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1.
Migmatites with sub‐horizontal fabrics at the eastern margin of the Variscan orogenic root in the Bohemian Massif host lenses of eclogite, kyanite‐K‐feldspar granulite and marble within a matrix of migmatitic paragneiss and amphibolite. Petrological study and pseudosection modelling have been used to establish whether the whole area experienced terrane‐wide exhumation of lower orogenic crust, or whether smaller portions of higher‐pressure lower crust were combined with a lower‐pressure matrix. Kyanite‐K‐feldspar granulite shows peak conditions of 16.5 kbar and 850 °C with no clear indications of prograde path, whereas in the eclogite the prograde path indicates burial from 10 kbar and 700 °C to a peak of 18 kbar and 800 °C. Two contrasting prograde paths are identified within the host migmatitic paragneiss. The first path is inferred from the presence of staurolite and kyanite inclusions in garnet that contains preserved prograde zoning that indicates burial with simultaneous heating to 11 kbar and 800 °C. The second path is inferred from garnet overgrowths of a flat foliation defined by sillimanite and biotite. Garnet growth in such an assemblage is possible only if the sample is heated at 7–8 kbar to around 700–840 °C. Decompression is associated with strong structural reworking in the flat fabric that involves growth of sillimanite in paragneiss and kyanite‐K‐feldspar granulite at 7–10 kbar and 750–850 °C. The contrasting prograde metamorphic histories indicate that kilometre‐scale portions of high‐pressure lower orogenic crust were exhumed to middle crustal levels, dismembered and mixed with a middle crustal migmatite matrix, with the simultaneous development of a flat foliation. The contrasting P–T paths with different pressure peaks show that tectonic models explaining high‐pressure boudins in such a fabric cannot be the result of heterogeneous retrogression during ductile rebound of the whole orogenic root. The P–T paths are compatible with a model of heterogeneous vertical extrusion of lower crust into middle crust, followed by sub‐horizontal flow.  相似文献   

2.
Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H2O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H2O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.  相似文献   

3.
The North Qaidam Orogenic Belt (NQOB), lying at the northern margin of the Tibet Plateau, records two orogenic cycles: A Proterozoic cycle related to the amalgamation and breakup of the supercontinent Rodinia, and an Early Palaeozoic cycle including oceanic subduction and continental deep subduction. At present, the only information about the Proterozoic cycle is the concurrent c. 1,000–900 Ma magmatic and metamorphic events, which limited the understanding of the Proterozoic evolution of NQOB and the relationship between the Qaidam Block and other Rodinia fragments. In this study, a kyanite‐bearing eclogite was identified in Yuka terrane. It has positive‐slope chondrite‐normalized rare earth element distribution patterns, similar to present‐day N‐MORB. LA–ICP–MS zircon U–Pb dating obtained a protolith age of 1,273 Ma and an eclogite facies metamorphic age of 437 Ma, which is similar to the continental deep subduction age of the Yuka terrane. Zircon Lu–Hf analysis show that the magmatic zircon cores have high εHf(t) of 8.36–15.98 and TDM1 of 1,450–1,131 Ma (M = 1,303 ± 55 Ma, consistent with its protolith age within error), indicating a juvenile crust protolith of the eclogite. The MORB‐like whole‐rock composition and zircon U–Pb and Lu–Hf analysis indicate that the protolith of the kyanite‐bearing eclogite was a Mesoproterozoic oceanic slice. P–T pseudosection analysis shows that the kyanite‐bearing eclogite experienced four metamorphic stages: (1) a prograde stage with the assemblage garnet+omphacite+talc+lawsonite+phengite+quartz at 22.4–23.2 kbar and 585°C; (2) a peak stage with the assemblage garnet+omphacite+lawsonite+phengite+coesite at 32.5 kbar and 670°C; (3) an early retrograde stage with the assemblage garnet+omphacite+kyanite+phengite+coesite/quartz±lawsonite at 27.1–30.0 kbar and 670–690°C; and (4) a late retrograde stage with the assemblage garnet+omphacite+epidote+hornblende+phengite+quartz at <18.0 kbar. The established clockwise P–T path is similar with other continental‐type eclogites in this area. On the basis of the geochemical and geochronological data, as well as the P–T path, we suggest that the protolith of the kyanite‐bearing eclogite was emplaced in the active margin of the Qaidam Block during the assembly of Rodinia and underwent continental deep subduction in the Early Palaeozoic. We conclude that (1) the Qaidam Block participated in the assembly of the Rodinia supercontinent. It was situated at or proximal to the margin of the supercontinent and probably close to India, east Antarctica and Tarim; and (2) both Mesoproterozoic and Early Palaeozoic oceanic crust slices occur in the NQOB. Thus, special caution is needed when using the metamorphic ages of oceanic affinity eclogites without protolith ages to constrain the evolution history of the North Qaidam UHPM belt.  相似文献   

4.
Jadeite‐bearing kyanite eclogite has been discovered in the Iratsu body of the Sanbagawa belt, SW Japan. The jadeite + kyanite assemblage is stable at higher pressure–temperature (P–T) conditions or lower H2O activity [a(H2O)] than paragonite, although paragonite‐bearing eclogite is common in the Sanbagawa belt. The newly discovered eclogite is a massive metagabbro with the peak‐P assemblage garnet + omphacite + jadeite + kyanite + phengite + quartz + rutile. Impure jadeite is exclusively present as inclusions in garnet. The compositional gap between the coexisting omphacite (P2/n) and impure jadeite (C2/c) suggests relatively low metamorphic temperatures of 510–620 °C. Multi‐equilibrium thermobarometry for the assemblage garnet + omphacite + kyanite + phengite + quartz gives peak‐P conditions of ~2.5 GPa, 570 °C. Crystallization of jadeite in the metagabbro is attributed to Na‐ and Al‐rich effective bulk composition due to the persistence of relict Ca‐rich clinopyroxene at the peak‐P stage. By subtracting relict clinopyroxene from the whole‐rock composition, pseudosection modelling satisfactorily reproduces the observed jadeite‐bearing assemblage and mineral compositions at ~2.4–2.5 GPa, 570–610 °C and a(H2O) >0.6. The relatively high pressure conditions derived from the jadeite‐bearing kyanite eclogite are further supported by high residual pressures of quartz inclusions in garnet. The maximum depth of exhumation in the Sanbagawa belt (~80 km) suggests decoupling of the slab–mantle wedge interface at this depth.  相似文献   

5.
Prograde P–T–t paths of eclogites are often ambiguous owing to high variance of mineral assemblages, large uncertainty in isotopic age determinations and/or variable degree of retrograde equilibration. We investigated these issues using the barroisite eclogites from the Lanterman Range, northern Victoria Land, Antarctica, which are relatively uncommon but free of retrogression. These eclogites revealed three stages of prograde metamorphism, defining two distinctive P–T trajectories, M1–2 and M3. Inclusion minerals in garnet porphyroblasts suggest that initial prograde assemblages (M1) consist of garnet+omphacite+barroisite/Mg‐pargasite+epidote+phengite+paragonite+rutile/titanite+quartz, and subsequent M2 assemblages of garnet+omphacite+barroisite+phengite+rutile±quartz. The inclusion‐rich inner part of garnet porphyroblasts preserves a bell‐shaped Mn profile of the M1, whereas the inclusion‐poor outer part (M2) is typified by the outward decrease in Ca/Mg and XFe (=Fe2+/(Fe2++Mg)) values. A pseudosection modelling employing fractionated bulk‐rock composition suggests that the eclogites have initially evolved from ~15 to 20 kbar and 520–570°C (M1) to ~22–25 kbar and 630–650°C (M2). The latter is in accordance with P–T conditions estimated from two independent geothermobarometers: the garnet–clinopyroxene–phengite (~25 ± 3 kbar and 660 ± 100°C) and Zr‐in‐rutile (~650–700°C at 22–27 kbar). The second segment (M3A–B) of prograde P–T path is recorded in the grossular‐rich overgrowth rim of garnet. Apart from disequilibrium growth of the M3A garnet, ubiquitous overgrowth of the M3B garnet permits us to estimate the P–T conditions at ~26 ± 3 kbar and 720 ± 80°C. The cathodoluminescence (CL) imaging of zircon grains separated from a barroisite eclogite revealed three distinct zones with bright rim, dark mantle and moderately dark core. Eclogitic phases such as garnet, omphacite, epidote and rutile are present as fine‐grained inclusions in the mantle and rim of zircon, in contrast to their absence in the core. The sensitive high‐resolution ion microprobe U–Pb dating on metamorphic mantle domains and neoblasts yielded a weighted mean 206Pb/238U age of 515 ± 4 Ma (tσ), representing the time of the M2 stage. On the other hand, overgrowth rims as well as bright‐CL neoblasts of zircon were dated at 498 ± 11 Ma (tσ), corresponding to the M3. Average burial rates estimated from the M2 and M3 ages are too low (<2 mm/year) for cold subduction regime (~5–10°C/km), suggesting that an exhumation stage intervened between two prograde segments of P–T path. Thus, the P–T–t evolution of barroisite eclogites is typified by two discrete episodes with an c. 15 Ma gap during the middle Cambrian subduction of the Antarctic Ross Orogeny.  相似文献   

6.
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   

7.
Abstract Eclogites are distributed for more than 500 km along a major tectonic boundary between the Sino-Korean and Yangtze cratons in central and eastern China. These eclogites usually have high-P assemblages including omphacite + kyanite and/or coesite (or its pseudomorph), and form a high-P eclogite terrane. They occur as isolated lenses or blocks 10 cm to 300 m long in gneisses (Type I), serpentinized garnet peridotites (Type II) and marbles (Type III). Type I eclogites were formed by prograde metamorphism, and their primary metamorphic mineral assemblage consists mainly of garnet [pyrope (Prp) = 15–40 mol%], omphacite [jadeite (Jd) = 34–64 mol%], pargasitic amphibole, kyanite, phengitic muscovite, zoisite, an SiO2 phase, apatite, rutile and zircon. Type II eclogites characteristically contain no SiO2 phase, and are divided into prograde eclogites and mantle-derived eclogites. The prograde eclogites of Type II are petrographically similar to Type I eclogites. The mantle-derived eclogites have high MgO/(FeO + Fe2O3) and Cr2O3 compositions in bulk rock and minerals, and consist mainly of pyrope-rich garnet (Prp = 48–60 mol%), sodic augite (Jd = 10–27 mol%) and rutile. Type III eclogites have an unusual mineral assemblage of grossular-rich (Grs = 57 mol%) garnet + omphacite (Jd = 30–34 mol%) + pargasite + rutile. Pargasitic and taramitic amphiboles, calcic plagioclase (An68), epidote, zoisite, K-feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites. This suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and/or amphibolite facies rocks with near-isothermal compression reflecting crustal thickening during continent–continent collision of late Proterozoic age. Equilibrium conditions of the prograde eclogites range from P > 26 kbar and T= 500–750°C in the western part to P > 28 kbar and T= 810–880°C in the eastern part of the high-P eclogite terrane. The prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part. Subsequent reactions, manifested by (1) narrow rims of sodic plagioclase or paragonite on kyanite and (2) symplectites between omphacite and quartz are interpreted as an effect of near-isothermal decompression during the retrograde stage. The conditions at which symplectites re-equilibrated tend to increase from west (P < 10 kbar and T < 580°C) to east (P > 9 kbar and T > 680°C). Equilibrium temperatures of Type II mantle-derived eclogites and Type III eclogite are 730–750°C and 680°C, respectively.  相似文献   

8.
The Kulet eclogite in the Kokchetav Massif, northern Kazakhstan, is identified as recording a prograde transformation from the amphibolite facies through transitional coronal eclogite to fully recrystallized eclogite (normal eclogite). In addition to minor bodies of normal eclogite with an assemblage of Grt + Omp + Qz + Rt ± Ph and fine‐grained granoblastic texture (type A), most are pale greyish green bodies consisting of both coronal and normal eclogites (type B). The coronal eclogite is characterized by coarse‐grained amphibole and zoisite of amphibolite facies, and the growth of garnet corona along phase boundaries between amphibole and other minerals as well as the presence of eclogitic domains. The Kulet eclogites experienced a four‐stage metamorphic evolution: (I) pre‐eclogite stage, (II) transition from amphibolite to eclogite, (III) a peak eclogite stage with prograde transformation from coronal eclogite to UHP eclogite and (IV) retrograde metamorphism. Previous studies made no mention of the presence of amphibole or zoisite in either the pre‐eclogite stage or coronal eclogite, and so did not identify the four‐stage evolution recognized here. P–T estimates using thermobarometry and Xprp and Xgrs isopleths of eclogitic garnet yield a clockwise P–T path and peak conditions of 27–33 kbar and 610–720 °C, and 27–35 kbar and 560–720 °C, respectively. P–T pseudosection calculations indicate that the coexistence of coronal and normal eclogites in a single body is chiefly due to different bulk compositions of eclogite. All eclogites have tholeiitic composition, and show flat or slightly LREE‐enriched patterns [(La/Lu)N = 1.1–9.6] and negative Ba, Sr and Sc and positive Th, U and Ti anomalies. However, normal eclogite has higher TiO2 (1.35–2.65 wt%) and FeO (12.11–16.72 wt%) and REE contents than those of coronal eclogite (TiO2 < 0.9 wt% and FeO < 12.11 wt%) with one exception. Most Kulet eclogites plot in the MORB and IAB fields in the 2Nb–Zr/4–Y and TiO2–FeO/MgO diagrams, although displacement from the MORB–OIB array indicates some degree of crustal involvement. All available data suggest that the protoliths of the Kulet eclogites were formed at a passive continent marginal basin setting. A schematic model involving subduction to 180–200 km at 537–527 Ma, followed by slab breakoff at 526–507 Ma, exhumation and recrystallization at crustal depths is applied to explain the four‐stage evolution of the Kulet eclogite.  相似文献   

9.
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   

10.
Lawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a prograde P–T path that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐T of ～520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ～10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived prograde P–T path associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐T subduction zone.  相似文献   

11.
In the Orlica–?nie?nik Dome (NE Bohemian massif), alternating belts of orthogneiss with high‐pressure rocks and belts of mid‐crustal metasedimentary–metavolcanic rocks commonly display a dominant subvertical fabric deformed into a subhorizontal foliation. The first macroscopic foliation is subvertical, strikes NE–SW and is heterogeneously folded by open to isoclinal folds with subhorizontal axial planes parallel to the heterogeneously developed flat‐lying foliation. The metamorphic evolution of the mid‐crustal metasedimentary rocks involved successive crystallization of chlorite–muscovite–ilmenite–plagioclase–garnet, followed by staurolite‐bearing and then kyanite‐bearing assemblages in the subvertical fabric. This was followed by garnet retrogression, with syntectonic crystallization of sillimanite and andalusite parallel to the shallow‐dipping foliation. Elsewhere, andalusite and cordierite statically overgrew the flat‐lying fabric. With reference to a P–T pseudosection for a representative sample, the prograde succession of mineral assemblages and the garnet zoning pattern with decreasing grossular, spessartine and XFe are compatible with a P–T path from 3.5–5 kbar/490–520 °C to peak conditions of 6–7 kbar/～630 °C suggesting burial from 12 to 25 km with increasing temperature. Using the same pseudosection, the retrograde succession of minerals shows decompression to sillimanite stability at ～4 kbar/～630 °C and to andalusite–cordierite stability at 2–3 kbar indicating exhumation from 25 km to around 9–12 km. Subsequent exhumation to ～6 km occurred without apparent formation of a deformation fabric. The structure and petrology together with the spatial distribution of the metasedimentary–metavolcanic rocks, and gneissic and high‐pressure belts are compatible with a model of burial of limited parts of the upper and middle crust in narrow cusp‐like synclines, synchronous with the exhumation of orogenic lower crust represented by the gneissic and high‐pressure rocks in lobe‐shaped and volumetrically more important anticlines. Converging P–T–D paths for the metasedimentary rocks and the adjacent high‐pressure rocks are due to vertical exchanges between cold and hot vertically moving masses. Finally, the retrograde shallow‐dipping fabric affects both the metasedimentary–metavolcanic rocks and the gneissic and high‐pressure rocks, and indicates that the ～15‐km exhumation was mostly accommodated by heterogeneous ductile thinning associated with unroofing of a buoyant crustal root.  相似文献   

12.
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   

13.
Relict eclogites and associated high-pressure rocks are present in the Eastern Segment of the SW Swedish gneiss region (the tectonic counterpart of the Parautochthonous Belt of the Canadian Grenville). These rocks give evidence of Sveconorwegian eclogite facies metamorphism and subsequent pervasive reworking and deformation at granulite and amphibolite facies conditions. The best-preserved eclogite relics suggest a clockwise P–T –t history, beginning in the amphibolite facies, progressing through the eclogite facies, decompressing and partially reequilibrating through the high- and medium-pressure granulite facies, before cooling through the amphibolite facies. Textures demonstrate the former coexistence of the plagioclase-free assemblages garnet+clinopyroxene+quartz+rutile+ilmenite, garnet+clinopyroxene+ kyanite+rutile, and garnet+kyanite+quartz+rutile. The former existence of omphacite is evidenced by up to 45 vol.% plagioclase expelled as small grains within large clinopyroxene. Matrix plagioclase is secondary and occurs expelled from clinopyroxene or in fine-grained, granulite facies reaction domains formed during resorption of garnet and kyanite. Garnet shows preserved prograde growth zoning with rimward increasing pyrope content, decreasing spessartine content and decreasing Fe/(Fe+Mg) ratio, but is partly resorbed and reequilibrated at the rims. P–T estimates from microdomains with clinopyroxene+plagioclase+quartz+garnet indicate pressures of 9.5–12 kbar and temperatures of 705–795 °C for a stage of the granulite facies decompression. The preservation of the prograde zoning suggests that the rocks did not reside at these high temperatures for more than a few million years, and chemical disequilibrium and ‘frozen’ reaction textures indicate heterogeneous reaction progress and overstepping of reactions during the decompression through the granulite facies. Together these features suggest a rapid tectonic exhumation. The eclogite relics occur within a high-grade deformation zone with WNW–ESE stretching and associated oblique normal-sense, top-to-the-east (sensu lato) displacement, suggesting that extension was a main cause for the decompression and exhumation. Probable tectonic scenarios for this deformation are Sveconorwegian late-orogenic gravitational collapse or overall WNW–ESE extension.  相似文献   

14.
The Vårdalsneset eclogite situated in the Western Gneiss Region, SW Norway, is a well preserved tectonite giving information about the deformation regimes active in the lower crust during crustal thickening and subsequent exhumation. The eclogite constitutes layers and lenses variably retrograded to amphibolite and is composed of garnet and omphacite with varying amounts of barroisite, actinolite, clinozoisite, kyanite, quartz, paragonite, phengite and rutile. The rocks record a five‐stage evolution connected to Caledonian burial and subsequent exhumation. (1) A prograde evolution through amphibolite facies (T =490±63 °C) is inferred from garnet cores with amphibole inclusions and bell‐shaped Mn profile. (2) Formation of L>S‐tectonite eclogite (T =680±20 °C, P=16±2 kbar) related to the subduction of continental crust during the Caledonian orogeny. Lack of asymmetrical fabrics and orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the L>S fabric was coaxial. (3) Formation of sub‐horizontal eclogite facies foliation in which the finite stretching direction had changed by approximately 90°. Disruption of eclogite lenses and layers between symmetric shear zones characterizes the dominantly coaxial deformation regime of stage 3. Locally occurring mylonitic eclogites (T =690±20 °C, P=15±1.5 kbar) with top‐W kinematics may indicate, however, that non‐coaxial deformation was also active at eclogite facies conditions. (4) Development of a widespread regional amphibolite facies foliation (T =564±44 °C, P<10.3–8.1 kbar), quartz veins and development of conjugate shear zones indicate that coaxial vertical shortening and sub‐horizontal stretching were active during exhumation from eclogite to amphibolite facies conditions. (5) Amphibolite facies mylonites mainly formed under non‐coaxial top‐W movement are related to large‐scale movement on the extensional detachments active during the late‐orogenic extension of the Caledonides. The structural and metamorphic evolution of the Vårdalsneset eclogite and related areas support the exhumation model, including an extensional detachment in the upper crust and overall coaxial deformation in the lower crust.  相似文献   

15.
The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre‐ to centimetre‐scale eclogite pods occur in blueschist facies metabasalt, marble and quartz‐rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P–T conditions: ～26 kbar, 500 °C (lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (lawsonite blueschist); and 15–16 kbar, 480–500 °C (lawsonite‐epidote blueschist). Pressures for the Sivrihisar lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of lawsonite ± epidote define P–T conditions and paths. For example, in some lawsonite‐bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by lawsonite suggest an anticlockwise P–T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the lawsonite stability field. Results of the P–T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P–T conditions were tectonically juxtaposed as kilometre‐scale slices and associated high‐P pods, although all shared the same exhumation path from ～9–11 kbar, 300–400 °C. Within the tectonic slices, alternating millimetre–centimetre‐scale layers of eclogite and blueschist formed together at the same P–T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in fO2 or other chemical factors.  相似文献   

16.
High‐P metamorphic rocks that are formed at the onset of oceanic subduction usually record a single cycle of subduction and exhumation along counterclockwise (CCW) P–T paths. Conceptual and thermo‐mechanical models, however, predict multiple burial–exhumation cycles, but direct observations of these from natural rocks are rare. In this study, we provide a new insight into this complexity of subduction channel dynamics from a fragment of Middle‐Late Jurassic Neo‐Tethys in the Nagaland Ophiolite Complex, northeastern India. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of a medium‐grade amphibolite tectonic unit within a serpentinite mélange, we establish two overprinting metamorphic cycles (M1–M2). These cycles with CCW P–T trajectories are part of a single tectonothermal event. We relate the M1 metamorphic sequence to prograde burial and heating through greenschist and epidote blueschist facies to peak metamorphism, transitional between amphibolite and hornblende‐eclogite facies at 13.8 ± 2.6 kbar, 625 ± 45 °C (error 2σ values) and subsequent cooling and partial exhumation to greenschist facies. The M2 metamorphic cycle reflects epidote blueschist facies prograde re‐burial of the partially exhumed M1 cycle rocks to peak metamorphism at 14.4 ± 2 kbar, 540 ± 35 °C and their final exhumation to greenschist facies along a relatively cooler exhumation path. We interpret the M1 metamorphism as the first evidence for initiation of subduction of the Neo‐Tethys from the eastern segment of the Indus‐Tsangpo suture zone. Reburial and final exhumation during M2 are explained in terms of material transport in a large‐scale convective circulation system in the subduction channel as the latter evolves from a warm nascent to a cold and more mature stage of subduction. This Neo‐Tethys example suggests that multiple burial and exhumation cycles involving the first subducted oceanic crust may be more common than presently known.  相似文献   

17.
A combined study of petrology and geochemistry was carried out for granulites from the Tongbai orogen in central China. The results reveal the tectonic evolution from collisional thickening to extensional thinning of the lithosphere at the convergent plate boundary. Petrographic observations, zircon U–Pb dating, and pseudosection calculations indicate that the granulites underwent four metamorphic stages, which are categorized into two cycles. The first cycle occurred at 490–450 Ma and involves high-P (HP) metamorphism (M1) at 785–815°C and 10–14 kbar followed by decompressional heating to 840–880°C and 8–9 kbar for medium-pressure granulite facies metamorphism (M2), defining a clockwise P–T path. The high pressure is indicated by the occurrence of inclusions of rutile+kyanite+K-feldspar in the garnet mantle. The second cycle occurred at c. 440 Ma and shows an anticlockwise P–T path with continuous heating to ultrahigh-temperature (UHT) metamorphism (M3) at 890–980°C and 9–11 kbar, followed by decompressional cooling to 740–880°C and 7–9 kbar (M4) till 405 Ma. The HP metamorphism is synchronous with the ultrahigh-pressure eclogite facies metamorphism in the Qinling orogen, indicating its relevance to the continental collision in the Cambrian. The UHT metamorphism took place at reduced pressures, indicating thinning of the collision-thickened orogenic lithosphere. Therefore, the Tongbai orogen was initially thickened by the collisional orogeny and then thinned, possibly as a result of foundering of the orogenic root. Such tectonic evolution may be common in collisional orogens where compression during continental collision switched to extension during continental rifting.  相似文献   

18.
The sequential growth of biotite, garnet, staurolite, kyanite, andalusite, cordierite and fibrolitic sillimanite, their microstructural relationships, foliation intersection axes preserved in porphyroblasts (FIAs), geochronology, P–T pseudosection (MnNCKFMASH system) modelling and geothermobarometry provide evidence for a P–T–t–D path that changes from clockwise to anticlockwise with time for the Balcooma Metamorphic Group. Growth of garnet at ～530 °C and 4.6 kbar during the N–S‐shortening event that formed FIA 1 was followed by staurolite, plagioclase and kyanite growth. The inclusions of garnet in staurolite porphyroblasts that formed during the development of FIAs 2 and 3 plus kyanite growth during FIA 3 reflect continuous crustal thickening from c. 443 to 425 Ma during an Early Silurian Benambran Orogenic event. The temperature and pressure increased during this time from ～530 °C and 4.6 kbar to ～630 °C and 6.2 kbar. The overprinting of garnet‐, staurolite‐ and kyanite‐bearing mineral assemblages by low‐pressure andalusite and cordierite assemblages implies ～4‐kbar decompression during Early Devonian exhumation of the Greenvale Province.  相似文献   

19.
A re‐evaluation of the P–T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet–clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to ～16 kbar and 750 °C, followed by near‐isothermal decompression to ～10 kbar. Associated with the eclogite are two types of occurrences of sapphirine‐bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1–2 m zone adjacent to the eclogite body with a peak assemblage of garnet–kyanite–quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine–plagioclase and spinel–plagioclase symplectites developed around kyanite. The second variety of sapphirine‐bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching ～7 kbar and 750 °C. These results indicate that the final reaction in these rocks occurred at mid‐crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to <5–10 Myr, indicating that ～25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of ～2–5 mm year?1 following eclogite facies metamorphism, after which the rocks resided at mid‐crustal levels for 80–100 Myr.  相似文献   

20.
Numerous lenses of eclogite occur in a belt of augen orthogneisses in the Gubaoquan area in the southern Beishan orogen, an eastern extension of the Tianshan orogen. With detailed petrological data and phase relations, modelled in the system NCFMASHTO with thermocalc , a quantitative P–T path was estimated and defined a clockwise P–T path that showed a near isothermal decompression from eclogite facies (>15.5 kbar, 700–800 °C, omphacite + garnet) to high‐pressure granulite facies (12–14 kbar, 700–750 °C, clinopyroxene + sodic plagioclase symplectitic intergrowths around omphacite), low‐pressure granulite facies (8–9.5 kbar, ～700 °C, orthopyroxene + clinopyroxene + plagioclase symplectites and coronas surrounding garnet) and amphibolite facies (5–7 kbar, 600–700 °C, hornblende + plagioclase symplectites). The major and trace elements and Sm–Nd isotopic data suggest that most of the Beishan eclogite samples had a protolith of oceanic crust with geochemical characteristics of an enriched or normal mid‐ocean ridge basalt. The U–Pb dating of the Beishan eclogites indicates an Ordovician age of c. 467 Ma for the eclogite facies metamorphism. An 39Ar/40Ar age of c. 430 Ma for biotite from the augen gneiss corresponds to the time of retrograde metamorphism. The combined data from geological setting, bulk composition, clockwise P–T path and geochronology support a model in which the Beishan eclogites started as oceanic crust in the Palaeoasian Ocean, which was subducted to eclogite depths in the Ordovician and exhumed in the Silurian. The eclogite‐bearing gneiss belt marks the position of a high‐pressure Ordovician suture zone, and the calculated clockwise P–T path defines the progression from subduction to exhumation.  相似文献   

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