Continental Flood Basalts and Mantle Plumes: a Case Study of the Northern Ethiopian Plateau

L. Beccaluva^1*, G. Bianchini^1,2, C. Natali¹ & F. Siena¹

¹Dipartimento di Scienze della Terra, Università di Ferrara, Italy; bcc@unife.it, bncglc@unife.it, ntlcld@unife.it, snr@unife.it
²School of Geology, Geography and the Environment, Kingston University, Kingston upon Thames,UK

Corresponding author: Luigi Beccaluva

This webpage is an abridged version of the paper: Beccaluva, L., G. Bianchini, C. Natali, and F. Siena, Continental Flood Basalts and Mantle Plumes: a Case Study of the Northern Ethiopian Plateau, J. Petrology, 50, 1377-1403; doi:10.1093/petrology/egp024

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Introduction

There has been renewed scrutiny of mantle plumes in the last decade. Many controversial hypotheses have been proposed regarding the depth of plume provenance, triggering mechanisms, shape and size, relationships with hotspots, large igneous provinces (LIPs), rift volcanism, and the very existence of plumes has been questioned (Ernst & Buchan, 2001; Foulger et al., 2005; Beccaluva et al., 2007b; Foulger & Jurdy, 2007). The East African rift system is of particular interest as large Continental Flood Basalts (CFB) were erupted during the Tertiary associated with regional uplift (Ethiopian and Kenyan domes), and they have been attributed to one or two distinct mantle plumes (Ebinger & Sleep, 1998; Rogers et al., 2000; Yirgu et al., 2006). The Early Oligocene Northern Ethiopia-Yemen volcanic province comprises the entire range of CFB magmas, from Low-Ti to High-Ti and very High-Ti basalts and picrites, erupted in a well-defined space-time interval, and overlain by younger alkaline basalts with peridotite xenoliths. These provide direct evidence of the nature of the mantle beneath the plateau. This CFB province corresponds to the region where Afro-Arabia continental break-up took place with the formation of the Red Sea-Gulf of Aden-East African rift system centred on the Afar triple junction, where a possible  deep mantle plume has been suggested on the basis of geophysical data (Hofmann et al., 1997; Davaille et al., 2005).

The Northern Ethiopian plateau (Figure 1) mainly consists of tholeiitic to transitional basaltic lavas (Mohr & Zanettin, 1988), which were erupted at ~ 30 Ma in a period of 1 Ma or less (Hofmann et al., 1997). The basalts cover an area of ~210,000 km² with a lava pile up to 2000 m thick in the central-eastern part, thinning to less than 500 m toward the northern and southern boundaries. On the eastern margin, neighbouring the Afar escarpment, rhyolitic volcanic rocks characterizing the upper part of the sequence have been interpreted as the differentiated products of basaltic magmas that mark the start of continental rifting (Ayalew et al., 2006). Similar nearly coeval CFB volcanism is recorded in the Yemen conjugate margin covering an area of ~80,000 km² (Baker et al., 1996; Ukstins et al., 2002). Plateau volcanism in Northern Ethiopia was preceded by uplift (at least since the Late Eocene, based on stratigraphical data: Bosellini et al., 1987) of the underlying basement by up to 1-2 km (Sengör, 2001). After the CFB emplacement, the area was affected by a further ~2 km uplift, as a result of isostatic and tectonic processes (Gani et al., 2007). Dyke swarms, that most probably fed CFB fissure eruptions (Mège & Korme, 2004; Figure 1), are subparallel to sea-floor spreading axes in the Gulf of Aden and Red Sea, suggesting multiple rifting and spreading processes radiating from the still active Afar zone (e.g., the volcano-seismic event of 2005; Ayele et al., 2007).

Figure 1. Sketch map of the Oligocene continental flood basalts (CFBs) of the Northern Ethiopia and the Yemen conjugate margins, based on the geological map by Merla et al. (1973), NASA STRM images, data from Baker et al. (1996), Pik et al. (1998), Kieffer et al. (2004) and this work. LT, Low-Ti tholeiitic basalts; HT1, High-Ti tholeiitic basalts; HT2, very High-Ti transitional basalts and picrites. Dyke swarms after Mége & Korme (2004); locations of mantle xenoliths hosted in Quaternary alkaline volcanic rocks in the plateau area are also indicated. Inset: regional geodynamic sketch map with plate motions and velocities after Bellahsen et al. (2003).

Geographical information system (GIS)-based digitization of the 1:2,000,000 geological map of the region (Merla et al., 1973), carried out as part of this study, permitted the original volume of the flood basalts in the Northern Ethiopian Plateau to be estimated as 250,000 km³, implying an eruption rate of the order of 0.2-0.3 km³/year, which is comparable with that of other CFB provinces (Farmer, 2003). The plateau basalts are overlain by huge shield volcanoes mainly composed of alkaline lavas (Mt. Choke and Gugugftu, 22 Ma; Mt. Guna, 10.7 Ma; Kieffer et al., 2004). In the southwestern part of the plateau highly alkaline lavas were erupted from a number of Quaternary volcanic centres. Basanite lavas from two of these volcanoes near Injibara and west of Nekemte (Dedessa River) carry abundant mantle xenoliths, which have also been considered in this study.

The new major and trace-element analyses of the Northern Ethiopian plateau basalts confirm the existence of three main magma types spatially zoned according to their TiO₂ content (Pik et al., 1998) and allow a more precise definition of their zonal arrangement (Figure 1). Low-Ti tholeiites (LT), in the NW, are quantitatively predominant (150,000 km³). High-Ti lavas (HT1) predominate southeastward. Ultra-titaniferous transitional basalts and picrites (HT2) are concentrated in the Lalibela area, closer to the centre of the Afar triangle. In both alkali vs silica (Figure 2) and TiO₂ vs MgO (Figure 3) diagrams distinctive compositional features of the three magma types can be observed. LT basalts mostly plot in the subalkaline field, showing the widest differentiation range with MgO 3-12 wt %, coupled with the lowest TiO₂ contents (1.0-3.5 wt %). The HT1 group is mainly composed of tholeiitic basalts with MgO 4-9 wt % and TiO₂ 2.5-4.8 wt %. HT2 transitional basalts and picrites cluster around the alkaline-subalkaline boundary with MgO 5-12 wt % and TiO₂ 3.5-5.9 wt % for basalts, and MgO 13-16 wt % and TiO₂ 2.6-4.5 wt % for picrites. Moreover, there is a broad correlation between TiO₂ and Fe₂O₃ tot, the latter increasing from 11-12 wt % in the least differentiated LT to 14-15 wt % in the HT lavas (not shown).

Figure 2. Total alkali-silica classification diagram (Le Bas et al., 1992) for the Northern Ethiopia CFBs (data from this work and Pik et al., 1998). For comparison, data from coeval Yemen CFBs have also been plotted (from Baker et al., 1996; and authors’ unpublished data). LT, Low-Ti tholeiites; HT1, High-Ti tholeiites; HT2, very High-Ti transitional basalts and picrites. The dashed line dividing alkalic and subalkalic series is after Irvine & Baragar (1971).

Figure 3. TiO₂ vs MgO (wt %) for the Northern Ethiopian CFBs (data from this study and Pik et al., 1998). For comparison data from coeval Yemeni CFBs have also been plotted (from Baker et al., 1996; and authors’ unpublished data). LT, Low-Ti tholeiites; HT1, High-Ti tholeiites; HT2, very High-Ti transitional basalts and picrites. Empirical intergroup boundaries are drawn to minimize misclassified samples.

The decreasing silica saturation from LT to HT2 is reflected in the parallel decrease in normative hypersthene from 12-23% in LT and HT1 tholeiites to < 10% in many HT2 transitional basalts and picrites. CFBs from the coeval Yemen plateau (Manetti et al., 1991; Baker et al., 1996; author’s unpublished data) are classified as HT1 and HT2, and show striking compositional similarities with the Northern Ethiopian High-Ti lavas facing the Afar triangle (Figures 2 and 3). This further supports the hypothesis that, before the opening of the Afar-Red Sea system, these volcanic districts would have been part of the same magmatic province extending ~700 km in an east-west direction. A regional comparison shows that the LT and HT1 lavas from the Ethiopia-Yemen province show close similarities to other Low- and High-Ti CFB provinces (e.g., Paranà, Piccirillo & Melfi, 1988; Deccan, Melluso et al., 1995). On the other hand, the distinctive compositions of the HT2 ultra-titaniferous basalts and picrites seem to be only partially comparable with those of some picrites from the Karoo igneous province (Sweeney et al., 1991; Cawthorn & Biggar, 1993; Ellam, 2006) and the Siberian Traps (meimechites: Campbell et al., 1992).

Petrogenetic modelling

Different approaches based on major elements were used to develop a comprehensive petrogenetic model for the least differentiated CFBs from the three groups, which all include (with the exception of HT1) samples with Mg# 0.68-0.72, that is, in equilibrium with mantle sources containing olivine Fo > 88 (Herzberg & O’Hara, 2002; Green & Falloon, 2005). Most of the Northern Ethiopian CFBs have lower Mg# and only a few HT2 picrites (non-olivine-cumulative) reach the threshold of Mg# 0.72 that was considered by Niu & O’Hara (2008) as the required value for true primary magmas. However, the relatively low Mg# values of CFBs from Northern Ethiopia are, at least in part, related to the distinctive Fe enrichment that appears to be a primary feature of these magmas. Therefore, model calculations, reported below, have been carried out on a large dataset (also including analyses by Pik et al., 1998) for basalts with MgO > 6%, which represents the compositional limit for olivine to be considered the main phase controlling the liquid line of descent (Herzberg & Asimow, 2008).

The empirical model of Niu & Batiza (1991) was first applied to estimate the degree of melting (F), and the initial (P0) and final (Pf) melting pressure of the Northern Ethiopian basalts, assuming that they formed along a melting column from mantle sources under decompression conditions. Basalt compositions out of the range of the experimental data on which the model was based were discarded and analyses with MgO < 8% were normalized to MgO = 8% by adding an appropriate amount of olivine Fo₈₇, which represents the most magnesian phenocryst composition observed in the Northern Ethiopian basalts. These data were integrated with the pressure (PA) and temperature (T) of melt segregation calculated using the algorithm of Albaréde (1992). For picrites, which fall outside the composition range investigated by Niu & Batiza (1991), only the Albaréde thermobarometric approach was used, and F=25-30% was assumed in accordance with experimental petrology data (Green & Falloon, 2005, and references therein).

Results are reported in the P-T petrogenetic grid of Figure 4. Temperatures of basalt segregation range from 1400°C for HT to 1200°C for LT. In picrites, higher segregation temperatures (up to 1500°C) are recorded in accordance with both the exceedingly high Ni content (1100-650 ppm in bulk-rocks) and the high Cr content of their olivine phenocrysts (up to 1000 ppm), which conform with plume-derived superheated magmas (Campbell, 2001).

Figure 4. Pressure (P)-temperature (T) petrogenetic grid depicting the melting conditions of Northern Ethiopian CFBs based on the model proposed by Niu & Batiza (1991) (P0 and Pf are initial and final pressures of melt formation) and Albarede (1992) (PA and T are pressure and temperature of melt segregation). Results from this study and Pik et al. (1998). Volatile (H-O-C)-bearing (1) and dry (2) lherzolite solidi, spinel (sp) and garnet (gt) stability fields, and adiabat (3) for mantle potential temperature Tp 1430°C are after Green & Falloon (2005). Shaded fields refer to the P-T equilibration conditions of lherzolite mantle xenoliths from Northern Ethiopia (this work and Conticelli et al., 1999), HT and LT initial melts. (See text for further explanation.)

Further temperature estimates for picrites have been obtained using olivine-liquid thermometers according to Ford et al. (1983) and Herzberg & O’Hara (2002), giving temperatures of ~ 1400°C (in equilibrium with Fo₉₀ olivine) and 1360-1365°C (in equilibrium with Fo₈₈ olivine) for samples LAL6 and LAL9, respectively. As expected, these olivine liquidus temperatures are systematically lower (by some 50-100°C) than those calculated for magma segregation from the mantle sources. This allows us to conclude that, even considering the possible H₂O effect in lowering the liquidus temperature (Falloon et al., 2007), the generation conditions of the hottest (picritic) magmas has to be ≥ 1350°C.

These results emphasize that the zonal arrangement of Northern Ethiopian CFBs from LT in the west to HT2 in the east broadly corresponds to a parallel increase in magma temperature. Moreover, the temperature difference between the hottest Ethiopian CFBs and the ambient mantle, represented by mantle xenoliths from the area (Figure 4), is at least > 300°C, which is compatible with the thermal anomaly induced by a deep mantle plume (Farnetani & Richards, 1994; Campbell, 2007). The geothermal regime seems to vary from nearly adiabatic (potential temperature Tp = 1430°C), with generation of the hottest picritic magmas over a pressure interval of 1.6-3.0 GPa, to non adiabatic, where LT magmas were formed. This implies that the mantle region that underwent partial melting had its deepest and hottest part centred in the east, close to the Afar triple junction, where a deep plume has been proposed on the basis of seismic tomography (Davaille et al., 2005). This is consistent with the petrological model proposed by Herzberg & O’Hara (2002) for plume-associated picritic magmas.

The interpolation of the P0 values of the initial melts for the LT and HT magmas defines two distinct near-solidus mantle arrays located between the dry and the volatile bearing peridotite solidi. The HT initial melting array is closer to the H-O-C-bearing lherzolite solidus, suggesting a more volatile-enriched nature for their mantle sources, as also indicated by the significant presence of hydrous phases in some HT basalts.

Major element mass-balance calculations between the basalts and the lherzolite xenoliths were also carried out to constrain further the composition of the mantle sources, using F estimates obtained from the model above. The modal mineral composition of the mantle sources, as well as the melting proportions, were defined by iterative least-squares calculations based on the major-element compositions of the constituent minerals plus additional mantle phases such as amphibole. Phlogopite, garnet and ilmenite were generally not present in the studied lherzolite xenoliths, but required by computation. A summary of the modelling results is given in Table 10 of Beccaluva et al. (2009), which relates each magma type with the source mineralogy, melting proportions and degree, and the P-T conditions of melting. This indicates the following features.

1. Chemical components corresponding to amphibole ± phlogopite are always required in the magma source, varying from 7% for LT tholeiites to 10 -12% for HT basalts and picrites, and are the predominant melting phases. These phases, or another chemically equivalent mineral assemblage stable at the relevant P-T conditions, are considered to be related to metasomatizing agents interacting with the original mantle parageneses, and confirm the hydrated nature of the mantle solidus hypothesized in Figure 4.
   
2. Titanium-rich minerals (e.g., ilmenite, rutile, armalcolite) are additional metasomatic phases required in the mantle sources of the ultra-titaniferous (HT2) magmas.
   
3. The presence of negative olivine in the melting proportions of the subalkaline melts, although decreasing from LT to HT1 tholeiites, reflects a significant incongruent melting of orthopyroxene in the source of the tholeiitic magmas (Beccaluva et al., 1998).
   
4. The significant presence of garnet required in both the mantle source and the melting proportions of the HT magmas confirms P0 estimates of their initial melting conditions in the garnet lherzolite stability field (Figure 4).

Further petrogenetic constraints are provided by the distribution of those incompatible trace elements that are widely recognized as indicators of OIB mantle sources in hotspot or plume regions. To evaluate quantitatively the incompatible-element distribution in the magma sources, batch melting modelling was carried out based on the basalt compositions, using the melting parameters defined in the previous section. Results show that to generate the entire range of Ethiopian CFB magmas, from LT to HT, the calculated mantle sources (SLT bas, SHT1 bas, SHT2 picr; Figure 5) should be of the order of 2-15 times richer in incompatible elements with respect to those of the most fertile Northern Ethiopian mantle xenoliths. Significantly, these source compositions satisfactorily match those (S1, S2, S3; Figure 5) resulting from lherzolite xenoliths GOJ26 and GOJ40A with the addition of an amount of metasomatic amphibole plus Ti phases approaching that required by the major element mass-balance calculations (Table 1). The lherzolite xenoliths have flat mantle normalized trace element patterns (0.7-1.0 x PM), showing little sign of incompatible element enrichment, in contrast to other xenolith occurrences in southern Ethiopia (Bedini et al., 1997) and along the Saharan Belt (Hoggar, Algeria, Beccaluva et al., 2007a; Gharyan, Libya, Beccaluva et al., 2008).

Figure 5. Primitive Mantle (PM)-normalized (Sun & McDonough, 1989) incompatible-element patterns of Northern Ethiopian LT and HT magmas and their inferred mantle sources. SLT bas, SHT1 bas and SHT2 picr refer to source compositions calculated by batch melting for LT basalt SIM15, HT1 basalt BLN4 and HT2 picrite LAL6, using partition coefficients (Kd) from the GERM website (http://earthref.org/GERM) and melting parameters as defined in Table 10 of Beccaluva et al. (2009). These theoretical compositions compare favourably with those of Ethiopian mantle xenoliths GOJ26 and GOJ40A (Table 1 of Beccaluva et al., 2009) with the addition of appropriate metasomatic phases: S1=lherzolite GOJ26 (ol 61, opx 22, cpx 14, sp 3) + 3% amphibole (from Zabargad lherzolites; Brooker et al., 2004); S2=lherzolite GOJ40A (ol 60, opx 22, cpx 15, sp 3) + 9% amphibole (from Kerguelen mantle xenoliths; Moine et al., 2001); S3=lherzolite GOJ40A + 12% amphibole and 1% ilmenite (from Kerguelen mantle xenoliths; Grégoire et al., 2000; Moine et al., 2001).

The consistency of the major- and trace-element modelling suggests that the Northern Ethiopian CFBs were generated from lherzolite mantle sources variably metasomatized by alkali-silicate hydrous melts enriched in high field strength elements (HFSE; e.g., Ti, Nb, Zr), low field strength elements (LFSE; e.g., Ba and Th), and LREE. These metasomatizing agents show compositional similarities to those recorded in mantle xenoliths from the Kerguelen Islands, where both amphibole and Ti-rich metasomatic phases (including rutile, ilmenite and armalcolite) have been reported (Grégoire et al., 2000).

The interpretation of this metasomatic enrichment is not straightforward, being related either to a contribution from the Earth’s metallic core (Humayun et al., 2004) or to the involvement of eclogitic materials recycled in the mantle (Sobolev et al., 2007). However, the distinctive Ti enrichment observed in the Northern Ethiopian CFBs implies that the metasomatizing agents were enriched in this element, favouring an origin from mantle sources that included Ti-rich eclogitic materials. These materials most probably result from the recycling of Ti-rich mid-ocean ridge basalt (MORB) protoliths in the mantle via ancient subduction.

The origin of the volatile components in the metasomatizing agents is a matter of considerable debate. The helium isotopic composition of Ethiopian CFBs shows Ra up to 20 for some High-Ti magmas, suggesting an origin from the undegassed lower mantle (Pik et al., 2006) or even from the D" layer at the core -mantle boundary (Tolstikhin & Hofmann, 2005). Analogous considerations are valid for the relatively high H₂O contents in the HT magmas. From the above it is evident that the inferred metasomatic agents could have integrated various mantle geochemical components with different provenance and mobility. These components probably pooled by scavenging and mixing of heterogeneous mantle materials during plume ascent (Farnetani et al., 2002).

Conclusions

The spatial distribution of the various magma types, together with their petrogenetic P-T-X constraints, are used to define the shape, size and location of the melting region in the underlying mantle where the magmas were generated. Our results suggest an onion-like melting region (Figure 6), most probably representing the head of an impinging deep mantle plume that induced a dramatic thermal anomaly, lithospheric bulging or faulting, and the Early Oligocene CFB volcanism in Northern Ethiopia-Yemen. The petrological model presented here fits the first-order requirements for a deep mantle plume hypothesis as predicted by laboratory and numerical modelling (Farnetani & Richards, 1994; Campbell, 2007); that is, shape, dimensions, temperature excess (ΔT > 300°C), lithosphere bulging, and CFB timing.

Figure 6. Schematic illustration showing the Afar plume impinging on the Afro-Arabian lithosphere and the generation of Oligocene Northern Ethiopia-Yemen CFBs from a thermally and compositionally zoned plume head. Mantle sources are affected by decreasing metasomatic effects from the hottest core of the plume head to the cooler outer zones. Metasomatic agents are envisaged as alkali-silicate melts enriched in multiple geochemical components (Ti, HFSE, LFSE, LREE, H₂O, noble gases, etc.) rising along the plume axial zone. The hypothesized provenance depth of mantle xenoliths entrained in Neogene-Quaternary alkaline volcanic rocks is also indicated. The inset shows mantle tomographic cross-sections beneath Afar, based on models for shear-wave velocity variations (Davaille et al., 2005). Low velocities are attributable to the presence of less dense or hotter material from a rising plume. Click here or on figure for enlargement.

In this scenario multiple geochemical components, pooled and upwelled along the plume stem, effectively accumulated and spread laterally in the plume head. The intensity of the metasomatism decreases from the plume axial zone towards the periphery, as reflected by the zonal arrangement of the erupted magmas, which requires progressively less metasomatized mantle sources in a westward direction. CFB generation in Northern Ethiopia and the conjugate Yemen margin was favoured by several factors:

1. lowered solidus temperatures of plume metasomatized mantle sources;
2. heat transfer by the plume buoyancy flux that raised the regional geotherm;
3. decompression of the upwelling mantle.

As illustrated in Figure 6, these factors can satisfactorily account for the generation, at comparable melting degrees, of LT tholeiites from the outer (westwards) comparatively less metasomatized mantle sources and of HT1 basalts from the more metasomatized mantle domains closer to the plume axis (eastwards). Ultra-titaniferous HT2 transitional basalts and picrites were generated in the innermost (hottest) part of the Afar plume from highly metasomatized mantle domains, which probably underwent nearly adiabatic decompression (in the range 3.0 -1.6 GPa) starting from the garnet-peridotite stability field.

Paleogeographical restoration of the Yemen CFBs, nearly exclusively represented by HT magmas, fits perfectly with the analogous HT lavas of the Northern Ethiopian plateau, and suggests an asymmetric shape and extension of the underlying plume head (Figure 6), as indicated by available seismic tomography data (Davaille et al., 2005). The distinctive composition and zonal arrangement of the Northern Ethiopia-Yemen CFBs centred on the Afar triple junction further supports a relationship with a mantle plume independent from the Kenyan plume (Rogers et al., 2000). The available Sr-Nd-Pb isotope data show a common isotopic signature for CFBs from the Northern Ethiopia and Yemen plateaux (Afar plume signature) and a distinct composition with respect to the Southern Ethiopian and Kenyan lavas (Rogers et al., 2000; Furman et al., 2006; Rogers, 2006). Further evidence for a distinct Afar plume is provided by recent shear-wave velocity data and relative stratification of anisotropy in the Afar region (Sicilia et al., 2008).

The geodynamic evolution of the region after the CFB activity indicates that the African plate was relatively stable with respect to the Afar plume, with extension mainly focused along the Red Sea-Gulf of Aden spreading system during Neogene-Quaternary times. This implies that, while the African plate was locked against the Alpine orogenic systems along the Mediterranean and Bitlis collisional margins, the Arabian plate could migrate northeastward, pulled by the active Makran subduction, thus favouring the current oceanic opening along the Red Sea-Afar-Gulf of Aden system (see inset in Figure 1).

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Discussion

Jerry Winterer, 23rd November, 2009

1. In this webpage, Beccaluva et al. never get below the upper part of the asthenosphere. The plume in the summary figure connecting to the deeper interior is gratuitous. Everything about the petrology works without a plume.
2. The citation for doming is Bosellini et al. (1987), which has nothing about doming. In fact, the exposure of pre-Oligocene strata is ascribed to a large Oligocene drop in sea level. The Bosellini data come from the Aden coast of Somaliland, not from the Afar region.
   

This article is the usual set of interesting and valuable observations about petrology, plus completely unrelated speculation about deep plumes tacked on, as a non sequitor.

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