Continental subduction and collision normally follows oceanic subduction,with the remarkable event of formation and exhumation of high-to ultra-high-pressure(HP-UHP)metamorphic rocks.Based on the summary of numerical ...Continental subduction and collision normally follows oceanic subduction,with the remarkable event of formation and exhumation of high-to ultra-high-pressure(HP-UHP)metamorphic rocks.Based on the summary of numerical geodynamic models,six modes of continental convergence have been identified:pure shear thickening,folding and buckling,one-sided steep subduction,flat subduction,two-sided subduction,and subducting slab break-off.In addition,the exhumation of HP-UHP rocks can be formulated into eight modes:thrust fault exhumation,buckling exhumation,material circulation,overpressure model,exhumation of a coherent crustal slice,episodic ductile extrusion,slab break-off induced eduction,and exhumation through fractured overriding lithosphere.During the transition from subduction to exhumation,the weakening and detachment of subducted continental crust are prerequisites.However,the dominant weakening mechanisms and their roles in the subduction channel are poorly constrained.To a first degree approximation,the mechanism of continental subduction and exhumation can be treated as a subduction channel flow model,which incorporates the competing effects of downward Couette(subduction)flow and upward Poiseuille(exhumation)flow in the subduction channel.However,the(de-)hydration effect plays significant roles in the deformation of subduction channel and overriding lithosphere,which thereby result in very different modes from the simple subduction channel flow.Three-dimensionality is another important issue with highlighting the along-strike differential modes of continental subduction,collision and exhumation in the same continental convergence belt.展开更多
During the past ten years, various types of HP-UHP metamorphic rocks have been discovered in the South Altyn Tagh, the North Qaidam and the North Qinling （秦岭） in the West and Middle Central China orogen. The UHP r...During the past ten years, various types of HP-UHP metamorphic rocks have been discovered in the South Altyn Tagh, the North Qaidam and the North Qinling （秦岭） in the West and Middle Central China orogen. The UHP rocks, as lentoid bodies in regional gneisses, include eclogite （garnet-bearing pyroxenite）, garnet peridotite and various pelitic or felsic gneisses. There are many records of minerals and microstructures of exsolution indicate the UHP metamorphism, such as coesite （or its pseudomorph）, diamond, exsolution of clinopyroxene/amphibole/＋rutile or rutile＋quartz＋apatite in garnet, exsolution of quartz in omphacite and exsolution of kyanite＋spinel in precursor stishovite.The discovery of microstructure evidence for the presence of precursor stishovite in typical Alrich gneiss from the South Altyn Tagh reveals continental subduction and exhumation to and from a depth of more than 350 km. It is the petrological record of the deepest subduction and exhumation of continental rock in the world. The in situ zircon U-Pb dating using LA-ICP- MS or SHRIMP methods shows that the metamorphic ages of the HP-UHP rocks in the South Altyn Tagh, the North Qaidam and the North Qinling are 475-509, 420--457, and 485-514 Ma, respectively. The metamorphic ages of HP-UHP rocks in the North Qaidam are 20-80 Ma younger than those in the South Altyn Tagh and the North Qinling, and the metamorphic ages do not systematically increase or decrease from the South Altyn Tagh through the North Qaidam to the North Qinling. The absence of time transgressive variety of the metamorphism in the three regions does not support the hypothesis that the HP-UHP rocks in these re. gions form the same HP-UHP metamorphic zone. And the HP-UHP rocks in these regions can not be simply correlated to the collision between the North China plate and the South China plate. At present, the study of the HP-UHP rocks in the West and Middle Central China orogen faces several key issues or challenges, such as： （1） the continental subduct展开更多
Zircon U-Pb thermal ionization mass-spectrometer (TIMS) and secondary ion mass-spectrometer (SIMS) dating, mica and amphibole 40Ar-39Ar dating and mineral Sm-Nd isotopic compositions of Huangzhen low temperature eclog...Zircon U-Pb thermal ionization mass-spectrometer (TIMS) and secondary ion mass-spectrometer (SIMS) dating, mica and amphibole 40Ar-39Ar dating and mineral Sm-Nd isotopic compositions of Huangzhen low temperature eclogite and country granitic gneiss are carried out. The zircon U-Pb weighted average SIMS age is (231.6?9.7) Ma for one eclogite. The mica 40Ar-39Ar isochron age is (232.6?2.1) Ma and the lowest plateau age is (221.7?2.4) Ma from same sample. U-Pb TIMS concordant ages from other eclogite zircons are from (221.3?1.4) Ma to (222.5?2.3) Ma. U-Pb SIMS low intercept age from country granitic gneiss is (221?35) Ma. The retrograde amphibole 40Ar-39Ar isochron age is (205.9?1.0) Ma. Except for mica, which may contain excess 40Ar, all the ages represent peak and retrograde metamorphism of low temperature eclogites. It is indicated that the Huangzhen low temperature eclogites differ from Xiongdian low temperature eclogites of north of the Northern Dabie Terrain in metamorphic ages. Huangzhen low temperature eclogites share one coherent HP-UHP terrain with high temperature eclogites from Southern Dabie Terrain and they may have differences in subduction depth and cooling rates during exhumation.展开更多
High/ultrahigh-pressure(HP/UHP)metamorphic complexes,such as eclogite and blueschist,are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones.Glaucophane eclogites have been rec...High/ultrahigh-pressure(HP/UHP)metamorphic complexes,such as eclogite and blueschist,are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones.Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary mélange in the Changning-Menglian suture zone,at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau.The authors report the result of petrological,mineralogical and metamorphism investigations of these rocks,and discuss their tectonic implications.The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists.The major mineral assemblage of the eclogites includes garnet,omphacite,glaucophane,phengite,clinozoisite and rutile.Eclogitic garnet contains numerous inclusions,such as omphacite,glaucophane,rutile,and quartz with radial cracks around.Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation.Four stages of metamorphic evolution can be determined:The prograde blueschist facies(M_(1)),the peak eclogite facies(M_(2)),the decompression blueschist facies(M_(3))and retrograde greenschist facies(M_(4)).Using the Grt-Omp-Phn geothermobarometer,a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658℃ was determined,which is typical of low-temperature ultrahigh-pressure metamorphism.The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic mélange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean.The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone.展开更多
The Chinese Tianshan belt of the southern Altaids has undergone a complicated geological evolution. Different theories have been proposed to explain its evolution and these are still hotly debated. The major subductio...The Chinese Tianshan belt of the southern Altaids has undergone a complicated geological evolution. Different theories have been proposed to explain its evolution and these are still hotly debated. The major subduction polarity and the way of accretion are the main problems. Southward, northward subduction and multiple subduction models have been proposed. This study focuses on the structural geology of two of the main faults in the region, the South Tianshan Fault and the Nikolaev Line. The dip direction in the Muzhaerte valley is southward and lineations all point towards the NW. Two shear sense motions have been observed within both of these fault zones, a sinistral one, and a dextral one, the latter with an age of 236-251 Ma. Structural analyses on the fault zones show that subduction has been northward rather than southward. The two shear sense directions indicate that the Yili block was first dragged along towards the east due to the cloclkwise rotation of the Tarim block. After the Tarim block stopped rotating, the Yili block still kept going eastward, inducing the dextral shear senses within the fault zones.展开更多
The kyanite-bearing garnet pelitic gneiss from the Jianggalesavi area in southern Altyn Tagh high pressure/ultra-high pressure belt was proved to have been experienced UHP metamorphism (>12 GPa) by the discovery of...The kyanite-bearing garnet pelitic gneiss from the Jianggalesavi area in southern Altyn Tagh high pressure/ultra-high pressure belt was proved to have been experienced UHP metamorphism (>12 GPa) by the discovery of kyanite and spinel exsolution microstructure in quartz (precursor stishovite). In this study, three stages of retrograded metamorphism (M2-M4) after the UHP metamorphism (Ml) were identified for the UHP pelitic gneiss. The HP granulite-facies stage (M2) was characterized by the mineral assemblage of garnet+kyanite+K?feldspar+nitile+qiiartz±ilinenite, recording the P-T condition of >1.12 GPa and ?850-930℃. The granulite-facies stage (M3) was represented by the mineral assemblage of garnet rim+K-feldspar+sillimanite (SillJ+biotite (Bti)+plagioclase (Pli)+ilmenite+quartz, and confined under P-T conditions of 0.5-0.8 GPa and^770-795℃. The late cooling stage M4 was accompanied by the appearance of fine-grained Pl2, Sill2 and Bt2 in the matrix, and the P-T conditions were 0.4-0.6 GPa and <675℃. A clockvvised P-T path was obtained for the pelitic gneiss in the P-T pseudosection, which showed a deep subduction/collision processes with subsequent exhumation and cooling. Com? bined with the corresponding multistage metamorphic assemblages, the age dating results implied that the zircons from the gneiss have integrated the recording peak metamorphic (Ml,484±3 Ma) and retrograded metamorphic ages (M2 to M3, 450±2 Ma). There was about 32 Ma interval during the first exhumation from the upper mantle depth (>350 km) to the lower crust depth (-40-20 km), resulting in an average exhumation rate of 9.11-9.70 mm/yr. In the southern Altyn Tagh region, the HP and UHP rocks from different areas had identical peak metamorphic ages. Therefore, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which can be interpreted by the model of subduction channel. The continental crustal were subducted to different depths along the direction of the subductio展开更多
Helium isotopic compositions are considered to be ideal tracers to identify whether mantle materials have been added to crustal rocks or fluids.In this paper,we present the helium isotopic compositions of the Songduo ...Helium isotopic compositions are considered to be ideal tracers to identify whether mantle materials have been added to crustal rocks or fluids.In this paper,we present the helium isotopic compositions of the Songduo eclogites in the Lhasa terrane,Tibet.We found that garnet and omphacite in the eclogites have different helium retention characteristics.The 4He content of most omphacite grains are about 10–20 times of that of garnet,suggesting that omphacite has a higher ability to capture 4He than garnet.Similarly,there is about 10–20 times difference in 3He content between omphacite and garnet in the same eclogite samples.The 3He/4He ratios of garnet and omphacite in these rocks range from 0.27 to 0.60 Ra(relative to the modern air 3He/4He ratio,1.4×10^-6).These ratios are within the range of both mantle-and crust-derived helium,suggesting mixed sources.The Songduo eclogites have much higher3He/4He ratios than those observed in the Dabie eclogites of eastern China.Such high ratios are typically thought to be associated with deep mantle sources.We cautiously conclude that deep mantle materials might have been involved during the formation of the Songduo eclogites.展开更多
The Hefei Basin of eastern China developed in response to uplift of the Dabie Orogen,and zircon dating can be used to assess the exhumation history of the orogen.Zircons were collected from samples of the Lower Jurass...The Hefei Basin of eastern China developed in response to uplift of the Dabie Orogen,and zircon dating can be used to assess the exhumation history of the orogen.Zircons were collected from samples of the Lower Jurassic Fanghushan Formation and Middle Jurassic Sanjianpu Formation in the southern Hefei Basin,and mica-quartz schist and biotite granite gneiss from the Susong Complex of the Dabie Orogen.The zircon U-Pb dating was undertaken using laser ablation-inductively coupled plasma-mass spectrometry.The detrital zircons from conglomerates of the Fanghushan Formation and from clasts within the conglomerates have age-frequency distributions with the main clusters between 2.0 and 1.8 Ga,similar to age data of the Susong Complex.On the other hand,the zircons of the Fanghushan Formation do not show the age cluster at 1000–900 Ma that characterizes zircons in the underlying metasediments of the lower Paleozoic Foziling Group.A cluster of Triassic zircon ages also appears in the arkosic sandstones of the Fanghushan Formation.These data indicate that the provenance of the Fanghushan Formation was a mixture of high-pressure(HP)and ultrahigh-pressure(UHP)Triassic metamorphic rocks,Paleozoic magmatic rocks,and the Susong Complex,but not the lower Paleozoic Foziling Group even though it directly underlies the sediments of the Hefei Basin.Two samples from the Sanjianpu Formation show zircon age clusters at 797 and 791 Ma(middle Neoproterozoic)and 226 Ma(Triassic),and again,these are markedly different from the age clusters that characterize the Foziling Group.It seems,therefore,that despite the Foziling Group being at the surface in the underwater depositional area of the Hefei Basin,it was not exposed in the source area of the Hefei basinal sediments during the Jurassic,and there are two possible reasons for this.First,the exhumation of the Dabie Orogen was directed partly towards the north,in the process of which some of the Foziling Group was covered.Second,the Susong Complex rocks became involved in the development of展开更多
In terms of petrology,thermomechanical simulation is an important frontier to study the geodynamic process of the exhumation and uplift of high pressure(HP)to ultrahigh pressure(UHP)metamorphic rocks in subduction zon...In terms of petrology,thermomechanical simulation is an important frontier to study the geodynamic process of the exhumation and uplift of high pressure(HP)to ultrahigh pressure(UHP)metamorphic rocks in subduction zones and collision orogenic belts.Based on the recent petrological studies and numerical modellings for the exhumation of HP to UHP metamorphic terranes,the exhumation mechanisms of HP to UHP metamorphic terranes can be roughly summarized into ten types:channel flow,diapiric exhumation,a coexistence mechanism of channel flow and diapiric exhumation,slab breakoff,multi-stage exhumation,divergent plate motion(including slab rollback and the upper-plate divergent motion away from the subducted plate),overthrust exhumation,overpressure mechanism,wedge-like extrusion and microplate rotation.The exhumation of high-density UHP oceanic eclogites is a relative controversial issue.Some of our recent researches on quantitatively determining the exhumation mechanism of UHP oceanic eclogites using thermomechanical and phase equilibrium modelling was introduced in details in this paper.We obtained the 3-D density evolutions of three-type subducted oceanic materials(MORB,serpentine and oceanic sediments)in P-T space by the methods of phase equilibrium and density calculation.According to the density difference between the metabasic and their surrounding rocks,the exhumed eclogites could be divided into two types.The first category,the self-exhumation eclogites(ρ_(MORB)<ρmantle),which can exhume driven by their own buoyancy,an example is the coesite-bearing oceanic eclogites from Southwest Tianshan.Another is the carried-exhumation eclogites(ρ_(MORB)>ρmantle),which can only be carried back to the surface with the assistance of low-density metasediments and serpentinite due to their negative buoyancy;the coesite-bearing UHP eclogites of Zermatt-Saas in the Western Alps is a typical example.Besides,we further explored the ultimate self-exhumation depth,exhumation mechanisms,the effect of the transition from high pre展开更多
The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record closure of the Iapetus Ocean and subsequent continental collision of Baltica and Laurentia.The Seve Nappe Complex(SNC)of the ...The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record closure of the Iapetus Ocean and subsequent continental collision of Baltica and Laurentia.The Seve Nappe Complex(SNC)of the Scandinavian Caledonides includes relics of the outermost Baltoscandian passive margin that were subducted to mantle depths.The earliest of the deep subduction events has been dated to ca.500-480 Ma.Evidence of this event has been reported from the northern exposures of the SNC.Farther south in the central and southern segments of the SNC,(ultra)high-pressure rocks have yielded younger ages in the range of ca.470-440 Ma.This study provides the first record of the early Caledonian event in the southern SNC.The evidence has been obtained by depth profiling of zircon grains that were extracted from the Tv?r?klumparna microdiamond-bearing gneiss.These zircon grains preserve eclogite facies overgrowths that crystallized at 482.6±3.8 Ma.A second,chemically-distinct zircon overgrowth records granulite facies metamorphism at 439.3±3.6 Ma,which corroborates previous geochronological evidence for granulite facies metamorphism at this time.Based on these results,we propose that the entire outer margin of Baltica was subducted in the late Cambrian to early Ordovician,but the record of this event may be almost entirely eradicated in the vast majority of lithologies by pervasive late Ordovician to early Silurian metamorphism.展开更多
基金supported by the Start-up Research Fund from Institute of Geology of CAGS(Grant No.J1219)National Natural Science Foundation of China(Grant No.40921001)China Geological Survey Projects(Grant Nos.1212011121275,1212011120161)and Sinoprobe Project
文摘Continental subduction and collision normally follows oceanic subduction,with the remarkable event of formation and exhumation of high-to ultra-high-pressure(HP-UHP)metamorphic rocks.Based on the summary of numerical geodynamic models,six modes of continental convergence have been identified:pure shear thickening,folding and buckling,one-sided steep subduction,flat subduction,two-sided subduction,and subducting slab break-off.In addition,the exhumation of HP-UHP rocks can be formulated into eight modes:thrust fault exhumation,buckling exhumation,material circulation,overpressure model,exhumation of a coherent crustal slice,episodic ductile extrusion,slab break-off induced eduction,and exhumation through fractured overriding lithosphere.During the transition from subduction to exhumation,the weakening and detachment of subducted continental crust are prerequisites.However,the dominant weakening mechanisms and their roles in the subduction channel are poorly constrained.To a first degree approximation,the mechanism of continental subduction and exhumation can be treated as a subduction channel flow model,which incorporates the competing effects of downward Couette(subduction)flow and upward Poiseuille(exhumation)flow in the subduction channel.However,the(de-)hydration effect plays significant roles in the deformation of subduction channel and overriding lithosphere,which thereby result in very different modes from the simple subduction channel flow.Three-dimensionality is another important issue with highlighting the along-strike differential modes of continental subduction,collision and exhumation in the same continental convergence belt.
基金supported by the National Basic Research Pro-gram of China (No. 2009CB825003)the National Natural Science Foundation of China (Nos. 40972128, 40572111)the Ministry of Science and Technology of China for the State Key Laboratory of Continental Dynamics of Northwest University
文摘During the past ten years, various types of HP-UHP metamorphic rocks have been discovered in the South Altyn Tagh, the North Qaidam and the North Qinling （秦岭） in the West and Middle Central China orogen. The UHP rocks, as lentoid bodies in regional gneisses, include eclogite （garnet-bearing pyroxenite）, garnet peridotite and various pelitic or felsic gneisses. There are many records of minerals and microstructures of exsolution indicate the UHP metamorphism, such as coesite （or its pseudomorph）, diamond, exsolution of clinopyroxene/amphibole/＋rutile or rutile＋quartz＋apatite in garnet, exsolution of quartz in omphacite and exsolution of kyanite＋spinel in precursor stishovite.The discovery of microstructure evidence for the presence of precursor stishovite in typical Alrich gneiss from the South Altyn Tagh reveals continental subduction and exhumation to and from a depth of more than 350 km. It is the petrological record of the deepest subduction and exhumation of continental rock in the world. The in situ zircon U-Pb dating using LA-ICP- MS or SHRIMP methods shows that the metamorphic ages of the HP-UHP rocks in the South Altyn Tagh, the North Qaidam and the North Qinling are 475-509, 420--457, and 485-514 Ma, respectively. The metamorphic ages of HP-UHP rocks in the North Qaidam are 20-80 Ma younger than those in the South Altyn Tagh and the North Qinling, and the metamorphic ages do not systematically increase or decrease from the South Altyn Tagh through the North Qaidam to the North Qinling. The absence of time transgressive variety of the metamorphism in the three regions does not support the hypothesis that the HP-UHP rocks in these re. gions form the same HP-UHP metamorphic zone. And the HP-UHP rocks in these regions can not be simply correlated to the collision between the North China plate and the South China plate. At present, the study of the HP-UHP rocks in the West and Middle Central China orogen faces several key issues or challenges, such as： （1） the continental subduct
基金supported by the National Natural Science Foundation of China(Grant Nos.40273028 and 40033010)the State Key Basic Research Project(Grant No.G1999075503).
文摘Zircon U-Pb thermal ionization mass-spectrometer (TIMS) and secondary ion mass-spectrometer (SIMS) dating, mica and amphibole 40Ar-39Ar dating and mineral Sm-Nd isotopic compositions of Huangzhen low temperature eclogite and country granitic gneiss are carried out. The zircon U-Pb weighted average SIMS age is (231.6?9.7) Ma for one eclogite. The mica 40Ar-39Ar isochron age is (232.6?2.1) Ma and the lowest plateau age is (221.7?2.4) Ma from same sample. U-Pb TIMS concordant ages from other eclogite zircons are from (221.3?1.4) Ma to (222.5?2.3) Ma. U-Pb SIMS low intercept age from country granitic gneiss is (221?35) Ma. The retrograde amphibole 40Ar-39Ar isochron age is (205.9?1.0) Ma. Except for mica, which may contain excess 40Ar, all the ages represent peak and retrograde metamorphism of low temperature eclogites. It is indicated that the Huangzhen low temperature eclogites differ from Xiongdian low temperature eclogites of north of the Northern Dabie Terrain in metamorphic ages. Huangzhen low temperature eclogites share one coherent HP-UHP terrain with high temperature eclogites from Southern Dabie Terrain and they may have differences in subduction depth and cooling rates during exhumation.
基金supported by the National Natural Science Foundation of China(92055314 and 41602091)the geological survey project of China Geological Survey(DD20160016).
文摘High/ultrahigh-pressure(HP/UHP)metamorphic complexes,such as eclogite and blueschist,are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones.Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary mélange in the Changning-Menglian suture zone,at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau.The authors report the result of petrological,mineralogical and metamorphism investigations of these rocks,and discuss their tectonic implications.The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists.The major mineral assemblage of the eclogites includes garnet,omphacite,glaucophane,phengite,clinozoisite and rutile.Eclogitic garnet contains numerous inclusions,such as omphacite,glaucophane,rutile,and quartz with radial cracks around.Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation.Four stages of metamorphic evolution can be determined:The prograde blueschist facies(M_(1)),the peak eclogite facies(M_(2)),the decompression blueschist facies(M_(3))and retrograde greenschist facies(M_(4)).Using the Grt-Omp-Phn geothermobarometer,a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658℃ was determined,which is typical of low-temperature ultrahigh-pressure metamorphism.The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic mélange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean.The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone.
基金supported by the Chinese Major State Basic Research Program (Grants Nos.2009CB825007,2007CB411307)National Natural Science Foundation of China(Grant Nos.40730314,40821002, 41230207,41390441,41190075)the Molengraaff Fund to MCS
文摘The Chinese Tianshan belt of the southern Altaids has undergone a complicated geological evolution. Different theories have been proposed to explain its evolution and these are still hotly debated. The major subduction polarity and the way of accretion are the main problems. Southward, northward subduction and multiple subduction models have been proposed. This study focuses on the structural geology of two of the main faults in the region, the South Tianshan Fault and the Nikolaev Line. The dip direction in the Muzhaerte valley is southward and lineations all point towards the NW. Two shear sense motions have been observed within both of these fault zones, a sinistral one, and a dextral one, the latter with an age of 236-251 Ma. Structural analyses on the fault zones show that subduction has been northward rather than southward. The two shear sense directions indicate that the Yili block was first dragged along towards the east due to the cloclkwise rotation of the Tarim block. After the Tarim block stopped rotating, the Yili block still kept going eastward, inducing the dextral shear senses within the fault zones.
基金supported by the National Natural Science Foundation of China (No. 41872053)the NSF of Shandong Province (No. ZR2019BD046)+2 种基金the Chinese Ministry of Science and Technology (No. 2015CB856103)the Opening Foundation of the State Key Laboratory of Continental Dynamics, Northwest University (No. 17LCD07)SDUST Research Fund (No. 2015TDJH101)
文摘The kyanite-bearing garnet pelitic gneiss from the Jianggalesavi area in southern Altyn Tagh high pressure/ultra-high pressure belt was proved to have been experienced UHP metamorphism (>12 GPa) by the discovery of kyanite and spinel exsolution microstructure in quartz (precursor stishovite). In this study, three stages of retrograded metamorphism (M2-M4) after the UHP metamorphism (Ml) were identified for the UHP pelitic gneiss. The HP granulite-facies stage (M2) was characterized by the mineral assemblage of garnet+kyanite+K?feldspar+nitile+qiiartz±ilinenite, recording the P-T condition of >1.12 GPa and ?850-930℃. The granulite-facies stage (M3) was represented by the mineral assemblage of garnet rim+K-feldspar+sillimanite (SillJ+biotite (Bti)+plagioclase (Pli)+ilmenite+quartz, and confined under P-T conditions of 0.5-0.8 GPa and^770-795℃. The late cooling stage M4 was accompanied by the appearance of fine-grained Pl2, Sill2 and Bt2 in the matrix, and the P-T conditions were 0.4-0.6 GPa and <675℃. A clockvvised P-T path was obtained for the pelitic gneiss in the P-T pseudosection, which showed a deep subduction/collision processes with subsequent exhumation and cooling. Com? bined with the corresponding multistage metamorphic assemblages, the age dating results implied that the zircons from the gneiss have integrated the recording peak metamorphic (Ml,484±3 Ma) and retrograded metamorphic ages (M2 to M3, 450±2 Ma). There was about 32 Ma interval during the first exhumation from the upper mantle depth (>350 km) to the lower crust depth (-40-20 km), resulting in an average exhumation rate of 9.11-9.70 mm/yr. In the southern Altyn Tagh region, the HP and UHP rocks from different areas had identical peak metamorphic ages. Therefore, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which can be interpreted by the model of subduction channel. The continental crustal were subducted to different depths along the direction of the subductio
基金supported jointly by the National Natural Science Foundation of China (Nos. 41373029, 41773029)the China Geological Survey (Nos.DD20190060, 12120114061501)
文摘Helium isotopic compositions are considered to be ideal tracers to identify whether mantle materials have been added to crustal rocks or fluids.In this paper,we present the helium isotopic compositions of the Songduo eclogites in the Lhasa terrane,Tibet.We found that garnet and omphacite in the eclogites have different helium retention characteristics.The 4He content of most omphacite grains are about 10–20 times of that of garnet,suggesting that omphacite has a higher ability to capture 4He than garnet.Similarly,there is about 10–20 times difference in 3He content between omphacite and garnet in the same eclogite samples.The 3He/4He ratios of garnet and omphacite in these rocks range from 0.27 to 0.60 Ra(relative to the modern air 3He/4He ratio,1.4×10^-6).These ratios are within the range of both mantle-and crust-derived helium,suggesting mixed sources.The Songduo eclogites have much higher3He/4He ratios than those observed in the Dabie eclogites of eastern China.Such high ratios are typically thought to be associated with deep mantle sources.We cautiously conclude that deep mantle materials might have been involved during the formation of the Songduo eclogites.
基金supported by the National Natural Science Foundation of China(Grant Nos.41572186&41872216)。
文摘The Hefei Basin of eastern China developed in response to uplift of the Dabie Orogen,and zircon dating can be used to assess the exhumation history of the orogen.Zircons were collected from samples of the Lower Jurassic Fanghushan Formation and Middle Jurassic Sanjianpu Formation in the southern Hefei Basin,and mica-quartz schist and biotite granite gneiss from the Susong Complex of the Dabie Orogen.The zircon U-Pb dating was undertaken using laser ablation-inductively coupled plasma-mass spectrometry.The detrital zircons from conglomerates of the Fanghushan Formation and from clasts within the conglomerates have age-frequency distributions with the main clusters between 2.0 and 1.8 Ga,similar to age data of the Susong Complex.On the other hand,the zircons of the Fanghushan Formation do not show the age cluster at 1000–900 Ma that characterizes zircons in the underlying metasediments of the lower Paleozoic Foziling Group.A cluster of Triassic zircon ages also appears in the arkosic sandstones of the Fanghushan Formation.These data indicate that the provenance of the Fanghushan Formation was a mixture of high-pressure(HP)and ultrahigh-pressure(UHP)Triassic metamorphic rocks,Paleozoic magmatic rocks,and the Susong Complex,but not the lower Paleozoic Foziling Group even though it directly underlies the sediments of the Hefei Basin.Two samples from the Sanjianpu Formation show zircon age clusters at 797 and 791 Ma(middle Neoproterozoic)and 226 Ma(Triassic),and again,these are markedly different from the age clusters that characterize the Foziling Group.It seems,therefore,that despite the Foziling Group being at the surface in the underwater depositional area of the Hefei Basin,it was not exposed in the source area of the Hefei basinal sediments during the Jurassic,and there are two possible reasons for this.First,the exhumation of the Dabie Orogen was directed partly towards the north,in the process of which some of the Foziling Group was covered.Second,the Susong Complex rocks became involved in the development of
基金the National Basic Research Program of China(Grant No.2015CB856105)the National Natural Science Foundation of China(Grant Nos.91755206,41520104004).
文摘In terms of petrology,thermomechanical simulation is an important frontier to study the geodynamic process of the exhumation and uplift of high pressure(HP)to ultrahigh pressure(UHP)metamorphic rocks in subduction zones and collision orogenic belts.Based on the recent petrological studies and numerical modellings for the exhumation of HP to UHP metamorphic terranes,the exhumation mechanisms of HP to UHP metamorphic terranes can be roughly summarized into ten types:channel flow,diapiric exhumation,a coexistence mechanism of channel flow and diapiric exhumation,slab breakoff,multi-stage exhumation,divergent plate motion(including slab rollback and the upper-plate divergent motion away from the subducted plate),overthrust exhumation,overpressure mechanism,wedge-like extrusion and microplate rotation.The exhumation of high-density UHP oceanic eclogites is a relative controversial issue.Some of our recent researches on quantitatively determining the exhumation mechanism of UHP oceanic eclogites using thermomechanical and phase equilibrium modelling was introduced in details in this paper.We obtained the 3-D density evolutions of three-type subducted oceanic materials(MORB,serpentine and oceanic sediments)in P-T space by the methods of phase equilibrium and density calculation.According to the density difference between the metabasic and their surrounding rocks,the exhumed eclogites could be divided into two types.The first category,the self-exhumation eclogites(ρ_(MORB)<ρmantle),which can exhume driven by their own buoyancy,an example is the coesite-bearing oceanic eclogites from Southwest Tianshan.Another is the carried-exhumation eclogites(ρ_(MORB)>ρmantle),which can only be carried back to the surface with the assistance of low-density metasediments and serpentinite due to their negative buoyancy;the coesite-bearing UHP eclogites of Zermatt-Saas in the Western Alps is a typical example.Besides,we further explored the ultimate self-exhumation depth,exhumation mechanisms,the effect of the transition from high pre
基金funded by the National Science Centre,Poland CALSUB project no.2014/14/E/ST10/00321financial support has been provided by the Polish National Agency for Academic Exchange Scholarship PPN/IWA/2018/1/00030/U/00001 granted to C.Barnes。
文摘The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record closure of the Iapetus Ocean and subsequent continental collision of Baltica and Laurentia.The Seve Nappe Complex(SNC)of the Scandinavian Caledonides includes relics of the outermost Baltoscandian passive margin that were subducted to mantle depths.The earliest of the deep subduction events has been dated to ca.500-480 Ma.Evidence of this event has been reported from the northern exposures of the SNC.Farther south in the central and southern segments of the SNC,(ultra)high-pressure rocks have yielded younger ages in the range of ca.470-440 Ma.This study provides the first record of the early Caledonian event in the southern SNC.The evidence has been obtained by depth profiling of zircon grains that were extracted from the Tv?r?klumparna microdiamond-bearing gneiss.These zircon grains preserve eclogite facies overgrowths that crystallized at 482.6±3.8 Ma.A second,chemically-distinct zircon overgrowth records granulite facies metamorphism at 439.3±3.6 Ma,which corroborates previous geochronological evidence for granulite facies metamorphism at this time.Based on these results,we propose that the entire outer margin of Baltica was subducted in the late Cambrian to early Ordovician,but the record of this event may be almost entirely eradicated in the vast majority of lithologies by pervasive late Ordovician to early Silurian metamorphism.
