Eclogite was firstly discovered at the Da Qaidam region (Yang,et al., 1998), and then in the Xitieshan and Dulan regions in 1999, constituting an over 350km long high\|pressure metamorphic belt in the northeastern Qin...Eclogite was firstly discovered at the Da Qaidam region (Yang,et al., 1998), and then in the Xitieshan and Dulan regions in 1999, constituting an over 350km long high\|pressure metamorphic belt in the northeastern Qinghai—Tibet plateau. Eclogites occur as pods in the garnet\|muscovite gneiss of the Dakendaban Group (or called Shaliuhe Group in Dulan) of Upper Proterozoic age. In general, the pods of eclogite vary in size; most of them are less than 20m×10m, some large ones up to about 100m×50m. The eclogite\|hosted gneiss is pale\|gray in color, consisting mainly plagioclase and quartz, and minor muscovite (5%～10% in vol.) and garnet (1%～2%). Some of the country rocks of eclogite are mica\|quartz\|(feldspar) schist, quartzite, and ultramafic rocks, the latter also occur in blocks.Over 50 pods were found in a belt of 10km×3km in the Da Qaidam region (No.1 location). Only a few pods of eclogite were found in the Xitieshan region in 1999 field expedition (No.2 location). Eclogite in Dulan occurs in the Proterozoic strata of Shaliuhe Group (same as the Dakendaban Group but with a different name). The eclogites in the Dulan region (No.3 location) expose about 10km wide in SN and an unknown length in EW, and can be subdivided into two belts, the North Eclogite Belt of Dulan (NEBD) and the South Eclogite Belt of Dulan (SEBD).展开更多
Asthemaintectoniccomponentofthe Himalayan–Tibetan orogen,the Lhasa terrane has received much attention as it records the entire history of the orogeny.The occurrence of high pressure eclogite in the Sumdo complex in ...Asthemaintectoniccomponentofthe Himalayan–Tibetan orogen,the Lhasa terrane has received much attention as it records the entire history of the orogeny.The occurrence of high pressure eclogite in the Sumdo complex in central Lhasa terrane has a significant importance on the understanding of the Paleo-Tethys subduction and plate itineration processes in this area.The petrological,geochemical and geochronological data of eclogite and associated blueschist and garnet-bearing mica schist from Sumdo,Jilang and Bailang area have been briefly reviewed to explore the origin and metamorphic evolution of this suture.Eclogites from the Sumdo complex have experienced low temperature,high pressure to ultrahigh pressure metamorphism,revealing a fastsubduction and exhumation process in a typical oceanic subduction zone.The large P-T range between different eclogites in the literature may be affected by the big error of unappropriated using geothermobarometry and may also because of slices of subducted blocks derived from different depths juxtapose together during exhumation.By summarizing the U-Pb,Lu-Hf and Sm-Nd ages of eclogites,the eclogite facies metamorphism is likely to occur in early Triassic during 245-225 Ma,but not the previously accepted late Permian at ca.260 Ma by the reinterpretation of the former geochronological data from literature.The opening of Paleo-Tethys Ocean between the Lhasa terrane initiate prior to ca.280 Ma and ultimate closure to integrate the Lhasa terrane was no earlier than225 Ma and may triggered by the initial subduction of Bangong-Nujiang Tethys Ocean in the north.展开更多
The Sulu ultra-high pressure(UHP)metamorphic belt in Eastern China is well known as the eastern extension of the Qingling-Dabie orogenic belt formed by subduction and collision between the Sino-Korean and Yangtze crat...The Sulu ultra-high pressure(UHP)metamorphic belt in Eastern China is well known as the eastern extension of the Qingling-Dabie orogenic belt formed by subduction and collision between the Sino-Korean and Yangtze cratons.The main hole of the Chinese Continental Scientific Drilling(CCSD)project is located at the southern segment of the Sulu UHP metamorphic belt(34°25′N/118°40′E),about 17 km southwest of Donghai County.Integrated geophysical investigations using gravity,magnetic,deep展开更多
Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh-pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP meta...Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh-pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP metamorphic belt in China, the Sulu eclogite belt is the product of A-subduction induced by strong compression of the Yellow Sea terrane to the Jiaodong-northereastern Jiangsu terrane during the interaction of the Eurasian plate and Palaeo-Pacific plate in the Indosinian. It stretches about 350 km and contains over 1000 eclogite bodies. Most eclogites in this belt belong to Groups B and C in the classification of Coleman et al., and commonly contain kyanite, while the Lanshantou eclogite belongs to Group A and contains coesite. The MgO, CaO and FeO contents in garnet and pyroxene show regular variation from the core to the rim, which reveals the PTt paths of progressive metamorphism during the Early Mesozoic (240-200 Ma) and retrogressive metamorphism during the Late Mesozoic and Cenozoic exhumation.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
Different scales of structural data reveal a complex deformation history of ultrahigh- pressure （UHP） rocks exposed in the Weihai-Rongcbeng area, NE Sulu （northern Jiangsu-eastern Shandong）, eastern China. Excludi...Different scales of structural data reveal a complex deformation history of ultrahigh- pressure （UHP） rocks exposed in the Weihai-Rongcbeng area, NE Sulu （northern Jiangsu-eastern Shandong）, eastern China. Excluding pre-UHP deformations, at least five major sequential deformational stages （D1-Ds） are recognized. The first deformation （DO produced a weak foliation and lineation in massive eclogites. The foliated eclogite with a dominant foliation containing a stretching and mineral lineation was developed during the I）2 deformation. Both the D1 and D2 deformations occurred under UHP metamorphic conditions, and are well preserved in the eclogite bodies. D3 structures which developed shortly after the formation of granulite/amphibolite facies symplectites are characterized by imbricated associations marked by a regional, steeply dipping foliation, compositional layering, eclogite boudinage, isoclinal folds and reverse ductile shear zones. The D3 deformation was accompanied by decompressional partial melting. A regional, gently dipping amphibolite facies foliation and stretching lineation, low-angle detachments, and dome- and arc-shaped structures formed during the D4 deformation stage dominate to some degree the map pattern of the Weihai-Rongcbeng UHP domain. The last stage of deformation （Ds） gave rise to the final exhumation of the UHP rocks. Ds is characterized by development of brittle-dominated high-angle faulting associated with emplacement of large volmnes of undeformed granite plutons and dykes dated at 134-100 Ma. The deformational and metamorphic sequence followed by the UHP rocks in the Weihai-Rongcheng area is similar to that studied in the entire Dabie-Sulu UHP and HP metamorphic belts from microscopic to mapping scale. Based on structural data, combined with available petrographic, metamorphic and geochronological data, a speculative tectonic evolutionary model for the Dabie-Sulu UHP and IIP belts is proposed, involving continental subduction/collision between the Sino-Korean and Yangtze cratons and subsequent polyphase exhumation histories of the UHP and IIP metamorphic rocks.展开更多
Although tectonic models were presented for exhumation of ultrahigh-pressure (UHP) metamorphic rocks during the continental collision, there is increasing evidence for the decoupling between crustal slices at various ...Although tectonic models were presented for exhumation of ultrahigh-pressure (UHP) metamorphic rocks during the continental collision, there is increasing evidence for the decoupling between crustal slices at various depths within deeply subducted continental crust. This lends support to the multi-slice successive exhumation model of the UHP metamorphic rocks in the Dabie-Sulu orogen. The available evidence is summarized as follows: (1) the low-grade metamorphic slices, which have geotectonic affinity to the South China Block and part of them records the Triassic metamorphism, occur in the northern margin of the Dabie-Sulu UHP metamorphic zone, suggesting decoupling of the upper crust from the underlying basement during the initial stages of continental subduction; (2) the Dabie and Sulu HP to UHP metamorphic zones comprise several HP to UHP slices, which have an increased trend of metamorphic grade from south to north but a decreased trend of peak metamorphic ages correspondingly; and (3) the Chinese Continental Science Drilling (CCSD) project at Donghai in the Sulu orogen reveals that the UHP metamorphic zone is composed of several stacked slices, which display distinctive high and low radiogenic Pb from upper to lower parts in the profile, suggesting that these UHP crustal slices were derived from the subducted upper and middle crusts, respectively. Detachment surfaces within the deeply subducted crust may occur either along an ancient fault as a channel of fluid flow, which resulted in weakening of mechanic strength of the rocks adjacent to the fault due to fluid-rock interaction, or along the low-viscosity zones which resulted from variations of geotherms and lithospheric compositions at different depths. The multi-slice successive exhumation model is different from the traditional exhumation model of the UHP metamorphic rocks in that the latter assumes the detachment of the entire subducted continental crust from the underlying mantle lithosphere and its subsequent exhumation as a whole. This also reveals the difference between the continental subduction and oceanic subduction. In addition, several important proposals concerning the multi-slice successive exhumation model are made for further studies.展开更多
Unusual polyphase inclusions of K-feldspar+quartz+titanite+solid salt and K-feldspar+albite+quartz+epidote with textures similar to the other K-feldspar+quartz inclusions were found in omphacite grains from the Sulu u...Unusual polyphase inclusions of K-feldspar+quartz+titanite+solid salt and K-feldspar+albite+quartz+epidote with textures similar to the other K-feldspar+quartz inclusions were found in omphacite grains from the Sulu ultrahigh pressure (UHP) eclogites. One of these inclusions contain square to round solid salt inclusions of KCl-NaCl composition. Such a mineral assemblage within K-feldspar-bearing inclusions hosted by UHP metamorphic phases suggests that (1) potassium granitic melts enriched in Cl components were presented during UHP metamorphism or at the early stage of rapid exhumation of deeply subducted continental slab; (2) they were resulted from reactions between the incoming granitic melts and quartz (or coesite); and (3) solid salt inclusions of NaCl-KCl were derived from dehydration and desiccation of Cl-bearing melts. Our new observations further demonstrate that during the tectonic evolution of UHP rocks, fertile components within deeply subducted continental materials could undergo partial melting, leading to the formation of Cl-bearing potassium granitic melts and substantial migration of fluid-conservative elements (e.g. Ti, Hf) within the UHP slab.展开更多
文摘Eclogite was firstly discovered at the Da Qaidam region (Yang,et al., 1998), and then in the Xitieshan and Dulan regions in 1999, constituting an over 350km long high\|pressure metamorphic belt in the northeastern Qinghai—Tibet plateau. Eclogites occur as pods in the garnet\|muscovite gneiss of the Dakendaban Group (or called Shaliuhe Group in Dulan) of Upper Proterozoic age. In general, the pods of eclogite vary in size; most of them are less than 20m×10m, some large ones up to about 100m×50m. The eclogite\|hosted gneiss is pale\|gray in color, consisting mainly plagioclase and quartz, and minor muscovite (5%～10% in vol.) and garnet (1%～2%). Some of the country rocks of eclogite are mica\|quartz\|(feldspar) schist, quartzite, and ultramafic rocks, the latter also occur in blocks.Over 50 pods were found in a belt of 10km×3km in the Da Qaidam region (No.1 location). Only a few pods of eclogite were found in the Xitieshan region in 1999 field expedition (No.2 location). Eclogite in Dulan occurs in the Proterozoic strata of Shaliuhe Group (same as the Dakendaban Group but with a different name). The eclogites in the Dulan region (No.3 location) expose about 10km wide in SN and an unknown length in EW, and can be subdivided into two belts, the North Eclogite Belt of Dulan (NEBD) and the South Eclogite Belt of Dulan (SEBD).
文摘Asthemaintectoniccomponentofthe Himalayan–Tibetan orogen,the Lhasa terrane has received much attention as it records the entire history of the orogeny.The occurrence of high pressure eclogite in the Sumdo complex in central Lhasa terrane has a significant importance on the understanding of the Paleo-Tethys subduction and plate itineration processes in this area.The petrological,geochemical and geochronological data of eclogite and associated blueschist and garnet-bearing mica schist from Sumdo,Jilang and Bailang area have been briefly reviewed to explore the origin and metamorphic evolution of this suture.Eclogites from the Sumdo complex have experienced low temperature,high pressure to ultrahigh pressure metamorphism,revealing a fastsubduction and exhumation process in a typical oceanic subduction zone.The large P-T range between different eclogites in the literature may be affected by the big error of unappropriated using geothermobarometry and may also because of slices of subducted blocks derived from different depths juxtapose together during exhumation.By summarizing the U-Pb,Lu-Hf and Sm-Nd ages of eclogites,the eclogite facies metamorphism is likely to occur in early Triassic during 245-225 Ma,but not the previously accepted late Permian at ca.260 Ma by the reinterpretation of the former geochronological data from literature.The opening of Paleo-Tethys Ocean between the Lhasa terrane initiate prior to ca.280 Ma and ultimate closure to integrate the Lhasa terrane was no earlier than225 Ma and may triggered by the initial subduction of Bangong-Nujiang Tethys Ocean in the north.
文摘The Sulu ultra-high pressure(UHP)metamorphic belt in Eastern China is well known as the eastern extension of the Qingling-Dabie orogenic belt formed by subduction and collision between the Sino-Korean and Yangtze cratons.The main hole of the Chinese Continental Scientific Drilling(CCSD)project is located at the southern segment of the Sulu UHP metamorphic belt(34°25′N/118°40′E),about 17 km southwest of Donghai County.Integrated geophysical investigations using gravity,magnetic,deep
文摘Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh-pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP metamorphic belt in China, the Sulu eclogite belt is the product of A-subduction induced by strong compression of the Yellow Sea terrane to the Jiaodong-northereastern Jiangsu terrane during the interaction of the Eurasian plate and Palaeo-Pacific plate in the Indosinian. It stretches about 350 km and contains over 1000 eclogite bodies. Most eclogites in this belt belong to Groups B and C in the classification of Coleman et al., and commonly contain kyanite, while the Lanshantou eclogite belongs to Group A and contains coesite. The MgO, CaO and FeO contents in garnet and pyroxene show regular variation from the core to the rim, which reveals the PTt paths of progressive metamorphism during the Early Mesozoic (240-200 Ma) and retrogressive metamorphism during the Late Mesozoic and Cenozoic exhumation.
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.
基金This study is funded by the Major State Basic Research Development Program （G1999075506）the National Natural Science Foundation of China （40372094 and 49972067）.
文摘Different scales of structural data reveal a complex deformation history of ultrahigh- pressure （UHP） rocks exposed in the Weihai-Rongcbeng area, NE Sulu （northern Jiangsu-eastern Shandong）, eastern China. Excluding pre-UHP deformations, at least five major sequential deformational stages （D1-Ds） are recognized. The first deformation （DO produced a weak foliation and lineation in massive eclogites. The foliated eclogite with a dominant foliation containing a stretching and mineral lineation was developed during the I）2 deformation. Both the D1 and D2 deformations occurred under UHP metamorphic conditions, and are well preserved in the eclogite bodies. D3 structures which developed shortly after the formation of granulite/amphibolite facies symplectites are characterized by imbricated associations marked by a regional, steeply dipping foliation, compositional layering, eclogite boudinage, isoclinal folds and reverse ductile shear zones. The D3 deformation was accompanied by decompressional partial melting. A regional, gently dipping amphibolite facies foliation and stretching lineation, low-angle detachments, and dome- and arc-shaped structures formed during the D4 deformation stage dominate to some degree the map pattern of the Weihai-Rongcbeng UHP domain. The last stage of deformation （Ds） gave rise to the final exhumation of the UHP rocks. Ds is characterized by development of brittle-dominated high-angle faulting associated with emplacement of large volmnes of undeformed granite plutons and dykes dated at 134-100 Ma. The deformational and metamorphic sequence followed by the UHP rocks in the Weihai-Rongcheng area is similar to that studied in the entire Dabie-Sulu UHP and HP metamorphic belts from microscopic to mapping scale. Based on structural data, combined with available petrographic, metamorphic and geochronological data, a speculative tectonic evolutionary model for the Dabie-Sulu UHP and IIP belts is proposed, involving continental subduction/collision between the Sino-Korean and Yangtze cratons and subsequent polyphase exhumation histories of the UHP and IIP metamorphic rocks.
基金the Chinese Academy of Sciences (Grant No. kzcx2-yw-131)the National Natural Science Foundation of China (Grant Nos. 40572035, 40634023 and40773013).
文摘Although tectonic models were presented for exhumation of ultrahigh-pressure (UHP) metamorphic rocks during the continental collision, there is increasing evidence for the decoupling between crustal slices at various depths within deeply subducted continental crust. This lends support to the multi-slice successive exhumation model of the UHP metamorphic rocks in the Dabie-Sulu orogen. The available evidence is summarized as follows: (1) the low-grade metamorphic slices, which have geotectonic affinity to the South China Block and part of them records the Triassic metamorphism, occur in the northern margin of the Dabie-Sulu UHP metamorphic zone, suggesting decoupling of the upper crust from the underlying basement during the initial stages of continental subduction; (2) the Dabie and Sulu HP to UHP metamorphic zones comprise several HP to UHP slices, which have an increased trend of metamorphic grade from south to north but a decreased trend of peak metamorphic ages correspondingly; and (3) the Chinese Continental Science Drilling (CCSD) project at Donghai in the Sulu orogen reveals that the UHP metamorphic zone is composed of several stacked slices, which display distinctive high and low radiogenic Pb from upper to lower parts in the profile, suggesting that these UHP crustal slices were derived from the subducted upper and middle crusts, respectively. Detachment surfaces within the deeply subducted crust may occur either along an ancient fault as a channel of fluid flow, which resulted in weakening of mechanic strength of the rocks adjacent to the fault due to fluid-rock interaction, or along the low-viscosity zones which resulted from variations of geotherms and lithospheric compositions at different depths. The multi-slice successive exhumation model is different from the traditional exhumation model of the UHP metamorphic rocks in that the latter assumes the detachment of the entire subducted continental crust from the underlying mantle lithosphere and its subsequent exhumation as a whole. This also reveals the difference between the continental subduction and oceanic subduction. In addition, several important proposals concerning the multi-slice successive exhumation model are made for further studies.
基金supported by the SinoProbe Project (SinoProbe-2-6)the National Natural Science Foundation of China (41073024 and 40872048)
文摘Unusual polyphase inclusions of K-feldspar+quartz+titanite+solid salt and K-feldspar+albite+quartz+epidote with textures similar to the other K-feldspar+quartz inclusions were found in omphacite grains from the Sulu ultrahigh pressure (UHP) eclogites. One of these inclusions contain square to round solid salt inclusions of KCl-NaCl composition. Such a mineral assemblage within K-feldspar-bearing inclusions hosted by UHP metamorphic phases suggests that (1) potassium granitic melts enriched in Cl components were presented during UHP metamorphism or at the early stage of rapid exhumation of deeply subducted continental slab; (2) they were resulted from reactions between the incoming granitic melts and quartz (or coesite); and (3) solid salt inclusions of NaCl-KCl were derived from dehydration and desiccation of Cl-bearing melts. Our new observations further demonstrate that during the tectonic evolution of UHP rocks, fertile components within deeply subducted continental materials could undergo partial melting, leading to the formation of Cl-bearing potassium granitic melts and substantial migration of fluid-conservative elements (e.g. Ti, Hf) within the UHP slab.
