Geological Setting
The Sesia-Lanzo Zone (SLZ) belongs to the Austroalpine Domain of the Western Alps (Fig. 1) and is interpreted as a slice of the Adria convergent continental margin (e.g. Polino et al. 1990) that recorded a pervasive Alpine HP metamorphism (Compagnoni et al. 1977). The SLZ is now bounded by relicts of the Ligurian-Piemontese Tethys to the north and by the Insubric-Tonale line (Canavese line) to the south; the latter divides the SLZ from the Southern Alpine Ivrea Zone (Fig. 1). During Alpine subduction, the SLZ was tectonically eroded from the upper continental margin (Polino et al. 1990) and subducted to depths > 60 km. During the Cretaceous, the SLZ records a pervasive eclogite-facies re-equilibration. During the Cretaceous, the SLZ records a pervasive eclogite-facies re-equilibration.
Afterwards, it was partially affected by a greenschist-facies event, associated with the exhumation to the uppermost part of the Tertiary nappe stack (Passchier et al. 1981; Inger et al. 1996; Duchene et al. 1997; Rubatto et al. 1999; Zucali et al. 2002b; Zucali and Spalla, 2011).
On the basis of its lithological composition and dominant metamorphic imprint, the SLZ has been subdivided into two tectonic units (Fig. 1): the upper unit, the II Dioritic Kinzigitic Zone, shows a dominant metamorphic imprint under amphibolite to granulite facies conditions of pre-Alpine age. The lower unit, which consists of metapelites, metabasics and metagranitoids locally preserving Permian igneous bodies (e.g. Monte Mucrone and Val Sermenza gabbro), is divided into three complexes that record a pervasive Alpine metamorphic imprint: the Gneiss Minuti Complex (GMC) that preserves a dominant metamorphic imprint under greenschist facies conditions, the Eclogitic Micaschist Complex (EMC) that preserves dominant metamorphic imprint under eclogite facies conditions and the Rocca Canavese Thrust Sheet (Pognante 1989b; Spalla and Zulbati 2004) where a lawsonite blueschists-facies metamorphic imprint characterizes the peak stage. The eclogitic imprint has been dated to Late Cretaceous-Early Palaeocene (Oberhänsli et al. 1985; Inger et al. 1996; Duchene et al. 1997; Ruffet et al. 1997; Rubatto et al. 1999; Handy et al. 2005; Babist et al. 2006; Konrad-Schmolke et al. 2006). The area of study is located in the EMC.
Within portions of the SLZ during its pre-Alpine evolution a penetrative amphibolite foliation was imprinted in metapelites and in basic granulites, overprinting granulite facies assemblages (Lardeaux and Spalla 1991). P-T estimates for this pre-Alpine evolution indicate an early metamorphic imprint at 0.6 ≤ P ≤ 0.9 GPa and T = 700 - 900°C, followed by a re-equilibration stage at P = 0.3 - 0.5 GPa and T = 570 - 670 °C. A greenschist-facies retrogression is recorded at P = 0.25 - 0.35 and T < 550°C (Lardeaux and Spalla 1991). Similar P-T conditions have been estimated for the crystallization of Permian intrusives (Zucali et al., 2002b).
In the Monte Mucrone-Mars area and around Alpe Toso, Val Sesia, pre-Alpine mineralogical and textural remnants are preserved, where amphibolite-facies pre-Alpine paraschists and metabasics are intruded by granitoids of Permian age (Oberhänsli et al. 1985; Paquette et al. 1989). In the Southern SLZ, acidic and mafic pre-Alpine granulites are well preserved and associated with granulitised gabbros, characterized by a similar pre-Alpine P-T evolutions (Rebay and Spalla 2001).
Alpine P-T conditions for the eclogitic stage have been estimated at P > 1.5 GPa and T = 500 - 600 °C (Desmons and O'Neil 1978; Koons 1986; Tropper et al. 1999; Zucali et al. 2002a, 2004, 2011). Prograde and retrograde P-T conditions are characterized by lawsonite-bearing assemblages in some localities, implying a low geothermal regime active during both prograde and retrograde evolution (Pognante 1989a, 1989b; Matsumoto and Hirajima 2004; Zucali et al. 2004; Zucali and Spalla, 2011).
The innermost part of Sesia-Lanzo Zone (EMC) was intruded by the Biella Oligocene Pluton between 29-31 Ma (Bigioggero et al. 1994; Romer et al. 1996; Zanoni et al. 2008) while the andesitic dykes, that crosscut all Alpine fabrics, have ages between 29 and 33 Ma (Scheuring et al. 1973; Kapferer et al. 2011).
The Lago della Vecchia area (Piemonte, Italy, UTM coordinates: 1413689.522 - 5058432.551;1413689.522 - 5058432.551) belongs to the SLZ and it is mainly constituted by variably deformed metagranitoids characterized by a eclogite facies metamorphic overprint.
Figure 1. The Sesia-Lanzo Zone
A) Tectonic outline of the Alpine chain. B) Geological outline of Western Alps (redrawn after Dal Piaz et al. 2001). C) Classical subdivision of the Sesia-Lanzo Zone. The red star indicates the investigated area. D) NW-SE geological cross-section along the Western Alps: SA= Southern Alps; SL= Sesia-Lanzo Zone; MR = Monte Rosa; DB= Dent-Blanche; PI= Penninic; SB= Grand Saint Bernard; MB= Mont Blanc; LPN= Lower Penninic; H= Helvetic; MO= Molasse.