Pan-African and early Paleozoic tectonothermal events in the Nyainrong microcontinent:Constraints from geochronology and geochemistry  

作　　者：XIE ChaoMing LI Cai SU Li WU YanWang XIE YaoWu 

机构地区：[1]College of Earth Sciences,Jilin University [2]The Geological Lab Center,China University of Geosciences [3]Regional Geological Survey Party,Tibet Bureau of Geology and Mineral Exploration and Development

出　　处：《Science China Earth Sciences》2013年第12期2066-2079,共14页中国科学（地球科学英文版）

基　　金：supported by National Natural Science Foundation of China(Grant Nos.41072166 and 41272240);Program of China Geological Survey(Grant Nos.1212011121248,1212011221093)

摘　　要：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.To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the constraints 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 northern 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.The206Pb/238U concordant age of zircon is505–517 Ma,corresponding to the Middle-Late Cambrian,which is the formation age of the protolith.The samples have characteristics 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×Ga/Al ranges from 2.12 to 2.41,and P2O5and Al2O3content decreased with SiO2increasing.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 regional data,the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process.The preliminary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andeantype orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.

关 键 词：Amdo gneiss zircon U-Pb dating GEOCHEMICAL Nyainrong microcontinent Tibetan Plateau 

分 类 号：P597.3[天文地球—地球化学]