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 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 garnet-chloritoid
micaschists. 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°C) (stage 1), up to the coesite stability
field (27-28 kbar 520-530°C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to
15 kbar 500-515°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.