Eo-Alpine HP metamorphism in the Permian intrusives from the steep belt of the central Alps (Languard-Campo nappe and Tonale Series)

Diorites and granitoids that intruded the Upper Austroalpine units of the central Alps during the Permian display mappable tectonic imprints and metamorphic transformations that were acquired during the Alpine tectonometamorphic cycle. Superposed heterogeneous deformations interacted with metamorphic re-equilibration stages and created a range of textural types that reflect local deformation gradients: coronitic transformations textures, normally foliated S-tectonites and mylonitic foliations. The three textural types are distinguished on maps recording foliation trajectories of successive deformation phases, which are correlated to the evolution of metamorphic assemblages. Tectonic deformation of Alpine age is represented by three generations of ductile syn-metamorphic structures. The mineral assemblages stable during the first Alpine deformation phase (DI) are Amp(II) + Pl(II) + white mica(I) + Zo/Czo + Grt + Qtz ± Mg-Chl ± Ilm in metadiorites and Pl(II) + white mica(I) + Zo/Czo + Grt + Amp(II) + Qtz ± Ilm/Ttn in metagranitoids; the successive foliations D2a and D2b are defined by greenschist facies minerals. Thermobarometric estimates allow T = 500-600 °C and P = 1.1 ± 0.2 GPa conditions to be determined during D1 and T ≤ 350 °C and P ≤ 0.5 GPa during D2. Relict igneous minerals in metadiorites allow to determine intrusive conditions of T = 879 ± 110 °C and P = 0.4-0.7 GPa. Radiometric ages and P/T ratio of Alpine P(max) T(Pmax) suggest that the inferred P-T-d-t path may represent the thermal state of the initial Alpine subduction stages. (C) 2000 Editions scientifiques et medicales Elsevier SAS.