The emplacement of the Montagne Noire axial zone (French Massif Central): New insights from petro-textural, geochronological and AMS studies
Résumé
In the southern French Massif Central, the Montagne Noire axial zone is a NE-SW elongated granitic-migmatitic dome surrounded by recumbently folded Palaeozoic sedimentary series with a southern vergence. From rim to core, the axial zone consists of micaschist, gneiss, augen orthogneiss, cordierite migmatite, and anatectic granites (Laouzas, Montalet). Syn- to late kinematic granitoids intrude the dome. The relative succession of events in Montagne Noire: 1-recumbent folding, 2-doming and 3-post-migmatitic granitoids intrusion is widely acknowledged. Conversely, the tectonic setting of the doming is disputed. The proposed models do not take into account the granitic and migmatitic structures, geochronology and dome core structure. Therefore, petro-textural and Magnetic Susceptibility Anisotropy studies and new monazite U-Th/Pb datings have been carried out in the Montagne Noire axial zone. The Montalet granite (333±4 Ma) is coeval with migmatization, and the Soulié granite (318±4 Ma) marks dome intrusion. In the dome axial zone rocks, magmatic, late-magmatic, tectonic and metamorphic textures have been recognized. Magnetic mineralogical study indicates that biotite is the main carrier of the magnetic susceptibility in granitic and migmatitic rocks, and that cordierite does not influence vectors directions. The attitude of the magnetic foliation complies with the preferred orientation of biotite, schlierens, enclaves and with migmatitic foliation observed in the field. The AMS fabric confirms the elliptical shape of the axial zone inferred from the migmatitic and gneissic envelope. The southward dipping foliation identified in both flanks is in agreement with an asymmetric dome. Moreover, the magnetic lineations exhibit a contrasting pattern with a dominant NE-SW trend, and a N-S one as well. The AMS features are in agreement with a diapiric component for the migmatite ascent. A model involving diapirism coeval with anticlinal stacking followed by extension discussed.