Comprehensive research on the restoration of scattered high-pressure metamorphic terranes in the Inner Zone of Southwest Japan

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The Sangun metamorphic belt is considered for a long time to be a major coherent glaucophanitic terrane of pre-Jurassic age, and to be paired with the low-pressure Hida metamorphic belt. However, recent studies on geochronology and tectonics have revealed that the belt is in fact comprised of three geologic units of different ages. A tentative threefold division is summarized as follows.1) Around 300 Ma (Carboniferous) group=Northern terrane: This terrane is confined to the northern marginal region (Wakasa, Nagato tectonic zone and Wakamiya) of the "Sangun belt", and corresponds to the western extension of the Hida marginal zone. Each distribution area of this terrane is bounded on adjacent units by distinctive faults and thrusts, and is also characterized by a dominance of serpentinite, metagabbroic and metagranitic rocks, which appear to compose dismembered ophiolite. These ophiolitic rocks have given radiometric ages of 470 to 380 Ma. It is worthy of note that the dates are identica … More l with those of the same kind rocks in the Hida marginal zone.2) Around 220 Ma (Triassic) group=Western terrane: This group occurs in the western region (north Kyushu to Yamaguchi Prefecture) of the "Sangun belt". In the Nishiki-cho area, K-Ar whole rock ages of weakly metamorphosed acidic tuffs are also dated as about 220 Ma, which are identical with Rb-Sr ages for the "Sangun pelitic schists". The acidic tuff and its adjacent siliceous slate yield Permian radiolarians (about 220 Ma). Thus, the 220 Ma would be considered to represent the final phase of the glaucophanitic metamorphism of the western terrane. This is consistent with geologic evidence that the "Sangun metamorphic rocks" are unconformably overlain by the Upper Triassic Mine Group.3) Around 180 Ma (Jurassic) group=Eastern terrane: Younger ages are widespread in the eastern region (Tottori, Okayama and Shimane Prefecture) of the "Sangun belt". In comparison with the western terrane, siliceous metamorphic rocks become conspicuously abundant, and some fensters composed of the equivalents of the Tamba Group are found in this terrane. Recently, Triassic conodonts and Jurassic type radiolarians were obtained from siliceous metamorphic rocks in the Motigase area. These may support a view that the metamorphic rocks of the eastern terrane are younger in age of their deposition as well as in metamorphic age than those of the western terrane. Less