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

Alliance Nicaise Saha-Fouotsa^1*, Rigobert Tchameni², Olivier Vanderhaeghe³, Armin Zeh⁴, Periclex Martial Fosso Tchunte², Aurélien Eglinger⁵, Emmanuel Negue Nomo⁶, Pierre Barbey⁷

¹Department of Earth Sciences, Faculty of Science, University of Maroua, Maroua, Cameroon.
²Department of Earth Sciences, Faculty of Science, University of Ngaoundéré, Ngaoundéré, Cameroon.
³GET, Université de Toulouse, UPS, CNRS, IRD, CNES, Toulouse, France.
⁴KIT-Karlsruher Institut für Technologie, Campus Süd, Institut für Angewandte Geowissenschaften, Abteilung Mineralogie und Petrologie, Karlsruhe, Germany.
⁵Université de Lorraine, CNRS, GeoRessources, Nancy, France.
⁶Department of Earth Sciences, Faculty of Science, University of Yaoundé I, Yaoundé, Cameroon.
⁷Université de Lorraine, CNRS, CRPG, Nancy, France.

DOI: 10.4236/gep.2021.99001                

Abstract

The Pan-African/Brasiliano orogenic belts are a part of numerous Neoproterozoic orogenic belts that belong to the long-lived orgenic cycle that distancing phase started at the Tonian around 1.0 Ga. The Tonian magmatism fairly documented in the Neoproterozoic belts of Borborema Province (NE-Brazil), seemed so far inexistent in the Central African Orogenic Belt (CAOB) although these two belts were geologically correlated. Through the Lu-Hf geochronological analysis on zircon of tonalite, the present work, coupled with the previous data, suggests the existence of a Tonian age magmatism in the Central Cameroon Domain of the CAOB although the latter is much reworked. The Nguesseck tonalite outcrops in the northern part of the Mbé-Sassa-Mbersi region, in the northern edge of Central Cameroon Domain of the CAOB and in the Tcholliré-Banyo shear zone (TBSZ). The Lu-Hf data obtain on the zircon grains of this tonalite reveal juvenile Hf_TDM age of ca. 1.0 Ga. This age, combined to the previous geochemical data suggests that the protholites of this tonalite would have extracted from the source during the distancing phase (rifting and oceanization) of the Pan-african/Brasiliano orogeny at the early Neoproterozoic.

Keywords

Mbé-Sassa-Mbersi, Central Cameroon Domain, Central African Orogenic Belt, Tonian magmatism, Lu-Hf Isotopic Data

1. Introduction

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.

2. General Geological Context

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

Figure 1. Gondwana Supercontinent (modifyed after Gray et al., 2008). Numbers 1 to 18 represent some Pan-African belt. 1: Paraguai (P); 2: Araguaia (Ag); 3: Brasiliano (Br); 4: Araçuai (Ac); 5: Kaoko (Ka); 6: Damara (Da); 7: Lufilien arc (La); 8: Katanga/Zambèze (K/Z); 9: Mozambicaine (Mo); 10: Sør Rondane Massifs (SR); 11: Yamato Mountain (Y); 12: Lützow-Holm bay (LF); 13: Madagascard (Mad); 14: Tanzanianne (Ta); 15: Hoggar (Ho); 16: Transahariennes/Dahomeides (Tr/Dh); 17: Centrale African Orogenic Belt (CAOB); 18: Borborema (Bo).

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

Figure 2. (a) Tectonic map of Western Gondwana showing the position of the Central African Orogenic Belt pinched in between the West-African craton, the Congo craton and the Sahara metacraton (modified after Küster and Liégeois, 2001; Toteu et al., 2004; Saha-Fouotsa et al., 2019). The position of the map at the scale of Africa is shown in the inset. (b) Geology of the Central African Orogenic in Cameroon and southern Chad (modified after Toteu et al., 2001, 2004; Pinna et al., 1994; Saha-Fouotsa et al., 2019). The location of the map is shown by a grey rectangle on Figure 2(a). Abbreviation: CD = Central Domain; SZ = Shear Zone.

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.

3. Geological Background of Studied Rocks

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

Figure 3. Geological map of the Mbé-Sassa-Mbersi area.

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.

4. Methodology

4.1. Sampling

Classical field methods were used in this study. Three samples were collected according to their distance on the field and apparent freshness, but only one has been geochronologically analyzed because of their petrographic similarity, both macroscopic and microscopic.

4.2. Lu-Hf Analytical Method

Rock sample was crushed with a big and a small jaw crusher at Géosciences Environnement Toulouse (GET) laboratory (France). Then, samples were sieved and washed, and the 50 µm to 500 µm fraction was retained. Zircon grains were concentrated by 1) using heavy liquids (bromoform: density = 2.84 g/cm³, and diiodomethane: density = 3.31 g/cm³), and 2) magnetic separation with a Frantz isodynamic separator. Subsequently the zircon grains were hand-picked under a binocular microscope, mounted in epoxy blocks, and polished to expose their centre parts. Internal textures and zoning were imaged using combined secondary electron microscopy (SEM), cathodoluminescence (CL) and back-scattered electron (BSE) imaging techniques obtained with a JEOL JSM-6490 instrument coupled with a Gatan MiniCL at the Goethe-Universität Frankfurt am Main (Germany). Subsequently, U-Pb and Lu-Hf isotopes analyses of zircon grains were carried out successively by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), using an Thermo-Finnigan ELEMENT and NEPTUNE mass spectrometer coupled to an RESONETIC 193 nm Excimer laser, all at the Goethe-Universität Frankfurt am Main, Campus Riedberg (Germany). Methods and instruments were the same as described in detail by Gerdes and Zeh (2006, 2009), with modifications explained in Zeh and Gerdes (2012). Laser spots for Lu-Hf (squared spots with an edge length of 40 μm) were placed mostly directly “on-top” of the U-Pb laser spots (from 20 to 30 μm in diameter with a typical penetration depth of ~15 - 20 μm), or within the same zone characterized by CL imaging. Calculations are shown in Table 1.

Table 1. LA-MC-ICPMS Lu-Hf isotope data of zircon from Tonalite sample ofthe Mbé-Sassa-Mbersi region.

5. LA-MC-ICPMS Lu-Hf Result of the Tonalite

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.

Figure 4. Cathodoluminescence (CL) images of analysed tonalite representative zircons of Mbé – Sassa-Mbersi region. The solid circle shows the location of U-Pb isotopic analysis spot and the dashed circle indicates the location of Lu-Hf isotopic analysis spot. The indications reported to de zircon image represent from top to bottom; the analysis numbers correspond to those in Table 1, the U-Pb ages, the ¹⁷⁶Hf/¹⁷⁷Hf(t) values and the ɛHf values.

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).

6. Discussion

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).

Figure 5. U-Pb ages versus εHf(t) values diagram of analysed tonalite of Mbé-Sassa-Mbersi region.

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).

7. Conclusion

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.

Acknowledgements

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.

Conflicts of Interest

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

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