Slab melt metasomatism as recorded in cpx and opx in mantle xenoliths from Cerro del Fraile (Argentina)

Ultramafic xenoliths found in Quaternary lava flows and diatreme breccias at Cerro del Fraile, 25 km east of the main AVZ (Austral Volcanic Zone, Patagonia), represent fragments of the mantle wedge above the subducting slab and provide valuable material for studying the nature of metasomatizing melt in suprasubduction setting. They mainly consist of lherzolites, with minor harzburgites and plagioclase-bearing orthopyroxenites. One composite sample is characterized by dunitic domains cut by orthopyroxenite veins. Host basalts are basaltic-trachyandesite, with mg# varying between 46.4 and 53.1. Compositionally they are very similar to the Nb-enriched arc basalts found in several volcanic arcs worldwide. Based on major and trace element features two groups of clinopyroxenes and orthopyroxenes can be distinguished in the lherzolites, and another group in the orthopyroxenite. Primary cpx and cpx in melt patches in peridotites have higher mg# than cpx in pyroxenites. Analogously mg# of opx in the peridotitic matrix range from 88.0 to 91.4 while opx in pyroxenites have lower and more variable mg#. Secondary cpx presents higher LREE, Th, U, Zr, Hf, and Sr contents than primary cpx. Analogously Group2 opx has higher LREE, Th, U and Sr contents than Group1 opx. Opx of pyroxenites are very different, with flat REE patterns at about chondritic values and Sr and Ti contents higher than those of peridotites’ opx. Ti content in both primary and secondary peridotitic minerals are always very low (Ti in opx<450 ppm, Ti in cpx<1100 ppm). These geochemical characteristics are consistent with interaction and hybridization with a Si-Na-rich melt, deriving from the melting of the subducting Antarctic plate. Slab melt metasomatism produced both Na and LREE enrichment in pyroxenes and positive Zr-Hf anomalies in cpx. The melts reacted with peridotites and reached the Moho, where they resided and fractionated orthopyroxenites. Successively the asthenospheric upwelling caused the mixing between OIB-like material and the already subduction-contaminated mantle wedge resulting in the formation of Nb-enriched trachybasalts that brought the xenoliths to the surface.