June 2020 LIP of the Month

The Cenozoic Magmatism of East Africa – a Brief Summary

Tyrone O. Rooney

Dept. of Earth and Environmental Sciences, Michigan State University, East Lansing, MI 48823, U.S.A.  Email: rooneyt@msu.edu


This contribution is a directed summary of a substantial body of work published in 2017 (Rooney, 2017) and 2020 (Rooney, 2020a, 2020b, 2020c, 2020d), and by necessity excludes much of the fascinating history of East Africa synthesized in that work. To gain more appreciation for this history, the interested reader is directed to the series of five volumes cited above where the Cenozoic Magmatism of East Africa is explored, thoroughly.

I am writing this article for ‘LIP of the Month’, thus my initial task is defining the spatial and temporal extent of the Large Igneous Province (LIP) within East Africa. Compilations of global LIPs typically focus upon the Oligocene-aged flood basalts that blanket the northwest Ethiopian Plateau and continue across the Red Sea into Yemen. This ‘African-Arabian LIP’ is thus spatially and temporally distinct from the younger East African Rift System, which dissects these earlier flood basalts and exhibits magmatism that is more focused within the developing rift valley. This commonly held perception of the spatial and temporal development of magmatism in East Africa is wrong.

Magmatism, rifting, and thermo-chemical anomalies

One of the more important outcomes of the synthesis series alluded to above is the recognition of a shared ‘plume’ component evident in the radiogenic isotope geochemistry of lava suites erupted throughout East Africa (Rooney, 2020d). This observation redefines: (1) the geographic extent of the LIP as extending throughout the footprint of the East African rift, (2) the temporal extent of the LIP as continuing throughout the Cenozoic. Such a temporally and spatially extensive LIP might be considered problematic if attributed to a singular mantle ‘plume’, however, our understanding has evolved as to how reservoirs other than the depleted upper mantle contribute to magmatism in this region (Barrat et al., 1998; Deniel et al., 1994; Feyissa et al., 2019; Furman et al., 2006; Hart et al., 1989; Meshesha and Shinjo, 2008; Rooney et al., 2012a; Schilling et al., 1992). The recognition of two large low shear velocity provinces (LLSVP) located at the core-mantle boundary beneath the Pacific Ocean and African continent (Garnero et al., 2016; Garnero and McNamara, 2008; Ritsema et al., 2011) has transformed our understanding as to how material may transfer from the deep mantle to the surface (e.g., Bastow et al., 2010; Mulibo and Nyblade, 2013). The synthesis series contends that the common occurrence of the same “C”-like isotopic signature (Hanan and Graham, 1996), which is best defined in Afar but is evident throughout East Africa, reflects mass contributions from the African LLSVP to the East African upper mantle.

The broad contribution from the African LLSVP to the East African upper mantle clarifies the observation of 3He/4He values that have been influenced by an undifferentiated mantle reservoir extending from Rungwe in the south, to Afar in the north (Fig. 1) (Halldórsson et al., 2014; Hilton et al., 2011; Marty et al., 1996; Nelson et al., 2019; Rooney et al., 2012a). Further support of the LLSVP model is evident from multiple studies that report a widespread elevated mantle potential temperature recorded in lavas from throughout East Africa. These studies show that mantle potential temperatures (TP) are most elevated above ambient upper mantle in lavas associated with the Eocene-Oligocene flood basalt episode (up to 1520°C), but critically, TP has remained elevated until the present day (Rooney et al., 2012b; Ferguson et al., 2013; Armitage et al., 2015). Collectively, the extant geophysical and geochemical evidence point to the same conclusion – East Africa has been, and continues to be, influenced by material upwelling from the African LLSVP.

Figure 1. Generalized map of the main rock units of East Africa showing basement age (yellow and pink) and the distribution of Cenozoic magmatic activity (black). The Tanzania craton is shown as a white shaded area. Cretaceous rifting is shown as a fine hatching and is typically NW-SE orientated; Cenozoic rifting is shown in coarser hatching and is broadly N-S orientated. Outcrop shapes are derived from the USGS Surficial Geology of Africa (geo7_2ag), rifts are from Purcell (2018), the extent of the craton is taken from Foley et al. (2012).

The origin of Cenozoic magmatism in East Africa is not, however, entirely controlled by contributions from deep mantle reservoirs. When considered in the context of other plume-influenced regions, the positive anomalies in mantle potential temperature and 3He/4He excesses over depleted mantle are modest. This points to an upper mantle beneath East Africa that is plume-influenced, and not plume-dominated. This conclusion complicates efforts to attribute magma generation to upwelling from the LLSVP and instead points to the continental lithosphere as central to the distribution and composition of magmatism in East Africa (Rooney, 2020a, 2020b). Decompression of the plume-influenced East African upper mantle by thinning of the continental lithosphere dominates both the generation mechanisms and distribution of magmatism in the region. The interaction of these melts with metasomatically-enriched continental lithospheric mantle profoundly influences the composition of the erupted lavas (Rogers et al., 1992, 1998; Nelson et al., 2019; Rooney, 2020d). Therefore, describing the processes that resulted in the formation of the East African LIP requires not only a focus on how deep upwellings influenced the intensive and extensive parameters of melting, but also how the development of the East African Rift destabilized the African lithosphere.

Lava compositions provide insight into the relative contributions from lithospheric and sub-lithospheric reservoirs to magmatism in East Africa as the LIP develops. Throughout East Africa, six magma families have been identified (Table 1) that provide insight into the sources of magmatism in the region (Fig. 2). Initial magmatic events associated with the flood basalts typically resulted in the eruption of lavas with compositions from Types 1, 2, and 3. These lava types suggest the flood basalts derive from the melting of an upwelling mantle thermo-chemical anomaly, combined with contributions from a metasomatized lithospheric mantle that may or may not have become detached (Baker et al., 1996; Beccaluva et al., 2009; Furman et al., 2016; Kieffer et al., 2004; Natali et al., 2016; Pik et al., 1999).

Magma Type



I – Incompatible element depleted family of magmas

Extreme depletions in most elements forming a relatively flat incompatible trace element normalized pattern. Subtype Ia exhibits positive anomalies in LILE (e.g. Ba).

Associated with Eocene and Oligocene Flood basalts and remains of uncertain origin.

II – OIB Family of magmas

Exhibits a typical OIB-like pattern in a primitive mantle normalized diagram. Type IIa extend to MgO-rich compositions and are less silica undersaturated. Type IIb have lower SiO2 and elevated CaO and incompatible trace elements

Origin of Type IIa is controversial and may be associated with material within the Afar plume, lithospheric material metasomatized by the plume, or delaminated. Type IIb are likely derived from melting of lithospheric mantle metasomes.

III – The Moderate Family of Magmas

Typified by a distinctive Ba peak, a U-Th trough, and a Nb-Ta peak. The slope of the REE are controlled by the depth and degree of melting.

The most common magma type within the East African Rift and is interpreted to be a melt of a plume-influenced upper mantle.

IV – Intermediate Composition

Typified by a pattern that is a simple mix of Type II and Type III magma

Contamination of a Type III magma as it passes through the lithosphere and either assimilates a metasome or mixes with a Type II melt.

V – Potassic Metasomes

Typified by a relatively flat pattern in the most incompatible trace elements within the primitive mantle normalized figure, with small negative anomalies in U and K, and a mild depletion in Zr-Hf.

Melts derived from a phlogopite-bearing lithospheric mantle metasome. Well-developed within the Virunga Province in the Western Branch.

VI – Depleted Source

Typified by extreme depletion in the most incompatible trace elements and resembles MORB, but with unusual positive anomalies in LILE

Uncertain – may be a plume component but more work needed.

Table 1: Classification of magma types erupting in the East African Rift. This table is taken from Parts II through IV of the synthesis. The magma types noted herein are predominantly identified on the basis of commonalities in patterns in primitive mantle normalized incompatible trace element diagrams. Other magma groups may emerge as further work is undertaken in the region and this list should therefore be viewed as preliminary and subject to addition in the future.

Figure 2. Primitive Mantle Normalized figure (Sun & McDonough, 1989) showing the magma types described in Table 1. For sources of data used to construct this figure, refer to Rooney (2020d).

Magmatism in East Africa that occurred subsequent to the flood basalt phase exhibits the strong overprint of the continental lithosphere. The characteristics of these post-flood basalt lavas exhibit a first order variability that correlates with proximity to the Tanzania craton (Fig. 1; 3) (Foley et al., 2012; Rogers et al., 2000; Rooney, 2020b). Ancient metasomes located within the lithospheric mantle of the craton have evolved exotic isotopic compositions which, when also combined with crustal assimilation, can exert a strong influence over the composition of erupted lavas (Hudgins et al., 2015; Rogers et al., 1998; Rosenthal et al., 2009). Within regions of the East African Rift that have developed upon the Neoproterozoic mobile belts (Fig. 1), the isotopic indicators of magma-lithosphere interaction are not as pronounced. However, radiogenic ingrowth over ca. 700 Ma has produced compositions that resemble the HIMU mantle reservoir (Rooney et al., 2017, 2014). In addition to this spatial heterogeneity in lava compositions, there is a commonly observed temporal trend wherein early magmatism within a region of the rift exhibits dominantly lithospheric characteristics (Type II magmas) (Fig. 2). Over time, this signature becomes transitional (Type IV) with contributions from the lithospheric mantle and underlying convecting mantle (Rooney, 2020b). Finally, in more mature rift sectors, Type III magmas predominate and are evidence of a largely sub-lithospheric source of magmatism (Rooney, 2020c).

Figure 3. Relative enrichment of lavas in East Africa using trace element ratios sensitive to the enrichment of the mantle source. Note La/Sm is a chondrite normalized ratio. Carbonatite values continue off the scale of the figure, which has been clipped to prevent compression. Warm colors are from the mobile belt, cooler colors are from the craton-influenced regions. These data are for modern basalts (>5 wt. % MgO - large symbols; <5 wt. % MgO or if information is missing – small symbols). See Rooney (2020d) for full information on the sources and reservoir citations.

Unified history of Cenozoic Magmatism in East Africa

Flood Basalt Episodes: Eocene Initial Phase (ca. 45 to 34 Ma) and Oligocene Traps Phase (ca. 33.8 to 27 Ma)

The earliest manifestations of Cenozoic volcanism in East Africa are in southern Ethiopia and northern Kenya at ca. 45 Ma (Ebinger et al., 1993; George et al., 1998). This Eocene Initial Phase was dominated by the Amaro Flood Basalts (45.2 – 39.8 Ma) and Gamo Flood Basalts (ca. 39.8 – ca. 34.1 Ma) (George and Rogers, 2002; Stewart and Rogers, 1996). Subsequent flood basalt magmatism shifted northward to the northwest Ethiopian Plateau and Yemen where the earliest high precision date of magmatism is reported from the Oligocene at 31.11 Ma (Prave et al., 2016), though this date is not from the base of the sequence. These Oligocene flood basalts have been divided based on composition into low, and high Ti groups (LT, HT1, and HT2) (Pik et al., 1999, 1998). The dominant phase of flood basalt activity lasted to ca. 29 Ma and is followed by bimodal or more silicic volcanism (Rooney, 2017). It is considered that magmatism during the flood basalt episodes is related to the impingement of an upwelling plume at the base of the lithosphere, interaction of the plume with the lithosphere, and the potential detachment and melting of the plume-enriched lithospheric mantle (Baker et al., 1996; Beccaluva et al., 2009; Furman et al., 2016; Kieffer et al., 2004; Natali et al., 2016; Pik et al., 1999).

Early Miocene Resurgence Phase (ca. 26 to 16 Ma)

This phase is signified by a period of shield building, with a peak in activity on the northwest Ethiopian plateau ca. 24 Ma (Kieffer et al., 2004; Rooney et al., 2014) and earlier activity on the southeastern Ethiopian plateau (Nelson et al., 2019) (Fig. 4). South of the Turkana Depression, this phase is expressed as the Samburu Event (ca. 20 Ma), during which time some of the first manifestations of volcanism occur within the western branch of the East African Rift (Pouclet et al., 2016; Roberts et al., 2012). Magmatism during this phase is diverse and includes melts of metasomatized SCLM, plume-influenced upper mantle, and mixtures of these two endmembers. The cause of this magmatic event is considered related to a pulse of widespread extension impacting the African lithosphere (Macgregor, 2015; Purcell, 2018; Rooney, 2020a).

Figure 4. Generalized distribution of magmatism during the Early Miocene Resurgence (including the Samburu Phase). Full details of the extent and units involved in this phase can be found in Rooney (2017; 2020a).

Flood Phonolites and Silicic Eruptives Phase (ca. 16 to 12 Ma).

In the eastern branch of the East African rift system, this phase of activity was most well-developed in the southern parts of the yet-to-form Kenya rift. Here it is postulated that the thick sheets of flood phonolites (Jones and Lippard, 1979; Smith, 1994) are the southern extension of the earlier Samburu Event. Further north, this phase is represented by less alkaline, silicic volcanism in Ethiopia (Chernet et al., 1998). The co-location of these silicic lavas with faults has been used to suggest a linkage between the development of the eastern branch of the East African Rift and these magma types (Ebinger et al., 1993).

Mid Miocene Resurgence Phase (12 to 9 Ma)

This phase of activity sees the return of widespread and voluminous basaltic magmatism centered on the margins of the Afar depression (including Yemen) and the Turkana depression; little activity is seen in the southernmost portion of the eastern branch of the East African Rift. The initial stages of more significant alkaline basaltic volcanism is seen in the western branch during this phase (Fontijn et al., 2012; Pouclet et al., 2016). The origin of magmatism during this phase is likely associated with a renewed period of extension centered on Afar and Turkana.

Early Rift Development Phase (9 to 4 Ma)

The now developing rifts impacted magmatic events following the Mid-Miocene Resurgence Phase. Magmatic activity during this phase was dominantly bimodal or silicic, and centered on both the eastern and western branch of the East African Rift (e.g., Abebe et al., 2005, 1998; Ebinger et al., 1989; Ivanov et al., 1998; WoldeGabriel et al., 1990; Wolfenden et al., 2004). Along the southern portions of the eastern branch of the East African Rift, there are the notable development of large volcanic edifices (e.g., Mt. Kenya, Sadiman, and Essimingor) (e.g., Mana et al., 2015, 2012). In Afar, this phase is coincident with the eruption of the dominantly basaltic Dalha/Dalhoid Series (Audin et al., 2004; Daoud et al., 2011; Le Gall et al., 2018; Varet, 2017; Wolfenden et al., 2005), which erupted contemporaneous with basin development.

The Stratoid Phase (ca. 4 to 0.5 Ma)

Volcanic activity in East Africa during this phase is diverse and widely distributed. Named for the recommencement of widescale stratiform basaltic activity in Turkana (the Gombe Group) (Erbello and Kidane, 2018; Gathogo et al., 2008; Haileab et al., 2004) and a contemporaneous phase of activity in Afar (Afar Stratoid Series) (Alene et al., 2017; Kidane et al., 2003; Lahitte et al., 2003), this phase also includes basaltic shield development and abundant alkaline volcanism in the Northern Tanzania Divergence and in the western branch of the East African Rift (Dawson, 2008; Mana et al., 2015). In Afar, localization of strain resulted in the progressive restriction of basaltic activity and the creation of a new magma group (The Gulf Series), which is found along the margin of the nascent zones of focused magmatic intrusion (Rooney, 2020c).

The Axial Phase (ca. 0.5 Ma to Present)

The specific timing of the commencement of this phase varies within the rift. This phase is best expressed within the more developed portions of the eastern branch of the East African Rift, where magmatism (bimodal in composition) has become progressively more restricted to axial grabens (e.g., Ayalew et al., 2016; Dunkley et al., 1993; Ebinger and Casey, 2001; Hutchison et al., 2016; MacDonald et al., 2001; Mohr, 1967; Rooney et al., 2007, 2005, 2011; Siegburg et al., 2018). Within the western branch and southern portions of the eastern branch, where magmatic activity has not yet focused upon an axis, the nomenclature is less instructive. In these less mature portions of the rift the current phase of activity often manifests as large central alkaline volcanoes (Barette et al., 2017; Dawson, 2008; Mana et al., 2015).

Figure 5. Map showing the distribution of Quaternary rocks from East Africa. Modified from Rooney (2020d).


As the community begins to synthesize ever larger datasets, our understanding of spatial and temporal extent of LIPs continues to evolve. East Africa provides us a beneficial case, as the upwelling of thermo-chemically anomalous material from the African LLSVP intersects plate tectonic processes in the form of a continental rift. To those working upon LIPs, a narrow focus on the flood basalt episode misses the rich history that is preserved in the ca. 30 Myr of activity that post-dated the LIP. Similarly, for communities interested in rift development, the existence of a mantle thermo-chemical anomaly has a profound influence on this plate-tectonic process.


This work was funded by the United States National Science Foundation grant numbers: 1219647, 1551872, 1850606. Andrew Bollinger and Sahira Cancel are thanked for providing feedback on a copy of this document.


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