Metamorphic evolution of the Brianconnais units along the ECORS-CROP profile (Western Alps): new data on metasedimentary rocks

Brianconnais units are squeezed between two Mesozoic eclogitic belts ( Piemont-Ligurian ocean and Valaisan ocean) along the ECORS-CROP seismic line in the Italian-French Western Alps ( France, Italy). The metamorphic evolution of this area plays a key role for understanding the evolution of the Western Alps and is discussed on the basis of detailed petrographic investigations carried out on weathered sediments issued from the erosion of the Hercynian belt, especially on lower Permian to Mesozoic sediments. In the Zone Houillere, as well in the Permo-Triassic cover of the Brianconnais basement, the index metamorphic mineral assemblage is mainly composed of white micas with varying chemical composition, chloritoid and garnet. This same assemblage occurs within different lithologies (metaarkose, metapelite, metasandstone). Consequently, equilibrium phase diagrams were computed for different whole rock compositions using DOMINO software. The results of the P-T investigations clearly show that each unit underwent a different sequence of metamorphic reactions. An increase in metamorphic grade from greenschist facies conditions in the Northwest ( Zone Houillere) to the transition between blueschist and eclogite facies conditions in the Southeast ( Internal Brianconnais) is observed. A major discontinuity in metamorphic grade is located at the contact between Zone Houillere and Ruitor unit, as documented by a pressure gap of similar to 7 kbar. In general, the observed metamorphic field gradient is inverted and is interpreted to represent different depths of burial during subduction, which correlates with the paleogeographic position of the different units.