Meso and microstructural evolution of the Mont Morion metaintrusive complex (Dent-Blanche nappe, Austroalpine domain, Valpelline, Western Italian Alps)

The Mont Morion complex of the Dent Blanche nappe is interpreted as a multi-stadial Alpine km-scale shear zone. Permian intrusive rocks are transformed into white mica chlorite-bearing or glaucophane-bearing gneisses along high-strain horizons (100m-thick), while within low-strain cores (100- to 1000m-thick), metaintrusives preserve igneous features and xenoliths of amphibolites, acid granulites and biotite-bearing gneisses. Ten superposed evolutionary stages (1 to 10) occurred: from pre-Permian (1 and 2, only in pre-intrusive host rocks) to Permian-Triassic (3 and 4) and Mesozoic-Cenozoic (5 to 10). Stages 1 and 2 developed under amphibolite to granulite-facies conditions. During stage 3, the Mont Morion complex was intruded (P-range = 0.25-0.70 GPa at T between 600-760°C). Stage 4 occurred under greenschist facies conditions. Stages 5 and 6 are associated with the development of tectonitic mylonitic km-scale domains under blueschist facies conditions. Stage 7 is represented by folding of S6 under greenschist-facies conditions. Metre-scale shear zones developed during stages 8, 9 and 10. The pre-Alpine part of the P-T-t-d path is interpreted as trace of post-collisional thinning Variscan continental crust, related to the upwelling of magmas (e.g. Mont Collon-Matterhorn gabbros; Arolla granites); the Alpine part of the P-T-t-d path is interpreted as record of continental crust subduction and exhumation, related to the Alpine oceanic-continental subduction and continental collision.