Rinkite, cerianite-(Ce), and hingganite-(Ce) in syenite gneisses from the Sushina Hill Complex, India: occurrence, compositional data and petrogenetic significance

摘要

Accessory rare earth element (REE) minerals occur in small quantities in agpaitic and miaskitic nepheline syenite gneisses of the Sushina Hill Complex, India. The REE-rich minerals restricted mainly to the agpaitic rocks are rinkite, cerianite-(Ce), and cerian thorite. Rinkite, formed at the ortho-magmatic stage predates other REE-rich phases and is the most Nd-F-rich rinkite (6.62—7.45 wt.% Nd_2O_3; 8.75—9.74 wt.% F) with very high Nd/Ce (>2.46) ratios reported to date. Hydrothermal cerianite-(Ce), formed by the decomposition of eudialyte in the agpaitic rocks, occurs as small rounded crystals rich in Ce (~63—74 wt.% CeO_2) and Y (6.03—11.69 wt.% Y_2O_3). The presence of cerianite-(Ce) indicates formation in an evolving hydrothermal fluid in an oxidizing milieu. Hingganite-(Ce) is present in the miaskitic unit and is considered to represent the superposition of an agpaitic mineral on an initial miaskitic assemblage. Hingganite-(Ce) is characterized by elevated contents of Ce (18.03—21.94 wt.% Ce_2O_3), and Nd (13.90—15.40 wt.% Nd_2O_3). Experimental data, coupled with the observed assemblage, suggest that the hingganite-(Ce) precipitated from the hydrothermal fluid between 400 and 300℃ followed by cerianite-(Ce) (<~300℃). This conclusion implies that eudialyte decomposition was probably initiated above 400℃.