This work carried out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on Late Paleozoic intrusions in the Xingxingxia region near the Xinjiang-Gansu...This work carried out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on Late Paleozoic intrusions in the Xingxingxia region near the Xinjiang-Gansu provincial boundary, western China, aiming to constrain the Late Paleozoic tectonic framework of the Xingxingxia region and the final closure time of South Tianshan Ocean in the East Tianshan. The Xingxingxia area is located in the east part of the Tianshan orogen, and adjacent to the north of the Tarim Basin. The Late Paleozoic magma activities in the Xingxingxia region can be mainly divided into three stages. The first stage includes intrusive magma activities under a collision setting between Late Ordovician to the Late Devonian. The second stage is intrusive magma activities under a subduction setting during（304±3）–（278±3） Ma, and the third stage involves intrusive magma activities under a collision and post-collision setting during（268±5）–（259.9±2.6） Ma. The final suture zone of South Tianshan Ocean should be between the Central Tianshan Block and South Tianshan accretionary complex. Based on previous work, both the first stage magma activities（i.e., intrusive magmatic activities between the Late Ordovician to Late Devonian） and the Hongliuhe ophiolitic complex indicate a close event between Central Tianshan Block and South Tianshan Accretionary Complex. The 304±3 Ma dioritic metamorphic gneiss of the XingX ingxia complex and the 278±3 Ma diorite are all island arc calc-alkaline rocks, the 289±3 Ma gabbro is island arc tholeiitic gabbro formed by magma from metasomatic enrichment mantle. All these results indicate that the second stage of magmatic activities is under a subduction setting. The third stage magma activities i.e. the granitic magma activities of（268±5）–（259.9±2.6） Ma occurred at a transitional setting from compressional to post-collision extensional tectonic setting. Thus, around（268±5）–（260±3） Ma, the final closure of the South Tianshan Ocean occurred and the Tianshan orogen shifted into the intracontinental evolution stage. During and after the closure process, a wide range of metamorphism and large dextral strike-slip faults developed.展开更多
Ultra-high pressure（UHP）eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhum...Ultra-high pressure（UHP）eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation（Hermann et al.,2000;Guillot et al.,2001）.Under normal subduction geotherms,serpentinites break down just before UHP conditions are reached and therefore most of the exhumed eclogites representing subducted oceanic crust formed under fore-arc conditions.We investigated eclogite blocks enclosed into serpentinites that occur in the southwestern Tianshan oceanic subduction,China.A previous study proved that the serpentinites derive from altered oceanic crust and experienced UHP metamorphism at low temperatures of 510-530°C（Shen et al.,2015）.Three relatively fresh eclogite samples were studied in detail.Sample 129-7 shows the retrograde mineral assemblage of amphibole+biotite+albite+chlorite+minor titanite and peak metamorphic relics of omphacite+garnet±chlorite.Sample C107-23 is mainly composed of amphibole+albite+chlorite+zoisite+muscovite+minor titanite as a retrograde assemblage and garnet+phengite as the peak metamorphic relics with omphacite only found as inclusions in garnet.Similar to sample C107-23,sample C11066 preserves large-grained euhedral to subhedral garnet relics with omphacite inclusions,and epidote,diopside,amphibole,muscovite,chlorite,albite and biotite are in the matrix belong to the retrograde assemblage.These three retrograde eclogite samples were modelled using thermodynamic calculations in the Mn NCKFMSHO（Mn O-Na;O-Ca O-K;O-FeO-Mg O-Al;O;-SiO;-H;O-Fe;O;）system.Based on the peak assemblage of omphacite+garnet and the crossing of the grossular and pyrope isopleths in garnet,peak P-T conditions of;60-470oC,28-29 kbar（129-7）,450-500oC,28-35 kbar（C107-23）,;75-505oC,26-29 kbar（C11066）were calculated.The retrograde assemblages indicate near isothermal decompression resulting in a clockwise P-T evolution of these eclogites.The peak metamorphic pressures at 500°C are well within UHP conditions（coesite stability field）and are within error the same as peak conditions of the host serpentinites（Shen et al.,2015）.This provides evidence that eclogites and serpentinites shared the same evolution.We infer that the subducted low-density serpentinites were assembled with the high-density eclogites during subdution and helped the latter to exhume back to the surface.The studied eclogites thus represent rare examples of relics of oceanic crust that was subducted to sub-arc depth.展开更多
Ultra-high pressure(UHP)eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation
South Tianshan–Solonker suture,is the largest and southernmost suture within the Central Asian orogenic belt(CAOB).It records the ultimate collision between Tarim–North China cratons and Siberia craton,and is common...South Tianshan–Solonker suture,is the largest and southernmost suture within the Central Asian orogenic belt(CAOB).It records the ultimate collision between Tarim–North China cratons and Siberia craton,and is commonly interpreted as marking the eventual closure of Paleo-Asian Ocean.South Tianshan suture belongs to the western segment of the suture zone,and its evolutionary features are important for defining the formation age of the South Tianshan–Solonker suture.In this paper,the authors review the geochronological,geochemical,petrographic,and paleontological evidence within South Tianshan suture to delineate its formation era and closure characteristics,and thus further revealing the ultimate evolutionary pattern of the western segment of Paleo-Asian Ocean.This suture records strong plate collision before Late Carboniferous,forming a series of high-pressure metamorphic rocks,characterized by the presence of blue schist,eclogite and mica schist.In Permian,the whole area was under a relatively stable post-orogenic setting,with the formation of bimodal volcanic rocks,post-collisional granites,and terrestrial molasses.Sedimentary facies gradually changed from marine to either lacustrine or fluvial during this period.An Early Permian granite dike crosscuts the HP metamorphic belt,and the HP metamorphic rocks also underwent retrograde metamorphism at this time,indicating the formation of the South Tianshan suture was earlier than Permian.Hence,the western section of Paleo-Asian Ocean closed during Late Carboniferous,and Tarim Craton moved northward to collide with Kazakhstan–Yili Block,leading to the formation of the South Tianshan suture.展开更多
Objective Siliceous rocks can form in different positions in the oceanic plate,for example,the oceanic spreading ridge center,the oceanic basin and the continental margin.The geochemistry of siliceous rocks is very se...Objective Siliceous rocks can form in different positions in the oceanic plate,for example,the oceanic spreading ridge center,the oceanic basin and the continental margin.The geochemistry of siliceous rocks is very sensitive to the depositional environment,which can thus be used to constrain the tectonic setting of related rocks(Garbán et al.,2017).展开更多
The adakites rocks are typically considered as direct diagnostic evidence for subduction and continental growth,as well as the important evidence to determine the location of the major oceanic basin and evolution hist...The adakites rocks are typically considered as direct diagnostic evidence for subduction and continental growth,as well as the important evidence to determine the location of the major oceanic basin and evolution history of lost oceanic basins,which are longstanding conundrums for understanding the tectonics of the Paleo-Asian Ocean(PAO).As the processor of the Altaids,the PAO was a long-lived ocean,and its multiple subductions led to substantial continental growth in the Central Asian continent.Here we report our newly discovered Paleozoic adakitic rocks(granite and dacite)developed in Jijitaizi in the Eastern Tianshan of the southern Altaids to address the above issues.The Jijitaizi granite and dacite contain high concentrations of Sr(203-343 ppm)and low concentrations of Y(3.00-6.36 ppm)and Yb(0.23-0.62 ppm),indicative of the adakitic affinity.Zircon U-Pb dating results show that the adakitic rocks were formed in three episodes at 372.26±1.51 Ma,356.02±1.11 Ma and 334.51±0.68 Ma from the Late Devonian to Carboniferous.The rocks are depleted in Nb,P and Ti,and possess variable Mg#values(28.51-57.22),high Na_(2)O/K_(2)O ratios(1.39-4.94),as well as relatively depleted Hf isotopic compositions,all of which point to a subduction origin from slab melting.Our new finding with the date from the Jijitaizi complex(ca.321 Ma)suggests continuing subduction of the Paleo-Asian oceanic slab beneath the Central Tianshan from the Late Devonian to Carboniferous.Our work provides a solid line of key evidence demonstrating that continuous oceanic subduction of the major PAO evolved into its mature stage following subducting beneath the Central Tianshan from ca.372 to 321 Ma.展开更多
The South Tianshan Orogen and adjacent regions of Central Asia are located in the southwestern part of the Central Asian Orogenic Belt. The formation of South Tianshan Orogen was a diachronous, scissors-like process, ...The South Tianshan Orogen and adjacent regions of Central Asia are located in the southwestern part of the Central Asian Orogenic Belt. The formation of South Tianshan Orogen was a diachronous, scissors-like process, which took place during the Palaeozoic, and its western segment was accepted as a site of the final collision between the Tarim Craton and the North Asian continent, which occurred in the late Palaeozoic. However, the post-collisional tectonic evolution of the South Tianshan Orogen and adjacent regions remains debatable. Based on previous studies and recent geochronogical data, we suggest that the final collision between the Tarim Craton and the North Asian continent occurred during the late Carboniferous. Therefore, the Permian was a period of intracontinental environment in the southern Tianshan and adjacent regions. We propose that an earlier, small-scale intraplate orogenic stage occurred in late Permian to Triassic time, which was the first intraplate process in the South Tianshan Orogen and adjacent regions. The later large- scale and well-known Neogene to Quaternary intraplate orogeny was induced by the collision between the India subcontinent and the Eurasian plate. The paper presents a new evolutionary model for the South Tianshan Orogen and adjacent regions, which includes seven stages： （I） late Ordovician-early Silurian opening of the South Tianshan Ocean; （11） middle Silurian-middle Devonian subduction of the South Tianshan Ocean beneath an active margin of the North Asian continent; （111） late Devonian-late Carboniferous closure of the South Tianshan Ocean and collision between the Kazakhstan-Yili and Tarirn continental blocks; （IV） early Permian post-collisional magmatism and rifting; （V） late Permian-Triassic the first intraplate orogeny; （Vt） Jurassic-Palaeogene tectonic stagnation and （VII） Neocene-Quaternary intraplate orogeny.展开更多
基金the Program of China Geological Survey(grant No.1212011220649)
文摘This work carried out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on Late Paleozoic intrusions in the Xingxingxia region near the Xinjiang-Gansu provincial boundary, western China, aiming to constrain the Late Paleozoic tectonic framework of the Xingxingxia region and the final closure time of South Tianshan Ocean in the East Tianshan. The Xingxingxia area is located in the east part of the Tianshan orogen, and adjacent to the north of the Tarim Basin. The Late Paleozoic magma activities in the Xingxingxia region can be mainly divided into three stages. The first stage includes intrusive magma activities under a collision setting between Late Ordovician to the Late Devonian. The second stage is intrusive magma activities under a subduction setting during（304±3）–（278±3） Ma, and the third stage involves intrusive magma activities under a collision and post-collision setting during（268±5）–（259.9±2.6） Ma. The final suture zone of South Tianshan Ocean should be between the Central Tianshan Block and South Tianshan accretionary complex. Based on previous work, both the first stage magma activities（i.e., intrusive magmatic activities between the Late Ordovician to Late Devonian） and the Hongliuhe ophiolitic complex indicate a close event between Central Tianshan Block and South Tianshan Accretionary Complex. The 304±3 Ma dioritic metamorphic gneiss of the XingX ingxia complex and the 278±3 Ma diorite are all island arc calc-alkaline rocks, the 289±3 Ma gabbro is island arc tholeiitic gabbro formed by magma from metasomatic enrichment mantle. All these results indicate that the second stage of magmatic activities is under a subduction setting. The third stage magma activities i.e. the granitic magma activities of（268±5）–（259.9±2.6） Ma occurred at a transitional setting from compressional to post-collision extensional tectonic setting. Thus, around（268±5）–（260±3） Ma, the final closure of the South Tianshan Ocean occurred and the Tianshan orogen shifted into the intracontinental evolution stage. During and after the closure process, a wide range of metamorphism and large dextral strike-slip faults developed.
文摘Ultra-high pressure（UHP）eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation（Hermann et al.,2000;Guillot et al.,2001）.Under normal subduction geotherms,serpentinites break down just before UHP conditions are reached and therefore most of the exhumed eclogites representing subducted oceanic crust formed under fore-arc conditions.We investigated eclogite blocks enclosed into serpentinites that occur in the southwestern Tianshan oceanic subduction,China.A previous study proved that the serpentinites derive from altered oceanic crust and experienced UHP metamorphism at low temperatures of 510-530°C（Shen et al.,2015）.Three relatively fresh eclogite samples were studied in detail.Sample 129-7 shows the retrograde mineral assemblage of amphibole+biotite+albite+chlorite+minor titanite and peak metamorphic relics of omphacite+garnet±chlorite.Sample C107-23 is mainly composed of amphibole+albite+chlorite+zoisite+muscovite+minor titanite as a retrograde assemblage and garnet+phengite as the peak metamorphic relics with omphacite only found as inclusions in garnet.Similar to sample C107-23,sample C11066 preserves large-grained euhedral to subhedral garnet relics with omphacite inclusions,and epidote,diopside,amphibole,muscovite,chlorite,albite and biotite are in the matrix belong to the retrograde assemblage.These three retrograde eclogite samples were modelled using thermodynamic calculations in the Mn NCKFMSHO（Mn O-Na;O-Ca O-K;O-FeO-Mg O-Al;O;-SiO;-H;O-Fe;O;）system.Based on the peak assemblage of omphacite+garnet and the crossing of the grossular and pyrope isopleths in garnet,peak P-T conditions of;60-470oC,28-29 kbar（129-7）,450-500oC,28-35 kbar（C107-23）,;75-505oC,26-29 kbar（C11066）were calculated.The retrograde assemblages indicate near isothermal decompression resulting in a clockwise P-T evolution of these eclogites.The peak metamorphic pressures at 500°C are well within UHP conditions（coesite stability field）and are within error the same as peak conditions of the host serpentinites（Shen et al.,2015）.This provides evidence that eclogites and serpentinites shared the same evolution.We infer that the subducted low-density serpentinites were assembled with the high-density eclogites during subdution and helped the latter to exhume back to the surface.The studied eclogites thus represent rare examples of relics of oceanic crust that was subducted to sub-arc depth.
文摘Ultra-high pressure(UHP)eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation
基金the National Natural Science Foundation of China(Nos.41730210,41888101).
文摘South Tianshan–Solonker suture,is the largest and southernmost suture within the Central Asian orogenic belt(CAOB).It records the ultimate collision between Tarim–North China cratons and Siberia craton,and is commonly interpreted as marking the eventual closure of Paleo-Asian Ocean.South Tianshan suture belongs to the western segment of the suture zone,and its evolutionary features are important for defining the formation age of the South Tianshan–Solonker suture.In this paper,the authors review the geochronological,geochemical,petrographic,and paleontological evidence within South Tianshan suture to delineate its formation era and closure characteristics,and thus further revealing the ultimate evolutionary pattern of the western segment of Paleo-Asian Ocean.This suture records strong plate collision before Late Carboniferous,forming a series of high-pressure metamorphic rocks,characterized by the presence of blue schist,eclogite and mica schist.In Permian,the whole area was under a relatively stable post-orogenic setting,with the formation of bimodal volcanic rocks,post-collisional granites,and terrestrial molasses.Sedimentary facies gradually changed from marine to either lacustrine or fluvial during this period.An Early Permian granite dike crosscuts the HP metamorphic belt,and the HP metamorphic rocks also underwent retrograde metamorphism at this time,indicating the formation of the South Tianshan suture was earlier than Permian.Hence,the western section of Paleo-Asian Ocean closed during Late Carboniferous,and Tarim Craton moved northward to collide with Kazakhstan–Yili Block,leading to the formation of the South Tianshan suture.
基金supported by the National Natural Science Foundation of China(grant Nos.42072267,41602229,41802234)the Fundamental Research Funds for the Central Universities,CHD(300102279201,300102279104)the Natural Science Foundation of Shaanxi Province,China(2019JQ-090,2019JQ-209)。
文摘Objective Siliceous rocks can form in different positions in the oceanic plate,for example,the oceanic spreading ridge center,the oceanic basin and the continental margin.The geochemistry of siliceous rocks is very sensitive to the depositional environment,which can thus be used to constrain the tectonic setting of related rocks(Garbán et al.,2017).
基金financially supported by the National Natural Science Foundation of China(41888101,41822204)One Hundred Talent Program of the Chinese Academy of Sciences(CAS)+4 种基金the Science and Technology Major Project of Xinjiang Uygur Autonomous Region,China(2021A03001)the National Key Research and Development Program of China(2017YFC0601201)the Chinese Ministry of Land and Resources for the Public Welfare Industry Research(201411026-1)the“Light of West China”Program of the CAS(2017-XBQNXZ-B013,2018-XBYJRC-003)a Project of the ChinaPakistan Joint Research Center on Earth Sciences of the CAS(131551KYSB20200021)。
文摘The adakites rocks are typically considered as direct diagnostic evidence for subduction and continental growth,as well as the important evidence to determine the location of the major oceanic basin and evolution history of lost oceanic basins,which are longstanding conundrums for understanding the tectonics of the Paleo-Asian Ocean(PAO).As the processor of the Altaids,the PAO was a long-lived ocean,and its multiple subductions led to substantial continental growth in the Central Asian continent.Here we report our newly discovered Paleozoic adakitic rocks(granite and dacite)developed in Jijitaizi in the Eastern Tianshan of the southern Altaids to address the above issues.The Jijitaizi granite and dacite contain high concentrations of Sr(203-343 ppm)and low concentrations of Y(3.00-6.36 ppm)and Yb(0.23-0.62 ppm),indicative of the adakitic affinity.Zircon U-Pb dating results show that the adakitic rocks were formed in three episodes at 372.26±1.51 Ma,356.02±1.11 Ma and 334.51±0.68 Ma from the Late Devonian to Carboniferous.The rocks are depleted in Nb,P and Ti,and possess variable Mg#values(28.51-57.22),high Na_(2)O/K_(2)O ratios(1.39-4.94),as well as relatively depleted Hf isotopic compositions,all of which point to a subduction origin from slab melting.Our new finding with the date from the Jijitaizi complex(ca.321 Ma)suggests continuing subduction of the Paleo-Asian oceanic slab beneath the Central Tianshan from the Late Devonian to Carboniferous.Our work provides a solid line of key evidence demonstrating that continuous oceanic subduction of the major PAO evolved into its mature stage following subducting beneath the Central Tianshan from ca.372 to 321 Ma.
基金supported by the National Natural Science Foundation of China (Grant Nos. 40772121, 40314141 and 40172066)China National Project No. 973 (2009CB219302)IGCP Project #592 "Continental construction in Central Asia" supported by UNESCO-IUGS
文摘The South Tianshan Orogen and adjacent regions of Central Asia are located in the southwestern part of the Central Asian Orogenic Belt. The formation of South Tianshan Orogen was a diachronous, scissors-like process, which took place during the Palaeozoic, and its western segment was accepted as a site of the final collision between the Tarim Craton and the North Asian continent, which occurred in the late Palaeozoic. However, the post-collisional tectonic evolution of the South Tianshan Orogen and adjacent regions remains debatable. Based on previous studies and recent geochronogical data, we suggest that the final collision between the Tarim Craton and the North Asian continent occurred during the late Carboniferous. Therefore, the Permian was a period of intracontinental environment in the southern Tianshan and adjacent regions. We propose that an earlier, small-scale intraplate orogenic stage occurred in late Permian to Triassic time, which was the first intraplate process in the South Tianshan Orogen and adjacent regions. The later large- scale and well-known Neogene to Quaternary intraplate orogeny was induced by the collision between the India subcontinent and the Eurasian plate. The paper presents a new evolutionary model for the South Tianshan Orogen and adjacent regions, which includes seven stages： （I） late Ordovician-early Silurian opening of the South Tianshan Ocean; （11） middle Silurian-middle Devonian subduction of the South Tianshan Ocean beneath an active margin of the North Asian continent; （111） late Devonian-late Carboniferous closure of the South Tianshan Ocean and collision between the Kazakhstan-Yili and Tarirn continental blocks; （IV） early Permian post-collisional magmatism and rifting; （V） late Permian-Triassic the first intraplate orogeny; （Vt） Jurassic-Palaeogene tectonic stagnation and （VII） Neocene-Quaternary intraplate orogeny.
