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From initiation to termination: a petrostratigraphic tour of the Ethiopian Low-Ti Flood Basalt Province

  • S. R. Krans
  • T. O. Rooney
  • J. Kappelman
  • G. Yirgu
  • D. Ayalew
Original Paper
  • 316 Downloads

Abstract

Continental flood basalts (CFBs), thought to preserve the magmatic record of an impinging mantle plume head, offer spatial and temporal insights into melt generation processes in large igneous provinces (LIPs). Despite the utility of CFBs in probing mantle plume composition, these basalts typically erupt fractionated compositions, suggestive of significant residence time in the continental lithosphere. The location and duration of residence within the lithosphere provide additional insights into the flux of plume-related magmas. The NW Ethiopian plateau offers a well-preserved stratigraphic sequence from flood basalt initiation to termination, and is thus an important target for study of CFBs. This study examines modal observations within a stratigraphic framework and places these observations within the context of the magmatic evolution of the Ethiopian CFB province. Data demonstrate multiple pulses of magma recharge punctuated by brief shut-down events, with initial flows fed by magmas that experienced deeper fractionation (lower crust). Broad changes in modal mineralogy and flow cyclicity are consistent with fluctuating changes in magmatic flux through a complex plumbing system, indicating pulsed magma flux and an overall shallowing of the magmatic plumbing system over time. The composition of plagioclase megacrysts suggests a constant replenishing of new primitive magma recharging the shallow plumbing system during the main phase of volcanism, reaching an apex prior to flood basalt termination. The petrostratigraphic data sets presented in this paper provide new insight into the evolution of a magma plumbing system in a CFB province.

Keywords

Flood basalt Ethiopia Stratigraphy Petrography Cumulophyric Megacrystic plagioclase Magma plumbing system 

Introduction

The most significant manifestation of the interaction between an upwelling thermo-chemical anomaly and the continental lithosphere is the formation of a continental flood basalt (CFB) province (Ernst 2014). CFBs are voluminous outpourings of dominantly basaltic lava (> 100,000 km2 area, ≥ 1-km thick) that erupt over geologically short-time intervals (1–5 Ma, with the greatest volume erupted over ~ 1 Ma) (Self et al. 1997; Jerram and Widdowson 2005). Much focus has been placed on constraining the intensive and extensive parameters of melt generation associated with upwelling plumes (Kogiso et al. 2003; Davaille et al. 2005; Herzberg and Asimow 2008; Kimura and Kawabata 2015). However, lavas erupted in CFBs are rarely primary, and exhibit geochemical evidence of extended residence time in the continental crust (Cox 1980; Villiger et al. 2004). It is thus apparent that CFB lavas, which are preserved as well-constrained temporal sequences, reveal insights into the development of the magmatic plumbing system of a large igneous province (LIP), and by extension, the mechanisms of plume–lithosphere interaction.

Constraints as to the processes active within the plumbing system of CFBs have relied upon the extensive geochemical data sets that now exist within most LIPs (Arndt et al. 1993; Lightfoot et al. 1990; Peng et al. 1994; Pik et al. 1998, 1999; Baker et al. 2000; Mahoney et al. 2000; Walker et al. 2002; Wolff and Ramos 2013). These geochemical data sets provide broad constraints on the processes that control magma evolution in CFBs, namely—recharge, assimilation, and fractional crystallization (RAFC) (Bohrson et al. 2014; Lee et al. 2014). In a complex magma plumbing system, with multiple pathways of differentiation, it becomes increasingly difficult to interpret the relative influence of RAFC on magma evolution (O’Hara and Herzberg 2002). Indeed, an assessment of existing phase assemblages and evaluation of which phases are in equilibrium with the liquid are needed to elucidate these processes. Thus, despite the utility of geochemical data sets in inferring magma differentiation processes within the magmatic plumbing system of LIPs, further refinement of these models is dependent on alternate constraints. The petrographic properties of a lava, which provide direct confirmation of phases present during the evolution of a given magma, provide such constraints.

While the utility of petrographic constraints in assessing CFB magma evolution is clearly evident, there remains a paucity of such data sets within the literature. Given the new geochemically defined frameworks for flood basalt provinces, it is timely to examine how petrographic data sets can enhance and refine conceptual models of magmatic plumbing systems in these environments. In this paper, we present a petrostratigraphic reconstruction of the western portion of the Ethiopian–Yemen flood basalt province with a detailed, flow-by-flow petrographic analysis from initiation to termination. Modal analyses indicate evidence of two fractionation regimes, one in the lower crust (dominated by the crystallization of clinopyroxene cumulates in the absence of plagioclase) and a second in the mid-to-shallow crust (dominated by plagioclase cumulates). The distribution of these cumulates through the flood basalt section facilitates a reconstruction of the magmatic plumbing system. We see distinct cycles in the evolution of the magmatic plumbing system that over time, result in the shallowing of magma storage from flood basalt onset to termination. Our model has broad application beyond the Ethiopian–Yemen flood basalts and provides an explanation for the frequent observation in CFBs of an initial phase of largely picritic flows followed by a main phase of compositionally homogenous flows, which is terminated by explosive silicic volcanism.

Background

Cenozoic plume-dominated magmatism in East Africa extends from Turkana in the south, the southeastern Ethiopian plateau in the east, Yemen in the northeast, Ethiopia and Eritrea in the north, and Sudan in the west, and preserving a rich stratigraphic record of magmatism over the past 45 Ma (Rooney 2017). Basaltic magmatism is divided into three time periods: Eocene initial phase (45–34 Ma), Oligocene trap phase (~ 33.9–27 Ma), and the early Miocene resurgence phase (~ 26.9–22 Ma) which is later followed by bimodal lavas and silicic volcanism (Rooney 2017). During the Eocene initial phase, volcanism was restricted to southern Ethiopia and northern Kenya and dominantly basaltic (George and Rogers 2002; Furman et al. 2006; Rooney 2017). Estimated magmatic flux from the Eocene initial phase is significantly lower than the Oligocene trap phase (~ 3 × 10−4 versus 6 × 10−2–1.5 × 10−1 km3/year, respectively) (Ebinger et al. 1993; George et al. 1998; Rooney 2017). During the Oligocene trap phase, the eruption of the Ethiopian–Arabian plateau flood basalts significantly increased the aerial extent of volcanism to include northern Ethiopia and Yemen (Baker et al. 1996; Hofmann et al. 1997; Rochette et al. 1998; Rooney 2017). There is some ambiguity as to the earliest onset of flood basalt activity in the Ethiopian–Yemen province, estimated at ~ 33 Ma in Turkana (Zanettin et al. 1983; Morley et al. 1992), 31.2 Ma (± 1.2 Ma) in NW Ethiopia (Hofmann et al. 1997; Rochette et al. 1998; Ukstins et al. 2002) and 30.9 Ma (± 0.24 Ma) (Baker et al. 1996), and it could be even earlier (Ukstins et al. 2002). The largest volume was extruded between 30 and 29 Ma (Baker et al. 1996; Hofmann et al. 1997; Abbate et al. 2014). Following the termination of the flood basalts, volcanic activity is relatively rare except for focused activity in Turkana (Brown and Mcdougall 2011) and sparse silicic activity and diking along the evolving Ethiopian–Yemen rift margin (Peate and Bryan 2008; Rooney et al. 2013). In the early Miocene, a resurgence in basaltic activity occurred in the form of shield building activity in the NW Ethiopian plateau, fissure eruptions on the SE Ethiopian plateau, and increased eruptions in the Turkana region (Kieffer et al. 2004; Furman et al. 2006; Brown and Mcdougall 2011; Rooney 2017). Recent estimates for total lava volumes erupted from 45 to 22 Ma in the African Arabian LIP are ~ 720,000 km3, placing this province between Emeishan and Deccan LIPs in terms of overall volume (Rooney 2017).

Stratigraphy

Initial stratigraphic studies, centered on the Northeastern region of the Ethiopian plateau, divided the Ethiopian flood basalts into four divisions from oldest to youngest: Ashange basalt, Aiba fissure basalt, Alaji basalts and rhyolites, and the Termaber basalts (Zanettin et al. 1980). The Ashange is characterized by thin (~ 5 m) dipping flows restricted to only a few kilometers in extent. These flows are locally zeolitized and are unconformably overlain by flat-lying flows. The Aiba Formation is characterized by massive flows (up to 100 m when ponded) that are generally olivine basalt with columnar jointing common. Locally, sparse-interbedded pyroclastic deposits of varying thickness are observed in this formation at Amba Aiba. This formation is overlain by the Alaji and Termaber Formations which are characterized by ignimbrites with sparsely interbedded basalt flows, and shield volcanism, respectively (Mohr and Zanettin 1988). This initial stratigraphic division between the lower Ashange and upper Aiba was recognized as being of limited regional utility due to the local characteristics of the type sections (deformed beds overlain by an unconformity) which are not observed elsewhere (Mohr 1983). Subsequent research by Hofmann et al. (1997) uses the 1950 m-thick Lima-Limo section north of Lake Tana to divide the province into upper and lower units based on morphological boundaries: (1) a lower 900 m-thick section of lavas capped by 150 m of differentiated products forming a significant regional terrace and (2) an upper 1000 m-thick section intercalated with silicic tuffs forming another erosional terrace. The work of Kieffer et al. (2004) assesses the transition from main-phase LT and HT flood basalt through the development of Miocene shield volcanoes, including a lower 1470-m thickness of flood basalt and nearly 300 m of overlying shield flows from three regions on the plateau. Kieffer et al. (2004) adopt the “lower” and “upper” stratigraphic divisions of Hofmann et al. (1997) and do not report significant compositional heterogeneity over time.

Spatial compositional variability

A spatial division based on geochemical differences has been assigned for flood basalts of the northwest Ethiopian plateau and includes a western Low-Ti (LT) sub-province (the focus of this study), and an eastern High-Ti (HT) sub-province (Pik et al. 1998). The LT basalts are tholeiitic with characteristically low TiO2 (1–2.6 wt%), low Nb/La (0.55–0.85), and higher SiO2 (47–51 wt%) (Pik et al. 1998). The LT basalts are characterized petrographically as having hypocrystalline coarser-grained (intergranular and aphyric–ophitic) textures, 0–10% phenocrysts that are predominately plagioclase ± olivine, and often glomerophyric (Pik et al. 1998; Beccaluva et al. 2009; Natali et al. 2016). Groundmass contains plagioclase, interstitial clinopyroxene (colorless in PPL), and Fe–Ti oxides (Pik et al. 1998); these characteristics are consistent among tholeiitic magmas. The HT basalts are further subdivided into HT1 and HT2, where HT1 lavas represent a continuum between LT and HT2 magma types. The HT1 basalts are aphyric to ol-phyric and range from microcrystalline to coarser-grained textures. Plagioclase is rare as a phenocryst phase and tends to exhibit sieve texture when present. Olivine is common in the groundmass, along with pinkish clinopyroxene (in PPL) and abundant Fe–Ti oxides, consistent with more alkaline magmas with respect to LT magmas. HT2 lavas are transitional-alkaline basalts and picrites with higher TiO2 (2.6–5 wt%), higher Nb/La (1.1–1.4), and lower SiO2 (44–48.3 wt%) (Pik et al. 1998). HT2 basalts are typically porphyritic with olivine as the dominant phenocryst phase ± pinkish Ti-rich clinopyroxene, Cr-spinel, and rare plagioclase, again consistent with transitional to alkaline lavas (Pik et al. 1998). The groundmass contains similar phases plus Ti magnetite with rare alkali feldspar and phlogopite (Beccaluva et al. 2009).

The clear distinction between LT and HT lava types has led to investigations of their magmatic origins using geochemical and isotopic data sets (Pik et al. 1999; Kieffer et al. 2004; Beccaluva et al. 2009; Natali et al. 2016). While the source of HT lavas has been attributed to melting of the Afar plume (Pik et al. 1999; Natali et al. 2016), the origin of LT lavas remains a topic of debate (Pik et al. 1999; Kieffer et al. 2004; Beccaluva et al. 2009). Fractional crystallization models using normative mineralogy originally proposed by Pik et al. (1998) suggested that LT magmas were derived through shallow fractionation processes. Subsequent trace element and isotopic analysis proposed a two-stage AFC process for LT magmas, indicating a lower crustal and upper crustal component influencing geochemical trends (Pik et al. 1999). Assimilation of continental crust has been assessed using trace-element ratios (Nb/La, Ce/Pb, and Ba/Th) and isotopic ratios (Sr–Nd–Pb systems) (Pik et al. 1999; Kieffer et al. 2004). While some studies attribute the low Nb/La and low 206Pb/204Pb of LT magmas to crustal contamination (Pik et al. 1999; Meshesha and Shinjo 2007) others suggest the low Nb/La and unradiogenic Pb could reflect source composition (Kieffer et al. 2004; Beccaluva et al. 2009). The issue regarding the origin of LT magmas remains largely unresolved due to the difficulty of constraining AFC processes using whole rock geochemistry. We present petrography of individual flows within the LT stratigraphy of the NW Ethiopian plateau and use these data sets to evaluate magma–lithosphere interactions.

Methods

Stratigraphic sampling

Samples were collected from the far western region of the northwest Ethiopian plateau erosional escarpment and lowlands over the course of two field seasons (2010 and 2014; Fig. 1b). Because flood basalt in this region are predominately flat-lying (Fig. 2a), the superpositional relationship of individually sampled flows was clear, especially when stacks of flows could be traced from one exposure to another, sometimes over several 100 m. Sample sites were recorded with a handheld GPS (see Table S1). Elevation was also recorded and used as a proxy to log stratigraphic sequences over short distances at the outcrop scale; however, given the large error (~ ± 10 m) inherent with most handheld GPS units, elevation was used as a rough proxy within a new series of exposed flows, and this value was paired with visual confirmation between flow contacts and height estimates of individual flows in the field over outcrop scales.

Fig. 1

a Map of Oligocene basalts in Ethiopia and Yemen (modified from Rooney 2017). The HT flood basalt provinces (purple) are restricted to the eastern half of the NW plateau and Yemen. The LT flood basalt province (blue) is restricted to the western half of the Ethiopian plateau. Previous studies of LT basalt are from the Lima-Limo section in the north and have been reported at the base of the flood basalt sequence near Sekota (Pik et al. 1998, 1999; Kieffer et al. 2004). This study is focused west and northwest of Lake Tana (b). b Topographic relief map showing sample locations in this study including exposure of the underlying Pan African granite (basement) and tuff near the base of the sequence. Symbols represent different stratigraphic transects. The upper limit of the Tana escarpment is shown by a dashed line. Gondar graben boarder fault near Aykel town (solid line) as noted in Mège and Korme (2004). The division between the escarpment and lowlands occurs just below 1000 masl

Fig. 2

Field photos: a view of the upper and middle flood basalts exposed on the NW escarpment looking east. Note that flows are flat-lying. Photo was taken at an elevation of ~ 1560 masl near sampling location of 3495. b Complex braided flow facies with inflated lobe structure, solid cores, and vesicular rims. Photo taken from middle group A along NW escarpment at ~ 980 masl in unsampled portion just below sample 3380. Note hammer for scale. c Columnar jointing observed in tabular flow facies near top of middle flood basalt sequence. Photo taken along NW escarpment from sampling site of 3362. Note person for scale. d Pahoehoe flow surface with hammer for scale. This photo was taken during the second field season at a location along a more southern transect (not reported here) and resembles flow surfaces observed in the NW escarpment. e Red flow contacts in stacked 1 m-thick flows from middle group B in the NW escarpment (~ 1465 masl). f Red paleosol (psol) at 1844 masl dividing the middle group C and upper flood basalt sequences in the NW escarpment. The paleosol is ~ 0.5-m thick, overlying sample 3361 and underlying sample 3360

Continuous sampling of individual flows within the 1635 m-thick sequence was performed at generally fresh road cuts along five transects (Fig. 1b). At higher elevations (> 950 masl), the deeply eroded upper flood basalt sequence forms steep competent slopes and cliffs along the Tana escarpment (see Kappelman et al. 2014, Fig. 2). Flood basalt flows are also found exposed across the top of the plateau but faulting near Aykel town and north of Lake Tana disrupt the stratigraphic relationship of these flows (Chorowicz et al. 1998; Mège and Korme 2004). For this reason, we elect to use the western top of the escarpment as the upper boundary of our stratigraphic transects. The NW–SE trending NW escarpment transect covers ~ 30 km from 2175 masl near Aykel town, down the erosional escarpment to 987 masl at the start of the lowlands. This section is well-preserved and exposes > 1100 m of lava flows. At ~ 1260 masl there is a relatively flat segment that extends for ~ 8 km along this transect; continuity in lava flows was traced along this segment and no samples were collected. At lower elevations (< 950 masl), the slope is much more gradual and outcrops are usually isolated (Fig. 1b; also see Kappelman et al. 2014, Fig. 2). The NW Lowlands transect continues west for another 65 km from 940 to 720 masl, ending just east of the Ethio-Sudan border. The Maganan transect trends NE–SW and covers ~ 30 km of lowland flows from 900 to 610 masl. The Shinfa transect trends E–W and covers ~ 21 km of the lowest flood basalt exposure (589–540 masl). The Galegu transect trends N–S and covers ~ 38 km from 932 to 564 masl.

A total of 190 of the freshest basalt lava flows were sampled from the LT province. A sledge hammer was used to break through exposed surfaces, and this sampling protocol served to avoid areas of alteration and weathering rinds. Flow facies mostly resemble compound-braided sequences of competent lobes in weathered vesicular basalt (Fig. 2b) with rare thick tabular flow facies with columnar jointing observed between 1770 and 1870 masl (Fig. 2c). Less vesicular, competent portions of flows (e.g., solid lobe cores or massive horizons) were preferentially sampled within highly vesicular flows to avoid alteration. Flow contacts were commonly seen as continuous red horizons within competent portions of flow stacks and typically resemble pahoehoe flow tops (Fig. 2d, e). In rare cases, rubbly flow tops (aa, or agglomerate) were observed. Thicker horizons of red, friable material were interpreted as weathering horizons. Similar red horizons seen in the Columbia River basalt province have been identified as paleosols (Sheldon 2003). Unlike the weathering horizons in the Columbia River basalts, we did not observe root structures or fossils, but we still use the term paleosol (psol) to describe these horizons, since they are thick (> 30) cm and composed of clay-size material with hackly jointing (Fig. 2f). These paleosols vary in color from red to green to tan. Because the basalts in some sections were more vesicular and weathered than others, flow contacts were not always preserved. In these cases, sampling occurred when changes in flow morphology, texture, and mineralogy were observed. In the lowlands, where outcrops were more isolated, sampling occurred whenever there was fresh exposure.

Proterozoic basement (Pan African granite) is exposed ~ 20 km west of Galegu at 622 masl but its surface varies in elevation. A thorough inspection of the far western landscape and especially the bottoms of the river channels that are exposed only during the dry season permitted the collection of a dozen flood basalt samples between 540 and 622 masl with these representing the base of the sequence.

Petrography

Thin sections were prepared for 178 samples and compared in terms of phenocryst abundances, mineralogical assemblages, and micro-textures. The Ethiopian LT flood basalts can be classified into three textural groups and three modal mineralogical groups based on hand sample and thin section observations. Two additional terms describing the presence of glomerocrysts and megacrysts in lava flows are used as modifiers. Many of these petrographic observations (plag-megacrystic, intergranular, aphyric–ophitic, and cpx-cumulophyric) have been reported in other CFBs including the Columbia River Basalts (Bondre and Hart 2008; Camp et al. 2013), the Mid-continent Rift (Huber 1973; Annells 1973; Berg and Klewin 1988), Siberian Traps (Lightfoot et al. 1990; Renne and Basu 1991), Deccan Traps (Beane et al. 1986; Hooper et al. 1988; Duraiswami et al. 2001), the Ethiopian flood basalt province (Beccaluva et al. 2009) and other older CFBs (Anderson and Dunham 1966; Emeleus and Bell 2005; Cheng et al. 2014; Sheth 2016). Existing literature on CFBs reveal a consistent nomenclature is required. For this reason, the nomenclature used to establish petrographic groups in this study is defined as follows:

Textures

Aphyric–microcrystalline

A rock that is aphyric in hand sample and microcrystalline in thin section (Fig. 3a). In hand sample, these rocks appear dark gray and are often glassy in appearance. In thin section, they contain few to no phenocrysts (< 3%) with indistinguishable mineralogy in groundmass (plagioclase 0.25 mm, other phases < 0.5 mm). Groundmass textures vary between felty and trachytic. Rare phenocrysts, when present, are subhedral to euhedral plagioclase less than 1 mm.

Fig. 3

Photomicrographs of representative textures and modal mineralogy in the LT flood basalts. All images are in XPL unless noted otherwise. a Aphyric–microcrystalline texture (3387-2). Sample is from lower flood basalt, NW Lowlands section (742 masl). b Aphyric–intergranular texture (3382 T). Sample is from middle flood basalt group A, NW lowlands section (925–930 masl). c Aphyric–ophitic texture with interstitial clinopyroxene up to 1 mm (3460 B). Sample is from middle flood basalt group B, NW escarpment (1240–1255 masl). d Plag-megacrystic texture with cm-size plagioclase and oscillatory zoning (3500). Sample is from middle flood basalt group B, NW escarpment (1605–1615 masl). e Cpx-cumulophyric texture with 5 mm clinopyroxene glomerocrysts (3433). Sample is from lower flood basalt, Galegu section (710 masl). f Plag-megacrystic texture in hand sample (3500, same as in d). g Olivine–phyric flow with 1–3 mm subhedral olivine in intergranular groundmass (3452). Sample is from middle flood basalt group A, NW escarpment (1160 masl). h Plagioclase–olivine–phyric flow with up to 5 mm sieved plagioclase, 0.5–1 mm subhedral olivine, and microcrystalline groundmass (3477). Sample is from middle flood basalt group B, NW escarpment (1450 masl). i Plagioclase-phyric flow with up to 3 mm plagioclase glomerocrysts in microcrystalline groundmass (3422). Sample is from middle flood basalt group A, Maganan section (900 masl). j Intergranular texture in transitional basalt from Shinfa (3443). Sample is from lower flood basalt (565 masl). k Olivine-rich transitional basalt from Shinfa (3515 A). Sample is from lower flood basalt (561 masl). l Same image as j, but in PPL. Note the pinkish-brown interstitial titaniferous augite and abundant Fe–Ti oxides

Aphyric–intergranular

A rock that is aphyric in hand sample but having discrete grains of olivine ± pyroxene filling the interstices between larger plagioclase lathes (Fig. 3b). In hand sample, these rocks appear coarser grained than aphyric–microcrystalline rocks, and similar range in grain size as aphyric–ophitic rocks. Phases often include larger plagioclase (0.3–1 mm) and smaller ol + cpx + oxides (0.1–0.5 mm).

Aphyric–ophitic

This texture is often gradational with aphyric–intergranular, but the term is used when plagioclase lathes are partially to fully enclosed by larger (> 0.3 mm) interstitial clinopyroxene (Fig. 3c), giving the rock a mottled appearance in thin section that can be visible in hand sample when texture is coarser grain (cpx > 1 mm). Porphyritic samples (plag ± ol-phyric) were occasionally observed with aphyric–ophitic groundmass.

Modal mineralogy

In this study, porphyritic rocks are termed -phyric when they contain > 3% phenocrysts in an aphyric groundmass. Volume % phenocrysts are approximations from visual inspection of thin sections and hand sample. We define the cut-off range for phenocrysts as 0.3 to < 1000 mm.

Ol-phyric

Porphyritic rock with > 70% olivine in the phenocryst phase (Fig. 3g). Total phenocryst abundances are 5–15% with groundmass from microcrystalline to intergranular, and in rare cases aphyric–ophitic. Olivines are 0.6–2 mm, commonly euhedral, and in rare cases skeletal. In many cases, the olivine is partially or completely altered to iddingsite (goethite + smectite). Completely altered olivine was only recognized in cases where the euhedral phenocryst shape was preserved and found pervasively throughout the sample.

Plag-ol-phyric

Porphyritic rock with nearly equal proportions of plagioclase and olivine in the phenocryst phase (Fig. 3h). Total phenocryst abundances are 5–25% and up to 40% in coarser-grained rocks. Groundmass textures alternate between microcrystalline and intergranular. Plagioclase ranges 0.3–3.5 mm, euhedral to subhedral with sieve texture common in larger subhedral grains. Olivine are generally smaller (0.3–1.5 mm), euhedral, and partially or completely altered to iddingsite.

Plag-phyric

Porphyritic rock with > 70% plagioclase in the phenocryst phase (Fig. 3i). Total phenocryst abundances are 5–25% with predominately microcrystalline groundmass and rare intergranular to aphyric–ophitic groundmass textures. Plagioclase range from 0.5–5 mm, euhedral to subhedral, and can occur in irregular or radial clusters. Plagioclase composition determined by the Michel Lévy method ranges from andesine to bytownite (An40–An70). Olivine micro-phenocrysts are sometimes included in larger plagioclase. Strong sieve texture is common in subhedral grains; in rare cases, sieved cores are mantled by euhedral rims.

Glomerocrysts and megacrysts

Glomerocrystic material and megacrysts are found in several lava flows in this study. Glomerocrysts are either clinopyroxene-rich ± olivine or plagioclase-rich ± olivine, while megacrysts are usually plagioclase. Below is a detailed description of these modifying terms:

Plag-megacrystic

This term is used as a modifier to describe flows containing megacrystic plagioclase > 1 cm and up to 4 cm in length (Fig. 3d, f). Flows containing megacrystic plagioclase are between 25 and 35 vol% phenocrysts. Plagioclase megacrysts are mostly euhedral to subhedral and exhibit extensive oscillatory zoning. Mineral and melt inclusions, when present, occur along growth zones. Sieve texture, when present, also occurs along growth zones and rarely in the core of plagioclase. The megacrysts can occur isolated or clustered and lack preferred orientation. In rare cases, clusters of plagioclase megacrysts are radial. Plagioclase composition for megacrysts determined by Michel Lévy is mostly labradorite (An64–An72) (see Table S2). Occasionally, clinopyroxene and olivine were observed within clusters of plagioclase megacrysts.

Cpx-cumulophyric

A term used when glomerocrysts of clinopyroxene ± olivine are observed (Fig. 3e). These glomerocrysts range in size from 1 to 4 mm in diameter, are dominantly clinopyroxene with characteristic augite twinning, and clear in PPL. The glomerocrysts do not appear to be in equilibrium with the groundmass (resorbed edges, undulose extinction, recrystallization). This term is used as a modifier, since these glomerocrysts are found in a variety of flow modes and textures.

Results

The majority of flows are petrographically consistent with LT magmas (as described in “Spatial compositional variability” above). Only a few intermittent flows (ten in total) restricted to the lower 200 m of the stratigraphy did not fall into the above categories (Fig. 3j–l). These flows are rich in euhedral to skeletal olivine that lacks alteration to iddingsite. They tend to have intergranular textures with pinkish-brown clinopyroxene (in PPL) and abundant Fe–Ti oxides. These observations are consistent with the more transitional-alkaline compositions of HT magmas (described in “Spatial compositional variability”) and are henceforth referred to as transitional basalts.

Stratigraphy of the NW LT flood basalts

The petrostratigraphic column is coded using the petrographic nomenclature described above, where color denotes mode and symbols denote textures (Fig. 4 and Table S2). The earlier published distinction between the Ashange, Aiba, Alaji, and Termaber flood basalts (Mohr and Zanettin 1988) is not entirely clear in these new data, probably because the characteristics used to define the upper and lower units relied on local observations that are not consistent across the entire flood basalt province. The morphological distinction between upper and lower flood basalt suggested by Hofmann et al. (1997) is an improvement but lacked petrological constraint. Based on petrographic differences observed in this study, we define three stratigraphic divisions: lower, middle, and upper flood basalts.

Fig. 4

Petrostratigraphic column for flood basalt transects in this study. Elevation in masl is listed on the left of column and sample numbers are listed on the right. Sample numbers with an asterisk indicate transitional basalt. a Galegu section showing basement topography. b Shinfa section with interbedded transitional basalt flows. A single flow (3408 A) at the base of the Maganan section appears petrographically similar to the uppermost flow of the Shinfa section (3407); they are separated by ~ 12 km distance. c Maganan section. Four transitional basalt flows are found interbedded with LT lavas between 650 and 750 masl. The boundary between lower and middle flood basalts is extrapolated at ~ 850 masl (psol and tuff). d The NW Lowlands section. Middle flood basalt sequence begins approximately when plag-megacrystic basalt appears (~ 865 masl). e The NW escarpment section. The boundary between the middle and upper flood basalts is observed at 1844 masl (psol). The boundaries within the middle flood basalt groups occur at ~ 1210 masl (A–B) and ~ 1550 masl (B–C) which is separated by a paleosol

Lower flood basalts (540–940 masl)

The lower flood basalt sequence consists of 400 m of flows and includes the Shinfa, Galegu, and Maganan sections, and lower half of the NW Lowland section. The precise number of flows is difficult to determine, because it is not possible to correlate the flows between and among these widely separated sections. The basal exposures of this sequence reflect the underlying basement topography and the elevation varies among stratigraphic sections (Fig. 4a–d). For example, and as noted above, the Proterozoic Pan African granite outcrops ~ 20 km west of the lowest flows at Galegu; both occur at 622 masl (Fig. 4a), and another flow is found 24 km SW of this area at 564 masl.

Shinfa (540–589 masl)

The lava flows of the Shinfa section (Fig. 4b) are among the lowest elevation flows observed (540–589 masl). The number of flows is estimated between 7 and 9 based on the observation of vesicular flow tops, a rare agglomerate flow top, and petrographic variation. A 2-m-thick dike crosscuts the flow unit directly below the agglomerate flow. Flow thicknesses are not well constrained due to an absence of exposed contacts, but are estimated between 5 and 15 m. Flows at Shinfa are dominantly ol-phyric, and alternate between tholeiitic and transitional magma types. Total phenocryst abundances range from 3 to 7% with intergranular to ophitic groundmass.

Galegu (564–932) masl

The Galegu section includes the exposure of the Pan African basement and serves to illustrate the variable topography that forms the base of the flood basalts (Fig. 4a). This exposure is one of the few examples where basement is exposed in close proximity (tens of meters) to the lowest exposed flows. The lower flows in this section range from aphyric (glassy to aphyric–ophitic) to plag-phyric. From 653 to 785 masl, flows are dominantly plag-ol-phyric and sometimes exhibit cpx-cumulophyric texture. A large hill near Galegu exposes over 200 m of continuous section that grades from porphyritic flows to aphyric–ophitic flows separated by thick packages (20–25 m) of weathered vesicular basalt and rare aphyric–microcrystalline flows (2–5-m thick). The upper portions of the hill (800–932 masl) are dominated by thick (10–18 m) aphyric–ophitic flows and capped by an eroded aphyric–intergranular flow.

Maganan (610–900 masl)

The Maganan section has sparse outcrops because of its undulating topography and is, therefore, difficult to characterize in terms of flow number, flow thickness, and absolute variation in flow composition and textures (Fig. 4c). The few exposures below 750 masl alternate between tholeiitic and transitional magma types and are mostly ol-phyric to aphyric with varying coarseness in groundmass (microcrystalline to aphyric). The lowest exposure (3408 A) is a rare ol-phyric flow with coarsely ophitic groundmass and is nearly identical petrographically to the upper eroded flow at Shinfa (3407). While these two exposures (3407 and 3408 A) are separated by ~ 12 km distance, they are suspected to be the same flow or originate from a similar eruptive event. Flows exposed above 800 masl are dominantly plag-phyric and often display columnar jointing. Two 30 cm paleosols are observed at 686 and 849 masl. A 4-m tuff is also observed at 853 masl and overlies the higher paleosol.

NW Lowlands (720–850 masl)

The NW Lowlands are separated by a ~ 50-m covered interval between sample 3405 and 3403 (Fig. 4d). Below the covered interval, inflated lobe structures are commonly observed with rare columnar jointing. The exact number of flows is difficult to determine due to complex inflated lobe structures, but ~ 22 flow packages are estimated based on variation in mode and textures. Flows are usually 2–5-m thick, alternating between porphyritic and aphyric–microcrystalline flow packages with rare cpx-rich glomerocrysts. Porphyritic flows are usually plag-phyric to plag-ol-phyric with 8–20% phenocrysts in a microcrystalline groundmass. A tuff of variable thickness (2–10 m) is observed at ~ 776 masl. A thick (0.5–1 m) paleosol is observed at 792 masl. The basalts above this paleosol are mostly aphyric flows up until the 50-m covered interval.

Middle flood basalts (850–1844 masl)

The middle flood basalts are the thickest portion of the LT flood basalts and include nearly 1000 m of flows, including the upper half of the NW Lowlands section (Fig. 4d) and most of the NW escarpment section (Fig. 4e). Plag-megacrystic flows first appear above 850 masl (sample 3402); a significant petrological change that defines the start of the middle flood basalts. The division between lower and middle flood basalt occurs roughly at the topographic transition from steep sloping escarpment to gradual sloping lowlands, similar to the morphological boundary in the Lima-Limo section described by Hofmann et al. (1997). The covered interval in the NW Lowlands section makes it difficult to place a precise division between lower and middle flood basalt, and therefore, an extrapolation is made using the neighboring Maganan section, beginning ~ 3 km south of the NW Lowlands section. In the Maganan section, a tuff and paleosol occur just 10 m below the elevation of the plag-megacrystic basalt observed in the NW Lowlands section (Fig. 4c, d). We have chosen to draw the division within the covered interval so that it coincides with the elevation of this tuff and paleosol.

The NW escarpment consists of ~ 1200 m of flood basalts with preserved flow contacts exposed in fresh road cuts that were densely sampled (Figs. 2a, 4e). Variation in flow thickness, mineralogy, and textures does not appear to vary systematically over short vertical intervals but can be grouped over large (250–350 m) intervals throughout the section. The boundary between middle and upper flood basalts occurs in this section at 1844 masl. We define three flow groups within the middle flood basalts based on the presence of glomerocrysts and megacrysts, modal cycling of flows, and flow thicknesses.

Group A occurs between 863 and 1212 masl and is defined by the first appearance of plag-megacrystic basalt followed by alternating ol-phyric to plag-phyric basalt. Group A includes the upper portion of the NW Lowlands section above the covered interval (Fig. 4d) up through the lower 1000 m of flows in the NW escarpment (Fig. 4e). Flow thicknesses are usually between 2 and 16 m but can be as thick as 24 m. Packages of thin (1–2-m thick) stacked flows and highly vesicular packages with solid cores are common. At the base of this group is a 25-m-thick plag-megacrystic flow overlying a thin aphyric–microcrystalline flow at ~ 860 masl (3403). Above the plag-megacrystic flow, flows are thinner (2–5 m), ol-phyric to aphyric with coarser groundmass textures (intergranular to ophitic) and separated by 10–20 m covered intervals with no exposed flows. Cpx-cumulophyric texture is more common between 1000 and 1040 masl, but then disappears. Alternating plag-phyric and plag-ol-phyric stacked thin flows and braided pahoehoe flow packages are found in the upper 150 m of this group.

Group B (Fig. 4e) occurs between 1216 and 1561 masl and is defined by the presence of thick aphyric–ophitic and intergranular basalt alternating with aphyric to porphyritic basalt. Aphyric–ophitic to aphyric–intergranular basalt flows separated by thick (25–50 m) packages of weathered vesicular basalt differentiate group B from group A. Plag-phyric, plag-ol-phyric, ol-phyric, and aphyric–microcrystalline basalt are also observed, with groundmass textures varying from microcrystalline to intergranular and ophitic. Cpx-cumulophyric texture occurs in the middle and top of this sequence. Flow thicknesses are usually between 4 and 11 m and up to 19 m, although packages of thin (≤ 1 m) stacked flows are also observed. Multiple 2–5-m-thick dikes are observed in this sequence. The sequence is terminated by a 30 cm paleosol.

Group C occurs between 1561 and 1844 masl and is defined by the recommencement of plag-megacrystic basalt (3496) after the 30 cm paleosol (Fig. 4e). Thick plag-megacrystic basalt and alternating aphyric basalt are common. Cpx-cumulophyric texture is observed in flows directly overlying paleosols. Flow thicknesses in this sequence increase from 4–10, to 15–20, to 37 m at the top. The capping flow is columnar jointed with a cpx-cumulophyric agglomerate flow top overlain by a 50 cm paleosol.

Upper flood basalts (1844–2175 masl)

The upper flood basalt sequence (Fig. 4e) includes ~ 330 m of flows and is truncated at the top of the plateau where the stratigraphic relationships are difficult to discern because of faulting north of Lake Tana near Aykel town. This sequence has 21 observed flows and one silicic tuff. The lower boundary of the upper sequence is defined by the disappearance of cpx-glomerocrysts, coincident with a 50 cm paleosol, and the disappearance of megacrystic plagioclase. Above the 50 cm paleosol (1844 masl), the first flows are thicker (10–21 m) and are aphyric–microcrystalline. Continuing up the sequence, flows are consistently 5–6-m thick and are dominantly plag-phyric to aphyric–microcrystalline. Coarse groundmass textures (ophitic and intergranular) are rare.

Discussion

Decoding magmatic processes from petrography and stratigraphy

The use petrography and stratigraphy to evaluate magmatic processes first requires an assessment of the parameters that control crystallization of phases in a basaltic magma. These parameters include: (1) the starting composition of the parental magma, (2) pressure of crystallization, and (3) the presence of volatiles. The composition of the parental magma controls the mode and order of crystallization along the liquid line of descent (Bartels et al. 1991; Shi and Libourel 1991), thus explaining the observation of ol + plag phenocrysts in LT lavas, and ol + cpx in HT lavas (Pik et al. 1998; Kieffer et al. 2004; Beccaluva et al. 2009; Natali et al. 2016). Within the NW Ethiopian plateau there is a limited diversity in magma compositions—defined by existing literature as either HT or LT provinces, which are spatially restricted to the eastern and western half of the NW Ethiopian plateau, respectively (Pik et al. 1998; Kieffer et al. 2004; Beccaluva et al. 2009). While a thin layer of LT lavas has been found, at the base of a predominately HT1 sequence in the NE portion of the Ethiopian plateau (Kieffer et al. 2004), there is no evidence to suggest cyclicity between HT and LT magma types throughout the duration of flood basalt activity (Pik et al. 1998; Kieffer et al. 2004; Beccaluva et al. 2009); therefore, a continuous sequence of flows with similar compositional properties can be treated as a single magma type from a petrographic perspective. On the basis of our current understanding of the behavior of magmas with a broadly basaltic composition, the crystallization of clinopyroxene is favored at higher pressures (10–20 kbar), at the expense of plagioclase and olivine (Morse 1980). The same magmas at lower pressures (< 5 kbar) will favor the crystallization of plagioclase over clinopyroxene (Morse 1980). Such observations are valid for relatively dry magmas, as the presence of water (> 3 wt%) can facilitate the crystallization of clinopyroxene over plagioclase at shallow crustal pressures (Feig et al. 2006). Since the LT flood basalts are reportedly dry (Kieffer et al. 2004), a preferred crystallization of clinopyroxene versus plagioclase can indicate a relative difference in depth of stalled magmas. Within the stratigraphy presented in this paper, the majority of flows are petrographically consistent with LT lavas. Only ten flows, restricted to the lower 200 m of the stratigraphy, are observed as transitional basalts similar to HT lavas. For the purposes of the discussion that follows, only LT lavas will be considered.

Conceptual model for the evolution of LT flood basalts

Petrographic observations presented in this study demonstrate three divisions within the Ethiopian flood basalt province, defined broadly as the lower flood basalt, which is dominantly ol-phyric with cpx-rich cumulates, the middle flood basalt, which exhibits plag-megacrystic flows and oscillation between plag-phyric and ol-phyric flows, and the upper flood basalt, which is dominantly plag-phyric and devoid of cumulates (Fig. 5a). We propose that these distinctive petrographic heterogeneities reflect differences in the depth of fractionation and magnitude of magma flux into the lithosphere.

Fig. 5

a Simplified petrostratigraphic column of the Ethiopian NW plateau LT flood basalts. Relative percentages of modal phases for each unit and group are shown on the right. Percentages for lower flood basalts are the mean of all stratigraphic sections (Lowlands, Maganan, Shinfa, and Galegu). b Modal variation between different stratigraphic sections in lower flood basalts

Lower flood basalt: deep fractionation and early evolution

The most distinctive feature observed in the lower flood basalt is the presence of ol-phyric flows and clinopyroxene-rich glomerocrysts. Cpx-cumulophyric texture occurs throughout the lower and middle flood basalt sequences with no correlation to modal mineralogy or flow texture. The clinopyroxene-rich glomerocrysts contain minor olivine and are devoid of plagioclase. Considering the clinopyroxene-rich glomerocrysts are of LT magma origin, their presence and phase assemblage suggest they crystallized at higher pressures more consistent with mid- deep crust rather than shallow crust. Initial melts derived from the impinging plume head would have migrated up through the sub-continental lithospheric mantle due to their buoyancy until the melts reached the density contrast where they would accumulate. The stalled, LT parental magma would begin to crystallize cpx + opx ± olivine. Existing seismic and gravity data supports the presence of a large mafic cumulate body beneath the NW Ethiopian plateau (Mackenzie et al. 2005; Cornwell et al. 2006, 2010; Stuart et al. 2006; Mammo 2013) overlain by large mafic dike swarms (Mège and Korme 2004). The presence of large mafic cumulates underlying CFBs has been shown in other LIPs (Holm and Prægel 2006; Ridley and Richards 2010). Recent petrologic studies of pyroxenite xenoliths from Cenozoic volcanism on the Ethiopian plateau show equilibrium pressures consistent with the crust/mantle boundary and suggest a linkage between opx + cpx-rich cumulates and LT flood basalt magmatism (Rooney et al. 2017). However, the cpx-glomerocrysts observed in this study lack orthopyroxene and are, therefore, further along the liquid line of descent. For this reason, we interpret the clinopyroxene cumulates observed in this study as having crystallized from stalled magmas in the lower to middle crust (Fig. 6). These lower to middle crust staging chambers are fed by more primitive magmas derived from lower in the lithosphere.

Fig. 6

Model for magmatism. a Lower flood basalt eruptive phase. Isolated eruptive centers exploit pre-existing weaknesses in the lithosphere, resulting in low magma flux and low-volume eruptions. Initial flows fill topographic lows and are ol-phyric with transitional basalt characteristics. Cpx-glomerocrysts found in flows are derived from stalled magmas in the mid- to lower crust. Transition toward plag-phyric is possibly related to early formation of sill complex in the shallow crust. b Lower flood basalt hiatus prior to middle flood basalt phase. Shallow reservoir forms plagioclase mush with oscillatory zoned plag-megacrysts during pause in flood basalt eruption. Silicic volcanism (tuff) results from fractionation in shallow reservoir. Paleosol develops due to weathering of flood basalts and/or tuff

The second most distinctive feature is the presence of paleosols that punctuate eruption of lava flows in the lower flood basalt. The paleosols represent weathering horizons of basalt (and/or silicic tuff) during pauses in flood basalt eruption. These pauses suggest a decrease in magma flux; magma may still be fluxing into the lithosphere, but the rate may not be high enough to cause them to erupt at the surface.

The transition from ol-phyric to plag-phyric flows in the lower flood basalt sequence appears to be a case for simple fractional crystallization of a single pulse of magmatism. However, paleosols and tuffs are found intercalated with these flows suggesting pulsed magma flux into the lithosphere and lower volume eruptive events. Likewise, modal variation between local stratigraphy (Fig. 5b) suggest isolated fractionation paths for magma traversing the lithosphere. It is more likely that the overall transition from ol-phyric to plag-phyric trend observed in the lower section reflects the early formation of a shallowing magmatic plumbing system in which initial lavas traversed the crust quickly, experiencing limited differentiation and producing ol-phyric flows (Fig. 6a). Over time, magmas began to stall in the shallow crust where they differentiated to form plag-phyric magmas prior to eruption (Fig. 6a). A pause in flood basalt extrusion, defined by a 30-cm-thick paleosol and 4-m-thick silicic tuff, occurs following the lower flood basalt emplacement (Fig. 6b).

Middle flood basalt: cyclicity, polybaric fractionation, and increasing flux

In the lower flood basalts, we observe an increase in plag-phyric flows interrupted by several pauses. This sequence was succeeded by eruption of the middle flood basalt. The most striking feature of the middle flood basalt is the appearance of plag-megacrystic flows with high An–plagioclase megacrysts (mostly labradorite) and strong oscillatory zoning. Sieve texture (although rare) is usually restricted to zones between the core and rim, suggesting some perturbation in equilibrium conditions midway through crystal growth. Thick plag-megacrystic flows often occur above paleosols and alternate with aphyric flows which can contain sparse plagioclase phenocrysts. Despite the high crystal cargo in plag-megacrystic flows (up to 35%) these flows are commonly observed with pahoehoe flow tops which suggest they are relatively low viscosity upon emplacement.

The occurrence of plagioclase-megacrystic basalt flows in other CFBs has been attributed to the presence of a shallow magma plumbing system that undergoes periods of longer residence between eruptive episodes (Hansen and Grönvold 2000; Sen 2001; Sen et al. 2006; Higgins and Chandrasekharam 2007; Borges et al. 2014) and the high An contents of plagioclase megacrysts has been explained by multiple recharge events into the shallow crystal mush zone (Óskarsson et al. 2017). The low viscosity nature of plag-megacrystic flows has been explained by processes related to disaggregation and eruption of the crystal cargo as a result of recharge (Óskarsson et al. 2017). From here on, we propose that the plag-megacrystic flows in this study are evidence of an established shallow mush zone undergoing frequent recharge.

Despite clear evidence of a shallowing fractionation system within the middle flood basalt, significant complexities in modal abundances require subdivisions of the middle group to resolve differentiation processes within this interval. On the flow-by-flow scale, there is no apparent systematic variation in modal mineralogy or flow thickness over time, indicating the magma plumbing system is too complex to assume simple differentiation by crystallization. However, when comparing large flow packages, a broader shift in plumbing system dynamics can be observed petrographically (e.g., middle flood basalt groups A–C). The alternating occurrence of cpx-cumulophyric and plag-megacrystic texture attest to varying contributions from the deeper and shallower staging chambers, respectively. The presence of cpx-glomerocrysts suggests pulses of magma recharge from the deeper staging chamber. Cyclical variation from ol-phyric → plag-ol-phyric → plag-phyric suggests either multiple episodes of magma differentiation due to recharge pulses or that the sequence of flows are tapping magmas from a variety of differentiation pathways within the magmatic plumbing system. We will attempt to deconvolute the complexity in cyclicity by addressing petrographic patterns observed in each sub-group of the middle flood basalt sequence.

Group A of the middle flood basalts represents a new pulse of magmatism that followed the lower flood basalt hiatus (Fig. 7a). It began with the extrusion of a thin aphyric flow immediately followed by a 24-m-thick plag-megacrystic flow. As the shallow reservoir was perturbed by newly injected magma, it disaggregated and mobilized the plagioclase mush, resulting in a plagioclase-megacrystic cargo with up to 35% plagioclase by volume (Fig. 7b). Ol-phyric flows were likely fed by dikes that bypassed the crystal mush, while aphyric flows may have bypassed the mush zone or erupted through feeders that previously cleared the loose crystal cargo from mush zones. Clinopyroxene glomerocrysts restricted to flows between 1000 and 1100 masl suggest an increase in magma supply feeding the shallow plumbing system from the lower crustal chamber. Alternating modal mineralogy of flows in this group reflect variation in magma differentiation by crystal fractionation which can either be explained by isolated feeder systems that have independent fractionation paths and/or by frequent injection of new magmas into the plumbing system (Fig. 7c).

Fig. 7

a Middle flood basalt: group A. Early eruption of thick plag-megacrystic basalt followed by ol-phyric and aphyric flows. Plag-megacrystic flows are produced when dikes from lower crustal magmas are captured by the shallow crystal mush zone, which triggers an eruption carrying the megacrystic cargo to the surface. Olivine–phyric flows are fed by dikes that by-pass mush zones. Aphyric flows may be fed by dikes directly from lower crustal magmas or after a pathway through the plagioclase mush zone is cleared. b Inset showing injection of new magma into plagioclase mush zone, mobilizing the zoned plagioclase megacrysts and producing a plag-megacrystic flow with up to 35% plag by volume. c Middle flood basalt: group A (continued). Continuing eruptions are fed by isolated fractionation pathways producing alternating flow packages of plag-ol-phyric and plag-phyric. Cpx-glomerocrysts are restricted to flows between 1000 and 1100 masl and suggest a continuing increase in magma flux. The shallow plumbing system continues to mature during influx of new magma. d Middle flood basalt: group B. Magmatic plumbing system achieves maturity and becomes an interconnected, homogenized, shallow dike and sill complex. Magma supply to surface is likely rapid to produces thick, insulated flows that form slow-cooling textures (aphyric–intergranular to aphyric–ophitic). Cpx-glomerocrysts are common in flows and occur in increasing frequency toward the top of this flow group, suggesting that magma recharging the lower crustal system is achieving an apex in flux. e Middle flood basalt hiatus between groups B and C. A temporary pause in flood basalt eruption and formation of a paleosol. Plagioclase mush develops in shallow reservoir and forms oscillatory zoned plagioclase megacrysts. Some portions of sill complex begin to freeze in the crust and no longer offer pathways for new magma. f Middle flood basalt: group C. New magma pulse re-mobilizes and disaggregates plagioclase mush and produces thick plag-megacrystic flows interbedded with thin aphyric flows fed by feeders that by-pass the mush zone. Cpx-glomerocrysts observed in this flow group suggest there are still significant volumes of magma recharging the lower crustal magma system. The final eruptions in this group produce very thick (> 35 m) aphyric flows with columnar jointing and cpx-glomerocrysts. This event represents the apex of flood basalt volcanism

Group B of the middle flood basalt represents both high magmatic flux and eruptive volume (Fig. 7d). These flows are predominantly thick, coarser-grained flows (8–16 m; intergranular to aphyric–ophitic textures) with stacked, thin, aphyric to plag ± ol-phyric flows. Cpx-glomerocrysts are common and occur with increasing frequency toward the top of the flow group, suggesting that magma flux into the lower staging chambers was high, while extrusion rates increased as the plumbing system reached maturity (e.g., wider conduits and an interconnected, homogenized magma plumbing system)(Fig. 7d). High extrusion rates resulted in thick flows with slow-cooling textures (aphyric–intergranular to aphyric–ophitic), compared to the more phyric and aphyric–microcrystalline flows of group A. These characteristics appear to indicate higher flux of magma into the plumbing system and higher extrusion rates than previously erupted groups.

A pause in flood basalt extrusion occurs between group B and group C of the middle flood basalts (Fig. 7e). During this hiatus, a plagioclase mush developed again in the shallow reservoir and formed oscillatory zoned plagioclase megacrysts. Some of the shallow reservoirs likely froze depending on volume and connectivity. A 30 cm paleosol formed at the surface of the middle flood basalt group B.

Group C of the middle flood basalt began with a new pulse of magma introduced to the shallow plumbing system from the deeper crustal magmas (Fig. 7f) and another episode of mush remobilization. This pulse produced alternating flows of plag-megacrystic, aphyric, and plag-phyric basalts. Cpx-glomerocrysts occurred at the beginning and in the middle of this flow group, suggesting there was still new magma recharging the deeper staging chambers. Flow thickness increases over time, from 8 to 10 m to nearly 40 m, with columnar jointing toward the top of the flow group. The dramatic increase in flow thickness seen in this flow group is not seen anywhere else in the stratigraphy presented here and attests to an apex in flood basalt eruption. There is an abrupt cessation in flood basalt eruptions immediately following group C (Fig. 8a). During this hiatus, a 50 cm paleosol forms on the uppermost surface of group C.

Fig. 8

a Hiatus between middle and upper flood basalts. Pause in flood basalt eruptions allows for the formation of a paleosol. Magma flux into the magmatic system has decreased, and the lower crustal stalled magmas begin to crystallize. As recharge to the shallow reservoirs shuts down, the plumbing system is no longer capable of sustaining a mush zone and the dike and sill complex freezes completely. b Upper flood basalt eruptive phase signifies the termination of flood basalts. Flows are uniformly thin (~ 5 m), dominantly plag-phyric or aphyric, and devoid of cpx-glomerocrysts and plag-megacrysts. The plag-phyric flows indicate that magma is once again stalling and fractionating in the shallow crust, but not long enough to produce large megacrystic plagioclase

Upper flood basalt: the death of a flood basalt

Above the 50 cm psol, the upper flood basalt sequence is characterized by an initial eruption of a 20-m-thick aphyric flow followed by predominantly plag-phyric and aphyric flows of consistent thickness (~ 5 m). The upper flood basalts began erupting after the hiatus and represent the onset of flood basalt termination (Fig. 8b). In addition to the fairly uniform modal composition and thickness, the upper flood basalts are devoid of cpx-glomerocrysts and megacrystic plagioclase. We hypothesize that lower flux of magma into the lithosphere could no longer produce the recharge volumes necessary to sustain the growth of large megacrystic plagioclase and allowed the shallow mush zones to freeze. The dominance of plagioclase in these flows also suggests lower flux, since a decrease in magma recharge would facilitate fractionation of the magmas residing in the lithosphere. The absence of cpx-glomerocrysts may also be explained by a decrease in flux into the deeper staging chambers, since higher volumes of newly injected magma would be more capable of disaggregating cumulate. As flood basalt activity terminated, volcanism transitioned to localized shield building activity across the plateau, beginning with the 30 Ma Simien shield near the LT flood basalts at Lima Limo (Kieffer et al. 2004). The transition from flood basalt eruptions to shield activity is further evidence of the decrease in magma flux associated with flood basalt termination.

Comparison with other CFBs

Continental flood basalt provinces are typically divided into three phases: (1) an initiation phase characterized by low-volume, transitional-alkaline lavas erupted via pre-existing weaknesses in the lithosphere; (2) a main phase characterized by high flux and large volume, fissure-fed, tholeiitic lavas with approximately 80–90% of the erupted volume extruded during this phase; and (3) a waning or termination phase characterized by a rapid reduction in volume and frequency of eruption, intercalated with silicic-explosive eruptions and more widely distributed volcanic centers (Jerram and Widdowson 2005). The transition between these phases reflects changes in the magmatic plumbing system in terms of the flux of magma entering and leaving the lithosphere.

During the initiation phase, early lavas tend to be more primitive, lower volume (in comparison to main phase), and intercalated with weathering horizons (Beane et al. 1986; Lightfoot et al. 1993; Jerram et al. 2000; Jerram and Stollhofen 2002; Sheldon 2003; Jerram and Widdowson 2005; Zhang et al. 2006; Beccaluva et al. 2009). These characteristics indicate that magma involved in early eruptions (1) experienced limited fractionation in comparison to later predominately plagioclase-phyric flood basalt eruptions, and (2) eruptions occur in short pulses separated by pauses. The observed variability in modal mineralogy found in the lower flood basalts in this study supports the idea of isolated feeder systems during early development of the magmatic plumbing system. As the magmatic plumbing system evolved, there is evidence of shallow fractionation in concert with evidence for deep fractionation. Geochemical and petrologic studies (Cox 1980; Mohr 1983; Lightfoot et al. 1990) provide evidence for polybaric fractionation in the evolution of CFB magmas and suggest that the depth of crustal storage shallows over time (Peate and Bryan 2008).

The largest volumes of basalt are erupted during main-phase volcanism (Self et al. 1997; Thordarson and Self 1998; Duraiswami et al. 2001; Bondre et al. 2004; Jerram and Widdowson 2005). This observation has led to debate over the rate of flood basalt eruption which has focused on dating, volume estimates, and flow morphology (Courtillot et al. 1986; Self et al. 1997; Rochette et al. 1998; Thordarson and Self 1998; Duraiswami et al. 2001; Bondre et al. 2004; Jerram and Widdowson 2005; Sen et al. 2006; Chenet et al. 2008; Barry et al. 2010; Reidel et al. 2013). Classic tabular flow facies with rubbly flow tops, thought to indicate high extrusion rates as in the Parana-Etendeka main phase (Jerram and Widdowson 2005), are generally rare in the NW Ethiopian flood basalt province. Conversely, braided lobe facies observed in the Deccan and Columbia River are thought to indicate prolonged, episodic eruptions (Self et al. 1997; Thordarson and Self 1998; Duraiswami et al. 2001; Bondre et al. 2004; Jerram and Widdowson 2005). Braided lobe facies were more common than classic tabular flow facies in the NW Ethiopian plateau, and an episodic main phase is supported by the oscillation of modal mineralogy and alternating occurrence of glomerocrysts and zoned megacrysts observed in this study. The growth of large zoned plagioclase megacrysts requires some duration of residence time in between eruptions, but frequent recharge into the mush is necessary to maintain crystal cargo mobility for future transport during eruptions. Similarly, the high An-content commonly observed in plagioclase megacrysts from CFBs requires frequent recharge of more primitive magmas into the mush zone. The occurrence of cpx-rich glomerocrysts is evidence of this recharge. Using the detailed petrostratigraphic analysis of consecutive flows documents a general increase in magma flux throughout the main phase, as evidenced by the increasing frequency of cpx-glomerocrysts and thicker lava flows that produce the greatest basalt volume prior to flood basalt termination.

During the termination phase, a general transition toward more evolved lavas is typically observed in CFBs; this is consistent with a magma plumbing system that is shutting down. For example, the late-stage eruptions of the Ethiopian–Yemen, Emeishan, and Parana-Etendeka CFBs are interbedded with silicic tuffs (Mohr and Zanettin 1988; Piccirillo et al. 1988; Ayalew et al. 1999, 2002; Ukstins et al. 2002; Zhang et al. 2006). In the upper flood basalts, we observe flows that are dominantly plag-phyric to aphyric and an absence of plagioclase megacrysts and cpx-glomerocrysts. This composition indicates two things: (1) the plumbing system feeding the upper flood basalt is dominated by shallow fractionation; and (2) recharge of new primitive magma into the shallow plumbing system is significantly reduced in comparison to the middle flood basalts. The tendency for flood lavas to be more plag-phyric toward the upper part of the sequence is consistent with observations made throughout the Ethiopian–Yemen CFB province (Mohr 1983; Pik et al. 1998; Kieffer et al. 2004) as well as the Deccan and Emeishan (Beane et al. 1986; Zhang et al. 2006). A decrease in magma flux is consistent with the transition from flood basalt eruption to shield activity in Ethiopia, and more broadly the termination of CFBs.

Conclusion

This study’s detailed petrographic analyses of a ~ 1600 m flow-by-flow stratigraphy provides critical insights into the evolution of the magmatic plumbing system of CFBs. Initial flows are fed by magmas that have experienced deeper fractionation (mid- to deep crust). Overtime, the fractionation became polybaric thus reflecting the stalling of magmas in both the shallow and deeper crust. Overall the flux of new primitive magma appears to have been continuous throughout the eruption of the middle flood basalts but its magnitude changed over time. Changes in flux magnitude control how much lava erupts at the surface as well as hiatuses in eruption. During hiatuses, megacrystic plagioclase with high An content (labradorite) and oscillatory zoning form, indicating constant feeding of the shallow plumbing system by more primitive magmas. In contrast, the very high extrusion rate witnessed during group B of the main eruptive phase suggests an apex in magma flux through the lithosphere. Shallowing of the magma plumbing system toward the end of the eruptive phase is consistent with observations of increased silicic volcanism in the Ethiopian flood basalt province and other CFBs. Furthermore, changes in magma flux are consistent with the typical observation that CFBs undergo a transition from fissure-fed flood basalt eruptions to magmatism centered on large volcanic shields.

The data presented here suggest that the main phase of flood basalt eruption is better explained by pulses of high flux, rather than continuous high flux. The origin of these pulses remains enigmatic and may relate to heterogeneities in plume composition, variation in upwelling rate, or mantle potential temperature. The results of this study provide new petrographic constraints that require revision of existing models of plume–lithosphere interaction in terms of pulsed versus continuous magma flux. Although the details of the magmatic plumbing model presented herein may prove to be unique to the Ethiopian–Yemen CFB province, these petrographic observations and lithologic associations are commonly found in other CFBs. These detailed petrographic analyses, framed within a well-constrained stratigraphy, have provided new geochemical insights into the initiation and evolution of the Ethiopian flood basalt province and offer a new approach for understanding magma evolution in other CFBs.

Notes

Acknowledgements

Thoughtful comments by Nicholas Arndt and an anonymous reviewer helped imrpove this manuscript. We thank Mark Ghiorso for careful editorial handling and helpful suggestions. We thank Gabre Meskel and other members of the field team for assisting us. We also thank the Ethiopian Ministry of Mines.

Funding

This work was supported by the United States National Science Foundation [EAR-1219647, EAR-1219459, BCS-0921009].

Supplementary material

410_2018_1460_MOESM1_ESM.xlsx (24 kb)
Supplementary material 1 Table S1. Sample locations, flow elevations and thickness, and general outcrop notes. Location information is reported using WGS84 in decimal degrees latitude and longitude (DD Lat and DD Long, respectively). Elevations are reported in meters above sea level. Minimum and maximum elevation reported when contacts between flows were observed in the field (unless specified otherwise). Flow thickness determined from minimum and maximum thickness. a Denotes minimum thickness of flow based on height of exposed outcrop where contacts were not observed (XLSX 24 KB)
410_2018_1460_MOESM2_ESM.xlsx (21 kb)
Supplementary material 2 Table S2. Petrographic information for individual samples. Textures and mode (modal mineralogy) are as described in results sections 4.1.1 and 4.1.2, respectively. The total volume % phenocrysts are approximations from visual inspection of thin sections and hand sample. The %An for plagioclase was determined petrographically by Michel Lévy method. NS= no sample collected (XLSX 20 KB)

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • S. R. Krans
    • 1
  • T. O. Rooney
    • 1
  • J. Kappelman
    • 2
  • G. Yirgu
    • 3
  • D. Ayalew
    • 3
  1. 1.Department of Earth and Environmental SciencesMichigan State UniversityEast LansingUSA
  2. 2.Department of Anthropology and Department of Geological SciencesUniversity of TexasAustinUSA
  3. 3.Department of Earth SciencesAddis Ababa UniversityAddis AbabaEthiopia

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