Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Kekem shoshonitic gabbro-norite association is part of the high-K calc-alkaline (HKCA) post-collisional magmatism, a major feature of the Pan-African Belt in Cameroon. LA-ICP-MS U-Pb zircon analyses provide an age of 576 ± 4 Ma for the Kekem complex. This age is interpreted as dating the emplacement of the massif during the waning stage of the Pan-African orogeny. The latter is related to dextral movements along the Central Cameroon Shear Zone (CCSZ). The REE patterns display enriched LREE (LaN/YbN = 14.2-23.5) while HREE present a nearly flat profile (DyN/YbN = 1.3-1.7), and the La/Sm and Sm/Yb ratios led to propose that the Kekem gabbro-norites have been derived from the partial melting of a garnet-spinel lherzolite mantle source. The negative Nb-Ta and Ti anomalies and the positive Pb anomalies indicate that this mantle source was modified by contribution of a subduction-related material. The low Ce/Pb (2.6-10.4) and Th/Yb ratios associated to high Ba/La ratios, indicate that source enrichment could be related to slab derived fluids. As a whole, the Kekem geochemical features suggest that primary gabbro-noritic magmas derived from a subduction-modified mantle source (metasomatised lithospheric mantle). Moderately high 86Sr/87Sr initial ratios (0.7068-0.7082), low εNd (−5 to −9) and old Nd TDM model ages (1.6-1.8 Ga) are interpreted to result from contamination of Neoproterozoic mantle by the Paleoproterozoic crust. The ca. 576 Ma movements along the CCSZ are related to a Neoproterozoic metacratonization of the northern margin of the Congo craton during the Pan-African orogeny. This metacratonization led to vertical planar lithospheric delamination along lithospheric transcurrent faults, asthenospheric uprise and partial melting of the Paleoproterozoic lithospheric mantle.
ISSN: 0899-5362

hal-00903665
https://hal.archives-ouvertes.fr/hal-00903665
DOI : 10.1016/j.jafrearsci.2013.03.010

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The N-S oriented Raghane shear zone (8 degrees 30') delineates the western boundary of the Saharan metacraton and is, with the 4 degrees 50' shear zone, the most important shear zone in the Tuareg shield. It can be followed on 1000 km in the basement from southern Air, Niger to NE Hoggar, Algeria. Large subhorizontal movements have occurred during the Pan-African orogeny and several groups of granitoids intruded during the Neoproterozoic. We report U-Pb zircon datings (laser ICP-MS) showing that three magmatic suites of granitoids emplaced close to the Raghane shear zone at c. 790 Ma, c. 590 and c. 550 Ma. A comprehensive and detailed (158 sites, more than 1000 cores) magnetic fabric study was performed on 8 plutons belonging to the three magmatic suites and distributed on 200 km along the Raghane shear zone. The main minerals in all the target plutons do not show visible preferential magmatic orientation except in narrow shear zones. The AMS study shows that all plutons have a magnetic lineation and foliation compatible with the deformed zones that are zones deformed lately in post-solidus conditions. These structures are related to the nearby mega-shear zones, the Raghane shear zone for most of them. The old c. 793 Ma Touffok granite preserved locally its original structures. The magnetic structures of the c. 593 Ma Ohergehem pluton, intruded in the Aouzegueur terrane, are related to thrust structures generated by the Raghane shear zone while it is not the case of the contemporaneous plutons in the Assode-Issalane terrane whose structures are only related to the subvertical shear zones. Finally, the c. 550 Ma granite group has magnetic structure related to the N-S oriented Raghane shear zone and its associated NNE-SSW structures when close to them, but NW-SE oriented when further. These NW-SE oriented structures appear to be characteristic of the late Neoproterozoic evolution of the Saharan metacraton and are in relation to the convergence with the Murzuq craton. This evolution reflects the rheological contrast existing along the Raghane shear zone marking the western boundary of the Saharan metacraton.
ISSN: 0040-1951

hal-00429354
https://hal.archives-ouvertes.fr/hal-00429354
DOI : 10.1016/j.tecto.2009.04.022

Éditeur(s) : HAL CCSD Cambridge University Press (CUP)
Résumé : International audience
The Amalaoulaou Neoproterozoic island-arc massif belongs to the Gourma belt in Mali. The metagabbros and pyroxenites forming the main body of this arc root show the pervasive development of garnet. In the pyroxenites, the latter has grown by reaction between pyroxene and spinel during isobaric cooling. By contrast, in the metagabbros, garnet textures and relations to felsic veins exclude an origin through solid-state reactions only. It is proposed that garnet has grown following dehydration and localized melting of amphibole-bearing gabbros at the base of the arc. The plagioclase-saturated melts represented by anorthositic veins in the metagabbros and by trondhjemites in the upper part of the massif provide evidence for melting in the deep arc crust, which locally generated high-density garnet clinopyroxene rutile residues. Garnet growth and melting began around 850 degrees C at 10 kbar and the tonalitic melts were most probably generated around 1050 degrees C at P >= 10 kbar. This HT granulitic imprint can be related to arc maturation, leading to a P-T increase in the deep arc root and dehydration and/or dehydration-melting of amphibole-bearing gabbros. Observation of such features in the root of this Neoproterozoic island arc has important consequences, as it provides a link to models concerning the early generation of continental crust.
ISSN: 0016-7568

hal-00420960
https://hal.archives-ouvertes.fr/hal-00420960
DOI : 10.1017/S0016756808005499

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In the Arokam Tenere, the three studied massifs of granitoids are located around the N-S oriented Raghane mega-shear zone, which separates two of the main domains of the Tuareg shield, the Saharan metacraton and the Central Hoggar. The field observations, AMS analyses and U-Pb zircon dating were completed by a study of Landsat images because of the scarcity of outcrops in several parts of the studied area.;The image analysis allows to distinguish Arokam-East and Abdou granitoids in the eastern plutonic complex. It has also shown that the western plutonic complex corresponds to two different intrusions: the Yvonne granite and granodiorite. This is confirmed by the magnetic fabric that presents different characteristics in the different granitoids. U-Pb zircon dating and field observations show that the Arokam-West basement granite is much older (1915 Ma) than the Hanane granodiorite (582 Ma) in the central plutonic complex. Arokam-East and -West granites then belong to the Eburnean basement. The magnetic fabric of these granites is mainly associated with a post-magmatic deformation, probably of Pan-African age. Yvonne granodiorite is likely contemporaneous of the main displacement along the Raghane shear zone. The Yvonne granite (594 Ma) and Hanane granodiorite have a fabric similar to that previously obtained in most plutons of the Tiririne area. This fabric is related to the regional stress field, associated with the activity of the Raghane shear zone, during the late-magmatic phase in the plutons. On the contrary, the magnetic fabric of the Abdou pluton is still reflecting only the magma flow.
ISSN: 0899-5362

hal-00617478
https://hal.archives-ouvertes.fr/hal-00617478
DOI : 10.1016/j.jafrearsci.2011.02.007

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Intraplate deformation is most often linked to major stress applied on plate margins. When such intraplate events are accompanied by magmatism, the use of several dating methods integrated within a multidisciplinary approach can bring constraints on the age, nature and source mobilized for generating the magma and in turn on the nature of the intraplate deformation. This study focuses on the large gabbro Arrikine sill (35 km in extension) emplaced within the Silurian sediments of the western margin of the Murzuq cratonic basin in southeastern Algeria. Its emplacement is dated during the early Devonian (415-400 Ma) through the determination of a reliable paleomagnetic pole by comparison with the Gondwana Apparent Polar Wander Path (APWP). This age can be correlated with deep phreatic eruptions before Pragian time thought to be at the origin of sand injections and associated circular structures in Algeria and Libya. For the sill, the K–Ar age of 325.6 ± 7.7 Ma is related to a K-rich aplitic phase that has K-enriched by more than 20% the Devonian gabbro. Laser-ICP-MS U–Pb method dates only inherited zircons mostly at c. 2030 Ma with additional ages at c. 2700 Ma and younger ones in the 766-598 Ma age range. The Arrikine sill is a high-Ti alkaline gabbro having the geochemical composition of a hawaiite akin to several intraplate continental and oceanic provinces, including the contemporaneous Aïr ring complexes province in Niger, but also to the Mauna Loa volcano in Hawaii. This peculiar composition akin to that of the contemporaneous Aïr province is in agreement with a lower Devonian age for the Arrikine sill.The lower Devonian Arrikine sill emplacement is related to a “Caledonian” transtensive reactivation of the western metacratonic boundary of the Murzuq craton. This event also generated in the Saharan platform the so-called “Caledonian unconformity” of regional extension, the Aïr ring complexes and magmatic rocks that produced sand injections. It could be related to rifting of the Hun terranes that occurred at the plate margin to the north (Stampfli and Borel, 2002, Blackey, 2008 and references therein). The mid-Carboniferous (c. 326 Ma) reactivation corresponds to Variscan compression on NW Africa generating aplitic fluids, but also to the major “Hercynian unconformity” of regional extension. The generation of the Arrikine magma is attributed to partial melting through adiabatic pressure release of uprising asthenosphere along tectonically reactivated mega-shear zones, here bordering the relictual Murzuq craton enclosed in the Saharan metacraton.
ISSN: 0899-5362

hal-01306637
https://hal.archives-ouvertes.fr/hal-01306637
DOI : 10.1016/j.jafrearsci.2015.11.019