Yanai District Research Articles

A high‐Tmetamorphic complex derived from the high‐PSuo metamorphic complex in the Omuta district, northern Kyushu, southwest Japan

AbstractNew U–Pbages of zircons from migmatitic pelitic gneisses in the Omuta district, northern Kyushu, southwest Japan are presented. Metamorphic zonation from the Suo metamorphic complex to the gneisses suggests that the protolith of the gneisses was the Suo metamorphic complex. The zircon ages reveal the following: (i) a transformation took place from the high‐PSuo metamorphic complex to a high‐Tmetamorphic complex that includes the migmatitic pelitic gneisses; (ii) the detrital zircon cores in the Suo pelitic rocks have two main age components (ca1900–1800 Ma and 250 Ma), with some of the detrital zircon cores being supplied (being reworked) from a high‐grade metamorphic source; and (iii) one metamorphic zircon rim yields 105.1 ±5.3 Ma concordant age that represents the age of the high‐Tmetamorphism. The high‐Pto high‐Ttransformation of metamorphic complexes implies the seaward shift of a volcanic arc or a landward shift of the metamorphic complex from a trench to the sides of a volcanic arc in an arc–trench system during the Early Cretaceous. The Omuta district is located on the same geographical trend as the Ryoke plutono‐metamorphic complex, and our estimated age of the high‐Tmetamorphism is similar to that of the Ryoke plutono‐metamorphism in the Yanai district of western Chugoku. Therefore, the high‐Tmetamorphic complex possibly represents the western extension of the Ryoke plutono‐metamorphic complex. The protolith of the metamorphic rocks of the Ryoke plutono‐metamorphic complex was the Jurassic accretionary complex of the inner zone of southwest Japan. The high‐Pto high‐Ttransformation in the Omuta district also suggests that the geographic trend of the Jurassic accretionary complex was oblique to that of the mid‐Cretaceous high‐Tmetamorphic field.

Evaluation of residual pressure in an inclusion-host system using negative frequency shift of quartz Raman spectra

Raman spectra of quartz inclusions in garnet hosts of low-pressure/temperature metamorphic rocks from the Yanai district in the Ryoke belt (around 0.1–0.3 GPa/500–600 °C), Southwest Japan, exhibit frequency (peak position) shifts toward lower wavenumbers as compared to those of a quartz standard measured at ambient conditions. The observed negative frequency shifts indicate that tensile normal stress is exerted on the quartz–garnet boundary and therefore, quartz inclusions are subjected to negative residual pressure. Elastic modeling that assumed the constant elastic properties of minerals cannot explain this negative residual pressure. This study estimated the residual pressure based on a new scheme of elastic modeling with equation of state (EOS) of quartz and garnet, which takes into account the pressure- and temperature-dependency of compressibility and expansivity. The calculated residual pressure was converted into frequency shifts of quartz Raman spectrum based on the experimentally determined relation. The results showed that the quartz inclusions in garnets retain residual pressure of about −0.3 GPa, and logically reproduced the observed frequency shifts in the direction of lower wavenumbers. The new elastic modeling also simulates positive frequency shifts retained by quartz inclusions in garnets of high-pressure metamorphic rocks from the Sambagawa metamorphic belt in Southwest Japan, and from the Motagua fault zone in Guatemala. The degree and direction of Raman frequency shifts of quartz inclusion in garnet depend on metamorphic conditions when the quartz was included in the host garnet. Conversely, the metamorphic conditions prevailing when a set of a quartz inclusion and garnet host was recrystallized can be inferred from Raman frequency shifts of quartz inclusion in garnet. The proposed Raman spectroscopic analysis should be a powerful and useful tool to decipher information at earlier stage of garnet growth even in samples of highly recrystallized matrix phases during exhumation and retrograde stages.

領家帯柳井地域新期花崗岩(木部岩体)のマグマ過程

領家帯柳井地域新期花崗岩(木部岩体)のマグマ過程

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.

Formation of a high‐temperature metamorphic complex due to pervasive melt migration in the hot crust

Abstract Detailed reviews of metamorphic thermal structures and spatial distributions of granitic rocks in the high‐T Higo metamorphic complex (HMC), central Kyushu, Japan, and Ryoke metamorphic complex in the Yanai district (RMC‐Y), southwest Japan, reveal that the high‐grade metamorphic rocks were associated with migmatites or syn‐metamorphic granitoids. The close association of these rocks suggests that transport and solidification of granitic melts are appreciable heat sources. Partial melting of the pelitic host resists heating of the crust due to its endothermic reaction (i.e. absorption of latent heat). However, the effects of slower thermal evolution produced by partial melting are limited by the boundary conditions assumed for the HMC and RMC‐Y. New thermal numerical modeling reveals that the volume fraction of the solidified melt products and the duration of melt migration required for crustal heating to a certain temperature decrease with increasing rates of melt migration. The model predicts that the melt migration rate during formation of the HMC is about three to 10 times higher than that of the RMC‐Y, and the corresponding duration of melt migration is about one order of magnitude shorter. The model described here also explains the apparently contradictory observations made in different structural units of the RMC‐Y: the metamorphic pressure–temperature (P‐T) ratio of the southern unit is higher than those of the central and northern units, even though the volume fraction of granitic rocks in the southern unit is larger. The model suggests that the melt migration rate in the northern and central units was faster than that in the southern unit. Overall, the model implies that variations of metamorphic field P‐T arrays and the spatial distributions of granitic rocks in high‐T metamorphic complexes may be explained simply by variations in the rate and duration of melt migration in the hot crust.

Carbonaceous matter in quartz veins and pelitic rocks from the Ryoke metamorphic belt in the Yanai district, SW Japan

Carbonaceous matter (CM) with various modes of occurrence has been observed in quartz veins in the biotite zone of the Ryoke metamorphic belt in the Yanai district, Yamaguchi Prefecture, SW Japan. Raman spectroscopic analysis of the CM revealed that high temperature fluids had percolated through fractures, which were now occupied by the quartz veins that had formed during the retrograde stages of metamorphism. These fluids may have also affected the degree of organization of the fine-grained CM particles in the host pelitic rocks and the coarse-grained particles that had precipitated from them at a later stage.

相対地質温度圧力計とギブス法による領家変成岩類の温度圧力条件と温度圧力経路

Thermodynamic consideration of equilibria among garnet, biotite, plagioclase, and quartz enables us to estimate pressure-temperature conditions of pelitic and siliceous rocks occurring in a wide range of metamorphic grades, as a difference from the condition of a reference sample. Application of this technique to the Ryoke metamorphic rocks in the Yanai district provides the thermobaric structure of the area in more detail than was available before.The Gibbs method applied to a low-grade siliceous rock of the area yields a decompressional pressure-temperature path. The validity of the result is still ambiguous and should be evaluated petrographically in terms of constancy of mineral assemblage during garnet growth.

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.

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.

Cooling and inferred exhumation history of the Ryoke metamorphic belt in the Yanai district, south‐west Japan: Constraints from Rb–Sr and fission‐track ages of gneissose granitoid and numerical modeling

Abstract The Ryoke metamorphic belt in south‐west Japan consists mainly of I‐type granitoids and associated low‐pressure/high‐temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb–Sr whole‐rock–mineral isochron ages and fission‐track ages of the gneissose granodiorite in the central structural unit. Four Rb–Sr ages fall in a range of ca 89–87 Ma. The fission‐track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1σ error), respectively. Combining the newly obtained ages with previously reported (Th–)U–Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (&gt; 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low‐angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large‐scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is ≥ 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1–0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis.

Cooling and inferred exhumation history of the Ryoke metamorphic belt in the Yanai district, south-west Japan: Constraints from Rb-Sr and fission-track ages of gneissose granitoid and numerical modeling

Cooling and inferred exhumation history of the Ryoke metamorphic belt in the Yanai district, south-west Japan: Constraints from Rb-Sr and fission-track ages of gneissose granitoid and numerical modeling

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, $$$$

Late Cretaceous U/Pb zircon ages and Precambrian crustal inheritance in Ryoke granitoids, Kinki and Yanai districts, Japan.

Ryoke granitoids on Ikoma-yama and Katsuragi-yama in the Kinki District, and on Yashiro-jima in the Yanai District, have Late Cretaceous zircon U/Pb isotopic crystallization ages. Older and Younger granites on Ikoma-yama were emplaced within a short time interval between 81 and 82 ± 1 Ma. Younger Ryoke granite on Katsuragi-yama, located south of Ikoma-yama, crystallized at 75 ± 3 Ma. The Gamano granodiorite and Kita-Oshima granite-gneiss on Yashiro-jima have overlapping crystallization ages of 95 to 96 ± 1 Ma. This result supports earlier ideas that the Gamano and Kita-Oshima rocks are cogenetic. An inherited component of Precambrian (1855 to 1953 Ma) radiogenic lead is present in some of the zircons from the Ryoke granitoids analyzed in this study. This result implies that older continental materials were assimilated into the granitic magmas. Possible sources include Precambrian basement or younger upper crustal rocks that contain recycled Precambrian detritus.

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.

柳井南部地域の領家変成帯からの含十字石泥質捕獲岩の発見とその変成史

柳井南部地域の領家変成帯からの含十字石泥質捕獲岩の発見とその変成史

Temperature–time path for the low‐pressure Ryoke metamorphism, Japan, based on chemical zoning in garnet

Garnet crystals from low‐pressure/high‐temperature (LPHT) Ryoke metamorphic rocks in the Yanai district, south‐western Japan, show several kinds of chemical zoning patterns that systematically vary with grain radius between c. 0.1 and 0.5 mm. Large grains (&gt;c. 0.4 mm) show normal zoning and small grains (&lt;c. 0.4 mm) show unzoned or reversely zoned cores. Observations of the chemical zoning and of the spatial and size distributions of the garnet grains between c. 0.1 and 0.5 mm in radius suggest that they were formed by continuous nucleation and diffusion‐controlled growth.A previously estimated temperature–time path (T –t path) for the Ryoke metamorphism, using 1‐D numerical simulation, is characterized by a rapid increase in temperature, 0.0017 °C yr−1 on average, and a period of high temperature (&gt;600 °C) shorter than 0.5 Myr, which was presumably caused by the intrusion of a granodiorite sheet. Chemical zoning of garnet grains with different radii simulated for the T –t path using a numerical model of continuous nucleation and diffusion‐controlled growth, in combination with intracrystalline diffusion, compares well with the observed zoning patterns in garnet grains with different radii. This is in spite of the fact that the simulated zoning patterns vary greatly, depending on subtle differences in the T–t history. Therefore, they suggest that the T–tpath gives a good explanation for the LPHT Ryoke metamorphism. Although this study only refers to the Ryoke metamorphism, the technique may be applicable to thermal modelling of other metamorphic terranes.

A new estimate of the conditions for transition from basal 〈 a〉 to prism [ c] slip in naturally deformed quartz

Quartz c-axis fabrics of naturally deformed quartz in metacherts from the Ryoke metamorphic belt in the Yanai district, southwestern Japan, suggest that the prism [ c], prism 〈 a〉 and basal 〈 a〉 slip systems were activated during the plastic deformation under high-temperature metamorphic conditions. With increasing temperature, quartz c-axis fabric patterns change from girdle type, through X-maximum with a faint crossed girdle, to X-maximum. The dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature, closely correlated with the change in quartz c-axis fabric. The fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size. Moreover, observations with transmission electron microscopy (TEM) clearly identify the dominant activation of [ c] slip through the presence of basal subgrain boundaries and free dislocations with the [0001] Burgers vector in the quartz from the metachert samples exhibiting X-maximum c-axis fabric. Based on these observations it is concluded that the mechanism switch from basal 〈 a〉 to prism [ c] slip systems occurred with increasing temperature in the metacherts. The transition temperature is petrologically estimated to be ca. 550–600°C at the natural strain rate.

Compositional zoning patterns of garnet during prograde metamorphism from the Yanai district, Ryoke metamorphic belt, southwest Japan

In the Yanai district, southwest Japan, garnet crystals in pelitic and siliceous rocks are normally zoned, reversely zoned or homogeneous. Normally zoned crystals show a continuous decrease in Mn content from the core towards the margin, whereas reversely zoned crystals show enrichment of Mn at the margin. Normally zoned garnet grew by one of two distinct coarsening mechanisms, either nucleation and growth or impingement and overgrowth. In pelitic rocks, there is a systematic change from normal zoning to homogeneous and reverse zoning with increasing metamorphic grade. This can be ascribed to the enhancement of internal diffusion with increasing metamorphic temperature. In the cordierite-I zone, with an estimated metamorphic temperature of 560 ± 30°, garnet in pelitic rocks with a grain size larger than 0.2 mm preserves its zoning pattern, whereas garnet in siliceous rocks with a similar grain size is homogeneous. This suggests that both rock type and grain size are factors that influence the degree to whole growth zoning of garnet is homogenized. The relative paucity of biotite in siliceous rock can explain the development of fine-grained garnet in this rock type, which is, therefore, easily homogenized. Coarse homogeneous garnet in the siliceous rocks was formed by impingement of small grains during prograde metamorphism. Such aggregates are easily homogenized by both internal and grain-boundary diffusion.

Heterogeneous biotite from Ryoke metamorphic rocks in the Yanai district, southwest Japan.

Heterogeneous biotite from Ryoke metamorphic rocks in the Yanai district, southwest Japan.