Astrophyllite in peralkaline granites from the Graciosa Province (S-SE Brazil): Insights for magmatic vs. post-magmatic crystallization environments

• Published in: Lithos 2022-12, Vol.432-433, p.106892, Article 106892
• Main Authors: Siachoque, Astrid, Vilalva, Frederico C.J., Vlach, Silvio R.F.
• Format: Article
• Language: English
• Subjects: Agpaitic mineralogy; Astrophyllite; Magmatic and post-magmatic crystallization environments; Peralkaline granites; REE partition coefficients
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2022.106892

Astrophyllite is a common “agpaitic” accessory mineral in evolved peralkaline alkali feldspar granites from the Mandira and Papanduva plutons in the Graciosa Province (S-SE Brazil). Textural and in-situ chemical (major and trace element) data provide new insights on the mechanism of formation and evolution of astrophyllite in magmatic and post-magmatic environments. Magmatic astrophyllite forms isolated or aggregates of micaceous crystals that, in part, co-precipitated with sodic amphiboles (arfvedsonite and riebeckite) and aegirine, and with TiZr and/or REE-rich accessories. It presents relatively Nb- and Zr-rich and Ti-poor compositions and the Mn contents are significantly higher in the Papanduva crystals. These crystals have significant REEs contents (up to 3949 ppm), and efforts were made to obtain rough estimates for the REEs partition coefficients (ast/meltDREE) between astrophyllite and modeled evolved melts to examine their controls. Except for Nd to Tb, the calculated ast/meltDREE are higher for the Na-richer crystals from the Mandira samples, suggesting a crystal chemical control on their partitioning, especially the preference of trivalent REEs to charge-balance Na+ incorporation by the coupled substitution Na+ + REE3+ ↔ 2Ca2+. Lattice strain modeling supports the entrance of the REEs mainly in the [10]B cationic site of the astrophyllite structure. Post-magmatic astrophyllite occurs mainly as acicular and radiating crystals by replacing magmatic arfvedsonite, riebeckite, and aegirine. They are Ti-rich, close to the ideal end member, with lower Zr, Hf, Th, U, Nb, Ta, Ga, Li, Sc, Y, and REE contents than the primary crystals. Their formation occurred mainly via disequilibrium processes and substitution reactions of early sodic amphiboles and clinopyroxenes triggered by the circulation of relatively K- and Ti-rich hydrothermal fluids. Compositional variations between both the astrophyllite groups reveal that the parameters Zr/(Zr + Ti) and Mn/(Mn + Fe) can be used as chemical indicators to discriminate magmatic and post-magmatic (hydrothermal) crystallization environments, as well as to evaluate their redox conditions. •First insights into astrophyllite REE partition coefficients.•Lattice strain modeling supports REE entrance into astrophyllite [10]B site.•Zr/(Zr + Ti) ratio distinguish magmatic and post-magmatic astrophyllite.