Fluorite alignments cutting metasomatic textures in trachyte feldspars from Oki-Dogo Island, Sea of Japan

Feldspar internal textures in a pyroclastic trachyte from Oki-Dogo, Sea of Japan, were examined to expand the understanding of feldspar reactions during the cooling through magmatic to hydrothermal stages, beyond the previous information of feldspars in Oki-Dogo alkaline lava and sheet rocks, using the methods of an electron microprobe and cathodoluminescence. Two types of micron-size internal microtextures were found to coexist in individual feldspar phenocrysts: clear domain textures, formed during a high-temperature magmatic stage, and turbid microperthitic textures, formed during a low-temperature subsolidus stage. The both microtextures are products of metasomatic replacement reactions. In addition, nano-size fluorite grains are aligned across the microtextures. The fluorite occurrence records the behavior of fluorine related to feldspar reactions. The first account of metasomatic microtextures crosscut by fluorite alignments in volcanic alkali feldspars expands our knowledge of feldspar reactions during the cooling and fluorine behaviors related to them in igneous rocks and shows the significance of the careful analysis of feldspar internal microtextures.