Introduction

The Bossangoa-Bossembélé area is located on the central part of the CAR in the North Equatorial Fold Belt (NEFB) (Danguene et al. 2014). Although the western extension towards the Yade-Adamawa area is still poorly known, the study of Yade-Adamawa region is of particular interest in deciphering the true nature of the fold belt as it corresponds to the transition between the northern margin of the Congo Craton (CC) and the Neoproterozoic domain. The zone towards the Yade-Adamawa region is considered to be a major structure which controlled its tectono-metamorphic/magmatic evolution since the Proterozoic times (Danguene et al. 2014; Mapoka et al. 2010; Nzenti et al. 1988). Clarifying the northward extension of the high-pressure Yaoundé and Nyong–Ogooué series, distinction of the different portions of crust with different age, and localization of the ca.2000 Ma relict zones on the fold belt is, therefore, needed to understand the orogenic assembly of the North Equatorial Fold Belt in CAR and allow a better understanding of the geodynamic significance of the ca. 2000 Ma relicts to the north of the Archean Congo-Sao Francisco Craton.

The Bossangoa-Bossembélé area characterized by the occurrence of well-exposed granulite facies rocks is a suitable site for studying the metamorphic evolution and their geodynamic significance in the NEFB in CAR. The relative timing of these metamorphic events is still unclear in many parts of the CAR. The field mapping carried out during this present study has revealed excellent exposures of the gneiss and has inspired this contribution. These good exposures of the granulitic unit, spanning a continuous range of chemical compositions (with about 58–82 wt.-% SiO2), make the Bossangoa-Bossembélé area a key site to study the formation of granulitic rocks in this part of the NEFB. This paper provides new data on the geology and the first SHRIMP U-Pb zircon age of the main representative gneiss and discusses the geological significance and petrogenesis of Paleoproterozoic high-grade rocks from the northern part of the CAR in the NEFB. A comparison of the studied granulite with similar rock types in this region is also provided.

Geological setting

The CAR has been divided into three major structural units (Cornacchia et al. 1989; Rolin 1992; Nzenti 1998). The southern unit represents a northern part of Congo Craton and consists of (i) micaschists and quartzites of Archean and Paleoproterozoic age (Poidevin 1991); (ii) metabasites (amphibolites, pyroxenites of Mbomou) of Archean age (2900 Ma; Lavreau et al. 1990); (iii) charnockites series and gneiss similar to those of the Congo Craton in Cameroon (Pin and Poidevin 1987); and (iv) Archean greenstones (komatiites), itabirites, greywacke, rhyodacitic tuffs, and granitoids (Cornacchia and Giorgi 1989). The study area (Figs. 1 and 2) belongs to the intermediate or central domain and consists of Archean gneisses, metabasites, granites, meta-sedimentary rocks, and migmatites (e.g., Danguene et al. 2014; Mapoka et al. 2010, and references therein). The northern part is composed of granulites, orthogneisses, and granites of Neoproterozoic age (833 ± 66 Ma). It corresponds to the western extension of the Pan-African fold belt in Cameroon.

Figure 1
figure 1

Palinspatic reconstruction of Africa and NE Brazil (Late-Precambrian) modified from Mapoka et al. (2010). ASZ: Adamawa shear zone (or Cameroon Central Shear Zone: CCSZ); SF: Sanaga fault; SL: São Luis Craton; Pa: Patos shear zone; Pe: Pernambuco shear zone; TBF: Tibati-Banyo- Foumban fault. BOF: Bétaré Oya Fault. The study area is marked by a rectangle

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Fig. 2
figure 2

Geological map of the Bossangoa-Bossembélé area

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Analytical methods

Whole-rock geochemistry

After the petrographic investigation of the rock units, 16 representative samples were selected for whole-rock chemical analysis. Samples were crushed and subsequently reduced to a very fine powder by grinding in a tungsten carbide ring mill. Major and trace element concentrations of whole-rock were analyzed by X-ray fluorescence spectrometry and LA-ICP-MS, at the University of Lausanne. Major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Cr, and Ni) were measured on fused lithium borate glass disks using a Philips PW2400 X-ray fluorescence spectrometer. Trace elements were measured from pressed powder pellets on the same XRF spectrometer (Nb, Zr, Y, Sr, U, Rb, Th, Pb, Ga, Ni, Cr, V, Ce, Ba, and La), and by LA-ICP-MS on glass disks (Be, Sc, Ti, V, Cr, Ni, Cu, Zn, Y, Zr, Nb, Cs, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Th, and U). Tests were made to assess the amount of trace element contamination, such as Ta and Nb, from the tungsten carbide mill. The samples analyzed in this study are relatively rich in these elements, therefore contamination is considered negligible. Laser ablation measurements were made with a 193-nm Lambda Physik Excimer laser (Geolas 200M system) coupled to a Perkin-Elmer 6100 DRC ICP-MS. Laser settings were 27 kV with a 10 Hz repetition rate, yielding a fluorescence of about 12 J/cm2 on the ablation site. Helium was used as carrier gas (1.1 l/mn) and NIST612 glass was used as the external standard, and Ca and Al as internal standards (on the basis of electron microprobe measurements on the ablation pit site). BCR2 basaltic glass was regularly used as a monitor to check for reproducibility and accuracy of the system. Results were always within ± 10% range of the values reported by Witt-Eickschen et al. (2003), while Rb, Cs, Y, and especially Cr were sometimes within the ± 10% range of the USGS’ recommended values for BCR2. Analytical uncertainties are currently better than 1% for major elements and 5–10% for trace element concentrations higher and lower than 20%, respectively. Analytical precision for rare earth elements (REE) is estimated at 5% when concentrations are >10 times chondritic and at 10% when lower.

Zircon U-Pb geochronology

One sample of sillimanite garnet gneiss (sample 7A) was selected for dating. Two kilograms of the sample was subjected by routine heavy mineral separation. Zircons were hand-picked, mounted in epoxy, and polished. Individual zircon grains were subjected to U-Pb isotopic analysis using the sensitive high-resolution ion microprobe (SHRIMP) at the Research School of Earth Sciences at the Australian National University, Canberra. Detailed SHRIMP analytical procedures have been reported by Compston et al. (1984) and Williams and Claesson (1987). The technique focuses a primary beam of negative oxygen ions in vacuo onto the zircon surface from which a small area (25–30μm diameter) of sputtered positive secondary ions is extracted. Secondary ions, which include Zr, Th, U, and Pb from the zircon, are passed through a curvilinear flight path in a strong magnetic field and then counted at a mass resolution of 6500 on a single collector using cyclic magnetic stepping. Isotopic ratios and inter-element fractionation are monitored by continuous reference to a standard Sri Lankan zircon (SL13), fragments of which are mounted with each sample. Progressive changes in the Pb/U ionic ratio during sputtering were corrected using an empirical quadratic relationship between Pb+/U+ and UO+/U+ determined for the standard zircon (Claoue-Long et al. 1995). A radiogenic 206Pb/238U ratio of 0.0928 for the standard zircon, corresponding to an age of 572 Ma, is obtained through standard isotope dilution analysis. Initial Pb isotope compositions of the analyzed zircons are assumed to be similar to that of model-derived average crustal Pb of similar age according to Cumming and Richards (1975). The analytical precision of Pb isotope ratios is controlled by machine counting statistics whereas the precision of Pb/U ratios is affected by uncertainties in the recommended Steiger and Jäger (1977) decay constants. Weighted mean 207Pb/206Pb ages were obtained for zircons exhibiting an obvious clustering and indistinguishable 207Pb/206Pb ages at a 95% confidence level. Individual analyses in the data tables and Concordia diagrams are presented at 1σ. Errors associated with scatter within the cluster are obtained by standard statistical techniques. Dispersion within the cluster is attributed to modern Pb loss.

Results

Petrography and mineral assemblages

The metamorphic rocks are made up of two distinct rock units. The first unit is of sedimentary parentage and mainly composed of pelitic granulites and the second of igneous origin is made up of mafic granulites. All these rocks outcrop as flagstones, lenses, boulders, or boudins (Fig. 3). Granoblastic microstructures prevail in all rock types, although flaser and mylonitic textures are also commonly observed.

Fig. 3
figure 3

Field occurrences of the pelitic granulites and mafic granulites. Flagstones (a) and boulder outcrops of pelitic granulites. Flagstones (c) and boudins (d) of mafic granulites

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Pelitic granulites are predominantly composed of sillimanite bearing garnet gneiss and minor garnet gneiss. They are medium-to coarse-grained layered rocks with alternating millimeter- to centimeter-thick alumina-rich and quartzo-feldspathic layers (Fig. 4a). Garnet frequently shows corona textures with plagioclase, quartz, and biotite rim. They are composed of quartz (20–30%), perthitic K-feldspar (15–20%), plagioclase An30-40 (25–30%) occasionally antiperthitic, garnet porphyroblasts (15–20%), biotite (5–10%), and sillimanite (4–8%). Accessory minerals are rutile, zircon, apatite, and graphite. Quartz forms either platy crystals or is in inclusion to garnet and sometimes in pressure shadow of porphyroblasts. Almond shape K-feldspar (5.2 × 3.8 to 6.2 × 3.8 mm) contains opaque minerals. Porphyroblastic garnet crystals (0.4 × 0.24 to 4.4 × 1.7 mm) show the pressure shadow zones filled by quartz, biotite, and sillimanite (Fig. 4a). Prismatic crystals of sillimanite (0.36 × 0.15 to 1.4 × 0.6 mm) are molded by mylonitic schistosity or disseminated on the rock (Fig. 4b). Oxides are included in the garnet porphyroblasts; the smaller ones are oriented elongated. Mineral assemblages are Qtz - Sil - Kfs - Grt - Ru (mineral abbreviations according to Kretz 1983).

Fig. 4
figure 4

Photomicrographs of the pelitic granulites and mafic granulites. a Porphyroblastic garnet crystals show the pressure shadow zones filled by quartz, biotite, and sillimanite; b prismatic crystals of sillimanite molded by mylonitic schistosity; c granoblastic microstructure of pyroxene gneiss; d composition of gneiss with pyroxene and garnet relicts; e kelyphitic rims of garnet composed of amphibole, apatite, quartz, and plagioclase. f Pyroxene crystals showing coronitic microstructure; g, h oxides crystals showing reactional coronas with garnet and hornblende

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The mafic granulites are composed of garnet-pyroxene gneiss associated with gneiss with pyroxene and garnet relicts. These rocks occur mainly as a large flagstone (0.1 to 4 km) along M’poko River, in Bourouma village, but they also occur as boudins within the meta-sedimentary unit (Fig. 3c, d). The mafic granulites are fine- to medium-grained, dark-colored, and granoblastic and display centimeter-thick alternating quartz-plagioclase and pyroxene-garnet ± biotite layers (Fig. 4c and d). It is composed of quartz (5–15%), clinopyroxene (40–50%), plagioclase (20–30%), almandine garnet (8–10%), and biotite (≥ 5%). Accessory minerals include apatite and oxides (Fig. 4g and h). These rocks show abundant retrograde poikiloblastic hornblende (20–30%) replacing pyroxene whereas some samples contain a significant proportion of titanite and oxides. Quartz forms either oriented platy crystals or small rounded ones (0.1–0.4 mm in diameter). Plagioclase grains are large sub-euhedral crystals (2–5mm) which show granulation. The clinopyroxene (augite in composition) crystals are large (2–6 mm), often zoned, and intensively transformed to green hornblende. Garnet (Fig. 4e) crystals are unzoned with a grain size that varies between 1 and 3 mm. The garnets are almandine type and associated with quartz and clinopyroxene. Orthopyroxene crystals (0.48 × 0.44 to 0.4 × 0.68 mm) display corona with single Qtz - Kfs - Grt rim or double rims (i) Qtz - Kfs - Grt and (ii) Grt - Kfs - Qtz (Fig. 4f). Flakes of biotite occur as secondary minerals in the rock matrix. Primary and secondary mineral assemblages are Qtz - Cpx - Pl - Grt (granulite facies) and Hbl - Bt -Qtz - Spn (amphibolite facies), respectively. The peak metamorphic mineral assemblages identified in the studied mafic granulites are Cpx - Opx - Grt - Qtz - Pl - Op.

Temperature and pressure were estimated from the various mineral assemblages using the mafic granulitic and KFMASH petrogenetic grid (Fig. 5) of Osanai et al. (2006) and Spear et al. (1999), respectively. The average P-T conditions of the peak granulite grade metamorphism are ca. 800°C, 10 Kbar (mafic granulites, Fig. 5a), and 7–9 Kbar, 750–800°C (pelitic granulites, Fig. 5b), consistent with temperatures defined from mineral phase assemblages.

Fig. 5
figure 5

Synthetic P-T diagrams for the Bossangoa-Bossembele area granulites. Mafic granulites grid (a); pelitic granulites in the KFMASH grid (b). Hornblende producing univariant reactions are from Spear (1993). Stability of Al-silicates from Holdaway (1971). Stability curves from Berman (1988) and Spear and Cheney (1989) with melting reactions from Vielzeuf and Clemens (1992). (2) Muscovite + quartz + Al2SiO5 = K-feldspar + liquid; (3): Muscovite + quartz + H2O = Al2SiO5 + liquid; (4) K-feldspar + Al2SiO5 + quartz = H2O + liquid; (8) Biotite + Al2SiO5 + quartz + garnet = cordierite + K-feldspar + liquid; (9) Biotite + garnet + quartz + cordierite = orthopyroxene + K-feldspar + liquid; (10) Garnet + sillimanite + spinel = cordierite + quartz

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Geochemistry

Sixteen representative samples (10 meta-sediments and 6 meta-igneous) were selected for chemical analysis and their whole-rock compositions are listed in Tables 1 and 2.

Table 1 Major (wt%) and trace (ppm) elements compositions for representative pelitic granulite samples from Bossangoa-Bossembélé area
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Table 2 Major (wt%) and trace (ppm) elements data for representative mafic granulite samples from Bossangoa-Bossembélé area
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Pelitic granulites

Meta-sedimentary rocks display the features of pelitic rocks with Fe2O3, K2O, MgO, and TiO2 closely correlated to Al2O3 (Fig. 6a, c, e). All these elements are anticorrelated with silica (Fig. 6b, d, f) suggesting that the protolith of the studied rocks was composed of a quartz-clay mixture. The TiO2 contents (0.62–0.79%) and TiO2/Al2O3 ratios (0.04–0.11) are high with respect to those of clay (0.04; Goldschimidt 1954; 0.03–0.05; Taylor and McLennan 1985), but correspond to values given for continental terrigene sediments (Van de Camp et al. 1976). The trace element contents (Table 1) are similar to those of shale except for Rb and Sr (Table 1). The Nb contents (1.30–4.04 ppm, average = 1.99 ppm) are low and show a significant variation. This average is very low compared to those of shales (Van de Camp et al. 1976). However, the K/Rb ratios range between 0.01 and 0.04 (average of 0.03), similar to those of the Yaoundé (Nzenti et al. 1988) and Santa Ynez (Van de Camp et al. 1976) meta-sediments. This ratio is slightly higher than that of the upper crust (Taylor and McLennan 1981). Their REE patterns (normalized to chondrite of Evensen et al. 1978) are fractionated, with a slight LREE depletion (CeN/SmN = 0.9–1.36), flat HREE (GdN/YbN = 1–1.29), and a strong positive Eu (Eu/Eu*= 4.35–18.59) anomalies (Fig. 7).

Fig. 6
figure 6

Binary plots of some major elements (versus Al2O3 and SiO2) for pelitic granulites

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Fig. 7
figure 7

Chondrite-normalized REE patterns of pelitic granulites

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Mafic granulites

Meta-igneous rocks have the composition of diorite (Table 2) with a silica content ranging from 58.04 to 61.71 wt% respectively and rather constant alumina content (17.16 to 17.75 wt%). They are enriched in Al2O3, Fe2O3, MgO, Ba, and Rb (except sample LERE2B). TiO2 contents and FeO*/MgO ratios vary from 1.1 to 1.39 wt% and 2.26–2.41 respectively. The iron enrichment with the values of titanium in these rocks is comparable to those of the tholeiitic series. Also, CaO/TiO2 (3.71–4.52) and Al2O3/TiO2 (>12) ratios correspond closely to those of oceanic tholeiites (Shibata et al. 1979; Leeman et al. 1980). The investigated samples are enriched in alkalis (4.98≤ Na2O + K2O ≤ 5.21%), and CaO (5.14–6.34%). Compared to the gabbros described by Le Maître (1976), they have similar contents of major elements. Ba (197–1098 ppm), Sr (182–344 ppm), Cr (95–1450 ppm), and Ni (60–467ppm) contents are variable and high while those of Nb (3 to 44 ppm), Rb (10–97 ppm), Y (22–60 ppm), Zn (83–187 ppm), and Zr (42–341 ppm) are variable and relatively low. The high contents of Cr, Ni, Sr, and Y are probably related to the fractionation of pyroxene and garnet. The binary element diagrams of some major elements versus MgO (Fig. 8a–f) show that these rocks have the same evolutionary tendency characterized by the decrease of SiO2 and a positive correlation in CaO, TiO2, Fe2O3, and P2O5. The Y/Nb ratios > 1 are similar to those of tholeiites (Pearce and Cann 1973). Plotting in the TiO2 vs. FeO*/MgO diagram (Miyashiro 1974; Fig. 9a), all these rocks fall in the tholeiites field. Their REE patterns (Figure 9b) are fractionated (LaN/YbN = 4.82–359.13); LREE enrichment (CeN/SmN = 1.76–2.68 and GdN/YbN = 0.34–1.68) relative to HREE, and show a negative Eu anomaly (Eu/Eu* = 0.44 to 0.82). These behaviors are intermediary between continental and oceanic tholeiites (Shibata et al. 1979; Leeman et al. 1980). Their overall trace element patterns (Fig. 9c) show Th, Nb, Sr, Zr, and Y anomaly, suggesting the participation of the continental crust and mantle in their genesis (Thompson et al. 1984).

Fig. 8
figure 8

Binary diagrams of some major elements versus MgO of mafic granulites

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Fig. 9
figure 9

a TiO2 vs. FeO*/MgO diagram; b REE patterns of mafic granulites; c multi-element patterns of mafic granulites

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SHRIMP U-Pb zircon age

The zircons from the study sample 7A show a wide range of shapes, sizes, and colors (Fig. 10). A dominant form, however, is the near-spherical, clear, “soccerball” type usually formed during high-T metamorphism (Vavra et al. 1996, 1999; Schaltegger et al. 1999; Hoskin and Black 2000; Kelly and Harley 2005). The grains show a variety of internal structures, included inherited cores, and no oscillatory zoning. The rounded soccer ball/metamorphic zircons are generally dark in CL and show limited zoning. All types and generations of zircon were analyzed (Table 3). The multifaceted to rounded forms with their wide external zones, poorer in uranium, suggest a metamorphic growth type (Pidgeon 1992; Kelly and Harley 2005). The Th/U ratios vary from 0.72 to 0.82 in the core to 0.08 to 0.19 in the external zone (Table 2). On the Concordia diagram (Fig. 11), the older zircon fractions yielded an upper intercept age of 1952 Ma, younger ones cluster on Concordia to give a weighted mean 206Pb/238U age of 640.8 ± 6.0 Ma.

Fig. 10
figure 10

Cathodoluminescence (CL) images of typical zircons from the pelitic granulites (7A). Note the near-spherical, clear, “soccerball” type usually formed during metamorphism. The grains show a variety of internal structures, included inherited cores and no oscillatory zoning, no sector- or planar zoned, no “ghost-like” or “bleached” for former oscillatory zones

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Table 3 SHRIMP U-Pb zircon data for the selected sample (7A) of the Bossangoa-Bossembélé pelitic granulites
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Fig. 11
figure 11

Concordia diagram showing all SHRIMP data points for old (a) and younger (b) zircon grains from the pelitic granulites (sample 7A)

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Discussion

The Bossangoa-Bossembélé area consists of both pelitic and mafic granulites. The pelitic granulites are derived from pelite and greywacke, while the mafic granulites display the chemical composition of intermediate to basic tholeiitic rocks. Petrography and mineral assemblages (Grt-Cpx-Pl-Qtz and Grt-Pl-Sill-Qtz) indicate peak metamorphic conditions under granulite facies at about 800°C, 10 Kbar (mafic granulites) and 8 kbar (pelitic granulites). From the CL imaging, the zircon grains extracted from the pelitic granulites display either no sector or planar zoned which are commonly attributed to high-T subsolidus growth and melt crystallization growth, respectively (Pidgeon 1992; Schaltegger et al. 1999; Ashwal et al. 1999; Hoskin and Black 2000; Corfu et al. 2003; Kelly and Harley 2005). In addition, complete transgression of older zones by sharp fronts or visible compositional zones were not observed on these zircons suggesting that in situ modification under high-T conditions can be ruled out. Furthermore, even partial modification commonly expressed by “ghost-like” or “bleached” for former oscillatory zones was noticed as all the zircons are blurred (e.g., Corfu et al. 1994; Vavra et al. 1996, 1999; Harley et al. 2001, 2007; Carson et al. 2002). These observations suggest that the studied zircon grains were not formed as the result of a high-T anatectic melt growth or late-stage mineral-fluid interactions at temperatures well below those of the high-T metamorphic event.

Based on the morphology and structure of zircons, the peak metamorphic mineral assemblages probably correspond to a pre-Pan-African HP-HT metamorphic cycle. Indeed, phase assemblages indicate that the conditions of metamorphism culminated during Paleoproterozoic time as revealed by the U-Pb zircon core upper intercept age of ca 1952 Ma. These rocks which have also suffered Pan-African orogeny at 640.8 ± 6.0 Ma form a polycyclic unit with a complex evolution corresponding to Paleoproterozoic HP-HT metamorphic cycle followed by Pan-African MP-MT tectono-metamorphic event. The older age (1952 Ma) is interpreted to represent the age of crystallization of the zircons during granulite metamorphic conditions. The younger age (640 Ma) corresponds to a younger Pan-African tectono-metamorphic event. The frequency of high-pressure granulite facies assemblages indicates that the base of the crust was reworked during the Pan-African orogeny. The HP-HT conditions during or close to peak metamorphic conditions are in accordance with previous studies in other Paleoproterozoic areas in the Pan-African fold belt in Cameroon (Tanko Njiosseu et al. 2005; Nzenti et al. 2007; Ganwa et al. 2008, 2011; Ndema Mbongue et al. 2014), suggesting that the whole central part of the belt which is presently exposed represents the same crustal level. The new findings suggest that a Paleoproterozoic continental crust was involved in the Pan-African North Equatorial fold belt from the south (Nyong and Ogooué series) to the north. In addition, strong similarities (granulitic rocks with similar composition and ages) exist between the studied rocks and high-grade gneisses of eastern Nigeria (Dada et al. 1989) and of the Transamazonian belt of NE Brazil (Van Schmus et al. 1999; Brito Neves et al. 2002; Neves et al. 2006). This implies that the Pan-African sedimentary series were deposited on an old Paleoproterozoic continental crust extending up to the north, in CAR, Chad, and Sudan. Thus the Bossangoa-Bossembélé and Nyong and Ogooué series are the extensions in central Africa of the Transamazonian belt.

Conclusion

The petrographic and geochronological investigations of the Bossangoa-Bossembélé area have led to the following conclusions:

  1. 1.

    The Bossangoa-Bossembélé consists of both pelitic and mafic granulites. The pelitic granulites were derived from pelite and greywacke, while the mafic granulites display the chemical composition of diorite.

  2. 2.

    The peak metamorphic conditions are indicated by Cpx - Opx - Grt - Qtz - Pl and Grt -Pl - Sill - Qtz granulitic mineral assemblages.

  3. 3.

    SHRIMP U-Pb zircon dating results indicated that the peak HP-HT metamorphic conditions were reached during the Eburnean/Transamazonian orogeny at ca. 1952 Ma. The studied rocks which were later affected by the Pan-African orogeny at 640.8 ± 6.0 Ma form a polycyclic unit with a complex evolution corresponding to Paleoproterozoic HP-HT metamorphic cycle followed by Pan-African MP-MT tectono-metamorphic event.

  4. 4.

    The Congo craton spreads not only far from the southern part of the North Equatorial fold belt in Cameroon as initially suggested but on the whole fold belt from south to the north (CAR, Chad, Sudan).

  5. 5.

    The Bossangoa-Bossembélé and Nyong and Ogooué series are the extensions in central Africa of the Transamazonian belt.