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

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The Ronda peridotite supplies one of the best objects to document subcontinental mantle deformation, but its internal deformation and exhumation mechanisms remain controversial. Here we provide new structural data and numerical results that constrain the Oligocene-Miocene deformation history of the Ronda massif. We first describe a mantle shear zone in the northern massif that developed subcrustal strain localization during decompression and related partial melting. The deformation regime of this mantle shear zone evolved from penetrative NE-SW stretching to sinistral shear highlighted by discrete shear bands. We then show structural observations that document a viscous deformation in the southern massif occurring prior to the thrust-assisted emplacement of the peridotite during large decompression. Finally, we performed numerical investigations that quantify a high temperature of 980°C for the basal peridotite lens at the time of its crustal emplacement. Our numerical results constrain the timing of ductile deformation of the peridotite just before 22 Ma, probably between 30 and 22 Ma. Altogether, these features led us to conclude that the deformation and exhumation of the Ronda peridotite results from lithosphere thinning subsequently inverted in the course of the Oligocene-Miocene. Among available models, our findings support the hypothesis of peridotite exhumation by the inversion of a thinned back-arc continental lithosphere during westward slab rollback through the Alboran region.
ISSN: 0278-7407

insu-00924180
https://hal-insu.archives-ouvertes.fr/insu-00924180
https://hal-insu.archives-ouvertes.fr/insu-00924180/document
https://hal-insu.archives-ouvertes.fr/insu-00924180/file/tect20062.pdf
DOI : 10.1002/tect.20062

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
For the first time, an albite orthogneiss has been recognised and dated within the HP-LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U-Pb age of 480.8 ± 4.8 Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6-647.4 Ma. Zircon and titanite U-Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U-Pb age of 366 ± 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP-LT peak metamorphism.
ISSN: 1437-3254

insu-00714570
https://hal-insu.archives-ouvertes.fr/insu-00714570
DOI : 10.1007/s00531-011-0732-5

Éditeur(s) : HAL CCSD
Résumé : International audience
The late stages of the Variscan orogeny in western and central Europe are characterized by the emplacement of numerous Carboniferous granitic intrusions. In the southern part of the Armorican massif, volcanic and plutonic rocks occur within the Carboniferous series of the Ancenis basin. Chemical analyses indicate that they belong to the same magmatic high-K calc-alkaline association of peraluminous composition, which likely derives from crustal melting of an igneous source. A LA-ICPMS U-Pb/zircon age of 319.3 ± 3.1 Ma dates, for the first time, this magmatic event and, by there, constrains the maximum age of both the end of the sedimentation and the northward tilting of the sediments in the Ancenis basin. This magmatic event is coeval with the sedimentation of the coal-bearing Namurian basins located along the Nort-sur-Erdre fault. These intrusions are interpreted as the feeding conduits of the volcano-sedimentary layers encountered in these basins. This would imply a minimal dextral displacement of ca. 20-25 km along the Nort-sur-Erdre fault.
Bulletin de la Socciété Géologique de France

insu-00656606
https://hal-insu.archives-ouvertes.fr/insu-00656606

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
High 3He/4He ratios in some basalts have generally been interpreted as originating in an incompletely degassed lower-mantle source1, 2, 3, 4, 5, 6, 7, 8, 9. This helium source may have been isolated at the core-mantle boundary region since Earth's accretion4, 5, 6. Alternatively, it may have taken part in whole-mantle convection and crust production over the age of the Earth7, 8, 9; if so, it is now either a primitive refugium at the core-mantle boundary8 or is distributed throughout the lower mantle7, 9. Here we constrain the problem using lavas from Baffin Island, West Greenland, the Ontong Java Plateau, Isla Gorgona and Fernandina (Galapagos). Olivine phenocryst compositions show that these lavas originated from a peridotite source that was about 20 per cent higher in nickel content than in the modern mid-ocean-ridge basalt source. Where data are available, these lavas also have high 3He/4He. We propose that a less-degassed nickel-rich source formed by core-mantle interaction during the crystallization of a melt-rich layer or basal magma ocean5, 6, and that this source continues to be sampled by mantle plumes. The spatial distribution of this source may be constrained by nickel partitioning experiments at the pressures of the core-mantle boundary.
ISSN: 0028-0836

hal-00941025
https://hal.archives-ouvertes.fr/hal-00941025
DOI : 10.1038/nature11771