Variscan P–T deformation paths from the southwestern Aiguilles Rouges massif (External massif, western Alps) and their implication for its tectonic evolution

Abstract

Petrological and structural investigations in metapelites and amphibolites were undertaken to unravel the Variscan tectonic and metamorphic evolution of the southwestern Aiguilles Rouges massif. The geometry of planar and linear structures indicates a transpressive deformation regime with a change from predominantly subhorizontal to subvertical movements during its evolution. In metapelite samples from gneiss areas, garnet zonation trends have been correlated with chemical variations in plagioclase and biotite by microstructural observations. Applied conventional cation-exchange and net-transfer geothermobarometers yielded anti-clockwise P–T-deformation paths. Clockwise P–T-deformation paths resulted from (Na, Ca)-amphibole zonations of amphibolite samples from a different lithotectonic unit. Notable differences in P–T conditions and the general shape of P–T paths for the gneisses and amphibolites indicate that the units do not represent a continuous metamorphic zonation and underwent no common tectono-metamorphic evolution throughout the entire deformation. The combination of P–T-deformation paths and structural investigations revealed first-stage thrust tectonics followed by a transpressive deformation which was responsible for the uplift of the southwestern Aiguilles Rouges massif.