The ultrahigh-pressure unit of the Dora-Maira Massif

The Dora-Maira Massif (DMM) is one of the internal crystalline massifs of the Western Alps; it was part of the margin of the European continent before its collision with the Adria microcontinent during the Oligocene, after the opening of an intervening ocean during the Mesozoic and its subduction during the Paleocene. Below eclogite-facies remnants of this oceanic crust – the Monviso meta-ophiolite, the DMM exposes a pile of slivers of upper continental crust that reached different metamorphic grades during the Eocene. In the southern part of the massif, one of these slivers, the Brossasco-Isasca Unit, is less than one-kilometer thick and consists of Permian granite and its country-rocks (metapelite and minor marble). The whole unit underwent an Alpine UHP metamorphism epitomized by the occurrence of coesite, a high-pressure polymorph of quartz, in various rock-types and by the presence of near-end member pyrope garnet (Py99) in ‘whiteschist’, a highly magnesian coesite - pyrope - kyanite - white mica +/- talc metasomatic rock occurring as lenses within metagranite. The stability of the kyanite-jadeite assemblage and the presence of the silicate ellenbergerite as inclusions in pyrope megacrysts (up to 25 cm, Schertl et al., 1991) are additional features attesting peak metamorphic pressures exceeding 3 GPa (i.e. = 100 km depth) at temperatures near 700-750°C.As “type locality” for UHP metamorphic rocks and terranes, the Dora-Maira Massif has been continuously attracting new studies: mineralogical (six new phases), petrological (benchmarking of new barometers), geochemical (Mg-metasomatism) or geochronological (continental subduction rate, peak metamorphism, very rapid exhumation), e.g. Gebauer et al. (1997); Rubatto and Hermann (2001).