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1.
Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.  相似文献   

2.
A sequence of mineral associations was examined in eclogitized basites of the Krasnaya Guba dike field in the Belomorian Mobile Belt. Two morphological types of eclogite and eclogite-like rocks were recognized: (1) eclogite rocks that developed after ferrogabbro dikes and completely replaced these dikes from contact to contact and (2) eclogite-like rocks that developed after gabbronorites in zones of ductile deformations and shearing. According to data mineral geothermobaromety, both rock types were formed within temperature and pressure ranges corresponding to high-pressure and high-temperature amphibolite facies at T = 700 ± 40°C and P = 10.0 ± 0.5 kbar. The peak metamorphic parameters of the host gneisses are analogous. The decompressional stage, which is unambiguously identified by reaction textures, occurred at 630–660°C and 7.9–8.2 kbar. As the temperature and, first of all, pressure decreased, the SiO2 activity in the fluid systematically varied. The eclogitization of the basites took place locally in relation to fluid fluxes, which were restricted to zones of intense deformations, at variable SiO2 activity. The rocks show evidence of two stages of post-eclogite amphibolization. Older amphibolization 1 was coeval with the late prograde metamorphic stage (T = 650°C, P = 10–11 kbar). Younger amphibolization 2 affected eclogitized basite dikes and unaltered gabbronorites (together with their host gneisses) over large areas. This process coincided with decompression (T = 580°C, P = 7–8 kbar) and was likely accompanied by the exhumation of deep zones of BMB to upper-middle crustal levels.  相似文献   

3.
ABSTRACT The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic-ultramafic blocks, and abundant Jurassic-Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high-pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene-bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near-isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase-amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite-amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km-1 at intermediate to low pressures, and may represent the Sino-Korean hangingwall during Triassic subduction for formation of the ultrahigh- and high-P units to the south. Post-collisional intrusion of a mafic-ultramafic cumulate complex occurred due to breakoff of the subducting slab.  相似文献   

4.
The paper reports the first data obtained by state-of-the-art analytical techniques on the composition of minerals and the metamorphic age and metamorphic parameters of eclogite-like rocks from Sidorov and Ileiki islands, Keret Archipelago, White Sea. The U-Pb zircon magmatic age of the metabasites lies within the range of 2400–2480 Ma. The rocks were subjected to eclogite metamorphism at 1870–1890 Ma, with this age estimate consistent with analogous estimates for eclogite metamorphism elsewhere within BMB. Simultaneously garnetite zones were produced. The metabasites were eclogitized at 12 kbar and 700°C, i.e., near the boundary between the eclogite and amphibolite facies of relatively high pressure, because of a local pressure increase during rock cooling in the presence of fluid. The retrograde metamorphic episode proceeded under pressures from 12 to 6.5 kbar at temperatures from 700 to 600°C. The contact amphibolization of the metabasites at a temperature close to 620°C and pressures of 2–2.5 kbar (low-pressure amphibolite facies) occurred at 1870 Ma and is pronounced in the form of an amphibolite rim around a boudin of eclogitized basite and in significant changes in the trace-element and REE composition of the eclogite zircon.  相似文献   

5.
《地学前缘(英文版)》2018,9(6):1795-1807
The high-to ultrahigh-pressure metamorphic rocks of the Atbashy complex were petrologically investigated. The eclogites of the Choloktor Formation show a prograde evolution from epidote-blueschist facies(P = 17-21 kbar and T = 450-515 ℃) to peak eclogite-UHP conditions(P = 26-29 kbar and T = 545-615 ℃) with a subsequent epidote-amphibolite and greenschist facies overprint. The micaschists of the Choloktor Formation also show a clockwise P-T path from blueschist/epidote-blueschist facies conditions through peak eclogite facies conditions(P = 21-23 kbar and T = 530-580 ℃) to retrograde epidote-amphibolite and greenschist facies stages. A comparison of the P-T paths in the eclogites and mica-schists of Choloktor Formation reveal that they may have shared their P-T history from peak to retrograde stages. The mica-schists of the Atbashy Formation record peak metamorphism of P = 10-12 kbar and T = 515-565 ℃, which indicates that the highest grade of regional metamorphism in the Atbashy Ridge was epidote-amphibolite facies.The newly obtained P-T conditions for the mica-schists of Choloktor Formation indicate that sheets of sedimentary rocks were brought to great depths along the subduction zone and they metamorphosed under eclogite facies HP conditions. The eclogite blocks were amalgamated with mica-schists of Choloktor Formation in the eclogite facies HP conditions and together they experienced isothermal decompression to ～40 km. During this path, the eclogites and mica-schists of Choloktor Formation docked with mica-schists of Atbashy Formation at 10-12 kbar and 515-565 ℃, and from this depth(～40 km) the whole sequence was exhumed together. These new results improve our understanding of high-pressure metamorphism in subduction-related accretionary prism zones and the exhumation processes of deeply-seated rocks in the Atbashy HP-UHP complex.  相似文献   

6.
Recently, a huge ultrahigh‐pressure (UHP) metamorphic belt of oceanic‐type has been recognized in southwestern (SW) Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, felsic garnet phengite schists, marbles and serpentinites. The well‐preserved coesite inclusions were commonly found in eclogites, garnet phengite schists and marbles. Ti‐clinohumite and Ti‐chondrodite have been identified in UHP metamorphic serpentinites. Based on the PT pseudosection calculation and combined U‐Pb zircon dating, the P‐T‐t path has been outlined as four stages: cold subduction to UHP conditions before ～320 Ma whose peak ultrahigh pressure is about 30 kbar at 500oC, heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600oC at 22kbar, then the early cold exhumation from amphibolite eclogite facies to epidote‐amphibolite facies metamorphism characterized by ITD PT path before 220 Ma and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn‐subduction arc‐like 333‐326 Ma granitic rocks and 280‐260 Ma S‐type granites in the coeval low‐pressure and high‐temperature (LP‐HT) metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed.  相似文献   

7.
The Strona-Ceneri Zone comprises a succession of polymetamorphic, pre-Alpidic basement rocks including ortho- and paragneisses, metasedimentary schists, amphibolites, and eclogites. The rock pile represents a Late Proterozoic or Palaeozoic subduction accretion complex that was intruded by Ordovician granitoids. Eclogites, which occur as lenses within the ortho-paragneiss succession and as xenoliths within the granitoids record a subduction related high-pressure event (D1) with peak metamorphic conditions of 710 ± 30 °C at 21.0 ± 2.5 kbar. After isothermal uplift, the eclogites experienced a Barrowtype (D2) tectonometamorphic overprint under amphibolite facies conditions (570-630 °C, 7-9 kbar). U-Pb dating on zircon of the eclogites gives a metamorphic age of 457 ± 5 Ma, and syn-eclogite facies rutile gives a 206Pb/238U age of 443 ± 19 Ma classifying the subduction as a Caledonian event. These data show that the main tectonometamorphic evolution of the Strona-Ceneri Zone most probably took place in a convergent margin scenario, in which accretion, eclogitization of MOR-basalt, polyphase (D1 and D2) deformation, anatexis and magmatism all occurred during the Ordovician. Caledonian high-pressure metamorphism, subsequent magmatism and Barrow-type metamorphism are believed to be related to subduction and collision within the northern margin of Gondwana. Editorial handling: Edwin Gnos  相似文献   

8.
Metabasic rocks from the Adula Nappe in the Central Alps record a regional high‐pressure metamorphic event during the Eocene, and display a regional variation in high‐pressure mineral assemblages from barroisite, or glaucophane, bearing garnet amphibolites in the north to kyanite eclogites in the central part of the nappe. High‐pressure rocks from all parts of the nappe show the same metamorphic evolution of assemblages consistent with prograde blueschist, high‐pressure amphibolite or eclogite facies conditions followed by peak‐pressure eclogite facies conditions and decompression to the greenschist or amphibolite facies. Average PT calculations (using thermocalc ) quantitatively establish nested, clockwise P–T paths for different parts of the Adula Nappe that are displaced to higher pressure and temperature from north to south. Metamorphic conditions at peak pressure increase from about 17 kbar, 640 °C in the north to 22 kbar, 750 °C in the centre and 25 kbar, 750 °C in the south. The northern and central Adula Nappe behaved as a coherent tectonic unit at peak pressures and during decompression, and thermobarometric results are interpreted in terms of a metamorphic field gradient of 9.6 ± 2.0 °C km?1 and 0.20 ± 0.05 kbar km?1. These results constrain the peak‐pressure position and orientation of the nappe to a depth of 55–75 km, dipping at an angle of approximately 45° towards the south. Results from the southern Adula Nappe are not consistent with the metamorphic field gradient determined for the northern and central parts, which suggests that the southern Adula Nappe may have been separated from central and northern parts at peak pressure.  相似文献   

9.
俯冲隧道是俯冲板片与上覆板块之间的剪切带,也是高压—超高压变质岩折返和深部流/熔体活动的通道。大别山超高压变质岩分布广泛,变形程度差异很大,是研究大陆俯冲隧道中岩石变质- 变形过程的理想地区。本文系统总结了前人对中大别双河地区超高压变质岩的岩石学和年代学研究成果,在双河地区开展了地质填图、应变分析和三维构造重建。通过将超高压变质岩的变形特征与P- T- t轨迹结合,识别出超高压变质岩折返过程中的三期韧性变形。在双河北部发现了一个上盘向NW剪切的千米尺度的榴辉岩相鞘褶皱,枢纽向SE倾伏,倾伏角约20°,与榴辉岩、片岩和长英质片麻岩的拉伸线理平行,表明超高压变质岩初始折返阶段的流体活动使榴辉岩的强度显著降低,榴辉岩与围岩一起发生韧性变形。该期变形被角闪岩相退变质阶段上盘向NW的剪切叠加,此时应变集中于片麻岩、片岩、大理岩等非能干层,强度较高的榴辉岩成为构造透镜体。而绿片岩相变质阶段上盘向SE方向的剪切与早白垩世北大别花岗片麻岩穹隆的形成有关。对双河南部弱变形花岗片麻岩的锆石U- Pb定年揭示了757±14 Ma的原岩年龄和 240~216 Ma的变质年龄,与双河北部含柯石英强变形花岗片麻岩类似,暗示其也经历了三叠纪超高压变质作用及随后的角闪岩相退变质作用。通过计算长英质片麻岩的有效黏度,发现无水碱长花岗片麻岩的有效黏度高于黑云斜长片麻岩,折返阶段的流体活动使超高压变质岩的强度显著降低,当局部的流体活动不足以弱化碱长花岗岩体时,应变集中于黑云斜长片麻岩。因此,大陆俯冲隧道中的应变分布受矿物组成、流体活动和岩体规模的共同影响。  相似文献   

10.
Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite–garnet glaucophane schists occur as lenticular or sheet‐like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (XJd 0.46–0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote–blueschist to eclogite facies. Metamorphic P–T conditions estimated using pseudosection modelling are 580–600 °C and 18–20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite–garnet glaucophane schists have low CaO (4.4–4.5 wt%) and MgO (2.1–2.3 wt%) bulk‐rock compositions. The P–T– pseudosections show that low XCa bulk‐rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low XCa bulk‐rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low‐T type and subsequent high‐T type eclogitic metamorphic events. The jadeite–garnet glaucophane schists experienced low‐T type eclogite facies metamorphism, and the P–T path is similar to lawsonite‐bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low‐T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high‐T type eclogitic metamorphism within a single subduction zone.  相似文献   

11.
We present the results of a comparative study of the geochemical changes of amphibolites and gneisses from the Belomorian mobile belt in response to plagiomigmatization, high-pressure metamorphism, two-feldspar migmatization, and secondary amphibolization during Paleoproterozoic (Svecofennian) tectonometamorphic activation. It is established that most of the Paleoproterozoic metamorphic processes are nonisochemical for major and trace elements, which is possibly caused by the interaction of protolithic rocks with metamorphic fluids. The finds of eclogites within the Belomorian mobile belt are spatially and genetically related to the large fields of apoamphibolite and apogneiss plagiomigmatites. Apoamphibolite eclogites, Grt–Aug eclogite-like rocks, apoamphibolite and apogneiss plagiomigmatites were formed by a single process initiated by the influence of an alkaline fluid on the amphibolite–gneiss complex. This was accompanied by the depletion of the rocks in HREE, enrichment in LREE, disappearance of the negative and formation of the positive europium anomaly. The formation of later two-feldspar migmatites was related to the reworking of the gneiss–migmatite–amphibolite complex by more acid fluids, which led to the depletion of microclinized rocks in LREE. Secondary amphibolization and epidotization of the basites and metabasites did not affect their REE distribution pattern.  相似文献   

12.
The metabasites of Chadegan, including eclogite, garnet amphibolite and amphibolite, are forming a part of Sanandaj–Sirjan Zone. These rocks have formed during the subduction of the Neo–Tethys ocean crust under Iranian plate. This subduction resulted in a subduction metamorphism under high pressuremedium temperature of eclogite and amphibolites facies condition. Then the metamorphic rocks were exhumed during the continental collision between the Afro–Arabian continent and the Iranian microcontinent. In the metabasite rocks, with typical MORB composition, garnet preserved a compositional zoning occurred during metamorphism. The magnesium (XMg) gradually increases from core to rim of garnets, while the manganese (XMn) decreases towards the rim. Chondrite–normalized Rare Earth Element patterns for these garnets exhibit core–to–rim increases in Light Rare Earth Elements. The chondrite–normalized REE patterns of garnets, amphiboles and pyroxenes display positive trend from LREEs to Heavy Rare Earth Elements (especially in garnet), which suggests the role of these minerals as the major controller of HREE distribution. The geochemical features show that the studied eclogite and associated rocks have a MORB origin, and probably formed in a deep–seated subduction channel environment. The geothermometry estimation yields average pressure of ~22 kbar and temperature of 470–520°C for eclogite fomation. The thermobarometry results gave T = 650–700°C and P ≈ 10–11 kbar for amphibolite facies.  相似文献   

13.
Abstract The D'Entrecasteaux Islands of eastern Papua New Guinea consist of a number of active metamorphic core complexes formed under an extensional tectonic setting related to sea-floor spreading in the west Woodlark Basin. The complexes are defined by mountainous domes (>2500 m high) of fault-bounded, high-grade metamorphic rocks (including eclogite facies) intruded by 2–4-Ma granodiorite plutons. Garnet–clinopyroxene exchange thermometers indicate that the temperature of equilibration of the eclogites was 730–900° C. The jadeite component of omphacite indicates minimum pressure of 21 kbar, suggesting depths of >70 km. The metamorphic rocks have undergone widespread retrogression to amphibolite facies. Retrogression of the metamorphic basement is associated with shearing and formation of the metamorphic core complexes. P–T conditions in the early stages of shear zone activity, determined using the garnet–biotite exchange thermometer and the GASP and GRIPS barometers, were 570–730° C and 7–11 kbar. A second phase of re-equilibration at much lower pressures appears to be related to the widespread intrusion of granodiorite plutons. One re-equilibrated gneiss indicated maximum temperature of 730° C at estimated pressures of approximately 4 kbar. This late, high-temperature metamorphism is also indicated by reactions involving the production of hercynite and corundum in aluminous gneisses and formation of sillimanite at the expense of kyanite. Two major episodes of granodiorite intrusion occurred during uplift and exhumation of the core complexes. Both closely coincide spatially with high-temperature metamorphic rocks, the onset of deformation in extensional shear zones and subsequent uplift of the metamorphic basement. These observations indicate a fundamental link between uplift and granodiorite intrusion during continental extension and the formation of the D'Entrecasteaux Islands metamorphic core complexes.  相似文献   

14.
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15.
The Flatraket Complex, a granulite facies low strain enclave within the Western Gneiss Region, provides an excellent example of metastability of plagioclase‐bearing assemblages under eclogite facies conditions. Coesite eclogites are found <200 m structurally above and <1 km below the Flatraket Complex, and are separated from it by amphibolite facies gneisses related to pervasive late‐orogenic deformation and overprinting. Granulites within the Flatraket Complex equilibrated at 9–11 kbar, 700–800°C. These predate eclogite facies metamorphism and were preserved metastably in dry undeformed zones under eclogite facies conditions. Approximately 5% of the complex was transformed to eclogite in zones of fluid infiltration and deformation, which were focused along lithological contacts in the margin of the complex. Eclogitisation proceeded by domainal re‐equilibration and disequilibrium breakdown of plagioclase by predominantly hydration reactions. Both hydration and anhydrous plagioclase breakdown reactions were kinetically linked to input of fluid. More pervasive hydration of the complex occurred during exhumation, with fluid infiltration linked to dehydration of external gneisses. Eclogite facies shear zones within the complex equilibrated at 20–23 kbar, 650–800°C, consistent with the lack of coesite and with the equilibration conditions of external HP eclogites. If the complex experienced pressures equivalent to those of nearby coesite eclogites (> 28 kbar), unprecedented metastability of plagioclase and quartz is implied. Alternatively, a tectonic break exists between the Flatraket Complex and UHP eclogites, supporting the concept of a tectonic boundary to the UHP zone of the Western Gneiss Region. The distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability, and is a function of the protolith.  相似文献   

16.
The central part of the Carolina terrane in western South Carolina comprises a 30 to 40 km wide zone of high grade gneisses that are distinct from greenschist facies metavolcanic rocks of the Carolina slate belt (to the SE) and amphibolite facies metavolcanic and metaplutonic rocks of the Charlotte belt (to the NW). This region, termed the Silverstreet domain, is characterized by penetratively deformed felsic gneisses, granitic gneisses, and amphibolites. Mineral assemblages and textures suggest that these rocks formed under high‐pressure metamorphic conditions, ranging from eclogite facies through high‐P granulite to upper amphibolite facies. Mafic rocks occur as amphibolite dykes, as metre‐scale blocks of coarse‐grained garnet‐clinopyroxene amphibolite in felsic gneiss, and as residual boulders in deeply weathered felsic gneiss. Inferred omphacite has been replaced by a vermicular symplectite of sodic plagioclase in diopside, consistent with decompression at moderate to high temperatures and a change from eclogite to granulite facies conditions. All samples have been partially or wholly retrograded to amphibolite assemblages. We infer the following P‐T‐t history: (1) eclogite facies P‐T conditions at ≥ 1.4 GPa, 650–730 °C (2) high‐P granulite facies P‐T conditions at 1.2–1.5 GPa, 700–800 °C (3) retrograde amphibolite facies P‐T conditions at 0.9–1.2 GPa and 720–660 °C. This metamorphic evolution must predate intrusion of the 415 Ma Newberry granite and must postdate formation of the Charlotte belt and Slate belt arcs (620 to 550 Ma). Comparison with other medium temperature eclogites and high pressure granulites suggests that these assemblages are most likely to form during collisional orogenesis. Eclogite and high‐P granulite facies metamorphism in the Silverstreet domain may coincide with a ≈570–535 Ma event documented in the western Charlotte belt or to a late Ordovician‐early Silurian event. The occurrence of these high‐P assemblages within the Carolina terrane implies that, prior to this event, the western Carolina terrane (Charlotte belt) and the eastern Carolina terrane (Carolina Slate belt) formed separate terranes. The collisional event represented by these high‐pressure assemblages implies amalgamation of these formerly separate terranes into a single composite terrane prior to its accretion to Laurentia.  相似文献   

17.
Eclogites from the Onodani area in the Sambagawa metamorphic belt of central Shikoku occur as layers or lenticular bodies within basic schists. These eclogites experienced three different metamorphic episodes during multiple burial and exhumation cycles. The early prograde stage of the first metamorphic event is recorded by relict eclogite facies inclusions within garnet cores (XSps 0.80–0.24, XAlm 0–0.47). These inclusions consist of relatively almandine‐rich garnet (XSps 0.13–0.24, XAlm 0.36–0.45), aegirine‐augite/omphacite (XJd 0.08–0.28), epidote, amphiboles (e.g. actinolite, winchite, barroisite and taramite), albite, phengite, chlorite, calcite, titanite, hematite and quartz. The garnet cores also contain polyphase inclusions consisting of almandine‐rich garnet, omphacite (XJd 0.27–0.28), amphiboles (e.g. actinolite, winchite, barroisite, taramite and katophorite) and phengite. The peak P–T conditions of the first eclogite facies metamorphism are estimated to be 530–590 °C and 19–21 kbar succeeded by retrogression into greenschist facies. The second prograde metamorphism began at greenschist facies conditions. The peak metamorphic conditions are defined by schistosity‐forming omphacites (XJd ≤ 49) and garnet rims containing inclusions of barroisitic amphibole, phengite, rutile and quartz. The estimated peak metamorphic conditions are 630–680 °C and 20–22 kbar followed by a clockwise retrograde P–T path with nearly isothermal decompression to 8–12 kbar. In veins cross‐cutting the eclogite schistosity, resorbed barroisite/Mg‐katophorite occurs as inclusions in glaucophane which is zoned to barroisite, suggesting a prograde metamorphism of the third metamorphic event. The peak P–T conditions of this metamorphic event are estimated to be 540–600 °C and 6.5–8 kbar. These metamorphic conditions are correlated with those of the surrounding non‐eclogitic Sambagawa schists. The Onodani eclogites were formed by subduction of an oceanic plate, and metamorphism occurred beneath an accretionary prism. These high‐P/T type metamorphic events took place in a very short time span between 100 and 90 Ma. Plate reconstructions indicate highly oblique subduction of the Izanagi plate beneath the Eurasian continent at a high spreading rate. This probably resulted in multiple burial and exhumation movements of eclogite bodies, causing plural metamorphic events. The eclogite body was juxtaposed with non‐eclogitic Sambagawa schists at glaucophane stability field conditions. The amalgamated metamorphic sequence including the Onodani eclogites were exhumed to shallow crustal/surface levels in early Eocene times (c. 50 Ma).  相似文献   

18.
Ｐｅｔｒｏｇｅｎｅｓｉｓ　ｏｆ　Ｅｃｌｏｇｉｔｅｓ　ｉｎ　ｔｈｅ　Ｌｉｇｈｔ　ｏｆ　ＰｕｎｃｔｕａｔｅｄＭｅｔａｍｏｒｐｈｉｃ　Ｅｖｏｌｕｔｉｏｎ　ｉｎ　Ｄａｂｉｅ　Ｔｅｒｒａｎｅ，Ｃｈｉｎａ￥ＹｏｕＺｈｅｎｄｏｎｇ；ＨａｎＹｕｊｉｎｇ；ＺｈｏｎｇＺ...  相似文献   

19.
TAKASU  AKIRA 《Journal of Petrology》1984,25(3):619-643
In the Besshi district of the Sambagawa metamorphic belt, thereare two types of eclogites: one occurring in the Sebadani metagabbromass and retrograded from high-temperature anhydrous ecologite,and the other in the basic schists and produced by prograde,dehydration of epidote amphibolite. The Sebadani metagabbro mass was originally layered gabbro,which was once equilibrated in the ecologite facies before emplacementinto the Sambagawa terrain as a hot eclogite mass. Basic schistssurrounding the Sebadani mass, which had suffered the Sambagawametamorphism of albite epidote amphibolite facies, were contact-metamorphosedat high pressure by the emplacement of the Sebadani mass. Asa result, the basic schists were dehydrated to form eclogiticbasic schists, i.e. garnet and omphacite porphyroblast-bearingbasic schists. Thus, two types of ecologite, retrograde andprograde, converged into the same metamorphic condition, 610–650?C, 7–17 kb, in a part of the ecologite facies duringthe Sambagawa metamorphism. Correspondingly, the values of distributioncoefficients of Fe and Mg between garnet and omphacite increasefrom core pairs to rim pairs in the retrograde eclogites anddecrease from core pairs to rim pairs in the prograde ecologites.After this stage, both the prograde and retrograde eclogitesshared a common metamorphic history; they were retrograded viathe epidote amphibolite facies to the greenschist facies. The Sebadani metagabbro mass, as a large tectonic block, hadbeen emplaced into a m?lange zone in the Sambagawa metamorphicbelt after the peak of the Sambagawa metamorphism, probablyfollowing initiation of uplift of the metamorphic rocks fromtheir deep-seated environment.  相似文献   

20.
The Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields <11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.  相似文献   

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