Short-scale variability of the SCLM beneath the extra-Andean back-arc (Paso de Indios, Argentina): Evidence from spinel-facies mantle xenoliths

Matilde, León and Chenque hills are some of the several occurrences of Cenozoic basalts carrying ultramafic xenoliths in Paso de Indios region, Argentina. The mantle xenoliths from the Chenque and León hills mainly present porphyroclastic texture, whereas the Matilde hill ones have coarse-grained to porphyroclastic textures. The equilibrium temperatures are in the range of 780 to 940°C, indicating a provenance from shallow sectors of the lithospheric mantle column that was subjected to a relatively low heat flux at Cenozoic Era. According to the modal compositions, the mantle columns beneath Matilde and León hills mostly record partial meting events larger than 22%, while less depleted peridotites occur in the Chenque suite (starting from 10% partial melting). Such an observation is confirmed by the partial melting estimates based on Cr#Sp, which vary from 8 to 14% for the selected Chenque samples and from 14 to 18% for the Matilde ones. The common melting trend is overlapped by short scale cross-cutting local trends. Local trends can be generated by open-system processes, such as open-system partial melting and/or post partial-melting metasomatic migration of exotic Na-Cr-rich melts. Petrographic survey evidences the occurrence of two main mineralogical reaction schemes due to channelled and/or pervasive melt extraction/migration. These are: i) pyroxenes dissolution and segregation of new olivine in olivine-rich peridotites, and ii) replacement of primary olivine by orthopyroxene±clinopyroxene in orthopyroxene-rich peridotites. Enhanced pyroxene dissolution is attributed to channelling of silica-undersaturated melts, whereas replacement of primary olivine by orthopyroxene±clinopyroxene points to reaction with silica-saturated melts. Late disequilibrium reactions identified in the xenoliths comprise: the breakdown of orthopyroxene in contact with the host basalt and (rarely) reaction coronae on orthopyroxene, clinopyroxene and spinel linked to glassy veins. Such features are apparently related to the injection of melt, likely during entrainment into the host basalts and ascent to the surface.