Granitoid emplacement during a thrusting event: structural analysis, microstructure and quartz c-axis patterns. An example from Hercynian plutons in the French Massif Central

Abstract

In the French Massif Central, the Rouergue–Albigeois area consists of three tectonic units stacked during the Hercynian orogeny. The structural analysis of the units and particularly the quartz 〈 c〉 axis, textural and structural observations in the syntectonic Pinet-type plutons allow us to determine the deformation history. A first thrusting event, D1, is responsible for a southwestward emplacement of the high-grade Lévézou nappe, above the Lower Gneiss Unit itself overlying the para-autochthonous micaschists. In Late Devonian–Early Carboniferous, this stack of nappes experienced a second event, D2, characterized by a top-to-the-NW emplacement under medium pressure/medium temperature conditions of the para-autochtonous unit upon the Lower Gneiss Unit. During D2, the Pinet-type plutons were deformed by a shearing consistent with the kinematics recorded in the country rocks. In contrast to previous works, which considered the Pinet-type plutons emplacement to be coeval with the D2 event, we argue that the emplacement of the plutons occurred during the top-to-the-SW D1 nappe stacking. This work emphasizes the importance of the quartz 〈 c〉 axis and microstructure analysis for the identification of the early structural history in polyphase deformed terranes.