A huge thrust system, the North Lhasa Thrust (NLT), was discovered in the northern Lhasa block of the Tibetan Plateau based on geological mapping of the Damxung region and its vicinity, the Deqen-Lunpola traverse and ...A huge thrust system, the North Lhasa Thrust (NLT), was discovered in the northern Lhasa block of the Tibetan Plateau based on geological mapping of the Damxung region and its vicinity, the Deqen-Lunpola traverse and the Amdo-Bam Co profile. The NLT consists of the Dongqiao-Lunpola thrust (DLT), the west Namco thrust (WNT) and the south Damxung thrust (SDT) and ductile shear zones, ophiolite slices and folds extending in a WNW direction. Major thrust faults of the NLT seem to merge into a single deep-seated detachment of the upper-crust and totally displaced southward as far as 100-120 km. Chronological analyses with 39Ar-40Ar of plagioclase and hornblende, Rb-Sr isochron of minerals and fission-tracks of apatite from mylonite within the WNT yield ages of 174-173 Ma, 109 Ma and 44 Ma, showing 3 periods of thrusting in the north Lhasa block caused by subduction of the Tethys oceanic plate and the India-Eurasia continental collision respectively.展开更多
The Qinghai–Tibet plateau is a composite continental fragment formed by collision of multiple terranes and island arcs.The Lhasa terrane,which is located in the central part of the plateau,is bounded by the Yarlung–...The Qinghai–Tibet plateau is a composite continental fragment formed by collision of multiple terranes and island arcs.The Lhasa terrane,which is located in the central part of the plateau,is bounded by the Yarlung–Zangbo suture to the south and Bangong-Nujiang suture to the north.An E–W–trending belt of(ultra)-high pressure eclogite was discovered in the Sumdo region of the Lhasa terrane.Careful field studies combined with petrological,geochemical and isotopic analyses show that the Sumdo eclogites mark a Carboniferous–Permian suture zone,at least 100 km long,containing ophiolite fragments,eclogites and Indosinian post–orogenic granitoids.This suture divides the Lhasa block into a northern and southern segment.Sumdo eclogite occurs about 200 km east of Lhasa city,and extends over 100km in an E–W direction.Sumdo eclogites were accompanied by garnet amphibolite and plagioclaseamphiboliteformedbyretrograde metamorphism of the eclogites.The eclogites were derived from oceanic basalts.LA–ICPMS U–Pb dating of zircon from the Sumdo eclogites indicates a Permian metamorphic age(260–270 Ma)and a Carboniferous protolith age of 303±4.8 MaThe ophiolite fragments in the Sumdo suture zone are composed of the ultramafic rocks,MORB–type basalt,OIB–type basalt and island arc basaltic andesite,some of which are intruded by post-collisional granites.The ultramafic body,a typical tectonic block in the suture zone,is completely serpentinized.Its geochemical features suggest that it is composed of harzburgite,typical of depleted mantle peridotite.The MORB and OIB–type basalts crop out in the Chasagang Formation,and the basaltic andesite crops out in the Leilongku Formation,both of which make up the Sumdo Group.Zircons from the OIB–type basalt with typical magmatic characteristics yield an average U–Pb age of306(95%)Ma,suggesting formation in a Paleo–Tethyan basin in the Carboniferous.U–Pb dating of zircon from the basaltic andesites yielded a concordant age of 265±3.1Ma,similar to the metamorphic age(266–270 Ma)of the eclogites,suggesting formation during subduction of the oceanic crust.Indosinian granodiorite with an age of194±4.3 Ma crops out north of the Sumdo suture.These granodiorites are similar to the late Indochina granites in the Lhasa block,and most likely formed during continent-arc collision or during closure of the Paleo-Tethyan Ocean.A four–stage model for the evolution of the Sumdo Paleo–Tethyan suture is proposed:1)From the Ordovician to the Devonian Gondwanaland was part of an epicontinental sea;2)In the Carboniferous,continental rifting produced a local basin,which then evolved into a Paleo-Tethyan ocean basin,dividing the Lhasa block into two segments;3)From the Permian to the early Triassic,Paleo-Tethyan oceanic crust was subducted northward and;4)In the middle Triassic and early Jurassic,the two fragments collided to form the modern Lhasa block.展开更多
Asthemaintectoniccomponentofthe Himalayan–Tibetan orogen,the Lhasa terrane has received much attention as it records the entire history of the orogeny.The occurrence of high pressure eclogite in the Sumdo complex in ...Asthemaintectoniccomponentofthe Himalayan–Tibetan orogen,the Lhasa terrane has received much attention as it records the entire history of the orogeny.The occurrence of high pressure eclogite in the Sumdo complex in central Lhasa terrane has a significant importance on the understanding of the Paleo-Tethys subduction and plate itineration processes in this area.The petrological,geochemical and geochronological data of eclogite and associated blueschist and garnet-bearing mica schist from Sumdo,Jilang and Bailang area have been briefly reviewed to explore the origin and metamorphic evolution of this suture.Eclogites from the Sumdo complex have experienced low temperature,high pressure to ultrahigh pressure metamorphism,revealing a fastsubduction and exhumation process in a typical oceanic subduction zone.The large P-T range between different eclogites in the literature may be affected by the big error of unappropriated using geothermobarometry and may also because of slices of subducted blocks derived from different depths juxtapose together during exhumation.By summarizing the U-Pb,Lu-Hf and Sm-Nd ages of eclogites,the eclogite facies metamorphism is likely to occur in early Triassic during 245-225 Ma,but not the previously accepted late Permian at ca.260 Ma by the reinterpretation of the former geochronological data from literature.The opening of Paleo-Tethys Ocean between the Lhasa terrane initiate prior to ca.280 Ma and ultimate closure to integrate the Lhasa terrane was no earlier than225 Ma and may triggered by the initial subduction of Bangong-Nujiang Tethys Ocean in the north.展开更多
Recently, an over 100 km long MORB-type eclogite belt of Permian was discovered in the Sumdo （松多） region of the Lhasa block, Tibet. A critical question thus is： what is the tectonic setting of the eelogite belt a...Recently, an over 100 km long MORB-type eclogite belt of Permian was discovered in the Sumdo （松多） region of the Lhasa block, Tibet. A critical question thus is： what is the tectonic setting of the eelogite belt and is it related to an unrecognized suture in the region？ Further investigations show that there are some mafic and ultramafic rocks spacially associated with the eclogite belt in the region. Three ultramafic massifs were recognized in the Sumdo region, and called the Luomaling （罗马岭）, Gongbupala （贡布爬拉） and Qiazhasumdo （卡扎松多） massifs. All the massifs are fault-contacted with greenschist （Chasagang （岔萨岗） Formation） or muscovite-quartz schist （Mabuku （马布库） Formation）, and individuals are about 100 m×50 m in size extending in EW as the regional structure. All the ultramafic rocks have been entirely serpentinized, and the Gongbupala massif has been selected for study in geochemistry. Eleven chemical analyses of the rocks from the Gongbupala massif show a narrow range in contents： SiO2 （35.97-40.63） wt.%, MgO （37.02-38.60） wt.%, TiO2 （0.01-0.08） wt.%, Al2O3 （0.80-1.64） wt.%, （Na2O＋K2O） less than 0.1 wt.%, with high volatile contents （H2O＋CO2） （11.24-14.91） wt.%. After recalculation without H2O＋CO2, the mean values are SiO2 45.24 wt.%, MgO 43.54 wt.%, FeOT （7.45-9.97） wt.% （8.55 wt.% in average）, （MgO＋FeOT） 52.09 wt.%, Mg# （100×Mg/（Mg＋Fe＊）, where Fe＊ represents total Fe）=89.42-90.08, （m＋f）/Si （（atomicity Mg＋atomicity Fe） /atomicity Si）=1.53-1.75 （1.59 in average）, respectively. The mean M/F （atomicity Mg/atomicity Fe） ratio of the rocks is 9.05, which is classified as magnesium enriched-type of ultramafic rocks. The compositional features, depleted in K, Na, Ca, AI and Ti and enriched in Mg#, indicate the characteristics of peridotite originated from a depleted mantle. The rocks have low REE with ∑（1.60-2.68）×10^-6 similar to those of the primitive mantle. The chondrite-normalized REE patterns of all samples show slightly enrichment in LREE, with （Ce/Yb）N 1.03-2.46, but a little depleted in HREE. Most samples show a slight negative anomaly in Eu, a feature in REE from a relic mantle and common features in highly serpentinized ultramafic rocks in the Yarlung-Zangbo （雅鲁藏布） ophiolite and the Bangong （班公）-Nujiang （怒江） ophiolite in Tibet. The primitive mantle-normalized spiderdiagram of trace elements for Gongbupala ultramafic rocks yields uniform distributed pattern. They are relatively enriched in Rb, Ba, La, P element （LHSE） and depleted in Sm, Ti, Y, Yb element （HFSE）, a feature of metasomatic mantle peridotite. The geochemical features of the rocks suggest that the protolith of Gongbupala serpentinite in Sumdo region is harzburgite, a typical depleted mantle rock, and may represent a dismembered ophiolite unit in the region.展开更多
文摘A huge thrust system, the North Lhasa Thrust (NLT), was discovered in the northern Lhasa block of the Tibetan Plateau based on geological mapping of the Damxung region and its vicinity, the Deqen-Lunpola traverse and the Amdo-Bam Co profile. The NLT consists of the Dongqiao-Lunpola thrust (DLT), the west Namco thrust (WNT) and the south Damxung thrust (SDT) and ductile shear zones, ophiolite slices and folds extending in a WNW direction. Major thrust faults of the NLT seem to merge into a single deep-seated detachment of the upper-crust and totally displaced southward as far as 100-120 km. Chronological analyses with 39Ar-40Ar of plagioclase and hornblende, Rb-Sr isochron of minerals and fission-tracks of apatite from mylonite within the WNT yield ages of 174-173 Ma, 109 Ma and 44 Ma, showing 3 periods of thrusting in the north Lhasa block caused by subduction of the Tethys oceanic plate and the India-Eurasia continental collision respectively.
文摘The Qinghai–Tibet plateau is a composite continental fragment formed by collision of multiple terranes and island arcs.The Lhasa terrane,which is located in the central part of the plateau,is bounded by the Yarlung–Zangbo suture to the south and Bangong-Nujiang suture to the north.An E–W–trending belt of(ultra)-high pressure eclogite was discovered in the Sumdo region of the Lhasa terrane.Careful field studies combined with petrological,geochemical and isotopic analyses show that the Sumdo eclogites mark a Carboniferous–Permian suture zone,at least 100 km long,containing ophiolite fragments,eclogites and Indosinian post–orogenic granitoids.This suture divides the Lhasa block into a northern and southern segment.Sumdo eclogite occurs about 200 km east of Lhasa city,and extends over 100km in an E–W direction.Sumdo eclogites were accompanied by garnet amphibolite and plagioclaseamphiboliteformedbyretrograde metamorphism of the eclogites.The eclogites were derived from oceanic basalts.LA–ICPMS U–Pb dating of zircon from the Sumdo eclogites indicates a Permian metamorphic age(260–270 Ma)and a Carboniferous protolith age of 303±4.8 MaThe ophiolite fragments in the Sumdo suture zone are composed of the ultramafic rocks,MORB–type basalt,OIB–type basalt and island arc basaltic andesite,some of which are intruded by post-collisional granites.The ultramafic body,a typical tectonic block in the suture zone,is completely serpentinized.Its geochemical features suggest that it is composed of harzburgite,typical of depleted mantle peridotite.The MORB and OIB–type basalts crop out in the Chasagang Formation,and the basaltic andesite crops out in the Leilongku Formation,both of which make up the Sumdo Group.Zircons from the OIB–type basalt with typical magmatic characteristics yield an average U–Pb age of306(95%)Ma,suggesting formation in a Paleo–Tethyan basin in the Carboniferous.U–Pb dating of zircon from the basaltic andesites yielded a concordant age of 265±3.1Ma,similar to the metamorphic age(266–270 Ma)of the eclogites,suggesting formation during subduction of the oceanic crust.Indosinian granodiorite with an age of194±4.3 Ma crops out north of the Sumdo suture.These granodiorites are similar to the late Indochina granites in the Lhasa block,and most likely formed during continent-arc collision or during closure of the Paleo-Tethyan Ocean.A four–stage model for the evolution of the Sumdo Paleo–Tethyan suture is proposed:1)From the Ordovician to the Devonian Gondwanaland was part of an epicontinental sea;2)In the Carboniferous,continental rifting produced a local basin,which then evolved into a Paleo-Tethyan ocean basin,dividing the Lhasa block into two segments;3)From the Permian to the early Triassic,Paleo-Tethyan oceanic crust was subducted northward and;4)In the middle Triassic and early Jurassic,the two fragments collided to form the modern Lhasa block.
文摘Asthemaintectoniccomponentofthe Himalayan–Tibetan orogen,the Lhasa terrane has received much attention as it records the entire history of the orogeny.The occurrence of high pressure eclogite in the Sumdo complex in central Lhasa terrane has a significant importance on the understanding of the Paleo-Tethys subduction and plate itineration processes in this area.The petrological,geochemical and geochronological data of eclogite and associated blueschist and garnet-bearing mica schist from Sumdo,Jilang and Bailang area have been briefly reviewed to explore the origin and metamorphic evolution of this suture.Eclogites from the Sumdo complex have experienced low temperature,high pressure to ultrahigh pressure metamorphism,revealing a fastsubduction and exhumation process in a typical oceanic subduction zone.The large P-T range between different eclogites in the literature may be affected by the big error of unappropriated using geothermobarometry and may also because of slices of subducted blocks derived from different depths juxtapose together during exhumation.By summarizing the U-Pb,Lu-Hf and Sm-Nd ages of eclogites,the eclogite facies metamorphism is likely to occur in early Triassic during 245-225 Ma,but not the previously accepted late Permian at ca.260 Ma by the reinterpretation of the former geochronological data from literature.The opening of Paleo-Tethys Ocean between the Lhasa terrane initiate prior to ca.280 Ma and ultimate closure to integrate the Lhasa terrane was no earlier than225 Ma and may triggered by the initial subduction of Bangong-Nujiang Tethys Ocean in the north.
基金supported by China Geological Survey (Nos. 1212010610107 and 1212010610105)the Institute of Geology, Chinese Academy of Geological Sciences (No. J0701)
文摘Recently, an over 100 km long MORB-type eclogite belt of Permian was discovered in the Sumdo （松多） region of the Lhasa block, Tibet. A critical question thus is： what is the tectonic setting of the eelogite belt and is it related to an unrecognized suture in the region？ Further investigations show that there are some mafic and ultramafic rocks spacially associated with the eclogite belt in the region. Three ultramafic massifs were recognized in the Sumdo region, and called the Luomaling （罗马岭）, Gongbupala （贡布爬拉） and Qiazhasumdo （卡扎松多） massifs. All the massifs are fault-contacted with greenschist （Chasagang （岔萨岗） Formation） or muscovite-quartz schist （Mabuku （马布库） Formation）, and individuals are about 100 m×50 m in size extending in EW as the regional structure. All the ultramafic rocks have been entirely serpentinized, and the Gongbupala massif has been selected for study in geochemistry. Eleven chemical analyses of the rocks from the Gongbupala massif show a narrow range in contents： SiO2 （35.97-40.63） wt.%, MgO （37.02-38.60） wt.%, TiO2 （0.01-0.08） wt.%, Al2O3 （0.80-1.64） wt.%, （Na2O＋K2O） less than 0.1 wt.%, with high volatile contents （H2O＋CO2） （11.24-14.91） wt.%. After recalculation without H2O＋CO2, the mean values are SiO2 45.24 wt.%, MgO 43.54 wt.%, FeOT （7.45-9.97） wt.% （8.55 wt.% in average）, （MgO＋FeOT） 52.09 wt.%, Mg# （100×Mg/（Mg＋Fe＊）, where Fe＊ represents total Fe）=89.42-90.08, （m＋f）/Si （（atomicity Mg＋atomicity Fe） /atomicity Si）=1.53-1.75 （1.59 in average）, respectively. The mean M/F （atomicity Mg/atomicity Fe） ratio of the rocks is 9.05, which is classified as magnesium enriched-type of ultramafic rocks. The compositional features, depleted in K, Na, Ca, AI and Ti and enriched in Mg#, indicate the characteristics of peridotite originated from a depleted mantle. The rocks have low REE with ∑（1.60-2.68）×10^-6 similar to those of the primitive mantle. The chondrite-normalized REE patterns of all samples show slightly enrichment in LREE, with （Ce/Yb）N 1.03-2.46, but a little depleted in HREE. Most samples show a slight negative anomaly in Eu, a feature in REE from a relic mantle and common features in highly serpentinized ultramafic rocks in the Yarlung-Zangbo （雅鲁藏布） ophiolite and the Bangong （班公）-Nujiang （怒江） ophiolite in Tibet. The primitive mantle-normalized spiderdiagram of trace elements for Gongbupala ultramafic rocks yields uniform distributed pattern. They are relatively enriched in Rb, Ba, La, P element （LHSE） and depleted in Sm, Ti, Y, Yb element （HFSE）, a feature of metasomatic mantle peridotite. The geochemical features of the rocks suggest that the protolith of Gongbupala serpentinite in Sumdo region is harzburgite, a typical depleted mantle rock, and may represent a dismembered ophiolite unit in the region.
