Geological setting
The Sesia-Lanzo Zone (SLZ, Fig. 1) represents the widest portion of continental crust in the Western Alps that underwent high pressure (HP) metamorphism during the Alpine subduction (e.g. Babist et al., 2006; Compagnoni et al., 1977; Dal Piaz et al., 1972; Meda et al., 2010; Pognante, 1991; Roda et al., 2012), preceding the collision between European and Adria continental plates.
The Alpine metamorphic history of the SLZ comprises the record of an eclogite-facies imprint followed by blueschist- and greenschist-facies re-equilibrations (e.g. Castelli & Rubatto, 2002; Compagnoni, 1977; Gosso, 1977; Lardeaux et al., 1982; Pognante, 1989a; Rebay & Messiga, 2007; Spalla et al., 1983; Zucali & Spalla, 2011). Mineral ages ranging between 90 and 65 have been related to the Alpine eclogite-facies peak (Cenki-Tok et al., 2011; Rubatto et al., 1999). The very low T/P ratio characterising this evolution assists the preservation of many pre-Alpine igneous and metamorphic relicts in rocks with an Alpine polyphasic recrystallization. P-T conditions for the early-Alpine HP imprints range between 500–625 °C and 1.3–2.5 GPa (see Roda et al., 2012 for a review of available PT estimates). The external margin of the SLZ is bounded by eclogitized ophiolitic relicts of the Liguria-Piedmont Ocean, the Piedmont Zone, and its internal margin is a thick mylonitic belt (the Canavese Line), separating SLZ from the lower crustal rocks of the Southalpine Ivrea Zone, which escaped the Alpine HP evolution (Bigi et al., 1990).
Figure 1. Tectonic sketch of Western Alps (modified after Dal Piaz, 1999) showing the location of the Sesia-Lanzo Zone (SL); red diamond locates the mapped area of Fig 2.
The two bottom insets locate the tectonic sketch in the frame of the European Alps. Legend: 1) Penninic continental nappes: MR = Monte Rosa, AB = Arcesa-Brusson, GP = Gran Paradiso, SB = Grand St. Bernard; 2a) Austroalpine nappes: DB = Dent Blanche (Vp = Valpelline lower crust, Ar = Arolla series), MM - P = Mt. Mary-Pillonet thrust system, AR = Acque Rosse, CH = Chatillon-St. Vincent, E = Etirol-Levaz, G = Grun, EM = Mt. Emilius, GR = Glacier-rafray, S = Santanel, TP = Tour Ponton, SL = Sesia-Lanzo Zone (Dk = Dioritic-kinzigitic upper element, Gm = Gneiss Minuti complex with (2) or without (1) eclogitic relics, Emc = Eclogitic Micaschists complex, Rct = Rocca Canavese Thrust Sheets); 2b) Mesozoic metasedimentary covers in Austroalpine Domain: R = Roisan Zone, Sc = Scalaro unit; 3) Ophiolitic Piedmont Zone: CO = Combin Zone, PCB = Pancherot-Cime Bianche, FC = Faisceau de Cogne, ZS = Zermatt-Saas Zone, MA = Mt. Avic, A = Antrona ophiolite, LM = Lanzo Massif; 4) Southalpine Domain: CA = Canavese Zone; 5) Oligocene Plutons: B = Biella, M = Miagliano, T = Traversella; 6) Lineaments: CL = Canavese Line, SF = Simplon Fault, AR = Aosta-Ranzola fault system.
A pre-Alpine polyphasic metamorphic evolution, from granulite- to amphibolite-facies conditions, is still preserved in marbles, metapelites, metagranitoids and metabasics (Castelli, 1991; Compagnoni et al., 1977; Lardeaux et al., 1982; Lardeaux & Spalla, 1991; Rebay & Spalla, 2001): the pre-Alpine T-climax has been constrained at T=730–830 °C P=0.7-0.9 GPa (Lardeaux & Spalla, 1991). Granulite- and amphibolite-facies imprints have been interpreted as the result of an extension-related uplift of a portion of the Variscan crust, occurred in Permian–Triassic times during the lithospheric thinning leading to the Tethys opening (e.g. Marotta & Spalla, 2007; Marotta et al., 2009).
This pre-Alpine tectono-metamorphic evolution is preserved in the metamorphic complexes forming the SLZ: the Eclogitic Micaschist Complex (EMC), the Gneiss Minuti Complex (GMC), the II Dioritic–Kinzigitic Zone (IIDK) and the Rocca Canavese Thrust Sheets (RCT) (e.g.: Compagnoni et al., 1977; Pognante, 1989a; Pognante, 1989b). The IIDK consists of kilometric lenses, lying between EMC and GMC, in which Alpine eclogitic assemblages are not described, though in the Vogna Valley the tectonic contact underlying the margin between IIDK and EMC is marked by eclogite-facies mylonites (Lardeaux, 1981; Lardeaux et al., 1982). Eclogitic parageneses are widely described both in EMC and GMC with a strong difference in the volume affected by the greenschist re-equilibration: the GMC, which lies along the tectonic boundary with the Piedmont Zone, is widely re-equilibrated under greenschist facies conditions, while the EMC, constituting the internal part of the SLZ, records the greenschist-facies re-equilibration mainly along discrete shear zones and shows a dominant metamorphic imprint under eclogite-facies conditions (e.g. Spalla, 1983; Spalla et al., 1991).
The EMC protoliths are high-grade paragneisses, granulites, amphibolites and minor marbles and quartzites, which are the country rocks of Permian granitoids and gabbros (Bussy et al., 1998; Callegari et al., 1976; Castelli, 1987; Cenki-Tok et al., 2011; Compagnoni et al., 1977; Oberhaensli et al., 1985; Zucali, 2011) and from which the Mt. Mucrone body is the most renown. The deformation history of EMC, in this area, comprises four generations of Alpine folds, two of which are associated with the high-pressure (HP) mineral assemblages, and two generations of shear zones synchronous with the blue- and greenschist-facies re-equilibrations, respectively (Hy, 1984; Zucali et al., 2002b). According to earlier studies (Zucali, 2002) D1 and D2 deformations are generally associated with axial plane foliations marked by Ph, ±Pg, Na-Cpx, Grt, Rt and Zo (mineral abbreviations are used according to Whitney & Evans, 2010); the same mineral assemblage is stable during D3 folding. D4 is accompanied by a widespread re-equilibration under blueschist-facies conditions, confined along metre-wide mylonitic shear zones, which locally developed at the boundary between metagranitoids and country rocks. D5 mega-scale folds, D6 centimetre-thick mylonitic shear zones and gentle folding, overprinting blueschist mylonites and the eclogitic foliations, are synchronous with the formation of a typical greenschist-facies mineral association of Qz, Ab, Wm, Ep, Ttn and green-Amp.