Shohei Banno

Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan

Summary The Cretaceous (pre-Japan Sea) Sanbagawa metamorphism affected the Japanese Jurassic complex south of the Median Tectonic Line in the regions now recognized as the Sanbagawa, Mikabu and Chichibu belts. The metamorphic peak (116 Ma) was reached and passed during the tectonic ‘D1’ deformation, corresponding to sinistral shear N30°E along the eastern margin of the Asian continent. This was followed by ‘D2’ (c. 85 Ma) fold and thrust deformation, the vergence of which is normal to the ‘D1’ trend. These deformational events established the present thermal structure. The final regional deformation formed upright ‘D3’ folds. The four metamorphic zones based on pelitic assemblages can be enhanced by using basic schists to subdivide the pelitic chlorite zone. Apparent Fe-Mg partition coefficients between chlorite and garnet show an essential regional continuity of metamorphism and that thrust-offsets do not juxtapose elements from different mineral zones. Peak conditions of metamorphism ranging from 250°C and 6 kbars to 600°C and 10 kbars are consistent with simple P-T-t loops which progress at higher pressures and return at lower pressures to the surface.

Pressure-temperature trajectory of the Sanbagawa metamorphism deduced from garnet zoning

Abstract Most garnets in a small volume of the pelitic Sanbagawa schists nucleated simultaneously, grew, and then ceased with their growth, maintaining near-equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism. Chemical trends of zoned garnets systematically shift from the pyrope-poor to pyrope-rich portions in a ternary MnFeMg system in order of ascending metamorphic grade. A ternary (MnFeMg) model system suggests that the higher-grade garnets started to grow at lower pressures and higher temperatures than the lower-grade ones during prograde metamorphism. It follows that high-grade metamorphism took place at lower pressures in the garnet to oligoclase-biotite zones of the Sanbagawa metamorphic terrain in central Shikoku.

Classification of eclogites in terms of physical conditions of their origin

The apparent FeMg distribution coefficient between garnet and clinopyroxene K′ is defined by K″=XFeXMggaXFeXMgcpx· The effects of pressure, temperature, and chemistry of the rocks on K′ were examined, using formula volumes of end members of these minerals, and the K′ values of natural eclogites. It is shown that K′ increases with increasing pressure, and decreases with increasing temperature, and that the effect of chemistry on K′ is usually not large. It is shown that the eclogite types defined by the geological mode of occurrences are also characterized by particular range of K′ values, and that the relative temperatures of eclogite crystallization as estimated by K′ and by ordinary petrological considerations are usually in harmony with each other. Further, K′ can be applied to distinguish the difference in temperatures among the eclogites of some types. The crystallization temperature of eclogites increases in the following order, which is shown by localities: Colombia, Ural and Guatemala, California, Alps and Japan, Bavaria and Spain, Norway, East Sudetes, and granulite facies eclogites. Eclogite inclusions in basalt and in kimberlite represents highest temperature but the former represents higher pressure than the later. The crystallization temperature of some eclogites which are not included above are also discussed.

Occurrence and field relationships of ultrahigh-pressure metagranitoid and coesite eclogite in the Su-Lu terrane, eastern China

Coesite eclogite is associated with metagranitoid in a 50×100 m2 outcrop within the regionally developed amphibolite-facies Su-Lu orthogneiss. Primary intrusive relationships between the metagranitoid and basic rocks and bulk-chemistry analyses show that together they represent a composite igneous body that has subsequently been strongly deformed and metamorphosed. The presence of rutile, sodie pyroxene, corona garnet, and possible pseudomorphs after coesite all suggest very high pressures of metamorphism in the metagranitoid. This is the first documented occurrence of ultrahigh-pressure (UHP) metagranitoid outside of the European Alps. The existence of UHP metagranitoid shows that low density of rocks does not necessarily prevent subduction to mantle depths. Even at peak metamorphic conditions the UHP composite igneous body reported here would have a bulk density less than the mantle. Buoyancy forces may, therefore, have been important in the early exhumation of this unit. Other outcrops of coesite eclogite in the Su-Lu region may also have been originally metamorphosed along with low-density granitoid rocks.

Decompression P–T path of coesite eclogite to granulite from Weihai, eastern China

Granulitized coesite-bearing eclogite from Weihai, northeastern part of the Shandong peninsula, eastern China was studied in detail to reveal the modification of mineral chemistry during decompression metamorphism. Considerable modification of chemical composition is recorded in clinopyroxene that occurs both as inclusions in garnet and as a matrix mineral. Careful examination of chemical variation with the change in microstructure made it possible to estimate the equilibrium composition of minerals at the coesite eclogite and garnet granulite stages. We were able to define three reference points on the P–T path, namely, coesite eclogite (3 GPa, 660±40°C), granulite (1 GPa, 700±30°C) and amphibolite (0.9 GPa, 600±20°C). The path thus obtained is similar to those obtained by previous workers and supports nearly isothermal decompression of coesite eclogite.

Phase petrology of eclogites and related rocks from the Motalafjella high-pressure metamorphic complex in Spitsbergen (Arctic Ocean) and its significance

Abstract A Caledonian eclogite suite is associated with lower-grade high-pressure rocks in Motalafjella, Oscar II Land, central-western Spitsbergen. A variety of mineral assemblages occur in the suite, depending on the bulk-rock composition; omphacite-paragonite-glaucophane-epidote-garnet in eclogites, jadeite-glaucophane-paragonite-quartz in siliceous schists, and MgFe chloritoid-paragonite-epidote-garnet in pelitic schists. Petrologic and paragenetic studies are presented to characterize this unique set of mineral assemblages in relation to other eclogite suites. Mineralogical data, mainly the distribution of Fe and Mg between coexisting clinopyroxene and garnet, and the existence of impure jadeite+quartz assemblage, give approximately 580–640°C and 18–24 kbar. New criteria for the eclogite classification are derived based on the mineral assemblages and a model petrogenetic grid. The Motalafjella eclogites associated with the glaucophane-epidote assemblage belong to the medium-temperature portion of the eclogite facies.

Petrology of peridotite and garnet clinopyroxenite of the Mt. Higasi-Akaisi mass, central Sikoku, Japan — Subsolidus relation of anhydrous phases

The mineralogy of anhydrous minerals of peridotite and garnet clinopyroxenite of the Mt. Higasi-Akaisi peridotite mass, Japan, is described. The subsolidus equilibria among garnet, clinopyroxene, orthopyroxene and olivine are discussed in terms of composition range of solid solutions and of Fe-Mg partition. It is concluded that the anydrous minerals of this mass equilibrated at lower temperatures than any of well studied peridotite-garnet clinopyroxenite association. The tentative estimation gives 5–600° C and 7–13 Kb as the physical conditions of equilibration. Comparison with similar associations from other occurrences and geological implications are briefly discussed.

Evolution of the Sambagawa metamorphic belt, Japan

Abstract The high-pressure Sambagawa metamorphic belt evolved within a Mesozoic accretionary prism that developed in the convergent margin along the east of the Eurasian continent. Although extensively researched, recent studies in this region have revealed a number of important features that allow more complete reconstruction of the tectonic history. (i) The grouping of radiometric ages suggests the existence of three major structural units with distinct metamorphic histories. One of these has since been completely eroded away. (ii) The difference of ages recorded by different minerals is, in many cases, small suggesting rapid exhumation after reaching peak metamorphic conditions. (iii) The dominant ductile deformation of the Sanbagawa belt representing major orogen-parallel flow is commonly related to retrograde metamorphic reactions. This shows that the associated fabric developed during exhumation not subduction. (iv) Kinematic studies suggest that the main ductile deformation caused major ductile thinning of the region which was probably a significant factor in causing exhumation of the region.

The Sulu UHP Terrane: A Review of the Petrology and Structural Geology

Petrological and isotopic evidence suggests that the protolith of the Sulu ultrahigh-pressure (UHP) terrane was Precambrian continental crust consisting of granite, granodiorite, gabbro, marble, and basic dikes, with local granulite-facies assemblages. Around 220 Ma this unit of continental rocks was buried to depths up of ∼120 km within the mantle. Structures formed during exhumation suggest highly mobile behavior of acidic rocks, even under conditions of very low water activity. Petrological studies show that the Sulu terrane underwent isothermal decompression, which implies relatively rapid exhumation, and suggests that the role of melting during exhumation may have been underestimated. The later stages of exhumation are associated with NW-SE-directed tectonic transport and the formation of at least one major normal detachment.

Kurosegawa zone and its bearing on the development of the Japanese Islands

Abstract The Kurosegawa zone in southwest Japan is a 600 km long serpentinite melange in the Chichibu terrains. It runs generally E-W but is slightly oblique to the subparallel arrangement of the Ryoke, Sanbagawa and Chichibu belts of Southwest Japan. A variety of geological units occurs in the Kurosegawa zone: 1. (1) granodiorite, gneiss and amphibolite of ca. 400 Ma, 2. (2) Siluro-Devonian formations, 3. (3) Upper Carboniferous to Jurassic formations, 4. (4) Upper Jurassic to Lower Cretaceous formations, 5. (5) serpentinite and 6. (6) low- to medium-grade metamorphic rocks of various baric types (ages, 220, 320, 360 and 420 Ma by K-Ar). The most widespread is a high-pressure intermediate group of metamorphic rocks. Serpentinite is emplaced along the faults between and within the constituent units. Rocks of the Kurosegawa zone represent a mature orogenic belt along a continental margin or an island arc. Its original site as constrained by paleomagnetism was near the equatorial area. Here, 400 Ma old paired metamorphism and related magmatism took place. The island arc or microcontinent migrated northward to collide with the Eurasia plate during Late Jurassic, thus consuming the intervening ocean.