Geochronology and geochemistry of the Fangcheng Neoproterozoic alkali-syenites in East Qinling orogen and its geodynamic implications

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 anomalies. 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 ε_Nd(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°C. 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 Neoproterozoic. 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. Supported by the National Natural Science Foundation of China (Grant Nos. 40472032, 40721063 and 40672070), the National Basic Research Program of China (Grant No. 2006CB403504) and the Innovative Project of Chinese Academy of Sciences (Grant No. KZCX2-YW-128)