ALLANITE AND CHEVKINITE IN A-TYPE GRANITES AND SYENITES OF THE GRACIOSA PROVINCE, SOUTHERN BRAZIL
Primary allanite shows complex zoning but is always rich in ferriallanite and TiO2, a compositional trait that distinguishes allanite in A-type rocks from allanite in calc-alkaline series. Normalized patterns show strong fractionation of heavy REE (LaN/NdN = 3-5, LaN/YN = 200-400). Hydrothermal allanite appears in both associations, and corresponds to ferriallanite-(Ce) with smaller REE fractionation (2 < LaN/NdN < 3) in the alkaline association, and allanite-(Y) with middle REE enrichment in the aluminous association. Chevkinite compositions are similar to those observed in granites and syenites worldwide. The abundance of REE+Y (43-49 wt. %) is twice that observed in allanite. Yet, the normalized REE patterns show similar LaN/NdN ratios and lower LaN/YN ratios (90-300). The early crystallization of chevkinite-(Ce) suggests that the parental magmas were close to REE-saturation. The compositions of primary allanite and chevkinite were variably modified by interaction with post-magmatic fluids and metamictization. Altered chevkinite shows high Ti and low FeT and REE, but the REE fractionation patterns were not significantly modified.
The textural evidence for early chevkinite crystallization and late allanite crystallization agrees well with the experimentally-determined stability fields for chevkinite and allanite. The two minerals only coexist in the less evolved syenites of the alkaline association, in which they reveal reaction textures, suggesting that (a) allanite and chevkinite may not be stable simultaneously, and (b) magma composition may control their stability.