Mineral growth within fluid-filled cracks: Example of polymineralic Veins from the Sanbagawa metamorphic belt, Japan

Sealed cracks in high-pressure metamorphic rocks are a direct evidence of fluid-filled cracks in subduction zones, but their implication to material transport remains unclear. The pelitic schists of the Sanbagawa metamorphic belt contain two types of veins at the exhumation stage, composed of quartz + albite + K-feldspar + chlorite (type I) and quartz + albite + calcite (type II). Type I veins contain elongate quartz and albite grains growing from vein walls, and show the mineral distribution related to compositional bands of host rocks. Type II veins are characterized by euhedral quartz grains with concentric zoning and by homogeneous mineral distribution along vein length. The microstructural variation of veins depends on the crack aperture of multiple crack-seal event; < 0.05 mm for type I and 0.5 - 3.0 mm for type II. These observations suggest a transition of the dominant transport mechanism of vein components with increasing crack aperture from diffusion from host rocks to fluid advection.