Geochemical constraints on the origin and tectonic setting of the serpentinized peridotites from the Paleoproterozoic Nyong series,Eseka area,SW Cameroon

Philomene Nga Essomba Tsoungui·Sylvestre Ganno·Evine Laure Tanko Njiosseu·Jean Lavenir Ndema Mbongue·Brice Kamguia Woguia·Landry Soh Tamehe,3·Jonas Didero Takodjou Wambo·Jean Paul Nzenti

Abstract Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon.Here,we present new field data,petrography,and first comprehensible wholerock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region.The studied rock samples are characterized by pseudomorphic textures,including mesh microstructure formed by serpentine intergrowths with cores of olivine,bastites after pyroxene.Antigorite constitutes almost the whole bulk of the rocks and is associated (to the less amount) with tremolite,talc,spinel,and magnetite.Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types.Type 1 has high MgO (＞40 wt%) content and high Mg# values (88.80) whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg# values(＜40 and 82.88 wt%,respectively).Both types have low Al/Si and high Mg/Si ratios than the primitive mantle,reflecting a refractory abyssal mantle peridotite protolith.Partial melting modeling indicates that these rocks were derived from melting of spinel peridotite before serpentinization.Bulk rock high-Ti content is similar to the values of subducted serpentinites(＞50 ppm).This similarity,associated with the high Cr contents,spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subductionrelated environment.The U-shaped chondrite normalized-REE patterns of serpentinized peridotites,coupled with similar enrichments in LREE and HFSE,suggest the refertilized nature due to melt/rock interaction prior to serpentinization.Based on the results,we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subductionrelated environment,especially in Supra Subduction Zone setting.These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the São Francisco craton in Brazil.

Keywords Serpentinites·Mantle peridotites·Melt/rock interaction·Paleoproterozoic suture zone·Eseka area·Nyong series·Congo/São Francisco cratons

1 Introduction

Fig.1 Geological map of Southwestern Cameroon(modified after Lerouge et al.2006).Inset illustrating the general map of Cameroon and the position of the SW Cameroon relative to the Congo craton in Africa

Ultramafic rocks are igneous and meta-igneous rocks bearing SiO2less than 45 wt%,MgO ＞18 wt%,FeO ＞9 wt%,and low content of K2O(＜1 wt%)with a total of mafic minerals ＞90 vol% (Ballhaus and Ryan 1995).These rocks,especially mantle peridotites and associated lower crustal plutonic rocks are a major component of the oceanic lithosphere.Hydrated peridotites,also known as serpentinites,form through the alteration of olivine-and pyroxene-dominated protoliths at temperatures lower than 650–700 °C (Evans et al.2013; Deschamps et al.2013;O’Hanley 1996).Serpentinites are particularly abundant and well exposed near and along ultra-slow spreading ridge settings that are characterized by low magma supply and complex tectonic activities associated with spreading and strike-slip faulting (Fruh-Green et al.2004).Furthermore,peridotite-dominated massifs characterized most of the non-transform offset environments.Over the last two decades,the processes of serpentinization have attracted much attention,and interest in these rocks is still growing.Many authors (e.g.Hattori and Guillot 2003,2007; Fruh-Green et al.2004;Hilairet et al.2007;Maffione et al.2014)have suggested that serpentinization is a fundamental process with significant geophysical,geochemical,and biological importance for the Earth’s dynamic and global geochemical cycle.However,deciphering the origin of the serpentinites and the serpentinization processes remains a challenge (Deschamps et al.2013).

In Cameroon,the Congo craton crops in the southern part of the country and forms the Ntem complex(Maurizot et al.1986; Toteu et al.1994; Pouclet et al.2007).This complex,as well as the other Archaean cratons in the world,is characterized by a conspicuous presence of greenstones belt which trends E–W and extends over 500 km from Mbalam to the East to Kribi to the West(Maurizot et al.1986; Suh et al.2008; Ganno et al.2018).Most of the greenstone rocks in Cameroon correspond to mantle peridotites that are generally serpentinized and crop out at Eseka,an area located at the northwestern edge of the Ntem Complex.Very little work exists on the petrogenesis of these serpentinized rocks,and their tectonic setting is not constrained yet.Moreover,the Eseka serpentinites are closely associated with eclogitic rocks genetically originating from basalts and basaltic andesites,with an upper mantle-derived tholeiitic trend (Bouyo Houketchang et al.2019).This occurrence suggests that the studied serpentinites may have originated from subducted oceanic lithosphere.In this paper,we present the first whole-rock major and trace element (including Rare Earth Elements,REE) geochemical data of serpentinized peridotites from the Eseka area with the aim to decipher their petrogenetic processes and discuss the possible geodynamic environment.

2 Geological setting

Fig.2 Detailed geological map of the study area

The Eseka area belongs to the Ntem Complex greenstones belt which is located at the northern margin of the Congo craton in Cameroon (Maurizot et al.1986; Nédélec et al.1990).The Congo craton is a large sub-circular mass with an area of about 5711000 km2and a diameter of 2500 km;including an Archean basement,a Meso Proterozoic belt,and a post-Proterozoic cover(Shang et al.2007,2010).The Ntem complex is a stable block of Archean and Paleoproterozoic rocks overlaid in the north by the Pan-African orogenic belt (Yaounde Group,Nzenti et al.1988; Tchameni et al.2001; Shang et al.2010).The Ntem Complex has been subdivided into three main units (Fig.1):Ntem unit,Nyong unit,and Ayina unit (Maurizot et al.1986;Lerouge et al.2006).The Ntem unit chiefly consists of magmatic charnockitic rocks (hypersthene bearing granites)in the central part,with non-charnockitic rocks which include granulitic gneisses in the southern part and tonalite–trondhjemite–granodiorite (TTG suite) in the northern part (Shang et al.2004,2007).Potassic-rich granites and late dolerite dykes crosscut the TTG–charnockitic suite and the granulite gneiss(Maurizot et al.1986;Vicat et al.1996;Pouclet et al.2007; Tchameni et al.2010; Shang et al.2010).The recent integrated in situ analyses of zircon U–Pb ages and Hf–O isotopes data from the Ntem Unit reveals that the charnockites crystallized at ca.2.92 Ga while the trondhjemites and associated amphibolite protoliths crystallized synchronously at around 2.87–2.86 Ga(Li et al.2016).

The Nyong unit,located at the NW border of the Archean Ntem Complex,is a ca.240 km long and 160 km wide NNE-SSW trending band of dominantly Paleoproterozoic metasedimentary and metaigneous rocks (Fig.1).Initially interpreted as being part of the Archean Congo craton reactivated during Paleoproterozoic and Pan-African orogenies (Lasserre and Soba 1976; Maurizot et al.1986),this unit is now considered as a Paleoproterozoic nappe thrust onto the Congo craton (Penaye et al.2004; Lerouge et al.2006),yielding both Archaean and Paleoproterozoic materials associated with various migmatitic gneisses of TTG composition,granitoids,and remnants of greenstone belts commonly made up of pyroxenites,serpentinites,banded iron formations (BIF),mafic and ultramafic metavolcanics and post-tectonic metadoleritic dykes(Ndema Mbongue et al.2014;Ganno et al.2017;Soh et al.2018).More recently,Loose and Schenk(2018)and Bouyo Houketchang et al.(2019) have reported the existence of well-preserved to variably retrogressed eclogite facies metamorphic rocks associated with greenstones belt in the Nyong unit.The SHRIMP U–Pb age of these eclogites was constrained at 2093 ± 45 Ma (Loose and Schenk 2018).This date which is similar to that of their host rocks(2050 Ma,Lerouge et al.2006) suggests that the Nyong Group underwent the same metamorphism by coupling field relationships with the contemporaneous zircon growth between both rock types during the Paleoproterozoic between 2100 and 2000 Ma (Bouyo Houketchang et al.2019).The new findings allowed the reinterpretation of the Paleoproterozoic Nyong unit as one of the oldest subducted oceanic slab or trace of a suture zone so far recorded within the West Central African Fold Belt.Moreover,ca 600 Ma Pan-African high-grade recrystallization and static overgrowth was recorded in the western part of the Nyong unit.This suggests that this unit underwent a polycyclic evolution (Toteu et al.1994; Lerouge et al.2006; Ndema Mbongue et al.2014).

Fig.3 Outcrop photograph showing the occurrences of the Eseka ultramafic complexes (a) and hand specimen view of the fresh sample (b).Pseudomorphs after orthopyroxene and olivine and development of mesh texture(c,d).Recrystallized serpentine minerals(antigorite)showing elongate interpenetrating blades (e,f).Mineral abbreviations after Whitney and Evans (2010):antigorite (Atg); clinopyroxene (Cpx); olivine(Ol); orthopyroxene (Opx); tremolite (Tr); serpentine (Ser); spinel (Spl),magnetite (mgt)

3 Sampling and analytical methods

Samples were collected from several outcrops of the study area.Proper care was taken to collect the best possible fresh samples.For petrographic studies,polished thin sections were prepared at Geotech Lab Vancouver(Canada).Detailed descriptions of the thin sections were done using conventional techniques at the University of Yaounde 1,Cameroon.After a detailed petrographic study,16 samples were selected for major-and trace-element analyses using the pulp.Whole-rock analyses were determined by Inductively Coupled Plasma-Atomic Emission(ICP-AES)for major elements and by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for trace elements and REE at ALS Minerals Global Group,Vancouver(Canada).The samples were pulverized to obtain a homogeneous sample out of which 0.2 g of rock powder was fused with 0.9 g LiBO2at 1000 °C,and then dissolved in 100 mm3of HNO3at 4%.REE concentrations are determined by ICP-MS from 0.25 g of rock powder fused in 4 digestives acid.Analytical uncertainties vary from 0.1 to 0.04% for major elements,0.1 to 0.5% for trace elements; and 0.01 to 0.5 ppm for rare earth elements (REE).Analysis precision for rare earth elements is estimated at 5% for concentrations ＞10 ppm and 10% when lower.Loss on ignition (LOI) was determined by weight difference after ignition at 1000 °C.Different standards were used and data quality assurance was established by applying these standards as unknown between samples.

4 Results

4.1 Petrography

The ultramafic rocks of the Eseka area are invariably serpentinized to varying degrees.The dominant lithological unit encountered is serpentinized peridotite that mainly occurs as blocks of varying sizes (Fig.3a),at Song-Hot village to the south of the Eseka town (Fig.2).These serpentinized peridotites are black to dark green in color,fibrous in appearance (Fig.3b) and are divided into two groups:type 1 and type 2 serpentinized peridotites.The first group is composed of pyroxene,serpentine (mainly antigorite),tremolite(abondant),talc,spinel and magnetite as accessories; while the second group is formed by primary olivine and orthopyroxene relics,serpentine (mainly antigorite),tremolite,talc,epidote,and opaque’s minerals.Talc minerals in both types are secondary phase resulting from the alteration of primary pyroxene and olivine minerals.They are characterized by pseudomorphic textures,including mesh microstructure formed by magnetite and serpentine intergrowths with cores of olivine,bastites after pyroxene (Fig.3c,d),depending on the degree of serpentinization.Few relics of primary minerals are observed.Only olivine and orthopyroxene were partially preserved at the core of serpentine aggregates.Such a mineralogical composition suggests that,before serpentinization,these rocks corresponded to peridotite with harzburgite composition.In the least altered samples,individual crystals of olivine are crosscut by fractures filled with serpentine.Antigorite (average grain size of 0.3 mm) constitutes almost the whole bulk of the rocks.It is derived from the hydration of olivine and pyroxene and appears as colorless aggregates of fibro lamellar nature that gives anomalous interference colors and parallel extinction (Fig.3e).These antigorites in the form of fibers are found in parallel to subparallel,at times they are randomly disposed in reticulate form,exhibiting interpenerating fabric (Fig.3f).Deformational features such as wavy extinction,and distorted exsolution lamellae are common.Some serpentine crystals host inclusions of magnetite.Pyroxene mostly occurs as subhedral porphyroclasts with irregular grain boundaries,scattering over the mineral constituents(Fig.3c,d).Tremolite occurs as flaky and fibrous—like colorless crystals,with a subhedral shape.In the more altered and deformed samples,the tremolite laths are aligned parallel to the schistosity (Fig.3d).Epidote alteration occurs along cleavage planes.Talc is another alteration mineral and generally occurs as fine crystals aligned along fractures and at the edges of pyroxene crystals.In some highly hydrated samples,primary minerals are not preserved and few accessory phases(zircon,magnetite)are observable.These rock types are characterized by abundant antigorite aggregates fibers,talc,tremolite; and magnetite(Fig.3f).In nearly all type 2 serpentinite samples the porphyroblastic texture is preserved,suggesting that serpentinization occurred under static conditions.

Fig.4 a AFM((Na2O+K2O)–FeO–MgO)and b ACM(Al2O3–CaO–MgO)ternary plots of the Eseka serpentinized peridotites.Fields of mafic cumulates,ultramafic cumulates,and metamorphic peridotites are after Coleman (1977)

Fig.5 a SiO2 versus Al2O3,b CaO versus Al2O3,c Fe2O3 versus Al2O3,d Na2O versus Al2O3,e MgO versus Al2O3,f TiO2 versus Al2O3 diagrams for the studied serpentinized periodotites.The light grey fields are of orogenic,ophiolitic and abyssal mantle peridotites(Bodinier and Godard 2003)

4.2 Geochemistry

4.2.1 Major element

Bulk rock major element composition of 16 representative samples of the Eseka serpentinized peridotites is shown in Table 1.The loss on ignition (LOI) values ranges from 10.70 to 14.15 wt% (average 12.11 wt%),reflecting extensive serpentinization of olivine and pyroxene as confirmed by the petrography.In order to compensate for variable serpentinization of these peridotites,the major element data was recalculated on an anhydrous basis to 100 wt%.This provides a much better comparison between peridotites having varying amounts of serpentinization(Coleman and Keith 1971; Niu 2004; Deschamps et al.2013).The studied rocks had a low content of SiO2,varying from 40.76 to 45.30% with an average of 43.61%(anhydrous),similar to the silica abundance of fresh peridotites (43.54%; Nockolds 1954).Most samples contained high MgO (36.80–44.07 wt%) and low Al2O3(1.71–3.37 wt%).Their Mg number[Mg#:cationic ratio of 100 × Mg2+/(Mg2++Fe2+)] varied from 81.57 to 90.38.Two types of serpentines have been distinguished based on the MgO contents and Mg#values:Type 1(9 samples)was characterized by ＞40 wt% MgO,Mg#:87.32–90.38 (average 88.80); and Type 2 (7 samples) displays ＜40 wt%MgO and Mg# ranging from 81.57 to 83.49,with an average of 82.88.On the AFM ((Na2O-K2O)-FeO-MgO)and ACM (Al2O3–CaO–MgO) ternary diagrams of Coleman (1977),the two types of serpentinized peridotites plot in the metamorphic peridotites field (Fig.4a,b).This suggests that the analyzed serpentinized peridotites correspond to depleted or residual mantle rocks.This result is also confirmed by their high Mg# values.Indeed,the average Mg# value of Type 1 serpentinized peridotites(88.80) is similar to that of the primitive mantle (89; Sun and McDonough 1989) and residual peridotites (Bonatti and Michael 1989; Bodinier and Godard 2003).

In terms of Mg/Si and Al/Si ratios,Type 1 samples showed higher Mg/Si (0.93–1.03,average 0.96) than primitive mantle (PM) values (0.85) but lower Al/Si(0.040–0.053)than the PM values(0.1).In contrast,Mg/Si value of Type 2 serpentinized peridotites ranges from 0.81 to 0.92 (average 0.86) similar to that of PM (～0.85) but their Al/Si values are slightly higher (0.068 in average)than those of type 1 serpentinized peridotites.In general,the Al/Si values of the studied serpentinized peridotites were much lower than the PM～0.1 (McDonough and Sun 1995),but higher than the highly depleted mantle residual harzburgites (～0.02).The lower Al2O3/SiO2ratios of the studied serpentinized peridotites relative to the PM values suggest that protoliths of these serpentinized peridotites had experienced partial melting before serpentinization (Niu 2004; Gamal El Dien et al.2016;Bhat et al.2019).Snow and Dick (1995) and Niu (2004)have proposed that SiO2and Al2O3are both immobile during serpentinization while CaO,as well as large-ion lithophile elements,are highly mobile.For this reason,Al2O3has been largely used in literature as a depletion index to evaluate the behavior of major elements(e.g.Bhat et al.2019 and references therein).Figure 5 shows major element oxide versus Al2O3diagrams in which the PM and the mantle peridotites compositions are plotted for comparison.SiO2shows contrastive behavior (positive correlation for Type 1 serpentinized peridotites and negative correlation for Type 2 serpentinized peridotites) with Al2O3,while MgO shows negative correlation for both types of serpentinized peridotites.The negative correlation of MgO with Al2O3is a characteristic commonly observed in residual mantle rocks (Niu 2004).TiO2co-vary with Al2O3,whereas no relation is observed for Fe2O3,CaO,and Na2O,suggesting their mobile nature.Also,the studied serpentinized peridotites plot within the field of mantle peridotites on the least-mobile major oxide versus Al2O3(Fig.5).The studied samples also contain low CaO(0.01–0.13 wt%for Type 1 and 0.12–0.79 wt%for Type 2)and since CaO is highly mobile,its depletion suggests that alteration of clinopyroxene is accompanied by dissolution of Ca (Hattori and Guillot 2007).

4.2.2 Trace and rare earth elements

The Eseka serpentinized peridotites are enriched in transition metals such as Cr (2620–3620 ppm) and V(39–118 ppm) reflecting their mantle origin.The large-ion lithophile elements (LILE) content is low except for Ba with concentration up to 54 ppm (Table 2).Depletion in high-field-strength elements (HFSE) is also observed,except for the Zr with content ranging from 5 to 20 ppm.In a trace element vs Al2O3variation diagram(Fig.6),all the data plot within the field of mantle peridotites.Both the two types of serpentinized peridotites show Cr enrichment and Zr depletion with respect to the primitive mantle.Zr and Yb correlate positively with Al2O3(Fig.6c),consistent with tectonically emplaced abyssal peridotites (Bodinier and Godard 2003).

The studied rocks have absolutely low rare earth element (REE) contents (∑REE=5.51–35.44 ppm) with Type 1 serpentinized peridotites displaying most REE depletion (average ∑REE=8.73 ppm).Chondrite-normalized REE patterns are nearly parallel and show slight light REE (LREE) enrichment and heavy REE (HREE)depletion.Type 1 serpentinized peridotites are more enriched in LREE (LaN/YbN=2.03–14.50) relative to HREE(GdN/YbN=1.19–1.93) and middle REE (MREE; CeN/SmN=0.43–1.07).Their REE patterns show both negative Ce (Ce/Ce*:0.09–0.28) and Eu (Eu/Eu*:0.29–0.28)anomalies(Fig.7a).Chondrite-normalized REE patterns of Type 2 serpentinized peridotites (Fig.7d) exhibit convexdownward patterns with a prominent negative Eu anomaly(Eu/Eu*:0.18–0.38) and flat HREE profiles.Weak negative Ce anomalies (Ce/Ce*=0.68–0.87) are presented in four samples,while the other two samples (RV17A and RV17B) lack Ce anomaly.The negative Eu anomaly observed in the two types of serpentinized peridotites is probably a result of a primary partitioning event occurred during partial melting,because any impregnation with a hydrothermal fluid would lead to a positive Eu anomaly(Bau and Dulski 1999; Bhat et al.2017).

On Primitive mantle (PM)-normalized multi-element diagrams (Fig.7b,d),the studied serpentinite samples show variable negative Nb,Rb,Sr,Ce and Eu,and positive U,Nd and La anomalies.Sr is highly depleted in both serpentinite types.Their incompatible immobile elements(Gd to Lu)are also depleted and show flat PM-normalized patterns,similar to abyssal peridotites (Niu 2004; Bhat et al.2017).

5 Discussion

5.1 Alteration/element mobility

The serpentinized rocks of this study were collected from the Eseka-Lolodorf paleoproterozoic greenstones belts.This belt underwent a polycyclic metamorphic evolution with three phases of deformation (Ndema Mbongue et al.2014;Ganno et al.2017;Bouyo Houketchang et al.2019).From field evidence and petrographic study,the studied serpentinized peridotites show varying degrees of talc-alteration overprint the serpentinite minerals.The replacement of serpentine by talc is a metasomatic process that indicates the substantial mass transfer of Mg,Fe,and Si(Bach et al.2004; Boschi et al.2006),following the reaction Mg3Si2O5(OH)4+2SiO2(aq)=Mg3Si4O10(-OH)2+H2O.Such a reaction is generally considered a prograde reaction taking place in the presence of SiO2-rich fluids at temperatures of about 300–350 °C (at low pressure,Bach et al.2004; Boschi et al.2006).The Eseka serpentinized peridotites are characterized by a secondary mineral assemblage of chlorite-talc-tremolite.In hydrothermally metamorphosed oceanic rocks,assemblages of talc,calcic amphibole,and chlorite are common and generally form at a broad range of temperatures under greenschist-facies conditions below 500 °C,as replacement of primary and secondary mafic minerals during multistage fluid/rock interaction or as direct precipitation during mixing of hydrothermal fluids and seawater(Boschi et al.2006).In the studied samples,talc occurs either as a replacement product of groundmass serpentine or together with tremolite ± chlorite as alteration product,suggesting interaction of serpentinite with a subsequent high-T,lowpH,low-MgO/SiO2fluid.

Fig.6 Selected trace elements binary plots against Al2O3(anhydrous wt%).Compositions of orogenic,ophiolitic and abyssal mantle peridotites(light grey fields) are after Bodinier and Godard 2003; primitive mantle values are from McDonough and Sun (1995)

The main effect of serpentinization of peridotites is the addition of water and the formation of serpentine phases that contain up to 15 wt% of H2O (Deer et al.1992;Deschamps et al.2013).For example,the loss on ignition(LOI)is commonly used as a good indicator for the degree of serpentinization if the samples do not contain other hydrous minerals (Deschamps et al.2013).The studied samples are made up of hydrous minerals,high LOI (average LOI=12.04 wt%),suggesting that they are strongly serpentinized.Thus they may have experienced sea-floor weathering leading to some element mobility and resetting of geochemical composition.Therefore,the recognition of the primary chemical composition of these rocks is somehow difficult due to the effects of metamorphism,hydrothermal alteration,and deformation.The interpretation of their petrogenetic processes in terms of whole-rock compositions requires an assessment of the potential effects of alteration on elemental mobility.Numerous studies have demonstrated that in rocks exposed to hydrothermal alteration and to metamorphism,the elements Al,Ti,Fe,P,HFSE (Th,Zr,Hf,Nb and Ta),REE and transition metals (Cr,Ni,Sc,V,Y and Co) are relatively immobile,while the elements Na,K,Ca,LILE (Cs,Rb,Ba,and Sr) and Pb tend to be mobile (Pearce 1982;Rollinson 1993;Arndt 1994; Polat et al.2002; Polat and Hofmann 2003; Deshmukh et al.2018).Therefore,in the following discussion using geochemical data,the features of immobile elements are applied.

5.2 Protolith and origin of the Eseka serpentinized peridotites

Fig.7 REE compositions(a,b)and trace element(c,d)patterns for serpentinized peridotites from Eseka area.Data for normalization are from Sun and McDonough (1989) and McDonough and Sun (1995) respectively for chondrite and primitive mantle

In their reviews on serpentinites associated with high-to ultrahigh-pressure metamorphic rocks,Hattori and Guillot(2007)have demonstrated that major element ratios of Mg/Si and Al/Si did not change during the hydration and metamorphism of peridotites,and also that the ratios reflect those of anhydrous peridotites.On the other hand,Evans et al.(2013) have proposed that in orogenic belts,the serpentinites could derive from various protolith compositions,ranging from cumulates to residual mantle peridotite (abyssal peridotite and mantle-wedge peridotite).Figure 8 illustrates the bulk-rock chemical variation of the studied serpentinized peridotites in MgO/SiO2–Al2O3/SiO2space.In this diagram,all the analyzed samples plot below the‘mantle terrestrial array’trend (magmatic depletion(or enrichment) trend from a primitive mantle to highly depleted harzburgitic composition (Jagoutz et al.1979;Hart and Zindler 1986).Also,most serpentinite samples(both Type 1 and Type 2) plotted in the area for abyssal peridotites,with the exception of two samples (RV3A and RV3B) of Type 1 serpentinized peridotites and three samples (RV16B,RV16E,and RV16H) of Type 2 serpentinized peridotites.These samples were characterized by relative high MgO/SiO2(＞1)and Al2O3/SiO2(＞0.06)values,respectively for Type 1 and Type 2 serpentinized peridotites.The two samples of Type 1 serpentinized peridotites plotted in the field of Cuban peridotites(Fig.8a).

Residues have higher Mg/Si and lower Al/Si,whereas melt has a lower Mg/Si and higher Al/Si(Deschamps et al.2010).According to Niu(2004),abyssal serpentinites have low Al2O3/SiO2(＜0.035) and MgO/SiO2ratios varying between 0.8 and 1.2; while for abyssal peridotites,those ratios ranging from 0.01 to 0.07; and from 0.75 to 1.05 respectively.Le Maître (1976) has proposed that the average MgO/SiO2and Al2O3/SiO2ratios are 0.83 and 0.05; 0.66 and 0.09; and 0.99 and 0.04 respectively for harzburgite,lherzolite,and dunite.The Eseka serpentinites showed MgO/SiO2values ranging from 0.98 to 1.03,average 0.97 (type 1) similar to dunites and from 0.81 to 0.92,average 0.86(type 2)and Al2O3/SiO2ratios between 0.04 and 0.05,average 0.04(type 1),and between 0.05 and 0.08,average 0.06 (type 2).Thus the studied serpentinites have similar characteristic as dunites (type 1) and harzburgites (type 2).The Eseka Type 1 serpentinized peridotites display a refractory nature,e.g.high MgO/SiO2,high Al2O3content (average=2.47 anhydrous wt%),similar to abyssal plagioclase peridotites of mid-ocean ridge setting (Coleman and Keith 1971; Niu et al.1997;Niu 2004).Type 2 serpentinized peridotites are more enriched in Al2O3(2.93 wt%) suggesting more refractory nature than Type 1.The refractory depleted mantle protolith of studied serpentinized peridotites is also confirmed by their Al/Si ratio (0.04–0.08) which is lower than the PM～0.1 (McDonough and Sun 1995),but higher than the highly depleted mantle residual harzburgites(～0.02).This suggests that the analyzed samples are refractory peridotite residues after partial melting because Al is mildly incompatible with mantle minerals and is depleted during partial melting (Hattori and Guillot 2007).

Fig.8 a MgO/SiO2 versus Al2O3/SiO2 (volatile free) of the studied serpentinized peridotite samples.Depleted mantle,primitive mantle,and abyssal peridotite are from Salters and Stracke (2004a,b),McDonough and Sun (1995) and Niu (2004),respectively.The‘terrestrial array’’ (large gray line) is after Jagoutz et al.(1979) and Hart and Zindler (1986).The compositional variation due to partial melting effect is shown with the red arrow.Fields of abyssal,Mariana forearc,Himalayan and Cuban peridotites Ishii et al.(1992),Niu(2004),Hottari and Guillot (2007) are plotted for comparison.b Dy/Yb versus La/Yb plot for the Eseka serpentinized peridotites showing their generation at shallower depths in spinel-peridotite stability field(Jung et al.2006)

In the Dy/Yb and La/Yb diagram,the studied samples plot in the spinel-peridotite mantle composition field,with the exception of four samples (RV10A,RV10E,RV10F,and RV10G) which display garnet-peridotite composition(Fig.8b).Dick and Bullen (1984) have proposed that plagioclase peridotites constitute～30 vol% of abyssal peridotites on the seafloor.However,plagioclase was not identified in all the studied samples.Considering the very low Ca (0.01–0.61 wt%) and Na (0.01–0.02 wt%) in our samples,we suggest that the original amount of plagioclase was low and was altered during serpentinization.

Our results have also revealed bulk-rock major and trace elements depletion,relative to PM values.The depletion of REE and HFSE relative to PM has been reported in orogenic,abyssal and ophiolite peridotites (Bodinier et al.1990; Bodinier and Godard 2003; Niu 2004) with their petrogenetic interpretation as being mantle melting residues.The chondrite-normalized REE patterns of Type 2 serpentinized peridotites display the convex-downward suggesting their melting-residual nature (Bhat et al.2017).

5.3 Effect of melt/rock interaction

When studied abyssal peridotites,Niu (2004) and Deschamps et al.(2013) have proposed that melt-rock interactions can cause enrichment in equal proportions of LREE and HFSE,since REE and HFSE have similar chemical behavior and solubility in mafic melts,whereas in aqueous solutions LREE are more easily mobilized than HREE and HFSE.Since hydration of ultra-mafic rocks is the main process of serpentinization,the conjoint enrichment of LREE and HFSE in serpentinites is the result of the interaction between refractory upper mantle rocks and a percolating,trace element-rich melt prior to serpentinization (Paulick et al.2006; Kodolányi et al.2011; Cannaòet al.2016).In Nb–La binary plot of Paulick et al.(2006)in which two positive trends (the light gray arrow representing the refertilization fingerprint after melt/rock interaction whereas,black arrow represents fluid/rock interaction)were defined (Deschamps et al.2010),the studied serpentinized peridotites display similar enrichments in LREE and HFSE and plot along and close to the melt-rock interaction trend (Fig.9a).This observation is confirmed by the Yb versus La/Yb plot (Fig.9b) in which the Eseka serpentinized peridotite samples show an increasing trend following the melt/rock interaction trend.The studied samples also show enrichment in HREE with respect to LREE (Fig.9c),so have refertilized nature due to melt/rock interaction prior to serpentinization.Niu (2004) has demonstrated that abyssal peridotites,subducted serpentinites,and mantle wedge serpentinites,which have suffered melt/rock interactions,present strong enrichment in U and deviate from the terrestrial array.In the Th versus U diagram (Fig.9d) samples from both Type 1 and Type 2 are plotted near the magmatic array,suggesting that enriched Th and U are most likely resulting from melt/rock interaction processes,although enrichment in compatible trace elements in subducted serpentinites by fluids during serpentinization is not negligible (Niu 2004).Such meltrock interactions prior to serpentinization are a common process and have been recently described in serpentinites from the Qinling orogenic belt,central China (Wu et al.2018) and in the serpentinized peridotites from the Suru Valley ophiolite slice Ladakh Himalaya(Bhat et al.2017).

Fig.9 a Nb versus La (ppm),b La/Yb ratio versus Yb (ppm),c Sm/Yb versus La/Sm and d U versus Th binary plots for the studied serpentinized peridotites.Melt/rock interaction trend is represented by the light gray arrow whereas,black arrow represents fluid/rock interaction trend in mantle peridotites(Paulick et al.2006).Compositional field of abyssal peridotites(Fig.9b)and gray ellipse of refertilized serpentinites(Fig.8c) plotted for comparison are after Niu (2004)

5.4 Tectonic implications

The serpentinization of mantle rocks generally occurred in two different environments:(1) during ocean-floor metamorphism/alteration at a mid-ocean ridge and (2) through dynamic metamorphism at a subduction zone(Kamenetsky et al.2001).Many authors (e.g.Mével 2003; Kodolányi et al.2011;Deschamps et al.2013;Wu et al.2018)argued that the geochemistry of serpentinites is intimately associated with specific tectonic settings:Ocean-floor metamorphism caused mobility of trace elements such as Rb and Sr,enrichment of Pb and Sb by carbonate precipitation,and U-enrichment due to hydrothermal alteration (Kodolányi et al.2011).Bulk rock Ti concentration is commonly used to determine the tectonic setting of serpentinites because Ti is not only immobile during serpentinization but also insignificantly enriched during melt/rock interactions.Indeed,mantle wedge serpentinites are characterized by low bulk rock Ti content (2–50 ppm),while abyssal serpentinites and subducted serpentinites have relatively high Ti concentration,10–130 ppm,and＞50 ppm,respectively(Deschamps et al.2013).The studied serpentinized peridotites display Rb and Sr depletion and are strongly serpentinized (average LOI=12.04 wt%) suggesting a seafloor alteration.However,their low U and Pb contents contrast with the ocean-floor alteration environment.Otherwise,their Ti/V ratios (average 11.49) similar to that of arc setting (≤30) suggest the subduction-related environment (Shervais 1982).Prominent negative Nb anomaly in the multi-element spider diagrams (Fig.7c,d) and the high Ti concentration (average:453 and 977,respectively for Type 1 and Type 2) similar to values of subducted serpentinites (Fig.10) clearly supported a subduction-related setting.Also,the high Cr contents and spinel protolith compositions suggest that the parent magmas were generated in a subduction environment,especially in Supra Subduction Zone setting (Parkinson and Pearce 1998).

Fig.10 a U(ppm),b Ba(ppm),c Sr(ppm)and d Ti(ppm)versus Yb(ppm)binary plots for the studied serpentinized peridotites.Compositions of the primitive mantle,global subducting sediments(GLOSS),sediments are from McDonough and Sun(1995),Plank and Langmuir(1998)and Li and Schoonmaker (2003),respectively

In their review on the fluid-mobile element during serpentinization,Peters et al.(2017) have noticed distinct behavior of Cs and U in mid-ocean ridge(MOR)and forearc (FA) serpentinizing fluids.They proposed discriminative element enrichment ratios of U:Cs ＞1 and U:Cs ＜1 for MOR serpentinites,and FA serpentinites respectively.The Eseka serpentinized peridotites (both type 1 and Type 2)were characterized by high U:Cs ratios(＞10),and they plotted in the MOR serpentinites field of the U–Cs enrichment diagram (Fig.11).Characteristic element fractionations are thereby governed by redox-dependent differential U mobility at mid-ocean ridges and in forearcs,and by high Cs input in forearcs due to fluids equilibrated with sediments (Peters et al.2017).

According to Pearce(2008),if the mantle-derived rocks were modified by subduction-derived fluids,it would be enriched in Th.As a result,the Th/Yb ratio would be higher in subduction-related components than that of the mantle array.When plotted in the Th/Yb versus Nb/Yb diagram(Fig.12),the Eseka serpentinized rocks fall in the volcanic arc array.Type 1 samples display backarc signatures whereas Type 2 samples plot within and close to the forearc field.Considering that the two studied serpentinite types crop in a relatively small area,the scattering of the samples may be due to the varied metasomatic effect or other melt/rock interactions,although possible contamination by subducted sediments is also to be envisaged.

Fig.11 U–Cs enrichment diagram showing the location of the Eseka serpentinites in the mid-ocean ridge serpentinites field.Mid-ocean ridge and Fore-arc serpentinites fields are from Petters et al.(2017).Primitive mantle (McDonough and Sun 1995),depleted mantle(Salters and Stracke 2004a,b),GLOSS (Plank and Langmuir 1998),and ocean water (Li 1991) values for comparison

Fig.12 The plot of Th/Yb versus Nb/Yb for the Eseka serpentinized peridotites.The mantle array includes constructive plate boundary magmas (NMORB:normal mid-ocean ridge basalts,E-MORB:enriched mid-ocean ridge basalts) and within-plate alkaline basalts(OIB:ocean island basalts).AUCC is Archean upper continental crust.Fields for convergent margin basalts include the tholeiitic(TH),calc-alkaline(CA),and shoshonitic(SHO)magma series.The vectors S,C,W,and f refer to subduction zone component,crustal contamination,within plate fractionation,and fractional crystallization respectively (after Pearce 2008).Forearc,arc,and backarc fields are of recent convergent margins fields from Metcalf and Shervais(2008)

The studied serpentinite samples are spatially associated with 2093 ± 45 Ma eclogite facies metabasite with MORB(mid-ocean-ridge basalt) affinity and interpreted as the oldest remnants of subducted Paleoproterozoic suture zone or oceanic crust in Africa(Loose and Schenk 2018;Bouyo Houketchang et al.2019).The subduction-related setting demonstrating in the present study further confirms that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the São Francisco craton in Brazil.This important finding confirms the fact that modern-style plate tectonic processes characterized by subduction of oceanic lithospheric plates were already in operation in Palaeoproterozoic times,as also proposed by Kusky et al.(2013) and Loose and Schenk (2018).

Although Cr-spinel mineral chemistry is a useful geotectonic indicator to discriminate between different tectonic settings and geodynamics of the ultramafic rocks(e.g.Kamenetsky et al.2001; Gamal El Dien et al.2016; Bhat et al.2017),the whole-rock major and trace elements data used in this study represent a preliminary data on the Eseka serpentinized peridotites and appear to be useful to put an initial constraint on the tectonic setting of this area.To better constrain the petrogenesis and tectonic settings of the Nyong series,future works will focus on mineral chemistry and stable isotope compositions.

6 Conclusions

Petrographic and first whole-rock geochemical data of the Eseka serpentinized peridotites within the Paleoproterozoic Nyong series have revealed that the studied samples are characterized by pseudomorphic textures,including mesh microstructure formed by serpentine intergrowths with cores of olivine,bastites after pyroxene.Antigorite constitutes almost the whole bulk of the rocks.Whole-rock geochemistry of the Eseka serpentinites led to the distinction of two types.The first one (Type 1) has high MgO(＞40 wt%) content and high Mg# values (88.80) whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg# values (＜40 wt% and 82.88,respectively).Both the two types of serpentinized peridotites have high LOI(mean 12.11 wt%)values,low Al/Si,and high Mg/Si ratios than the primitive mantle,reflecting a refractory abyssal mantle peridotite protolith.Partial melting modeling indicates that these rocks were derived from melting of spinel peridotite before serpentinization.The studied mantle peridotites display similar enrichments in LREE and HFSE suggesting their refertilized nature due to melt/rock interaction prior to serpentinization.Moreover,bulk rock high-Ti content similar to the values of subducted serpentinites (＞50 ppm) together with the high Cr contents and spinel-peridotite protholith compositions suggest that the parent magmas were generated in a subduction-related environment,especially in Supra Subduction Zone setting.Hence,the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the São Francisco craton in Brazil.

AcknowledgementsThis study is a part of the senior authors PhD thesis research work at the University of Yaoundé I.We gratefully acknowledge the editors for the handling of the manuscript.This is the contribution of ICGP-Y 646 project.