Compositional Differences between Felsic Volcanic Rocks from the Margin and Center of the Northern Main Ethiopian Rift

Pliocene felsic rift margin and Quaternary rift center volcanic rocks from the northern Main Ethiopian Rift (MER) exhibit contrasts in major and trace element contents and Sr-Nd isotopic ratios. Quaternary rift center felsic volcanic rocks are mainly peralkaline trachytes and rhyolites, whereas Pliocene felsic rift margin volcanic rocks are represented by benmoreites, weakly peralkaline trachytes and rare rhyolites. Most of the felsic rift margin volcanic rocks have greater Al2O3, K2O, Nb, Zr, Rb, and Sr, and lesser CaO, Zr/Nb, and CaO/Al2O3 than rift center volcanic rocks. These contrasts may have been inherited from differences in the compositions of their parental basic magmas, which were produced by variable degrees of partial melting. In both series, the felsic volcanic rocks generally have higher initial Srisotopic (0.7038-0.7073) ratios than their basic equivalents (0.7035-0.7046). Ndisotopic ratios of most felsic rift center samples (0.5129-0.5126) are similar to their associated basic volcanic rocks. In contrast, the Ndisotopic ratios (0.5128-0.5124) of felsic rift margin volcanic rocks are commonly lower than their companion basic volcanic rocks (0.512806-0.512893), and are relatively lower than rift center equivalents. The elemental and Sr-Nd isotopic compositions of the volcanic rocks suggest that fractional crystallization from differing basic parents accompanied by a limited assimilation (AFC) was the dominant process controlling the genesis of the MER felsic volcanic rocks.


INTRODUCTION
The Main Ethiopian Rift (MER), the Southwestern Ethiopian Rift Zone (SWERZ), the Tana Rift and the Afar region represent the northernmost part of the East African Rift System (Fig. 1).The Ethiopian volcanic province is dominated by up to 300,000 km 3 of generally fissure-fed Mid-Tertiary basic (SiO 2 < 53 wt.%) volcanic rocks, and minor associated felsic (SiO 2 > 53 wt.%) products.However, the proportion of felsic products in the Ethiopian rift valley itself is high, reaching about 90% of the total volume (Mohr, 1992).Between 45 and 22 Ma, volcanic activity in the Ethiopian plateau (Figs.1a and b) was characterized by fissural flows.Central volcanoes covered the fissural flows beginning at about 30 Ma and 15-13 Ma, and erupted intermittently into the Pleistocene (Morton et al., 1979;WoldeGabreil et al., 1990;Wolde, 1996;Stewart and Rogers, 1996;Chernet et al., 1998;Pik et al., 1998;Ayalew et al., 2002).Recent geochemical and isotopic studies have focused on the Oligocene-Miocene to Quaternary basic-felsic volcanism that accompanied the formation of the MER.These studies have proposed the involvement of distinct mantle components in various proportions, and the importance of the Afar mantle plume and lithospheric mantle in the sources of the basic lavas (Fig. 1).
Figure 1.Maps showing the location of the study area.a) Index map showing surface expression of the Ethiopian rift system and volcanic cover (Wolde, 1996) and the Ethiopian Plateau (EP, Stewart and Rogers, 1996).b) Volcanic and tectonic structures of the MER.The white ellipses are the study areas.N (northern), C (central), and S (southern) sectors of the MER; SWERZ is the southwestern Ethiopian Rift Zone.c) Sample sites, felsic volcanic centers and volcanic and tectonic structures of the Addis-Nazreth region.YTVL Yerer Tulu Welel Volcano Tectonic Lineament from Abebe et al. 1998.Although the previous studies have (Fig. 1) (Gasparon et al., 1993;Peccerillo et al., 1995;Trua et al., 1999;Abebe et al., 1998;Boccaletti et al., 1999), on volcanic rocks which are mostly younger than 2 Ma.The felsic volcanic products at the rift margin (Addis Ababa) and rift center (Nazreth) regions and their compositional variations are not yet well studied.This paper presents new elemental and Sr-Nd isotope data for selected volcanic sequences from the northern sector of the MER (Fig. 1).Based on the petrographic, geochemical, and isotopic characteristics of the volcanic units, we describe the compositional differences between the volcanic rocks at the rift margin and rift center, and discuss the petrogenetic relationships between basic and felsic magmas in order to assess the influence of basic parents and continental crust in the genesis of the felsic melts.

GEOLOGICAL SUMMARY OF THE ADDIS ABABA AND NAZRETH REGIONS
The study area is located in the center of the Ethiopian dome (Fig 1 ), and contains volcanic sequences that are directly related to the northern Main Ethiopian Rift (MER) activity.Reevaluation of seismic refraction data for the region by Makris and Ginzburg (1987) revealed thinning of the crust from 44 km thick at Addis Ababa to 30-26 km in the center of the MER to the east.This led Wolde (1996) to regard the volcanic sequences at Debre Zeit and to its east as rift center, and Miocene to Pliocene volcanic rocks in the Addis Ababa region, west of Debre Zeit as rift margin.
On the basis of whole-rock and mineral K-Ar and 40 Ar-39 Ar ages, Morton et al. (1979) and Chernet et al. (1998) found that the volcanic cover extending from Addis Ababa to Nazreth showed age progression from 22.8 Ma in plateau basalts to 0.21 Ma in the rift center volcanic rocks.At the base of the rift margin volcanic rocks, alkaline-transitional basalt (22.8 Ma) of the Plateau unit is in fault contact with the overlying Intoto unit (22.2-22Ma) (Morton et al. 1979;Chernet et al. 1998).The Intoto unit consists of trachyte-rhyolite flows and associated ignimbrites at its base (22.2Ma) and plagioclase phyric basalt (22 Ma) in its upper part.The Early Miocene Plateau and Intoto units represent bimodal volcanic rocks, which were formed during a localized terminal episode following the massive Oligocene fissural basalt activity of the northwestern Ethiopian plateau and are present at the rift margin in the Addis Ababa region (Morton et al., 1979;Begazi et al., 1993;Chernet et al., 1998).Because of their close realationship, the Plateau and Intoto units are here collectively referred to as the Plateau unit © Mekelle University ISSN: 2073-073X 4 .The geochemical data for the Plateau unit is not discussed in this paper because they are pre-rifting eruptions.
Volcanic activity resumed at the rift margin after a considerable hiatus between 22-10 Ma, by eruption of transitional-alkaline basalts of the Addis Ababa unit (9-7 Ma).This was followed by the welded trachytic tuffs (5.1 and 3.3 Ma) of the Nazreth unit, which are thought to have been derived from the mostly trachytic Pliocene (4.6-3.09Ma) Wechecha, Menagesha, Furi, and Yerer volcanoes (Fig. 1).These centers are collectively designated as the Wechecha unit (Chernet et al., 1998).
Predominantly felsic volcanic products were erupted in the Debre Zeit and Nazreth regions in the rift center between 2.0-0.20 Ma.This sequence consists of the Nazreth, Keleta, Boku-Tede, Bofa, Dera-Sodere, Gedemsa, Boseti, and Melkasa units in ascending order (Fig. 1; Boccaletti et al., 1999).For simplicity, the felsic volcanic rocks of Nazreth, Keleta, Boku-Tede, and Dera-Sodere units are here grouped as the Nazreth unit, whereas the basaltic rocks of the Bofa and Melkasa units are collectively termed the rift center basic volcanic rocks.

CLASSIFICATION AND PETROGRAPHIC SUMMARY
Most of the samples investigated here were fresh, and collected from lava flows, except for a few welded ignimbrites from the rift center (Table 1).According to the TAS classification diagram (Le Bas et al., 1986) the rock types in both areas range from basalt to rhyolite, but trachytes and rhyolites predominate (Fig. 2).The rocks fall in numerous compositional fields, making terminology cumbersome.Therefore, a restricted set of terms has been adopted here.Rock samples with SiO 2 < 53 wt.% are defined as basic, whereas samples with SiO 2 > 53 wt.% are classed as felsic.In common with many bimodal suites in continental rifts and oceanic islands, the basic rocks at the rift margin are generally alkaline or transitional, whereas rift center equivalents are typically transitional (Wolde, 1996) (Fig. 2).Strongly felsic alkaline rocks at the rift margin and center are weakly peralkaline and strongly peralkaline, respectively (Table 1).
According to Macdonald (1974), most of the peralkaline felsic volcanic rocks are commendites with rare pantellerites.However, most of the Gedemsa unit samples are pantelleritic (Peccerillo et al., 1995) Petrographically, the basic rocks in both rift margin and center are usually aphyric.Porphyritic samples on the other hand are rare and contain about 15-25% phenocrysts.The phenocryst minerals are olivine, clinopyroxene, and plagioclase with or without opaques.The groundmass of the basic lavas consists of the above phenocryst phases and accessory glass, zircon, apatite, and titanite, along with secondary alteration products including sericite/carbonate, iddingsite, and hematite.Loss on ignition values in these samples differ little from the unaltered samples (Table 1).
Figure 2. Classification of volcanic rocks of the northern MER according to the scheme of Le Bas et al. (1986).The alkaline-sub-alkaline boundary is from Irvine and Baragar (1971).Additional data for the volcanic rocks from Abebe et al. 1998), Chernet (1995) and Peccerillo et al. (1995).
The felsic volcanic rocks in the rift margin are relatively aphyric compared to the basic rocks, and generally show trachytic textures.
The felsic rift center volcanic rocks contain similar type of minerals as their rift margin equivalents.Majority of the samples are aphyric, but few samples containing up to 35% phenocryst phases with both felsic and ferromagnesian minerals forming the phenocryst and groundmass phases.Felsic minerals are usually sanidine, quartz, sodic plagioclase, and anorthoclase, whereas the ferromagnesian minerals are aegirine-augite, sodic-amphibole, olivine, and opaques.Both the lavas and ignimbrites contain the same minerals, but differ in their textures.The lavas commonly show perlitic cracks/spherulitic groundmass textures, whereas ignimbrites show eutaxitic textures with dominantly vitrophyric fiamme groundmasses.

Analytical Procedures
Out of 125 collected samples, 75 selected samples from each unit were analyzed for major and trace elements and Sr-Nd isotopes (Table 1) in the Department of Geoscience, Shimane University, Japan.Samples were crushed in a tungsten carbide ring mill (Roser et al., 1998), and dried at 110 0 C for 24 hours.No significant Nb or Ta contamination was present in the carbide ring mill compared to that ground in agate (Roser et al., 1998).The remaining XRF data will be provided upon request.Major and trace element analyses were performed using glass beads prepared either by fusing 0.7g of rock powder mixed with 3.5g of Li 2 B 4 O 7 (Norrish and Hutton, 1969) or by mixing 1.8g sample with 3.6g alkali flux (LiBO 2 :LiB 4 O 7 = 1:4) (Kimura and Yamada, 1996).Analyses were made using a Rigaku RIX 2000 X-ray fluorescence spectrometer, using conventional peak/background methods, with calibration against a suite of Geological Survey of Japan (GSJ) and USGS rock standards.The reproducibility was monitored with appropriate international standards JB01, JB02, and JB03, and was within +10% for all elements with concentrations higher than 10 ppm.

Major Element Data
Whole rock analyses and Sr-Nd isotope data of representative samples are listed in Table 1.The remaining data are available upon request.The basic (SiO 2 < 53 wt.) rocks have MgO contents between 4 and 14 wt.%,Mg-numbers (mg#) between 50 and 74, Ni ≤ 380 ppm and Cr ≤ 822 ppm.Most of these values suggest that very few rocks represent the primary mantle melts (Sato, 1977;Wilson, 1989).Majority of the samples having mg# between 44 and 50 suggest that they do not represent primary magma compositions (Table.1).However, Na 2 O contents increase from about 2 wt.% in the basic volcanic rocks to about 7 wt.% in the felsic rocks (at 65 wt.%SiO 2 ), and then decreases to 4.2 wt.% increasing SiO 2 (>70%) in most of the felsic samples.In the Boseti unit, however, Na 2 O continues to increase in some of the felsic rocks.Al 2 O 3 contents in basic samples range from 13.0 to 21.3 wt.%, maintain similar values (13.3-17.5 wt.%) in felsic samples through to 65 wt.%SiO 2 , and then decrease to values as low as 9.2 wt.% at 75 wt.%Compositional differences exist between the rift margin and rift center volcanic rocks (Table 1; Fig. 3).For example, CaO contents in the rift center basic rocks (9.4-11.3wt.%) are generally higher than most of the rift margin equivalents (7.7-10 wt.%), except in few clinopyroxenephyric samples from the Addis Ababa unit that are younger (10.9-12.5 wt.%).In contrast, Al 2 O 3 contents of most rift margin basic volcanic rocks are higher (14.8-18.6 wt.%) compared to the rift center equivalents (13-17.5 wt.%).
The contrasts in CaO and Al 2 O 3 seen in the basic lavas of the two regions are also apparent in the felsic volcanic rocks.Al 2 O 3 contents are greater in most of the felsic rift margin volcanic rocks than in the rift center, whereas CaO contents are generally greater in the rift center (Fig. 3).K 2 O concentrations are generally greater and ∑FeO contents lesser in the rift margin felsic volcanic rocks than in the rift center.However, the above compositional differences become less pronounced in samples with SiO 2 contents > 67 wt.%.Rift center felsic samples are generally richer in SiO 2 than most from the Wechecha (Pliocene) unit at the rift margin (Fig. 3)

Trace Element Data
In both the rift margin and rift center volcanic sequences, compatible elements such as Ni, Co, and Cr decrease in abundance with increasing SiO 2 (Table 1).This indicates the influence of olivine, clinopyroxene, and Fe-Ti oxide fractionation.Zr, Nb, Y, and Rb, being incompatible elements show increase in their concentration with fractionation within each rock series, albeit with some scatter (Fig. 4).Sr and Ba contents in the felsic volcanic rocks generally decrease with fractionation, representing compatible behavior, though they too show wide variations.
Most rift margin basic lavas have greater Zr and Nb contents than do the rift center basic lavas.Moreover, at given SiO 2 content Sr is much more enriched in rift margin basic lavas than in their rift center equivalents (Fig. 4, Table 1).Zr, Nb, Sr, and Rb are also generally more enriched in their rift margin felsic rocks than in Quaternary rift center felsic equivalents.However, Y and Ba abundances mostly overlap, as does Sr at > 65 wt.%SiO 2 .Among the Wechecha felsic samples, high Zr and Nb are observed in the Menagesha and Wechecha mountain samples (Table 1), whereas relatively low Nb and Zr contents characterise samples from the Furi and Yerer mountains (Fig. 4).

Nd-and Sr-Isotope Data
Twenty-eight samples spanning the compositional range from the least fractionated MgO-rich rocks to the most fractionated rhyolite samples were analysed for Sr-and Nd-isotopes.Isotopic © Mekelle University ISSN: 2073-073X 10 ratios as shown in table 1 are plotted in figure 5, together with data for 15 samples from Chernet (1995) and Abebe et al. (1995).Initial 143 Nd/ 144 Nd ratios in the basic lavas range from 0.512812 to 0.51289, and from 0.51237 to 0.51286 in the felsic volcanic rocks.Initial 87 Sr/ 86 Sr ratios range from 0.70349 to 0.70456 in the basic lavas, and from 0.70446 to 0.70783 in the felsic volcanic rocks.The rift margin basic volcanic rocks have higher Nd-and lower Sr-isotopic ratios than rift center equivalents (Table 1; Fig. 5).
In contrast, felsic volcanic rocks from the rift margin have lower Nd-isotopic ratios than most of their rift center equivalents.However, their Sr-isotopic compositions are equally variable.The felsic volcanic rocks extend from the isotopic range of the basic volcanic rocks towards higher 87 Sr/ 86 Sr (Fig. 5).Most of the felsic rift center volcanic rocks lie above the Debre Zeit field (Gasparon et al. 1993), whereas the felsic rift margin volcanic rocks partly overlie it.Rift center felsic volcanic rocks overlie the field defined by equally felsic lavas from northern Kenya (Kabeto et al., 2001) (Fig. 5).

DISCUSSION
It is well established that compositional differences in parental basaltic magmas are reflected in the compositions of felsic melts (Wilson et al., 1995;Panter et al., 1997).Fractional crystallization of basaltic magmas with some crustal assimilation and partial melting of basic lower crust/underplated igneous rocks were proposed as dominant processes for generation of felsic melts in the MER rift center (Fig. 1; Gasparon et al., 1993;Abebe et al., 1998;Peccerillo et al., 1995;Trua et al., 1999;Boccaletti et al., 1999).The influence of different parental magma compositions and the processes involved in the genesis of felsic melts in the northern sector of the MER are discussed below.

Influence of Parental Magma Compositions
The general, elemental contrasts between the rift margin and rift center volcanic rocks discussed above (Figs.3 and 4; Table 1) are clearly evident on CaO/Al 2 O 3 vs SiO 2 , Zr vs Nb, and Zr/Nb vs Zr plots (Fig. 6).At a given SiO 2 content most of the basic rocks (SiO 2 < 53 wt.%) from the rift center are displaced towards higher CaO/Al 2 O 3 ratios (Fig. 6a).Few basic rocks from the rift margin have CaO/Al 2 O 3 ratios comparable with rift center equivalents, and in both groups the ratio decreases with fractionation.CaO/Al 2 O 3 ratios remain higher in the felsic rift center rocks (SiO 2 > 53 wt.%) than in rift margin equivalents, but a few felsic samples from the Wachecha (Furi and Yerer Mts.) unit overlap (Table 1).This may be due to similar fractionating phases controlling their evolution, or indicate that they were derived from compositionally similar basic parents.
Zr and Nb contents show a well defined linear correlation (Fig. 6b), and both being incompatible elements increase with fractionation (Kamber and Collerson, 2000;Kabeto et al., 2001b).
Constancy of trace element ratios between basic and felsic melts (e.g., Zr/Nb; Fig. 6b and c) is often cited as strong evidence that fractional crystallization has been the dominant process in their evolution (Weaver, 1977;Wilson, 1989).At given Zr content most rift margin samples show higher Nb contents than do the rift center samples.Kamber and Collerson (2000) have indicated that Nb is more sensitive to variations in degrees of partial melting than Zr, and hence can be used to decipher the influence of variable degrees of melting.In this regard, some rift margin samples from the Furi and Yerer Mountains have Nb contents as low as the rift center samples (Table 1; Figs. 4 and 6b).It is evident that Yerer and Furi mountains are compositionally closer to rift center composition than the Wechecha and Menagesha Mountains.Furthermore, samples from Yerer and Furi are younger and have a narrower age range (2.03-4.04Ma) than the Wechacha and Mengasha Mountains samples (3.09-6.63Ma) (Chernet et al., 1998) (Fig. 6).
Plotting Zr/Nb vs Zr (Fig. 6c) also shows that felsic volcanic rocks of the study area fall into two clusters.Based on their Zr/Nb ratios, most samples from the rift margin plot along Zr/Nb ≤ 5, whereas rift center volcanic rocks, and the few samples from the Yerer and Furi Mountains with low Nb contents cluster along Zr/Nb ≥ 6.Similarly, Zr/Nb ratios in the basic rocks of the two sequences also vary (Fig. 6c).The rift center basic rocks have Zr/Nb ratio ≥ 5, whereas most of the basalts from the Addis Ababa area that are thought to be parental to the rift margin felsic volcanic rocks, and a few rift center basic rocks; have Zr/Nb ≤ 5.
Zr/Nb ratios in volcanic rocks may also reflect crustal contamination,titanite fractionation or variation in degree of partial melting (Wilson et al., 1995;Kamber and Collerson, 2000;Kabeto et al., 2001b).For example, basaltic sample ET1201 from the Plateau unit (Table 1) has a Zr/Nb ratio of 13.0.ET1201 has a very low initial Nd-isotopic ratio (0.51222 ± 8) compared to other basic lavas with lower Zr/Nb ratios, which may indicate that the higher Zr/Nb ratio reflects crustal involvement (Kabeto et al. 2001).Hence, we consider the rift center volcanic rocks (Zr/Nb > 6), and those rift margin volcanic rocks with Zr/Nb ratios > 5, to reflect either crustal input, titanite fractionation (Fig. 6c) or different sources.
The clear differences in crustal thickness, extensional tectonics, age of volcanic activity (Morton et al., 1979;Makris and Ginzburg, 1987;Wolde, 1996;Abebe et al., 1998;Boccaletti et al., 1999), and compositions of volcanic rocks in the two regions suggest that they were derived from different parantal magmas.The role of parental basic magma compositions in the rift margin and center felsic melts must be considered in the light of elevated or depleted absolute trace and major element abundances and the degree of silica saturation.Furthermore, the effects of crustal contamination must be accounted for.For example, the silica saturation in the felsic volcanic rocks in the rift center could be produced by substantial contamination of the basic magma that is parental to most of the Pliocene rift margin eruptives by silica-rich crust.
However, this is an unsuitable mechanism to produce the felsic volcanic rocks from the rift center, because open-system behavior would produce higher incompatible trace element abundances in the more contaminated series (DePaolo, 1981;Nelson and Davidson, 1993).This is not observed here.Moreover, the Nd-isotopic compositions of the felsic rift center volcanic rocks lie within the range of basic volcanic rocks in the region (Fig. 5).The lowest Nd-isotopic ratios are noted only in those felsic volcanic rocks at the rift margin that show higher degree of contamination.Davidson and Wilson (1989).Symbols as in Fig. 2.
Alternatively, contamination of basic magma, thought to be the parent for rift center felsic products by bulk assimilation of silica deficient crustal material (amphibolitic or basic crust?) must also be considered.This would make the rift margin Wechecha unit magmas more contaminated than rift center felsic magmas (Nelson and Davidson, 1993).We consider this to be unlikely, as it cannot explain the relative depletion of the compatible elements MgO and CaO in the Wechecha samples (Fig. 3).
It appears that differing degrees of partial melting of the mantle source provide the best explanation for the compositional differences seen between the two volcanic sequences (Nelson and Davidson, 1993).A lower degree of melting, deeper partial melting (Kushiro, 1968;O'Hara, 1968) or melting in the presence of CO 2 (Davies and Macdonald, 1987) could easily have produced incompatible element-enriched magma that differentiated to produce the most felsic rift margin samples.In contrast, higher degrees of melting could produce incompatible element depleted magmas that subsequently differentiated to produce the rift center felsic volcanic rocks.
It has been suggested that basic lavas produced by low degrees of mantle partial melting have high incompatible element contents (e.g., Zr, Nb, Y, K, and Rb), high Al 2 O 3 , and low SiO 2 and CaO (Tatsumi and Kimura, 1991;Wolde, 1996;Kabeto et al., 2001b).In contrast, basic lavas that are produced by high degrees of partial melting have lower incompatible element and Al 2 O 3 contents, but higher CaO and SiO 2 .Wolde (1996) has shown that alkali basalts produced by a low degree of partial melting are common in the western part of the rift and the margin, and are found only locally east of Debre Zeit (the rift center).In contrast, transitional basalts which originate from higher degrees of partial melting are commonly found within the rift center, where thinning of the crust has been identified from seismic refraction studies (Makris and Ginzburg, 1987).Moreover, Abebe et al. (1995) have suggested that the degree of alkalinity in basaltic melts increases away from the rift center.
In the northern MER, the felsic melts in the rift center generally have lower Al 2 O 3 , K 2 O, Zr, Nb, Y, Rb, and Sr and higher SiO 2 and CaO than most rift margin equivalents (Figs. 3 and 4).Hence, the major and trace element contrasts in the felsic products between the two regions could originate from compositional differences in their basaltic parents.Wilson et al. (1995) demonstrated that compositional differences between silica-undersaturated and oversaturated felsic melts in the continental magmatism of the Central Massif (France) were controlled by subtle compositional differences between their respective basic magmas.In line with this suggestion Kabeto et al. (2001) have considered that silica-saturated (basalt-trachyte) and silicaundersaturated (basanite-phonolite) lineages in the northern Kenyan rift sector originated from compositional differences in their parental basic magmas, and both felsic by differentiation combined with a little assimilation.This is also likely to be the case here.

Fractional Crystallization and/or Degree of Partial Melting
The general decrease of Ni, Cr, Sr, Ba, MgO, CaO, ∑FeO, TiO 2 , and P 2 O 5 with increasing SiO 2 (Figs. 3 and 4; Table 1) indicates that the geochemical evolution of these volcanic rocks was governed by fractionation of olivine, clinopyroxene, Fe-Ti oxide, feldspars, and apatite.TiO 2 and P 2 O 5 also show to decrease at the same SiO 2 content in all rock suites, indicating simultaneous apatite and Fe-Ti oxide fractionation.Moreover, the general increase in Zr, Nb, Rb, Y, and K concentrations with increasing SiO 2 is also consistent with fractional crystallization from a similar basic parent, to produce the mugearites, benmoreites, trachytes, and rhyolites of both regions.The well-defined linear correlation between Zr and Nb contents in the sequences (Fig. 6b and c) also suggests that fractional crystallization was a dominant process.
The Sr-and Nd-isotopic ratios of the felsic lavas (Fig. 5) also do not lie wholly within the isotopic range of their associated basic lavas, indicating that fractional crystallization was not the only process responsible for their genesis.
Genesis of the felsic MER volcanic rocks by anatexis of the upper continental crust must be discarded on the basis of geochemical characteristics.The Afro-Arabian continental crust displays a wide range of isotopic composition (Davidson and Wilson, 1989;Hegner and Pallister, 1989;Möller et al., 1998) which is dissimilar to the felsic MER volcanic rocks (Table 1).The Nd-isotopic compositions of most felsic MER volcanic rocks are also more radiogenic than mean upper crust.Hence, partial melting of the upper continental crust cannot be the source for the felsic melts.Low degrees of partial melting of basic lower crust and/or underplated basic magmas as possible source for the felsic melts can be tested by batch melting modeling calculations (Shaw, 1970;Skjerlie and Johnston, 1993) performed on Rb vs Sr (not shown).
Based on the Sr-isotopic variations between the basic (0.70349 to 0.70456) and felsic (0.70446 to 0.70783) volcanic rocks (Table 1; Fig. 5), it is possible that fractional crystallization might not be the only process responsible for the generation of felsic lavas.Even if the data appears to be explained well by fractional crystallization from a basaltic parent, the fractionation stage could be an open system, and hence some assimilation of crustal material is possible.The estimation of the extent of contamination by crustal material is complicated by the diversity shown by Arabo-African basement rocks (Hegner and Pallister, 1987;Davidson and Wilson, 1989;Möller et al., 1998).

© Mekelle University ISSN: 2073-073X 16
The Sr-Nd isotopic compositions of the felsic volcanic rocks from the rift margin also suggest that assimilation of crustal material has occurred may be limited.Initial Nd-isotopic ratios, which are insensitive to small degrees of contamination, are variable and range from 0.51276 to 0.51237.Hence, we favor fractional crystallization from different basic parents, combined with assimilation of crustal materials, over a combined partial melt and fractionation origin for the felsic volcanic rocks.

Assimilation and Fractional Crystallization (AFC)
A conventional way of identifying crustal contamination in a suite is to show that 87 Sr/ 86 Sr and 143 Nd/ 144 Nd initial ratios vary systematically (increase and/or decrease) with increasing degree of differentiation.Using SiO 2 as a fractionation index, most of the felsic rocks here show an overall increase in Sr-isotopic ratio (Fig. 7b).Most intermediate samples from the Wechecha and Boseti units exhibit lower Nd-isotopic ratios than the highly felsic samples, suggesting higher rates of contamination occurred in the intermediate lithotypes (Kabeto et al., 2001).The felsic volcanic samples from the study area apparently plot along AFC trend (Fig. 7a), with some scatter.
Figure 7. Plots of (a) initial Sr-and (b) Ndisotopic ratios against SiO 2 for the northern MER volcanic rocks.The felsic rocks plot on two apparent AFC trends.Apparent AFC and differentiation trends (FC) are also shown.The Nd-isotopic ratios also show variations with SiO 2 (see text for discussion).Symbols as in Fig. 2.
Few mineral aggregates and resorbed feldspar with melt inclusions were observed during our petrographic investigation.
Although this could indicate magma mixing or assimilation, complete mixing can be excluded, because no straight line relationships exist on simple binary plots such as SiO 2 -MgO (Fig. 3).Therefore, we consider that magma mixing is a minor process in the genesis of the felsic volcanic rocks.
The D values used are similar to those in the compilation of Rollinson (1995).An example of AFC calculation performed from the average of the Sudanese upper crust (Davidson and Wilson,   1989, 87 Sr/ 86 Sr = 0.72797, Sr = 426 ppm, and 143 Nd/ 144 Nd = 0.51137, Nb = 9.6, Nd = 24 ppm) is shown in Fig. 8a and b, with basalt sample ET1602 taken as the basic parent.Assimilation of the assumed crustal rock by strongly differentiated trachytes and rhyolites at low mass assimilation to mass crystallization rates (R = 0.001-0.6),and moderate to high F values (> 0.1 on AFC curves) can produce the samples which exhibit high 87 Sr/ 86 Sr, low Sr (< 185 ppm), low 143 Nd/ 144 Nd and high Nb (> 200 ppm).
87 Sr/ 87 Sr ratios plotted against Sr concentration (Fig. 8a) clearly show the effects of AFC.The data describes a curved differentiation trend with a sharp inflection around the highly felsic compositions, reflecting the strong influence of plagioclase fractionation with concomitant decrease in Sr (Figs. 4 and 8).The hypothetical trend describes the general differentiation trend among the felsic rocks, along which Ni, Cr, CaO, and MgO contents broadly decrease.Such hypothetical curves were also tested in the Jebel Marra area of the Sudan (Davidson and Wilson, 1989) and were successfully applied to northern Kenyan felsic lavas (Kabeto et al., 2001), to examine the evolution of basaltic to trachytic and phonolitic magmas.
Relatively higher rates of assimilation are calculated for intermediate rocks (Fig. 8a and b).This may be explained by differentiation of intermediate magmas at deeper levels in the crust, where higher ambient wallrock temperatures and presence of hot basic magmas would facilitate higher rates of assimilation (Davidson and Wilson, 1989;Macdonald et al. et al., 1995;Panter et al., 1997).Furthermore, high rates of assimilation (0.3-0.6) are evident for most felsic volcanic rocks from the rift margin (Fig. 8b), indicating a greater degree of contamination than at the rift center.This is also in agreement with the thicker crust at the rift margin than at the rift center (Makris and Ginzburg, 1987).

CONCLUSIONS
Studies of the northern Main Ethiopian Rift (MER) volcanic rocks offer insight into the genetic relations of basic and felsic volcanic rocks, and establish that compositional contrasts occur in equivalent volcanic rocks at rift margin and center magma series within a single intraplate continental setting.Higher Al 2 O 3 , K 2 O, Zr, Nb, Sr, and Rb and lower CaO, CaO/Al 2 O 3 , and Zr/Nb concentrations in the rift margin felsic volcanic rocks erupted mostly in the Pliocene compared to the Quaternary equivalents in the rift center reflect inheritance from their basic parents.The spatial and temporal distinctions between the volcanic suites in the study area and their markedly different geochemistry are explained by evolution along separate magma trends.
Hence, alkaline basaltic melts produced by lower degrees of partial melting are a possible source for most of the felsic volcanic rocks at the rift margin.In contrast, transitional basaltic melts produced at high degrees of partial melting are thought to be the parent for the felsic volcanic rocks in the rift center.
Modeling of the geochemical variations suggests that crystal-liquid fractionation processes within the shallow reservoirs were dominant during most trachyte-rhyolitic production in the rift center, along with less well-developed AFC processes.AFC appears to play a greater role in the genesis of intermediate rift center rocks and felsic rocks at the rift margin.We consider this to partly be a function of depth of fractionation of the magmas, implying that intermediate and rift margin magmas differentiated at deeper levels, whereas the more felsic trachytes and rhyolites of the rift center originated at shallow crustal levels.This is also in agreement with known variation in crustal thickness, variation as thinner crust is present in the rift center than at the rift margin.

ACKNOWLEDGMENTS
We thank Prof. S. Iizumi for his help with isotope analyses and Dr. T. Bary for AFC model calculation and discussion.K. Kabeto acknowledges financial support from Japan Society for Promotion of Science (JSPS) during post doctoral research at Shimane University.

Figure
Figure 3. Plots of selected major elements (wt.%) against differentiation index (SiO 2 ) for northern MER volcanic rocks.Symbols as in Fig. 2.
P 2 O 5 and TiO 2 contents also tend to increase from basic to intermediate compositions (up to 55 wt.% SiO 2 ), and then sharply decrease up to 65 wt.%SiO 2 and remain flat (Fig.3).

Figure 4 .
Figure 4. Plots of selected trace elements (ppm) against differentiation index (SiO 2 wt.%) for northern MER volcanic rocks.The lines on the Zr, Nb, Sr and Rb versus SiO 2 illustrate the compositional contrasts between the volcanic rocks in the rift center and margins.The shaded area on the Zr and Nb plots indicate where samples from the Furi and Yerer Mountains overlap samples from the rift center.Symbols as in Fig. 2.

Figure 6 .
Figure 6.(a) Plot of CaO/Al 2 O 3 ratio against SiO 2 , showing the two apparent evolution trends for the northern MER.Most rift margin volcanic rocks plot at lower ratios.(b) Linear correlation between Zr and Nb contents.Samples generally plot along Zr/Nb = 5 and Zr/Nb = 7, which may indicate different sources or crustal input (see text for discussion).(c) Zr/Nb vs Zr plot for the volcanic rocks.Arrows show assumed AFC and differentiation trends (FC) for rift margin and center sequences from different basic parent.The shaded area on (b) and (c) indicate samples from Furi and Yerer Mountains overlapping the samples from the rift center (see text for discussion).Data for crustal rocks fromDavidson and Wilson (1989).Symbols as in Fig.2. .