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Etude structurale (ASM), pétrologique et géochimique (Rb-Sr, Sm-Nd, U-Pb) des plutons panafricains de Negash et de Konso (Ethiopie) : place des mélanges acides-basiques dans la construction des plutons calco-alcalins

Abstract : This multidisciplinary work has two major objectives: (i) understanding the structures and textures related to solidification of mafic-felsic magma interactions (mixing and mingling) and their implications ta mechanisms of magma emplacement and pluton construction and (ii) understanding the geodynamic and temporal relationships between two major structures of the Pan-African orogeny in Northeast Africa which met in Ethiopia : the Arabian Nubian Shleld (ANS) and the Mozambique Belt (MB). based on a comparative study of Negash (ANS) and Konso plutons (MB). Malic-felsic interactions. Two major types of mafic-feisic magma interactions characterize the Negash pluton: (i) mingling and mixing at the lower mafic-felsic interfaces and (il) strongly mixed and mingled monzodiorites. Structural, mineralogical and chemical data suggest that the pluton was construcled by a successive assemblage of four differentiated magma batches (at intermediate chambers), which were forcefully injectedinto pre-existing foliated country rocks and that the different types of hybridisation are not random but are related ta the dynamics and mechanismof pluton emplacement: (i) monzogranitic followed by granodioritic magmas; (ii) injection of a malt-rich monzodioritic/monzogabbroic tangue into the partially crystaliized felsic magmas resulting in inverted density gradient which in tum led to interactions at the lower interfaces; and (iii) simultaneous rising of granodioritic and monzodioritic magmas along the same conduit resulting in the strongly hybridised magma which represents the last intrusion. The hybrid rocks are not results of simple binary mixing but involve atleast three end-members (monzoqabbres, monzodiorites and monzogranites/granodiorites). All these data suggest emplacement of differentiated, low viscosity magma batches and that the pluton was constructed by an assembly of successive melt-rich catches (dyking) ANS and MS. The ANS, which is composed of calc-alkaline and juvenile magmatic rocks without any significant pre-Pan-African continental crust, is essentially a zone of accretion. The MB, consisting of juvenile magmatic rocks gneissic-granulitic assemblages, is not only a zone of accretion but also a zone of recycling of older crustal material. Both terrains display similar succession of contemporaneous events and both are zones of accretion, but correspond to distinct geodynamic environments. The Negash and Konso plutons represent late stages of the geodynamic evolution of the Pan-African orogeny in Ethiopia, representing different stages of the Wilson cycle where the Negash pluton is related ta late-tectonic magmatism during the closing stages of arc-arc accretion in the ANS, while the emplacement of the Konso pluton marked a transition from a post-collisional ta anorogenic intra-plate magmatism in the MB. The ANS and the MB in Ethiopia cduld be considered as contemporaneous and representing the same Wilson cycle but they reilect distinct geodynamic contexts and different levels of erosion.
Mots-clés : Magmatisme Orogenèse
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Submitted on : Thursday, March 29, 2018 - 10:43:09 AM
Last modification on : Monday, October 19, 2020 - 3:51:31 PM


  • HAL Id : tel-01746712, version 1



Asfawossen Asrat. Etude structurale (ASM), pétrologique et géochimique (Rb-Sr, Sm-Nd, U-Pb) des plutons panafricains de Negash et de Konso (Ethiopie) : place des mélanges acides-basiques dans la construction des plutons calco-alcalins. Sciences de la Terre. Université Henri Poincaré - Nancy 1, 2002. Français. ⟨NNT : 2002NAN10049⟩. ⟨tel-01746712⟩



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