Conglomerates of the Kuma Group, central Shikoku, southwest Japan contain Sanbagawa schist clasts with a variety of metamorphic grades and lithologies. K-Ar and 40 Ar/ 39 Ar dating of phengite show all the pelitic schist clasts from low-to high-grade zones have similar phengite ages (82-84 Ma) that are significantly older than those from the in situ Sanbagawa sequence of central Shikoku. This is because the Kuma-Sanbagawa sequence was exhumed earlier than the in situ Asemi sequence with an exhumation process intermediate between those for the Kanto Mountains and the in situ Asemi sequences. 40 A/ 39 Ar plateau ages (103 and 117 Ma) of phengite in amphibolites indicate the timing of the early stage of the exhumation of the metamorphic pile, probably close to the peak metamorphic age.

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 d 002 spacing of carbonaceous materials is 3.590-3.437 Å , 3.415-3.390 Å and 3.387-3.364 Å , respectively. The age-d 002 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-temperaturestructure 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.

The Sakuma-Tenryu district consists mainly of pelitic and basic schists. Its metamorphic sequence has been divided into two units, the Shirakura and the Sejiri units. We carried out K-Ar analyses of phengite separates and X-ray diffraction analyses of carbonaceous materials from the pelitic schists of both units. The age-d 002 relationships show that the ages become older (66-73 Ma) in the Shirakura unit and younger in the latter with increasing metamorphic temperature. The former has a positive relationship observed in the Sanbagawa meta-Accretionary Complex (meta-AC) (Sanbagawa metamorphic belt sensu stricto) in central Shikoku and the latter, a negative one in the Shimanto meta-AC (a subunit of traditional Sanbagawa belt) of the Kanto Mountains. These contrasting age-temperature relationships are due to different tectonic styles relating to the exhumation of the metamorphic sequences. The duration from the peak metamorphism to the closure of the phengite K-Ar system was significantly different between the two metamorphic sequences: longer than 31 my in the Sanbagawa meta-AC and shorter than 13 my in the Shimanto meta-AC. The different natures of subducted plate boundaries may cause the different exhumation processes of metamorphic belts.

The Samnua Depresion Zone (SNDZ) extends NW SE for more than 400 km from Northern Vietnam to Southwestern China. It represents the northern edge of the Indochina block and is separated from The groundmass fraction of the CBR yields an age of 1.9 Ma. Geochronological comparison and using fossil records suggest that the Rb Sr ages of Mesozoic igneous rocks provide apparent young age, a pseudo isochron age. The major igneous activity of the SNDZ took place during Late Permian Triassic time to form granitic and felsic volcanic rocks, which were related to the continental volcanic arc magmatism. Minor intraplate igneous activity occurred in the Paleogene. This suggests that the tectonic environment of the SNDZ changed from a convergent plate margin to a matured continental margin in the time span between the Triassic and Paleogene.