Structure and petrography of the southwestern margin of the Biella pluton, Western Alps

Abstract

This work presents a new form surface map of the southwestern margin of the Biella pluton at the scale 1:10,000. The Biella pluton is part of the Periadriatic intrusives of the Alps and is emplaced in the continental Sesia-Lanzo Zone of the western Austroalpine domain. The country rocks consist of metapelites and different metagranitoids. Pre-intrusive HP (high pressure) mineral assemblages are dominant in country rocks with the exception of metagranitoids dominated by HT (high temperature) assemblages. The plutonic rocks consist of monzonite with minor syenite. The ductile polyphasic deformation of the country rocks predates the pluton emplacement, with the exception of syn-intrusive folding and shearing, which were recorded in the country rocks of the deeper part of the pluton. Synintrusive deformation may be represented by brittle structures that bear mineralisation; however, the majority of the brittle deformation postdates the final emplacement of the pluton. The orientation of the pervasive foliation in the country rocks controls the space available for magma intrusion and possible magmatic flow during emplacement. The inferred diffusion of the thermal aureole in the country rocks is based on the variation in contact metamorphic minerals, which is described by microscopic analysis. The extent of the aureole appears to be controlled by the type of dominant mineral assemblages, rock permeability, and the orientation of the regional foliation in country rocks with respect to the pluton margin. The multiscale structural analysis reveals that the Biella pluton emplaced at a depth as shallow as the greenschist facies conditions or shallower.

3D reconstruction of fabric and metamorphic domains in a slice of continental crust involved in the Alpine subduction system: the example of Mt. Mucrone (Sesia–Lanzo Zone, Western Alps)

“…The Alpine metamorphic history of the SLZ comprises the record of a prograde blueschist-facies imprint, overprinted by eclogite-facies imprint, and followed by blueschist-and greenschist-facies re-equilibrations (e.g. Compagnoni and Maffeo 1973;Compagnoni 1977;Gosso 1977;Reinsch 1979;Pognante et al 1980;Lardeaux 1981;Lardeaux et al 1982aLardeaux et al , 1983Spalla et al 1983;Pognante 1989a;Venturini et al 1994;Castelli and Rubatto 2002;Zucali et al 2004;Babist et al 2006;Rebay and Messiga 2007;Zanoni 2010;Zucali and Spalla 2011;Giuntoli and Engi 2016;Giuntoli et al 2018a;Roda et al 2018aRoda et al , 2019b. Mineral ages ranging between 90 and 65 have been related to the Alpine eclogite-facies peak (Oberhänsli et al 1985;Duchêne et al 1997;Rubatto et al 1999;Cenki-Tok et al 2011;Regis et al 2014;Giuntoli et al 2018b;Halama et al 2018).…”

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

“…The Lago della Vecchia-Valle d'Irogna area is located within the EMC (Figure 1(B)), in the Intermediate Unit according to the classification of Regis et al (2014), just to the north of the Oligocene Biella pluton, far from its contact aureole (Berger, Thomsen, Ovtcharova, Kapferer, & Mercolli, 2012;Romer, Schärer, & Steck, 1996;Zanoni, 2016;Zanoni, Bado, Spalla, Zucali, & Gosso, 2008).…”

“…The SLZ is classically divided into four main complexes ( Figure 1(B); Compagnoni et al, 1977;Gosso, 1977): (1) the II Dioritic-Kinzigitic Zone (IIDK) preserves a pervasive pre-Alpine HT metamorphic imprint; (2) the Gneiss Minuti Complex (GMC) and (3) the EMC display a dominant Alpine metamorphic imprint and locally preserve pre-Alpine HT relicts (Compagnoni et al, 1977;Gosso, Messiga, Rebay, & Spalla, 2010;Lardeaux, Gosso, Kiénast, & Lombardo, 1982;Manzotti et al, 2014;Zanoni, 2016); (4) the Rocca Canavese Thrust Sheets are characterized by a blueschist Alpine imprint with no eclogite-facies relicts (Cantù, Spaggiari, Zucali, Zanoni, & Spalla, 2016;Pognante, 1989;Spalla & Zulbati, 2003). The EMC and GMC consist of metapelite, paragneiss, metagranitoid, metabasite, and impure marble recording Alpine polyphasic deformation and eclogite-facies peak conditions.…”

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

“…Although in EMC the eclogitic Alpine metamorphism is dominant, tracers of pre-Alpine metamorphism and magmatism are preserved as relict kinzigites, granites, and gabbro in microscopic up to a few square kilometres domains. Such relicts are preserved in variably deformed and transformed domains [2,9,21,30,40,47,49,77,78]. Given the strong Alpine imprint in the EMC, the pre-Alpine textures and metamorphic assemblages are better preserved in the rocks volumes that were poorly deformed during the Alpine convergence (e.g., [9]).…”

“…This HT event was ascribed to the Permian-Triassic lithospheric thinning that affected Pangea and consequent asthenospheric upwelling [22][23][24][25][26][27]. [19,40,55,74,78,82,83]) with the location of sampling areas: Lago della Vecchia (LdV), Monte Mucrone (MM), and Rocca Canavese (RCT); (c) simplified geological cross-section through the Western Alps modified after [84]. PL = Periadriatic lineament; PF = Penninic front.…”