Formation processes of fibrolitic sillimanite seams and sillimanite inclusion-rich quartz by fluid-rock interaction (Wazuka-Kasagi area, Ryoke Belt, SW Japan)

Abstract

Fluid-rock interaction between pelitic gneiss and fluids released from a post-tectonic granite in the low pressure/temperature (P/T) type Ryoke metamorphic belt (Wazuka-Kasagi area, SW Japan) has been shown to have formed fibrolitic sillimanite-quartz seams in close association with discordant granitic veins crosscutting the host gneiss. The sillimanite seams are characterized by the presence of cathodoluminescence (CL)-dark quartz grains including sillimanite needles ± muscovite and the absence of feldspars in and around the seams. Thermodynamic calculations showed that the reaction between feldspars and aqueous fluid with low values of log(aK⁺/aH⁺) < ∼ 3.6 and log(aNa⁺/aH⁺) < ∼ 4.1 can produce sillimanite ± muscovite + quartz at 3 kbar, 600 °C. Feldspars close to the fluid path were consumed and CL-dark quartz grains with sillimanite inclusions were formed by the high aH⁺ fluid-rock interaction. The abundance of sillimanite inclusions in quartz decreases with distance from the seam, and quartz grains far from the seam typically have CL-bright and sillimanite-free cores, reflecting the decreasing aH⁺ with distance from the seam as a result of the progress of the fluid-rock interaction. The sillimanite seams associated with CL-dark quartz enclosing sillimanite ± muscovite and the absence of feldspars in the vicinity are microstructural indicators of fluid-rock interaction by high aH⁺ fluids and represent fossilized pathways for such fluids.