Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Strain localization in upper mantle shear zones by grain size reduction and the activation of grain size sensitive deformation mechanisms (grain boundary sliding, diffusion creep) is closely linked to phase mixing. With its mylonitic grain sizes (50–100 µm) and well mixed phase assemblage, the km-scale shear zone at the northwestern boundary of the Ronda peridotite is in this respect no exception. In transects across the “mylonitic” into the less deformed “tectonic” part of this shear zone four dominant microstructural domains were identified: olivine-rich matrix, mixed matrix and neoblast tails of clino- and orthopyroxene. In these, phase mixing quantities, its formation processes and its impact on strain localization were analyzed by a combined microstructural (EBSD) and geochemical (EPMA) analysis. Dominant microstructure of all samples is the mixed matrix composed of olivine, ortho- and clinopyroxene. Its homogenous distribution of interstitial, and/or wedge-shaped pyroxenes contradicts mechanical mixing. In general, high (> 60 %) phase boundary percentages in all four microstructural domains indicate extensive phase mixing independent from microstructural domain and distance to the deformational center of the shear zone located at the NW boundary of the peridotite massif. The constant grain sizes with local variations independent on the distance the deformational center indicate a broad scale deformation with ± constant stresses in the entire mylonitic area. Decreasing Mg# and increasing Ti contents with increasing distance to the NW shear zone boundary, highly lobate phase boundaries, homogenous phase mixing and secondary phase distribution in all samples corroborate a metasomatic phase mixing by melt-rock reactions and crystallization of pyroxenes in the entire shear zone transect. Consistent geochemistry and phase assemblage in mylonites and tectonites but a change from equiaxial (tectonites) to wedge-shaped pyroxenes aligned in the foliation (mylonites) indicate a pre- to syn-deformational melt infiltration. Following the geochemical gradient, the potential source of melt is below a structurally deeper “melting” front which separates the sheared peridotites from coarse granular peridotites. The presence of mixed matrix in the entire shear zone, its microstructural and geochemical consistency indicate that the melt-infiltration was fundamental for the formation of and the strain localization in the major shear zone of the NW Ronda peridotite.
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