Magmatic processes forming replacement textures with fluorite alignments in feldspars in an evolved trachyte from Oki‐Dogo Island, Sea of Japan

Abstract Internal microtextures of ternary alkali feldspars in sanidine trachyte from Oki‐Dogo Island were examined using an electron microprobe analyzer, a scanning electron microscope, a transmission electron microscope and cathodoluminescence instruments, to develop the understanding of volcanic processes of alkaline magmas related to feldspar crystallization. The examined trachyte is an evolved rock of the Oki‐Dogo Pliocene trachyte group. Its phenocryst feldspars are commonly associated with lamellar‐wavy‐domain textures with scales approximately from 100 nm up to several hundreds of μm that show complex and gradual variations in composition: however, anti‐rapakivi zoning textures common in other Oki‐Dogo alkaline rocks are almost completely absent in the trachyte. These textures are produced by extensive magmatic ion‐exchange replacement reactions progressively advanced in the evolved magma. Characteristic braided fluorite alignments are developed consistently with lamellar‐wavy‐domain textures in phenocryst feldspars, and similar braided alignments are also present in groundmass feldspars with complicated microtextures. Most of fluorite grains are <100 nm in diameter, and the patterns of braided fluorite alignments vary greatly in individual feldspars. The whole occurrence of the feldspar microtextures represents an extreme example of diffusion‐controlled replacement reactions, progressively advanced in the dry (relatively anhydrous) trachyte magma. The genetic processes forming fluorite alignments in feldspars are related to magma compositions, especially F and P contents, and the crystallization of F‐bearing minerals, especially of fluorapatite.