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 Lajishan Mountain fault zone consists of two NE_protruding arcuate faults, i.e. the northern and southern margin fault of Lajishan Mountain with the fault length of 230km and 220km respectively. The fault zone is ...The Lajishan Mountain fault zone consists of two NE_protruding arcuate faults, i.e. the northern and southern margin fault of Lajishan Mountain with the fault length of 230km and 220km respectively. The fault zone is located in the large_scale compressional structure zone and tectonic gradient zone in_between the NNW_trending right_lateral strike_slip Reshui_Riyueshan fault zone and the NWW_trending left_lateral strike_slip northern margin of west Qinling Ranges fault zone is also an important boundary fault zone, separating the Xining_Minhe basin and the Xunhua_Hualong basin at the southern and northern sides of the Lajishan Mountain respectively. Geologic geomorphic evidences of new activity revealed by field investigations indicate that the latest movement of the Lajishan fault zone was in late Epipleistocene (only a few segments were active in early Holocene) and is mainly of compressive thrusting with slightly left_lateral strike_slip component. The above movement has possibly resulted in the occurrence of about 20 moderate earthquakes of magnitude around 5.0. The Lajishan region can therefore be regarded as a seismotectonic window to reflect tectonic movement and earthquake activity.展开更多
The Qilian orogenic belt,as an important component of the central orogenic system,can be divided into three tectonic units from north to south:the North Qilian tectonic belt,the Central Qilian tectonic belt and the So...The Qilian orogenic belt,as an important component of the central orogenic system,can be divided into three tectonic units from north to south:the North Qilian tectonic belt,the Central Qilian tectonic belt and the South Qilian tectonic belt.The Lajishan ophiolitic mélange zone is an important part of the South Qilian tectonic belt.The study of the formation age and tectonic attributes of the different rock units in the Lajishan is of great significance for reconstructing the tectonic evolution of the south Qilian tectonics belt and the Proto-Tethys Ocean.The Ayishan formation is an important unit of the Lajishan mélange belt,but its stratigraphic age and formation environment are still not constrained,which limits the reconstruction of the early Palaeozoic tectonic evolution of the South Qilian tectonic belt and the closure time of the Proto-Tethys Ocean.In recent years,a set of rock assemblages consisting of andesite,rhyolite,sandstone and pebbly sandstone have been identified in the Ayishan formation exposed in the Lajishan through detailed geological mapping in the field.They are in fault contact with the underlying ophiolite mélange.Regionally,a volcano-sedimentary succession consisting of andesite,rhyolite,volcanic breccia,and volcaniclastic sandstone nonconformably overlies the Cambrian arc-accretionarycomplex system in the Ayishan area.These rocks were originally assigned to the Ayishan formation and attributed an Early Ordovician age(BGMRQP,1964,1991).Our rhyolite samples from the Ayishan formation yielded a weighted mean 206Pb/238U age of 447 Ma using zircon U-Pb dating,which can be interpreted as the eruption age of the volcanic rocks.These volcanic rock assemblages are distributed in sandstone,pebbled sandstone and conglomerate in the form of interlayers.Therefore,the zircon UPb age of the volcanic rocks can constrain the age of Ayishan formation that it should belong to the late Ordovician rather than early Ordovician.Additionally,we propose that the Ayishan rhyolites were formed in collision-rela展开更多
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.
文摘The Lajishan Mountain fault zone consists of two NE_protruding arcuate faults, i.e. the northern and southern margin fault of Lajishan Mountain with the fault length of 230km and 220km respectively. The fault zone is located in the large_scale compressional structure zone and tectonic gradient zone in_between the NNW_trending right_lateral strike_slip Reshui_Riyueshan fault zone and the NWW_trending left_lateral strike_slip northern margin of west Qinling Ranges fault zone is also an important boundary fault zone, separating the Xining_Minhe basin and the Xunhua_Hualong basin at the southern and northern sides of the Lajishan Mountain respectively. Geologic geomorphic evidences of new activity revealed by field investigations indicate that the latest movement of the Lajishan fault zone was in late Epipleistocene (only a few segments were active in early Holocene) and is mainly of compressive thrusting with slightly left_lateral strike_slip component. The above movement has possibly resulted in the occurrence of about 20 moderate earthquakes of magnitude around 5.0. The Lajishan region can therefore be regarded as a seismotectonic window to reflect tectonic movement and earthquake activity.
基金granted by the National Natural Science Foundation of China(Grant Nos.41272221,41772228,41702239)the Geological Survey Program of China(Grant Nos.1212011120159,DD20160201-04)
文摘The Qilian orogenic belt,as an important component of the central orogenic system,can be divided into three tectonic units from north to south:the North Qilian tectonic belt,the Central Qilian tectonic belt and the South Qilian tectonic belt.The Lajishan ophiolitic mélange zone is an important part of the South Qilian tectonic belt.The study of the formation age and tectonic attributes of the different rock units in the Lajishan is of great significance for reconstructing the tectonic evolution of the south Qilian tectonics belt and the Proto-Tethys Ocean.The Ayishan formation is an important unit of the Lajishan mélange belt,but its stratigraphic age and formation environment are still not constrained,which limits the reconstruction of the early Palaeozoic tectonic evolution of the South Qilian tectonic belt and the closure time of the Proto-Tethys Ocean.In recent years,a set of rock assemblages consisting of andesite,rhyolite,sandstone and pebbly sandstone have been identified in the Ayishan formation exposed in the Lajishan through detailed geological mapping in the field.They are in fault contact with the underlying ophiolite mélange.Regionally,a volcano-sedimentary succession consisting of andesite,rhyolite,volcanic breccia,and volcaniclastic sandstone nonconformably overlies the Cambrian arc-accretionarycomplex system in the Ayishan area.These rocks were originally assigned to the Ayishan formation and attributed an Early Ordovician age(BGMRQP,1964,1991).Our rhyolite samples from the Ayishan formation yielded a weighted mean 206Pb/238U age of 447 Ma using zircon U-Pb dating,which can be interpreted as the eruption age of the volcanic rocks.These volcanic rock assemblages are distributed in sandstone,pebbled sandstone and conglomerate in the form of interlayers.Therefore,the zircon UPb age of the volcanic rocks can constrain the age of Ayishan formation that it should belong to the late Ordovician rather than early Ordovician.Additionally,we propose that the Ayishan rhyolites were formed in collision-rela
