The petrological research on the ultra high pressure metamorphism (UHP) of collisional orogen indicates that the upper crustal rocks is subducted to depths exceeding 100 km, and returned to the surface rapidly. In thi...The petrological research on the ultra high pressure metamorphism (UHP) of collisional orogen indicates that the upper crustal rocks is subducted to depths exceeding 100 km, and returned to the surface rapidly. In this study, we investigate the thermal structure of collisional orogen as a slab of continental lithosphere being subducted beneath an overriding wedge of continental lithosphere by the 2 D finite element method. The advection heat transfer due to the accretion of orogenic wedge is considered. The wedge is composed of the upper crust materials through the accretion from the down going plate to the upper plate. For identifying the significance of the geometric and/or kinetic factors on the thermal structure of continental subduction, the different combinations of parameters, including dip angle of subduction zone, accretion or erosion rates, and the convergence velocity etc., are used in modelling. The time span of continental subduction in our calculation is less than 30 Ma, according to the short duration of ultra deep subduction of continental slab suggested by the preservation of metastable pre peak low pressure mineralogy assemblage in the garnet of UHP rocks. Therefore, the steep dip angle of down going plate and/or low rate of accretion favour the ultra deep subduction of upper crust materials, especially for the slower down going slab. Meanwhile, taking the erosion rate as the level of exhumation rate of UHP rocks in some orogens (i.e., 1-2 km/Ma or more) does not result in the anatexis melting of crust of the overriding plate, due to the cooling effect of the rapid down going slab. However, the temperature structures of all models are generally cooler than those recovered by thermobarometric studies of the UHP rocks. This implies the significant increase of temperature after the rapid subduction of continental slab. Following the method of Davies and von Blackenburg (1998), we show that the slab breakoff can occur at the depth exceeding 100 km. Thermal modelling on the post subduction stage shows the heating related to the plate breakoff can cause the higher temperature recorded by the exhumed UHP rocks. The higher geotherm during post subduction stage leads to the weak strength of the orogenic wedge, and favours the faster upward movement of the UHP rock slices as ductile agents. The lower temperature gradient of the subduction slab predicted by modelling suggests the cold subducting slab could have transported significant fluids to mantle depth, not released during subduction. Accordingly, the absence of coeval calc alkalic magmatism in UHP orogens might resulted from the lower temperature as well as the fluid free circumstance, both are related to the rapid subduction of cold plate. Therefore, shear heating is not needed for explanation the thermal evolution of UHP orogen. On the other hand, the post collisional or late stage granitic plutonism is closely related to the deep seated heat producing materials of the accretion wedge.展开更多
Up to now it is known that almost all ultrahigh-pressure (UHP) metamorphism of non-impact origin occurred in continent-continent collisional orogenic belt, as has been evidenced by many outcrops in the eastern hemisph...Up to now it is known that almost all ultrahigh-pressure (UHP) metamorphism of non-impact origin occurred in continent-continent collisional orogenic belt, as has been evidenced by many outcrops in the eastern hemisphere. UHP metamorphic rocks are represented by coesite- and diamond-bearing eclogites and eclogite facies metamorphic rocks formed at 650-800℃ and 2.6-3.5 GPa, and most of the protoliths of UHP rocks are volcanic-sedimentary sequences of continental crust. From these it may be deduced that deep subduction of continental crust may have occurred. However, UHP rocks are exposed on the surface or occur near the surface now, which implies that they have been exhumed from great depths. The mechanism of deep subduction of continental crust and subsequent exhumation has been a hot topic of the research on continental dynamics, but there are divergent views. The focus of the dispute is how deep continental crust is subducted so that UHP rocks can be formed and what mechanism causes it to be subducted to great depths and again exhumed to the shallow surface. Through an analysis of the continental process and mechanical boundary conditions of the Dabie collisional belt-an UHP metamorphic belt where the largest area of UHP rocks in the world is exposed, this paper discusses the variations of viscous stresses and average pressure in the viscous fluid caused by tectonism with rock physical properties and the contribution of the tectonic stresses to production of UHP. Calculation indicates that the anomalous stress state on the irregular boundary of a continental block may give rise to stress concentration and accumulation at local places (where the compressional stress may be 5-9 times higher than those in their surroundings). The tectonic stresses may account for 20-35% of the total UHP. So we may infer that the HP (nigh-pressure)-UHP rocks in the Dabie Mountains were formed at depths of 60-80 km. Thus the authors propose a new genetic model of UHP rocks-the point-collision model. This model conforms to the basic principles of the mechanics and also to the geologic records and process in the Dabie orogenic belt. It can explain why UHP rocks do nol exist along the entire length of the collisional orogen but occur in some particular positions. The authors also propose that the eastern and western corners of the Himalaya collision zone are typical point-collision areas and that almost all UHP metamorphism of continental crustal rocks occurred in the two particular positions.展开更多
Based upon fluid effects, retrograde metamorphism of eclogites in the Dabieregion can be divided into the fluid-poor, fluid-bearing and fluid-rich stages. The fluid-poor stageis marked by polymorphic inversion, recrys...Based upon fluid effects, retrograde metamorphism of eclogites in the Dabieregion can be divided into the fluid-poor, fluid-bearing and fluid-rich stages. The fluid-poor stageis marked by polymorphic inversion, recrystallization and exsolution of solid solutions, and isthought to represent eclogite-facies retrograde environments. The fluid-bearing stage is likely tohave occurred at the late stage of ecologite-facies diaphthorosis and is represented by kyaniteporphyroblasts, rutile, and sodic pyroxene in association with high-pressure hydrous minerals suchas phengite and zoisite (clinozoisite) without significant amount of hydrous minerals such asamphibole, epidote and biotite. The fluid-rich stage might have commenced concomitantly with loweramphibolite-facies diaphthoresis and persisted all the way towards the near-surface environment. Theproduct of this stage is characterized by plentiful hydrous and volatile-bearing phases.The dissemination-type rutile mineralizations in eclogites might have formed by preferentialshearing-induced pressure solution of gangue minerals at the fluid-bearing stage. The accompanyingvein rutile was precipitated from fluids of this stage after local transport and concentration, andmay hence represent proximal mobilization of titanium from the eclogite. Therefore, rutile veins canbe used as an exploration indicator for dissemination-type rutile deposits.展开更多
A detailed tectonic analysis demonstrates that the present observed regional tectonic configuration of the ultrahigh-pressure metamorphic terrane in the Dabie massif was mainly formed by the extension processes of the...A detailed tectonic analysis demonstrates that the present observed regional tectonic configuration of the ultrahigh-pressure metamorphic terrane in the Dabie massif was mainly formed by the extension processes of the post-Indosinian continent-continent oblique collision between the Sino-Korean and Yangtze cratons and ultrahigh-pressure metamorphism (UHPM). The configuration is characterized by a regional tectonic pattern similar to metamorphic core complexes and by the development of multi-layered detachment zones. On the basis of the identification of compressional and extensional fabrics, it is indicated that the exhumation and uplift of ultrahigh-pressure (UHP) metamorphic rocks from the mantle depth to the surface can be divided into at least three different decompression retrogressive metamorphism and tectonic deformation stages, in which the subhorizontal crustal-scale extensional flow in the middle-lower crust under amphibolite facies conditions is an important geodynamic process in the exhumation of UHP metamorphic rocks. Moreover, the extensional flow is probably driven by delamination and magmatic underplating of thickened lithospheric mantle following the continental oblique collision.展开更多
The transitional pressure of quartz coesite under the differential stress and highly strained conditions is far from the pressure of the stable field under the static pressure. Therefore, the effect of the different...The transitional pressure of quartz coesite under the differential stress and highly strained conditions is far from the pressure of the stable field under the static pressure. Therefore, the effect of the differential stress should be considered when the depth of petrogenesis is estimated about ultrahigh pressure metamorphic (UHPM) rocks. The rheological strength of typical ultrahigh pressure rocks in continental subduction zone was derived from the results of the laboratory experiments. The results indicate the following three points. (1) The rheological strength of gabbro, similar to that of eclogite, is smaller than that of clinopyroxenite on the same condition. (2) The calculated strength of rocks (gabbro, eclogite and clinopyroxenite) related to UHPM decreases by nearly one order of magnitude with the temperature rising by 100 ℃ in the range between 600 and 900 ℃. The calculated strength is far greater than the faulting strength of rocks at 600 ℃, and is in several hundred to more than one thousand mega pascals at 700-800 ℃, which suggests that those rocks are located in the brittle deformation region at 600 ℃, but are in the semi brittle to plastic deformation region at 700-800 ℃. Obviously, the 700 ℃ is a brittle plastic transition boundary. (3) The calculated rheological strength in the localized deformation zone on a higher strain rate condition (1.6×10 -12 s -l ) is 2-5 times more than that in the distributed deformation zone on a lower strain rate condition (1.6×10 -14 s -1 ). The average rheological stress (1 600 MPa) at the strain rate of 10 -12 s -1 stands for the ultimate differential stress of UHPM rocks in the semi brittle flow field, and the average rheological stress (550-950 MPa) at the strain rate of l0 -14 - 10 -13 s -l stands for the ultimate differential stress of UHPM rocks in the plastic flow field, suggesting that the depth for the formation of UHPM rocks is more than 20-60 km below the depth estimated under static pressure condition due to the effect of the differential stress.展开更多
The high-pressure metamorphic belt （HPMB） of eclogite-blueschist in Central Qiangtang （羌塘） lies in the Longmu Co （龙木错）-Shuanghu （双湖） suture zone. To the west, the HPMB extends 500 km from Hongjishan （...The high-pressure metamorphic belt （HPMB） of eclogite-blueschist in Central Qiangtang （羌塘） lies in the Longmu Co （龙木错）-Shuanghu （双湖） suture zone. To the west, the HPMB extends 500 km from Hongjishan （红脊山） to Caiduochaka （才多茶卡）, east of Shuanghu; to the east it extends to Baqing （巴青） and Jitang （吉塘） in Qamdo （昌都）, and then bends southward to Yunnan （云南） Province. Including the Lancangjiang （澜沧江） blueschist belt, the entire HPMB is about 2 000 km long. In Central Qiangtang, the belt is mainly composed of blueschist and eclogite, whereas in West Yunnan it contains only blueschist. The Baqing-Jitang segment is dominated by garnet phengite schist. 40Ar-39Ar dating of glaucophane and phengite from the blueschists yielded plateau ages ranging from 223 to 215 Ma, whereas SHRIMP U-Pb dating of zircon from the eclogites gives metamorphic ages of 243-217 Ma. The calculated metamorphic conditions for the blueschists are 410-460 ℃ and 0.67-0.75 GPa, and for the eclogites, 〈500℃ and 1.56-2.35 GPa. The metamorphic ages suggest that the Longmu Co-Shuanghu suture closed in the Late Triassic. The region south of the Longmu Co-Shuanghu- Lancang suture consists of the pan-African basement overlain by Condwana sedimentary and metasedimentary rocks, whereas the region north of the suture is dominated by the Jinning （晋宁） basement and Yangtze sedimentary and metasedimentary rocks. The Qiangtang HPMB marks the closure of the paleo-Tethys Ocean.展开更多
The Dabie Mountains are believed to be a collisional orogenic belt between the Yangtze amd Sino-Koreancontinental plates. It is composed of the foreland fold-thrust zone, the subducting cover and basement of theYangtz...The Dabie Mountains are believed to be a collisional orogenic belt between the Yangtze amd Sino-Koreancontinental plates. It is composed of the foreland fold-thrust zone, the subducting cover and basement of theYangtze continental plate, the coesite- and diamond-bearing ultra-high pressure metamorphic zone and themeta-ophiolitic melange zone in the subducting basement, the fore-arc flysch nappe and the back thrust zoneoccurring respectively on the southern and northern margins of the Sino-Korean continental plate and the in-herited basin with molassic deposits on the northern margin. When the palaeo-Dabie oceanic plate subductednorthward in the Early Palaeozoic, volcanic arc and back arc basin probably formed on the southern margin ofthe Sino-Korean continental plate. The Sm / Nd isotopic dating of the strata and eclogite which were drawn in-to the foreland fold-thrust zone indicates that the intense collision of the two continental plates took place inthe Early Mesozoic.展开更多
文摘The petrological research on the ultra high pressure metamorphism (UHP) of collisional orogen indicates that the upper crustal rocks is subducted to depths exceeding 100 km, and returned to the surface rapidly. In this study, we investigate the thermal structure of collisional orogen as a slab of continental lithosphere being subducted beneath an overriding wedge of continental lithosphere by the 2 D finite element method. The advection heat transfer due to the accretion of orogenic wedge is considered. The wedge is composed of the upper crust materials through the accretion from the down going plate to the upper plate. For identifying the significance of the geometric and/or kinetic factors on the thermal structure of continental subduction, the different combinations of parameters, including dip angle of subduction zone, accretion or erosion rates, and the convergence velocity etc., are used in modelling. The time span of continental subduction in our calculation is less than 30 Ma, according to the short duration of ultra deep subduction of continental slab suggested by the preservation of metastable pre peak low pressure mineralogy assemblage in the garnet of UHP rocks. Therefore, the steep dip angle of down going plate and/or low rate of accretion favour the ultra deep subduction of upper crust materials, especially for the slower down going slab. Meanwhile, taking the erosion rate as the level of exhumation rate of UHP rocks in some orogens (i.e., 1-2 km/Ma or more) does not result in the anatexis melting of crust of the overriding plate, due to the cooling effect of the rapid down going slab. However, the temperature structures of all models are generally cooler than those recovered by thermobarometric studies of the UHP rocks. This implies the significant increase of temperature after the rapid subduction of continental slab. Following the method of Davies and von Blackenburg (1998), we show that the slab breakoff can occur at the depth exceeding 100 km. Thermal modelling on the post subduction stage shows the heating related to the plate breakoff can cause the higher temperature recorded by the exhumed UHP rocks. The higher geotherm during post subduction stage leads to the weak strength of the orogenic wedge, and favours the faster upward movement of the UHP rock slices as ductile agents. The lower temperature gradient of the subduction slab predicted by modelling suggests the cold subducting slab could have transported significant fluids to mantle depth, not released during subduction. Accordingly, the absence of coeval calc alkalic magmatism in UHP orogens might resulted from the lower temperature as well as the fluid free circumstance, both are related to the rapid subduction of cold plate. Therefore, shear heating is not needed for explanation the thermal evolution of UHP orogen. On the other hand, the post collisional or late stage granitic plutonism is closely related to the deep seated heat producing materials of the accretion wedge.
基金the keyfundamentalgeologicalresearch project (No.9501102-3) the Ninth Five-Year Plan supported by the Ministry of Land and Resources a projectsupported by National Natural Science Foundation ofChina grant 19972064.
文摘Up to now it is known that almost all ultrahigh-pressure (UHP) metamorphism of non-impact origin occurred in continent-continent collisional orogenic belt, as has been evidenced by many outcrops in the eastern hemisphere. UHP metamorphic rocks are represented by coesite- and diamond-bearing eclogites and eclogite facies metamorphic rocks formed at 650-800℃ and 2.6-3.5 GPa, and most of the protoliths of UHP rocks are volcanic-sedimentary sequences of continental crust. From these it may be deduced that deep subduction of continental crust may have occurred. However, UHP rocks are exposed on the surface or occur near the surface now, which implies that they have been exhumed from great depths. The mechanism of deep subduction of continental crust and subsequent exhumation has been a hot topic of the research on continental dynamics, but there are divergent views. The focus of the dispute is how deep continental crust is subducted so that UHP rocks can be formed and what mechanism causes it to be subducted to great depths and again exhumed to the shallow surface. Through an analysis of the continental process and mechanical boundary conditions of the Dabie collisional belt-an UHP metamorphic belt where the largest area of UHP rocks in the world is exposed, this paper discusses the variations of viscous stresses and average pressure in the viscous fluid caused by tectonism with rock physical properties and the contribution of the tectonic stresses to production of UHP. Calculation indicates that the anomalous stress state on the irregular boundary of a continental block may give rise to stress concentration and accumulation at local places (where the compressional stress may be 5-9 times higher than those in their surroundings). The tectonic stresses may account for 20-35% of the total UHP. So we may infer that the HP (nigh-pressure)-UHP rocks in the Dabie Mountains were formed at depths of 60-80 km. Thus the authors propose a new genetic model of UHP rocks-the point-collision model. This model conforms to the basic principles of the mechanics and also to the geologic records and process in the Dabie orogenic belt. It can explain why UHP rocks do nol exist along the entire length of the collisional orogen but occur in some particular positions. The authors also propose that the eastern and western corners of the Himalaya collision zone are typical point-collision areas and that almost all UHP metamorphism of continental crustal rocks occurred in the two particular positions.
文摘Based upon fluid effects, retrograde metamorphism of eclogites in the Dabieregion can be divided into the fluid-poor, fluid-bearing and fluid-rich stages. The fluid-poor stageis marked by polymorphic inversion, recrystallization and exsolution of solid solutions, and isthought to represent eclogite-facies retrograde environments. The fluid-bearing stage is likely tohave occurred at the late stage of ecologite-facies diaphthorosis and is represented by kyaniteporphyroblasts, rutile, and sodic pyroxene in association with high-pressure hydrous minerals suchas phengite and zoisite (clinozoisite) without significant amount of hydrous minerals such asamphibole, epidote and biotite. The fluid-rich stage might have commenced concomitantly with loweramphibolite-facies diaphthoresis and persisted all the way towards the near-surface environment. Theproduct of this stage is characterized by plentiful hydrous and volatile-bearing phases.The dissemination-type rutile mineralizations in eclogites might have formed by preferentialshearing-induced pressure solution of gangue minerals at the fluid-bearing stage. The accompanyingvein rutile was precipitated from fluids of this stage after local transport and concentration, andmay hence represent proximal mobilization of titanium from the eclogite. Therefore, rutile veins canbe used as an exploration indicator for dissemination-type rutile deposits.
文摘A detailed tectonic analysis demonstrates that the present observed regional tectonic configuration of the ultrahigh-pressure metamorphic terrane in the Dabie massif was mainly formed by the extension processes of the post-Indosinian continent-continent oblique collision between the Sino-Korean and Yangtze cratons and ultrahigh-pressure metamorphism (UHPM). The configuration is characterized by a regional tectonic pattern similar to metamorphic core complexes and by the development of multi-layered detachment zones. On the basis of the identification of compressional and extensional fabrics, it is indicated that the exhumation and uplift of ultrahigh-pressure (UHP) metamorphic rocks from the mantle depth to the surface can be divided into at least three different decompression retrogressive metamorphism and tectonic deformation stages, in which the subhorizontal crustal-scale extensional flow in the middle-lower crust under amphibolite facies conditions is an important geodynamic process in the exhumation of UHP metamorphic rocks. Moreover, the extensional flow is probably driven by delamination and magmatic underplating of thickened lithospheric mantle following the continental oblique collision.
文摘The transitional pressure of quartz coesite under the differential stress and highly strained conditions is far from the pressure of the stable field under the static pressure. Therefore, the effect of the differential stress should be considered when the depth of petrogenesis is estimated about ultrahigh pressure metamorphic (UHPM) rocks. The rheological strength of typical ultrahigh pressure rocks in continental subduction zone was derived from the results of the laboratory experiments. The results indicate the following three points. (1) The rheological strength of gabbro, similar to that of eclogite, is smaller than that of clinopyroxenite on the same condition. (2) The calculated strength of rocks (gabbro, eclogite and clinopyroxenite) related to UHPM decreases by nearly one order of magnitude with the temperature rising by 100 ℃ in the range between 600 and 900 ℃. The calculated strength is far greater than the faulting strength of rocks at 600 ℃, and is in several hundred to more than one thousand mega pascals at 700-800 ℃, which suggests that those rocks are located in the brittle deformation region at 600 ℃, but are in the semi brittle to plastic deformation region at 700-800 ℃. Obviously, the 700 ℃ is a brittle plastic transition boundary. (3) The calculated rheological strength in the localized deformation zone on a higher strain rate condition (1.6×10 -12 s -l ) is 2-5 times more than that in the distributed deformation zone on a lower strain rate condition (1.6×10 -14 s -1 ). The average rheological stress (1 600 MPa) at the strain rate of 10 -12 s -1 stands for the ultimate differential stress of UHPM rocks in the semi brittle flow field, and the average rheological stress (550-950 MPa) at the strain rate of l0 -14 - 10 -13 s -l stands for the ultimate differential stress of UHPM rocks in the plastic flow field, suggesting that the depth for the formation of UHPM rocks is more than 20-60 km below the depth estimated under static pressure condition due to the effect of the differential stress.
基金supported by the National Natural Science Foundation of China (Nos. 40872146, 40672147, 40773020)China Geological Survey (No. 1212010818015)
文摘The high-pressure metamorphic belt （HPMB） of eclogite-blueschist in Central Qiangtang （羌塘） lies in the Longmu Co （龙木错）-Shuanghu （双湖） suture zone. To the west, the HPMB extends 500 km from Hongjishan （红脊山） to Caiduochaka （才多茶卡）, east of Shuanghu; to the east it extends to Baqing （巴青） and Jitang （吉塘） in Qamdo （昌都）, and then bends southward to Yunnan （云南） Province. Including the Lancangjiang （澜沧江） blueschist belt, the entire HPMB is about 2 000 km long. In Central Qiangtang, the belt is mainly composed of blueschist and eclogite, whereas in West Yunnan it contains only blueschist. The Baqing-Jitang segment is dominated by garnet phengite schist. 40Ar-39Ar dating of glaucophane and phengite from the blueschists yielded plateau ages ranging from 223 to 215 Ma, whereas SHRIMP U-Pb dating of zircon from the eclogites gives metamorphic ages of 243-217 Ma. The calculated metamorphic conditions for the blueschists are 410-460 ℃ and 0.67-0.75 GPa, and for the eclogites, 〈500℃ and 1.56-2.35 GPa. The metamorphic ages suggest that the Longmu Co-Shuanghu suture closed in the Late Triassic. The region south of the Longmu Co-Shuanghu- Lancang suture consists of the pan-African basement overlain by Condwana sedimentary and metasedimentary rocks, whereas the region north of the suture is dominated by the Jinning （晋宁） basement and Yangtze sedimentary and metasedimentary rocks. The Qiangtang HPMB marks the closure of the paleo-Tethys Ocean.
文摘The Dabie Mountains are believed to be a collisional orogenic belt between the Yangtze amd Sino-Koreancontinental plates. It is composed of the foreland fold-thrust zone, the subducting cover and basement of theYangtze continental plate, the coesite- and diamond-bearing ultra-high pressure metamorphic zone and themeta-ophiolitic melange zone in the subducting basement, the fore-arc flysch nappe and the back thrust zoneoccurring respectively on the southern and northern margins of the Sino-Korean continental plate and the in-herited basin with molassic deposits on the northern margin. When the palaeo-Dabie oceanic plate subductednorthward in the Early Palaeozoic, volcanic arc and back arc basin probably formed on the southern margin ofthe Sino-Korean continental plate. The Sm / Nd isotopic dating of the strata and eclogite which were drawn in-to the foreland fold-thrust zone indicates that the intense collision of the two continental plates took place inthe Early Mesozoic.
