The earliest evidence for modern-style plate tectonics recorded by HP–LT metamorphism in the Paleoproterozoic of the Democratic Republic of the 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
Journal of geoscience and environment protection, 2024
The West Congo Belt contains in its rocks of Neoproterozoic age from Nemba complex outcropping in the Moumba River. This West Congo belt is made up of a crustal segment of the Arcuaï-West Congo orogen which extends from southwest Gabon to the northeast of Angola. This study aims to constrain the geochemical signature Nemba complex of West Congo belt from the petrograhic and geochemical study on the whole rock. The petrographic data from this study show the Moumba metabasites are made up of amphibolites, metagabbros, epidotites and greenschists interstratified in the Eburnean metasediments and affected by mesozonal to epizonal metamorphism characterized by the retromorphosis of intermediate amphibolite facies minerals into greenschist facies. Whole-rock geochemical data indicate that these metabasites are continental flood basalts (CFB) of basic nature and transitional affinity emplaced in intraplate context. These continental flood basalts are generated from magma originating from a significantly enriched shallow mantle plume and this magma then contaminated by the continental crust during their ascent. The reconstruction of tectonic signature suggests that West Congo belt would result from closure of an ocean basin with subduction phenomena. This collision would be marked by the establishment of ophiolite complex. We show that this model is incompatible with the CFB nature of metabasites and the orogenic evolution of Neoproterozoic. It does not seem that we can evoke a genetic link with a subduction of oceanic crust, because How to cite this paper: Matiaba-Bazika,
New ion microprobe U-Pb zircon ages, as well as some geochemical and isotopic analyses, for key igneous units within the central part of the West Congo belt are integrated with geological information to provide an updated geological map (1:1000000 scale) and a synthetic type cross-section of the belt, as well as an updated lithostratigraphic chart of the 'West Congo Supergroup'. Three Neoproterozoic units are recognised, from oldest to youngest, the Zadinian, Mayumbian and West Congolian 'Groups'. Emplacement of early Zadinian peralkaline granites (Noqui massif, 999 97 Ma) and rhyolites (Palabala) was accompanied by incipient rift sedimentation, corresponding to the onset of transtensional rifting, preferentially in a transverse mega-shear setting along the margin of the Congo craton. Subsequent upper Zadinian magmatism produced a thick (1600-2400 m) basaltic sequence (Gangila), which has geochemical characteristics typical of continental flood basalts (CFBs). The Gangila basalts, associated with major pull-apart rifting, were followed rapidly by the 3000-4000 m thick Mayumbian rhyolitic lavas, dated at 920 9 8 Ma at the base and 912 97 Ma at the top. The felsic lavas are intruded by coeval high-level (micro)granites, whose emplacement is dated at 924 925 Ma (Mativa body) and at 917 914 Ma (Bata Kimenga body) in the Lufu massif. This voluminous bimodal magmatic province is similar to the Paraná and Deccan provinces, and shares similar lithospheric sources. It corresponds to the initial, transtensional rifting stage along the western edge of the Congo craton before Rodinia breakup. The early Neoproterozoic rocks of the West Congo Supergroup rest unconformably on a ca. 2.1 Ga Palaeoproterozoic polycyclic basement (Kimezian Supergroup). No Mesoproterozoic events are recorded in the area. Following the initial, transtensional early Neoproterozoic (ca. 1000-910 Ma) rifting stage, Bas-Congo behaved as a passive margin of the Congo craton, as indicated by deposition of ca. 4000 m of Neoproterozoic (pre-Pan-African) platform sediments (lower part of West Congolian Group) preceding ca. 2000 m of Pan-African molasse-type sediments (upper part of West Congolian Group). In the late Neoproterozoic, during Pan-African assembly of Gondwanaland, the Bas-Congo passive margin, which was largely protected by thick lithosphere of the Congo craton, collided with a western active margin to form the Brasiliano-Araçuaí belt, now : S 0 3 0 1 -9 2 6 8 ( 0 1 ) 0 0 1 9 2 -9 L. Tack et al. / Precambrian Research 110 (2001) 277-306 278 preserved adjacent to the São Francisco craton of Brazil. This collision, which ended in Bas-Congo at ca. 566 Ma, induced relatively limited effects in the West Congo belt, which experienced no late Neoproterozoic magmatic activity.
Sedimentology, 2018
The Kamoa sub-basin, in the southeastern part of the Democratic Republic of Congo, is a rift basin that hosts a world-class stratiform copper deposit at the base of a very thick (1.8 km) succession of matrix-supported conglomerates (diamictite) (Grand Conglomérat Formation) that has been interpreted by some as the product of deposition in the aftermath of a planet-wide glaciation. Newly This article is protected by copyright. All rights reserved. available subsurface data consisting of more than 300 km of drill core throws new light on the origin of diamictite and associated facies types, and their tectonic, basinal and palaeoclimatic setting. Initiation of rifting is recorded by a lowermost subaqueous succession of fault-related mass flow conglomerates and breccias (the 'Poudingue') with interdigitating coeval and succeeding sandstone turbidites (Mwashya Subgroup). Overlying diamictites of the Grand Conglomérat were deposited as subaqueous debrites produced by mixing and homogenization of antecedent breccias and gravel from the Poudingue and Mwashya sediments with basinal muds. Failure of over-steepened basin margins and debris flow was likely to be triggered by faulting and seismic activity and was accompanied by syn-depositional subaqueous basaltic magmatism recorded by peperites and pillow lavas within diamictites. The thickness of diamictites reflects recurring phases of faulting, volcanism and rapid subsidence allowing continued accommodation of rapidly deposited resedimented facies well below wave base. A distal or glacial influence in the form of rare dropstones and striated clasts is evident, but tectonically-driven mass flow destroyed any primary record of glacial climate originally present in basin margin sediments. Such basin-margin settings were common during Rodinia rifting and their stratigraphy and facies record a dominant tectonic, rather than climatic, control on sedimentation. Deposition occurred on tectonic time scales inconsistent with a Snowball Earth model for Neoproterozoic diamictites involving a direct glacial contribution to deposition.
Do Cenozoic analogues support a plate tectonic origin for the Earth’s earliest continental crust?
Archean continental crust largely comprises the trondhjemite, tonalite, and granodiorite/ dacite (TTG/D) suite of igneous rocks. Formation of the earliest Archean (>3.5 Ga) TTG/Ds is controversial, being attributed to either subduction zone processes with active plate tectonics or thermochemical mantle convection with no plate tectonic processes. A suite of Cenozoic adakite-like lavas in Jamaica has geochemical compositions comparable to early Archean TTG/D. The data indicate that the adakites were generated by underthrusting (or subducting) and partial melting of oceanic plateau crust beneath Jamaica. This setting is analogous to proposed plate tectonic processes in the early Archean where hot, thick, and more buoyant Archean oceanic crust underthrusts adjacent plates. The new adakite data imply that earliest Archean TTG/D continental crust could have formed above primitive subduction zones.
Modern-style plate subduction preserved in the Palaeoproterozoic West African craton
Nature Geoscience, 2011
The timing of onset of modern-style plate tectonics is debated. The apparent lack of blueschist metamorphism 1 -a key indicator of modern plate subduction 2 -in rocks aged more than about 1 billion years calls into question the existence of plate tectonics during the Archaean and Palaeoproterozoic eras 3,4 . Instead, plate tectonics and subduction could have either not occurred at that time 5 , or could have proceeded differently 6 owing to warmer conditions in the early Earth mantle 7 . Here we use thermodynamic models 8-10 to investigate the formation conditions of metamorphic minerals in the 2.2-2.0 Gyr old West African metamorphic province. We find a record of blueschist metamorphism in these rocks. We show that minerals such as chlorite and phengite formed at high pressures of 10-12 kbar, low temperatures of 400-450 • C and under a geothermal gradient of 10-12 • C km −1 . These conditions are typical of modern subduction zones. We therefore suggest that modern-style plate tectonics existed during the Palaeoproterozoic era. We conclude that ancient blueschist metamorphism may exist in other parts of the world, but the identification of these rocks has so far been hampered by methodological problems associated with deciphering their pressure and temperature evolution.
A Proterozoic-rift origin for the structure and the evolution of the cratonic Congo basin
Earth and Planetary Science Letters, 2011
Keywords: intracratonic basins "Cuvette Centrale" of Congo continental rifting sediment backstripping We interpret the cratonic Congo basin, a large circular "Cuvette Centrale" filled with up to 9 km of Proterozoic to Neogene sediments, as the consequence of a Neo-Proterozoic rift. Firstly, the magnitude and the long-term subsidence are consistent with the thermal time-constant of a 200-250 km thick lithosphere inferred from several tomographic studies. Secondly, the surface accumulation of sediments is compensated at depth by crustal thinning, whose magnitude can be estimated from the analysis of the surface gravity: after backstripping the effect of the sediments, a residual NW-SE positive and narrow gravity anomaly is observed across the "Cuvette Centrale" and is interpreted as the remaining crustal thinning associated with this rift. Assuming that isostasy is governed by a necking level and a flexural response to sediment loads, we have estimated the combination of the depth of necking and the equivalent elastic thickness of the lithosphere that provide the best fit with the residual gravity, i.e. 10 km and 100 km respectively. The corresponding uplift of the upper mantle is in the continuity of the Mbuji-Mayi Supergroup to the SE and the Liki-Bembian Group to the NW. These two groups represent older stages of rifting in the Congo craton, which shows that rifting has periodically affected and weakened the "Cuvette Centrale" during a long period of time.
Filling in the juvenile magmatic gap: Evidence for uninterrupted Paleoproterozoic plate tectonics
Earth and Planetary Science Letters, 2014
Despite several decades of research on growth of the continental crust, it remains unclear whether the production of juvenile continental crust has been continuous or episodic throughout the Precambrian. Models for episodic crustal growth have gained traction recently through compilations of global U-Pb zircon age frequency distributions interpreted to delineate peaks and lulls in crustal growth through geologic time. One such apparent trough in zircon age frequency distributions between ∼2.45 and 2.22 Ga is thought to represent a pause in crustal addition, resulting from a global shutdown of magmatic and tectonic processes. The ∼2.45-2.22 Ga magmatic shutdown model envisions a causal relationship between the cessation of plate tectonics and accumulation of atmospheric oxygen over the same period. Here, we present new coupled U-Pb, Hf, and O isotope data for detrital and magmatic zircon from the western Churchill Province and Trans-Hudson orogen of Canada, covering an area of approximately 1.3 million km 2 , that demonstrate significant juvenile crustal production during the ∼2.45-2.22 Ga time interval, and thereby argue against the magmatic shutdown hypothesis. Our data is corroborated by literature data showing an extensive 2.22-2.45 Ga record in both detrital and magmatic rocks on every continent, and suggests that the operation of plate tectonics continued throughout the early Paleoproterozoic, while atmospheric oxygen rose over the same time interval. We argue that uninterrupted plate tectonics between ∼2.45 and 2.22 Ga would have contributed to efficient burial of organic matter and sedimentary pyrite, and the consequent rise in atmospheric oxygen documented for this time interval.