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

Abstract

Three samples of meta-acidic rocks with pre-Alpine metamorphic relicts from the Sesia-Lanzo Zone eclogitic continental crust were investigated using stepwise controlled elemental maps by means of the Quantitative X-ray Maps Analyzer (Q-XRMA). Samples were chosen with the aim of analysing the reacting zones along the boundaries between the pre-Alpine and Alpine mineral phases, which developed in low chemically reactive systems. The quantitative data treatment of the X-ray images was based on a former multivariate statistical analytical stage followed by a sequential phase and sub-phase classification and permitted to isolate and to quantitatively investigate the local paragenetic equilibria. The parageneses thus observed were interpreted as related to the pre-Alpine metamorphic or magmatic stages as well as to local Alpine re-equilibrations. On the basis of electron microprobe analysis, specific compositional ranges were defined in micro-domains of the relict and new paragenetic equilibria. In this way calibrated compositional maps were obtained and used to contour different types of reacting boundaries between adjacent solid solution phases. The pre-Alpine and Alpine mineral parageneses thus obtained allowed to perform geothermobarometry on a statistically meaningful and reliable dataset. In general, metamorphic temperatures cluster at 600–700 ∘C and 450–550 ∘C, with lower temperatures referred to a retrograde metamorphic re-equilibration. In all the cases described, pre-Alpine parageneses were overprinted by an Alpine metamorphic mineral assemblage. Pressure-temperature estimates of the Alpine stage averagely range between 420 to 550 ∘C and 12 to 16.5 kbar. The PT constraints permitted to better define the pre-Alpine metamorphic scenario of the western Austroalpine sectors, as well as to better understand the influence of the pre-Alpine metamorphic inheritance on the forthcoming Alpine tectonic evolution.

The emplacement, alteration, subduction and metamorphism of metagranites from the Tso Morari Complex, Ladakh Himalaya

“…The Tso Morari metagranites underwent partial transformation in the eclogite facies. Examples of igneous textural domains, preserved despite later replacement of the igneous minerals, are found in low-strain igneous rocks such as the previously mentioned Monte Mucrone metaquartz diorite (Dal Piaz et al, 1972;Oberhänsli et al, 1985;Koons et al, 1987, Zucali et al, 2021, the Brossasco metagranites (Schertl et al, 1991;Biino and Compangnoni., 1992) and the Yankou metagranite (Wallis et al, 2007), as well as in the Kokchetav Massif in Kazakhstan . Additionally, in the Flaktraket mangeritegranulite complex in the Western Gneiss region of Norway, >95% of the complex is thought to have preserved metastable igneous textures and coronas around the magmatic orthopyroxene during eclogite-facies conditions (Wain et al, 2001).…”

“…The Tso Morari metagranites underwent partial transformation in the eclogite facies. Preservation of igneous textural domains, despite further replacement of the igneous minerals, is observed in low-strain igneous rocks such as the Yankou metagranite of the Sulu province (Wallis et al, 1997), the Monte Mucrone meta-quartz diorite of the Sesia Zone, Alps (Dal Piaz et al, 1972;Oberhänsli et al, 1985;Koons et al, 1987, Zucali et al, 2021, the Brossasco metagranites of the Dora Maira (Schertl et al, https://doi.org/10.1180/mgm.2022.121 Published online by Cambridge University Press 1991; Biino and Compangnoni., 1992), the Kokchetav Massif and granulite-facies rocks in the Western Gneiss Region of Norway (Wain et al, 2001). In the Flaktraket mangerite-granulite complex of the Western Gneiss Region of Norway, > 95% of the complex is thought to have preserved metastable igneous textures and coronas around the magmatic orthopyroxene during eclogite-facies conditions (Wain et al, 2001).…”

Metamorphic Remnants of the Variscan Orogeny across the Alps and Their Tectonic Significance

“…In the Permian-Triassic period, a high temperature-low pressure metamorphism and intense mafic to acidic igneous activity affected the Variscan continental crust of the Penninic, Austroalpine, and Southalpine domains. No similar metamorphic features have been described in the rocks of the Helvetic-Dauphinois-Provençal domain [62,140].…”