Mantle metasomatism recorded upon bimodal chromitites (E. Chalkidiki, Greece): a tool to unravel metasomatic processes

Nea Roda and Gomati ultramafic bodies (east Chalkidiki, north Greece) consist of both Cr- and Al- podiform chromitites, which are highly altered. Their PGE geochemistry and subsequently PGE-mineralogy (PGM) demonstrate abnormal element concentrations with an enrichment in PPGE (Pd, Pt), leading to high Pd/Ir ratios. Secondary PGM and base metal assemblages are dominated by Sb and As, whereas primary phases form sulphides. At a more mature stage, desulphurization of the aforementioned phases led to formation of native metals. Diopside hosted within diopsidite and chromitite show both an alkaline melt- and a fluid- rock interaction, depicted by LREE enrichment. The temperature of the metasomatic fluids was lower than 600^oC, as recorded by chlorite and garnet geothermometry. A raise in fluid mobile elements (FME: B, Sb, Li, As, Cs, Pb, U, Ba and Sr) is noted in the whole rock and clinopyroxene analysis. All these characteristics along with the distinctive spinel textures (porous, zoned grains) point to a metasomatic event during subduction that led to the post-magmatic modification of the chromitites and the mantle section causing a LREE, Pb, As, Sb, Pd and Pt enrichment.