ALBERT

Description: A detailed petrological study of mafic and ultramafic xenoliths from Cerro del Fraile (Southern Patagonia, Argentina) was developed to highlight (1) the mineralogical and geochemical composition of the lithospheric mantle beneath the area, (2) the nature of the metasomatizing agents that infiltrate the mantle wedge above the Antarctic plate, (3) the processes that allow the mantle to be refertilized, and (4) the nature of the material dragged down in the subduction zone and recycled within the South Patagonian sub-arc mantle. Major and trace element analyses of clinopyroxene and orthopyroxene in peridotitic and pyroxenitic rocks suggest that a proto-adakite, deriving from the melting of the subducting Antarctic plate, was responsible for the metasomatic features of the peridotitic rocks and the crystallization of the pyroxenites. A few composite xenoliths bridge the two processes—peridotite enrichment and pyroxenite crystallization—indicating that the variously depleted mantle reacts with the incoming melt to generate a newly fertile mantle domain. This reaction occurs under reducing conditions [log f O 2 (QFM) –1·21 to –0·34], unusual for mantle wedge settings. Heavy rare earth elements and Al 2 O 3 and MgO contents in pyroxenes indicate a partial melting degree varying from 10 to 25%. The peculiar enrichment in Zr (–Hf), Th and U of the pyroxenes indicates the melting of oceanic sediments, which include manganese nodules and, possibly, organic matter, in agreement with the estimated low oxygen fugacity conditions. Some geochemical analogies have been found between the calculated metasomatic melts and the Austral Volcanic Zone adakites. In this case, the amount of sediment involved in the genesis of the infiltrating melts is larger than that previously proposed for the genesis of the erupted Patagonian adakites. Chemical–physical conditions favouring the upward percolation through the mantle wedge of these SiO 2 -rich and viscous melts are also discussed.