Quartz-albite-calcite veins in metamorphic rocks of the Sanbagawa belt, Japan, contain sugary mosaics of equant quartz grains <0.01–1.6 mm in size (blocky texture). The quartz crystals show euhed ral to subhedral double-terminated crystal shape, and concentric growth zoning, indicating suspension in aqueous fluid during crystal growth until formation of a framework within the crack. Such a crystallization process is only possible when downward crystal settling is balanced by upward fluid flow. Application of crystal settling theory to the crystal size distributions within the veins reveals an extremely high rate of fluid ascent during blocky vein formation (10−2 to 10−1 m/s) due to the low viscosity of the hydrothermal fluid (~10−4 Pa·s). Such a high rate of ascent suggests the injection of fluids from mobile hydrofractures within deeper levels of the subduction zone.