1. A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif(Western Alps)
- Author
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Manzotti, Paola, Schiavi, F., Nosenzo, Francesco, Pitra, Pavel, Ballèvre, Michel, Stockton University (Stockton), Université Clermont Auvergne (UCA), Stockholm University, Géosciences Rennes (GR), Université de Rennes (UR)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire des Sciences de l'Univers de Rennes (OSUR), Université de Rennes (UR)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Rennes 2 (UR2)-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Rennes 2 (UR2)-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE)-Centre National de la Recherche Scientifique (CNRS), Czech Geological Survey [Praha], Società Geologica Italiana, and Dubigeon, Isabelle
- Subjects
[SDU.STU.TE]Sciences of the Universe [physics]/Earth Sciences/Tectonics ,[SDU.STU.TE] Sciences of the Universe [physics]/Earth Sciences/Tectonics ,chloritoid ,[SDU.STU.PE]Sciences of the Universe [physics]/Earth Sciences/Petrography ,[SDU.STU.PE] Sciences of the Universe [physics]/Earth Sciences/Petrography ,Dora-Maira ,coesite - Abstract
International audience; The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of primeimportance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in thesouthern Dora-Maira massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now inthe 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-chloritoidmicaschists. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P-Tevolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusionsin the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record ofgrowth during the prograde increase in P and T (25-27 kbar 470-500°C) (stage 1), up to the coesite stabilityfield (27-28 kbar 520-530°C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to15 kbar 500-515°C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, beganto develop during this stage, and was then overprinted by chlorite-ilmenite (stage 4). The Chasteiran Unit isdiscontinuously 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, muchcolder, P-T evolution, showing that different slices were detached from the downgoing subduction slab. TheChasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P-Tconditions are comparable to the ones of the Tian Shan coesite-chloritoid-bearing rocks.
- Published
- 2022