Geological and geochemical variations in Mid-Tertiary Ethiopian Flood Basalt Province , Maychew , Tigray Region , Ethiopia

The paper presents the results of a comprehensive major element, trace element, and Sr-Nd-PbHf isotopic study of Mid-Tertiary volcanic sequences from the northwestern flood basalt province in Ethiopia. The volcanic rocks studied range in composition from basanites, alkaline basalts, and ankaramites, which form the 1st three sequences at the base associated with basaltic agglomerate (sequence 1, 2 and 3) to transitional and tholeiitic basalts and picrites confined to the upper three sequences (sequences 4, 5 and 6). Sequence 5 is bimodal with intermediate-felsic pyroclastic rocks intercalating the transitional-tholeiitic basalts. There is a good correlation of sequences with geochemical enrichments, such as an increase La/LuN with TiO2, and decrease in Al2O3 and SiO2 towards the base. The smooth increase of La/LuN ratios in lower sequences reflect the general decrease of degree of partial melting that sampled heterogeneous packages of mantle plume materials. In the Sr-Nd isotopic diagram two clusters are formed one with restricted and lowSr(0.70356-0.70345) and Nd(0.51290-0.51284) isotopic compositions, defined by sequence 1, and the other cluster with relatively higher Sr/Sr (0.7052-0.7036) and Nd/Nd (0.51296-0.5127) isotopic compositions defined by samples from sequence 2, and most other sequences with little scatter. The Pb-isotopic compositions vary systematically within each sequence (from base to top), and each sequence clustered systematically in a different Pb– Pb– isotopic space with the highest Pb/Pb (19.10-19.30) and Pb/Pb (15.60-15.65) measured in sequence 1, quite differently from other sequences and from previously reported for northwestern Ethiopian flood basalt province, and the lowest Pb/Pb (18.20-18.56) and Pb/Pb (15.51-15.55) measured in sequence 4. Sequence 2 and 3 lavas display a similar Pb/Pb and Pb/Pb ranges falling between sequence 1 and 4. In contrast, sequence 6 samples displayed towards higher Pb/Pb than sequence 2, 3, and 4, but with lower and higher Pb/Pb than sequence 1 and 5 respectively. The lavas of sequence 1 and 4 have relatively less radiogenic Hf/Hf than other sequences with slight scatter. The systematic geochemical variations in lavas are remarkable and reflects three mantle end component mixing with minimal crustal in put as a fourth component. The enriched sequence 1 has very similar geochemistry to HIMU-type ocean island basalts (OIBs), and this end member ascribed to be the most enriched Afar plume component 1, which is the most enriched Afar Plume component during initial continental break. The second enriched component is defined by sequence 2 and 3 samples and may reflect the second enriched component in Ethiopian flood basalt, previously reported for the high-Ti2 basalts assumed to be the enriched Afar Plume component, whereas the third component is the depleted component defined by sequence 4 partly overlap the range previously reported for low-Ti basalts.


INTRODUCTION
The Mid-Tertiary (~30 Ma) Ethiopian continental flood basalts form part of the larger Afro-Arabian Igneous Province, which is related to the Afar plume and the Red Sea-Gulf of Aden -Ethiopian Rift triple junction.Maychew, the area, of present study (Fig. 1) forms part of the high-Ti basalts of Ethiopian flood basalt province.Rocks here are composed of alkaline lavas (basanites, basalts, and ankaramites, with basaltic agglomerate) at the base, felsic volcanics in the middle, and transitional to tholeiitic mafic lavas at the top of the transitional sequence.Pik et al. (1998Pik et al. ( , 1999) ) have classified the north-western Ethiopian flood basalts into three distinct geochemical groups based on trace element and Ti concentrations: low-Ti basalts (LT), high-Ti1 (HT1) basalts and high-Ti2 (HT2) basalts.They recognized a suite of 'low-Ti' (LT) basalts restricted to the northwestern part of the province (Fig. 1) assumed to be derived from depleted mantle.They are characterized by relatively flat REE patterns and lower Ti and incompatible trace element concentrations.Alkali basalts found to the south and east of the province on the other hand show higher concentrations of incompatible elements and more fractionated REE patterns and related to the so-called 'high-Ti' basalts (HT1 and HT2).
In Western Ethiopian Plateau, up to 2 km thick predominantly mafic lavas with minor felsic pyroclastics (Fig. 1) make up the northern part of the Ethiopian flood basalt sequence (e.g., Hoffman, 1997;Ayalew et al., 2002) and cover an area of ~ 106 km 3 (Rochette et al., 1998).The Maychew area is located in the northern eastern corner of the western Ethiopian plateau and covers an area of about 100 km 2 .At present there is no geochronological data available for Maychew lavas to verify the sequential variations, but the earlier K-Ar whole-rock age dating of basalts collected from 110 m and 140 m above base in Maychew area gave 26.2 Ma and 25.7 Ma (Jones, 1976), respectively.However, such a young age compared to the current refined and accepted mineral 40 Ar/ 39 Ar age of 30.9 and 29.2 Ma for the northwestern Ethiopian continental flood lavas (Hofmann et al., 1997;Coulie et al., 2003;Kieffer et al., 2004) could be related to the unrecognized argon loss or gain in these intensely weathered formations.Hence, at present 31-29 Ma is accepted as a reasonable estimate for the Maychew flood basalt sequence.As shown in figure 1, the Maychew area is located between the Adigrat area and Chinese road section (A and C, in Fig. 1) and the samples collected from these areas are dated between 31-29 Ma.Moreover, recently Kieffer et al. (2004), dated plagioclase separate from alkaline picrite from Bora Volume 2 (1): 2010 © CNCS Mekelle University 6 Mountain (B in Fig. 1, close to Maychew area) at 30.99±0.13Ma.The dated sample is taken from lateral variation of ankaramite/ picrite of sequence 2 (Fig. 2).The younger age of 29.2 Ma (Hofmann et al., 1997) is obtained from ignimbrite layers exposed at the topper part of the sequences, which is equivalent to sequence 5 described at Maychew area.
Figure 1.Location map of NW Ethiopian Plateau, Afar Rift and Main Ethiopian Rift (from Kuster et al., 2005).The approximate broken line separates the low-Ti and High-Ti flood basalt province (Pik et al., 1998).
Despite a relatively large chemical database for the southern and western Ethiopian plateau volcanics, there are still many parts of the plateau magmatism that are not studied.Most studies on Ethiopian rift and plateau magmatism have been carried out at a regional scale (Pik et al., 1999, Kieffer et al., 2004), and detailed investigations on single sections of the plateau are scarce.Moreover, lack of a detailed geological map of any particular section of the Ethiopian flood basalt province makes it difficult to correlate much of the existing high quality geochemical data from various parts of the Ethiopian flood basalt province.This paper presents

Kurkura, K (MEJS)
Volume 2 (1): 4-25, 2010 © CNCS Mekelle University 7 geology map of part Maychew area dominated by high-Ti basalts (Figs.1;2 & 3).Systematic rock sampling of selected sections was carried out.They are analysed for whole-rock major and trace elements, Sr, Nd, Pb, and Hf isotopic data using suitable procedures (Nakamura et al., 2003) at Pheasant Memorial Laboratory of the Institute for study the earth's Interior, Okayama University at Misasa.The large data set of the volcanics thus obtained is discussed in the paper in addition to geology and stratigraphic sequence of the flows.

STRATIGRAPHIC SEQUENCES, PETROGRAPHIC DESCRIPTION OF VOLCANICS OF MAYCHEW AREA
Geology of Maychew area covered in this paper includes many lithological units (Figs. 2 & 3) and the volcanic successions which reveal six cycles (referred as sequence, Kabeto et al., 2004).
Out of six, four are related to mafic-agglomerate volcanism (exposed between 1800-2900 m a.s.l, sequences 1, 2, 3, and 4), in which basanites, and alkaline basalts and transitional ankaramites occupy the base (sequences 1, 2 and 3) followed by silica rich tholeiitic to transitional sequence 4.This is followed by sequence 5 of mafic-felsic volcanism (2900-3450 m) and that is covered  (1987) used to differentiate the deformed lower and upper undeformed formations, however, such evidence was not observed in the northwestern part of the plateau (Kieffer et al., 2004).
In Maychew area, there is a marked change in the morphology.Sequence 1, 2, and 3 are marked by deformed and subdued topography.However, sequence 2 also display gently undulating morphology and the steepness of the slope is passing from sequence 2 to 3 and the first sharp cliff marks sequence 4. The exposure of sequence 5 is marked by broken flat surface where the andesitic pyroclastics nearly eroded to flat surface (suitable agricultural land) and a resistant pediment made up of picrite and ankaramite. 2 sharp cliffs one in the middle and the other on top is marked by 15 to 10 m thick consolidated tuff and ignimbrite partly showing columnar joints.
The last sequence forms tabular facies, in which flow units are about 12 m thick, alternate with braided lobes of pahoehe of ~1 to 3 m thick.The sequence 2 and 3 resemble the high-Ti2 and the other sequences (4, 5, and 6) might be considered as high-TiO 2 1 of Pik et al. (1998,1999).
However, sequence 1 is described for the first time in the Ethiopian Plateau Geology (Kabeto et al., 2004(Kabeto et al., , 2006)).There is systematic variation in the characteristics of the dominant volcanic rocks from the base (1800 m, Mehoni plain) of the Maychew volcanic sequence to the summit at the Tsibet Mountain (3780 m) (Figs. 2 & 3) and these sequences are described below.

Sequence 4:
Cliff forming aphyric to phyric basalts and basaltic agglomerate intercalations (~2700-2900 m), and has a maximum thickness of 200 m (Fig. 2).The transitional to tholeiitic basaltic flows typically comprise many horizontally stratified flows of 1-5 m thick separated by patchy basaltic agglomerate (0.5-1 m).The lava flows consist of three litho-types, aphanitic, and slightly porphyritic basalts, and basaltic agglomerate.Flows in this sequence typically contain up to 10% plagioclase phenocryst in a pilotaxitic groundmass of predominantly plagioclase microlites, pyroxene, and Fe-Ti oxides and glass now altered partly to chlorite and epidote.
Trachytic texture is common, and few samples contain microphenocryst of brownish clinopyroxene, Fe-Ti oxides, and olivine.
Greenish to grey varieties form the basal part of the pyroclastic flow deposits, whereas welded variety form the top part even grading to thinly banded rhyolite (20-35 cm) flows at the top of the ignimbrite layers (Fig. 2).In between the basal andesitic tephera and the top ignimbrite and rhyolite flows, phyric to aphyric basalts and picrite-ankaramite occur (Fig. 2).Greenish andesitic tephera variety predominates and they form crystal, rock, and glass fragments in glassy rarely devitrified and epidotized groundmass.The crystals are commonly plagioclase, aegirine-augite, and opaques possibly Fe-Ti oxides.In the ignimbrites, aegirine-augite and abundant sanidine with few opaque and anorthoclase, and rare rock fragments together form the fragmental part.

SAMPLING STRATEGY AND ANALYTICAL METHODS
Geological mapping activity and samples of all possible fresh volcanic rocks exposed within the study area were collected during three separate field seasons.Seven different transects were selected within 100 km 2 , to enable detail mapping, and sampling a wide, a diversity of temporal and spatial chemical variations as possible in a high-Ti zone of the flood basalt sequence.
The samples were analyzed for whole-rock major and trace elements, and Sr-, Nd-, Pb-, and Hfisotopic data at the Pheasant Memorial Laboratory (PML), Institute for study the earth's Interior, Okayama University at Misasa.A total of 89 whole-rock samples were crushed using jaw crusher to coarse chips of 3-5 mm size, from which fresh chips were carefully hand-picked.They were rinsed with deionized water in an ultrasonic bath at least three times, and then they were dried at 100 o C for 12 hours.The washed and dried chips were ground using an alumina puck mill.Major elements, Ni and Cr data were obtained using X-ray fluorescence spectrometer (XRF) (Phillips PW2400) on glass beads containing a lithium tetraborate flux (10 to 1 dilution of samples) (Takei, 2002).Loss on ignition (LOI) was obtained gravimetrically.Trace elements were determined by isotope dilution (ID) and analyzed by inductively coupled plasma mass spectrometry (ICP-MS) using a Agilents 7500cs system fitted with a flow injection system (Makishima & Nakamura, 1997;Makishima et al., 1997Makishima et al., , 1999;;Yokoyama et al., 1999;Moriguti et al., 2004).Isotopes data for Rb, Sr, Sm and Nd were generated for the samples selected for Sr-Nd isotopic measurement, by isotope dilution thermal ionization mass spectrometry using a modified Finningan MAT261 instrument with NBS983 standard.Trace element concentrations in CaO-rich samples were measured by the Al-addition methods as suggested by Tanaka et al.
(2007 & reference therein).All of the major and trace elements analyses were duplicated for each sample, and replicate analyses had <0.5 relative % and 3-5 relative % difference, for major and trace elements, respectively.
The analytical procedures for chemical separation and mass spectrometry followed in the study are from Yoshikawa & Nakamura (1993) for Sr isotope measurements; Makishima & Nakamura (1991) for Nd; Kuritani & Nakamura (2002) for Pb;and Lu et al. (2007) for Hf.To remove the effect of secondary alterations (after petrographic examinations) all powders for Pb-and Hf-and some samples for Sr-Nd-isotopic measurements were leached with 6N HCl at 70 o C for about 9 hours, before acid digestions.

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Mass spectrometry was carried out with TIMS system in static multi-collection mode.
Normalizing factors to correct isotopic fractionation during spectrometer analysis are 86 Sr/ 88 Sr = 0.1194 for Sr and 146 Nd/ 144 Nd = 0.7219 for Nd.The isotopic composition of NIST SRM987 and LaJolla standards are 87 Sr/ 86 Sr = 0.710190 and 143 Nd/ 144 Nd = 0.511863, respectively (Makishima & Masuda, 1993) and are reported relative to long-term (over 2 years) laboratory averages.For Pb isotope measurements the correction of mass fractionation was carried out by the normal double spike method using a 207 Pb-204 Pb spike, as described by Kuritani & Nakamura (2003).

Classification of the volcanic flows/sequences
According to the total alkali-SiO 2 classification diagram (Fig. 4), mafic rock types range in composition from basanite, picrite-ankaramite, basalt to basalt-trachy-andesite.But transitional to sub-alkaline basalts predominate.In contrast with many continental flood basalts, the mafic rocks at Maychew area do not entirely show tholeiitic affinities and also do not fall in the basalt field (Fig. 4).Based on the petrographic data and lithological assemblages the different mafic lavas are subdivided in to six sequences from base to top.General characteristics of the mafic rocks are illustrated in the total alkalis-silica (TAS) diagram (Fig. 4).
The Maychew sequences have compositions that plot, with some exceptions, in the alkaline and sub-alkaline field, respectively.According to the proposed classification of Ethiopian flood basalts (Piccerillo et al., 1979), the sequences 1 and 2 are alkaline in composition, whereas sequences 3 and 4 lavas have compositions that plot between alkaline (few) and sub-alkaline field, indicating transitional character.However, the sequence 4 shows higher SiO 2 contents and/or higher Na 2 O + K 2 O (up to 5.2 wt.%) contents and some samples plot in the alkali field when compared to sequence 5 and 6 samples, which have lower SiO 2 and Na 2 O + K 2 O (< 3.5

Major element compositions of mafic rocks from the Maychew flood basalt section
General characteristics of the mafic rocks are illustrated in the total alkalis-silica (TAS) diagram (Fig. 4), and plots of the selected major and trace elements versus MgO shown in figure 5.The lavas from sequence 1 and 2 are referred as basanites because with two or three exceptions majority samples plot in the alkaline field.On the other hand, the lavas from sequences 3 and 4 are referred to as transitional ankaramite and basalt respectively as they plot in the transitional and Al 2 O 3 than the sequences upwards, but display higher FeO total and TiO 2 contents.On the contrary, sequences 5 and 6 show lower values for TiO 2 and the highest for Al 2 O 3 with few exceptions that overlap sequences 4 and 3. K 2 O content is variable in most sequences, it is relatively higher in sequence 3 than sequence 4 at a given MgO content except with few overlap.Similar difference can be inferred from contents of CaO and TiO 2 .Major element chemical variations within each sequence can be largely explained by crystal fractionation involving olivine, clinopyroxene, iron-titanium oxides, and plagioclase, however, difference in TiO 2 , CaO, K 2 O, Al 2 O 3 and to some extent in SiO 2 and FeO total among the six sequences reflect source variations or/and variable degrees of partial melting.
In figure 5, major element vs. MgO, majority elements clearly show existence of compositional variations among sequences.However, in some cases there is an overlap e.g.sequence 3 and 2 overlap in more mafic compositions, but their evolved varieties differ in TiO 2 , FeO total , Al 2 O 3 , and SiO 2 content.Similarly, the sequences 3 and 4 indicate overlap in their evolved varieties in MgO vs. Al 2 O 3 , SiO 2 , FeO total and TiO 2 , but they do differ in K 2 O and CaO contents (Fig. 5d).

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The sequence 1 and 2 show the greatest relative enrichment in P 2 O 5 , FeO total and TiO 5 .In contrast, the upper sequences 3, 4, 5, and 6 shows relatively low P 2 O 5 , FeO total , and TiO 2 , but highly enriched in SiO 2 and Al 2 O 3 .Between 1 & 2, sequence1 shows lower SiO 2 (45-42 Wt. %), Al 2 O 3 (12-8.5Wt.%) and TiO 2 (5.4-3.6 Wt.%) and higher CaO (15-12 Wt.%) and FeO total (18.2-16Wt.%) (Figs. 4 & 5).Compared to 4, sequence 3 shows higher P 2 O 5 , K 2 O and TiO 2 and lower Al 2 O 3 and CaO than, however, their SiO 2 , Na 2 O (not shown) and FeO total concentrations appear equally variable and scattered in both sequences.Moreover, CaO concentrations in sequence 4 lavas are higher than that of sequences 3 and 2 lavas with respect to MgO content.In the case of Na 2 O and K 2 O, they are scattered considerably in all the sequences and suggest secondary redistribution of Na and K.However, the extent of secondary alteration has also been evaluated using K 2 O/P 2 O 5 , which is > 1(not shown) in all types except for the three basanite samples from sequence 1, indicating minimum influence of subaerial weathering.K 2 O/P 2 O 5 ratio is used in evaluating the effect of subaerial weathering in mafic lavas, where K is easily leached by secondary alteration and P is not.

Trace element compositions of mafic rocks from the Maychew flood basalt section
The compatible elements Ni and Cr (Fig. 6a & b) show perfect linear relationship with MgO and suggest the control of olivine, clinopyroxene, and Cr-spinel.Presence of olivine, clinopyroxene or/and Cr-spinel well explains high contents of MgO, Ni, and Cr in picrite and ankaramite (Seq. 5,3 & 2).They are possibly mantle-derived melts having ~11% MgO (Skovgaard et al., 2001).
Crystal fractionation is the preferred mechanism for Cr depletion as its abundance is not expected to vary under conditions of increasing partial melting (Pearce and Norry, 1979).The incompatible elements La,Nb,Lu,Sm,Sr and Rb (Figs. 6c,d,e,h & f) with some scatter increase with decreasing MgO.The basanites (Seq.1) at the base of Maychew flood basalt section has the highest relative enrichments of incompatible trace elements (La & Nb).The sequence 2, alkaline basalts and ankaramites, has intermediate contents and in the upper sequence 3, 4, 5 and 6 the trace elements sequentially depleted upwards with some scatter.
by tholeiitic to transitional basalts of sequence 6 (3450-3780 m); and represent the last stage of flood basalt volcanism in the Eastern part of northwestern Ethiopian plateau flood basalts.The lower most mafic sequence (Sequence 1) is in fault contact with Cretaceous Amba Aradam Sandstone, indicating that the base of the sequence is not exposed (Figs.2 & 3).The sequences lack major unconformities; although intercalations of 10-20 cm thick paleosols are common in some sequences otherwise volcanism and depositions were continuous (Fig.3).The lower three sequences (1, 2 & 3) are more disturbed and tilted at 4-25 o towards SE than upper 3 sequences (4, 5 & 6).However, along NNE-SSW, NNW-SSE, and ENE-WSW striking normal faults they are also weakly tilted at about 3-4 o , and disturbed.The altitude sub-division used above may vary depending on these normal faults throw, which is clear at Bekura and Tsibet mountains (Figs.2 & 3).There is evidence of deformation of the type thatMerla (1979)  and Brehe et al.

2. 1 .
Sequence 1: Deeply weathered and often tilted basanite and basaltic agglomerate intercalations (~between 1800 and 2500 m).Sequence 1 ranges in maximum thickness from 100-150 m and mark the base of exposed flood basalt province (at Mehoni plain and at the base of Bekura Mt.), and comprise many individual flows.Typically occur as a series of stacked flows 2-6 m thick separated at places by paleosol horizons (5-20 cm).Basanitic agglomerate layers are also common.The massive flows contain very few microphenocrysts (< 2%) of greenish elongated clinopyroxene (diopsidic) and Fe-Ti oxides with rare olivine mostly altered, in the groundmass of clinopyroxene, phlogophite, opaque, glass and rare plagioclase.2.2.Sequence 2: It ranges in thickness from 450-600 m (~ 2000-2500 m) and comprise many individual flows.It is strongly weathered and tilted.The alkaline basalt and ankaramite flows typically occur as a series of stacked flows 1-10 m thick separated commonly by patchy basaltic agglomerate (1-10 m) layers.The mafic lava flows of this sequence consist of four litho-types, ankaramite, aphanitic basalts, porphyritic basalts and basaltic agglomerates.Porphyritic basalts and ankaramite predominates and often contain phenocryst (10-80%) of greenish and brownish clinopyroxene (zoned) and olivine, which are commonly iddingsitised.Fe-Ti oxides, plagioclase, clinopyroxene and glass form the microphenocryst and groundmass phases.Few ankaramites contain abundant olivine phenocryst compared to clinopyroxene.The aphanitic flows are generally microcrystalline with few pyroxenes and altered olivine microphenocrysts and often Volume 2 (1): 4-25, 2010 © CNCS Mekelle University 9 form the base and top of the sequence.2.3.Sequence 3: It consists of ankaramite, phyric basalts and agglomerate intercalations (~2500-2700 m) and ranges in thickness from 150-200 m.It comprises many individual massive cumulative ankaramites and porphyritic basalts (Fig. 3).The lavas typically occur as a series of massive to blocky bodies of 30-40 m thick separated by patchy agglomerate (~2 m) and phyric basalt (2-3 m) layers.The ankaramites range in composition from pyroxene-rich at base to plagioclase-rich in the top and olivine gabbros at the middle that form intergranular/gabbroic texture consisting of dominantly Ti-augite (3-6 mm), followed by plagioclase (2-3 mm), olivine and Fe-Ti oxides (1-2 mm), forming seriate texture.The top part of sequence 3 is plagioclase dominant basalt.
weathered and partly welded glassy ash forms the groundmass.Olivine rich ankaramite show intergranular texture consists of dominantly olivine, clinopyroxene, and plagioclase.Samples taken from Aygi section is commonly olivine picrites, where olivine predominates over clinopyroxene and plagioclase, whereas from Tsibet mountain section (Figs.2 &3), clinopyroxene and plagioclase predominate over olivine and at places range to olivine gabbro composition similar to sequence 3 ankaramites.

Figure 2 .
Figure 2. Schematic columnar section of Maychew area (modified from Kabeto et al., 2004).Numbers on the left side are elevations in meter above sea level.Abbreviation: Sq1 = Sequence1.The scale on the right of lithological column is in hundreds of meters.

Figure 3 .
Figure 3. Geological map of Maychew area (Numbers on the map indicate sequences as shown in Figure 2. Bora is 20 Km NW and Alage is 20 Km north of the map area).
sequences 5 and 6 are referred to as tholeiitic picrites-ankaramite and basalts as they plot though within the transitional field but towards tholeiitic affinity.The Maychew lava sequences can be broadly grouped from base to top as strongly alkaline, transitional, and tholeiitic and these subdivisions also well reflected in major oxides, trace element, and Sr-, Nd-, Pb-, and Hf-isotopic compositional variations.The mafic lavas span a large range in MgO, from primitive (~23 Wt. %) to evolved (~4 Wt. %) compositions.In general, the ankaramites and picrites from sequences 3 and 5 show highest MgO content and the transitional basalts of sequence 4 the lowest.Sequence 3 displays a wide variation in MgO content followed by sequences 5 and 2. SiO 2 content (Figs.4 &5) for the Maychew mafic lavas ranges from 42 to 54 Wt. %, basanites < 45 Wt.% (Seq.1) and transitional basalts > 48 Wt.% (Seq.4).SiO 2 , Al 2 O 3 , TiO 2 , and K 2 O (not shown) contents display broad negative correlations with MgO, whereas CaO content in sequences 2, 3 and 5 shows initial negative correlation with MgO till the content reached 12 Wt.%,afterwards the correlation becomes positive.Sequences 1, 4 and 6 (if MgO rich sample is excluded) on the other hand display positive correlation compared to others indicating their differentiated nature.Basanite (Seq.1)shows significant compositional difference than other sequences i.e. lowest in SiO 2 and Al 2 O 3 , highest in FeO total , CaO and MnO (not shown) contents.Further, alkaline basalt of sequence 2, also shows lower values for SiO 2

Figure 6 .
Figure 6.MgO vs. selected trace elements for Maychew lavas.The HT1 (small crossed rectangle), HT2 (shaded dots) and LT (small open circle) basalts are shown for comparison on c, e & f.HT2 overlaps fields of sequence 2 & 3, HT1 overlaps fields of evolved sequence 3, Sequence 4, 5 and 6, whereas LT basalts do not overlap any of Maychew sequences (when incompatible elements such as Nb & La are considered).
increase in (La/Lu) N with TiO 2 and a general decrease in Al 2 O 3 and SiO 2 towards the base (Figs.6 & 7).

•
. CONCLUSION • The HT1+HT2 zone of northwestern Ethiopian flood basalt province show sequential compositional variations (Seq 1-Seq 6).The sequence 2 & 3 are similar to HT2 of Pik et al (1998), whereas Sequence 4 overlaps the ranges of HT1 basalts of Pik et al (1998).• • geochemical variations in lavas are interpreted to reflect mixing of three mantle components, with minimal crustal in-put as a fourth component.basanites is described for first time in this study and enriched in incompatible elements more than the HT2 basalts.Sequence 5 and 6 show different chemical variations in isotopic ratios than HT1 and HT2.Smooth increase of (La/Lu) N ratios down the sequence reflect the general decrease of degree of partial melting of heterogeneous packages of mantle materials.The systematic The enriched sequence 1 has very similar geochemistry to HIMU-type ocean island basalts, and this ascribed as end member t the ost enriched Afar plume component 1 involved in the initial continental break-up.• The second enriched component is defined on the basis of the geochemistry of the samples from sequences 2 & 3 which reflect ponent in Ethiopian previously described (Pik et al., 1999) as high-Ti2 basalts assumed to be the Afar Plume component.• The third component would be the depleted one as defined by samples from sequence 4, magmatic province in W and SE Ethiopia.J. Coulie`t ive K-Ar and Ar/Ar dating of Ethiopianand Yemenite Oligocene volcanism: Hofmann, C., Courtillot, V., Feraud, G., Rochette, P., Yirgu, G., Ketefo, E & Pik, R. 1997.ones, P. 1976.Age of the lower flood basalts of the Ethiopian plateau.Nature, 261: 567-569.which partly overlaps the range previously reported for low-Ti basalts (Fig. 8; Pik et • The differenc variable lithospheric signatures, with the 2 nd enriched Afar plume component having more lithospheric material.The forth component is crustal signature (Fig. 8d) which invariably affected all the lava sequences.Crustal signatures are quite prominent in sequences 3 & 4 compared to others.NOWLEDGEMENTS rku a Kabeto acknowledges financial support from Japanese Society for Promotion of Science and also by the program for the 'Center of Excellence for the 21st Centu I, kayama University.I thank Prof. Talat Ahmad and Dr. K. Bheemalingeswara, and the r.Tadesse for their constructive comments on the manuscript.ERENCES D., Barrey, P., Marty, B., Reisberg, L., Yir timing of giant ignimbrite deposits associated with Ethiopian continental flood basalts.Geochemica et Cosmochimica Acta, 66: 1429-1448.S.M., Desta, B., Nicoletti, M & Tefera, M. 1987.Geology, geochronology and geodynamic implications Geol.Soc.London, 144: 213-226., E., Quidelleur, X., Gillot, P.Y., Courtillot, V., Lefevre, J.C & Chiesa, S. 2003.Compara implications for timing andduration of the Ethiopian traps.Earth and Planetary Science Letters, 206: 477-492.Timing of the Ethiopian flood basalt event and implications for plume birth and global Change.Nature, 389: 838-841.J