. Japan. Assoc. Min. Petr. Econ. Geol. 78, 11-20, 1983 K-Ar ages of Sangun metamorphic rocks in Yamaguchi Prefecture and their geologic significance YUJIRO NISHIMURA Institute of Earth Sciences, Faculty of Liberal Arts , Yamaguchi University, Yamaguchi 753, Japan. Eizo NAKAMURA Department of Geology, University of Toronto, Toronto, Ontario M5S 1A1, Canada. IKUO HARA Institute of Geology and Mineralogy, Faculty of Science, Hiroshima University, Hiroshima 730, Japan. K-Ar ages have been determined on white micas from four Sangun pelitic schists in the Nishiki-cho and Toyoga-dake areas, Yamaguchi Prefecture. The dates from the Nishiki-cho area are 172 and 175 Ma, while those from the Toyoga-dake area are 264 and 274 Ma. They represent a wide range of time, 172 to 274 Ma, which is nearly identical with that of eight ages, 165 to 264 Ma, reported previously from the Sangun metamorphic rocks. The twelve available dates on the Sangun metamorphic rocks can be divided into two age clusters, an older age cluster around 260 Ma (middle Permian) and a younger cluster around 170 Ma (middle Jurassic). It is suggested that the two age clusters may coincide with the two stages of uplift for the Sangun metamorphic rocks. Namely, the age cluster around 260 Ma may indicate the event in which the Sangun metamorphic rocks were emplaced as nappes from the deep tectonic position of glaucophanitic metamorphism into a shallow tectonic position and the metamorphism had ceased. On the other hand, the age cluster around 170 Ma may represent an event related to the beginning of the formation of the pile nappe structure in which the Sangun metamorphic rocks tectonically overlie the Jurassic non-metamorphic equivalents of the Tamba Group. (regional metamorphic) belt in the Inner Introduction Zone of Southwest Japan, called the Sangun The Sangun metamorphic rocks com- (metamorphic) belt or the Sangun-Chugoku posed of schists and phyllites are exposed belt (Nishimura et al., 1977; Fig. 4). It in scattered small areas from north Kyushu has been demonstrated that the original to northwest Kinki. The Paleozoic forma sediments of the belt were probably deposit tions and the Yakuno ophiolitic complex ed in Devonian to Permian time and associated with them are also weakly underwent the Sangun metamorphism. The metamorphosed. They were all affected by metamorphosed rocks in some areas were one and the same cycle of metamorphism of unconformably covered by Lower Triassic the glaucophane-schist facies series, called sediments of neritic facies (Nakazawa and the Sangun metamorphism, and constitute a Shiki, 1954) or Upper Triassic sediments (Manuscript received October 7, 1982) J 12 Yujiro Nishimura, Eizo Nakamura and Ikuo Hara of molasse facies (Hase, 1950). Thus, a younger limit on the time of the Sangun metamorphism has been considered on the basis of geologic evidence to be late Permian to early Triassic (e.g. Nishimura, 1977). On the other hand, Permian to Triassic conodont-bearing strata of chert facies (mainly Upper Triassic) have been found recently from several areas in and around the Sangun-Chugoku belt (Toyohara, 1977; Kimura, 1977; Tanaka, 1980; Hayasaka and Fig, 1. Locations of the Nishiki-cho and Toyoga Hara, 1981). More recently, radiolarian dake areas in Yamaguchi Prefecture. fossils of Early to Middle Jurassic age have also been discovered from some parts of results and discuss their geologic signif the strata (Hayasaka and Hara, 1982; icance. Isozaki and Matsuda, 1982). These strata are lithologically characterized by a dom Geological outline inance of banded cherts and pebbly mud 1. Nishiki-cho area stones and are correlated with the Tamba Nishimura (1971) has described the Group which is widespread in the Tamba geology and metamorphism of the Nishiki zone on the east of the Sangun-Chugoku cho area in the east of Yamaguchi Pre belt. The Sangun metamorphic rocks fecture. The area is mainly occupied by the always overlie these strata (Hara et al., Sangun metamorphic rocks (Tsuno Group) 1980). The Paleozoic formation and the and overlying weakly metamorphosed rocks Yakuno ophiolitic complex in the Maizuru (Nishiki Group) (Fig. 2). The Tsuno Group zone are also thrust over the Tamba Group consists mostly of pelitic, psammitic and of the Tamba zone (Hayasaka and Hara, basic schists being accompanied by siliceous 1980). Therefore, it can be pointed out and calcareous schists. Serpentinties also that the Sangun-Chugoku belt is made up occur subordinately. The total thickness of the nappe group and its underlying non- is estimated to be 1,700 meters or more. metamorphic strata (= equivalents of the The Nishiki Group is typified by a dom Tamba Group). inance of alternating beds of psammite, These new geologic observations de pelite and acidic tuff, but contains inter mand reexamination of the commonly ac calated chert, limestone and conglomerate. cepted theory of the geologic development The total thickness amounts to 2,900 to of the Sangun-Chugoku belt. Geochrono 3,900 meters. The latter Group yields oc logical data may provide a clue to solve this casional Permian fusulinid fossils, whereas problem. However, available radiometric the Tsuno Group has not yet been dated by dates on the belt are very meagre. A K-Ar fossil evidence. dating program was on the Sangun meta The metamorphic terrain is divided into morphic rocks from the Nishiki-cho and the following three zones on the basis of the Toyoga-dake areas, Yamaguchi Prefecture metamorphic mineral assemblages of basic (Fig. 1). In this paper we present the rocks: zone A (pumpellyite-chlorite) , zone K-Ar ages of Sangun metamrphic rocks 13 and represents the typical facies series of the high-pressure intermediate group (Miya shiro, 1961). The rocks were folded to form anticlinal and synclinal structures, the axes of which are roughly horizontal with nearly E-W trend through the Tsuno and Nishiki Groups. Thus, rocks of the Tsuno Group are exposed in the anticlinal zones and those of the Nishiki Group in the synclinal zones. Along the boundary be tween the two Groups is found a kind of tectonic slide. On the south of this area, the Sangun metamorphic rocks are in thrust fault con tact with the Permian Mikawa Group (= equivalents of the Tamba Group), and further south the Mikawa Group is thrust over the Triassic to Jurassic Kuga Group (=equivalents of the Tamba Group) (Kojima, 1953; Toyohara, 1977; Hara et al., 1979). The vergence of the thrusting is southward. Therefore, it must be concluded that the Nishiki-cho-Kuga area is characterized by the prevalence of pile nappe structures. Fig. 2. Geological map of the eastern part of 2. Toyoga-dake area the Nishiki-cho area (Extracted from Nishimura, 1971). The Nagato. tectonic zone is a narrow Dots represent the localities of the fault zone trending from NNE to SSW in dated specimens. D: Deal, K: Kokabe, M: Miyanokushi. the western part of Yamaguchi Prefecture and is accompanied by various kinds of B (pumpellyite-actinolite) and zone C tectonic blocks of old basement rocks. (epidote-glaucophane). The boundary Tectonic blocks of orthogneisses, trondhje surfaces between the zones are nearly mites and amphibolites of about 400 Ma in parallel to bedding planes, and the grade of radiometric age mainly occur in the metamorphism increases downwards in the northern areas of the zone, e.g. Dai and stratigraphic succession. Zone A comprises Misumi, whereas blocks of schists, meta the whole of the Nishiki Group, zone B gabbroic rocks and Paleozoic rocks are coincides with the uppermost horizons of widespread in the southern areas, e.g. the Tsuno Group and zone C corresponds Toyoga-dake and Ozuki (Murakami and to the lower horizons of the Tsuno Group. Nishimura, 1979). The progressive metamorphism from zone A Fig. 3 shows the geological map of the to zone C is called the Sangun metamorphism Toyoga-dake area, The schists are exposed 14 Yujiro Nishimura Eizo Nakamura and Ikuo Hara metamorphic conditions grading from the pumpellyite-actinolite facies through the glaucophane-schist facies to the barroisite zone, corresponding to the generalzied zona tion of the Sangun metamorphic rocks (Hashimoto, 1972; Nishimura et al., 1977). Therefore, it has been proposed that the schists in the present area may be tectonic blocks derived from the Sangun meta morohic rocks (Nishimura, 1976). The main part of the Toyoga-dake area is composed of weakly metamorphosed pelites, psammites, greenstones and lime stone conglomerates, called the Toyohigashi Group. The limestone pebbles in the con glomerates yield foraminifera fossils rang ing from Carboniferous to Middle Permian in age. Thus, the Group may be correlated with the Upper Permian. However, it is difficult to establish the stratigraphy and the geologic structure for the Group in detail, as it has been faulted and subjected to intense cataclasis. Metagabbros, metadiabases and ser Fig. 3. Geological map of the Toyoga-dake area pentinites also occur as small lenticular (Modified from Nishimura, 1976). bodies intercalated in the Toyohigashi Dots represent the localities of the dated Group. A K-Ar age of hornblende from specimens. the metagabbro has been reported as 233 H: Higashinagano, N: Nakano, T: To yoga-dake. Ma (Shibata et al., 1979). The schists and the Paleozoic rocks of along the eastern and western margins of this area are unconformably covered by the the tectonic zone and made up of pelitic, Lower Jurassic Higashinagano Formation of basic and siliceous schists. Because they the Toyora Group on the west of the area, are extremely variable in degree of and are in fault contact with the Upper recrystallization and deformation, the Triassic Aso Formation of the Mine Group metamorphic terrain can not be zonally clas on the east. In the Asa area situated to sified. Representative mineral assemblages the southwest, some outcrops of the of all the basic schists observed are as Sangun metamorphic rocks are found.