Chasteiran micaschist outcrop, Roure, Metropolitan City of Turin, Piedmont, Italyi

Regional Level Types
Chasteiran micaschist outcrop	Outcrop
Roure	Commune
Metropolitan City of Turin	Metropolitan City
Piedmont	Region
Italy	- not defined -

Outcrop of micaschist of the so-called Chasteiran Unit, a thin (10 to 50 m thick) layer of garnet–chloritoid micaschist discontinuously exposed along the contact between the Pinerolo and Muret Units (northern Dora-Maira Massif), from the village of Villaretto in the Chisone Valley to the crest of Truc Lausa in the Germanasca Valley. The micaschist has a peculiar appearance, due to the numerous, strongly flattened, blackish lenses essentially consisting of chloritoid and metamorphosed carbonaceous material (graphite), alternating with whitish, quartz- and mica-rich lenses. Garnet is dispersed in the rock as idioblastic or sub-idioblastic grains 1 to 5 mm in diametre.

The rock sampled at the hamlet of Chasteiran is dark grey in colour and consists of muscovite (20–30%), quartz (15–20%), chloritoid (15–20%), garnet (5–10%), chlorite (5–10%), paragonite (5–10%), graphite (5–10%), as well as minor rutile, ilmenite, epidote, albite and, importantly, coesite. Accessory tourmaline also occurs (Manzotti et al., 2022).

Garnet grains are zoned and are classifiable as almandine (from 80 to 67 mol%). Inclusions are mainly concentrated in garnet inner cores, whereas they are less abundant in garnet outer cores and rims. Garnet inner cores contain monocrystalline inclusions of chloritoid, rutile, quartz, and tourmaline, and polymineralic aggregates of paragonite + minor epidote ± muscovite. These aggregates may be interpreted as pseudomorphs after lawsonite, as already reported in other high-pressure metapelite from the Dora-Maira Massif. Garnet outer cores contain inclusions of coesite, polycrystalline aggregates of quartz, and rare monomineralic inclusions of chloritoid, graphite, and rutile. A few polymineralic inclusions consisting of paragonite + minor epidote are also observed in this domain. Rare monomineralic inclusions of rutile, quartz, tourmaline, and epidote occur in garnet rims.

Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P–T evolution. 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 510–530 °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).

According to the results exposed by Manzotti et al. (2022), 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.

References

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