ORIGINAL PAPER

Pierre Barbey Æ Dereje Ayalew Æ Gezahegn Yirgu

Insight into the origin of gabbro-dioritic cumulophyric aggregates fromsilicic ignimbrites: Sr and Ba zoning profiles of plagioclase phenocrystsfrom Oligocene Ethiopian Plateau rhyolites

Received: 23 September 2004 / Accepted: 6 December 2004 / Published online: 9 February 2005� Springer-Verlag 2005

Abstract The Were Ilu ignimbrites are unlike other Oli-gocene rhyolites from the Ethiopian continental floodbasalt province, in that they consist of plagioclase (An19–54), augite, pigeonite and Ti-magnetite, instead of an-orthoclase, sodic sanidine, aegirine-augite and ilmenite.The minerals occur as (micro-)phenocrysts isolatedwithin a glassy matrix or forming gabbroic and dioriticcumulophyric clots. Plagioclase is partially re-melted(sieve-textures with infilling glass). It is zoned withsudden changes in composition. However, the bulkzoning is normal with An-rich core (An45–54) and moresodic rim (An19–28). Ba and Sr concentration profiles oftwo plagioclase phenocrysts show a bulk rimward in-crease with compositions ranging from 250 ppm to1,060 ppm and from 400 ppm to 1,590 ppm, respec-tively. The matrix glass has low CaO content (0.1–0.5 wt.%), a peralkalinity index of 0.79–1.04 and aver-age Sr and Ba contents of 48±22 and 525±129 ppm,respectively. Geochemical modelling of Ba and Sr zon-ing profiles of plagioclase, based on experimental con-straints, suggests that the cumulophyric clots can bederived from fractional crystallisation associated withlimited assimilation (8 wt.%) from melts slightly lessevolved than their rhyolitic matrix glass. These clots arenot witnesses of intermediate magmas allowing the DalyGap to be filled, but are cumulates differentiated fromrhyodacitic melt. This indicates that parental magmaswere stored in crustal magma chambers where theydifferentiated before being erupted at the surface.

Introduction

The close spacial and temporal association of basalt andrhyolite (bimodal suites), without significant volume ofrocks of intermediate composition, is prominent incontinental flood basalt (CFB) provinces. Geochemistryof silicic volcanic rocks in these provinces (e.g. Karoo,Parana, Deccan, Yemen, Ethiopia) led some authors tosuggest that the rhyolites are anatectic product of crustalor mantle-derived materials (Cleverly et al. 1984;Lightfoot et al. 1987; Garland et al. 1995; Kar et al.1998), whereas others contend that rhyolites are closelyrelated to the erupted basalts by fractional crystallisa-tion (FC) with variable crustal contamination (Betton1979; Walter et al. 1987; Davidson and Wilson 1989;McCulloch et al. 1994; Chazot and Bertrand 1995;Garland et al. 1995; Baker et al. 2000; Peccerillo et al.2004). Concurrently, hypotheses have been put forwardto explain the magmatic gaps (Chayes 1963) in thosesuites supposed to be derived from FC: (1) physico-chemical properties of intermediate magmas lesseningtheir eruptibility (Baker et al. 1977; Marsh 1981; Turnerand Campbell 1986; Brophy 1991; Macdonald et al.1995); (2) long residence times and extensive crystalli-sation of Fe–Ti oxides enhancing the silica content andyielding residual rhyolitic liquids (Clague 1978; Garlandet al. 1995); and (3) large thermal and chemical bifur-cation induced by small variations in magma residencetime and cooling rate, leading to bimodal eruptedproducts (Bonnefoi et al. 1995).

The two rival interpretations (i.e. partial meltingversus FC) relevant to the origin of the bimodal volcanicsequences and, therefore, to the nature of the Daly Gap,led authors to look for the presence of intermediateproducts coexisting with rhyolites (e.g. former interme-diate melts or solid assemblages resulting from theirdifferentiation). For example, Bellieni et al. (1986) con-sider that the Parana silicic volcanics were generated bypartial melting of lower crustal rocks and interpret re-sorbed labradoritic plagioclase phenocrysts as representing

Editorial Responsibility: J. Hoefs

P. Barbey (&) Æ D. AyalewCRPG-CNRS,B.P. 20, 54501 Vandœuvre-les-Nancy, FranceE-mail: barbey@crpg.cnrs-nancy.frTel.: +33-3-83594234Fax: +33-3-83511798

D. Ayalew Æ G. YirguDepartment of Geology and Geophysics,Addis Ababa University,P. O. Box 1176, Addis Ababa, Ethiopia

Contrib Mineral Petrol (2005) 149: 233–245DOI 10.1007/s00410-004-0647-2

‘‘restites’’. On the other hand, in a comprehensive petro-logical and geochemical study on the Quaternary Gedemsavolcano in the Main Ethiopian Rift, Ayalew et al. (2001)and Peccerillo et al. (2004) show the coexistence of inter-mediate magma with rhyolite, and consider porphyriticmafic-intermediate enclaves as a link between basalt andrhyolite, through FC. These two examples, among manyothers, raise the question of the significance of the mafic tointermediate inclusions found in silicic ignimbrites.Do theyreally represent witnesses of intermediate melts allowingthe Daly Gap to be filled, and can they be used to decipherthe processes involved in the generation of bimodal basalt–rhyolite suites?

Samples selected for this study consist of anorthosi-tic, gabbroic and dioritic cumulophyric aggregates,which come from Oligocene silicic ignimbrites of theEthiopian Plateau (Were Ilu area (WI); Fig. 1). Wepresent mineralogical, textural and chemical data, anduse experimental data together with partitioning of Srand Ba between plagioclase and silicate melt to provideconstraints on the processes involved in their geneses.We discuss their possible position in the basalt–rhyolitesuite and their significance in the processes involved inthe generation of the rhyolites.

Geological background

The Oligocene Afro-Arabian volcanic province, associ-ated with continental break-up (Red Sea, Gulf of Adenand Ethiopian Rift), was produced by impingement of ahot mantle plume at the base of the lithosphere (Marty

et al. 1996). Volcanism occurred from approximately45 Ma ago to the present, with peak activity at around30 Ma (Ebinger et al. 1993; Hofmann et al. 1997;George et al. 1998; Rochette et al. 1998; Ayalew et al.2002; Ukstins et al. 2002). The distribution and timing ofvolcanism has been explained by the lateral flow ofplume material in zones of pre-existing lithosphericthinning (Mesozoic–Palaeogene Rifts and passive mar-gins; Ebinger and Sleep 1998). Extension in the regionbegan in the late Oligocene at ca. 26 Ma (Menzies et al.1997; Ukstins et al. 2002), i.e. after the onset of floodbasalt volcanism. The formation of the nearest oceaniccrust was much more protracted and occurred at ca.10 Ma in the Gulf of Aden (Cochran 1981) and at ca.5 Ma in the Red Sea (Girdler and Styles 1974).

In the Yemen and Ethiopian CFB province, Oligo-cene rhyolites are spatially and temporally associatedwith basalts in a suite which lacks intermediate rocks.Petrological and geochemical studies (Baker et al. 2000;Ayalew et al. 2002; Ayalew and Yirgu 2003) suggest,however, that the rhyolites are possibly derived from FCof basaltic magmas similar in composition to the ex-posed flood basalts, with crustal contribution ( £ 25%).The volcanic rocks of the WI area overlie an approxi-mately 1,200-m sequence of Mesozoic sediments cover-ing unconformably upper Paleozoic continentalformations (Russo et al. 1994). This succession rests on aNeoproterozoic basement (950–500 Ma), which corre-sponds to a juvenile crust formed by arc accretion duringthe Pan-African orogeny (e.g. Teklay et al. 1998; Asratet al. 2001). The WI volcanic succession consists of floodbasalts overlain by about 150 m of rhyolite ignimbrites.The most recent date for the WI rhyolites, on top of theplateau, is 29.00±0.03 Ma (40Ar–39Ar; Coulie et al.2003), in agreement with whole-rock and internal Rb–Srisochron ages (30.1±0.4 Ma) obtained on the WegelTena section (Ayalew et al. 2002). The WI type rhyolitesrepresent only a restricted fraction (approximately10 vol.%) of the Ethiopian Plateau ignimbrites.

Rock types, isotopic data and phase chemistry

The WI ignimbrites are layered, densely welded rocks,which have a glassy appearance and a well-developedcolumnar jointing. They are distinguished from the restof the Oligocene Ethiopian Plateau rhyolites in that theyare free of alkali feldspar, aegyrine–augite, amphibole(richterite/eckermanite) and ilmenite, but carry appre-ciable amounts of (micro-)phenocrysts and cumulophy-ric aggregates consisting of plagioclase (8 vol.%), augite(6%), pigeonite (2%) and Ti-magnetite ( £ 4%). Whole-rock major and trace element data of the studied rocksare found in Ayalew et al. (2002; Appendix 3, samples5798, 5898, AD74, AD76, AD78). WI ignimbrites (67.1–75.6 wt.% SiO2) have peralkalinity indices (Na+K/Al)within the range of the other plateau rhyolites (0.8–1.0 vs.0.9–1.5) but higher CaO contents (1.3–1.7 vs.<1.0 wt.%).Rhyolite from WI yielded 87Sr/86Sr and 143Nd/144Nd

Fig. 1 a Location of the Ethiopia–Yemen continental flood basaltprovince and the Afar Rift triple junction (dashed line denotesextent of Ethiopian dome). b Simplified geological map of thewestern Ethiopian Plateau showing the main volcanic lithologies(adapted fromMerla et al. 1979) and location of the WI area.MERmid-Ethiopian Rift

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initial ratios of 0.70535 and 0.51276, which are wellwithin the range of the initial Sr–Nd isotope ratios of theEthiopian Plateau rhyolites (0.70369–0.70649 and0.51261–0.51289; Ayalew et al. 2002). They are alsocomparable with those of the underlying basalts(0.70334–0.70649 and 0.51271–0.51296; Hart et al. 1989;Ayalew et al. 1999; Pik et al. 1999).

Plagioclase

Plagioclase displays three main occurrences: (1) stronglyzoned phenocrysts ( £ 3 mm in length) forming cumul-ophyric clots with pyroxene and Ti-magnetite (Fig. 2a–c); (2) small grains forming rounded microgranularaggregates in association with Ti-magnetite (Fig. 2d);and (3) zoned or unzoned isolated phenocrysts (Fig. 2e–h) occurring as large euhedral crystals (2–3 mm inlength) or small fragmented grains ( £ 1 mm in length).

Plagioclase phenocrysts commonly have embayed mar-gins and sieved-textures with abundant glass patches,which cut across the zoning pattern (Fig. 2a, e, h). Thisresembles zones of partial resorption obtained in meltingexperiments (Johannes 1989) or reported for long inplagioclase phenocrysts from andesites (e.g. Pailuc1932). The overall distribution in An content is normal,in spite of some spikes in the mole fraction (XAn). Earlierzones may be partly resorbed and overgrown by newmore sodic zones (Fig. 2c). The most calcic composi-tions occur in the core of crystals forming the cumulo-phyric clots (An45–51; Fig. 2b, c), and in microgranularaggregates (An £ 54, Fig. 2d; Table 1). Compositionsof rims are the most frequently in the range An19–28, butmay be as high as An36 in a few cases. The Or compo-nent is always low (Or £ 12; Fig. 3a). Plagioclase fromWI is clearly distinguished from feldspar of other pla-teau rhyolites, which consists exclusively of anorthoclaseand sodic sanidine (An £ 3, Or25–46; Fig. 3a). Sr and Ba

Fig. 2 Microphotographsshowing the main texturalfeatures of the WI phenocrysts.a Cumulophyric aggregates ofplagioclase, clinopyroxene andTi-magnetite (note the sieve-textures of plagioclase withinfilling glass). b andc Cumulophyric aggregates ofaugite and zoned plagioclasewith An-rich cores (solidlines refer to chemical profilesof Fig. 4); note the resorptionof darker zones (arrow).d Microgranular aggregate ofplagioclase and Ti-magnetite.e and f Unzoned orconcentrically zoned resorbedplagioclase phenocrysts withinfilling glass. g Roundedinclusions of plagioclase withina partly resorbed plagioclasephenocryst. h Zoned partlyresorbed plagioclase phenocrystdisplaying An-rich coresurrounded by more sodic rim.Scale bar=0.5 mm. Cpx Augiteand pigeonite, pl plagioclase,mag Ti-magnetite

235

contents were determined in zoned plagioclase pheno-crysts free of glass patches and occurring in cumulo-phyric clots. Analytical conditions are given inAppendix 1. Two examples (samples 5798 and 5898) arepresented in Fig. 4 (see also Appendix 2). Plagioclasegrains display significant zoning with sudden variationsfrom one zone to another. Core–rim concentrations varyby a factor 2 to 4 with a rimward increase of 247–1,060and 371–906 ppm for Ba, and of 407–1,083 and 614–1,590 ppm for Sr.

Pyroxene

Pyroxene occurs as prismatic to rounded grains (0.5–2 mm in length), commonly included within plagioclase,or isolated in the matrix. It corresponds to augite andpigeonite (Table 1), both of which occur as separategrains or coexist in single crystals. Augite shows weakchemical zoning toward Fe-richer rims (up to 1.8 wt.%FeO; Fig. 3b), and pigeonite has a relatively Fe-richcompositions (Wo10En37Fs53), as observed in andesitesand rhyodacites associated with flood basalts (Bellieniet al. 1984). Both pyroxenes have very low Cr2O3 con-tents (below the microprobe detection limit of 90 ppm).They have low Na and Ti contents (0.05<Na2O<0.42;0.22<TiO2<0.98 wt.%) with respect to other plateaurhyolites, which contain dominantly aegirine–augite(10.8<Na2O<13.5; 1.1<TiO2<3.6%). Some augiticclinopyroxenes (Na2O<1.6; TiO2<0.4 wt.%) were ob-served in the other plateau rhyolites, but they have lower

Al2O3 contents than those of WI (0.36–0.61 vs. 0.86–1.13 wt.%, respectively). Augite–pigeonite pairs yieldedequilibration temperatures of 900�C using the calibra-tion of Lindsley (1983).

Ti-magnetite

Ti-magnetite occurs as subhedral to anhedral, isolatedgrains ( £ 1 mm in diameter), or as inclusions in plagio-clase and pyroxene. It shows high ulvospinel contents(Table 1), which vary from 53 to 55 mol.% in somesamples to 61–69% in others. This suggests fO2 condi-tions ranging from FMQ�1.5 to FMQ+0.5 for a 800–1,000�C temperature range, according to the calibrationof Andersen and Lindsley (1988). In other plateau rhyo-lites, Fe–Ti-oxides are mainly ilmenite (0.85<Ilm<0.98),locally associated with Ti-magnetite (0.34<Usp<0.47).

Matrix

Matrix is glassy with perlitic cracking and eutaxitic tex-ture. Matrix glass composition (72.0–75.4 wt.% SiO2;Table 2) has low CaO contents (0.1–0.5 wt.%) with aperalkalinity index of 0.79–1.04 (average 0.97±0.05). Na/K ratios range from 0.48 to 0.87 and are significantly lowerthan those of glasses from other plateau rhyolites(Fig. 5b). It cannot be excluded that these lowNa/K ratiosreflect, at least partly, Na loss by secondary hydration.Nevertheless, the Zr contents of WI ignimbrites

Table 1 Average composition (wt.%) of pyroxenes and examples of composition of Ti-magnetite and plagioclase from WIignimbritesa

Sample Augite Pigeonite Magnetite Plagioclase

5798 (Fig. 2d) AD76 (Fig. 2f) AD76 (Fig. 2h)

Core Rim Core Rim Core Rim

n 46 25 1 1 1 1 1 1 1SiO2 50.65 (0.37) 50.24 (0.39) 0.12 55.21 59.52 61.74 60.79 61.96 58.42TiO2 0.44 (0.12) 0.26 (0.02) 19.10 0.07 0.03 0.02 0.04 0.03 0.06Al2O3 0.99 (0.31) 0.35 (0.16) 1.06 28.11 25.42 24.08 24.19 23.69 25.72FeOtot 17.50 (1.66) 28.64 (0.66) 72.64 0.50 0.56 0.41 0.40 0.42 0.43MnO 1.41 (0.29) 2.43 (0.20) 1.42 0.05 0.00 0.00 0.00 0.00 0.00MgO 10.69 (0.66) 12.28 (0.40) 0.86 0.32 0.03 0.03 0.00 0.03 0.03CaO 17.05 (1.10) 4.49 (0.24) – 10.97 7.53 5.38 5.89 5.03 7.74Na2O 0.33 (0.04) 0.10 (0.03) – 4.99 6.90 7.78 7.47 7.87 6.79K2O – – – 0.34 0.70 1.17 1.04 1.39 0.72Total 99.06 98.78 95.20 100.56 100.70 100.60 99.83 100.43 99.91En (%) 31.8 (1.9) 37.2 (1.1)Fs (%) 31.7 (3.2) 52.9 (1.1)Wo (%) 36.5 (2.3) 9.8 (0.5)XFe 0.53 (0.04) 0.43 (0.0)FeO/MgO 1.6±0.3 2.3±0.1Fe2O3 30.07FeO 45.70XUsp 0.56An (%) 53.8 36.1 25.8 28.5 24.0 37.1Ab (%) 44.3 59.9 67.5 65.5 68.1 58.9Or (%) 2.1 4.0 6.7 6.0 7.9 4.0

aStandard deviations in brackets; FeOtot= total iron as Fe2+; – below detection limits; n number of analyses

236

(608–725 ppm; average 682±44 ppm) are, on thewhole, lower than those of the other plateau rhyolites(610–1576 ppm; average 872±222 ppm). The sum ofoxides (95.3–97.8 wt.%, 96.3 on average) is consistentthough a bit low compared to whole-rock loss on igni-tion values (2.3–3.1 wt.% in the same samples). WIglasses have high normative Quartz and Orthoclasecontents (35.6±1.8 and 32.7±1.4 wt.%), and very lownormative Anorthite and Diopside ( £ 2.3 wt.%). Srand Ba contents in glass in the vicinity of the two studiedcumulophyric aggregates were determined by ionmicroprobe (Appendix 1, Table 2). They range respec-tively from 17 ppm to 88 ppm (average 48±22) andfrom 324 ppm to 766 ppm (average 525±129). A pre-liminary study of glass inclusions in plagioclase pheno-crysts (Ayalew 1999) shows that they are of two types:

1. in sieve-textured plagioclase phenocrysts; large glasspatches (Fig. 2a, e, h) show major element composi-tion similar to that of the matrix, with the exceptionof slightly higher FeO contents (Table 2)

2. in phenocrysts free of sieve-texture; small glassinclusions (5–10 lm) with a gas bubble show variableSiO2 concentrations, ranging from those of the ma-trix glass down to about 70 wt.% (Table 2).

Discussion

Investigation of textures and zoning patterns of plagio-clase grains helps us to place constraints on the processesinvolved in silicic magma generation and to discuss thesignificance of the cumulophyric aggregates found in theWI ignimbrites. Morphologies of pyroxene and plagio-clase phenocrysts (fragmented and rounded grains,sieve-textured plagioclase with infilling glass) suggestpartial resorption due to remelting probably related toadiabatic decompression, as liquidus temperatures maybe lowered with decreasing pressure in water-undersat-urated melts. Although this indicates that the pheno-crysts were carried up with the silicic magma during itsascent and eruption, they are unlikely to represent xe-nocrysts inherited from the underlying rocks. The sub-jacent flood basalts have less evolved plagioclase andpyroxene compositions (An48–85 and Wo33–47En42–54Fs2–16), contain olivine and lack pigeonite (Pik et al. 1998).Neither represent xenoliths of underplated mafic mate-rial sampled by rhyolitic melt. The very high FeO/MgOratios of augite and pigeonite (1.6 and 2.3, respectively)show that they cannot have been in equilibrium with abasaltic melt considering the mineral/melt Kd value(0.23 according to Grove and Bryan 1983). This is fur-ther corroborated by their very low Cr2O3 contents(<0.09%) compared to those of clinopyroxenes fromthe flood basalts (0.13–1.09 wt.%, average 0.41±0.33;Pik et al. 1998). Moreover, the microgranular texture ofsome cumulophyric aggregates (Fig. 2d) suggests localrapid cooling inconsistent with an origin from the lowercrust. Lastly, the basement rocks consist dominantly ofPan-African low-grade metabasalts, metagraywackesand calc-alkaline granitoids (Woldehaimanot and Be-hrmann 1995), which are mineralogically inconsistentwith the phase assemblage found in the WI ignimbrites.In any case, the mantle-like isotopic signature of the WIrhyolites precludes significant incorporation of oldergranitic material.

The WI phenocrysts are more likely to representcumulate phases grown from less evolved parentalmagma and subsequently carried up by rhyolitic melt;this is in agreement with (1) the bulk anorthositic, gab-broic and dioritic modal composition of the aggregatesand (2) the overall normal An zoning patterns of pla-gioclase phenocrysts. Such an assemblage is common inrhyodacites and rhyolites associated with flood basaltseries, as for instance in the Parana province whereplagioclase (An59–39), augite, pigeonite and Fe–Ti oxideare the typical phenocryst phases (Bellieni et al. 1984).The coexistence of pigeonite with augite and theirintermediate iron content are also considered to becommon features of intermediate volcanics (Gill 1981).

Ab Or

Were Ilu rhyolites(n = 207)

Other plateau rhyolites

50

50

An

Pigeonite

FerroaugiteAugite

HedenbergiteDiopside

Enstatite Ferrosilite50A

A

AR

RB

(a)

(b)

Fig. 3 Mineral composition of the WI rhyolites (open circles):a Albite–Anorthite–Orthoclase diagram showing the compositionof WI plagioclase (open circles) compared to alkali feldspar fromother plateau rhyolites (open diamonds). b The pyroxene quadri-lateral (Morimoto et al. 1988) illustrating the composition of thehigh- and low-Ca pyroxenes (open circles) compared to augite (opendiamonds) and aegyrine (filled diamonds) from other plateaurhyolites. Shown for comparison: fields of Ca-poor and Ca-richpyroxenes from the Parana volcanic rocks (A andesites, Rrhyodacites) from Bellieni et al. (1984), and of Ca-rich pyroxenesfrom the Ethiopian Plateau flood basalts (B) from Pik et al. (1998)

237

It has also been suggested that the presence of pigeonitein continental tholeiitic series can be considered as anevidence of host–rock assimilation during differentiation(Campbell 1985). Experiments on plagioclase crystalli-sation from basaltic to andesitic melts (e.g. Panjasa-watwong et al. 1995) show that plagioclase An contentincreases with melt Ca# (=100Ca/Ca+Na), tempera-ture and water content, but decreases with increasingpressure. The presence of An40–54 plagioclase (corecomposition of WI plagioclase) would imply melt withCa# of 30–50 for a pressure range of 1–10 kbar. Thisis far beyond the values of both the matrix glass(4.0<Ca#<18.6) and bulk composition (1.3<Ca#<23.1)of the WI rhyolites. As far as these experimental data can

be applied in our case, this indicates that the cores crys-tallised from less differentiated melts.

The phase assemblage of the WI rhyolites, distinctfrom that of the other Oligocene Ethiopian Plateaurhyolites (alkali feldspar, aegyrine–augite, amphiboleand ilmenite), can be related to distinct melt composi-tions and temperatures. In spite of similar low CaOcontents, the WI glass compositions are distinguishableby their lower peralkalinity values, Na/K ratios (Fig. 5)and Zr contents. Experimental investigations show thatmagnetite, Fe-hedenbergite or aegirine–augite and alkalifeldspar are the liquidus phases in peralkaline rhyolites(Scaillet and Macdonald 2001), whereas in metaluminousA-type silicic magmas they are augite, orthopyroxene,

50

20

30

40

600 800 1000 1200400200

600

600

1000

1400

1000

1400

1800

200

200

(a)XAn

corerim

Sr(ppm)

0

(b)

Ba(ppm)(c)

58-98

50

20

30

40

600 800400200

600

600

1000

1400

1000

1400

1800

200

200

(d)XAn

corerim

Sr(ppm)

0

(e)

Ba(ppm)(f)

57-98Fig. 4 Variation of XAn (molefraction) and of Ba and Srconcentrations (ppm) acrossplagioclase phenocrysts fromcumulophyric aggregates ofFig. 2b (5898) and Fig. 2c(5798). Solid circles electronmicroprobe data, crosses ionmicroprobe data

238

plagioclase, quartz and alkali feldspar (Dall’Agnol et al.1999). Scaillet and Macdonald (2001) suggest that thelack of plagioclase in peralkaline rhyolitic magma couldbe closely related to the peralkaline character of meltrather than to its low CaO content. Besides, the absenceof sanidine and quartz in the WI phenocryst phaseassemblage compared to the composition of melt, whichis Quartz- and Orthoclase-normative (ca. 35% for each)with high K2O contents (5.5±0.3 wt.% on average),implies that melt temperatures were above both the alkalifeldspar and quartz liquidi.

Lastly, accounting for the slow diffusivity of CaAland NaSi pairs in plagioclase (e.g. Baschek and Johan-nes 1995), the gradual evolution of plagioclase pheno-cryst composition towards the Albite end-member alongwith sudden changes in plagioclase An contents (Fig. 4)

can be considered as resulting from melt differentiationunder decreasing temperature accompanied with possi-ble recharge or assimilation or solid-state re-equilibra-tion. All these data along with recent experimentsshowing that fractional crystallisation of mildly alkalinebasalts is likely to produce differentiated series frombasalts to rhyolite (Nekvasil et al. 2004) led us to modelthe evolution of Sr and Ba profiles of plagioclaseassuming differentiation along a crystallisation path.

Modelling Sr and Ba profiles of plagioclase

Though physicochemical parameters cannot easily beconstrained for the parental magma in equilibrium withthe WI plagioclase phenocryst cores, we attempted tomodel the compositional evolution of the two zonedplagioclases studied by using experimental constraints.First, we have to know whether Sr and Ba contents ofthe different zones of plagioclase phenocrysts representcomposition equilibrated with the melt or whether theycorrespond to subsequent solid-state re-equilibrationupon cooling. As discussed by Zellmer et al. (1999), equi-librium partitioning of Sr and Ba between two plagioclasezones a and b can be deduced from mineral/melt partitioncoefficients, which depend on temperature and anorthitecontent (Blundy and Wood 1991):

Da=bi ¼ Ca

i

Cbi¼ exp

�wi X aAn � X b

An

� �

RT

� �ð1Þ

where wi is a constant depending on trace element i (e.g.38,200 J mol�1 for Ba), XAn is the mole fraction ofAnorthite, R is the gas constant (8.314 J�1 mol�1 K�1)and T is the temperature in K. Deviation between DBa

and DSr values calculated from An contents and esti-mated from concentrations varies between 0 and 37%suggesting that the concentration profiles are not fully

Table 2 Average compositions (wt.%) of matrix glass for two WI ignimbrite samples, average compositions of glass infilling sieve-textured plagioclase phenocrysts and composition of small glass inclusions (5–10 lm) in plagioclase phenocrystsa

Matrix glass Glass inclusions in plagioclase

Glass infilling sieve-texturedphenocrysts

Glass in smallinclusionsb

Sample 5798 5898 5798 5898 AD76 AD76n 12 15 5 5 1 1SiO2 74.65 (0.30) 74.42 (0.43) 75.34 (0.55) 75.03 (0.7) 70.8 73.8TiO2 0.34 (0.02) 0.33 (0.02) 0.38(0.02) 0.41 (0.03) 0.5 0.5Al2O3 10.90 (0.16) 10.69 (0.42) 11.40(0.33) 11.53 (0.45) 10.2 10.6FeOtot 1.84 (0.10) 1.78 (0.10) 2.23 (0.23) 2.47(0.13) 5.0 2.2MnO 0.08 (0.05) 0.05 (0.04) 0.07(0.07) 0.14 (0.12) 0.3 0.1MgO 0.07 (0.07) 0.10 (0.08) 0.19(0.09) 0.18(0.14) 0.3 0.1CaO 0.38 (0.02) 0.37 (0.04) 0.52(0.05) 0.55(0.03) 0.8 0.4Na2O 2.77 (0.29) 2.51 (0.35) 2.23(0.13) 2.14(0.12) 1.8 2.6K2O 5.56 (0.09) 5.56 (0.17) 5.07(0.20) 5.00(0.19) 5.7 5.7Total 96.58 95.82 97.43 97.44 95.4 96.0Ba (ppm) 525 (129) 495 (103)Sr (ppm) 48 (22) 45 (18)

aStandard deviations in brackets; FeOtot=total iron as Fe2+; n number of analysesbData from Ayalew (1999)

(Na

+ K

) / A

l

0.2

0.4

0.6

0.8 1.2 1.6 2.0

1.0

1.4

1.8

Na / K

Were Ilu glasses

whole rock compositionsof other rhyolites

glasses fromother rhyolites

whole-rock composition of WI ignimbrites

Fig. 5 Peralkalinity index versus Na/K ratio for WI glass matrix(open circles) and related whole-rock compositions (filled squares),compared to other plateau rhyolites (Wegel Tena, Molale andDebre Birhan; data from Ayalew et al. 2002, and unpublished)

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re-equilibrated. We shall see below that the Ba and Srprofiles cannot be accounted for by intracrystalline dif-fusion. The profiles are, therefore, assumed to representplagioclase/melt partitioning and to be suitable fordeciphering the processes involved in their generation.

Considering that the aggregates correspond tocumulate phases, we assume that three possible pro-cesses may have led to their generation: closed-system

equilibrium crystallisation (EC), or FC, or assimilation-fractional crystallisation (AFC; De Paolo 1981). Owingto preservation of whole-rock mantle-like Sr–Nd isoto-pic ratios in rhyolites, we suppose that the possiblecontaminant corresponded to rocks having low Rb/Srratios and low initial 87Sr/86Sr isotopic ratios. Regionalgeological data suggest that they are likely to be maficmetavolcanic and plutonic rocks of the Pan-AfricanBasement. Their average Sr and Ba contents are310±370 and 45±35 ppm, respectively, with an averageRb/Sr ratio of 0.013 and an initial 87Sr/86Sr isotopicratio of 0.70309 (Woldehaimanot and Behrmann 1995;Teklay et al. 1998).

The fractionating solid is supposed to correspond tothe cumulophyric clots, which consist of plagioclase, Ca-rich and Ca-poor pyroxenes and Ti-magnetite in therough weight fractions 0.5, 0.3, 0.1 and 0.1, respectively(Table 3). Solid-melt partitioning of Sr and Ba is con-sidered to be controlled by plagioclase only, and to obeythe model of Blundy and Wood (1991). The lack of

Table 3 Results of petrogenetic modelling showing the major element composition (100% anhydrous) calculated for the cumulate and theparent melta

Pl Aug Pgt Ti-mag Cumulate Residual melt Contaminant Parent melt (1) (2) (3)

SiO2 57.3 51.0 50.4 0.9 49.1 75.0 53.2 72.7 72.1 76.4 71.6TiO2 0.1 0.4 0.2 20.7 2.2 0.6 0.6 0.9 0.8 0.1 0.7Al2O3 26.6 0.9 0.4 1.2 13.7 11.3 15.0 11.4 13.4 12.1 12.6FeOtot

b 0.3 17.9 29.4 74.8 16.0 3.5 10.2 4.9 3.7 1.7 4.8MnO 0.0 1.5 2.4 1.4 0.8 0.2 0.3 0.3 0.0 0.1MgO 0.0 10.6 12.4 1.0 4.5 0.2 6.6 0.4 0.7 0.0 0.9CaO 9.0 17.3 4.6 0.0 10.1 0.9 11.6 1.5 2.2 0.3 2.1Na2O 6.2 0.3 0.1 0.0 3.2 3.0 2.3 3.1 3.8 4.4 2.7K2O 0.5 0.0 0.0 0.0 0.2 5.3 0.2 4.9 3.4 4.9 4.5Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Fig. 6 Modelling of the evolution of Sr and Ba concentrations(ppm) in plagioclase phenocrysts assuming AFC, compared tophenocrysts 5798 (crosses) and 5898 (solid circles). Trends ofequilibrium crystallisation (EC, dots), fractional crystallisation(FC, open circles) and solid-state re-equilibration (SR, opentriangles) shown for comparison. The equation used for AFC isfrom De Paolo (1981): Cm/Cm

0=F� z+(r/r�1)(Ca/Cm0 )ln F, where

Cm, Cm0 and Ca are the element concentrations in the initial melt,

the residual melt and the material assimilated, F is the fraction ofresidual melt and r is the assimilated/crystallised mass ratio.Numbers refer to the mass fraction of residual melt. Solid-statediffusion trends are modelled from Eq. (1), assuming a core with500 and 600 ppm Ba and Sr. See text for further details

aShown for comparison: (1) A-type granitic glass from Dall’Agnollet al. (1999), (2) slightly peralkaline comenditic glass from Scailletand Macdonald (2001), and (3) Parana rhyodacite from Bellieniet al. (1986). The mass fraction of plagioclase (Pl), augite (Aug),

pigeonite (Pgt) and Ti-magnetite (Ti-mag) in the cumulate are 0.5,0.3, 0.1 and 0.1, respectively. F=0.85 and Y=0.08; see text forfurther explanationbFeOtot=total iron as Fe2+

Ba(ppm)

An(mol%)

FC

EC

AFC

1000

1400

600

20020 40

An(mol%)

20 40 60 60

1400

1000

600

200

Sr(ppm)

AFC

FCEC

1.00

0.85

0.85

(a) (b)

plagioclase phenocrysts

r = 0.55

1.00

5898 5798

SR

SR

240

quartz, K-feldspar and amphibole, along with the two-pyroxene thermometric estimates (900�C) and experi-mental data on metaluminous A-type and peralkalinesilicic magmas (Dall’Agnol et al. 1999; Scaillet andMcDonald 2001) suggest that crystallisation occurredmainly in the 1,000–850�C temperature range, i.e. frombelow the liquidus of both plagioclase and pyroxenes toabove the liquidus of quartz and alkali feldspar.According to experimental data (Dall’Agnol et al. 1999)and in agreement with phenocryst composition, plagio-clase composition is assumed to vary from An48 to An24.Scaillet et al. (1997) have shown that silicic magmas stillcontain up to 80% melt down to temperatures 15–20�Cabove the solidus. For metaluminous compositions and850<T<1,000�C, the modelled crystallisation pathsshow that the wt.% crystals ranges from 10 to 20 de-pending on fO2 conditions (Scaillet et al. 1997). Thefraction of melt (F) is therefore assumed to vary between1.0 and 0.85.

Sr and Ba concentrations of the parent melt are un-known. However, the data of Bellieni et al. (1986) forandesitic to rhyolitic low-Ti volcanic rocks of the ParanaCFB province allow the range of Sr and Ba contents tobe very roughly constrained at 100<Sr<240 ppm and400<Ba<710 ppm.

Modelling results

Modelled compositions for Ba and Sr are compared toplagioclase phenocrysts 5798 and 5898 (Fig. 6). The lowproportion of plagioclase in the solid extracted is a pre-requisite to obtain a significant Sr and Ba enrichment,whatever the model of crystallisation chosen. The twocontrolling parameters are the fraction of residual melt(F) and the proportion of plagioclase in the crystallisedsolid (XPl). The fraction of plagioclase observed in theaggregates (XPl�0.5) therefore implies high values of F(‡0.85) in agreement with experimental data for thetemperature range considered (Scaillet et al. 1997). Thecompositional range of the plagioclase phenocrysts isreproduced satisfactorily by AFC for samples 5898 (bothBa and Sr) and 5798 (Ba, only), whereas FC seems tobetter reproduce the Sr trend for sample 5798. However,besides the fact that the Sr plot is less discriminant (theFC and AFC trends are closer to each other than in theBa plot), it should be emphasized that the petrogeneticmodel assumes a single value for the mass of materialassimilated. However, the two samples may representdistinct magma batches and, therefore, the amount ofcontamination may be lower for sample 5798 as sug-gested by the trends in both Ba and Sr plots. The Sr andBa contents of the parent melt (175 and 450 ppm,respectively) and the value of the assimilated/crystallisedmass ratio (r £ 0.55, degree of contamination £ 8 wt.%),adjusted to fit the trace element profiles of the plagioclasephenocrysts, are consistent with the data of Bellieni et al.(1986) and with the range of contamination envisaged byAyalew et al. (2002) for the Ethiopian Plateau rhyolites

(<10%). The maximum change in the initial 87Sr/86Srisotopic ratio of the melt induced by AFC is <1.5·10�3,i.e. well within the range of the isotopic ratios observed forthe plateau rhyolites (0.70369–0.70649). The modelledcomposition of the residual melt (Sr=71 ppm andBa=441 ppm) in equilibrium with a An24 plagioclase rimis consistent with the average composition of the hostmatrix glass for Sr (48±22 ppm) but somewhat low forBa (525±129 ppm).

Ba and Sr distribution by solid-state re-equilibrationthrough diffusion (Eq. (1)) cannot explain the trendsobserved in the two plagioclase phenocrysts (Fig. 6b).Moreover, re-equilibration of Sr by intracrystalline dif-fusion is probably unlikely because (1) it is inconsistentwith the presence of several sharp spikes in An contentacross the phenocrysts, and (2) the time span necessaryto re-equilibrate the whole crystal appears too high (ofthe order of 107 years using the Sr diffusion data ofZellmer et al. 1999) compared to the duration of theEthiopian volcanic event (<1 Ma according to Rochetteet al.. 1998) and to the duration of differentiation inmagma chambers (103–105 years according to Cawthornand Walraven 1998, and to Hawkesworth et al. 2000).

Despite some uncertainties on geochemical modelling,the anorthositic, gabbroic and dioritic cumulophyricclots observed in the WI ignimbrites appear likely to bederived by fractional crystallisation plus limited con-tamination from a melt less differentiated than the meltnow represented by the rhyolitic matrix glass. Never-theless, they are unlikely to represent intermediate meltcompositions as suggested by experimental constraintand mass balance calculation. Major element composi-tion of parent melt can be modelled from the equation:

C0i ð1� Y Þ þ CA

i Y ¼ Cliqi F þ Csol

i ð1� F Þ ð2Þ

where Ci0, Ci

A, Ciliq and Ci

sol are the contents of element(i) in the parent magma, assimilated rocks, residual meltand fractionated solid, and Y is the mass fraction ofassimilated host rock. A rough estimate can be made(Table 3) considering that the assimilated rocks are Pan-African metavolcanics, and assuming F=0.85 andY=0.08. The model gives a parental melt with a SiO2

content 2–3% lower than the residual melt. This showsthat even though we consider the silica content of thematrix glass (75% recalculated at 100% anhydrous) as amaximum value for the residual melt, the silica contentof the parent melt is unlikely to have been significantlylower than 70%. The composition calculated for theparent melt is similar to that of the A-type granitic glasscomposition used by Dall’Agnol et al. (1999) in theircrystallisation experiments. It also resembles the com-position of Parana rhyodacites (Bellieni et al. 1986).Moreover, our model is consistent with the compositionof the small glass inclusions found in WI plagioclasephenocrysts, which show silica contents ranging from70% to 74% (Ayalew 1999).

On the whole, the anorthositic, gabbroic and dioriticcumulophyric aggregates cannot be witnesses of inter-

241

mediate andesitic magmas. Nevertheless, they suggestthat the WI rhyolites result from differentiation byfractional crystallisation of rhyodacitic melts associatedwith limited assimilation, in agreement with previousmodels based on whole-rock isotopic and trace elementdata (Baker et al. 2000; Ayalew et al. 2002; Ayalew andYirgu 2003).

A last point which could be dealt with is the possiblelink between the WI metaluminous rhyolites and theother mildly peralkaline Ethiopian Plateau rhyolites.Considering Sr–Nd isotopic compositions and emplace-ment in a very short time span (<1 Ma for both basaltsand rhyolites), we can suppose that both types of rhyolitehave a common parent and involved rapid differentiationprocesses. The WI rhyolites are distinguishable from theothers by a phase assemblage of higher temperature withclinopyroxenes having higher Al2O3 contents, and bylower Zr and higher Sr contents (Sr=146–222 ppm,average 170±35 vs. 8–262 ppm, average 97±84). Bar-beri et al. (1975), in their study of the Boina Centreconsisting of a basalt–pantellerite suite including inter-mediate members, reported (1) a fractionating phaseassemblage consisting (for F<0.45) of plagioclase (an-desine–oligoclase), Fe-rich clinopyroxene and Fe–Tioxides, followed by anorthoclase which may containoligoclase cores; and (2) a sudden decrease in Sr contentfor F<0.20. They further suggest that the transition toperalkalinity is linked to the ‘‘plagioclase effect’’ (Bowen1945), i.e. fractionation of a sodic plagioclase from a li-quid with extremely low normative Anorthite content.The WI ignimbrites show these characteristics, i.e. cu-mulophyric aggregates consisting of plagio-clase + pyroxene + Ti-magnetite, plagioclasephenocrysts with An £ 20 rims, glass with low norma-tive Anorthite contents ( £ 2 wt.%, average 0.70±0.67)and a strong decrease in Sr for F<0.20 (Ayalew et al.2002). All these data are consistent with (but does notprove) a genetic link between the WI metaluminousrhyolites (which appear to be less differentiated) and theother Ethiopian Plateau rhyolites.

Conclusions

Textural and chemical study of the (micro)-phenocrystphase assemblage of the WI rhyolites leads us to thefollowing conclusions:

1. Contrary to most Ethiopian Plateau rhyolites, theycontain plagioclase as the only feldspar, together withaugite, pigeonite and Ti-magnetite. The minerals oc-cur as anorthositic, gabbroic and dioritic cumulo-phyric clots. This composition can be attributed todistinct melt composition and temperature.

2. Compositional zoning profiles of plagioclase grainsshow an overall rimward decrease in An mole frac-tion (from An50 to An19) associated with an increasein Ba and Sr contents, suggesting an overall differ-entiation under decreasing temperature.

3. Textures and geochemical modelling suggest thatthese cumulophyric clots are not witnesses of andesiticmelts and, therefore, cannot be used to fill the DalyGap. They are likely to correspond to the differenti-ation products of rhyodacitic melts by fractionalcrystallisation (plus limited assimilation) in crustalmagma chambers. This shows that great care shouldbe taken when interpreting mafic to intermediateinclusions in silicic volcanics as witness of intermedi-ate melts, without careful evaluation of geochemicalaffinities between inclusions and their host magma.

4. Whatever the model invoked for the origin of the WItype ignimbrites of the Ethiopian Plateau (melting ofa mafic lower crust or FC of basaltic magma), theirgeneration involved a stage of differentiation incrustal magma chambers.

Acknowledgements Thanks to J. Ravaux for assistance in electronmicroprobe analyses, and to E. Deloule for ion probe data. We aregrateful to B. Scaillet and an anonymous reviewer for their criticalreview of the manuscript. This research was founded by the InstitutNational des Sciences de l’Univers (INSU-CNRS) and Universityof Addis Abeba. CRPG contribution 1721.

Appendix 1: analytical conditions

Major element composition of minerals and glass ma-trix, and Ba and Sr concentration profiles of plagioclasephenocrysts were determined by using a CAMECA SX-50 electron microprobe (Henri Poincare University,Nancy). Operating conditions were: (1) 10-nA samplecurrent, 15-kV accelerating potential, counting times of20 s and a beam diameter of 1 lm for plagioclase,pyroxene and Ti-magnetite; (2) 10 nA, 15 kV, 10 s and a5 lm defocussed beam for matrix glass; and (3) 100 nA,15 kV, 20 s and beam diameter of 1 lm for Sr, Ba andmajor elements in plagioclase. Calibration was made ona combination of silicates and oxides. Data reductionswere performed by using the PAP correction procedure(Pouchou and Pichoir 1991).

Ba and Sr contents of the matrix glass were deter-mined and plagioclase Ba and Sr profiles of sample 5898were controlled by ion microprobe. Measurements wereperformed on the CRPG Cameca IMS 3f ion microprobe.A 10-kV O� primary beam of 15- to 20 nA intensity wasfocussed to a spot of 20 lm diameter. Secondary ionswere accelerated to 4,500 eV and analysed at a massresolution of �500 with an energy filtering at �80±10 V.The background, 30Si, 86Sr, 88Sr, 137Ba and 138Ba weremeasured by peak switching, with counting times of 3 son each peak. Successive measurements were cumulatedfor 15 min on each sample position. Secondary ion cur-rents are normalised to Si, and secondary yields relativeto Si determined on three standard glasses (NBS 614,BHVO, BCR2G) ranging in composition from 49.9 to72.0 wt.%. There is no observable variation of the rela-tive secondary yields of Sr and Ba for these standards,implying that the calibration is not dependent on thechemical composition of silicate samples.

242

Appendix 2

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