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 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 °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). 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.

First finding of coesite in the eclogitic continental crust of the Sesia Zone, Italian Western Alps

“…[ 2 ] in the Eclogitic micaschist complex of the Sesia Zone, Argand's ‘root zone’ of Dent Blanche nappe [ 3 , 4 ]. Among them, Chen's paper [ 2 ] deserves to be highlighted for its original contribution to the geochronology, geochemistry and petrology on leucocratic metagranitoids of the Sesia eclogitic complex in the lower Aosta valley: this unit underwent UHP metamorphism at 76.0 ± 1.0 Ma (U-Pb zircon ages), 2.8–3.3 GPa and 450–520°C [ 2 ], corroborating previous regional dating and P-T estimates [ 5 , 6 and references therein]. In this view, all the continental and oceanic units of the Austroalpine-Penninic collisional wedge [ 7 ] experienced subduction and a similar UHP metamorphism [ 2 , 5 , 6 ].…”

“…Two other coesite sites have been recently discovered by Manzotti et al. [ 5 ], in the northern Dora-Maira, and by Chen et al. [ 2 ] in the Eclogitic micaschist complex of the Sesia Zone, Argand's ‘root zone’ of Dent Blanche nappe [ 3 , 4 ].…”

Thermal Structure of the Paleo‐Continental Subduction Zone: Insights From Quantitatively Constrained Prograde P–T Paths of Exhumed LT/UHP Eclogites in the Dabie Orogen

“…First, the pressures calculated by QuiG barometer may be biased to lower values as a result of underestimation of P trap for quartz inclusions induced by (1) relaxation due to the exposition of quartz inclusion (e.g., Angel et al., 2014; Manzotti et al., 2022; Y. X. Zhang, 1998); (2) elastically anisotropic quartz inclusion (e.g., Angel et al., 2015; Murri et al., 2019); (3) the large grain size or the near surface position of inclusion (Campomenosi et al., 2018; Mazzucchelli et al., 2018, 2019; Zhong et al., 2020); (4) viscous relaxation of the inclusion pressure (Dabrowski et al., 2015; Zhong et al., 2020); and (5) approximation of linear Equations of State (EOS) of inclusion‐host system (Angel et al., 2017). For this study, the quartz inclusions selected for measurements of Raman shifts have small diameters of 5–10 μm and are located over three times their radial to the surface or fracture; thus, the effects of cases (1) and (3) are limited.…”

Metamorphism and linked deformation in understanding tectonic processes at varied scales

“…• The finite metamorphic field gradient is in the range of 5-10 °C/km, thus similar to the previously presented circumpacific ones, compatible with long-lasting oceanic subduction (ca. 55 Ma) between Upper Cretaceous [Handy et al, 2010, Rebay et al, 2018 [ Groppo et al, 2019, Manzotti et al, 2022, in the "Schistes Lustrés" unit located west of the Gran-Paradiso Massif [Bousquet, 2008], or in the Sesia-Lanzo Zone [Giuntoli et al, 2018, Roda et al, 2020, Zucali and Spalla, 2011. In Zermatt zone or Monviso meta-ophiolites these heterogeneities are still a matter of debate.…”

“…• Since the discovery of coesite in the Dora-Maira unit [Chopin, 1984], the identification of UHP conditions in the Western Alps is progressively increasing. UHP or nearly UHP conditions are now documented in continental meta-sediments and meta-granitoids [Chopin et al, 1991, Kienast et al, 1991, Groppo et al, 2019, Manzotti et al, 2022, Chen et al, 2023, in oceanic meta-sediments [Reinecke, 1991] in oceanic meta-basalts and meta-gabbros [Angiboust et al, 2012, Bucher et al, 2005, Ghignone et al, 2022, Luoni et al, 2019 as well as in oceanic mantle serpentinites [Rebay et al, 2012[Rebay et al, , 2018]. • Slices of both oceanic and continental crusts were subducted during Alpine orogeny.…”