西南日本内帯220 Ma高圧変成岩の原岩年代と変成履歴 : 山口県錦町地域の例  [in Japanese] Depositional age and metamorphic history of 220 Ma high P/T type metamorphic rocks : an example of the Nishiki-cho area, Yamaguchi Prefecture, Southwest Japan  [in Japanese]

The Nishiki-cho area in eastern Yamaguchi Prefecture is mainly occupied by the Sangun crystalline schists (Tsuno Group) and overlying weakly metamorphosed Permian rocks (Nishiki Group). They are bounded by adistinctive surface of tectonic discontinuity, defined in this paper as the Kitayama thrust. Paleontological and sedimentological studies on the Nishiki Group have revealed that the group is not a coherent sedimentary sequence but a subduction-related sedimentary complex accumulated in trench environs, and that the age of the deposition and/or rearrangement is limited in Late Permian to earliest Triassic time. In spite of the lack of fossil evidence, the Tsuno Group is belived to have also been accumulated in an ancient subduction zone during the Middle Triassic, judging from a K-Ar relict hornblende age (239±13 Ma) of an ophiolitic block. The metamorphic area underlain by the two groups is divided in to the following three zones on the basis of the metamorphic mineral assemblages in basic rocks: zone A (pumpellyite-chlorite), zone B (pumpellyite-actinolite) and zone C (epidote-glaucophane). Zone boundaries are nearly parallel to bedding schistocity planes, and the grade of metamorphism increasesd ownwards in the succession. Zone A comprises the whole sequence of the Nishiki Group, zone B coincides with the uppermost horizons of the Tsuno Groupand zone C corresponds to its lower hori zones. The apparent interplanar spacing d_<002> values of carbonaceous material from the pelitic metamorphic rocks, which were systematically sampled along the Okuhata River route, progressively decrease from the upper to the lower horizones. The decrease of the values is correlated with the advance of graphitization, showing the increase of metamorphic temperature from zone A through zone B to zone C. K-Ar ages were determined on 14 white micas from the samples examined for the d_<002> values. The ages obtained are collectively concentrated in a range of 220±7 Ma. These features clearly demonstrate that the all rocks of the Nishiki and Tsuno Groups have undergone a single episode of glaucophanitic metamorphism and constitute a facies series of the high-pressure intermediate group.