We have dated 1060 detrital zircon grains from the Neoproterozoic to lower Palaeozoic Volta basin, Ghana, and from sandstones in the adjoining thrust sheets belonging to the Pan-African Dahomeyide ...belt. All dated zircons in the lower Voltaian Bombouaka Group are older than ~
1000 Ma, while samples from the middle Voltaian Oti Group and the upper Voltaian Obosum Group contain numerous zircons of 600–1000 Ma. The samples we have studied from the Dahomeyide thrust sheets (Buem and Togo structural units) have zircon age spectra similar to those from the Bombouaka Group, confirming a correlation of the investigated sandstones with lower Voltaian strata. The Bombouaka Group was deposited between ~
1000 and 600 Ma, perhaps shortly after 1000 Ma, as suggested by earlier Rb–Sr data on clay minerals. Deposition of the Oti and Obosum Groups took place shortly after 600 Ma, perhaps continuing into the lower Palaeozoic. Most samples contain Palaeoproterozoic zircons with ages of 2000–2200 Ma that probably have been derived from the surrounding crystalline (Birimian) basement. Archaean zircons, present in smaller proportions, may have come from Archaean rocks in the West-African craton. Most zircons of 1000–1900 Ma were probably derived from sources outside the West-African craton, the Amazonian craton being a plausible source region. Zircons with ages around 1200 Ma are believed to have been derived from Grenvillian orogenic belts, perhaps those that fringe the Amazonian craton. If this is so, no major seaways could have been present between these belts and the West-African craton during the early Neoproterozoic. Zircons younger than 1000 Ma were probably derived from an eastern continental block that collided with the West-African craton during the Pan-African orogeny ~
600 Ma ago.
•Zircon U–Pb is using to date the Kekem gabbro-norite pluton.•Mineralogical assemblage of gabbro and norite is determined.•Geochemistry and isotopic data were performed to determine petrogenesis of ...this pluton.•Using combination of geochronogical and geochemical data to propose geodynamical setting.
The Kekem shoshonitic gabbro-norite association is part of the high-K calc-alkaline (HKCA) post-collisional magmatism, a major feature of the Pan-African Belt in Cameroon. LA-ICP-MS U–Pb zircon analyses provide an age of 576±4Ma for the Kekem complex. This age is interpreted as dating the emplacement of the massif during the waning stage of the Pan-African orogeny. The latter is related to dextral movements along the Central Cameroon Shear Zone (CCSZ).
The REE patterns display enriched LREE (LaN/YbN=14.2–23.5) while HREE present a nearly flat profile (DyN/YbN=1.3–1.7), and the La/Sm and Sm/Yb ratios led to propose that the Kekem gabbro-norites have been derived from the partial melting of a garnet-spinel lherzolite mantle source. The negative Nb–Ta and Ti anomalies and the positive Pb anomalies indicate that this mantle source was modified by contribution of a subduction-related material. The low Ce/Pb (2.6–10.4) and Th/Yb ratios associated to high Ba/La ratios, indicate that source enrichment could be related to slab derived fluids. As a whole, the Kekem geochemical features suggest that primary gabbro-noritic magmas derived from a subduction-modified mantle source (metasomatised lithospheric mantle). Moderately high 86Sr/87Sr initial ratios (0.7068–0.7082), low εNd (−5 to −9) and old Nd TDM model ages (1.6–1.8Ga) are interpreted to result from contamination of Neoproterozoic mantle by the Paleoproterozoic crust.
The ca. 576Ma movements along the CCSZ are related to a Neoproterozoic metacratonization of the northern margin of the Congo craton during the Pan-African orogeny. This metacratonization led to vertical planar lithospheric delamination along lithospheric transcurrent faults, asthenospheric uprise and partial melting of the Paleoproterozoic lithospheric mantle.
The Fomopéa plutonic complex in West Cameroon comprises three petrographical units: a biotite–hornblende granitoid (BHG) including some diorite, a biotite monzogranite (BmG) and an edenite ...syenogranite (EsG). Amphibolites occur within each unit. The massif was emplaced into a Pan-African amphibolite-facies metamorphic basement. All rocks display igneous textures and are chemically calc-alkaline, the BHG being metaluminous (ASI
<
1) the BmG (ASI
=
0.98–1.06) and EsG (ASI
=
0.94–1.1) being metaluminous to weakly peraluminous. U–Pb dates on zircon give a Pan-African age of 620
±
3
Ma for a diorite and 613
±
2
Ma for a quartz–monzodiorite, both belonging to the BHG. Sr–Nd isotopic data indicate the mixing between a juvenile source, probably the mantle (nearest Fomopéa pole:
ε
Nd(620 Ma)
=
+4 and
87Sr/
86Sr
(620Ma)
=
0.703) and a Palaeoproterozoic to Archaean lower continental crust (nearest Fomopéa pole:
ε
Nd(620Ma)
=
−16 and
87Sr/
86Sr
(620Ma)
=
0.709; Nd
T
DM
=
2.9
Ga) through a contamination process or through a bulk mixing event at the base of the crust. Evidence for both processes is provided by the coexistence of mafic enclaves and gneissic xenoliths within the granitoids. We propose a model whereby linear lithospheric delamination occurred along the Central Cameroon shear zone (CCSZ) in response to post-collisional transpression. This delamination event induced the partial melting of the mantle and old lower crust, and facilitated the ascent of the magmas. The emplacement of numerous post-collisional Neoproterozoic plutons along the CCSZ during the Pan-African orogeny indicates that this process was of paramount importance. The continental signature and geophysical data from the area indicate that the CCSZ corresponds to the northern lithospheric boundary of the Archaean Congo craton, and that the events recorded here correspond to the metacratonic evolution of the northern boundary of the Congo craton.
The alkaline magmatism from Niger–Nigeria to Cameroon forms large scale magmatic provinces across the African plate. It displays a N–S trend from Aïr in Niger to Jos Plateau in Nigeria changing ...southeastwards towards Cameroon. We have compiled recent petrological, geochemical and structural data on these magmatic provinces. The data show that although there is a general age decrease from one province to another (407
±
8
Ma in Aïr to ⩽66
Ma in Cameroon), there is no age migration in any given province, except in the Nigeria province (Younger Granites) where a rough NE–SW age decrease is observed. The relationship between these different magmatic provinces that share similar geochemical data, added to the SW–NE parallel trends of Nigeria, Benue Trough and Cameroon Line, is difficult to explain in terms of a simple northward motion of the African plate over a single hotspot. In the light of recent tectonic models, we suggest complex interaction between, on the one hand, at least two mantle plumes acting in succession (including the St. Helena mantle plume) and, on the other hand, lithospheric fractures that induce oblique alignments of new magmatic complexes.
The Neoproterozoic Era includes some of the most extreme ice ages in Earth history. The exact number of glaciations is unknown, although there were at least two events of global reach and possibly an ...equal number of lesser advances. Neoproterozoic glacial deposits in West Africa have proven particularly difficult to correlate with better constrained successions elsewhere. In most West African successions, only a single glaciogenic deposit is present, generally within a stratigraphic ‘Triad’of tillite, carbonate, and bedded chert; the age of these deposits and their synchroneity across the craton remain controversial. We report isotopic data from carbonates that cap tillites in the Volta Basin (Sud-Banboli Group). In the three sections measured,
∂
13
C
(‰ PDB) begins at ∼−2 to −3 and drops to −5 to −8, correlated with a drop in both
∂
18
O
and Mg/Ca. This pattern—both the drop in
∂
13
C
and its correlation with
∂
18
O
and lithology—has been observed in Neoproterozoic cap carbonates on other cratons, suggesting that it reflects primary depositional variations. In addition, the strong positive correlation among lithology,
∂
13
C
, and
∂
18
O
suggests that lithological variation may have exerted a principal control on isotopic variation, implying that the dolomite formed in isotopic equilibrium with seawater, as a primary precipitate or very early diagenetic replacement. This would require low levels of sulfate (estimated to be ∼1
mM or less), consistent with available S-isotopic data, the presence of barite crusts in association with some cap carbonates, and the conditions of seawater chemistry hypothesized in the aftermath of a ‘snowball Earth.’
The stratigraphic pattern of carbon isotope variation in the Volta Basin cap carbonates suggests that underlying tillites are likely Marinoan (∼650–600
Ma) in age. This interpretation is supported by distinctive lithological characters observed in Volta Basin cap carbonates and accepted Marinoan correlatives. Tillites elsewhere on the craton have been assigned latest Proterozoic or Early Cambrian ages. Either the basis for these proposals requires reexamination, or West African tillites record more than one glacial event.
•First geochronological study of detrital zircons in Neoproterozoic (meta-) sedimentary (Mayombe and West Congo Supergroups) of the (Araçuai-) West Congo foldbelt in Congo Brazzaville.•U–Pb data ...(LA-ICP-MS) on 1674 zircon grains from 15 samples permit a subdivision into (1) those in which the youngest zircons are ca. 900Ma, and (2) those that also contain late Neoproterozoic (600–700Ma) zircons.•Sedimentary rocks of group 1 were deposited during the rift and drift phases of the Araçuai – West Congo orogenic cycle, while those of group 2 were formed during collision and uplift.•On the basis of these results a revision of the Neoproterozoic stratigraphy for Congo Brazzaville and the Democratic Republic of Congo is proposed.
The African part of the Neoproterozoic Araçuai-West Congo orogen forms a more than 1400km long belt, running from Gabon in the north to Angola in the south, parallel to the South Atlantic coast. It consists of the West Congo fold belt in the west and a largely undeformed sedimentary foreland basin in the east. In the Republic of Congo (Congo Brazzaville) these are known as, respectively, the ‘Mayombe fold belt’ and the ‘Niari basin’. The Mayombe fold belt is largely made up of strongly deformed metasedimentary rocks of the Mayombe Supergroup and its reworked granitoid basement complex. The Niari basin comprises the sedimentary strata of the West Congo Supergroup, which contain a unit of diamictite and associated cap-carbonate, related to the Marinoan (ca. 635Ma) glaciation. In this paper detrital zircon U–Pb data are employed to investigate the relationships between the Mayombe and West Congo Supergroups in Congo Brazzaville, as well as the relationships with similar stratigraphic units in the Democratic Republic of Congo (DR Congo). Unfortunately, major differences in stratigraphical nomenclature exist between Congo Brazzaville and DR Congo which, together with geological differences, complicate comparisons. The terminology used in this paper follows the Congo Brazzaville tradition.
U–Pb data have been obtained by LA-ICP-MS on 1674 zircon grains from samples of the Mayombe and West Congo Supergroups (seven and eight samples, respectively). Two of the samples have Palaeoproterozoic ages. The other samples are of Neoproterozoic age. Zircons from the latter fall into four main age groups: Group 1, 500–800Ma, forming 8% of the total population; Group 2, 900–1200Ma (29%); Group 3, 1800–2300Ma (19%); and Group 4, 2500–3100Ma (31%). Only ca. 13% of the zircons fall outside of these groups.
Most zircons of Group 1 (600–800Ma) are interpreted to have been derived from late Neoproterozoic magmatic arc rocks and syn- to post-collisional granites of the Araçuai orogen in Brazil, since rocks of these ages are rare in the Congo craton to the east. Most of Group 2 zircons have probably been derived from early Neoproterozoic (1000–900Ma) rhyolitic and granitic rocks exposed in DR Congo. The Palaeoproterozoic and Archaean zircons of Groups 3 and 4 may have been derived both from eastern and western source regions.
None of the stratigraphical units beneath the Marinoan diamictite contain the 500–800Ma zircons of Group 1. This is interpreted to indicate deposition of these units during the earlier Neoproterozoic rift and drift phases in the history of the Araçuai-West Congo orogen, while the strata that do contain Group 1 zircons were deposited during the collisional phases and post-collisional uplift. A significant hiatus is likely to be present beneath the Marinoan diamictite. Based on these results, a revision of the stratigraphy of the Mayombe and West Congo Supergroups is recommended.
Three granitoid rocks and two metasedimentary samples from the reworked basement complex have also been investigated, yielding Palaeoproterozoic ages, 2050–2100Ma for the granites, and Palaeoproterozoic and Archaean detrital zircon ages for the metasedimentary rocks.
Sm–Nd isotope data have been obtained on a number of (meta-)argilitic and siltstone samples. All samples yielded Palaeoproterozoic model ages, indicating mixing of sediments of different ages, as also observed in the zircon age distributions.
► First modern zircon U–Pb geochronology on granitoid and metasedimentary rocks from Benin, West Africa. ► Zircon U–Pb, Rb–Sr, and Sm–Nd isotope data demonstrate that most of Benin consists of ...Neoproterozoic rocks. ► Detrital and metamorphic zircon in metasedimentary rocks differentiated on the basis of Th/U ratios. ► Precise correlation with the Borborema Province, Brazil, suggested by previous authors, questioned.
The westernmost part of the Pan-African (c. 600Ma) Dahomeyide orogen in Togo and Benin consists of a complex stack of thrust sheets, mainly composed of Palaeoproterozoic (‘Eburnean’, c. 2000Ma) granitoid rocks and younger volcanic and metasedimentary successions. This thrust stack comprises the suture between the West-African craton and the Benino-Nigerian province to the east. This eastern province consists largely of migmatites, supracrustals and granitoid rocks. Rb–Sr and Sm–Nd isotope data for four granites from the western domain confirm the Palaeoproterozoic age of these rocks found by earlier studies. One of these granites yielded a zircon U–Pb age of c. 2060Ma. Twenty samples from the eastern region yield Rb–Sr and Sm–Nd data indicating that both migmatites and granitoid rocks are of Neoproterozoic (Pan-African) age. Zircon U–Pb data on six of these samples yielded ages ranging from c. 650 to c. 550Ma. Suggestions that parts of the migmatites might be of Eburnean age could not be confirmed. The Neoproterozoic granitoid rocks may represent a Pan-African arc overlying an eastward dipping subduction zone, and Sm–Nd data indicate that they contain significant proportions of older crustal material. Age probability diagrams for detrital zircons from three metasedimentary rocks (one from Nigeria) and one S-type quartz diorite from the eastern region show prominent peaks at c. 1900Ma and 2400–2200Ma. A provenance of the zircons from sources in the Borborema province in NE Brazil or lateral equivalents thereof is suggested. A few Neoproterozoic zircons are present in three of the samples. On the basis of Th/U ratios a distinction can be made between Neoproterozoic zircons of metamorphic and detrital origin. The zircon age probability diagram of a quartzite from northern Benin exhibits one peak, at c.1900Ma, different from the other metasedimentary samples. This quartzite might be correlated with sedimentary strata of the Volta basin that overlie the Eburnean basement in Ghana, Togo and Burkina Faso. Correlation of the rocks in the study area with those in the Borborema province is discussed.
This paper attempts to describe the Neoproterozoic–Cambrian lithostratigraphic successions occurring on the West African craton and in the surrounding Pan-African fold belts, with special reference ...to glacial or glacially influenced deposits. It provides a brief synthesis of these terrains in order to propose inter-regional correlations, and to place the glacial events already described in the literature within the tectonic framework of this part of the world. Correlations are based on facies associations and isotopic databases, and supported by the occurrence of glacial deposits when these are ascribed to continental-scale glaciation. As expected, there is a diachronism of the main tectonic events around the craton when the mobile belts display a roughly similar overall facies trend reflecting the successive stages of the Pan-African orogenic cycle from rifting to collision. Contrary to most of the Neoproterozoic glacial strata elsewhere, which consist generally of marine diamictites preserved in marginal basins, West Africa displays the cratonic counterpart (tillites and associated terrestrial facies) deposited on exposed land surface by continental ice sheets. Lithostratigraphic correlations and a combination of relative dates on sedimentary rocks and on tectonic markers show that a major West African glaciation occurred between 630 and 610
Ma and can be correlated with the Marinoan ice age. This major climatic event is contemporaneous with the final stages of the Pan-African orogenic cycle. Under favourable climatic conditions (mid to high latitudes), the presence of surging reliefs at the rim of a wide cratonic platform may account for the development of the West African Marinoan ice sheet. The diachronism around the craton of collision-surrection events may also account for the occurrence of unrelated local mountain-type glaciation that could be mistaken with the craton-scale glaciation.
This faulting tectonics analysis concerns the southernmost segment of the Dahomeyide Orogen and the West-African craton eastern margin in southeast Ghana. The analysis of strike-slip faults in the ...frontal units of the Dahomeyide Belt indicates that four distinct compressive events (NE-SW, ENE-WSW to E-W, ESE-WNW to SE-NW and SE-NW to SSE-NNW) originated the juxtaposition of the Pan-African Mobile Zone and the West-African craton. These paleostress systems define a clockwise rotation of the compressional axis during the structuring of the Dahomeyide Orogen (650–550 Ma). The SE-NW and SSE-NNW to N-S compressional axes in the cratonic domain and its cover (Volta Basin) suggest that the reactivation of the eastern edge of the West African craton is coeval with the last stages of the Pan-African tectogenesis in southeast Ghana. An extensional episode expressed as late normal faulting is also recorded in this study. This E-W to SE-NW extension, which is particular to the southernmost part of the Dahomeyide Belt, appears to be post-Pan-African. This extension probably contributed to the formation of a major Jurassic rifting zone that originated the Central Atlantic and the Benue Trough.
Les travaux menes des les annees 60 dans les formations sedimentaires de la region du Beli n'ont pas permis letablissement d'une echelle lithostratigraphique. Dans la presente etude, qui s'appuie sur ...une approche terrain a travers des leves de coupes geologiques et sur des etudes lithologique, petrographique et structurale, une triade, ainsi qu'une formation molassique ont ete mises en evidence pour la premiere fois dans cette portion du bassin. Ainsi, a l'instar des formations sedimentaires des bassins de Taoudeni et des Volta, celles de la region du Beli, localisees sur la bordure sud du bassin du Gourma (Nord-Est du Burkina Faso), comportent une association Tillite-calcaire-silexite qui a ete correlee a la triade du bassin de Taoudeni. L'echelle lithostratigraphique de la region du Beli comprend huit formations. La triade repose sur des gres-quartzites et est surmontee successivement par des shales, des dolomies, un ensemble metamorphique allochtone constitue de phyllades et par la formation molassique. Tout comme les formations carbonatees, certaines roches du complexe silexitique renferment des stromatolites ainsi que des micro-organismes. La deformation panafricaine se materialise par des plissements, une schistosite et par un reseau de failles. On y reconnait les phases compressives Dl, D3 et D4 des episodes tectoniques de l'orogenese panafricaine et on soupconne une reactivation des fractures au cours du cenozoique. Les caracteristiques des episodes tectoniques permettent de distinguer deux unites structurales: l'unite structurale du Nord-Beli (allochtone), caracterisee par une deformation de type ductile et un anchimetamorphisme, est charriee sur l'unite structurale du Sud-Beli (para-autochtone). Celle-ci est marquee par une deformation a dominante cassante, n'a subi aucun metamorphisme et est chevauchant sur le socle eburneen. La formation silexitique de la triade contient une mineralisation polymetallifere probablement enrichie lors des differents episodes tectoniques qui ont affecte cette formation. (C) 2017 Elsevier Ltd. All rights reserved.
PDF