Amphibole Compositional Trends in Oversaturated and Undersaturated Alkaline Plutonic Ring.Complex

Abstract

France ABsrRAcr Amphiboles from different alkaline ring-com-plexes, representing both silica-oversaturated and silica-undersaturated petrographic associations, have been studied in relation to their host rocks. Tex-tural, optical and chemical evidence emphasizes major amphibole compositional trends related to host-rock chemistry. In rocks whose agpaitic co-efficient is less than 0.9, the Ca + Alt" content of amphiboles is more than 2.5, whereas it is less than 2.5 in agpaitic rocks (NazO*K2O,/ALO" > 0.9). This feature is shown firstly by the presence of solid-solution series from kaersutite to horn-blende or hastingsite with substitutions I Ti € NaAAl. Ti + O NaFe2+ and CaAlt" <2 Na Si. The first trends are related to early magmatic stages, and the second to late mag-matic stages. The absence of (Ca + Alb)-rich amphiboles in agpaitic rocks and an observed break between (Ca + Alr")-rich amphiboles and (Ca .-1-Ali")-poor amphiboles suggest that (Ca + Alt')-rich amphibole stability is controlled by magma alkalinity. One argument is based on the description of a reaction, involving hastingsite and a residual liquid, which results in the crystalliza-tion of clinopyroxene and Ti-magnetite, and in the addition of potential analcime to the residual liquid. This alkalinization of the liquid ultimately pro-duces a peralkaline concentrate,