Detecting thermal anomalies using thermochronometry around hydrothermal veins in the Kii Peninsula, southwest Japan – An attempt

Abstract

Fluid-inclusion and thermochronometric analyses have been applied to hydrothermal veins and their host rocks outcropping in the Hongu area in the Kii Peninsula, southwestern Japan in an attempt to detect thermal anomalies related to hydrothermal events and to quantify the thermal effects on the host rocks. Hydrothermal events at ~150°C and ~200°C were identified by fluid-inclusion microthermometry of quartz veins. Zircon yielded U–Pb ages of ~74.7–59.2 Ma in the youngest population, fission-track ages of ~27.2–16.6 Ma in the youngest population, and (U–Th)/He single-grain ages of ~23.6–8.7 Ma. Apatite yielded pooled fission-track ages of ~14.9–9.0 Ma. The zircon U–Pb ages constrain the maximum depositional ages of the samples. However, the fission-track and (U–Th)/He dates show no clear trend as a function of distance from the veins. Namely, no thermal anomaly was detected in the surrounding host rocks based on thermochronometry data patterns. The fission-track and (U–Th)/He dates are thought to record the regional thermal and exhumation histories rather than a direct thermal imprint of fluid flow, probably because the duration of such activity was too short or occurred before subsequent regional cooling events when they were thermally overprinted. Apatite fission-track ages of ~10 Ma might reflect regional mountain uplift and exhumation related to rapid subduction of the Philippine Sea slab which was associated with clockwise rotation of the Southwest Japan Arc.