REE-BEARING MINERALS IN THE ALBITITES OF CENTRAL SARDINIA, ITALY

Recent studies on albitite rocks located in the granodiorite complex o f Central Sardinia have revealed that epidote has a widespread occurre nce as a light rare-earth element (LREE)-bearing accessory common phas e. Titanite has been recorded as a heavy rare earth element (HREE)-bea ring mineral. The Hercynian granodiorite complex of Central Sardinia i s composed chiefly of quartz, Ca-plagioclase, K-feldspar and biotite a nd of a wide variety of secondary assemblages, mainly allanite, titani te and zircon. Albitic plagioclase and quartz are the main mineral com ponents of the albitites. Additional minerals include, besides allanit e and epidote, a more calcic-plagioclase (oligoclase), K-feldspar, chl orite, titanite and more rarely muscovite. The mineral assemblages and REE-bearing minerals of albitites were analysed by wavelength dispers ive spectrometry (WDS). Chemical data suggest that there is a near com plete solid-solution between epidote and allanite whereas little varia tions in HREE of titanites were detected. In epidote-group minerals a pronounced zoning in REE was observed while titanite was recorded unzo ned. Textural relations were studied by SEM to distinguish primary fro m secondary epidotes. Chemical criteria to recognize magmatic from alt eration epidotes were also applied. The alteration epidotes mainly occ ur and generally originate from plagioclase alteration and from leachi ng of magmatic allanite. Comparison of textures using both the SEM tec hnique and EPMA data showed that the characteristic 'patchy zoning', o bserved in epidotes, corresponds with different amounts of REE in thes e minerals. The schematic model proposed for the epidote-forming react ions during the metasomatic processes that affected the granodiorites involves: (i) the instability of the anorthitic component of plagiocla se; (ii) the simultaneous formation of albite; (iii) the leaching of t he magmatic allanite with a redistribution of REE in the epidotes of t he albitites.