Structural, AMS and geochronological study of a laccolith emplaced during Late Variscan orogenic extension: the Rocles pluton (SE French Massif Central)

Abstract

International audienceIn the southern French Massif Central, the Rocles leucogranite of Variscan age consists of three petrographic facies; textural analysis shows that they experienced the same subsolidus deformation. New chemical U-Th-Pb dating on monazite yielded 324 ± 4 Ma and 325 ± 5 Ma ages for muscovite-rich and biotite-rich facies respectively. AMS-study results agree with petrostructural observations. The magnetic planar and linear fabrics, which correspond to the preferred orientation of biotite and muscovite, are consistent with the foliation and lineation defined by the preferred mineral orientation. This fabric developed during pluton emplacement. The accordance of this granite foliation with that observed in the host rock, suggests that the Rocles pluton is a laccolith, but its present geometry resulted from post-emplacement southward tilting due to the uplift of the Late Carboniferous Velay dome. Restoration of the primary geometry of the pluton and its country-rocks to a flat-lying attitude places the granite lineation close to the trend measured in other plutons of the area. This restoration further supports the interpretation of the Rocles laccolith as a pluton emplaced along a tectonic contact reactivated during the late-orogenic collapse of the Variscan Belt

Carboniferous normal faults in the Eastern Pyrenees: evidences and age constrains of syn-orogenic Variscan extension

“…This situation is comparable with that of the French Massif Central where two different extension episodes have been documented [7,18]. In this case however, they are separated by a longer span of time, due to the fact that in the French Massif Central syn-orogenic extension started earlier, during Late Visean-Serpukhovian [19,4]. In this segment of the Variscan orogen, syn-orogenic extension migrates southwards, towards the external zones: Late Visean-Serpukhovian in the Northern French Massif Central, Serpukhovian-Bashkirian (NamurianWestphalian) in the Montagne Noire [19] and Moscovian (Late Westphalian) in the Pyrenees, following the main Variscan deformation, which exhibits a dyachronous character.…”

Emplacement of the Lavadores granite (NW Portugal): U/Pb and AMS results

“…Granite bodies record the tectonic events that are related to their emplacement during magma cooling, through the orientation of the magmatic foliation and lineation, and trough solid-state deformation at high and low temperature that record the kinematics of deformation as crystallization progress (Be Mezeme et al, 2007;Gleizes et al, 2006;Verner et al, 2009). Based on petrographic overview and AMS data, we argued that the Lavadores granite structure indicates fabrics developed at a magmatic stage during magma crystallization.…”

Gravity inversion, AMS and geochronological investigations of syntectonic granitic plutons in the southern part of the Variscan French Massif Central

“…It is already well established that numerous granodioritic and leucogranitic plutons of Namurian to Westphalian age (325-315Ma) throughout the southern part of the FMC are syntectonic bodies that recorded the late orogenic ductile NW-SE extension (Faure et al 1992;Faure 1995;Talbot et al 2004Talbot et al , 2005aTalbot et al , 2005bBe Mezème et al 2006b). The Margeride pluton is one of the largest porphyritic monzogranitic pluton in the FMC.…”

“…Monazite U-Th-Pb chemical dating of granitic massifs is now increasingly used to address geochronological questions as it provides accurate and fast age determination that allow to constrain tectonic belt evolution and pluton emplacement (Cocherie and Albarede 2001;Cocherie et al 2005;Be Mezème et al 2006a, 2006b). Granitic bodies often record tectonic regimes developed during a short time interval.…”