Geochemistry and zircon U-Pb ages of the Paleoproterozoic ultramafic rocks of the Mbi Valley,Boali area,Central African Republic

Evine Laure Njiosseu Tanko·Prince Emilien Danguene·Philomene Nga Essomba Tsoungui·Sylvestre Ganno·Jonas Didero Takodjou Wambo·Bertin Guy Takam Tchoupe·

Gus Djibril Kouankap Nono5·Timoleon Ngnotue1·Boniface Kankeu6·Jean Biandja2·Jean Paul Nzenti4

Abstract This paper investigates the geochemistry of bulk rock and infers the petrogenesis of ultramafic rocks in the Boali Precambrian terrane in Mbi Valley,in the North of the Central African Republic(CAR).The studied rocks comprise coarse primary olivine and orthopyroxene relics(dominant phase),magnesio-hornblende,magnetite,and antigorite.Whole-rock analysis indicates low SiO2(average of 43.14 wt%)and high MgO(19.84-26.98 wt%)contents and their Mg number(Mg#)ranges from 74 to 82.They display high Ni (526-865 ppm), Cr(1500-3680 ppm)contents.AFM(Na2O-K2O)-FeO-MgO)and ACM (Al2O3-CaO-MgO)ternary diagrams have revealed that the studied samples correspond to arc-related ultramafic cumulates.Chondrite-normalized REE plots display an increasing trend from La to Sm(CeN/SmN:0.74-1.81),weak negative to no Eu(Eu/Eu*=0.72-1.05)and strong negative Ce(Ce/Ce*=0.33-0.98)anomalies.Primitive mantle normalized of multi-element diagrams exhibit LREE enrichment and large ion lithophile elements(LILE)relative to high field strength elements(HFSE),and notable negative anomalies in Nb.This suggests the generation of the parent melt by slab dehydration and wedge melting processes.In addition,incompatible trace element composition and ratios assumed that the source magma had an enhanced mantle source associated with a prominent influence of continental crust.Metasomatism of mantle wedge by plate-dehydrated,LILE-rich fluids and the incorporation of sediments derived from subduction explain the enhancement of the source.Integrated major and trace element compositions jointly with the tectonic reconstruction of this region and LA-ICP-MS U-Pb data on zircon constrain the emplacement age at ca.2099 Ma in a continental margin arc setting involving subduction of an oceanic plate beneath the continental lithosphere,dehydration of the slab and mantle wedge melting.This result intimate that the remnants of Paleoproterozoic oceanic crust or subduction event and subsequent basins closure extended from the Congo craton in Cameroon to CAR and NE Brazil.

Keywords Atlantic supercontinent·Central African Republic·Ultramafic cumulate·LILE-enriched fluids·Arc setting

1 Introduction

Precambrian mafic magmatism has occurred all over the globe(Heaman 1997;Srivastava and Gautam 2009)and provides valuable insights on the development(or recasting)of the Earth’s crust,crustal growth processes,the physicochemical conditions of mantle melting,evolving magma compositions and the nature of paleotectonic environments,all of this can have wider implications when it comes to constraining the tectono-magmatic framework of Precambrian terranes(e.g.Taylor and McLennan 1985;Rudnick 1995;Heaman 1997;Srivastava and Gautam 2009;Stern and Johnson 2010;).The spatiotemporal relationship between mafic magmatism and crustal evolution during the Precambrian times has been an important subject of petrological interest.Ultramafic magmatism is a single geological event,which periodically records the geological archives on each continent,leading to widespread mantle melting processes and major thermal events(i.e.Stern 1994;Dostal et al.1994;Srivastava and Ahmad 2008;Ahmad et al.2008;Reagan et al.2008;Srivastava and Gautam 2009;Berger et al.2011).However,the source and original character of these ultramafic rocks are largely covered by the metamorphism they have undergone,their tectonic disturbance,and their isolation within more siliceous rocks.Therefore,their origin is poorly constrained,and five possible origins are proposed in the literature:(i)ultramafic cumulates(Kenneth and McTaggart,1971);(ii)picritic lava flows(Eggins 1993;Zhaochong et al.2006);(iii)tectonised subcontinental mantle(Arndt et al.2009);(iv)mantle residues(Kimura and Sakae 2012),or(v)melt-mantle reaction products(Kimura et al.2016,2017).

Well exposed ultramafic rocks have been discovered in the Boali area in CAR.This area forms part of the North Equatorial Fold Belt(NEFB)that extends from Nigeria to the CAR through Cameroon and can be correlated with the Borborema Province of NE Brazil(Caby et al 1991).The NEFB is affected by the Central Cameroon Shear Zone(CCSZ),which is a major lineament of the Pan-African Orogen of Central Africa.This transcontinental shear zone is a major crustal discontinuity extending from northeastern Brazil into CAR(Fig.1).The study area corresponds to the continuity of the Archean-Paleoproterozoic domain of the northern border of the Congo craton.The geochemical characteristics,ages,and geodynamic significance of the ultramafic magmatism in the North Equatorial Fold Belt(NEFB)in CAR are not yet constrained.Therefore,clarifying the northward extension of the Nyong-Ogooue´ultramafic series,the distinction of the different portions of crust/mantle with different ages,and localization of the Archean-Paleoproterozoic relict zones on the fold belt are needed for a better understanding of the geodynamic reconstruction of the Archean Congo/Sao Francisco craton.

Fig.1 Late-Precambrian palinspatic reconstitution of Africa and NE Brazil modified from Mapoka et al.(2010).ASZ:Cameroon Central Shear Zone=CCSZ;SF:Sanaga fault;SL:Sa~o Luis Craton;Pa:Patos shear zone;Pe:Pernambuco shear zone;TBF:Tibati-Banyo-Foumban fault.BOF:Be´tare´Oya Fault.Red rectangle:study area

In this paper,we report mineral chemistry,whole-rock geochemical data,and the first and new LA-ICP-MS U-Pb zircon age of the main representative ultramafic rock samples collected along the Mbi Valley River in the Boali area,the northern part of the CAR.These data are used to discuss the petrogenesis and geodynamic significance of these Paleoproterozoic ultramafic rocks.

2 Geological setting

2.1 Regional geological setting

The CAR has been divided into three major structural units(Cornacchia et al.1989;Rolin 1992;Nzenti 1994),from south to north:1.Southern unitrepresents the Northern part of Congo Craton and consists of(i)micaschists and quartzites of Archean and Paleoproterozoic age(Poidevin 1991);(ii)metabasites of Archean age(2900 Ma;Lavreau et al 1990);(iii)charnockites series and gneisses similar to those of the Congo Craton in Cameroon(Pin and Poidevin 1987),and(iv)Archean komatiites,itabirites,greywacke,rhyodacitic tuffs and granitoids(Cornacchia and Giorgi 1989).(v)The central part of this domain is occupied by an intermediate rock series(2100 Ma,Lavreau et al.1990),which consists of quartzites,amphibolites,and orthogneisses.2.The intermediate or central domainconsists of Archean gneisses,metabasites,granites,and Paleoproterozoic metasedimentary rocks and migmatites.According to Rolin(1992),these rocks are separated from the Neoproterozoic gneisses by a ductile shear zone.3.The Northern partis composed of granulites,orthogneiss,and granite of Neoproterozoic age(833±66 Ma:Lavreau 1990).It corresponds to the western extension of the Pan-African fold belt in Cameroon.This domain is bordered in the south by a late Pan-African shear zone(Cornacchia and Dars 1983).The inquiry area(Figs.1 and 2)pertains to the central domain which corresponds to the western prolongation of the fold belt in Cameroon.This domain consists of Archaean metabasites,metasedimentary rocks,migmatites,and granites(Danguene et al.2014;Tanko Njiosseu et al.2021).

2.2 Field geology

The inquiry rock samples correspond to ultramafic cumulates which outcrop in the Mbi Valley River,west of the Bakou and at Karango villages,as blocks of varying sizes(2 to 200 m across)(Fig.3a),or kilometric slabs.The huge outcrop is located to the west of Bakou village,and it’s about 5 km wide and 12 km long.This outcrop is hosted by metapyroxenite and gneiss.Contact between intrusion and the country rocks is underlined by the lenses of mylonitic rocks,displaying sinistral shear movement.This assumed that the investigated mantle rocks have been intruded through the N130°NE-trending shear zone(Fig.2).Ultramafic rocks of the Mbi Valley area are slightly serpentinized.The serpentinization mostly affects the olivine and

Fig.2 Geological map for Mbi River Valley area after Gerard and Gerard(1953)

orthopyroxene.The dominant lithological unit encountered is coarse-to medium-grained peridotite.This rock is black to dark grey(Fig.3b),showing the spheroidal weathering characteristic of igneous rocks weathering.

2.3 Petrography

The ultramafic rocks are made up of coarse primary olivine and orthopyroxene relics(dominant phase),amphibole,magnetite,and serpentine.Serpentine minerals in both types are secondary phases resulting from the alteration of primary pyroxene and olivine minerals.These minerals are typified by pseudomorphic textures(Fig.3c).In most samples,individual crystals of olivine are crosscut by fractures filled with serpentine.The relict of cumulus texture,olivine surrounded by pyroxene is still visible(Fig.3d).Amphibole crystals are of both primary and secondary types,with the secondary amphibole coming from the orthopyroxene as evident from their relict cleavage.The majority of pyroxene crystals are also altered to chlorite,serpentine,and siderite.Antigorite(average grain size of 0.2 mm)constitutes the main serpentine mineral of the rock and appears as colorless aggregates of fibrolamellar nature that give anomalous interference colors and parallel extinction(Fig.3e).Magnetite represents a minor constituent in most analyzed samples.

Fig.3 Representative field and thin-section photographs for ultramafic intrusions in the Mbi River Valley area.a The dominant lithological unit encountered is peridotite which mainly occurs as boulders of varying sizes showing onion skin alteration b macroscopic view of the fresh sample showing pseudomorphic textures.c Coarse-grained texture dominated by orthopyroxene,olivine,Mg-hornblende,and secondary chlorite and serpentine.d relict cumulus texture e Olivine crystals showing transformation to serpentine+magnetite.Opx orthopyroxene,Ol olivine,Mg-Hbl magnesio-hornblende,Mag magnetite,Serp serpentine,Chl chlorite

2.4 Sample preparation

Ultramafic samples as fresh as possible have been collected in the field and twenty-two representatives of them were selected for petrographic studies.Polished thin sections were made at Canada Vancouver Geotech Lab using conventional techniques.Optical microscopy allowed us to determine the texture and mineralogy of these samples at the University of Yaounde´1,Cameroon.

3 Analytical methods

3.1 Electron probe microanalysis

Large samples were collected in the terrain to obtain representative whole-rock chemical analyses.Appropriate precautions have been taken to collect the sample as fresh as possible and of good quality and efforts have been made to minimize crosscontamination and external contaminants of the samples during transport and preparation.Twenty-two representative ultramafic samples were selected for geochemical studies.Chemical-mineralogical analyzes were obtained using the fvie spectrometers JEOLJXA-8200 electronic microprobe fromthe Geological and Planetary Sciences Division(GPS),California Institute of Technology,USA.Surfaces were vacuum-coated with about 15 nm of carbon.Operating conditions were 15 kV,25 nA,1μm beam,20 s on-peak counting times,and the CITZAF matrix correction routine.Backgrounds were subtracted using the mean-atomic number working curve method in place of offpeak counting.The analytical standards used for analyses were synthetic forsterite,fayalite,Mn-olivine,anorthite,TiO2,and Cr2O3;Amelia albite,Asbestos microcline,and Durango apatite.

3.2 Whole-rock geochemistry

Whole-rock for major elements was analyzed by ICP-AES(Inductively Coupled Plasma-Atomic Emission),and for both trace elements,REE by ICP-MS(Inductively Coupled Plasma Mass Spectrometry)at ALS Minerals Global Group,Vancouver(Canada).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 REE.The analysis precision for REE is 5%for concentrations of 10 ppm and 10%for lower concentrations.The LOI was calculated using the weight difference after ignition at 1000°C.Standards procedures were applied,and reliable data quality was established by treating these standards as unknown between samples.

3.3 LA-ICP-MS U-Pb dating of in-situ zircons

Zircon grains were extracted from the sample HUB-3 by heavy liquid and magnetic separation at the Langfang Rock Detection Technology Services in Hebei(China),before being handpicked under a binocular microscope for mounting in epoxy resin.To identify their internal structure and to choose potential target sites for the U-Pb analysis,cathodoluminescence (CL) images, transmitted and reflected light microphotographs were obtained using a scanning electron microscope at Beijing Kehui Testing International Co.Ltd.Zircon U-Pb geochronology was conducted in the same laboratory by LA-ICP-MS.Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as a description by Soh Tamehe et al.(2021).Each analysis began with a 15-s background acquisition(gas blank)followed by a 45-s data acquisition from the sample.The ICPMSDataCal 8.4 software was used for fractionation correction and calculation of the dating results(Liu et al.2010).Zircon GJ1 was used as an external standard for UPb dating and according to Liu et al.(2010).Better U-Th-Pb isotopic ratios used for GJ1 are from Jackson et al.(2004).The external standard GJ1 has for uncertainty value of 0.5%;this value,the latter was propagated to the final results of the samples.NIST 610 was used to calibrate the U,Th,and Pb concentrations in all of the zircon grains studied.Isoplot/Ex ver3 was used to create Concordia diagrams and calculate weighted mean calculations.The zircon Plesovice is dated as unknown samples and yielded a weighted mean206Pb/238U age of 337±2 Ma(2SD,n=12),which is close to the recommended206Pb/238U age of 337.13±0.37 Ma(2SD)(Sla´ma et al.2008).

4 Results

4.1 Mineral chemistry

The chemical compositions of orthopyroxene,olivine,amphibole,magnetite,serpentine,chlorite,and siderite from the Mbi Valley ultramafic rocks are mentioned in the Electronic Supplementary Material Table S1 to S6.

The orthopyroxenes(Table S1)are all of the group of Ca-Fe-Mg pyroxene,named enstatite or clinoenstatite according to the classification(Fig.4a)of Morimoto(1989).They are poor in calcium(0.02-0.94 wt%),Na2O(0.0-0.72 wt%),Al2O3(0.92-1.70 wt%)and TiO2(0.0-0.13 wt%),and enriched in MgO(32-36.58 wt%)(Table S1).Their high Mg#values,where Mg#=MgO/[MgO+TFeO],ranging from 89 to 91 together with variable Al2O3and TiO2contents resemble those of arc-related cumulates(Yuan et al.2017).Mostly,the orthopyroxenes show a remarkably uniform composition and a restricted compositional range(En88-90Fs9-11Wo0-2)and are mainly enstatite.

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Fig.4 a OPX Q-J classification scheme(Morimoto 1989)and b classification and name of amphibole(Leake et al.1997)

Olivine grains are made up of forsterite(average Fo values of 86-87,Table S2).They are poor in Al2O3(0.64-1.16 wt%)and display high MgO(43.84-45.80 wt%)and high Mg#(85-87)values.

Data on the amphiboles mineral chemistry is provided in Table S3.The amphiboles are mainly magnesio-hornblende(Leake et al 1997;Fig.4b)in composition(XMg-=0.99-1.19).Serpentine(Table S6)partially substitutes orthopyroxene and shows average composition of both lizardite(XMg=0.88-0.89)and antigorite(XMg=0.95).In the studied samples,magnetite and siderite(Table S4)are the main iron minerals,while chlorites are typified by MgO content ranging from 28.78 to 29.87 wt%,Al2O3from 17.05 to 20.87 wt%,and FeO between 6.44 and 7.13 wt%(Table S6).

4.2 Whole-rock geochemistry

4.2.1 Major elements

Geochemical whole-rock data of twenty-two ultramafic rock samples are mentioned in Tables 1 and 2.Samples show high LOI values(5.6-8.5 wt%)reflecting moderate serpentinization of olivine and pyroxene as evidenced by the petrography.The low SiO2content(40.95-43.56 wt%:average of 43.14 wt%)and MgO content(23.92-27.08 wt%)of the rocks suggests a pyroxene-rich protolith,than a more olivine-or hornblende-rich protolith that would have ultrabasic chemistry(Wyllie 1967).All the studied samples show low SiO2(40.95-43.56 wt%;average of 43.14 wt%),and high MgO(23.92-27.08 wt%)concentrations.Their Mg number(Mg#)varied from 80 to 82.Al2O3varies from 5.9 to 7.9 wt%,TiO2from 0.12 to 0.2 wt%and CaO vary from 2.78 to 6.37 wt%,with CaO/Al2O3ratio below unity for all samples.Total alkalis(Na2O+K2O)account for＜1 wt%.LOI values are moderate to high(5.6-8.2 wt%)and reflect some degree of the alteration as revealed by the serpentinization of olivine and pyroxene in most samples(Fig.3e).In major element variation diagrams(Fig.5),Al2O3,TiO2,CaO,concentrations decrease with increasing MgO content whereas Fe2O3tot increases with increasing the MgO content.Because of the high measured LOI values,major element oxides were recalculated to 100%total on a volatile-free basis(Rollinson and Pease 2021).On the AFM[(Na2O-K2O)-FeO-MgO]and ACM(Al2O3-CaO-MgO)ternary diagrams of Coleman(1977),the studied ultramafic rock samples occupied the ultramafic cumulates field(Figs.6a,b).Binary plots of Mg#against SiO2,Al2O3,FeOT,and TiO2(Fig.7)indicate that these rocks belong to the continental arc-cumulate type(Chin et al.2018).

Fig.5 Harker diagrams of selected Major elements of the Mbi Valley ultramafic rocks

Fig.7 Mg#vs.the major oxide plots.a SiO2(wt%);b Al2O3(wt%);c FeO*(wt%);and d TiO2(wt%).Fields for continental arc cumulate and mid-oceanic ridge cumulate are taken from Chin et al.(2018)

Table 1 Major element composition(wt%)and ratio of the Mbi Valley ultramafic cumulates

HUB25 HUB24 HUB23 HUB22 HUB21 HUB20 HUB19 HUB18 HUB17 HUB16 HUB15 1.2 1 0.3 0.3 0.4 0.2 0.2 0.3 0.2 0.2 0.7 7.1 6.7 10.6 8.5 8.6 8 8.3 6.9 7 7.8 6.8 29 26 7 7 5 9 6 5 9 4 26 3.8 3.6 4.8 5.5 3.9 5.6 6 6.4 6.7 5.7 3.6 7.9 7.5 5 5.6 4.1 4.8 5.1 5.5 5.8 5.1 6.9 0.2 0.2 0.2 0.2 0.2 0.1 0.2 0.2 0.2 0.1 0.3 0.3 0.3 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 2670 2760 1580 3270 1820 3290 3070 3680 3630 3410 2980 686 694 553 750 529 782 810 865 898 794 685 80.4 79.1 93.1 86.2 94 90.4 90.8 103.5 105.2 94.7 81.5 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 62 60 68 55 69 59 50 59 59 57 62 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 HUB25 HUB24 HUB23 HUB22 HUB21 HUB20 HUB19 HUB18 HUB17 HUB16 HUB15 2.4 1.7 1.8 2.3 2.3 2.6 2.5 2.7 2.5 2.5 2.3 2.91 2.8 3.4 3.4 3.6 2.9 2 3.2 3.8 2.7 3.4 0.71 0.5 0.57 0.51 0.55 0.74 0.78 0.82 0.76 0.77 0.53 3.1 2.1 2.5 2.5 2.5 3.1 3.3 3.6 3.5 3.2 2.3 0.81 0.56 0.56 0.46 0.48 0.74 0.77 0.91 0.94 0.84 0.55 0.27 0.19 0.17 0.17 0.19 0.25 0.26 0.24 0.28 0.26 0.18 0.84 0.64 0.66 0.61 0.63 0.87 0.97 1.12 1.15 0.97 0.6 0.15 0.12 0.11 0.12 0.12 0.15 0.17 0.2 0.21 0.18 0.11 0.91 0.77 0.72 0.71 0.69 0.98 1.08 1.28 1.34 1.05 0.72 0.2 0.16 0.15 0.16 0.16 0.2 0.23 0.27 0.28 0.22 0.15 0.64 0.48 0.49 0.47 0.46 0.59 0.65 0.82 0.78 0.68 0.41 0.08 0.06 0.08 0.08 0.06 0.08 0.1 0.11 0.11 0.09 0.06 0.55 0.42 0.49 0.42 0.42 0.56 0.62 0.72 0.77 0.61 0.39 0.08 0.06 0.07 0.07 0.07 0.08 0.1 0.11 0.11 0.09 0.06 13.65 10.56 11.77 11.98 12.23 13.84 13.53 16.1 16.53 14.16 11.76 HUB25 HUB24 HUB23 HUB22 HUB21 HUB20 HUB19 HUB18 HUB17 HUB16 HUB15 30 30 20 10 10 10 10 10 10 10 20 440 460 200 210 220 290 200 320 380 270 340 0.02 0.02 0.05 0.05 0.05 0.03 0.05 0.03 0.03 0.04 0.03 40 38 25 28 21 48 26 28 29 51 23 14 16 11 9 9 6 7 6 6 6 13 26.3 25 25 56 41 48 51 55 58 51 34.5 6.0 7.7 14.0 24.0 17.0 26.0 25.0 27.0 25.0 25.0 11.5 96.7 86.7 35.0 70.0 50.0 90.0 60.0 50.0 90.0 40.0 130.0 4.0 3.3 1.5 3.0 4.0 2.0 2.0 3.0 2.0 2.0 3.5 Table2Traceelementcompositions(wt%)andratiosoftheMbiValleyultramaficcumulates HUB14 HUB13 HUB12 HUB11 HUB10 HUB8 HUB7 HUB6 HUB5 HUB4 HUB2 Sample 0.6 0.7 0.6 0.5 0.6 0.5 0.5 0.5 1.1 0.8 0.8 Rb 6.8 6.7 7.3 6.3 7.7 6.4 7.1 6.2 7.4 7.3 9.8 Sr 6 9 5 4 4 3 4 2 14 13 6 Ba 4.2 4.1 3.6 3.9 4.6 3.6 5.1 4.6 3.5 4.7 6.8 Y 6.9 5.6 5.6 5.5 6.7 5.3 6.1 4.9 5.8 6.2 7 Zr 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.3 Hf 0.1 0.1 0.3 0.3 0.2 0.2 0.2 0.1 0.2 0.4 0.7 Nb 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Ta 2700 2830 2870 2810 3000 3030 2790 2850 2780 2910 2900 Cr 704 712 685 690 699 672 725 722 678 742 777 Ni 80.8 83.7 85.9 79.9 81.1 81.6 79.6 78.5 79.9 82.7 85.4 Co 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 U 59 62 59 59 68 61 66 61 60 67 77 V 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Th 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Cs HUB14 HUB13 HUB12 HUB11 HUB10 HUB8 HUB7 HUB6 HUB5 HUB4 HUB2 Sample 1.8 1.9 1.6 1.9 2.1 1.9 2.3 2 1.6 2.2 2.9 La 2.1 3.6 2.2 2.1 2 2.1 1.8 1.8 3.5 2.2 2.1 Ce 0.51 0.49 0.44 0.47 0.54 0.45 0.53 0.46 0.46 0.59 0.71 Pr 2.1 2.2 2 1.9 2.2 2.1 2.2 1.8 1.8 2.5 2.9 Nd 0.52 0.49 0.5 0.47 0.51 0.52 0.52 0.46 0.47 0.54 0.68 Sm 0.18 0.17 0.15 0.16 0.2 0.17 0.18 0.15 0.17 0.18 0.22 Eu 0.65 0.63 0.6 0.66 0.78 0.61 0.72 0.62 0.6 0.68 0.9 Gd 0.11 0.11 0.11 0.11 0.13 0.11 0.11 0.1 0.1 0.12 0.15 Tb 0.71 0.75 0.71 0.74 0.79 0.69 0.8 0.69 0.62 0.84 0.91 Dy 0.17 0.15 0.16 0.16 0.18 0.14 0.17 0.14 0.12 0.17 0.2 Ho 0.43 0.47 0.41 0.46 0.51 0.42 0.46 0.43 0.41 0.53 0.61 Er 0.06 0.06 0.07 0.06 0.07 0.06 0.07 0.06 0.06 0.07 0.08 Tm 0.43 0.4 0.39 0.42 0.43 0.36 0.38 0.33 0.36 0.46 0.46 Yb 0.07 0.07 0.07 0.07 0.07 0.06 0.06 0.05 0.06 0.08 0.07 Lu 9.84 11.49 9.41 9.68 10.51 9.69 10.3 9.09 10.33 11.16 12.89 SumREE HUB14 HUB13 HUB12 HUB11 HUB10 HUB8 HUB7 HUB6 HUB5 HUB4 HUB2 Sample 10 10 30 30 20 20 20 10 20 40 70 Nb/Ta 210 360 220 210 200 210 180 180 350 220 210 Ce/Ta 0.05 0.03 0.05 0.05 0.05 0.05 0.06 0.06 0.03 0.05 0.05 Th/Ce 35 28 28 28 34 27 31 25 29 31 23 Zr/Hf 13 11 11 12 13 10 12 11 12 11 10 Zr/Sm 69 56 18.7 18.3 33.5 26.5 30.5 49 29 15.5 10 Zr/Nb 18.0 19.0 5.3 6.3 10.5 9.5 11.5 20.0 8.0 5.5 4.1 La/Nb 60.0 90.0 16.7 13.3 20.0 15.0 20.0 20.0 70.0 32.5 8.6 Ba/Nb 6.0 7.0 2.0 1.7 3.0 2.5 2.5 5.0 5.5 2.0 1.1 Rb/Nb

Table2continued HUB25 HUB24 HUB23 HUB22 HUB21 HUB20 HUB19 HUB18 HUB17 HUB16 HUB15 HUB14 HUB13 HUB12 HUB11 HUB10 HUB8 HUB7 HUB6 HUB5 HUB4 HUB2 Sample 0.33 290 0.06 16.11 0.33 3.21 1.14 180 0.20 0.61 0.87 2.96 1.24 23.1 1.00 0.53 0.33 260 0.04 11.30 0.33 4.79 1.14 230 0.24 0.71 1.21 2.75 1.23 12.2 0.97 0.72 0.50 70 0.04 2.50 0.50 6.22 0.75 280 0.17 0.33 1.47 2.50 1.09 12.5 0.85 0.81 1.00 70 0.04 2.92 1.00 3.81 1.15 240 0.18 0.18 1.78 3.72 1.18 9.5 0.98 0.67 1.00 50 0.06 2.94 1.00 3.54 1.14 170 0.24 0.24 1.81 3.72 1.21 10.3 1.05 0.71 1.00 90 0.04 3.46 1.00 3.51 1.32 260 0.18 0.18 0.95 3.15 1.26 21.9 0.95 0.49 1.00 60 0.04 2.40 1.00 3.25 1.24 250 0.16 0.16 0.63 2.74 1.27 25.4 0.92 0.35 1.00 50 0.04 1.85 1.00 2.97 1.26 270 0.14 0.14 0.85 2.55 1.26 34.6 0.72 0.51 1.00 90 0.04 3.60 1.00 2.66 1.22 250 0.13 0.13 0.98 2.21 1.21 36.7 0.82 0.65 1.00 40 0.04 1.60 1.00 2.98 1.38 250 0.16 0.16 0.78 2.78 1.29 27.7 0.88 0.47 0.50 260 0.04 11.30 0.50 4.18 1.41 230 0.26 0.51 1.49 4.01 1.24 11.2 0.96 0.68 1.00 60 0.06 3.33 1.00 3.46 1.21 180 0.23 0.23 0.98 2.84 1.22 11.5 0.94 0.52 1.00 90 0.05 4.74 1.00 3.88 1.23 190 0.25 0.25 1.77 3.23 1.27 10.5 0.93 0.85 0.33 50 0.06 3.13 0.33 3.20 1.28 160 0.26 0.77 1.06 2.79 1.24 10.2 0.83 0.60 0.33 40 0.05 2.11 0.33 4.04 1.12 190 0.24 0.71 1.08 3.07 1.27 10.5 0.88 0.51 0.50 40 0.05 1.90 0.50 4.12 1.19 210 0.23 0.47 0.95 3.32 1.47 13.5 0.97 0.43 0.50 30 0.05 1.58 0.50 3.65 1.44 190 0.28 0.56 0.98 3.59 1.37 10.8 0.92 0.50 0.50 40 0.04 1.74 0.50 4.42 1.37 230 0.26 0.53 0.84 4.11 1.53 12.7 0.90 0.36 1.00 20 0.05 1.00 1.00 4.35 1.39 200 0.30 0.30 0.94 4.12 1.52 9.7 0.86 0.42 0.50 140 0.06 8.75 0.50 3.40 1.31 160 0.28 0.56 1.80 3.02 1.35 9.6 0.98 0.98 0.25 130 0.05 5.91 0.25 4.07 1.17 220 0.22 0.87 0.98 3.25 1.20 12.5 0.91 0.45 0.14 60 0.03 2.07 0.14 4.26 1.48 290 0.22 1.52 0.75 4.28 1.58 20.8 0.86 0.33 Th/Nb Ba/Th Th/La Ba/La Th/Nb La/Sm Sm/Yb La/Ta Th/Yb Nb/Yb(Ce/Sm)N(La/Yb)N(Gd/Yb)N(Gd/Sm)N Eu/Eu*Ce/Ce*

4.2.2 Trace elements

Trace elements concentrations of ultramafic rocks for the Mbi Valley are mentioned in Table 1.All samples have relatively high contents of V (59-77 ppm), Co(78.5-105.2 ppm), Ni (529-898 ppm), and Cr(1580-3680 ppm)confirming their ultramafic origin.However,Ni and Cr concentrations are low relative to typical mantle peridotites(Hess 1989).The average Zr/Y(1.27)and Nb/Y(0.02-0.1)ratios resemble those seen in continental igneous suites(Condie 2003).

Chondrite-normalized REE patterns(Sun and McDonough 1989)for the Mbi Valley ultramafic rocks(Fig.8a)resemble each other and are marked by slightly LREE enrichment(LaN/YbN:2.21-4.28)relative to HREE(GdN/YbN:1.09-1.58),weakly negative to zero anomalies in Eu(Eu/Eu*=0.78-1.05)and negative anomalies(Ce/Ce*=0.33-0.98)in Ce.The multi-element spectra display negative anomalies in Nb,Ta,Sr,Zr,and P,and overall 0.3-4 times enriched trace element contents than chondritic values(Fig.8b).

Fig.8 REE patterns of normalized Chondrite(a,b),primitive mantle(c,d)of the Mbi valley ultramafic rocks

4.3 Zircon U-Pb geochronology

Data of zircon U-Pb for sample HUB-3 is signalized in Table 2 and shown in Figs.9 and 10.CL images of zircon crystals show a large range of shapes,sizes,and colors(Fig.9).A dominant form,however,is the chunky type with the original euhedral to subhedral shape and most grains in CL images exhibit oscillatory zoning,despite the thin outer zone attributable to metamorphic growth(rounded terminations).Another type of zircon crystal is represented by multifaceted to rounded shapes exhibiting wide external zones,poorer in uranium,suggesting a metamorphic growth type(Pidgeon 1992).All types of zircon morphology were selected for analysis.Thirty-three zircon spots were analyzed and 21 exhibited good concordance(%concordance＞90)were selected for age calculation(Table 2).These spots display Th and U variable contents ranging from 39 to 191 ppm and from 40 to 766 ppm,respectively,and low to high Th/U ratios(0.05-3.37)(Table 3).The spots define an upper intercept age 2099±30 Ma(MSWD=2.3;n=21),and a lower intercept age of 553±22 Ma(MSWD=2.3)on the Discordia diagram(Fig.10).By the zircon morphology,structure,and Th/U ratios,the age of the upper intercept is interpreted as the magmatic age for the ultramafic cumulate rocks.This age is similar to the age emplacement of the Tamkouro plutonic rocks at 2069±9.6 Ma(Danguene et al.2014).The lower intercept ties into the Pan-African/Braziliano tectono-metamorphic event.

Fig.9 Cathodoluminescence(CL)images show a large set of shapes and sizes of zircon crystals from the ultramafic cumulates of the Mbi Valley area

Fig.10 LA-ICP-MS Discordia(a)and Pro-density(b)diagram showing upper intercept and lower intercept ages

5 Discussion

5.1 Geochemical fingerprinting

The measured LOI values are fairly high(5 to 8 wt%),confirming intense alteration of the studied rocks,and thus indicative of some elements’mobility.Moreover,the Mbi Valley ultramafic rocks have experienced low-grade metamorphism and hydrothermal alteration.In such conditions,LILEs(K,Ba,Rb)are mobile while HFSE(Th,Nb,Zr,Ta,and Hf),REEs,and transitional elements(i.e.Ni,Cr)tend to remain immobile(Winchester and Floyd 1977;Pearce et al.1992;Polat et al.2002;Rollinson 2014).Pearce et al.(1992)have proposed using the connection between Zr and other elements to gauge the mobility of these elements during alteration and low-grade metamorphism.In the investigated samples,Ce,Rb,Sm,Nb,Sr,and Yb vary systematically with Zr,attesting to their relative immobility(Fig.11a-f),suggesting that the rocks have kept their primary chemical compositions.Therefore,in the following sections,the behaviors of the less mobile trace elements are used to investigate the age,petrogenesis,and tectonic setting of the Mbi River ultramafic rocks.

Fig.11 Binary plots of selected trace elements against Zr

5.2 Nature of parental magma

The geochemistry of ultramafic rocks shows some fluctuations for their major and trace element contents which could probably reflect the source mantle heterogeneity or the crustal contamination effect(i.e.Pearce and Norry 1979;Pearce 1983).The Mbi Valley ultramafic rock samples show high Al2O3/TiO2(＞22:29.50 to 57)ratios and the little TiO2(＜0.3 wt%:0.16-0.2)contents,consistent with a significant refractory nature of their mantle source(Hickey and Frey 1982;Duncan and Green 1987).Such a refractory mantle source assumes that the studied samples represent mantle cumulate,which by nature is refractory because orthopyroxene and/or olivine are poor in incompatible trace elements(Khanna et al.2016).All the samples have higher Mg#of 80-82 and MgO(＞20 wt%)contents,which are indicative of a cumulate origin.Their elevated Cr(1500-3680 ppm),Ni(533-898 ppm)contents are also coherent with those awaited for ultramafic rocks of a cumulate origin(Cr＞1000 ppm,Ni＞300 ppm;e.g.,Roberts et al.2000;Polat et al.2011,2012).

Conc.(%)rho±1σ 94 99 96 96 98 91 91 98 94 99 96 96 99 96 92 96 97 97 96 90 99 0.3664 0.7101 0.5677 0.6962 0.7851 0.3670 0.5167 0.8870 0.6005 0.7695 0.5083 0.7863 0.6654 0.5206 0.4597 0.6590 0.3392 0.3634 0.7148 0.4368 0.7076 7 27 19 24 28 7 20 43 21 27 10 35 31 23 23 23 8 11 24 9 11 206Pb/238U Age(Ma)588 1995 1946 1849 2034 604 1785 1976 1906 2026 603 1913 2018 1914 1861 1929 597 608 1933 541 552±1σ 16 20 18 19 18 16 21 25 19 18 16 23 24 24 27 18 18 24 18 17 13 207Pb/235U Age(Ma)621 2012 2008 1971 2063 656 1936 2016 2016 2032 622 1973 2038 1986 1999 2000 613 622 2009 592 556±1σ 72 29 30 38 29 61 71 27 35 29 78 33 42 37 42 31 82 102 34 83 67 207Pb/206Pb Age(Ma)746 2027 2069 2103 2091 835 2098 2054 2126 2035 702 2032 2054 2053 2135 2069 672 665 2084 1200 589 Table3LA-ICP-MSzirconU-PbdatafortheultramaficcumulatesintheMbiValleyarea 206Pb/238U 207Pb/235U 207Pb/206Pb Th/U U Th Spot±1σ Ratio±1σ Ratio±1σ Ratio ppm ppm 0.0012 0.0058 0.0040 0.0050 0.0060 0.0012 0.0041 0.0090 0.0044 0.0058 0.0017 0.0072 0.0066 0.0049 0.0048 0.0047 0.0013 0.0018 0.0051 0.0015 0.0019 0.0955 0.3627 0.3525 0.3322 0.3709 0.0981 0.3190 0.3588 0.3440 0.3694 0.0980 0.3456 0.3676 0.3457 0.3347 0.3489 0.0971 0.0989 0.3497 0.0875 0.0895 0.0288 0.1411 0.1250 0.1284 0.1368 0.0297 0.1423 0.1781 0.1333 0.1302 0.0291 0.1586 0.1728 0.1646 0.1903 0.1269 0.0325 0.0432 0.1278 0.0301 0.0222 0.8441 6.2550 6.2217 5.9649 6.6269 0.9078 5.7283 6.2833 6.2807 6.3989 0.8446 5.9793 6.4426 6.0706 6.1621 6.1663 0.8292 0.8450 6.2350 0.7922 0.7298 0.0022 0.0020 0.0022 0.0028 0.0021 0.0020 0.0028 0.0020 0.0026 0.0021 0.0023 0.0024 0.0026 0.0027 0.0032 0.0023 0.0024 0.0029 0.0021 0.0026 0.0018 0.0641 0.1249 0.1279 0.1304 0.1295 0.0669 0.1299 0.1267 0.1321 0.1254 0.0628 0.1252 0.1267 0.1268 0.1328 0.1279 0.0619 0.0617 0.1289 0.0659 0.0594 0.77 0.09 0.18 0.18 0.51 0.95 0.18 0.24 0.51 0.05 1.37 0.34 0.17 0.15 0.49 0.11 0.88 1.05 0.15 1.26 0.84 149 651 244 268 296 110 573 784 312 643 85 564 304 311 338 360 57 61 766 97 126 115 60 44 49 150 104 103 186 159 29 116 191 53 45 165 39 50 64 115 123 107 HUB3-03 HUB3-04 HUB3-05 HUB3-06 HUB3-08 HUB3-09 HUB3-10 HUB3-14 HUB3-15 HUB3-16 HUB3-17 HUB3-19 HUB3-22 HUB3-23 HUB3-24 HUB3-25 HUB3-26 HUB3-27 HUB3-29 HUB3-31 HUB3-33

In the molar element ratio diagrams(Fig.12a and b;Pearce 1968),the Mbi Valley ultramafic rocks essentially plot sub-parallel to orthopyroxene-controlled fractionation trend with a consistent slope of 1.1.This inferred that the Mbi rocks are products of the accumulation of orthopyroxene and olivine.Moreover,the studied samples display very low Nb contents(0.1-0.7 ppm)relative to N-MORB(mean Nb:2.3 ppm),and a high Fe/Mn(49-96)ratio than those of MORB(55-58)(Le Roux et al.2010).Their Zr/Nb(10-69)values resemble those seen in continental margin arc magmas from the Andes(10.45-42.10)and are interpreted as indicative of an enriched mantle source(Ewart 1982;Marriner and Millward 1984;Thrope et al.1984;Hickey et al.1986;Wilson 1989).Therefore,it is suggested that the mantle source for the ultramafic rocks of Mbi Valley is enriched relative to N-MORB.Several incompatible trace element ratios(Table 2,Weaver 1991)insinuate that the origin magma of the rocks comes from a source of the enriched mantle(EMI)associated with a prominent influence of continental crust.Depletion in Nb-Ta relative to LREE and LILE and flat HREE pattern on chondrite-normalized REE profile(Fig.8)are features of low state partial melting of a subduction-modified enriched mantle source above the garnet-stability field,which in this case,is represented by a sub-arc lithospheric mantle wedge of spinel peridotite composition.The conjunction of low Nb/Ta with high Zr/Hf and Zr/Sm ratios is in accordance with residual amphibole in the source,suggestive of source enrichment by hydrous metasomatism of the mantle wedge(Munker et al.2004;Manikyamba et al.2009).Peate et al.(1997)argued that during subduction in a continental margin arc setting,hydrous fluxing of incompatible trace elements,such as LILE and LREE,from the dehydrated slab alters the overlying lithospheric mantle edge leading to the enrichment of LILE and LREE.Indeed,the patterns of incompatible trace elements defined by negative Nb,Ti,and Ta anomalies are diagnostic features of HFSE depleted rocks generated by subduction zone magmatic processes.The(Nb/La)PMvs.(Th/Nb)PMbinary plot(Fig.13a)depicts the negative anomaly in Nb for the studied rocks and they follow the subduction component and crustal assimilation trends in the Ce/Nb vs.Th/Nb diagram(Fig.13b)testifying their continental margin arc character.

Fig.12 Molar proportions of major element concentrations for ultramafic cumulates after Pearce(1968)

Well-documented elemental ratios commonly used to monitor potential fluid or sediment contributions to magma sources,include Ba/La and Th/La(McCulloch and Gamble 1991),Ta/La and Hf/Sm(La Fle`che et al.1998),Sr/Th and Th/Ce(Turner et al.2012).The Th/Ce(0.01-0.06)ratio of the rocks compared with OIB(0.052)and MORB(0.016)is a piece of evidence for mantle enrichment by subductionderived sediments(Taylor and McLennan 1985;Stein and Hofmann 1994;Condie 1998;Brunet and Machetel 1998).This interpretation is corroborated by Ba/Th vs.Th/Nb diagram(Fig.13c).Furthermore,the Ta/La(0.038-0.079),Hf/Sm(0.10-0.33)ratios may reflect the existence of sediment-derived fluid.The source mineralogy of such metasomatized mantle can be characterized using some distinctive REE abundances and ratios due to their progressive increase in incompatibility from LREE to HREE(Green 2006;Aldanmaz et al.2000).With a La/Sm ratio of 1.66-3.12 and Sm/Yb ratio of 1.11-1.73,the Mbi Valley ultramafic samples fall in the spinel lherzolite melting trajectory in both the La/Sm vs.Sm/Yb and Sm/Yb vs.Sm diagrams(Fig.14a,b).This indicates the existence of a spinel-dominated mineral assemblage in their source region and varying states of partial melting of a fertile subcontinental lithosphere source.

Fig.13 a(Nb/La)PM vs.(Th/Nb)PM,b Ce/Nb vs.Th/Nb,and c Ba/Th vs.Th/Nb diagram for ultramafic cumulates from the Mbi Valley area.Data source in c:Average Pelitic Sediments(APS)(Taylor and McLennan 1985);N-MORB(Sun and McDonough 1989)

Fig.14 a Sm/Yb vs.La/Sm and b Sm/Yb vs.Sm plots for ultramafic cumulates from the Mbi Valley area.DMM(depleted MORB mantle),RSC(residual slab component),SDC(subduction component),UCC(upper continental crust),SS(south Sandwich and Marianas arc basalts)(Saunders et al.1991);Superior Province tholeiites(Kerrich et al.1999).In b and c,the melt curves were calculated with different starting materials(garnet lherzolite,garnet-spinel lherzolite,and spinel lherzolite)using the non-modal batch melting equations of Shaw(1970).The solid and lines dashed are enriched subcontinental lithospheric mantle(SCLM,Sm=0.6 ppm and Sm/Yb=0.96,Aldanmaz et al.2000)and the melting trends for depleted mantle(DM,Sm=0.3 ppm and Sm/Yb=0.86,McKenzie and O’Nions 1991),respectively.Partition factors used for modeling are from the compilation of McKenzie and O’Nions(1991).Partial melting level based on a given source is marked beside the curves

From the above discussion,it can be inferred that(i)metasomatism of mantle source by subduction-derived LILE-rich fluids,and(ii)source contamination by subducted sediments,is the source enrichment processes that explain the source features of the Mbi Valley ultramafic rocks.

5.3 Tectonic setting

Proterozoic mafic magmatism has played a significant role in the development of the Earth involving oceanic plates subduction,the disappearance of oceans,growth,and accretion of intra-oceanic arcs developed in domains of plate-convergence and aggregation of cratonic blocks leading to supercontinent assemblies (e.g.,Rodinia,Gondwana:Weil et al.1998;Barr et al.1999;Teixeira and Cordani,2008).Many authors(i.e.,Condie 1998;Manikyamba et al.2004)have proposed that mantle-derived ancient magmatic rocks which have been modified by subduction processes in an arc-related tectonic environment preserve the main geochemical signatures thus providing clues for magmato-tectonic reconstructions.To understand the tectonic setting for the emplacement of Mbi Valley Paleoproterozoic ultramafic cumulates,various tectonic discrimination diagrams based on immobile trace elements were used.In the Nb/Th vs.Y diagram,the studied samples exhibit arc-related tectonic settings(Fig.15),while they are placed into the arc-related ultramafic cumulate domain in the AFM diagram(Fig.6).This inferred or assumed that these rocks formed in Mid Oceanic Ridge to supra subduction zone tectonic settings(SSZ).The cartoon diagram in Fig.16 shows the Tectonic settings of this SSZ.

Fig.6 a AFM((Na2O+K2O)-FeO-MgO)and b ACM(Al2O3-CaO-MgO)ternary diagrams of the Mbi Valley ultramafic rocks.Domains of mafic cumulates,ultramafic cumulates,and metamorphic peridotites are after Coleman(1977)

Fig.15 Tectonic discrimination diagrams:a Nb/Th vs.Y plot(Jenner et al.1991).The solid line at-7.4 is the reference line for Nb/Th in the earliest mantle.‘transition’’zone in Nb/Th ratios between those samples with well-developed arc signatures(Nb anomalies)on normalized primitive mantle plots and those(non-arc volcanics)which lack such a signature

Fig.16 Cartoon showing the tectonic setting of ultramafic cumulates

The geochemical data for Mbi Valley ultramafic cumulates provide a significant view into the overall tectonic development of the central domain of the Pan-African North Equatorial Fold Belt in the Proterozoic time frame and its correlation with the development of the Gondwana supercontinent.Indeed,petrogenetic characters and tectonic affiliation for ultramafic cumulates brought out in this study point towards their generation and emplacement at ca.2099±30 in a geodynamic regime involving oceanic plates subduction in a continental margin arc environment.Remnants of Paleoproterozoic oceanic crust or subduction event and subsequent basins closure were recently discovered in the Nyong Group at the NW border of the Archean Congo craton(Loose and Schenk 2018;Bouyo Houketchang et al.2019;Nga Essomba et al.2020)and in Bafia group(Tchouankoue´et al.2021)in Cameroon and South Sa~o Francisco Craton of Brazil(Aguilara et al.2017).Thus,the amalgamation of the Atlantic supercontinent(e.g.Nigeria,Cameroon,CAR,and Brazil)was formed through current plate tectonics processes(Rogers 1996).This finding suggests that an event of subduction preceded the Paleoproterozoic collision.Therefore,the present plate tectonic actions characterized by oceanic lithospheric plate’s subduction already operated in Paleoproterozoic times,as also proposed by Kusky et al.(2013)and Loose and Schenk(2018).The Paleoproterozoic terranes at the border of the Sa~o Francisco and Congo cratons were deeply reworked by Pan-African/Braziliano orogeny(Nzenti et al.2007;Caxito et al.2020;Nzepang Tankwa et al.2021;Owona et al.2021).

6 Conclusions

The petrological and geochemical study of ultramafic samples from Mbi Valley gives the following conclusions:

1. The ultramafic rocks correspond to ultramafic cumulate composed of orthopyroxene relics(dominant phase)and,coarse primary olivine,magnesiohornblende,magnetite,and antigorite.

2. The proposed petrogenetic model involves the subduction and subsequent dehydration of the oceanic lithosphere to release LILE-enriched fluids into the above mantle edge which metasomatized the lithospheric mantle wedge and induced partial melting at a shallower depth corresponding to the constancy field of the spinel peridotite.

3. Whole-rock major and trace element compositions with the tectonic reconstruction of this region and LA-ICPMS U-Pb data on zircon constrain the emplacement age at ca 2099 Ma in a continental arc setting.This result shows that remnants of Paleoproterozoic oceanic crust or subduction event and subsequent basins closure extended from Congo craton in Cameroon to CAR and NE Brazil.

Supplementary InformationThe online version contains supplementary material available at https://doi.org/10.1007/s11631-022-00540-3.

AcknowledgementsThe authors acknowledge financial support of SCAC(Service de la Coope´ration et d’Action Culturelle)Bangui and support from Professor Paul Asimow of California Institute of Technology,USA for providing EPMA data,and from Dr.Landy Soh Tamehe of the School of Geosciences and Info-Physics,Central South University,China for helping in zircon U-Pb data acquisition.

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Declarations

Conflict of interestThe authors declare that they have no conflict of interest.