The garnet amphibolites from the newly identified Wanhe ophiolitic mélange in the Changning-Menglian suture zone(CMSZ)provide a probe to elucidate the evolution of the Triassic Palaeo-Tethys.An integrated petrolo...The garnet amphibolites from the newly identified Wanhe ophiolitic mélange in the Changning-Menglian suture zone(CMSZ)provide a probe to elucidate the evolution of the Triassic Palaeo-Tethys.An integrated petrologic,phase equilibria modeling and geochronological study of the garnet amphibolites,southeast Tibetan Plateau,shows that the garnet amphibolites have a peak mineral assemblage of garnet,glaucophane,lawsonite,chlorite,rutile,phengite and quartz,and a clockwise P-T path with a prograde segment from blueschist-facies to eclogite-facies with a peak-metamorphic P-T conditions of 2000–2100 MPa and 495–515℃,indicating a cold geothermal gradient of about 240–260℃/GPa.Theretrograde metamorphic P-T path is characterized by nearly isothermal decompression to lower amphibolite-facies and subsequent cooling to greenschist-facies.The metamorphic zircons have fractionated HREE patterns and significant negative Eu anomalies,and therefore the obtained zircon U-Pb age of 231±1.5 Ma is interpreted to be the timing of the amphibolite facies metamorphism occurrence.The present study probably indicates that the garnet amphibolites in the Wanhe ophiolitic mélange was the retrograded highpressure eclogite-facies blueschist,instead of the previously proposed eclogites,and the garnet amphibolites recorded the subduction and exhumation process of the Palaeo-Tethys Oceanic crust in the Triassic.展开更多
Discontinuous ophiolite suites along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet representing remnants of the Neo-Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Creta...Discontinuous ophiolite suites along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet representing remnants of the Neo-Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Cretaceous.Older ophiolite or ophiolitic mélange(e.g.Triassic)to reveal the initial evolution the Neo-Tethyan ocean within the YZSZ have rarely been documented so far.The western YZSZ extending from the Saga to Ladakh area are composed of the northern ophiolitic subbelt,the Zhongba terrane and the southern ophiolitic sub-belt.In this study,we document structural,petrological and geochronological data of mafic intrusions from the Mayoumu massif in the southern ophiolitic sub-belt of the western YZSZ.Two lithos-tectonic sub-units,the southern ophiolitic complex and the northern ophiolitic mélange,are recognized in the Mayoumu massif in terms of structural pattern and petrology.LA-ICP-MS zircon U-Pb dating of gabbro from the ophiolitic mélange yields an age of 243±1 Ma with zirconεHf(t)values of+7.9 to+13.2.Two samples of diabase samples from the ophiolite yield ages of 131±1 Ma and 124±1 Ma with zirconεHf(t)values ranging from+10.8 to+15.0 and+12.3 to+15.4,respectively.Geochemically,rare earth elements(REEs)patterns show that these mafic intrusions are similar to those of the normal mid-ocean-ridge basalt(N-MORB).Enrichment of fluidmobile elements(e.g.Rb and Ba)and depletion of Nb,and Ta suggest that these intrusions were possibly originated from melting of a depleted mantle source influenced by subducted slab.Our data strongly confirms that the Neo-Tethyan ocean between the Lhasa terrane and Gondwana had been existed since the Middle Triassic at least.The ages of the two diabases from the ophiolitic complex reveal that mafic magma activities within the Neo-Tethyan ocean could be as a continuing process during 120–130 Ma.Emplacement and preservation of older ophiolitic recorder during evolution of the Neo-Tethyan Ocean may be closely related to the occurrence of the Zhongba micro-terrane within the western YZSZ.展开更多
The evolution and final closure of the Neo-Tethys Ocean are one of the most important geological events that have occurred on Earth since the Mesozoic.However,the evolution of the Neo-Tethys is not well constrained,in...The evolution and final closure of the Neo-Tethys Ocean are one of the most important geological events that have occurred on Earth since the Mesozoic.However,the evolution of the Neo-Tethys is not well constrained,in particular whether its opening occurred in the Permian or the Triassic and whether a plume was involved with its opening or not.In this study,we present geochronological and geochemical data for mafic igneous rocks in mélanges along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet to constrain the timing and mechanism of opening the Neo-Tethys Ocean.Based on field observations,the YZSZ mélanges can be divided into three segments.The western(west of Zhongba)and eastern(Sangsang-Renbu)segments are composed of ocean plate stratigraphy representing accretionary complexes that formed during subduction of Neo-Tethyan oceanic lithosphere beneath the southern margin of the Asian continent.Mélanges in the central segment(Zhongba-eastern Saga)typically have a siliciclastic matrix,and represent Tethyan Himalayan strata that were structurally mixed with the southern margin of the Asian continent.Based on our and previously published geochemical data,the mafic rocks in the YZSZ mélanges are ocean island basalt(OIB)-like,with ages in the Late Permian-Middle Triassic,the Middle-Late Jurassic,and the Early Cretaceous,respectively.An OIB-like block with an age of ca.253 Ma is identified from the Zhongba mélanges in the western segment,and it is the oldest OIB lithology yet identified in the YZSZ mélanges related to the evolution of the Neo-Tethys Ocean.Geochemical features indicate that this OIB-like block is distinct from typical OIBs and would be formed during continental rifting to incipient seafloor spreading.In the framework of plate divergent-convergent coupling systems and based on literature data for early Middle Triassic seamounts,radiolarian cherts,and normal mid-ocean ridge basalt-like oceanic crust,we conclude that opening of the Yarlung Zangbo Neo-Tethys Ocean would mainly occur at~250–243 Ma in the Early Triassic,not later than the early phase of Middle Triassic.In addition,a mantle plume was not involved in opening the Yarlung Zangbo Neo-Tethys Ocean.On the other hand,we have also identified a suite of ca.160 Ma OIB-like basaltic sills from the Bainang mélanges in the eastern segment,which is the same age as the OIB lithologies previously reported in the Zhongba mélanges.Based on the sill-like occurrence and absence of plume-related rock associations in this region,the Bainang OIB-like rocks might result from Middle-Late Jurassic continental rifting in northern Gondwana.Magmatism related to this tectonic event is preserved in both the YZSZ mélanges and Himalayan strata,but its tectonic significance requires further investigation.Based on this study of the YZSZ mélanges and the previous studies of YZSZ ophiolites,Gangdese belt igneous rocks,and sedimentary rocks,we have reconstructed the entire Wilson Cycle of the Yarlung Zangbo Neo-Tethys Ocean,mainly involving continental rifting and ocean opening,subduction initiation,ultraslow-spreading ridge-trench conversion,subduction re-initiation,and oceanic closure and initial India-Asia collision for the tectonic emplacement of ophiolites.These processes were associated not only with magmatic flare-ups and lulls in the Gangdese belt but also with two stages of ophiolite obduction.Our data therefore provide new insights into the evolution of the Yarlung Zangbo Neo-Tethys Ocean and related Tethyan geodynamics.展开更多
基金supported by the National Natural Science Foundation of China(41802071,41773026 and 41303028)the Geological Survey of China-Nepal railway(202008000000180117).
文摘The garnet amphibolites from the newly identified Wanhe ophiolitic mélange in the Changning-Menglian suture zone(CMSZ)provide a probe to elucidate the evolution of the Triassic Palaeo-Tethys.An integrated petrologic,phase equilibria modeling and geochronological study of the garnet amphibolites,southeast Tibetan Plateau,shows that the garnet amphibolites have a peak mineral assemblage of garnet,glaucophane,lawsonite,chlorite,rutile,phengite and quartz,and a clockwise P-T path with a prograde segment from blueschist-facies to eclogite-facies with a peak-metamorphic P-T conditions of 2000–2100 MPa and 495–515℃,indicating a cold geothermal gradient of about 240–260℃/GPa.Theretrograde metamorphic P-T path is characterized by nearly isothermal decompression to lower amphibolite-facies and subsequent cooling to greenschist-facies.The metamorphic zircons have fractionated HREE patterns and significant negative Eu anomalies,and therefore the obtained zircon U-Pb age of 231±1.5 Ma is interpreted to be the timing of the amphibolite facies metamorphism occurrence.The present study probably indicates that the garnet amphibolites in the Wanhe ophiolitic mélange was the retrograded highpressure eclogite-facies blueschist,instead of the previously proposed eclogites,and the garnet amphibolites recorded the subduction and exhumation process of the Palaeo-Tethys Oceanic crust in the Triassic.
基金granted by the National Key Basic Research Program from the Chinese Ministry of Science and Technology(Grant No.2015CB856101)Foundation of Research Center for Tibetan Plateau Geology(21201010000150014-29)the China Geological Survey(No.1212011221066)
文摘Discontinuous ophiolite suites along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet representing remnants of the Neo-Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Cretaceous.Older ophiolite or ophiolitic mélange(e.g.Triassic)to reveal the initial evolution the Neo-Tethyan ocean within the YZSZ have rarely been documented so far.The western YZSZ extending from the Saga to Ladakh area are composed of the northern ophiolitic subbelt,the Zhongba terrane and the southern ophiolitic sub-belt.In this study,we document structural,petrological and geochronological data of mafic intrusions from the Mayoumu massif in the southern ophiolitic sub-belt of the western YZSZ.Two lithos-tectonic sub-units,the southern ophiolitic complex and the northern ophiolitic mélange,are recognized in the Mayoumu massif in terms of structural pattern and petrology.LA-ICP-MS zircon U-Pb dating of gabbro from the ophiolitic mélange yields an age of 243±1 Ma with zirconεHf(t)values of+7.9 to+13.2.Two samples of diabase samples from the ophiolite yield ages of 131±1 Ma and 124±1 Ma with zirconεHf(t)values ranging from+10.8 to+15.0 and+12.3 to+15.4,respectively.Geochemically,rare earth elements(REEs)patterns show that these mafic intrusions are similar to those of the normal mid-ocean-ridge basalt(N-MORB).Enrichment of fluidmobile elements(e.g.Rb and Ba)and depletion of Nb,and Ta suggest that these intrusions were possibly originated from melting of a depleted mantle source influenced by subducted slab.Our data strongly confirms that the Neo-Tethyan ocean between the Lhasa terrane and Gondwana had been existed since the Middle Triassic at least.The ages of the two diabases from the ophiolitic complex reveal that mafic magma activities within the Neo-Tethyan ocean could be as a continuing process during 120–130 Ma.Emplacement and preservation of older ophiolitic recorder during evolution of the Neo-Tethyan Ocean may be closely related to the occurrence of the Zhongba micro-terrane within the western YZSZ.
基金supported by the National Natural Science Foundation of China (Grant Nos.42025201,92155001,41802062,41888101)the Second Tibetan Plateau Scientific Expedition and Research Program (Grant No.2019QZKK0801)。
文摘The evolution and final closure of the Neo-Tethys Ocean are one of the most important geological events that have occurred on Earth since the Mesozoic.However,the evolution of the Neo-Tethys is not well constrained,in particular whether its opening occurred in the Permian or the Triassic and whether a plume was involved with its opening or not.In this study,we present geochronological and geochemical data for mafic igneous rocks in mélanges along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet to constrain the timing and mechanism of opening the Neo-Tethys Ocean.Based on field observations,the YZSZ mélanges can be divided into three segments.The western(west of Zhongba)and eastern(Sangsang-Renbu)segments are composed of ocean plate stratigraphy representing accretionary complexes that formed during subduction of Neo-Tethyan oceanic lithosphere beneath the southern margin of the Asian continent.Mélanges in the central segment(Zhongba-eastern Saga)typically have a siliciclastic matrix,and represent Tethyan Himalayan strata that were structurally mixed with the southern margin of the Asian continent.Based on our and previously published geochemical data,the mafic rocks in the YZSZ mélanges are ocean island basalt(OIB)-like,with ages in the Late Permian-Middle Triassic,the Middle-Late Jurassic,and the Early Cretaceous,respectively.An OIB-like block with an age of ca.253 Ma is identified from the Zhongba mélanges in the western segment,and it is the oldest OIB lithology yet identified in the YZSZ mélanges related to the evolution of the Neo-Tethys Ocean.Geochemical features indicate that this OIB-like block is distinct from typical OIBs and would be formed during continental rifting to incipient seafloor spreading.In the framework of plate divergent-convergent coupling systems and based on literature data for early Middle Triassic seamounts,radiolarian cherts,and normal mid-ocean ridge basalt-like oceanic crust,we conclude that opening of the Yarlung Zangbo Neo-Tethys Ocean would mainly occur at~250–243 Ma in the Early Triassic,not later than the early phase of Middle Triassic.In addition,a mantle plume was not involved in opening the Yarlung Zangbo Neo-Tethys Ocean.On the other hand,we have also identified a suite of ca.160 Ma OIB-like basaltic sills from the Bainang mélanges in the eastern segment,which is the same age as the OIB lithologies previously reported in the Zhongba mélanges.Based on the sill-like occurrence and absence of plume-related rock associations in this region,the Bainang OIB-like rocks might result from Middle-Late Jurassic continental rifting in northern Gondwana.Magmatism related to this tectonic event is preserved in both the YZSZ mélanges and Himalayan strata,but its tectonic significance requires further investigation.Based on this study of the YZSZ mélanges and the previous studies of YZSZ ophiolites,Gangdese belt igneous rocks,and sedimentary rocks,we have reconstructed the entire Wilson Cycle of the Yarlung Zangbo Neo-Tethys Ocean,mainly involving continental rifting and ocean opening,subduction initiation,ultraslow-spreading ridge-trench conversion,subduction re-initiation,and oceanic closure and initial India-Asia collision for the tectonic emplacement of ophiolites.These processes were associated not only with magmatic flare-ups and lulls in the Gangdese belt but also with two stages of ophiolite obduction.Our data therefore provide new insights into the evolution of the Yarlung Zangbo Neo-Tethys Ocean and related Tethyan geodynamics.
