Faculté des sciences

Melt migration and deformation in the upper mantle : an example of the Lanzo peridotite massif (Western Alps, Italy)

Kaczmarek, Mary-Alix ; Müntener, Othmar (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2007 ; Th.1955.

Mantle domains from present-day and ancient ocean-continent transition zones display signs of melt/rock reaction, but the relationships of the deformation processes during magma-starved periods of (ultra-)slow spreading are poorly understood. The transition from melt-poor to -rich regions is likely to be an important rheological boundary. The mylonitic mantle shear zones are commonly found in... More

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    Summary
    Mantle domains from present-day and ancient ocean-continent transition zones display signs of melt/rock reaction, but the relationships of the deformation processes during magma-starved periods of (ultra-)slow spreading are poorly understood. The transition from melt-poor to -rich regions is likely to be an important rheological boundary. The mylonitic mantle shear zones are commonly found in peridotite massifs and in the oceanic lithosphere and the grain size reduction is considered as an important process, but the role of melt-enhanced reactions is less well known. This thesis is the result of field, petrologic and geochemical studies on a high temperature mantle shear zone in the Lanzo peridotite massif from the Western Alps (Northern Italy), and its interaction with melt migration are presented. The mantle shear zone between the northern and the central bodies of the Lanzo massif displays five rock types with different deformation fabrics: coarse grained secondary granular (CGSG), fine grained secondary granular (FGSG), proto-mylonite, mylonite, hydrous-mylonite and ultra-mylonite. The spatial distribution of deformation is asymmetric with respect to the mylonite, increases from SW to NE, and the northern body is composed of CGSG rocks. Discordant mafic dikes are asymmetrically distributed and concentrated in the southern part, which is interpreted as the footwall of the large mantle shear zone. The high Al pyroxene porphyroclastic cores, indicate high temperatures (1100-1030°C). The low Al neoblasts, display substantially lower final equilibration temperatures at ~860°C. Spinel Cr# (molar Cr/Cr+Al) and TiO2 concentrations are extremely variable and cover the entire range from spinel to plagioclase peridotite. The variability does not correlate with microstructures in a simple way. However, the CGSG spinel compositions from the central body are more variable than spinel from mylonite, hydrous-mylonite and spinel compositions from the northern body. The spinel compositions indicate local disequilibrium, argue for a rapid exhumation of the central relative to the northern peridotite body. Melt impregnation textures such as dissolution of clinopyroxene and cotectic crystallization of orthopyroxene and plagioclase, and replacement of olivine by orthopyroxene are common in the massif. Melt impregnation textures largely disappear in the mylonite, indicating that deformation along the mylonite zone outlasted melt-rock reactions. Melt-rock reaction may cause grain size reduction, which in turn led to localization of deformation. The actively deforming peridotite mylonite acted as a permeability barrier for migrating liquid, and thus inhibiting the propagation of gabbroic dikes. This is supported by the observation that gabbros are asymmetrically distributed with respect to the shear zone and concentrate in the footwall of the high-temperature peridotite mylonite zone. The whole rock major and trace element data of variably deformed peridotite display a large compositional variation from "super-fertile" (e.g. higher than primitive upper mantle [PUM]) to refractory peridotite in the CGSG and, a homogeneous fertile composition in the deformed rocks, which can be explained by grain size reduction. The incompatible elements show an enrichment in the order of 10-20% in the deformed rocks (proto-mylonite, mylonite and hydrous-mylonite) compared to the CGSG from the northern body. This supports the hypothesis that fine-grained rocks acted as a permeability barrier. The (Ce/Yb)N ratio display little variation and reinforces the hypothesis that the geochemical signature is not defined by fractionation processes. Simple models indicate that the plagioclase peridotite composition is more consistent with the refertilization processes. It can qualitatively be estimated, that about 5 to 10% of MORB-type liquid must be added to explain the variability of the Lanzo plagioclase peridotites. The nrichment of peridotites in the footwall of a major mantle shear zone indicates that melt accumulation must have been important: consistent with the permeability hypothesis. Gabbroic rocks from the Lanzo peridotite massif form a cumulate sequence with about 3 orders of magnitude enrichment of incompatible elements. Fe-Ti gabbros dated from the central and the southern part of the massif provided middle Jurassic ages of 161 ± 2, 158 ± 2 and 163 ± 1 Ma, which argues for magmatic activity over a few millions of years. Zircon crystals are characterized by high but variable Th/U ratios, REE patterns enriched in HREE, pronounced positive Ce and negative Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition coupled with whole rock chemistry indicate that a number of gabbros crystallized in-situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model, in which the emplacement mechanism of gabbroic rocks in ocean continent transition zones evolves from "early" in-situ crystallization to "late" stratified crystallization with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the igneous crystallization is probably related to the thermal evolution of the underlying mantle lithosphere.