In situ EMPA and SIMS investigation of mineral geochemistry of mantle xenoliths from Cerro Nemby (25°24' S, 57°32' W), Paraguay

Mantle xenoliths are hosted in lavas localized in the Nemby area (25°24' S, 57°32' W; Asunciòn-Sapucai-Villarrica graben, ASV, central Paraguay: Fig.1), where a small melanephelinitite plug (Cerro Nemby), with elliptical topography (800 x 500 m, about 100 m above the plain), contains very abundant mantle xenoliths (10-15% by volume of the plug) together with crustal xenoliths (Comin-Chiaramonti et al., 2001). According to Le Bas (1987), lavas consist of nephelinite and subordinately of ankaratrite (CIPW Ab < 5 wt% e Ne > 20 wt%). The average size of the mantle xenoliths (10-12 cm, max 45 cm, i.e. the largest observed in ASV) and the compositional range (lherzolite to dunite to pyroxenite) make these xenoliths particularly suitable for a study regarding metasomatic processe(s) affecting the Sub-Continental Lithospheric Mantle of central Paraguay. The dunite results to be the most abundant xenolith type in such lavas. In-situ geochemical characterization was performed on silicates and glasses from very fresh xenoliths, which document a large variety of rock types. Five samples were investigated, namely: i) dunite 3209; ii) spinel harzburgite 3284; iii) spinel lherzolite 3293; iv) olivine websterite 3253 and v) olivine clinopyroxenite 3270. The analyses were directly carried out on thin petrographic sections (30 µm thick) of the selected samples.