ABSTRACT

The border of Cameroon and Chad is characterized by a Precambrian basement straddling the Touboro-Baïbokoum area; this basement is made up of gneiss, amphibolite and granitoids (granite, syenite, granodiorite). The studied rocks display high-K calk-alcalcaline to shoshonitic characteristic. Granitoids are metaluminous. Rocks under study derived from partial melts from metabasaltic to metatonalitic sources, and partial melts from metagreywackes. They shear the same origin as many granitoids describe westward in the Meiganga area and west Cameroon. This shows that the basement straddling the Touboro-Baïbokoum area belongs to the Adamawa-Yadé Domain of the Central African Fold Belt.

Keywords

Precambrian Basement, Touboro-Baïbokoum, Cameroon, Chad, Adamawa-Yade Domain, Central African Fold Beld

1. Introduction

The main geological feature of central Africa is the predominance of Precambrian basement units. Much of this basement is constituted by the formations of the Panafrican Central Africa Fold Belt (CAFB) [1] , also known as the North Equatorial Panafrican Fold Belt [2] . The Congo craton to the south is considered as the foreland of the belt [3] . Previous works [4] -[7] involved in regional correlation divided the Central Africa Fold Belt into three main domains: 1) the Yaoundé Domain (YD), a huge nappe unit thrusted southward onto the Congo craton; 2) the Adamawa-Yadé Domain (AYD), a Paleoproterozoic basement which was dismembered during the Panafrican orogeny and which extends east of the Tcholliré-Banyo shear zone; 3) the Western Cameroon Domain (WCD) located west of the Tcholliré-Banyo shear zone (TBSZ) and which extends along the western border of Cameroon (Figure 1). Only limited cross-border geological research has been undertaken between Cameroon and Chad; one can cited: petrographic and structural studies on the rhyolitic domes of Dandi-Hadjer El Hamis, to the south of Lake Chad, considered as the extension of the Cameroon volcanic Line [8] ; zircon dating of the Mayo Kebbi area regarded as the equivalent of the Poli series in Cameroon [9] . Geological correlation between the two countries is often based on the geographical continuation than on any geological consideration. This paper presents the results of a petrography and geochemistry study of the Precambrian basement straddling the Chad and Cameroon border in the Toboro-Baibokoum area. It appears that this area belongs to the AYD of the Central African Fold Bet (Figure 1).

2. Geological Setting

The Touboro-Baïbokoum area (Figure 2) belongs to the Adamawa Yadé Domain of the Central Africa Fold Belt, which stretches ENE through Cameroon, southern Chad and also possibly the Northern Central Africa Republic. This central part of the CAFB, better studied in Cameroon, is characterized by the wide spread occurrences of granitoids hosted in orthoand para-gneiss, and associated with regional transcurrent shear zone [10] . Granitoids of the AYD have been classified into syntectonic, late tectonic and post tectonic, with respect to their solid state of deformation [6] [11] -[13] . Granitoids are made up of granodiorites and granites of various compositions. In the Baïbokoum area, the metamorphic rocks form enclaves in the granitoids. The rocks are made up of schist, quartzite, and gneiss with intercalation of amphibolite [14] . Granitoids can be divided into postectonic granitoids, which cross cut the old granitoids and considered as the latest magmatic manifestation of the Panafrican orogeny [1] . Old granitoids are made up of granites, granodiorite and monzonite, which show syntectonic

stretching parallel to the schistosity of the metamorphic rocks. These old granitic rocks were considered as old Precambrian crust reactivated during the Pan-African orogeny [15] until Toteu et al. [16] demonstrate in the Poli area (Cameroon): their syn-tectonic emplacement at 630 Ma, their relatively low ⁸⁷Sr/⁸⁶Sr initial ratios and conclude that they represent material newly added to the crust during the Pan-African orogeny.

3. Analytical Methods

Petrography of 30 thin sections covering all rock types has been made. Chemical analyses were carried out on 14 samples. Major and trace elements were analyzed by X-ray fluorescence at the University of Tübingen for 9 samples and at the MIPROMALO (Mission of Promote Local Materials), Yaoundé for 5 samples. Analytical uncertainties are estimated at ±1% for major elements and 5% - 10% for most trace elements.

4. Petrography of the Studied Samples

Samples under study are made up of plutonic and metamorphic rocks. Pyroxene-amphibole-biotite bearing granite (PABG) forms a NE-SW band (Figure 2) in the middle of the investigated area, and crops out as flagstone (Figure 3(a)) or boulders. The tardi-tectonic nature of the PABG is highlighted by a weak schistosity marked by the alignment of biotite and amphibole. The rock is porphyritic in texture, with megacrysts of K-feldspar (2 - 3 cm) in a groundmass of quartz, plagioclase, K-feldspar, amphibole, biotite and clinopyroxene. Accessories minerals such as zircon, titanite and opaque minerals are present. Clinopyroxene appears as remnant or rimed by the amphibole (Figure 3(b)), which is a green hornblende associated to the biotite; biotite also appears along the cleavage plane of the hornblende (Figure 3(c)).

The amphibole-biotite granite (ABG) occupies the NW of the studied area (Figure 2) and forms hills cover by boulders of various sizes (Figure 4(a)). Enclaves of amphibolite of various sizes are present in the ABG at the level of the Vina River at Touboro (Figure 4(b)). It has a granular magmatic texture. Amphibole is regularly associated to biotite and both highlight a faint schistosity in the rock. The K-feldspar exhibits distinct carlsbad twins and the contact with plagioclase crystals is occasionally characterized by a myrmekitic texture. Plagioclase sometime portrays optical zoning. Quartz occurs as subhedral crystals with undulose extinction. Accessories minerals are titanite and opaque minerals.

Granodioritic rocks (G) crop out to the East of the studied area; it form NE-SW stretching hills cover by blocs of various sizes. At the out crops scale, granodiorite is characterized by phenocrystal of K-feldspar with inclusion of amphibole (Figure 5(a)). In thin section, the studied sample portrays mark of deformation with protomylonitic texture (Figure 5(b)). Amphibole is a green hornblende forming pœcilitic crystal with inclusions of quartz, plagioclase and zircon. Biotite is a retrograde transformation of hornblende. In the mylonitic portion of the rocks, biotite is recrystallized and associated to the ribbon of quartz. Quartz is anhedral with undulose extinction and usually forms poly crystals ribbon. K-feldspar forms phenocryst bordered by small sizes crystals of quartz and plagioclase. Plagioclase as far as K-feldspar shows undulose extinction. Accessories minerals are zircon, apatite and opaque minerals.

Syenite forms a NE-SW elongated body at the center of the studied area constituted by boulders of various sizes (Figure 6(a)). The rock is granular in texture (Figure 6(b)) and made up of Feldspar; amphibole; pyroxene, biotite, quartz, and accessories minerals (zircon, apatite). Pyroxene is represented by both ortho and clino pyroxene, which confers to the syenite a charnockitic character. They are xenomorphic; orthopyroxene is clearly distinct from then clinopyroxene by a well define (010) sharp cleavage. Amphibole is a green hornblende growing along the cleavage of pyroxene or at their rims (Figure 7). Amphibole crystals always show concentration of opaque minerals along their cleavage. Biotite with brow pleochroism is associated to amphibole. It contains inclusion of zircon and sometime apatite. K feldspar is characterized by crystals showing flame perthitic albite intergrowth. Plagioclase is partially alternated into sericite. Quartz is interstitial between others minerals and portrays undulose extinction. Accessory minerals are made up of zircon, apatite, titanite and opaque minerals.

Pyroxene and garnet bearing gneiss (PGGn) is located at the border of Cameroon and Chad, and extends from the southern border of the study area till the confluence between Mbere and Vina. In thin section, it shows grano to nematoblastic texture with pyroxene, amphibole, garnet, biotite, plagioclase, quartz, apatite; zircon, opaque minerals. Clinoptroxene is transformed onto amphibole along the cleavage and also appears as relic in amphibole crystals. Amphibole is a green hornblende associated to the biotite and opaque minerals. Biotite is characterized by it brownish color and many inclusions of zircon and/or monazite. Garnet is xenomorphic, associated to feldspar and opaque minerals, and shows crystallization of biotite along its cracks (Figure 8(a)). Plagioclase form crystals often affected by deformation. Some crystals exhibit bent twin lamellas and the others deformational kink-bands. Quartz is characterized by an undulose extinction and also form polycrystalline aggregate surrounding others minerals.

Amphibole, biotite gneiss (ABGn) occupies the SE corner of the study area, cropping out as flagstone in the savanna (Figure 8(b)). The rock is grano-lepidoblastic in texture with biotite; amphibole, feldspar, quartz, zircon, opaque minerals. Amphibole is a green hornblende associated to brown biotite which shows concentration

of opaque minerals along the cleavage. Biotite is sometime transformed into chlorite and regularly contains inclusion of zircon. Feldspar is represented by plagioclase and microcline with fine plain twinning. Plagioclase is sometime being transformed into sericite and is often pigmented by submicroscopic opaque minerals. Feldspar is associated to quartz and all show undulose extinction.

Banded amphibolite appears as layer of 30 to 35 cm thick in the gneiss. The banded structure is due to the presence of leucocratic thin layers, product of migmatization parallel to the foliation of the rock.

5. Geochemistry

Chemical analyses of the study rocks are represented in the Table 1. The SiO₂ vs. (Na₂O + K₂O) diagram [17] is

used for chemical classification of the samples (Figure 9). Granites are ploted in the field of Alkali granite, with one gneiss sample in the field of diorite and two amphibole-biotite granite samples in the fields of granodiorite and syenite respectively. Studied rocks are acid to slightly intermediate in composition (SiO₂ > 60%). Studied rocks span a narrow compositional range in silica content from 60.5 wt% to 73.6 wt%. They have high values of alkali oxides, with K₂O = 1.7 wt% - 8.0 wt% and Na₂O = 2.5 wt% - 5.4 wt%, and of Fe₂O₃* (total Fe) = 1.0 wt% - 7.0 wt%, but low abundance in MnO = 0.0 wt% - 0.1 wt%, MgO = 0.2 wt% - 3.6 wt%, CaO = 0.5 wt% - 4.9 wt%, P₂O₅ = 0.0 wt% - 0.8 wt%. The TiO₂ content is moderate (0.1 wt% - 1.1 wt%). Al₂O₃ ranges from 11.3 wt% to 16.3 wt%.

Plots of major element abundances against SiO₂ (Figure 10) do not show a well-defined linear covariation with respect to increasing silica contents. A slight decrease in TiO₂, MgO, Fe₂O₃, MnO, and a scattered distribution of Al₂O₃, Na₂O, CaO and P₂O₅ are shown in Figure 10. This could mean that the studied rocks derive from magma of different sources and perhaps of different ages.

In the K₂O vs SiO₂ diagram [18] (Figure 11(a)), the Touboro-Baibokoum rocks portray high-K calc-alkaline (gneiss) to shoshonitic character (amp-Biot granite, Biot Granite, Syenite), with however one banded amphibolite sample in the high calc-alkaline suite and the other in the medium-K calc-alkaline suite. The general highpotassic character of the studied rocks is concordant with their Al₂O₃ content higher than 9% [19] [20] and with their noticeable abundance of K-feldspar (and hence of K₂O).

In the TAS diagram [21] offering a classification in alkaline, mid-alkaline and sub-alkaline (calc-alkaline + tholeiitic), studied rocks are in the field of mid-alkaline series (Figure 11(b)). ASI ratios (A/CNK = [Al₂O₃/ (CaO + Na₂O + K₂O) mole%]) (Table 1) vary from 0.55 to 1.05 revealing their metaluminous to slightly peraluminous character as far as their I-type (Figure 12(a), [23] [24] ). This character is confirmed in the diagram of Maniar and Piccoli [25] , which employs the Shand’s molar parameter, A/NK versus A/CNK (Figure 12(b)). This diagram also clearly separates the metaluminous granitoids, which have A/NK ratios defined as molar Al₂O₃/(NaO + K₂O) greater than 1, and the peralkaline granitoids (A/NK < 1). One amphibole biotite granite, one syenite and the biotite granite plot into the peralkaline field. The position of these samples could be explain by their mineralogy; they are rich in alkali mineral (e.g. K feldspar) which shift down the Al₂O₃/(NaO + K₂O) ratio.

The studied rocks show a wide range of large ion lithophile element (LILE) abundances, Ba (289 - 3540 ppm), Rb (60 - 282 ppm), Sr (259 - 1783 ppm). The high-field-strength elements (HFSE) also vary considerably within the analyzed samples: Y (1 - 66 ppm), Zr (72 - 743 ppm), Ce (46 - 225 ppm), Nb was present in three samples and show a concentration of 31.3, 39.5 and 44.4 ppm respectively. Selected variation diagrams for trace elements (Zn, Cr, Rb, Sr, Zr and Ba) as shown in Figure 13 lack any continuous trend. Nevertheless, a poor-defined with gentle decrease for Rb, Zr and Ba with the increasing concentration of SiO₂ can be observed.

6. Discussion and Conclusion

The precambrian basement of Touboro-Baïbokoum is made up of rocks wich petrography and chemical characters portray characteristic of high-K to schoshonitic suites [18] with feature of I-type granitoids [23] [26] . The high-K content and calc-alcaline nature of these rocks suggest that the most suitable protholiths could be ancient volcanic rocks [27] . Partial melting of various source rocks, such as amphibolites, tonalitic gneiss, metapelite, and metagreywackes in terms of molar Al₂O₃/(MgO + FeOt)-CaO/(MgO + FeOt), has been discussed by Alther et al. [28] ; in the corresponding diagram for the rocks of the Touboro-Baïbokoum area (Figure 14), most of the studied samples derive from the partial melts from metabasaltic to metatonalitic sources. Metabasaltic to metatonalitic sources have been demonstrated for the I-type granite from the Lachan Fold Belt, SE Australia [26] . One sample of pyroxene-amphibole-biotite granite is in the field of the rocks derivate from partial melts from metagrewackes. This distribution could portray the contribution of the crustal material to its source. Such granite of crustal origin and the contribution of crustal material to the source of granitoids have been demonstrated in the Cameroonian portion of the Adamawa-Yade Domain [12] [33] . Many I type granite e.g. Ngondo, Batié and Fomopéa [34] -[36] of west Cameroon show the same characteristics. This suggests the presence of a sedimentary component either in their source, or incorporated by the granitoid magma. It is likely that biotite granite and gneiss shear the same protholite as it has been shown in the Meiganga area [37] that Neoproterozoic leucogranite was formed by melting of the crustal material similar to the inferred protolith of amphibole biotite gneiss. The presence of biotite and amphibole in the pyroxene-amphibole-biotite granite is consistent with the water saturation of the magma. One wonders how Magma of such composition could lead to crystallization of pyroxene. The presence of such an anhydrous mineral in a water saturated environment can be understood only if it is considered as a foreign body. The fact that clinopyroxene is rimed by amphibole portrays the instability of this mineral in the granitic magma. Clinopyroxene is thus a mineral of the basement, probably the pyroxene-garnet gneiss, incorporated into the magma during its emplacement. Xenocrystals of clinopyroxene rimed by amphibole have been reported in the granite to the south of Meiganga in Cameroon [33] .

The high-K concentration of the studied rocks is reflected in their high content of biotite and K-feldspar minerals. Intermediate to felsic composition of granitoid is consistent with a derivation from the ancient source of tonalitic to granodioritic composition [38] . Syenite of Baïbokoum shows charnockitic character with presence of both

ortho and clinopyroxene. Due to this, it has been considered as part of the Archeans formation by earlier authors (see [1] ). In Meiganga area, pyroxene-amphibole-bearing gneiss shows the geochemical characteristics of archean TTG associations [39] and the REE content allowed considering some samples as derived from Archean

crust. Nevertheless, geochronological and isotopical study are necessary to more unravel the history of the Touboro-Baïbokoun area.

The rocks of the Touboro-Baïbokoum area show petrographic and chemical features as the rocks of westward areas of Meiganga and Ngaoundere. This similitude, in addition to the outcrop feature, strengths the consideration that these areas are parts of the same domain of the Central African Fold belt (CAFB), the Adamawa Yadé Domain (AYD).

Acknowledgements

Thanks go to anonymous reviewers for their critical reviews of the manuscript. The second author (GAA) is

highly indebted to the WFW (Fonds zur Förderung der wissenschaftlichen Forschung) for support of his postdoctoral research at University of Vienna (Austria).

References

NOTES

^*Corresponding author.