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

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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. It is suggested that the 5000 km wide Pan-African domain of Saharan Africa suffered regional continental lithospheric mantle delamination during the early stages of this orogeny. Delamination 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. -from Authors
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... Garfunkel 1999;Johnson and Woldehaimanot 2003;El-Bialy 2010;Johnson et al. 2011;Morag et al. 2011;Moreno et al. 2012;Fritz et al. 2013). The concluding chapter in the Precambrian history of the ANS involved extensive alkaline volcanism (580-550 Ma) marking the transition to a within-plate setting that was prevalent throughout the Phanerozoic (Bentor 1985;Black and Liégeois 1993;Stern 1994;Garfunkel 1999;Genna et al. 2002;Eyal et al. 2010;El-Bialy and Hassen 2012;Golan and Katzir 2017). These late Precambrian alkaline volcanics occur as minor, though widespread, exposures throughout the ANS (Harris 1982). ...
... At this pivotal stage, magmatism switched from calc-alkaline to alkaline and the geodynamic regime in which these magmas intruded/ extruded varied from subduction-related to post-collision, to intra-plate extension and ultimately to a stable platform (e.g. Bentor 1985;Black and Liégeois 1993;Mushkin et al. 2003;Samuel et al. 2007;Eyal et al. 2010;El-Bialy and Hassen 2012;Azer et al. 2014;Robinson et al. 2015;Abbo et al. 2020;El-Bialy 2020;Litvinovsky et al. 2021). The Abu Durba alkaline volcanics-shallow intrusions (ADV) were emplaced through an anorogenic stage in which the at present rigid ANS massif was subjected to extensional stresses and intraplate rifting (Figure 11). ...
Article
Anorogenic magmatism in the northern Arabian-Nubian Shield occurred during a long-lasting crustal extension event (<580 Ma), which succeeded the formation of the Pan-African orogenic belt in NE Africa and Arabia. Late Neoproterozoic anorogenic felsic volcanic-subvolcanic rocks, along with post-collisional granitoid suites, are exposed in the Gabal Abu Durba mountain range along the eastern flank of the Gulf of Suez in Sinai. These rocks include granite and rhyolite porphyries, metaluminous to weakly peralkaline with distinct potassic alkaline and ferroan affinities. Major and trace element characteristics such as K2O/MgO >16, total alkalis >8.5 wt.%, (K2O + Na2O)/CaO >10, agpaitic index (NK/A) >0.85 and Zr (>250 ppm), Nb (>20 ppm), Y (>80 ppm), Zn (>100 ppm) and Ce (>100 ppm), and 10,000 × Ga/Al >2.6 and [Zr + Nb + Y + Ce] >350 ppm are suggestive of an A-type granite genesis. Zircon U-Pb dating of two representative samples returned crystallization ages of 569 ± 2.6 Ma and 561.7 ± 3.2 Ma for granite and rhyolite porphyries, respectively. The melting temperatures estimated at 997–1020°C are consistent with high-temperature liquidus conditions. Fractional crystallization, coupled with less significant crustal assimilation, was likely the main mechanism of formation of these rocks from a common primitive asthenospheric mantle-derived trachytic magma in an anorogenic intra-plate setting. The A-type alkali granite and rhyolite porphyries of Gabal Abu Durba are manifestations of mantle upwelling and lower crust underplating in the late Ediacaran anorogenic stage. These felsic subvolcanic intrusions (≈ 569–562 Ma) along with their volcanic counterparts constitute a new unmatched rock unit that most probably represents the last chapter of the anorogenic magmatism in the Neoproterozoic Arabian-Nubian Shield crust before the beginning of Phanerozoic era.
... Abdelrahman [12] identified ophiolitic fragments in the Keraf area. The western and the southern extensions of the Nile Craton are not precisely defined yet, therefore many names apart from 'Nile Craton' are [13][14][15]. Pre-Keraf structures are best preserved in the Nubian and Bayuda Deserts to the west of the Keraf shear zone [5]. ...
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In the present study, two regions of the NE, N areas of Sudan have been investigated through gravity probing. A recognized number of anomalies have been observed, interpreted in the light of the plate tectonics theory models and the related structural deformation of the Wilson cycle in the western part of the Nubian-Arabian Shield. These gravity data have been acquired by many foreign and geological institutions contoured to prepare two gravity maps of the whole country, and N Sudan regions. The derived models uncover a new region with mantle up-welling and mantle wedge along the Red Sea offshore and central NE Sudan as a possible extension of S of Keraf Shear Zone, respectively, while the others have shown crustal thickening decorated by ophiolitic shear zones. High anomalies indicate a higher densityrock appraised to exist from the 3D gravity model and buried within the vicinity of Atbara and Shandi cities and are being referred to as buried ophiolitic sinusoids, low-grade metamorphic rocks of greenschist facies with tectonic mélange of a subduction zone. Their possible source according to the anomalies is surmised to be the Serpentinized - Mantle wedge of the Neoproterozoic subduction zone formed beneath the southern extension of Keraf Shear Zone, which is buried under the recent thinner and lighter geological cover in the Shendi-Atbra vicinity. A crustal thickening texture with various undulations in the Mohos’ interface has been observed from the gravity map that occurs in the lithospheric length between Nile valley and the Red Sea offshore.
... The crust formed during the Cryogenian stage includes four main lithologic units, which are metavolcano-sedimentary successions, dismembered ophiolitic nappes and ophiolitic mélange, metagabbrodiorite complexes and quartz diorite-trondhjemite-tonalitegranodiorite intrusions (older granites). The Ediacaran age (post-collisional episode) starts after the cessation of collision between the east and west Gondwana (Stern, 1994;Abdelsalam and Stern, 1996;Stoeser and Frost, 2006) as a result of extensional collapse of the thickened ANS crust (Greiling et al., 1994;Blasband et al., 2000;Be'eri-Shlevin et al., 2009a, 2009b, probably due to lithospheric delamination (Black and Liégeois, 1993;Moghazi, 2003;Avigad and Gvirtzman, 2009;Farahat and Azer, 2011;Castillo, 2021). The post-collision stage of the ANS evolution was characterized by the eruption of K-rich Dokhan volcanic rocks, deposition of molasse-type sediments and emplacement of shallow level felsic A-type younger granite intrusions (Beyth et al., 1994;Jarrar et al., 2003;Johnson, 2003;Eyal et al., 2010;Moghazi et al., 2012). ...
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Three Mo-bearing granitic intrusions, namely: Gattar, Abu Harba, and Abu Marwa occur in the Gattar area in the North Eastern Desert of Egypt. These late tectonic granites intrude Neoproterozoic rocks of island arc metavolcanics and metagabbro-diorite complexes in addition to post-collision molasse-type sediments (Hammamat Group) and Dokhan-type volcanic rocks. U–Pb zircon geochronology yielded ages of 630 ± 27 Ma, 600.1 ± 8.5 Ma, and 601.1 ± 2.4 Ma for the Gattar, Abu Harba, and Abu Marwa granite intrusions, respectively. Whole-rock geochemical data showed the granites to be alkaline to peralkaline post-collision A-type granites. The three granite intrusions are considered co-magmatic as indicated by their similar ages and the continuous variation of major and trace elements on the variation diagrams. The inferred crystallization temperatures, as calculated from zircon geothermometry according to Watson et al., 2006, fall in the range of 616-690°C (average = 662°C), 592-720°C (average = 653°C), and 592-894°C (average = 706°C) for the Gattar, Abu Harba, and Abu Marwa granite intrusions, respectively. This indicates that the Abu Marwa granite is the least evolved, followed by the Gattar and Abu Harba granites. The calculated logfO2 values for zircons from all the studied granites show highly oxidized magma features. The Abu Marwa granite (average logfO2 = -12.62) has the strongest oxidized state, whereas Gattar granite (average logfO2 is -15.46) and Abu Harba granite (average logfO2 = -15.99) have moderate logfO2 values implying moderate oxidized states. Guided by the tectonic evolution of the post-collision stage in the Arabian-Nubian Shield, it is suggested that the original magma of the granites in the Gattar area was formed by partial melting of an underplated crustal source, which had been fertilized by alkali-, HFSE-, and REE-rich fluids released and metasomatized by an underplating mantle-derived magma. Fractional crystallization and fluid differentiation/metasomatism were major processes in the petrogenetic evolution of these granite intrusions. Fluorine-complexing of HFSEs and REEs was involved in the early stages of fractional crystallization (i.e., crystallization of the Abu Marwa granite intrusion), resulting in a HFSE- and REE-rich residual melt that form the more evolved alkali feldspar granite of Gattar and Abu Harba intrusions. The late magmatic ore-forming fluids get the Gattar granite intrusion to be enriched in molybdenite relative to Abu Harba and Abu Marwa granites. The reason for the different degrees of molybdenite enrichment in the three intrusions depended on many factors (e.g. degree of fractionation and oxygen fugacity of the granitic magma).
... Un assemblage de régions hétérogènes où les corrélations sont difficiles. L'ensemble a été comprimé, au cours du même cycle orogénique, entre deux grands continents convergents, aboutissant à deux grandes collisions, l'une avec le craton Est Africain, l'autre avec le craton Ouest-Africain Black et Liégeois 1993 ;Caby et al. 1981 ;Liégeois et al. 1987). ...
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Known for its gold mineralization, Amesmessa is a very important region located in the southern part of In Ouzzal in Hoggar (south of Algeria). In this area, outcrops a particular and rather enigmatic rock, carbonatite, the latter is a magmatic rock composed of various carbonates, and which is associated with alkaline syenites, the combination is intersecting an Archaean and granulitic basement of In Ouzzal. Carbonatite is often characteristic of significant enrichment of rare earths and other rare metals. In this present study, a bibliographic synthesis is provided on the subject, supported by petrographic observations, the use of remote sensing, a research guide, recommendations and a discussion. The results obtained are promising and will prompt further research into the region. Keywords: Carbonatite, Syénite, Rare Earth Elements, Amesmessa, In Ouzzal, Hoggar. Connue pour sa minéralisation aurifère, Amesmessa est une région très importante qui se situe dans la partie méridionale de l’In Ouzzal au Hoggar (Sud Algérien). Dans cette zone affleure une roche particulière et assez énigmatique, la carbonatite, cette dernière est une roche magmatique composée de divers carbonates, et qui est associée à des syénites alcalines, le tout recoupant un socle Archéen et granulitique de l’In Ouzzal. La carbonatite est souvent caractéristique d’un enrichissement significatif en terres rares et autres métaux rares. Dans cette présente étude, une synthèse bibliographique est fournie sur le sujet, soutenue par des observations pétrographiques, l’utilisation de la télédétection, un guide de recherche, des recommandations et une discussion. Les résultats obtenus sont prometteurs et inciteront à d’éventuelles recherches plus approfondies sur la région. Mots-clés: Carbonatite, Syénite, Éléments de Terre Rare, Amesmessa, In Ouzzal, Hoggar.
... Globally, A-type granites occur in Precambrian and Phanerozoic terrains, mainly in extensional tectonic settings (e.g. Loiselle & Wones, 1979;Eby, 1990Eby, , 1992Black & Liégeois, 1993). A-type granites are distinct from other granite types and are defined as relatively anhydrous with high SiO 2 , high Na 2 O þ K 2 O, low MgO and high incompatible trace element contents including REE, Zr, Y, Nb and Ta (e.g. ...
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We report a new zircon U–Pb age of 1257 ± 6 Ma for the Punugodu granite (PG) pluton in the Eastern Dharwar Craton (EDC), Southern India. The Mesoproterozoic PG is alkali feldspar hypersolvus granite emplaced at shallow crustal level, as evident from the presence of rhyodacite xenoliths and hornfelsic texture developed in the metavolcanic country rocks of the Neoarchaean Nellore Schist Belt (NSB). Geochemically, the PG is metaluminous, ferroan and alkali-calcic, and is characterized by high SiO 2 and Na 2 O + K 2 O, Ga/Al ratios >2.6, high-field-strength elements and rare earth element (REE) contents with low CaO, MgO and Sr, indicating its similarity to anorogenic, alkali (A-type) granite. The highly fractionated REE patterns with negative europium anomalies of PG reflect its evolved nature and feldspar fractionation. Mafic (MME) to hybrid (HME) microgranular enclaves represent distinct batches of mantle-derived magmas that interacted, mingled and undercooled within the partly crystalline PG host magma. Felsic microgranular enclaves (FME) having similar mineralogical and geochemical characteristics to the host PG most likely represent fragments of marginal rock facies of the PG pluton. The PG appears to be formed from an oceanic island basalt (OIB)-like source in an anorogenic, within-plate setting. The emplacement of PG ( c. 1257 Ma) in the vicinity of Mesoproterozoic Kanigiri Ophiolite ( c. 1334 Ma) shows an age gap of nearly 77 Ma, which probably suggests PG emplacement in an extensional environment along a terrain boundary at the western margin of the Neoarchaean NSB in the EDC.
... Our data show an E-W age migration trend from the Zuku (186 Ma) through the Banke (180 Ma) to the Dutsen Wai (177 Ma) complexes (Figure 2a). These trends have been interpreted as resulting from subordinated transcurrent faults linked to the N-S-trending mega-shear zone extending from northern Niger to northcentral Nigeria (Rahaman et al. 1984;Black et al. 1985;Black and Liegeois 1993;Girei et al. 2019). Varied structural emplacement trends seen in the NaYG province (e.g. the E-W emplacement trend) and in other parts of the Trans-Saharan alkaline province (e.g. the Hoggar massifs; Guiraud et al. 1987) suggest strong structural controls on emplacement of anorogenic magmatism. ...
Article
Landsat-7 ETM + satellite imagery integrated with corrected airborne geophysical data (radiometric, gravity, and aeromagnetic) anomalies maps, and field investigations were used to discriminate lithologies and related structures in Mayo-Belwa and Jada-Ganye-Toungo areas (Adamawa Massif), northeastern Nigeria. Field investigations portray the granite-gneisses as the oldest rock units, and have sequentially been invaded by the syn-, late-to post-collisional Older granites. Subsurface linear structures in the area display dominant NE-SW, NW-SE, NNE-SSW, and ENE-WSW trends, reflective of variations in pre-existing structural orientations, and near-surface expressions of reactivated zones of weakness in the basement crust. The geodynamic systems revealed that most elongated lineaments are expressed as axial part of valleys trending NE-SW, ENE and NNW, forming diagonal grid to the NW-SE stresses. Abnormally high total radioactivity gravitates to areas of deflection (alluvium), and late- and post-tectonic granites. The gravity intensity provides a spectrum of anomalous responses including abnormally high positive gravity (volcanic rocks), moderately high positive gravity (granite-gneisses and late-to post-tectonic and syn-tectonic granites), background gravity and low negative gravity (granite-gneisses, alluvium and proluvium). Aeromagnetic anomalies discriminate the Older granites (intense positive anomalies), granite-gneisses, sediments (moderate positive, near background, and moderate-intensive negative anomalies), syn-tectonic granites (moderate positive anomalies), banded-gneiss and amphibolite (intensive negative anomalies), volcanic rocks (intense negative anomaly), and alluvium/proluvium (low positive anomalies). This present study led to the development of up-to-date and accurate litho-structural and geophysical maps of the geologically-rugged and hardly accessible Adamawa Massif, north-eastern Nigeria and further constrain the geochemical (High-K calc-alkaline, peraluminous) and tectonic (syn-to post-collisional) characteristics of typical Pan-African granitoids.
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Shale gas reservoirs witnessed an increasing interest in the last few years, due to their abundant reserves and the increasing of worldwide demand on energy. Algeria is an oil-rich country and hosts one of the largest unconventional shale gas reservoirs in the world. This study focuses on unraveling paleoclimatic and paleoenvironmental conditions of lower Silurian formation (Llandovery) from the Ahnet basin (south of Algeria), being considered as one of important prospects under-evaluation for unconventional shale gas resources. In this study, a detailed mineralogical investigation of shale minerals using various techniques such as X-ray dif-fraction (XRD) on oriented aggregates clays and scanning electron microscopy (SEM-EDX) in addition to spectroscopy natural gamma (SNG). The results show that the dominant facies is black shale with a ratio of Th/U (˂ 4) deposited in a marine sedimen-tation environment manifested by a low Th/K and U/K values (average = 10.40 ppm/pct and 14.41 ppm/pct, respectively). The mineralogical assemblage of black shales deduced from Th versus K cross-plots represented illite and kaolinite. The XRD analysis performed on oriented slides showed the trend of kaolinite, illite, and chlorite. The chlorite mineral is relatively constant (~ 5%). The distribution of kaolinite and illite shows an inverse change. An increase in the percentage of kaolinite and decrease in illite from upper Llandoverian (illite ~ 73 to ~ 68%, kaolinite: ~ 19 to ~ 28%) to lower Llandoverian (illite ~ 64 to ~ 48%; kaolinite ~ 31 to ~ 46%) reflects the evolution to a deep marine environment and humid climatic conditions; moreover, it confirmed by SEM-EDX results, they have further argued the evolution toward deep marine environments, as they show a dispersion of sulfides which can be shown with the mineral pyrite on the other hand absence of carbonate minerals. Paleoenvironmental studies of the lower Silurian period of the Ahnet basin show that the shales of the Llandoverian epoch characterized by adequate conditions allow the preservation of organic matter and the generation of hydrocarbons. In addition, the mechanical characteristics of the stable structures of illite and kaolinite allow maintaining the hydraulic fracturing in good quality.
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Field observations carried out in the northwestern edge of the Paleoproterozoic Nyong Complex in Edéa Region and their micropetrographic study show that the sets of syntectonic and pre-syntectonic granitoids in the area recorded a heterogeneous polyphase tectonic deformation influenced by the Sanaga fault. The structural analysis highlights the succession of four deformation phases (D1–D4). The first three are plastic strain, while the last is brittle and responsible for the current structural features. The D1 deformation phase is characterized by a general flattening induced by a shortening oriented NW–SE to E-W in a pure shear regime leading to the development of P1 folds, L1 lineations, strands B1 of N-S elongation and S1 foliation, and axial plane folds P1, which performs a differential shift towards the west. D2 deformation phase is an important continuous strain by simple shearing. It is at the origin of the P2 folding, the folds of which build the Ekitté duplexes with warping towards the south and the old dextral and sinistral shears ductile on the one hand, and ductilo-brittle in echelon on the other hand. It is the E-W major branch responsible for dumping the P1 fold eastward. The third phase of deformation D3 corresponds to a tightening underlined by a double subvertical shortening and E-W to NW–SE coupled to a very intense N-S stretch. It is also rotational by simple shear with a transpressive character marked by composite surfaces in which the C/S structures are parallel as in Mbanda, thus materializing the transposition of S1 by S3 and C3. The structures of this plastic strain bear witness to the horizontal movements associated with tectonics of collision between Congo and Sao Francisco cratons with the closure of an oceanic domain. The D4 phase is brittle and affects all plastic strain episodes. It then encompasses the whole deformation sequence, from the D1 phase to the neotectonic phase where it is prevalent. It affects the two previously defined fault systems: (1) the Sanaga-Batignol system made up of directional faults of E-W orientation, the westward movement of which is related to the opening of the South Atlantic and (2) the Ekitté system which consists of N-S faults with normal faults or strike-slip faults due to a differential northward slide generating rocks of tectonic origin at Pout Loloma.
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Precambrian terranes and units of the Dahomeyan (now Beninian belt) from Togo, Benin, and southwest Nigeria are described and compared with those from the northernmost part of the Brasiliano belt. The major phases of deformation and metamorphism occurred about 600 Ma in both continents. Foreland nappes derived from passive margin sedimentary beds of the West African craton display high-pressure metamorphism. Parautochthons include slices of an ophiolitic-type assemblage. The suture zone metabasic rocks are only recorded in the Beninian belt. They may represent the granulitized and eclogitized mafic root of ensimatic arc terranes, and have no equivalent in northeast Brazil. The Nigerian Province includes to the west a narrow elongated belt of highpressure granulites, also found in the northernmost part of the Borborema Province of northeast Brazil. Two main lithostratigraphic units can be recognized in both provinces: (1) gray gneisses that derive from Archean plutonic rocks, thoroughly deformed, recrystallized, and remobilized during the Pan-African-Brasiliano thermo-tectonic events; and (2) Proterozoic monocyclic units displaying the same petrostructural evolution as reworked Archean, and in which we tentatively recognize a Lower Proterozoic group consisting of aluminous metaquartzites and pelitic schists, which were intruded by 2(?) to 1.8 Ga anorogenic granites, and a younger disconformable flysch-type unit of assumed Late Proterozoic age. Large-scale horizontal movements responsible for a flatlying foliation and nappes affected several domains of the Nigerian and Borborema Provinces, whereas steep structures formed penecontemporaneously along synmetamorphic shear zones. Second-order late-metamorphic steep shear zones are considered as trans-continental lithospheric fractures that overprinted collisional structures. Among these, the 4°50' Kandi fault and the Sobral fault represent a particularly good correlation of northeast Brazil and southwest Nigeria-Benin, and are used to propose a rigorous Precambrian fit across the south Atlantic Ocean.
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The trends of heat flow density in West Africa are discussed in relation to the large-scale structural framework. The heat flow density is low in the stable Archean nuclei (30–40 mW m−2) and increases to 50–60 mW m−2 in the surrounding mobile belts affected by the Pan-African orogeny (600 Ma). In the northern part of West Africa, the high heat flow density (70–100mW m−2) is interpreted as resulting from a rejuvenation of the lithosphere during Mesozoic and Cenozoic in connection with several tectonic processes (Hercynian orogeny, Cretaceous rifting, Cenozoic volcanism and Alpine orogeny).
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A review of the recent literature from NE Africa and re-evaluation of available isotopic age data was carried out in order to demonstrate its evolutionary history during Pan-African times. -from Authors