Abstract

The Ishigaki high-pressure schist belt in the southern Ryukyu Arc is correlated with the Suo high-pressure schist belt in southwest Japan. The former metamorphic sequence is composed mainly of basic and pelitic schists and is subdivided into three zones, the lower-grade zone A, the medium-grade zone B and the high-grade zone C, based on the mineral assemblages of the basic schists. The K–Ar phengite age gives 188–205 Ma for zone A, 196–206 Ma for zone B and 208–220 Ma for zone C, while the apparent d002 spacing of carbonaceous materials is 3.590–3.437 Å, 3.415–3.390 Å and 3.387–3.364 Å, respectively. The age–d002 relationships suggest that the ages become older with increasing metamorphic temperature. This positive age–temperature relationship in the Ishigaki area contrasts with a negative relationship in the Nishiki area in the Suo belt. The two areas also display a contrasting thermal structure with the former area having an inverted metamorphic gradient and the latter displaying a normal thermal structure. These contrasting age–temperature–structure relationships in the metamorphic belt could be due to different tectonic styles relating to the exhumation of the metamorphic sequences. We suggest that the ages obtained are related directly to the ductile deformation history of the matrix phengite below the closure temperature (500 °C) during exhumation of the host rocks. The duration from the beginning of exhumation to the apparent resetting of the phengite K–Ar system was different between the two metamorphic sequences, and significantly longer in the Ishigaki than the Nishiki.

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