Ryoke Belt Research Articles

領家帯に記録された東アジア東縁の白亜紀イベント

In the Ryoke belt formed at the eastern margin of East Asia during mid-Creataceous time, the most important geological record is that of felsic igneous activity, which may have been caused by the subduction of a young (hot) oceanic lithosphere. The mantle-derived arc magmas formed due to subduction of the hot lithosphere underplated the lower crust. Continuous underplating of the mantle-derived magmas resulted in partial melting of the lower crustal mafic to intermediate rocks to produce granite magmas. The granite magmas rose and were emplaced at a mid-crustal depth. They caused thermal perturbation around rocks of the Jurassic accretionary complex (Mino–Tamba belt) and formed the resultant low-pressure/high-temperature regional contact metamorphism (Ryoke metamorphism). At the time of Ryoke metamorphism, during subduction of a hot oceanic lithosphere with high obliquity, a low-angle detachment fault formed at the forearc region of the mid-Cretaceous SW Japan arc. Following this event, subduction of an inactive oceanic ridge or a very hot oceanic lithosphere with low obliquity occurred; a compressional stress regime occurred at the forearc region, forming granite mylonites and large-scale upright folds associated with crustal displacement along the detachment fault. After this event, the angle of subduction decreased gradually; the location of the felsic igneous activity moved toward the continent.

20万分の1地質図幅「伊勢」と領家帯細粒苦鉄質岩

20万分の1地質図幅「伊勢」と領家帯細粒苦鉄質岩

Simultaneous measurements of compressional wave and shear wave velocities, Poisson's ratio, and Vp/Vs under deep crustal pressure and temperature conditions: Example of silicified pelitic schist from Ryoke Belt, Southwest Japan

AbstractThe ultrasonic technique for measuring travel times of compressional and shear waves using dual‐mode transducers was adapted to a piston cylinder apparatus, allowing simultaneous measurements of travel times of compressional and shear waves of island arc samples under the high pressure and temperature conditions of island arcs. This method enables us to determine elastic properties and their pressure and temperature derivatives simultaneously. Furthermore, Vp/Vs can be directly determined from travel times of compressional and shear waves independently of length change due to compression or thermal expansion of rock samples under deep crustal conditions, providing more accurate Vp/Vs values than those determined from individual measurements of travel times of both elastic wave types using single‐mode transducers. Experimental techniques and results are demonstrated using data on silicified pelitic schist from the Ryoke Belt to 0.6 GPa. The simultaneous measurement gives Vp = 5.60 km/s, ∂Vp/∂P = 0.090 (km/s)/GPa, Vs = 3.37 km/s, ∂Vs/∂P = 0.05 (km/s)/GPa, σ = 0.216, and Vp/Vs = 1.66 at ambient conditions. The temperature derivatives were constrained from fitting using linear functions of temperature, yielding ∂Vp/∂T = −0.518 × 10−3 (km/s)/K and ∂Vs/∂T = −0.182 × 10−3 (km/s)/K. Performing simultaneous measurements of travel times of compressional and shear waves using dual‐mode transducers, it is possible to accurately determine Vp/Vs and Poisson's ratio of crustal minerals and rocks at deep crustal conditions to study the composition of the crustal interior, e.g. rock types and fluids below the hypocentral region of earthquakes or around bright spots.

三波川変成帯中の新たな独立した広域変成帯の存在―白亜紀から第三紀の日本における造山運動―

The Sanbagawa metamorphic belt in SW Japan was previously considered to extend in the E-W direction from the Kanto Mountains to Kyushu Island, a distance > 800 km. However, Aoki et al. (2007) recently demonstrated that protoliths of metamorphic rocks in the Oboke area of the belt in central Shikoku accumulated at the trench after ca. 90-80 Ma. Furthermore, Aoki et al. (2008) showed that these rocks suffered blueschist metamorphism at 66-61 Ma, which differs from the timing of the Sanbagawa metamorphism. Thus, these results show that the Sanbagawa belt in Shikoku is a composite metamorphic belt. We, therefore, redefine the traditional Sanbagawa belt; the structurally upper part is the Sanbagawa metamorphic belt (sensu stricto). It formed as an accretionary complex at ca. 140-130 Ma and subsequently experienced BS-EC facies metamorphism at ca. 120-110 Ma (Okamoto et al., 2004). By contrast, the structurally lower segment termed the Shimanto BS facies metamorphic belt, formed as an accretionary complex after ca. 90-80 Ma and experienced peak metamorphism at ca. 60 Ma. Our observations have important implications for the lateral extension of these two metamorphic belts in SW Japan. The accretionary ages of the traditional Sanbagawa belt in the Kanto Mountains are younger than the Sanbagawa peak metamorphic age (Tsutsumi et al., 2009), clearly indicating that the entire region of Kanto Mountains Sanbagawa must belong to the Shimanto metamorphic belt. The same timing relationships were also found for the Sanbagawa belt on Kii Peninsula (Otoh et al., 2010). These results, therefore, indicate that the Shimanto metamorphic belt is exposed in Shikoku, Kii, and Kanto, thus the spatial distribution of Sanbagawa belt (ss) is less than half of its previous extent. The metamorphic grade of the Kanto Mountains in the Shimanto metamorphic belt ranges from pumpellyite-actinolite facies to epidote-amphibolite facies. Therefore, the higher-grade rocks of the Shimanto metamorphic rocks are exposed in the Kanto Mountains in comparison with Shikoku and Kii Peninsula. Hence, these two distinct BS-EA-EC (?) metamorphic belts are virtually equivalent in terms of spatial distribution, metamorphic range of grade, and facies series. Pacific-type orogenic belts typically comprise accretionary complex, high-P/T metamorphic belt, fore-arc sediments, and batholith belt landward from the trench (Maruyama et al., 1996). In SW Japan, the Sanbagawa belt (ss) is paired with the Ryoke low-P/T metamorphic belt and with the ca. 120-70 Ma Sanyo TTG batholith belt. Furthermore the related fore-arc basin may have developed penecontemporaneously with the Shimanto BS-EA orogeny, which is paired with the late Cretaceous to early Tertiary San-in TTG belt, which extending along the Japan Sea coast. In-between the intervening Izumi Group, a fore-arc basin deposit formed during the Campanian to Maastrichtian. Thus, these two groups of orogenic units, which formed during independent orogenies were both extensively modified during the opening of the Japan Sea ca. 20 Ma. The southward thrusting of the Ryoke and Cretaceous TTG belts over the Sanbagawa extended beyond the southern limit of the Sanbagawa, leading the up-down relationship of the Sanbagawa (ss) and the Ryoke belts.

Comparison of petrofabrics with composite magnetic fabrics of S–C mylonite in paramagnetic granite

An anisotropy of magnetic susceptibility (AMS) analysis was conducted for a typical S–C mylonite in a small-scale ductile shear zone derived from Late Cretaceous magnetite- free granite in the Ryoke Belt, Teshima Island, southwest Japan. In such mylonites, paramagnetic minerals such as biotite and hornblende, which define foliations, are assumed to control the AMS. Accordingly, we attempted to measure the orientations of both minerals to correlate the S–C–C′ fabric to the AMS using techniques of microscopic and back-scattered electron (BSE) image analyses. A measured magnetic fabric of the S–C mylonite that is expressed by a K min normal plane ( K max– K int plane) approximated S foliation, K max orientation coincided well with the orientations of mean resultant vectors for long axes of biotite and minor hornblende, whereas K max did not coincide well with those of the mean resultant vectors for long axes of the brighter domain (aggregates of biotite and minor hornblende) from the binary BSE image. These results show that the magnetic fabric fairly reflects the shape preferred orientation of individual grains of paramagnetic monoclinic minerals, especially biotite, which forms the S–C–C′ fabric.

Initial 87Sr/86Sr ratio heterogeneity in Kamihara Tonalite, Ryoke belt, southwest Japan: Evidence from strontium isotopic analysis of apatite

Strontium isotopic analysis was carried out on magmatic apatite in the Kamihara Tonalite obtained from the Cretaceous Ryoke belt, southwestern Japan. 87Sr/86Sr isotopic compositions of euhedral apatite indicated that the initial 87Sr/86Sr ratio (SrI) in the tonalite varies from 0.70695 to 0.70790. This provides direct evidence of SrI heterogeneity in the Kamihara Tonalite. The SrI heterogeneity is inherited to Rb-Sr whole-rock age determination, and makes the age discrepancy between the Rb-Sr age and CHIME, K-Ar ages. By combining the results of the isotopic analysis, knowledge on the whole-rock chemistry, and field observations, it can be inferred that the SrI heterogeneity in the Kamihara Tonalite was caused by the assimilation of high SrI country metamorphic rocks of the Ryoke belt.

Decompressional microstructure from garnet-bearing mafic granulite in Yanai district, Ryoke belt, Southwest Japan

Newly found garnet-bearing mafic granulite in Yanai district, Ryoke belt occurs as blocks in gneissose tonalite. This granulite consists mainly of brown hornblende, orthopyroxene and plagioclase, with trace amounts of garnet, biotite and quartz. Garnet porphyroblast up to 3 cm in diameter is locally contained, and is surrounded by orthopyroxene and plagioclase symplectites that coexist with quartz. On the basis of microstructure and mineral assemblage in the garnet-bearing mafic granulite, it reveals that the reaction of garnet + quartz→orthopyroxene + plagioclase occurred at a decompressional process under the granulite facies condition.

Ductile deformation and development of andalusite microstructures in the Hongusan area: Constraints on the metamorphism and tectonics of the Ryoke Belt

Abstract The main mesoscopic features of the ductile deformation in the Hongusan region of the Ryoke Belt can be explained in terms of a single deformation phase, Dm. The associated stretching lineation is roughly east–west and has an associated top‐to‐the‐west sense of shear. This information fills an important gap in our knowledge about the deformational history of the Ryoke metamorphic belt. Andalusite porphyroblasts are well‐developed in the Hongusan area and their microstructure can be used to link the deformational history with the metamorphism. However, the andalusite porphyroblasts show both intertectonic and post‐tectonic microstructures. Some porphyroblasts have an intertectonic core and post‐tectonic rim. Analysis of associated metamorphic minerals and bulk‐rock chemistry suggests that the distinct types of microstructure can be explained in terms of distinct formation reactions related to regional and contact metamorphism. The intertectonic microstructure shows the presence of a deformation pre‐dating Dm, referred to as Dc. These two deformation stages are sufficient to account for all the main deformation features. The distribution of post‐tectonic andalusite growth shows a very broad thermal effect around an intrusion in the area. This can help place constraints on the thermal history of the Ryoke metamorphic belt.

四国西部・領家帯花崗岩類のRb-Sr全岩アイソクロン年代とU-Pbジルコン年代

四国西部・領家帯花崗岩類のRb-Sr全岩アイソクロン年代とU-Pbジルコン年代

Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan

Research Article| July 01, 2006 Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan Miharu Otani; Miharu Otani 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Search for other works by this author on: GSW Google Scholar Simon Wallis Simon Wallis 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Miharu Otani 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Simon Wallis 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Publisher: Geological Society of America Received: 14 Nov 2005 Revision Received: 13 Feb 2006 Accepted: 03 Apr 2006 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2006) 34 (7): 561–564. https://doi.org/10.1130/G22430.1 Article history Received: 14 Nov 2005 Revision Received: 13 Feb 2006 Accepted: 03 Apr 2006 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Miharu Otani, Simon Wallis; Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan. Geology 2006;; 34 (7): 561–564. doi: https://doi.org/10.1130/G22430.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Quartz c-axis lattice preferred orientation (LPO) patterns have been widely used to analyze different aspects of mid-crustal ductile deformation. However, despite the common presence of annealed microstructures in high-grade metamorphic belts, the effects of static recrystallization on such quartz LPOs are unclear. A suite of samples of quartz schist from the Nukata area of the Ryoke metamorphic belt was collected along profiles of several kilometers that reach the postorogenic Shinshiro tonalite. The samples show an increase in grain size and changes in microstructure, such as straightening of grain boundaries and formation of quartz-in-quartz inclusions that represent annealing related to contact metamorphism. In contrast to the clear microstructural changes, quartz c-axis LPO patterns show only very minor modifications even after strong annealing. This result implies that quartz LPO patterns should not be obliterated by static recrystallization and quartz-rich tectonites that lack such LPO patterns deformed by processes other than dislocation creep. Likely processes in high-grade rocks are grain boundary sliding assisted by melt migration and diffusion creep, both of which are characterized by Newtonian rheologies. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Provenance of Paleozoic–Mesozoic sedimentary rocks in the Inner Zone of Southwest Japan: An evaluation based on Nd model ages

Nd model ages using depleted mantle (TDM) values for the sedimentary rocks in the Inner Zone of the SW Japan and western area of Tanakura Tectonic Line in the NE Japan allow classification into five categories: 2.6–2.45, 2.3–2.05, 1.9–1.55, 1.45–1.25, and 1.2–0.85 Ga. The provenance of each terrane/belt/district is interpreted on the basis of the TDMs, 147Sm / 144Nd vs. 143Nd / 144Nd relation, Nd isotopic evolution of the source rocks in East China and U–Pb zircon ages. The provenance of 2.6–1.8 Ga rocks, which are reported from Hida–Oki and Renge belts and Kamiaso conglomerates, is inferred to be the Sino–Korean Craton (SKC). The 2.3–1.55 Ga rocks, mostly from Ryoke, Mino and Ashio belts, are originally related with the SKC and/or Yangtze Craton (YC). The provenances of the sedimentary rocks with 1.45–0.85 Ga, from the Suo belt, Higo and some districts in the Mino and Ashio belts, are different from the SKC and YC. Especially, the Higo with 1.2–0.85 Ga is considered as a fragment of collision zone in East China. Akiyoshi belt probably belongs to the youngest age category of 1.2–0.85 Ga. Some metasedimentary rocks from the Ryoke belt have extremely high 147Sm / 144Nd and 143Nd / 144Nd ratios, whose main components are probably derived from mafic igneous rocks within the Ryoke belt itself and from the adjacent Tamba belt.

Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan

Quartz c-axis lattice preferred orientation (LPO) patterns have been widely used to analyze different aspects of mid-crustal ductile deformation. However, despite the common presence of annealed microstructures in high-grade metamorphic belts, the effects of static recrystallization on such quartz LPOs are unclear. A suite of samples of quartz schist from the Nukata area of the Ryoke metamorphic belt was collected along profiles of several kilometers that reach the postorogenic Shinshiro tonalite. The samples show an increase in grain size and changes in microstructure, such as straightening of grain boundaries and formation of quartz-in-quartz inclusions that represent annealing related to contact metamorphism. In contrast to the clear microstructural changes, quartz c-axis LPO patterns show only very minor modifications even after strong annealing. This result implies that quartz LPO patterns should not be obliterated by static recrystallization and quartz-rich tectonites that lack such LPO patterns deformed by processes other than dislocation creep. Likely processes in high-grade rocks are grain boundary sliding assisted by melt migration and diffusion creep, both of which are characterized by Newtonian rheologies.

中部領家帯に分布する領家花崗岩類の巨視き裂構造と弾性波速度との関係

Based on geological field survey on granitoids distributed in the Chubu Ryoke Belt, orientation and occurrence of macroscopic (outcrop scale) fractures were analyzed to characterize the anisotropic fracture network system in the study area. Variation of P-wave velocity was measured to identify preferred orientation of weak planes developed in the granitic rocks. In northeast part of the Inagawa granite, parallelepiped block structure composed from fractures with three preferred orientation was observed. The structure is inferred to be derived from tensile stress field in the granitic body. On the contrary, in southwest part of the Inagawa granite and the old Ryoke granitic rocks, complex sheared fractures were dominant. Predominant orientations of those macroscopic fractures were close to the directions of major and representative active fault systems of the Chubu Ryoke area such as the Median Tectonic Line (NE -SW strike) and the Atera fault (NW-SE strike).P-wave velocity measurement revealed that anisotropy of the P-wave velocity was closely related to the distribution of parallelepiped fractures, however, shear fractures had no clear relation to variation of the P-wave velocity.P-wave velocity measurement has an advantage on prediction of major orientations of parallelepiped joint system developed in granitic rocks, however in the case of highly stressed rock mass, better approximation on preferred orientations of fractures is likely to be achieved with consideration for occurrence of major faults and tectonic stress regime distributed in the area.

The use of apatite as a record of initial 87Sr / 86Sr ratios and indicator of magma processes in the Inagawa pluton, Ryoke belt, Japan

Sr isotopic analysis of apatite in the extensive Inagawa pluton of southwestern Japan reveals the spatial distribution of initial 87Sr / 86Sr (SrI) within this body with an unprecedented resolution. While the SrI values in older units show the largest variation (0.7093–0.7107), the younger ones have relatively homogeneous SrI values around 0.7096. The older units also contain numerous mafic microgranular enclaves, which are absent in the younger units. Combining these results with bulk rock analyses and field observations suggests the wide variation in petrology and bulk chemistry in the Inagawa pluton is not solely due to fractional crystallization as previously thought. Mixing between mafic and felsic melts has also played a significant role. The lack of enclaves and homogeneous SrI values in the younger units suggest that mixing was complete by this time.

Mafic rocks from the Ryoke Belt, southwest Japan: implications for Cretaceous Ryoke/San-yo granitic magma genesis

ABSTRACTMafic rocks in the Ryoke belt, the Cretaceous granitic province in Southwest Japan, occur in two modes: (1) as mafic dykes and pillow-shaped enclaves, and (2) as isolated kilometresized bodies of gabbroic cumulate. The dykes and pillows have fine-grained textures with thin radiating plagioclase laths, indicative of quenching. The gabbroic cumulates are predominantly coarse-grained and commonly lithologically layered.SHRIMP zircon U-Pb ages of both types of mafic rocks are in the range 71–86 Ma, late Cretaceous. The mafic rocks become younger eastwards, matching the along-arc age trend of the associated Cretaceous granites (Nakajima et al. 1990). Both types of mafic rocks were apparently generated during the same magmatic event that produced the Ryoke/San-yo granites. The mafic dykes and pillows are aphyric basaltic-andesites to andesites (SiO2 54–60 wt.%), with microphenocrysts of biotite and hornblende. They have a composition which is similar to mafic rocks from the northern Sierra Nevada, and also to medium-K calc-alkaline rocks from present-day arc volcanics. The gabbroic cumulates are mostly pyroxene-hornblende gabbros (SiO2 43–52 wt.%). Their bulk-rock chemical compositions are mostly unlike any magma compositions.Both types of mafic rocks from the Ryoke belt have relatively high 87Sr/86Sr initial ratios (SrI), 0·7071–0·7097, which are similar to those of the associated granites. The granites were formed either by fractional crystallisation of the mafic magmas, or by partial melting of newly formed mafic rocks at depth. The high SrI indicates that the mafic magmas were derived from enriched mantle or mixed with enriched crustal materials. Even if the mixing occurred between primitive basaltic magma and metasedimentary rocks, then the basaltic andesite–andesite magmas must have contained more than 60% mantle-derived components. The Cretaceous magmatism in Southwest Japan represents a major episode of crustal growth by additions from the upper mantle in an arc setting.

Mafic rocks from the Ryoke Belt, southwest Japan: implications for Cretaceous Ryoke/San-yo granitic magma genesis

Mafic rocks from the Ryoke Belt, southwest Japan: implications for Cretaceous Ryoke/San-yo granitic magma genesis

Nd-isotopic composition of Phanerozoic sediments in the Inner Zone of Southwest Japan Arc : implications on provenance characteristics and contribution to formation of mature island arc system

The Nd-isotopic data on sedimentary and metamorphic rocks of SW Japan Arc allow their discrimination into five different depleted mantle (TDM) model age clusters, 2.6-2.45 Ga, 2.3-2.05 Ga, 1.9-1.55 Ga, 1.45-1.25 Ga, 1.2-0.85 Ga. The 2.6-2.45 Ga and 1.9-1.55 Ga model ages are also coincident with U-Pb inherited zircon ages of the above two epochs as well as the major magmatic activity in the Sino-Korean Craton (SKC). The 2.3-2.05 Ga model ages can be considered as the initial formation ages for the precursors of sedimentary rocks. The Nd-isotopic data suggest that the Hida Belt was most likely formed as a part of the SKC. The mantle underlying the Ryoke Belt had continental lithospheric signature during Triassic-Jurassic period. The 1.9-1.55 Ga model ages, especially 1.8 Ga~, can be associated with the formation of this belt. The source material for the sedimentary rocks occurring in the accretionary terrane of northeastern areas in the SW Japan Arc was probably in and around the SKC of the Ryoke Belt itself. The sedimentary rocks occurring in southwestern areas of the Arc were mainly composed of materials derived from a relatively younger source (1.45-0.85 Ga). (Communicated by Syun-iti AKIMOTO, M.J.A., Jan. 13, 2004)

Competence contrast between pelitic schist and silicified pelitic schist in the Iwakuni—Yanai area of the Ryoke Belt, southwest Japan.

Pelitic metamorphic rocks of the Ryoke Belt are distributed in the Rokuroshi area, southern part of Iwakuni district. Dark-brown biotite schist was locally silicified and decolorized to form milky-white “silicified domain”. Quartz veins were developed in both of the biotite schist and silicified domain. The veins in biotite schist are generally parallel to the schistosity and form boudinage due to ductile flow of the host rock. The veins in silicified domain are oblique to the schistosity with medium to high angles and have not undergone ductile deformation except for some schistosity-parallel veins. The mode of occurrence of these veins indicates that the silicified domain is much more competent than the biotite schist. Ductile deformation after the silicification was accommodated by viscous flow of biotite schist. The silicification probably results from dissolution-precipitation processes which may have raised pore pressure to cause hydraulic fracturing.

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, % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! <![CDATA[% MathType!MTEF!2!1!+- % feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeiCaiaabM % hacaqGYbGaae4BaiaabchacaqGLbGaaGjbVlabgUcaRiaaysW7caqG % NbGaaeOCaiaab+gacaqGZbGaae4CaiaabwhacaqGSbGaaeyyaiaabk % hacaaMe8Uaey4kaSIaaGjbVlaabwgacaqGHbGaae4CaiaabshacaqG % VbGaaeOBaiaabMgacaqG0bGaaeyzaiaaysW7cqGHRaWkcaaMe8Uaae % yCaiaabwhacaqGHbGaaeOCaiaabshacaqG6bGaaGjbVlabg2da9iaa % ysW7caqGWbGaaeiAaiaabYgacaqGVbGaae4zaiaab+gacaqGWbGaae % yAaiaabshacaqGLbGaaGjbVlabgUcaRiaaysW7caqGHbGaaeOBaiaa % b+gacaqGYbGaaeiDaiaabIgacaqGPbGaaeiDaiaabwgaaaa!780C! $$ {\text{pyrope}}\; + \;{\text{grossular}}\; + \;{\text{eastonite}}\; + \;{\text{quartz}}\; = \;{\text{phlogopite}}\; + \;{\text{anorthite}} $$ Pressure and pressure gradient increased with increasing temperature such that pressures of high-grade areas exceeded that of the triple point of aluminosilicates. The thermobaric structure of the study area shows that pressure increased up to 5 kbar with southward increase in metamorphic temperature up to the highest-grade area, the garnet-cordierite zone. Further south, pressure was almost the same as that of the garnet-cordierite zone and temperature decreased. This asymmetric distribution of metamorphic conditions on both sides of the garnet-cordierite zone can explain the asymmetric distribution of metamorphic zones; the K-feldspar-cordierite zone and sillimanite-K-feldspar zone on the north and south sides of the garnet-cordierite zone, respectively. The breakdown reaction of muscovite and quartz defines the beginning of both the K-feldspar-cordierite zone and sillimanite-K-feldspar zone, which took place under low and high pressures, respectively. These thermobaric structures suggest that temperature varied laterally at mid-crustal level during the peak of metamorphism.

Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U–Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts

In this paper we present new U–Pb zircon ages determined with Sensitive High-Resolution Ion MicroProbe (SHRIMP) for nine plutonic rocks or orthogneisses and one paragneiss from the Higo and Maana belts (here referred to as the paleo-Ryoke belt) in southwest Japan, and the South Kitakami belt in northeast Japan. Both belts are Paleozoic–Mesozoic continental terranes discontinuously distributed along the Median Tectonic Line (MTL), and structurally lying on the Sambagawa belt in the Japanese Islands. Three groups of U–Pb zircon ages showing the timing of different magmatic events were determined for the plutonic rock or orthogneiss samples: ca. 500 Ma (Early Ordovician, two samples), a mean age of 292.0±12.4 Ma (Late Carboniferous–Early Permian, one sample) and ca. 110 Ma (Early Cretaceous, six samples). Furthermore, zircons with core–rim microstructures from a garnet–biotite–cordierite paragneiss in the Higo belt were also dated with SHRIMP. The cores yielded 2155–184 Ma inherited U–Pb ages (mostly varying between 330 and 184 Ma), while the rims yielded a mean age of 116.5±18.7 Ma (Early Cretaceous) U–Pb recrystallization ages comparable with the igneous ages, limiting the sedimentary age of the protolith to between Early–Middle Jurassic (ca. 180 Ma) and Early Cretaceous time. These SHRIMP U–Pb ages of plutonism, metamorphism and sedimentation, together with previously reported radiometric ages, have revealed a strong similarity in the tectonic histories of the paleo-Ryoke and South Kitakami and Abukuma belts, suggesting that these belts can be correlated. At present the paleo-Ryoke belt and the South Kitakami and Abukuma belts are separated by more than 1000 km. However, these belts may have originally comprised a continuous continental terrane that was later displaced and juxtaposed, together with the underlying Sambagawa belt, against the Ryoke belt by large-scale sinistral faulting along the MTL and associated major strike–slip faults during the Latest Cretaceous.