Zoning and sectoriality of monazite-(Ce) from granite pegmatites of the central and South Urals

Abstract

Zonal and sectorial monazite-(Ce) crystals from ceramic granite pegmatites of the Adui pluton are enriched in Ce and La, whereas monazite from the miarolitic gemstone pegmatites at the western contact of the pluton are enriched in Nd, Sm, Gd, and Y. This difference is caused by earlier crystallization of ceramic pegmatites and higher temperatures of their formation (650–600°C for ceramic pegmatites and 550–300°C for miarolitic pegmatites). Monazites from ceramic and miarolitic pegmatites of the Adui pluton differ in La and Nd contents, but their compositional trend in La-Nd coordinates is similar to the variation in monazite composition from the early to late granite pegmatites in the Ilmeny Mountains. It is suggested that decrease in temperature is a factor controlling REE contents in monazite. Heterovalent P ↔ Si and REE ↔ Th, Ca isomorphism in the consecutive zones of growth pyramids in monazite is both unidirectional and wave-like. Monazite from granite pegmatites of the Adui pluton and Ilmeny Mountains crystallized mainly under slightly alkaline conditions.