Tectonic Evolution Of The Nyainrong Microcontinent, Tibet

Located in the juncture of the South Qiangtang block and Gangdise block, theNyainrong microcontinent is the only tectonic block that has a large outcrop ofcrystalline basement, and it is one of the most favorable areas for understanding andresearching early formation and evolution of the Qinghai－Tibet Plateau. Althoughthe exploration into the tectonical attributes of microcontinent and PrecambrianNyainrong Group has started long ago, different understandings and names of themremain. However, more and more researches indicate that there are theNeoproterozoic－early Paleozoic crystalline basement rocks in the Nyainrongmicrocontient. Currently, the study of crystalline basement is limited, mainly aimingat the geochronology of basement gneisses, and the obtained ages are not consistent，especially lack of the petrogenesis research。To better understand the tectonothermalevent of the Pan－African and early Paleozoic in the Nyainrong microcontinent andconstraints on its tectonic evolution, we based our study on detailed field observations,precise zircon U－Pb dating, and geochemical study of basement orthogneiss inAmdo area. We discussed the original age, nature, and origin.Combined with the existing research data and results to discuss the basiccharacteristics of the Nyainrong microcontinent from Neoproterozoic to Mesozoic ineach evolution stage, These newly obtained data gave important constraints on thespatio－temporal evolution and geodynamic setting of the Nyainrong microcontinent.We study the zircon U－Pb dating of the Nyainrong gneiss which widelyexposed on the Nyainrong microcontinent. The field geological characteristics,petrology and geochemistry of all dating samples show that its protolith is acid ormedium－acid intrusive rocks. morphological and composition characteristics of Gneiss zircon indicated their mantle origins.Three samples have206Pb/238U ages ranging from817±20to842±13Ma, in theNeoproterozoic; Six samples have206Pb/238U ages ranging from517±4to488±4Ma,in the Cambrian. Combined with the existing chronological study, Nyainrongmicrocontinent tectonic－magmatic evolution geochronological framework couldbe established. The magmatic evolution in the Nyainrong microcontinent can bedivided into three main evolution stages: Neoproterozoic, Cambrian, Mesozoic.Geochemical studies have shown that the composition of Neoproterozoic igneousrocks belong to sub－alkaline metaluminous－peraluminous diorite and granite－diorite rocks, with low silicon, high aluminum, alkali－poor, abound of light REE.LREE fractionation is more significant, Eu negative anomalies, strongly depleted inSr, Nb and Ta is loss relative to La. The view is that the magmatism was the productof stretch contextin beginning of the Rodinia supercontinent breakup。The composition of Precambrian gneiss rock protolith belongs to granite andgranodiorite. The genetic types of the Amdo gneiss protolith is the differentiation ofsubalkaline over aluminum I－type granite. The rocks with varying degrees ofenrichment of LREE, the degree of fractionation of HREE and LREE is strong, Theprimitive mantle－normalized spider diagram exhibits the characteristics of beingstrongly depleted in high field strength elements, showed that the tectonic setting isvolcanic island arc granite. The preliminary view is that the Middle－Late Cambrianmagmatic 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.This paper reports the results of U－Pb dating of the zircon in high－pressuregranulite. There are two types of zircon in High－pressure granulites, the first type isa core－edge microstructure, the zircon residualed cores with typical features ofmagmatic zircon, zircon U－Pb age is541±8Ma~834±11Ma; the second zirconwith typical structural characteristics of the causes metamorphic zircon, zircon U－Pbage is179±2Ma. Age data show granulite had experienced the Pan－Africanorogeny, and occurred peak high－pressure metamorphism transformation in theEarly－Middle Jurassic. Ar－Ar dating results by laser stepwise heating indicate that the basement orthogneiss class and Jurassic granite plateau age concentrated in166~176Ma, represents the final deformation of the basement gneiss and Jurassicmagmatic cooling times.Combined with the existing results, we have study on Nyainrong microcontinentand the global super－continent event in this paper. The author thinks that theNyainrong microcontinent was more affinity with the Yangtze plate from810Ma to910Ma, and Neoproterozoic magmatic event was the product of stretch contextinbeginning of the Rodinia supercontinent breakup。With the Rodinia supercontinentbreakup, Nyainrong microcontinent is likely to be cleaved and freed from the edge ofthe Yangtze plate, and converge to the northern margin of Gondwana in earlyCambrian. Be impact by the Pan－African Movement in early Cambrian, with thecontinuing converge of microcontinent to the northern margin of the Indian continentresulting in the Cambrian I－type granite emplacement, and the microcontinentcrystalline basement eventually formed in the early Ordovician.Combined with the results of research on the microcontinent since the Paleozoic,thik that the affect of the late Precambrian tectonic movement to microcontinentprobably lasts to the Permian.More than200Ma geological history, Nyainrongmicrocontinent abnormal bulge and lack of Paleozoic sedimentary. Early Jurassicstrong tectonic deformation events of the ancient basement rocks on Nyainrongmicrocontinent, with the large－scale granite development of the early Jurassic hasunified dynamic background in the region.