Pan-African Fold Belt Research Articles

Potentially toxic metals contamination in stream sediments of Mbal area (Pan-African fold belt, Cameroon)

The Mbal area is situated within the Neoproterozoic Lom Basin in Cameroon. The objective of this paper was to determine the level of contamination and risk associated of potentially toxic metals (PTMs) in stream sediments of Mbal. Sampling was performed by panning and collected sediment were analysed using an inductively coupled plasma-mass spectrometer. The sediment samples showed significant enrichment in mercury, above 10000 ppb. The other transition metals including manganese, zinc, lead were also elevated, ranging from 825 to 1950, 50.80 to 97 and 9.80 to 87.20 ppm, respectively. Multivariate statistical analyses revealed that the enrichment of the PTMs were attributed both to geogenic and anthropogenic sources. Pollution indicators such as degree of contamination, modified degree of contamination and pollution load index yielding 10809.33–200005, 982.67–18182.33 and 0.59–1.18 respectively, suggest high degree of sediments contamination from natural and anthropogenic sources. Geo-accumulation index and enrichment factor revealed moderate to extreme levels of metallic pollution. Potential ecological risk indices show extreme ecological risk index of PTMs suggesting ecological effects to biota and humans in the ecosystem. Mercury, lithology, mining, domestic and agricultural activities were major sources of PTMs pollution. Toxic metals may cause chronic health problems such as cardiovascular diseases, nervous system disorder, kidney damage and gastrointestinal disorder. This study underlines the importance of continuous monitoring and adaptive management for the preservation of the biotic and non-biotic environment. The results may assist mining companies and local populations in setting realistic targets for monitoring and remediation programs.

Petrochemical characterization of metasedimentary rocks in the Nkolkosse area, Yaoundé domain of the Pan-African fold belt in Cameroon: constrain on their provenance, protolith, and tectonic setting

Petrochemical characterization of metasedimentary rocks in the Nkolkosse area, Yaoundé domain of the Pan-African fold belt in Cameroon: constrain on their provenance, protolith, and tectonic setting

Contamination, sources and risk assessments of metals in stream sediments of Pouma area, Pan-African Fold Belt, Southern Cameroon

Contamination, sources and risk assessments of metals in stream sediments of Pouma area, Pan-African Fold Belt, Southern Cameroon

Time relationship between emplacement, fabric development and regional deformation of the Manchi granitic pluton (western - Cameroon domain)-an integrated AMS, CPO and microstructural investigation

In this study, we integrate anisotropy of magnetic susceptibility (AMS) and microstructural data including quartz crystallographic preferred orientation (CPO) from SEM-EBSD from the Manchi Pan-African granitic pluton with the objective of deciphering the time relationship between fabric development, emplacement and regional tectonics. The Manchi pluton, located in the western Cameroon domain of the Pan-African Fold Belt of Central Africa in Cameroon, is made of hornblende-biotite granite (HBtG), biotite granite (BtG) and protomylonites of HBtG. The pluton intruded the high grade country rocks made of banded gneiss and amphibolites. AMS of the pluton is controlled dominantly by paramagnetic phases such as, Fe-bearing silicates (biotite and hornblende). This is inferred from the low Km values (<500 × 10−6 SI units) as well as from hysteresis data. The AMS ellipsoid shape is mostly oblate, implying that flattening component of the strain was important during granite emplacement. Microstructural observations reveal presence of magmatic to sub-magmatic and solid-state deformation textures developed at moderate- to low-temperatures. This is consistent with the presence of rhomb <a> and basal <a> slip systems established from quartz c-axis data through EBSD analysis. The pluton recorded top-toward SW to top-toward south sense of shear as established by the kinematic markers. This indicates a component of simple shear during tectonic evolution of the pluton. We infer that the magnetic fabrics recorded in the Manchi granitic pluton is a manifestation of the D2 regional deformation. The dominance of flattening strain and the concentric pattern of magnetic data are indicative of ballooning being an important mechanism under transpressive regime.

Petrographic and geochemical study of Doua granitoids (adamawa-yade domain, center Cameroon): Petrogenesis and geodynamic implication

The region Doua is located in the central area or Adamawa-Yade domain of the Pan-African fold belt in Cameroon. This work focuses on the petrography and geochemistry of granitoids in this region. The study area is made up of plutonic rocks (amphibole and biotite granite, biotite and muscovite granite, syeno-diorite, syenite. The geochemical study of the granitoids in the area shows that the rocks are magnesian and slightly iron-bearing, potassic to hyper-potassic calc - alkaline, metaluminous and belongs to type I granitoids. The profiles of the rare earths (REE) show a negative anomaly in Eu. The multi-element spectra show positive anomalies, or negative anomalies for some samples in Ba and Negative anomalies in Nb-Ta, Sr and Ti. The process which led to the formation of these rocks is fractional crystallization. These rocks are setup in a syn-collisional subduction environment (volcanic arc granitoids).  Â

Petrography and Geochemistry of Ba&amp;#239;bokoum-Touboro-Ngaoundaye Granitoids on the Chad-Cameroon-RCA Borders (Adamawa-Yade Domain)

The Baïbokoum-Touboro-Ngaoundaye area is located in the Adamoua-Yade domain of the Pan-African Fold Belt in Central Africa. The present work concerns the petrographic and geochemical study. The study area is made up of plutonic rocks including two amphibole granites, four biotite and muscovite granites, two syenites and one monzonite sample. These rocks have a granular porphyroid texture, with a mylonitic tendency. The primary paragenesis consists of Opx + Cpx + Amp + Bt + Pl + Or + Mic + Qtz + Ap + Sph + Zr and the secondary paragenesis consisting of Ch + Op. Plutonic rocks are shoshonitic to calc-alkaline with a strong potassic affinity. The REE profiles of these rocks show a negative anomaly of Eu in the granites and no Eu anomaly in syenites and monzonite (syeno-diorite). Multielement spectra of plutonic rocks present a negative anomaly in Nb-Ta and Ti. Fractional crystallization is the process that made these rocks of study area. These rocks are placed in a subduction to syn-collisional (VAG + Syn-COLG) and of intraplate granitoids (WPG) environment.

Lineaments analysis of Bafoussam (West-Cameroon) using remote sensing and vertical electrical sounding: Hydrogeological implication

The study proposes to map the tectonic accidents existing in Bafoussam locality using the remote sensing and vertical electrical sounding methods. The principal component analysis and directional filtering applied respectively to Landsat8 and SRTM images allowed to extract remote sensing lineament map. The interpretative rose diagram shows a distribution of lineaments following four directions: NE-SW, NW-SE, N-S and E-W. The main directions are from N30°E to N80°E; these are those of large strike-slip of the Central Pan-African Fold Belt (CAFB) in Cameroon and from the extension of the Central Cameroonian Shear Zone (CCSZ) in the studied area. The superimposition of these lineaments on the hydrographic network map enabled establishing genetic links because rivers flow following the main N30°E and the secondary N120°E lineament directions. The superimposition of the iso-resistivity maps resulting allowed to extract the geophysical lineament map which has been confirmed by achieving resistivity sections in depth. These lineaments are N40°E to N60°E directed comparable to the network fracture and foliation direction on orthogneiss flagstones within the area. These lineaments superimposed on those automatically extracted by remote sensing show a good correlation following the two methods used. The both mapping methods therefore allowed to extract the fracturing map zone. As groundwater is generally stored in the fracture network of hard rocks, the results of this study will thus provide progress in the course of wells and boreholes implantation. The second generation of fractures with a N140°E striking direction is the most appropriate for the hydrogeological studies in Bafoussam.

Morphotectonic Analysis of Mboula Area in Relation with Central Cameroon Shear Zone (CCSZ) and Lithology Using Remote Sensing and Field Data

The field studies, the Landsat 8 OLI/TIRS processing and the digital elevation model of the Alos Palsar images enabled us to map the lineaments of the Mboula area, in the Adamawa-Yadé domain of the Central Pan-African Fold Belt. The use of the OLI 8 band on Landsat 7 × 7 window made it possible to highlight these lineaments. The result depicted in more than 12,417 segments of lines and curves corresponding to potential fractures and geological contacts. Note that the maximum number of main lineaments after treatment (3,274 lineaments) are oriented NE-SW. The second lineament trend in the study area is NNE-SSW and NNW-SSE respectively. The DEM of the Alos Palsar sensor made it possible to establish four geomorphological units in the study area: hilltop, mountain, plateau and alluvial plain. By superimposing these on the lithological map and tectonic, the coincidence is quite obvious. Thus, field and remote sensing analysis reveal that lithology and the CCSZ have influenced significantly the geomorphological evolution of the Mboula area. These results also illustrate the interest of remote sensing analysis in mapping tectonic structures, with benefit of time saving and precision.

Interpretation of Aeromagnetic Data to Investigate Crustal Structures of the Contact Congo Craton - Pan-African Belt at the Eastern Cameroon

The collision between the Congo Craton and the Pan African fold belt of Central Africa had great impacts on the geological and tectonic points of view, notably the installation of several tectonic accidents such as faults, fractures, dikes, folds, domes. This aeromagnetic study is based on Paterson&amp;#39;s aeromagnetic data interpretations through the use of multiple operators. These data were processed by Oasis Montaj software. The total magnetic intensity map reduced to the equator (RTE-TMI) shows important anomalies features the major important regional anomalies. Maps of the vertical gradient, analytical signal and tilt angle maps have meanwhile highlighted several short wavelength anomalies assimilated to folding, dykes, fractures or faults. The map of maxima upward to 2 km allowed to establish the structural map of the study area. It turns out that the different types of geological accidents follow ENE-WSW, ESE-WNW, NE-SW, NW-SE and even E-W and N-S directions. All these directions are very similar to the geological history of the area. Anything that seems to confirm that the study area was the scene of intense tectonic movements resulting from the collision between the Congo Craton and the Central Africa Fold Belt.

Contrasting Ba-Sr Granitoids from Bamenda Area, NW Cameroon: Sources Characteristics and Implications for the Evolution of the Pan African Fold Belt

The basement rocks of Bamenda town are mainly covered by volcanic rocks and are made up of two distinct types of granitoids: granites and leucogranites. These basement rocks belong geochemically to the same granite field. The studied granitoids are silica rich, with concentrations ranging from 70 to 73% in granite and 73 to 76% in leucogranite. Both belong to high-K calc-alcaline series with granite being magnesian and more potassic (Na2O/K2O 2O/K2O >1). Leucogranites are low Ba-Sr granitoids, strongly peraluminous (A/CNK > 1.1) and plot in the field of S-type granites, while granites are high Ba-Sr granitoids, slightly peraluminous and plot in the field of I-type granitoids. The chondrite-normalized REE patterns show more LREE enrichment in granites (>100*chondrite) than in leucogranites (>10*chondrite). Major and trace element compositions of the leucogranites and granites indicate crustal derivation from the partial melting of metapelite and metagreywacke respectively. The REE patterns display pronounced negative Eu anomalies (Eu/Eu* = 0.23-0.36) in leucogranites due to low degree of partial melting of a plagioclase depleted crustal source, and weakly negative to null Eu anomalies (Eu/Eu* = 0.81-1.08) in granites due to high degree of partial melting. The estimated temperatures of Bamenda granitoids magma range between 800°C and 950°C. Bamenda I-type granites are syn-tectonic and are similar to the other granitoids of the central domain of the Pan-African North-Equatorial Fold belt in Cameroon. Bamenda S-type peraluminous leucogranites are post-tectonic and chemically similar to the Himalayan peraluminous leucogranites. Furthermore, decompression model may be related to Bamenda leucogranite formation. Tectonically, the studied granitoids are related to crustal delamination characterizing the post-collisional event within the Pan African Fold Belt.

Mineralogy and geochemistry of Neoproterozoic siliceous manganese formations from Ntui–Betamba (Cameroon Pan-African Fold Belt): implications for mineral exploration

The siliceous manganese formations from Ntui–Betamba area have been studied with the aim to constrain their mineralogy, geochemistry, and genesis. Neoproterozoic manganese-bearing rocks at Ntui–Betamba include garnet–galaxite-bearing quartzite and galaxite-bearing quartzite that—besides a characteristic quartz, spessartine-garnet, and galaxite (MnAl2O4) assemblage—comprises also graphite and rutile. Galaxite crystals show chemical zoning from core to rim due to the change in composition from galaxite to cryptomelane. Bulk rock geochemistry reveals that the studied rocks are characterized by high SiO2 content (52.83 wt%). They are enriched in Fe2O3 and Al2O3 and display Mn/Fe ratio of 1.2. MnO concentrations are high in galaxite-bearing quartzite (16 wt%), and moderate (8.03 wt%) in garnet–galaxite-bearing quartzite. The transition trace metal (Co, Cu, and Ni) contents are lower, with Co and Cu markedly depleted relative to Ni, and variable contents of Zn. The REE pattern of garnet–galaxite-bearing quartzite shows Ce-positive anomaly similar to the hydrogenous sediment, but differs from the later with strong Eu-negative anomaly and the lack of weak positive anomaly in Yb, whereas the galaxite-bearing quartzite is characterized by the lack of Ce anomaly and the weak Eu-negative anomaly. Both rocks show HREE content close to that of hydrothermal sediment and exhibit flat HREE trend. The high contents of silica, iron, and aluminum, together with the Co/Zn ratios (~0.3) and the contrasting behavior of REE, suggest that the studied manganese-bearing rocks are of sedimentary origin probably derived from a mixed hydrothermal–hydrogenous source. The high content of Mn-bearing minerals in these rocks represents indicators of concealed or buried mineralization and could be used as exploration guide.

Geochemical and Isotope Sr-Nd Character of Dschang Biotite Granite: Implications for the Pan-African Continental Crust Evolution in West-Cameroon (Central Africa)

The biotite granite of Dschang composing of quartz, feldspar and biotite, is a haplogranite I-type calc-alkalinegranitoid. It is weakly peraluminous with a ferroan affinity and exhibits fractionated feature. This granite wasgenerated at the minimum temperature and pressure of crystallization of 650°C/4 Kb. Its emplacement issyn-central Cameroon shear zone like most of post-collisional granitoids of the Pan-African fold belt in centralAfrica, and it displays N-S to NNE-SSW magmatic foliation dipping 30° to 60° towards W or WNW. Theevolution of the major elements shows a generalized decrease correlation trends look firstly like fractionalcrystallization. The depleted Sr content related to decreasing Al2O3 and CaO concentrations, indicate the roleplayed by plagioclase during differentiation. The trace elements normalized to the primitive mantle, shownegative anomalies out of Nb, Ta, Sr and Ti, and positive anomalies out of Ba, Rb, Th, comparable with those ofthe continental crust. The isotopic data Sr-Nd are rather homogeneous suggesting a single source for the biotitegranite of Dschang. ?Nd600Ma (- 16 to -18) and TDM (1.9 to 2.1 Ga) confirm the formation of the magma of thebiotite granite of Dschang starting from an old Paleoproterozoic crust by partial melting around 600 Ma. Thismelting which seems to have spread in the area and at several levels of the continental lithosphere, hencemagmas were mixing throughout their evolution, considering the diversity of the granites and their sources, iscertainly caused by great movements along CCSZ, at the post-collisional period of Pan-African orogenesis incentral Africa.

Delineation of the northern limit of the Congo Craton based on spectral analysis and 2.5D modeling of aeromagnetic data in the Akonolinga-Mbama area, Cameroon

Aeromagnetic data of the Akonolinga-Mbama region are analyzed in order to elucidate the subsurface geology of the area. The available data in the form of a residual aeromagnetic map is interpreted as a vast magnetically quiet region, and complex zones which do not correlate with the surface geology of the region.Within the magnetically quiet zone, a high negative circular elongated anomaly zone seems to represent an intrusion of a plutonic rock into the metamorphic formations of the region. Spectral analysis and two-and-a-half dimensional (2½-D) modeling are used to estimate the depth of the causative bodies and determine the source rocks along three profiles crossing the suspected areas. Models from various zones of granitic intrusions are obtained, thereby proposing some shallow fault lines along zones of contact. This permits us to mark out the northern margin of the Congo Craton, thus enabling us to distinguish the cratonic formations from the Pan African fold belt. Part of the belt has been thrust over the northern portion of the Congo Craton in Cameroon.

Pre-collisional geodynamic context of the southern margin of the Pan-African fold belt in Cameroon

We reassess the geodynamic context close to the Congo craton during the pre-collisional period of the Pan-African orogeny from whole-rock major and trace element compositions and isotopic data obtained in the westward extension of the Yaounde series (Boumnyebel area, Cameroon). The series consists of metasediments (micaschists, minor calc-silicate rocks and marbles) and meta-igneous rocks (hornblende gneisses, amphibolites, metagabbros, pyroxenites and talcschists) recrystallized under high-pressure conditions.Chemically, the micaschists correspond to shales and greywackes similar to the Yaounde high-grade gneisses. 87Sr/86Sr initial ratios (0.7084–0.7134), moderately negative εNd(620Ma) values (−5.75 to −7.81), Nd model ages (1.66<TDM<1.74Ga) and radiometric ages point to the conclusion that the Yaounde basin was filled with siliciclastic sediments derived from both reworked older continental crust (Palaeoproterozoic to Archaean in age) and Neoproterozoic juvenile volcanogenic material. This occurred in the same time span (625–1100Ma) as the deposition of the Lower Dja, Yokadouma, Nola and Mintom series (Tonian–Cryogenian). Dolomitic marble associated with mafic/ultramafic rocks and characterized by high Cr (854–1371ppm) and Ni (517–875ppm) contents, are considered to result from chemical precipitation in relation with submarine magmatic activity.Talcschists (orthopyroxenitic to harzburgitic in composition) show primitive-mantle-normalized multi-element patterns with significant negative Nb–Ta anomalies, and slopes similar to that of average metasomatically altered lithospheric mantle. These rocks could be mantle slices involved in the collision tectonics. Amphibolites show the compositions of island-arc basalts with systematic negative Nb–Ta anomalies, 87Sr/86Sr initial ratios mostly <0.7047 and positive εNd(620Ma) values (+1.41 to +6.58). They are considered to be the expression of incipient oceanisation to the north of the Congo craton during the early Neoproterozoic. Hornblende gneisses show andesitic compositions, with high 87Sr/86Sr initial ratios (0.7105 and 0.7125) and low εNd(620) values (−14.0 and −20.7) suggesting that their genesis involved juvenile and recycled older crustal materials. Syn-metamorphic metagabbro (Mamb) and metadiorite (Yaounde) intrusions show negative Nb–Ta negative anomalies but enrichment in light rare-earth and large-ion lithophile elements, suggesting a metasomatized mantle source. Overall, meta-igneous rocks seem to be representative of distinct magmatic events that accompanied the evolution of the Yaounde sedimentary basin, from opening and oceanisation to convergence and closure in relation with the collisional process.These data suggest that the Yaounde basin should not be considered as a back-arc basin, but more likely represents the expression of extensional processes to the north of the Congo craton, which led to rifting, fragmentation and limited oceanisation. In this view, the Adamawa-Yade block may represent a micro-continent detached from the Congo craton during the early Neoproterozoic.

On the occurrence of gold mineralization in the Pala Neoproterozoic formations, South-Western Chad

The Pala region, in southwestern Chad, belongs to the northern part of the Central African Pan-African Fold Belt. It is made up of greenschist-facies schists and is characterized by bimodal, mainly mafic, magmatism. This schist unit named Goueigoudoum Series is intruded by pre- to post-tectonic plutonic rocks dated between 737 and 570Ma and dykes of quartz. Gold is mined artisanally from alluvial deposits and primary chalcopyrite–pyrite-bearing quartz veins, brecciated and silicified zones and shear zones. The majority of the mineralized shear zones and some quartz veins generally trend N–S to NNE–SSW or NW–SE and are interpreted as extensional shear fractures related to regional NE–SW-trending sinistral strike–slip shear zones. The geological context of the Pala region clearly indicates hydrothermal fluids formed along active continental margins during collisional orogenesis, and subsequent associated fluid migration typically occurred during strike–slip events. Although the origin of fluids may be varied (magmatic, metamorphic or meteoric fluids, Proterozoic seawater, or sedimentary basin formation waters), the distribution of the mineralizations along the granitoid intrusions suggests that magmatism played a major role in the dynamics of the mineralizing fluids.

Geometric analysis of superposed folds in the Kimbi area (Cameroon Pan-African fold belt) based on the Fold Profiler method

Fold shape analyses using the program Fold Profiler method are presented for folds with interference in the Kimbi area with the aim of deciphering their geometry. Two types of functions, conic sections and cubic Bézier curves, were fitted to the natural fold profiles to analyse the fold shape. Two dominant families have been distinguished: parabolic folds for F 1 folds and parabolic to elliptical folds for F 2 and F 3 folds. We also use Ramsay’s scheme ( t ′ a versus α plot) for classifying the geometry of the folded layers. In this classification, F 1 and F 3 folds belong to class 1C and class 3 folds while F 2 are of classes 1B, 1C and 3. Class 1B folds are formed by buckling while class 1C folds are formed by flattening of parallel folds. The variation of fold shape can be explained by the variation in the rheological properties of the folded layers as a result of changes in physical conditions, or by changes in the kinematics of the folding processes. These data, which are the first ever folding results in this region, can be integrated into the tectonic evolution models Northwestern Cameroon’s Pan-African fold belt.

Petrogenesis of peraluminous magmas from the Akum-Bamenda Massif, Pan-African Fold Belt, Cameroon

Peraluminous intrusives of the Akum-Bamenda Massif, Pan-African Belt, Central Cameroon, were synkinematically emplaced in a Pan-African sinistral strike-slip shear zone. The rock sequences consist of medium-grained leucogranites, fine-grained leucogranites, and orthogneisses of biotite granite composition; in aggregate, they cover a range from about 65 to 74 wt.% SiO2, defining a continuous chemical evolutionary trend and displaying characteristics of the high-K and medium calc-alkaline series. Leucogranites are strongly peraluminous (A/CNK > 1.1) and plot in the field of S-type granites, whereas orthogneisses are metaluminous and plot in the field of I-type granitoids. Major and trace element compositions and the Rb/Sr isotopes of the leucogranites indicate crustal derivation by remelting of a composite metapelite + metagreywacke protolith similar to the metasedimentary rocks of the central domain of the Cameroon Pan-African North-Equatorial fold belt.

Geochemistry of magmatic rocks and time constraints on deformational phases and shear zone slip in the Méiganga area, central Cameroon

The Méiganga area is situated in the Adamawa–Yadé domain (AYD) of the Pan-African fold belt in Cameroon. The AYD is characterized by abundant plutonic rocks that intruded Palaeoproterozoic gneisses. It is cut by the transcurrent Central Cameroonian Shear Zone (CCSZ). The studied area is made up of metadiorite (MD), pyroxene-bearing granite (PGr) and biotite-muscovite granite (BMGr), hosted in a metasedimentary sequence of amphibole-biotite gneisses. The granitoids are metaluminous to slightly peraluminous, and mainly of I-type (ASI ≤ 1.1), representing a high-K calc-alkaline to shoshonitic suite. They were derived from crustal protoliths (BMGr), from rocks of mantle origin (MD, PGr), and/or from the remelting of metabasalt or metatonalite (MD, PGr). Four successive deformational phases (D1, D2, D3, and D4) are present in the Méiganga area. The S1 foliation is formed by tectonic transposition of the S0 primitive surface represented by contacts between the gneiss and intercalated amphibolites. The synmigmatitic D2 deformational phase is characterized by S2 banded schistosity, S2/C2 sinistral shear planes, and F2 folds with axes parallel to a L2 stretching lineation. Imprints of the D2 and subsequent deformational phases are similar in the metadiorite and host rocks, implying syn-D2 emplacement and crystallization of the metadiorite; therefore 614–619 Ma 207Pb/206Pb zircon evaporation ages obtained for this rock type date the syn-D2 magmagenesis. Similarly, the D3 phase of the PGr is 601 ± 1 Ma, dated by the 207Pb/206Pb evaporation method. D4 is a late-stage brittle deformational phase. Sinistral movement of the CCSZ is associated with D2, whereas its latest activity, characterized by dextral slip, cannot be older than emplacement of the 558 ± 2 Ma BMGr (207Pb/206Pb zircon evaporation age).

Evidence for symmetric extension and exhumation of the Yaounde nappe (Pan-African fold belt, Cameroon)

It is commonly assumed that the Yaounde nappe was deformed and emplaced onto the Congo craton during two compressional Pan-African deformation phases ( D 1 and D 2). A detailed field and microstructural study of the mica schists, gneisses and pyriclasites of the Yaounde nappe in the Yaounde area was carried out and it appeared, however, that the D 2 deformation phase more likely corresponds to a major N–S and moderate E–W ductile pure shear extension, rather than compression. This was inferred from the study of various strain indicators, including the geometry of shear zones, the shape of boudins, the shape of pressure shadows, the orientation of deformed veins, of fold axes, the mineral lineations, as well as the preferred orientation pattern of quartz c-axes. The P– T conditions pertaining during this extensional phase were variably recorded by the mica schists (675 °C and 9 kbar), gneisses (800 °C and 12 kbar) and pyriclasites (750 °C and 10 kbar). Pure shear extension turned into minor top-to-the-south simple shear at lower temperature during retrogression into the greenschist facies. We therefore suggest that the syn- D 2 extension associated with subsequent uplift and erosion led to (Pan-African) exhumation of the Yaounde nappe. The D 1 phase of deformation is poorly known due to strong D 2 phase overprinting. However, the high P– T conditions indicated by the metasedimentary rocks in the area suggest that D 1 was compressional. The peak conditions of the related prograde metamorphism correspond to D 1– D 2 transition at the amphibolite–granulite interface. We propose that D 1 and D 2 respectively represent the compressional and extensional phases of a Pan-African tectono-metamorphic cycle within the Yaounde nappe.

The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton

Since 1980, several diverging models for the Pan-African fold-belt of the Anti-Atlas mountains have been proposed. Differences mainly result from diverging interpretations of geological, geochemical and geochronological data. However, the fundamental point is the lack of a detailed lithostratigraphic framework of the Neoproterozoic successions, supported by geochronological data and regional correlations. Furthermore, these models did not evolve from multidisciplinary studies which are essential in the case of this belt, whose structure is complex and diversified. We discuss here some points taken as a base by Ennih and Liegeois to conclude that the entire Anti-Atlas is underlain by Eburnian crust and that the northern limit of the West African craton (WAC) and its passive margin is located at the South Atlas Fault (SAF). This led the authors to propose a new model of the Anti-Atlas belt, which also explains the origin of the Khzama-Bouazzer ophiolite. 1. Evidence for a Palaeoproterozoic basement in Saghro?