Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure （UHP） terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphi...Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure （UHP） terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited （detrital） and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence （CL） imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones （domains） to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure （HP） metamorphic conditions of T = 571-668℃ and P =1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite eclogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782-849℃ and P 〉 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600- 710℃ and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245 ± 4 Ma for domain 1, 235 ± 3 Ma for domain 2 and 215± 6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244 ± 4 Ma, 233 ± 4 Ma and 214 ± 5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10-11 Ma and exhumation of the rocks occurred over a period of 19-20 Ma. Thus, subduction from - 55 km to 〉 160 km deep mantle depth took place at rates of approximately 9.5-10.5 km/Ma and exhumation from depths 〉160 km to the base of the crust at -30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin lithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.展开更多
The Yanji area,northeastern China,a part of the orogenic collage between the North China Block in the south and the Jiamusi-Khanka Massifs in the northeast,is the most likely location where the Pacific Plate subductio...The Yanji area,northeastern China,a part of the orogenic collage between the North China Block in the south and the Jiamusi-Khanka Massifs in the northeast,is the most likely location where the Pacific Plate subduction・related magmatic activities and subsequent exhumation processes occurred.Here,we report new low-temperature thermochronology of apatite and zircon data from the granitoid samples in the Yanji area.The exhumation rates of Tianfozhishan,Yanji area,were〜0.049 and〜0.073 mm/year,interpreted from the elevations and apatite and zircon fission track ages,respectively.The exhumation,integrated with the geological setting,suggested that the paleogeothermal gradient of the Tianfozhishan,even extending to the Yanji area,was possibly to be greater than 35℃/km in the Late Cretaceous.The thermal history modeling of the data indicates a basically similar pattern,but the various timing for different samples between the Oligocene-Early Miocene and the Middle Miocene in the Yanji area.We hence conclude that a fourstages of cooling,from〜6.7℃/Ma(during the Late Cretaceous),to〜0.8℃/Ma(during the Late Cretaceous to the Oligocene-Early Miocene),then to〜2-3℃/Ma with varied styles(between the Oligocene-Early Miocene and the Middle Miocene),and finally to<0.2℃/Ma(since the Middle Miocene),has taken place through the exhumation of the Yanji area.The maximum exhumation is>3 km under a reasonable paleogeothermal gradient(>35℃/km),speculated from the possible exhumation rate of Tianfozhishan.Combined with the tectonic setting,this exhumation,including two stages of pronounced tectonic uplift and denudation and two stages of weak exhumation driven by the low regional erosion rate,is possibly related to the subduction of the Pacific Plate beneath the Eurasian Plate since the Late Cretaceous.This study used more robust evidence to propose higher paleogeothermal gradient(>35℃/km),reflecting exhumation of>3 km in the Yanji area since the Late Cretaceous.展开更多
High-pressure(HP)metamorphic terrane in the Tongbai orogen comprises two HP slices(I and II)and a tectonic mélange zone in the northeast and a blueschist-greenschist zone in the southwest.HP slice I is represente...High-pressure(HP)metamorphic terrane in the Tongbai orogen comprises two HP slices(I and II)and a tectonic mélange zone in the northeast and a blueschist-greenschist zone in the southwest.HP slice I is represented by the northern and southern eclogite zones on the two sides of the Tongbaishan antiform.HP slice II is represented by retrograded eclogite-bearing metamorphic enclaves in Cretaceous gneissic granites in the Tongbai Complex.U-Pb,Lu-Hf,Rb-Sr and40Ar/39Ar multichronometric data indicate that the peak metamorphism of HP slice I took place at^255 Ma,whereas the metamorphic ages of HP slice II are as young as232–220 Ma.By contrast,the tectonic mélange zone near the suture was metamorphosed at^256 Ma.Such a diachroneity of different slices across the direction of the orogen in the Hong’an-Dabie-Sulu HP/UHP terrane is ubiquitous,and it can be interpreted by a syn-subduction detachment/exhumation model.Furthermore,the metamorphic age of HP slice I in the Tongbai orogen is older than that of the equivalent HP slice in the Hong’an orogen by^15 Ma,suggesting that the diachroneity may have also existed along the direction of the orogen.A seesaw-type subduction/exhumation model is proposed to explain this age disparity and the subduction of the South China Block becomimg shallower towards the west.展开更多
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
Two fresh types of eclogites, namely the massive eclogite and foliated eclogite, are dis- cernible in large eclogite bodies surrounded by country rock gneisses from the Dabie Sulu UHP metamorphic zone. They are diffe...Two fresh types of eclogites, namely the massive eclogite and foliated eclogite, are dis- cernible in large eclogite bodies surrounded by country rock gneisses from the Dabie Sulu UHP metamorphic zone. They are different in mineral assemblage, texture and structure at various scales. The massive eclogite has a massive appearance with a metamorphic inequigranular and grano- blastic texture, which consists mainly of nominally anhydrous minerals such as garnet, omphacite, rutile with inclusions of coesite and rare microdiamond. Massive eclogites which formed at the peak UHP metamorphic conditions （~3.1-4.0 GPa, 800~50 ） within the coesite to diamond stability field recorded the deep continental subduction to mantle depths greater than 100 km during the Triassic （-250-230 Ma）. The diagnostic UHP minerals, mineral assemblages and absence of notable macro- scopic deformation indicate the peak metamorphic ＇forbidden-zone＇ P-T conditions, an extremely low geothermal gradient （〈7 ＂C＇kma） and low differential stress. The foliated eclogite is composed of garnet＋omphacite＋rutile＋phengite＋kyanite＋zoisite＋talc＋nybtite^coesite/quartz pseudomorphs after coesite. It is quite clear that the foliated eclogite bears relatively abundant hydrous mineral, and shows well-developed penetrative foliation carrying mineral and stretching lineation reflecting intense plastic deformation or flow of eclogite minerals. The foliatcd eclogite occurred at mantle levels and recorded the earliest stages of exhumation of UHP metamorphic rocks. At a map scale, the foliated eclogites de- fine UHP eclogite-facies shear zones or high-strain zones. Asymmetric structures are abundant in the zones, implying bulk plane strain or dominant non-coaxial deformation within the coesite stability field. The earliest stages of exhumation, from mantle depths to the Moho or mantle-crust boundary layering, were characterized by a sub-vertical tectonic wedge extrusion, which occurred around 230-210 Ma. The three- dimensional relationship between the massive and foliated eclogites is well displayed a typical ＇block-in-matrix＇ rheological fabric pattern in- dicating the partitioning of deformation and metamorphism in the UHP petrotectonic unit. The existing data support the now widely accepted con- cept of deep continental subduction/collision and subsequent exhumation between the Yangtze and Sino-Korean cratons. The pressure is a constitutive geological variable. The influence of tectonic over- presure on UHP metamorphism is rather limited.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 pressure to ultra-high pressure on the exhumation process of oceanic eclogites and the spatial distribution of exhumed HP-UHP metamorphic terranes.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
The discovery of ultrahigh-pressure(UHP)index minerals such as coesite and micro-diamond in supracrustal rocks indicate that a large amount of crustal rocks were subducted to mantle depths and subsequently returned to...The discovery of ultrahigh-pressure(UHP)index minerals such as coesite and micro-diamond in supracrustal rocks indicate that a large amount of crustal rocks were subducted to mantle depths and subsequently returned to the Earth’s surface.However,exhumation mechanism of the deeply subducted crustal rocks has been a hot topic in geodynamic community and remains controversial.Previous studies have been performed by petrologic observations,analogue experiments and numerical simulations for exhumation processes and mechanisms of deeply subducted crustal rocks,and a series of important results have been achieved.In this paper,we summerize in detail recent progresses and existing problems on types of subduction zones,exhumation processes and mechanisms of deeply subducted crustal rocks,and related factors.Based on these,three forthcoming prospects are presented:(1)The geological processes within the exhuming crust;(2)the interaction between the exhuming crust and the surface process;(3)the interaction between the exhuming crust and the mantle.展开更多
基金the National 973 Project of Chinese Ministry of Science and Technology （Grant No. 2003CB716502） the Natural Science Foundation of China （Grant No. 40399143） +1 种基金 the German Science Foundation （DFG grant No. GE 1152/2-2 , WE2850/3- 1）.
文摘Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure （UHP） terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited （detrital） and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence （CL） imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones （domains） to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure （HP） metamorphic conditions of T = 571-668℃ and P =1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite eclogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782-849℃ and P 〉 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600- 710℃ and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245 ± 4 Ma for domain 1, 235 ± 3 Ma for domain 2 and 215± 6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244 ± 4 Ma, 233 ± 4 Ma and 214 ± 5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10-11 Ma and exhumation of the rocks occurred over a period of 19-20 Ma. Thus, subduction from - 55 km to 〉 160 km deep mantle depth took place at rates of approximately 9.5-10.5 km/Ma and exhumation from depths 〉160 km to the base of the crust at -30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin lithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.
基金supported by the DREAM project of MOST China (2016YFC0600406)the National Natural Science Foundation of China (Grant Nos. 41072158, 41372227)
文摘The Yanji area,northeastern China,a part of the orogenic collage between the North China Block in the south and the Jiamusi-Khanka Massifs in the northeast,is the most likely location where the Pacific Plate subduction・related magmatic activities and subsequent exhumation processes occurred.Here,we report new low-temperature thermochronology of apatite and zircon data from the granitoid samples in the Yanji area.The exhumation rates of Tianfozhishan,Yanji area,were〜0.049 and〜0.073 mm/year,interpreted from the elevations and apatite and zircon fission track ages,respectively.The exhumation,integrated with the geological setting,suggested that the paleogeothermal gradient of the Tianfozhishan,even extending to the Yanji area,was possibly to be greater than 35℃/km in the Late Cretaceous.The thermal history modeling of the data indicates a basically similar pattern,but the various timing for different samples between the Oligocene-Early Miocene and the Middle Miocene in the Yanji area.We hence conclude that a fourstages of cooling,from〜6.7℃/Ma(during the Late Cretaceous),to〜0.8℃/Ma(during the Late Cretaceous to the Oligocene-Early Miocene),then to〜2-3℃/Ma with varied styles(between the Oligocene-Early Miocene and the Middle Miocene),and finally to<0.2℃/Ma(since the Middle Miocene),has taken place through the exhumation of the Yanji area.The maximum exhumation is>3 km under a reasonable paleogeothermal gradient(>35℃/km),speculated from the possible exhumation rate of Tianfozhishan.Combined with the tectonic setting,this exhumation,including two stages of pronounced tectonic uplift and denudation and two stages of weak exhumation driven by the low regional erosion rate,is possibly related to the subduction of the Pacific Plate beneath the Eurasian Plate since the Late Cretaceous.This study used more robust evidence to propose higher paleogeothermal gradient(>35℃/km),reflecting exhumation of>3 km in the Yanji area since the Late Cretaceous.
基金supportedby the National Basic Research Program of China(2009CB825006)
文摘High-pressure(HP)metamorphic terrane in the Tongbai orogen comprises two HP slices(I and II)and a tectonic mélange zone in the northeast and a blueschist-greenschist zone in the southwest.HP slice I is represented by the northern and southern eclogite zones on the two sides of the Tongbaishan antiform.HP slice II is represented by retrograded eclogite-bearing metamorphic enclaves in Cretaceous gneissic granites in the Tongbai Complex.U-Pb,Lu-Hf,Rb-Sr and40Ar/39Ar multichronometric data indicate that the peak metamorphism of HP slice I took place at^255 Ma,whereas the metamorphic ages of HP slice II are as young as232–220 Ma.By contrast,the tectonic mélange zone near the suture was metamorphosed at^256 Ma.Such a diachroneity of different slices across the direction of the orogen in the Hong’an-Dabie-Sulu HP/UHP terrane is ubiquitous,and it can be interpreted by a syn-subduction detachment/exhumation model.Furthermore,the metamorphic age of HP slice I in the Tongbai orogen is older than that of the equivalent HP slice in the Hong’an orogen by^15 Ma,suggesting that the diachroneity may have also existed along the direction of the orogen.A seesaw-type subduction/exhumation model is proposed to explain this age disparity and the subduction of the South China Block becomimg shallower towards the west.
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 Natural Science Foundation of China(Nos.40372094 and 49972067)
文摘Two fresh types of eclogites, namely the massive eclogite and foliated eclogite, are dis- cernible in large eclogite bodies surrounded by country rock gneisses from the Dabie Sulu UHP metamorphic zone. They are different in mineral assemblage, texture and structure at various scales. The massive eclogite has a massive appearance with a metamorphic inequigranular and grano- blastic texture, which consists mainly of nominally anhydrous minerals such as garnet, omphacite, rutile with inclusions of coesite and rare microdiamond. Massive eclogites which formed at the peak UHP metamorphic conditions （~3.1-4.0 GPa, 800~50 ） within the coesite to diamond stability field recorded the deep continental subduction to mantle depths greater than 100 km during the Triassic （-250-230 Ma）. The diagnostic UHP minerals, mineral assemblages and absence of notable macro- scopic deformation indicate the peak metamorphic ＇forbidden-zone＇ P-T conditions, an extremely low geothermal gradient （〈7 ＂C＇kma） and low differential stress. The foliated eclogite is composed of garnet＋omphacite＋rutile＋phengite＋kyanite＋zoisite＋talc＋nybtite^coesite/quartz pseudomorphs after coesite. It is quite clear that the foliated eclogite bears relatively abundant hydrous mineral, and shows well-developed penetrative foliation carrying mineral and stretching lineation reflecting intense plastic deformation or flow of eclogite minerals. The foliatcd eclogite occurred at mantle levels and recorded the earliest stages of exhumation of UHP metamorphic rocks. At a map scale, the foliated eclogites de- fine UHP eclogite-facies shear zones or high-strain zones. Asymmetric structures are abundant in the zones, implying bulk plane strain or dominant non-coaxial deformation within the coesite stability field. The earliest stages of exhumation, from mantle depths to the Moho or mantle-crust boundary layering, were characterized by a sub-vertical tectonic wedge extrusion, which occurred around 230-210 Ma. The three- dimensional relationship between the massive and foliated eclogites is well displayed a typical ＇block-in-matrix＇ rheological fabric pattern in- dicating the partitioning of deformation and metamorphism in the UHP petrotectonic unit. The existing data support the now widely accepted con- cept of deep continental subduction/collision and subsequent exhumation between the Yangtze and Sino-Korean cratons. The pressure is a constitutive geological variable. The influence of tectonic over- presure on UHP metamorphism is rather limited.
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 pressure to ultra-high pressure on the exhumation process of oceanic eclogites and the spatial distribution of exhumed HP-UHP metamorphic terranes.
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the National Basic Research Program of China(Grant No.2015CB856104)the National Natural Science Foundation of China(Grant No.41773020).Author information。
文摘The discovery of ultrahigh-pressure(UHP)index minerals such as coesite and micro-diamond in supracrustal rocks indicate that a large amount of crustal rocks were subducted to mantle depths and subsequently returned to the Earth’s surface.However,exhumation mechanism of the deeply subducted crustal rocks has been a hot topic in geodynamic community and remains controversial.Previous studies have been performed by petrologic observations,analogue experiments and numerical simulations for exhumation processes and mechanisms of deeply subducted crustal rocks,and a series of important results have been achieved.In this paper,we summerize in detail recent progresses and existing problems on types of subduction zones,exhumation processes and mechanisms of deeply subducted crustal rocks,and related factors.Based on these,three forthcoming prospects are presented:(1)The geological processes within the exhuming crust;(2)the interaction between the exhuming crust and the surface process;(3)the interaction between the exhuming crust and the mantle.
