At the Frisson Lakes (FL), a small layered mafic sequence surrounded by migmatitic orthogneisses is exposed as a tectonic window of the crystalline basement of the Gesso-Stura Terrain (GST) of the Argentera Massif (Western Alps, Italy) within its Meso-Cenozoic sedimentary cover. The mafic sequence is characterised by Pl-rich and Pl-poor layers of HP granulites consisting of porphyroclasts of Omp, Grt, Amp, Pl, and rare Qtz embedded in a fine-grained mylonitic matrix of neoblastic Amp, Pl, Cpx, Grt and minor Bt and Qtz. Symplectites of Amp + Pl partly replace porphyroclastic Grt and Omp. Chemical compositions of the same minerals from the Pl-rich and Pl-poor layers are different, suggesting very limited element diffusion between layers during metamorphic evolution. All samples show the following features: (i) porphyroclasts of Grt and Cpx characterised by cores rich in exsolved rutile and albite, respectively, (ii) zoned Cpx with Jd- and CaTi-Ts-richer core, and (iii) a systematic decrease in Al(VI) of Amp from the earliest porphyroclasts to the latest mylonitic neoblasts.

Geological and petrological data indicate that the FL mafic sequence derived from the mylonitic deformation of a former layered mafic intrusion, which occurred before Carboniferous migmatization. The mafic sequence records a continuous metamorphic evolution characterised by: (i) a metamorphic peak (stage A) at HP granulite-facies conditions (735 ± 15 LC and 1.38 ± 0.05 GPa) in which coarse-grained minerals developed statically; (ii) an early decompression stage (stage B), which occurred at the transition between HP granulite- and HP-HT amphibolite-facies (709 ± 2 °C and 1.10 ± 0.02 GPa); (iii) a HT–MP amphibolite-facies stage (stage C) (665 ± 15 °C and 0.85 ± 0.15 GPa), which was accompanied by pervasive mylonitic deformation; and (iv) a final non-pervasive “symplectitic” stage (stage D) (500 < T < 625 °C and P < 0.59 GPa), which developed at shallow crustal levels, most likely favoured by a moderate influx of hydrous fluids.

The P–T path reconstructed for the FL layered mafic sequence, for the first time quantitatively estimated for the Argentera Massif, allows to conclude that the granulites and eclogites previously reported in the GST belong to a single metamorphic stage, which developed at HP granulite-facies conditions. This metamorphic history, which appears to be similar to that recorded in other External Crystalline Massifs, is consistent with a subduction process followed by relatively rapid exhumation.

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