To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the con- straints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating an...To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the con- straints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in north- ern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The 2~6pbF38U concordant age of zircon is 505-517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have char- acteristics of high silicon, alkali-rich, alkalic rate AR =1.73-3.7, the differentiation index DI = 70.78-90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 x Ga/AI ranges from 2.12 to 2.41, and P205 and A1203 content decreased with SiO2 increasing. Th and Y contents have a good positive correlation with Rb content; the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with re- gional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The prelimi- nary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andean- type orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.展开更多
基金supported by National Natural Science Foundation of China(Grant Nos.41072166 and 41272240)Program of China Geological Survey(Grant Nos.1212011121248,1212011221093)
文摘To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the con- straints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in north- ern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The 2~6pbF38U concordant age of zircon is 505-517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have char- acteristics of high silicon, alkali-rich, alkalic rate AR =1.73-3.7, the differentiation index DI = 70.78-90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 x Ga/AI ranges from 2.12 to 2.41, and P205 and A1203 content decreased with SiO2 increasing. Th and Y contents have a good positive correlation with Rb content; the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with re- gional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The prelimi- nary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andean- type orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.
