The central Cameroon Domain is the part of Central African Orogenic Belt which is correlated with the Pan-African/Brasiliano orogenic belts of the Borborema Province in NE-Brazil (Figure 1; Neves, 2003; Guimarães et al., 2012). The Proterozoic evolution in the Boeborema Province would have started from the assembly of a supercontinent named Atlantica (Rogers, 1996) until its fragmentation during the late Mesoproterozoic to early Neoproterozoic (Neves, 2003; Van Schmus et al., 2008; Guimarães et al., 2012). There were initially several failed attempts of this fragmentation towards the end of Paleoproterozoic (~1.8 - 1.7 Ga) and the beginning of Mesoproterozoic (~1.52 Ga) before intensification in the beginning of Neoproterozoic ca. 1 Ga (Souza & Moura 1995; Van Schmus et al., 1999; Kozuch, 2003; Santos et al., 2010; Guimarães et al., 2012). The Cariris Velhos belt is a part of the Borborema Province which exhibits a petrographic complex dated at the Tonian (1.0 - 0.85 Ga) (Santos, 1995; Brito Neves et al., 2001; Tack et al., 2001; Kozuch, 2003; Santos et al., 2010; Guimarães et al., 2012). In spite of the similarity between the Borborema Province (NE-Brazil) and Central African Orogenic Belt (Van Schmus et al., 2008), the Tonian age rocks presented in the Cariris Velhos suite have not yet been described in the Cameroon part of Central African Orogenic Belt, though the biotite granite of the Ouaddaï (Chad) yield concordant inherited zircon cores crystallised around 1.05 Ga (Djerossem et al., 2020). These rocks were already described in the West Congo Belt (Araçuai Orogen) where thick rift-related alkaline volcanic-sedimentary basin and anorogenic granites of 1000 - 910 Ma are exposed (Tack et al., 2001; Van Schmus et al., 2008; Guimarães et al., 2012). However, ca. 1.0 Ga dolerite dykes with continental tholeiite composition outcropped in the Paleoproterozoic formation and 1167 ± 61 Ma granite (Yobé) in far south-east of Cameroon and South-west of Central African Republic. These dolerites are assigned to the pre-Panafrican extension phase (Vicat & Pouclet 1995; Vicat et al., 1997, 2001). This paper presents Lu-Hf isotopic data on zircon from the Nguessek Tonalite near the Tcholliré-Banyo shear zone in the Mbé-Sassa-Mbersi region. These data provide additional information concerning the evolution of the early Neoproterozoic in the central Cameroonian domain of the Central Africa orogenic belt.

The similarities in geology, gold mineralization and old fluviodeltaic deposits in the São-Francisco/Congo, Amazonian, West African and Rio de la Plata cratons

allowed some authors to suggest that, these cratons formed a same continent named Atlantica, at the end of the Transamazonian/Eburnean orogeny (Ledru et al., 1994; Bossière et al., 1996; Rogers, 1996; Teixeira et al., 1999; Norcross et al., 2000; Neves, 2003). Paleomagnetic poles for mafic rocks in Amazonia allowed to suggest that Atlantica was the part of Rodinia supercontinent at the late Mesoproterozoic (Tohver et al., 2002). Atlantica would be the basement of the Pan-African Borborema, Araguia, Nigeria and Cameroon orogenic domains (Neves, 2003; Guimarães et al., 2012). Excluding continental rifting stages between 1.8 and 1.5 Ga mark by ubiquitous taphrogenesis, this continent remained unaffected for about 1 Ga (Neves, 2003). But, around 1 Ga at the early Neoproterozoic (Tonian), this portion of the crust is witness of an important magmatic event in Borborema province and Araguaia Group correlates with important rifting episodes in central-western Africa and anorogenic magmatism in the São Francisco, Congo, and Amazonian cratons (Vicat & Pouclet 1995; Souza & Moura 1995; Vicat et al., 1997, 2001; Neves, 2003; Guimarães et al., 2012; Viana & Battilani 2014).

The various studies carried out show that Tonian magmatism takes place between 1000 and 850 Ma in Brazil and Africa (Santos, 1995; Vicat & Pouclet 1995; Vicat et al., 1997, 2001; Van Schmus et al., 1999, 2008; Da Silva Filho et al., 2000; Brito Neves et al., 2001; Tack et al., 2001; Kozuch, 2003; Neves, 2003; Santos et al., 2010; Guimarães et al., 2012). This magmatism which is materialized in the outcrops by volcano-sedimentary formations associated with meta-mafic and felsic volcanic and plutonic rocks, orthogneisses and dolerite dykes show continental tholeiite and alkaline signature (Souza & Moura 1995; Vicat et al., 2001; Neves, 2003; Guimarães et al., 2012; Viana & Battilani 2014). In the Central Cameroon Domain in Central African Orogenic Belt (Figure 2(a) and Figure 2(b)), the tonian ages are mentioned in the biotite-muscovite granitoids (~926 Ma) and amphibole-biotite gneiss (~889 Ma) respectively in the Ngaoundere cliff and Meiganga area (Tchameni et al., 2006; Ganwa et al., 2011). But these ages have been interpreted as inherited and of no geological significance, probably due to the lack of more detailed studies.

The Central Cameroon Domain is characterised by the emplacement of magmatic rocks during the Pan-African orogeny that display a variety of petrological and geochemical signatures (Figure 2(b)). Plutonic rocks consist of diorite, tonalite, trondhjemite, granodiorite and granite (Toteu et al., 2001, 2004; Njanko et al., 2006; Tchameni et al., 2006; Ganwa et al., 2008; Naïmou et al., 2014; Saha-Fouotsa 2018; Saha-Fouotsa et al., 2018, 2019). They have been dated from ca. 650 to 570 Ma and their calc-alkaline signatures, to high-K calc-alkaline for the youngest ones, have been attributed to mixing between a juvenile source and magmas issued from partial melting of Archean to Paleoproterozoic crust (Tchameni et al., 2006; Kwékam et al., 2010; Mbassa et al., 2016; Tchakounté et al., 2017; Saha-Fouotsa et al., 2019). With regard to the juvenile source, two

propositions have been made: 1) an enriched mantle or partial melting triggered by post-collisional delamination of the subcontinental lithospheric mantle (Tchameni et al., 2006; Kwékam et al., 2010); and 2) partial melting of Neoproterozoic oceanic crust tectonically accreted with the continental terranes (Mbassa et al., 2016; Saha-Fouotsa 2018; Saha-Fouotsa et al., 2019).

In this paper, we bring new isotopic data which allows to reconsider the interpretation about tonian ages in Cameroon and to discuss the source of the juvenile magmatism at Neoproterozoic in order to improve the reconstruction of the tectono-magmatic history of the Mbé-Sassa-Mbersi region and therefore that of the Central Cameroon Domain.

The Mbé-Sassa-Mbersi region is situated at the northern edge of the Central Cameroon domain, along the Tcholliré-Banyo shear zone (TBSZ) between the Tcholliré and Banyo towns, about 60 km north of the Ngaoundéré city (Figure 3). Saha-Fouotsa (2018) and Saha-Fouotsa et al. (2018, 2019) identify in this region, a diversity of lithological formations including: 1) mafic rocks in which

some present concordant U-Pb ages of Pan-African.; 2) paragneisses displaying ages with Archean, Paleoproterozoic and Neoproterozoic sources and calc-silicate gneiss; 3) heterogeneous, migmatitic gneisses with an intermediate to felsic composition, and 4) monzodioritic to granitic plutons and dikes more or less affected by solid-state deformation, showing I-type chemical signature with three series which are tonalites-trondhjemites with a magnesian medium-K signature, hornblende-biotite granites with a magnesian high-K signature and biotite-granite with a ferroan shoshonitic signature. All these formations are affected by ductile polyphase deformation marked by tectonic nappe verging toward SSE (D1), simple shear dominated transpression associated to the Tcholliré-Banyo shear zone evolution (D2) and strain partitioning induced by transpression and links to the Central Cameroon shear zone evolution (D3) (Saha-Fouotsa et al., 2018). The minimum age obtained on metamorphic rims of paragneiss constrain the development of ductile fabric occurring from around 582 ± 4 Ma under granulite to amphibolite facies metamorphic conditions as constrained by a peak at ca. 1.3 GPa and ca. 770˚C (Saha-Fouotsa et al., 2019).

The tonalite analysed here, outcrops in the Nguessek locality in the North of the Mbé-Sassa-Mbersi region. The rock is deformed showing alternating quartz-feldspar and amphibole rich layers and preferred orientation of grains (Saha-Fouotsa et al., 2018). The deformation of this tonalite is described by Saha-Fouotsa et al. (2018) as belonging to the first deformation phase in the Mbé-Sassa-Mbersi region, which is linked to tectonic nappe verging toward SSE. It showing magnesian medium-K and I-type chemical signature and it emplacement period is constrained by zircon U-Pb data at 651 ± 3 Ma (Saha-Fouotsa et al., 2019). The Rare Earth Elements (REE) patterns fractionation with low heavy REE concentrations, has been interpret by Saha-Fouotsa et al., (2019) as a possible origin by partial melting of mafic material involving fractionation of amphibole and garnet. However, the absence of additional isotopic data did not allow further interpretation of the source of this tonalite. For this paper, we use Lu-Hf isotopic data obtained on the same analysis points of sample of tonalite used for U-Pb isotopic data on zircon to improve the interpretation of the source of this rock.

The studied tonalite sample is representative of a kilometre-scale pluton elongated in the HT foliation at the regional scale. Zircon grains from this sample are mainly subhedral, prismatic with slightly rounded terminations. The grain size varies from 200 to 500 µm in length, and CL images show oscillatory zoning suggesting a magmatic origin (Figure 4). Taking into account the large size of the grains and the texture of the zircon analysed, several points of analysis were made on mineral cores and edges to have a better analysis and interpretation of the ages obtained.

Eighteen analyses were thus made on eleven grains of zircon yielding ¹⁷⁶Lu/¹⁷⁷Hf ratios vary from 0.00027 to 0.00105 and the homogeneous ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.282592 to 0.282640. The εHf(t) values and T_DM ages varying from +7.6 to +8.9 and from 1.0 to 1.09 Ga, respectively (Table 1; Figure 4).

The Tonian extensional events were recorded in the São Francisco Congo Craton and surroundings (Silva et al., 1995; Correa-Gomes & Oliveira, 2000; Tupinambá et al., 2007). These events all mark by the mafic dyke were subdivided into two major phase of continental rifting. The early episode occurred between 1100 - 1000 Ma (Tupinambá et al., 2007) and while the second episode was expressed between 930 - 900 Ma (Silva et al., 1995; Correa-Gomes & Oliveira, 2000; Tupinambá et al., 2007).

The Lu-Hf isotopic data of ca. 1.0 Ga from the Nguesseck tonalite in the Mbé-Sassa-Mbersi region belong to this early episode of continental rifting in the São Francisco Congo Craton. Age obtained in this work is comparable to concordant inherited age of 1.05 Ga, got by Djerossem et al. (2020) to the magmatic zircon core of biotite granite in the Ouaddaï (Chad) which is part of Central African Orogenic Belt and continuity of the Central Cameroon Domain (Djerossem et al., 2020, 2021).

Although some authors have interpreted the Tonian episode as failed attempts of dislocation of the Atlantica Paleoproterozoic continent (Renné et al., 1990; Schobbenhaus, 1996; Uhlein et al., 1998; Alvarenga et al., 2000; Tack et al., 2001), the T_DM ages obtained by Da Silva Filho et al. (2000) in the northern edge of the Sergipano belt were interpreted as an accretion of some juvenile material to the continental lithosphere and suggesting either formation of oceanic crust or underplating of mafic magmas during active rifting. The local occurrence of C-type eclogites near the Pernambuco shear zone (PSZ) in the central domain of the Borborema province (Beurlen et al., 1992) and obduction of ultramafic rocks with pillow basalts in the Araguaia belt (Alvarenga et al., 2000) are in favour with existence of an oceanic crust form around the early and middle Neproterozoic.

Geochemical data (magnesian medium-K series, fractionated REE patterns with low heavy REE concentrations, volcanic arc granite …) of the Nguesseck tonalite (651 ± 3 Ma) obtained by Saha-Fouotsa et al. (2019) in the Mbé – Sassa-Mbersi region suggest a possible origin by partial melting of mafic material involving fractionation of amphibole and garnet. The Lu-Hf data of this work suggest that, this mafic material would have formed around 1Ga taking into account the HfT_DM ages obtained on all of the zircon grains analysed in the studied sample (Table 1; Figure 4 and Figure 5).

Thus, taking into account the positive values of εHf(t) (+7.6 to +8.9) from tonalite analysed zircon grains, we can suggest that juvenile tonian magmatism episode would have happened at the northern edge of the Central Domain of the Central African Orogenic belt. Which can be corroborate by concordant U/Pb inherited age (1.05 Ga) obtained by Djerossem et al. (2020) in the Ouaddaï (Chad). Geochemical data of the Nguesseck tonalite got by Saha-Fouotsa et al. (2019) with Hf positive value (Figure 5), could suggest that Hf_TDM ages ca. 1.0 Ga obtained in this work, would therefore correspond to the age of extraction of the mafic magma at the origin of the protholite of this tonalite. These Hf_TDM ages could characterize a Neoproterozoic rifting stage with oceanic crust formation. The latter would therefore have subducted during the convergent phase of the Pan-African/Brasiliano orogeny, partially melted and gave Nguesseck tonalite magma which crystallized at around 651 Ma (Saha-Fouotsa et al., 2019).

The time between 1.0 Ga and 651 Ma fits well into the estimated period for the formation of the old long-lived orogenic belts (1000 - 600 Ma) to which the Central African Orogenic belt belongs (Kröner, 1975; Bayer & Lesquer, 1978; Drake, 1980; Chikhaoui et al., 1980; Lesquer et al., 1981; Regan & Marsh, 1982; Nzenti et al., 1984; Poidevin, 1985; Caby, 1987; Barbey et al., 1990; Fernandez et al., 1992; Penaye et al., 1993). Similar age (1011 ± 86 Ma, 1051 ± 22 Ma, 1048 ± 11 Ma and 1056 ± 21) is obtained on zircons from alkaline syenitic gneisses by Pb/Pb and U/Pb method on zircon and interpreted as evidence of the beginning of the rifting process in the Araguaia Group in Brazil (Souza & Moura 1995; Viana & Battilani 2014).

Combined Lu-Hf isotopic result obtained in this work (εHf(t) (+7.6 to +8.9) and Hf_TDM ages ca. 1.0 Ga) and previous petrographical, geochemical and U-Pb geochronological data suggest that the Nguessek tonalite would be the result of a partial melting of a 1.0 Ga juvenile rock formed in a context of distancing at the beginning of the Neoproterozoic. This new data obtained in the Mbé-Sassa-Mbersi region, part of the Central Cameroon Domain which belongs to the CAOB, allows to have other points of similarity between this orogenic bel and the Borborema Province in the NE of Brazil.

This work is integrated in the URCO research project financially supported by the CNRS-NEEDS program and the AUF program (Project N˚ 51110SU201). We acknowledge fruitful discussions with Djerossem Nenadji Félix and Tsozué Désiré. Jean-François Ména is thanked for the help provided during the preparation of zircon grain for analysis at Geoscience Environment Toulouse laboratory (GET). The authors also thank the reviewers who made it possible to improve the quality of this paper.

The authors declare no conflicts of interest regarding the publication of this paper.

Saha-Fouotsa, A. N., Tchameni, R., Vanderhaeghe, O., Zeh, A., Tchunte, P. M. F., Eglinger, A., Nomo, E. N., & Barbey, P. (2021). Lu-Hf Isotopic Data of the Mbé-Sassa-Mbersi Tonalite (Central Cameroon Domain): Indicator of ca. 1.0 Ga Juvenile Tonian Magmatism in the Region. Journal of Geoscience and Environment Protection, 9, 1-19. https://doi.org/10.4236/gep.2021.99001