Blueschist mylonitic zones accommodating syn-subduction exhumation of deeply buried continental crust: the example of the Rocca Canavese Thrust Sheets Unit (Sesia–Lanzo Zone, Italian Western Alps)

Abstract The Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of Sesia–Lanzo Zone (SLZ), bounded by Periadriatic (Canavese) Lineament separates Alpine subduction from Southalpine domain. RCTU limited to south Lanzo Massif (LM) and east Eclogitic Micaschists Complex (EMC). Particularly tectonic contact area RCTU, adjacent neighbouring SLZ LM characterised 100–200-m-thick mylonitic ultra-mylonitic zone (MZ) was active under blueschist-to greenschist-facies conditions. Despite dominant structure, some rocks (garnet-bearing gneiss, garnet-free gneiss orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. scarcity superposed structures small size relicts impose detailed microstructural analysis supported chemical investigation reconstruct tectono-metamorphic history MZ. Therefore, we integrated classical meso- approach with novel quantitative technique based on Quantitative X-Ray Map Analyzer (Q-XRMA), used classify rock-forming minerals starting an array X-ray elemental maps, both at whole thin section micro-domain scale, as well calibrate maps for pixel-based end-member component relevant more robust conventional geothermobarometer application calculating reliable PT pseudosections. Pre-Alpine are garnet white mica porphyroclasts garnet-bearing biotite K-feldspar orthogneiss, respectively, providing no constraints. evolution MZ rocks, has been subdivided three deformation metamorphic stages. first structural equilibration stage (D1 event) occurred pressure ca. 1.25–1.4 GPa temperature 420–510 °C, i.e. blueschist-facies D2 event, foliation pervasive MZ, 0.95–1.1 380–500 epidote-blueschist-facies conditions similar those predicted blocks constitute mélange, they overlap exhumation paths EMC units. became coupled together during event. This coupling different units belonging continental oceanic lithosphere relatively cold thermal regime, typical zone, pre-dating collision.