Tourmaline breakdown in the migmatite zone of the Ryoke metamorphic belt,SW Japan

A sharp line delimitating the distribution of tourmaline (termed as a ‘tourmaline‐out isograd’) is defined in the migmatite zone of the Ryoke metamorphic belt, Japan. The trend of the tourmaline‐out isograd closely matches that of the isograds formed through the regional metamorphism, suggesting that it represents the breakdown front of tourmaline during regional metamorphism. This is confirmed by the presence of the reaction textures of tourmaline to sillimanite and cordierite near the tourmaline‐out isograd. The breakdown of tourmaline would release boron into associated melts or fluids and be an important factor in controlling the behaviour of boron in tourmaline‐bearing high‐temperature metamorphic rocks. Near the tourmaline‐out isograd, large tourmaline crystals occur in the centre of interboudin partitions containing leucosome. In the melanosome of the intervening matrix, reaction textures involving tourmaline are locally observed. These observations imply that tourmaline breakdown is related to a melting reaction and that the boron in the leucosome is derived from the breakdown of tourmaline in the melanosome during prograde metamorphism. Boron released by tourmaline breakdown lowers both the solidus temperature of the rock and the viscosity of any associated melt. Considering that the tourmaline‐out isograd lies close to the schist–migmatite boundary, these effects might have enhanced melt generation and segregation in the migmatite zone of the Ryoke belt. The evidence for the breakdown of tourmaline and the almost complete absence of any borosilicates throughout the migmatite zone suggest that boron was effectively removed from this region by the movement of melt and/or fluid. This implies that the tourmaline‐out isograd can reflect a significant amount of mass transfer in the anatectic zones.     

Ductility contrast induced by silicification in pelitic schist of the Ryoke metamorphic belt,Japan

Contrasting ductility is recognized in the rocks of Cretaceous Ryoke metamorphic belt in Iwakuni area, southwest Japan. Pelitic schist is ubiquitous in the region and differences in mineral assemblages mark increase in metamorphic grade. The area has been graded as chlorite-biotite zone in the north progressing into biotite- and muscovite-cordierite zones in the south. Pelitic schist near the boundary between the biotite- and muscovite-cordierite zones has undergone partial silicification to form whitish silicified schist layers which contain two types of quartz veins: those parallel to foliation in the host rock are called schistosity-concordant veins, and those inclined to host rock foliation, schistosity-discordant veins. In this study we examined the quartz structure in the silicified schist and in both types of veins to understand the ductility contrast induced by the silicification process. Crystallographic orientations of quartz in the veins and silicified schist rocks were studied using the Scanning Electron Microscopy (SEM) based Electron Back Scatter Diffraction (EBSD) technique. Quartz c-axis orientations in the silicified schist are nearly random, demonstrating an absence of post-silicification ductile deformation. Quartz grains in the schistosity-concordant veins have preferred c-axis orientations perpendicular to the schistosity indicating ductile shortening. In contrast, schistosity-discordant veins display distinct quartz c-axis fabric than that found in the schistosity-concordant veins. This is because the two types of host rocks exhibit a difference in ductility during deformation. The presence of deformed quartz veins in the undeformed silicified schist indicates transformation of the ductile pelitic schist into the brittle silicified schist at mid-crustal levels where these rocks originate, hence forming contrasting rock layers. Schistosity-concordant veins in the biotite-rich pelitic schist deformed with its host rock in a ductile manner while the schistosity-discordant veins in the neighboring silicified schist were left intact. Silicification of the pelitic schist may have been caused by the silica-rich geofluids produced by subsurface processes. Geofluids responsible for the occurrence of such mechanically contrasting layers mark an increase in seismic reflectivity at mid-crustal depths and may be potential reflectors of seismic waves giving rise to the so-called “bright spots”.     

Oxygen isotope study of metamorphic and granitic rocks of the Yanai district in the Ryoke belt,Japan

¹⁸O/¹⁶O ratios have been obtained for 134 whole-rocks and minerals from metamorphic and granitic rocks of the Yanai district in the Ryoke belt, Southwest Japan. The ¹⁸O/¹⁶O ratios of pelitic rocks of the marginal metamorphic zone decrease progressively with increasing metamorphic grade. In the gneiss-granite complex (zone of migmatite [1]), the most characteristic feature of the rocks is that oxygen isotopic homogenization proceeds on both local and regional scales in parallel with “granitization” or chemical homogenization. Granitic rocks of various origin are fairly uniform in isotopic composition with δ ¹⁸O of quartz of 12 to 14‰ (SMOW) and δ ¹⁸O of biotite of 7 to 9‰ and are about 3 to 4‰ enriched in ¹⁸O compared to other Cretaceous granites of non-metamorphic terranes in Japan. The high ¹⁸O/¹⁶O ratios of granitic rocks of this district were discussed in relation to the ¹⁸O-depletion in metasediments. Oxygen isotopic fractionations among coexisting minerals from various rock-types of the gneiss-granite complex indicate that these minerals were formed under near isotopic equilibrium at a temperature of about 600 to 700° C. Some abnormal fractionations of quartz-biotite pairs also were obtained for rocks which had undergone a progressive ¹⁸O-depletion or ¹⁸O-enrichment. This is due to high resistivity of quartz and contrastive susceptibility of biotite to isotopic exchange during metamorphism and “granitization”.     

Carbonaceous material in the Ryoke metamorphic rocks, Kinki district, Japan

Carbonaceous material in the Ryoke pelitic metamorphic rocks in the Wazuka area, Kinki district, Japan, has been studied by X-ray diffraction (XRD) analysis. Detrital graphite in the lower-grade rocks is recognized in both X-ray diffractograms and transmission electron micrographs. Progressive graphitization is considered to have proceeded continuously on the basis of the XRD data of the bulk concentrates of carbonaceous material, and a conspicuous asymmetric shape of XRD peaks in lower-grade samples is ascribed to the mixture of carbonaceous materials with different crystallinities. Fully-ordered graphite does not occur until the highest-grade part of the chlorite-biotite zone. The variation of degree of crystallinity of carbonaceous materials inferred from XRD data is consistent with the prograde mineral zones. The temperatures for the development of fully-ordered graphite are estimated to be between 410° and 440°C in regional metamorphism through comparing the XRD data from low-, medium- and high-pressure types of metamorphic terrains.     

Pressure-temperature conditions of the Ryoke metamorphic rocks in Yanai district,SW Japan

Pressure-temperature conditions of metamorphism in the Yanai district, Ryoke belt, SW Japan, have been determined using garnet-biotite thermometry in combination with an empirically calibrated barometer in the assemblage common in pelitic and siliceous rocks, garnet + biotite + plagioclase + quartz. The barometer estimates pressure difference between a well-established sample and unknown samples based on the reaction,

Dissolution and precipitation processes in deformed amphibolites: an example from the ductile shear zone of the Ryoke metamorphic belt, SW Japan

The granitic mylonite zone in the Cretaceous Ryoke metamorphic belt contains deformed amphibolites as thin layers. The amphibolite layers do not exhibit pinch‐and‐swell or boudinage structures, even when contained in a high‐strain granitic mylonite. This mode of occurrence suggests that they were deformed as much as the surrounding granite mylonite. In the highly deformed zone, strongly foliated amphibolites contain Ti‐rich brown amphibole porphyroclasts rimmed by Ti‐poor green amphibole, titanite and chlorite. These porphyroclasts are elongated, forming shear surfaces defined by preferential distribution of the chlorite and titanite. Porphyroclastic plagioclase in the strongly foliated amphibolites consists of two components: an anorthite‐rich core and an anorthite‐poor rim. Based on these observations, the mass‐balanced reaction occurring during deformation is defined as As the reaction products form a weak interconnected matrix, the strain rate of the amphibolites may be controlled by the rate of dissolution–precipitation through fluids. Weakly foliated amphibolites in the low‐strain zone exhibit cataclastic microstructures, whereas the strongly foliated amphibolites do not exhibit such features. These microstructural and chemical changes suggest that high‐strain amphibolites were initially deformed by cataclasis, followed by deformation through metamorphic reactions. During the metamorphism/deformation, old plagioclase grains with high X_an were not stable and dissolved, and new plagioclase grains with low X_an crystallized at the old plagioclase rim. Dissolution of old plagioclase and precipitation of new plagioclase occurred normal to and parallel to the foliation, respectively, reflecting incongruent pressure solution due to differential stress and changes in P–T–H₂O conditions. The development of incongruent pressure solution is attributed to increased fluid flux in the strongly foliated amphibolites, as evidenced by the greater abundance of hydration‐reaction products in the strongly foliated amphibolites than in the weakly foliated ones.     

Boron in metapelites controlled by the breakdown of tourmaline and retrograde formation of borosilicates in the Yanai area,Ryoke metamorphic belt,SW Japan

Tourmaline-out isograd formed by the breakdown of tourmaline is defined in the upper amphibolite-facies metapelites in the Yanai area, Ryoke metamorphic belt, SW Japan. The rim composition of tourmaline progressively becomes aluminous with ascending metamorphic grade, and the chemical zoning of tourmaline is controlled by ^X□AlNa_–1Mg_–1 and MgTi^YAl_–2 vectors in low- to medium-grade zones where muscovite is stable, whereas it is controlled by Mg(OH)^YAl_–1O_–1, CaMgO^X□_–1 ^YAl_–1(OH)_–1 and MgTi^YAl_–2 vectors in further higher–grade, muscovite-unstable zones. The size of tourmaline increases drastically where breakdown of muscovite+quartz takes place, probably due to the growth of tourmaline during breakdown of muscovite. On the high-temperature side of the tourmaline-out isograd, depletion of whole-rock boron is observed. Escape of boron-bearing melt or the fluid evolved from the melt during its crystallization probably caused this depletion, although locally trapped, boron-bearing melt or fluid formed irregularly shaped tourmaline and dumortierite during retrograde metamorphism.     

Behavior of zircon in the upper-amphibolite to granulite facies schist/migmatite transition, Ryoke metamorphic belt, SW Japan: constraints from the melt inclusions in zircon

Behavior of zircon at the schist/migmatite transition is investigated. Syn-metamorphic overgrowth is rare in zircon in schists, whereas zircon in migmatites has rims with low Th/U that give 90.3 ± 2.2 Ma U–Pb concordia age. Between inherited core and the metamorphic rim, a thin, dark-CL annulus containing melt inclusion is commonly developed, suggesting that it formed contemporaneous with the rim in the presence of melt. In diatexites, the annulus is further truncated by the brighter-CL overgrowth, suggesting the resorption and regrowth of the zircon after near-peak metamorphism. Part of the zircon rim crystallized during the solidification of the melt in migmatites. Preservation of angular-shaped inherited core of 5–10 μm in zircon included in garnet suggests that zircon of this size did not experience resorption but developed overgrowths during near-peak metamorphism. The Ostwald ripening process consuming zircon less than 5–10 μm is required to form new overgrowths. Curved crystal size distribution pattern for fine-grained zircons in a diatexite sample may indicate the contribution of this process. Zircon less than 20 μm is confirmed to be an important sink of Zr in metatexites, and ca. 35-μm zircon without detrital core are common in diatexites, supporting new nucleation of zircon in migmatites. In the Ryoke metamorphic belt at the Aoyama area, monazite from migmatites records the prograde growth age of 96.5 ± 1.9 Ma. Using the difference of growth timing of monazite and zircon, the duration of metamorphism higher than the amphibolite facies grade is estimated to be ca. 6 Myr.     

Garnet-amphibolites at Yokokawa in the Abukuma metamorphic belt,Japan

The mineralogy and petrochemistry of the garnet-amphibolites from the highgrade part of the Abukuma metamorphic belt have been studied, using five analyses of rocks, five of hornblendes, three of garnets and one analysis of cummingtonite, Garnetiferous amphibolites are rich in Fe, whereas non-garnetiferous ones are rich in Mg, especially in cummingtonite-amphibolite. The chemical composition of hornblendes associated with garnet is pargasitic and rich in FeO and poor in CaO, but that of non-garnetiferous rocks is rich in MgO. The garnets are rich in almandine molecule. Mg/Mg + Fe²⁺ ratios of both hornblendes and garnets correspond with those of the host rocks. The development of garnet in the Adirondack metabasites belonging to the upper almandine-amphibolite and granulite facies is observed in Mg-rich rocks as well as in Fe-rich rocks, in which both garnet and hornblende are rich in Mg respectively. However, under the conditions of the andalusite-sillimanite type metamorphism as shown in the Abukuma Plateau, Fe-rich garnet occurs in Fe-rich basic rocks, but cummingtonite occurs in Mg-rich ones instead of Mg-rich garnet. Finally, the problem of polymetamorphism is discussed. The cummingtonite-amphibolite may be the product of polymetamorphism, and Mg-rich garnet which had been present previously was decomposed to cummingtonite and plagioclase by the subsequent regional metamorphism of andalusite-sillimanite type.     

Ammonium in biotite from metamorphic and granitic rocks of Japan

Ammonium contents of biotites from metamorphic and granitic rocks of Japan have been determined, and correlated with the ${}^{18}\text{O}{}^{16}\text{O}$ ratios of the rocks.NH₄ contents of biotites averaged 22 ppm in granitic rocks of non-metamorphic terranes, 67 ppm in granitic rocks in the Ryoke metamorphic belt, and 279 ppm in metamorphosed sedimentary rocks of the Ryoke belt. In granitic rocks, enrichment of NH₄ in biotites is a result of the interaction between the granitic magma and surrounding sedimentary rocks. In metasedimentary rocks, the high NH₄ content in biotites is due to inheritance from original organic material in sedimentary rocks.Biotites from migmatites of the Ryoke belt contain more NH₄ (average, 475 ppm) than those from metasedimentary rocks. This suggests the existence of a metamorphic fluid or anatectic magma enriched in NH₄.     

Migmatite tectonics of the hidaka metamorphic belt, Hokkaido, Japan

The Hidaka metamorphic belt is situated at the junction of the Honshu and Kuril arcs in the axial zone of Hokkaido in northern Japan. Various migmatites, which occupy the core of the metamorphic belt, are classified as lens, sheet, falling star and dome facies on the basis of composition, scale and form as proposed by Harland (1956). Each facies is produced progressively. Movement is first lateral and then upwards at the sheet facies stage, followed by the development of the diapiric falling star and dome facies. Subsequently, the granitic phase starts to form from the lens facies, again within the migmatite sheets, leading to the emplacement of granitic plutons. The movement of the migmatite and granite bodies is controlled by the tangential stress field, as well as by the buoyancy in the gravitational field.     

The higher grade metamorphic zonation of the Sambagawa metamorphic belt in central Shikoku, Japan

The higher grade metamorphic zonation of the Sambagawa (= Sanbagawa) belt is established for the first time for the whole area of central Shikoku. As discontinuous reactions to define the isograd are absent, the metamorphic grade is primarily determined by the Mg-Fe partitioning between garnet and chlorite along representative traverses. However, for regional mapping, mineralogical features of the pelitic schists, such as using mineral assemblages of more than divariant equilibrium, the modal garnet to chlorite ratio, and the optical properties of chlorite, are employed as auxiliary criteria.
The presence of the highest grade mineral zone in the middle of the structural level is confirmed, but its spatial distribution is far more complex than hitherto accepted. Thermal axes are now confirmed at three different structural levels. A model is presented in which the stacking of thrust sheets of different grade took place while metamorphic reactions were in progress. Thermal readjustment brought a continuous metamorphic temperature gradient across and within the thrust sheets. Tectonic blocks of metagabbro and ultramafic rock were emplaced synchronously with thinning and subsequently also re-equilibrated. Local anomalies of metamorphic grade, represented by mixing of schists of different metamorphic grade, exist, but they are due to a later stage event.     

The transition from brittle to ductile deformation in the Sambagawa metamorphic belt, Japan

The dominant deformation mechanism during the Sambagawa metamorphism changes from brittle to ductile with increasing metamorphic temperature. The magnitude of plastic strains inferred from the shapes of deformed radiolaria in metachert increases sharply across the boundary between the epidote-pumpellyite-actinolite zone and the epidote-actinolite zone. The synmetamorphic crack density of metachert is an indicator of the contemporaneous brittle strain of rocks, and it decreases sharply as the grade reaches the epidote-actinolite zone. Hence, the ratio of the ductile strain to the brittle strain of metachert decreases rapidly across the transition to the epidote-actinolite zone of the Sambagawa metamorphic belt.
The sharp change of the ductile strain magnitude also takes place at the epidote-actinolite grade in the Shimanto metamorphic belt of Japan, an example of the intermediate pressure facies series of metamorphism. It is concluded that the transition from brittle to ductile deformation takes place at about 300-400°C. and is independent of pressure of metamorphism.     

Denudation history of the high T/P Ryoke metamorphic belt, southwest Japan: constraints from CHIME monazite ages of gneisses and granitoids

CHIME (chemical Th–U-total Pb isochron method) monazite ages were determined for gneisses and granitoids from the eastern and western parts of the Ryoke belt separated by about 500 km. The monazite ages for the gneisses are concentrated between 102 and 98  Ma, and are interpreted as the time of monazite formation under lower amphibolite facies conditions. The peak metamorphism seems to be contemporaneous with the emplacement of the geologically oldest plutons that are dated at c . 95  Ma in both the eastern and western parts. In the eastern part plutonism continued from c . 95  Ma to c . 68  Ma at intervals of 2–10  Ma, whereas in the western part it ceased at c . 85  Ma. The CHIME monazite ages agree well with the relative age of granitoids derived from intrusive relationships of granitoids in both parts. These lines of evidence are incompatible with a current view that the plutonometamorphism in the Ryoke belt becomes younger towards the east. The CHIME monazite ages, coupled with available data on the depth at which the Ryoke metamorphism took place and the emplacement of individual plutons, show that the western part was eroded more rapidly (about 1.5  mm year⁻¹) than the eastern part (about 0.8  mm year⁻¹) over the time span from 91 to 85  Ma. The denudation rates agree well with those in active orogenic belts like the Alps and Himalayas.     

Tertiary deep crustal ultrametamorphism in the Hidaka metamorphic belt, northern Japan

The Main Zone of the Hidaka metamorphic belt is an imbricate stack of crustal material derived from an island arc in which a sequence of units with increasing metamorphic grade from low to high structural levels is exposed. The basal part of the metamorphic sequence underwent granulite facies metamorphism with peak P–T conditions of 7kbar, 870°C. In this zone pelitic granulite includes leucosomes which consist mainly of orthopyroxene-plagioclase-quartz.
To test whether the leucosome was derived by partial melting of the surrounding pelite, melting experiments of the pelitic granulite were carried out for water-saturated and dry systems at 7 kbar and 850°C. The chemical composition of the leucosome produced during these runs shows a peraluminous S-type tonalitic affinity and is located very close to the tie-line between the average melts produced in water-saturated systems and the average composition of the residual orthopyroxene + plagioclase. This therefore suggests that the lecosome in pelitic granulite was formed by incipient anatexis at close to the highest P–T condition of the Main Zone.
The age of the crustal anatexis is determined by the Rb-Sr whole rock isochron method for garnet-cordierite-biotite gneiss (host rock), garnet-orthopyroxene-cordierite gneiss (restite) and S-type tonalite (melt). This gives an age of 56.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.705711. The S-type tonalite magmas that form large intrusive masses in the Main Zone were probably generated by crustal anatexis in deeper parts of the crust at the same time (late Palaeocene).     

Non-stoichiometry of interlayer cations in micas from low- to middle-grade metamorphic rocks in the Ryoke and the Sanbagawa belts,Japan

Micas in 17 pelitic (K-feldspar-free) and 8 psammitic (K-feldspar-bearing) rocks from the Wazuka and the Asemi-gawa areas in the Ryoke and the Sanbagawa metamorphic terrains, respectively, were analyzed on an electron-probe microanalyzer. The deficiency of alkali cations in the low- to middle-grade metamorphic micas is ascribed to the illite substitution, K^XII+Al^IV=^XII (vacancy)+Si^IV.At the same metamorphic grade, the deficiency of interlayer cations in micas from the pelitic rocks is greater than that from the psammitic rocks. However, it decreases with rising temperature in both rock-types, irrespective of the pressure of metamorphism.K-feldspar and biotite buffer the illite substitution. Two reactions are proposed to explain the decrease of the alkali-cation deficiency in both muscovite and biotite.     

Phase equilibria and the pressure-temperature path of the highest-grade Ryoke metamorphic rocks in the Yanai district, SW Japan

The garnet-cordierite zone, the highest-grade zone of the Ryoke metamorphic rocks in the Yanai district, SW Japan, is defined by the coexistence of garnet and cordierite in pelitic rocks. Three assemblages in this zone are studied in detail, i.e. spinel + cordierite + biotite, garnet + cordierite + biotite and garnet + biotite, all of which contain quartz, K-feldspar and plagioclase. The Mg/(Fe + Mg) in the coexisting minerals decreases in the following order: cordierite, biotite, garnet and spinel. Two facts described below are inconsistent with the paragenetic relation in the K₂OFeOMgOAl₂O₃SiO₂H₂O (KFMASH) system in terms of an isophysical variation. First, garnet and biotite in the last assemblage have Mg/(Fe + Mg) higher than those in the second. Second, the first two assemblages are described by the reaction,

Carbon isotopic study on coexisting calcite and graphite in the Ryoke metamorphic rocks,northern Kiso district,central Japan

Carbon isotope fractionation between coexisting calcite and grpahite ( ¹³C_cc-gr) has been determined in metamorphosed limestones and calc-silicate rocks from the Ryoke metamorphic belt in the northern Kiso district. In this district, the Ryoke metamorphic rocks, ranging from the lower greenschist facies to the upper amphibolite facies, are widely distributed. The fractionation of ¹³C/¹²C between calcite and graphite decreases regularly with increasing metamorphic grade and is independent of absolute ¹³C values of calcite. This evidence suggests that carbon isotopic exchange equilibrium has been attained during metamorphism even in the greenschist facies and isotopic modification, possibly caused by retrogressive metamorphism, is not distinguished. For T=270–650° C, the fractionation is expressed by the following equation: ¹³C_cc-gr=8.9×10⁶T^–2–7.1 (T in °K).This equation has a slope steeper than the current results on the ¹³C_cc-gr versus 10⁶T^–2 diagram. It can be used as a potential geothermometer for almost the entire temperature range of metamorphism. ¹³C values of carbonaceous matter in unmetamorphosed limestones in this district are approximately –22, due to its biogenic origin. Graphite from metamorphosed limestones is also considered to be of biogenic origin but shows enrichment of ¹³C due to isotopic exchange with calcite. ¹³C values of graphite as well as ¹³C_cc-gr confirm that zone II represents the lowest grade zone of Ryoke metamorphism. The maximum equilibrium fractionation of ¹³C between calcite and graphite is considered to be approximately 23%, which corresponds to 270° C. Below this temperature, it seems that carbon isotopic exchange between the minerals does not occur.Calcite in marble from the higher grade zones has relatively lower ¹³C and ¹⁸O values. The depletion of heavy isotopes is considered to be caused by the loss of ¹³C and ¹⁸O enriched carbon dioxide during decarbonation reactions. For oxygen, it is considered that isotopic exchange with metamorphic fluids plays an important role in lowering the ¹⁸O value of calcite in some higher grade marbles.     

Unpairing metamorphic belts: P–T paths and a tectonic model for the Ryoke Belt, southwest Japan

Southwest Japan is divided into Outer and Inner Zones by the Median Tectonic Line (MTL), a major transcurrent fault. The Outer Zone is composed of the Sambagawa (high-pressure intermediate or high P/T type metamorphism), Chichibu and Shimanto Belts. In the Inner Zone, the Ryoke Belt (andalusite– sillimanite or low P/T type metamorphism) was developed mainly within a Jurassic accretionary complex. This spatial relationship between high P/T type and low P/T type metamorphic belts led Miyashiro to the idea that metamorphic belts were developed as ‘paired’ systems. Textural relationships and petrogenetically significant mineral assemblages in pelites from the Ryoke Belt imply peak P–T conditions of ≈5 kbar and up to 850 °C in migmatitic garnet–cordierite rocks from the highest-grade metamorphic zone. It is likely that the thermal anomaly responsible for metamorphism of the Ryoke Belt was related to a segment of the Farallon–Izanagi Ridge as it subducted under the eastern margin of the Asian continent during the Cretaceous. The sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dP/dT slope, inferred to have been generated by a nested set of hairpin-like ‘clockwise’P–T paths. These P–T paths are characterized by limited prograde thickening, minor decompression at peak-T , and near-isobaric cooling, features that may be typical of P–T paths in low P/T type metamorphic belts caused by ridge subduction. A ridge subduction model for the Ryoke Belt implies that juxtaposition of the high-P/T metamorphic rocks of the Sambagawa Belt against it was a result of terrane amalgamation. Belt-parallel ductile stretching, recorded as syn-metamorphic, predominantly constrictional strain in both Ryoke and Sambagawa Belt rocks, and substantial sinistral displacement on the MTL are consistent with left-lateral oblique convergence. Diachroneity in fast cooling of the Ryoke Belt is implied by extant thermochronological data, and is inferred to relate to progressive SW to NE docking of the Sambagawa Belt. Thus, an alternative interpretation of ‘paired’ metamorphic belts in Japan is that they represent laterally contemporaneous terranes, rather than outboard and inboard components of a trench/arc ‘paired’ system. Amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of ‘paired’ metamorphic belts in the geological record.     

Pseudotachylite from the Main Zone of the Hidaka metamorphic belt, Hokkaido, northern Japan

Pseudotachylite veins have been found in the mylonite zone of the Hidaka metamorphic belt, Hokkaido, northern Japan. They are associated with faults with WNW-ESE to ENE-WSW or NE-SW trends which make a conjugate set, cutting foliations of the host mylonitic rocks with high obliquity. The mylonitic rocks comprise greenschist facies to prehnite-pumpellyite facies mineral assemblages. The mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near-surface environments. An analysis of the deformational and metamorphic history of the Hidaka Main Zone suggests that the ambient rock temperature was 200–300° C immediately before the formation of the Hidaka pseudotachylite. Three textural types of veins are distinguished: cryptocrystalline, microcrystalline and glassy. The cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline-type veins. The microcrystalline type is largely made up of quench microlites of orthopyroxene, clinopyroxene, biotite, plagioclase and opaque minerals with small amounts of amphibole microlites. The interstices of these microlites are occupied by glassy and/or cryptocrystalline materials. The presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km. The bulk chemical composition of the pseudotachylite veins is characterized by low SiO₂ and a high water content and is very close to that of the host mylonitic rocks. This indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases. Local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts. Pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high Al contents. The presence of pigeonite and high-Al pyroxene microlites, of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins. The melt temperatures were estimated to be up to 1100° C using a two-pyroxene geothermometer. Using published data relating water solubilities in high-temperature andesitic magmas to pressure, a depth estimate of about 4 km is inferred for the Hidaka pseudotachylites. Evidence derived from pseudotachylites in the Hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals.