Ryoke Metamorphic Research Articles

Recognition of Detrital Carbonaceous Material in the Ryoke Metamorphic Belt by Using Raman Thermometry: Implications for Thermal Structure and Detrital Origin

Abstract Raman spectroscopy of carbonaceous material (RSCM) thermometry is an effective method to determine the thermal structure of metasedimentary domains with high spatial resolution. The mean value of numerous measurements in a single sample is used for temperature estimation. A potential complicating factor is the presence of detrital carbonaceous material (CM) that can introduce bias in the results. Detrital CM generally shows a high degree of crystallinity making it difficult to discriminate detrital and metamorphic CM in high-grade metamorphic domains. Application of the RSCM thermometry to 100 metamudstone samples from the Hongusan area of the Ryoke metamorphic belt reveals multimodal degrees of crystallinity of CM in many samples indicating the common presence of detrital CM grains. Application of a new procedure to filter out the effects of detrital CM reveals a thermal structure that can be explained as the result of the overprinting of two distinct thermal regimes related to regional and subsequent contact metamorphism. The ability to recognize the presence of detrital CM and measure its crystallinity can be used to infer the metamorphic grade present in the hinterland from which the detrital material is derived. A combination of this information on metamorphic temperature with geochemical and radiometric age data reported in the literature indicates that the detrital CM was derived from basement rocks of the Korean Peninsula.

Late Cretaceous uplift history of the Cretaceous volcanic arc in Southwest Japan: Provenance analysis of the Yuasa–Aridagawa basin based on U–Pb zircon ages

AbstractThe Ryoke Metamorphic complex has undergone low‐P/T metamorphism and was intruded by granitic magmas around 100 Ma. Subsequently, the belt was uplifted and exposed by the time deposition of the Izumi Group began. The tectonic history of uplift, such as the timing and processes, are poorly known despite being important for understanding the spatiotemporal evolution of the Ryoke Metamorphic Belt. U–Pb zircon ages from sedimentary rocks in the forearc and backarc basins are useful for constraining uplift and magmatism in the provenance. U–Pb dating of detrital zircons from 12 samples (four sandstones and eight granitic clasts) in the Yuasa–Aridagawa basin, a Cretaceous forearc basin in the Chichibu Belt of Southwest Japan, gave mostly ages of 60–110 Ma. Granitic clasts contained in conglomerate suggest that granitic intrusions predate the formation of Coniacian and Maastrichtian conglomerate. Emplacement ages of granitic bodies originated from granitic clasts in Coniacian conglomerate are (110.2 ±1.3) Ma, (106.1 ±1.8) Ma, (101.8+5.8–3.8) Ma, and (95.3 ±1.4) Ma; for granitic clasts in Maastrichtian conglomerate, (89.6 ±1.8) Ma, (87.3+2.4–1.8) Ma, (85.7 ±1.2) Ma, and (82.7 ±1.2) Ma. The results suggest that detrital zircons in the sandstones were mainly derived from volcanic eruptions contemporaneous with depositional age, and plutonic rocks of the Ryoke Metamorphic Belt. Zircon ages of the granitic clast samples also indicate that uplift in the provenance began after Albian and occurred at least during the Coniacian to Maastrichtian. Our results, together with the difference of provenance between backarc and forearc basins suggest that the southern marginal zone of the Ryoke Metamorphic Belt was uplifted and supplied a large amount of clastic materials to the forearc basins during the Late Cretaceous.

三郡・三波川・領家各変成帯相互の関係

三郡・三波川・領家各変成帯相互の関係

Metamorphic condition of a regional metamorphic complex in the Omuta district in northern Kyushu, southwest Japan

AbstractA high‐temperature (T) metamorphic complex occurs in the Omuta district, northern Kyushu, Japan. Three metamorphic zones are defined based on pelitic mineral assemblage, i.e. chlorite–biotite zone, muscovite–andalusite zone and sillimanite–K‐feldspar zone with ascending metamorphic grade from north to south. Two isograds trend approximately east–west, which is oblique to the boundary between the metamorphic complex and the Tamana Granodiorite located on the southeast. The metamorphic condition of two pelitic rocks that occur in the muscovite–andalusite zone and sillimanite–K‐feldspar zone are estimated as 510 ±30 °C, 300 ±60 MPa and 720 ±30 °C, 620 ±60 MPa, respectively. Thermodynamic consideration reveals that use of the same geothermobarometer enables precise determination of the difference in pressure between the samples as 320 ±10 MPa. This indicates that the pelitic samples were metamorphosed at different depth by 11–12 km that is significantly larger than the geographic distance of 6.8 km between the sample localities. This also suggests that crustal thinning took place after the high‐T metamorphism. The high‐T metamorphic complex is, therefore, not of static contact metamorphism but of dynamic regional metamorphism. The present result combined with petrological and chronological similarities implies that this complex suffered the regional Ryoke metamorphism.

伊那領家変成帯,日曽利花崗岩の岩石化学的特徴 : 領家変成帯におけるアダカイト質花崗岩( 日本列島における白亜紀アダカイト質花崗岩類の起源)

伊那領家変成帯,日曽利花崗岩の岩石化学的特徴 : 領家変成帯におけるアダカイト質花崗岩( 日本列島における白亜紀アダカイト質花崗岩類の起源)

岩石学における非平衡過程の速度論的研究

Non-equilibrium textures in metamorphic rocks contain a great deal of information on reaction kinetics and mass transfer in metamorphic processes. The reaction zone between chemically incompatible minerals (or rocks) is a good example of such non-equilibrium textures, and hence it is worth studying it in detail. Korzhinskii's chemical potential diagram is a useful method to understand the zonal sequence in the reaction zone. It can explain the variability in the zonal sequence by specifying the corresponding diffusion path on the chemical potential diagram. However, the method cannot tell us why the specific diffusion path is realized in nature. Fisher-Joesten's steady diffusion model can be considered as a quantification of Korzhinskii's chemical potential diagram, which introduces flux-ratio equations corresponding to specification of the diffusion path on the chemical potential diagram. The steady diffusion model successfully describes the stability fields of zonal sequences in the L-ratios (phenomenological coefficient ratios) diagram. Application of the steady diffusion model to an olivine-plagiocalse corona in a norite from the Ryoke Metamorphic Rock is presented. Extension of the steady diffusion model to open system is also discussed for the case of a metabasite-marble reaction zone in the Hirao Limestone and for myrmekites from the Okueyama granite. Experimental studies of the formation of reaction zones in the system dolomite-quartz-H2O reveal temporal change in zonal sequence and mineral species of the reaction zones, showing a transition from an initial transient state to a steady state. Finally, I discuss periodic structures in rocks representing a pattern formation in a mineral-fluid system, taking an orbicular marble from the Hirao Limestone as an example. Although our understanding on such a ‘nonlinear structure’ is insufficient, I believe that there are some unknown physico-chemical processes behind such a structure.

領家帯柳井地域新期花崗岩(木部花崗岩)のマグマ過程

Cretaceous granitic rocks in the Chugoku district, Southwest Japan arc, are divided into the following three groups; Older Ryoke granitoids, Younger Ryoke granitoids and San'yo granites. The Kibe Granite, a representative Younger Ryoke granitoids in the Yanai area, Yamaguchi Prefecture, occurs as an elliptic outcrop associated with a coeval quartz diorite, and intruded the high-grade Ryoke metamorphic rocks. The Kibe Granite is dominated by homogeneous biotite-granite that contain euhedral alkali-feldspar phenocrysts. These rocks also contain local micro-magmatic enclaves and have migmatitic structures in contact zones with the pelitic gneisses in the marginal parts of the granite. The mixing and mingling structures are developed at the contact between the granite and quartz diorite. The major and trace element variations of the Kibe Granite can be explained by a subtraction of plagioclase together with minor biotite and alkali-feldspar from a parental magma. The Kibe Granite was formed through the following processes: assimilation of the pelitic gneisses, magma mixing with the quartz diorite and fractional crystallization of a parental magma. These processes took place at the exhumation stage of the Ryoke metamorphism following its isothermal decompression path.

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.

Petrography of cuttings from spa borings and their tectonic setting upon the Kanto Plain at Shimohara, Tsukuba city and at Hanai, Noda city

This report describes the subsurface geology, based on the petrography of rock cuttings taken from bore holes at spa sites, at Shimohara, Tsukuba City, Ibaraki Prefecture, and at Hanai, Noda City, Chiba Prefecture, both upon the Kanto Plain. The cutting samples from Tsukuba (four samples from 1241–1210 m depth) are hornfels, biotite-muscovite schist, and sillimanite-andalusite gneisses that resemble the Tsukuba metamorphic rocks. A single cutting sample from Noda (1295–1300 m depth) is mylonitized biotite granite that is similar in lithology to the Late Cretaceous Name Granite in the Shimonita area, Gunma Prefecture. These results provide new information on the eastern extension of the Ryoke Metamorphic Belt and the southern extension of the Tsukuba Metamorphic Belt. The occurrence of mylonite from Noda also provides important information in terms of constraining the eastern extension of the Median Tectonic Line.

Development of migmatites and the role of viscous segregation in high- T metamorphic complexes: Example from the Ryoke Metamorphic Complex, Mikawa Plateau, Central Japan

The Ryoke Metamorphic Complex of the Mikawa Plateau, Central Japan, contains well-preserved geological and thermal structures formed during Cretaceous high-temperature (high- T) metamorphism. These structures can be explained by a pervasive melt migration model. However, the estimated flux of melt is much higher than that expected for compaction flow of felsic melt. An additional tectonic force and/or melt segregation to network of veins or layers is therefore required. This paper presents the results of numerical modeling of the structural evolution of immiscible viscous fluids under simple-shear deformation, and discusses the role of viscous segregation in enabling pervasive melt flow in hot crust. The results of numerical modeling of simple-shear deformation of immiscible viscous fluids reveal that low-viscosity fluids segregate into lenses and layers. This segregation is a type of viscous segregation that takes place to minimize the dissipation of energy in the system. The rate of energy dissipation is proportional to viscosity. Therefore, this process is expected to occur universally in immiscible viscous fluids with large viscosity contrasts, and is an important process in focusing melt in partially melted rocks. The structures produced by the numerical simulation are similar to those of migmatites. The simulation results suggest that melts distributed randomly in partially melted rock will segregate into lenses or layers with progressive simple-shear deformation. During deformation-enhanced segregation, a melt network will form dynamically. If this process occurs at small to large scales in the hot crust, melts will be effectively transported upward due to buoyancy forces. It is possible that pervasive migmatites at low structural levels within high- T metamorphic complexes may represent “fossil” melt pathways formed by crustal-scale viscous segregation.

5万分の1地質図幅「御油」・「豊橋及び田原」地域の領家変成コンプレックス

5万分の1地質図幅「御油」・「豊橋及び田原」地域の領家変成コンプレックス

Geological relationship between Anyui Metamorphic Complex and Samarka terrane, Far East Russia

AbstractThe Anyui Metamorphic Complex (AMC) of Cretaceous age is composed of metachert, schist, gneiss, migmatite and ultramafic rocks, and forms a dome structure within the northernmost part of the Jurassic accretionary complex of the Samarka terrane. The two adjacent geological units are bounded by a fault, but the gradual changes of grain size and crystallinity index of quartz in chert and metachert of the Samarka terrane and the AMC, together with the gradual lithological change, indicate that at least parts of the AMC are metamorphic equivalents of the Samarka rocks. Radiolarian fossils from siliceous mudstone of the Samarka terrane indicates Tithonian age (uppermost Jurassic), and hence, form a slightly later accretion. This signifies that the accretionary complex in the study area is one of the youngest tectonostratigraphic units of the Samarka terrane. The relationship between the Samarka terrane and AMC, as well as their ages and lithologies, are similar to those of the Tamba–Mino–Ashio terrane and Ryoke Metamorphic Complex in southwest Japan. In both areas the lower (younger) part of the Jurassic accretionary complexes were intruded and metamorphosed by Late Cretaceous granitic magma. Crustal development of the Pacific‐type orogen has been achieved by the cycle of: (i) accretion of oceanic materials and turbidites derived from the continent; and (ii) granitic intrusion by the next subduction and accretion events, accompanied by formation of high T/P metamorphic complexes.

Chemical Evolution of Zircons in the Paleogene Naegi Granite, Central Japan

AbstractThe chemical composition of zircons from S‐ and I‐type ilmenite‐series granitic rocks in the Chubu district is summarized based upon recent electron probe microanalysis. Zircons in S‐type Busetsu granite of the Ryoke Metamorphic Belt have a homogeneous composition with minimum impurities, whereas those of I‐type Naegi granite in the non‐metamorphic Sanyo Belt are enriched in Hf, Y, REE, Th and U along the crystal rims. Similar enrichment has been observed in a variety of zircon called Naegite. These minor components are concentrated in the F‐rich fluid phase of residual melts of the Naegi granite magma, and are crystallized in pegmatites during the latest magmatic stage. High values of Nb and Ta in some Naegite reported previously are attributed to micro‐inclusion of fergusonite.

Structural evidence for large‐scale top‐to‐the‐north normal displacement along the Median Tectonic Line in southwest Japan

Abstract The Median Tectonic Line (MTL) is a first‐order tectonic boundary that separates the Sanbagawa and Ryoke Metamorphic Belts. Strike‐slip movements on the MTL have been well documented by many workers. New field based structural studies in the Sanbagawa Belt close to the MTL reveal a large number of secondary faults and semibrittle shear bands indicating a top‐to‐the‐north normal sense of displacement. The strikes of these shear zones and their spatial distributions suggest that development of these structures is related to movements on the MTL. These results imply that the MTL has a large‐scale normal shear component on a regional scale that can help account for the exhumation of the Sanbagawa Belt. Our proposed history of the MTL can also account for changes in the geometry of folds in the Sanbagawa Belt.

Geology and metamorphic zonation of the Ryoke Metamorphic Belt on Kasado-jima Island, SW Japan

The Cretaceous low-pressure/high-temperature Ryoke Metamorphic Belt contains abundant Ryoke and San-yo Granitoids that intruded to various crustal depths during or after formation of the belt. To clarify the tectono-metamorphic history of the belt, it is necessary to evaluate the thermal effects of the intrusion of the different types of granitoids. The geology of Kasado-jima Island, SW Japan, consists of metasedimentary rocks, massive biotite granite, and migmatitic biotite tonalite. The granite and tonalite occur in the central and southern areas of the island, respectively. Within the central area, granite intrudes metasedimentary rocks and cuts across bedding-parallel schistosity within the metapelites; this indicates that intrusion of the granite postdates Ryoke metamorphism. In contrast, tonalite in the southern area was emplaced parallel to the schistosity. On the basis of the field occurrences and petrographic characteristics of these granitoids, we correlate the biotite tonalite and biotite granite with the Older Ryoke and San-yo Granitoids, respectively. Based on the paragenesis of schistosity-forming minerals within metapelites, we identify two mineral zones in the island: the biotite and cordierite zones. The biotite zone is characterized by a mineral assemblage of muscovite + biotite ± garnet, and the cordierite zone is characterized by the assemblage muscovite + biotite + K-feldspar + cordierite or garnet. On the basis of mineral chemistry, we estimate peak metamorphic conditions for the biotite and cordierite zones to be ∼ 470 °C (reference pressure: 200 MPa) and ∼ 550 °C at 250 MPa, respectively. In the central part of the island, a contact aureole adjacent to the biotite granite is identified by the occurrence of randomly oriented muscovite and very large dendritic cordierite porphyroblasts. These randomly oriented porphyroblasts may have formed under a high degree of supersaturation associated with the thermal perturbation of the shallow intrusion of the biotite granite into cooler crust. In the southern area, most metamorphic minerals are of similar size, and their long axes are aligned to form a schistosity; this indicates a small degree of thermal perturbation and/or a high strain rate. These observations indicate that the biotite tonalite on Kasado-jima Island, as a deep-seated granitoid, was related to regional low-pressure/high-temperature metamorphism.

Regional occurrence of orthopyroxene‐bearing basic rocks in the Yanai district, southwest Japan: Evidence for granulite‐facies Ryoke metamorphism

Abstract The present paper is reporting on the regional occurrence of orthopyroxene‐bearing basic rocks from the Ryoke Metamorphic Belt in the Yanai district, southwest Japan. Their localities are confined to the highest‐grade zone of the area (i.e. the garnet–cordierite zone, where garnet coexists with cordierite, K‐feldspar and biotite in pelitic rocks). Orthopyroxene coexists with quartz and hydrous minerals such as biotite, cummingtonite and hornblende, and in some cases with clinopyroxene, suggesting that the highest grade of the Ryoke metamorphism reached a low‐temperature subfacies of the granulite facies, contrary to the upper amphibolite facies as previously asserted.

Thermal evolution of the Ryoke metamorphic belt, southwestern Japan: Tectonic and numerical modeling: Reply

The Ryoke metamorphic belt of southwestern Japan is composed of Cretaceous Ryoke granitoids and associated metamorphic rocks of low-pressure facies series. The Ryoke granitoids are divided into sheet-like bodies (e.g. Gamano granodiorite) and stock-like bodies. The Gamano granodiorite intruded concordantly into the high-grade metamorphic rocks without development of a contact metamorphic aureole, and the intrusion ages of the granodiorite are similar to the ages of thermal peak of the low pressure (low-P) metamorphism. It is suggested that the low-P Ryoke metamorphism resulted from the intrusion of the Gamano granodiorite. In this study, a simple 1-D numerical model of conductive heat transfer was used to evaluate the thermal effects of emplacement of the Gamano granodiorite. Calculated temperature-time (T-t) paths are characterized by a rapid increase of metamorphic temperature and a relatively short-lived period of high temperature. For example, the 5"-t path at the 15-km depth is characterized by a rapid average increase in temperature of 1.4 x 10-3"C/year and high temperatures for < ca 0.5 Ma. The calculated peak temperature for each depth is nearly equal to the petrologically estimated value for each correlated metamorphic zone. The results suggest that the magma-intrusion model is one possible thermal model for low-pressure facies series metamorphism.

The Ryoke Granitoid and Metamorphic Belt, Southwest Japan: A Middle-Crustal Section of the Eurasian Continental Margin

The Ryoke Granitoid and Metamorphic Belt, Southwest Japan: A Middle-Crustal Section of the Eurasian Continental Margin

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.

Progressive sequence of reactions of the Ryoke metamorphism in the Yanai district, southwest Japan: the formation of cordierite

The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low‐ to medium‐grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K‐feldspar have a higher K component in an A′KF diagram than those in rocks lacking K‐feldspar. This fact reflects an increase in the K2O content in muscovite, but in biotite it reflects an increase of not only the K2O content but also of the octahedral vacancy.At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K‐feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al‐poor biotite+phengitic muscovite=K,Al‐rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium‐grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low‐pressure type metamorphism. Some textures in one of the high‐grade areas, the K‐feldspar‐cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K‐feldspar+waterand andalusite=sillimanite.quartz=cordierite+K‐feldspar+waterThis implies that this zone probably has a P–T path involving isobaric heating.