Deformation Style of Slickenlines on Mélange Foliations and Change in Deformation Mechanisms Along Subduction Interface: Example from the Cretaceous Shimanto Belt, Shikoku, Japan

Accretionary complexes record the histories of changes in physical properties of sediments from unlithified sediments to lithified rocks through the deformation processes along subduction interface. The trench sediment suffered various deformation of particulate flow, pressure solution deformation and cataclastic faultings from ductile to brittle regime during accretion in subduction zone. Tectonic mélange is a characteristic rock in on-land accretionary complexes. The dominant deformation mechanism of tectonic mélange formation is pressure solution on the basis of microscopic observation. However, brittle slickenlines are also commonly observed on mélange foliations at the outcrop scale. Although the slickenlines as a brittle failure is common on the surface of the pressure solution foliation, the relationship of their kinetic are still uncertain. Detailed observations of slickenlines suggest that they are formed by reactivation of the mélange foliations, which indicates that the slickenlines are developed after formation of block in matrix texture characterized in mélange. In addition, mélange foliations are cut by faults related to underplating of oceanic materials. Therefore, formation of slickenlines occur before underplating in a relatively deep portion along subduction interface. On the basis of P-T conditions reported from other parts of the Cretaceous Shimanto Belt, the mélange formation and underplating is inferred to have occurred around the seismic front or within the seismogenic zone. The change in deformation mechanisms from pressure solution to brittle failure may be the first change in physical properties from plastic to brittle around seismic front.     

Isotopic evidence for channeled fluid flow in low-grade metamorphosed Jurassic accretionary complex in the Northern Chichibu belt, western Shikoku, Japan

The low‐grade metamorphosed Jurassic accretionary complex of the Northern Chichibu Belt, Hijikawa area, western Shikoku, is divided into two units, the Hijikawa and Kanogawa units, that are separated by an out‐of‐sequence thrust (OOST), the Ozu‐Kawabegawa Fault. The Kanogawa unit south of the Ozu‐Kawabegawa fault consists mainly of sandstone, shale, broken formation of alternating sandstone and shale, greenstone, chert, and pelitic melange, while the Hijikawa unit is characterized by a stack of subunits separated by small‐scale thrusts. The subunits are mainly made up of basic, pelitic and psammitic semischists, schistose pelitic melange, and chert. Petrological and mineralogical constraints suggest peak metamorphic conditions of 204–247 °C at 1–3 kbar in the Kanogawa unit, and 228–289 °C at 3–5.6 kbar in the Hijikawa unit. Quartz and quartz‐calcite veins are widely developed in the study area, especially in the Hijikawa unit. Regional variations in stable isotopic data show that the δ¹⁸O_quartz and δ¹⁸O_calcite values in veins tend to increase towards the Ozu‐Kawabegawa Fault. The δ¹⁸O_{whole rock} values are remarkably high in some subunits close to OOSTs within the Hijikawa unit. Oxygen isotopic compositions from vein quartz indicate that a higher δ¹⁸O fluid migrated upward from depth along the Ozu‐Kawabegawa Fault within the accretionary prism during prehnite‐pumpellyite facies metamorphism. The fluid source is inferred to be pelitic rocks at higher temperatures and greater depths within the accretionary prism.     

Tectonic incorporation of the upper part of oceanic crust to overriding plate of a convergent margin: An example from the Cretaceous–early Tertiary Mugi Mélange, the Shimanto Belt, Japan

A tectonic mélange exposed on land is examined to reveal relationships between mélange formation, underplating, and deformation mechanisms, focusing on the deformation of basaltic rocks. The studied Mugi Mélange of the Shimanto Belt is composed of a shale matrix surrounding various blocks of sandstone, pelagic sediments, and basalts. The mélange was formed during Late Cretaceous to early Tertiary times in a subduction zone under P–T conditions of 150–200 °C and 6–7 km depth as estimated from vitrinite reflectance and quartz veins fluid inclusions. The mélange represents a range of deformation mechanisms; pressure solution with micro-scale cataclasis in the shale matrix, brittle tension cracking in the blocks, and ubiquitous strong cataclasis in the basal portion of basaltic layers. The cataclastic deformation in the basalts suggests a breakage of a topographic high in the seismogenic depth.     

西南日本黒濑川带的解剖：晚二叠世和早侏罗世增生混杂岩的构造-地层学与浅变质作用

The Kurosegawa belt forms a relatively narrow terrane that can be traced continuously throughout southwest and central Japan. The major constituent continental fragments of the Kurosegawa belt include Early Paleozoic granitic rocks, high-grade metamor-phic rocks, Carboniferous metamorphic rocks (epi-dote-amphibolite facies), Triassic-Early Jurassic metamorphic rocks (pumpellyite-actinolite facies), serpentinites of unknown age, Silurian–Devonian volcanoclastic rocks intercalated with limestones, and Permian–Jurassic shallow marine sediments (e.g., Ichikawa et al., 1956; Maruyama et al., 1984; Faure, 1985; Yoshikura et al., 1990; Aitchison et al., 1991, 1996; Hada et al., 1992, 2001; Isozaki et al., 1992). These diverse rock suites are highly disrupted, form-ing lenticular bodies within the Late Permian accre-tionary complex (AC) which collectively are covered by younger (Cretaceous) marine to brackish water sediments (e.g., Aitchison et al., 1991; Isozaki et al., 1992). We characterize the tectono-stratigraphic ar-chitecture and low-grade metamorphism of the accre-tionary complex preserved in the Kurosegawa belt of the Kitagawa district in eastern Shikoku, Southwest Japan, in order to understand its internal structure, tectono-metamorphic evolution, and assessments of displacement of continental fragments within the complex.     

Low-grade metamorphism of the Mikabu and northern Chichibu belts in central Shikoku, SW Japan: implications for the areal extent of the Sanbagawa low-grade metamorphism

In central Shikoku, SW Japan, the Mikabu belt is bounded to the north by the Sanbagawa belt, and to the south by the northern (N) Chichibu belt. The N-Chichibu belt can be further subdivided into northern and southern parts. There is no apparent difference in the overall geology, structure, or fossil and radiometric ages between the Mikabu belt and the northern part of the N-Chichibu belt. Greenstones from the Mikabu belt and the northern part of the N-Chichibu belt show evidence for similar low-grade metamorphism, and include the following mineral assemblages with albite+chlorite in excess: metamorphic aragonite, sodic pyroxene+quartz, epidote+actinolite+pumpellyite, glaucophane+ pumpellyite+quartz, and lawsonite (not with actinolite or glaucophane). These similarities suggest that the Mikabu belt and the northern part of the N-Chichibu belt belong to the same geological unit (the MB-NNC complex). The mineral assemblages also indicate that the MB-NNC complex belongs to a different metamorphic facies from the low-grade part of the Sanbagawa belt, that is, the former represents lower temperature/higher pressure conditions than the latter. Structural and petrological continuity between the MB-NNC complex and Sanbagawa belt has not yet been confirmed, but both have similar radiometric ages. It is therefore most likely that the MB-NNC complex and Sanbagawa belt belong to the same subduction complex, and were metamorphosed under similar but distinct conditions. These two units were juxtaposed during exhumation. In contrast, the southern part of the N-Chichibu belt is distinct in lithology and structure, and includes no mineral assemblages diagnostic of the MB-NNC complex and the Sanbagawa belt. Thus, the southern part of the N-Chichibu belt may represent a different geological unit from the MB-NNC complex and Sanbagawa belt.     

Provenance of Jurassic accretionary complex: Mino terrane, inner zone of south-west Japan – implications for palaeogeography of eastern Asia

Provenance and tectonic history of the Jurassic accretionary complex, Mino terrane, located in the Inner Zone of south‐west Japan, were studied using sandstone framework composition and mudrock geochemistry. Modal analysis of sandstones shows that the tectonic setting of the source area for the studied Mino terrane clastic rocks was uplifted basement, largely dominated by high‐grade metamorphic terrain composed of quartz and feldspar, especially plagioclase. The textural and mineralogical immaturity, extent of alkali and alkaline earth element leaching, low chemical index of alteration values and depleted rare earth element (REE) contents suggest rapid uplift and erosion within the source terrain and a relatively weak weathering intensity. Factor analysis revealed that grain‐size effects governed compositional heterogeneity in the studied sediments. Provenance of the sediments is interpreted as being plagioclase‐enriched felsic basement rock, such as granodiorite, within a continental margin and evolved arc tectonic setting rather than active volcanic arc. Lack of a contribution from active volcanic arcs may have resulted from the cessation of volcanism during the reorganization of the subducting plate system and/or erosion of arc volcanics and exposure of basement. Considering the previous studies on palaeogeography and palaeocurrent reconstruction, the north‐eastern part of the Yeongnam massif in the Korean Peninsula is interpreted as the most probable source area for the studied turbidites. The results of mixing calculation for Mino terrane sediments suggest that Precambrian leucocratic granite and the basement rock of the Cretaceous Gyeongsang Basin shed large amounts of sediments to the Mino trench, whilst Precambrian granitic gneiss and the Triassic pluton supplied lesser amounts. The results of this study reveal that, although active subduction–accretion processes were occurring, the Mino trench was bordered by continental basement rocks. This knowledge contributes to enhanced understanding of the Jurassic palaeogeography of the east Asia continental margin.     

Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south-west Japan: evidence from 40Ar/39Ar whole-rock phyllite ages

Abstract Five whole-rock ⁴⁰Ar/³⁹Ar plateau ages from low-grade sectors of the Sambagawa belt (Besshi nappe complex) range between 87 and 97 Ma. Two whole-rock phyllite samples from the Mikabu greenstone belt record well-defined ⁴⁰Ar/³⁹Ar plateau ages of 96 and 98 Ma. Together these ages suggest that a high-pressure metamorphism occurred in both the Sambagawa and Mikabu belts at c. 90–100 Ma. The northern Chichibu sub-belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north ( c. 110 Ma) to south ( c. 215 Ma). The northern Chichibu sub-belt is correlated with the Kuma nappe complex (Sambagawa belt). Two whole-rock phyllite samples from the Kurosegawa terrane display markedly older metamorphic ages than either the Sambagawa or the Chichibu belts.
Accretion of Sambagawa-Chichibu protoliths began prior to the middle Jurrasic. Depositional ages decrease from middle Jurassic (Kuma-Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units. The Kurosegawa belt and the geological units to the south belong to distinctly different terrances than the Sambagawa-Chichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous.     

Flow kinematics in the deeper part of a subduction channel, as inferred from inclusion trails in plagioclase porphyroblasts from the Sambagawa metamorphic rocks, south-west Japan

The subduction and exhumation of accretionary prism metasedimentary rocks are accompanied by large‐strain ductile deformations which may be recorded in microstructures. Porphyroblast microstructures have been a key to unravel the kinematics in such deformed belts. Shape‐preferred orientation (SPO) of epidote and amphibole inclusions that define S‐shaped trails in prograde cores of plagioclase porphyroblasts were analysed from the high‐P/T Sambagawa metamorphic rocks. Inclusions are found to be elongate parallel to the [010] and [001] directions, respectively, and their long‐axis orientations define an internal foliation S_i (best‐fit great circle) and lineation L_i (maximum on the S_i). S‐shaped inclusion trails in the orthogonal sections do not exhibit the same geometries, but rather are grouped into two types, where the foliation intersection axes (FIAs) are nearly perpendicular and parallel to L_i, respectively. These two types of S‐shaped inclusion trails are seen in the sections inclined at low and high angles to the L_i, respectively. However, the latter type commonly consists of composite trails, where the S_i is first rotated about an FIA perpendicular to the L_i (i.e. unique axis), and then about an FIA parallel to the L_i. The S‐shaped inclusion trails are interpreted to have formed by the successive overgrowth of matrix minerals and rotation of the plagioclase porphyroblast cores about a unique axis in non‐coaxial deformation. The rotation of S_i about an FIA nearly parallel to the L_i is perhaps an apparent rotation, caused by the deflection of foliation around the growing prismatic plagioclase grain prior to inclusion into the porphyroblast. This study has for the first time documented the 3‐D geometry of S‐shaped inclusion trails in porphyroblasts from accretionary prism metasedimentary rocks and identified their origin, which helps to understand the flow kinematics in the deeper part of a subduction channel.     

The role of dewatering in the progressive deformation of a sandy accretionary wedge: constraints from direct imagings of fluid flow and void structure

Geological investigation of the deformation structures and sedimentary setting of the Emi Group, a Miocene sand-rich accretionary complex, central Japan, revealed a six stage-structural evolution during shallow level accretion in a subduction zone. The early deformation (stage 1) is characterized by independent particulate flow in layer parallel faults, scaly cleavages and web structures, and upward dewatering in dish-and-pillar structures and breccia injections, while later deformation (stages 2–6) involve mappable scale folding, meso- to macro-scopic thrusts and web structures with cataclastic flow. Based on microscopic analyses of these structures, the early faulting with independent particulate flow (stage 1 deformation) is associated with dilatancy and preferred orientation of void space, whereas the later faulting with cataclastic flow (stage 2 deformation) occurs with compaction and crude preferred orientation. The former features imply more permeable fluid migration pathways, supported by the permeability measurements and direct imaging of fluid flow by X-ray CT. On the other hand, the later fault zone has lower permeability and porosity than intact rock, and plays as fluid sealing. Thus, in the early stage (stages 1), fluid flow occurs as focused flow through dilatant fault zones with independent particulate flow or fluid migration by upward dewatering forming dish-and-pillar structures and breccia injections, whereas no evidence of fluid flow is recognized at the later stages (stages 2–6). Namely the fault zones focus fluid flow during primary accretion in shallow levels, and the fluid flow is strongly controlled by the deformation mechanism. Furthermore, the change of the deformation mechanism could be effected by progressive increment of the confining pressure, accompanied with accretion and lithification in the accretionary prism. In the shallow, dilatant-faulting regime where the deformation mechanism is independent particulate flow, focused flow dominates, whereas in the deep, cataclastic regime distributed flow may play a main conduit rather than the focused flow.     

Prograde pressure–temperature paths in the pelitic schists of the Sambagawa metamorphic belt, SW Japan

Prograde P–T paths recorded by the chemistry of minerals of subduction‐related metamorphic rocks allow inference of tectonic processes at convergent margins. This paper elucidates the changing P–T conditions during garnet growth in pelitic schists of the Sambagawa metamorphic belt, which is a subduction related metamorphic belt in the south‐western part of Japan. Three types of chemical zoning patterns were observed in garnet: Ca‐rich normal zoning, Ca‐poor normal zoning and intrasectoral zoning. Petrological studies indicate that normally‐zoned garnet grains grew keeping surface chemical equilibrium with the matrix, in the stable mineral assemblage of garnet + muscovite + chlorite + plagioclase + paragonite + epidote + quartz ± biotite. Pressure and temperature histories were inversely calculated from the normally‐zoned garnet in this assemblage, applying the differential thermodynamic method (Gibbs' method) with the latest available thermodynamic data set for minerals. The deduced P–T paths indicate slight increase of temperature with increasing pressure throughout garnet growth, having an average dP/dT of 0.4–0.5 GPa/100 °C. Garnet started growing at around 470 °C and 0.6 GPa to achieve the thermal and baric peak condition near the rim (520 °C, 0.9 GPa). The high‐temperature condition at relatively low pressure (for subduction related metamorphism) suggests that heating occurred before or simultaneously with subduction.     

Deformation and fluid flow during orogeny at the palaeo-Pacific active margin of Gondwana: the Early Palaeozoic Robertson Bay accretionary complex (north Victoria Land, Antarctica)

Structural investigations, integrated with X‐ray diffraction, fluid inclusion microthermometry and oxygen‐stable isotope analyses are used to reconstruct the deformation history and the palaeo‐fluid circulation during formation of the low‐grade, turbidite‐dominated Early Palaeozoic Robertson Bay accretionary complex of north Victoria Land (Antarctica). Evidence for progressive deformation is elucidated by analysing the textural fabric of chronologically distinct, thrust‐related quartz vein generations, incrementally developed during progressive shortening and thickening of the Robertson Bay accretionary complex. Our data attest that orogenic deformation was mainly controlled by dissolution–precipitation creep, modulated by stress‐ and strain‐rate‐dependent fluid pressure cycling, associated with local and regional permeability variations induced by the distribution and evolution of the fracture network during regional thrusting. Fracture‐related fluid pathways constituted efficient conduits for episodic fluid flow. The dominant migrating fluid was pre‐to‐syn‐folding and associated with the migration of warm (160–200 °C) nitrogen‐ and carbonic (CO₂ and CH₄)‐bearing fluids. Both fluid advection and diffusive mass transfer are recognized as operative mechanisms for fluid–rock interaction and vein formation during continuous shortening. In particular, fluid–rock interaction was the consequence of dissolution–precipitation creep assisted by tectonically driven cooling fluids moving through the rock section as a result of seismic pumping. The most likely source of the migrating fluids would be the frontal part of the growing accretionary complex, where fluids from the deep levels in the hinterland are driven trough channelization operated by the thrust‐related fracture (fault) systems.     

冷俯冲带深部的板块断裂过程——以日本中部侏罗纪燧石-碎屑岩杂岩体中的断裂带为例

近年来,深海大洋钻探已证实在洋陆俯冲带的浅部,板块边界断裂高度集中在特征性岩层中,然而对于这种断裂高度集中化现象是否发生在俯冲带深部仍然缺乏充分的认识,因此对日本中部侏罗纪燧石-碎屑岩杂岩体中一个露头宽度约50 m的断裂带进行了调查。该断裂带沿古俯冲带深部的板块边界延伸而出的逆冲断层发育,断裂带中的剪切变形集中在数层黑色有机质黏土岩中,表现为沿交织鳞片状叶理的分散剪切,剪切方向分为左行走滑和逆冲走滑,可能形成于俯冲带中的滑动分异或左行斜俯冲过程。体现滑动集中化的拆离面切割了黑色有机质黏土岩层,拆离面表现为左行滑动。利用碳物质拉曼光谱测量了拆离面上及黑色有机质黏土岩层中的碳物质。结果表明,相对于黑色有机质黏土岩,拆离面中的碳物质具有更高的成熟度,表明在左行滑动过程中产生了快速的摩擦升温。野外观测和实验结果表明,这一古断裂带记录的俯冲带深部板块边界断裂高度集中在黑色有机质黏土岩层中,发育于其中的拆离面的左行剪切可能产生于深部地震性滑动。     

Use of fracture filling mineral assemblages for characterizing water-rock interactions during exhumation of an accretionary complex: An example from the Shimanto Belt,southern Kyushu Japan

Various fracture filling minerals and secondary minerals in fracture walls were formed by fluid-rock interaction during the exhumation of the Palaeogene Shimanto Belt of Kyushu, Japan, which is located in an accretionary complex. Each mineral formed under favourable geological conditions and can be used to estimate the conditions of accretion and formation of the related rock sequences. Petrographic observations, mineralogical and geochemical analyses were made on fracture filling minerals and secondary minerals from boreholes of ca. 140 m depth, drilled in the Shimanto Belt. Results reveal that the secondary minerals were formed in three major stages distinguished by the sequential textural relationships of the minerals and the interpreted environment of mineral formation. Filling mineral assemblages show that the studied rock formation has been subducted to a depth of several km and the temperature reached was ca. 200–300 °C. After the subduction, the rock formation was uplifted and surface acidic water penetrated up to 80 m beneath the present ground surface. The acid water dissolved calcite fracture filling minerals to form the present groundwater flow-paths, which allowed recent wall rock alteration to occur. The results shown here imply that filling mineral assemblages can be an effective tool to evaluate the environmental changes during exhumation of an accretionary complex.     

Comparative characteristics of the Samarka (Sikhote-Alin) and Ultra-Tamba (Japan) terranes as grounds for correlating fragments of the Jurassic accretionary prism in two regions

Comparative data on tectono-stratigraphic complexes of the Ultra-Tamba terrane (Inner Zone of Japan) and upper structural level of the Samarka terrane in the Jurassic accretionary prism of Sikhote Alin are considered. Structural, lithological, petrographic data and age constraints characterizing rock associations of the terranes show that the latter are similar to a great extent, and consequently the Ultra-Tamba terrane can be regarded as an element of the Tamba-Mino-Ashio accretionary prism of the Jurassic but not Permian age, as it was thought earlier. The considered data substantiate confident structural correlation of both fragments of the Jurassic prism and of two regions in general.     

班公湖-怒江洋的扩张脊俯冲: 宗白增生杂岩中侏罗世辉长岩脉地球化学和Sr-Nd同位素特征

丁青蛇绿岩位于班公湖-怒江缝合带东段,被宗白增生杂岩分为东、西2个蛇绿岩体,面积分别约为400 km2和150 km2。宗白增生杂岩由异地体亚宗混杂岩和上部原地体陆缘碎屑沉积岩组成。亚宗混杂岩由低变质岩带、砾岩带、玄武质凝灰岩夹薄层泥硅质岩带和作为主要基质的泥页岩夹薄层杂砂岩组成,其中泥页岩基质被中侏罗世辉长岩脉侵入。辉长岩脉发育双侧冷凝边,走向90°~110°不等,主量、稀土和微量元素成分均介于富集型洋中脊玄武岩和洋岛玄武岩之间,结合εNd（t）=1.88~2.41和（87Sr/86Sr）t=0.70912 ~ 0.70919,指示宗白辉长岩岩浆为大洋岩石圈地幔底部地震波低速带（LVZ）顶部富集挥发分和不相容元素的洋岛玄武岩型熔体,与源自LVZ下部亏损软流圈地幔的正常洋中脊玄武岩型熔体混合的产物,形成于弧前扩张脊与俯冲带相互作用的板片窗环境。     

Anisotropy of magnetic susceptibility and paleomagnetic studies in relation to the tectonic evolution of the Miocene–Pleistocene accretionary sequence in the Boso and Miura Peninsulas, central Japan

Anisotropy of magnetic susceptibility (AMS) and paleomagnetic methods have been applied on the middle Miocene–Pleistocene sedimentary sequence in the Boso and Miura Peninsulas of central Japan in order to identify the invisible regional deformation sense as well as the intensity of deformation of sediments. The southern sequences of the two peninsulas were subjected to syn-sedimentary deformation of folding and faulting generated in compressional tectonics. A previous result of the AMS experiment on the sequences shows a development of a strong magnetic lineation. Thus, it is conceivable that the lineation had to be generated during the process of deformation, and in a direction perpendicular to the shortening. However, the orientation of the magnetic lineations is inconsistent among the different tectonic domains in the southern sequence. The paleomagnetic declination in each domain reveals a clockwise rotation in various degrees. Reconstructed directions of the magnetic lineations show a consistent pattern in the east–west direction, suggesting that the sedimentary sequence was subjected to a north-southward compression. In contrast, the compressive direction of the sediment cover on the Pliocene–Pleistocene sequence reveals a northwest direction. Our results suggest that the Philippine Sea Plate had been subducting northward during the middle Miocene–Pliocene, and changed its direction during the Pliocene.     

Progress of actinolite-forming reactions in mafic schists during retrograde metamorphism: an example from the Sanbagawa metamorphic belt in central Shikoku, Japan

Hydration reactions are direct evidence of fluid–rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite → hornblende → actinolite in the high‐grade zone, and winchite → actinolite in the low‐grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass‐balance relations suggest that: (1) the actinolite‐forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H₂O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of actinolite to total amphibole, Y_act, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Y_act < 0.5), but it increases drastically in the high‐grade part of the garnet zone (Y_act > 0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Y_act > 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H₂O and the surface area of hornblende. In contrast to high‐grade rocks, the winchite breakdown reaction is well advanced in the low‐grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low‐grade rocks may be due to small size of winchite before the reaction.     

Reply to comments by H. Mashima on ‘Evolution of the eastern margin of Korea: constraints on the opening of the East Sea (Japan Sea)’ by Kim et al. [Tectonophysics 436 (2007) 37–55]

We thank H. Mashima for his interest in our recent article in Tectonophysics [Kim, H.J., Lee, G.H., Jou, H.T., Cho, H.M., Yoo, H.S., Park, G.T., Kim, J.S., 2007, Evolution of the eastern margin of Korea: Constraints on the opening of the East Sea (Japan Sea). Tectonophysics 436, 37–55.] and welcome the opportunity to respond to his comments. In our article we suggested that the southern part of the East Sea (Japan Sea) opened principally in the southeast direction in response to the northwestward subduction of the Pacific Plate beneath the Japan Arc. In contrast, Mashima claims that the opening of the East Sea was achieved in the south–southeast direction. However, there are many crucial things in his comments that we find scientifically unconvincing and misleading. In this reply, we give a detailed response to his comments.     

The homogenization temperature in the fluid inclusions of Ordovician halite and paleoclimatic implication

正1 Introduction The homogenization temperature of fluid inclusions reflects the temperatures of the brines from which halite crystals grew.Therefore,it is a powerful mean to reveal the paleoclimate.Northern Shaanxi Salt Basin is located in the central and eastern of Ordos Basin.We have detail petrographical research and the homogenization     

Metamorphic record of the Asemi‐gawa eclogite unit in the Sanbagawa belt,southwest Japan: Constraints from inclusions study in garnet porphyroblasts

The Sanbagawa belt is one of the famous subduction‐related high‐pressure (HP) metamorphic belts in the world. However, spatial distributions of eclogite units in the belt have not yet satisfactorily established, except within the Besshi region, central Shikoku, southwest Japan because most eclogitic rocks were affected by lower pressure overprinting during exhumation. In order to better determine the areal distribution of the eclogite units and their metamorphic features, inclusion petrography of garnet porphyroblasts using a combination of electron probe microanalyser and Raman spectroscopy was applied to pelitic and mafic schists from the Asemi‐gawa region, central Shikoku. All pelitic schist samples are highly retrogressed, and include no index HP minerals such as jadeite, omphacite, paragonite, or glaucophane in the matrix. Garnet porphyroblasts in pelitic schists occur as subhedral or anhedral crystals, and show compositional zoning with irregular‐shaped inner segments and overgrown outer segments, the boundary of which is marked by discontinuous changes in spessartine. This feature suggests that a resorption process of the inner segment occurred prior to the formation of the outer segment, indicating discontinuous crystallization between the two segments. The inner segment of some composite‐zoned garnet grains displays Mn oscillations, implying infiltration of metamorphic fluid during the initial exhumation stage. Evidence for an early eclogite facies event was determined from mineral inclusions (e.g., jadeite, paragonite, glaucophane) in the garnet inner segments. Mafic schists include no index HP minerals in the matrix as with pelitic schists. Garnet grains in mafic schists show simple normal zoning, recording no discontinuous growth during crystal formation. There are no index HP mineral inclusions in the garnet, and thus no evidence suggesting eclogite facies conditions. Quartz inclusions in garnet of the pelitic and mafic schists show residual pressure values (?ω₁) of >8.5 cm^?1 and <8.5 cm^?1 respectively. The combination of Raman geobarometry and conventional thermodynamic calculations gives peak P–T conditions of 1.6–2.1 GPa at 460–520°C for the pelitic schists. The ?ω₁ values of quartz inclusions in mafic schists are converted to a metamorphic pressure of 1.2–1.4 GPa at 466–549°C based on Raman geothermometry results. These results indicate that a pressure gap definitely exists between the mafic schists and the almost adjacent pelitic schists, which have experienced a different metamorphic history. Furthermore, the peak P–T values of the Asemi‐gawa eclogite unit are compatible with those of Sanbagawa eclogite unit in the Besshi region of central Shikoku, suggesting that these eclogite units share a similar P–T trajectory. The Asemi‐gawa eclogite unit exists in a limited area and is composed of mostly pelitic schists. We infer that these abundant pelitic schists played a key role in buoyancy‐driven exhumation by reducing bulk rock density and strength.