U–Pb ages of zircons from metamorphic rocks in the upper sequence of the Hidaka Metamorphic Belt, Hokkaido, Japan: identification of two metamorphic events and implications for regional tectonics

AbstractThis paper presents quantitative data on the finite strain, quartz crystal fabric, geometry of flow and deformation temperatures in deformed quartzite samples to characterize the ductile deformation along the thrust sheets constituting the Sanandaj–Sirjan Metamorphic Belt within the Zagros Mountains of Iran. The results of this study emphasize the heterogeneous nature of deformation in this belt, showing a spatial variation in strain magnitude and in degree of non-coaxiality. A dominant top-to-the-SE sense of shear is indicated by the asymmetry of microstructures and quartz c-axis fabrics. Quartz c-axis opening angles suggest deformation temperatures range between 435° ± 50°C and 510° ± 50°C, which yield greenschist to amphibolite facies conditions during the ductile deformation. Mean kinematic vorticity number (Wm) measured in the quartzite samples ranges between 0.6 and 0.9 with an average of 0.76, which indicates that extrusion of the metamorphic rocks of the region was facilitated by a significant component of pure shear strain. Traced towards the basal thrust of the Zagros Thrust System from northeast to southwest, the quartz grain fabrics change from asymmetric cross-girdle fabrics in the internal part of the deformation zone to an asymmetric single-girdle fabric at distances close to the basal thrust. This variation may depend on the structural depth and on the geometry of the ductile deformation zone. The observed increase in strain and vorticity within the study area is comparable with patterns recorded within metamorphic rock extrusions within other orogens in the world.

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Rb–Sr Geochronology of the Hida metamorphic belt, Japan

Canadian Journal of Earth Sciences ◽

10.1139/e70-134 ◽

1970 ◽

Vol 7 (6) ◽

pp. 1383-1401 ◽

Cited By ~ 21

Author(s):

K. Shibata ◽

T. Nozawa ◽

R. K. Wanless

Keyword(s):

Granitic Rocks ◽

Metamorphic Event ◽

Crystalline Schist ◽

Metamorphic Rocks ◽

Schist Belt ◽

Metamorphic Belt ◽

Early Mesozoic ◽

Mineral Isochron ◽

Middle Paleozoic ◽

Later Phase

Rb–Sr whole-rock and mineral isochron ages have been determined for metamorphic and granitic rocks of the Hida metamorphic belt. The results indicate that an extensive metamorphic event together with plutonic activity took place within the belt during the latest Paleozoic – early Mesozoic period. The older ages of 220–250 m.y. represent an earlier phase of the metamorphism, whereas the younger ages of 170–180 m.y. represent a later phase. The Funatsu granitic rocks yielded a whole-rock isochron age of 176 m.y. with an initial 87Sr/86Sr ratio of 0.7056. This age is believed to indicate the time of original emplacement, and the rocks are considered to represent late-kinematic intrusion in the Hida belt.Some information on the middle Paleozoic metamorphism in the Hida Mountains was obtained from the isochron study. The whole-rock isochron age of 412 m.y. for the metamorphic rocks of the Fujibashi area may be considered, although not confirmed, to indicate the time of older metamorphism. The Omi Schist of the Circum–Hida crystalline schist belt, which belongs to the glaucophanitic type of metamorphism, gave a mineral isochron age of 350 m.y. thereby providing evidence of mid-Paleozoic metamorphism.The initial 87Sr/88Sr ratios for the whole-rock samples of the Hida metamorphic belt are found to be generally low, i.e. 0.705–0.708. This is especially so for the metamorphic rocks from the northern part of the belt where the lowest values were found.

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Variety and genesis of the pyroxene-bearing S- and I-type granitoids from the Hidaka Metamorphic Belt, Hokkaido, northern Japan

Earth and Environmental Science Transactions of the Royal Society of Edinburgh ◽

10.1017/s0263593300000997 ◽

2004 ◽

Vol 95 (1-2) ◽

pp. 161-179 ◽

Cited By ~ 16

Author(s):

Toshiaki Shimura ◽

Masaaki Owada ◽

Yasuhito Osanai ◽

Masayuki Komatsu ◽

Hiroo Kagami

Keyword(s):

Partial Melting ◽

Granulite Facies ◽

Mafic Granulite ◽

Granitic Rocks ◽

Metamorphic Rocks ◽

Metamorphic Belt ◽

Model Calculations ◽

Magmatic Arc ◽

River Region ◽

Northern Japan

ABSTRACTThe high-dT/dP-type Hidaka Metamorphic Belt in Hokkaido, northern Japan, represents a tilted crustal section of a magmatic arc of Tertiary age. The highest metamorphic grades reached are granulite facies, and the syn-metamorphic granitic rocks are widely distributed in this metamorphic terrane. The granitic rocks are mainly tonalitic and granodioritic in composition, and are classified into peraluminous (S-type) and metaluminous (I-type) granitoids. A large amount of pyroxene-bearing S-type tonalites (garnet-orthopyroxene tonalite) is distributed in the Niikappu river region in the northern part of the Hidaka Metamorphic Belt. Pyroxene-bearing I-type tonalite (two-pyroxene hornblende tonalite) bodies are also distributed in this area.The pyroxene-bearing tonalites are classified into several sub-types on the basis of their field occurrence, texture, mineral assemblage and geochemical features. Homogeneous IH- and SH-type tonalite are thought to represent original magmas, i.e. those which have been generated by partial melting of mafic metamorphic rocks and pelitic-psammitic metamorphic rocks, respectively. Model calculations assuming batch partial melting indicate that possible restites are garnet-two-pyroxene mafic granulite for IH-type and garnet-orthopyroxene aluminous granulite for SH-type. The unexposed lowermost crust of the ‘Hidaka crust’ is thought to be composed of garnet-two-pyroxene mafic granulite, garnet-orthopyroxene aluminous granulite and metagabbros.

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