A new occurence of pyrophanite from the amphibolite-hosted skarn in Western Kimmeria, Xanthi, Northern Greece

Περίληψη

A new occurrence of pyrophanite (MnTiO3) in Greece is described from the garnetclinopyroxene- wollastonite exoskarn of western Kimmeria. Hostrock of the pyrophanite- bearing skarn is amphibolite (retrogressed eclogites) of the Nestos suture zone in the Rhodope massif. In addition endo- and exo-skarns occur both in the Xanthi granodiorite and the surrouding marbles and consinst of diopside-vesuvianitequartz and wollastonite-garnet-calcite-quartz respectively. Pyrophanite formed in the prograde stage, together with garnet, pyroxene, wollastonite, titanite, zircon, rutile, thorite and ouranothorianite. Retrograde minerals are quartz, epidote, calcite, hematite, and sulfides. Chlorite-geothermometry revealed temperatures in the range from 300 to 350 oC for the retrograde event. Calcic garnets (andraditegrossular solid solutions) from the prograde amphibolite-hosted skarn are chemically zoned with either isotropic or anisotropic cores surrounded by non-cubic anisotropic rims, thus suggesting chemical disequilibrium or disordering phenomena during crystal growth. The pyrophanite crystals are included in titanite and are accompanied by U-rich thorium minerals (uranothorianite, thorite) and by rutile. Electron microprobe analyses indicate that the studied pyrophanites are solid solutions between end-member pyrophanite and ilmenite, with isomorphous substitution of Mn2+ by Fe2+. No Mg has been detected (geikielite end-member). The alteration of pyrophanite and rutile to titanite suggests an increase of fO2 values of the fluids, probably due to increased incursion of meteoric waters to the system. The assemblage thorite, uranothorianite, pyrophanite and zircon indicates a magmatic contribution from the Xanthi granodiorite.