiro (1993). It seems reasonable to applysamples from Dalat (DT), Phuoc Long (PH), Pleiku (PL), Buon Ma these relationships to the natural melt compositions,Thuot (BMT), Xuan Loc (XL) and Re Island (RE), distinguishing early

assuming the latter were derived from sources of similar(I) and late (II) eruptive series.fertility, segregated over a similar pressure interval, andrepresent a similar range of melt fractions to those of the

(McKenzie & O’Nions, 1991; Watson & McKenzie, experiments.1991).

We adopted the first approach and used experimentalmelts produced in ‘sandwich’ capsules (avoiding reaction

Comparison of natural and experimentalbetween experimental charge and capsule) (e.g. Stolper,melts1980; Kushiro, 1990) and using ‘diamond aggregate’

methods (allowing equilibrium between melt and solid In general, the Vietnamese melts are closer in compositionto FeO∗-rich melts generated from HK-66 than to thosewhile avoiding problems of quench crystallization) (e.g.

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Fig. 7. Plots of 87Sr/86Sr vs MgO/FeO∗ (wt %) for representative basalts from Phuoc Long (SW sector), Pleiku, Buon Ma Thuot, Song Cau,Quang Ngai and Re Island (central sector), Xuan Loc, Dalat and Ile des Cendres (SE sector), and Dien Bien Phu, Khe Sanh and Con Coisland (northern centres), indicating the effects of crustal wallrock reaction [see Hoang et al. (1996) for details].

Fig. 8. Plots of Sm/Nd vs mg-number [Mg× 100/(Mg+ Fe2+)] for clinopyroxenes separated from spinel lherzolites [data published by Hoanget al. (1998)]. Η: northern and central sectors; Α: Dalat; open crosses: Ile des Cendres.

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Fig. 9. Plots of (a) FeO∗ vs SiO2, (b) FeO∗ vs TiO2, (c) FeO∗ vs K2O and (d) FeO∗ vs P2O5 (in wt %) for basalt chemical type averagesnormalized to Mg-15 for Xuan Loc (XL), Pleiku (PL), Buon Ma Thuot (BMT), Dalat (DT), Phuoc Long (PH), Ile des Cendres (Cendres),Quang Ngai (QN), Re Island (RE) and Con Co (CC). Compositions of early (I) and late (II) series are outlined where possible.

from refractory KBL-1 (Fig. 10). However, the inverse of ~2–3°C/km (Table 1). The prominence of quartztholeiite, shallow Fe-15 vs Si-15 slope, and high juvenilecovariance of Fe-15 and Si-15 in the natural melts is less

steep than that of anhydrous HK-66 melts and resembles H2O contents in Vietnamese basalts, are not typical ofoceanic and continental intraplate basalts and stronglythose of melts equilibrated with fertile peridotite under

hydrous conditions (Fig. 10). According to the anhydrous suggest a hydrous source. Hydrous melt segregationdepths of between ~45 and 110 km correspond closelycalibrations, melt segregation pressures range from nearly

4 GPa and temperatures of ~1470°C (e.g. alkali basalts to estimates for basalts in Iceland (Nicolson & Latin,1992), Skye (Scotland) (Scarrow & Cox, 1995), and thefrom Xuan Loc) to <0·5 GPa and ~1400°C (quartz

tholeiites from Dalat and Phuoc Long), representing a East African Rift (Latin et al., 1993). Mantle adiabatsinterpolated from H2O-undersaturated melt segregationpressure–temperature slope of ~0·75°C/km. In contrast,

segregation conditions based on the H2O-undersaturated conditions may thus offer useful insights concerning am-bient mantle T p, topology of the MBL–TBL interface,calibrations range from <3·5 GPa pressure and

temperatures of ~1450°C to ~1·5 GPa and and the relationship between convecting asthenosphereand extended, conductively cooled lithosphere.1350–1400°C, representing a pressure–temperature slope

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Fig. 10. Comparison of Mg-15-normalized Vietnamese chemical type averages with experimental melt compositions in terms of SiO2 vs FeO∗(wt %). Anhydrous experimental partial melts are of: fertile peridotite HK-66 (anhydrous) and refractory peridotite KLB-1 (Hirose & Kushiro,1993). H2O-undersaturated experimental melts were equilibrated with ‘sandwiches’ of olivine, orthopyroxene and clinopyroxene from HK-66(Kushiro, 1990).

the peridotite solidus (Gallagher & Hawkesworth, 1992)MELTING DYNAMICS AND MANTLEor mobilization of relict melt fractions (‘mafic com-

BOUNDARY LAYER MODEL ponents’) (Harry & Leeman, 1995). Harry & LeemanA petrogenetic model for Vietnamese basalts needs to (1995) have contended that H2O does not play a sig-explain the relatively high volume basalt plateaux oc- nificant role in generating magma, as they believe thatcupying extensional pull-apart nodes, the common pro- during initial stages of extension volatile fractions aregression from tholeiite-dominated early series, tapping quickly exhausted by early-formed melts. They suggested,refractory lithosphere-like sources, to later alkali basalt- rather, that significant melt fractions are produced atdominated series, tapping relatively fertile, asthenospheric the solidi of ‘mafic components’ present in the lowersources, a T p of ~1440°C and adiabatic gradient of lithosphere, depending on the amount and duration of>2°C/km, and LILE- and H2O-rich asthenosphere. In lithospheric extension. However, despite the presence ofthe absence of data for lithosphere thickness, we de- pyroxenite (etc.) in peridotite restites, geothermal gra-veloped a simple boundary layer model for Indochina dients based on xenolith thermobarometry (e.g. Ionov etthat is consistent with the petrologic and geochemical al., 1998) differ significantly from adiabats of the typedata and general considerations of rheology and thermal indicated by Vietnamese melt segregation conditions,state. suggesting non-ductile MBL is not decompressed to the

The progression from FeO∗-poor to FeO∗-rich (OIB- extent needed to produce magma. In contrast, con-like) basalts in continental settings is commonly in- sideration of mantle dehydration reactions supports aterpreted to reflect a change from lithospheric to as- key role for H2O in melting refractory lithospheric mantle.thenospheric mantle sources (e.g. Perry et al., 1987; Water is held in amphibole or mica at most lithosphericDePaolo, 1988; Kempton et al., 1991; Gallagher & pressures (Lambert & Wyllie, 1970; Gallagher & Hawke-Hawkesworth, 1992; Turner & Hawkesworth, 1995). sworth, 1992). Beyond the stabilities of these phases,However, the problem of melting relatively non-ductile, small melt fractions saturated with H2O and fusible maficrefractory MBL has been raised as an objection to this components will be able to form if temperatures exceed

the H2O-saturated solidus. However, even where suchinterpretation and led to models invoking H2O to lower

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As a framework for reconciling petrologic, thermal andgeodynamic constraints, we adopt a lithospheric MBLwhose mantle component is isotopically depleted andrefractory, and rigidly attached to the crust (McKenzie& Bickle, 1988; Hawkesworth et al., 1990). Below this,the TBL is hypothetically characterized by attenuatedshear-wave velocities and taken to mark the rheologictransition from dominantly convective to conductive heattransfer. It also marks the closest approach of the inflectedthermal gradient to the peridotite solidus, allowing thehighest potential for melting, and is assumed to beweak and isotopically enriched (Anderson, 1995). Theasthenosphere is generally held to be isotopically depleted,unless lithospheric components have been added—eitherrecycled via plumes or delaminated directly from above(e.g. Storey et al., 1989; Hart et al., 1992)—and residualto the extraction of primordial crust (Hofmann, 1988).

Hydrous phase breakdown would also produce changesin mantle rheology at similar depths (with or withoutmelting) such that the TBL also marks the intersectionof geotherm and dehydration curves (Lambert & Wyllie,1970; Anderson, 1995). It is thus helpful to look at thecombined effects of T p and H2O on peridotite solidi,which for a particular thermal gradient determine thedepth and extent of melting. These relationships areshown in Fig. 12 along with the mantle adiabat in-terpolated for Vietnamese melts. The depth of phlogopitedehydration ranges from ~3·8 GPa (~110 km depth) atnormal T p to <3 GPa (<90 km depth) at T p >1400°C(Fig. 12). The interpolated Indochina T p (~1440°C) issignificantly higher than normal for subcontinental as-thenosphere (e.g. McKenzie & Bickle, 1988; Wilson,1993) suggesting an ambient MBL thickness of ~80 km(~2·6 GPa) (Fig. 12). A conductive type 1 geotherm isbelieved to be typical for small or negligible lithosphericstretching factors (b) and is consistent with xenoliththermobarometric data from analogous settings (e.g.Ionov et al., 1998). It also agrees with thermal gradientsFig. 11. Plots of (a) FeO∗ (wt %) vs temperature (°C), and (b) SiO2computed as a function of uniform stretching (Latin &(wt %) vs pressure (GPa) for experimental partial melts produced under

hydrous and anhydrous conditions. Open circles, anhydrous melts of White, 1990), which suggest type 2 geotherms (Fig. 12)fertile peridotite HK-66 (Hirose & Kushiro, 1993); crossed circles, H2O- result at b values of 2–3 (Fig. 13).undersaturated melts equilibrated with HK-66 ‘sandwiches’ (Kushiro,

An important implication of the model is that uniform1990). For H2O-undersaturated experimental results (e.g. Kushiro,1990) only runs with <5 wt % added H2O and K D(Fe/Mg)

OL–MELT 3[0·27 stretching causes changes in MBL bulk composition aswere used. The regressions used in the melt segregation P–T calculations a result of its partial conversion to TBL. This effect isare shown (see text).

illustrated in Fig. 12, where stretching (producing type 2geotherms) causes upward migration of the rheologicboundary, separating solid from partially molten regions,melts remain in situ (i.e. in equilibrium) the extent ofrelative to the pre-stretching (type 1 geotherm) boundarymelting necessarily remains small, buffered by the smallwhich was both rheologic and compositional. In otheramount of H2O present. Similarly, fractional meltingwords, the upward migration of dehydration depth re-resulting from decompression could not advance beyondsulting from a local increase in b converts the lowermostinitial increments and pooled fractions would be enrichedlithospheric mantle into rheologically weak TBL. In-in SiO2, LILE, HFSE and LREE, whereas in contrast,corporation of refractory peridotite into the ductile regionsignificant (magma-producing) equilibrium melts could

only form close to dry or H2O-undersaturated solidi. produces a ‘macro’-heterogeneous column, which, with

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Fig. 12. Schematic asthenosphere–lithosphere boundary model showing experimental solidus for fertile peridotite HK-66 (Hirose & Kushiro,1993), stability fields of phlogopite and amphibole in peridotite (Modreski & Boettcher, 1973; Milhollen et al., 1974), spinel–garnet transition,and an H2O-undersaturated solidus (assuming all H2O in hydrous phases). H2O-undersaturated melt segregation conditions of ‘primitive’ chemicaltype averages, estimated using the regressions in Fig. 10, are plotted, and suggest an asthenosphere potential temperature (T p) of ~1440°C. TheP–T field of incipient, H2O-saturated melting is shaded. At T p= 1440°C and b3 ~1·0 ambient conductive heat flow may be ~70–90 mW/m2

(Pollack & Chapman, 1977) characterizing lithospheric mantle and producing geotherms of type 1. Adiabatic asthenospheric geotherms of type2 may result from uniform stretching at b3 ~2·5, causing significant melting at pressures of between ~1–5 and 3·5 GPa. Mantle sections to theright show the dependence of MBL–TBL interface depth, spinel lherzolite transition and potential enrichment of the TBL, on the geothermtype (hence b) for a given T p.

continued stretching and decompression, will yield ad- & Hai, 1991) and high heat flow (Duchkov et al., 1992;Uyeda & Nagao, 1994) in south–central Vietnam.vanced partial melts. According to Fig. 12, the column

ranges from garnet lherzolite to spinel lherzolite, andpresumably undergoes continuous subsolidus reactionduring diapiric uprise. Although simplified, this model

CONCLUSIONSavoids the requirement for magma production within theMBL, and reconciles the apparent conflict between low (1) Cenozoic basalt plateaux in southern and centralmelt segregation pressures (<1·5 GPa) and MBL Vietnam appeared over a total area of ~23 000 km2 asthicknesses > ~80 km. part of a widespread regional volcanic episode. Eruptions

We therefore propose that ambient lithospheric thick- at discrete centres appear to have involved at least twonesses of 80–100 km are reasonable at T p of ~1440°C, episodes separated by thick palaeosols, referred to asand that thinning at transtensional ‘nodes’ leads to the ‘early’ and ‘late’ series. The bi-episodal pattern is re-penetration by and advanced polybaric melting of low- cognized at the Dalat, Phuoc Long, Pleiku, Buon Maviscosity TBL and asthenosphere. Decompression melting Thuot, Xuan Loc and Re Island centres, and probablywould yield plateau basalt sequences of low-pressure, other offshore localities, although at Buon Ma Thuot thelarge-fraction tholeiites and high-pressure, low-fraction compositional trend is ‘inverted’.alkali basalts and basanites. This conclusion is consistent (2) K–Ar and Ar–Ar age data indicate activity occurredwith element inversions on the Vietnamese basalt data over the following intervals: Dalat (17·6–7·9 Ma), Phuocconducted by D. McKenzie at Cambridge University Long (<8–3·4 Ma), Buon Ma Thuot (5·8–1·67 Ma),which indicate a T p of ~1450°C and partial melt column Pleiku (4·3–0·8 Ma), Xuan Loc (0·83–0·44 Ma) and Ileextending between garnet lherzolite and spinel lherzolite des Cendres (0·8–0 Ma), and reflects clockwise rotationfacies. Our conclusion that a major thermal anomaly of transtensional fractures. Palaeomagnetic data indicateexists beneath Indochina is supported by geomorphologic little or no tectonic rotation since the India–Asia collision,

although this can be reconciled with the extrusion modelevidence for ~600 m uplift since the late Neogene (Bao

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Fig. 13. Adiabatic upwelling as a result of differential stretching of a convective geotherm generated from a 100 km thick MBL for an interiorpotential temperature 1440°C (after Latin & White, 1990). Geotherms for respective stretching factor values (b) (continuous lines) and the locusof melt fraction (F ) of 0·25 (dot–dashed lines) are shown together with the dry peridotite solidus (bold dashed line) and typical H2O-undersaturatedsolidus (fine dashed line) [adapted from McKenzie & Bickle (1988) and Latin & White (1990)]. Xenolith P–T equilibration estimates reflectingconductive thermal gradients are expected to correspond to b values between 1 and ~1·5 (type 1 geotherm in Fig. 12). Melt segregation P–Testimates based on H2O-undersaturated experiments (Fig. 11) indicate adiabats consistent with b values between 2 and 3 (type 2 geotherm inFig. 12) for T p = 1440°C. No melting would be expected for this b range at T p = 1280°C (see Latin et al., 1993).

if Indochina behaved as a non-rigid plate. The basalts result from collision-extruded asthenosphere rather thana deep plume.probably reflect stretching associated with the change

(5) Phlogopite stability provides a possible model forfrom left- to right-lateral motion on the Ailao Shan–Redthe base of the lithosphere MBL. Given the P–T shapeRiver shear zone.of phlogopite dehydration interpolated potential tem-(3) With the exception of Buon Ma Thuot, early seriesperatures suggest ambient MBL thicknesses of ~80 km.basalts comprise high-SiO2 and low-FeO∗ quartz andThinning of the MBL at transtensional ‘nodes’ leads toolivine tholeiites, tapping a relatively refractory (litho-penetration and advanced polybaric melting of low-sphere-like) source, and a later series of low-SiO2 andviscosity TBL and asthenosphere. Decompression meltinghigh-FeO∗ olivine tholeiites, alkali basalts and basanites,of ‘macro’-heterogeneous columns yields plateau basalttapping a fertile (asthenosphere-like) source.sequences of low-pressure, large-fraction tholeiites and(4) Comparison of Mg-15-normalized basalt com-high-pressure, low-fraction alkali basalts and basanites.positions with experimental melts allowed estimation of

melt segregation pressures and temperatures: (a) an-hydrous conditions: <4 GPa and ~1470°C (for alkalibasalts from Xuan Loc) to <0·5 GPa and ~1400°C

ACKNOWLEDGEMENTS(quartz tholeiites from Dalat and Phuoc Long), and (b)H2O-undersaturated conditions: <3·5 GPa and ~1450°C We thank sponsors of the Indochina Research Con-to ~1·5 GPa and 1350–1400°C. Hydrous rather than sortium (Agip, Amerada Hess, Amoco, Arco, British Gas,anhydrous conditions are favoured by: (a) high basaltic BHP, British Petroleum, DuPont–Conoco, Chevron, ElfH2O contents, (b) minimum melt segregation pressures Aquitaine, Enterprise, Mobil, Petrofina, Petronas, Phil-consistent with melting below the thinned MBL, and (c) lips, Occidental, Shell, Total and Unocal) for support,interpolated mantle adiabats of 2–3°C/km (compared and Dr Nguyen Trong Yem of the Institute of Geologywith <1°C/km for anhydrous conditions) consistent with of the National Centre for Science and Technology

(Hanoi) and Dr Nguyen Xuan Bao of the Geologicalfluid dynamic models. High potential temperatures may

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DePaolo, D. J. (1988). Neodymium Isotope Chemistry. An Introduction. NewInstitution of Washington. We thank Keith Cox and York: Springer-Verlag, 181 pp.Ikuo Kushiro for invaluable discussions, and Colin Devey, Duchkov, A. D., Yem, N. T., Toan, D. V. & Bak, C. V. (1992). FirstSamuel Makasa and Godfrey Fitton for reviews. Dan estimates of heat flow in Vietnam. Soviet Geology and Geophysics 33,

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