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1.
W. Cui  X. Wang 《Island Arc》1995,4(4):347-361
Abstract According to field occurrence and P-T condition, eclogites of southern Henan and northern Hubei Provinces can be divided into two types: medium temperature (MT) and low temperature (LT) eclogites. MT eclogite occurs as layers or lenticular bodies within migmatized gneiss of the Dabie Group. This study is the first to report an occurrence of the assemblages coesite and kyanite + talc in this area. Garnet exhibits a distinct prograde compositional zoning and has mineral inclusions with rotational textures indicating syntectonic growth. Five evolutionary stages are outlined. (1) Pre-eclogite stage, determined by the inclusions of barroisite + zoisite + quartz in the cores of zoned garnets. (2) Eclogite stage, characterized by garnet + omphacite + kyanite ± talc + coesite + rutile, represents the peak metamorphism. The peak conditions are estimated to be T = 600-700°C, P >27 kb. (3) Glaucophane stage, without an appearance of plagioclase, is assigned to a transitional stage. Blades of glaucophane form rims around garnet grains as a result of the reaction talc + jadeite = glaucophane. This marks the beginning of retrograde metamorphism. (4) Symplectite stage, where eclogitic minerals break down, and Amp + Pl symplectite develops around garnet or omphacite; (5) Later retrograde stage is represented by epidote-amphibolite assemblages. Low temperature eclogite appears as blocks in the Qijiaoshan Formation (part of the Susong Group). Four stages can be identified: (1) Pre-eclogite stage, amphibole + epidote + sphene inclusions occur in garnet core; (2) Eclogite stage, consists of garnet + omphacite + rutile + quartz + phengite + glaucophane + zoisite. The peak conditions are T = 490-560°C, P <15 kb; (3) Symplectitic stage, is characterized by the breakdown of eclogitic minerals; (4) Greenschist facies stage, is recorded by a greenschist facies assemblage. The difference between the two types of eclogites suggests contrasting processes. A model is proposed whereby partial melting of continental crust and the emplacement of tonalite occurs during the exhumation of ultrahigh-pressure eclogite terrain.  相似文献   

2.
Poly-deformed and poly-metamorphosed glaucophane-eclogite mega-boudins beneath the Samail Ophiolite, Oman record an early subduction-related high-P metamorphism as well as subsequent overprinting deformation and metamorphism related to exhumation. Previously published Rb/Sr ages of 78 Ma and 40Ar/39Ar ages of 82-79 Ma record the major NE-directed shearing event that partially exhumed the eclogites to a shallower crustal level. New Sm/Nd garnet-garnet leachate-whole rock isochron data from garnet-bearing eclogite assemblages in the As Sifah subwindow in NE Oman are 110±9 Ma (DG02-87D); 5-point isochron) and 109±13 Ma (DG02-86E; 3-point isochron). On the basis of microfabric and field structural relationships these ages are interpreted to reflect the timing of prograde, peak high-P metamorphism in the rocks structurally beneath the Samail Ophiolite. This metamorphism clearly predates the age of formation of the obducted Samail oceanic lithosphere (97-94 Ma) as well as the subsequent obduction onto the margin (80-70 Ma). A U-Pb SHRIMP zircon age from small (<200 μm in length) zircons with herring-bone textured zoning from DG02-87D indicate that rapid zircon growth associated with high-Si phengites occurred at 82±1 Ma. Zircon growth is possibly related to liberation of Zr on garnet breakdown during decompression metamorphism under high-P conditions with exhumation. These data require that crustal stacking models attendant with ophiolite obduction are inappropriate to explain the Oman high-P metamorphism.  相似文献   

3.
Abstract The amphibolites occur sporadically as thin layers and blocks throughout the Sulu Terrane, eastern China. All analyzed amphibolite from outcrop and drill cores from prepilot drill hole CCSD‐PP1 and CCSD‐PP2, Chinese Continental Scientific Drilling Project in the Sulu Terrane, are retrograded eclogites overprinted by amphibolite‐facies retrograde metamorphism, with characteristic mineral assemblages of amphibole + plagioclase + epidote ± quartz ± biotite ± ilmenite ± titanite. However, coesite and coesite‐bearing ultrahigh‐pressure (UHP) mineral assemblages are identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from these amphibolites. In general, coesite and other UHP mineral inclusions are preserved in the cores and mantles of zircons, whereas quartz inclusions occur in the rims of the same zircons. The UHP mineral assemblages consist mainly of coesite + garnet + omphacite + rutile, coesite + garnet + omphacite, coesite + garnet + omphacite + phengite + rutile + apatite, coesite + omphacite + rutile and coesite + magnesite. Compositions of analyzed mineral inclusions are very similar to those of matrix minerals from Sulu eclogites. These UHP mineral inclusion assemblages yield temperatures of 631–780°C and pressures of ≥2.8 × 103 MPa, representing the P–T conditions of peak metamorphism of these rocks, which are consistent with those (T = 642–726°C; P ≥ 2.8 × 103 MPa) deduced from adjacent eclogites. These data indicate that the amphibolites are the retrogressive products of UHP eclogites.  相似文献   

4.
Dislocation microstructures in naturally deformed silicate garnets   总被引：1，自引：0，他引：1  
Dislocation microstructures of naturally deformed silicate garnets and olivines in garnet-peridotites and silicate garnets in eclogites from four localities have been observed with a transmission electron microscope (TEM) to clarify the dislocation characteristics of silicate garnets. We have obtained the following results: (1) dislocation densities of garnets in all the garnet-peridotites (ρ = 105−107 cm−2) are always nearly an order of magnitude lower than those of co-existing olivines; (2) dislocation densities of garnets in eclogites (ρ = 105−108 cm−2) which are embedded in garnet-peridotites are almost an order of magnitude higher than those of garnets in the surrounding garnet-peridotites; (3) the dominant Burgers vector, b, of mainly edge dislocations in garnet is 100 for specimens with dislocation density ρ = 105−106 cm−2, while b=1/2111 for specimens with ρ = 107−108 cm−2. Result (1) indicates that the observed dislocations in garnets were formed by plastic deformation under the same stresses as for co-existing olivines, and that there is a similar relationship between applied stress and dislocation density for garnets as for olivines. Result (2) suggests that the stress concentration occurred around eclogites embedded in garnet-peridotites, and the resulting differential stress in garnets in eclogites was further elevated by the interlocking of neighboring hard garnet grains. Finally, result (3) indicates that the dominant Burgers vector of mainly edge dislocations in garnet changes from 100 to 1/2111 with increasing applied differential stress.  相似文献   

5.
Lower crustal high grade metamorphic rocks have been successively found at Pamirs nearby the western Himalayan syntaxis, Namjagbarwa and Dinggye nearby the eastern Himalayan syntaxis and the central segment of the Himalayan Orogenic Belt, respec-tively[1―4]. In particular, some researchers deduced that there were probably eclogites at some locations[5]. Moreover, some geochronological data of these lower crustal granulites also have been accumulated. For example, the high-pressure granulit…  相似文献   

6.
The strongly deformed eclogites are well developed in ultra-high pressure jadeite-quartzite zone of the Dabie Mountains, Eastern China, and garnets had been deformed strongly. Observations by transmission electron microscopy identified not only structure of plastic deformation occurring as free dislocation, dislocation loops and dislocation walls, but also clusters of water molecules present in the deformed garnet. Using infrared spectroscopy, two types of hydrous components are identified as the hydroxyl and free-water in the garnet. Based on analysis of microstructure mechanism of deformation in garnets, and experimental data of petrology, the clusters of water molecules were considered to lead strong plastic deformation of garnet by dislocations because of mechanical weakening.  相似文献   

7.
The strongly deformed eclogites are well developed in ultra-high pressure jadeite-quartzite zone of the Dabie Mountains, Eastern China, and garnets had been deformed strongly. Observations by transmission electron microscopy identified not only structure of plastic deformation occurring as free dislocation, dislocation loops and dislocation walls, but also clusters of water molecules present in the deformed garnet. Using infrared spectroscopy, two types of hydrous components are identified as the hydroxyl and free-water in the garnet. Based on analysis of microstructure mechanism of deformation in garnets, and experimental data of petrology, the clusters of water molecules were considered to lead strong plastic deformation of garnet by dislocations because of mechanical weakening.  相似文献   

8.
Yong-Jiang  Liu  Franz  Neubauer  Johann  Genser  Akira  Takasu  Xiao-Hong  Ge Robert  Handler 《Island Arc》2006,15(1):187-198
Abstract   Pelitic schists from Qingshuigou in the Northern Qilian Mountains of China contain mainly glaucophane, garnet, white mica, clinozoisite, chlorite and piemontite. Isotopic age dating of these schists provides new constraints on the formation of the high-grade blueschists at Qingshuigou. White mica 40Ar/39Ar ages range from 442.1 to 447.5 Ma (total fusion age of single grain) and from 445.7 to 453.9 Ma (integrated age of white mica concentrates). These ages (442.1–453.9 Ma) represent the peak metamorphic ages or cooling ages of the blueschists during exhumation shortly after peak metamorphism. The 40Ar/39Ar dates in the present study are similar to ages previously reported for eclogites and blueschists in the area; this suggests that both the eclogites and pelitic sediments underwent high-grade metamorphism during the same subduction event. From this chronological evidence and the presence of well-developed Silurian remnant-sea flysch and Devonian molasse, it is concluded that the Northern Qilian Ocean had closed by the end of the Ordovician, and rapid orogenic uplift followed in the Devonian.  相似文献   

9.
Eclogites and omphacite-bearing blueschists have been newly found in the eastern segment of the southwest Tianshan orogenic belt,Xinjiang,northwest China.After detailed petrological study,three samples including one fresh eclogite TK003,one blueschist sample TK026-8 and one retrograded eclogite TK027,were selected for phase equilibrium modeling under NC(K)MnFMASHO(N2O-CaO-K2O-MnO-FeO-MgO-Al2O3-SiO2-H2O-O)system,by thermocalc 3.33 software.Composition analyses of garnets in these three samples show typical growth zoning with Xpy and Xgrs increasing,Xspss decreasing from core to rim.Pseudosection modeling of the garnet zonation reflects that the eclogites and blueschist experienced a similar P-T evolution trajectory,with a near iso-baric heating in the early stage,and reached eclogite facies metamorphic field with peak P-T regime of 480–515°C,2.00–2.30 GPa.Subsequently the rocks experienced an early iso-thermal decompression retrograde stage with P-T conditions of 515–519°C,1.78–1.93 GPa.Variations of mineralogy and modes of these rocks are probably due to different retrograde paths as a consequence of different bulk-rock composition,as well as a variation in fluid activity during exhumation.P-T calculation and a peak geothermal gradient of 6–7°C/km indicate HP rocks in the Kekesu Valley experienced cold subducted eclogite facies metamorphism.Thus a huge oceanic subduction eclogite facies metamorphic belt in southwest Tianshan has been recognized,extending from the Kekesu Valley in the east to the Muzhaerte Valley in the west for nearly200 km.However,UHP evidence has not been found in the Kekesu terrane,perhaps because the slab in east part of southwest Tianshan did not subduct into such a great depth.  相似文献   

10.
δ18O values of coexisting garnet, clinopyroxene and phlogopite for twelve compositionally and texturally diverse Roberts Victor eclogite xenoliths range from +3.8 to +7.1, +4.0 to +7.4 and +5.9 to +7.4, respectively. Differences between theδ18O values of coexisting garnets and clinopyroxenes are normally zero; however, there is some variation in theδ18O values of different fractions of the same mineral in four samples which suggests the presence of isotopic zonation and inhomogeneity, possibly resulting from the introduction of a secondary fluid which metasomatized the eclogites and resulted in the formation of phlogopite, amphibole and celsian. Theδ18O value of the metasomatic fluid is generally buffered by the isotopic composition of the primary garnet and clinopyroxene, as indicated by a correlation between the isotopic composition of phlogopite and the primary pyroxene and garnet.The large range inδ18O values of the eclogites and the similarity in the isotopic composition of coexisting pyroxene and garnet support the interpretation that the Roberts Victor eclogites represent metamorphosed, altered basalts. The eclogites were subjected to infiltration metasomatism in the mantle prior to their incorporation in the kimberlite, and the source of this fluid was probably unrelated to the eclogite.  相似文献   

11.
Parallel studies on rock magnetic properties, petrology and mineralogy were conducted on 16 eclogite samples from the ZK703 hole and magnetic susceptibilities and densities of 41 eclogite samples with different degree of retrograde metamorphism (from fresh eclogite to fully-retrograded eclogite) from the Chinese Continental Scientific Drilling (CCSD) near the ZK703 hole, located at Donghai, southern Sulu ultrahigh-pressure metamorphic belt, eastern China. Results show: (1) that the high-field slopes obtained from the hysteresis loops (the paramagnetic fraction χpara) and density have a positive correlation with the volume concentration of garnet + omphacite, a typical mineral assemblage used to semi-quantify the degree of retrograde metamorphism. The low-field slopes obtained from hysteresis loops (the ferrimagnetic susceptibility fraction χferri), saturation isothermal remanent magnetization Mrs and saturation magnetization Ms have a positive correlation with the volume concentration of symplectite, a mineral related to retrograded metamorphism. Therefore they could be potential indicators for quickly semi-quantifying the degree of retrograde metamorphism of the eclogite units. (2) The dominant magnetic carriers in retrograded eclogites are magnetite particles in pseudo-single domain grain size region. (3) The P–T conditions during the retrograde (decompressional) process could first increase the concentration of magnetite, which can reach up to 3% for extensively retrograded eclogite and then was dissolved for fully-retrograded eclogite. Therefore, change in the magnetite contents during the retrograde process is the major factor controlling the magnetism of retrograde eclogites.  相似文献   

12.
Diancangshan metamorphic massif is one of the four metamorphic massifs developed along the Ailaoshan-Red River strike-slip fault zone, Yunnan, China. It has experienced multi-stage metamorphism and deformation, especially since the late Oligocene it widely suffered high-temperature ductile shear deformation and exhumation of the metamorphic rocks from the deep crust to the shallow surface. Based on the previous research and geological field work, this paper presents a detailed study on deformation and metamorphism, and exhumation of deep metamorphic rocks within the Diancangshan metamorphic massif, especially focusing on the low-temperature overprinted retrogression metamorphism and deformation of mylonitic rocks. With the combinated experimental techniques of optical microscope, electron backscatter diffraction attachmented on field-emission scanning electron microscopy and cathodoluminescence, our contribution reports the microstructure, lattice preferred orientations of the deformed minerals, and the changes of mineral composition phases of the superposition low-temperature retrograde mylonites. All these results indicate that: (1) Diancangshan deep metamorphic rock has experienced early high-temperature left-lateral shear deformation and late extension with rapid exhumation, the low-temperature retrogression metamorphism and deformation overprinted the high-temperature metamorphism, and the high-temperature microstructure and texture are in part or entirely altered by subsequent low-temperature shearing; (2) the superposition of low-temperature deformation-metamorphism occurs at the ductile-brittle transition; and (3) the fluid is quite active during the syn-tectonic shearing overprinted low-temperature deformation and metamorphism. The dynamic recrystallization and/or fractures to micro-fractures result in the strongly fine-grained of the main minerals, and present strain localization in micro-domians, such as micro-shear zones in the mylonites. It is often accompanied by the decrease of rock strength and finally influences the rheology of the whole rock during further deformation and exhumation of the Diancangshan massif.  相似文献   

13.
Abstract Eclogites occur in three districts of the northern and southern parts of Tien-Shan. Three eclogites collected from the Aktyuz, Makbal and Atbashy districts were analyzed; the P-T paths of three eclogites were estimated by analyzing compositional growth zoning and retrograde reaction of garnet and omphacite. Aktyuz and Makbal eclogites have not preserved the prograde path. An Aktyuz eclogite that underwent a quartz eclogite facies metamorphism (about T = 600°C, P = 12 kbar) has recorded three stages of retrograde metamorphism. Four stages of retrograde metamorphism were recognized in a Makbal eclogite; the garnet-omphacite geothermometer gave about T = 560°C at 20 kbar as the highest metamorphic condition. Garnet from a garnetchloritoid-talc schist of the Makbal district includes quartz pseudomorphs after coesite; some units evidently underwent a low-temperature part of coesite eclogite fades metamorphism. Prograde and retrograde paths were recognized in an Atbashy eclogite; five stages of metamorphic reaction were observed in the Atbashy sample. The prograde path from stage I to stage III has been recorded in garnet and omphacite in which quartz pseudomorphs after coesite are included. The peak metamorphism of stage III took place at about 660°C at 25 kbar. The stages IV and V are retrograde. UHP eclogite facies metamorphism took place twice in Kyrghyzstan. The Aktyuz and Atbashy eclogites gave Rb-Sr mineral-isochron ages of about 750 Ma and 270 Ma, respectively. The K-Ar age of paragonite from the Makbal eclogite is about 480 Ma.  相似文献   

14.
Abstract The Maksyutov Complex, situated in the southern Ural Mountains of Russia, is the first location where quartz aggregates within garnets exhibiting radial fractures were identified as coesite pseudomorphs (Chesnokov & Popov 1965). The complex consists of two tectonic units: a structurally lower eclogite-bearing schist unit and an overlying meta-ophiolite unit. Both units show evidence for multiple stages of metamorphism and deformation. The high-pressure metamorphism of the eclogite-bearing schist unit, discussed in this report, is suspected to be related to a collision between the Russian platform and a fragment of the Siberian continent during the early Cambrian. At least three stages of metamorphism (M1-3) and two stages of deformation (S1 and S2) were observed in thin sections: M1) garnet (Alm55-60, Prp22-28, Grs16-20) + omphacite (Jd46-56) + phengite (Si ≅ 3.5) + rutile; M2) garnet + glaucophane ± lawsonite + white mica; and M3) epidote + chlorite ± albite ± actinolite + white mica. Observed mineral parageneses define a retrograde P-T path for the eclogite. Mineral assemblages within the most representative eclogite from the lower unit of the Maksyutov Complex indicate minimum peak pressures of 15 kbar at temperatures of approximately 600°C. If the presence of coesite pseudomorph is confirmed, the peak ultrahigh-pressure metamorphism may be as high as 27 kbar at 615°C.  相似文献   

15.
Garnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks.  相似文献   

16.
H. Tabata  S. Maruyama  & Z. Shi 《Island Arc》1998,7(1-2):142-158
The ultrahigh- and high-pressure (UHP–HP) metamorphic belt of the Dabie Mountains, central China, formed by the Triassic continental subduction and collision, is divided into four metamorphic zones; from south to north, the greenschist facies zone, epidote amphibolite to amphibolite facies zone, quartz eclogite zone, and coesite eclogite zone, based on metabasite mineral assemblages. Most of the coesite-bearing eclogites consist mainly of garnet and omphacite with homogeneous compositions and have partially undergone hydration reactions to form clinopyroxene + plagioclase + calcic amphibole symplectites during amphibolite facies overprinting. However, the least altered eclogites sometimes contain garnet and omphacite that preserve compositional zoning patterns which may have originated during their growth at peak temperature conditions of ∼ 750 °C, suggesting a short duration of UHP metamorphic conditions and/or consequent rapid cooling during exhumation. Systematic investigation on peak metamorphic temperatures of coesite eclogite have revealed that, contrary to the general trend of metamorphic grade in the southern Dabie unit, the coesite eclogite zone shows rather flat thermal structure (T = 600 ± 50 °C) with the highest temperature reaching up to 850 °C and no northward increase in metamorphic temperature, which is opposed to the previous interpretations. This feature, along with the preservation of compositional zonation, implies complicated differential movement of each eclogite mass during UHP metamorphism and the return from the deeper subduction zone at mantle depths to the surface.  相似文献   

17.
The Sanbagawa high-pressure schists from central Shikoku in Southwest Japan have experienced high-strain ductile deformation during exhumation and cooling. This study examines the effects of high-strain ductile deformation on K–Ar ages of phengites on the basis of fabric, chemistry and K–Ar ages of phengites from the pelitic, psammitic and quartzose (or albitic) schists collected from the same outcrop in the albite–biotite zone. Phengites in the pelitic and psammitic schists generally occur forming aggregates consisting of fine-grained phengite crystals and are extremely fine-grained in domains close to relatively rigid garnet and albite porphyroblasts, indicating that deformation-induced grain-size reduction had taken place in phengite during the ductile deformation accompanying the exhumation of host schists. We suggest that the grain-size reduction of phengite is due to strain-induced recrystallization or dynamic recrystallization. The matrix phengites in schists are chemically heterogeneous on the thin-section scale but the phengites from pelitic and psammitic schists from the same outcrop have similar chemical range. Phengite included in garnet has a high Si value and its Na/(Na + K) and Mg/(Mg + Fe) ratios are significantly low in comparison with those in matrix. The phengite included in garnet records the chemistry in equilibrium with other major silicate phases during the higher pressure stage of the P–T–t history of the schists. In contrast, the matrix phengites having low Si values are likely to have been formed during retrograde metamorphism in extremely restricted equilibrium domains. The two or three different types of schists from the same outcrop, which have a similar grain size of phengite, have similar K–Ar ages, suggesting that the closure temperature does not depend on chemistry. However, the hematite-rich quartzose schist with strong grain-size reduction of both phengite and quartz has a significantly younger K–Ar phengite age than the pelitic and quartzose schists in the same outcrop that do not show grain-size reduction. We suggest that the exhumation tectonics of the schists, which have experienced strong ductile deformation at temperatures less than ~350°C, played an important role resulting in the observed variation in age.  相似文献   

18.
Abstract In northern Kazakhstan the WNW striking Kokchetav megamélange includes different crustal sequences with high‐pressure/ultrahigh‐pressure (HP/UHP) remnants of their 540–520 Ma subduction metamorphism. Two domains separated by the north‐east trending Chaglinka fault are distinguished. The western domain exhibits NE–SW structures within a single Kumdy–Kol megaunit of diamond‐bearing UHP metasediments and high‐temperature (HT) eclogites. The eastern domain consists of the composite Kulet megaunit with the Kulet UHP unit (coesite‐bearing metasediments, whiteschists and eclogites), the Enbek–Berlyk medium‐pressure (MP) unit (kyanite‐bearing, high‐alumina rocks with interleaved coronitic metagabbro), and ortho‐ and paragneisses with eclogites and amphibolites included. All eclogites in the eastern domain are of the relatively low temperature (LT) type. Sillimanite is common and appears after kyanite in the sheared MP unit. A regional and moderately ESE plunging linear fabric coincides with the fold‐axis of the foliation poles from the eastern domain. Whether this also reflects a regional top to the WNW transport, as inferred from the dextral strike‐slip on steeply to SSW dipping foliation, needs further study. Top to the WNW shear is shown by weakly inclined low pressure (LP) cordierite rocks that flank the eastern domain in the south. Some new 39Ar/40Ar mica cooling ages (519, 521 Ma) from the Kulet UHP micaschists reflect the same early stage evolutionary event as was previously shown for the Kumdy–Kol UHP rocks (515, 517 Ma) in the west. Similar 39Ar/40Ar ages (500, 517 Ma) are recorded by micas and amphibole that outline a top to NNW shear fabric in the non‐subducted Proterozoic basement, north of the megamélange. A 447 Ma overprint of the MP sequences is considered to reflect the strike‐slip deformation with sillimanite and the reworking of an early kyanite‐bearing tectonite. Biotites from the LP cordierite rocks yielded approximately 400 Ma 39Ar/40Ar ages. In case they reflect the WNW shear deformation, the latter is considered to be associated with a regional granite magmatism (420–460 Ma) extending south of the eastern domain. In their present different structural domains the Kulet and Kumdy–Kol UHP units display a similar early stage event. Subsequent LP deformation, which is likely to be associated with regional granite magmatism (420–460 Ma), is assumed to have obliterated any common or uniform early exhumation structure for the whole megamélange. The north‐east structured Kumdy–Kol domain is assumed to have preserved the most information about the early stage exhumation. This domain is at an angle to the regional WNW strike of the megamélange.  相似文献   

19.
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 (> 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.  相似文献   

20.
Yasuo  Miyagi  Akira  Takasu 《Island Arc》2005,14(3):215-235
Abstract   Prograde eclogites occur in the Tonaru epidote amphibolite mass in the Sambagawa Metamorphic Belt of central Shikoku. The Tonaru mass is considered to be a metamorphosed layered gabbro, and occurs as a large tectonic block (approximately 6.5 km × 1 km) in a high-grade portion of the Sambagawa schists. The Tonaru mass experienced high- P /low- T prograde metamorphism from the epidote-blueschist facies to the eclogite facies prior to its emplacement into the Sambagawa schists. The estimated P – T conditions are T  = 300–450°C and P  = 0.7–1.1 GPa for the epidote-blueschist facies, and the peak P – T conditions for the eclogite facies are T  = 700–730°C and P  ≥ 1.5 GPa. Following the eclogite facies metamorphism, the Tonaru mass was retrograded to the epidote amphibolite facies. It subsequently underwent additional prograde Sambagawa metamorphism, together with the surrounding Sambagawa schists, until the conditions of the oligoclase–biotite zone were reached. The high- P /low- T prograde metamorphism of the eclogite facies in the Tonaru mass and other tectonic blocks show similar steep d P /d T geothermal gradients despite their diverse peak P – T conditions, suggesting that these tectonic blocks reached different depths in the subduction zone. The individual rocks in each metamorphic zone of the Sambagawa schists also recorded steep d P /d T geothermal gradients during the early stages of the Sambagawa prograde metamorphism, and these gradients are similar to those of the eclogite-bearing tectonic blocks. Therefore, the eclogite-bearing tectonic blocks reached greater depths in the subduction zone than the Sambagawa schists. All the tectonic blocks were ultimately emplaced into the hanging wall side of the later-subducted Sambagawa high-grade schists during their exhumation.  相似文献   

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