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1.
The Sidi Flah and Ougnat inliers are located in the eastern Anti-Atlas antiform between the Anti-Atlas Major Fault (AAMF) and South Atlas Fault (SAF). They consist of many granitoid intrusions emplaced into Neoproterozoic metasedimentary rocks and surmounted by upper Neoproterozoic A-type granites. The Sidi Flah (Saghro) and Ougnat granitoids are part of the Neoproterozoic magmatic activity related to northwards subduction of an oceanic plate beneath the Saghro continental margin. They are post-orogenic I- and S-type granitoids related to the ending of the compressional deformation in this Pan-African belt. A petrographic, geochemical and zircon typology study leads us to subdivide these rocks into three magmatic groups: (1) a medium- to high-K calc-alkaline group formed by quartz diorites and amphibole granodiorites is found in both Sidi Flah and Ougnat inliers; (2) a high-K calc-alkaline group is present in Sidi Flah. These two groups have a (deeper and) hybrid mantle-crust origin; (3) a peraluminous group in Ougnat is linked to the post-collisional setting and has a shallow crustal source. On a primitive mantle-normalized trace-element diagram, almost all of these rocks show a significant Nb depletion relative to K and La, which is typical of the calc-alkaline magmatism from the subduction-zone environment. Absence of structural marks of thrusting upon the West African craton (WAC) of this arc system and the ophiolitic suite in Bou-Azzer, and the presence of Imiter muscovite-bearing granite as part of Pan-African belt do not support the localization of northern limit of WAC at the level of SAF.  相似文献   

2.
《Comptes Rendus Geoscience》2018,350(6):279-288
Between the High Atlas and the Saharan platform, the Anti-Atlas of Morocco offers large exposures of Precambrian rocks beneath the moderately folded Paleozoic series. These inliers allow reconstructing a segment of the Pan-African Belt and of its foreland at the northern outskirts of the West African Craton (WAC). From ∼ 885 Ma to ∼ 540 Ma, three periods are recognized in the Pan-African cycle. The Tonian–Cryogenian period ends with the obduction of supra-subduction ophiolite and oceanic arc material at ∼ 640 Ma. The Early Ediacaran period is marked by the development and subsequent closure of a wide marginal basin next to a likely Andean-type arc. The Late Ediacaran period is recorded by subaerial molasse deposits associated with post-collisional high-K calc-alkaline to shoshonitic magmatism. Although a wide consensus has been reached based on the number of new robust datings, several questions still remain pending, which we address taking into account relevant African and European correlations.  相似文献   

3.
Aeromagnetic data of the Anti-Atlas Mountains show an important magnetic anomaly along the ‘Major Anti-Atlas Fault’, produced by different mafic and ultramafic rocks of a Neoproterozoic ophiolite complex. The magnetic modelling of Bou Azzer–El Graara ophiolitic suture shows a deep-seated anomaly through the upper continental crust corresponding to a north-dipping subduction. The polarity of the Pan-African subduction in the Anti-Atlas is therefore compatible with the contemporaneous Pan-African orogenic belts, where polarity of subduction dipped away from the West African Craton during the amalgamation of Western Gondwana. To cite this article: A. Soulaimani et al., C. R. Geoscience 338 (2006).  相似文献   

4.
We report a new regional correlation for the Ediacaran succession in the Anti-Atlas belt on the northwestern margin of Gondwana, based on U-Pb LA-ICP-MS zircon geochronology of volcanic rocks in the NE edge of the Saghro inlier. The thick volcano-sedimentary succession comprises a diverse suite of rhyolitic-ignimbrite, basaltic to andesitic lava fields, rhyolitic lava, mafic hydroclastic complex, fallout and surge deposits, pyroclastic dyke, interbedded clastic sediment and subvolcanic bodies.Ten volcanic rocks yield crystallization ages ranging from 573 to 547 Ma, consistent with a lower and upper Ouarzazate Supergroup affinity respectively. Inherited zircon ages range from 623 Ma to 600 Ma, analogous to zircon peaks in the older volcano-sedimentary rocks of the Bou Salda, and Saghro groups in the Anti-Atlas, suggesting the continuity of the Saghro Group beneath the Ouarzazate Supergroup at the NE edge of the Saghro inlier.Rocks with a lower Ouarzazate Supergroup affinity include lithic-poor ignimbrites which yield ages of 573.6 ± 1.9 Ma, 571.8 ± 4.2 Ma, 571.3 ± 2.6 Ma, and 567.4 ± 2.9 Ma, two fallout deposits which yield ages of 563.5 ± 2.1 Ma and 569.2 ± 1.9 Ma, a surge deposit dated at 571.6 ± 2.8 Ma and a rhyolite lava dated at 562.5 ± 3.1 Ma. Two lithic-poor ignimbrites from the upper Ouarzazate Supergroup are dated at 557.3 ± 2.6 Ma and 547.9 ± 3.1 Ma.Volcanic activity at the NE edge of the Saghro inlier is related to West African Cadomian orogenic (WACadomian) activity between 620 and 560 Ma. During this period the Saghro and Bou Salda groups were deposited, followed by the lower Ouarzazate Supergroup. Later extension along the Gondwanan margin took place close to Ediacaran – Cambrian boundary, contemporaneous with upper Ouarzazate Supergroup deposition.  相似文献   

5.
The Saghro Group consists of a thick volcanic-sedimentary sequence with intercalated basaltic lavas, the first magmatic event in eastern Saghro area. Nd isotopes of basaltic pillow lavas show TDM model ages ranging from 640 to 580 Ma, which represent a maximum age for basalt eruption.Granitoids within the Saghro Group consist of a charnockitic suite, tonalites, granodiorites and monzogranites. They are high-K calc-alkaline (HKCA) with a post-collisional character, and were emplaced at high-levels in the crust. Their ages of emplacement are within the 580–560 Ma bracket, implying that the entire Saghro Group is slightly older than or partly coeval to granitoid emplacement and implying a common geodynamical setting. Sr–Nd isotopic compositions and Nd TDM model ages point to a mixed origin, combining a juvenile mantle source and an Eburnean crustal component, which could be the West African Craton (WAC). The juvenile component in the Saghro granitoids could be the depleted upper mantle that has sourced the earlier basalts.Field observations, geochemical and geochronological data together support that, during the Pan-African orogeny, the Anti-Atlas was subjected to a regional transpressional to transtensional event. This event would have been responsible for the dissection of the northern margin of the WAC into several blocks, the development of deep sedimentary basins and the emplacement of HKCA magmas.  相似文献   

6.
New geochronological analyses (U–Pb SIMS zircon ages) have yielded ages of 552 ± 5 Ma for the Bou Madine rhyolitic dome (Ougnat, eastern Anti-Atlas), 543 ± 9 Ma for the Tachkakacht rhyolitic dyke (Saghro–Imiter, eastern Anti-Atlas), and 531 ± 5 Ma for the Aghbar trachytic sill (Bou Azzer, central Anti-Atlas). Inherited zircon cores from the Aghbar trachytic sill and from the Bou Madine rhyolitic dome have been shown to be of Statherian age (ca. 1600–1800 Ma) and Palæoproterozoic (>2100 Ma) age, respectively, suggesting that a significantly older protolith underlies the Pan-African rocks in the Central and Eastern Anti-Atlas. Granodiorites and rhyolites from the Saghro–Imiter area have similar low 87Sr/86Sr (0.702–0.706) and 143Nd/144Nd (0.5116–0.5119) initial ratios, suggesting a mixture of mantle and lower crust sources. This can also be inferred from the low 187Os/188Os ratios obtained on pyrite crystals from the rhyolites.A recently published lithostratigraphic framework has been combined with these new geochemical and geochronological data, and those from the literature to produce a new reconstruction of the complex orogenic front that developed at the northern edge of the Eburnian West African craton during Pan-African times. Three Neoproterozoic magmatic series can be distinguished in the Anti-Atlas belt, i.e., high-K calc-alkaline granites, high-K calc-alkaline to shoshonitic rhyolites and andesites, and alkaline-shoshonitic trachytes and syenites, which have been dated at 595–570, 570–545 and 530 Ma, respectively.The accretion of the Pan-African Anti-Atlas belt to the West African super continent (WAC) was a four-stage event, involving extension, subduction, moderate collision and extension. The calc-alkaline magmatism of the subduction stage was associated with large-scale base metal and gold mineralisation. Metallogenic activity was greatest during the final extensional stage, at the Precambrian–Cambrian boundary. It is characterised by world-class precious metal deposits, base–metal porphyry and SEDEX-type occurrences.  相似文献   

7.
In the Tifnoute Valley, three plutonic units have been defined: the Askaoun intrusion, the Imourkhssen intrusion and the Ougougane group of small intrusions. They are made of quartz diorite, granodiorite and granite and all contain abundant mafic microgranular enclaves (MME). The Askaoun granodiorite and the Imourkhssen granite have been dated by LA-ICP-MS on zircon at 558?±?2 Ma and 561?±?3 Ma, respectively. These granitic intrusions are subcontemporaneous to the widespread volcanic and volcano-detrital rocks from the Ouarzazate Group (580–545 Ma), marking the post-collisional transtensional period in the Anti-Atlas and which evolved towards alkaline and tholeiitic lavas in minor volume at the beginning of the Cambrian anorogenic intraplate extensional period. Geochemically, the Tifnoute Valley granitoids belong to an alkali-calcic series (high-K calc-alkaline) with typical Nb-Ta negative anomalies and no alkaline affinities. Granitoids and enclaves display positive εNd-560Ma (+0.8 to +3.5) with young Nd-TDM between 800 and 1200 Ma and relatively low 87Sr/86Sr initial ratios (Sri: 0.7034 and 0.7065). These values indicate a mainly juvenile source corresponding to a Pan-African metasomatized lithospheric mantle partly mixed with an old crustal component from the underlying West African Craton (WAC). Preservation in the Anti-Atlas of pre-Pan-African lithologies (c. 2.03 Ga basement, c. 800 Ma passive margin greenschist-facies sediments, allochthonous 750–700 Ma ophiolitic sequences) indicates that the Anti-Atlas lithosphere has not been thickened and was never an active margin during the Neoproterozoic. After a transpressive period, the late Ediacaran period (580–545 Ma) is marked by movement on near vertical transtensional faults, synchronous with the emplacement of the huge Ouarzazate Group and the Tifnoute Valley granitoids. We propose here a geodynamical model where the Tifnoute Valley granitoids as well as the Ouarzazate Group were generated during the post-collisional metacratonic evolution of the northern boundary of the West African craton. The convergence with the peri-Gondwanan active margin produced brittle fracturing of the cratonic boundary without thickening, allowing rising of magmas. The Tifnoute Valley granitoids display a metasomatized lithospheric mantle source mixed with a minor ancient (2 Ga) continental crust component from the underlying WAC.  相似文献   

8.
《Geodinamica Acta》2001,14(6):373-385
The Early Cryogenian groups of Sidi Flah, Kelaat Mgouna and Boumalne (Saghro, Anti-Atlas, Morocco) are constituted by turbiditic deposits and interbedded lavas, accumulated in tectonic basins. At Sidi Flah, volcanics are transitional showing initial rift tholeiites (IRT) fingerprint and alkali basalts of oceanic island basalt (OIB) compositions. At Kelaat Mgouna, volcanics consist of low-Nb continental tholeiites. At Boumalne, basalts are of IRT composition. The volcanic and sedimentary formations belong to a nascent rift caused by thermal doming along a SW-NE axis, the Saghro rift. The continental break-up occurred in the early Neoproterozoic and during the Rodinia supercontinent dislocation, within a continent called “Ibero-saharian Craton” which was in front of the West-African Craton located near the South Pole. The Saghro rift is contemporaneous to the opening of an oceanic domain represented by Central Anti-Atlas ophiolites and related to the extension of the Brazialiano Ocean.  相似文献   

9.
Cambro-Ordovician palaeogeography and fragmentation of the North Gondwana margin is still not very well understood. Here we address this question using isotopic data to consider the crustal evolution and palaeogeographic position of the, North Gondwana, Iberian Massif Ossa–Morena Zone (OMZ). The OMZ preserves a complex tectonomagmatic history: late Neoproterozoic Cadomian orogenesis (ca. 650–550 Ma); Cambro-Ordovician rifting (ca. 540–450 Ma); and Variscan orogenesis (ca. 390–305 Ma). We place this evolution in the context of recent North Gondwana Cambro-Ordovician palaeogeographic reconstructions that suggest more easterly positions, adjacent to the Sahara Metacraton, for other Iberian Massif zones. To do this we compiled an extensive new database of published late Proterozoic–Palaeozoic Nd model ages and detrital and magmatic zircon age data for (i) the Iberian Massif and (ii) North Gondwana Anti-Atlas West African Craton, Tuareg Shield, and Sahara Metacraton. The Nd model ages of OMZ Cambro-Ordovician crustal-derived magmatism and Ediacaran-Ordovician sedimentary rocks range from ca. 1.9 to 1.6 Ga, with a mode ca. 1.7 Ga. They show the greatest affinity with the Tuareg Shield, with limited contribution of more juvenile material from the Anti-Atlas West African Craton. This association is supported by detrital zircons that have Archaean, Palaeoproterozic, and Neoproterozoic radiometric ages similar to the aforementioned Iberian Massif zones. However, an OMZ Mesoproterozoic gap, with no ca. 1.0 Ga cluster, is different from other zones but, once more, similar to the westerly Tuareg Shield distribution. This places the OMZ in a more easterly position than previously thought but still further west than other Iberian zones. It has been proposed that in the Cambro-Ordovician the North Gondwana margin rifted as the Rheic Ocean opened diachronously from west to east. Thus, the more extensive rift-related magmatism in the westerly OMZ than in other, more easterly, Iberian Massif zones fits our new proposed palaeogeographic reconstruction.  相似文献   

10.
The boundaries of the West African Craton mark the location of a continuous suture zone that records Neoproterozoic to Early Cambrian oceanic closure. The western part of the circum-West African suture zone extends through the line of outcrop of the Mauritanide, Bassaride and Rokelide mountain belts. Our geochemical analyses are consistent with the idea that igneous and metamorphic rocks of the Rokelide and Southern Mauritanide mountain belts of West Africa occupy a suture zone that records the closing of a Neoproterozoic to Early Cambrian ocean basin during the Pan-African orogeny and final assembly of Gondwana. The closing of that basin was marked by the collision between Archean rocks of the Leo massif of the West African Craton and reactivated Archean and Paleoproterozoic rocks that now outcrop nearer to the coast of Africa in Sierra Leone and Liberia. Within the Rokelides, the geochemistry of the Kasewe Hills volcanic rocks and Marampa amphibolite indicate that remnants of an arc system are caught up in the suture zone. The geochemistry of Guingan schists that outcrop along strike of the Rokelides is compatible with the idea that the metamorphosed equivalents of the Marampa and Kasewe Hills arc volcanic rocks extend through the Bassarides and into the Southern Mauritanides.  相似文献   

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