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Petrogenesis of silicic peralkaline rocks in the Ethiopianrift: Geochemical evidence and volcanological implications

A. Peccerillo a,*, C. Donati a, A.P. Santo b, A. Orlando c, G. Yirgu d, D. Ayalew d

a Dipartimento di Scienze della Terra, University of Perugia, Piazza Universita, I – 06100 Perugia, Italyb Dipartimento di Scienze della Terra, University of Florence, Via La Pira 4, Florence, Italy

c Istituto di Geoscienze e Georisorse, CNR, Via La Pira 4, Florence, Italyd Department of Earth Sciences, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia

Received 1 June 2005; received in revised form 28 October 2005; accepted 1 June 2006Available online 24 February 2007

Abstract

Major, trace element and isotopic data for mafic to peralkaline silicic volcanic rocks from the northern sector of the main Ethiopianrift are discussed with the aim of placing constraints on processes of magma genesis and evolution and to present models for magmaplumbing systems of rift volcanoes. Basalts straddle the subalkaline–alkaline boundary and exhibit important variations of incompatibleelement abundances and ratios. Silicic rocks consist of dominant pantellerites and minor comendites and trachytes, although some vol-canoes along the rift shoulders consist entirely or predominantly of trachytes. Rocks with intermediate compositions are very scarce.Mafic and silicic rocks exhibit similar values as some basalts for many incompatible element and radiogenic isotopic ratios. Geochemicaland petrological modelling shows that the most likely petrogenetic process for rift magmatism is a derivation of rhyolites from basalts bydominant fractional crystallisation occurring at shallow depths. Variations of incompatible element ratios and radiogenic isotopes in thebasalts suggest heterogeneous sources and significant interaction with the crust. In contrast, the role of crustal assimilation during evo-lution of silicic magmas is negligible. It is suggested that large amounts of basalts were emplaced both into the lower continental crust,and at shallow depths. Shallow level fractional crystallisation generated zoned magma chambers with rhyolitic melts accumulating at thetop, and mafic magmas ponding at the bottom. Volcanic activity was fed preferentially by the upper rhyolitic layer, whereas mafic mag-mas were erupted only accidentally, when extensional faults intersected the bottom of shallow reservoirs or tapped directly the deepmagma chambers. The presence of trachytic volcanoes along the rift shoulders could result from clinopyroxene-dominated high-pressurebasalt fractionation, which did not allow melts to reach rhyolitic compositions. Satellite imagery and field studies reveal the occurrence ofa large number of caldera collapses in the main Ethiopian rift, suggesting that several magma chambers have been formed at shallowdepths, possibly favoured by block tilting and strike-slip faulting. This explains the huge amounts of silicic rocks along the northernEthiopian rift. The occurrence of huge magma reservoirs is also supported by positive gravity anomalies detected by previous studiesbeneath several silicic volcanic centres.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Ethiopian rift; Geochemistry; Petrogenesis; Fractional crystallization; Peralkaline rocks

1. Introduction

The latest stages of the Ethiopian rift opening have beenmarked by eruption of huge amounts of volcanic products(Fig. 1), with a large prevalence of silicic rocks, minor bas-

alts and scarcity or absence of intermediate compositions(e.g., Mohr, 1971; Merla et al., 1979; Berhe et al., 1987;Mohr and Zanettin, 1988; Kampunzu and Lubala, 1991;Hart et al., 1989; Yemane et al., 1999; Peccerillo et al.,2003). Such a bimodal distribution of the volcanism is acommon feature of many volcanoes, especially in the con-tinental rift, but its genesis is still debated (e.g., Kampunzuand Lubala, 1991; Thompson et al., 2001).

1464-343X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jafrearsci.2006.06.010

* Corresponding author.E-mail address: pecceang@unipg.it (A. Peccerillo).

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Journal of African Earth Sciences 48 (2007) 161–173

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Silicic rocks dominate the rift floor sequences and makeup the bulk of silicic volcanoes along the rift axis. Theyconsist of dominant peralkaline rhyolites and minortrachytes, which were emplaced mostly under the form ofpyroclastic flow and fall deposits, with minor lavas.Trachytes are mostly confined to the rift shoulders wheresome volcanoes (e.g., Yerer and Zuqala) are reported tobe formed entirely by these rocks (Gasparon et al., 1993).Mafic rocks are mainly represented by transitional basaltsforming rows of cinder cones and associated lava flows,with minor tuff cones and tuff rings.

The composition of Ethiopian rift magmatism, thebimodal distribution of magma types and the scarcity ofintermediate rocks (Daly gap) have been the subject of sev-eral studies (e.g., Gasparon et al., 1993; Peccerillo et al.,2003 and references therein). However, there is still muchdebate on these issues, both for the Ethiopian rift and otherareas of continental breakup (e.g., Kampunzu and Mohr,1991; Peccerillo et al., 2003 with references). In particular,it is not clear for the main Ethiopian rift: (a) whether silicicrocks derive from associated basalts by fractional crystalli-sation or they represent independent melts from the crust;(b) the reason for the scarcity or absence of intermediaterocks; and (c) the dominance of peralkaline rhyolites alongthe rift floor and the presence of trachytic volcanoes alongthe rift shoulders.

In this paper, these issues are discussed using new andliterature petrological and geochemical data. Geochemicalmodelling will be discussed to evaluate petrogenetic pro-cesses and to place constraints on physical conditions ofmagma evolution and on the structure of the plumbing sys-tems of silicic volcanoes. It will be shown that fractionalcrystallisation of transitional basalts is the most likely pro-cess for the generation of peralkaline rhyolites in the rift.These occurred at low pressure within large and chemically

zoned magma chambers. Eruptions tapped preferentiallythe upper silicic layer of these reservoirs, whereas maficmelts were not allowed to rise to the surface because ofthe higher density with respect to silicic melts. The presenceof trachytic magmas along the rift shoulders is attributed tohigh-pressure fractional crystallisation of transitionalbasaltic melts, which did not reach rhyolitic compositionsbecause of dominant separation of clinopyroxene withrespect to olivine and feldspar.

2. Geological setting

The Oligocene to present volcanism of Ethiopia coversan area greater than 600,000 km2 (e.g., Merla et al.,1979). It is dominated by basaltic lavas and by rhyoliticand trachytic pyroclastic products and minor lavas. Thevolcanic rocks rest upon Mesozoic marine sedimentarysequences or directly on the Precambrian metamorphicbasement. Volcanism took place almost continuously fromOligocene to present, and a maximum of basalt outpouringhas been recognised at about 31 Ma (Hofmann et al.,1997). Classical studies on Ethiopian volcanism distin-guished three main stages of volcanic activity (e.g., Kazminet al., 1980; Mohr and Zanettin, 1988). The first one is Oli-gocene to Miocene in age and was characterised by erup-tion of large flood lava sequences (known as Ashangeand Aiba Basaltic Formations) forming the Ethiopianbasaltic plateau. Latest phases of this stage were character-ised by alternating basalt and ignimbritic eruption, formingthe so-called Alaji Rhyolitic Formation (e.g., Merla et al.,1979).

A second stage of activity is Miocene in age and is char-acterised by the construction of huge basaltic shield volca-noes (Termaber Basalt Formation). Rock composition isdominated by transitional to Na-alkaline basalts withminor benmoreites and trachytes. Some nephelinitic rocksalso occur (e.g., Piccirillo et al., 1979).

The third stage is Pliocene to Quaternary in age and isdirectly related to the main phases of opening of the mainEthiopian rift and Afar. In the Ethiopian rift, volcanism isdominated by peralkaline rhyolitic ignimbrites and pumice-and ash-fall deposits, with minor lava flows. Silicic rocksare associated with volumetrically subordinate basalticproducts that form cinder cones and lava flows mostlyaligned along extensional faults of the Wonji fault belt(e.g., Mohr, 1971). Most recent acidic products in thenorthern Ethiopian rift have been erupted from central vol-canoes with large summit calderas, such as Gedemsa, Koneand Fantale. Fissural eruptions of silicic rocks have beeninferred, but rarely demonstrated by field studies, for thelowest exposed silicic rocks forming the rift-floor ignimb-rites (e.g., Di Paola, 1972; Boccaletti et al., 1999).

3. Petrography

Mafic rocks from the Ethiopian rift include lavas,strombolian scoriae and hydrovolcanic lapilli and ashes.

Stratovolcano

Uncertain caldera

Caldera

Fault

Zuqala

Fantale

Kone

Boseti-Gudda

Gedemsa

Nazret

Metahara

Debre Zeit

Addis Ababa

Yerer

ZwayLake Chilalo

Basaltic centreEthiopia

Gulf of AdenAddisAbaba

9˚

8˚

39˚ 40˚38˚

Fig. 1. Satellite image of the northern sector of the main Ethiopian rift,with indication on the main structural and volcanological features.

162 A. Peccerillo et al. / Journal of African Earth Sciences 48 (2007) 161–173

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Lavas and scoriae have a porphyritic texture with pheno-crysts of olivine, plagioclase and clinopyroxene surroundedby a microcrystalline to hypocrystalline groundmass. Totalphenocryst abundance is very variable and mostly rangesbetween 5% and 20% by volume; however, some stronglyporphyritic rocks with up to 40–50 vol% phenocrysts andmegacrysts are also found. Mg-olivine occurs nearly alwaysas euhedral crystals, sometimes zoned and frequently trans-formed to iddingsite. Plagioclase phenocrysts, often zoned,are mostly bytownitic–labradoritic in composition. Augiticclinopyroxene crystals are not always present as pheno-crysts but are ubiquitous in the groundmass. Some lavasand scoriae (e.g., near Debre Zeit) contain megacrysts ofclinopyroxene and plagioclase up to a few cm large. Thegroundmass is composed of plagioclase, olivine, clinopy-roxene and Fe–Ti oxides.

Intermediate rocks are rare (Brotzu et al., 1974; Pecce-rillo et al., 2003; Authors’ unpublished data). Their textureranges from almost aphyric to seriate porphyritic with phe-nocrysts and megacrysts of plagioclase, olivine and clino-pyroxene. Plagioclase is generally dominant and isstrongly zoned with compositions ranging from bytowniteto oligoclase. Olivine is often transformed to iddingsite; itoccurs often as corroded Fo-rich crystals. Clinopyroxeneranges from colourless diopside to green salite.

Trachytes and rhyolites include lavas, pumices andwelded ignimbrites. Trachytic lavas are generally porphy-ritic with phenocrysts of anorthoclase, plagioclase, fayaliticolivine and ferrous clinopyroxene. The groundmass con-tains alkali-feldspar, pyroxene and Fe–Ti oxides and vari-able abundances of glass (e.g., Gasparon et al., 1993;Barberio et al., 1999; Peccerillo et al., 2003 and referencestherein). The rhyolitic rocks range from hypocrystallineporphyritic to hypoyaline. Phenocrysts include quartz,sanidine, anorthoclase, fayalite and hedenbergite. Micro-phenocrysts of opaque minerals, alkali-amphibole, alkali-pyroxene and aenigmatite are common.

4. Geochemistry

Representative data of volcanic rocks along the north-ern sector of the Ethiopian rift are given in Table 1. Thealkali-silica diagram is reported in Fig. 2. Variation dia-grams of major and some key trace elements are shownin Figs. 3 and 4. In the latter diagrams data for the Boinavolcano in Afar are reported for comparison (Barberi et al.,1975).

Alkali vs. silica diagram indicates that the rift rockshave a transitional to weakly alkaline character and fallalong the boundary separating the alkaline from the sub-alkaline series of Irvine and Baragar (1971). It also revealsa scarcity of rocks with intermediate silica contents, whichis a main feature of the Ethiopian rift magmatism.

Major element variations show considerable scattering.Overall, there is an increase in alkalies and incompatibleelements (e.g., Zr, Th) and a decrease in MgO, TiO2,CaO, P2O5, ferromagnesian elements (e.g., Ni, Cr) and

Sr with increasing silica. Ba increases from mafic to inter-mediate rocks to decrease in the rhyolites. Very high con-centrations of Ba in some trachytes have been attributedto accumulation of Ba-rich alkali feldspars (Peccerilloet al., 2003). A wide range of concentrations in TiO2 isobserved among mafic rocks, a feature also found in bas-alts from the Ethiopian plateau (Pik et al., 1998). P2O5

also shows large variations in the mafic rocks, with twodistinct trends and some of the highest values beingobserved in the Chilalo volcanic area, situated on theeastern margin of the main Ethiopian rift (Trua et al.,1999). In contrast, Na2O exhibits a large compositionalrange in the silicic rocks, although some of the lowestconcentrations are the effect of loss during secondary pro-cesses (Peccerillo et al., 2003). Note that several majorand trace elements, especially ferromagnesian elements,show curved trends on variation diagrams, which are typ-ical of fractional crystallisation processes. Rocks from theBoina centre plot along these trends.

Inter-element plots (Fig. 5) display positive correlationbetween pairs of incompatible elements (e.g., Zr vs. Ce,Ta, etc.), although with some scattering which decreasessharply if single volcanic areas are considered separately.Sr, Ni and other compatible elements define stronglycurved hyperbolic distribution with steep negative trendsin the mafic rocks and very low values in the silicic compo-sitions. Plots of Zr vs. incompatible element ratios (e.g.,La/Yb, Rb/Nb) display strong variations in the maficrocks, whereas silicic rocks are less variable and fall inthe field of mafic volcanics.

Sr isotopic ratios are rather scattered (Fig. 4f). However,there is an overall increase in radiogenic Sr with increasingsilica contents (e.g., Gibson, 1972; Gasparon et al., 1993;Trua et al., 1999; Peccerillo et al., 2003). The mafic rockshave variable values, with a poorly defined negative trendof 87Sr/86Sr vs. Ni (correlation coefficient, r = �0.6; Fig. 6).

5. Discussion

Petrological and geochemical data on volcanic rocks canbe used to infer physical conditions of magma crystallisa-tion, and to elaborate models for the plumbing systemsof volcanoes (e.g., Hawkesworth et al., 2000; Frezzottiand Peccerillo, 2004). Such a potentiality stems from thefact that major and trace element variation of evolvingmagmas change as a function of the type of fractionatingphases. These, in turn, depend on physical conditions ofmagma cooling and crystallisation, as established by exper-imental petrology studies (e.g., Hay and Wendlandt, 1995;Scaillet and Macdonald, 2001). In the following para-graphs, we will first discuss geochemical-petrological mod-els for magma genesis and evolution in the northern sectorof the main Ethiopian rift. Successively, the implicationsfor physical conditions of magma ascent and crystallisationwill be examined.

The most important petrologic problems of rift magma-tism include: (a) the genesis of the widespread peralkaline

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rhyolitic magmatism, (b) the reasons of the Daly gap, and(c) the processes responsible for trace element variationsobserved in the mafic rocks. Some of these issues have beendiscussed in detail for the volcano of Gedemsa (Peccerilloet al., 2003). Here, we will basically lean on this study totest whether the same conclusions can be applied at theregional scale, i.e. for the whole magmatism occurringalong the northern sector of the main Ethiopian rift.

5.1. Genesis of silicic magmas

The genesis of peralkaline acidic rock is an unsolvedproblem of igneous petrology (see Scaillet and Macdonald,2003). Basically, three main different mechanisms havebeen proposed (e.g., Lowenstern and Mahood, 1991; Black

et al., 1997; Peccerillo et al., 2003): (a) melting of the oldcontinental crust; (b) melting of basaltic rocks emplacedat the base of the crust by underplating, possibly accompa-nied by alkali-fluorine enrichment by gaseous transfer and(c) fractional crystallization plus possible crustal assimila-tion starting from intermediate or basic magmas.

Melting of Precambrian rocks with a composition asthose cropping out at the margins of the Ethiopian volcanicprovince, is unlikely. Ethiopia Precambrian rocks generallyhave higher 87Sr/86Sr and Large Ion Lithophile Elementsover High Field-Strength Elements (LILE/HFSE; e.g.,Rb/Nb, Th/Ta) than the peralkaline silicic volcanics fromthe rift (Fig. 7; Peccerillo et al., 1998; Alene et al., 2000).It has been demonstrated that melts formed by melting ofcrustal rocks preserve or increase LILE/HFSE ratios, since

Table 1Major, trace element and isotopic data for selected volcanic rocks from the northern sector of the Ethiopian rift

Volcano Chilalo-Zway Chilalo-Zway Gedemsa Gedemsa Zuqala Gedemsa Bede Gebabe Kone

Source of data 2 2 3 3 1 3 1 4SiO2 47.38 49.06 52.03 59.27 67.38 69.72 72.78 71.96TiO2 1.96 2.69 1.77 1.92 0.43 0.42 0.25 0.19Al2O3 16.29 20.40 16.52 13.78 14.54 10.22 9.36 9.19Fe2O3 6.56 7.69 4.43 6.07 5.22 6.4 2.70 1.77FeO 4.86 2.58 5.34 3.39 - 0.59 3.48 2.13MnO 0.17 0.17 0.16 0.22 0.22 0.25 0.19 0.08MgO 9.05 2.26 7.74 2.84 0.22 0.06 0.03 0.11CaO 9.32 8.29 6.98 3.76 0.98 1.09 0.25 0.21Na2O 2.63 3.68 2.83 4.94 6.09 5.41 6.20 3.89K2O 0.72 0.87 1.25 2.28 4.53 4.36 4.04 3.77P2O5 0.31 0.56 0.36 0.69 0.05 0.02 0.01 4.19LOI 0.74 1.76 0.59 0.86 0.34 1.46 0.71 2.53Cs 0.2 0.1 0.1 2.42 0.43 2.7Rb 9 108 17 54 123 117 216 230Sr 437 1047 378 345 49 68 1 5Y 23 50 26 145 99 95 163 163Zr 130 196 150 325 816 707 1339 1672Nb 18 29 24 55 175 111 144 62Ba 313 430 323 2954 1096 349 24 39La 19.6 57 23 101 116 80 185 129Ce 47 67 50 203 192 174 39 266Nd 23.1 52 25 85 81 73 132 119Sm 4.9 10.3 5.46 18.7 15.8 15.1 28.3 23Eu 1.72 3.3 1.77 6.23 3.3 3.4 1.5 1.86Tb 0.7 1.44 0.66 2.97 2.2 2.32 3.7 3.57Yb 1.89 2.8 1.98 8.73 9.4 9.16 14.8 15.4Lu 0.27 0.41 0.25 1.2 1.5 1.1 2.3 2.3Hf 3 5 4.18 8.96 19.0 15.9 29 35.0Ta 1.54 1.74 1.57 3.44 10.6 6.32 15 14.2Th 2.2 2 3.2 13.8 19.3 15.6 29.2 27U 0.59 0.79 2.7 6.7 5.3Cr 334 7 81Ni 63 8 72 7 6 2V 278 202 252 121 13 3Sc 27 15 28.8 24.2 2.6 187Sr/86Sr 0.70392 0.70444 0.704047 0.704438 0.704547 0.704068 0.707621 0.70671143Nd/144Nd 0.51278 0.51282 0.512795 0.5128 0.512796 0.512731 0.51273206Pb/204Pb 18.190 18.588 18.296 18.276207Pb/204Pb 15.562 15.574 15.576 15.549208Pb/204Pb 38.307 38.662 38.512 38.458

Source of data: 1, Gasparon et al. (1993); 2, Trua et al. (1999); 3, Peccerillo et al. (2003); 4, Authors, this work (see Peccerillo et al., 2003 for analyticalprocedure and precision). A more complete list of data is available from authors on request.

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LILE are more incompatible than HFSE (Ta, Nb and Zr)during crustal anatexis (e.g., Beard and Lofgren, 1991;Ayres and Harris, 1997). Therefore, both Sr-isotope andtrace element ratios rule out a derivation of recent silicicrift volcanism by melting of Precambrian rocks.

Melting of underplated young basaltic crust is a muchmore likely mechanism, which does not conflict with the sim-ilar incompatible element and radiogenic isotopic ratios ofbasalts and rhyolites. It also provides a nice explanationfor the Daly gap, since basalts and rhyolites would representmagmas generated within two distinct sources (basaltswithin the mantle and rhyolites within the underplated basal-tic crust). It has been calculated that small degrees of melting(some 5%) of a transitional basalt can give silicic melts, leav-ing residues dominated by plagioclase and pyroxene (e.g.,Thy et al., 1990; Beard and Lofgren, 1991; Garland et al.,1995; Hay and Wendlandt, 1995). However, some geochem-ical data argue against such a hypothesis.

Trace element data on magmatic clinopyroxene (e.g.,Drexler et al., 1983; Mungall and Martin, 1995; Streckand Grunder, 1997) show that this phase heavily discrimi-nates between heavy REE and light REE, i.e. it has differ-ent mineral/liquid partition coefficients for the two groupsof elements (KLu,Y = 0.8 and KLa = 0.1 for the Gedemsarocks; Peccerillo et al., 2003). Mass balance calculationsbased on major elements indicate that some 25–30% ofclinopyroxene is left as residual phase during basalt meltingand rhyolite genesis. This would produce a strong fraction-ation of REE in the silicic liquids; such a feature is notobserved in the rift silicic rocks, whose La/Yb values showa tendency to fall at the lower end of rift basalt composi-tional range (Fig. 5g). Moreover, melting of basaltic rocksare unable to generate liquids with very low abundances ofcompatible elements (Ni, Cr, V, Sc and Sr; e.g., Hanson,

1978), such as those of the Ethiopian rift rhyolites. Thisis shown by geochemical modelling of batch meltingreported in Fig. 8 where different types of mafic and inter-mediate rocks are used as possible sources. However, atwo-stage process of basalt melting followed by fractionalcrystallisation, could explain the low compatible elementabundances of rhyolites (Peccerillo et al., 2003).

Fractional crystallisation starting from basalts to givesilicic rock is strongly suggested by curved trends for sev-eral major and trace element diagrams. Fractional crystal-lisation has been tested quantitatively using both massbalance calculations and thermodynamic modellingthrough the use of MELTS software package (Ghiorsoand Sack, 1995; Asimow and Ghiorso, 1998). Mass balancecalculation shows that trachytic compositions can bereached after about 70% fractional crystallisation of a tran-sitional basalt. The fractionating mineral assemblage isdominated by plagioclase (about 50%), with minor olivine(about 20%), clinopyroxene (�20%), Ti-magnetite (�10%)and accessory apatite. An additional 50–60% fractionationof dominant alkali feldspar and minor clinopyroxene(around 8%), plus accessory fayalite and Fe–Ti oxides isnecessary to obtain the most silicic composition. Overall,the total amount of fractionation from basalt to rhyoliteis about 90%.

MELTS thermodynamic simulations have been per-formed at variable pressures (i.e., 0.1 and 0.8 GPa) andoxygen fugacity, starting from two different basaltic com-positions, characterised by distinct enrichments in alkalies.The two starting parental melts are average compositionsof weakly alkaline and subalkaline basalts. They have beencalculated from available analyses of unaltered rift basalts(SiO2 < 50 wt%; MgO > 5 wt%; Na2O + K2O > 2.5 wt%;LOI < 2 wt%), separately for rocks plotting above or belowthe divide between the alkaline and subalkaline fields ofIrvine and Baragar (1971). Major elements of these meltsare given in Table 2.

Results of MELTS calculations are shown in Fig. 9a,and b as liquid lines of descent on TAS diagrams for sub-alkaline and weakly alkaline basalt starting compositions,calculated at 0.1 and 0.8 GPa. Fig. 9c–f indicate propor-tions of mineral phases separating at different pressuresfrom the two parental melts. Oxygen fugacity has beenfixed at QFM, but its modification has not been found toaffect substantially liquid lines of descent.

The main conclusions arising from MELTS simulationscan be summarised as follows.

1. Rhyolitic compositions can be reached by low-pressurefractional crystallisation, starting from both the subal-kaline and weakly alkaline basalt. The trend of the sub-alkaline basalt better fits the real data for rhyolites,whereas trachytes are better modelled by the weaklyalkaline trend. High-pressure fractional crystallisationseems unable to give silica-rich compositions, and resid-ual liquids have trachytic compositions after 90% frac-tional crystallisation.

35 40 45 50 55 60 65 70 750

2

4

6

8

10

12

14

16

Na

2O

+K

2O

wt%

SiO wt%2

BasaltBasalticandesite

AndesiteDacite

Rhyolite

Trachyte

Mugearite

Benmoreite

TephriteBasanite

Phono-Tephrite

Hawaiite

Nazreth-MetaharaZuqalaChilalo-ZwayFantale

GedemsaBoseti-GuddaDebre ZeitNazreth-Bofa-Wonji

Fig. 2. Alkali vs. silica classification diagram for volcanic rocks from thenorthern sector of the main Ethiopian rift. The dashed line is the dividebetween the subalkaline and the alkaline field of Irvine and Baragar(1971). Data from: Brotzu et al. (1974); Gibson (1972); Gasparon et al.(1993); Boccaletti et al. (1995, 1999); Trua et al. (1999); Peccerillo et al.(2003); authors’ this work.

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2. Peralkaline compositions are attained only for simula-tions performed at 0.1 GPa; in particular, the peralka-linity (vertical dashed line in Fig. 9a–d) is achieved atlesser silica contents when simulation is performedwith weakly alkaline starting composition (Fig. 9aand b).

3. The predominant phases to crystallise at low pressureare feldspars (Fig. 9c and d). This may play a role toget peralkalinity melts (plagioclase effect). At high pres-sure consistent cpx crystallization (Fig. 9e and f) takesplace in the early stages of simulation, which inhibitsmagma evolution to high silica contents (e.g., Lowen-stern and Mahood, 1991).

The greatest amounts of solid separation take place dur-ing the intermediate stage of the fractionation at low-pres-sure. This generates a rapid increase in silica for theintermediate magmas, which is graphically expressed bythe large interval between successive temperature stepsreported as full squares on trends in Fig. 9a and b.

5.2. The Daly gap

It has been long stated that the Daly gap is betterexplained by assuming a separate origin for coexistingbasaltic and rhyolitic magmas along the northern Ethiopianrift (i.e. mantle melting for the basalts and melting of under-

12

17

22

Al2O3

0

5

10

15

MgO

0

5

10

15

FeO

0

1

2

3

4

TiO2

0

2

4

6

8

10

Na2O

0

5

10

15

CaO

40 50 60 70 800

2

4

6

8

K2O

SiO2

40 50 60 70 800.0

0.5

1.0

1.5

2.0

P2O5

SiO2

total

a b

d

e

h

f

c

g

Fig. 3. Major element variation diagrams for volcanic rocks from the northern sector of the main Ethiopian rift. Source of data and symbols as in Fig. 2.Full diamonds represent compositions of the Boina center, Afar (Barberi et al., 1975).

166 A. Peccerillo et al. / Journal of African Earth Sciences 48 (2007) 161–173

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pyplated basalts for silicic rocks; e.g., Boccaletti et al., 1995).However, such a hypothesis conflicts with curved trendsof major and trace elements, which support continuousfractional crystallisation of basalts, as explained earlier.Thermodynamic models of fractional crystallisation pro-cesses furnish an alternative explanation for the Daly gap.It has been shown that transition of residual melts at theintermediate compositions during low-pressure fraction-ation is a very fast process, which is related to the largeamounts of phases crystallising at the same time. Thus,the composition of the liquid changes abruptly over theintermediate stage, especially at low pressure. Therefore, ifa constant cooling rate of the magma is supposed, the tem-poral interval in which intermediate composition liquids(trachy-basalts to trachy-andesites) exist is restricted. Inother words, fractional crystallisation processes discrimi-nate against intermediate compositions whose abundanceis intrinsically low during these processes.

Formation of zoned magma chambers can play animportant additional role in the generation of the Dalygap. Liquids formed in fractionating magma chambershave variable densities, which make mafic melts to pond

at the bottom and acid ones to rise to the top. The two sep-arate portions of zoned reservoirs may evolve rather inde-pendently (e.g., Wolff and Storey, 1984; Turner andCampbell, 1986). Radiogenic isotope and oxygen isotopicsystematics for some rift volcanoes (e.g. Gedemsa; Pecce-rillo et al., 2003) suggest that silicic magmas evolved byfractional crystallisation and very little interaction withthe wall crustal rocks. Such a hypothesis does not conflictwith the large variation of Sr isotopic ratios of rhyolites,since this is an effect of very low concentration of elementalSr, which makes silicic magmas very sensitive to Sr-isotopevariation also for very low assimilation rate. By contrast,the lower mafic zone evolved by dominant mixing withnew magma coming from depth and some crustal assimila-tion. Continuous arrival and mixing of fresh magma pre-served a broadly mafic composition for melts ponding atthe bottom of the magma chamber. Therefore, two distinctcompositions are hosted by a single body, with little inter-mediate melts. These can be generated in small amounts atthe interface between silicic and mafic layers, e.g. by mixingor chemical diffusion (e.g., Snyder and Tait, 1998). The for-mation of these zoned bodies makes the silicic magmas to

0

1000

2000

3000

4000

Zr

0

1000

2000

3000

4000

Ba

0

100

200

300

Ni

0

200

400

600

800

1000

1200

Sr

40 50 60 70 800

10

20

30

40

50

Th

SiO2

40 50 60 70 800.702

0.704

0.706

0.708

0.710

SiO2

ba

c

e f

d

87S

r/86

Sr

Fig. 4. Trace element and 87Sr/86Sr vs. SiO2 variation diagrams for volcanic rocks from the northern sector of the main Ethiopian rift. Source of data andsymbols as in Figs. 2 and 3.

A. Peccerillo et al. / Journal of African Earth Sciences 48 (2007) 161–173 167

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be erupted preferentially, which explains the dominance ofacid rocks in the main Ethiopian rift.

In conclusion, the fractional crystallisation hypothesisfor the genesis of silicic melts in the Ethiopian rift explainsthe scarcity of intermediate rocks in the field as the com-bined effect of the scarcity of intermediate melts generatedduring fractional crystallisation, and of the zoned structureof magma chambers. The prevalence of rhyolitic rocks withrespect to basalts does not reflect real proportions ofmagma in the rift environment, but is rather an effect ofthe particular structure of the volcano plumbing systems,which favours eruption of silicic melts accumulated at thetop of zoned reservoirs.

5.3. Geochemical variability of basalts

The basalts from the northern Ethiopian Rift exhibitwide variations for incompatible element abundances andratios, and for isotopic signatures. LILE/LILE andLILE/HFSE ratios (i.e., Th/Ta, Rb/Nb, and Ba/Rb) varyby more than one order of magnitude in the mafic rocks(MgO > 5 wt% and SiO2 < 52 wt%; Fig. 10). In principle,these variations may depend either on source heterogeneityand/or on shallow level evolutionary processes.

Within the limits of the scarcity of data, the rough positivetrend of Ni vs. Sr isotope ratios (see Fig. 6) supports a role ofcrustal contamination. Ba/Rb, Rb/Nb and Th/Ta variation

0

100

200

300

400

500

600

Nb

0

100

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500

Ce

0 1000 2000 3000 40000

10

20

30

La/Yb

Zr

0

500

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1500

Sr

0

50

100

150

200

250

300

Ni

0

100

200

300

400

500

V

0 1000 2000 3000 40000

1

2

3

4

5

Rb/Nb

Zr

0

5

10

15

20

Ta

a b

d

f

hg

e

c

Fig. 5. Inter-element variation diagrams for volcanic rocks from the northern sector of the main Ethiopian rift. Source of data and symbols as in Figs. 2and 3.

168 A. Peccerillo et al. / Journal of African Earth Sciences 48 (2007) 161–173

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in basaltic rocks also define trends between Rb-depletedcompositions and the Ethiopian Precambrian rocks(Fig. 10). These trends, however, cannot be modelled byassimilation of continental crust by magmas with the lowestBa/Rb and Rb/Nb ratios. Such a process would require alarge amount of contamination (about 40% of average Ethi-opian basement rocks) to explain the whole range of Ba/Rb,starting from the most depleted basaltic compositions. Thisconflicts with the high MgO content of the rocks and the lackof any correlation of incompatible element ratios vs. MgO(not shown). Therefore, most of the variation of incompati-ble element ratios basically reflects mantle heterogeneity,although crustal contamination may have contributedconsiderably to basalt compositional heterogeneity.

An interesting feature emerging from diagram reportedin Fig. 10, is that, whereas silicic rocks have large varia-tions of Ba/Rb due to feldspar fractionation, the rangeof Rb/Nb is rather narrow and corresponds to the highestvalues shown by basalts. In the light of the genetichypotheses for silicic magmas discussed above, this seemsto indicate that only some of the basalts (possibly themost contaminated) gave silicic liquids. This peculiarity

10 1000.703

0.704

0.705

0.706

0.707

87S

r/86

Sr

Ni

Fig. 6. Ni vs. 87Sr/86Sr variation diagram for mafic rocks (MgO > 5 wt%)from the norther sector of the Ethiopian rift. Source of data and symbolsas in Figs. 2 and 3.

0 100 200 300.01

.1

1

10

100

Rb/

Nb

Rb

0.700 0.720 0.740 0.760 0.780.1

1

10

100

1000

Th/

Ta

87Sr/86Sr

a b

Fig. 7. (a) Rb vs. Rb/Nb and (b) 87Sr/86Sr vs. Th/Ta diagrams for volcanic rocks from the northern sector of the main Ethiopian rift. Data from thesouthern Ethiopia Precambrian basement (asterisks) are shown for comparison. Source of data and symbols as in Figs. 2 and 3. Data on Precambrianbasement are from Peccerillo et al. (1998).

0 100 200 300 400 5000

1000

2000

3000

4000

Zr

V

10

10

D = 5D = 0.01

V

Zr

10

20

Batch melting trends

Fractional crystallisation trend

Silicic rocks

Basalts

Fig. 8. Zr vs. V variation diagram for volcanic rocks from the northernsector of the main Ethiopian rift. Batch melting and fractional crystal-lisation models starting from different parental compositions are shown. Acompatible and an incompatible behaviour for V (Ds/l = 5) and Zr (Ds/

l = 0.01) has been assumed based on variation of these elements in theinvestigated rock suites. Note that, contrary to fractional crystallisation,batch melting of mafic rocks is unable to give magmas with strongdepletion in compatible elements (V, Ni, Cr, Sr, etc.).

Table 2Composition of starting parental magmas in MELTS simulations

Average

1 2

SiO2 47.96 47.02TiO2 1.95 2.25Al2O3 16.88 17.02Fe2O3 5.52 5.20FeO 5.46 6.07MnO 0.17 0.17MgO 7.14 7.27CaO 9.80 9.66Na2O 2.97 3.17K2O 0.83 0.99P2O5 0.37 0.47LOI 0.88 0.75

1, average composition of rift basalts falling in the subalkaline field ofIrvine and Baragar (1971); 2, average composition of rift basalts falling inthe alkaline field.

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45 50 55 60 65 70 75 80

4

6

8

10

12

Mildly Alkaline

0.8

GPa

0.1GPa

45 50 55 60 65 70 75 80

4

6

8

10

12

Sub-alkaline

0.8 GPa 0.1GPa

SiO wt%2

45 50 55 60 65 70 75 800

2

4

6

8

10

Ab48-Or50

Ab39

-Or5

7

An13An24

An34

An44

An55

An66

An75

ap

sp

feld

cpx

ol

45 50 55 60 65 70 75 800

2

4

6

8

A

Ab41-

Or56

An14

An36

An45

An54

An64

An21

An73

ap

sp

feld

cpx

ol

45 50 55 60 65 70 75 800

2

4

6

8

P=0.8 GPa P=0.8 GPa

P=0.1 GPaP=0.1 GPa

Ab48-Or49

An34

An24

An14

An45

An54

An5

9

ap grtsp

feld

cpx

45 50 55 60 65 70 75 800

2

4

6

8

10

Ab49-Or46

An18

An29

An40

An50

An5

5

ap grtsp

feld

cpx

Fra

ctio

nate

d so

lids

(wt%

)

Fra

ctio

nate

d so

lids

(wt%

)

Fra

ctio

nate

d so

lids

(wt%

)

Fra

ctio

nate

d so

lids

(wt%

)

SiO wt%2

SiO wt%2

SiO wt%2

SiO2 wt%SiO2 wt%

K2O

+N

a2O

wt%

K2O

+N

a2O

wt%

ba

dc

e f

Fig. 9. TAS (a,b) diagrams of Ethiopian rift rocks (small circles) showing fractional crystallisation models calculated at 0.1 and 0.8 GPa, using theMELTS software package (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998). Oxygen fugacity is fixed at QFM buffer. A subalkaline and amildly alkaline basalt, have been chosen as starting compositions. Proportions of separating phases are also shown in (c–f) for different fractionationmodels (cpx, clinopyroxene; feld, feldspar; ol, olivine; ap, apatite; grt, garnet; sp, spinel). Vertical dashed lines in (a–d) indicate the transition to thesilicic peralkaline field. In (a,b) the symbols along the trends indicate liquid compositions at temperature intervals of 25 �C, starting from liquidustemperature (1180 �C and 1269 �C, subalkaline composition at 0.1 and 0.8 GPa, 1188 �C and 1246 �C, alkaline composition at 0.1 and 0.8 GPa). Theend points of each trend represent conditions at which 10% of residual melt is left. In (c–f) the numbers along the feldspar line (feld) represent theanorthite content in plagioclase (An%) and the albite and orthoclase (Ab–Or) contents in K-feldspar. For sake of clarity minor mineral phases andwater have been omitted.

170 A. Peccerillo et al. / Journal of African Earth Sciences 48 (2007) 161–173

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needs additional studies to be confirmed and, eventually,to be understood.

5.4. Modes of magma ascent and geophysical-volcanological

constraints on shallow-level intrusions

Petrological and geochemical modelling of magma gen-esis and evolution allows to propose the following scenariofor magma generation, ascent and ponding beneath theEthiopian rift volcanoes (Fig. 11). Weakly alkaline andtransitional magmas are generated within a heterogeneousupper mantle, characterised by distinct incompatible ele-ment compositions. Mafic magmas may pond in the lowercrust or at the Moho (e.g., Corti et al., 2003) undergoinghigh-pressure evolution with formation of trachytic mag-mas. Such a process preferentially occurs along the riftshoulders, where several trachytic volcanoes are situated.In the axial zone of the rift, mafic magmas ascent more

readily to shallow levels, possibly because of intensive nor-mal and oblique rifting (Bonini et al., 1997; Boccalettiet al., 1999; Corti et al., 2003). Magmas intruded at shallowdepths form large magma chambers, where intensive frac-tional crystallisation produces zoned reservoirs with a per-alkaline silicic upper layer and basalts at the bottom.Eruptions preferentially tap the silicic layer, whereas maficmelts reach the surface only when fractures intersect thelower layer of the shallow chamber or reach the under-plated basalts in the deep reservoirs. Small amounts ofmafic melts can also be erupted as enclaves within silicicmelts, when large silicic eruptions drain extensively theupper layer of shallow magma chambers. This is observedat Gedemsa, where mingling between magmas with differ-ent compositions appears during syn-caldera ignimbriteeruption and becomes a prominent feature of the post-cal-dera activity (Peccerillo et al., 2003).

Such a model is supported by volcanological and geo-physical evidence. Fig. 1 reports a schematic distributionof caldera rims in the northern sector of the Ethiopian rift,as inferred from satellite imagery and field observation.Large caldera depressions appear to be a very common fea-ture in this sector. Most of these calderas are cut by recentWonji faults and are partially covered by younger volcanicactivity; therefore, their number is possibly higher thanrecognised in the present study. Particularly interestingareas are the Gedemsa volcano, where a large caldera iscut by the younger Wonji faults, the active Fatale volcano,and Kone. In the latter, the occurrence of several nestedcollapses inside a wide depression, some 30 km in diameter,is observed. An interesting feature of this large caldera isthat young faults related to regional extension deviatearound the eastern border of Kone depression rather thancutting it (Fig. 1). This probably indicates the presence of alarge rigid body of intrusive rocks, which is able to guideregional extensional faulting.

Further evidence in favour of large magma chambersbeneath the Ethiopian rift comes from regional gravitystudies. Mahatsente et al. (1999) revealed several positiveanomalies with a circular shape along the northern Ethio-pian rift. Many of these anomalies occur beneath largesilicic volcanoes. According to Mahatsente et al. (1999),these anomalies reveal the presence of high-density intru-sive bodies at variable depths. Three-dimensional model-ling of gravimetric data was interpreted as evidence forthe occurrence of several intrusions along the rift, whosedensity was estimated to range from about 3000 to3100 kg/m3, decreasing from bottom to top within eachbody. These intrusions were found to be rooted in the man-tle, and to cross the entire crust reaching a depth of lessthan 4 km. According to Mahatsente et al. (1999), thesebodies represent intrusion of mantle material within thecrust and the upward decrease in density testifies to interac-tion with upper crustal material. We argue, however, thatthe intrusion of dense mantle material into the middleand upper crust is unlikely. A much more plausible expla-nation for gravity data is that intrusive bodies represent

.01 .1 1 10 100.01

.1

1

10

100

1000

Ba/Rb

Rb/Nb

SouthernEthiopiaPrecambrianbasement

Silicic rocks

Basalts60

40

Fig. 10. Ba/Rb vs. Rb/Nb diagram for the basaltic rocks (MgO > 5 wt%)from the main Ethiopian rift. Symbols as in Fig. 2. The thick line is a bulkassimilation trend of average Precambrian basement by basaltic magma.Numbers along the line represent amounts of assimilated crust. Data fromthe southern Ethiopia Precambrian basement are from Peccerillo et al.(1998).

Upper mantle

Trachyticvolcano

Moho

Basalticeruptions

Siliciceruptions

Basaltunderplating

Fig. 11. Schematic cross section (roughly in a NE–SW direction) showinga possible model for the distribution of magma chambers along thenorthern sector of the main Ethiopian rift, as inferred from fieldobservation, geochemical and geophysical studies, and satellite imagery.For further explanation, see text.

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crystallised mafic magmas and their cumulates. The match-ing between circular gravity anomalies and silicic centressupport the idea that large intrusive magma bodies crystal-lised in shallow magma chambers beneath these centres.

In summary, the overall picture that emerges from thepresent study is that magmatic activity has contributedsubstantially to the composition and structure of the crustin the Ethiopian rift. Much of the crustal rocks may consistof intrusive bodies, which derived from cooling and crystal-lisation of large quantities of mafic to silicic magmas. Asketch model for the rift structure is shown in Fig. 11.

6. Conclusions

Quaternary volcanism in the northern Ethiopian riftconsists of dominant peralkaline rhyolites and minortrachytes, and transitional to mildly alkaline basalts. Rockswith intermediate compositions are very scarce or absent.Some trachytic volcanoes occur on the margins of the rift.Basalts exhibit geochemical variations, which are inter-preted to highlight both heterogeneous mantle sourcesand crustal assimilation.

Geochemical and petrological modelling shows that themost likely petrogenetic process for the generation of silicicrocks is a derivation from basalts by fractional crystallisa-tion and minor interaction with the crust. Low-pressureevolution processes gave peralkaline rhyolitic derivativemelts, whereas trachytic volcanoes along the rift marginscould have derived by high-pressure fractional crystallisa-tion from a same type of parental magmas as the peralka-line rhyolites.

The model that better explains petrological, geochemicaland field data for rift volcanism suggests that largeamounts of basalts have been emplaced in the lower crustand at shallow level. Extensive low-pressure fractionalcrystallisation generated compositionally zoned magmachambers, with an upper rhyolitic zone and basaltic mag-mas ponding at the bottom. Eruptions preferentiallytapped the upper rhyolitic layer, giving dominant silicicexplosive volcanism. The few basaltic lavas erupted in therift were extruded through regional faults that occasionallycut the lower part of the shallow reservoirs, or tap directlythe deep reservoirs formed by magma underplating at theMoho.

Acknowledgements

The research on Ethiopian magmatism has been finan-cially supported by the Ministry of University and Scien-tific Research of Italy (MIUR) and by funds from theUniversity of Perugia.

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