Geochemical and Petrological Insights into the Neoproterozoic Moumba Metabasites: Implications for Crustal Processes in the West Congo Belt (Republic of Congo) (original) (raw)
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Journal of African Earth Sciences, 2019
High-to ultrahigh-pressure metamorphic assemblages consisting of garnet-omphacitic clinopyroxene bearing mafic rocks have been identified within the Paleoproterozoic Nyong Group in SW Cameroon, at the northwestern margin of the Archean Congo craton. These rocks were investigated in detail and for the first time evidence for eclogite facies metamorphism at ca 25 kbar and 850°C is provided. A clockwise P-T path with nearly isothermal decompression (ITD) is deduced from mineral zoning and textural relationships characterized by mineral recrystallization and multi-layered coronitic overgrowths of plagioclase and clinopyroxene surrounding garnet porphyroblasts. These P-T conditions imply a burial depth greater than 90 km, at lower geothermal gradient of ca 10°C/km. The geochemical signature of ten representative rock samples show that two groups of eclogite facies rocks genetically originate from mostly basaltic and basaltic andesite compositions, with a characteristic upper mantle-derived tholeiitic trend. Moreover, their chondrite and MORB normalized REE and trace element concentrations are characterized by nearly flat REE patterns with very little to no Eu anomaly, (La/Sm) N ≥ 1 and Zr/Nb ≤ 10, as well as a gradual depletion from LREE to HREE with also very little to no Eu anomaly, but (La/Sm) N < 1, Zr/Nb > 10 and negative anomalies in Th, K, Nb, Ta, Sr, Zr and Ti consistent with midocean ridge basalt (MORB) contaminated by a subduction component or by a crustal component. Previous available geochronological data coupled with our new petrological, mineralogical and geochemical findings clearly indicate that the eclogite facies metabasites from the Eburnean Nyong Group between 2100 and 2000 Ma represent one of the oldest subducted oceanic slab or trace of a suture zone so far recorded within the West Central African Fold Belt (WCAFB). The geodynamic implications of these eclogites suggest a subduction-related
2016AbbasseneTectonophysics.pdf
Bougaroun is the largest pluton (~200 km 2 ) in the 1200 km-long Neogene magmatic belt located along the Mediterranean coast of Maghreb. New U-Pb dating on zircons and K-Ar ages on whole rocks and separated minerals document its emplacement at 17 Ma within the Lesser Kabylian basement, a continental block that collided with the African margin during the Neogene. This Upper Burdigalian intrusion is therefore the oldest presently identified K-rich calc-alkaline massif in the whole Maghrebides magmatic lineament and marks the onset of its activity. The Bougaroun peraluminous felsic rocks display a very strong crustal imprint. Associated mafic rocks (LREE-enriched gabbros) have preserved the "orogenic" (subduction-related) geochemical signature of their mantle source. Older depleted gabbros cropping out at Cap Bougaroun are devoid of clear subduction-related imprint and yielded Ar-Ar hornblende ages of 27.0 ± 3.0 Ma and 23.3 ± 3.2 Ma. We suggest that they are related to the Upper Oligocene back-arc rifted margin and Early Miocene oceanic crust formation of the nearby Jijel basin, an extension of the Algerian basin developed during the African (Tethyan) slab rollback. The fact that the Bougaroun pluton intrudes exhumed Kabylian lower crustal units, mantle slices and flysch nappes indicates that the Kabylian margin was already stretched and in a post-collisional setting at 17 Ma. We propose a tectono-magmatic model involving an Early Miocene Tethyan slab breakoff combined with delamination of the edges of the African and Kabylian continental lithospheres. At 17 Ma, the asthenospheric thermal flux upwelling through the slab tear induced the thermal erosion of the Kabylian lithospheric mantle metasomatized during the previous subduction event and triggered its partial melting. We attribute the strong trace element and isotopic crustal signature of Bougaroun felsic rocks to extensive interactions between ascending mafic melts and the African crust underthrust beneath the Kabylie de Collo basement.
This study focuses on the metabasite rocks of the Nemba Complex of the Mayombe belt, an African segment of Araçuaï-West Congo Orogen (A-WCO) extending from the southwest of Gabon to the northwest of Angola. These metabasite rocks outcrops are in southwestern Congo along the Loukounga river. The Nemba complex is of Neoproterozoic age and represents the lower part of the west congolian Supergroup. The objective of this study is to constrain the geodynamic context of the Nemba complex from the petrology and geochemistry of the metabasites sampled in the Loukounga River. The observed rocks are composed of amphibolites, metagabbros, epidotites and greenschists and are affected by folding accompanied by flux schistosity and crenulation schistosity. Geochemical analyzes show that the rocks have a basic to ultrabasic chemical composition with SiO 2 contents between 41.85% and 58.23%. The geochemical composition of the major and traces elements shows that the rocks are basalts, basaltic andesites and andesites. The magma shows enrichment in LREE, LILE and depletion in HREE and HFSE. The multielement spectra show negative anomalies in Nb-Ta, Ti and a relatively low Nb/La ratio which characterize a lithospheric source contaminated by continental crust. Traces elements discrimination plots show that Loukounga metabasites are emplaced in intraplate geodynamic context like that associated with the basalts of the trap-types continental shelves and are possibly derived from mantle plumes contemporaneous with or slightly prior to magmatism.
Tholeiitic magmatism associated with continental rifting in the Lufilian Fold Belt of Zambia
Journal of African Earth Sciences, 1999
Metabasic rocks form a small but geologically important component of the geology of the base metal-rich Neoproterozoic Copperbelt of central Africa. The disposition of the metabasic rocks follows the 150 km long, arcuate, structural trend of the thrust belt. Despite their structural disposition, the metabasic rocks played a passive role during the thrusting due to their rigidity and lithological contrast with the host sedimentary rocks. The main thrust horizons are located along evaporite layers below the position of the metabasic rocks. The metabasic rocks form part of an allochtonous unit overlying para-autochtonous rocks of the Upper and Lower Katangan sequences. A petrological and geochemical study of the metabasic rocks indicates that they crystallised from genetically related, tholeiitic magmas. Fractionation of olivine, pyroxene and plagioclase played an important role in the generation of the range of mafic compositions. The rocks are enriched in incompatible trace elements with chondrite-normalised (La/Yb), values ranging from 4 to 7. Incompatible trace element ratios (e.g. La/Nbl.5;
International Journal of Geosciences
The West Congo belt contains Paleoproterozoic and Neoproterozoic Units which are deformed during Panafrican event. The Neoproterozoic Unit contains rift and post rift deposits. The rift formation is made of metabasite, volcanic and volcano-sedimentary rocks. The metabasite constitutes the Nemba Complex which is considered put into place in a continental or oceanic context. Samples from Nemba Complex collected along the "Congo ocean railway realignment" are analyzed. Major elements indicate a low potassic subalkaline affinity of the samples. Classification diagrams based of major elements shows that the rocks are a tholeiitic serie constituted of andesitic basalts, sub-alkaline andesitic basalts and a sub-alkaline to alkaline basalts. Some samples with a calc-alkaline signature indicate a crustal contamination. The traces elements plots indicate that the rocks are the MORB. The spectrum of multi-elements is characteristic of continental rocks with a lithospheric origin. This is an agreement with many works on the Nemba Complex and the alkali plutons and acid volcanic rocks associated which attribute there a rift context.
Geologic evolution of the neoproterozoic Zambezi orogenic belt in Zambia
Journal of African Earth Sciences, 1994
The Neoproterozoic Zambezi belt links with the Mozambique belt, Lufilian arc, and the inland branch of the Damara belt within the regional Pan-African tectonic framework of southern Africa. The belt contains a thick supracrustal sequence deposited on older sialic basement and penetratively deformed with it during Neoproterozoic (Pan-African) orogenesis. In Zambia, where the entire width of the orogen is exposed, local bimodal volcanic rocks at the base of the sequence are overlain by psammites and pelites, which in turn are succeeded by extensive carbonate and calc-silicate rocks. Abundant scapolite in metamorphic assemblages within the belt is taken as evidence for the original presence of evaporites. The nature of the rock types and the inferred stratigraphic sequence are consistent with deposition in an intracontinental rift basin invaded by marine waters. Available isotopic age brackets for the timing of supracrustal deposition show that the basin developed between 880 and 820 Ma.
Journal of the Cameroon academy of sciences, 2009
The Kekem area (southwestern part of the central domain of the Cameroon North Equatorial Fold Belt) is composed of high-grade migmatitic gneisses in which two lithological units are distinguished: (i) a metasedimentary unit (garnet-sillimanite-biotite-gneisses and garnet-biotite-gneisses) interpreted as a continental series; and (ii) meta-igneous rocks comprising mafic pyroxene gneisses, amphibolites, and orthogneisses. These units recrystallised under HT-MP conditions (T=700-800°C, P ≥ 0.5-0.8GPa) and were deformed in relation to a major tangential tectonic event with the NNE-SSW kinematic direction. The lithological association and its tectono-metamorphic evolution show striking similarities with the Banyo and Maham III gneisses, suggesting that the extensional depositional environment envisaged for this formation can be extended farther west. P-T calculations in this contribution provide new data on the Pan-African structural and metamorphic evolution of the metapelites and metabasites in the basement of the Kekem area. The results show two distinct events: (1) crystallization during a Pan-African high temperature metamorphic event and, (2) subsequent deformation and high temperature mylonitization. The data imply a high-temperature amphibolite-facies metamorphism along a clockwise P-T path. The recorded P-T involves a marked variation in pressure, which is typical of collisional crustal thickening. The contrasted metamorphic evolution between areas located to the south of the Central Cameroon Shear Zone (CCSZ; high pressure: Yaoundé, Ntui-Betamba), and those located to the north (low pressure: Banyo, Tibati), along with widespread remains of Paleoproterozoic crust, suggest important crustal thickening during Pan-African tangential tectonics in southern Cameroon. As a consequence, the CCSZ is not simply a late Pan-African transpressive shear zone but appears to have been formerly a major intracontinental thrust zone.
Precambrian Research, 1989
The Archaean granite-gneiss terrain of central Gabon comprises medium-to high-grade gneisses metamorphosed at 3.15 Ga, forming the Monts Cristal and underlying the Mitzic region, intruded by and enclosed as enclaves in granitoids 3.0-2.6 Ga old that form the extensive Chaillu and North Gabon blocks. These rocks are unconformably overlain in the east of Gabon by Early Proterozoic Francevillian sedimentary rocks, between 2.3 and 2.0 Ga old, forming the Franceville and Boou~ Basins. The Francevillian is overthrust on its west side by the Ogoou~ Metamorphics, themselves considered to be of Early Proterozoic age. These rocks crop out in a broad synclinorium consisting essentially of paragneisses, with orthogneisses appearing locally at the base. The overall tectonic setting is discussed in terms of the evolution of the various domains from west to east. (1) On the west flank of the synclinorium is a migrnatitic-granulitic dome, the Abami~ Dome, in which structures diverge away from the core. Metamorphism is of high-temperature, intermediate-to low-pressure type. (2) On the east flank of the Ogoou$ synclinorium the medium-grade Ogoou~ Metamorphics (sillimanite-biotite, kyanite-staurolite, staurolite-muscovite-biotite) have been thrust eastwards on to the low-grade (kyanite-sericite) Francevillian. (3) The Francevillian is parautochthonous, being widely detached from its Archaean basement owing to the effects of the same eastward-directed tangential tectonics. (4) Locally, slices of Archaean basement have been tectonically incorporated. The structural cross-section illustrates a history of diapiric uprise passing eastward into tangential tectonics that is typical of classical collision belts. The suture marking the collision is interpreted to be located in the Abarni~ migmatite dome, where the nappes are rooted. The Gabon Orogenic Belt is correlated with the collision belts of about the same age on the eastern margin of the Congo craton and with the Trans-Amazonian Orogenic Belt of the Guyana Shield.