A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)

Abstract The distribution of ultrahigh-pressure metamorphism ( UHP ) at the scale a mountain belt is prime importance for deciphering its past subduction history. In Western Alps, coesite has been recognized in southern Dora-Maira Massif, lens-shaped Brossasco-Isasca Unit, but not found up to now other parts massif. We report discovery new unit northern Massif (Western Alps), named Chasteiran Unit. It only few tens metres thick and consists graphite-rich, garnet–chloritoid micaschists, whose protoliths may be black shales Silurian age. Garnet inclusions (chloritoid, rutile) growth zoning allow precisely model P – T evolution. Coesite crystals, which are pristine or partially transformed palisade quartz occur as garnet outer cores. According thermodynamic modelling, displays continuous record during prograde increase (25–27 kbar 470–500 °C) (stage 1), stability field (27–28 510–530 2), well sub-isothermal decompression about 10 (down 15 500–515 3). main regional, composite, foliation, marked by chloritoid rutile, began develop this stage, was then overprinted chlorite–ilmenite 4). Unit discontinuously exposed immediate hangingwall Pinerolo it located far away from, without physical links classic Moreover, records different, much colder, P–T evolution, showing that different slices were detached from downgoing slab. fourth coldest Alpine known so entire belt. Its conditions comparable ones Tian Shan coesite–chloritoid-bearing rocks.