2. the Biella Volcanic Suite

Swiss J Geosci (2012) 105:67–84 DOI 10.1007/s00015-012-0092-6 Dating emplacement and evolution of the orogenic magmatism in the internal Western Alps: 2. The Biella Volcanic Suite Notburga Kapferer • Ivan Mercolli • Alfons Berger • Maria Ovtcharova • Bernhard Fu¨genschuh Received: 1 September 2011 / Accepted: 12 March 2012 / Published online: 29 April 2012 Ó Swiss Geological Society 2012 Abstract The high-pressure metamorphic rocks of the Valle del Cervo Pluton at 30.39 ± 0.50 Ma sets therefore Sesia–Lanzo zone are partly covered by a volcano-sedi- the lower time limit for tectonic processes responsible for mentary unit, the Biella Volcanic Suite. Calc-alkaline and the tilting and burial. After the burial, the Biella Volcanic shoshonitic lavas extruded sub-aerially on the Oligocene Suite remained for around 20 million years between the surface. Uranium–Lead zircon dating yields 32.44– zircon and the apatite partial annealing zone. The apatite 32.89 Ma for the eruption of the calc-alkaline lavas and fission track ages spread between 16 and 20 Ma gives the therefore fixes a minimum age for the paleosurface. The time frame for the second exhumation of these units. The Biella Volcanic Suite consists mainly of epiclastic rocks Biella Volcanic Suite and the adjacent rocks of the Sesia– deposited in a high-energy fluvial environment and minor Lanzo zone were the second time exhumed to the surface in lava flows. The rocks of the suite display widespread post- Messinian times, after a long residence time within the eruption transformations. Illite and chlorite thermometry as apatite partial annealing zone. well as fission track dating suggest a thermal overprint related to burial of the Biella Volcanic Suite. An upper Keywords Biella volcanic rocks Á Sesia–Lanzo zone Á crustal rigid block tilting in the area causes this burial. U–Pb zircon dating Á Fission track dating Á Hydrothermal tourmaline and ankerite veins related to the Very low grade metamorphism intrusion of the nearby Valle del Cervo Pluton crosscut the already tilted Biella Volcanic Suite. The intrusion age of 1 Introduction Editorial Handling: E. Gnos. The most internal part of the Sesia–Lanzo zone along the Canavese line is covered by a Tertiary volcano-sedimen- N. Kapferer Á I. Mercolli Institute of Geological Sciences, University of Bern, tary series. These rocks have been studied since long time Baltzerstrasse 1?3, 3012 Bern, Switzerland following different perspectives (Bianchi and Dal Piaz 1963; Carraro 1966; Ahrendt 1969; Scheuring et al. 1974; & A. Berger ( ) Zingg et al. 1976; De Capitani et al. 1979; Lanza 1979; Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, Callegari et al. 2004). Based on mineralogical and geo- 1350 Copenhagen, Denmark chemical arguments on fresh samples, Callegari et al. e-mail: [email protected] (2004) described the origin and evolution of the volcanic suite. They called this unit the ‘‘Cover Series of the Sesia M. Ovtcharova Section of Earth and Environmental Sciences, Zone’’. To emphasize its essentially volcanic origin and its University of Geneva, Rue de Maraıˆchers 13, restricted regional distribution, we suggest calling this unit 1205 Geneva, Switzerland the Biella Volcanic Suite (BVS). We intend to contribute to the precise dating of the emplacement and of the sub- B. Fu¨genschuh Institute of Geology and Paleontology, University of Innsbruck, sequent low temperature alteration during burial of these Innrain 52, 6020 Innsbruck, Austria rocks. A dating approach was already presented in earlier 68 N. Kapferer et al. papers using K/Ar methods, which may be influenced by Fig. 1 Geological map of the Biella area and profile along the Valle c alteration of the rocks (Scheuring et al. 1974; Zingg et al. del Cervo. Compiled after: Malaroda et al. (1966); Bigioggero et al. (1994); Callegari et al. (2004); Rossetti et al. (2007); Zanoni et al. 1976). Such an overprint may include several steps of (2008); Wissmann (1985); Puschnik (2000); Caviezel (2007) and different alteration processes (ancient and modern surface paleomagnetic data of dykes after Lanza (1977, 1979) weathering, syn-emplacement volcanic hydrothermal alteration, burial metamorphism). We tried to decipher this volcanic rocks of the Biella Volcanic Suite on a preserved problem by combining the description of the mineralogical paleosurface on top of the gneisses of the Sesia–Lanzo zone changes with radio-isotopic and fission track (FT) dating. (Schmid et al. 1989; Kapferer et al. 2011). From that The U/Pb system in zircon should not be strongly affected moment, in the studied area, the Canavese line acts by such a low temperature overprint and have therefore the essentially as brittle fault system accommodating upper potential to record the crystallisation of zircon close to the crustal tectonics (fault rocks 3 and 4; Berger et al. 2012a, b). eruption time of the magmas. In contrast, FT data are The Sesia–Lanzo zone occurs to the northwest of the sensitive to thermal overprint and should give insights into Canavese line, and consists of a poly-metamorphic frag- the timing of the metamorphic overprint. Based on the ment of continental crust derived from the Adriatic Plate combination of these two methods, we will propose a (e.g., Dal Piaz 1999). It forms a lenticular body, oriented temporal scheme of the evolution of the BVS. The BVS is SW–NE, between the Canavese line in the south and the spatially and geochemically related to the coeval magmatic Piemonte ophiolite nappes to the north. activity in this area, i.e., Valle del Cervo Pluton, Miagliano The investigated volcano-sedimentary rocks lie on top Pluton and Traversella Pluton (Fig. 1) and the numerous of the high-pressure rocks of the Mombarone nappe, part of dykes intruding the rocks of the Sesia–Lanzo zone and Ivrea- the Sesia–Lanzo zone (Babist et al. 2006; more or less Verbano zone. More detailed data on these plutonic rocks are equivalent to ‘‘Eclogitic Micaschist Complex’’ of Com- given in a companion contribution (Berger et al. 2012b). The pagnoni et al. 1977). The BVS are restricted to a narrow high precision time–temperature evolution of the BVS and belt (ca. 20 km long and of variable thickness) along the related magmatic rocks will serve as a base to unravel the above described Canavese line between the village of near surface tectonic evolution during Neogene times. Donato and the upper Val Sessera (Fig. 1). Frequently, the epiclastic members of the volcanic suite contain components of the underlying rocks of the Sesia–Lanzo zone 2 Geological setting (Fig. 2; Bianchi and Dal Piaz 1963). The composition of the volcanic rocks ranges from basalt to andesite in the Oligocene, syn-orogenic volcanic and plutonic rocks out- high-K calc-alkaline suite and from trachyandesite to crop together with high-pressure metamorphic rocks of the trachydacite in the shoshonitic suite (Callegari et al. 2004). Sesia–Lanzo zone along a narrow region in the internal part Bianchi and Dal Piaz (1963) characterized the volcano- of the Western Alps along the Canavese line (Fig. 1). The sedimentary cover of the Sesia–Lanzo zone as a complex Canavese line is the western branch of the Periadriatic Lin- volcanic sequence of porphyries, coarse grained con- eament, a first order boundary in the Alps (e.g., Schmid et al. glomerates, volcanic breccia and thin tuffitic layers. The 1989). The Canavese line separates the Sesia–Lanzo zone epiclastic conglomerates form layered sequences of locally from the Ivrea-Verbano zone (e.g., Ahrendt 1972, 1980; variable thickness (up to 400–500 m). The different layers Schmid et al. 1987, 1989; Biino and Compagnoni 1989; are characterized by the variation of the grain size, the Zingg and Hunziker 1990; Handy et al. 2005; Siegesmund relative abundance of the different types of volcanic et al. 2008). It forms a mylonite belt of variable thickness components and the variable amount and size of the Sesia (10–300 m) of strongly deformed slices of rocks (cataclastic clasts. Within such layers neither grading nor sorting of the rocks, mylonites and locally fault gouge) derived from the components can be observed. Bianchi and Dal Piaz (1963) Sesia–Lanzo, Ivrea-Verbano and Sesia Zones. Four major observed clasts of the eclogitic rocks from the Sesia–Lanzo Alpine deformation events are superimposed on the different zone within the volcano-sedimentary unit, and assumed a fault rocks along the Canavese line. In late Cretaceous to Permian age for the porphyries. Carraro (1966) described Paleocene times, a mylonite belt 1 (fault rocks 1 or mylonite the basal layers of the epiclastic rock containing eclogitic belt 1 of Schmid et al. 1989) developed during subduction clasts as a non-metamorphic psephitic continental series of and exhumation of the Sesia–Lanzo zone. Further conver- upper Carboniferous age. In contrast, Scheuring et al. gence in the Sesia–Lanzo zone produces new steeper Sesia-, (1974) estimated a tertiary age for the volcaniclastic rocks Ivrea- and Canavese-derived mylonites (Schmid et al. based on paleobotanical data and K–Ar total rock ages of 1987, 1989). This stage predates the emplacement of the andesitic volcanic rocks between 29 and 33 Ma. The Biella Volcanic Suite 69 70 N. Kapferer et al. Fig. 2 Typical aspects of different lithologies in the BVS. with abundant clasts from the Sesia–Lanzo zone (Sordevolo), and a Epiclastic conglomerate with strong heterogeneous clasts size d ankerite and tourmaline veins crosscutting andesitic epiclastite distribution (Valle, Val Oropa), b internal depositional layering in (Passobreve, Valle del Cervo) epiclastic conglomerate (Valle, Val Oropa), c epiclastic conglomerate 3 Methods furnace at 900 °C for approximately 48 h. Subsequently, zircons from each sample were transferred into 3 ml screw- The samples for zircon U/Pb dating have been crushed top Savillex vials together with ca.

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