ALLANITE AND CHEVKINITE IN A-TYPE GRANITES AND SYENITES OF THE GRACIOSA PROVINCE, SOUTHERN BRAZIL

The Graciosa Province is a large province of A-type granites and syenites in southern Brazil, formed during the final stages of the Brasiliano Cycle (ca. 580 Ma). Two main associations are present: (1) an alkaline association, including metaluminous alkali feldspar syenites with fayalite and hedenbergite that vary continuously towards peralkaline alkali feldspar granites with sodic amphibole; (2) an aluminous association, characterized by metaluminous to weakly peraluminous biotite ± hornblende syeno/monzogranites. Allanite and chevkinite are the most important REE phases in the Graciosa Province. Allanite-(Ce) is ubiquitous in the aluminous association, while chevkinite-(Ce) is typical in the alkaline association.

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 (La_N/Nd_N = 3-5, La_N/Y_N = 200-400). Hydrothermal allanite appears in both associations, and corresponds to ferriallanite-(Ce) with smaller REE fractionation (2 < La_N/Nd_N < 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 La_N/Nd_N ratios and lower La_N/Y_N 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 Fe_T 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.