Phase relations of a F-enriched peraluminous granite: an experimental study of the Kymi topaz granite stock, southern Finland

Abstract

Phase relations of F-rich Kymi equigranular topaz granite have been investigated experimentally at 100–500 MPa as a function of water activity and F content. Fluorite and topaz can crystallize as liquidus phases in F-rich peraluminous systems, but the F content of the melt should exceed 2.5–3.0 wt% for the crystallization of topaz. In peraluminous F-bearing melts containing more than 1 wt% F, topaz and muscovite are expected to be the first F-bearing phases to crystallize at high pressure, whereas fluorite and topaz should crystallize first at low pressure. Following reaction models the saturation of fluorite and topaz: CaAl2Si2O8 (plagioclase) + 2[AlF3]melt = CaF2 (fluorite) + 2Al2SiO4F2 (topaz). The obtained partition coefficient for F between biotite and glass D(F)Bt/glass ranges from 1.89 to 0.80 (average value 1.29) and can be used as an empirical fluormeter to determine the F content of coexisting melts. Combined petrological and experimental studies of the Kymi equigranular topaz granite indicate that plagioclase was the liquidus phase at nearly water-saturated (fluid-saturated) conditions and that the F content of the melt was at least 2 wt%. The mean F content of natural biotite (3.92 wt%) suggests that the late-stage crystallization of biotite occurred in melts containing about 3 wt% F. The early crystallization of biotite and the presence of muscovite in our experiments at 200 MPa contrasts with the late-stage crystallization of biotite and the absence of muscovite in the natural assemblage, indicating that crystallization pressure may have been lower than 200 MPa for the granite.