Variation in mantle lithology and composition beneath the Ngao Bilta volcano, Adamawa Massif, Cameroon volcanic line, West-central Africa

Journal Pre-proof Variation in mantle lithology and composition beneath the Ngao Bilta volcano, Adamawa Massif, Cameroon volcanic line, West-central Africa T.E.M.D.J.I.M. Robert, N.J.O.M.B.I.E.W.A.G.S.O.N.G. Merlin Patrick, N.Z.A.K.O.U.T.S.E.P.E.N.G. Arnold Julien, F.F.O.L.E.Y. Stephen PII:

S1674-9871(19)30156-2

DOI:

https://doi.org/10.1016/j.gsf.2019.08.002

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GSF 877

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Geoscience Frontiers

Received Date: 24 April 2019 Revised Date:

28 July 2019

Accepted Date: 5 August 2019

Please cite this article as: Robert, T., Merlin Patrick, N.W., Arnold Julien, N.T., Stephen, F.F., Variation in mantle lithology and composition beneath the Ngao Bilta volcano, Adamawa Massif, Cameroon volcanic line, West-central Africa, Geoscience Frontiers, https://doi.org/10.1016/j.gsf.2019.08.002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Variation in mantle lithology and composition beneath the Ngao Bilta volcano, Adamawa Massif, Cameroon volcanic line, West-central Africa

Robert TEMDJIM a, Merlin Patrick NJOMBIE WAGSONG a, Arnold Julien NZAKOU TSEPENG a and Stephen F. FOLEY b*

a

Department of Earth Sciences, Faculty of Sciences, University of Yaounde I, P.O. Box 812 Yaoundé - Cameroon.

b

Department of Earth and Planetary Sciences and ARC Centre of Excellence for Core to Crust Fluid Systems, Macquarie University, North Ryde, NSW 2109, Australia.

*

Corresponding author.

E-mail addresses TEMDJIM R.: [email protected] NJOMBIE WAGSONG P. M.: [email protected] NZAKOU TSEPENG J. A.: [email protected] FOLEY S.F: [email protected]

ABSTRACT Mantle peridotites entrained as xenoliths in the lavas of Ngao Bilta in the eastern branch of the continental Cameroon Line were examined to constrain mantle processes and the origin and nature of melts that have modified the upper mantle beneath the Cameroon Line. The xenoliths consist mainly of lherzolite with subordinate harzburgite and dunite. They commonly contain olivine, orthopyroxene, clinopyroxene and spinel although the dunite is spinel-free. Amphibole is an essential constituent in the lherzolites. Mineral chemistry differs between the three types of peridotite: olivines have usual mantle-like Mg# of around 90 in lherzolites, but follow a trend of decreasing Mg# (to 82) and NiO (to 0.06 wt.%) that is continuous in the dunites. Lherzolites also contain orthopyroxenes and/or clinopyroxenes with low-Mg#, indicating a reaction that removes Opx and introduces Cpx, olivine, amphibole and spinel. This is attributed to reaction with a silica-undersaturated silicate melt such as nephelinite or basanite, which originated as a low-degree melt from a depleted source as indicated by low Al2O3 and Na2O in Cpx and high Na2O/K2O in amphibole. Thermobarometric estimates place the xenoliths at pressures of 11–15 kbar (35–50 km) and temperatures of 863–957˚C, along a dynamic rift geotherm and shallower than the region where carbonate melts may occur. The melt/rock reactions exhibited by the Ngao Bilta xenoliths are consistent with their peripheral position in the eastern branch of the Cameroon Volcanic Line in an area of thinned crust and lithosphere beneath the Adamawa Uplift.

Keywords: Cameroon Volcanic Line; Adamawa Volcanic Massif; peridotite xenoliths; partial melting; melt-rock reactions.

1. Introduction Numerous mantle xenoliths occur in many volcanic provinces of the Cameroon volcanic Line (CVL), both in the oceanic sector (Caldeira and Munhá, 2002; Matsukage and Oya, 1

2010; Fig.1) and in the continental sector (Dautria and Girod, 1986; Lee et al., 1996; Teitchou et al., 2007, 2011; Wandji et al., 2009; Matsukage and Oya, 2010; Temdjim, 2012; Nguihdama et al., 2014; Tamen et al., 2015; Liu et al., 2017; Mbowou et al., 2017; Njombie Wagsong et al., 2018; Fig.1). We focus in this study on peridotite xenoliths hosted in basaltic lavas of the Ngao Bilta volcano in the Adamawa Volcanic Massif (AVM), which is located in the eastern branch of the continental sector of the Cameroon Line (Fig.1). These xenoliths provide the opportunity to investigate mantle processes beneath the continental domain of the Cameroon Line, and may yield important information about the evolution of the mantle beneath the AVM, which may be related to magma origin. Previous studies show an inhomogeneous distribution of xenolith types over a relatively small area. Spinel lherzolites, pyroxenites and wehrlites may show variable Mg# (100×Mg/(Mg+Fe)) from the usual mantle values around 90 to much lower values in the low 80s (Mbowou et al., 2017; Tedonkenfack et al., 2019), whereas in other areas lherzolites dominate and pyroxenites are missing (Nkouandou and Temdjim, 2011; Njombie Wagsong et al., 2018). The migration of partial melts and melt-rock reactions involved in metasomatism and refertilization modify the composition and microstructures of mantle rocks (Lenoir et al., 2001; Tommasi et al., 2004; Le Roux et al., 2007). These processes play a key role in the formation, and in the mineralogical and chemical evolution of the sub-continental lithospheric mantle (SCLM; Tommasi et al., 2008; Zong and Liu, 2018). Mantle processes beneath the AVM have been studied in previous works (Lee et al., 1996; Nkouandou and Temdjim, 2011; Nkouandou et al., 2015; Njombie Wagsong et al., 2018). The migration of different melts have been held responsible for the modifications to the mantle rocks, varying from basaltic melts, through hydrous silicate melts to carbonatites (Njombie Wagsong et al., 2018; Tedonkenfack et al., 2019). This paper is a first report on three different types of mantle xenoliths found in the Ngao Bilta volcano, aimed at illuminating mantle processes and lithological variation beneath the AVM. 2

2. Geological setting The Ngao Bilta volcano is located in the Ngaoundere Plateau, an uplifted section of the Adamawa Volcanic Massif (Fig. 1). The AVM is bordered to the north and south by PanAfrican faults oriented N70°E (the Adamawa and Mbere-Djerem faults). The tectonic evolution of these faults has been linked to reactivation of shear zones since Cretaceous times (Moreau et al., 1987; Genik, 1993; Guiraud et al., 2005) following the opening of the Southern Atlantic Ocean. The basement rocks of the Ngaoundere Plateau belong to the Pan-African fold belt and comprise mainly granitoids and metamorphic rocks. The granitoids yield ages of 615 ±27 Ma to 575±27 Ma (Tchameni et al., 2006; Ganwa et al., 2008) and were intensively deformed and metamorphosed under amphibolite-facies conditions. The oldest metamorphic rocks with ages of ~ 2.1 Ga (Penaye et al., 2004) were reworked during the Pan-African orogeny (Toteu et al., 2001). The basement rocks are crosscut and partially covered by numerous types of Oligocene to Pleistocene volcanic rocks with alkaline to peralkaline affinities (Temdjim et al., 2004; Nkouandou et al., 2008; Fagny et al., 2012). Some of these volcanic rocks contain various types of mantle xenoliths (Dautria and Girod, 1986; Lee et al., 1996; Matsukage and Oya, 2010; Nkouandou and Temdjim, 2011; Nguihdama et al., 2014; Mbowou et al., 2017; Njombie Wagsong et al., 2018). Geophysical studies indicate the Moho depth beneath the Adamawa Plateau to be ~33 km (Stuart et al., 1985; Poudjom Djomani et al., 1997; Nnange et al., 2000; Goussi Ngalamo et al., 2017) with uplifted areas associated with thinner crust (Guidarelli and Aoudia, 2016).

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3. Analytical methods The modal compositions of mantle xenoliths from Ngao Bilta volcano (Table 1) were determined from images of Na, Mg, Al and Ca distributions (Multispec software; Biehl and Landgrebe, 2002) over the full area of each thin section. The chemical compositions of minerals were determined by a Jeol JXA 8900 RL electron microprobe at the Department of Geosciences, University of Mainz. Operating conditions were15 kV acceleration voltage and 12 nA probe current using a focused beam size of 2 µm. Counting time was 20–30 s depending on element abundance.

4. Petrography and mineral chemistry 4.1. Petrographic features of the Ngao Bilta mantle peridotites Mantle xenoliths were collected from basaltic lava flows associated with the scoria cone of the Ngao Bilta volcano. Our attention was focused on the more common smaller samples, which have an average size of 1.2 up to 6.0 cm in diameter. They vary from angular to subrounded shapes in contact with the host basalts. Modal compositions of the eight mantle xenoliths are reported in Table 1. Three main types of peridotites can be identified on the basis of mineral assemblage; lherzolite, harzburgite and dunite. The most abundant xenolith types at Ngao Bilta are spinel lherzolite and dunite; spinel harzburgite is subordinate. Although some mantle xenoliths are weathered, minerals can usually be recognized; we focused our attention on the freshest samples, so that our analyses and conclusions are not compromised by the effects of weathering. They consist of yellowish-green olivine (Ol), emerald-green clinopyroxene (Cpx), brown orthopyroxene (Opx) and dark spinel (Spl) with metallic lustre.

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4.1.1. Spinel-bearing lherzolite (SL) Five representative samples (DB-01, DB-06, BJ-01, N49-02 and N47-02; Table 1) were selected for study from 13 lherzolites collected. SL exhibit protogranular texture and are composed of olivine, orthopyroxene, clinopyroxene and spinel, with minor amphibole. They have variable abundances of olivine (53–66 vol.%), orthopyroxene (18–28 vol.%) and clinopyroxene (12–19 vol.%) with Opx/Ol ratios varying from 0.28 to 0.50. Spinel (0.9–3.6 vol.%) occurs as intergranular grains (Fig. 2e). Amphibole (0.24–3.6 vol%; Table 1) is the only hydrous phase in the lherzolites. Olivines are subhedral to anhedral and 1–5 mm in diameter. They are cracked and sometimes show kink band structures (Fig. 2a, b). Orthopyroxene forms subhedral crystals with grain sizes ranging from 1 to 3 mm and shows kink band structures. Orthopyroxene sometimes has exsolution lamellae of clinopyroxene (Fig. 2c). Clinopyroxenes have an average size below 3 mm and show polygonal grain boundaries with olivine and orthopyroxene, forming triple-point junctions. This feature indicates textural equilibrium between Cpx, Opx and olivine (Lee et al., 1996), discounting the possibility of later introduction of Cpx, which is evident from low dihedral angles in mantle lherzolites elsewhere (Glaser et al., 1999; Rehfeldt et al., 2008). In some samples, clinopyroxene crystals exhibit exsolution lamellae of orthopyroxene (Fig. 2b, d). Pale brown to brownish spinels with 0.5 mm diameter occur interstitially between olivine and pyroxenes and often occur in groups (Fig. 2e, f). The spinel is anhedral and more rarely vermicular. A trace amount of amphibole is found in SL, occurring as small (< 0.5 mm) thin pleochroic lamellae, spatially associated with spinels (Fig. 2f).

4.1.2. Spinel-bearing harzburgite (SH) Only one fresh sample of harzburgite (DB-02) was analysed, but the modal abundances of minerals are similar in other, unanalysed, samples. This sample has protogranular texture 5

(Fig. 3a) and a modal abundance of olivine of 80 vol.%, higher than in the lherzolites, whereas that of orthopyroxene (16%), clinopyroxene (3.4%) and spinel (0.44%) are lower (Table 1). This results in a lower Opx/Ol ratio (0.19) in SH than in SL (0.28–0.50). Olivines are euhedral to subhedral with grain sizes up to 3 mm in diameter. They commonly show strain features such as cracks (Fig. 3b) but are not kinked as in the SL. Opx crystals (> 5 mm) are subhedral and exhibit kink bands. They often show exsolution lamellae of clinopyroxene that disappear near the crystal borders (Fig. 3c). Cpx crystals (0.5–2.0 mm) are subhedral to anhedral, and occasionally contain fine parallel exsolution lamellae of Opx (Fig. 3d). Coarse euhedral to subhedral crystals of clinopyroxene may exhibit triple-grain junctions approaching 120° with olivine and orthopyroxene. The SH rarely contains brown spinel, which occur as very small intergranular anhedral grains (<0.8 mm; Fig. 3b, e). Fine-grained neoblasts of olivine, Opx and Cpx occur rarely (Fig 3f).

4.1.3. Spinel-free dunite (Dunite) Dunites (samples DB-03 and DB-04) consist of 93–97 vol.% olivine and 2.4–5.8 vol.% orthopyroxene. Clinopyroxene is always present but scarce (0.5–1 vol.%), whereas spinel is notably absent. In thin section, dunites show a strongly deformed mixed porphyroclastic to equigranular texture, which is inhomogeneous on a small scale (Fig. 4a). It consists of a finegrained matrix of recrystallized olivine, Opx and Cpx surrounding porphyroclasts of the same minerals (Fig. 4a, b). Olivine porphyroclasts (5–8 mm) form elongated grains with a weak preferred orientation and exhibit kink band structures (Fig. 4b), and are surrounded by aggregates of olivine neoblasts (Fig. 4a, b). Recrystallized olivine neoblasts at the borders of porphyroclasts show undulose extinction. Neoblasts are polygonal and sometimes show triple junctions approaching 120°. Orthopyroxenes also show two generations: porphyroclasts (2–4 mm), commonly with kink bands (Fig. 4c), and neoblasts (Fig. 4d). Subordinate clinopyroxenes are smaller (1–3 mm) than coexisting olivine and Opx (Fig. 4c, d): Cpx 6

porphyroclasts exhibit undulose extinction (Fig. 4c) and are surrounded by clinopyroxene neoblasts.

4.2. Mineral chemistry Representative analyses of mineral phases of Ngao Bilta mantle are reported in Supplementary Tables S1–S5, including analyses for cores, rims, neoblasts and porphyroclasts.

4.2.1. Olivine Olivines from all three groups of Ngao Bilta mantle xenoliths are variable in composition (Fig. 5; Supplementary Table S1). They are most homogeneous in spinel lherzolites (SL), with most between Mg# 89.5 and 90.9, which is typical for primitive mantle olivines (Mg# 86–90; Chen et al., 2003) and to Type I xenoliths (Mg# 86–91; Frey and Prinz., 1978). A smaller group with Mg# 87–88 (Fig.5a, b) are neoblasts. CaO contents are low (< 0.20 wt.%) but form two groups at <0.08 wt.% and mostly 0.14–0.20 wt.% (Fig. 5b); the former group is typical for spinel lherzolites in the mantle (Köhler and Brey, 1990; Foley et al., 2013). NiO, however, is uniformly in the mantle range of 0.24–0.36 wt.%. These minor element contents, including 0.05–0.21 wt.% MnO, are similar to olivines in spinel lherzolites elsewhere in the CVL, including both oceanic (Caldeira and Munhá, 2002) and continental sectors (Lee et al., 1996; Matsukage and Oya, 2010; Nkouandou et Temdjim, 2011; Teitchou et al., 2011; Temdjim, 2012; Nguihdama et al., 2014; Nkouandou et al., 2015; Njombie Wagsong et al., 2018). Olivines in spinel harzburgites (SH; Fig. 5; Supplementary Table S1) have Fo (84.3–90.2), MnO (0.12– 0.26 wt.%), CaO (0.02–0.22 wt.%) and NiO (0.07–0.37 wt.%). Here there are two distinct groups: Mg# >90 for primary crystals, and Mg# 84–85 for neoblasts, the latter group with notably lower Mg# than their counterparts in SL. The harzburgitic olivines are 7

similar to those of harzburgites reported from Sao Tomé (Caldeira and Munhá, 2002) in the oceanic sector of the CVL and from Lake Nyos (Teichou et al., 2011; Temdjim, 2012) in the continental sector. Some grains vary in composition with a moderate increase in CaO from core to rim. This may be due either to partial re-equilibration at low pressure (Stormer; 1973) or to diffusive processes caused by heating (Takahashi, 1980). The olivine porphyroclast and neoblast compositions in dunites (Supplementary Table S1) are much lower in Mg# (82.2–88.0) and NiO (0.10–0.28 wt.%), and higher in CaO (0.12– 0.31 wt.%) than most olivines in SL and SH whereas the MnO (0.06-0.26 %wt) contents are similar. The dunites contain no olivines with Mg# >88 and thus so show no primitive mantle compositions. Similar olivines have been documented in dunites from Mount Cameroon Volcano in the (continental CVL; Wandji et al., 2009). Olivines show slight intra-grain heterogeneity in dunites, with cores similar to the rims of olivine in SL. Dunitic olivines describe a more continuous trend than those in SL and SH (Fig. 5), on which neoblasts in SL and SH lie. These trends describe a broad positive correlation of Mg# with NiO (Fig. 5a) and negative correlation with CaO (Fig. 5b). These trends are similar to those expected from fractional crystallization of a mafic melt.

4.2.2. Orthopyroxene Orthopyroxenes in all Ngao Bilta mantle peridotite suites have compositions that differ more significantly from one rock to another than the olivines, and form much more distinct groups (Fig. 6; Supplementary Table S2). Orthopyroxenes in SL form two distinct groups with Mg# 89.8–91.0 and 83.3–85.1, but which do not differ in CaO (Fig. 6a; Supplementary Table S2). Orthopyroxenes in SH show only a slight compositional variation (Mg# 90.2– 90.9; Supplementary Table S2), and are missing the group with values in the mid-80s. Their Mg# are typical of type I ultramafic xenoliths (Mg# 90–92; Frey and Prinz, 1978) and are comparable to those of orthopyroxenes in the harzburgites of Lake Nyos (Mg# = 88.3–91.0; 8

Teitchou et al., 2011; Temdjim, 2012) in the CVL, and Ngaoundere (Mg# = 90–91; Nkouandou et al., 2015) elsewhere in the Adamawa Volcanic Massif. Minor elements in SH orthopyroxene are CaO (0.50–1.8 wt.%), Al2O3 (2.6–2.9 wt.%) and TiO2 (<0.13 wt.%). Despite the uniform Mg#, they form two groups with CaO 0.50–0.56 wt.% and 1.40–1.78 wt.% (Fig. 6a, b), whereby TiO2 correlates positively with CaO (Fig. 6b). Opx is only a trace phase in the dunites (max. 5.8 modal%) and is restricted to the lower Mg# group (83.6–84.1). This is similar to Opx in dunites from Mount Cameroon (Wandji et al., 2009). The CaO contents (0.32–0.48 wt.%; Supplementary Table S2) of orthopyroxenes in dunites are close to those of orthopyroxenes (0.30–0.62 wt.%; Supplementary Table S2) in lherzolites, but much lower than those some harzburgites (up to 1.78 wt.%; Supplementary Table S2), which is counter-intuitive for equilibrium in mantle peridotites. TiO2 contents (0.07–0.12 wt.%; Supplementary Table S2) in dunite Opx are in the same range than those in both SL (0.01– 0.15 wt.%; Supplementary Table S2) and SH (0.02–0.13 wt.%; Fig. 6b). All Ngao Bilta peridotites have orthopyroxenes with TiO2≤ 0.15 wt.%, comparable to those registered in mantle xenoliths throughout the CVL (Lee et al., 1996; Caldeira and Munhá, 2002; Matsukage and Oya, 2010; Nkouandou et Temdjim, 2011; Teitchou et al., 2011; Temdjim, 2012; Nguihdama et al., 2014; Nkouandou et al., 2015; Njombie Wagsong et al., 2018).

4.2.3. Clinopyroxene The chemical compositions of clinopyroxene in SL and SH are reported in Supplementary Table S3. Clinopyroxenes in SL are diopside to augite with Mg# ranging from 86.0 to 92.5. These form three groupings, of which two are magnesian (Mg# 90.0–92.5 and 91.5–92.0), but differ slightly in Al2O3 and strongly in TiO2 (Fig. 7a, b). These two magnesian groups have Mg# slightly higher than those of coexisting olivines and orthopyroxenes. The wide ranges of SL clinopyroxenes in Al2O3 (3.71–7.83 wt.%), TiO2 (0.08–0.57 wt.%), Na2O (0.74–2.27 wt.%), CaO (19.4–22.1 wt.%) and Cr2O3 (0.38–1.07 wt.%) also reflect these three 9

groupings. One group with Mg# 90.0–92.5, Al2O3 5.5–5.9 wt.%, TiO2 0.44–0.57 wt.%, Cr2O3 0.79–1.07 wt.%, and Na2O 1.51–2.17 wt.% is similar to clinopyroxenes from spinel lherzolites in continental lithosphere (Pearson et al., 2003). The other magnesian group has mostly lower Cr2O3 (Fig.7c) and Na2O (Fig.7d) as well as TiO2 and Al2O3. Samples with the low-TiO2 group of clinopyroxenes coexist with amphibole. The third group has distinctly lower Mg# (86.0–87.5): it occurs in the SL and encompasses all harzburgitic clinopyroxenes (Fig.7a). This group has lower contents of Al2O3 (3.63–3.98 wt.%) and Na2O (0.78–0.89 wt.5; Fig. 7d), but similar Cr2O3 and intermediate TiO2 (Fig.7b). This group corresponds to the latest generation (neoblasts). SL clinopyroxenes have Cr# (Cr/(Cr+Al)) with higher values than those reported in spinel lherzolite xenoliths from Hosséré Garba (Cr#cpx= 6.5–7.2; Nguihdama et al., 2014) and Youkou (Cr#cpx= 4–9.5; Njombie Wagsong et al., 2018) in the Adamawa Volcanic Massif, which may indicate to a higher degree of depletion (Pearson et al., 2003). The depleted SL clinopyroxenes have AlVI between 0.048 and 0.120: their AlVI/AlIV ratios (0.43–1.95) are similar to those reported from mantle peridotites from CVL (Lee et al., 1996; Caldeira and Munhá, 2002; Matsukage and Oya, 2010; Nkouandou et Temdjim, 2011; Teitchou et al., 2011; Temdjim, 2012; Nguihdama et al., 2014; Nkouandou et al., 2015; Njombie Wagsong et al., 2018). The SH clinopyroxenes’ Mg# differ from those in harzburgite from Lake Nyos (Mg# 89.8–91.0; Teitchou et al., 2011; Temdjim, 2012) and Sao Tomé (Mg#cpx = 86.9–93.8; Caldeira and Munhá, 2002), and especially from those in harzburgite from Adamawa plateau (Mg#cpx = 95–98; Nkouandou et al., 2015).

4.2.4. Spinel The chemical compositions of spinel crystals in SL are presented in Supplementary Table S4. The correlation seen in Fig.8a indicates three distinct groups of spinel in SL in terms of their Cr#, Mg# and TiO2 contents. This variation has not been documented before in spinel 10

lherzolite xenoliths from the Adamawa Volcanic Massif, from which only the high-Mg# group has been identified before (Fig.8a). The two samples with lower Mg# spinels (Fig.8a) correspond to those with different olivine compositions, accounting for the two groups with higher CaO (0.14–0.20 wt.%) in Fig.5b. Spinels in sample DB-01 have high Cr# (39–40) in the upper range typical for continental spinel lherzolite mantle, which together with olivine Mg# of 89.9–90.9, gives them a central position in the OSMA array (Olivine-Spinel Mantle Array; Arai, 1994). They have the lowest modal spinel abundances of all lherzolite samples, which is consistent with the trend of increasing Cr2O3 as spinel modal abundance falls during partial melting (Kurat et al., 1980), which supports an origin as residues after partial melting. Coexisting orthopyroxenes and clinopyroxenes, however, have low Mg# (83.3–85.1 and 87.0–87.4, respectively), indicating a lack of equilibrium with the olivine and spinel. The second anomalous group has lower Mg# (67.9–68.8) and higher TiO2 (0.67–0.69 wt.%) than the main group (Fig.8a, b). Coexisting olivines are the group that overlaps with the most Mg-rich dunitic olivines, with Mg# 87.3–90.4 and CaO 0.15–0.20 wt.% (Fig. 5); coexisting orthopyroxenes (Mg# 90.5– 90.7) and clinopyroxenes (Mg# 91.0–92.5; Na2O 1.5–2.2 wt.%) have unremarkable, mantlelike compositions. This sample (BJ-01) has the highest modal abundance of amphibole. The third, most abundant, spinel group has low Cr# (9.6–13.2) and TiO2 (<0.1wt.%) and the highest Mg# (75.9–76.5; Fig. 8) and Al2O3 (54.9–58.3 wt.%). These samples have uniform, low-CaO (<0.08 wt.%) olivines with Mg# 89.4–90.5: some have Opx (89.9–91.0) and Cpx (91.5–92.0) with high Mg# in keeping with equilibrium, whereas others have lower values for both (≈84 vs. 86.0–87.4). The Al-rich spinel group (Fig. 8) is similar to those in spinel lherzolites from Lake Nyos (Temdjim, 2012; Pintér et al., 2015) and Barombi Mbo (Teitchou et al., 2011; Pintér et al., 2015) in the continental sector of the CVL.

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4.2.5. Amphibole Major element contents of amphiboles in SL are presented in Supplementary Table S5. They are mainly pargasite but also include pargasitic hornblende (Leake et al., 1997). Mg# varies from 82.6 to 89.9; in one sample all are >89, whereas in another the full range (82.6– 89.4) is present. These amphiboles are similar to those in lherzolites elsewhere in the CVL (Lee et al., 1996; Princivalle et al., 2000; Caldeira and Munhá, 2002; Matsukage and Oya, 2010; Nkouandou et Temdjim, 2011; Teitchou et al., 2011; Temdjim, 2012; Nguihdama et al., 2014; Pintér et al., 2015; Njombie Wagsong et al., 2018). All coexisting olivines have typical peridotitic Mg# (89.4–90.9) as do clinopyroxenes (91.5–92.0). Spinels belong exclusively to the high Al2O3 group, and both spinels and clinopyroxenes in these samples (N47-02 and N49-02) belong to the groups with lowest TiO2 contents. Amphiboles show heterogeneous compositions with high values of Al2O3 (13.75–15.56 wt.%), Na2O (3.24–4.12 wt.%), TiO2 (1.79–5.45 wt.%), and K2O (0.08–0.94 wt.%), but low Cr2O3 (0.58–1.47 wt.%). These compositions are similar to those of metasomatic amphiboles in veins, or disseminated in mantle peridotites (Ionov et al., 1997).

5. Thermobarometry of Ngao Bilta xenoliths The Ngao Bilta xenoliths contain mineral assemblages made up of olivine, orthopyroxene, clinopyroxene and spinel that offer mineral pairs that can usually be used for temperature and pressure estimates. However, this assumes equilibrium between all phases, which is demonstrably not the case for several mineral pairs in some of the xenoliths. Fertile upper mantle peridotites have bulk rock Mg# ≈90, which is due principally to olivine of Mg# 90 dominating the modal mineralogy of the rock (63%–64% on average in off-craton xenoliths; Pearson et al., 2003). Orthopyroxene has a similar Mg#, whereas that of clinopyroxene is slightly higher (91–92). Spinel has considerably lower and more variable Mg#, which correlates negatively with Cr# (100×Cr/(Cr+Al); Arai, 1994), but this has little effect on rock 12

Mg# because it is present in only trace amounts. Most of the Ngao Bilta xenoliths diverge from these expectations, indicating disequilibrium, which we assign to magmatic overprinting of the original metamorphic equilibrium assemblages. Nevertheless, many minerals have remnants of their original compositions that can be used to estimate pressure-temperature conditions of origin before the magmatic overprinting. The two-pyroxene thermometer of Brey and Köhler (1990; BK2-px) is preferred for spinel lherzolites in which there is no magmatic overprinting of Mg#. Single-pyroxene thermometers can be used for cases where one of the pyroxenes has been modified: here, the Ca-in-Opx thermometer of Brey and Köhler (1990; BKOpx) or the Cpx thermometer of Nimis and Taylor (2000) can be used, whereby we note that the latter is intended for garnet peridotites. Results for all three are listed in Table 2, together with temperatures from an old calibration of the 2-pyroxene thermometer (Wells, 1977), which is considered suitable for pyroxenes with lower Mg# (Xu et al., 1998). Although all barometers for spinel lherzolites fall behind the precision of those for garnet peridotites (O’Reilly et al., 1997; Xu et al., 1998; Foley et al., 2006), the best available uses Ca exchange between olivine and Cpx (Brey and Köhler, 1990; BKP): for Ngao Bilta xenoliths, results are listed only for samples with equilibrium Mg# Ol+Cpx pairs. The two-pyroxene barometer of Putirka (2008) is also listed in Table 2 and gives remarkably similar results, offering confidence in the accuracy of both. Using the Mg# relationships noted above to indicate equilibrium conditions, samples N4902 and BJ-01 have olivine and both pyroxenes that may reflect pre-magmatic conditions. These result in a temperature range of 908–957˚C (BK-2px) and a restricted pressure range of 12.1–14.6 kbar, well above the spinel/garnet transition pressure in peridotites (20–25 kbar; Glaser et al., 1999; Kempton et al., 1999; Foley et al., 2006). These points fit a dynamic rift geotherm (Fig. 9), in keeping with shallow magmatic activity beneath the Cameroon Line. Sample DB-06 confirms this result (919 ˚C at 14.8 kbar) using high-Mg# cores.

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In the other lherzolite samples, at least one of the pyroxenes has been overprinted by melts, causing a reduction in Mg# (84–87), meaning that temperatures can only be estimated with a single-mineral thermometer. The Ca-in-Opx thermometer (Brey and Köhler, 1990) gives similar temperatures of 879–904 ˚C for samples with overprinted Opx; their pressures of 13.2–14.8 kbar may represent conditions predating the magmatic overprint. The spinel harzburgite (DB-02) gives a similar temperature of 908 ˚C with BKOpx, whereas the dunite with minor Opx (DB-03; Mg# 84) gives a lower temperature of 863 ˚C. Pressures cannot be constrained for the last two due to the lack of Cpx. The Nimis and Taylor (2000) thermometer gives results generally lower than the preferred BK thermometers, as does the Wells (1977) thermometer, despite being recommended for pyroxenites.

6. Discussion 6.1. Origin of spinel peridotites and magmatic overprinting The silicate minerals of the spinel lherzolites of Ngao Bilta volcano display Mg# pairs that are characteristic for spinel lherzolite xenoliths and massif peridotites from the upper mantle. All SL contain olivines with Mg# of 90: where these coexist with Opx of Mg# 90 and Cpx with Mg# 91, they correspond to equilibrium lherzolite assemblages. They are only mildly depleted with respect to primitive mantle (ca. 10%; Matsukage and Oya, 2010), as indicated by the olivine Mg# of 90 coupled with low Cr# in spinel (Fig. 8). Only one sample (DB-01) has higher Cr# of 39–40 indicative of a higher degree of depletion (≈25%; Matsukage and Oya, 2010), which is consistent with the low modal abundance of spinel (Kurat et al., 1980). The inhomogeneity in the degree of depletion may indicate either local variations in partial melting, or that most samples have been re-fertilized, meaning they had Mg# that were higher before magmatic re-enrichment (Zhang et al., 2007). The pyroxenes in sample DB-01 have Mg# lower than expected (Opx 84, Cpx 87) which is interpreted to be evidence of magmatic overprinting (see below). The rare samples with high Cr# spinels may be remnants of once 14

widespread depleted upper mantle that is now common only at Bioko, in the oceanic sector of the Cameroon Line close to the coast (Matsukage and Oya, 2010). The modal abundance of clinopyroxene in the Ngao Bilta spinel lherzolites (12%–19%) is significantly higher than the average for off-craton continental mantle (12%; Pearson et al., 2003), and the spinel mode reaches up to 3.6%, twice the continental mantle average. High clinopyroxene modes are known elsewhere in the Cameroon Line, sometimes resulting in pyroxenites and websterites (Princivalle et al., 2000; Temdjim et al., 2012; Nkouandou et al., 2015; Njombie Wagsong et al., 2018). This is interpreted as evidence for widespread refertilization of previously more depleted upper mantle peridotites due to repeated melting events beneath the Cameroon Line. Mineral chemistry allows further characterization of the refertilization process. Two groups of clinopyroxenes with low (3.6–4.0 wt.%) and high (4.7–5.9 wt.%) Al2O3 are seen, whereby the low-Al2O3 group also have low Mg# (86–88; Fig. 7a). Two groups were also seen in xenoliths at nearby Youkou, but without low Mg# (Njombie Wagsong et al., 2018). The low Mg# Cpx also have low Na2O (<0.9), whereas TiO2 is in the intermediate group (0.3–0.45; Fig. 7b,c). Low Al and Na are usually thought of as indicating depletion, but the low Mg# discounts this: clinopyroxenes with low Mg# (≤87) and low Na2O have been interpreted as resulting from reaction of peridotitic minerals with percolating melt (Zhang et al., 2007), meaning that the low Na2O in Cpx may be due to it having grown from other phases with low-Na2O such as Opx or olivine. The low Mg# (84) orthopyroxenes in samples DB-01 and N47-02 have low CaO (0.3–0.6 wt.%) and Al2O3 (Fig. 6c), which are also typical for slightly depleted lherzolites (Pearson et al., 2003). The high CaO in some harzburgitic Opx (Fig. 6a) are exceptional, not the main population of low-CaO Opx in SL. The spinel harzburgite xenolith DB-02 has orthopyroxenes with average Mg# 90.6, in equilibrium with first generation olivines (Mg# 90.1 average), which are overprinted by a second generation with Mg# 84–85. This demonstrates overprinting by late low-Mg# melts, 15

with which the rare clinopyroxenes (3 vol.%; Mg# 87.1) are associated. Low NiO in the later olivines (Fig. 5a) further indicates more advanced fractionation of these melts. The Cpx belong to the low Al2O3 and Na2O groups, but have intermediate TiO2 (Fig. 7). Despite its uniform Mg# (90.2–90.9), Opx in the harzburgite falls into two groups with low and high CaO (0.52–0.57 wt.% vs. 1.4-–1.8 wt.%), with the high CaO group also showing high TiO2 (up to 0.13 wt.%; Fig. 6b).

6.2. Origin of the dunites from Ngao Bilta volcano by melt-rock reaction Three possible hypotheses are commonly invoked to explain the origin of dunites: (1) they are the residues of high degrees of partial melting of fertile peridotite (Gurney and Harte, 1980); (2) they are magmatic cumulates of liquidus crystals from mafic to ultramafic magmas (Rehfeldt et al., 2007); and (3) they are reaction products, representing the product after removal of Opx by reaction with passing melts (Quick, 1981; Kelemen et al., 1995; Akizawa and Arai, 2009). Any textural information that may have helped to decide between these origins for the Ngao Bilta dunite xenoliths has been lost because the xenoliths were later deformed (Fig. 4). The first origin can be eliminated: strong depletion would be expected to leave residual olivines with high Mg# (92–94; Bernstein et al., 1998) and a small proportion of Cr-rich spinels should survive (Kurat et al., 1980). In contrast, the xenolith dunites have low Mg# (88–82) – none as high as fertile mantle olivines – and no spinel is observed. Some dunite samples also contain a small proportion of orthopyroxene (2%–6%): in these cases the Opx has similarly low Mg# (83.6–84.1) together with low CaO and Al2O3, but TiO2 in the higher half of the range for SL (Fig. 6). The second possible origin, from fractionated melts, is unlikely because this would require olivine to remain the liquidus phase following extensive degrees of fractionation. Olivine is usually the first silicate mineral to crystallise from melts that originate by meting of peridotite 16

when they move to lower pressures, but the liquidus phase field remains narrow if melts crystallise when still within the melt. By the time more extensive fractionation, which is required to lower the Mg# to the mid-80s, had occurred, crystallisation from hydrous alkaline melts would be dominated by pyroxenes (Foley, 1990). The third mechanism of reactive flow is preferred here. Low-pressure analogues of dunite formation by the infiltration of mafic melt are well known from dunite veins in ophiolites (Quick, 1981; Kelemen, 1990): they result as interaction products of the incongruent melting of orthopyroxene and precipitation of olivine (Payot et al., 2009), process that may induce the formation of high-porosity dunite for efficient melt extraction from the mantle (Aharonov et al., 1995; Spiegelman et al., 2001). A similar process has been shown to operate at pressures of 5 GPa (ca. 170 km) where melts have very low SiO2 contents (Pinter, 2019): this offsets the tendency of Opx to replace olivine as the product of incongruent melting (Walter, 2003). At the pressures relevant to the Ngao Bilta xenoliths (1.3–1.5 GPa), this mechanism would favour olivine formation. The products of melt/rock reaction of this type depend on whether the melt flows through completely, leaving dunite in the passageways it took, or whether the melts stops and effectively adds its chemical components to the rock. The former case is interpreted to form the dunites, whereas the second is seen in the lherzolites in the addition of Cpx, amphibole and spinel in addition to olivine. The formation of dunites by flow-through reaction is also consistent with the higher TiO2 in the low-Mg# olivine of sample DB-11 (Supplementary Table S5; 0.08–0.11 wt.%), which may indicate a hydrous component in the olivine. Hydrous, low-Mg# olivines have been noted from cratonic xenoliths in Siberia (Doucet et al., 2014). The formation of dunite by flow-through reaction is consistent with the lower Mg# of neoblasts and the lack of spinel: if the olivine were residual, it would be expected to have higher Mg# of around 90 (Sano and Kimura, 2007) and Cr-enriched spinel should occur (Kurat et al., 1980).

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6.3. Temperature-pressure conditions The pressures obtained for the Ngao Bilta SL and SH of 12–15 kbar are well within the spinel peridotite field and correspond to 40–50 km depth, not far below the Moho which is at 23–30 km beneath this area (Browne and Fairhead, 1983; Poudjom Djomani et al., 1997; Nnange et al., 2000; Goussi Ngalamo et al., 2018). Together with the temperatures for the spinel lherzolites of 908–957 ˚C by the BK2px thermometer and 879–904 ˚C by the singlemineral BKOpx, these fit a dynamic rather than a static rift geotherm (Fig. 9). They do not define a good geotherm because of the poor resolution from barometers, but all samples are clearly above a geotherm of 60 W/m2, indicating unusually hot mantle at this depth. These results are similar to those obtained with spinel lherzolite from Youkou volcano in the Adamawa Volcanic Massif (Njombie Wagsong et al., 2018). The only temperatures obtainable for the dunites are from Opx which, however, has low Mg# similar to the olivines: these are lower (863 ˚C), possibly corresponding to a reequilibration temperature after crystallization from a slightly fractionated melt that was probably hydrous, according to the low temperature. The pressure-temperature range of the Ngao Bilta xenoliths is similar to those already documented in peridotites from elsewhere in the Adamawa Volcanic Massif (Dautria and Girod, 1986; Lee et al., 1996; Matsukage et al., 2010; Nkouandou and Temdjim, 2011; Nguihdama et al., 2014; Nkouandou et al., 2015; Njombie Wagsong et al., 2018) and for other mantle peridotites in the CVL (Teitchou et al., 2007; Matsukage and Oya, 2010; Temdjim, 2012; Pintér et al., 2015). In summary, the xenoliths document shallow movement of melts in mantle that is unusually hot at shallow levels beneath the continental extremity of the Cameroon Line.

6.4. Magmatic overprints (mantle metasomatism) beneath the Ngao Bilta volcano The peridotite xenoliths of Ngao Bilta volcano show both petrographical and chemical evidence of magmatic overprinting by metasomatic processes. The pressure-temperature 18

estimates restrict this to the uppermost mantle (40–50 km), and low temperatures imply hydrous melt compositions. The evidence for magmatic overprinting is seen as changes in modal mineralogy as well as hidden in mineral chemistry. Mineralogically, the spinel lherzolites are enriched in clinopyroxene (up to 19%), which has inherited low Na2O and Al2O3 from precursor phases, and amphibole and spinel are also introduced in spatial association with each other (Fig.2e, f). As discussed above, all rocks types contain minerals reflecting magmatic overprints. The high Ca in dunitic olivines is further evidence for a low-pressure process, as Ca in olivine is inversely proportional to pressure (Simkin and Smith, 1970). The magmatic signature in Cpx is expressed as low Mg# correlated with lower Al and Na than in normal spinel lherzolites, and Al2O3 is also low compared to refertilized peridotites at Lherz (Le Roux et al., 2007). The two SL containing Cpx with the lowest Mg# have the highest modal Cpx (16–19%), confirming the correlation of modal enrichment in Cpx with low Mg#. The low-Mg# Opx are also not characterised by high CaO. Amphibole and spinel occur spatially associated with the introduced Cpx, indicating that the infiltrating melts were hydrous. The amphiboles range in Mg# from 89 to 78, which is too low for equilibrium as amphibole-bearing spinel lherzolite (Mengel and Green, 1989), confirming their relationship with pyroxenes with low Mg#. A stronger magmatic overprint (i.e. lower Mg#) in amphiboles correlates with higher TiO2 (up to 5.5 wt%) and lower Cr2O3. Na2O/K2O ratios (4–44) vary in amphiboles with ‘peridotitic’ Mg# (88–90), but are uniformly low (3.6–6.2) in those with lower Mg#. The low TiO2 are typical for amphiboles in peridotites, Na2O/K2O ratios are lower in peridotite experiments than in the Cameroon xenoliths (Mengel and Green, 1989) but these coexist with phlogopite. Amphiboles in spinel peridotites at Pali Aike are similar to the low-TiO2 group at Ngao Bilta and also have higher K2O (Kempton et al., 1999), as do amphiboles in spinel-garnet lherzolites at Bereya (Vitim, Siberia; Glaser et al., 1999). These comparisons suggest that the infiltrating melt at Ngao 19

Bilta had a high Na2O/K2O and TiO2 signature. Spinels have low Cr# consistent with introduction of melts, and TiO2 contents are low relative those in Vitim peridotites, but similar to those in spinel peridotites in the Lambert-Amery rift of Antarctica (Foley et al., 2006). Taken together, the mineral chemistry of the introduced phases indicates that the infiltrating melt was a silica-undersaturated silicate melt and not a carbonate-rich melt as has been proposed in some regions of the Cameroon Line (Tedonkenfack et al., 2019) and held responsible for wehrlitization reactions in other areas (Rudnick et al., 1993; Yaxley and Green, 1998; Wang et al., 2010). Xenoliths from elsewhere in the Cameroon Line have high Mg# minerals despite the modal introduction of Cpx (Nkouandou and Temdjim, 2011; Nguihdama et al., 2014), indicating that the melts affecting the Ngao Bilta xenoliths, characterised by Mg# as low as 82, may have been relatively fractionated, as seen also at other localities (Wandji et al., 2009; Tedonkenfack et al., 2019). Increased modal clinopyroxene, olivine and spinel at the expense of orthopyroxene – as observed here in the Ngao Bilta lherzolites – has been interpreted elsewhere as a signal for the action of carbonatite melts (Wallace and Green, 1991; Yaxley et al., 1991; Rudnick et al., 1993). Water is clearly also needed to stabilize amphibole, but amphibole coexists in experiments with carbonatite melts at 2–3 GPa (Wallace and Green, 1988). The chemical characteristics of the amphiboles also show that the metasomatism was probably caused by silicate melts. The TiO2, Cr2O3, K2O contents and Mg# show a wide variation, but carbonatite melt should not cause an increase in TiO2. The question of silicate or carbonate melt involvement can be tested further with reference to the Ca/Al ratio in Cpx: ratios below 5 are characteristic for equilibrium with silicate melts, whereas carbonatites show consistently higher values that may be as high as 70 (Fig. 10; Zong and Liu, 2018). Ngao Bilta clinopyroxenes have a maximum Ca/Al of 5.6 (Fig. 10), suggesting silicate melt metasomatism (Yaxley and Green, 1998; Wang et al., 2010). The increase in modal Cpx and 20

spinel at the expense of Opx requires a desilication reaction that can be administered by a low-SiO2 silicate melt such as basanite or nephelinite and does not require it to be carbonaterich (Ionov et al., 2005). A higher-degree melts such as tholeiite is unlikely however, as this would introduce lower amounts of alkalies and considerably less water. Referring back to Fig. 9, the enrichment process in the Ngao Bilta xenoliths occurred at too shallow a depth for carbonatite metasomatism, which is likely to be restricted to pressure >18 kbar (beige zone in Fig. 9). Our results from Ngao Bilta xenoliths can now be related to other information about the upper mantle from xenoliths along the Cameroon Line. The similarity in trace elements and isotopes across the oceanic and continental sectors indicate melt origin beneath the lithosphere in both sectors and that the unmetasomatised continental lithosphere is similar to oceanic lithosphere (Lee et al., 1996; Déruelle et al., 2007; Teitchou et al., 2007). Peridotites with the least metasomatic overprint are found in the oceanic sector and close to the coast in the continental sector around the Mt Cameroon area (Fig. 1; Lee et al., 1996; Caldeira and Munha, 2002; Matsukage and Oya, 2010; Pintér et al., 2015). Overprinting by more fractionated melts occurs throughout the continental sector, but appears to dominate further inland, both before the branching on the CVL and in the eastern branch in which the Adamawa plateau lies (Fig. 1; Wandji et al., 2009; Nguihdama et al., 2014; Mbowou et al., 2017; Njombie Wagsong et al., 2018; Tedockenfack et al., 2019). There is no marked age progression along the Cameroon Volcanic Line, and both fertile and depleted mantle rocks are sampled at the inland location of Kapsiki Plateau (Tamen et al., 2015). There may be a tendency for higher xenolith temperatures in central positions beneath the main rift (Matsukage and Oya, 2010), which may explain the apparent rarity of amphibole-bearing peridotites at localities such as Barombi relative to more marginal and distal positions such as Nyos and the Adamawa Plateau (Pintér et al., 2015; Njombie Wagsong et al., 2018). Indeed,

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earlier depletion may have dried out the mantle prior to recent re-fertilization by infiltrating, low-degree melts (Pintér et al., 2015). The Ngao Bilta volcano is situated on the Adamawa uplift, which gravity anomalies may indicate is at an early stage of rifting (Poudjom Djomani et al., 1997). The magmatic overprinting seen in the xenoliths may be related to shallow, low-degree melting (Fig. 9; hence silica-undersaturated compositions) allowed by lithosphere delamination and channelized mantle flow, which localises melting beneath the rift (Goussi Ngalamo et al., 2018). Abundant shallow melt movements are also indicated by gravity indications for intrusions at shallow lithosphere and crustal depths (Nnange et al., 2000).

7. Conclusions Peridotites from Ngao Bilta volcano can be classified into three types: lherzolites are predominant over harzbugites and dunites. Most of the Ngao Bilta peridotite xenoliths from volcano were depleted by melt loss in the spinel stability field. Thermobarometric estimates constrain pressures to conditions of 11–15 kbar and temperatures to 863–957 ˚C. Lherzolites and harzburgites contain mineralogical evidence for reaction with fractionated low-silica melts. This includes low Mg# Cpx and Opx, an increase in modal abundance of Cpx at the expense of Opx, and the introduction of amphibole and spinel. The dunites are interpreted as the products of flow-through reaction of the melts, whereas many lherzolites are enriched by the addition of melt components. The melt that reacted with peridotites in the uppermost mantle (35–50 km depth) was a silica-undersaturated silicate melt such as nephelinite or basanite and not a carbonate melt. The features of the Ngao Bilta xenoliths correspond to their peripheral position in the Cameroon Volcanic Line and the early stage of rifting beneath the Adamawa Uplift in its eastern branch.

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Acknowledgments Thick polished sections (150–200 µm) and microprobe analyses were performed during the stay of the first author (R. Temdjim) at the Institute of Geosciences of the Johannes Gutenberg University in Mainz (Germany), financed by the Germany Academic Exchange Organisation the DAAD (Deutscher Akademischer Austauschdienst). SF is funded by ARC grant FL180100134.

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Figure captions Figure 1: Geological map of the Ngao Bilta region in the Adamawa volcanic massif. Upper left inset: Cameroon Volcanic Line after Déruelle et al. (2007); red point = position of Ngao Bilta. Upper right inset: African cratons after Kampunzu and Poppof (1991). Figure 2: Photomicrographs showing mineral textures of spinel lherzolites from Ngao Bilta volcano. (a) Large olivine exhibiting kink bands; (b) cracked olivine and clinopyroxene showing exsolution of orthopyroxene; (c) Opx containing Cpx exsolution; (d) Spinel and olivine bordering Cpx; (e, f) spatial association of amphibole (brown) and spinel (dark brown). Abbreviations: Ol =olivine, Opx = orthopyroxene; Cpx = clinopyroxene; Spl = spinel; Amp = amphibole. Figure 3: Photomicrographs showing mineral texture in spinel harzburgite from Ngao Bilta. (a) Protogranular texture; (b) cracked olivine crystal; (c) Opx containing Cpx exsolution; (d) Cpx with Opx exsolution; (e) relationship of spinel to olivine and Cpx; (f) second-generation neoblasts of olivine and orthopyroxene. Figure 4: Photomicrographs of spinel-free dunite from Ngao Bilta. (a) Typical porphyroclastic texture; (b) strained primary olivine porphyroclast; (c) strained orthopyroxene porphyroclast; (d) small mosaic grains of secondary Ol, Opx and Cpx. Figure 5: Compositions of olivines in lherzolites (blue diamonds), harzburgites (red squares) and dunites (green triangles). Lherzolites and harzburgites contain both primitive mantle-like (Mg#≈90) and lower Mg# olivines, whereas dunites are exclusively lower Mg#. Smaller symbols are for CVL analyses from the literature (Lee et al., 1996; Wandji et al., 2009; Nkouandou and Temdjim, 2011; Temdjim; 2012; Nguihdama et al., 2014; Nkouandou et al., 2015; Njombie Wagsong et al., 2018) whereby the same symbol shapes signify the same rock types. Figure 6: Compositions of orthopyroxenes in Ngao Bilta xenoliths. The low Mg# minerals show similar CaO and Al2O3 but variable TiO2 relative to primitive Mg#90 minerals. The few

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anomalous high-CaO Opx do not belong to the low-Mg# group. Symbols as in Fig.5, also for literature data. Figure 7: Compositions of clinopyroxenes in Ngao Bilta xenoliths. Lower Mg# Cpx have consistently low Al2O3 (a), TiO2 (c) and Na2O (d), and slightly lower Cr2O3 (b). The Cpx with lowest TiO2 coexist with amphibole. Symbols as in Fig.5. Figure 8: Compositions of spinels in Ngao Bilta spinel lherzolites. High Cr# sample DB-01 may represent original depleted mantle compositions, whereas all introduced spinels are low Cr# and high Mg#. Symbols are for individual xenoliths: orange diamonds = DB-01; blue squares = BJ-01; green circles = N49-02; red triangles = DB-06; yellow diamonds = N47-02. Open black squares are literature data from the CVL (see Fig.5 for references). Figure 9: Pressure-temperature conditions estimated by the two-pyroxene and Ca-in-Opx thermometers in combination with the Ol-Cpx barometer (see Table 2; Brey and Köhler, 1990). Static and dynamic rift geotherms from Chapman (1986). Other Adamawa xenoliths (squares) from Nkouandou et al. (2015) and Njombie et al. (2018). Blue line is peridotite solidus with CO2 and H2O; red line with H2O saturation, and orange line = dry solidus of peridotite (Foley and Pintér, 2018). Figure 10: Ca/Al and Mg# of Ngao Bilta clinopyroxenes compared to Cpx from metasomatism by carbonatite and silicate melts (coloured backdrops; Zong and Liu, 2018). Data for natural carbonatite metasomatism (green circles) from Yaxley et al. (1998) and Neumann et al. (2002); experimental carbonate melt/peridotite from Klemme et al. (1995), Brey et al. (2008) and Gervasoni et al. (2017); silicate melt metasomatism (grey squares) from Yaxley and Green (1998) and Wang et al. (2010). Residues after partial melting from Walter (2003). Ngai Bilta Cpx show no strong indication of carbonate melt metasomatism.

Table captions Table 1:

Modal abundances of minerals in mantle xenoliths from Ngao Bilta (vol.%)

Table 2:

Temperature and pressure estimates for Ngao Bilta xenoliths

Supplementary table captions Supplementary Table 1:

Electron microprobe analyses of olivines

Supplementary Table 2:

Electron microprobe analyses of orthopyroxenes 33

Supplementary Table 3:

Electron microprobe analyses of clinopyroxenes

Supplementary Table 4:

Electron microprobe analyses of spinels

Supplementary Table 5:

Electron microprobe analyses of amphiboles

34

Research Highlights *

Ngao Bilta volcano in the Adamawa Plateau, Cameroon, sampled xenoliths from the uppermost mantle at 35-50km depth.

*

The mantle beneath the eastern branch of the Cameroon line experienced magmatic overprinting at <50km depth.

*

Melts that overprint the mantle are SiO2-undersaturated silicate melts and not carbonatites.

Table 1 Rock Sample Olivine Orthopyroxene Clinopyroxene Spinel Amhpibole Cpx/Opx

Modal abundances of minerals in mantle xenoliths from Ngao Bilta (vol.%) lherzolite DB-01 54.9 27.6 16.0 0.9 0.7

lherzolite DB-06 55.6 22.3 18.5 2.6 0.9

lherzolite BJ-01 60.9 19.0 12.9 3.6 3.7

lherzolite N47-02 65.9 20.4 12.3 1.2 0.2

lherzolite N49-02 62.8 17.8 15.9 2.9 0.7

harzburgite DB-02 80.4 15.8 3.4 0.4

dunite DB-03 93.3 5.8 0.95

dunite DB-04 97.1 2.4 0.54

0.58

0.83

0.68

0.60

0.89

0.22

0.16

0.23

Table 2 Temperature and pressure estimates for Ngao Bilta xenoliths Bold = preferred values (see text for explanation) Rock type Sample P (BK1990) P (P2008) T T T T

(2px: BK1990) (Cpx: NT2000) (Ca Opx: BK1990) (2-px: W77)

kbar kbar ˚C ˚C ˚C ˚C

SL DB-01 13.2

SL DB-06 14.8 13.3

SL BJ-01 14.6 15.0

781 818 879 838

919 885 912 902

957 908 905 915

SL N47-02 14.8

874 904 874

SL N49-02 12.1 11.0 908 904 896 913

SH DB-02

Dunite DB-03

833 908 879

863