Journal of Petrology | Volume 45 | Number 4 | Pages 793-834 | 2004
Journal of Petrology 45(4) © Oxford University Press 2004; all rights reserved.
Flood and Shield Basalts from Ethiopia: Magmas from the African Superswell
1 LABORATOIRE DE GÉODYNAMIQUE DES CHAÎNES ALPINES, UMR 5025 CNRS, BP 53, 38041 GRENOBLE CEDEX, FRANCE
2 ISTEM, CC 066, UNIVERSITÉ MONTPELLIER II, PLACE E. BATAILLON, 34095 MONTPELLIER CEDEX 05, FRANCE
3 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, SCIENCE FACULTY, ADDIS ABABA UNIVERSITY, P.O. BOX 1176, ETHIOPIA
4 DÉPARTEMENT DES SCIENCES DE LA TERRE ET DE L'ENVIRONNEMENT, UNIVERSITÉ LIBRE DE BRUXELLES 50, AV. F. D. ROOSEVELT, B-1050, BRUSSELS, BELGIUM
5 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF DURHAM, SOUTH ROAD, DURHAM DH1 3LE, UK
RECEIVED JULY 31, 2002; ACCEPTED SEPTEMBER 22, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING STRATEGY AND ANALYTICAL...STRATIGRAPHY AND CONSTITUTION OF...PETROGRAPHY OF MAFIC ROCKS...GEOCHEMICAL DATAISOTOPIC COMPOSITIONSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The Ethiopian plateau is made up of several distinct volcanic centres of different ages and magmatic affinities. In the NE, a thick sequence of 30 Ma flood basalts is overlain by the 30 Ma Simien shield volcano. The flood basalts and most of this shield volcano, except for a thin veneer of alkali basalt, are tholeiitic. In the centre of the province, a far thinner sequence of flood basalt is overlain by the 22 Ma Choke and Guguftu shield volcanoes. Like the underlying flood basalts, these shields are composed of alkaline lavas. A third type of magma, which also erupted at 30 Ma, is more magnesian, alkaline and strongly enriched in incompatible trace elements. Eruption of this magma was confined to the NE of the province, a region where the lava flows are steeply tilted as a result of deformation contemporaneous with their emplacement. Younger shields (e.g. Mt Guna, 10·7 Ma) are composed of Si-undersaturated lavas. The three main types of magma have very different major and trace element characteristics ranging from compositions low in incompatible elements in the tholeiites [e.g. 10 ppm La at 7 wt % MgO (=La7), La/Yb = 4·2], moderate in the alkali basalts (La7 = 24, La/Yb = 9·2), and very high in the magnesian alkaline magmas (La7 = 43, La/Yb = 17). Although their Nd and Sr isotope compositions are similar, Pb isotopic compositions vary considerably; 206Pb/204Pb varies in the range of
17·9–18·6 in the tholeiites and 19·0–19·6 in the 22 Ma shields. A conventional model of melting in a mantle plume, or series of plumes, cannot explain the synchronous eruption of incompatible-element-poor tholeiites and incompatible-element-rich alkali lavas, the large range of Pb isotope compositions and the broad transition from tholeiitic to alkali magmatism during a period of continental rifting. The lithospheric mantle played only a passive role in the volcanism and does not represent a major source of magma. The mantle source of the Ethiopian volcanism can be compared with the broad region of mantle upwelling in the South Pacific that gave rise to the volcanic islands of French Polynesia. Melting in large hotter-than-average parts of the Ethiopian superswell produced the flood basalts; melting in small compositionally distinct regions produced the magmas that fed the shield volcanoes. KEY WORDS: Ethiopia; flood basalts; shield volcanism; superswell
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INTRODUCTION |
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According to Hofmann et al. (1997)
, most of the Ethiopian flood basalts erupted 30 Myr ago, during a short 1 Myr period, to form a vast volcanic plateau. Immediately after this peak of activity, a number of large shield volcanoes developed on the surface of the volcanic plateau, after which subsequent volcanism was largely confined to regions of rifting (Mohr, 1983a; Mohr & Zanettin, 1988). The rift that opened along the Red Sea and Gulf of Aden separated the Arabian and African continents, and isolated a small portion of the volcanic plateau in Yemen and Saudi Arabia (Chazot & Bertrand, 1993; Baker et al., 1996a; Menzies et al., 2001). Volcanic activity continues to the present day along the Ethiopian and Afar rifts.
The Ethiopian flood basalts are the youngest example of a major continental volcanic plateau. Because of their relatively young age and their eruption in a region where plate movements are slow, we find a complete record from the initial, high-flux, flood volcanic phase through to its shutdown and the onset of continental rifting, and finally the initiation of sea-floor spreading. The region represents an ideal situation to study the nature of the mantle source of continental flood volcanism and the manner in which magmas derived from this source interacted with the continental lithosphere, as has been done by researchers such as Mohr & Zanettin (1988), Baker et al. (1996a), Hofmann et al. (1997), Pik et al. (1998, 1999) and Baker et al. (2000, 2002).
In this study we have focused on the large shield volcanoes and compared their compositions with those of the flood volcanics. We have traced the variations in eruption style and magma flux and studied the petrology, geochemistry and isotopic compositions of lavas with ages ranging from 30 to 10 Ma, or from the peak of flood volcanism to the onset of major rifting in the northern part of the volcanic plateau. This information has allowed us first to evaluate the roles of crustal contamination and lithosphere melting during the evolution of the province, and then to test conventional models in which flood volcanism is attributed to melting in the head of a large mantle plume.
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GEOLOGICAL BACKGROUND |
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The Ethiopian flood basalts (or traps) cover an area of about 600000 km2 with a layer of basaltic and felsic volcanic rocks (Fig. 1). The thickness of this layer is highly variable but reaches 2 km in some regions. The total volume of volcanic and shallow intrusive rocks has been estimated by Mohr (1983b)
and Mohr & Zanettin (1988) at about 350 000 km3.
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The mineralogical and chemical composition of the flood basalts is relatively uniform. Most are aphyric to sparsely phyric, and contain phenocrysts of plagioclase and clinopyroxene with or without olivine. Most have tholeiitic to transitional compositions (Mohr, 1983a
; Mohr & Zanettin, 1988; Pik et al., 1998). Interlayered with the flood basalts, particularly at upper stratigraphic levels, are felsic lavas and pyroclastic rocks of rhyolitic, or less commonly, trachytic compositions (Ayalew et al., 1999).
Pik et al. (1998, 1999) divided the basaltic rocks into several types on the basis of trace element and Ti concentrations. They recognized a suite of ‘low-Ti’ basalts (LT) characterized by relatively flat rare earth element (REE) patterns and low levels of Ti and incompatible trace elements. According to Pik et al. (1998), these rocks are restricted to the northwestern part of the province, as shown in Fig. 1. Alkali basalts with higher concentrations of incompatible elements and more fractionated REE patterns—the so-called ‘high-Ti’ basalts (HT1 and HT2)—are found to the south and east. The HT2 basalts are slightly more magnesian than the HT1 basalts and commonly are rich in olivine ± clinopyroxene phenocrysts. They have higher concentrations of incompatible elements and show extreme fractionation of the REE.
Although the post-trap volcanism in the regions of active rifting around Addis Ababa and Djibouti has been the subject of numerous publications (e.g. Justin Visentin et al., 1974; Zanettin et al., 1978; Barrat et al., 1990; Deniel et al., 1994), little attention has been paid to the shield volcanoes. These volcanoes are a conspicuous feature of the Ethiopian plateau and distinguish it from other well-known, but less well-preserved, flood basalt provinces such as the Deccan and Karoo. The shield volcanoes have been described only in overview papers by Mohr (1983b) and Mohr & Zanettin (1988) and in a few short specialized papers (Mohr, 1967; Zanettin & Justin Visentin, 1974, 1975; Piccirillo et al., 1979; Zanettin, 1993; Wolde & Widenfalk, 1994; Barberio et al., 1999). The shields are described as being made up predominantly of volcanic rocks with alkaline compositions. The basal diameters of the shields range from 50 to 100 km and the highest point in Ethiopia, the 4533 m high peak of Ras Dashan, is the present summit of the eroded Simien shield. This peak soars almost 2000 m above the top of the flood basalts, which lies at about 2700 m in the northern part of the plateau. If an additional 500 m of eroded material is taken into account (Mohr, 1967), a total original height of about 3 km is estimated for this volcano. Although smaller in diameter, the summits of many of the other shield volcanoes also exceed 4000 m. Mt Choke, the second shield we studied, has a basal diameter of over 100 km and rises to 4052 m, some 1200 m above the surrounding flood volcanics. Guguftu, the third shield, is more highly eroded and its original form is difficult to discern. The 3859 m peak of Mt Uorra is the summit of the present volcano.
A trachytic unit on the flank of the Simien shield has been dated by Rochette et al. (1998) at 29·7 ± 0·05 Ma by 40Ar/39Ar and by Coulié et al. (2003) by K–Ar at 29·1 ± 0·4 Ma. Coulié et al. also dated the basalt ETH 199 (labelled EH99 in their paper) from near the present summit by K–Ar at 29·9 ± 0·4 Ma. All these ages are within error of Hofmann et al.'s (1997) age of the underlying flood basalts. Most other shields are significantly younger. New 40Ar/39Ar ages for the Choke and Guguftu volcanoes (Fig. 1, discussed below) indicate that both erupted around 22 Myr ago (Table 1); another new result for Mt Guna, which is located between Simien and Choke, provides an age of 10·7 Ma. Zanettin (1992) and Ukstins et al. (2002) reported that shield volcanoes farther to the south have ages between 20 and 3 Ma.
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The lava flows of the shield volcanoes are thinner and less continuous than the underlying flood basalts. They also are more porphyritic, containing abundant and often large phenocrysts of plagioclase and olivine. Like the flood volcanics, the shield volcanoes are bimodal and contain sequences of alternating basalts, rhyolitic and trachytic lava flows, tuffs and ignimbrites, particularly near their summits. The compositions of the lavas in some of the younger volcanoes (e.g. Mt Guna, Fig. 1) are more variable and include nephelinites and phonolites (Zanettin, 1993
) in addition to alkali basalts. |
SAMPLING STRATEGY AND ANALYTICAL METHODS |
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The samples analysed in this study come from three regions. Most of our attention was focused on the Simien shield in the northern part of the plateau and its underlying flood basalts (Fig. 1). Other samples were collected from the Choke and Guguftu shields, a little farther to the south. To provide information about the geographical distribution of different magma types, supplementary samples were collected in the Alem Ketema region, just north of Addis Ababa and the Sekota region in the NE (Fig. 1). Reference is also made to the analytical data of Zanettin et al. (1976)
, Zanettin (1992, 1993), Pik et al. (1998, 1999) and Rochette et al. (1998). Figure 2 is a map of the Simien volcano. Figures 3 and 4 show the geological relations, the stratigraphy and the rock types collected along three sampled sections within the flood basalts and overlying shield. About 70 lava samples were collected from the principal Lima Limo section, which extends from Zarema, to the north of the area shown in Fig. 2, along the Lima Limo road, from near the base of the flood basalts to the upper flank of the shield. One other section (Aman Amba) is centred on the transition from plateau to shield volcanism; the third (Simien Main Series) extends out from near the present summit of the shield volcano towards its lower flank (Figs 2 and 3). Samples of peridotite xenoliths and clinopyroxene megacrysts from an alkali basalt flow on the flank of the volcano were also collected and analysed for major and trace elements and Nd isotopes.
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A total of 17 samples were collected along the road from the base to the summit of Choke volcano, and 16 samples were taken from the road that bisects Guguftu volcano. The location of these samples is shown in Fig. 5. Petrographic descriptions and locations of all samples collected in this study are given in the Electronic Appendix, which can be downloaded from the Journal of Petrology website at http://www.petrology.oupjournals.org.
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Samples were ground in an agate mortar. Major elements were analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES) and Ni, Cr and V by inductively coupled plasma mass spectrometry (ICP-MS) at the Centre de Recherche Pétrographique et Géologique in Nancy. The error on these data is less than 5% relative. Other trace elements were analysed by ICP-MS at the University of Grenoble following the procedure of Barrat et al. (1996
, 2000). For the trace elements Rb, Sr, Ba, Hf, Zr, Ta, Nb, U, Th, Pb, Y and REE, the accuracy is better than 5%; for Cs it is 10%. Table 2 contains the major and trace element data, along with measurement of standards BIR-1, BHVO-1 and RGM-1. Additional analyses are listed in the Electronic Appendix. The data are plotted in the figures on a volatile-free basis.
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Nd, Sr and Pb isotopic analyses (Tables 3 and 4) were carried out at the Université Libre de Bruxelles on a VG 54 mass spectrometer using the procedure described by Weis et al. (1987)
. All samples were leached for 10 min in 2N HCl in an ultrasonic bath, then a second time in dilute HF to remove secondary minerals. To evaluate the effects of leaching, two duplicate analyses were run on unleached samples. Sr isotopic ratios were normalized to 86Sr/88Sr = 0·1194 and Nd isotopic data were normalized using 146Nd/144Nd = 0·7219. The average 87Sr/86Sr value for NBS 987 Sr standard was 0·710278 ± 12 (2
m on the basis of 12 samples). Analyses of the Rennes Nd standard yielded 143Nd/144Nd = 0·511970 ± 7 (2m on the basis of 12 samples), which is within error of the recommended value of 0·511961 (Chauvel & Blichert-Toft, 2001). The recommended value corresponds to a value of 0·511856 for the La Jolla Nd standard. Chemical preparation for Pb isotopic analyses was carried out at the University of Montpellier. Powdered samples were weighed to obtain 100–200 ng of lead, and the samples were then leached with 6N HCl for 30 min at 65°C. Samples were dissolved for 36–48 h on a hot plate with a mixture of concentrated distilled HF and HNO3. Lead was separated using a procedure modified from Manhès et al. (1978). Blanks were less than 40 pg and are considered negligible for the present analyses. Lead isotopic compositions were analysed by multicollector ICP-MS using a VG model Plasma 54 magnetic sector system at the École Normale Supérieure de Lyon and the Tl normalization method described by White et al. (2000). This technique gives highly accurate and reproducible analyses, with an internal precision better than 50 ppm (2) and an external precision of 100–150 ppm (2).
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Ar–Ar ages were obtained at the University of Oregon (four samples, analyst R. A. Duncan) and at the Université Blaise Pascal in Clermont Ferrand (four samples, analyst N. Arnaud). At Oregon State University, splits of separated plagioclase or glass, or whole-rock samples were loaded in evacuated quartz vials and irradiated for 6 h at 1 MW power at the Oregon State University TRIGA reactor. The biotite standard FCT-3 (27·55 ± 0·12 Ma, equivalent to 513·9 Ma for hornblende Mmhb-1; Lanphere et al., 1990
) was used to monitor the neutron fluence. The isotopic composition of Ar was determined for each of nine temperature steps using an AEI MS-10S mass spectrometer [see Duncan et al. (1997) for experimental details]. Samples run at Clermont Ferrand were irradiated in the Mélusine reactor in Grenoble. Fish Canyon sanidine was used as a standard (27·55 ± 0·08 Ma). Figure 6 shows the age spectra and isochrons; complete analytical data can be found in Table E2 of the Electronic Appendix.
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STRATIGRAPHY AND CONSTITUTION OF THE VOLCANIC SEQUENCES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING STRATEGY AND ANALYTICAL...STRATIGRAPHY AND CONSTITUTION OF...PETROGRAPHY OF MAFIC ROCKS...GEOCHEMICAL DATAISOTOPIC COMPOSITIONSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Simien shield volcano and Lima Limo flood basalts
The lowermost sample of flood basalt was collected at an altitude of 1200 m near the town of Zarema, near the base of the sequence (Fig. 1). The upper contact, between flood basalts and the shield volcano, lies at an altitude of 2670 m on the Lima Limo road. This contact was defined by the appearance of highly porphyritic plagioclase-phyric lavas, the presence of a distinctive layer of mafic breccias, and by a change in chemical composition, as discussed below. The breccia layer is easily followed in cliff faces and river valleys along the northern escarpment of the plateau, and, in such exposures, the contact is seen to have a distinct dip, at about 3–4°, radially away from the summit of the Simien volcano (Fig. 3).
Except for the uppermost part of the sequence, where small differences in dip are observed, the flood basalts are flat lying. There is no evidence of deformation of the type that Merla et al. (1979) and Berhe et al. (1987) used in other parts of the plateau to distinguish between a lower deformed formation (Ashangi) and an upper undeformed formation (Aïba). Although a marked change in geomorphology—from subdued relief to steep cliffs—is observed midway through the sequence, this change does not correspond to any observable petrological or chemical characteristic of the flows (Violle, 1999). For these reasons, like Pik et al. (1998), we avoid the formation names Ashangi and Aïba, and will speak only of the upper and lower flood basalt formations (Figs 3 and 4).
We describe the basaltic flow units using the terminology of Jerram (2002). Classic-tabular facies, in which the flow units are about 20 m thick, alternate with compound-braided facies made up of many thin (1–2 m) pahoehoe lobes in packages up to 30 m thick. Thin scoriaceous zones and lighter-coloured tuff-rich zones are found at the tops of the units, and soils are relatively common. Thicker columnar-jointed units, which may represent ponded flows, are present, although rare, in most regions. Sequences of thicker, cliff-forming flows can be traced for several kilometres along strike in parts of the escarpment to the north of Debark (Fig. 2).
A conspicuous 100 m thick layer of felsic volcanic rocks, mainly rhyolitic tuffs and ignimbrites at an elevation of 2200 m (Figs 3 and 4), and two thinner felsic units higher in the sequence, have been described by Ayalew et al. (1999).
Flows within the Simien shield dip at 4–6° radially away from the summit. Except for a thin felsic unit on the western flank of the volcano, all rocks of the shield in the regions we studied have basaltic compositions. The lava flows are thinner and less continuous along strike than the flood basalts. Some are massive throughout, but most contain 1–3 m thick massive lobes distributed within thicker sequences of scoriaceous breccia.
The petrology of the flood basalts has been well described by Pik et al. (1998, 1999) and will not be repeated here. Within the shield, four petrological groups are distinguished using a combination of geographical location, petrology and geochemistry (Fig. 4).
Basalts of the Main Series of the Simien shield
The term ‘Main Series’ describes the rocks that constitute the bulk of the Simien volcano. They extend from the basal contact to the present summit and were sampled in sections east of Sankaber camp (Figs 2 and 3). They consist of aphyric to sparsely plag ± cpx-phyric basalts (with rare highly phyric units) whose petrology and geochemistry differ little from those of the underlying flood basalts.
Highly porphyritic trachybasalts
These rocks are restricted to a small region on the western flank of the volcano above the Lima Limo section and around the town of Debark (Figs 3 and 4). They contain abundant, very large phenocrysts of plagioclase, less abundant and smaller phenocrysts of clinopyroxene, and little to no olivine. Two trachybasaltic units (samples 141–142) occur in the flood basalt sequence at an altitude of 2450 m, some 300 m below the basal contact of the shield volcano (Fig. 4). These units have distinctive volcanic structures (one is a highly vesicular volcanic breccia) that show that they are not younger sills intruding the flood basalts. Their presence within the flood basalt sequence indicates that trachybasaltic volcanism of the type that formed the western flank of the shield volcano started before the flood volcanism had ceased.
Felsic volcanic rocks
A thin (10 m) unit of felsic volcanic rocks (samples 228–230) is present on the western flank. Although this unit lies at an altitude of only 3250 m, about 1200 m below the eroded summit of the volcano, when the dips of the volcanic strata are taken into account, the felsic sequence is seen to be near the top of the stratigraphic sequence, as shown in Fig. 3. Two rock types are present: (1) friable, poorly exposed quartz-phyric rhyolitic crystal tuff; (2) massive irregular lobes of feldspar-phyric or aphyric trachyte that in places intrude the rhyolitic tuff.
Alkali basalt
Several flows of alkali basalt (samples 232 and 610) directly overlie the felsic volcanic rocks. The lowest flow is massive, in places columnar-jointed; the second is petrologically similar and distinguished, locally, by the presence of small lherzolite xenoliths and megacrysts of clinopyroxene and spinel. 39Ar/40Ar dating of an alkali basalt (FB16) gave an age of 18·65 ± 0·19 Ma (Table 1 and Fig. 6).
Choke and Guguftu shield volcanoes
Choke is one of three major shield volcanoes enclosed within a large meander of the Blue Nile (Figs 1 and 5). It is a broad flat symmetrical shield made up dominantly of lava flows that extend radially out from the central conduit with dips of less than 5°. Most of the volcano consists of massive basaltic flows that are morphologically similar to those of the Simien volcano (Fig. 7). Some are aphyric, others porphyritic with phenocrysts of plagioclase ± clinopyroxene ± olivine. Felsic rocks—trachytic and rhyolitic flows and fragmental units—are limited to the upper 300–400 m of the present volcano.
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A rhyolite sample (ETH 238) from an altitude of 3870 m, about 200 m below the present summit, gave a plateau age of 22·4 ± 0·3 Ma and an isochron age of 23·0 ± 0·3 Ma (Table 1 and Fig. 6). A basalt sample (ETH 254) from 2680 m, near the base of the shield, yielded a plateau age of 23·01 ± 0·23 Ma and an isochron age of 19·6 ± 1·53 Ma. The age of the underlying flood volcanics is poorly known, being constrained only by ages of 29·4 ± 0·3 Ma to 26·9 ± 0·7 Ma reported by Hofmann et al. (1997)
for basalts from the Blue Nile valley at a location about 80 km east of the volcano. If we accept the older age, the Choke volcano erupted some 4 Myr after the underlying flood volcanics. Guguftu is located about 40 km south of the town of Desse (Fig. 5), close to the western margin of the Ethiopian rift and in a region of considerable deformation and erosion. For this reason the overall form of the volcano is not easily established. Most rocks are highly porphyritic basalts containing phenocrysts of olivine, clinopyroxene and plagioclase (Fig. 7). As in the Choke volcano, trachytic volcanic units form a minor component of the volcanic pile in the upper part of the sampled sequence. A lahar composed of all rock types in the region occurs in the middle of the sequence. A feature that distinguishes Guguftu from the other volcanoes is the presence of numerous mafic and felsic dykes oriented between 000° and 045°.
A feldspar-phyric rhyolite (ETH 265) from an altitude of 3405 m at the top of the shield sequence gave a plateau age of 23·3 ± 0·3 Ma and an isochron age of 23·8 ± 0·3 Ma (Table 1 and Fig. 6). ETH 262, a plag–cpx–ol-phyric basalt from a volcanic plug that intrudes basaltic flows in the lower part of the shield, yielded a 18·58 ± 0·24 Ma plateau age. Another plag–cpx–ol-phyric basalt (ETH 255), from an altitude of 2585 m and presumably in the upper part of the flood volcanic sequence, gave a plateau age of 19·1 ± 1 Ma; the last steps of the spectra, however, gave values between 23 and 30 Ma, because of excess Ar. The isochron age is 23·5 ± 1 Ma. This sample was collected 5 m from a felsic dyke and this intrusion may have perturbed the Ar–Ar system. The nearest previously dated samples in the flood basalts, from the Wegel Tena section about 50 km north from Guguftu, yield ages from 30·2 ± 0·1 Ma to 28·2 ± 0·1 Ma (Hofmann et al., 1997). On the other hand, Ukstins et al. (2002) obtained ages around 25 Ma in the upper part of the basaltic sequence near Dessie, which suggests that this part of the volcanic plateau had a protracted volcanic history.
Other regions
The village of Alem Ketema is situated on the left bank of the Blue Nile, about 100 km north of Addis Ababa (Fig. 1). In this region the flood basalts are far thinner than to the north, being limited to several 50–200 m thick sequences of relatively thin flows that alternate with felsic volcanics and clastic sedimentary rocks. The basalts are petrologically similar to flows of the Choke shield. Coulié (2001) and Coulié et al. (2003) obtained K–Ar ages between 20·7 and 23·5 Ma for felsic volcanics interlayered with these basalts.
The region between the towns of Sekota, Lalibela and Bora (Fig. 1) contains abundant picrites, ankaramites and alkali basalts with chemical characteristics that correspond to the HT2 magma type of Pik et al. (1998, 1999). Unlike the other regions, the rocks from this area are tilted, folded and cut by numerous normal faults; dips up to 40° are common. The thinner flows are massive with scoriaceous tops; thicker units (10–30 m) are internally differentiated with lower parts enriched in phenocrysts of olivine and clinopyroxene. We dated two samples of the HT2 rock type (Table 1 and Fig. 6). Sample ETH 679, from the gabbroic-textured upper part of a differentiated flow, gave a 40Ar/39Ar age of 30·86 ± 0·12 Ma. The second, ETH 633, a glassy olivine–clinopyroxene– phyric hyaloclastite that probably formed when a lava flow entered a shallow lake, gave an age of 30·99 ± 0·13 Ma.
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PETROGRAPHY OF MAFIC ROCKS OF THE SHIELD VOLCANOES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING STRATEGY AND ANALYTICAL...STRATIGRAPHY AND CONSTITUTION OF...PETROGRAPHY OF MAFIC ROCKS...GEOCHEMICAL DATAISOTOPIC COMPOSITIONSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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In this study, emphasis has been placed on the mafic rocks of the shield volcanoes because the flood basalts have been well described in previous studies by Mohr & Zanettin (1988)
, Zanettin (1993) and Pik et al. (1998, 1999).
Simien shield and underlying flood basalts
There is little systematic variation in the characteristics of the dominant basaltic rocks from the base of the flood volcanic sequence to the summit of the shield volcano. Instead, we see a seemingly random alternation of aphyric, sparsely phyric and highly phyric lavas (Figs 3 and 4). The phenocryst phases are plagioclase and clinopyroxene (ubiquitous) and olivine (present in most samples). Because we sampled the massive flow interiors, most of our samples are sparsely vesicular or non-vesicular.
Plagioclase-phyric basalts (>20% phenocrysts) occur at 1800–1900 m and 2000–2150 m. These rocks are petrologically similar to the porphyritic trachybasalts on the upper western flank of the Simien volcano, but they do not contain the remarkably high abundance of phenocrysts of the latter rocks, nor do they have their distinctive chemical compositions. Olivine-phyric basalts also occur throughout the stratigraphic sequence, particularly at the base and around 2500 m and 2700 m.
The trachybasalts contain abundant (up to 60%), very large (up to 3 cm long) phenocrysts of plagioclase and less abundant smaller phenocrysts of clinopyroxene. Olivine phenocrysts are present in some samples. The matrix consists of plagioclase, clinopyroxene, opaque minerals and olivine.
The alkali basalts of the uppermost series on the upper western flank of Simien are distinguished by the presence, within the volcanic groundmass, of small equant olivine grains. Samples from the second flow in this series, the unit that contains lherzolite xenoliths, are characterized by abundant olivine xenocrysts and megacrysts of clinopyroxene and spinel.
Choke and Guguftu shields
Almost all our samples from the Choke and Guguftu shields are plagioclase-phyric and contain olivine and clinopyroxene phenocrysts. They are distinguished from the basalts of the Simien shield by the presence of small grains of olivine in the matrix, and distinctly pink–brown clinopyroxene grains, features indicating that these basalts belong to the alkaline magma series.
Other regions
The flood basalts from the Alem Ketema region are alkali basalts with characteristics very like the flows of the Choke and Guguftu shields. The HT2 flows from the Sekota–Lalibela–Bora region are sparsely to highly phyric lavas characterized by abundant (up to 50%) rounded to euhedral phenocrysts of olivine and clinopyroxene. The groundmass consists mainly of clinopyroxene, abundant oxides and accessory biotite and apatite; plagioclase is rare or absent. The olivines are rich in forsterite (Fo76–87) providing evidence of crystallization from relatively magnesian magmas.
Alteration
Samples from all three shields are variably altered. Olivine, plagioclase and glass in the matrix are partially or, in some samples, completely replaced by secondary minerals; phases such as chlorite, carbonate, zeolites and clay minerals fill vesicles and fractures.
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GEOCHEMICAL DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING STRATEGY AND ANALYTICAL...STRATIGRAPHY AND CONSTITUTION OF...PETROGRAPHY OF MAFIC ROCKS...GEOCHEMICAL DATAISOTOPIC COMPOSITIONSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Major and trace element compositions of mafic rocks from the Simien shield and underlying flood basaltic sequence
The general characteristics of the basaltic rocks are illustrated in the total alkalis–silica (TAS) diagram (Fig. 8). Flood basalts, and basalts of the Simien Main Series, have compositions that plot, with one or two exceptions, in the subalkaline field or slightly within the alkaline field. To simplify subsequent discussion they will be referred to as tholeiitic basalts. Almost all the other basalts—those from the western flank of the Simien volcano and almost all the rocks from the Choke and Guguftu shields—have lower SiO2 contents and/or higher Na2O + K2O contents and plot in the alkali field. The distinction extends to other incompatible major and trace elements: the trachybasalts and the alkali basalts of Simien volcano, for example, have moderately high TiO2 and very high Nb, La and Th, compared with the flood basalts and the Simien Main Series (Figs 9 and 10).
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In MgO variation diagrams (Fig. 9), the flood basalts and basalts of the Simien Main Series have a relatively restricted range of compositions. Accumulation of plagioclase explains part of the more evolved (low-MgO, high-Al2O3) compositions of the trachybasalts. It cannot explain, however, their high contents of TiO2 and low contents of CaO, elements that are incompatible and compatible, respectively, in plagioclase. Instead, the data show that these rocks formed from relatively evolved, incompatible-element-enriched magmas. The alkali basalts are distinguished from the other lavas by relatively high MgO and low SiO2 and very high contents of incompatible elements.
Mantle-normalized trace element patterns for the flood basalts and the Simien Main Series are very similar (Fig. 10). They have relatively flat light REE (LREE), moderately sloping heavy REE (HREE), and pronounced negative Th and Ti anomalies (Fig. 10c and d). In these incompatible-element-enriched rocks, Nb/La ratios are distinctly sub-chondritic; were it not for the pronounced Th depletion, the patterns would show large negative Nb anomalies. The trachybasalts from the western flank of the shield have higher contents of the more incompatible elements and steeper patterns (Fig. 10b). Their negative Th, Nb and Ti anomalies are even larger than in the flood basalts. In the alkali basalts (Fig. 10a), the trend to steeper REE patterns persists: concentrations of LREE are higher and concentrations of HREE are lower than in other basalts. Anomalies of Th and Nb are absent, but the negative Ti anomalies remain.
Stratigraphic variations of chemical compositions within the flood basalts and Simien shield
The relatively constant composition of flood and shield basalts, from the base of the sequence to the top of the shield, is illustrated in Fig. 11. Silica contents range between 48 and 53% and show no up-sequence trends, apart for a poorly defined increase in the interval 1500–1800 m. A change in composition is more apparent for TiO2, K2O and Nd, which increase significantly in this interval, and for MgO, for which the concentration decreases.
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The four samples of trachybasalts from the lower portion of the shield volcano in the Lima Limo section (in the interval 2700–3000 m), and also the two lava flows intercalated with the flood basalts (samples 141 and 142, between 2300 and 2400 m), are distinguished from the flood basalts by low MgO, high TiO2, Al2O3, P2O5 and K2O, and high Ba, Sr, Zr, La/Sm and Sm/Yb.
The two samples of alkali basalts are plotted at their current altitude of about 3200 m, well below the summit of the volcano and below the older basalts of the Simien Main Series. They lie in this position because they form part of a later veneer on the flanks of the volcano (Fig. 3). These rocks have moderate to high MgO contents (reflecting their high olivine contents) and low SiO2 contents; levels of alkalis and incompatible trace elements are comparable with or higher than those of the other basalts.
Choke and Guguftu volcanoes
The petrological and geochemical characteristics of mafic lavas from the two younger shields are identical to those of the alkali basalt on the Simien shield. Samples from Choke volcano are petrologically more evolved and their compositions are influenced by the accumulation of plagioclase phenocrysts, but despite these differences, the geochemical features that characterize the alkali basalts remain. These features include high concentrations of incompatible elements, steeply sloping trace element patterns, and positive Nb anomalies (Figs 12 and 13). Also notable are low contents, relative to elements of similar compatibility, of Th and Ti (Fig. 13). Although our sampling was not sufficiently detailed to investigate stratigraphic variations in the compositions of the volcanoes, the 16–17 samples from each volcano reveal no systematic up-section variations.
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Other regions
The flood basalts from Alem Ketema have alkaline compositions and are enriched in incompatible trace elements, very like the alkali basalts of the Choke and Guguftu shields. The HT2 lavas from the Sekota– Lalibela–Bora region are very different (Figs 12 and 13). They contain moderate to high MgO and low SiO2, in accord with their abundant olivine and clinopyroxene phenocrysts. Their Al2O3 contents are low and their TiO2, P2O5 and incompatible trace element contents are very high. The HREE are strongly fractionated, and Nb anomalies are positive or absent.
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ISOTOPIC COMPOSITIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING STRATEGY AND ANALYTICAL...STRATIGRAPHY AND CONSTITUTION OF...PETROGRAPHY OF MAFIC ROCKS...GEOCHEMICAL DATAISOTOPIC COMPOSITIONSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Our new Nd and Sr isotopic data are illustrated in Fig. 14. The basaltic samples have a relatively limited range of compositions: 143Nd/144Nd = 0·51279–0·51296,
Nd(T) = +3·6 to +7·0; 87Sr/86Sr = 0·70337–0·70439. Flood basalts from the Lima Limo section have compositions similar to the trachybasalts and Main Series tholeiites from the Simien shield. Their 87Sr/86Sr ratios are slightly higher than samples from the Choke shield, but when Pik et al.'s (1999) data are included, there is almost complete overlap. Samples from the Guguftu shield are displaced to higher 87Sr/86Sr and lower 143Nd/144Nd, and plot largely separately from samples from the Choke shield. However, once again, when the Pik et al. (1999) data are included, there is little to discriminate between flood and shield basalts. Most felsic samples have slightly lower 143Nd/144Nd than the basalts, but their 87Sr/86Sr ratios are similar. An exception is a trachyte sample from the Guguftu shield, which has slightly lower 143Nd/144Nd and higher 87Sr/86Sr.
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Lead isotopic compositions, in contrast, vary widely (Fig. 15). The Lima Limo flood basalts have relatively low 206Pb/204Pb ratios, from 18·1 to 18·7, and, with the exception of one sample, low 207Pb/204Pb and 208Pb/204Pb. Trachybasalts from the Simien shield plot in a single group at still lower 206Pb/204Pb. The shield basalts have more radiogenic compositions. The thin 18·7 Ma veneer of alkali basalt on the Simien shield has intermediate 206Pb/204Pb ratios, and samples from the 23 Ma shields have higher ratios. Choke basalts have a combination of high 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb and low 87Sr/86Sr that distinguishes them from all other samples from the Ethiopian volcanic province. Guguftu basalts have slightly lower Pb isotope ratios but higher 87Sr/86Sr. Each volcano or volcanic sequence in the region has a distinct Pb–Sr isotopic composition.
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Lavas from the Ethiopian shield are characterized, therefore, by relatively constant Nd–Sr isotopic compositions but wide ranges in Pb isotope and in trace element concentrations and ratios. In the LT and HT2 lavas, for example, differences of almost an order of magnitude in the concentrations of elements such as Th or La, and major differences in Pb isotope ratios, are present in rocks with almost identical ranges of Nd and Sr isotope compositions.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING STRATEGY AND ANALYTICAL...STRATIGRAPHY AND CONSTITUTION OF...PETROGRAPHY OF MAFIC ROCKS...GEOCHEMICAL DATAISOTOPIC COMPOSITIONSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Spatial and temporal distribution of magma types in the northern Ethiopian plateau
The information presented above reveals a complex evolution, in time and space, of volcanism in the northern Ethiopian plateau. The Simien shield appears unique in that it consists predominantly of incompatible-element-poor LT tholeiites, like the flood basalts in this part of the plateau. Although Pik et al. (1999)
proposed that rocks of the LT series are restricted to the northwestern part of the region, our sampling, and the analyses reported by Zanettin et al. (1976), indicates a slightly wider distribution. Thin sequences of LT tholeiites are present to the north of the town of Sekota (Fig. 1), where our unpublished analyses show that the lowest 300 m of the volcanic sequence consists of LT tholeiites, and in the Nile Valley, where Zanettin et al. (1976) suggested the presence of LT tholeiites at the base of the sequence, immediately overlying the sedimentary rocks of the basement. Although the LT type of basalt erupted mainly in the northwestern part of the province, it is not entirely restricted to this region. Wherever the LT rocks have been dated, they give ages very close to 30 Ma.
Our analyses of the HT2 flood basalts from the Sekota– Lalibela–Bora region complement the data reported by Pik et al. (1998). These rocks, which have elevated concentrations of incompatible trace elements and strongly fractionated trace element patterns, appear to be restricted to a small region in the northeastern part of the plateau bounded approximately by the towns of Sekota, Bora and Desse. They are among the oldest volcanic rocks in the northeastern plateau, having erupted around 31 Ma.
Figures 12 and 13 show that, in terms of their trace element contents, the basalts from the Choke and Guguftu shields, the Alem Ketema flood basalts, and the late veneer of alkali basalt on the Simien shield all resemble Pik et al.'s (1998) HT1 type of flood basalt. These rocks share an alkaline magmatic character with relatively high concentrations of incompatible trace elements and moderately fractionated trace element patterns. As in the Simien shield, the magmatic character of the Choke and Guguftu shields matches that of the underlying flood basalts. The ages of HT1 rocks range from about 30 Ma in the north of the province to far younger in the south. The Choke and Guguftu shields and the Alem Ketema flood basalts are about 22 Myr old, and the late veneer of alkali basalt on the Simien shield is 18·7 Myr old.
In the central part of the plateau it is difficult to distinguish between flood and shield basalts: the Alem Ketema basalts, for example, had been mapped as flood basalts (Zanettin et al., 1976), but they are better interpreted as peripheral parts of a 23 Ma shield. The general picture seems to involve protracted but sporadic eruption of alkali basalt, from 30 Ma to <10 Ma, with migration of the centre of eruption from north to south.
The silica-undersaturated rock types (basanites, nephelinites and phonolites) in some of the younger shield volcanoes are still younger (25 to <5 Ma; Zanettin & Justin Visentin, 1974; Zanettin et al., 1976). Mt Guna, which erupted between the Choke and Simien shields, has an Ar–Ar age of 10·7 Ma (Table 1 and Fig. 6).
Is the Ethiopian plateau a typical flood basalt province?
The image of the Ethiopian volcanic province given in the literature, and supported by our study, is very different from that of a ‘normal’ flood basalt province. Type examples of continental flood basalts, such as those of the Deccan and Karoo provinces, are described as thick, monotonous sequences of thick, continuous, near-horizontal flows of tholeiitic basalt. In contrast, the descriptions of the Ethiopian province by Mohr & Zanettin (1988), and our own field observations and geochemical data, reveal a series of flood basalts overlain by large and conspicuous shield volcanoes. The magmatic character varies from north to south and within each region the character of the shield volcanoes matches that of the underlying flood basalts.
Are the differences between the Ethiopian plateau and the other flood volcanic provinces real, or are they merely apparent, a consequence of differing extents of erosion and degrees of preservation, compounded by incomplete sampling of these vast volcanic structures?
One undeniable difference is the relatively young age of the Ethiopian province. If the small-volume (170 000 km3, Courtillot & Renne, 2003), and in many ways unusual, Columbia River basalts are excluded, Ethiopia is the youngest major flood basalt province. It is one of the least deformed, and probably the only one in which the uppermost volcanic units are well preserved. Because of the uplift associated with most flood basalt provinces, their upper levels typically are strongly eroded. In the Karoo Province, for example, kimberlite pipes have been eroded to their root zones. However, in both the Deccan and Karoo provinces, there is solid evidence that flat-lying tholeiitic basalt flows erupted from start to finish of the volcanic event, and that shield volcanoes never formed. Cox & Hawkesworth (1985) measured inclinations along a 650-km-long section of the Deccan Traps and concluded that dips were extremely low, between 0·25 and 0·3°. Widdowson (1997) found that a flat-lying laterite at the top of the sequence represents the original, almost horizontal upper surface of the lava pile. Widdowson & Cox (1996) stated that: ‘individual flows and formations within the Deccan are known to cover huge areas with only relatively gradual changes of thickness ... After the cessation of the eruptions, the landscape must have been almost devoid of any important topographic features.’ Marsh et al. (1997), writing of the Karoo province, stated: ‘the present structure ... is one of a broad basin with a slight inward dip of the flows’, and ‘the constancy of thickness ... (of the volcanic formations) ... suggests that the bulk of the lava pile was built on a generally planar surface. There is nothing in the stratigraphy that suggests the presence of geographically focused lava eruption sites ...’
On the other hand, in the Paraná–Etendeka and North Atlantic igneous provinces, shield volcanoes, igneous centres and large volcanic disconformities have been mapped. Jerram & Robbe (2001) described an early shield volcano in the Etendeka province in Namibia. The North Atlantic province contains the well-known intrusive complexes of Skye, Mull a