Central African Orogenic Belt Research Articles

The deformed alkaline Balda granite (Northern Cameroon): A witness of back-arc basin in the northern part of Central African Orogenic Belt

Located on the border between the NW Cameroon and Mayo-Kebbi domains in northern Cameroon, the Balda pluton is a deformed alkaline granite. Due to its unique location and geological features, it holds crucial information for understanding the evolution of the Central African Orogenic Belt (CAOB), which remains poorly understood. This multidisciplinary study combines field data, microscopic observations, Anisotropy of Magnetic Susceptibility (AMS), and U-Pb zircon dating to constrain the geodynamic evolution of the CAOB.The Balda pluton is a NNE-SSW elongated pluton consists mainly of aegirine-riebeckite granite and leucogranite. It displays diverse magnetic susceptibility values (Km) spanning from 0.19 x 10-3 to 21.98 x 10-3. Notably, 93 % of the Km values exceed 5 × 10-4SI, indicating a prevalence of mixed paramagnetic and ferromagnetic mineralogy (such as magnetite-rich). This observation is supported by microscopic examinations and K-T curves. An analysis of AMS unveils high P% values (2.8 to 58.7 %, with a mean of 15 %) and prevalent oblate magnetic fabrics (68 %). The magnetic foliation generally aligns with the field foliation and has steep dips ranging from NNE to NE or from SSW to SW and magnetic lineations exhibit shallow plunges (<35°) from SSW to SW. Microscopic analysis reveals high-temperature, solid-state deformation microstructures, indicating post-emplacement deformation processes and the presence of kinematic markers consistent with sinistral shear. Zircon grain analysis reveals two distinct types: “bright” and “dark.” These types differ in both their appearance under cathodoluminescence and their chemical composition. Bright zircons yield a U-Pb age of 732.7 ± 7.5 Ma, interpreted as the pluton emplacement age. Dark zircons are younger (ca. 680 Ma), suggesting a later tectono-metamorphic or alteration event.The integration of these results in a larger geodynamic context suggests that the Balda pluton formed within a syn-orogenic extensional back-arc basin. This emplacement was followed by significant post-emplacement deformation characterized by sinistral simple shear-dominated transpression likely related to the continental collision (ca. 680 Ma) of the NW Cameroon and Mayo-Kébbi domains.

Understanding the crustal architecture beneath the Bangui magnetic anomaly and its interactions with central African tectonic megastructures based gravity and magnetic analysis

The Bangui magnetic anomaly (BMA) in Central Africa is one of the largest continental magnetic anomalies on Earth in terms of amplitude and lateral size. Determining the sources of the BMA can lead to an increased understanding of the crustal dynamic in the Central African sub-region and the African continent as a whole. Magnetic and gravity analysis-based derivative, two-dimensional forward modelling and a Curie isothermal depth, showed that (a) the bottoms of the magnetic sources were between 15 and 35 km; (b) the BMA is a coalescence of several anomalies that trend E-W and roughly NE-SW. These directions coincide with regional Pan African-aged shear zones along the Central African orogenic belt and to thrust sheets at the northern edge of the Congo Craton. The depth of magnetization does not exceed 35 km with the amplitude of magnetization becoming smaller in the Central African Republic. The potential magnetic susceptibility sources have an average density of 2850 kg/m3 and magnetic susceptibilities between 0.06 and 0.25 SI. The BMA is interpreted to be a combination of middle and lower crustal bodies that are not continuous and consist of magnetic mineral rich granulites and banded iron formations. The gravity and magnetic modelling indicate that the entire crust was involved in the Pan African collisional event similar to what is seen in the Mozambique belt in East Africa. Combined with geological and geochemical studies, the models add evidence that one or two subduction zones were involved in accreting terranes on the northern edge of the Congo Craton. The tectonic accretions caused a crustal remobilization along major shear zones that has locally contributed to a probable circulation of fluids enriched in ferromagnesian minerals during late Neoproterozoic magmatism that created the BMA sources.

Litho-structural mapping and structural evolution of the Bocaranga pluton, northwest Adamawa-Yadé domain, Central African Republic

The Bocaranga pluton, in the northern domain of the Central African Orogenic Belt (CAOB) in the Central African Republic (CAR), is located in the Adamawa-Yadé domain, near the Mbéré Shear Zone (MBSZ). This study provides new results of the satellite image processing, the detailed structural and petrographic studies of Bocaranga pluton which helps to understand the emplacement mechanism of this pluton. Image processing of the satellite data and field showed the occurrence and spatial distribution of (1) amphibole-pyroxenites, (2) garnet-biotite rich amphibolite, (3) migmatite, (4) hornblende-biotite and hornblende-garnet gneiss, (5) hornblende-biotite granite (HBG), (6) two micas granite (TMG), (7) fine-grained biotite granite (FBG), and (8) coarse-grained biotite granite (CBG) rock types. The mapping of regional lineaments was carried out by automatic and manual lineament extraction methods. NE-SW direction corresponds to the direction of S2 and S3 foliations, faults, mineral lineation, quartz vein, dyke (aplitic granitic or pegmatitic). NW-SE direction also corresponds to the direction of S1 foliation, F1 folds, faults, quartz veins, dykes (aplitic granitic or pegmatitic). In the absence of absolute geochronological data in study area, it has been possible to propose a polyphase structural evolution, deduced from the relative chronology of the Bocaranga granitoid. Three phases of deformation can be defined in this study. D1 phase is represented by early NW-SE trending foliation (S1) that has been progressively rotated parallel to S2 overprinting foliation occurring in gneiss, amphibolites and hornblende-biotite granite (HBG) intrusion as well. D2 phase of ductile sinistral sense from NE-SW to ENE-WSW is responsible for an emplacement of Bocaranga granitic pluton. The scarcity of structure orientation in the TMG and FBG (except the contact zones), would imply that these granites would have been crystallized in the absence of any stress. D3 is a phase of brittle deformation with three main directions corresponding to the main direction of the shear zones in the Northern domain of the CAR Pan-African fold belt.The mylonitic deformation observed in study area illustrates the intensity of the deformation. The dominance of ellipsoidal structures observed in Bocaranga area suggests that the shear mechanism is the main controlling structure of Bocaranga granite emplacement, which can be correlated with the emplacement of Tamkoro-Bossangoa granitoid in CAR and the Adamawa-Yadé domain in Cameroon, Chad and NE Brazil.These results imply a syntectonic emplacement of Bocaranga pluton during sinistral simple shear regime related to deformation along the Central Cameroon Shear Zone (CCSZ).

Petrology of the late Pan-African Guider syenite pluton (Cameroon): Implications for magma genesis and emplacement

The Guider pluton, located in the eastern border of the North-West Cameroon domain close to the Mayo-Kebbi domain, is one of the rare high potassic magmatic bodies in the northern portion of Central African Orogenic Belt. Previously studied for its structural and geochronological aspects, its petrogenesis remained uncertain. Petrography, mineralogy, and whole rock geochemistry data are presented. The pluton is made of pink and grey quartz syenites and contains numerous enclaves of quartz diorites. These enclaves are highly concentrated at the center of pluton, previously identify as the pluton's feeder zone. The transition between syenitic types is gradational while the contact between grey quartz syenite and quartz diorite is sharp. Geochemically, they display a high-K signature. Our results including field observations, petrography, mineral chemistry and geochemistry suggest that these rocks derive from a single parental magma differentiated by fractional crystallization and sedimentation in a magmatic chamber. These results also suggest that the parental magma was generated by partial melting of metasomatized lithospheric mantle. Combining previous results with former structural data, we envisage that the Guider pluton derive from two successive steps: (i) a static step of fractional crystallization and sedimentation in a deep magmatic chamber; and (ii) a dynamic step during which the stratified magma ascent toward the upper crust, the evolved syenitic magma being tapped first and the diorite later. The magmatic structures are consistent with this magma emplacement scenario.

Anatexis of metadiorite from the Yaoundé area, Central African Orogenic Belt in Cameroon: implications on the genesis of in-source granodiorite leucosomes

Anatexis of metadiorite from the Yaoundé area, Central African Orogenic Belt in Cameroon: implications on the genesis of in-source granodiorite leucosomes

The enigmatic continental crust of North-Central Africa: Saharan Metacraton or Central Sahara Shield?

AbstractThe continental crust of North-Central Africa between the Tuareg and Arabian-Nubian shields and south to the Central African Orogenic Belt is enigmatic due to the few bedrock exposures especially within the central region. The current understanding, based on a review of geochronology and isotope geochemistry, is that the central Sahara region is a large, coherent craton that was ‘highly remobilized’ during the Late Neoproterozoic amalgamation of Gondwana and referred to as the Saharan Metacraton. However, new data from the Guéra, Ouaddaï, and Mayo Kebbi massifs and the Lake Fitri inlier of Chad suggest that it may be a composite terrane of older cratonic blocks or microcontinents with intervening Mesoproterozoic to Neoproterozoic domains and referred to as the ‘Central Sahara Shield’. It is postulated that the older crust and juvenile crust were sutured together along a Pan-Gondwana collisional belt (Central Sahara Belt) that bisects the central Sahara region. The ‘Central Sahara Shield’ hypothesis suggests the Chad Lineament, a narrow arcuate gravity anomaly within central Chad, could be a collisional belt suture zone and that it may explain the existence of the relatively juvenile crust that typifies southern and eastern Chad. The new data improves upon the existing knowledge and challenges the lithotectonic paradigm of the Saharan Metacraton. Further investigations are required to fully characterize the crust of the central Sahara region and to test the contrasting hypotheses.

Late Ediacaran post-collisional magmatism in the Guéra Massif, South-Central Chad

ABSTRACT The Guéra Massif of South-Central Chad is composed of collisional and post-collisional intrusive rocks related to the Central African Orogenic Belt. Collisional granites were emplaced at 595–590 Ma but there are a series of post-collisional intrusions that were emplaced at 570–560 Ma. The older granites have zircon U-Pb weighted-mean ages of 569 ± 6 Ma, 568 ± 7 Ma, and 568 ± 6 Ma, whereas the younger granites are 556 ± 7 Ma and 561 ± 6 Ma but all ages are within their maximum uncertainty. Both groups of post-collisional granites are peraluminous, ferroan, and alkali calcic to calc-alkalic but dioritic microgranular enclaves were observed within the older granites. The chemical variability of each group can be explained by fractional crystallization and it is likely that their parental magmas were derived by partial melting of the lower crust. The Nd isotopes of the older rocks (εNd(t) = −9.5 to −1.2) are slightly less enriched than the younger rocks (εNd(t) = −13.1 and −9.1). Moreover, the Nd isotopic compositions appear to indicate the existence of two isotopically distinct regions of the Guéra Massif with one centred on the Mongo (west) region and the other near the El Hidjer to Zan (east) region. The older group was emplaced during a period of crustal thinning after oblique collision and shearing related to the final suturing of the Congo-Saõ Francisco Craton and the Saharan Metacraton. The younger group was likely emplaced after a period of regional WNW-ESE directed crustal shortening. The punctuated nature of deformation/crustal relaxation cycles in the southern Saharan Metacraton suggests that there may have been many small terranes rather than larger cratonic blocks involved in the lithotectonic evolution of the Central African Orogenic Belt north of the Congo-Saõ Francisco Craton.

Lu-Hf Isotopic Data of the Mb&amp;#233;-Sassa-Mbersi Tonalite (Central Cameroon Domain): Indicator of ca. 1.0 Ga Juvenile Tonian Magmatism in the Region

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 Mbe-Sassa-Mbersi region, in the northern edge of Central Cameroon Domain of the CAOB and in the Tchollire-Banyo shear zone (TBSZ). The Lu-Hf data obtain on the zircon grains of this tonalite reveal juvenile HfTDM 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.

Neoproterozoic syn-collision magmatism in the Nkondjock region at the northern border of the Congo craton in Cameroon: Geodynamic implications for the Central African orogenic belt

The Nkondjock area located in Western Cameroon is part of the Neoproterozoic Central African orogenic belt, at the northern border of the Congo craton. Rocks of the Nkondjock area are: two-mica orthogneisses, two-pyroxene orthogneisses and mylonitic orthogneisses. The protolith of two-mica, two-pyroxene and mylonitic orthogneisses crystallized at 607 ± 4, 624.4 ± 3.5 and 621 ± 9.1 Ma, respectively, and are thus consistent with the Central African Orogenic belt. Inherited zircons (2043 ± 27 Ma) of mylonitic orthogneisses are interpreted as an inherited age of the metacratonization of the northern border of the Congo craton. S-type affinity of two-mica orthogneisses together with two-pyroxene orthogneisses (Mabombé) display geochemical affinities typical of adakite, including high La/YbN (29.7–55.6) and low Y (3.43–6.69 ppm) with low Mg# (30.8–55), εNd(t) (−8.6 to −6.5), high 87Sr/86Sri (0.70684–0.71063) and positive εHf(t) (+3 to +4.9), similar to those adakites formed by partial melting of thickened lower crust with some mantle input. Two-pyroxene orthogneisses (Ndogboni) and mylonitic orthogneisses display I-type granitoids affinity with A/CNK (0.93–1.06) and are depleted in Nb, Ta, Sr, P, Ti and enriched in Rb, Th, and Pb. They also have low εNd(t) (−10.3 to −11), high 87Sr/86Sri (0.71017–0.71063) and negative εHf(t) (−19.1 to −1) similar to melts derived from mature continental crust. Sr-Nd-Hf isotopes confirm heterogeneous source for these orthogneisses with bimodal distribution on εHf(t) and εNd(t) values, suggesting mixing between juvenile mafic melt and a mature continental crust during the evolution of the Central African Orogenic belt. We suggest that the protolith of these orthogneisses (S- and I-type granitoids) were formed during syn-collisional setting between the Congo and the West African craton which followed a subduction through partial melting of a thickened lower continental crust due to a thermal anomaly induced by upwelling of asthenosphere through slab break-off.

An Assessment of the Magmatic Conditions of Late Neoproterozoic Collisional and Post-collisional Granites From the Guéra Massif, South-Central Chad

The Guera Massif, in South-Central Chad hosts granitic rocks that were emplaced during distinct intervals (595-590 Ma; ~570 Ma; ~560 Ma) of the Late Ediacaran Central African Orogenic Belt. To the northwest of the Guera Massif, younger post-collisional granites (554-545 Ma) are found near Lake Fitri. The older ( 590 Ma) rocks have geochemical characteristics of collisional granites whereas the younger ( 570 Ma) rocks are similar to post-collisional granites. Biotite and amphibole were analyzed to constrain the magmatic conditions of the granites. The biotite from the collisional granites tends to have higher Al and Ti and lower Fe# (Fe#average  0.67) than the post-collisional granites (Fe#average  0.88). The average crystallization temperatures range from 696 ± 37oC to 612 ± 8oC, with the average pressure of crystallization from 0.25 ± 0.09 GPa to 0.13 ± 0.02 GPa, and redox conditions between the nickel-nickel oxide (NNO) and quartz-fayalite-magnetite (QFM) buffers. The biotite crystallization temperatures of the post-collisional rocks are generally lower than the collisional rocks (570 Ma = 630 ± 26oC to 619 ± 30oC, 560 Ma = 626 ± 20oC to 607 ± 4oC, 550 Ma = 639 ± 18oC to 612 ± 13oC), but the crystallization pressures are similar (0.27 ± 0.05 GPa to 0.14 ± 0.04 GPa). The redox conditions transition from the QFM buffer to the wustite-magnetite (WM) buffer. In contrast, the biotite from the Lake Fitri post-collisional granites crystallized at higher pressure (0.39 ± 0.08 GPa to 0.35 ± 0.03 GPa) but similar redox conditions. The amphiboles in the younger (~590 Ma) collisional granites and the post-collisional granites yielded crystallization pressure estimates that are generally higher (~0.6 GPa to ~0.1 GPa) than the biotite estimates but there is overlap. The difference in pressure may be due to the timing of crystallization and/or crystal redistribution. Overall, there appears to be a secular change from high to low temperature and pressure whereas the redox conditions appear to be spatially related. The biotite crystallization pressure of the Lake Fitri granites suggests they were likely emplaced into a different domain/terrane of the Saharan Metacraton than the Guera Massif.

Uranium mineralization associated with late magmatic ductile to brittle deformation and Na–Ca metasomatism of the Pan-African A-type Zabili syntectonic pluton (Mayo-Kebbi massif, SW Chad)

The Mayo-Kebbi massif (Chad) exposes a Neoproterozoic juvenile crustal segment that has been tectonically accreted in the Central African Orogenic Belt and reworked during the Pan-African orogeny. It comprises a syntectonic high-K magmatic suite including the Zabili A-type granitic pluton. The Zabili pluton is made of a highly differentiated granite generated by fractional crystallization of a magma formed by partial melting of a Neoproterozoic juvenile protolith. Syn- to post-magmatic ductile to brittle deformation of the Zabili pluton is associated with metasomatism and deposition of uranium. Primary magmatic U-bearing minerals are zircon, monazite, and uranothorite. Late-magmatic deformation and Na-metasomatism are marked by the development of R’ antithetic shear zones with high-temperature dynamic recrystallization of K-feldspar and Ca-plagioclase phenocrysts coeval with crystallization of albite along deformation bands and grain boundaries, and crystallization of interstitial amphibole, calcite epidote and albite also affected by intracrystalline deformation. At this stage, U-bearing minerals are monazite, uraninite, brannerite (pseudomorphosed in ekanite). This late-magmatic event is dated by U–Th–Pb on monazite at 599 ± 4 Ma. Brittle deformation and Ca-metasomatism are marked by cataclastic zones and veins containing albite, epidote, calcite, chlorite, apatite, metamict zircon, pitchblende, U-silicates, and iron oxides. U-bearing minerals are altered and/or remobilized in ekanite, kasolite, and uranophane. These data suggest that the uranium mineralization hosted by the Zabili pluton records a superposition of processes and traces extreme crustal differentiation of a Neoproterozoic juvenile crustal segment reworked during the Pan-African orogeny.

Rutile LA-ICP-MS U–Pb geochronology and implications for tectono-metamorphic evolution in the Yaoundé Group of the Neoproterozoic Central African Orogeny

Tectono-metamorphic evolution within the Cameroonian Yaoundé Group portion of the Central African Orogenic Belt (CAOB) was previously assessed from both metamorphic zircon, garnet and monazite and whole rock dates. In this contribution, and for the first time, robust rutile LA-ICP-MS U–Pb dates were obtained. As rutile is ubiquitous in the low-to high-grade metamorphic rocks therein, our new data allowed to refine the age of the Neoproterozoic metamorphism characterized by four main rutile growth events at 719 ± 32 Ma, 661 ± 21 Ma, 630 ± 6 Ma, and 561 ± 8 Ma in the Yaoundé Group. An age of 522 ± 6 Ma attributed to Pb loss during Ediacaran tectonism is also defined. It appears from these rutile ages that the metamorphic evolution of the Yaoundé Group occurred, not in the time span of ca. 616 Ma to ca. 540 Ma as previously considered, but between 719 Ma and ca. 561 Ma, as revealed by U–Pb rutile dating. A comparison between these rutile dates and mineral and whole rock ages at the regional scale suggests the same time span for Neoproterozoic metamorphism in the Cameroon segment of the CAOB. It follows that the latter resulted from a protracted (>150 Myr) tectono-metamorphic and magmatic event.

Geochemistry of rutile from the Pan-African Yaoundé metamorphic group: Implications for provenance and conditions of formation

Morphological and geochemical features of both in-rock and alluvial rutile within the Yaoundé Group of the Pan-African Central African Orogenic Belt (CAOB) were assessed, using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) in order to assess its provenance and condition of formation. Rutile in gneiss and micaschist is fine grained (<0.2 × 0.8 mm) and occurs as euhedral to subhedral crystals, either within the matrix or included in garnet, biotite, kyanite and plagioclase. The alluvial rutile is coarse-grained and ranges in size from several millimetres to 5 x 2 cm. The alluvial rutile shapes (angular to sub-blunt) indicate local sedimentation or more or less important hydromechanical transport. Alluvial rutile is Al, Fe and Sb-rich compared to in-rock rutile. The Cr and Nb abundances and the ≤0.02 232Th/238U ratios suggest a common origin (metapelitic and metamafic) for both alluvial and in-rock rutile. The Zr-in-rutile thermometer indicates that coarse-grained alluvial rutile formed under blueschist and greenschist facies temperature conditions (514–561 °C), while the in-rock rutile was formed under amphibolite to granulite facies temperature conditions (614–726 °C). Alluvial rutile temperatures are symptomatic of pegmatites and high-grade hydrothermal bodies formed during late-stage tectono-metamorphic episodes of the Pan-African Orogeny. The temperature evolution depicted from in-rock rutiles indicates a regional metamorphism dominated by retrogression from granulite to amphibolite facies from central to coastal Cameroon.

Two series of Ediacaran collision-related granites in the Guéra Massif, South-Central Chad: Tectonomagmatic constraints on the terminal collision of the eastern Central African Orogenic Belt

The Guéra Massif of South-Central Chad is one of the least studied regions of the Saharan Metacraton. The oldest known rocks were emplaced during the Ediacaran and have geochemical characteristics of collision-related granites. The granites can be subdivided into two series on the basis of their compositions: 1) ferroan and alkalic to alkali calcic, and 2) magnesian and alkali calcic to calc-alkalic. The ferroan rocks (589 ± 6 Ma, 589 ± 6 Ma, 593 ± 7 Ma, 590 ± 8 Ma) tend to be younger than the magnesian rocks (595 ± 8 Ma) indicating there was a secular compositional change. The chemical variability within each series can be explained by fractional crystallization, but the ferroan series requires lower water content (~1 wt%) and relatively reducing conditions (ΔFMQ −1) compared to the magnesian series (H2O = ~2 wt%; ΔFMQ + 0.7). The older magnesian series is isotopically chondritic to moderately enriched (ISr = 0.7037 to 0.7073; εNd(t) = −3.2 to + 0.2) and was likely derived from a subduction-modified mantle source at an ‘Andean-type’ margin. In contrast, the ferroan series is isotopically enriched (ISr = 0.7048 to 0.7176; εNd(t) = −9.4 to −14.1) and has a significant amount of inherited Mesoproterozoic zircons (206Pb/238U = 1039 ± 24 Ma), indicating that it was likely derived by partial melting of an older crustal source. The changing nature of the granites likely records the transition from an active margin to a collisional zone between the Congo- São Francisco Craton and the Saharan Metacraton.

The geologic record of the exhumed root of the Central African Orogenic Belt in the central Cameroon domain (Mbé – Sassa-Mbersi region)

We present structural, petrological, geochemical and geochronological data compiled in a new geological map on the high-grade metamorphic rocks exposed in the Mbé – Sassa-Mbersi region located along the Tcholliré-Banyo Shear Zone, at the northern edge of the Central Cameroon domain.The region exposes a complex assemblage of metabasic (hornblendite, metagabbro, amphibolite) and metasedimentary (garnet paragneiss, calc-silicate gneiss) rocks alternating with ubiquitous migmatitic intermediate to felsic gneisses. U-Pb ages on detrital zircon in paragneiss point to the contribution of Archean, Paleoproterozoic and Neoproterozoic sources, and suggest a maximum deposition age of 725 ± 12 Ma for their sedimentary protolith. The protolith of some amphibolites was emplaced during the Neoproterozoic as indicated by U–Pb ages on magmatic zircon at 600 ± 4 and 599 ± 6 Ma. Plutonic rocks with I-type chemical signatures were emplaced discontinuously over a period of ca. 80 Ma. The magmatic evolution started with the intrusion of magnesian medium-K tonalites and trondhjemites at 651 ± 3 Ma, followed by magnesian high-K hornblende-biotite granites and ferroan shoshonitic biotite granites until 599 ± 2 Ma. Finally, pegmatite dykes were emplaced along dextral shear zones at 573 ± 2 Ma. The metamorphic rocks show a penetrative shallow-dipping foliation partially to totally transposed in a NE–SW trending steep-dipping foliation bearing a shallow-dipping lineation. These ductile syn-migmatitic fabrics, developed under high-grade metamorphic conditions in the presence of melt, between the metamorphic peak at ca. 1.3 GPa–770 °C, and the following isothermal decompression down to ca. 0.8 GPa. In situ U–Pb dating of metamorphic zircon rims from a garnet migmatitic paragneiss yields an age of 582 ± 4 Ma. Localization of high-grade deformation is indicated by the development of the Tcholliré-Banyo Shear Zone, characterized by mutually crosscutting subvertical and shallow-dipping mylonitic to ultramylonitic shear zones associated with sinistral and top-to-the SW kinematic criteria, respectively. These data indicate that the Central Cameroon domain exposed in the Mbé – Sassa-Mbersi region, represents the exhumed mid-to lower part of the former orogenic root of the Pan-African Central Africa Orogenic Belt that has undergone partial melting, lateral flow and intrusion of mafic to felsic calc-alkaline magmas between 650 and 580 Ma. The waning stage of the Pan-African orogeny is marked by the emplacement of syntectonic pegmatitic granite at ca. 575 Ma in subvertical E-W trending retrogressive dextral shear zones.

The Adamawa-Yadé domain, a piece of Archaean crust in the Neoproterozoic Central African Orogenic belt (Bafia area, Cameroon)

New field observations, ages and geochemical data are presented for the Adamawa-Yadé domain, forming part of Central African Orogenic Belt in Cameroon. This belt delineates the northern margin of the Congo craton and is part of the Braziliano/Pan-African orogenic system, which was formed during amalgamation of the West Gondwana continent at the end of the Neoproterozoic.U–Pb dating indicates that the protoliths of some orthogneisses from the Adamawa-Yadé domain were emplaced during three distinct periods. Tonalite-Trondhjemite-Granodiorite (TTG) suites in the Makénéné area intruded at ca. 3.0–2.5Ga, and were affected by partial melting at ca. 2.08–2.07Ga, during the Eburnean orogeny, and by a magmatic event with a metamorphic overprint at 0.64–0.61Ga, during the Pan-African orogeny. A dioritic orthogneiss from the Maham area, north of Makénéné, was emplaced at 2.07Ga and also reworked at 0.62–0.61Ga. The youngest granitic orthogneisses, sampled in the vicinity of Bafia, Bep and Ngaa-Bapé, yield intrusion ages of 0.64–0.63Ga. Some of these Neoproterozoic plutonic rocks show shoshonitic affinity and reveal εNd(630Ma) values ranging from −1.2 to −6.9 (1.7>TDM>1.1Ga), and 87Sr/86Sr initial ratios of 0.706 and 0.704, providing evidence for crustal reworking during the Pan-African collision. The isotopic geochemical characteristics provide evidence that potassic magmas result from the interaction between the Archaean TTG continental crust and magmas derived from a mantle enriched in silica and incompatible elements during subduction underneath a continental margin. The Bafia group appears as a complex geological entity comprised of an Archaean/Palaeoproterozoic basement and its Neoproterozoic sedimentary cover, both deformed and recrystallized at ca. 0.61Ga.The crustal evolution of the Adamawa-Yadé domain is very similar to that inferred for the northern margin of the Congo craton (the Ntem complex and Nyong series). This and the sedimentary record suggest that the Adamawa-Yadé domain represents an Archaean/Palaeoproterozoic microcontinent, which was detached from the northern margin of the Congo craton in the early Neoproterozoic, but became re-accreted together with the Mayo Kebbi (magmatic) arc during the Pan-African orogeny.

Pan-African metamorphic evolution in the southern Yaounde Group (Oubanguide Complex, Cameroon) as revealed by EMP-monazite dating and thermobarometry of garnet metapelites

Garnet-bearing micaschists and paragneisses of the Yaounde Group in the Pan-African Central African Orogenic Belt in Cameroon underwent a polyphase structural evolution with the deformation stages D 1–D 2, D 3 and D 4. The garnet-bearing assemblages crystallized in course of the deformation stage D 1–D 2 which led to the formation of the regional main foliation S 2. In X Ca– X Mg coordinates one can distinguish several zonation trends in the garnet porphyroblasts. Zonation trends with increasing X Mg and variably decreasing X Ca signalize a garnet growth during prograde metamorphism. Intermineral microstructures provided criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The syndeformational P– T path sections calculated from cores and rims of garnets in individual samples partly overlap and align along clockwise P– T trends. The P– T evolution started at ∼450 °C/7 kbar, passed high-pressure conditions at 11–12 kbar at variable temperatures (600–700 °C) and involved a marked decompression toward 6–7 kbar at high temperatures (700–750 °C). Th–U–Pb dating of metamorphic monazite by electron microprobe (EMP-CHIME method) in eight samples revealed a single period of crystallization between 613 ± 33 Ma and 586 ± 15 Ma. The EMP-monazite age populations between 613 ± 33 Ma enclosed in garnet and 605 ± 12 Ma in the matrix apparently bracket the high temperature–intermediate pressure stage at the end of the prograde P– T path. The younger monazites crystallized still at amphibolite-facies conditions during subsequent retrogression. The Pan-African overall clockwise P– T evolution in the Yaounde Group with its syndeformational high pressure stages and marked pressure variations is typical of the parts of orogens which underwent contractional crustal thickening by stacking of nappe units during continental collision and/or during subduction-related accretionary processes.

Geochemistry of the Bafoussam Pan-African I- and S-type granitoids in western Cameroon

The Bafoussam area in western Cameroon is part of the Central African Orogenic Belt. It is dominated by granitoids which belong to the Pan-African syn- to post-collisional post-650 Ma group. Syenogranites are predominant, but alkali-feldspar granite, monzogranite, quartz-monzonite and quartz-monzodiorite occur as well. Four granitoid suites, biotite granitoids and deformed biotite granitoids with amphibole, megafeldspar granitoids with megacrysts and two-mica granitoids with primary muscovite and igneous garnet are distinguished. The granites can be assigned to high-K calc-alkalic to shoshonitic series. The partly shoshonitic biotite granitoids are metaluminous to weakly peraluminous and can be labelled as a highly fractionated I-type suite. The megafeldspar granitoids are weakly peraluminous with I-type character whereas the two-mica granitoids are weakly to strongly peraluminous and belong to an S-type suite. Emplacement ages at 558–564 Ma for the two-mica granitoids have been dated from monazite by the EMP Th–U–Pb method. The REE in the biotite granitoids are moderately fractionated with (La/Lu) N = 23–38. Enrichment of Nb and Ta varies by one order of magnitude. The megafeldspar granitoids show homogeneous and strongly fractionated REE patterns with (La/Lu) N = 27–42. The primitive mantle-normalized element patterns are homogeneous with marked negative Ba, Nb, Ta, Sr, Eu and Ti anomalies. The two-mica granitoids are characterized by low to moderate total REE contents with strongly fractionated REE expressed by (La/Lu) N ranging from 7 to 59. The negative Nb and Ta anomalies are less significant. Nd and Sr whole-rock isotope data confirm different sources for the granitoid suites. The source of the I-type biotite granitoids was probably a juvenile mantle which has been variably metasomatized. The source of the I-type megafeldspar granitoids is characterized by juvenile mantle and lower crust components. Anatectic melts of the upper continental crust with variable contribution of lower continental crust or mantle melts can explain the heterogeneous isotopic signatures of the S-type two-mica granitoids. It is suggested that the melting of these sources was successively initiated by the rising isotherms during a syn- to post-collisional setting which followed a subduction.

Tectonic evolution of the Yaoundé segment of the Neoproterozoic Central African Orogenic Belt in southern Cameroon

The deformation history of the Neoproterozoic Central African Orogenic Belt in southern Cameroon is well recorded in the low- to high-grade rocks outcropping in the area around Yaoundé. The fabrics in these rocks are consistent with two main ductile deformation events D1 and D2. D1 predated emplacement of calc-alkaline dioritic bodies and caused the formation of nappes that resulted in high-pressure granulite metamorphism of soft sediments. A strong overprinting of these nappes during D2 symmetric extension, probably associated with large-scale foliation boudinage and (or) gneissic doming and intense magmatic underplating, gave rise to regional flat-lying fabrics. The latter were further buckled by D3 and D4 folding phases defining a vertical constriction occurring with a major east–west to NW–SE shortening direction. The corresponding F3 and F4 folds trend north–south to NE–SW and east–west to NW–SE, respectively, and represent the main regional strain patterns. Based on the east–west to NW–SE maximum shortening orientation indicated by F3 folds, it is proposed that the nappe-stacking phase D1 occurred in the same direction. The deformation history in the area can thus be described as corresponding principally to alternating east–west to NW–SE contractions and north–south to NE–SW orogenic-parallel extensions. At the regional scale, this could be due to the Transaharan east–west collisional system.