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Listwaenite evolution in the ophiolite mélange of Iti Mountain (continental Central Greece)

University of Patras, Department of Geology, Section of Earth Materials, GR-265 00 Patra, Greece

^* Corresponding author, e-mail: v.tsikouras{at}upatras.gr

Rare listwaenite occurrences with the assemblage quartz + calcite + magnetite + goethite + hematite ± talc ± ankerite have been found within the ophiolite mélange of the Iti Mountain. They also contain relic Cr-spinel, and have retained mesh and hourglass textures indicating an origin from serpentinite. Geochemical investigation, using isocon analysis and assuming Ti as immobile element, reveals conservation of mass. During the listwaenite-forming metasomatic event there was influx of Si, Ca, Ba, Zr, light rare-earth elements (LREE), Ir and Pt, while Mg, Mn, heavy rare-earth elements (HREE) and Pd were removed from the system. Al, Cr, Ni and Fe^tot behaved as immobile elements. The release of Mg and formation of quartz may have occurred from breakdown of the serpentine minerals at low pressure, involving very high X_CO2 in the fluid phase. Two distinct fluids are thought to have been involved in the alteration process. Fractionation of REE is explained by their mobility mainly as REE-carbonate complexes, which are favoured by a CO₂-rich and mildly alkaline fluid. Ca influx in the listwaenite is attributed to this fluid, which is likely related to the serpentinization of the peridotites. Transportation of SiO₂, as well as of platinum-group elements (PGE), was favoured by another low-pH, highly-oxidized, saline fluid, at low temperatures. Mobility of REE probably persisted, but likely as chloride complexes. The latter fluid may have been also responsible for oxidation of Cr-spinel to goethite and of magnetite to hematite. The presence of goethite and calcite in the listwaenite account for the enrichment of LREE, given that these minerals can selectively absorb the LREE into their structures. The HREE, which were released during the breakdown of serpentine and olivine, were subsequently removed from the system. At very low temperatures the two fluids were miscible and the interaction of the acid, oxidized fluid with the calcite probably led to precipitation of Pt and Ir in the listwaenite. Direct dissolution of olivine relics has likely occurred at a late stage and further depleted the listwaenite in Mg.

Key-words: listwaenite, metasomatism, isocon plot, REE, PGE.