The Jiangaidarina granitic mass(JM) is an important part of the magmatic belt in Longmu CoShuanghu Suture Zone(LSSZ) in the central Tibetan Plateau. An integrated research involving wholerock geochemistry, zircon LA-I...The Jiangaidarina granitic mass(JM) is an important part of the magmatic belt in Longmu CoShuanghu Suture Zone(LSSZ) in the central Tibetan Plateau. An integrated research involving wholerock geochemistry, zircon LA-ICP-MS U-Pb ages and Hf isotopic compositions was carried out to define the timing, genesis and tectonic setting of the JM. Zircon LA-ICP-MS U-Pb ages have been obtained ranging from 210 to 215 Ma, rather than the Early Jurassic as previously thought. Fifteen granite samples contain hornblendes and show a negative correlation between POand SiO, indicating that the JM is an I-type granite. All the granites are enriched in LREE relative to HREE, with negative Eu anomalies(Eu/Eu*=0.56-0.81), and have similar trace elements patterns, with depletion of Ba, Nb, Sr and P. These suggest that the JM was fractionated, and this is also proved by the characteristic of negative correlations between oxide elements(TiO, MgO, FeOt, MnO, CaO) and SiO. Almost all ε(t) values of the granites are between-10.3 and-5.8, implying that the JM has a crustal source intimately related with the South Qiangtang Block(SQB), except for one(+10.2), showing a minor contribution from mantle source.Moreover, relatively low NaO/KO ratios(0.42-0.93) and high A/CNK values(0.91-1.50) reflect that the JM was predominately derived from the medium-high potassium basaltic crust, interacted with greywacke. Our new geochemical data and geochronological results imply that the Late Triassic magmas were generated in a post-collisional tectonic setting, probably caused by slab break-off of the Longmu Co-Shuanghu Tethyan Ocean(LSTO). This mechanism caused the asthenosphere upwelling, formed extension setting, offered an enormous amount of heat, and provided favorable conditions for emplacement of voluminous felsic magmas. Furthermore, the LSTO could be completely closed during the Middle Triassic, succeed by continental collision and later the slab broke off in the Late Triassic.展开更多
The Nantinghe ophiolite is located in the northern part of the Changning-Menglian suture zone in southeast Tibet. It is composed of meta-peridotite, cumulative gabbro, meta-gabbro, plagioclase amphibolite and meta-bas...The Nantinghe ophiolite is located in the northern part of the Changning-Menglian suture zone in southeast Tibet. It is composed of meta-peridotite, cumulative gabbro, meta-gabbro, plagioclase amphibolite and meta-basalt. Zircon U-Pb dating of the cumulative gabbro gives concordant ages of 473.0±3.8 Ma and 443.6±4.0 Ma respectively, indicating the early and late episodes of mafic magmatisms during the Paleo-Tethys oceanic rifting. The 16 LA-ICPMS zircon U-Pb analyses of meta-gabbro yield a weight mean age of 439±2.4Ma. The gabbro shows relatively low contents of SiO2 (46.46%-52.11%), TiO2 (0.96%-1.14%) and K2O (0.48%-0.75%). Its trace element distribution patterns are partly similar to those of the mid-ocean ridge basalts, and part is depleted in high field strength elements such as Nb, Ta, Zr, Hf and Ti. These features suggest that the mafic rocks were probably formed in a MORB-like or backarc rift basin setting. The zircon U-Pb age of gabbro is consistent with a late crystallization age of the cumulative gabbro from the Nantinghe ophiolite, suggesting that the Paleo-Tethys oceanic basin was opened during 444-439 Ma, possibly as a backarc basin. It is the first precise age which defines the formation time of the early Paleozoic ophiolite in the Changning-Menglian suture zone. These geochronological and geochemical characteristics of the Nantinghe ophiolite are consistent with those from the Guoganjianianshan and Taoxinghu of the Longmu Co-Shuanghu suture in the Qiangtang region. Thus, we suggest that the both Changning-Menglian and Longmu Co-Shuanghu sutures were probably transformed from the relic oceanic crust of the uniform Paleo-Tethys, which likely represents the original and main Paleo-Tethys oceanic basin.展开更多
To better understand the lithosphere mantle collision tectonics between the India plate and Asia plate, we determine three dimensional P wave velocity structure beneath western Tibet using 27,439 arrival times from 2,...To better understand the lithosphere mantle collision tectonics between the India plate and Asia plate, we determine three dimensional P wave velocity structure beneath western Tibet using 27,439 arrival times from 2,174 teleseismic events recorded by 182 stations of Hi-CLIMB Project and 16 stations in the north of Hi-CLMB. Our tomographic images show the velocity structure significantly difference beneath northern and southern Qiangtang, which can further prove that the Longmu Co-Shuanghu ophiolitic belt is a significant tectonic boundary fault zone. There are two prominent high velocity anomalies and two prominent low velocity anomalies in our images. One obvious high velocity anomalies subduct beneath the Tibet at the long distance near 34°N, whereas it is broke off by an obvious low velocity anomaly under the IYS. We interpret them as northward subducting Indian lithosphere mantle and the low velocity anomanly under IYS likely reflects mantle material upwelling triggered by tearing of the northward subduction Indian lithosphere. The other prominent high velocity anomaly was imaged at a depth from 50 km to 200 km horizontal and up to the northern Qiangtang with its southern edge extending to about 34°N through Hoh Xil block. We infer it as the southward subducting Asia lithosphere mantle. The other widely low velocity anomaly beneath the Qiangtang block lies in the gap between the frontier of India plate and Asia plate, where is the channel of mantle material upwelling.展开更多
The high-pressure metamorphic belt （HPMB） of eclogite-blueschist in Central Qiangtang （羌塘） lies in the Longmu Co （龙木错）-Shuanghu （双湖） suture zone. To the west, the HPMB extends 500 km from Hongjishan （...The high-pressure metamorphic belt （HPMB） of eclogite-blueschist in Central Qiangtang （羌塘） lies in the Longmu Co （龙木错）-Shuanghu （双湖） suture zone. To the west, the HPMB extends 500 km from Hongjishan （红脊山） to Caiduochaka （才多茶卡）, east of Shuanghu; to the east it extends to Baqing （巴青） and Jitang （吉塘） in Qamdo （昌都）, and then bends southward to Yunnan （云南） Province. Including the Lancangjiang （澜沧江） blueschist belt, the entire HPMB is about 2 000 km long. In Central Qiangtang, the belt is mainly composed of blueschist and eclogite, whereas in West Yunnan it contains only blueschist. The Baqing-Jitang segment is dominated by garnet phengite schist. 40Ar-39Ar dating of glaucophane and phengite from the blueschists yielded plateau ages ranging from 223 to 215 Ma, whereas SHRIMP U-Pb dating of zircon from the eclogites gives metamorphic ages of 243-217 Ma. The calculated metamorphic conditions for the blueschists are 410-460 ℃ and 0.67-0.75 GPa, and for the eclogites, 〈500℃ and 1.56-2.35 GPa. The metamorphic ages suggest that the Longmu Co-Shuanghu suture closed in the Late Triassic. The region south of the Longmu Co-Shuanghu- Lancang suture consists of the pan-African basement overlain by Condwana sedimentary and metasedimentary rocks, whereas the region north of the suture is dominated by the Jinning （晋宁） basement and Yangtze sedimentary and metasedimentary rocks. The Qiangtang HPMB marks the closure of the paleo-Tethys Ocean.展开更多
The origin of the central Qiangtang metamorphic belt(CQMB)has long been in debate,which is not clear whether this belt is the exhumed Jinsha oceanic plate that had been subducted and underthrusted beneath the Qiangtan...The origin of the central Qiangtang metamorphic belt(CQMB)has long been in debate,which is not clear whether this belt is the exhumed Jinsha oceanic plate that had been subducted and underthrusted beneath the Qiangtang Block,or the in situ Longmu Co-Shuanghu suture that separated the south and north Qiangtang blocks.Here we report field observations,zircon U-Pb ages and Lu-Hf isotopes,as well as whole rock geochemistry and Sr-Nd isotopes of the Late Triassic volcanic rocks near the Chabo Co within the southern margin of the CQMB.The ca.229 Ma Chabo Co volcanic rocks and limestones possess characteristic lithologies of a seamount.Their geochemical and isotopic compositions are similar to OIB-type lavas.Unlike other metabasalts(eclogites and blueschists)in the CQMB,the Chabo Co volcanic rocks are OIB-type lavas that did not experience high-grade metamorphism;this is likely because that the Chabo Co seamount was detached from the subducting Longmu Co-Shuanghu oceanic slab.This work provides new solid evidences for an in situ origin of the CQMB.展开更多
基金financially supported by the geological survey project of China Geological Survey (Grant No. DD20160161)
文摘The Jiangaidarina granitic mass(JM) is an important part of the magmatic belt in Longmu CoShuanghu Suture Zone(LSSZ) in the central Tibetan Plateau. An integrated research involving wholerock geochemistry, zircon LA-ICP-MS U-Pb ages and Hf isotopic compositions was carried out to define the timing, genesis and tectonic setting of the JM. Zircon LA-ICP-MS U-Pb ages have been obtained ranging from 210 to 215 Ma, rather than the Early Jurassic as previously thought. Fifteen granite samples contain hornblendes and show a negative correlation between POand SiO, indicating that the JM is an I-type granite. All the granites are enriched in LREE relative to HREE, with negative Eu anomalies(Eu/Eu*=0.56-0.81), and have similar trace elements patterns, with depletion of Ba, Nb, Sr and P. These suggest that the JM was fractionated, and this is also proved by the characteristic of negative correlations between oxide elements(TiO, MgO, FeOt, MnO, CaO) and SiO. Almost all ε(t) values of the granites are between-10.3 and-5.8, implying that the JM has a crustal source intimately related with the South Qiangtang Block(SQB), except for one(+10.2), showing a minor contribution from mantle source.Moreover, relatively low NaO/KO ratios(0.42-0.93) and high A/CNK values(0.91-1.50) reflect that the JM was predominately derived from the medium-high potassium basaltic crust, interacted with greywacke. Our new geochemical data and geochronological results imply that the Late Triassic magmas were generated in a post-collisional tectonic setting, probably caused by slab break-off of the Longmu Co-Shuanghu Tethyan Ocean(LSTO). This mechanism caused the asthenosphere upwelling, formed extension setting, offered an enormous amount of heat, and provided favorable conditions for emplacement of voluminous felsic magmas. Furthermore, the LSTO could be completely closed during the Middle Triassic, succeed by continental collision and later the slab broke off in the Late Triassic.
基金supported by the National Basic Research Program of China (2009CB421003)the National Natural Science Foundation of China (41073033 and 40872055)the Chinese Geological Survey Projects (1212011121259 and 1212011085119)
文摘The Nantinghe ophiolite is located in the northern part of the Changning-Menglian suture zone in southeast Tibet. It is composed of meta-peridotite, cumulative gabbro, meta-gabbro, plagioclase amphibolite and meta-basalt. Zircon U-Pb dating of the cumulative gabbro gives concordant ages of 473.0±3.8 Ma and 443.6±4.0 Ma respectively, indicating the early and late episodes of mafic magmatisms during the Paleo-Tethys oceanic rifting. The 16 LA-ICPMS zircon U-Pb analyses of meta-gabbro yield a weight mean age of 439±2.4Ma. The gabbro shows relatively low contents of SiO2 (46.46%-52.11%), TiO2 (0.96%-1.14%) and K2O (0.48%-0.75%). Its trace element distribution patterns are partly similar to those of the mid-ocean ridge basalts, and part is depleted in high field strength elements such as Nb, Ta, Zr, Hf and Ti. These features suggest that the mafic rocks were probably formed in a MORB-like or backarc rift basin setting. The zircon U-Pb age of gabbro is consistent with a late crystallization age of the cumulative gabbro from the Nantinghe ophiolite, suggesting that the Paleo-Tethys oceanic basin was opened during 444-439 Ma, possibly as a backarc basin. It is the first precise age which defines the formation time of the early Paleozoic ophiolite in the Changning-Menglian suture zone. These geochronological and geochemical characteristics of the Nantinghe ophiolite are consistent with those from the Guoganjianianshan and Taoxinghu of the Longmu Co-Shuanghu suture in the Qiangtang region. Thus, we suggest that the both Changning-Menglian and Longmu Co-Shuanghu sutures were probably transformed from the relic oceanic crust of the uniform Paleo-Tethys, which likely represents the original and main Paleo-Tethys oceanic basin.
基金supported by the National Key Research and Development Program of China(Grant No.2016YFC0600301)the Geological Investigation Project of China Geological Survey(Grant No.DD20190448,DD20190370)the Natural Science Foundation of China(Grant No.41374101,4171101169,41274095)。
文摘To better understand the lithosphere mantle collision tectonics between the India plate and Asia plate, we determine three dimensional P wave velocity structure beneath western Tibet using 27,439 arrival times from 2,174 teleseismic events recorded by 182 stations of Hi-CLIMB Project and 16 stations in the north of Hi-CLMB. Our tomographic images show the velocity structure significantly difference beneath northern and southern Qiangtang, which can further prove that the Longmu Co-Shuanghu ophiolitic belt is a significant tectonic boundary fault zone. There are two prominent high velocity anomalies and two prominent low velocity anomalies in our images. One obvious high velocity anomalies subduct beneath the Tibet at the long distance near 34°N, whereas it is broke off by an obvious low velocity anomaly under the IYS. We interpret them as northward subducting Indian lithosphere mantle and the low velocity anomanly under IYS likely reflects mantle material upwelling triggered by tearing of the northward subduction Indian lithosphere. The other prominent high velocity anomaly was imaged at a depth from 50 km to 200 km horizontal and up to the northern Qiangtang with its southern edge extending to about 34°N through Hoh Xil block. We infer it as the southward subducting Asia lithosphere mantle. The other widely low velocity anomaly beneath the Qiangtang block lies in the gap between the frontier of India plate and Asia plate, where is the channel of mantle material upwelling.
基金supported by the National Natural Science Foundation of China (Nos. 40872146, 40672147, 40773020)China Geological Survey (No. 1212010818015)
文摘The high-pressure metamorphic belt （HPMB） of eclogite-blueschist in Central Qiangtang （羌塘） lies in the Longmu Co （龙木错）-Shuanghu （双湖） suture zone. To the west, the HPMB extends 500 km from Hongjishan （红脊山） to Caiduochaka （才多茶卡）, east of Shuanghu; to the east it extends to Baqing （巴青） and Jitang （吉塘） in Qamdo （昌都）, and then bends southward to Yunnan （云南） Province. Including the Lancangjiang （澜沧江） blueschist belt, the entire HPMB is about 2 000 km long. In Central Qiangtang, the belt is mainly composed of blueschist and eclogite, whereas in West Yunnan it contains only blueschist. The Baqing-Jitang segment is dominated by garnet phengite schist. 40Ar-39Ar dating of glaucophane and phengite from the blueschists yielded plateau ages ranging from 223 to 215 Ma, whereas SHRIMP U-Pb dating of zircon from the eclogites gives metamorphic ages of 243-217 Ma. The calculated metamorphic conditions for the blueschists are 410-460 ℃ and 0.67-0.75 GPa, and for the eclogites, 〈500℃ and 1.56-2.35 GPa. The metamorphic ages suggest that the Longmu Co-Shuanghu suture closed in the Late Triassic. The region south of the Longmu Co-Shuanghu- Lancang suture consists of the pan-African basement overlain by Condwana sedimentary and metasedimentary rocks, whereas the region north of the suture is dominated by the Jinning （晋宁） basement and Yangtze sedimentary and metasedimentary rocks. The Qiangtang HPMB marks the closure of the paleo-Tethys Ocean.
基金financially supported by the National Natural Science Foundation of China (Nos. 41702212,41672054)the Fundamental Research Funds for Central Universities,China University of Geosciences (Wuhan) (Nos. CUGL170816 and CUGQYZX1745)
文摘The origin of the central Qiangtang metamorphic belt(CQMB)has long been in debate,which is not clear whether this belt is the exhumed Jinsha oceanic plate that had been subducted and underthrusted beneath the Qiangtang Block,or the in situ Longmu Co-Shuanghu suture that separated the south and north Qiangtang blocks.Here we report field observations,zircon U-Pb ages and Lu-Hf isotopes,as well as whole rock geochemistry and Sr-Nd isotopes of the Late Triassic volcanic rocks near the Chabo Co within the southern margin of the CQMB.The ca.229 Ma Chabo Co volcanic rocks and limestones possess characteristic lithologies of a seamount.Their geochemical and isotopic compositions are similar to OIB-type lavas.Unlike other metabasalts(eclogites and blueschists)in the CQMB,the Chabo Co volcanic rocks are OIB-type lavas that did not experience high-grade metamorphism;this is likely because that the Chabo Co seamount was detached from the subducting Longmu Co-Shuanghu oceanic slab.This work provides new solid evidences for an in situ origin of the CQMB.
