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J G Liou - One of the best experts on this subject based on the ideXlab platform.
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Ultrahigh Pressure Metamorphism
Access Science, 2014Co-Authors: J G LiouAbstract:Ultrahigh-Pressure (UHP) Metamorphism is the mineralogical and structural modification of predominan…
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high and Ultrahigh Pressure Metamorphism past results and future prospects
American Mineralogist, 2008Co-Authors: W G Ernst, J G LiouAbstract:Fifty years ago, geologic conditions attending the formation of blueschists, eclogites, and garnet lherzolites were not known. But, with the advent of high-Pressure phase-synthesis equipment and precise calorimetry, minerals like jadeite, aragonite, pyrope, and the dense polymorphs of SiO 2 and C were shown to be stable at elevated Pressures and relatively low temperatures. Metamorphic conditions required by P - T stabilities of these minerals reflect the operation of plate tectonics, lithospheric subduction, and inferred mantle convection. Integration of phase equilibria with dynamic tectonic processes has illuminated the petrogenesis of the crust. Combined with geochemical, geophysical, and isotopic data, high-Pressure phase equilibria are also providing new constraints on the constitution and evolution of the mantle. Circumpacific blueschists and eclogites occur in penetratively sheared nappes that are overturned seaward, indicating 30–50 km descent of an oceanic plate during Metamorphism before partial exhumation of mainly low-density crustal material. Neoblastic coesite and microdiamond inclusions in tough, rigid host minerals show that continental collision involves fragmentary recovery of subducted rocks from depths of 100–130 km, far deeper than traditionally thought. Even more surprising, garnet peridotites from the central Alps, western Norway, Bohemia, and China display intergrowths and exsolution lamellae reflecting the former existence of majoritic garnet, stishovite, and other phases requiring depths of origin >300 km. Exsolved nanominerals attest to the decompression of precursor phases that had formed at profound depths preceding mantle upwelling. Times of deep-seated storage and rates of exhumation remain as major problems. Fluid-rock and lithosphere-asthenosphere interactions have recycled volatiles to the deep Earth through subduction of both hydrous and nominally anhydrous minerals. Mantle petrochemistry and plume-plate dynamics control the evolving architecture of the Earth’s crust and the interdependent biosphere. Applications of advanced technologies to condensed materials are leading to a fuller understanding of the planetary interior in time and space.
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epidote minerals in high p t metamorphic terranes subduction zone and high to Ultrahigh Pressure Metamorphism
Reviews in Mineralogy & Geochemistry, 2004Co-Authors: Masaki Enami, J G Liou, Chris G. MattinsonAbstract:Epidote minerals—the monoclinic epidote group minerals together with the orthorhombic polymorph zoisite—are important Ca-Al-silicates in many metabasites, metapelites and metacherts that are characterized by high P / T ratios. Such high P / T ratios are typical for subduction zones and the high-Pressure (HP) and Ultrahigh-Pressure (UHP) Metamorphism during continent-continent collisions (e.g., Liou 1973, 1993). All of these P-T conditions can be described by geothermal gradients between 5 and 20°C/km, that therefore provide a rough framework for the P-T conditions covered by this review (Fig. 1⇓). Depending on the actual thermal structure of a subduction zone, the subducting plate will encounter subgreenschist, greenschist, blueschist, epidote-amphibolite, amphibolite, HP granulite, and/or eclogite facies conditions during its travel down into the mantle (Fig. 1⇓). The P-T regime of the eclogite facies can further be subdivided into amphibole eclogite, epidote eclogite, lawsonite eclogite, and dry eclogite facies (Fig. 1⇓). HP Metamorphism refers to metamorphic Pressure in excess of ~1.0 GPa and includes parts of the blueschist, epidote-amphibolite, and HP granulite facies as well as the eclogite facies (Fig. 1⇓). UHP refers to the Metamorphism of crustal rocks (both continental and oceanic) at P high enough to crystallize the index minerals coesite and/or diamond. HP and UHP Metamorphism are separated conveniently by the quartz-coesite equilibrium which implies a minimum P > 2.7 GPa at T > 600°C for UHP Metamorphism (Fig. 1⇓). The equilibrium boundary for the graphite-diamond transition can be used to further subdivide the UHP region into diamond-grade and coesite-grade. The stability of coesite and other UHP minerals in a metamorphic regime requires abnormally low temperatures at depths greater than 100 km. Such environments can be attained only by the subduction of cold oceanic crust-capped lithosphere ± pelagic sediments or of continental crust. Figure 1. P-T regimes of UHP …
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Ultrahigh Pressure Metamorphism in the forbidden zone the xugou garnet peridotite sulu terrane eastern china
Journal of Metamorphic Geology, 2003Co-Authors: R Y Zhang, J G Liou, Jingsui Yang, Kunhui YeAbstract:The Xugou garnet peridotite body of the southern Sulu Ultrahigh-Pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo91), enstatite (En92−93), garnet (Alm20−23Prp53−58Knr6−9Grs12−18), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high-Mg number, very low CaO, Al2O3 and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp44−58) clinopyroxenites are foliated. Except for rare kyanite-bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm29−34Prp32−50Grs15−39) + omphacite (Jd24−36) + minor rutile. Clinopyroxenite and eclogite exhibit LREE-depleted and LREE-enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high-Pressure crystal accumulation (+ variable trapped melt) from melts resulting from two-stage partial melting of a mantle source. Recrystallized textures and P–T estimates of 780–870 °C, 5–7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP Metamorphism in the P–T forbidden zone with an extremely low thermal gradient (< 5 °C km−1), and multistage retrograde recrystallization during exhumation. Develop of prehnite veins in clinopyroxenite, eclogite, felsic blocks and country rock gneiss, and replacements of eclogitic minerals by prehnite, albite, white mica, and K-feldspar indicate low-temperature metasomatism.
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eclogite facies relics and inferred Ultrahigh Pressure Metamorphism in the north dabie complex central eastern china
American Mineralogist, 2000Co-Authors: Chinho Tsai, J G LiouAbstract:Mineral assemblages and microstructures of a newly identified (retrogressed) eclogite (sensu lato) in the North Dabie Complex (NDC), central-eastern China, indicate early very high-Pressure metamorphic relics preserved in a dominant amphibolite-facies host, where no eclogite (sensu stricto) has been positively identified before. The investigated eclogitic rock shows distinct multistage recrystallization, with granulite- and amphibolite-facies assemblages overprinting eclogite-facies relics. The minimum temperature for the eclogite-facies Metamorphism is estimated to be ~800−820 °C. A spectacular microstructure of oriented quartz needles (~2–20 μm wide, ~5–200 μm long) in matrix Ca-Na clinopyroxene implies the prior existence of a non-stoichiometric “supersilicic” omphacite stabilized at Ultrahigh-Pressure (UHP, ≥25 kbar) conditions, although no coesite or coesite pseudomorphs have been found in the samples. The absence of coesite may be due to the lack of free silica at UHP conditions or the consumption of silica during retrograde reactions. The inferred UHP conditions Metamorphism is further supported by Sm-Nd ages that are equivalent to Triassic metamorphic ages from UHP eclogites in the southeastern Dabie Mountains. This finding expands the UHP terrane northward about 50 km; spatial distribution of subduction/collision-related UHP rocks includes parts of the NDC.
Yuanyuan Xiao - One of the best experts on this subject based on the ideXlab platform.
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trace element budgets and re distribution during subduction zone Ultrahigh Pressure Metamorphism evidence from western tianshan china
Chemical Geology, 2014Co-Authors: Yuanyuan Xiao, Huaikun Li, Huichu Wang, Jon P DavidsonAbstract:We have conducted an LA-ICP-MS in situ trace element study of garnet, epidote group minerals, phengitic muscovite and paragonite in rocks of basaltic and sedimentary protolith from an Ultrahigh Pressure metamorphic belt along Western Tianshan, China. The data are used to evaluate the capacity of these minerals for hosting incompatible elements in response to subduction-zone Metamorphism (SZM). The results confirm existing studies in that the presence and stability of these minerals largely control the geochemical behaviors of elements during SZM. We found that redistribution of rare earth elements (REEs), Th and U into newly-formed minerals during progressive SZM precludes the release of these elements from the down-going ocean crust, which contradicts the common perception in models of slab-dehydration and flux-melting. This suggests that additional processes, such as the involvement of supercritical fluids or hydrous melts formed at depth are required to supply these elements to the mantle wedge for arc magmatism. In addition, the ready release of large ion lithophile elements (LILEs) by different minerals, and the high immobility of REEs in rocks of basaltic protolith indicate that the contribution of altered ocean crust after SZM may not be responsible for the correlated Sr–Nd (Hf) isotope systematics observed in oceanic basalts. That is, subducted ocean crust that has gone through SZM cannot be the major source material for ocean island basalts.
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trace element transport during subduction zone Ultrahigh Pressure Metamorphism evidence from western tianshan china
Geological Society of America Bulletin, 2012Co-Authors: Yuanyuan Xiao, Shaun Lavis, Julian A Pearce, Huaikun Li, Huichu Wang, Jon P DavidsonAbstract:Subduction-zone Metamorphism is considered to be a major chemical filter for both the arc magmatism and mantle compositional heterogeneity. To understand the element transport during this process, we conducted a petrographic and geochemical study of the bulk-rock blueschists and eclogites from the western Tianshan Ultrahigh-Pressure metamorphic belt, northwest China. By examining correlations among incompatible elements, we show that high field strength elements (HFSEs), rare earth elements (REEs), Th, and U are relatively immobile, whereas Pb and Sr are mobile in both basaltic and sedimentary protoliths during subduction-zone Metamorphism. K, Rb, Cs, and Ba are mobile in rocks of basaltic protolith but immobile in rocks of sedimentary protolith because of the presence and persistent stability of white mica throughout their petrologic history. The commonly observed enrichment of some immobile elements (e.g., U, light [L] REEs) in arc magmas may thus not be caused by subduction-zone aqueous fluids. The lack of Rb/Sr-Sm/Nd correlation in these metamorphosed rocks is inconsistent with the observed first-order Sr-Nd isotope correlation in oceanic basalts, suggesting that the residual ocean crust that has undergone subduction-zone Metamorphism cannot be the major source material for oceanic basalts, as widely believed, although it can contribute to mantle compositional heterogeneity in general.
Jon P Davidson - One of the best experts on this subject based on the ideXlab platform.
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trace element budgets and re distribution during subduction zone Ultrahigh Pressure Metamorphism evidence from western tianshan china
Chemical Geology, 2014Co-Authors: Yuanyuan Xiao, Huaikun Li, Huichu Wang, Jon P DavidsonAbstract:We have conducted an LA-ICP-MS in situ trace element study of garnet, epidote group minerals, phengitic muscovite and paragonite in rocks of basaltic and sedimentary protolith from an Ultrahigh Pressure metamorphic belt along Western Tianshan, China. The data are used to evaluate the capacity of these minerals for hosting incompatible elements in response to subduction-zone Metamorphism (SZM). The results confirm existing studies in that the presence and stability of these minerals largely control the geochemical behaviors of elements during SZM. We found that redistribution of rare earth elements (REEs), Th and U into newly-formed minerals during progressive SZM precludes the release of these elements from the down-going ocean crust, which contradicts the common perception in models of slab-dehydration and flux-melting. This suggests that additional processes, such as the involvement of supercritical fluids or hydrous melts formed at depth are required to supply these elements to the mantle wedge for arc magmatism. In addition, the ready release of large ion lithophile elements (LILEs) by different minerals, and the high immobility of REEs in rocks of basaltic protolith indicate that the contribution of altered ocean crust after SZM may not be responsible for the correlated Sr–Nd (Hf) isotope systematics observed in oceanic basalts. That is, subducted ocean crust that has gone through SZM cannot be the major source material for ocean island basalts.
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trace element transport during subduction zone Ultrahigh Pressure Metamorphism evidence from western tianshan china
Geological Society of America Bulletin, 2012Co-Authors: Yuanyuan Xiao, Shaun Lavis, Julian A Pearce, Huaikun Li, Huichu Wang, Jon P DavidsonAbstract:Subduction-zone Metamorphism is considered to be a major chemical filter for both the arc magmatism and mantle compositional heterogeneity. To understand the element transport during this process, we conducted a petrographic and geochemical study of the bulk-rock blueschists and eclogites from the western Tianshan Ultrahigh-Pressure metamorphic belt, northwest China. By examining correlations among incompatible elements, we show that high field strength elements (HFSEs), rare earth elements (REEs), Th, and U are relatively immobile, whereas Pb and Sr are mobile in both basaltic and sedimentary protoliths during subduction-zone Metamorphism. K, Rb, Cs, and Ba are mobile in rocks of basaltic protolith but immobile in rocks of sedimentary protolith because of the presence and persistent stability of white mica throughout their petrologic history. The commonly observed enrichment of some immobile elements (e.g., U, light [L] REEs) in arc magmas may thus not be caused by subduction-zone aqueous fluids. The lack of Rb/Sr-Sm/Nd correlation in these metamorphosed rocks is inconsistent with the observed first-order Sr-Nd isotope correlation in oceanic basalts, suggesting that the residual ocean crust that has undergone subduction-zone Metamorphism cannot be the major source material for oceanic basalts, as widely believed, although it can contribute to mantle compositional heterogeneity in general.
R Y Zhang - One of the best experts on this subject based on the ideXlab platform.
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Ultrahigh Pressure Metamorphism in the forbidden zone the xugou garnet peridotite sulu terrane eastern china
Journal of Metamorphic Geology, 2003Co-Authors: R Y Zhang, J G Liou, Jingsui Yang, Kunhui YeAbstract:The Xugou garnet peridotite body of the southern Sulu Ultrahigh-Pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo91), enstatite (En92−93), garnet (Alm20−23Prp53−58Knr6−9Grs12−18), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high-Mg number, very low CaO, Al2O3 and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp44−58) clinopyroxenites are foliated. Except for rare kyanite-bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm29−34Prp32−50Grs15−39) + omphacite (Jd24−36) + minor rutile. Clinopyroxenite and eclogite exhibit LREE-depleted and LREE-enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high-Pressure crystal accumulation (+ variable trapped melt) from melts resulting from two-stage partial melting of a mantle source. Recrystallized textures and P–T estimates of 780–870 °C, 5–7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP Metamorphism in the P–T forbidden zone with an extremely low thermal gradient (< 5 °C km−1), and multistage retrograde recrystallization during exhumation. Develop of prehnite veins in clinopyroxenite, eclogite, felsic blocks and country rock gneiss, and replacements of eclogitic minerals by prehnite, albite, white mica, and K-feldspar indicate low-temperature metasomatism.
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Ultrahigh-Pressure Metamorphism in the forbidden zone: the Xugou garnet peridotite, Sulu terrane, eastern China
Journal of Metamorphic Geology, 2003Co-Authors: R Y Zhang, Juhn G. Liou, Jingsui Yang, Kunhui YeAbstract:The Xugou garnet peridotite body of the southern Sulu Ultrahigh-Pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo91), enstatite (En92−93), garnet (Alm20−23Prp53−58Knr6−9Grs12−18), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high-Mg number, very low CaO, Al2O3 and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp44−58) clinopyroxenites are foliated. Except for rare kyanite-bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm29−34Prp32−50Grs15−39) + omphacite (Jd24−36) + minor rutile. Clinopyroxenite and eclogite exhibit LREE-depleted and LREE-enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high-Pressure crystal accumulation (+ variable trapped melt) from melts resulting from two-stage partial melting of a mantle source. Recrystallized textures and P–T estimates of 780–870 °C, 5–7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP Metamorphism in the P–T forbidden zone with an extremely low thermal gradient (
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petrogenetic grid for Ultrahigh Pressure Metamorphism in the model system cao mgo sio2 co2 h2o
Island Arc, 1995Co-Authors: Yoshihide Ogasawara, J G Liou, R Y ZhangAbstract:Petrogenetic grids for Ultrahigh-Pressure (UHP) Metamorphism were calculated at different Xco2 conditions in the model system CaO-MgO-SiO2-CO2-H2O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P-T grids at Xco2= 0.01 and 0.5 are described. In the calculated P-T grid at Xco2= 0.01, four out of 10 stable invariant points, Co-En-Ms-Tc, Co-Di-En-Tc-Tr, Co-Di-Ms-Tc-Tr and Di-En-Ms-Tc-Tr lie within the stability field of coesite. If the fluid phase has Xco2= 0.5, no invariant point is stable under UHP conditions. Some magnesite-bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO2, Ms + Tr = Do + Fo + CO2+ H2O and Ms + Tc = Fo+ CO2+ H2O at Xco2= 0.01 and by reaction Ms + Tc = Fo + CO2+ H2O together with these three at Xco2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low Xco2 conditions include the following: Co-Do-Ms, Co-Di-Ms, Co-Di-Tc, Di-Ms-Tc, Di-En-Tc-, Di-En-Ms, Co-Di-En, Di-En-Fo, Di-Fo-Ms, Di-Do-Fo. Among them, talc-bearing assemblages are restricted to Xco2 < 0.02 and their high-P limit is 31.7 kb (749°C) at Xco2= 0.01. Dolomite-magnesite-silica assemblages have large P-T stability fields even if Xco2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite-dolomite assemblage is restricted only to a calc-silicate rock interlayered with marble where Xco2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low Xco2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of Xco2 on the stability of coesite-dolomite-magnesite, diopside-enstatite-magnesite, diopside-talc assemblages is examined and the occurrence of coesite-dolomite, magnesite-bearing and talc-bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P-T grids.
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Petrogenetic grid for Ultrahigh‐Pressure Metamorphism in the model system CaO‐MgO‐SiO2‐CO2‐H2O
Island Arc, 1995Co-Authors: Yoshihide Ogasawara, Juhn G. Liou, R Y ZhangAbstract:Petrogenetic grids for Ultrahigh-Pressure (UHP) Metamorphism were calculated at different Xco2 conditions in the model system CaO-MgO-SiO2-CO2-H2O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P-T grids at Xco2= 0.01 and 0.5 are described. In the calculated P-T grid at Xco2= 0.01, four out of 10 stable invariant points, Co-En-Ms-Tc, Co-Di-En-Tc-Tr, Co-Di-Ms-Tc-Tr and Di-En-Ms-Tc-Tr lie within the stability field of coesite. If the fluid phase has Xco2= 0.5, no invariant point is stable under UHP conditions. Some magnesite-bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO2, Ms + Tr = Do + Fo + CO2+ H2O and Ms + Tc = Fo+ CO2+ H2O at Xco2= 0.01 and by reaction Ms + Tc = Fo + CO2+ H2O together with these three at Xco2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low Xco2 conditions include the following: Co-Do-Ms, Co-Di-Ms, Co-Di-Tc, Di-Ms-Tc, Di-En-Tc-, Di-En-Ms, Co-Di-En, Di-En-Fo, Di-Fo-Ms, Di-Do-Fo. Among them, talc-bearing assemblages are restricted to Xco2 < 0.02 and their high-P limit is 31.7 kb (749°C) at Xco2= 0.01. Dolomite-magnesite-silica assemblages have large P-T stability fields even if Xco2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite-dolomite assemblage is restricted only to a calc-silicate rock interlayered with marble where Xco2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low Xco2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of Xco2 on the stability of coesite-dolomite-magnesite, diopside-enstatite-magnesite, diopside-talc assemblages is examined and the occurrence of coesite-dolomite, magnesite-bearing and talc-bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P-T grids.
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an introduction to Ultrahigh Pressure Metamorphism
Island Arc, 1994Co-Authors: J G Liou, R Y Zhang, W G ErnstAbstract:Ultrahigh-Pressure (UHP) Metamorphism refers to mineralogical and structural readjustment of supracrustal protoliths and associated mafic-ultramafic rocks at mantle Pressures greater than ∼ 25 kbar (80-90 km). Typical products include metapelite, quartzite, marble, granulite, eclogite, paragneiss and orthogneiss; minor mafic and ultramafic rocks occur as eclogitic-ultramafic layers or blocks of various dimensions within the supracrustal rocks. For appropriate bulk compositions, Metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals with unusual compositions. Thus far, at least seven coesite-bearing eclogitic terranes and three diamond-bearing UHP regions have been documented. All lie within major continental collision belts in Eurasia, have similar supracrustal protoliths and metamorphic assemblages, occur in long, discontinuous belts that may extend several hundred kilometers or more, and typically are associated with contemporaneous high-P blueschist belts. This paper defines the P-T regimes of UHP Metamorphism and describes mineralogical, petrological and tectonic characteristics for a few representative UHP terranes including the western gneiss region of Norway, the Dora Maira massif of the western Alps, the Dabie Mountains and the Su-Lu region of east-central China, and the Kokchetav massif of the former USSR. Prograde P-T paths for coesite-bearing eclogites require abnormally low geothermal gradients (approximately 7°C/km) that can be accomplished only by subduction of cold, oceanic crust-capped lithosphere ± pelagic sediments or an old, cold continent. The preservation of coesite inclusions in garnet, zircon, omphacite, kyanite and epidote, and microdiamond inclusions in garnet and zircon during exhumation of an UHP terrane requires either an extraordinarily fast rate of denudation (up to 10 cm/year) or continuous refrigeration in an extensional regime (retreating subduction zone).
Lifei Zhang - One of the best experts on this subject based on the ideXlab platform.
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metamorphic evolution of Ultrahigh Pressure rocks from chinese southwestern tianshan and a possible indicator of uhp Metamorphism using garnet composition in low t eclogites
Journal of Asian Earth Sciences, 2014Co-Authors: Jinxue Du, Lifei Zhang, Thomas Bader, Tingting ShenAbstract:Abstract How to identify Ultrahigh-Pressure (UH P ) Metamorphism in the absence of coesite is a key problem to gain the correct P–T history for an orogenic belt. In this study, garnet composition combined with the pseudosection approach was used to identify Ultrahigh-Pressure Metamorphism and to determine P–T paths for eclogites and metapelites from Chinese southwestern Tianshan. Porphyroblastic garnets from both eclogites and metapelites develop pronounced chemical core-rim textures: the relatively homogeneous core with low pyrope [Prp; Mg/(Ca + Mn + Mg + Fe 2+ ) × 100] and grossular [Grs; Ca/(Ca + Mn + Mg + Fe 2+ ) × 100] content is overgrown by a thin rim with sharply increased Prp and Grs. Phase equilibria modeling indicates that the Ultrahigh-Pressure rocks have undergone a clockwise P–T path characterized by heating during early exhumation with peak P–T at 31–33 kbar and 490–520 °C. The P–T pseudosections for eclogites show that isopleths of Prp and Grs strongly depend on temperature and Pressure, respectively, especially in the stability fields of glaucophane–lawsonite-bearing eclogite facies assemblages. This indicates that garnet composition provides robust thermobarometric constraints. Consequently, we propose a Prp–Grs diagram which is subdivided into a high-Pressure region and an Ultrahigh-Pressure region by the quartz–coesite–transition curve. Those garnet compositions which fall into the Ultrahigh-Pressure region are regarded to have experienced Ultrahigh-Pressure Metamorphism. This approach is expected to be a useful tool to qualitatively identify Ultrahigh-Pressure Metamorphism for glaucophane–lawsonite-bearing eclogites and its particular strength is the quick examination of large datasets comprising samples with similar bulk composition. Using this method, garnet compositions of eclogites and mafic blueschists from Chinese southwestern Tianshan and lawsonite eclogites worldwide are plotted in the Prp–Grs diagram and several possible Ultrahigh-Pressure eclogite occurrences are newly identified.
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Developing the plate tectonics from oceanic subduction to continental collision
Chinese Science Bulletin, 2009Co-Authors: Yongfei Zheng, Kai Ye, Lifei ZhangAbstract:The studies of continental deep subduction and Ultrahigh-Pressure Metamorphism have not only promoted the development of solid earth science in China, but also provided an excellent opportunity to advance the plate tectonics theory. In view of the nature of subducted crust, two types of subduction and collision have been respectively recognized in nature. On one hand, the crustal subduction occurs due to underflow of either oceanic crust (Pacific type) or continental crust (Alpine type). On the other hand, the continental collision proceeds by arc-continent collision (Himalaya-Tibet type) or continent-continent collision (Dabie-Sulu type). The key issues in the future study of continental dynamics are the chemical changes and differential exhumation in continental deep subduction zones, and the temporal-spatial transition from oceanic subduction to continental subduction.
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Ultrahigh Pressure Metamorphism in eclogites from the western tianshan china reply
American Mineralogist, 2003Co-Authors: Lifei Zhang, David J Ellis, Samansa Williams, Wenbo JiangAbstract:We thank Dr. Klemd for his comments on our recent papers about Ultrahigh-Pressure Metamorphism in Western Tianshan, China (Zhang et al. 2002a, 2002b). It is good to have this opportunity to respond to his comments and to clarify some of our statements in detail. First of all, we wish to point out that we have never concluded that “all rocks from Chinese western Tianshan have undergone Ultrahigh-Pressure Metamorphism” as claimed by Dr. Klemd, but rather “some eclogites of studied samples have undergone Ultrahigh-Pressure Metamorphism (UHPM)”. It should be noted that samples we studied were collected from different outcrops and localities from those considered by Klemd and Gao, although we worked in the same area of Western Tianshan, China. His comments are mainly related to three aspects of mineralogic evidence for UHP in the eclogites from Western Tianshan (Zhang et al. 2002a, 2002b): (1) coesite pseudomorphs as inclusions in garnet; (2) quartz exsolution lamellae in omphacite; and (3) metamorphic magnesite in eclogite. Our responses are as follows. The characteristics of quartz pseudomorphs after coesite as inclusions in garnet with radial fractures in eclogites from Western Tianshan are quite different from those described by Wendt et al. (1993) in metapelites from the southeastern Saih Hatat tectonic window, Northeastern Oman Mountains. In contrast to the Oman metapelites, there are no apatite inclusions surrounded by radial cracks in garnet, and no separation between core and rim of the garnet after radial cracking in the Tianshan eclogites. The most important difference between the Oman and Tianshan eclogites is that, in the latter, the quartz inclusions surrounded by radial cracks are fine-grained polycrystalline aggregates of quartz with a “palisade” texture, i.e., the fine quartz grains are elongated perpendicular to the center in Tianshan eclogites (Figs. 3c and 3d in Zhang et al. 2002a), whereas …
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Ultrahigh-Pressure Metamorphism in eclogites from the western Tianshan, China—Reply
American Mineralogist, 2003Co-Authors: Lifei Zhang, David J Ellis, Samansa Williams, Wenbo JiangAbstract:We thank Dr. Klemd for his comments on our recent papers about Ultrahigh-Pressure Metamorphism in Western Tianshan, China (Zhang et al. 2002a, 2002b). It is good to have this opportunity to respond to his comments and to clarify some of our statements in detail. First of all, we wish to point out that we have never concluded that “all rocks from Chinese western Tianshan have undergone Ultrahigh-Pressure Metamorphism” as claimed by Dr. Klemd, but rather “some eclogites of studied samples have undergone Ultrahigh-Pressure Metamorphism (UHPM)”. It should be noted that samples we studied were collected from different outcrops and localities from those considered by Klemd and Gao, although we worked in the same area of Western Tianshan, China. His comments are mainly related to three aspects of mineralogic evidence for UHP in the eclogites from Western Tianshan (Zhang et al. 2002a, 2002b): (1) coesite pseudomorphs as inclusions in garnet; (2) quartz exsolution lamellae in omphacite; and (3) metamorphic magnesite in eclogite. Our responses are as follows. The characteristics of quartz pseudomorphs after coesite as inclusions in garnet with radial fractures in eclogites from Western Tianshan are quite different from those described by Wendt et al. (1993) in metapelites from the southeastern Saih Hatat tectonic window, Northeastern Oman Mountains. In contrast to the Oman metapelites, there are no apatite inclusions surrounded by radial cracks in garnet, and no separation between core and rim of the garnet after radial cracking in the Tianshan eclogites. The most important difference between the Oman and Tianshan eclogites is that, in the latter, the quartz inclusions surrounded by radial cracks are fine-grained polycrystalline aggregates of quartz with a “palisade” texture, i.e., the fine quartz grains are elongated perpendicular to the center in Tianshan eclogites (Figs. 3c and 3d in Zhang et al. 2002a), whereas …
