The Amdo microcontinent is located within the middle of Bangong-Nujiang suture（BNS） zone in the shape of lens. The basic geological research restricts geologists from understanding the histories of tectonic evolutio...The Amdo microcontinent is located within the middle of Bangong-Nujiang suture（BNS） zone in the shape of lens. The basic geological research restricts geologists from understanding the histories of tectonic evolution of BNS and regional geology more deeply. This paper systematically studies the geochronology and geochemistry of granitic gneisses from Amdo basement. These data provide constraints on formation age, source characteristics and tectonic setting of their protolith. The SHRIMP zircon U-Pb dating is operated for granitic gneisses. Samples AGS-2 and AGS-3（granitic gneiss） yield average zircon U-Pb ages of 485±14 and 487±6 Ma, respectively. These ages should represent the formation age of protolith and indicate that they are formed in the Early Ordovician. Granitic gneisses are characterized by high SiO2, Na2O, K2O and Al2O3, low Fe and Mg, enrichment in light rare earth elements（LREEs） and large ion lithophile elements（LILEs）, depletion in heavy rare earth elements（HREEs） and high field strength elements（HFSEs）, with negative Eu anomaly. The Rittmann index（σ） is 1.77 to 2.60, less than 3.3. The aluminum saturation index（A/CNK） values range from 0.88 to 1.26. These features suggest that protolith of granitic gneisses from Amdo basement show characteristics of calc-alkaline and S-type granite, and they could be derived from partial melting of metamorphic greywackes in the upper crust of low maturity. The tectonic setting is syn-collision. These all suggest that the formation of protolith of granitic gneisses from Amdo are caused by the Early Paleozoic orogeny, which could be related to proto-Tethyan oceanic subduction along Gondwana continental margins, and does not result from the production of Pan-African orogenesis.展开更多
Perhaps no region on earth presents such a confusing array of place names as does the area made up of the former provinces of Kham and Amdo in historic Tibet (Xizang). Within these areas, cities, towns, villages, moun...Perhaps no region on earth presents such a confusing array of place names as does the area made up of the former provinces of Kham and Amdo in historic Tibet (Xizang). Within these areas, cities, towns, villages, mountains, lakes and other geographic features have at least two names applied to them, one Tibetan, the other Chinese. Overlying this indigenous nomenclature are the names applied by outsiders, mostly Europeans, each of whom used their native language to transliterate the names they heard or read from Tibetan or Chinese script, and names in the languages of other ethnic minorities who live within the area. Adding to the confusion are the conflicting “standards” for transliterating Tibetan and Chinese names. To resolve the inconsistencies and uncertainties of place names in this region, a multilingual gazetteer and thesaurus was prepared. The gazetteer-thesaurus was compiled to be used as a tool for correlating the often radically different names assigned to a single place or feature and to provide the geographic coordinates for each. The impetus for this project was the need to assign geographic coordinates to plant specimens collected in the area since the latter part of the 19th century up until the advent and widespread use of Global Positioning System (GPS) receivers in the 1990s. Georeferencing specimens is necessary for plotting the historical distribution of species and for more completely understanding the information contained on specimen labels. Knowing the distribution of plants is important for answering phylogenetic questions, determining local and widespread biogeographical patterns, identifying areas of unusually high diversity or endemism, and determining areas in need of special protection. The value of such a gazetteer, however, extends well beyond the field of botany. It is intended to be of use to anyone with a desire to know the nomenclatural history of places in the area and for pinpointing with a fair degree of accuracy the location of each of those places.展开更多
High-pressure mafic granulites with a peak mineral assemblage of garnet + clinopyroxene + rutile + quartz were found in the Amdo basement,central Tibet. Two kinds of symplectites were identified that are composed of o...High-pressure mafic granulites with a peak mineral assemblage of garnet + clinopyroxene + rutile + quartz were found in the Amdo basement,central Tibet. Two kinds of symplectites were identified that are composed of orthopyroxene + plagioclase ± spinel and hornblende + plagioclase around garnet,which were interpreted to develop during the retrogressing stages in the granulites. P-T estimates suggested that peak metamorphic conditions were about 860-920°C and 1.46-1.56 GPa,which retrogressed from post-peak phase at 820-890°C and 0.88-1.15 GPa to amphibolite facies at 550-670°C and 0.52-0.65 GPa. These three stages define a clockwise P-T path with near-isothermal decompression and cooling following the peak high-pressure metamorphism. This suggests that the Amdo granulites underwent an initial subduction to a deep crustal level of ～50 km and then were rapidly exhumed to a shallow crustal level (～20 km). The formation of Amdo granulites is considered to result from the arc-continent collision between the Amdo basement and the Qiangtang terrane in the middle Jurassic,which is a crucial step to the tectonic evolution of the Tibetan Plateau.展开更多
基金supported by the geological survey project of China Geological Survey (Nos. 1212011120185, 1212011221111 and 1212011120182)
文摘The Amdo microcontinent is located within the middle of Bangong-Nujiang suture（BNS） zone in the shape of lens. The basic geological research restricts geologists from understanding the histories of tectonic evolution of BNS and regional geology more deeply. This paper systematically studies the geochronology and geochemistry of granitic gneisses from Amdo basement. These data provide constraints on formation age, source characteristics and tectonic setting of their protolith. The SHRIMP zircon U-Pb dating is operated for granitic gneisses. Samples AGS-2 and AGS-3（granitic gneiss） yield average zircon U-Pb ages of 485±14 and 487±6 Ma, respectively. These ages should represent the formation age of protolith and indicate that they are formed in the Early Ordovician. Granitic gneisses are characterized by high SiO2, Na2O, K2O and Al2O3, low Fe and Mg, enrichment in light rare earth elements（LREEs） and large ion lithophile elements（LILEs）, depletion in heavy rare earth elements（HREEs） and high field strength elements（HFSEs）, with negative Eu anomaly. The Rittmann index（σ） is 1.77 to 2.60, less than 3.3. The aluminum saturation index（A/CNK） values range from 0.88 to 1.26. These features suggest that protolith of granitic gneisses from Amdo basement show characteristics of calc-alkaline and S-type granite, and they could be derived from partial melting of metamorphic greywackes in the upper crust of low maturity. The tectonic setting is syn-collision. These all suggest that the formation of protolith of granitic gneisses from Amdo are caused by the Early Paleozoic orogeny, which could be related to proto-Tethyan oceanic subduction along Gondwana continental margins, and does not result from the production of Pan-African orogenesis.
基金Supported by the U. S. National Science Foundation, Grant No. DEB-0321846, and the National Natural Science Foundation of China, Grant No. 40332021.
文摘Perhaps no region on earth presents such a confusing array of place names as does the area made up of the former provinces of Kham and Amdo in historic Tibet (Xizang). Within these areas, cities, towns, villages, mountains, lakes and other geographic features have at least two names applied to them, one Tibetan, the other Chinese. Overlying this indigenous nomenclature are the names applied by outsiders, mostly Europeans, each of whom used their native language to transliterate the names they heard or read from Tibetan or Chinese script, and names in the languages of other ethnic minorities who live within the area. Adding to the confusion are the conflicting “standards” for transliterating Tibetan and Chinese names. To resolve the inconsistencies and uncertainties of place names in this region, a multilingual gazetteer and thesaurus was prepared. The gazetteer-thesaurus was compiled to be used as a tool for correlating the often radically different names assigned to a single place or feature and to provide the geographic coordinates for each. The impetus for this project was the need to assign geographic coordinates to plant specimens collected in the area since the latter part of the 19th century up until the advent and widespread use of Global Positioning System (GPS) receivers in the 1990s. Georeferencing specimens is necessary for plotting the historical distribution of species and for more completely understanding the information contained on specimen labels. Knowing the distribution of plants is important for answering phylogenetic questions, determining local and widespread biogeographical patterns, identifying areas of unusually high diversity or endemism, and determining areas in need of special protection. The value of such a gazetteer, however, extends well beyond the field of botany. It is intended to be of use to anyone with a desire to know the nomenclatural history of places in the area and for pinpointing with a fair degree of accuracy the location of each of those places.
基金supported by the National Natural Science Foundation of China (40972056 and 40672051)the project entitled "The evolution of Tethys suture in south Tibet" of the Institute of Geology,Chinese Academy of Geological Sciences (1212010918013)
文摘High-pressure mafic granulites with a peak mineral assemblage of garnet + clinopyroxene + rutile + quartz were found in the Amdo basement,central Tibet. Two kinds of symplectites were identified that are composed of orthopyroxene + plagioclase ± spinel and hornblende + plagioclase around garnet,which were interpreted to develop during the retrogressing stages in the granulites. P-T estimates suggested that peak metamorphic conditions were about 860-920°C and 1.46-1.56 GPa,which retrogressed from post-peak phase at 820-890°C and 0.88-1.15 GPa to amphibolite facies at 550-670°C and 0.52-0.65 GPa. These three stages define a clockwise P-T path with near-isothermal decompression and cooling following the peak high-pressure metamorphism. This suggests that the Amdo granulites underwent an initial subduction to a deep crustal level of ～50 km and then were rapidly exhumed to a shallow crustal level (～20 km). The formation of Amdo granulites is considered to result from the arc-continent collision between the Amdo basement and the Qiangtang terrane in the middle Jurassic,which is a crucial step to the tectonic evolution of the Tibetan Plateau.
