TEXTURES, GEOCHEMISTRY AND SIGNIFICANCE OF SHEAR ZONE IN THE PERIDOTITIC MASSIF OF LANZO (ALPS-ITALY)

Abstract

The localization of deformation in the lithosphere is well known from the study of seismic reflexion profiles (e.g. Flack et al., 1990; Baird et al., 1995), peridotite mylonite in mantle xenoliths (e.g. Nielson and Schwarzman, 1977; Harte, 1983) and peridotite massifs (e.g. Drury et al., 1990; Newman et al., 1999; Dijkstra et al., 2002). The deformation conditions of such mylonites indicate the strain localization in the lithosphere mantle peridotite because the basal part of subducted or obducted ophiolite is often characterized by strongly deformed peridotite (e.g., Vissers et al., 1991; Suhr, 1993; Boudier and Nicolas, 1995). The Lanzo peridotite massif in NW-Italy is characterized by fresh peridotite, well preserved high-temperature shear zones (Boudier, 1978) and many features indicating pervasive magma flow (Bodinier et al., 1986, Muntener and Piccardo, 2003). The presence of melt is known to lower the resistance to stress in solid state flow (Hirth & Kohlstedt, 1995a, 1995b). To determine constraints on the rheological weakening of mantle lithosphere, we investigate the mutual relationships between high-temperature deformation, meltrock reaction and emplacement of plutonic rocks in the plagioclase peridotite massif of Lanzo. Mapping of the Lanzo massif, in the northern and central parts, has revealed the exposed dimension of the peridotite mantle shear zone, which is at least 250 meters wide with a sub-vertical foliation generally dipping to the NE. Fieldwork, microscopic observation and preliminary grain size analyses highlight 4 types of microstructures from “coarse grained secondary protogranular texture (CGSPT)” (according to Mercier and Nicolas 1975) to “mylonite”. CGSPT is characterized by weakly deformed porphyroclastic zones and clear domains of igneous recrystallization with olivine (ol), orthopyroxene (opx), plagioclase (pl), ± clinopyroxene (cpx) and ± spinel (sp). The mylonite is characterized by porphyroclastic opx extremely stretched (aspect ratios up to 10:1) and a fine grained matrix (grain size: to 2 km) and an anastomosing texture in accordance to the high temperature foliation. Fieldwork allowed to localize a series of discordant gabbroic dikes ranging from ol-gabbro to kaersutite-bearing diorites, which are abundant on the southern side of the mylonite and are generally weakly deformed. Some cpx porphyroclasts show signs of previous reaction textures with a melt (cpx1 +liq -> opx + plg ± ol), a texture, which is common in the southern Lanzo massif (Muntener and Piccardo, 2003). This indicates that the mylonite formation postdates cpx-corroding melt/rock reactions. Thermobarometry of porphyroclastic mineral assemblages in the peridotite indicate high temperatures (1100°C-1150°C and two pyroxene thermometry of neoblast indicate final equilibration temperatures close to 850°C. Compositionalprofiles in porphyroclastic cpx from a CGSPT sample infiltrated by melt are heterogeneous and zoned. The core of some cpx contains high values of Al (0.3 Al per formula unit) and low values in Ti (0.015 p.f.u.), which corresponds to relic spinel peridotite. The rim is zoned with a decrease in Al (0.13 p.f.u), an increase in Ti (0.027 p.f.u.) and a Cr increase from the core to the rim. This variation corresponds to the equilibration in plagioclase facies. The heterogeneity of the profile and the increase of Ti towards cpx rims suggest melt/rock reaction during the transition from spinel to plagioclase facies conditions. Cr# (molar Cr/Cr+Al) and TiO2 concentrations in spinel from strongly deformed plagioclase peridotites show an extreme variability and do not seem to be microstructurally controlled. This indicates that once the plagioclase peridotites completely crystallized, exhumation to shallower depth must have been rapid, in order to preserve disequilibrium chemical compositions. Our results indicate that melt migration and high temperature deformation are juxtaposed both in time and space. Melt-rock reaction may cause grain size reduction, which in turn led to localization of deformation. Observations indicate that actively deforming peridotite mylonite might suppress brittle failure and that ascending gabbros might terminate and crystallize along actively deforming shear zones. This is supported by the observation that gabbros are asymmetrically distributed with respect to the shear zone. If true, a ‘gabbro-rich zone’ might be the footwall of high-temperature mylonitic shear zone.