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1.
杨建军 《岩石学报》2015,31(9):2465-2476
近六十多年来,关于造山带高压变质作用发生的原因一直存在争议。当前的主流观点认为高压变质岩形成于俯冲带深部以后折返到地壳。虽然早就认识到蓝片岩相变质作用总是与逆冲断裂相伴并且变质压力朝着冲断面增高,而标志地震的榴辉岩相假玄武玻璃也已经发现了二十多年,但是一般认为断裂或剪切带的作用只是引进流体促进深部岩石的变质反应。几年前,苏文辉等人(Su et al.,2006)通过实验演示了石英经过非晶化可以在柯石英稳定的压力下迅速转变为柯石英,提出无需深俯冲,浅源地震即可形成柯石英。本文介绍笔者等人对中国苏鲁超高压变质带仰口榴辉岩的部分工作结果,从地质观察角度为这一论点提供证据。例如,粒间柯石英仅出现在榴辉岩相角砾岩的角砾而非胶结物中,它必须在相当于地震的时间尺度快速降压冷却才能得以保存。变辉长岩中的榴辉岩相碎裂岩脉也记录了应力导致的瞬时高压和高温。角砾岩和碎裂岩中包含大量高压矿物的石榴石和针状蓝晶石以及局部出现的"显微花岗岩"都是类似地震熔体淬火的结果。它们指示震后高压矿物没有再生长。而如果地震发生在深部,流体浸入后矿物应当在高压下持续结晶,从而消除淬火结构。碎裂岩脉中星散的铬铁矿微粒极可能是附近超镁铁岩通过断裂迸溅进入基性岩的,是地震中不同岩石机械混合的证据;环绕它们的富铬榴辉岩矿物记录着一次无可争议的瞬时高压结晶事件。碎裂岩脉相对围岩更缺乏流体的事实反映应力而非流体在高压相变中的关键作用。榴辉岩相角砾岩和碎裂岩脉形成的时间尺度太小,来不及完成俯冲和折返过程。它们更可能是辉长岩在地壳原地因地震波引起的高压发生了榴辉岩化作用。目前的地震力学模型不能导出高压相变所需要的压力,只是因为建立模型时没有考虑涉及高压相变的地震。需要强调的是,假说或理论需要观察事实的检验而不是相反。已有资料显示,瞬时熔融和非晶化可能是震击高压相变的路径。  相似文献   

2.
The Ballantrae ophiolite in southern Scotland includes a NEE–SWW-trending serpentinite mélange that contains blocks of mafic blueschist and high-pressure, granulite facies, metapyroxenite (Sm–Nd metamorphic age: 576 ± 32 and 505 ± 11 Ma). Tectonic blocks of mafic schist are less than 3 × 3 m in size, and have greenschist, blueschist or epidote amphibolite facies assemblages corresponding to the high-pressure intermediate-type metamorphic facies series.Adjacent rocks of the serpentinite mélange are hydrothermally-altered MORB-like ophiolitic basalt (prehnite–pumpellyite facies), dolerite (actinolite–oligoclase sub-facies) and gabbro (amphibolite facies), all with assemblages that are diagnostic of the low-pressure metamorphic facies series.The difference in metamorphic facies series and parageneses of minerals between the high-pressure mafic blocks and the adjacent, low-pressure ophiolitic meta-basic rocks suggests that the former were exhumed from > 25 km depth within a cold subducted slab, and were juxtaposed with the latter, the bottom of a MORB-like ophiolite in the hanging wall of a trench. An ENE–WSW-trending, 501 ± 12 Ma volcanic arc belt extends for 3 km south of the serpentinite mélange. We suggest that ridge subduction associated with a slab window created arc-related gabbro (483 ± 4 Ma) at Byne Hill and within-plate gabbro (487 ± 8 Ma) at Millenderdale. Final continental collision created the duplex structure of the Ballantrae complex that includes the HP blocks and serpentinite mélange. These relations define diapiric exhumation in the Caledonian orogen of SW Scotland.  相似文献   

3.
In the southern Adula nappe (Central Alps), two stages of regional metamorphism have affected mafic and pelitic rocks. Earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite-hornblende eclogites was followed by a Tertiary overprint which varied from greenschist to high-grade amphibolite facies. Despite a common metamorphic history, contrasting equilibration conditions are often recorded by high-pressure mafic eclogite and adjacent predominantly lower-pressure pelite assemblages. This pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading. The rates are controlled by a mechanism in which dehydrating metapelites provide the H2O required for simultaneous overprinting of enclosed mafic eclogites by hydration.Quantitative mass balance modelling based on corona textures is used to show that overprinting of metapelites during unloading involved dehydration reactions. The relatively rapid rate of dehydration reactions led to nearly complete reequilibration of metapelites to amphibolite facies assemblages.After the formation during high-pressure metamorphism of mafic eclogites, later lower-pressure reequilibration by hydration to amphibolites was slow, and therefore incomplete, because it depended on large scale transport of H2O from adjacent, dehydrating metapelites.The facies contrast observed between rocks of different bulk composition is thus a consequence of the general tendency of metamorphic rocks to retain the most dehydrated assemblage as the final recorded state.  相似文献   

4.
Fluid availability during high‐grade metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid‐absent metamorphism commonly resulting in neither zircon growth/recrystallization for U‐Pb dating nor Sm‐Nd isotopic resetting for isochron dating. While peak ultra‐high pressure (UHP) metamorphism is characterized by fluid immobility, high‐pressure (HP) eclogite‐facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from the Sulu orogen of China indicates zircon growth at 216 ± 3 Ma as well as mineral Sm‐Nd and Rb‐Sr reequilibration at 216 ± 5 Ma, which are uniformly younger than UHP metamorphic ages of 231 ± 4 to 227 ± 2 Ma as dated by the SHRIMP U‐Pb method for coesite‐bearing domains of zircon. O isotope reequilibration was achieved between the Sm‐Nd and Rb‐Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. The HP eclogite‐facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. Based on available experimental diffusion data, the consistent reequilibration of U‐Pb, Sm‐Nd, Rb‐Sr and O isotope systems in the eclogite minerals demonstrates that time‐scale for the HP eclogite‐facies recrystallization is c. 1.9–9.3 Myr or less. This provides a maximum estimate for duration of the fluid‐facilitated process in the HP eclogite‐facies regime during the exhumation of deeply subducted continental crust.  相似文献   

5.
A Nappe system south to southwest of the São Francisco Craton represents the southern extension of the Brasília belt and describes an inverted metamorphic pile of greenschist facies toward amphibolite facies. The Aiuruoca-Andrelândia nappe is one of the nappes of this system. The hind portion of the Aiuruoca-Andrelândia nappe, south of Caxambu and Aiuruoca (MG), consists of a structural-metamorphic domain transported toward the E-NE. There is a metamorphic transition, from the kyanite zone to kyanite and sillimanite coexistence, until the sillimanite zone. Metapelitic rocks preserve high-pressure parageneses (Rt–Ky–Grt–Ms–Bt–Pl–Qtz) and contain retrograde eclogitic rocks. Sil–Pl–Qtz coronitic intergrowths around garnets are common decompressive textures. Kyanite schists register the Pmax of 11 kbar at 660 °C and define a decompressive path until 6–7.5 kbar at 650 °C. These P–T conditions represent the equilibrium in S2 schistosity (amphibolite facies) and the beginning of the cooling path in the Ky–Sil transition. The decompressive path suggests an extrusional process, immediately after burying at about 60 km. Exhumation controlled by convergent events, related to the São Francisco Plate subduction and tectonic erosion, took these units, isothermally, to higher levels (20–33 km). Later, the metamorphic path shifted toward near-isobaric cooling.  相似文献   

6.
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   

7.
Pseudotachylyte in the Cima di Gratera ophiolite, Alpine Corsica, is distributed in the peridotite unit and in the overlying metagabbro unit and was formed under blueschist to eclogite metamorphic facies conditions, corresponding to a 60–90 km depth range. Peridotite pseudotachylyte is clustered in fault zones either beneath the tectonic contact with overlying metagabbros or at short distance from it. Fault zones are either parallel to the contact or make an angle of 55° to it. Displacement sense criteria associated with fault veins indicate top-to-the-west or top-to-the-northwest reverse senses. Cataclasite flanking most veins was formed before or coevally with frictional melting and likely mechanically weakened the peridotite, facilitating subsequent seismic rupture. In the basal part of the metagabbro unit, post-mylonitization pseudotachylyte can be distinguished from pre-mylonitization pseudotachylyte formed earlier. In the equant metagabbro above the mylonitic sole, only one episode of pseudotachylyte formation can be identified. Kinematics associated with metagabbro pseudotachylyte remain unknown. The geometry and kinematics of the pseudotachylyte veins from the peridotite unit and to a lesser extent from the metagabbro unit are similar to modern seismic ruptures of the upper parts of the Wadati-Benioff zones such as in the Pacific plate beneath NE Japan.  相似文献   

8.
The considered part of ductile shear zones, which are widespread in the area of the village of Gridino in the central part of the Belomorian Mobile Belt, is accompanied by the high-temperature eclogitization of basites. The paper reports examples of eclogitization in rocks of various age, mostly in rocks of the Early Proterozoic lherzolite-gabbronorite and coronite gabbro complexes. The degrees of structural and mineralogical transformations in unequally deformed bodies of gabbronorites and olivine gabbronorites of the lherzolite-gabbronorite complex are correlated with the degrees of deformations of these rocks and their fluid recycling. The relatively weakly deformed rocks have massive and apomagmatic textures with garnet and omphacite reaction rims at boundaries between grains of magmatic plagioclase and pyroxenes. These rims are typical of the domainal equilibrium stage, which is an intermediate eclogitization stage between a magmatic rock and completely equilibrated eclogite. The enhancement of rock deformation results in the development of equilibrium eclogite with anisotropic metamorphic textures and structures. Textural transformations simultaneous with intense deformations are manifested involved the recrystallization of magmatic minerals and the development of linear fine-grained mineral aggregates of the Pl-Opx-Grt-Omp eclogite mineral assemblage and the systematically oriented growth of much larger mineral neoblasts in amphibole eclogite assemblages.  相似文献   

9.
Eclogite occurs within the southern domain of the East Athabasca mylonite triangle in northern Saskatchewan. Situated at the boundary between the Archean Rae and Hearne Provinces of the western Canadian Shield, the East Athabasca mylonite triangle is a fundamental exposure of the ~3,000-km-long Snowbird tectonic zone. The eclogite occurs in association with a variety of lower crustal high-pressure granulites that record a complex metamorphic history from 2.6 to 1.9 Ga. Temperatures of the eclogite facies metamorphism are constrained by garnet-clinopyroxene exchange thermometry at 920–1,000 °C. Minimum pressure conditions are recorded by the jadeite+quartz=albite geobarometer at 1.8–2.0 GPa. A near-isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene-plagioclase and pargasite-plagioclase. U-Pb IDTIMS zircon geochronology of the eclogite yields a weighted mean 207Pb/206Pb date of 1,904.0±0.3 Ma, which we interpret as the time of peak eclogite facies metamorphism. SHRIMP in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207Pb/206Pb date of 1,905±19 Ma. Inclusion suites of high-pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism. Zircon from one eclogite sample has older cores that are 2.54 Ga, which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith. U-Pb rutile data indicate slow cooling at ~1°C/Ma below ~500 °C from 1.88 to 1.85 Ga. The formation and exhumation of the eclogites at ca.1.9 Ga has important implications for the tectonic significance of the Snowbird tectonic zone during the Paleoproterozoic. The eclogites described here are consistent with transport of continental crust to mantle depths during the Paleoproterozoic, followed by rapid buoyancy-driven exhumation to normal lower crustal depths.Editorial responsibility: T.L. Grove  相似文献   

10.
Abstract Several small bodies of metabasite (maximum dimensions of 1000 m x 500 m) are included in the metamorphic rocks of the Nevado-Filabride Complex in the Betic Cordilleras (Almeria Region). The body of 400 m x 100 m, located 200 m due west of the Lubrin village, contains troctolitic gabbro with well-preserved igneous textures and mineral compositions, wholly amphibolitized gabbro, garnet-bearing metagabbro eclogite. Along with the textural and mineral changes, sensible and regular geochemical variations can be observed, where the content of MgO decreases from 24% to 11%, while that of CaO and Na2O increases from 7% to 11% and from 2% to 3%, respectively. In addition, the content of some minor elements such as Sr, Y, Nb, Zr and Sc increases while that of Ni and Cr decreases from troctolitic gabbro to the eclogite. The amphibolitized gabbro shows values scattered around those of the troctolitic gabbro. These geochemical variations are ascribed to inherited differences in the pre-metamorphic protolith, i.e. a fractionated gabbro which varies from olivine-rich to clinopyroxene-rich gabbro. Nevertheless, some metasomatism affected the Lubrin body without changing the main chemical trends, as documented by the significantly different 87Sr/86Sr ratios of each rock-type. This points to a metasomatism which involved the introduction of crustal radiogenic strontium. The petrographical and mineral chemical features are interpreted to be the result of syn-metamorphic fluid circulation possibly combined with deformation by shearing. The igneous texture and mineral chemistry have been retained wherever both fluid circulation and shearing were ineffective. On the contrary, where both events were effective, the formation of eclogite occurred. Later, the entire body underwent a retrogressive amphi-bolitic stage under greenschist facies conditions, which was probably responsible for the formation of the amphibolitized gabbro portion and for the retrogression of the eclogite.  相似文献   

11.
Rb-Sr and U-Pb isotopic data for granulite facies rocks, forming textural relics with respect to eclogite facies metamorphism in the Western Gneiss Region (WGR) of Norway, highlight the importance of textures and mineral reaction kinetics for the interpretation of geochronological data. Studied rocks from Bårdsholmen, southern WGR, were subjected to granulite facies metamorphism at 955 ± 3 Ma (U-Pb, zircon). Later on, they experienced a subduction-related, kinetically stranded eclogitization (T > 650 °C at ∼20 kbar) at 404 ± 2 Ma (Rb-Sr multimineral internal isochron data), followed by exhumation through amphibolite facies conditions. Full conversion of granulite to eclogite was restricted to zones of fluid infiltration and deformation. Despite the fact that metamorphic temperatures vastly exceeded the commonly assumed ‘closure temperature’ for Rb-Sr in submillimeter-sized biotite for several Ma during eclogite facies overprint, Sr-isotopic signatures of relic biotite have not been fully reset. Large biotite crystals nearly record their Sveconorwegian (Grenvillian) crystallization age. Sr signatures of other granulite facies phases (feldspar, pyroxenes, amphibole) remained unchanged, with the exception of apatite. The results imply that isotopic signatures much closer correspond to the P, T conditions of formation recorded by a dated phase and its paragenesis, than to a temperature history. In texturally well-equilibrated high-grade rocks which experienced no mineral reactions and remained devoid of free fluids during exhumation, like granulites or eclogites, isotopic resetting during cooling is either kinetically locked, or restricted to sluggish intermineral diffusion which demonstrably does not lead to full isotopic homogenization. In texturally unequilibrated rocks, textural relics are likely to represent isotopic relics. It is shown that for both high-grade rocks and for rocks with textural disequilibria, geologically meaningful isotopic ages based on isochron methods can only be derived from sub-assemblages in isotopic equilibrium, which have to be defined by analysis of all rock-forming minerals. Conventional two-point ‘mica ages’ for such rocks are a priori geochronologically uninterpretable, and valid multimineral isochron ages a priori do not record cooling but instead date recrystallization-inducing processes like fluid-rock interaction.  相似文献   

12.
Large volumes of pseudotachylyte (an intrusive, fault-related rock interpreted to form by a combination of cataclasis and melting) occur in Tertiary normal faults and accommodation zones along 400 km of the East Greenland volcanic rifted margin. Analysis of representative pseudotachylyte samples reveals a wide range of mesoscopic and microscopic textures, mineralogies, and chemistries in the aphanitic pseudotachylyte matrix. Three distinct types of pseudotachylyte (referred to as angular, rounded and glassy) are identified based on these characteristics. Angular pseudotachylyte (found primarily in dike-like reservoir zones) is characterized by angular grains visible on all scales, with micron-scale fragments of mica and amphibole. Its matrix is enriched in Fe2O3, MgO, and TiO2 relative to the host rock, with minor increases in CaO, K2O, and small decreases in Na2O. Rounded pseudotachylyte is found in reservoir zones, injection veins (pseudotachylyte-filled extension fractures), and fault veins (small faults with pseudotachylyte along their surfaces). It is characterized by smooth-surfaced, compacted grains on microscopic scales, and encloses rounded, interpenetrative lithic clasts on outcrop scale. Its matrix is enriched in Fe2O3, MgO, TiO2, and Al2O3 relative to the host rock, with minor depletion in Na2O and K2O. Glassy pseudotachylyte is found primarily along fault surfaces. Its matrix is characterized by isotropic, conchoidally fractured material containing microscopic, strain-free amphibole phenocrysts, and is enriched in TiO2, Al2O3, K2O, Fe2O3, MgO, CaO, and Na2O relative to the host rock. These observations suggest that angular pseudotachylyte was produced by cataclasis, with enrichment in metallic oxides resulting from preferential crushing of mechanically weak amphibole and mica minerals found in the gneissic host rock. Cataclasis and concomitant frictional heating resulted in the textural and chemical modification of angular pseudotachylyte by sintering or melting, producing rounded and glassy pseudotachylyte, respectively. Compositional and textural observations constrain the temperatures reached during frictional heating (700–900°C) which in turn delimit the amount of frictional heat imparted to the pseudotachylytes during slip. Our results suggest that the East Greenland pseudotachylytes formed during small seismic events along faults at shallow crustal levels. Consistent relative ages and widespread occurrence of pseudotachylyte-bearing faults in East Greenland suggest that widespread microseismicity accompanied the early development of this volcanic rifted margin.  相似文献   

13.
High-pressure granulite facies rocks of the Bacariza Formation (Cabo Ortegal, NW Spain) were syn-metamorphically deformed at the contacts with the bounding units (peridotite and eclogite massifs). This enabled the formation of meter-thick, spectacular shear zones with reworked and transposed foliations and lineations. The texturally stable mineral assemblage of the new fabrics records an intense, ductile deformation of the mineral aggregate at temperatures of 700–800 °C associated with amalgamation of eclogite, high-pressure granulitic rocks and ultramafic sheets in deep portions of a subduction channel. The lattice preferred orientation of the main constituent minerals (garnet, augite, amphibole, plagioclase, quartz and biotite) discloses the active deformation mechanisms at the scale of the mineral grains and the relationships with the deformation at larger scales. Overprinting relationships of the metamorphic assemblages demonstrates that partitioning and deformation localization occurred at different scales under similar high-grade conditions. Complete macroscopic transposition in the shear zones was complementary to meso and microscopic partitioning of deformation intensity and mechanisms between different lithological layers and mineral species.  相似文献   

14.
Recent U–Pb age determinations and P–T estimates allow us to characterize the different levels of a formerly thickened crust, and provide further constraints on the make up and tectono-thermal evolution of the Grenville Province in the Manicouagan area. An important tectonic element, the Manicouagan Imbricate zone (MIZ), consists of mainly 1.65, 1.48 and 1.17 Ga igneous rocks metamorphosed under 1400–1800 MPa and 800–900 °C at 1.05–1.03 Ga, during the Ottawan episode of the Grenvillian orogenic cycle, coevally with intrusion of gabbro dykes in shear zones. The MIZ has been interpreted as representing thermally weakened deep levels of thickened crust extruded towards the NW over a parautochthonous crustal-scale ramp. Mantle-derived melts are considered as in part responsible for the high metamorphic temperatures that were registered.New data show that mid-crustal levels structurally above the MIZ are represented by the Gabriel Complex of the Berthé terrane, that consists of migmatite with boudins of 1136±15 Ma gabbro and rafts of anatectic metapelite with an inherited monazite age at 1478±30 Ma. These rocks were metamorphosed at about the same time as the MIZ (metamorphic zircon in gabbro: 1046±2 Ma; single grains of monazite in anatectic metapelite: 1053±2 Ma) and under the same T range (800–900 °C) but at lower P conditions (1000–1100 MPa). They are mainly exposed in an antiformal culmination above a high-strain zone, which has tectonic lenses of high P–T rocks from the MIZ and is intruded by synmetamorphic gabbroic rocks. This zone is interpreted as part of the hangingwall of the MIZ during extrusion. A gap of 400 MPa in metamorphic pressures between the tectonic lenses and the country rocks, together with the broad similarity in metamorphic ages, are consistent with rapid tectonic transport of the high P–T rocks over a ramp prior to the incorporation of the mafic lenses in the hangingwall.Between the antiformal culmination of the Gabriel Complex and the MIZ 1.48 Ga old granulites of the Hart Jaune terrane are exposed. They are intruded by unmetamorphosed 1228±3 Ma gabbro sills and 1166±1 Ma anorthosite. Hart Jaune Terrane represents relatively high crustal levels that truncate the MIZ-Gabriel Complex contact and are preserved in a synformal structure.Farther south, the Gabriel Complex is overlain by the Banded Complex, a composite unit including 1403+32/−25 Ma granodiorite and 1238+16/−13−1202+40/−25 Ma granite. This unit has been metamorphosed under relatively low-P (800 MPa) granulite-facies conditions. Metamorphic U–Pb data, limited to zircon lower intercept ages (971±38 Ma and 996±27 Ma) and a titanite (990±5 Ma) age, are interpreted to postdate the metamorphic peak.The general configuration of units along the section is consistent with extrusion of the MIZ during shortening and, finally, normal displacement along discrete shear zones.  相似文献   

15.
Calc-silicate rocks occur as elliptical bands and boudins intimately interlayered with eclogites and high-pressure gneisses in the Münchberg gneiss complex of NE Bavaria. Core assemblages of the boudins consist of grossular-rich garnet, diopside, quartz, zoisite, clinozoisite, calcite, rutile and titanite. The polygonal granoblastic texture commonly displays mineral relics and reaction textures such as post kinematic grossular-rich garnet coronas. Reactions between these mineral phases have been modelled in the CaO-Al2O3-SiO2-CO2-H2O system with an internally consistent thermodynamic data base. High-pressure metamorphism in the calc-silicate rocks has been estimated at a minimum pressure of 31 kbar at a temperature of 630d? C with XH2, O ≥ 0.03. Small volumes of a CO2-N2-rich fluid whose composition was buffered on a local scale were present at peak-metamorphic conditions. The P-T conditions for the onset of the amphibolite facies overprint are about 10 kbar at the same temperature. XCo2 of the H2O-rich fluid phase is regarded to have been <0.03 during amphibolite facies conditions. These P-T estimates are interpreted as representing different stages of recrystallization during isothermal decompression. The presence of multiple generations of mineral phases and the preservation of very high-pressure relics in single thin sections preclude pervasive post-peak metamorphic fluid flow as a cause of a re-equilibration within the calc-silicates. The preservation of eclogite facies, very high-pressure relics as well as amphibolite facies reactions textures in the presence of a fluid phase is in agreement with fast, tectonically driven unroofing of these rocks.  相似文献   

16.
Zaw Win Ko  M. Enami  M. Aoya   《Lithos》2005,81(1-4):79-100
The Sanbagawa metamorphic rocks in the Besshi district, central Shikoku, are grouped into eclogite and noneclogite units. Chloritoid and barroisite-bearing pelitic schists occur as interlayers within basic schist in an eclogite unit of the Seba area in the Sanbagawa metamorphic belt, central Shikoku, Japan. Major matrix phases of the schists are garnet, chlorite, barroisite, paragonite, phengite, and quartz. Eclogite facies phases including chloritoid and talc are preserved only as inclusions in garnet. P–T conditions for the eclogite facies stage estimated using equilibria among chloritoid, barroisite, chlorite, interlayered chlorite–talc, paragonite, and garnet are 1.8 GPa/520–550 °C. Zonal structures of garnet and matrix amphibole show discontinuous growth of minerals between their core and mantle parts, implying the following metamorphic stages: prograde eclogite facies stage→hydration reaction stage→prograde epidote–amphibolite stage. This metamorphic history suggests that the Seba eclogite lithologies were (1) juxtaposed with subducting noneclogite lithologies during exhumation and then (2) progressively recrystallized under the epidote–amphibolite facies together with the surrounding noneclogite lithologies.

The pelitic schists in the Seba eclogite unit contain paragonite of two generations: prograde phase of the eclogite facies included in garnet and matrix phase produced by local reequilibration of sodic pyroxene-bearing eclogite facies assemblages during exhumation. Paragonite is absent in the common Sanbagawa basic and pelitic schists, and is, however, reported from restricted schists from several localities near the proposed eclogite unit in the Besshi district. These paragonite-bearing schists could be lower-pressure equivalents of the former eclogite facies rocks and are also members of the eclogite unit. This idea implies that the eclogite unit is more widely distributed in the Besshi district than previously thought.  相似文献   


17.
Eclogites from the Jæren nappe in the Caledonian orogenic belt of SW Norway contain aragonite, magnesite and dolomite in quartz‐rich layers. The carbonates comprise composite grains that occur interstitially between phases of the eclogite facies assemblage: garnet + omphacite + zoisite + clinozoisite + quartz + apatite + rutile ± dolomite ± kyanite ± phengite. Pressure and temperature conditions for the main eclogite stage are estimated to be 2.3–2.8 GPa and 585–655 °C. Published ultrahigh pressure (UHP) experiments on CaO‐, MgO‐ and CO2‐bearing systems have shown that equilibrium assemblages of aragonite and magnesite form as a result of dolomite breakdown at pressures >5 GPa. As a result, recognition of magnesite and aragonite in eclogite facies rocks has been used as an indicator for UHP conditions. However, petrological testing showed that the samples studied here have not experienced such conditions. Aragonite and magnesite show disequilibrium textures that indicate replacement of magnesite by aragonite. This process is inferred to have occurred via a coupled dissolution–precipitation reaction. The formation of aragonite is constrained to eclogite facies conditions, which implies that the studied rocks have experienced metasomatic, reactive fluid flow during their residence at high pressure (HP) conditions. During decompression, the bimineralic carbonate aggregates were overgrown by rims of dolomite, which partially reacted with aragonite to form Mg‐calcite. The well‐preserved carbonate assemblages and textures observed in the studied samples provide a detailed record of the reaction series that affected the rocks during and after their residence at P–T conditions near the coesite stability field. Recognition of the HP mechanism of magnesite replacement by aragonite provides new insight into metasomatic processes that occur in subduction zones and illustrates how fluids facilitate HP carbonate reactions that do not occur in dry systems at otherwise identical physiochemical conditions. This study documents that caution is warranted in interpreting aragonite‐magnesite associations in eclogite facies rocks as evidence for UHP metamorphic conditions.  相似文献   

18.
Retrograded eclogites from the central part of the northern margin of the North China Craton, Hebei Province, China occur as separate tectonic lenses or boundins within garnet–biotite–plagioclase gneisses of the Paleoproterozoic Hongqiyingzi Complex characterized by amphibolite facies paragneisses. The petrographic features and mineralogical compositions represent three main metamorphic stages: (1) the peak eclogite facies stage (P > 1.40–1.50 GPa, T = 680–730 °C), (2) the granulite facies stage and (3) the amphibolite facies stage (P = 0.67–0.81 GPa, T = 530–610 °C) formed during decompression. The major and trace element and Sm–Nd isotopic data suggest that most of the retrograded eclogite samples had protoliths of tholeiitic oceanic crust with geochemical characteristics of mid-ocean ridge basalt (MORB) or island arc tholeiite (IAT) environment, and were contaminated by crustal components during subsequent subduction. Zircon SHRIMP isotopic dating of two different textural varieties of retrograded eclogite defines a weighted mean age of 325 Ma, which is interpreted as the peak metamorphic age of the eclogites and reflects the occurrence of eclogite facies metamorphism related to subduction of Paleo-Asian Oceanic crust beneath the North China Craton during the Late Paleozoic. Finally, we show that the retrograded eclogite from Hebei Province is not related to the Baimashi retrograded eclogite at the northern foot of the Heng Mountains, approximately, 300 km to the southwest.  相似文献   

19.
A coesite-bearing eclogite breccia is reported here for the first time at Yangkou in the Chinese Su–Lu ultrahigh-pressure (UHP) metamorphic belt. It is thrusted over a coesite-bearing coronitic eclogite and is gradational to a foliated eclogite at the contact. The coronitic eclogite is characterized by garnet coronas between fine-grained high-pressure mineral aggregates forming pseudomorphs after plagioclase, ilmenite, biotite, and pyroxene in a gabbroic protolith. The breccia consists of fine-grained cataclastic eclogite fragments (garnet + omphacite + coesite/quartz ± phengite ± kyanite) and a coarser-grained matrix schist (garnet + quartz + phengite + kyanite). The foliated eclogite consists of intercalating bands of the cataclastic eclogite and a schist similar to the fragments and the matrix, respectively, in the breccia. The igneous fabric of the eclogitized gabbro is increasingly obliterated from the coronitic eclogite through the foliated eclogite to the breccia. Micropoikilitic amoeboid garnet containing numerous inclusions of omphacite and other high-pressure minerals is characteristic of eclogite facies pseudotachylytes and suggests flash melting and rapid crystallization. In the breccia and foliated eclogite, quartz + K-feldspar ± albite aggregates are included in garnet or form strings cutting across the cataclasites. In some aggregates, quartz grains are cemented by K-feldspar and vesicular albite, also implying crystallization from melts in a rapid cooling and decompression process from the UHP condition. The field context, the locally preserved igneous fabric in the breccia, the similar whole-rock compositions, as well as the complementary mineral assemblages in the fragments and the matrix with respect to the coronitic eclogite, suggest that the breccia was formed by cataclasis and segregation of minerals in a former coronitic eclogite in response to a sudden pressure release. Intergranular coesite is found only in the eclogite cataclasites and may have survived via the rapid cooling event, as coesite converts to quartz completely in a few years when being cooled slowly at lower pressures. Such rate information is incompatible with the presumed deep subduction/exhumation process but implies transient UHP metamorphism coeval with the seismic event.  相似文献   

20.
Recent petrological studies on high‐pressure (HP)–ultrahigh‐pressure (UHP) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P–T loop for the HP–UHP metamorphism and amphibolite–granulite facies overprint in the Moldanubian Zone. Here further evidence is presented that the granulite facies metamorphism occurred after the HP–UHP rocks had been exhumed to different levels of the middle or upper crust. A medium‐temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well‐preserved eclogite facies assemblage with omphacite and prograde zoned garnet. Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of ~600 °C at 2.0–2.2 GPa. The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca‐rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel. Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine‐spinel equilibria and pseudosection calculations indicate temperatures of ~850–900 °C at pressure below 0.7 GPa. The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short‐lived process. The new results point to a geodynamic model where HP–UHP rocks are exhumed to amphibolite facies conditions with subsequent granulite facies heating by mantle‐derived magma in the middle and upper crust.  相似文献   

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