Analysis of fabric evolution and metamorphic reaction progress at Lago della Vecchia-Valle d’Irogna, Sesia-Lanzo Zone, Western Alps

Abstract

The Lago della Vecchia-Valle d'Irogna rocks are part of the Eclogitic Micaschists Complex (EMC) of the Sesia-Lanzo Zone, western Austroalpine domain. The 1:10,000 scale map includes metaintrusive, minor micaschist, banded gneiss, and metabasic boudins. The multiscale structural analysis reveals successive magmatic and tectono-metamorphic stages: during M0 the metaintrusive protoliths emplaced; D1 took place under eclogite-facies conditions; during D2 stage, a pervasive foliation developed under retrograde blueschist-facies conditions; D3-D4 and D5 structures developed under greenschist-facies conditions; during M6 andesitic dykes intruded. The mapped degree of fabric evolution (FE) and metamorphic transformation (MT) related to D2-foliation shows that the MT was not only controlled by bulk rock and mineral compositions, but also by FE. The development of a pervasive blueschist-facies D2-foliation is in contrast with the eclogitic dominant fabric generally recorded in the EMC. This difference suggests that FE and MT are potentially responsible for km-scale heterogeneities in the tectono-metamorphic record.ARTICLE HISTORY

Integrating X-Ray Computed Tomography With Chemical Imaging to Quantify Mineral Re-crystallization From Granulite to Eclogite Metamorphism in the Western Italian Alps (Sesia-Lanzo Zone)

“…Similar contrasting relations between degree of fabric development and mineral associations have been described for eclogitized rocks in the same area, where Permian intrusive preserve their igneous fabrics, estimated as >90% of the rock fabric (Zucali et al, 2002;Zucali, 2011), but the mineral association related to the magmatic stage is almost totally replaced by eclogite facies minerals, up to 80-90% of the investigated portions (Koons, 1982;Zucali et al, 2002;Corti et al, 2017).…”

“…The C-related structure comprises 23% of grains laying at 34-46 • from the pre-S1 orientation. These S-C fabrics may be compared to pre-alpine features described by previous authors at the map-scale (Zucali, 2002;Corti et al, 2017) and preserved in similar low-strain volumes of the Sesia-Lanzo Zone. Still remains unclear how those meter-scale domains might preserve such clean fabrics even where no mechanical barriers have been described at the map-scale.…”

“…The EMC protoliths, composed by high-grade paragneisses, granulites, amphibolites and minor marbles and quartzites, constitute the country rocks of the Permian granitoids and gabbros (Compagnoni and Maffeo, 1973;Callegari et al, 1976;Compagnoni et al, 1977;Lardeaux, 1981;Oberhänsli et al, 1985;Castelli, 1987;Bussy et al, 1998;Cenki-Tok et al, 2011;Zucali, 2011;Corti et al, 2017); the Mt. Mucrone is one of these Permian bodies and the most renown.…”

“…The knowledge of crust and mantle dynamics is mostly based on the study of fabric elements, metamorphic assemblages and their relationships within the rock-volume, now exposed after the subduction-and collision-related processes. The metamorphic basements are usually affected by strong heterogeneities in the degree of fabric and metamorphic evolution (Salvi et al, 2010;Corti et al, 2017), mainly due to the strain partitioning and consequent heterogeneous distribution of metamorphic assemblages (e.g., Myers, 1970;Mørk, 1985;Spalla and Zucali, 2004;Spalla et al, 2005;Pearce and Wheeler, 2010). Detailed multiscale correlation between deformation and metamorphism has been shown to be of primary importance to constrain the relationships between fabric and metamorphism (e.g., Turner and Weiss, 1963;Park, 1969;Hobbs et al, 1976Hobbs et al, , 2010Williams, 1985;Lardeaux and Spalla, 1990;Passchier et al, 1990;Johnson and Vernon, 1995;Spalla et al, 1999;Salvi et al, 2010;Gosso et al, 2015;Corti et al, 2017).…”

Strain partitioning in host rock controls light rare earth element release from allanite-(Ce) in subduction zones

“…SLZ is a slice of continental crust that experienced HP-LT conditions during the Alpine subduction (e.g., Compagnoni et al, 1977;Spalla et al, 1991;Zucali et al, 2002;Gosso et al, 2010;Roda et al, 2012;Giuntoli et al, 2016;Roda et al, 2018;. Metagranitoids occur as metre-to kilometre-sized bodies in tectonic contact with micaschists and gneisses (Corti et al, 2017). They spectacularly preserve magmatic textures and igneous minerals, within metre to decametre domains.…”