Finding of Neoproterozoic syenites at Fangcheng in the northern Qinling region of the East Qinling orogen provides an important constraint on timing of tectonic transformation to extensional regime. The alkaline pluton consists mainly of nepheline syenite,aegirine syenite,and alkali-feldspar syenite. The syenites are of intermediate (SiO2=54%―62%),rich in alkali (K2O+Na2O=12%―15%),aluminum (Al2O3=16.81%―23.26%) and large ion lithophile elements (LILE),without any obvious Nb,Ta,Zr,and Hf anoma-lies. The Fangcheng syenites are geochemically characterized by relative enrichment of LREE,minor differentiation of HREE,significant negative Eu anomalies (Eu=0.13―0.23),less negative εNd(t) values of -1.37 to -3.90,young Nd model ages of 1364 to 1569 Ma,and high zircon saturation temperatures of 915 to 1044℃. The syenitic magmas probably originated from small proportion melting of upper mantle in an extensional regime of intraplate-anorogenic tectonic setting,and have been slightly contaminated by crustal materials during ascending and/or emplacement. LA-ICP-MS zircon U-Pb dating yields ages of 844.3±1.6 Ma (MSWD=0.86),suggesting that the Fangcheng alkaline syenites formed in the early Neo-proterozoic. They are the oldest Neoproterozoic alkaline rocks ever recognized in the Qinling orogen as well as in South China. This implies that the tectonic regime of the Qinling region would have transformed from post-collisional stretch to intraplate-anorogenic extension no later than 844 Ma.
BAO ZhiWeiWANG QiangBAI GuoDianZHAO ZhenHuaSONG YaoWuLIU XiaoMing
Mantle plume or superplume activities have often been invoked as a cause for the breakup of the Neoproterozoic...
Xuan-Ce Wang Xian-Hua Li~+ State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,P.O.Box 9825, Beijing 100029,China Key Laboratory of Isotope Geochronology and Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences, Guangzhou 510640,China Wu-Xian Li Key Laboratory of Isotope Geochronology and Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences, Guangzhou 510640,China Zheng-Xiang Li Institute for Geoscience Research,Department of Applied Geology,Curtin University of Technology,GPO Box U1987,Perth,WA 6845, Australia Ying Liu Key Laboratory of Isotope Geochronology and Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences, Guangzhou 510640,China Yue-Heng Yang State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,P.O.Box 9825, Beijing 100029,China Xi-Rong Liang Xiang-Lin Tu Key Laboratory of Isotope Geochronology and Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences, Guangzhou 510640,China
Secondary ion mass spectroscopy U-Pb zircon ages and mineralogical,geochemical,and Nd isotopic data are report...
Xian-Hua Li,Wei-Guang Zhu,~(1,2) Hong Zhong,~1 Xuan-Ce Wang, De-Feng He,~1 Zhong-Jie Bai,~1 and Feng Liu~3 State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China. 1 State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550002,China. 2 Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China. 3 Chengdu University of Technology,Chengdu 610059, China.
