2.46 Ga kalsilite and nepheline syenites from the Awsard pluton, Reguibat Rise of the West African Craton, Morocco. Generation of extremely K-rich magmas at the Archean–Proterozoic transition

The Awsard pluton in the Moroccan part of the Reguibat Rise, West African Craton, contains the oldest kalsilite-bearing rocks discovered to date, with a SHRIMP zircon U/Pb age of 2.46 ± 0.01 Ga. The pluton is composed of nepheline syenites, kalsilite syenites and minor silica-saturated syenites, all with the same primitive Sr and Nd initial isotope compositions and Nd model age that cluster around 87Sr/86Sr2.46Ga ≈ 0.7029, ɛNd2.46Ga ≈ −1.4, and TDM ≈ 2.75 Ga. Silica-saturated syenites are in fact contact fenites that grade into nepheline syenites, but the two feldspathoidal syenites are true magmatic rocks that crystallized from two coeval highly fractionated K-rich magmas with sharply different chemical compositions. Chemical, isotopic and textural evidence suggests that the two magmas originated by liquid immiscibility within an already fractionated alkaline potassic magma of asthenospheric origin that split in two phases, a nephelinitic melt rich in HFSE + REE, and a kalsilitic melt poor in HFSE + REE. The Awsard pluton, spatially associated to carbonatites and other alkaline rocks, does not mark a Late Archean fossil subduction zone but represents the first manifestation of a Paleoproterozoic alkaline province located in the Reguibat Rise, the full extent and importance of which is yet to be determined

Multiscale simulation and experimental analysis of damping in CFRP structures containing rubber

International audienceThis paper gives an intuitive numerical multi-scale method to estimate damping in anisotropic viscoelastic hybrid composite structures using finite element analysis. Different CFRP-R (CFRP with Rubber) architectures, both microstructural and macrostructural, are studied and compared in order to maximize damping but also to minimize rigidity loss. Homogenization by virtual DMA in frequency domain is performed on representative volume elements (RVE) to obtain the effective viscoelastic behaviour of every hybrid microstructure. The effective behaviours are used to define mechanical behaviour of laminates on which vibratory simulations are performed. Interesting and advanced simulations are discussed regarding materials parameters and geometrical aspects and are compared to experimental results

[Geochemical features and tectonic setting of Late Neoproterozoic Vendian volcanism in the western High Atlas, Morocco]

Late Neoproterozoic Vendian volcanic and volcaniclastic rocks are widely distributed in the western High Atlas. They are located north of the Tizi n'Test Fault, separating the West African Craton from a northerly adjacent craton. These volcanic rocks overlie a sernipelitic formation, which represents the equivalent of the Tidilline and Anzi Formations,of the Anti-Atlas. The geochemical characteristics of these volcanic rocks' suggest a calc-alkaline active margin environment associated with the post Pan-African tectonics. They differ from those of the Anti-Atlas by their lower content of K2O. The later rock type was generated by a melting process of the crust subducted beneath the northern craton. A carbonate-shale unit, which contains examples of interstratified calc-alkaline dacite, overlies the volcanic succession, demonstrating that the volcanic activity continued sporadically until Early Cambrian times. (C) 2002 Elsevier Science Limited. All rights reserved

The southern and central parts of the “Souttoufide” belt, Northwest Africa

International audienceThe Souttoufide belt is situated between the High-Atlas in the north, the Mauritanides in the south and the West African Craton as well as the Tindouf basin in the east. Thus, this belt is a part of the West African fold belt system which surrounds the West African Craton. Outcrops occur only in three areas, while the rest of the Souttoufide belt is covered by Meso- to Cenozoic sediments. These areas are, from the south to the north, the Adrar Souttouf Massif, the Smara Zemmour area, and the western part of the Anti-Atlas. Because of the continuing processing and numerous publications, the Anti-Atlas cannot be dealt with in detail. Fundamental geological work in both of the other areas was mainly done during the second third of the 20th century, followed by several decades of inactivity. Numerous studies were done in the Adrar Souttouf massif during the last decade, while the Smara-Zemmour area still remains poorly investigated. Most of them were focused on geochronological analyses and geochemistry. Accordingly, the obtained data led to a new interpretation. The Adrar Souttouf is interpreted as a Pan-African belt exhibiting a Neoproterozoic island arc reworked by the Variscan collision between the North American Craton (NAC) and the West African Craton (WAC). The Smara-Zemmour area is considered as the external part of this Variscan belt thrusted onto the Palaeozoic Tindouf basin. The western Anti-Atlas as considered as a Pan-African active margin reworked during the Variscan orogen. Together, these three areas, lead to the hypothesis of a Late Pan-African belt capped by the Early and Middle Palaeozoic covers and then affected by the Middle Carboniferous tectonic event (330 Ma) related to the Pangaean assembly. Correlations with adjacent belts (Mauritanides and Anti-Atlas) are considered

Geodynamic evolution of the northwestern Paleo-Gondwanan margin in the Moroccan Atlas at the Precambrian-Cambrian boundary

International audienceIn the southern Moroccan Atlas, abundant volcanic and sedimentary formations, dated from the Ediacaran to Cambrian time, were set at the northwestern Paleo-Gondwanan margin, after the main Pan-African orogenic event. The Precambrian-Cambrian geodynamic transition is characterized by an Early Cambrian marine transgression. We examine the tectonic conditions of this transgression and the magmatic signatures of the volcanic rocks that were produced just before and around the Precambrian-Cambrian boundary. Significant angular unconformities are evidenced, between the Late Neoproterozoic formations and the Cambrian deposits, in the central and eastern Anti-Atlas, which are due to a late Ediacaran NNE-SSW compressional event. The Late Neoproterozoic formations are related to an intracontinental volcanic chain of andesitic to rhyolitic lavas dated to the Ediacaran period. These calc-alkaline rocks were generated by melting of the mantle, previously metasomatized during the Pan-African orogenic stage, and of continental crust. The Late Ediacaran to Early Cambrian formations are analyzed in the Agoundis-Ounein and Toubkal areas, southwest of the old block of High-Atlas. An important basaltic pile unconformably overlies the Ediacaran rhyolitic formation and is overlain by Tommotian sediments. These basalts are continental tholeiites generated by melting of a normal subcontinental mantle. They outpoured from an important N 30°-trending fissural system over a basin floor. Some lherzolite fragments have been sliced along southwest-northeast faults, in the Lower Cambrian sediments. They originated from a transitional mantle between continental and oceanic domains. Farther east of the central Anti-Atlas, the Tommotian Djbel Boho volcano exhibits olivine basalts having an intraplate enriched asthenospheric signature type of ocean island basalt. The magmatic characteristics of the Late Ediacaran to Early Cambrian volcanic rocks, the structural features, and the presence of lherzolite fragments are consistent with a volcanic passive margin rift setting in a WNW-ESE extension regime. The meaning of this extensional event is discussed in relationships to the opening of a Cambrian basin and the drifting of the Avalonian terranes