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1.
Abstract The microstructure of quartz in metacherts of the Ryoke metamorphic belt in central Japan develops from polygonal, through duplex to irregular with increasing metamorphic grade. The polygonal microstructure is composed of small (mostly 90–160 μm), equant, equigranular, polygonal quartz grains, whereas the irregular microstructure is characterized by large (>300 μm) grains with irregular grain boundaries. The duplex microstructure is a mixture of small polygonal and large irregular grains. The development of these microstructures is interpreted as being due to secondary recrystallization. The size of polygonal grains is greatly influenced by the presence of second-phase minerals, such as mica, whereas that of large irregular grains is unaffected by second-phase minerals. There seems to be a critical grain size for quartz to occur as polygonal aggregates: no polygonal aggregates occur in rocks with larger than the critical grain size. The size (about 140 μm) decreases slightly with increasing volume fraction of mica. The mean grain sizes of polygonal quartz ( D ) and coexisting mica ( d ) in the duplex microstructure are systematically related to the volume fraction of mica ( f ) by D = 0.728 d (1/ f )0.629.  相似文献   

2.
Abstract The magnitudes of plastic strains of 104 metacherts were determined from the deformed shape of initially spherical radiolarians in the Sambagawa high- P type metamorphic belt of Western Shikoku, Japan. The strain magnitude increases with increasing metamorphic temperature from several per cent to 250%. The a2/a3 ratio of strain ellipsoids in the higher metamorphic grades decreases with increasing metamorphic grade while the a1/a2 ratio increases rapidly. The long axis of the strain ellipsoid for every grade is nearly parallel to the length of the metamorphic belt, suggesting that the flow direction of the synmetamorphic deformation was uniform along the belt. A map of strain zones within the Sambagawa high- P type metamorphic belt reveals that the metamorphic belt underwent a progressive bulk inhomogeneous shear deformation and that the high-grade zones represent a deep-seated boundary shear zone on the accretionary wedge between a subducting oceanic plate and the immobile rigid continental plate.  相似文献   

3.
The dominant deformation mechanism during the Sambagawa metamorphism changes from brittle to ductile with increasing metamorphic temperature. The magnitude of plastic strains inferred from the shapes of deformed radiolaria in metachert increases sharply across the boundary between the epidote-pumpellyite-actinolite zone and the epidote-actinolite zone. The synmetamorphic crack density of metachert is an indicator of the contemporaneous brittle strain of rocks, and it decreases sharply as the grade reaches the epidote-actinolite zone. Hence, the ratio of the ductile strain to the brittle strain of metachert decreases rapidly across the transition to the epidote-actinolite zone of the Sambagawa metamorphic belt.
The sharp change of the ductile strain magnitude also takes place at the epidote-actinolite grade in the Shimanto metamorphic belt of Japan, an example of the intermediate pressure facies series of metamorphism. It is concluded that the transition from brittle to ductile deformation takes place at about 300-400°C. and is independent of pressure of metamorphism.  相似文献   

4.
Evidence is presented of a lateral variation in differential stress during metamorphism along a regional metamorphic belt on the basis of the proportion of microboudinaged piemontite grains (p) in a quartz matrix in metacherts. It is proposed that p is a practical indicator of relative differential stress. Analysis of 123 metacherts from the 800 km long Sambagawa metamorphic belt, Japan, reveals that p‐values range from < 0.01 to 0.7 in this region. Most samples from Wakayama in the mid‐belt area have p‐values of 0.4–0.6, whereas those from western Shikoku have p‐values of < 0.1. This difference cannot be explained by variations in metamorphic temperature, and is instead attributed to a regional, lateral variation in differential stress during metamorphism.  相似文献   

5.
Southwest Japan is divided into Outer and Inner Zones by the Median Tectonic Line (MTL), a major transcurrent fault. The Outer Zone is composed of the Sambagawa (high-pressure intermediate or high P/T type metamorphism), Chichibu and Shimanto Belts. In the Inner Zone, the Ryoke Belt (andalusite– sillimanite or low P/T type metamorphism) was developed mainly within a Jurassic accretionary complex. This spatial relationship between high P/T type and low P/T type metamorphic belts led Miyashiro to the idea that metamorphic belts were developed as ‘paired’ systems. Textural relationships and petrogenetically significant mineral assemblages in pelites from the Ryoke Belt imply peak P–T conditions of ≈5 kbar and up to 850 °C in migmatitic garnet–cordierite rocks from the highest-grade metamorphic zone. It is likely that the thermal anomaly responsible for metamorphism of the Ryoke Belt was related to a segment of the Farallon–Izanagi Ridge as it subducted under the eastern margin of the Asian continent during the Cretaceous. The sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dP/dT slope, inferred to have been generated by a nested set of hairpin-like ‘clockwise’P–T paths. These P–T paths are characterized by limited prograde thickening, minor decompression at peak-T , and near-isobaric cooling, features that may be typical of P–T paths in low P/T type metamorphic belts caused by ridge subduction. A ridge subduction model for the Ryoke Belt implies that juxtaposition of the high-P/T metamorphic rocks of the Sambagawa Belt against it was a result of terrane amalgamation. Belt-parallel ductile stretching, recorded as syn-metamorphic, predominantly constrictional strain in both Ryoke and Sambagawa Belt rocks, and substantial sinistral displacement on the MTL are consistent with left-lateral oblique convergence. Diachroneity in fast cooling of the Ryoke Belt is implied by extant thermochronological data, and is inferred to relate to progressive SW to NE docking of the Sambagawa Belt. Thus, an alternative interpretation of ‘paired’ metamorphic belts in Japan is that they represent laterally contemporaneous terranes, rather than outboard and inboard components of a trench/arc ‘paired’ system. Amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of ‘paired’ metamorphic belts in the geological record.  相似文献   

6.
The convex form of subduction-stage pressure–temperature ( P–T ) paths up to c. 2.0 GPa implies the Sambagawa high- P metamorphic belt, Japan, formed a few million years before ridge subduction. Additional compilation of P–T conditions for higher- P Sambagawa rocks ( c. 2.0–2.5 GPa) reveals that the thermal profile along the slab surface shows a remarkable high- T -ward warping at c. 2.0 GPa ( c. 65 km). Previous thermal models indicate that this warping corresponds to the onset of induced mantle flow towards the subducting slab. If a normal thickness continental crust of c. 30 km was present, this implies the hangingwall region between 30 and 65 km depth was occupied by serpentinized wedge mantle isolated from large-scale mantle flow. Subsequent arrival of the spreading ridge, reheating and dehydration of the serpentinized wedge probably supplied the water necessary for causing granitic magmatism in the Ryoke high- T metamorphic belt, which is paired with the Sambagawa belt.  相似文献   

7.
This paper describes a microstructural sequence of quartz schists (metamorphosed chert) in the Asemi river region of the Sambagawa metamorphic terrain in central Shikoku, southwest Japan. The Asemi river region is divided into three areas on the basis of characteristics of microstructures of quartz schists observed under the optical microscope: areas I, II and III, in ascending order of metamorphic grade. Microstructures in area I consist of finer, equant, equidimensional and polygonal quartz grains free from internal deformation features. Microstructures in area II are characterized by oblate or elliptical grains with remarkable undulatory extinction surrounded by serrated grain boundaries. Microstructures in area III consist mainly of coarser and equant grains without distinct internal deformation features.The formation conditions of these microstructures are discussed in the light of recent experimental results.  相似文献   

8.
Abstract Five whole-rock 40Ar/39Ar plateau ages from low-grade sectors of the Sambagawa belt (Besshi nappe complex) range between 87 and 97 Ma. Two whole-rock phyllite samples from the Mikabu greenstone belt record well-defined 40Ar/39Ar plateau ages of 96 and 98 Ma. Together these ages suggest that a high-pressure metamorphism occurred in both the Sambagawa and Mikabu belts at c. 90–100 Ma. The northern Chichibu sub-belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north ( c. 110 Ma) to south ( c. 215 Ma). The northern Chichibu sub-belt is correlated with the Kuma nappe complex (Sambagawa belt). Two whole-rock phyllite samples from the Kurosegawa terrane display markedly older metamorphic ages than either the Sambagawa or the Chichibu belts.
Accretion of Sambagawa-Chichibu protoliths began prior to the middle Jurrasic. Depositional ages decrease from middle Jurassic (Kuma-Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units. The Kurosegawa belt and the geological units to the south belong to distinctly different terrances than the Sambagawa-Chichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous.  相似文献   

9.
Rocks from two parts of the Ultradauphinois Zone of the external French Alps have been examined, and the mechanisms by which they were deformed have been assessed from petrographic data. Jurassic and Triassic limestones deformed by pressure solution, dissolving non-ferroan-calcite and precipitating ferroan-calcite. Calcite pressure shadows are usually less elliptical in shape than pyrite pressure shadows, and grain boundary sliding is therefore thought to have played a significant role. Eocene rocks deformed by a variety of mechanisms. Limestones show mylonitic textures, whereas limestone conglomerates with a quartz-sandstone matrix deformed by pressure solution of calcite and grain boundary sliding of quartz. A model for enhanced diffusion of silica along mica seams is proposed to account for planar quartz-mica boundaries. Greywackes deformed by incongruent pressure solution, involving the metamorphic reaction of feldspar to mica and quartz, coupled with the replacement of feldspar by calcite.  相似文献   

10.
H. Honma  H. Sakai 《Lithos》1976,9(3):173-178
18O/16O ratios have been obtained for 99 minerals from rocks of the Hiroshima granite complex and adjacent Ryoke granites. Zonal distribution of oxygen isotopes is observed on a regional scale almost parallel to the extension of the Ryoke plutono-metamorphic belt, granites in or around the metamorphic belt being 2–3%0 richer in 18O than those farther away from the belt. Isotopic fractionations among coexisting minerals indicate that isotopic zonation existed at a magmatic stage. The zonal enrichment of 18O in the granite magma in the Ryoke belt and its periphery is a result of isotopic interaction between country rocks and the magma through fluid media. Genetic relationship between granites of the Ryoke and Chugoku belts are discussed with regard to the geological situation of the former belt.  相似文献   

11.
Contrasting ductility is recognized in the rocks of Cretaceous Ryoke metamorphic belt in Iwakuni area, southwest Japan. Pelitic schist is ubiquitous in the region and differences in mineral assemblages mark increase in metamorphic grade. The area has been graded as chlorite-biotite zone in the north progressing into biotite- and muscovite-cordierite zones in the south. Pelitic schist near the boundary between the biotite- and muscovite-cordierite zones has undergone partial silicification to form whitish silicified schist layers which contain two types of quartz veins: those parallel to foliation in the host rock are called schistosity-concordant veins, and those inclined to host rock foliation, schistosity-discordant veins. In this study we examined the quartz structure in the silicified schist and in both types of veins to understand the ductility contrast induced by the silicification process. Crystallographic orientations of quartz in the veins and silicified schist rocks were studied using the Scanning Electron Microscopy (SEM) based Electron Back Scatter Diffraction (EBSD) technique. Quartz c-axis orientations in the silicified schist are nearly random, demonstrating an absence of post-silicification ductile deformation. Quartz grains in the schistosity-concordant veins have preferred c-axis orientations perpendicular to the schistosity indicating ductile shortening. In contrast, schistosity-discordant veins display distinct quartz c-axis fabric than that found in the schistosity-concordant veins. This is because the two types of host rocks exhibit a difference in ductility during deformation. The presence of deformed quartz veins in the undeformed silicified schist indicates transformation of the ductile pelitic schist into the brittle silicified schist at mid-crustal levels where these rocks originate, hence forming contrasting rock layers. Schistosity-concordant veins in the biotite-rich pelitic schist deformed with its host rock in a ductile manner while the schistosity-discordant veins in the neighboring silicified schist were left intact. Silicification of the pelitic schist may have been caused by the silica-rich geofluids produced by subsurface processes. Geofluids responsible for the occurrence of such mechanically contrasting layers mark an increase in seismic reflectivity at mid-crustal depths and may be potential reflectors of seismic waves giving rise to the so-called “bright spots”.  相似文献   

12.
Mitsuhiro Toriumi 《Lithos》1979,12(4):325-333
The process of shape-transformation of quartz inclusions from polyhedral to spherical grains in albite single crystals during metamorphism is mainly controlled by the grain boundary diffusion of oxygen along the quartz/albite interface to reduce the interfacial free energy. The rate of the process, which is represented by the growth rate of the curvature of the edge surface of the grain, depends significantly on temperature and on the grain size of the quartz inclusion. The relations between temperature, T, the time, tr, and the critical radius, Rc, which is equal to the radius of maximum spherical grains, are given by log Rc = −0.11Eb/RT + 0.25log tr + C, in which Eb is the activation energy of the grain boundary diffusion of oxygen along the quartz/albite interface and C is a material constant.

The mean critical radius of spherical quartz inclusions in albite is 5 μm for the upper chlorite zone and garnet zone, 10 μm for the lower biotite zone, and 20 μm for the upper biotite zone in the Sambagawa metamorphic terrain. The mean values of the critical radii of spherical quartz inclusions in oligoclase of the Ryoke metamorphic rocks is about 5 μm for the chlorite zone and about 10–20 μm for the sillimanite zone.

Assuming temperatures of about 350°C for the upper chlorite and garnet zones, 400°C for the lower biotite zone, 550°C for the upper biotite zone, and 700°C for the sillimanite zone, the activation energy for the grain boundary diffusion of oxygen along the quartz/plagioclase interfase is estimated to be about 30 kcal/mol.  相似文献   


13.
Summary A forward model is proposed to reproduce the formation of garnet under conditions of sluggish diffusion transport in the matrix. Starting from a matrix consisting of chlorite and quartz, the amount of garnet growth and the chemical composition was calculated at each P–T increment in the system MnO–FeO–MgO–Al2O3–SiO2–H2O. Sluggish diffusion transport was introduced considering the local equilibrium between garnet surface and the matrix within a given diffusion distance (equilibration volume). Varying the diffusion distance, calculations were performed along the prograde P–T path of the Sambagawa metamorphic belt, Japan. The final size of the garnet grains was largely proportional to the diffusion distance. In contrast to the model without diffusion limitations, a shorter diffusion distance resulted in a rise of the Mg/(Mg + Fe) ratio in garnet before Mn approached zero. These results indicate that the chemical composition trend in zoned garnet from the Sambagawa belt is consistent with growth under sluggish material transport. The calculated amount of garnet growth increases dramatically with temperature. The amount of newly grown natural garnet in the Sambagawa metamorphic rocks was plotted against temperatures, where chemical compositions of garnet were calibrated against temperatures with the Gibbs’ method. This trend was also consistent with the modelled garnet behaviour.  相似文献   

14.
The interior of the Tauern Window exposes underplated Penninic continental lithosphere and the overlying obducted Penninic oceanic crust within a large antiformal dome in the internal zone of the Eastern Alps. These units have been affected by a polyphase deformation history. Generally, three deformation events are distinguished. D1 is related to underplating of, and top-to-the-N nappe stacking within, the Penninic continental units of the Tauern Window. Deformation stage D2 is interpreted to reflect the subsequent continent collision between the Penninic continental units and the European foreland, D3 is related to the formation of the dome structure within the Tauern Window. During thickening of continental lithosphere and nappe stacking (D1), and subsequent intracontinental shortening (D2), these tectonic units have been ductilely deformed close to a plane strain geometry. Conditions for the plastic deformation of the main rock-forming mineral phases (quartz, feldspar, dolomite, calcite) have prevailed during all three phases of crustal deformation. Generally, two types of quartz microstructures that are related to D1 are distinguished within the Tauern Window: (a) Equilibrated and annealed fabrics without crystallographic preferred orientations (CPO) have only been observed in the central part of the southeastern Tauern Window, corresponding with amphibolite-grade metamorphic conditions. (b) In the northeastern and central part of the Tauern Window microstructures are characterized by quartz grains that show equilibrated shape fabrics, but well preserved CPO with type-I cross girdle distributions, indicating a deformation geometry close to plane strain. During D2, two types of quartz microstructures are distinguished, too: (a) Quartz grains that show equilibrated shape fabrics, but well-preserved CPO. The c-axes distributions generally are characterized by type-I cross girdles, locally by type-II cross girdles, and in places, oblique single girdle distributions. (b) A second type of quartz microstructure is characterized by highly elongated grains and fabrics typical for dislocation creep and grain-boundary migration, and strong CPO. This type is restricted to the southern sections of the western and eastern Tauern Window. The c-axis distributions show type-I cross girdles in the western part of the Tauern Window and single girdles in the southeastern part. In the western part of the Tauern Window, a continuous transition from type (b) microstructures in the south to type (a) microstructures in the north is documented. The microstructural evolution also documents that the dome formation in the southeastern and western Tauern Window has already started during D2 and has continued subsequent to the equilibration during amphibolite to greenschist facies metamorphism. D3 is restricted to distinct zones of localized deformation. D3-related quartz fabrics are characterized by the formation of ribbon grains; the c-axes show small-circle distributions around the Z-axis of the finite-strain ellipsoid. During exhumation and doming (D3), deformation occurred under continuously decreasing temperatures.  相似文献   

15.
In the Sambagawa schist, southwest Japan, while ductile deformation pervasively occurred at D1 phase during exhumation, low-angle normal faulting was locally intensive at D2 phase under the conditions of frictional–viscous transition of quartz (c. 300 °C) during further exhumation into the upper crustal level. Accordingly, the formation of D2 shear bands was overprinted on type I crossed girdle quartz c-axis fabrics and microstructures formed by intracrystalline plasticity at D1 phase in some quartz schists. The quartz c-axis fabrics became weak and finally random with increasing shear, accompanied by the decreasing degree of undulation of recrystallized quartz grain boundaries, which resulted from the increasing portion of straight grain boundaries coinciding with the interfaces between newly precipitated quartz and mica. We interpreted these facts as caused by increasing activity of pressure solution: the quartz grains were dissolved mostly at platy quartz–mica interface, and precipitated with random orientation and pinned by mica, thus having led to the obliteration of existing quartz c-axis fabrics. In the sheared quartz schist, the strength became reduced by the enhanced pressure solution creep not only due to the reduction of diffusion path length caused by increasing number of shear bands, but also to enhanced dissolution at the interphase boundaries.  相似文献   

16.
江西武功山东区大型韧性剪切带的显微构造特征   总被引：2，自引：0，他引：2       下载免费PDF全文
吉磊 《地质科学》1995,30(1):95-103
武功山东区存在一条大型韧性剪切带。鞘褶皱倒向以及旋转变形构造(如S-C面理组构、旋转碎斑系、雪球构造和粒内显微破裂构造等)显示此剪切带为由南向北逆冲推覆性质。砾石、黄铁矿还原斑和石英斑晶的有限应变分析表明剪切带西段和东段岩石分别以收缩型椭球和压扁型椭球变形为特征。剪切带的主要变形时代是早古生代,可能与早古生代华夏陆块和扬子陆块之间的碰撞造山作用有关。  相似文献   

17.
AOYA  MUTSUKI 《Journal of Petrology》2001,42(7):1225-1248
Both structural and petrological data can be used to constrainthe P–T path of an eclogitic schist unit (the Seba basicschist) in the Sambagawa belt of SW Japan. The relationshipsbetween these two sets of data are well defined by porphyroblasticand other microstructures. The derived P–T path for theSeba basic schist has an overall clockwise trajectory with thedecompression, or exhumation-related, path taking place undera lower P/T gradient than the burial, or subduction-related,path. The clockwise nature of the P–T path is qualitativelysupported by chemical zoning of amphibole coexisting with eclogiticminerals. The significant feature of the P–T path is thepresence of two temperature maxima, the first in the eclogitefacies and the second in the epidote-amphibolite facies. Theexistence of two temperature maxima gives a simple explanationfor the observation that metamorphic zonal boundaries postdatingthe eclogite facies metamorphism cross-cut the distributionof the main eclogite bodies in the Sambagawa belt. Estimatesof metamorphic pressure using the jadeite content of clinopyroxenein the Seba area demonstrate the existence of a tectonic discontinuitybetween the eclogitic schist and surrounding non-eclogitic schist.Structural studies show that although these two units have experiencedvery different peak metamorphic conditions, they became juxtaposedduring a single ductile deformation affecting both units. Thisdeformation is related to exhumation of the eclogitic schistand subduction of the non-eclogitic schist, indicating thatboth were formed during the same subduction event. The presenceof a major tectonic boundary between two units with a similarorigin as subducted and accreted material, but contrasting metamorphichistories, can be interpreted in terms of nappe tectonics, andthe existence of an ‘eclogite nappe’, the thirdnappe of the Sambagawa belt, is proposed. KEY WORDS: deformation stage; dual thermal maxima; eclogite; P–T–D path; Sambagawa belt  相似文献   

18.
Quantification of discrete pressure–temperature domains in deformed chlorite + white mica‐bearing metapelites was undertaken on mineral compositions derived by two‐dimensional microprobe compositional mapping of selected areas of rock thin sections. In order to achieve compositional information at sufficient analytical precision, spatial resolution and sample coverage within a typical analysis time of 1 day, an optimization of measurement methods was necessary. The method presented here allows collection of raw counts for eight different element concentrations at an analytical precision of ～1–2 wt%. X‐ray intensity multiplane maps (one map per measured chemical element) are translated into concentration multiplane maps, utilizing selected conventionally measured spot analyses combined with the Castaing approximation for each mineral. As this step requires identification of the different minerals present in the mapped area, a statistical clustering technique to identify different groups of composition was developed, guided by simple petrographic inspection of the thin section, to delineate the important minerals in the mapped area. Finally, the compositions of each pixel are translated into a mineral structural formula thus yielding a new kind of image with a high content of petrological information. The reliability of the mineral composition images was emphasized by carrying out precision tests on the analytical data. The possible use of chemical maps to infer the P–T–deformation history of metamorphic rocks is illustrated with two samples from the Spitzbergen and the Sambagawa blueschist facies belts. In both samples, a strong correlation between structures and chemistry is observed. Qualitative estimates of P–T conditions from the Si‐content of mica and chlorite are in good agreement with their location in microstructures that formed at different times. Therefore, the combination of chemical maps with microstructural observations is a very powerful approach to understand both the evolution of complex metamorphic rocks and the control by deformation of mineral reactivity.  相似文献   

19.
A sharp line delimitating the distribution of tourmaline (termed as a ‘tourmaline‐out isograd’) is defined in the migmatite zone of the Ryoke metamorphic belt, Japan. The trend of the tourmaline‐out isograd closely matches that of the isograds formed through the regional metamorphism, suggesting that it represents the breakdown front of tourmaline during regional metamorphism. This is confirmed by the presence of the reaction textures of tourmaline to sillimanite and cordierite near the tourmaline‐out isograd. The breakdown of tourmaline would release boron into associated melts or fluids and be an important factor in controlling the behaviour of boron in tourmaline‐bearing high‐temperature metamorphic rocks. Near the tourmaline‐out isograd, large tourmaline crystals occur in the centre of interboudin partitions containing leucosome. In the melanosome of the intervening matrix, reaction textures involving tourmaline are locally observed. These observations imply that tourmaline breakdown is related to a melting reaction and that the boron in the leucosome is derived from the breakdown of tourmaline in the melanosome during prograde metamorphism. Boron released by tourmaline breakdown lowers both the solidus temperature of the rock and the viscosity of any associated melt. Considering that the tourmaline‐out isograd lies close to the schist–migmatite boundary, these effects might have enhanced melt generation and segregation in the migmatite zone of the Ryoke belt. The evidence for the breakdown of tourmaline and the almost complete absence of any borosilicates throughout the migmatite zone suggest that boron was effectively removed from this region by the movement of melt and/or fluid. This implies that the tourmaline‐out isograd can reflect a significant amount of mass transfer in the anatectic zones.  相似文献   

20.
相山地区变质岩划分为４个变质岩带，由南往北依次分布，构成典型的递增变质带。十字石的出现表明变质作用已达到低角闪岩相。获得十字石云母片岩及斜长角闪片岩ＲｂＳｒ等时年龄分别为７１９．７、７２７．６Ｍａ。首次厘定相山变质岩形成于新元古代，而非加里东期变质。其原岩的成岩时代应为前震旦纪  相似文献   

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