Pan-African Dahomeyide Orogen Research Articles

The architecture of the Buem Structural Unit: Implications for the tectonic evolution of the Pan-African Dahomeyide Orogen, West Africa

The Buem Structural Unit (BSU) occurs about 50 km west of the high-pressure (HP) rocks of the Pharusian suture zone (PSZ) of the Dahomeyide Orogen, West Africa. Shale and sandstone intercalations form the base of the BSU, followed by serpentinised peridotite, gabbro, massive and pillow basalts (fragment of oceanic crust), jasper/chert, and limestone/dolostone, with sandstone and diamictite forming the top of the BSU. The package was deformed at lower greenschist-facies conditions during the Pan-African orogeny. The N-S-trending BSU has been affected by three deformational events. The main deformation event D2, which transposed bedding-parallel D1 fabrics, produced a steeply ENE-dipping S2 tectonic foliation. S2 is axial planar to upright or west-verging isoclinal and chevron F2 folds. Top-to-the-west D2 shear zones in the sandstone, shale and serpentinised peridotite are associated with a down-dip ENE-plunging L2 stretching lineation. The mafic-ultramafic rocks occur as D2 tectonic slices, marking the boundaries between thrust sheets within the BSU. The D2-related N-S trending structural and short-wavelength (<1 km) magnetic grain of the BSU is superimposed on ENE-trending long-wavelength (>1 km) magnetic anomalies that are continuous along trend with the regional structures of the West African Craton (WAC) and are interpreted as signals from the basement rocks of the BSU. D2 deformation is interpreted to result from overthrusting of the BSU onto the southeastern margin of the WAC during collision of the WAC and Benino-Nigerian Shield (BNS). Meter-scale, steep and open F3 kink and crenulation folds with steeply SE-dipping axial planes are sporadically observed throughout the entire BSU. D3 deformation is inferred to reflect a period of NW-SE-shortening. Deformation history of the BSU shares structural similarities with the one of the PSZ. We, therefore, interpret the BSU as an allochthonous fold and thrust belt with a geodynamic evolution involving subduction of the WAC beneath the BNS and the ensuing collision between the two terranes. The tectonic framework of the BSU suggests a geodynamic evolution of the Dahomeyide Orogen evolving from initial accretion of oceanic crust and deposition of passive margin sedimentary sequences, and their inversion due to continent-continent collision during the Pan-African orogeny.

Two suites of gabbros in the Buem Structural Unit, of the Pan-African Dahomeyide orogen, southeastern Ghana: Constraints from new field and geochemical data

The Buem Structural Unit (BSU) of the Pan-African Dahomeyide orogenic belt, southeastern Ghana, is characterized by the occurrence of clastic and chemical sedimentary rocks, volcanic rocks and mafic-ultramafic rocks. The mafic rocks, comprising mainly gabbros occur in smaller proportion within the BSU. Representative samples of the gabbros have been analyzed petrographically and geochemically to infer their mode of emplacement, petrogenesis and tectonic setting. Two distinct gabbro types have been identified from petrography and whole-rock major and trace elements geochemistry, namely the B1 and B2 gabbros. The gabbros are generally phaneritic, holocrystalline and composed of plagioclase and clinopyronexe which have been replaced either completely or partially by sericite, chlorite and epidote. On the whole, B1 gabbros are deformed, altered, alkaline in nature, show no significant effect of crustal contamination and has affinity to OIB/E-MORB. The B2 gabbros, are relatively less deformed, subalkaline, akin to N-MORB and show arc signatures with minimal crustal contamination. The geochemical characteristics of the B1 gabbros are similar to the Buem volcanic rocks. The similarity of the B1 gabbros to the volcanic rocks may suggest a similar source and tectonic setting. Thus, the B1 gabbros may be related to rifting and emplacement at the eastern margin of the West African Craton (WAC). The effect of minimal crustal contamination and the arc nature of the B2 gabbros may suggest subduction related magmatism. Taken together, the occurrence of the gabbros may be related to rifting and subduction-collision at the eastern margin of the WAC.

The Aledjo Quartzitic Ruins in Northern Togo: Genesis and Structural Geomorphology

In northern Togo, the Aledjo Mountains represent a NE-SW to ENE-WSW trending segment of the Pan-African Dahomeyide orogen. This mainly quartzitic topography belongs to the external zone of the belt and, more precisely, to the eastern assemblage of the Atacora Structural Unit. It corresponds to a ruiniform landscape which resulted from an erosive sculpturing that started at the end of the Pan-African tectogenesis within the Dahomeyide Belt, at about 550 Ma. A first interpretation of these spectacular sculptures attributed their great variety to predisposition factors related to the petrographic heterogeneity of the quartzites and to the Pan-African tectogenesis. With reference to these genetic factors, the Aledjo quartzitic ruins are subdivided into two morphostructural groups related either to a combination of folding and erosion or fracturing and erosion. Moreover, the Aledjo ruiniform landscape may be considered as an attractive touristic target which can contribute to regional economic development in northern Togo. Keywords: Quartzitic Ruins, Aledjo Montains, Dahomeyide, Northern Togo, Morphogenesis

Geochemistry and Sm–Nd isotopic composition of the Agou Igneous Complex (AIC) from the Pan-African orogen in southern Togo, West Africa: Geotectonic implications

Major, trace element concentrations and Nd, Sr isotopic data are reported for Neoproterozoic high-pressure (HP) mafic granulites from the Agou Igneous complex (AIC), which marks the suture zone of the Pan-African Dahomeyide orogen in southwestern Togo. The main rock types consist of garnet-free and garnet-bearing metagabbronorites (GFM and GBM) that are cross cut by subordinate leucocratic to melanocratic veins and dykes of garnet-bearing microgabbroic rocks. Geochemical data suggest that the HP granulites may have preserved the geochemical imprints of their igneous protoliths. They display either cumulitic features or liquid compositions varying from primitive to evolved basalts and andesites and then to more differenciated dacitic magmas as supported by major and trace element contents. Primary basalts display LREE and LILE enriched patterns and no Eu anomaly, but are depleted in Y and Yb suggesting a slightly enriched mantle source or a weak crustal contamination. Mafic veins and dykes display REE flat patterns (LaN/YbN=0.58–1.7) and a slightly negative Eu anomaly and ∑REE content ranging between 10 and 20. These features are consistent with protoliths of N- to E-MORB affinities. Mafic cumulates and andesitic basalts are characterized by various REE patterns which could be compared to those of the primary and/or the evolved basalts, probably identifying various igneous suites. The fractionated REE patterns (Ce/YbN≈11.0–17.9) with slight LREE enrichment (Ce/SmN≈3.1–3.8) and HREE depletion (Sm/YbN≈3.6–4.8) could be induced by a garnet residue in the melt source. Neodymium isotopic data show that most mafic cumulative rocks have similar 143Nd/144Nd ratios between 0.5127 and 0.51289, corresponding to initial εNd(T) values between +2.26 and +6.33 that yielded TDM ages from 1.2 to 0.94Ga. The mafic mesocratic vein displays 143Nd/144Nd values of 0.51227, corresponding to initial εNd(T) values of +6.87 that yielded TDM=0.94Ga, and constrains the minimum age. However primitive basalts samples A17 (GFM) and other mafic cumulate Ag224 (GBM) give an older depleted mantle source derivation age of 2.31 and 1.68Ga respectively, suggesting crustal contamination by the surrounding gneissic Paleoproterozoic basement. The Nd isotopic compositions suggest that the parental magmas of the Agou Igneous Complex in southern Togo were derived from time-integrated depleted mantle source. Such geochemical features are consistent with an active continental margin environment and suggest that the AIC represents a magmatic body emplaced and cooled at the base of a continental crust above a subduction zone.

Geochemical characteristics and U–Pb zircon LA-ICPMS ages of granitoids from the Pan-African Dahomeyide orogen, West Africa

The Dahomeyide orogen in southeastern Ghana and adjoining parts of Togo and Benin records the suture of West African craton (WAC) into northwest Gondwana. The suture zone is characterized by distinctive high-pressure mafic rocks that mark the collision of WAC with exotic blocks to the east. Granitoids to the east of the suture zone, which are postulated to be juvenile crust representing the arc terrane that formed during subduction and oceanic closure, were investigated. We compiled a map of granitoid gneisses in Ghana, Togo, and Benin which allows us to distinguish between two main rock types: (1) migmatitic gneisses with biotite as the dominant mafic mineral and (2) dioritic gneiss characterized by hornblende as the main mafic mineral and with occasional garnet. These rocks types are separated by a shear zone inferred to be a splay of Kandi shear zone, a segment of the Trans-Saharan shear zone that extends for ∼2500km from the Sahara to the Gulf of Guinea. The granitoids are characterized by I-type signatures, display variable amounts of LREE enrichment, Eu depletion and Sr/Y ratios that are consistent with a subduction origin. The migmatitic gneiss unit, within a zone of highly strained granitoids (straight gneiss) that forms a sliver just east of the suture zone, yielded U–Pb zircon ages ranging from 615 to 589Ma. This age range is similar to the metamorphic crystallization age of the suture zone rocks. The zircons analyzed display concentric zoning (in CL images) and are inferred to be magmatic. On the other hand, zircon separates from the dioritic gneiss, with grains also showing igneous zoning in CL images, yielded U–Pb zircon crystallization ages ranging from 2.19 to 2.14Ga. These are the first U–Pb zircon ages to document Paleoproterozoic rocks in the postulated juvenile terrane, west of the Kandi fault zone. Nd model ages of rocks in the suture zone are also consistent with the existence of widespread involvement of older crust in the Pan-African collision zone, similar to the isotopic data from the correlative Medio Coreau domain of the northwest Borborema province in Brazil.

Lithostratigraphy, sedimentation and evolution of the Volta Basin in Ghana

We present a revised lithostratigraphy for the Voltaian Supergroup of Ghana, based on a review of existing literature, interpretations of remotely sensed data and reconnaissance field survey of the Volta Basin. These strata thicken eastwards, to a maximum of between 5 and 6 km adjacent to the Pan-African Dahomeyide orogen. They began to accumulate some time after about 1000 Ma, along the margin of an epicontinental sea. Initial sedimentation, comprising the age-equivalent Kwahu and Bombouaka Groups, shows a cyclical mode of deposition controlled by eustatic changes in sea-level that produced a range of nearshore marine, littoral and terrestrial environments. A major erosional interval was followed by deposition of the 3–4 km thick Oti-Pendjari Group. Basal tillites and associated sandy diamictons are correlated with the Marinoan (end-Cryogenian) glaciation, indicating a maximum depositional age of about 635 Ma. The overlying cap carbonates and tuffs were deposited within a shallow epeiric sea bordered by a volcanically active rift system. The main part of the group records the transition from a rifted passive margin to a fully developed foreland basin receiving marine flysch in the form of argillaceous strata interbedded with highly immature wacke-type sandstones and conglomerates. Maximum accommodation space was developed within a foredeep adjacent to the Dahomeyide belt. Towards the end of the orogenic phase, the foredeep succession became partially inverted and then was buried under coarse terrestrial, red-bed molasse of the Obosum Group.

Petrology and geochemistry of nepheline syenite and related carbonate-rich rocks in the Pan-African Dahomeyide orogen, southeastern Ghana, West Africa

Nepheline syenite and carbonate-rich rocks known as the Kpong complex occur as deformed layers within the Pan-African Dahomeyide suture zone, in southeastern Ghana. The dominant phases in the nepheline syenite are nepheline and feldspar, whereas the carbonate-rich rock is dominated modally by Sr-rich calcite and annitic biotite. Biotite, calcite and nepheline have similar compositions in the nepheline syenite and the carbonate-rich rock. The rocks of the Kpong complex are enriched in alkalis, Ba, Sr, and Nb, have high LREE/HREE ratios, and show large depletions in Zr and Hf relative to primitive mantle. These features are characteristic of carbonatitic and related alkaline rocks worldwide. Whole-rock major elements in the Kpong complex with respect to SiO 2 in bivariate plots show coherent variations with linear trends. However, based on the low Mg number, low Cr and Ni contents in whole rocks, and on overall high TiO 2, FeO, rocks of the Kpong complex appear to have crystallized from an evolved parent, rather than primary mantle-derived magma. The ubiquitous disaggregated enclaves within the carbonate-rich horizons, the linear compositional arrays defined in binary variation diagrams, and overlaps in major modal mineral compositions all suggest magma assimilation of country rock was involved in the evolution of the Kpong complex.

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

AbstractSeismic 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.

U-Pb zircon age of deformed carbonatite and alkaline rocks in the Pan-African Dahomeyide suture zone, West Africa

It has been proposed that the association of deformed carbonatite and alkali rocks (DARC) can be used to map ancient suture zones. The hypothesis considers the intrusion of such alkali rocks as indicating an episode of lithospheric extension that preceded the suture-forming orogeny whereas the deformation evident in DARCs records the collisional event. As such, DARCs are predicted to provide samples that can date both the beginning and the late stages in a Wilson Cycle. The occurrence of carbonatite and nepheline syenite in the Pan-African Dahomeyide orogen of southeastern Ghana has been cited as an example of such rocks. To test this prediction, we have dated carbonatite and nepheline syenite from the Kpong complex that decorate the sole thrust of the suture zone in southeastern Ghana. ID-TIMS analyses of zircon from the carbonatite yield a U-Pb age of 592 ± 2 Ma and those from nepheline syenite an age of 594 ± 4 Ma. Titanite in the carbonatite defines an age range between 586 and 578 Ma. The identical zircon ages are distinctly younger than 612–605 Ma U-Pb zircon ages of the high-pressure mafic granulites inferred to date peak metamorphism and mark the suturing event, but older than 40Ar/ 39Ar hornblende age of 582 ± 2 Ma from the same unit interpreted as dating exhumation. Consideration of zircon characteristics and field relations suggest that the U-Pb zircon ages most likely define the time of magmatic crystallization of the Dahomeyide carbonatite and nepheline syenite, implying that they formed during syn-orogenic extension related to the collisional orogeny at the margin of the West African craton. Thus, DARCs may not always be useful in constraining the initiation of a Wilson Cycle heralded by lithospheric breakup.

The role of Pan-African structures in intraplate seismicity near the termination of the Romanche fracture zone, West Africa

The distribution of epicenters of both historic earthquakes and recent seismic events in southeastern Ghana, compiled from local and teleseismic networks, show strong correlation with the Pan-African structures onshore and indicate an alignment with disruptions on seismic sections offshore. The seismic reflection sections reveal basement structures of the external zone of the Pan-African Dahomeyide orogen and these structures can be traced to offsets of shelf strata and seabottom reflectors, providing direct evidence, for the first time, for neotectonic activity that may be responsible for seismicity in the area. The deep structure of the external zone consists of moderately-dipping reflectors inferred to represent high-strain zones in the variably deformed margin of the West African craton. Taken together, the available data suggest that active tectonics in this intraplate environment may involve inversion of the Pan-African thrust structures but that this activity is apparently not related to reactivation of the nearby Romanche Fracture Zone.

Geochemistry of high-pressure granulites from the Pan-African Dahomeyide orogen, West Africa: constraints on the origin and composition of the lower crust

Major and trace element concentrations in high-pressure (HP) mafic granulites which mark the suture zone of the Pan-African Dahomeyide orogen have been determined. The data suggest that the HP granulites may have preserved the geochemical imprints of their magmatic protoliths where: (i) rocks characterized by island arc tholeiitic (IAT) imprints predominate, and (ii) those with oceanic crust (N-MORB) imprints occur but are subordinate. Rocks with eclogitic mineral assemblages tend to have MORB imprints. We interpret the association of IAT rocks with those having MORB compositions as evidence for a Pan-African subduction zone where arc-related oceanic crust formed either in back-arc or intra-arc basins. The HP granulites display trace element patterns that are very similar to mafic lower crust (LC) composition such as: (i) nearly identical concentrations of Ti, Mn and P, (ii) strong depletions in Cs, Rb, K and Th, and (iii) enrichment in Ba, Ta and Sr. But the HP granulites also differ from LC in that (i) rocks with IAT imprints show peculiar LREE enrichment with flat (La/Sm) N, (ii) those with oceanic crust geochemical signatures are slightly depleted in LREE and have low (La/Yb) N relative to LC, and (iii) they show no Eu anomaly. The data suggest that the Dahomeyide suture zone rocks may represent accessible LC formed by HP metamorphism of subduction zone basalts.

Chronology of nappe assemblage in the Pan-African Dahomeyide orogen, West Africa: evidence from 40Sr/39 mineral ages : Attoh K., Dallmeyer R.D. & Affaton P., Precambrian research, 1997, 82/1–2 (153–171)

Permian–Triassic boundary strata cropping out in the southern Junggar Basin contain a continuous and varied fossil record, making this fluvio-lacustrine succession a potential candidate for a non-marine boundary stratotype. Abundant, diverse and well preserved miospores are recorded and illustrated from the upper part of the Guodikeng Formation and the basal part of the Jiucaiyuan Formation, comprising 92 species and 48 genera, as well as some acritarchs. Seven new species (Anapiculatisporites decorus Ouyang et Norris, sp. nov., Baculatisporites uniformis Ouyang et Norris, sp. nov., Kraeuselisporites varius Ouyang et Norris, sp. nov., Lapposisporites echinatus Ouyang et Norris, sp. nov., Klausipollenites angustus Shu et Norris, sp. nov., Alisporites exilis Ouyang et Norris, sp. nov., Pilasporites perreticulatus Ouyang et Norris, sp. nov.) and 8 new combinations are proposed. Pteridophytic spores and gymnospermous pollen are almost equally diverse, comprising 45 species in 20 genera, and 43 species in 25 genera, respectively. Pteridophytes are numerically more important, notably Limatulasporites, Kraeuselisporites, Lundbladispora, Anapiculatisporites and Verrucosisporites, but also prominent are bisaccate pollen of Falcisporites, Klausipollenites and Alisporites. Less abundant taeniate pollen are largely bisaccate. These palynofloras are grouped together as the Lundbladispora–Lunatisporites–Aratrisporites Assemblage, argued to be Early Triassic (early Induan), and comprising both newly evolved Mesozoic taxa (herbaceous and shrubby lycopsids, coniferous trees, pteridosperms) and relict Paleozoic taxa (sphenophyllids, cordaitaleans and possibly Paleozoic lycopods). The lower boundary of the Triassic in the section is redrawn somewhat lower than by previous authors. The transitional nature of the Permian–Triassic palynofloras and of the paleophytoprovinces is discussed; neither indicates a sudden catastrophic event affecting terrestrial floras at the P–T boundary on a global scale. The Early Triassic vegetation appears to have grown under humid sub-tropical conditions in lowland flood plains and marshes dominated by hydrophilous and mesophilous pteridophytes and pteridosperms, but with locally drier upland environments supporting xerophilous and mesophilous conifers and pteridosperms. The Induan vegetation is closely comparable to that of the Angara, especially the Subangara Province, although it contains some elements of the Euramerican Province in association with a few Gondwanan, Cathaysian and cosmopolitan taxa.

Chronology of nappe assembly in the Pan-African Dahomeyide orogen, West Africa: evidence from [formula omitted] mineral ages

The Pan-African Dahomeyide orogen represents the southwestern segments of the eastern tectonic zone along which the West African Craton was incorporated into Gondwana. Orogenic contraction produced nappe complexes comprised of passive margin sediments and accreted exotic magmatic rocks in Ghana and Togo. External nappes include the Atacora, composed of lower amphibolite facies quartzite and mica schist which were structurally imbricated with mylonitic gneiss derived from c. 2.0 Ga foreland basement. The suture zone is represented by high-pressure granulite facies garnet homblende gneiss. We have analyzed muscovite and homblende concentrates from external and suture-zone nappes in incremental heating 40Ar 39 Ar experiments. Muscovites which define the penetrative foliation in the Atacora nappes and a shear foliation in the basement parautochthon record 40Ar 39 Ar plateau ages of 579.4 ± 0.8 Ma and 578.1 ± 0.5 Ma respectively. In contrast, c. 600 km to the north, muscovite in quartz schist near the base of the Atacora nappes displays a spectrum which gives a minimum age of 608.1 ± 1.2 Ma, whereas, muscovite in the overthrust Kara nappes yields a plateau age of 633.8 ± 0.5 Ma. The c. 30 m.y. difference between the Atacora nappes in the northern Dahomeyides and the younger dates of the southern external nappes indicate out of sequence thrust imbrication, however, forward-imbricating thrusting is also suggested by the older age of Kara nappes relative to Atacora nappes in the northern segment. The youngest ages are inferred to date orogen parallel nappe transport at c. 575 Ma. Hornblende from the structurally lower sections of the suture-zone nappes record a 36Ar 40 Ar versus 39Ar 40 Ar isotope correlation age of 587 ± 4.3 Ma. Hornblende from a tonalitic vein in the host garnet-mafic gneiss yields an isotope correlation age of 581.9 ± 2.4 Ma. These ages are interpreted to date the exhumation of the suture-zone nappes and thus provide a minimum age of crustal thickening in the Dahomeyide orogen. The age data are consistent with coeval forward- and hindward- propagating thrusting and nappe imbrication, and constrain the amalgamation in northwest Gondwana to have occurred in the latest Proterozoic rather than Cambrian.