The Hida Gaien Belt, central Japan, is one of the oldest belts in the pre-Neogene tectonic divisions of the Japanese Islands. It consists of the Ordovician mafic to ultramafic rocks, Ordovician to Permian clastic, volcaniclastic and carbonate rocks, 300-400 Ma HP metamorphic rocks with blueschists, serpentinites and the Mesozoic to Cenozoic covers. In this article, recent progress and perspectives in the research of the Palaeozoic-Mesozoic tectonics of the Hida Gaien Belt are reviewed and discussed. About 100 literatures, published in 1951-2004, mostly in the last two decades, are selected and introduced.
K-Ar age determination of 21 samples of high P/T metamorphic rocks were carried out on phengite from the Omi, Hakuba-Happo, Gamata and Ise areas of the Hida Gaien belt. K-Ar ages of 19 samples range from 283 to 338Ma. Though these rocks occur as isolated small geological units at present, the age range confirms that the Hida Gaien belt diagnostically includes high P/T metamorphic rocks of ca. 300Ma subduction zone metamorphic belt as the Renge belt (Nishimura, 1998) in the Southwest Japan. Absence of ca. 200 Ma subduction zone metamorphic rocks, the Suo belt (Nishimura, 1998), discards the idea that the Hida Gaien belt is a simple eastward extension of the pre-Jurassic geologic units in the Chugoku province of the Southwest Japan. U-Th-Pb EMP dating was carried out on zircon, monazite and thorite in samples from 3 granitic bodies of the Gamata, Hakuba-Happo and Naradani areas. Serpentinite in the Gamata and Hakuba-Happo areas was metamorphosed thermally by the Horadani and the Ariake granites, respectively. The Horadani granite shows ca. 100 Ma and the latter granite does ca. 60 Ma, suggesting that the serpentinite was exhumed after the ages of the contact metamorphism. Serpentinite also occurs as olistoliths in the Permian accretionary complex in the Hida Gaien belt, and also as a basement of the Kuruma-Group of Jurassic age. The exhumation of serpentinite thus took place at various stages from the Permian to at least Paleogene because of its low density and plasticity in nature. The Hida Gaien belt is characteristic of the association of high P/T subduction zone metamorphic rocks with age of ca. 300 Ma and serpentinite.
Ultramafic rocks of the Omi serpentinite melange are extensively serpentinized. However, newly discovered Cr-spinels in chromitite and massive serpentinite provide a key to decipher the origin and metamorphism of serpentinites. Cr-spinels show distinct chemical zoning: the cores have high Cr#[=Cr/(Cr+Al)atomic ratio](0.70-0.77); Mg#[=Mg/(Mg+Fe2+)atomic ratio]decreases remarkably towards the rim through a transition zone between core and ferritchromite rim. Moreover, the cores of zoned Cr-spinel in chromitite contain abundant tiny inclusions of mainly pargasitic amphibole (up to 3.8 wt.% Na2O) and rare diopside. These inclusions were completely replaced by secondary tremolitic amphibole and chlorite at the rim, and by dolomite at the Ti-enriched metasomatized rim. Although the Mg# of the core (0.20-0.43) is significantly lower than those of Cr-spinel in both Alpine and abyssal peridotites, the observed mineral inclusions and the analysed Cr# at the core are interpreted to be relics of igneous stage. Compositional characteristics and the occurrence of hydrous mineral inclusions of the zoned Cr-spinel exhibit critical features that suggest a supra-subduction origin. Serpentinites of the Omi serpentinite melange may have derived from the mantle wedge above a subduction zone and subsequently metamorphosed at low- to mid-temperature condition of probably eclogite or amphibolite facies.
Structural studies of the Kuzuryu area of the Hida Gaien belt, central Japan, have revealed that most Paleozoic geologic units in the area are bounded by shear zones. Among the shear zones, at least four brittle-ductile shear zones have dextral strike-slip component, whereas the cataclastic shear zone that marks the southern boundary of the Hida Gaien belt has sinistral strike-slip component. The dextral shear zones cut the Motodo Formation, which yield Early Jurassic granite pebbles, but do not cut the Early Cretaceous Tetori Group, whereas the sinistral shear zone cuts the Early Cretaceous Tetori Group. It is suggested that the dextral shear zones were active in Early Jurassic to Early Cretaceous times, whereas the sinistral shear zone was active in Cretaceous time.
Spatial and temporal deformation sequences based on the deformation facies analysis were revealed in the high-pressure Omi schists in serpentinite melange of the Hida Gaien Belt. The spatial deformation sequence (deformation zoning) represents four deformation zones defined by assemblage of deformed minor structures through schist blocks. High-mean-ductility-facies structures and low-mean-ductility-facies structures appear in every deformation zone, but some high- to moderate-mean-ductility-facies structures show characteristic development in each deformation zone. On the other hand, the temporal deformation sequence indicates that the deformed minor structures had been formed under conditions of decreasing mean ductility. Taking the relationship between development of the deformed minor structures and their lithologic control into consideration, fabric analysis suggests that the deformation zoning is not dependent on lithologic control, but reflects the kinematic history of the schists. Above analyses lead to early, middle and late deformation phases corresponding to each stage during exhumation of the serpentinite melange. That is the early phase represented by high-mean-ductility-facies structures reflecting the layer-parallel shear of the schist along deep subduction zone, the middle phase, high- to moderate-mean-ductility-facies structures, individual progress of ductile deformation in each zone of the exhuming schist blocks, and the late phase, low-mean-ductility-facies structures, brittle deformation in the shallower level of the crust.
Silurian and Devonian radiolarian biostratigraphy was studied for tuffaceous clastic rocks developed in the Fukuji - Hitoegane and Kuzuryu Lake - Upper Ise River areas of the Hida Gaien belt, central Japan. The objective of the study was to improve chronostratigraphic calibration of these intervals and to discuss local correlation among the Middle Paleozoic strata in Japan. Radiolarian zones characterized by eight distinctive assemblages have been defined in the seven measured sections, as follows: Haplotaeniatum tegimentum - Syntagentactinia excelsa (middle to upper Llandovery), Fusalfanus osobudaniensis - Secuicollacta itoigawai (lower Ludlow), Zadrappolus spinosus - Praespongocoelia parva (lower or middle Ludlow), Stylosphaera (?) magnaspina (upper Ludlow), Pseudospongoprunum (?) tauversi (lower to middle Pridoli), Futobari solidus - Zadrappolus tenuis (lower or middle Pridoli to Lower Devonian), Palaeoscenidium ishigai - Deflantrica furutanii (Lochkovian or Pragian to lower Emsian), and Pactarentinia intermedia - Pactarentinia igoi (lower Emsian) Assemblage Zones. Age control is based on the stratigraphic relationship between the zones, other fossils, and correlation with other radiolarian zones established in the southern Urals, west Texas, and the Kurosegawa belt, Southwest Japan. Biostratigraphic correlations clearly show that Ludlow to Emsian tuffaceous clastic rocks of deep-water origin in the Hida Gaien belt are exceedingly similar to those in the Kurosegawa and South Kitakami belts in depositional age. It provides a constraining link between these terranes during the early evolutionary stage of their geologic history.
Since the 1950s, rocks of the Hida Gaien belt have been subdivided into four units: (1) Middle to Upper Paleozoic shallow marine formations (including their metamorphosed facies of the Unazuki belt); (2) mafic to ultramafic igneous rocks including high P/T metamorphic rocks older than 300 Ma; (3) Upper Paleozoic limestone, chert and clastic rocks; and (4) Permian mafic igneous rocks and clastic formations. In this subdivision, the units are identified by their major lithology and their tectonic position. Over the last two decades, detailed knowledge of the lithology, age and stratigraphic relationships of these rocks has increased to such an extent that units (2), (3) and (4) are today correlated with rocks not of the Hida Gaien belt, but of the Sangun-Renge, Akiyoshi and Maizuru belts respectively. In this paper, the remaining unit (1) is correlated with the newly revised Hida Gaien belt. Unit (1) rocks are distributed in the Unazuki, Shiroumadake, Fukuji-Hongo-Furukawa, Naradani, Itoshiro and Asahi-Ise-Ohno areas. The type locality is designated as the Fukuji-Hongo-Furukawa area.