Low-pressure Metamorphism in the Ryoke Metamorphic Belt in the Yanai District, Southwest Japan

The Ryoke metamorphic belt is one of the typical low-pressure type metamorphic belts in the world. It is composed of granitoids (Older and Younger Ryoke granitoids) and associated metamorphic complex (Ryoke metamorphic rocks) of Cretaceous age. The Ryoke metamorphic rocks in the Yanai district, southwest Japan, show three different phases of ductile deformation. During the first phase (D1), a distinct foliation parallel to lithologic layering was formed under the thermal peak conditions of the low-pressure facies series metamorphism. The second phase deformation (D2) led to the formation of mylonitic shear zones and nappes. Deformation of the third phase (D3) was responsible for the formation of the upright folds with E-W trending axes. The movement picture of D1 during and immediately before the intrusion of the Older Ryoke granitoids was of extension tectonics. After D1, the nappes and upright folds of the metamorphic rocks and granitoids were formed during D2 and D3 probably under compressional stress field. The regional Ryoke metamorphism has been divided into two phases, M0 and Ml. The metamorphism of M0 was of nearly medium-pressure facies series (ca. 30°C /km) and that of M1 was of low-pressure facies series (ca. 40 ～ 50°C / km). On the basis of the mineral assemblages crystallized under M1, the Ryoke metamorphic rocks are divided into four metamorphic zones: biotite zone, cordierite zone (460 ～ 590°C, 2.5 ～ 3.5 kbar), sillimanite zone (630 ～ 690°C, 3 ～ 5 kbar), and garnet zone (730 ～ 770°C, 5.5 ～ 6.5 kbar). Because the intrusion of the Older Ryoke granitoids has a strong time and spatial association with M1, it is suggested that the heat sources of M1 are the emplacement of the Older Ryoke granitoids. By using 1-D numerical simulation, the thermal model for M1 was developed by heat conduction with fluid advection caused by intrusion of a granodiorite sheet at intermediate crustal levels. The results of the thermal model nearly consist with the petrologically estimated highest metamorphic temperatures during M1. Garnet crystals from the sillimanite zone are chemically zoned and show several kinds of zoning patterns. The observed overall zoning patterns in the garnets with different radii are well reproduced by the numerical analysis. These results suggest that the temperature-time path gives a good explanation for M1. Therefore, it can be said that the sheet-like Older Ryoke granitoids intruded at intermediate crustal levels(≈ 15-km-depth) are a heat source of M1. In conclusion, the Ryoke metamorphic rocks firstly were heated under medium-pressure facies conditions, and then they were further heated under low-pressure facies conditions caused by the intrusion of the Older Ryoke granitoids

Emplacement Mechanism of the Older Ryoke Granites in the Yanai District, Southwest Japan, with Special Reference to Extensional Deformation in the Ryoke Metamorphic Belt

The Ryoke metamorphic rocks in the Yanai district, southwest Japan show geological and rock structures produced by three different phases of ductile deformation. During the first phase (D1), a distinct foliation parallel to lithologic layering was formed under the thermal peak conditions during the low-pressure facies series Ryoke metamorphism, which is ascribed to the sheet-like intrusion of the older Ryoke granites. The second phase deformation (D2) led to the formation of mylonitic shear zones and nappes. The deformation of the third phase (D3) was responsible for the formation of the upright folds with E-W trending axes. In the metamorphic rocks of the Tsuzu area, which is placed in the northern part of the Yanai district, are often found melt-filled fractures of minor scales, which cut across their foliation. The deformation related to the formation of these melt-filled fractures resulted commonly from the foliation parallel extension. The overall movement picture inferred from the melt-filled fractures appears to be of the shear sense for the top to the north. The formation of the melt-filled fractures was responsible for the intrusion of the older Ryoke granites. Asymmetric textures such as extensional crenulation cleavage (ECC) and rotation of porphyroblasts, which grew under the thermal peak of the low-pressure facies series metamorphism, are also formed in the metapelites. The shear sense read from the asymmetric textures is the top to the north, though fairly dispersed. This is harmonic with the overall movement picture inferred from the melt-filled fractures. Therefore it can be said that the overall movement picture of the D1. deformation of the metamorphic rocks in the Tsuzu area during and immediately before the intrusion of the older Ryoke granites appears to have been of the same style of extension tectonics. Consequently, the D1 deformation occurred under extensional stress regime and the older Ryoke granites intruded in extensional fracture zones at intermediate crustal depths. The nappes of the metamorphic rocks and older granites were formed during the D2 deformation probably under compressional stress regime

Sinistral En Echelon Folding of the Sambagawa Schists and Its Tectonic Implication

The folds of the Sambagawa schists, which were produced during the last phase (Hijikawa—Oboke phase = Dh phase) of their folding history, are developed as a series of sinistral en echelon upright folds with half wavelength of less than 20 Km (Hara et al.,1977,1992). The Dh phase folds in Shikoku are accompanied with two culminations, Oboke culmination and Nakashichiban culmination, placed near the MTL. Their movement picture during the formation process of such the Dh phase folds has been analyzed on the basis of orientation pattern of parasitic folds and quartz microtextures. It has been clarified that the Dh phase folds were produced by left—lateral shear under N —S compression, being accompanied by the southward tectonic emplacement of two rigid bodies which gave rise to the Oboke and Nakshichiban culminations. These bodies can be assumed to be granitic and/or high—temperature metamorphic rocks tectonically derived from the Kurosegawa—Koryoke continent, as judged from the seismic refraction data in the Oboke district after Ichikawa (1968)

Sulphur-isotopic composition of the deep-sea mussel Bathymodiolus marisindicus from currently active hydrothermal vents in the Indian Ocean

Sulphur-isotopic composition of soft tissues from bathymodiolus marisindicus collected from hydrothermal vents in the indian ocean was reported. the [delta]34s values of the soft tissues (+3[similar]+5‰ vs cañyon diablo troilite) were nearly identical to those from the associated hydrothermal fluid and chimney sulphides (+5 to +8‰), but were significantly different from that of the common seawater sulphate (+21‰), which suggested that the endosymbiotic bacteria used sulphide in the fluid as an energy source. transmission electron microscopic observation of the endosymbionts also suggested that the symbiont is a thioautotroph. bathymodiolus species, which depend on either sulphide or methane oxidation, or both, have a worldwide distribution. bathymodiolus marisindicus from the indian ocean has a close relationship with congeners in the pacific ocean as evidenced by form of symbiosis. biogeography and migration of the genus bathymodiolus based on the relevant data are briefly discussed.</p

Evolution of Chitradurga Shear Zone; Some insights to the current interpretations in the tectonic context of Dharwar Craton, South India

第3回極域科学シンポジウム/第32回極域地学シンポジウム 11月30日（金） 統計数理研究所 ３階セミナー

西南日本柳井地域領家変成帯における低圧型変成作用

The Ryoke metamorphic belt is one of the typical low-pressure type metamorphic belts in the world. It is composed of granitoids (Older and Younger Ryoke granitoids) and their associated metamorphic complex (Ryoke metamorphic rocks) of Cretaceous age. The Ryoke metamorphic rocks in the Yanai district, southwest Japan, show three different phases of ductile deformation. During the first phase (D1), a distinct foliation parallel to lithologic layering was formed under the thermal peak conditions of the low-pressure fades series metamorphism, which is probably ascribed to the sheet-like intrusion of the Older Ryoke granitoids. The second phase deformation (D2) led to the formation of mylonitic shear zones and nappes. Deformation of the third phase (D3) was responsible for the formation of the upright folds with E-W trending axes. In the metamorphic rocks of the Tsuzu area, which is placed in the northern part of the Yanai district, there are many melt-filled fractures of minor scales, which cut across their foliation. The deformation related to the formation of these melt-filled fractures resulted commonly from foliation parallel shearing under extensional stress field. The overall movement picture inferred from the melt-filled fractures appears to be top to the N sense of shear, and the deformation related to the formation of the melt-filled fractures was responsible for the formation of the normal fault zones, along which the intrusion of the Older Ryoke granitoids occurred. Asymmetric textures such as extensional crenulation cleavage and rotation of porphyroblasts, which grew under the thermal peak of M1, are also formed in the metapelites. The shear sense read from the asymmetric textures is the top to the north. This is harmonic with the overall movement picture inferred from the melt-filled fractures. Therefore it can be said that the overall movement picture of D1 during and immediately before the intrusion of the Older Ryoke granitoids was of extension tectonics. As inferred from the dislocation densities in quartz grains deformed during D1, strain rate for D1 appears to be high (≈ 10-10 ~ 10-7 s-1). After D1, the nappes and upright folds of the metamorphic rocks and granitoids were formed during D2 and D3 probably under compressional stress field. The regional Ryoke metamorphism has been divided into two phases, M0 and M1. The metamorphism of M0 was of nearly medium-pressure facies series (ca. 30°C/km) and that of M1 was of low-pressure facies series (ca. 40 ~ 50°C/km). On the basis of the mineral assemblages crystallized under M1, the Ryoke metamorphic rocks are divided into four metamorphic zones: biotite zone, cordierite zone (460 ~ 590°C, 2.5 ~ 3.5 kbar), sillimanite zone (630 ~ 690°C, 3 ~ 5 kbar), and garnet zone (730 ~ 770°C, 5.5 ~ 6.5 kbar). Because the intrusion of the Older Ryoke granitoids has a strong time and spatial association with M1, it is suggested that the heat sources of M1 are the emplacement of the Older Ryoke granitoids. By using 1-D numerical simulation, the thermal model for M1 was developed by heat conduction with fluid advection caused by intnision of a granodiorite sheet at intermediate crustal levels. The calculated temperature-time path (T-t path) for the sillimanite zone during M1 is characterized by a rapid increase of temperature, 0.0017°C/year on average, and a short-period of high-temperature condition (> 600°C), shorter than 0.5 Ma. The results of the thermal model nearly consist with the petrologically estimated highest metamorphic temperatures during M1. Garnet crystals from the sillimanite zone are chemically zoned and show several kinds of zoning patterns. The patterns systematically vary with grain size, which are between ca. 0.1 and 0.5 mm in radius. Large grains (> ca. 0.4 mm) show normal zoning and small grains (< ca. 0.4 mm) show unzoned or reversely zoned profile in their cores. Observations of the chemical zoning and of the spatial and size distributions of the garnets between ca. 0.1 and 0.5 mm in radius suggest that the garnets have been formed by continuous nucleation and diffusion-controlled growth. To examine the validity of the T-t path for M1, the chemical zonings of garnets with different radii are simulated for the T-t path using a numerical model of continuous nucleation and diffusion-controlled growth, in combination with intracrystalline diffusion, and are compared with the observed ones. The observed overall zoning patterns in the garnets with different radii are well reproduced by the numerical model, in spite of the fact that the simulated zoning patterns greatly change responding to the subtle changes in the T-t history. Therefore, these results suggest that the T-t path gives a good explanation for M1. Therefore, it can be said that the sheet-like Older Ryoke granitoids intruded at intermediate crustal levels (≈ 15-km-depth) are a heat source of M1. In conclusion, the Ryoke metamorphic rocks firstly were heated under medium-pressure facies conditions, and then they were further heated under low-pressure facics conditions caused by the intrusion of the Older Ryoke granitoids.ABSTRACT / p2 CONTENTS / p4 CHAFFER 1:INTRODUCTION / p1 CHAPTER 2:OUTLINE OF GEOLOGY / p7 　2.1:Large-scale structures and deformation events / p7 　2.2:Regional metamorphism / p9 CHAPTER 3:STRUCTURAL ANALYSIS OF Dl DEFORMATION / p23 　3.1:Movement picture of Dl deformation in the Tsuzu area / p23 　3.2:Deformational conditions of Dl deformation as inferred from naturally deformed quartz in metacherts / p27 　3.3:Concluding remarks for the analysis / p38 CHAPTER 4:STRUCTURAL ANALYSIS OF D2 DEFORMATION / p58 　4.1:Kitaoshima granodiorite / p58 　4.2:Geological and deformation structures in the Hirarehana peninsula / p59 　4.3:Movement picture of D2 deformation and uplift tectonics of the Ryoke metamorphic belt / p60 CHAPTER 5:THERMAL MODELING FOR Ml METAMORPHISM / p68 　5.1:Tectonic and thermal models for numerical analysis / p68 　5.2:Numerical analysis / p69 　5.3:Discussion for the analysis / p74 　5.4:Concluding remarks for the analysis / p76 CHAPTER 6:AN EXAMINATION FOR THE THERMAL MODELING BASED ON CHEMICAL ZONING IN GARNETS / p85 　6.1:Experimental procedure / p86 　6.2:Chemical zoning in garnet / p86 　6.3:Nucleation and growth mechanisms of garnet / p88 　6.4:Numerical analysis / p90 　6.5:Discussion for the analysis / p97 　6.6:Concluding remarks for the analysis / p99 CHAPTER 7:GENERAL DISCUSSION AND CONCLUSIONS / p112 REFERENSES / p117広島大学(Hiroshima University)博士(理学)Physical Sciencedoctora