Dating emplacement and evolution of the orogenic magmatism in the internal Western Alps: 1. The Miagliano Pluton

Abstract

The Canavese Line in the Western Alps represents the position in the Alpine chain, where alkaline and calc-alkaline magmatism occur in close spatial and temporal association. In addition to available data on the alkaline Valle del Cervo Pluton, we present petrological and geochemical data on the Miagliano tonalite. The latter is of special interest, because it is located in the southeastern side of the Canavese Line, in contrast to most Periadriatic Plutons. The dioritic to tonalitic rocks of the Miagliano Pluton represent an intermediate stage of a calcalkaline differentiation, demonstrated by relics of two different pyroxenes as well as the texture of allanite. Hornblende barometry indicates pressures of *0.46 GPa consistent with the presence of magmatic epidote. Field relationships between the two Plutons, the volcanic and volcaniclastic rocks of the Biella Volcanic Suite and numerous dykes cross-cutting the different units, allow reconstruction of a more refined chronology of the calcalkaline and alkaline magmatic series. High precision zircon geochronology yields an age of 33.00 ± 0.04 Ma for the central tonalitic part of the Miagliano Pluton and 30.39 ± 0.50 Ma for the granitic core of the Valle del Cervo Pluton. The difference in age combined with cooling data and intrusion depth indicates dissimilar tectonic transport east and west of the Canavese Line. The earlier emplaced Miagliano Pluton has to be exhumed from an intrusion depth of *12-15 km, whereas the neighbouring and younger Valle del Cervo Pluton is exhumed from a depth of 5-7 km. This tectonic scenario is related to upper crustal rigid block rotation responsible for the burial of the lowermost Rupelian paleosurface of the Sesia-Lanzo Zone. Thus, the new ages constrain the paroxysm of the orogenic magmatism in the internal Western Alps to an extremely short lapse of time in the first half of the Rupelian.

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).…”

Dating emplacement and evolution of the orogenic magmatism in the internal Western Alps: 2. The Biella Volcanic Suite

“…The nearby Miagliano tonalite overlaps in age with these zircons and is part of the same calc-alkaline evolution (Berger et al 2012b). The chemical variations of some of the plutonic, subvolcanic and volcanic rocks indicate calcalkaline evolution developed in a narrow time interval.…”

“…1 for details) with the distribution of the fission track data of this study (grey shaded boxes) and from the literature (white boxes) N s number of spontaneous tracks counted on the sample, q s density of spontaneous tracks, N i number of induced tracks counted on the glimmer detector, q i density of induced tracks, v 2 result of the statistical test in %, Dpar measured long axis of etch pits on the polished surface in lm, Mean track length mean length of horizontal confined tracks in lm given with 1r error 1994). This magmatic evolution and the relative timing of emplacement seem to rule out a direct magmatic genetic relationship between the BVS and the Valle del Cervo Pluton (compare Callegari et al 2004;Berger et al 2012b). The volcanism in Biella is coeval and similar in composition as the relicts of volcanic clasts found in the Taveyannaz sandstone formation of the Helvetic Flysch system (Vuagnat 1983;Fischer and Villa 1990;Sinclair 1992;Ruffini et al 1997).…”

“…1) and the numerous dykes intruding the rocks of the Sesia-Lanzo zone and IvreaVerbano zone. More detailed data on these plutonic rocks are given in a companion contribution (Berger et al 2012b). The high precision time-temperature evolution of the BVS and related magmatic rocks will serve as a base to unravel the near surface tectonic evolution during Neogene times.…”

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

“…These magmas rose along the Periadriatic line, which is the main Alpine crustal break (Figure 1 (a)), and subsequently emplaced in the form of plutons, dykes, and volcanites (Berger, Thomsen, Ovtcharova, Kapferer, & Mercolli, 2012;Bernardelli, Castelli, & Rossetti, 2000;Bigioggero, Colombo, Del Moro, Gregnanin, Macera, & Tunesi, 1994;Callegari, Cigolini, Medeot, & D'Antonio, 2004;Carraro & Ferrara, 1968;Dolenec, 1994;Fodor et al, 2008;Kapferer, Mercolli, Berger, Ovtcharova, & Fügenschuh, 2012;Mayer, Cortiana, Dal Piaz, Deloule, De Pieri, & Jobstraibizer, 2003;Müller, Mancktelow, & Meier, 2000;Oberli, Meier, Berger, Rosenberg, & Gieré, 2004;Pamić & Palincaš, 2000;Romer, Schärer, & Steck, 1996;Romer & Siegesmund, 2003;Rosenberg, 2004;Rossetti, Agangi, Castelli, Padoan, & Ruffini, 2007;Stipp, Fügenschuh, Gromet, Stünitz, & Schmid, 2004;Van Merke de Lummen & Vander Auwera, 1990;Von Blanckenburg, Früh-Green, Diethelm, & Stille, 1992;Von Blanckenburg et al, 1998;Zanoni, 2010;Zanoni et al, 2008). The Periadriatic line nucleated due to the contrasting rheology between rocks of the Southalpine domain and rocks of the Austroalpine and Penninic domains, during Alpine nappe structural development (Figure 1(a)).…”