Kilometre-scale strain partitioning of the Mont Morion Permian metaintrusives during Alpine subduction (Dent-Blanche Unit, Valpelline - Western Italian Alps)

The structural and metamorphic evolution of Mont Morion Complex lithotypes, from pre-Permian to present, is presented. We identified pre-Permian rocks (e.g. amphibolite, biotite-bearing gneiss and acid granulite) preserved as roof pendants (i.e. xenoliths) within Permian intrusives. Permian intrusives and xenoliths are re-equilibrated during the Alpine time and coronitic to mylonitic metaintrusives are produced, due to meter to kilometer-scale fabric gradients, with associated white mica-, glaucophane-bearing gneiss. The tectono-metamorphic map shows the traces of the superimposed foliations and the fold axial planes. The foliations are distinguished on the basis of their relative chronology and mineralogical support. Using these structural data, reported on a single map, the successive stages of this portion of the Adria plate can be reconstruct: (i) a pre-Alpine extension, recorded by granulite- to amphibolite-facies xenolits; (ii) the Permian magmatic phase (e.g. Mont Morion, Mont Collon and Matterhorn intrusives); (iii) the Alpine subduction and subsequent collision.

La presente carta mostra l’evoluzione strutturale e metamorfica dei litotipi dal Permiano ad oggi. Le rocce pre-Permiane preservate come incassanti all’interno degli intrusivi Permiani. Durante la riequilibrazione Alpina tali rocce sono state trasformate in metaintrusivi da coronitici a milonitici associati a gneiss a mica bianca e glaucofane. Sulla carta sono riportate le tracce delle foliazioni e dei piani assiali, distinte sulla base della loro cronologia relativa e mineralogia caratteristica. Tali informazioni raccolte in una singola carta permettono di ricostruire i diversi stadi evolutivi di tale porzione crostale, dall’estensione pre-Alpina, seguita dalla fase intrusiva Permiana, fino alla subduzione e relativa collisione Alpina.