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_Journal of Structural Geology 33 (2011) 381-398_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Prograde lawsonite during the flow of continental crust in the Alpine subduction: Strain vs. metamorphism partitioning, a field-analysis approach to infer tectonometamorphic evolutions (Sesia-Lanzo Zone, Western Italian Alps) Michele Zucali a,*, Maria Iole Spalla a,b a Dipartimento di Scienze della Terra “Ardito Desio”, Università degli Studi di Milano, Via Mangiagalli 34, I-20133 Milano, Italy b CNR e Istituto per la Dinamica dei Processi Ambientali, Via Mangiagalli 34, I-20133 Milano, Italy Article info Article history: Received 19 April 2010; Received in revised form 26 October 2010; Accepted 5 December 2010; Available online 17 December 2010 Keywords: Sesia-Lanzo Zone Subduction Lawsonite Prograde Strain partitioning Abstract Detailed mapping of superposed fabrics and their mineral support allows for reconstruction of the tectonometamorphic evolution of the Ivozio Complex, within the inner portion of the Sesia-Lanzo Zone (Western Italian Alps). The resulting evolution is characterized by a multi-stage structural and metamorphic reequilibration during Alpine subduction, starting from the pre-Alpine igneous association (Amp0 ? Cpx0). The prograde associations begin with S1a marked by AmpI ? ZoI which pre-date the growth of GrtI (S1b); successive increase in pressure stabilizes a second generation of Amp ? Grt (S1c AmpII ? ZoI ? GrtII). The growth of prograde lawsonite and omphacite occur during S1d (OmpI ? Lws ? GrtII ? AmpII) within lawsonite-bearing eclogites, while S1e is associated with the breakdown of lawsonite, producing the association OmpI ? Ky ? ZoII ? GrtII ? AmpII (lws-bearing eclogites); S1d-e stages are associated with AmpII ? ZoI ? GrtII ? OmpI in eclogites. The second generation of penetrative foliation (S2), describing the retrograde evolution, is divided into S2a (AmpII ? GrtII ? Pg ? ZoII) and S2b (Chl ? AmpIII ? Pg ? Ab). The comparison between the reconstructed evolution of the Ivozio Complex and PeT paths inferred in the Southern Sesia-Lanzo Zone suggests a non-uniqueness of the Sesia-Lanzo Zone continental crust, during the Alpine subduction. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction In order to discriminate between mechanisms active during the subduction burial and exhumation of continental crust (e.g., tectonic erosion, ablative subduction), it is fundamental to determine the Pressure-Temperature relative time of deformation trajectories followed by the rocks within the subduction wedge. Moreover, the actual dimension and shape of the slices of crust acting as independent objects within the subduction system are also crucial. Pressure and temperature variations in time, deduced from numerical modelling and their comparison with thermobarometric and geochronological natural data, suggest that crustal particles involved in the subduction-collision lithosphere-scale processes may be recycled within the subduction wedge prior to exhumation and final, stable structural packaging (Gerya et al., 2002; Meda et al., 2010; Roda et al., 2010). The occurrence of lawsonite-bearing rocks is the marker of a highly depressed geothermal gradient characterizing the thermal state of very cold subduction zones (Cloos, 1982, 1993). Therefore, discrimination between prograde and retrograde lawsonite within subducted rocks is also crucial to identify the highly depressed geothermal gradient active during burial and/or exhumation paths. Generally, lawsonite develops within the subducting oceanic lithosphere under extreme LTeHP conditions (Tsujimori et al., 2006; Cetinkaplan et al., 2008; Ghent et al., 2009), while it is rarely reported in continental crust (Sesia-Lanzo Zone, Italy: Compagnoni et al., 1977; Pognante, 1989b; Zucali et al., 2004; Dabie, China: Li et al., 2004; Calabria - Southern Italy: Piccarreta, 1981; Turkey: Okay, 2002; Okay and Whitney, 2010). Therefore, the last occurrence implies that mechanisms promoting the subduction of crustal slices from the overriding continental plate before continental collision may be active. The Sesia-Lanzo Zone of the Western Italian Alps is a continental crust fragment (100 ? 10 km-sized) involved in the Alpine subduction and collision. It extensively recorded eclogite-facies low-temperature assemblages in its internal parts, known as the Eclogitic Micaschists Complex. Lawsonite-bearing assemblages were also found in Sesia-Lanzo Zone eclogites and were mainly interpreted as retrograde (in the EMC: Caron and Saliot, 1969; Compagnoni et al., 1977; Pognante et al., 1980, 1988; Spalla and Zulbati, 2003; Zucali et al., 2004; in the IIDK: Miletto, 1984; in the Gneiss Minuti: Pognante et al., 1987). The use of PeT-dep paths, supported by detailed regional scale field mapping, has been successfully applied to contour tectonic units with a coherent metamorphic evolution during a time interval (i.e., the tectonometamorphic units, Spalla et al., 2005). Thus, the discrimination of prograde or retrograde lawsonite assemblages is fundamental to identify contrasted PeT-dep paths characterizing different tectonometamorphic units in the Sesia-Lanzo Zone. Strain partitioning and mineral transformation occurring during a multiphase tectonometamorphic evolution may generate different fabrics, partially or completely underlined by newly grown minerals or mineral assemblages. These fabrics may be distinguished on the basis of the strain state: low strain (coronitic fabric domains), intermediate strain (tectonitic fabric domains) and high strain (mylonitic fabric domains) (Lardeaux and Spalla, 1990; Spalla and Zucali, 2004). Coronitic fabrics may contain structural and metamorphic mineral relics, allowing for preservation of earlier metamorphic assemblages. In tectonitic and mylonitic fabrics, the metamorphic growth sequence may be linked to the superimposed planar linear fabrics. Mylonitic fabrics may represent the end-members of both structural and mineralogical re-equilibration, where all mineral and structural relics of preexisting stages are completely erased. The grain size scale model of deformation partitioning (Bell et al., 1986) enables granular scale deformation stages to be related to successive kinematic stages, from crenulation to complete obliteration of the original fabric (Bell and Rubenach, 1983; Bell and Hayward, 1991; Salvi et al., 2010). The kinematic stages can be related to the evolution of reaction products of metamorphic transformations, distinguishing between fabrics dominantly supported by old minerals, slightly replaced by new minerals (coronitic microstructure of the fabric), and fabrics entirely marked by the new metamorphic assemblages (S or S L tectonite to mylonite). The field-analysis and microstructural correlation of progressive strain states (coronitic, tectonitic and mylonitic fabrics) and the related reacting volumes are here used as the basis of correlation for the tectonometamorphic history (Spalla et al., 1999, 2000; Gazzola et al., 2000; Zucali et al., 2002b; Hobbs et al., 2010; Salvi et al., 2010). In this contribution, this approach will be applied in order to decipher the tectonometamorphic evolution of the metabasites of the Ivozio Complex. The results will be compared with other Ключевые слова: gosso, evolution, range, bearing, characterized, petet path, qtz, high-pressure stability, reconstructed evolution, lawsonite growth, case history, ky ?, undulose extinction, scale, metamorphic, grti, eclogites, alps, time, schist, pervasive foliation, holland, lithos, ivozio metabasites, mechanism promoting, western alps, ivozio, lws, metabasites, mineralogist, ab, reaction, layer, marked, powell, spalla journal, metamorphic re-equilibration, journal structural, grtii rim, vein, schematic view, fabric, foliation, paola, gpa, schmidt okamoto, sesia lanzo zone, elsevier, high, retrograde stage, epidote, geological processes, parallel, zo, exhumation, poli, content, grtii, continental collision, ky, american, zone western, rebay, gerya, lawsonite, di, ivozio complex, science, complex, zucali spalla, sesia, garnet, zo ?, foliation marked, journal metamorphic geology, mineralogy, pognante, amphibole, zucali, pg ?, deformation, ep, grain, alpine, grti core, preferred orientation, rhombohedral shape, sesia-lanzo, petrographic map, relationship, numerical modelling, fabric evolution, dip angle, spo, spalla, assemblage, microstructural evolution, zoi, ompi, tectonometamorphic, journal, eclogitic, eclogite-facies condition, journal structural geology, compagnoni, retrograde lawsonite, omp, multi-stage structural, table, temperature, lawsonite-bearing rock, mylonitic fabric, continental, elongated parallel, metamorphism, ampi, eclogite, omphacite, foliated fabric, retrograde, ravna, dip azimuth, gure legend, zone, deformation partitioning, structural geology, facies, ? pg, path, tectonometamorphic unit, sciences, pg, metamorphic evolution, strain, alpine subduction, mineral, crust, jadeite, tectonics, zoii, subduction, structural, ampiii, peteted path, work, ? ky, correlation, metamorphic reaction, grt, lawsonite-bearing eclogites, metamorphic geology, geological, continental crust, rock, phase, modelling, ? zo, shape, mineral assemblage, holland powell, lanzo, ampii, glaucophane, pressure range, amp, retrograde evolution, geology, penetrative foliation, slice, western, pressure, italy, stability, geothermal gradient, stage, prograde, lawsonite-bearing, cloos, journal metamorphic, mineral growth, growth, rubatto, hydrous phase, petrologia compagnoni, white mica, crustal slice, condition, metamorphic complex, ompi krogh-ravna, doi, chl, main foliation, sized, sesia lanzo, mineral support, sesia-lanzo zone, subduction wedge, chemical composition, foliation figs, connolly, successive stage, pre-alpine evolution, tectonometamorphic evolution, crystal, composition, ghent, lanzo zone, map, analysis, orientation, pet, microstructural analysis, microstructural