Déformation et mise en place des granites (360-300Ma) dans un segment de la Chaîne Varisque (Plateau de Millevaches, Massif Central)

Abstract : The Limousin area is situated in the North West French Massif central and is characterized by large granitic plutons emplaced between 360 and 290 Ma. These plutons display close relationships with normal faults and strike slip faults forming the SE extension of the South-Armorican shear zone. The large N-S trending granitic complex of the Millevaches, limited by wrench faults and normal faults, displays structures characteristic of a emplacement within a strike-slip tectonic context.
Structural and microstructural analyses, AMS, gravity analysis and geochronology (40Ar/39Ar et U/Pb) allow us to propose a emplacement model for the Millevaches granite.
Magma ascent proceeds through vertical narrow ducts by successive injections along the NS-oriented principal axis of thePradines fault. Magmas are then trapped and channeled in the previously-formed flat-lying micaschist foliation which constitutes a major mechanical anisotropy of the middle crust. Synkinematic plutons emplaced in the dextral wrenching Pradines fault record N-S trending deformation trajectories in the Pradines fault and NW-SE on both sides of it The magma rising towards the roof of the laccolithe induces an oblate coaxial deformation accommodated by flat-lying normal faults. The crystallization of synkinematic leucogranites of the Millevaches dated at 313 ± 4 Ma, is coeval with the granulitic metamorphism which affected the surrounding micaschists.
Within the Limousin area, the onset of wrench tectonic is dated around 350 Ma and deformation ends around 300 Ma. We suggest that the two generations of granite (granodiorite-monzogranite and leucogranite) were emplaced around 350 Ma in a tectonic belt resulting from a transpressive tectonic context in response to the continental collision between Laurentia and Gondwana. Ductile shear zones constitute branch of a large and single wrenching lithospheric system similar to a NW-SE trending dextral “pop-up structure” going from the south Armorican Massif to the Limousin.
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Applied geology. Université Montpellier II - Sciences et Techniques du Languedoc, 2004. French


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Submitted on : Tuesday, February 22, 2005 - 8:57:29 AM
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Aude Gébelin. Déformation et mise en place des granites (360-300Ma) dans un segment de la Chaîne Varisque (Plateau de Millevaches, Massif Central). Applied geology. Université Montpellier II - Sciences et Techniques du Languedoc, 2004. French. <tel-00008553>

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