Abstract
The Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of the Sesia–Lanzo Zone (SLZ), bounded by the Periadriatic (Canavese) Lineament that separates the Alpine subduction complex from the Southalpine domain. The RCTU is limited to the south by the Lanzo Massif (LM) and to the east by the Eclogitic Micaschists Complex (EMC). Particularly the tectonic contact area of the RCTU, adjacent to the neighbouring SLZ and the LM is characterised by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ) that was active under blueschist-to greenschist-facies conditions. Despite the dominant mylonitic structure, some rocks (garnet-bearing gneiss, garnet-free gneiss and orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. The scarcity of superposed structures and the small size of relicts impose a detailed microstructural analysis supported by chemical investigation to reconstruct the tectono-metamorphic history of the MZ. Therefore, we integrated the classical meso- and microstructural analysis approach with a novel quantitative technique based on the Quantitative X-Ray Map Analyzer (Q-XRMA), used to classify rock-forming minerals starting from an array of X-ray elemental maps, both at whole thin section and micro-domain scale, as well as to calibrate the maps for pixel-based chemical analysis and end-member component maps, relevant for a more robust conventional geothermobarometer application as well for calculating reliable PT pseudosections. Pre-Alpine relicts are garnet and white mica porphyroclasts in the garnet-bearing gneiss and biotite and K-feldspar porphyroclasts in garnet-free gneiss and orthogneiss, respectively, providing no PT constraints. The Alpine evolution of the MZ rocks, has been subdivided in three deformation and metamorphic stages. The first Alpine structural and metamorphic equilibration stage (D1 event) occurred at a pressure of ca. 1.25–1.4 GPa and at a temperature of ca. 420–510 °C, i.e. under blueschist-facies conditions. The D2 event, characterised by a mylonitic foliation that is pervasive in the MZ, occurred at ca. 0.95–1.1 GPa and ca. 380–500 °C, i.e. under epidote-blueschist-facies conditions. The D2 PT conditions in the MZ rocks are similar to those predicted for the blocks that constitute the RCTU mélange, and they overlap with the exhumation paths of the EMC and LM units. Therefore, the RCTU, EMC and LM rocks became coupled together during the D2 event. This coupling occurred during the exhumation of the different tectono-metamorphic units belonging to both continental and oceanic lithosphere and under a relatively cold thermal regime, typical for an active oceanic subduction zone, pre-dating Alpine continental collision.
Highlights
The Sesia–Lanzo Zone (SLZ) has been divided into four complexes, which heterogeneously recorded the Alpine evolution (Fig. 1b): Eclogitic Micaschists (EMC), Gneiss Minuti (GM), II Dioritico-Kinzigitica (IIDK) and Rocca Canavese Thrust Sheets Unit (RCTU)
This study focuses on the tectonic contact zone of the RCTU with the neighbouring EMC and the Lanzo Massif (LM) units that is mapped as Mylonitic Zone (MZ) (Fig. 1c)
The tectonic contacts of the RCTU with the adjacent LM and the EMC are characterized by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ, Fig. 1c) that developed under blueschist- to greenschist-facies conditions (Spalla and Zulbati, 2003; Cantù et al 2016; Roda et al 2018)
Summary
The rocks belonging to different complexes of the Austroalpine domain in the Western Alps, originally part of the thinned continental margin of the Adria plate, recorded heterogeneous metamorphic climax conditions and P–T–d–t paths during the Alpine evolution (e.g. Dal Piaz et al 1972; Compagnoni 1977; Gosso 1977; Dal Piaz et al 2003; Babist et al 2006; Dal Piaz 2010; Roda et al 2012; Lardeaux 2014; Zucali et al 2020). The SLZ has been divided into four complexes, which heterogeneously recorded the Alpine evolution (Fig. 1b): Eclogitic Micaschists (EMC), Gneiss Minuti (GM), II Dioritico-Kinzigitica (IIDK) and Rocca Canavese Thrust Sheets Unit (RCTU) The latter (Fig. 1c) consists of a mixture of meter- to hundreds of metersscale slices of mantle- and continental crustal-derived rocks, enclosed in a serpentinite-rich matrix (Fig. 1c; Pognante, 1989a; Spalla and Zulbati 2003; Barnes et al 2014; Cantù et al 2016; Roda et al 2018, 2020). The RCTU is considered an example of a subductionrelated mélange formed during the subduction of the Alpine Tethys below the Adria plate (Roda et al 2018, 2020) This interpretation is based on the contrasting tectono-metamorphic evolution recorded by different rock slices enclosed in the serpentinites.
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