Melt-assisted strain localization in the upper mantle shear zone of northwestern Ronda (Spain)

On the northwestern boundary of the world’s largest exposure of subcontinental mantle a major shear zone crops out. A microstructural transect from mylonites to tectonites (0‑700 m distance to the shear zone front) reveals a metasomatic history of multiple melt pulses with major impact on deformation and shear localization. Fe/Ti rich melts coming from a structurally deeper melting front inside the peridotite body, re‑fertilized the entire shear zone area up to its present-day front and precipitated interstitial pyroxenes and spinel. The resulting mixed matrix of recrystallized olivine and interstitial grains localized the deformation in the entire melt-affected region. Microstructural characteristics are elongated olivines with film/wedge-shaped pyroxenes along the olivine grain boundaries and interstitial spinel. Melt-rock reaction textures such as indentations, highly irregular grain shapes and lobate boundaries are commonly present. Strong CPOs (olivine A-type) for all phases and the absence of amphibole in the mixed matrix indicate deformation under relatively dry conditions predominantly accommodated by dislocation creep. However, P-T estimates (1‑2 GPa, 800‑900 °C), average grain sizes of ~100 µm and the localization of deformation in the mylonitic area also indicate the activity of an additional grain size sensitive mechanism. Within the matrix, reacting pyroxene porphyroclasts form tails of pyroxene, olivine, amphibole and spinel neoblasts. Their similar or coarser grain sizes compared to the matrix and an equiaxial grain shape indicate no further strain localization in these tails. Their predominant B- or AG- CPO-types of olivine and the presence of amphibole suggest the presence of fluids during the tail formation.