Magmatic crystallization processes inferred from microtextures in anti-rapakivi feldspars: A case study of ferro-augite trachyte feldspars from Oki-Dogo, Japan

Abstract

Internal microtextures and compositional variations of both phenocryst and groundmass feldspars in ferro-augite trachyte from Oki-Dogo, Sea of Japan, were investigated to elucidate the magmatic processes controlling feldspar crystallization. Anti-rapakivi feldspars with strongly resorbed An-rich cores and Or-rich mantles with various outlines are associated with characteristic lamellar, wavy, and patchy textures that comprise An-rich, Or-poor (andesine–oligoclase–anorthoclase) and Or-rich, An-poor (sanidine–anorthoclase) feldspars through cores and mantles. Groundmass feldspars (microphenocrysts and microlites) grown continuously from phenocrysts with ragged boundaries also show variable microtextures similar to those in phenocrysts and have sector (hourglass and columnar) zoning. The compositional ranges of phenocryst and groundmass feldspars almost completely overlap. It is inferred from these characteristics, which have not hitherto been reported for volcanic feldspars, that the crystallization of phenocryst and groundmass feldspar proceeded with a close genetic relationship to each other during magma ascent, and was associated with kinetically controlled resorption (replacement) reactions. Crystallization occurred under H2O-undersaturated and H2O-unbuffered conditions. Subsolidus exsolution cryptoperthites that formed after crystallization are preserved as pristine states in intermediate alkali feldspars (sanidine–anorthoclase). This study established the entire feldspar crystallization process in a trachytic magma by analyzing feldspar internal textures in both phenocrysts and groundmass, and by doing so provides new insights into volcanic magmatic processes.