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 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.

Quantitative X-ray Maps Analaysis of Composition and Microstructure of Permian High-Temperature Relicts in Acidic Rocks from the Sesia-Lanzo Zone Eclogitic Continental Crust, Western Alps

“…The GMC and the EMC display a dominant greenschist and eclogitic Alpine metamorphic imprint, respectively [9,11,[55][56][57][58][59]. The Rocca Canavese Thrust Sheets Unit (RCTU) is characterised by a dominant blueschist facies Alpine imprint with no eclogite facies relicts [11,19,[60][61][62][63].…”

“…In GMC, metapelites and acidic and basic granulites (Figure 2 and Table 1) preserved high-temperature Permian mineral assemblages that are overprinted by Alpine coronitic assemblages developed under greenschist to eclogite facies conditions [12,80]. In RCTU only microscale relicts were reported, such as porphyroclasts of white mica and garnet in garnet-bearing gneiss and K-feldspar and biotite porphyroclasts in orthogneiss [19]. As in the EMC, Grt occurs as a pluri-millimetre-sized porphyroclast surrounded by foliation, with a fractured Fe and Mg-rich core and Alpine Ca-rich rims [19].…”

“…This limitation, well known in most of the pre-Alpine relicts metamorphic complexes of western Alps, is normally overcome by means of a detailed local analysis that can be performed combining microstructural analysis with X-ray elemental maps, obtained by Electron Microprobe Analyzer (EMPA). This already routinely performed approach was recently reinforced using increasingly efficient processing tools aimed at extracting numerical petrological constraints [5,[13][14][15][16][17][18][19].…”

“…We selected three localities, two in the EMC and one in the RCTU: Lago della Vecchia (LdV), Monte Mucrone (MM), and Rocca Canavese (RCT). The three localities were chosen where previous studies described metre-scale relicts of pre-Alpine protoliths and fabrics associated with well-preserved metamorphic minerals [9,19,21,29,30], but no detailed evolution and pressure-temperature estimates were reported. The three localities differ in lithologic and texture types, where LdV are metaintrusives preserved in Coronite Alpine fabric, MM-high temperature metapelites wrapped in Tectonite Alpine fabric, RCT are metapelite relicts within a Mylonite Alpine domain.…”

Metamorphic Facies and Deformation Fabrics Diagnostic of Subduction: Insights From 2D Numerical Models

“…These thresholds are considered effective for different rock types and therefore we use them as reference to evaluate the probability to develop coronitic, tectonitic and mylonitic textures, both in the continental and in the oceanic crust. In the Alps, rocks deeply involved in the subduction complex and recording P‐T conditions compatible with blueschist and eclogite metamorphic facies can be observed in the axial zone of the chain, and generally show heterogeneous deformation with dominant tectonitic or mylonitic fabrics (e.g., Gasco et al., 2011; Lardeaux, 2014a; Malatesta et al., 2012; Oberhänsli & Goffé, 2004; Roda et al., 2021; Zucali et al., 2020). These rocks are often associated with serpentinites (e.g., Assanelli et al., 2020; Gasco et al., 2011; Lardeaux, 2014a; Malatesta et al., 2012; Manzotti et al., 2014; Roda et al., 2020), which have a crucial role in exhumation mechanisms (Gerya et al., 2002; Guillot et al., 2009; Luoni et al., 2020; Meda et al., 2010; Roda et al., 2012; Schwartz et al., 2001; Tamblyn et al., 2020; Warren et al., 2008; Yamato et al., 2007).…”