Fragments of Proto-Tethyan oceanic lithosphere are well-preserved along the southern belt of the North Qilian suture,and the origin and emplacement of these ophiolites have become subjects of intense debate.In this st...Fragments of Proto-Tethyan oceanic lithosphere are well-preserved along the southern belt of the North Qilian suture,and the origin and emplacement of these ophiolites have become subjects of intense debate.In this study,we integrate field observations,mineralogical and geochemical analyses,zircon U-Pb dating,and isotopic data to investigate the Yanglong ophiolite.The Yanglong ophiolitic rocks are found as tectonic slices resting on the Neoproterozoic sedimentary and volcanic rocks.These rocks are composed of Cambrian serpentinized peridotite,gabbro,dolerite,and rodingite.The spinels in the serpentinized peridotites have variable Cr^(#)values(21,38-46,and 59-61)and display affinity to those in abyssal and forearc peridotites.The dolerites show slight enrichment in Th and have elevated(La/Sm)_(N) ratios(1.19-2.01),indicating a subduction-related geochemical affinity.The Yanglong ophiolitic rocks have positive zirconεHf(t)values(+10.3 to+18.4)and whole-rockεNd(t)values(+5.3 to+6.7)indicating derivation from partial melting of a depleted mantle source.These results,together with the regional geology,collectively suggest that the Yanglong ophiolite was generated in a forearc setting during the Early Cambrian northward intra-oceanic subduction.It was emplaced onto the Central Qilian Block during the subsequent arc-continent collision,no later than the Early Ordovician.展开更多
The South Qilian belt mainly comprises an early Paleozoic arc-ophiolite complex, accretionary prism, microcontinental block, and foreland basin. These elements represent accretion-collision during Cambrian to Silurian...The South Qilian belt mainly comprises an early Paleozoic arc-ophiolite complex, accretionary prism, microcontinental block, and foreland basin. These elements represent accretion-collision during Cambrian to Silurian time in response to closure of the Proto-Tethyan Ocean in the NE of the present-day Tibet Plateau. Closure of the Proto-Tethyan Ocean between the Central Qilian block and the Oulongbuluke block and the associated collision took place from NE to SW in a zipper-like style. Sediment would have been dispersed longitudinally SW-ward with a progressive facies migration from marginal alluvial sediments toward slope deep-water and deep-sea turbidites. This migration path indicates an ocean basin that shrank toward the SW. The Balonggongga'er Formation in the western South Qilian belt represents the fill of a latest Ordovician-Silurian remnant ocean basin that separated the Oulongbuluke block from the Central Qilian block, and records Silurian closure of the Proto-Tethyan Ocean and subduction beneath the Central Qilian block. However, alluvial deposits in the Lajishan area were accumulated in a retro-foreland basin, indicating that continent-continent collision in the eastern South Qilian belt occurred at c. 450–440 Ma. These results demonstrate that the Proto-Tethyan Ocean closed diachronously during early Paleozoic time.展开更多
Walegen Au deposit is closely correlated with granitic intrusions of Triassic age, which are composed of granite and quartz porphyries. Both granite porphyry and quartz porphyry consist of quartz, feldspar and muscovi...Walegen Au deposit is closely correlated with granitic intrusions of Triassic age, which are composed of granite and quartz porphyries. Both granite porphyry and quartz porphyry consist of quartz, feldspar and muscovite as primary minerals. WeaMy peraluminous granite porphyry （A/ CNK=1.10-1.15） is enriched in LREE, depleted in HREE with Nb-Ta-Ti anomalies, and displays subduction-related geochemistry. Quartz porphyry is strongly peraluminous （A/CNK=1.64-2.81） with highly evolved components, characterized by lower TiO2, REE contents, Mg#, K/Rb, Nb/Ta, Zr/Hf ratios and higher Rb/Sr ratios than the granite porphyry. REE patterns of quartz porphyry exhibit lanthanide tetrad effect, resulting from mineral fractionation or participation of fluids with enriched F and CI. LA- ICP-MS zircon U-Pb dating indicates quartz porphyry formed at 233±3 Ma. The ages of relict zircons from Triassic magmatic rocks match well with the detrital zircons from regional area. In addition, εHf（t） values of Triassic magmatic zircons from the granite and quartz porphyries are -14.2 to -9.1 （with an exception of ＋4.1） and -10.8 to -8.6 respectively, indicating a crustal-dominant source. Regionally, numerous Middle Triassic granitoids were previously reported to be formed under the consumption of Paleotethyan Ocean. These facts indicate that the granitic porphyries from Walegen Au deposit may have been formed in the processes of the closing of Paleotethyan Ocean, which could correlate with the arc-related magmatism in the Kunlun orogen to the west and the Qinling orogen to the east.展开更多
Identification and anatomy of oceanic arcs within ancient orogenic belt are significant for better understanding the tectonic framework and closure process of paleo-ocean basin.This article summarizes the geological,g...Identification and anatomy of oceanic arcs within ancient orogenic belt are significant for better understanding the tectonic framework and closure process of paleo-ocean basin.This article summarizes the geological,geochemical,and geochronological characteristics of upper crust of Proto-Tethyan Lajishan intra-oceanic arc and provides new data to constrain the subduction evolution of the South Qilian Ocean.The intra-oceanic arc volcanic rocks,including intermediate-mafic lava,breccia,tuff,and minor felsic rocks,are distributed along southern part of the Lajishan ophiolite belt.Geochemical and isotopic compositions indicate that the intermediate-mafic lava were originated from depleted mantle contaminated by sediment melts or hydrous fluids,whereas the felsic rocks were likely generated by partial melting of juvenile mafic crust in intra-oceanic arc setting.Zircons from felsic rocks yield consistent and concordant ages ranging from 506 to 523 Ma,suggesting these volcanic rocks represent the relicts of upper crust of the Cambrian intra-oceanic arc.Combined with the Cambrian forearc ophiolite and accretionary complex,we suggest that the Cambrian intra-oceanic arc in the Lajishan ophiolite belt is belonging to the intra-oceanic arc system which was generated by south-directed subduction in the South Qilian Ocean at a relatively short interval between approximately 530 and 480 Ma.展开更多
The Qilian orogenic belt is the northernmost orogen of the Tethyan domain and connects the Altaids to the north. It contains an assembly of Precambrian micro-continental fragments, early Paleozoic island arcs, accreti...The Qilian orogenic belt is the northernmost orogen of the Tethyan domain and connects the Altaids to the north. It contains an assembly of Precambrian micro-continental fragments, early Paleozoic island arcs, accretionary complexes, ophiolites, forearc and backarc basins, and high-pressure(HP) metamorphic rocks, indicating a long history of accretionary processes. Spatially, this orogen is adjacent to the Tarim, Qaidam, and North China blocks, which also extends into accretionary orogenic belts to the east and SW such as the Qinling and Kunlun belts. Abundant ophiolites in this orogen record the closure of an early Tethyan Ocean and amalgamations between micro-continents of North China, Qaidam, and Tarim. Thus, the ages and tectonic settings of these ophiolites within this belt provide important information regarding evolution of the Proto-Tethys Ocean and assembly of micro-continental blocks, which aids understanding of the spatial and temporal relationship of this orogen within the Tethyan realm. Dismembered ophiolites sporadically crop out along the northern margin of the South Qilian belt, and, from east to west, are locally referred to as the Lajishan, Gangcha, Muli, and Dadaoerji ophiolites. Much attention had been paid to these ophiolites, and several competing models for the tectonic evolution of this belt have been suggested. Considerable disagreement remains in respect of the temporal and spatial framework of the Qilian Orogen and details such as timing of subduction(s) and associated polarities, early collision events, and final closure of oceanic basins. In particular, the formation age and tectonic setting of Muli arc-ophiolite complex remains unknown, which limits understanding of the tectonics of the South Qilian belt and the history of the Proto-Tethys Ocean. The Muli arc-ophiolite complex is distributed over 20 km^2 west of the township of Muli in the western segment of the South Qilian Belt and consists of serpentinite, dunite, cumulate gabbro, basalt, plagiogranite, and chert. Field mapping results demonstrate that these units have been largely destroyed by faulting and generally occur as blocks/slices. They are tectonically interlayered with Upper Ordovician – Lower Silurian siliciclastic turbidite. Arc-ophiolite rocks are intruded by 470–450 Ma subduction-related granitoid plutons and are unconformably overlain by shallow marine to non-marine sediments of Permian-Jurassic age. Basalts show typical subduction-related calc-alkaline geochemical affinity, representing portions of an island arc. Geochemical results for plagiogranites and spinels from serpentinite demonstrate that the Muli arc-ophiolite complex represents a super-subduction zone(SSZ)-type ophiolite. U-Pb zircon data indicate formation associated with southward subduction of the Proto-Tethyan Ocean during a short interval between 539–522 Ma. Voluminous Late Ordovician-Early Silurian deep-water marine siliciclastic and volcaniclastic turbidites and volcanic arc rocks are exposed to the south of the Muli arc-ophiolite complex, whereas fluvial coarse-grained sandstones and conglomerates unconformably overlie the Cambrian-Middle Ordovician ophiolite-arc systems in the eastern South Qilian Belt. These indicate that closure of the Proto-Tethys Ocean was diachronous during the early Paleozoic.展开更多
基金the National Natural Science Foundation of China(Grant Nos.42230308,42072266,42102032,42362006)Bureau of Geological Exploration and Development of Qinghai Province(Grant Nos.[2023]33,[2022]32)China Geological Survey(Grant Nos.DD20221649,DD20242935,12120114079701).
文摘Fragments of Proto-Tethyan oceanic lithosphere are well-preserved along the southern belt of the North Qilian suture,and the origin and emplacement of these ophiolites have become subjects of intense debate.In this study,we integrate field observations,mineralogical and geochemical analyses,zircon U-Pb dating,and isotopic data to investigate the Yanglong ophiolite.The Yanglong ophiolitic rocks are found as tectonic slices resting on the Neoproterozoic sedimentary and volcanic rocks.These rocks are composed of Cambrian serpentinized peridotite,gabbro,dolerite,and rodingite.The spinels in the serpentinized peridotites have variable Cr^(#)values(21,38-46,and 59-61)and display affinity to those in abyssal and forearc peridotites.The dolerites show slight enrichment in Th and have elevated(La/Sm)_(N) ratios(1.19-2.01),indicating a subduction-related geochemical affinity.The Yanglong ophiolitic rocks have positive zirconεHf(t)values(+10.3 to+18.4)and whole-rockεNd(t)values(+5.3 to+6.7)indicating derivation from partial melting of a depleted mantle source.These results,together with the regional geology,collectively suggest that the Yanglong ophiolite was generated in a forearc setting during the Early Cambrian northward intra-oceanic subduction.It was emplaced onto the Central Qilian Block during the subsequent arc-continent collision,no later than the Early Ordovician.
基金the National Natural Science Foundation of China(Grants 41672221,41872241)China Geological Survey(Grants DD20190006,DD2016020104)IGGCAS Open Research Foundation(SKLK201702)。
文摘The South Qilian belt mainly comprises an early Paleozoic arc-ophiolite complex, accretionary prism, microcontinental block, and foreland basin. These elements represent accretion-collision during Cambrian to Silurian time in response to closure of the Proto-Tethyan Ocean in the NE of the present-day Tibet Plateau. Closure of the Proto-Tethyan Ocean between the Central Qilian block and the Oulongbuluke block and the associated collision took place from NE to SW in a zipper-like style. Sediment would have been dispersed longitudinally SW-ward with a progressive facies migration from marginal alluvial sediments toward slope deep-water and deep-sea turbidites. This migration path indicates an ocean basin that shrank toward the SW. The Balonggongga'er Formation in the western South Qilian belt represents the fill of a latest Ordovician-Silurian remnant ocean basin that separated the Oulongbuluke block from the Central Qilian block, and records Silurian closure of the Proto-Tethyan Ocean and subduction beneath the Central Qilian block. However, alluvial deposits in the Lajishan area were accumulated in a retro-foreland basin, indicating that continent-continent collision in the eastern South Qilian belt occurred at c. 450–440 Ma. These results demonstrate that the Proto-Tethyan Ocean closed diachronously during early Paleozoic time.
基金financially supported by the National Natural Science Foundation of China (41372207)China Geological Survey (12120115069401, DD20160201-04, DD20160022-02, and DD20160101)the China Scholarship Councilprogram (201408110143)
文摘Walegen Au deposit is closely correlated with granitic intrusions of Triassic age, which are composed of granite and quartz porphyries. Both granite porphyry and quartz porphyry consist of quartz, feldspar and muscovite as primary minerals. WeaMy peraluminous granite porphyry （A/ CNK=1.10-1.15） is enriched in LREE, depleted in HREE with Nb-Ta-Ti anomalies, and displays subduction-related geochemistry. Quartz porphyry is strongly peraluminous （A/CNK=1.64-2.81） with highly evolved components, characterized by lower TiO2, REE contents, Mg#, K/Rb, Nb/Ta, Zr/Hf ratios and higher Rb/Sr ratios than the granite porphyry. REE patterns of quartz porphyry exhibit lanthanide tetrad effect, resulting from mineral fractionation or participation of fluids with enriched F and CI. LA- ICP-MS zircon U-Pb dating indicates quartz porphyry formed at 233±3 Ma. The ages of relict zircons from Triassic magmatic rocks match well with the detrital zircons from regional area. In addition, εHf（t） values of Triassic magmatic zircons from the granite and quartz porphyries are -14.2 to -9.1 （with an exception of ＋4.1） and -10.8 to -8.6 respectively, indicating a crustal-dominant source. Regionally, numerous Middle Triassic granitoids were previously reported to be formed under the consumption of Paleotethyan Ocean. These facts indicate that the granitic porphyries from Walegen Au deposit may have been formed in the processes of the closing of Paleotethyan Ocean, which could correlate with the arc-related magmatism in the Kunlun orogen to the west and the Qinling orogen to the east.
基金supported by the China Geological Survey(Grant No.DD20221649)National Natural Science Foundation of China(Grant Nos.42230308,42072266)+3 种基金Bureau of Geological Exploration and Development of Qinghai Province(Grant No.[2022]32)the Xingdian Scholar Fund of Yunnan Province(Grant No.C6213001155)China Postdoctoral Science Foundation(Grant No.2021M691702)High-level Talents Project of Qinghai Province.
文摘Identification and anatomy of oceanic arcs within ancient orogenic belt are significant for better understanding the tectonic framework and closure process of paleo-ocean basin.This article summarizes the geological,geochemical,and geochronological characteristics of upper crust of Proto-Tethyan Lajishan intra-oceanic arc and provides new data to constrain the subduction evolution of the South Qilian Ocean.The intra-oceanic arc volcanic rocks,including intermediate-mafic lava,breccia,tuff,and minor felsic rocks,are distributed along southern part of the Lajishan ophiolite belt.Geochemical and isotopic compositions indicate that the intermediate-mafic lava were originated from depleted mantle contaminated by sediment melts or hydrous fluids,whereas the felsic rocks were likely generated by partial melting of juvenile mafic crust in intra-oceanic arc setting.Zircons from felsic rocks yield consistent and concordant ages ranging from 506 to 523 Ma,suggesting these volcanic rocks represent the relicts of upper crust of the Cambrian intra-oceanic arc.Combined with the Cambrian forearc ophiolite and accretionary complex,we suggest that the Cambrian intra-oceanic arc in the Lajishan ophiolite belt is belonging to the intra-oceanic arc system which was generated by south-directed subduction in the South Qilian Ocean at a relatively short interval between approximately 530 and 480 Ma.
基金granted by the National Natural Science Foundation of China(Grant No.41872241 and 41672221)
文摘The Qilian orogenic belt is the northernmost orogen of the Tethyan domain and connects the Altaids to the north. It contains an assembly of Precambrian micro-continental fragments, early Paleozoic island arcs, accretionary complexes, ophiolites, forearc and backarc basins, and high-pressure(HP) metamorphic rocks, indicating a long history of accretionary processes. Spatially, this orogen is adjacent to the Tarim, Qaidam, and North China blocks, which also extends into accretionary orogenic belts to the east and SW such as the Qinling and Kunlun belts. Abundant ophiolites in this orogen record the closure of an early Tethyan Ocean and amalgamations between micro-continents of North China, Qaidam, and Tarim. Thus, the ages and tectonic settings of these ophiolites within this belt provide important information regarding evolution of the Proto-Tethys Ocean and assembly of micro-continental blocks, which aids understanding of the spatial and temporal relationship of this orogen within the Tethyan realm. Dismembered ophiolites sporadically crop out along the northern margin of the South Qilian belt, and, from east to west, are locally referred to as the Lajishan, Gangcha, Muli, and Dadaoerji ophiolites. Much attention had been paid to these ophiolites, and several competing models for the tectonic evolution of this belt have been suggested. Considerable disagreement remains in respect of the temporal and spatial framework of the Qilian Orogen and details such as timing of subduction(s) and associated polarities, early collision events, and final closure of oceanic basins. In particular, the formation age and tectonic setting of Muli arc-ophiolite complex remains unknown, which limits understanding of the tectonics of the South Qilian belt and the history of the Proto-Tethys Ocean. The Muli arc-ophiolite complex is distributed over 20 km^2 west of the township of Muli in the western segment of the South Qilian Belt and consists of serpentinite, dunite, cumulate gabbro, basalt, plagiogranite, and chert. Field mapping results demonstrate that these units have been largely destroyed by faulting and generally occur as blocks/slices. They are tectonically interlayered with Upper Ordovician – Lower Silurian siliciclastic turbidite. Arc-ophiolite rocks are intruded by 470–450 Ma subduction-related granitoid plutons and are unconformably overlain by shallow marine to non-marine sediments of Permian-Jurassic age. Basalts show typical subduction-related calc-alkaline geochemical affinity, representing portions of an island arc. Geochemical results for plagiogranites and spinels from serpentinite demonstrate that the Muli arc-ophiolite complex represents a super-subduction zone(SSZ)-type ophiolite. U-Pb zircon data indicate formation associated with southward subduction of the Proto-Tethyan Ocean during a short interval between 539–522 Ma. Voluminous Late Ordovician-Early Silurian deep-water marine siliciclastic and volcaniclastic turbidites and volcanic arc rocks are exposed to the south of the Muli arc-ophiolite complex, whereas fluvial coarse-grained sandstones and conglomerates unconformably overlie the Cambrian-Middle Ordovician ophiolite-arc systems in the eastern South Qilian Belt. These indicate that closure of the Proto-Tethys Ocean was diachronous during the early Paleozoic.
