Dark cataclastic veins at the mouth of the Akirigami river in Tokunoshima, Southwest Japan

Abstract

Dark veins occur in a massive outcrop of pelitic–psammitic schists at the mouth of the Akirigami river in western Tokunoshima in the Amami Islands, southwest Japan. The dark veins are 10–20 cm thick and roughly parallel to the schistosity of the host rocks. Irregular branches and stepwise traverses across the schistosity are observed in places. The veins are filled with rock fragments (pelitic schist, psammitic schist, and rare amphibolite) and fine-grained matrix. A power-law size–frequency relationship for the clasts in the dark vein and powder X-ray diffraction profiles of the host pelitic schist and the matrix of the dark vein indicate that the dark veins are filled with crushed pelitic–psammitic schists and hardly include amorphous material. In the matrix of the dark veins, very fine anatase (TiO₂) grains surround calcite (CaCO₃) and titanite (CaTiSiO₅) forming rings. This suggests the decomposition of titanite owing to the infiltration of a CO₂–H₂O fluid. We propose that the dark veins were derived from intensive cataclasis and fluidization enhanced by excess hydrostatic pressure.