Petrogenesis of Oligocene volcanic rocks of the Lake Tana area,Ethiopian large Igneous Province

Ayenachew Alemayhou Desta ? Asfawossen Asrat? Minyahl Teferi Desta

Abstract The Lake Tana area is located within a complex volcano-tectonic basin on the northwestern Ethiopian plateau.The basin is underlain by a thick succession of Oligocene transitional basalts and sub-alkaline rhyolites overlain in places, particularly south of the lake, by Quaternary alkaline to mildly transitional basalts, and dotted with Oligo-Miocene trachyte domes and plugs.This paper presents the results of integrated field, petrographic, and major and trace element geochemical studies of the Lake Tana area volcanic rocks, with particular emphasis on the Oligocene basalts and rhyolites.The studies reveal a clear petrogenetic link between the Oligocene basalts and rhyolites.The Oligocene basalts are: (1) plagioclase, olivine,and/or pyroxene phyric; (2) show an overall decreasing trend in MgO, Fe2O3, and CaO with silica; (3) have relatively low Mg#, Ni and Cr contents and high Nb/La and Nb/Yb ratios; and (4) show LREE enriched and generally flat HREE patterns.All these imply the origin of the Oligocene basalts by shallow-level fractional crystallization of an enriched magma sourced at the asthenospheric mantle.The Oligocene rhyolites: (1) are enriched in incompatible while depleted in compatible trace elements, P and Ti; (2)show a strong negative Eu anomaly; (3) contain appreciable amounts of plagioclase, apatite, and Fe-Ti oxides; and(4) show clear geochemical similarity with well-constrained rhyolites from the Large Igneous Province(LIP)of the northwestern Ethiopian plateau.Low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers explains the origin of these rhyolites.Our study further shows that the Oligocene basalts and rhyolites are co-genetic and the felsic rocks of the Lake Tana area are related differentiates of the flood basalt volcanism in the northwestern Ethiopian plateau.

Keywords Large igneous province ?Oligocene basalt ?Oligocene rhyolite ?Fractional crystallization ?Lake Tana

1 Introduction

Large igneous provinces (LIPs) are considered to be products of mantle plume impingement.They usually cover an area of >100,000 km2and a volume of>100,000 km3where ～75% of the volume could have been produced within a relatively short period of 1-5 Ma(Campbell 2005;Bryan and Ernst 2008).The Afro-Arabian Cenozoic volcanic activity represents one of the youngest LIPs.It resulted from the impingement of the Afar plume,subsequent continental breakup, and rifting along the East African Rift System (EARS) separating the Nubian and Arabian plates (Mohr 1983; Mohr and Zanettin 1988;White and McKenzie 1989; Menzies et al.1992; Baker et al.1996;Marty et al.1996;Hofmann et al.1997;Ukstins et al.2002; Kieffer et al.2004).The impingement of the Afar mantle plume on the Afro-Arabian lithosphere since the Late Eocene caused up to ～3 km high regional uplift and doming (Gani et al.2007; S?eng?r and Natal’in 2001)accompanied by extension and faulting that favoured rapid continental flood basalt (CFB) ascent and eruption.The CFB now constitutes the northwestern and southeastern Ethiopian highlands on either side of the Main Ethiopian Rift(MER)and the Afar Rift.The Eocene-Oligocene CFB volcanism was followed by Oligo-Miocene shield volcanism, forming prominent shields over the nearly flat-topped CFB volcanic plateau.Some of the major shield volcanoes on the northwestern Ethiopian plateau include the Choke,Guna, and Siemen shields (Kieffer et al.2004), which surround the Lake Tana basin (Fig.1).The CFBs on the northwestern Ethiopian plateau form a ～2-3 km thick succession of massive basalt flows, scoriaceous basalts,volcanic agglomerates, scoria, felsic volcanic products(trachyte and rhyolite flows and pyroclastics)and abundant paleosols separating various levels of the CFBs (Asrat 2017).

This CFB and shield magmatic activity erupted 350,000 km3of basaltic and rhyolitic lavas and pyroclastic products covering an area of at least ～600,000 km2(Baker et al.1996)in the horn of Africa(Ethiopia,Eritrea,Djibouti)and southwest Yemen and Saudi Arabia forming the Afro-Arabian continental LIP.However, recent estimates have doubled the total volume of the Afro-Arabian LIP volcanism erupted during the period 45-22 Ma to ～720,000 km3(Rooney 2017).

The Ethiopian LIP is characterized by bimodal(basaltic and rhyolitic)magmatism,that lacks intermediate products(Ayalew et al.2002; Ayalew and Ishiwatari 2011), and was mainly erupted within a ～1 Myr period between 30 and 29 Ma (Berhe et al.1987; Hofmann et al.1997;Rochette et al.1998; Ukstins et al.2002; Kieffer et al.2004).Most of the CFBs and shield volcanic rocks were formed during the Oligo-Miocene.However, some Eocene-age volcanic rocks are also recorded from the southwestern Ethiopian highlands (e.g., Ebinger et al.1993).The flood basalts in the northwestern Ethiopian plateau have been roughly classified based on their TiO2content as low-Ti (LT) and high-Ti (subdivided as HT1 and HT2 basalts) (Fig.1; Pik et al.1998, 1999).Beccaluva et al.(2009) and Natali et al.(2016) subsequently demonstrated that the HT basalts, mostly exposed close to the Main Ethiopian and Afar rifts, were sourced from mostly plume-metasomatized mantle sources, with the HT2 basalts generated above the sub-lithospheric mantle plume head while the HT1 basalts were generated towards the peripheries.In addition, localized picritic basalts are exposed near the Western Afar Rift margin (Beccaluva et al.2009; Desta et al.2014).The LT basalts mostly exposed away from the rift margin, on the other hand,originated from a mantle source that was not significantly metasomatized by a plume.

Fig.1 Schematic map of the northwestern Ethiopian plateau showing the locations of the Oligocene Low-Ti tholeiitic basalts (LT); High-Ti tholeiitic basalts (HT1); very High-Ti transitional basalts (HT2), the Oligo-Miocene shield volcanoes and Quaternary volcanic centers(adopted from Desta et al.2014).The Lake Tana study area (Fig.2) is also marked

Fig.2 Geological map of the Tana Lake area; location of sampling sites are marked and Ar-Ar dates from Prave et al.(2016) are reported

Fig.3 Exposures of Tana area volcanic rocks: a Oligocene basalt with megacrysts of olivine (Ol) and b plagioclase (Pl) near Hamusit;c volcanic ash at the top of the Oligocene basalt unconformably overlain by Quaternary basalt flow; d flow-banded rhyolitic lava;e Oligocene pumaceous ignimbrite; f trachytic plug around Addis Zemen; g vesicular, calcite and zeolite amygdaloidal Quaternary basalt; and h exposed scoria cone from the Bahir Dar area

Some petrological and geochemical data on the magmatic rocks of the Lake Tana area are available.The region has been investigated as part of regional-scale geophysical(e.g., Bastow et al.2008; Keranen et al.2009) and petrological-geochemical studies of the northwestern Ethiopian plateau(e.g.,Pik et al.1998,1999;Ayalew et al.2002; Ayalew and Yirgu 2003; Kieffer et al.2004; Krans et al.2018).In addition, some studies on the regional and local tectonic setting (e.g., Chorowicz et al.1998; Me`ge and Korme 2004;Hautot et al.2006;Sembroni et al.2016)and the geological evolution of the Oligocene (e.g., Prave et al.2016) and Quaternary volcanic rocks and mantle xenoliths (e.g., Abate et al.1998; Ferrando et al.2007)exist.

In this work,we present the results of an integrated field,petrological, and major and trace element geochemical study of a suite of volcanic rocks from the Lake Tana area.The Oligocene felsic and basaltic rocks are the focus of this study while data on the Oligo-Miocene trachytes and the Quaternary basalts and scoria are also presented.The study aims to constrain the magma sources and petrogenesis of the Oligocene volcanic rocks of the Tana area and discusses their origin within the context of the regional LIP formation.

2 Geological setting

The study area - the Tana basin that encompasses Lake Tana at its center - is located on the northwestern Ethiopian plateau(Fig.2).Lake Tana,with a diameter of 70 km,is the largest lake in the country.The basin forms a faulted depression between an erosional escarpment perched above the western Ethiopian and Sudan lowland plains to the west and a tectonic escarpment of plate margin above the Afar depression to the east (Chorowicz et al.1998).

The Lake Tana area is located at the center of the northwestern Ethiopian plateau straddling the boundary between the LT and HT basaltic suites.Earlier studies indicated that the CFB on the northwestern Ethiopian plateau erupted over a ～1 Myr period centered at around 30 Ma (Hofmann et al.1997), while most of the felsic volcanic rocks were extruded just after the eruption of the Oligocene flood basalts (Ayalew et al.2002).Recent geochronological data (Prave et al.2016) define four episodes of volcanism in the Lake Tana area: (1) flood basalt volcanism at ～34 Ma; (2) extensive felsic ignimbrites and rhyolites with a minimum volume of 2000-3000 km3at 31.1-30.8 Ma; (3) basaltic volcanism at 28.9-23.8 Ma;and (4) localized scoriaceous basalt volcanism at 0.033 ± 0.005 Ma.

The formation of Lake Tana has traditionally been attributed to the damming of the Blue Nile River by the Quaternary basaltic lava flows, in the southern part of the present-day Lake (Merla et al.1979; Poppe et al.2013), that erupted along a set of north-south faults forming the Lake Tana Rift (e.g., Mohr 1961; Kazmin 1979;Hofmann et al.1997).The Lake Tana basin has been considered a complex rift basin formed at the junction of structural grabens on the Ethiopian plateau (e.g.,Chorowicz et al.1998; Hautot et al.2006), where numerous radial and concentric dike swarms contemporary to basin formation have been identified (Chorowicz et al.1998; Me`ge and Korme 2004).Chorowicz et al.(1998)showed the Lake Tana basin is located at the junction of the buried Dengel Ber graben to the south, the erosionally exposed Gondar graben to the north, and the reactivated Debre Tabor graben to the east of the present-day Lake Tana.Chorowicz et al.(1998) further elaborated that the structural complex was active during the Oligocene flood basalt volcanism, while fault reactivation accompanied by predominantly basaltic volcanism occurred during the Late Miocene-Quaternary,with crustal subsidence centered on the modern morphologic basin.It has also been suggested that the Lake Tana rifting and magmatism occurred above the inferred western side of the Afar mantle plume-head(Chorowicz et al.1998), consistent with its location at the boundary between the LT and HT basalts (Pik et al.1998, 1999; Beccaluva et al.2009; Natali et al.2016).Prave et al.(2016) on the other hand favoured a predominantly volcanic origin for the basin.Prave et al.(2016)argued that the extensive felsic volcanism at ～31 Ma was the product of super-eruptions that created a 60-80 km diameter caldera marked by km-scale caldera-collapse fault blocks and a steep-sided, sediment-filled basin where the present-day Lake Tana is located (Fig.2).

3 Field occurrence and petrography

The Lake Tana basin exposes various lithologies including Oligocene basalts and rhyolites, Oligo-Miocene trachyte domes and plugs, and Quaternary basalts and scoria, as well as recent fluvio-lacustrine sediments (Fig.2).The basaltic lava flows (～34-24 Ma; Prave et al.2016) are exposed on the elevated ridges around Lake Tana.The felsic rocks(intercalated lapilli tuff,flow-banded,massive,and feldspar-phyric rhyolites) (～31 Ma; Prave et al.2016) are exposed in generally NE-SW-trending, elongated, low-elevation rhyolitic lava flows and domesintercalated with extensive pyroclastic flow deposits in the southwestern,northern and northeastern shores of the lake.The Oligocene felsic units are possibly present beneath the Quaternary basalt flows and scoria cones which are extensively exposed on the southern and southeastern shores of the lake (Fig.2).The Quaternary lava field extends several tens of kilometers to the south of the lake.Thick fluvio-lacustrine sediments cover the eastern,northern, and northwestern wetlands surrounding the lake(Fig.2).Several N-S-, NW-SE- and NE-SW-trending lineaments, normal faults, and associated joints are observed in the northern, eastern and southern parts of the basin as manifestations of the three structural grabens.

Based on the lithological associations and published ages (Prave et al.2016), the major volcanic rock units in the Tana basin are (1) the Oligocene basalt flows; (2) the Oligocene rhyolite lava flows and pyroclastic deposits; (3)the Oligocene-Miocene trachytic domes and plugs;and(4)the Quaternary basalts and scoria cones (Fig.2).

3.1 Oligocene basalt flows

Fig.5 Classification of the Tana area volcanic rocks using the Total Alkali-vs-Silica classification diagram (Le Bas et al.1986; Middlemost 1994);the dashed line represents the boundary between alkaline and sub-alkalic rocks from Irvine and Baragar (1971)

The Oligocene flood basalt unit forms horizontally stratified lava flows on the generally flat to gently-sloping plateau surrounding Lake Tana, except the immediate southern and southeastern parts of the lake where the flood basalt is mostly covered by Quaternary basalt flows.The Oligocene basalt is usually silicified and it is commonly affected by spheroidal weathering.Irregular and vertical to sub-vertical fractures commonly affect this unit.In some sections,the unit is an intercalation of dark grey,aphanitic,massive, and epidotized basalt with pahoehoe lava structures, interlayered pink volcanic agglomerate and breccia with sub-rounded pebbles, and some blocks of aphanitic and vesicular basalt, and pinkish red-weathered basalt containing calcite amygdales.It is also overlain by trachytes and rhyolites, and pyroclastic tuff is interlayered with basalt towards the top and bottom of the unit.The basalts are dark grey, aphanitic to porphyritic, and rarely vesicular olivine-plagioclase phyric, pyroxene-plagioclase phyric, olivine phyric, and pyroxene phyric.In places,megacrysts of olivine and plagioclase laths ranging in size from 0.3 to 5.0 cm are observed (Fig.3a and b).The vesicles are usually small and partially filled by silica,calcite, and zeolite amygdales.The flood basalts are unconformably overlain by Quaternary basalt flows, in places marked by a volcanic ash unit (Fig.3c).

Petrographic analysis shows that the porphyritic basalts contain olivine (10 vol%), clinopyroxene (15 vol%), and plagioclase (23 vol%) phenocrysts set in a groundmass composed of fine-grained inter-granular pyroxene, fine laths of plagioclase and opaque minerals(Fe-oxide),where plagioclase and pyroxene show sub-parallel orientation.Anhedral to subhedral olivine phenocrysts have been strongly altered to iddingsite, which is fused within plagioclase phenocrysts as inclusions (Fig.4a and b).

3.2 Oligocene rhyolitic flows and pyroclastic deposits

The rhyolitic lava and pyroclastic deposits are exposed as NE-SW- to N-S-trending elongated domes southwest and west of Lake Tana in the vicinities of Yismala,Kunzila and Dengel Ber, and to some extent east of the lake in the vicinity of Yifag,Addis Zemen,and south of the lake in the vicinity of the Zege peninsula (Fig.2).In the Yismala -Kunzila area, this unit is prominently exposed as～200 m-thick intercalations of flow-banded, columnarly jointed, strongly altered (to kaolinite in many places) light grey to pinkish rhyolite flows(Fig.3d)and friable,whitish,thinly bedded and laminated pyroclastic deposits(tuffs and volcanic ash), pumaceous, lithic fragment-rich ignimbrites(Fig.3e).The rhyolites contain coarse orthoclase, quartz,and mica crystals,while the ignimbrites are densely welded and contain vitrophyric fiamme and fine to coarse lithic fragments.

Fig.6 Major element variation diagrams of the Tana area volcanic rocks:SiO2 versus a Fe2O3;b MgO;c Al2O3;d CaO;e Na2O;f K2O;g TiO2 and h P2O5.Shaded areas are reference fields for Oligocene to Pliocene basalts from the northwestern Ethiopian plateau and adjoining rift margins(Feyissa et al.2017;Ayalew et al.2018),the rift margins of the southeastern Ethiopian plateau(Ayalew et al.2018;Nelson et al.2019)and the Bale mountains on the southeastern Ethiopian plateau(Nelson et al.2019);MER and Afar rift basalts(Rooney et al.2007;Ayalew et al.2016); and Oligocene to Miocene felsic volcanic from the southeastern Ethiopian plateau (Nelson et al.2019)

Petrographically, the rhyolite lava samples display hypocrystalline to vitrophyric and perlitic (glassy), and occasionally porphyritic textures with sanidine phenocrysts(Fig.4c).The dominant phenocrysts are sanidine (12 vol%), quartz (3 vol%), plagioclase (1 vol%), opaque minerals(2 vol%)and rarely apatite set in a groundmass of microlithic sanidine laths, volcanic glass and quartz.Subhedral quartz phenocrysts up to 0.34 mm in size have partially resorbed textures.Some spherulitic textures with fibrous alkali feldspar and quartz microphenocrysts radiating within a glassy groundmass are also observed.

3.3 Oligocene-Miocene trachytic domes and plugs

A trachyte dome is exposed at Gorgora, located along the northern shore of the lake (Fig.2).The dome drapes over the Oligocene basalt flows, and it is mostly covered by fluvio-lacustrine sediments.The trachyte flow is greenishgrey, fine to medium-grained, massive, cryptocrystalline,and locally porphyritic with sanidine phenocrysts set in a matrix with aegrine-augite and quartz.

In addition, several trachyte plugs are common in the Lake Tana basin.The ～70 m-high Addis Zemen trachytic plug,which extruded through the relatively flat topography underlain by the Oligocene basalts, is one such example(Fig.3f).The plug is irregularly jointed, being broader at the base and narrower at the top.It is composed of light grey to greenish grey, fine-grained trachyte containing sanidine phenocrysts in a fine-grained matrix.Petrographically, the trachytes show trachytic texture with phenocrysts of large tabular crystals of sanidine (26 vol%),plagioclase (4 vol%) showing oscillatory zoning, sodic pyroxene (1 vol%), and opaque minerals (1 vol%)embedded in a dominantly glassy groundmass with some microlithic sanidine, hornblende and opaque minerals(Fig.4d).

3.4 Quaternary basalt flows and scoria cones

The Quaternary basalts and associated scoria cones are prominently exposed in the immediate south and southeast shores of Lake Tana and extend for several tens of kilometers to the southwest.Dek Island, the largest island on Lake Tana,is also entirely built from the Quaternary basalt flows.The Quaternary basalts, which are massive lava flows to highly vesiculated scoriaceous basalt flows,occupy a generally low-lying, flat to gentle topography,covered by 5-10 m-thick regolith in most places.In other places, intact lava flow fronts form a slightly elevated topography above the Oligocene basalts.In such places,the basalts outcrop as small hollow tubes or blisters, and the basalt flows show the original ‘‘pahoehoe’’ and ‘‘aa’’ lava flow structures.The basalts are dark-grey to greenish-grey olivine phyric and pyroxene-plagioclase phyric flows,strongly vesicular to scoriaceous, with rounded to elongated vesicles, filled in most cases by silica, calcite, and zeolite amygdales(Fig.3g)up to 6 cm diameter that shows strong alignment or elongation parallel to the flow layering.The basalt flows become less vesicular and more massive towards the bottom of the layering.Petrographically, the Quaternary basalt flows show typical porphyritic and in some cases glomeroporphyritic textures with larger euhedral to anhedral plagioclase laths (40 vol%), pyroxene (10 vol%) and olivine (7 vol%) phenocrysts set in a matrix containing plagioclase laths and olivine (Fig.4e g).

Some scoria cones are scattered immediately south of Lake Tana on top of the Quaternary basaltic lava flows,and show a general NE-SW-trending regional alignment.One of the prominent islands, Daga Estifanos, is a scoria cone.The scoria cones have moderate to steep slopes that preserve the original volcanic morphology, however, they very often have a half breached morphology and most are quarried (Fig.3h).The craters range in diameter from a few tens of meters to ～200 m.They are made up of horizontally stratified scoria falls, and the pyroclasts are variously coloured ranging from black to red, are loosely compacted varying from lapilli to bomb size in a variety of spherical to elliptical shapes including rod-like and spindle-like and contain some rock fragments of pebble to cobble size.In some localities, the pyroclastic bombs contain ultramafic xenoliths (or mantle nodules of olivine crystals and ultramafic rock inclusions consisting of pyroxene, olivine, and plagioclase aggregates).Petrographically, the scoria clasts are composed of microphenocrysts of plagioclase, 0.3-1 mm euhedral to anhedral olivine (6 vol%), and pyroxene (10 vol%) set in a groundmass composed of acicular plagioclase, olivine,clinopyroxene, and opaque minerals.

4 Analytical methods and results

4.1 Analytical methods

Fig.7 Incompatible trace element variation diagrams of the Tana area volcanic rocks:SiO2 versus a Sr;b Ba;c La;d Y; e Nb;f Ta;g Zr and h Hf.Shaded areas are reference fields for Oligocene to Pliocene basalts from the northwestern Ethiopian plateau and adjoining rift margins(Feyissa et al.2017;Ayalew et al.2018),the rift margins of the southeastern Ethiopian plateau(Ayalew et al.2018;Nelson et al.2019)and the Bale mountains on the southeastern Ethiopian plateau(Nelson et al.2019);MER and Afar rift basalts(Rooney et al.2007;Ayalew et al.2016);and Oligocene to Miocene felsic volcanic from the southeastern Ethiopian plateau(Nelson et al.2019).Note that a logarithmic scale is used on the Y-axis(Hf concentration)of Figure h to show the low concentration of the Hf values of the Tana area volcanic rocks compared to the values of the reference rocks

Fig.8 Incompatible versus incompatible trace element diagrams of the Tana area volcanic rocks:Zr versus a Ba;b Sr;c La;d Yb;e Nb and f Ta

Twenty-two rock samples have been collected for petrographic and geochemical analyses during a detailed geological mapping of the Lake Tana area (Fig.2).The selected rock samples represent rocks distributed in the mapped area.Whole rock geochemistry was determined at the Analytical Testing Services (ALS, Ireland), using Multi-Element Inductively Coupled Plasma-06 - MEICP06 (major elements) and Multi-Element Mass Spectrometry-81 - ME-MS 81 (trace elements).Trace element solutions were prepared using three digestions, i.e., a lithium borate fusion for resistive elements, a four-acid digestion for base metals, and an aqua regia digestion for volatile Au-related trace elements (ALS 2017).A 0.200 g sample was added to a lithium metaborate flux,mixed,and fused in a furnace at 1000 °C, and the resulting melt was cooled and dissolved in 100 mL of 4% HNO3/2% HCl solution.The extracted ion solution was analyzed by 7700x Agilent ICP-MS for 31 trace elements including the rare Earth elements (REE), and by ICP-AES for the remaining elements.The results were finally corrected for spectral inter-element interferences.Major element oxide concentrations were calculated from the elemental analysis(Supplementary Table 1).Analytical precisions are estimated at 2%for major elements,and 5%and 10%for trace element concentrations higher or lower than 20 ppm,respectively.The detection capacity of the methods is 0.01-100% for major elements and 0.01-10,000 ppm for most trace elements.

4.2 Major element variations and rock classification

Major element compositions are given in Supplementary Table 1.The samples generally have low LOI values ranging from 0.3 to 2.3 wt%.Except for one Oligocene basalt sample which shows high alkaline content, the majority of the Oligocene and Quaternary basalts are transitional and stradle the boundary between alkaline and tholeiitic fields of the Total Alkali - Silica (TAS)classification diagram (Le Bas et al.1986; Middlemost 1994; Fig.5).The majority of the felsic samples are subalkaline rhyolites,while two samples are alkaline trachytes(Fig.5).The total alkalis to Al2O3ratio (Le Bas et al.1986)ranges from 0.4 to 0.9 and shows that all of the Tana felsic rocks are sub-alkaline.

Fig.9 Primitive mantle-normalized trace element patterns of the Tana volcanic rocks: a Oligocene basalts; b Oligocene rhyolites; c Oligo-Miocene trachytes; and d Quaternary basalts.Normalizing values are from McDonough and Sun (1995).Shaded areas are reference fields for a Oligocene to Pliocene basalts from the northwestern Ethiopian plateau and adjoining rift margins(Feyissa et al.2017;Ayalew et al.2018),the rift margins of the southeastern Ethiopian plateau(Ayalew et al.2018;Nelson et al.2019)and the Bale mountains on the southeastern Ethiopian plateau(Nelson et al.2019);b Oligocene to Miocene felsic volcanic from the Ethiopian plateau(Ayalew and Yirgu 2003);c Miocene to Pliocene trachytes and phonolites from the Northern Ethiopian plateau at Axum(Hagos et al.2010);and d MER and Afar rift basalts(Rooney et al.2007;Ayalew et al.2016)

Major elements show consistently similar variations for both the Oligocene and Quaternary basalts (Fig.6).The SiO2(43.0-50.0 wt%), Fe2O3(11.0-16.0 wt%) and MgO(4.1-10.0 wt%) contents of these basalts are quite variable with Mg# ranging between 35 and 65.On the other hand,there are narrower ranges in their Al2O3(14.0-18.0 wt%),CaO (7.8-11.0 wt%), Na2O + K2O (3.5-6.0 wt%), TiO2(1.4-3.7 wt%) and P2O5(0.30-0.80 wt%) contents.Reference fields for Oligocene to Pliocene basalts from the northwestern and southeastern Ethiopian plateaus (data from Feyissa et al.2018; Ayalew et al.2018 and Nelson et al.2019) and the MER and the Afar rifts (data from Rooney et al.2007 and Ayalew et al.2016)are also shown(Fig.7).The Tana area Oligocene and Quaternary basalts largely fall within or close to the reference fields for all the major elements.There is also significant overlap between the Oligocene and Quaternary basalts with the reference fields (plateau and rift basalts).

There is a marked silica gap between the basaltic and felsic rocks, amounting to 16.0 wt% between the basalts and trachytes and 23.0%between the basalts and rhyolites.The trachytes and rhyolites similarly show a significant gap in their silica contents (～8.0 wt%).The trachytes and rhyolites respectively, show clustering in their SiO2(66.0-67.0 wt% and 73.0-77.0 wt%), and total alkali(Na2O + K2O)contents(11.0-13.0 wt%and 7.7-8.8 wt%)as well as all the other major elements, except for the variable Al2O3(14.0 and 17.0 wt%) contents of the trachytes (Fig.6 and Supplementary Table 1).The Tana area Oligocene rhyolites are more felsic than the reference values representing the felsic volcanic rocks from the southeastern Ethiopian plateau (yellow shaded area in Fig.7; data from Nelson et al.2019).

Fig.10 Chondrite-normalized REE patterns of the Tana volcanic rocks:a Oligocene basalts;b Oligocene rhyolites;c Oligo-Miocene trachytes;and d Quaternary basalts.Normalizing values are from Sun and McDonough (1989).Shaded areas are reference fields for a Oligocene to Pliocene basalts from the northwestern Ethiopian plateau and adjoining rift margins(Feyissa et al.2017;Ayalew et al.2018),the rift margins of the southeastern Ethiopian plateau(Ayalew et al.2018;Nelson et al.2019)and the Bale mountains on the southeastern Ethiopian plateau(Nelson et al.2019);b Oligocene to Miocene felsic volcanic from the Ethiopian plateau(Ayalew and Yirgu 2003);c Miocene to Pliocene trachytes and phonolites from the Northern Ethiopian plateau at Axum(Hagos et al.2010);and d MER and Afar rift basalts(Rooney et al.2007;Ayalew et al.2016)

Although the basalts show clustering with no clear trends in their major element contents against silica(Fig.6),there is a clear fractionation trend with increasing silica within the felsic rocks and from the basaltic to the felsic rocks.There is a marked decrease in Fe2O3, MgO,CaO, and TiO2, and a marked increase in K2O with increasing silica, while Al2O3and Na2O show generally subdued decreasing and increasing trends, respectively.

4.3 Trace element variations

Binary diagrams of selected generally incompatible elements(such as La,Nb,Zr)versus silica(Fig.7)and mildly compatible (such as Ba, Sr) and incompatible elements versus a strongly incompatible element (Zr) (Fig.8) are plotted to show the evolution of the trace elements along magma differentiation lines.Though the basalts show a general clustering of the trace elements owing to their limited silica variation, there is a discernible decreasing trend in Sr and an increasing trend in Ba with increasing silica (Fig.7).On the other hand, the trachytes and rhyolites show an appreciable range in Ba and the strongly incompatible elements (Y, Zr, Hf, and Nb) with no clear trend, while Nb and Ta do not display distinctive trends.The Tana area Oligocene and Quaternary basalts largely fall within or close to the reference fields for the plotted trace elements,though the Tana area basalts have generally more clustered and lower concentrations of the incompatible trace elements particularly Zr and Hf (Fig.7).

Fig.11 Zr versus a Ti, b Sm, and c Gd variation diagrams of the Tana basalts

The basalts show a strong clustering in the incompatible trace element variation diagrams (Fig.8), while the trachytes and rhyolites show some discernible increasing trends in La, Yb, Nb, and Ta and a decreasing trend in Ba with increasing Zr.

4.4 Multi-element and rare Earth element (REE)variations

The primitive mantle-normalized multi-element variation diagrams(Fig.9) show parallel to sub-parallel patterns for each group of samples.Reference fields from a range of Oligocene to Quaternary volcanic rocks from the Ethiopian plateau, the MER, and the Afar rift are added in the variation diagrams.These reference fields indicate that the Tana volcanic rocks show consistent element variations and trends with the corresponding reference rocks.The basalts (Fig.9a and d) show a slightly decreasing trend from the incompatible, light lithophile elements (LILE) to the more compatible high field strength elements (HFSE),with some samples showing a slight positive anomaly for K and Ti.The Quaternary basalts (Fig.9d) show a nearly homogenous trend, whereas the Oligocene basalts are slightly more scattered (Fig.9a).The trachytes and rhyolites,on the other hand,show a higher concentration of the incompatible LILE in comparison with the more compatible HFSE (Fig.9b and c).In addition, the trachytes and rhyolites show a marked trough in Ba,Sr,P,and Ti,while the rhyolites also show an additional trough in Eu.

The chondrite-normalized REE patterns (Fig.10) show a tight clustering of the trends in each group of samples,where both the basalts and felsic rocks show uniform slopes with smooth parallel trends from the light REE(LREE) to the heavy REE (HREE).The basalts (Fig.10a and d) show a slight enrichment in the LREE compared to the HREE.The rhyolites and trachytes have slightly more enriched REE contents in comparison with the basalts but they similarly show LREE enrichment compared to the HREE (Fig.10b and c).The rhyolites show a slight to strong negative Eu anomaly.The Tana volcanic rocks show generally consistent rare Earth element variations and trends with the corresponding reference rocks, though the Oligocene basalts and rhyolites are slightly LREE depleted compared with the reference rocks (Fig.10a and b).

Fig.12 Ti/Y versus Nb/Y variation diagram of the Oligocene basalts of the Tana area; line boundaries for the Low-Ti (LT), High-Ti 1(HT1),and High-Ti 2(HT2)basalts from the northwestern Ethiopian plateau are based on REE data from Pik et al.(1999).The Quaternary basalts are also plotted for comparison.Shaded areas are reference fields for Oligocene to Pliocene plateau basalts from the northwestern Ethiopian plateau and adjoining rift margins (Feyissa et al.2017;Ayalew et al.2018), the rift margins of the southeastern Ethiopian plateau (Ayalew et al.2018; Nelson et al.2019) and the Bale mountains on the southeastern Ethiopian plateau(Nelson et al.2019);and MER and Afar rift basalts (Rooney et al.2007; Ayalew et al.2016)

5 Discussion

5.1 Element mobility

Element mobility due to post-magmatic alteration should be characterized and any petrogenetic interpretations should rely on unaltered or least altered samples(Polat et al.2003;Polat and Hofmann 2003).The Tana volcanic rocks are relatively young(Oligo-Mioceneand Quaternary)withnoobvious postmagmatic deformation and alteration.The LOI values of the Tana volcanic rocks are generally low (＜2.0 wt%) in most samples (Supplementary Table 1), which could be a rough approximation of the low degree of alteration of the rocks.In addition,most samples show parallel to sub-parallel patterns on the primitive mantle-normalized multi-element diagrams,with no Ce anomaly(Fig.9),which were considered as some of the criteria to identify the least altered mafic rocks by Polat and Hofmann(2003).Moreover,on Zr versus Ti,Sm,and Gd diagrams the Tana area basalts display strong correlations(Fig.11)indicating low mobility of these elements and their insignificant disturbance by alteration (Polat and Hofmann 2003;Polat et al.2003).

5.2 Magma source

Fig.13 La/Yb versus Nb/La ratio diagram to discriminate the magma source of the Tana basalts.The boundaries are from Smith et al.(1999)

The Oligocene basalts of the Tana area plot in the High-Ti 1(HT1)and Low-Ti(LT)fields defined by the Ti/Y versus Nb/Y variations,except for one sample that has High-Ti 2(HT2)affinities (Pik et al.1998, 1999; Fig.12).Additional data from Oligocene and Pliocene basalts from the northwestern Ethiopian plateau and rift margins (Feyissa et al.2017;Ayalew et al.2018;Nelson et al.2019)and from Miocene to Quaternary basalts from the MER and the Afar rift(Rooney et al.2007;Ayalew et al.2016)are also included.These plots(Fig.12)show that most of the Oligocene plateau basalts fall in the HT1 and some in the HT2 fields of Pik et al.(1998;1999) consistent with their location east of the Lake Tana area,i.e.,east of the LT-HT boundary.

The geochemical characteristics of LT and HT1 lavas are consistent with shallow-level fractionation of plagioclase and various amounts of olivine and clinopyroxene(Pik et al.1998).The fractionating mineral assemblages common in these basalts are compatible with the sequence of fractionation and phase proportions observed experimentally at low pressure for similar basalt compositions(Cox and Bell 1972).On the other hand, the HT2 basalts are mainly transitional to alkaline and sourced from the highly metasomatized mantle that experienced high-temperature adiabatic decompression melting in the garnetperidotite stability field (Ayalew and Gibson 2009; Beccaluva et al.2009; Desta et al.2014).

Fig.14 La/Yb vs.Dy/Yb ratios of the Oligocene and Quaternary basalts of the Tana area.The fields of Oligocene basalts from the northwestern Ethiopian plateau (data from Pik et al.1999; Kieffer et al.2004;Beccaluva et al.2009)as reported in Ayalew et al.(2018)and those of Pliocene flood basalts of the eastern and western walls of the MER(data from Ayalew et al.2018)are included for comparison.All model conditions and source rock compostion are as reported by Ayalew et al.(2018) who used a non-modal batch melting model(Shaw 1970) using La/Yb and Dy/Yb ratios; the composition and modal mineralogy of mantle xenoliths sampled from beneath the MER used as source (Beccaluva et al.2011; Bianchini et al.2014;Meshesha et al.2011), and mineral-melt partition coefficients are adopted from Jung et al.(2012).Numbers on model curves indicate the percent melting

The Nb/La ratio(Fig.13)is used to decipher the magma source (Smith et al.1999) of the basalts.Nb and Ta are depleted in the lithospheric mantle compared to LREEs implying the Nb/La ratio is low in the lithospheric mantle(＜0.5) and higher (>1) in the asthenospheric mantle.Generally, the Oligocene (and Quaternary) basalts have a high Nb/La ratio, suggesting an enriched asthenospheric mantle source, however, lithospheric - asthenospheric mantle interaction might have been significant(Fig.13).In addition,the gentle slope in HREE(see Fig.10)supports a garnet free source for the basalts (Wilson 1989).Furthermore, the La/Yb vs.Dy/Yb ratio of the Oligocene and(Quaternary) basalts superimposed on a non-modal batch melting model of the mantle by Ayalew et al.2018(Fig.14) shows that the Oligocene (and Quaternary)basalts of the Tana area largely overlap with the Oligocene flood basalts of the northwestern Ethiopian plateau and Pliocene flood basalts from the eastern and western walls of the MER,and generally align along the melting curve of the amphibole-spinel peridotite.However, the Tana area basalts show some variation in Dy/Yb (1.7-2.9) similar to other Oligocene flood basalts from northwestern Ethiopia,indicating that amphibole-garnet peridotite might have played some role in magma generation of these flood basalts (Ayalew et al.2018).Ayalew et al.(2018) further argued that the presence of a small proportion of melt from the garnet-facies mantle may indicate the generation of melt from an ascending deeper mantle material at low pressure due to lithospheric extension.

Fig.15 SiO2 versus Th/Yb diagram to discriminate the dominant process forming the Tana area basalts.The boundaries are from Pearce (2008)

Although it has been previously argued that the metasomatized lithospheric mantle did not contribute significantly to the formation of the Oligocene basalts in the northwestern Ethiopian plateau (Kieffer et al.2004),recent studies (e.g., Beccaluva et al.2009, Natali et al.2016, 2017) have shown that most plume-metasomatized mantle sources are responsible for the formation of the HT1 and HT2 basalts of the northwestern Ethiopian plateau.These studies further indicated that the HT2 basalts are generated above the sub-lithospheric mantle plume head, while the HT1 basalts are generated towards the peripheries.The LT basalts are sourced from the least plume-metasomatized mantle.Furthermore, the isotopic ratios of Sr,Nd,and Pb exhibit two distinct magma sources for the petrogenesis of the northwestern Ethiopian plateau basalts, where the HT2 basalts correspond to an OIB-like mantle component while the LT basalts show the melting of a depleted mantle constituent (Pik et al.1999).Moreover, Natali et al.(2016) suggested that the Ethiopian-Yemeni LIP is the result of a hot mantle plume that entrained eclogitic components that led to a plume head with decreasing thermo-chemical effects over a radial extension of ～400 km.The regional LT/HT1 boundary passes across the Tana basin (see Fig.1) and the distribution of the majority of the Oligocene basalts in the LT and HT1 fields (Fig.12) confirms the validity of this boundary(Pik et al.1998, 1999) between the peripheral HT1 Oligocene basalts to the east of Lake Tana and the LT basalts to the west of the Lake.

5.3 Petrogenesis of the Oligocene basalts and rhyolites

Fig.16 The two-proxy (crustal input vs.residual garnet) diagrams after Pearce et al.(2021) combining TiO2/Yb against Nb/Yb a and Th/Nb against TiO2/Yb b for discriminating magma sources of LIP basalt suites through a plume- versus lithospheric-influenced derivation.SZLM: Subduction Modified Lithospheric Mantle; OPB:Oceanic Plateau Basalt; MORB- Mid-Oceanic Ridge Basalt; OIB:Oceanic Island Basalt;IAB:Island Arc Basalt;EM:Enriched Mantle Sources

The incompatible trace element ratios Th/Yb and Nb/Yb are considered to be important proxies for magma sources and crustal contamination (Pearce 2008).Though the Oligocene basalts of northwestern Ethiopia have been considered to be derived through an upwelling mantle plume and considered to be prone to crustal contamination (Pik et al.1999;Kieffer et al.2004;Meshesha and Shinjo 2007),the silica versus Th/Yb diagram (Fig.15) shows that crustal contamination was insignificant during the fractionation of the Oligocene (and Quaternary) basalts of the Tana area.The Oligocene (and Quaternary) basalts have Nb/Ta (16.5) and Zr/Hf (37.1) ratios typical of mantlederived magmas with insignificant contamination (McDonough and Sun 1995).

In addition, the LIP discrimination diagrams of Pearce et al.(2021) indicate that the Ologicene (and Quaternary)basalts plot within the plume array (Fig.16a and b).This explains the melting of an asthenosphere (mantle plume)source with insignificant interaction with the lithosphere or metasomatized mantle.Primary magma compositions plotting in the OIB-OPB segment of the plume array can be explained by the occurrence of little to no deep melting in the garnet stability field (Pearce 2008; Pearce et al.2021).

The Oligocene (and Quaternary) basalts have relatively low Ni (＜175 ppm) and Cr (＜400 ppm) contents,implying shallow-level fractionation of olivine ± clinopyroxene, the dominant phenocrysts observed in these basalts.The Mg# (35-65) of the Oligocene basalts are also lower than those of primary basaltic magmas from the mantle (Wilson 1989), suggesting the fractionation of enriched mantle magmas to form these basalts.The overall decreasing trend in MgO, Fe2O3, CaO(though the slight variation in SiO2content leads to clustering of the samples),as well as Cr and Ni,with increasing SiO2also supports the fractionation of ferromagnesian(olivine ± clinopyroxene) minerals.

The enrichment of the Oligocene rhyolites in the incompatible trace elements (such as Rb, Th, LREE, Nb,Ta, Zr, and Hf) and marked troughs in the slightly compatible trace elements (such as Ba and Sr) as well as the compatible P and Ti (see Fig.9b) attest to their formation by fractionation of plagioclase, apatite, and Fe-Ti oxides(Rollinson 1993).This is supported by the presence of an appreciable amount of these minerals in the studied rhyolite samples, which implies the differentiation of basaltic magma(s).The markedly negative Eu anomaly (see Fig.10b) in the rhyolites further suggests late-stage fractionation of plagioclase (Hanson 1980).In addition, the inflected trend of P2O5and Al2O3from the basalts to the rhyolite samples indicates late-stage fractionation of apatite and plagioclase (Hiluf and Asrat 2021).Furthermore, the Oligocene rhyolites of the Tana area show comparable Th(5.1-43.0 ppm)and Nb(27.2-153 ppm)contents as well as Rb/Nb(0.53-2.63)and La/Nb(0.12-1.79)ratios with those of the Lima Limo, Wegel Tena and Aiba rhyolites, which have been modeled to originate from low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers, with non-existent or limited amounts of crustal contamination(Ayalew and Yirgu 2003;Hiluf and Asrat 2021).Furthermore,radiogenic Sr isotopic ratios of rhyolites from the northwestern Ethiopian plateau indicate insignificant crustal contamination (Ayalew et al.2002).This is in line with many studies(e.g.,Ayalew et al.2002; Ayalew and Yirgu 2003; Ayalew and Gibson 2009;Natali et al.2011) which indicated that the northwestern Ethiopian plateau rhyolites evolved from the associated basalts through fractionation processes.

In conclusion,the clear stratigraphic intercalation of the Oligocene basalts and rhyolites of the Tana area and their presumed co-genesis from a common magma source by shallow-level fractional crystallization, as well as their compositional similarity to the rhyolites in the wider CFB of the northwestern Ethiopian plateau,strongly suggest the formation of the rhyolites as part of the flood basalt volcanism.However, detailed geochronologic and radiogenic isotopic investigations to constrain the sequence of the basaltic and rhyolitic successions are required to be conclusive on the exact petrogenetic relationship between the various levels of the Oligocene basalts and rhyolites of the Tana area.

6 Conclusions

Field, petrographic as well as major and trace element geochemical investigation of the Tana Lake area reveals a complex volcano-tectonic basin where a thick succession of Oligocene alkaline basalts and sub-alkaline rhyolites of the Ethiopian CFB are overlain in places,particularly south of the lake, by Quaternary alkaline to mildly transitional basalts.

The petrographic association (plagioclase, olivine, and pyroxene phenocrysts) and the geochemical signatures(including an overall decreasing trend in MgO, Fe2O3,and CaO with silica, high Nb/La and Nb/Yb ratios, LREE enriched and generally flat HREE patterns, relatively low Mg#, Ni, and Cr contents) of the Oligocene basalts imply that they originated by shallow-level fractional crystallization of an enriched magma sourced from the asthenospheric mantle with possible lithospheric - asthenospheric mantle interaction.This is generally consistent with the distribution of the LT and HT1 Oligocene basalt samples of the Tana area within the presumed zone of the LT and HT1 basalts of the CFB.Irrespective of their age difference,the Quaternary basalts of the Tana area show similar petrographic associations and geochemical signatures with those of the Oligocene basalts,suggesting their possible common source.

The Oligocene rhyolites of the Tana area are markedly enriched in incompatible elements and depleted in compatible trace elements, P and Ti, show strong negative Eu anomalies, and contain an appreciable amount of plagioclase,apatite,and Fe-Ti oxides,suggesting their formation by late-stage fractionation of these minerals from basaltic magma.This is supported by the geochemical similarity of the Oligocene rhyolites of the Tana area with well-constrained rhyolites from the CFB of northwestern Ethiopia,which have evolved by low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers.

Through detailed geochronological and radiogenic isotopic studies that are required to ascertain the specific cogenetic basaltic-felsic layers in the succession, the Oligocene basalts and rhyolites of the Tana area show clear geochemical links suggesting their common magma source and their co-evolution by shallow-level fractional crystallization.Therefore, the felsic rocks of the Tana area form an integral part of the flood basalt volcanism in the northwestern Ethiopian plateau.

Supplementary InformationThe online version contains supplementary material available at https://doi.org/10.1007/s11631-023-00634-6.

AcknowledgementsThis work has been conducted as part of the MSc thesis of AAD.Bahir Dar University is acknowledged for supporting AAD to conduct his MSc at Addis Ababa University under the supervision of AA.Fieldwork and analytical works have been partly funded by Addis Ababa University and the School of Earth Sciences of Bahir Dar University.The critical reviews of Paul Sotiriou and four anonymous reviewers were very helpful in improving our manuscript.

Declarations

Conflict of interestThe authors declare no conflict of interest.