Short-scale variability of the SCLM beneath the extra-Andeanback-arc (Paso de Indios, Argentina): Evidence from spinel-faciesmantle xenoliths

Abstract

Abstract Cenozoic basalts carrying ultramafic mantlexenoliths occur in the Matilde, León and Chenque hillsin the Paso de Indios region, Argentina. The mantle xenolithsfrom the Chenque and León hills mainly presentporphyroclastic textures, whereas the Matilde hill xenolithshave coarse-grained to porphyroclastic textures. Theequilibrium temperatures are in the range of 780 to 940ºC,indicating a provenance from shallow sectors of the lithosphericmantle column that were subjected to a relativelylow heat ffiux at Cenozoic Era. According to the modal compositions of xenoliths, themantle beneath Matilde and León hills was affected bygreater than 22% partial melting, while less depleted peridotitesoccur in the Chenque suite (starting from 10%partial melting). Such an observation is confirmed by thepartial melting estimates based on Cr#Sp, which vary from8 to 14% for the selected Chenque samples and from 14 to18% for the Matilde ones. The common melting trend is overlapped by small-scalecross cutting local trends that may have been generated byopen-system processes, such as open-system partial meltingand/or post partial-melting metasomatic migration ofexotic Na-Cr-rich melts. The two main mineralogical reaction schemes are: i) thedissolution of pyroxenes and the segregation of newolivine in olivine-rich peridotites, and ii) the replacementof primary olivine by orthopyroxene±clinopyroxenein orthopyroxene-rich peridotites. These were producedby channelled and/or pervasive melt extraction/migration. Enhanced pyroxene dissolution isattributed to channelling of silica- undersaturatedmelts, whereas the replacement of primary olivine byorthopyroxene±clinopyroxene points to reaction withsilica-saturated melts. Late disequilibrium reactions identified in the xenolithscomprise: the breakdown of orthopyroxene in contact withthe host basalt, and (rarely) reaction coronae on orthopyroxene,clinopyroxene and spinel linked to glassy veins.Such features are apparently related to the injection ofmelt, likely during entrainment into the host basalts andascent to the surface.