Chemical characteristics and trapping P-T conditions of fluid inclusions in quartz veins from the Sanbagawa metamorphic belt, SW Japan

Abstract

Microthermometry and micro-Raman spectroscopy are performed on fluid inclusions within lenticular quartz veins elongated parallel to the main foliation of a pelitic schist of the Sanbagawa metamorphic belt, Saganoseki, Kyushu, Japan. The fluid inclusions are isolated or are arranged along the planes, which are developed subparallel (∼ 20°) to the main foliation of the quartz veins. Both types of fluid inclusions show similar homogenization temperatures and compositions in the H₂O-NaCl-CH₄ system. The trapping P-T conditions of the isolated fluid inclusions are determined to be 3.7-6.5 kbar and 320-450 °C by combining microthermometry and the pressure dependence of CH₄ Raman shift. The estimated P-T range is consistent with the peak metamorphic conditions of the host rock (6-8 kbar, 300-400 °C) or bit lower, suggesting that the fluid inclusions are primary or secondary ones trapped at a relevant depth and they are potential evidence on the fluid composition derived from metamorphic reactions at 10-20 km depths.