Allanite-(Ce) ed epidoti a Terre Rare nelle albititi della Sardegna centrale (Italia)

Recent investigations have pointed out that epidotes and allanite-(Ce) are the main REE-bearing minerals in the albitites located in the granodiorite facies of Central Sardinia. Previous works on albitites hypothesize that they originated from Na and Mg-metasomatic alteration of granitoids. Albitic plagioclase, quartz and K-feldspar are the main mineral components of the albitites. Additional minerals include oligoclase, chlorite, epidote, allanite and titanite. Muscovite and biotite are rare. The mineral assemblages and REE-bearing minerals within the albitites were studied by EPMA, in wavelenght dispersive spectrometry (WDS). These studies have emphatised that epidotes are mainly LREE (light rare earth elements)-bearing while HREE (heavy rare earth elements) are mainly concentrated in titanites. Texture and textural relations were studied by SEM to discriminate magmatic and alteration epidotes. Chemical criteria to distinguish the magmatic from alteration epidotes were also applied. The newly-formed epidotes generally derive from plagioclase alteration and from leaching of magmatic allanite. The comparison of textural observations combining SEM technique with EPMA data has also showed that the observed characteristic «patchy-zoning» in epidotes correspond to different accomodation of REE in these minerals. The schematic model proposed for the epidote forming-reactions involves the instability of the anorthitic component of plagioclase, the simultaneous forma-tion of albite, the leaching of the magmatic allanite with a redistribution of REE in the newly-formed epidotes during the metasomatic processes that affected the granodiorites.