Ophiolites represent on-land fragments of paleooceanic crust and have been recognized as one of the key markers of suture zones.Here,we provide new insights into the emplacement of ophiolitic mélanges based on de...Ophiolites represent on-land fragments of paleooceanic crust and have been recognized as one of the key markers of suture zones.Here,we provide new insights into the emplacement of ophiolitic mélanges based on detailed geological mapping and structural analysis in the West Junggar and Songpan-Ganzi-Bayan Har orogens(Fig.1 and Fig.2).The results show that some ophiolitic mélange belts cannot be regarded as suture zones.The distribution of these ophiolitic mélange belts are usually associated with the structural processes during the closure of remnant oceanic basins.After the remnant-oceanic basin is filled with thick clastic deposit,the oceanic lithosphere material as the base of the remnant basin can be injected into the overlying sedimentary strata through various faultings under the regional compressive stress,forming the remnant oceanic basintype ophiolitic mélange system with dispersive distribution characteristics.Combining with previous researches,the emplacement mechanism of ophiolitic mélanges can be divided into four categories:subduction type which oceanic lithosphere subducted beneath active continental margin,obduction type which oceanic lithosphere obducted over passive continental margin,collision type between two continental lithospheres,and closure type of remnant-oceanic basin(Fig.3).These different types of ophiolitic mélange belts will be superimposed and even re-emplacement by the tectonic processes of post-plate convergence,complicating their distribution.Therefore,identifying the emplacement mechanism type of ophiolitic mélange belts formed in different tectonic processes and backgrounds is of importance for understanding the process of ocean-continental transition and the evolution of orogenic belts.展开更多
The Nadingcuo high-K calc-alkaline rocks mainly composed of trachyte and trachyandesite are the largest outcrop area of volcanic rocks in southern Qiangtang terrane in the Tibetan plateau. However,their exact source a...The Nadingcuo high-K calc-alkaline rocks mainly composed of trachyte and trachyandesite are the largest outcrop area of volcanic rocks in southern Qiangtang terrane in the Tibetan plateau. However,their exact source and peterogenesis are still debated.^（40）Ar-^（39）Ar and LAM-ICPMS zircon U-Pb isotopic dating confirm that these rocks erupted in Eocene.In addition,the Nadingcuo volcanic rocks are characterized by high Sr/Y content ratios,similar with the adakite derived from partial melting of oceanic crust.They can be further classified as high Mg~#（Mg~#=48-57） and low Mg~# （Mg~#=33-42） subtypes.The Nadingcuo adakitic rocks have relatively low（^（87）Sr/^（86）Sr）_i and highε_（Nd）（t）, showing a trend of similarity to the Dongcuo ophiolite present in the Bangong-Nujiang oceanic crust. Simple modeling indicates that the Nadingcuo adakitic rocks are a mix resulting from the basalt of Bangong-Nujiang Ocean with 10%-20%crustal material of Lhasa terrane.On these bases we suggest that the low Mg~# Nadingcuo adakitic rocks are the product of partial melting of remnant oceanic crust with small sediment,and the high Mg~# rocks are the result of reaction between rising melt of remnant oceanic crust with subducted sediment and mantle wedge.Therefore,the origin of Nadingcuo adakitic rocks may be related to intracontinental subduction triggered by collision of India-Asia during Cenozoic.展开更多
Remnant ocean basin is a key to understand the plate suturing and subsequent uplift and erosion of orogen. The Bay of Bengal Basin （BOBB） provides a typical example to analyze the remnant ocean basin structures, evo...Remnant ocean basin is a key to understand the plate suturing and subsequent uplift and erosion of orogen. The Bay of Bengal Basin （BOBB） provides a typical example to analyze the remnant ocean basin structures, evolution, and relationships between depositional filling and uplifting of the Himalayan Orogen. Thirty-nine seismic profiles as well as interval velocities of well BODC3 were used to compile isopach maps of the basin. Among the seismic data, 26 seismic profiles were applied to estab- lish 8 cross sections. The cross sections suggest the basin is asymmetric, bounded to the west by the eastern continental margin of India （ECMI） with graben-horst and to the east by the Sunda conver- gence margin dominated by trench-arc system. The BOBB is characterized by a prominent down flex- ure structures caused by huge amount of Bengal fan turbidite sediments accumulation. Our isopach maps and chronology data collected from adjacent regions reveal the initial development and fast southward growth of the Bengal fan were related to the early and major stage uplift and erosion of the Himalayan Orogen, respectively. The BOBB has experienced a critical transition from an ocean basin to a remnant ocean basin at Late Oligocene. Such basin structures and evolution features indicate the BOBB provides whole records of oblique convergence of the India and Asia plates, and the early and major stage evolution of the Himalayan Orogen.展开更多
Located in the southern Qinling Mountains of central China, the Guanjiagou Formation has been a con-troversial issue with regard to its depositional age and tec-tonic implications. Being comprised of an approximately ...Located in the southern Qinling Mountains of central China, the Guanjiagou Formation has been a con-troversial issue with regard to its depositional age and tec-tonic implications. Being comprised of an approximately 2050 m thick succession of texturally and compositionally immature, presumed marine turbiditic sandstones and con-glomerate, the Guanjiagou Formation consists of an overall prograding- and coarsening-upward megasequence. Al-though bounded by faults on both its northern and southern margins, it is weakly metamorphosed and deformed. To the north is the Devonian Sanhekou Group and to the south is the Neoproterozoic Hengdan Group. The lower portion of the sequence contains granitic and volcanic clasts (Guanjiagou conglomerate). The feldspars from these clasts were dated using the 40Ar/39Ar method. Two cooling ages of 219.690.49 and 216.460.59 Ma, for K-feldspar from a granitic clast and plagioclase from a volcanic clast, respectively, were obtained. These ages are identical to the time of regional igneous ac-tivities (ca. 240—220 Ma) and are interpreted as the prod-ucts of magmatism associated with collision in the Qinling orogenic belt in the Early Mesozoic, suggesting that the Guanjiagou Formation was deposited in the Norian of the Late Triassic, ca. 220 Ma. Therefore, 40Ar/39Ar and sedimen-tary analyses suggest that the Guanjiagou Formation con-tains sediments that may have filled in a remnant ocean ba-sin, which might be part of the Anyemaqen-Mianle ocean, or Tethys on the southern side of Central Orogenic Belt in China during the Late Paleozoic to Early Mesozoic.展开更多
基金granted by the China Geological Survey(Grand No.1212011220245,DD20179607,DD20160060)
文摘Ophiolites represent on-land fragments of paleooceanic crust and have been recognized as one of the key markers of suture zones.Here,we provide new insights into the emplacement of ophiolitic mélanges based on detailed geological mapping and structural analysis in the West Junggar and Songpan-Ganzi-Bayan Har orogens(Fig.1 and Fig.2).The results show that some ophiolitic mélange belts cannot be regarded as suture zones.The distribution of these ophiolitic mélange belts are usually associated with the structural processes during the closure of remnant oceanic basins.After the remnant-oceanic basin is filled with thick clastic deposit,the oceanic lithosphere material as the base of the remnant basin can be injected into the overlying sedimentary strata through various faultings under the regional compressive stress,forming the remnant oceanic basintype ophiolitic mélange system with dispersive distribution characteristics.Combining with previous researches,the emplacement mechanism of ophiolitic mélanges can be divided into four categories:subduction type which oceanic lithosphere subducted beneath active continental margin,obduction type which oceanic lithosphere obducted over passive continental margin,collision type between two continental lithospheres,and closure type of remnant-oceanic basin(Fig.3).These different types of ophiolitic mélange belts will be superimposed and even re-emplacement by the tectonic processes of post-plate convergence,complicating their distribution.Therefore,identifying the emplacement mechanism type of ophiolitic mélange belts formed in different tectonic processes and backgrounds is of importance for understanding the process of ocean-continental transition and the evolution of orogenic belts.
基金supported by the following projects:National Basic Research Program of China (2009CB421004,2009CB421003)Natural Science Foundation of China(41073033,40872055,and 40930316)+1 种基金Chinese Academy of Sciences(KZCX2-YW-Q04)China Geological Survey(1212010818098)
文摘The Nadingcuo high-K calc-alkaline rocks mainly composed of trachyte and trachyandesite are the largest outcrop area of volcanic rocks in southern Qiangtang terrane in the Tibetan plateau. However,their exact source and peterogenesis are still debated.^（40）Ar-^（39）Ar and LAM-ICPMS zircon U-Pb isotopic dating confirm that these rocks erupted in Eocene.In addition,the Nadingcuo volcanic rocks are characterized by high Sr/Y content ratios,similar with the adakite derived from partial melting of oceanic crust.They can be further classified as high Mg~#（Mg~#=48-57） and low Mg~# （Mg~#=33-42） subtypes.The Nadingcuo adakitic rocks have relatively low（^（87）Sr/^（86）Sr）_i and highε_（Nd）（t）, showing a trend of similarity to the Dongcuo ophiolite present in the Bangong-Nujiang oceanic crust. Simple modeling indicates that the Nadingcuo adakitic rocks are a mix resulting from the basalt of Bangong-Nujiang Ocean with 10%-20%crustal material of Lhasa terrane.On these bases we suggest that the low Mg~# Nadingcuo adakitic rocks are the product of partial melting of remnant oceanic crust with small sediment,and the high Mg~# rocks are the result of reaction between rising melt of remnant oceanic crust with subducted sediment and mantle wedge.Therefore,the origin of Nadingcuo adakitic rocks may be related to intracontinental subduction triggered by collision of India-Asia during Cenozoic.
基金financially supported by the National Science and Technology Major Project of China(No.2011ZX05030-002-003)
文摘Remnant ocean basin is a key to understand the plate suturing and subsequent uplift and erosion of orogen. The Bay of Bengal Basin （BOBB） provides a typical example to analyze the remnant ocean basin structures, evolution, and relationships between depositional filling and uplifting of the Himalayan Orogen. Thirty-nine seismic profiles as well as interval velocities of well BODC3 were used to compile isopach maps of the basin. Among the seismic data, 26 seismic profiles were applied to estab- lish 8 cross sections. The cross sections suggest the basin is asymmetric, bounded to the west by the eastern continental margin of India （ECMI） with graben-horst and to the east by the Sunda conver- gence margin dominated by trench-arc system. The BOBB is characterized by a prominent down flex- ure structures caused by huge amount of Bengal fan turbidite sediments accumulation. Our isopach maps and chronology data collected from adjacent regions reveal the initial development and fast southward growth of the Bengal fan were related to the early and major stage uplift and erosion of the Himalayan Orogen, respectively. The BOBB has experienced a critical transition from an ocean basin to a remnant ocean basin at Late Oligocene. Such basin structures and evolution features indicate the BOBB provides whole records of oblique convergence of the India and Asia plates, and the early and major stage evolution of the Himalayan Orogen.
文摘Located in the southern Qinling Mountains of central China, the Guanjiagou Formation has been a con-troversial issue with regard to its depositional age and tec-tonic implications. Being comprised of an approximately 2050 m thick succession of texturally and compositionally immature, presumed marine turbiditic sandstones and con-glomerate, the Guanjiagou Formation consists of an overall prograding- and coarsening-upward megasequence. Al-though bounded by faults on both its northern and southern margins, it is weakly metamorphosed and deformed. To the north is the Devonian Sanhekou Group and to the south is the Neoproterozoic Hengdan Group. The lower portion of the sequence contains granitic and volcanic clasts (Guanjiagou conglomerate). The feldspars from these clasts were dated using the 40Ar/39Ar method. Two cooling ages of 219.690.49 and 216.460.59 Ma, for K-feldspar from a granitic clast and plagioclase from a volcanic clast, respectively, were obtained. These ages are identical to the time of regional igneous ac-tivities (ca. 240—220 Ma) and are interpreted as the prod-ucts of magmatism associated with collision in the Qinling orogenic belt in the Early Mesozoic, suggesting that the Guanjiagou Formation was deposited in the Norian of the Late Triassic, ca. 220 Ma. Therefore, 40Ar/39Ar and sedimen-tary analyses suggest that the Guanjiagou Formation con-tains sediments that may have filled in a remnant ocean ba-sin, which might be part of the Anyemaqen-Mianle ocean, or Tethys on the southern side of Central Orogenic Belt in China during the Late Paleozoic to Early Mesozoic.
