Age (K–Ar phengite)–temperature–structure relations: a case study from the Ishigaki high-pressure schist belt, southern Ryukyu Arc, Japan

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 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.

Evolution of the Sanbagawa and Shimanto high-pressure belts in SW Japan: Insights from K–Ar (Ar–Ar) geochronology

“…Moreover, two pelitic schist samples from a same outcrop with different grain size yielded a same age within error. Thus, Itaya and Takasugi (1988) addressed that argon in phengite was depleted during low-T deformation, and the extent of argon depletion depends on (Nuong et al, 2008). The Ishigaki metamorphic sequence indicates that the age becomes progressively older with increasing metamorphic temperature and the thermal structure is inverted so that the highest-grade zone occurs in the upper most parts of the apparent stratigraphic succession.…”

Triassic high-strain shear zones in Hainan Island (South China) and their implications on the amalgamation of the Indochina and South China Blocks: Kinematic and 40Ar/39Ar geochronological constraints

“…Therefore, muscovite is often considered to be the most suitable minerals for constraining the timing of mylonitization using 40 Ar/ 39 Ar dating methods. Available data show that muscovite porphyroclasts might be incompletely reset in the mylonization condition of less than 500°C (e.g., Villa, 1998;Gouzu et al, 2006;Itaya et al, 2009;Nuong et al, 2008Nuong et al, , 2009. Taking account into the dynamic recrystallization temperature of 300-500°C for our samples (Table 1), muscovite separates out of the matrix were selected, in this study, for constraining the deformational events.…”

K–Ar and 40Ar/39Ar phengite ages of Sanbagawa schist clasts from the Kuma Group, central Shikoku, southwest Japan

“…Brittle deformation depends on variables including lithology, pressure, strain rate, and the presence or absence of fluids. Nuong et al. (2008) suggest that the onset of brittle deformation depends mainly on the strain rate of deformation during the exhumation of the metamorphic pile.…”