Granitoid emplacement controlled by a thrust fault : structural analysis, microtextural evidences and quartz C-axis patterns. An example from hercynian plutons in the Frech Massif Central.
Résumé
The Hercynian belt results of the collision between Gondwana, Armorica and Laurussia from South to North respectively. The French Massif Central which belongs to Gondwana experienced a complex kinematic history with several low-angle ductile and syn-metamorphic compressional and extensional events. The stack of nappes recognized in the whole Massif Central is also observed in Eastern Rouergue. In this area, the early nappe emplacement (named D1 event) is accommodated by a southward shearing which causes the superposition of the Upper Gneiss Unit above the Lower Gneiss Unit which itself overlies the Para-autochthonous Unit. A second event, D2, reworks the nappe pile with a top-to-the NW shearing. Porphyritic monzogranite plutons, called Pinet-type granites, record the D1 and D2 events. A petrostructural study of the Pinet-type plutons has been undertaken in order to understand the structural framework of the granite emplacement, and to better constrain the tectonic history of the Rouergue area. In order to characterize the relative timing and deformation mechanisms of the Pinet-type plutons, studies of textural relationships associated with quartz , and axes measurements have been realized. The D1 event is characterized by high temperature quartz axis sub-fabrics (600C-700C). The quartz axis sub-fabric are complex and do not provide clear kinematic data in agreement with other shear criteria. Two main mechanisms, active at high temperature near the subsolidus stage, explain the quartz c-axis development: oriented crystallisation process during the deformation and intracrystalline prism glide system. The D2 event is characterized by post-solidus quartz fabric with prism and basal glide systems active around 350C-450C. The shear sense given by the asymmetric fabric is in good agreement with the other shear criteria and demonstrates a top-to-the NW shearing.