Pan-African Domain Research Articles

Deep crustal architecture of the eastern limit of the West African Craton: Ougarta Range and Western Algerian Sahara

Satellite gravimetric data is considered one of the most powerful and effective tool for regional geodynamic-structural zonation. The limit over the West African Craton to the Pan-African domain discussed in this paper has intricate geodynamic features that include collision tectonics, subduction and rift opening events, polyphased structural patterns made up of oceanic-continental reversed blocks of different ages, and a mosaic of several gravity and magnetic anomalies. This region is endowed with natural gas, coal and a variety of economic mineral deposits. In this study, a comprehensive analysis of the gravity data was used to construct models and maps to delineate localized deep suture zone and key proprieties of the lower crust within the Ougarta Range, Western Algerian Sahara. The results of density distribution constrained using several geological information was utilized to produce a new integrated tecto-geodynamic map of the Ougarta area. Analysis of transformed gravity maps and geological indicators show that the West African Craton and Pan-African domain converged obliquely with continuous eastward subduction. The observed layout of the suture zone in this study corresponds to the trajectory mapped since 2050 My. Guided evaluation of the resulting structural maps indicate the presence of positive and negative flower structures, typical of strike-slip movements along the eastern border of the West African Craton.

Mineralogical and geochemical signatures of surface stream sediments from Dibamba River basin, SW Cameroon: Implications for provenance, weathering, and tectonic setting

The Dibamba River (DR) basin is one of the major fluvial basins of the Cameroon coastal area. The current work presents the very first systematic study of elemental geochemical characteristics, including REE, of surface sediments collected from the river bed at different parts of the river courses to determine the intensity of weathering, provenance, and tectonic settings, and evaluate the features that most faithfully mirror the nature of the source terrane. The sediments are dominantly fine and coarse sands. The framework components mainly consist of quartz. The clay minerals comprise kaolinite and illite. Opaque minerals, associated with kyanite, zircon, sillimanite, garnet, and hypersthene, are present as common accessory minerals. The DR sediments are classified geochemically as Fe-shale and Fe-sand. The CIA (72.04 ± 7.1), PIA (66.1–89.2), CIW, and the A–CN–K and (A-K)-C-N plots indicate moderate to high intensity of chemical weathering in the source areas. The SiO2/Al2O3, Al2O3/Na2O, K2O/Na2O ratios, and ICV indicate that the samples were predominantly texturally and compositionally mature. The ZTR index and the ternary Al2O3–TiO2–Zr and binary Th/Sc versus Zr/Sc diagrams suggest recycling and sorting of almost all the sediments. The REE patterns and the size of Eu, and various ternary and bivariate plots such as Zr versus TiO2, Th/Co versus La/Sc, and Cr/V versus Y/Ni indicate the contribution of sediments from intermediate and felsic source rocks. The tectonic discriminant-function-based diagrams revealed a rift or a passive margin setting for the source region. This is consistent with the tectonic history of the Pan-African domain and the Douala Cretaceous sub-basin, which are the probable source terranes of the studied stream sediments.

Crustal thickness, depth to the bottom of magnetic sources and thermal structure of the crust from Cameroon to Central African Republic: Preliminary results for a better understanding of the origin of the Bangui Magnetic Anomaly

The Bangui Magnetic Anomaly (BMA) is one of the largest magnetic anomalies in the world whose origin is still not known. This research investigated the crustal thickness, Curie depths and thermal structures in the Central African sub-regions – Cameroon, Central African Republic and adjacent countries – which are largely characterized by the Bangui Magnetic Anomaly. To achieve a better understanding and clearer idea of the location of the possible sources of the BMA, analyses of geothermal structures were conducted. Two potential methods were used: gravity to evaluate the crustal thickness and magnetics for geothermal analysis. Spectral analysis of gravity data shows that crustal thickness range between 14 and 55 km. The highest depths were found in Central African Republic. The lower values of crustal thickness were obtained in South-Chad basin with a minimum of roughly 14 km. Geothermal analysis is carried out using the Curie point depth, thermal gradient and heat-flow evaluations. The results show that the BMA is related to a thick crust of roughly 40 km. Depth to the bottom of possible sources does not exceed the lower crust. The mean Curie point depth estimated is 38 km with an error of ±2 km. Geothermal results also show the difference in the thermal behaviour between the crust in the Pan African and Precambrian domain. The mobile zone which constitutes the Pan African domain is associated with a thin crust of high heat-flow values of 65 mW/m2. However, the Precambrian domain beneath the BMA is associated with a thick crust with lower heat-flow values (roughly 45 mW/m2). The difference between crustal thickness and Curie point depths shows that all the sources of the BMA are crustal. The present results are in favour of a geological origin for the Bangui Magnetic anomaly.

Mineralogy and geochemistry of sands of the lower course of the Sanaga River, Cameroon: implications for weathering, provenance, and tectonic setting

This study focuses on the mineralogy and bulk chemical composition of 19 sediment samples, collected from the Sanaga River bed, between Nanga-Eboko and the Atlantic Ocean in Cameroon, to infer provenance, weathering, and tectonic setting. The textural analysis revealed that these sediments are mainly coarse-grained sands. X-ray diffraction and heavy mineral analyses show that these sands are made of quartz, microcline, plagioclase, muscovite, ilmenite, anatase, magnetite, opaques, and epidote. The Index of Chemical Variability values (1.07–2.68) indicates that the sands are immature. The Chemical Index of Alteration (53–66 %), Plagioclase Index of Alteration (PIA; 57–75 %), and Mafic Index of Alteration (54–67 %) values revealed a moderate intensity of weathering for these sediments. According to the compositional maturity diagram, the sands are mainly litharenites. The studied sands show enrichment in light rare earth elements relative to heavy rare earth elements and a negative anomaly in Eu (Eu/Eu* = 0.32–0.83). These sands were derived from felsic metamorphic rocks dated Meso to Neoproterozoic from the Pan-African (700–1000 Ma) domain of south Cameroon. Th/U ratios (mean = 5.40; n = 19) reveal that these sands originating from felsic source rocks and are low to moderately recycled. On the tectonic discrimination diagrams, all the samples are plotted in the rift and passive margin domains, which is consistent with the tectonic history of Pan African in southern Cameroon.

Improved climatological precipitation characteristics over West Africa at convection-permitting scales

The West African climate is unique and challenging to reproduce using standard resolution climate models as a large proportion of precipitation comes from organised deep convection. For the first time, a regional 4.5 km convection permitting simulation was performed on a pan-African domain for a period of 10 years (1997–2006). The 4.5 km simulation (CP4A) is compared with a 25 × 40 km convection-parameterised model (R25) over West Africa. CP4A shows increased mean precipitation, which results in improvements in the mature phase of the West African monsoon but deterioration in the early and late phases. The distribution of precipitation rates is improved due to more short lasting intense rainfall events linked with mesoscale convective systems. Consequently, the CP4A model shows a better representation of wet and dry spells both at the daily and sub-daily time-scales. The diurnal cycle of rainfall is improved, which impacts the diurnal cycle of monsoon winds and increases moisture convergence in the Sahel. Although shortcomings were identified, with implications for model development, this convection-permitting model provides a much more reliable precipitation distribution than its convection-parameterised counterpart at both daily and sub-daily time-scales. Convection-permitting scales will therefore be useful to address the crucial question of how the precipitation distribution will change in the future.

Evidence of Major Structural Features over the Pan-African Domain in the Bertoua-Mbangue Area (East Cameroon) from a Multiscale Approach of Modeling and Interpretation of Aeromagnetic Data

The aim of this study is to investigate crustal structures from East Cameroon, using aeromagnetic data. The modeling of aeromagnetic data is conducted using the Oasis Montaj 8.0 software. The total magnetic intensity map reduced to the equator (RTE-TMI) shows important anomalies features, namely, the Northern East magnetic anomalies of high amplitude, the Southwest where very low values of the magnetic intensity were observed, and a corridor with negative values relatively high, separating the anomalies. The horizontal gradient map shows on the one hand brittle and folded structures carried out in the area of study and on the other hand various rectilinear, narrow, and short-wave anomalies that can be classified as a family of little faults. The maxima observed on the RTE-TMI maps are correlated to intrabasement contacts; and the map derived from Euler’s solutions permitted to evaluate the depth of the geological accidents observed from the other filters. This map also reveals new faults with a depth greater than 5000 m. The lineaments identified in the Southwestern part could be linked to the Pan-African orogeny and seem to correspond to deep-seated basement structures, which are referred to the tectonic boundary between Congo Craton and the Pan-African orogeny belt. A 23/4-D modeling confirmed the observations derived from the RTE-TMI and HGM maps analyses. It shows intrusive bodies composed of gneiss and porphyroid granite and some domes with their roof situated at various depths not exceeding 1800 m from the surface. The structural map of the study area shows the trending of the structural features observed, namely, NE-SW, NW-SE, ENE-WSW, and WNW-ESE, respectively, while the E-W and N-S are secondary orientation of the observed tectonic evidence. Moreover, circular anomalies observed over the area are assimilated to intrusions of high magnetic materials or to granitic domes.

Crustal Structure and Tectonic Setting over the Panafrican Domain in Loum-Minta Area (Centre-East Cameroon) from Aeromagnetic Analysis

The present work aims to determine the geological structure, to highlight and to determine the characteristics of the fault system responsible for the current structure of the study area through the interpretation of available aeromagnetic data. Total magnetic intensity anomaly (TMI) was critically interpreted using several analysis techniques including Reduction to Equator (RTE), First Vertical Derivative, upward continuation, spectral analysis and 2D3/4 modelling. All results obtained from the interpretation process were combined together to draw an interpretative geological map of the area and allow the general view of the surface and sub-surface structures. The interpretative geological map reveals that the geological formations of the studied area appear to be intensely fractured by an E-W, ENE-WSW and NE-SW main orientation fault system. The lineaments identified in the area study could be linked to the Pan-African orogeny and seem to correspond to deep-seated basement structures, which are referred to the tectonic boundary between Congo Craton and the Pan-African orogeny belt. According to spectral analysis results, the depths of the sources of superficial and deep magnetic anomalies are 2500 m and 12,000 m respectively. The 2D3/4 modelling of one magnetic profile plotted on the reduced residual map at the equator was performed to approximate the geometry and depth of the sources of magnetic anomalies, the model suggests the intrusion of a large body of high susceptibility during the continental collision. The results of this study can be used to better understand deep-seated basement structures and to support decisions with regard to the development of industrial areas, as well as of hydrogeological and/or mining investigations to be undertaken in the study area.

Re-Os isotopic constraints on the evolution of the Bangong-Nujiang Tethyan oceanic mantle, Central Tibet

Geochemical (including Re-Os isotopic) studies of the mantle rocks of ophiolites in the Bangong-Nujiang suture zone in central Tibet have provided a coherent picture of the evolution of the Bangong-Nujiang Tethyan oceanic mantle from mid-ocean ridge (MOR) to subduction-zone (SSZ) settings.Clinopyroxene (cpx)-harzburgites and lherzolites in the Bangong Lake ophiolite were formed in a MOR setting, as demonstrated by the Cr# of spinels (<0.60) and whole-rock LREE-depleted patterns. Suprachondritic 187Re/188Os ratios (up to 1.833) of cpx-harzburgites and their spinels can be explained by interaction with melts derived from high Re/Os sources. Re-depletion (TRD) model ages (0.48-0.55Ga) suggest these rocks may represent a Pan-African domain beneath the Gondwana continent. High TiO2 contents of spinels and whole-rock samples imply that the lherzolites were formed through a refertilization process. Similarly, Re-Os isotopic systematics of sulfides in the lherzolites (187Re/188Os: 0.173-1.717, 187Os/188Os: 0.12646-0.17340) demonstrate that they are mixtures of primary and secondary sulfides. 187Os/188Os ratios (0.1211-0.1226) of whole-rock lherzolites give TRD ages of 0.73-0.97Ga, indicating the presence of Neoproterozoic lithospheric mantle under the spreading ridges.Mantle rocks in the SSZ-type ophiolites from Bangong Lake, Dongqiao and Nagqu reflect the complex evolution of the Bangong-Nujiang oceanic mantle during the SSZ stage. Most harzburgites from the Bangong Lake ophiolite give TRD ages of 1.0-1.5Ga, possibly representing relics of a Mesoproterozoic lithospheric mantle. However, three samples have both high Os contents (1.32-4.45ppb) and near-chondritic 187Os/188Os (0.1260-0.1297), and may represent Mesozoic oceanic lithospheric mantle. 187Os/188Os ratios of dunites and harzburgites from the Dongqiao and Nagqu ophiolites vary from 0.1174 to 0.1316 and give TRD ages up to 1.43Ga, also suggesting the existence of a Mesoproterozoic lithospheric mantle which has experienced mantle refertilization and later metasomatism caused by percolation of several generations of melts and/or fluids.We interpret the old mantle domains in the Bangong-Nujiang ophiolite belt as relics of ancient sub-continental lithospheric mantle (SCLM), which may have survived during the opening of the Bangong-Nujiang Tethyan ocean basin, and subsequently were incorporated into Mesozoic oceanic lithospheric mantle.

Heat-flow and subsurface temperature history at the site of Saraya (eastern Senegal)

Abstract. New temperature measurements from eight boreholes in the West African Craton (WAC) reveal superficial perturbations down to 100 m below the alteration zone. These perturbations are both related to a recent increase in the surface air temperature (SAT) and to the site effects caused by fluid circulations and/or the lower conduction in the alterites. The ground surface temperature (GST), inverted from the boreholes temperatures, increased slowly in the past (~0.4 °C from 1700 to 1940) and then, more importantly, in recent years (~1.5 °C from 1940 to 2010). This recent trend is consistent with the increase of the SAT recorded at two nearby meteorological stations (Tambacounda and Kedougou), and more generally in the Sahel with a coeval rainfall decrease. Site effects are superimposed to the climatic effect and interpreted by advective (circulation of fluids) or conductive (lower conductivity of laterite and of high-porosity sand) perturbations. We used a 1-D finite differences thermal model and a Monte-Carlo procedure to find the best estimates of these site perturbations: all the eight borehole temperature logs can be interpreted with the same basal heat-flow and the same surface temperature history, but with some realistic changes of thermal conductivity and/or fluid velocity. The GST trend observed in Senegal can be confirmed by two previous borehole measurements made in 1983 in other locations of West Africa, the first one in an arid zone of northern Mali and the second one in a sub-humid zone in southern Mali. Finally, the background heat-flow is low (31±2 mW m−2), which makes this part of the WAC more similar with the observations in the southern part (33±8 mW m−2) rather than with those in the northern part and in the Pan-African domains where the surface heat-flow is 15–20 mW m−2 higher.

Zircon ages and Hf isotopic systematics reveal vestiges of Mesoproterozoic to Archaean crust within the late Neoproterozoic–Cambrian high-grade terrain of southernmost India

Late Neoproterozoic to Cambrian ages have been widely recorded from the Southern Granulite Terrain (SGT) of India and were correlated with global tectono-thermal (Pan-African) events associated with amalgamation of the Gondwana supercontinent. We report results of single zircon dating from a charnockite and a granitic augen–gneiss from the SGT using SHRIMP and evaporation techniques, complemented by zircon Hf isotopic systematics. The SHRIMP data for zircons in the charnockite are spread along a well-defined discordia line suggesting variably strong lead–loss during the late Neoproterozoic Pan-African event. A concordant grain yielded a 207Pb/206Pb age of 1893±13Ma, and the upper intercept of the discordia line for all analyses with Concordia is at 1881±14Ma. The concordant grain age is considered to most closely reflect the time of emplacement of the charnockite protolith. The imprecise lower Concordia intercept at 572±25Ma is likely to reflect Pb-loss during granulite-facies metamorphism. Evaporation of four zircons yielded consistent 207Pb/206Pb isotopic ratios at high evaporation temperatures of ca. 1598°C, suggesting an age of ca. 1450Ma. This date is clearly much younger than the SHRIMP result and suggests that the evaporated grains no longer contained their original, magmatic 207Pb/206Pb ratios but record strong Pb-loss as also shown by the SHRIMP data. Lu–Hf isotopic analysis of ten spots on zircon grains previously analyzed by SHRIMP II yielded strongly negative εHf(t)-values of −6.1 to −9.2 and mean Hf crustal residence ages of 2.95 to 3.14Ga.Eight grains from the augen–gneiss analyzed on SHRIMP II also show variable but considerable Pb-loss. A concordant grain yielded an age of 1557±16Ma, and the other grains define a discordia line with an upper Concordia intercept at 1553±21Ma. Two unzoned grains produced a combined mean 206Pb/238U age of 583±6Ma and reflect the high-grade late Neoproterozoic metamorphic event. A concordant 207Pb/206Pb age of 2043±13Ma was obtained for a xenocryst. Evaporation of five large zircons from this sample yielded a precise mean 207Pb/206Pb age of 1556.0±0.5Ma, identical to the result for the SHRIMP-dated concordant grain. Lu–Hf isotopic analysis of ten spots on zircon grains from where SHRIMP data were gathered yielded strongly negative εHf(t)-values of −6.0 to −17.0 and mean Hf crustal residence ages of 2.68 to 3.37Ga.Our results suggest that not all rocks in the so-called Pan-African domain of southern India are Neoproterozoic in age, and that there could be more Mesoproterozoic to Archaean crust hidden in the “sea” of Neoproterozoic granulites, granitoids and migmatites than so far recognized. The Hf isotopic data reveal both granitoid samples to be derived from crustal melts generated from Archaean continental crust, and the chemistry of these rocks is inherited from their ancient source. We compare the zircon ages with those of similar rocks in the Highland Complex of Sri Lanka and discuss their relevance as vestiges of an older continental terrain incorporated within the late Neoproterozoic–Cambrian collision belt of East Gondwana. In view of rock and age similarities we tentatively suggest links between the Indian Trivandrum and Madurai Blocks and the Sri Lankan Highland and Wanni Complexes.

Pan-African strike–slip tectonics in eastern Cameroon—Magnetic fabrics (AMS) and structure in the Lom basin and its gneissic basement

Field, microstructural, and anisotropy of magnetic susceptibility (AMS) or magnetic fabric studies were applied to identify the sequence and character of the Pan-African structures in the basement of Eastern Cameroon at both sides of the regional scale Bétaré-Oya Shear Zone (BOSZ). The NE-SW trending BOSZ separates older gneisses and migmatites towards SE (domain I) from the younger rocks of the Lom meta-volcano-sedimentary basin towards NW (domain II). In domain I, early, ductile compressional deformation occurred in two events, D1 and D2, under relatively high T conditions. During subsequent cooling, strain partitioned between the competent basement gneisses with only mild compression and the bordering shear zone (BOSZ) with intense simple shear-wrenching (D3). Strain in the less competent rocks of domain II is dominated by simple shear, strike-slip wrenching (D3), with an earlier stage of compressional deformation preserved only in some low strain pods. Magnetic fabrics (AMS) document a progressive change from oblate ellipsoids towards prolate ellipsoids in domain I, when proceeding from the south towards the BOSZ. Foliations are mostly steep but define a girdle with a pole plunging gently towards WSW. The magnetic lineations also plunge mostly towards WSW at shallow angles. These fabrics indicate a compression approximately normal to the BOSZ, which is also the SE margin of the Lom Basin. In the Lom metasediments (domain II), AMS ellipsoids are typically oblate. Foliations trend NE-SW with mostly steep dips. Magnetic lineations plunge gently NE or SW. This fabric with foliations mostly steep and subparallel with the major BOSZ, combined with generally subhorizontal lineations implies the BOSZ as a Pan-African strike–slip shear zone with a subordinate component of compression. At a larger scale, the area is part of a continent-scale shear zone, separating external Pan-African domains of compression along the northern margin of the Congo craton from internal domains dominated by high-angle strike–slip and transpressional deformation. Together with published data, the present study thus demonstrates that transpression is a regional phenomenon in the Pan-African orogen of central and eastern Cameroon.

Deep structure of the southeastern margin of the West African craton from seismic reflection data, offshore Ghana

Abstract Seismic reflection data, acquired as part of an oil exploration survey offshore Ghana, have been reprocessed to reveal the deep structure of the Proterozoic continental crust on the continental shelf, off West Africa. The seismic profiles presented are located across the Romanche transform margin, which transects the West African craton (WAC) margin where it is bounded by the Pan-African Dahomeyide orogen. The seismic data reveal highly reflective middle–lower crust of the WAC but a nearly transparent upper crust of c . 10 km thickness. Bundles of discontinuous reflections are located at the base of the unreflective upper crust and locally define the trough of a synformal structure interpreted as the keel of a Palaeoproterozoic (Birimian) greenstone belt exposed onshore. The deepest crust imaged is characterized by more continuous, subhorizontal reflections that are very similar to those observed in the lower crust of cratonic areas such as the Superior Province. Near the WAC margin the subhorizontal reflections are truncated by east-dipping reflections in the Pan-African domain, which are correlated with cratonward directed fold-and-thrust structures formed during the c . 600 Ma Pan-African orogeny. These seismic observations represent, to date, the first deep seismic reflection images of the West African craton margin and the continental lithosphere in the region.

Archaean-Cambrian crustal development of East Antarctica: metamorphic characteristics and tectonic implications

Abstract The East Antarctic Shield consists of a variety of Archaean and Proterozoic-Cambrian high-grade terranes that have distinct crustal histories and were amalgamated at various times in the Precambrian-Cambrian. High-grade Pan-African tectonism at 600–500Ma is recognized from four distinct belts: the Dronning Maud Land, Lützow-Holm Bay, Prydz Bay and Denman Glacier Belts. These high-grade belts juxtapose distinct Mesoproterozoic and Neoproterozoic crustal provinces (Maud, Rayner and Wilkes), the Rauer Terrane, and have also marginally affected Archaean cratonic remnants in the Napier Complex and southern Prince Charles Mountains. The Wilkes Province experienced its principal tectonothermal events prior to 1130Ma and was not affected by the younger events that characterize the Maud Province (1150 and 1030–990Ma), the Rayner Province (990–920Ma) and the Rauer Terrane (1030–990Ma). These differences between the isotopic/event records of the basement provinces now separated by the Pan-African belts require that the older provinces were not formerly parts of a continuous ‘Grenville’ belt as proposed in the SW US-East Antartic model. East Antarctica was not a single unified crustal block within either East Gondwana or Rodinia until the Cambrian, which is now demonstrated to be the key phase of high-grade and ultrahigh-temperature (UHT) metamorphism associated with supercontinent assembly. The high-grade Pan-African tectonism is characterized by extensive infracrustal melting, clock-wise P-T paths, rapid post-peak exhumation along isothermal decompression paths to shallow- or mid-crustal levels by 500 Ma and the generation, at least locally, of UHT conditions. A significant flux of heat from the mantle into the deep and initially overthickened crust is required to produce these observed metamorphic effects. Whilst the thermal evolution can be explained by models that invoke the removal of most of the lithospheric mantle following crustal thickening and prior to rapid extension of the remaining crust, these one-dimensional models are inconsistent with present crustal thicknesses of 25–35km in the Pan-African domains of the East Antarctic Shield.

Cratons, mobile belts, alkaline rocks and continental lithospheric mantle: the Pan-African testimony

Several late-collision and intraplate features are not entirely integrated in the classical plate tectonic model. The Pan-African orogeny (730–550 Ma) in Saharan Africa provides some insight into the contrasting behaviour of cratons and mobile belts. Simple geophysical considerations and geological observations indicate that rigidity and persistence of cratons are linked to the presence of a thick mechanical boundary layer, the upper brittle part of the continental lithospheric mantle, well attached to an ancient weakly radioactive crust. The surrounding Pan-African mobile belts, characterized by a much thinner mechanical boundary layer and more radioactive crust, were the locus of A-type granitoids, volcanism, tectonic reactivation and basin development during the Phanerozoic. During oceanic closures leading to the assembly of Gondwana, lithosphere behaviour was controlled by its mechanical boundary layer, the crust being much less rigid. We suggest that the 5000 km wide Pan-African domain of Saharan Africa, a collage of juvenile and old reactivated basement terranes, has suffered regional continental lithospheric mantle delamination during the early stages of this orogeny, as has been postulated for the more recent Himalayan orogeny in Tibet. Delamination of the continental lithospheric mantle and juxtaposition of crust against hot asthenosphere can explain many features of the late Pan-African (around 600 Ma): reactivation of old terrains, abundant late-tectonic high-K calc-alkaline granitoids, high temperature-low pressure metamorphism, important displacements along mega-shear zones and mantle-derived post-tectonic granitoids linked to a rapid change in mantle source. Recycling into the asthenosphere of large amounts of continental lithospheric mantle delaminated during the Pan-African, can provide one of the reservoirs needed to explain the isotopic compositions of ocean island basalts. Lastly, the lithospheric control over the location of the alkaline rocks enjoins us to consider the thermal boundary layer (the lower ductile part of the continental lithospheric mantle) as a major mixing source zone for these rocks.

U-Pb geochronology of potassic syenites from Southwestern Nigeria and the timing of deformational events during the Pan-African Orogeny

Two syenite bodies from south-western Nigeria considered to be pre to syntectonic (Iwo) and late tectonic (Okeiho) with respect to the dominant N-S trending structures that characterize the Nigerian Percambrian basement were dated by the U-Pb zircon method. However, they yield Pan-African emplacement ages of 610 ± 7 Ma and 618 ± 4 Ma respectively which are indistinguishable within limits of the experimental errors. This suggests that the Pan-African deformation in the region was either migratory or heterogeneous in nature. Comparison of these results with data from other parts of the Pan-African domain east of the West African craton provides further evidence of the synchronous nature of the peak of Pan-African magmatism (630-600) in the area.

The Togo-Benin-Nigeria Shield: evidence of crustal aggregation in the Pan-African belt

The importance of “suspect” or “exotic” (i.e. allochthonous) terranes as a major element in collisional orogenic belts is becoming well established. We propose that the southern Pan-African domain in West Africa is an aggregation or “ mosaic” of island arcs, interarc (ensimatic) basins and continental fragments. A fracture zone in northwestern Nigeria, already identified as a possible crustal suture, is shown to separate two contrasted basement terranes. Elsewhere in the shield are flat-lying structures characteristic of those associated with crustal convergence, lying within or near to major fractures. Many small ultramafic/mafic bodies occur in the shield and some of them may be remnants of ophiolites caught up in suture zones. An aggregation of allochthonous terranes (island arcs, sedimentary basins and continental blocks) would help to explain: 1. (a) the great width of the Pan-African belt; 2. (b) the spread of ages within the “Pan-African” range (c. 750-450 Ma), also the relict Liberian and Eburnian ages (c. 2700 and 2000 Ma respectively); and the enigmatic Kibaran “event” (c. 1100 Ma); 3. (c) the contrasted volcano-sedimentary characteristics of the different supracrustal belts.

Late Proterozoic to Early Palaeozoic supracrustal succession of Sierra Leone: an aulacogen at the western margin of the West African Craton

Three parallel zones of Upper Proterozoic to Lower Palaeozoic sedimentary-volcanic cover rocks have been recognized in western Sierra Leone. The Kasila Group m the SW ist metamorphosed to the granulite facies and grades into the Marampa Schist Group, and this in turn grades into the unmetamorphosed Rokel River Group in the NE. Prior to this investigation, these rocks were thought to belong to different ages; the most highly metamorphosed was thought to be the oldest and the unmetamorphosed, the youngest. This view is unrealistic since field mapping has failed to reveal any sharp contacts between them and all available age data indicate their involvement in the Pan-African (ca. 550 Ma) Orogeny A model is proposed involving an intracontinental graben (aulacogen) extending from western Sierra Leone into the adjacent territories developed about 1000 Ma ago. During the Pan-African Orogeny a portion of the aulacogen fill overlying the remobilized basement was subjected to a regional metamorphism and deformation. The grade of metamorphism and the degree of deformation decreased steadily from the centre of the Pan-African domain to the contact with the West African craton. The unmetamorphosed and mildly deformed to undeformed equivalent rests on the craton.

The geodynamic significance of alkaline magmatism in the western Mediterranean compared with West Africa

Alkaline magmatic provinces occur globally in regions subjected to cratonization. Therefore, they may appear just after the end of an orogenic episode, when collision between two continental plates caused rapid uplift and unroofing of orogenic batholiths by isostatic adjustment under intense stress fields. A reversal in the sense of movement in a pre-existing fault system provoked alkaline magmatism. As reversals in the sense of movement are signs of global tectonics, it can be shown that at each orogeny an alkaline magmatism developed, essentially when relative plate motions became stationary. The Corsican alkaline province is a part of a larger western Mediterranean province, emplaced at the end of the Hercynian orogeny. Lithospheric faults along which melts from the mantle have ascended through the continental crust may subsequently act as preferential lines of weakness for oceanic basin development. In West Africa, alkaline provinces are essentially emplaced in Pan-African domains. A number of provinces are Cambrian in age, forming just after the termination of the Pan-African orogeny and located near major suture zones. Unlike the western Mediterranean province, the lithospheric faults, responsible for the ascent of the alkaline magmas, have not been reactivated in West Africa and no subsequent oceanic opening has occurred. In addition, Pan-African domains have been the sites of later alkaline provinces, during various time periods since the end of the Pan-African orogeny. Subsolvus granites predominate as early as 10 Ma after the last compressive tectonic phase, then hypersolvus granites supersede them during a 500 Ma period. Lastly, mixed and undersaturated complexes developed with increased proportions of carbonatites.

Permian alkaline undersaturated and carbonatite province, and rifting along the West African craton

Recent work1,2 carried out in north-east Mali, which showed the Pan-African orogeny (600 Myr) to be the result of a collision between the passive margin of the West African craton and the active margin of a continental mass situated to the east, has also outlined an entire province of alkaline undersaturated ring complexes and carbonatites, the first to be discovered in West Africa excluding Morocco3,4 (Fig. 1). This province is quite distinct petrologically and structurally from the Iforas alkaline younger granite province situated in the Pan-African domain east of the suture which has yielded Rb/Sr whole-rock ages of between 560 and 520 Myr (ref. 5). The undersaturated province is located along a well-defined rift on the edge of the West African craton which is totally devoid of Pan-African magmatism. We report here that the Permian radiometric age obtained for one of these intrusions shows the existence of a hitherto unsuspected important tectono-magmatic event in this part of Africa, which may have far-reaching structural implications as well as economic interest.

Metallogenesis of the Pan-African domain of the anti-Atlas, Morocco

Antérieurement à la déformation panafricaine, le socle paléoprotérozoïque et sa couverture calcaréo-quartzitique néoprotérozoïque de la boutonnière précambrienne de Zenaga (Anti-Atlas), sont recoupés par un complexe filonien de dolérites. Ces dolérites sont plus au moins transformées. La paragenèse primaire est formée par le plagioclase, le clinopyroxène, très rarement l'orthopyroxène, les oxydes ferro-titanés, l'apatite, la micropegmatite et parfois la hornblende et la biotite. L'étude minéralogique et géochimique montre que les dolérites sont des tholéiites continentales. Deux groupes ont été distingués en fonction des teneurs en terres rares, P2O5, Zr, Th, Ba et Sr, relativement plus élevées dans le groupe I que dans le groupe II, relativement plus riche en V. Le groupe I comprend les dykes orientés N-S et NW-SE, alors le groupe II correspond à ceux orientés NE-SW et E-W qui recoupent les premiers. Les variations chimiques s'expliquent par une fusion plus importante de la source mantélique pour le groupe II tardif. La mise en place des tholéiites continentales antérieurement aux plissements panafricains s'est effectuée durant une phase tectonique distensive qui a affecté la boutonnière de ZenagaBefore the Pan-African Orogeny, the Palæoproterozoic basement and its Neoproterozoic cover (limestones and quartzites) of the Zenaga Inlier were cross-cut by a swarm of doleritic dykes. They are more or less altered. The primary mineral assemblage consists of plagioclase, clinopyroxene, very rare orthopyroxene, ilmenite, apatite, micropegmatite and sometimes hornblende and biotite. Mineralogical and geochemical studies indicate that the dolerites are continental tholeiites. Two groups of dykes have been distinguished. Accordingly, rare earth elements, P2O5, Zr, Th, Ba and Sr contents are higher in group I than in group II, which is richer in V. Group I comprises the north-south and northwest-southeast swarms, while group II corresponds to northeast-southwest and east-west swarms, which were emplaced later. These geochemical variations may be explained by a higher degree of melting of the mantle source for the later group II. Doleritic dykes of Zenaga had been emplaced during an extensional episode, prior to Pan-African folding.