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

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