Geochemistry and zircon-apatite U-Pb geochronology of mafic dykes in the Shuangxiwu area: Constraints on the initiation of Neoproterozoic rifting in South China

The Shuangxiwu area of northwestern Zhejiang Province preserves the most complete magmatic records of the Neoproterozoic orogenic cycle at the eastern segment of the Jiangnan Orogen, South China. However, the transition from orogenesis to post-orogenic extension or rifting still remains debated and this led to great confusion on the breakup of Rodinia supercontinent. In this study, we carried out LA-ICP-MS zircon and apatite U-Pb dating and whole-rock geochemical analyses for the mafic dykes that intruded the Shuangxiwu arc volcanic rocks and the Heshangzhen Group in northwestern Zhejiang Province. The U-Pb dating results indicate that the mafic dykes formed at ∼760 Ma, rather than ca. 860-850 Ma as previously suggested. All the rocks are tholeiitic in compositions and can be divided into three subgroups based on their geochemistry. Group-1 has the lowest TiO2 contents (0.7-1.0 wt%) and intermediate Mg# values (40-47), and they show enrichments in large ion lithophile elements and depletions in high field strength elements, resembling typical continental arc basalts. Group-2 has the highest TiO2 contents (1.5-3.6 wt%) and Mg# values (50-55), and they do not show anomalies in P, Zr and Hf, similar to ocean island basalts. Group-3 has elevated SiO2 of 58.6-61.2 wt%, intermediate TiO2 contents of 1.2-1.5 wt%, the lowest Mg# values of 21-30, and negative anomalies in Nb, Ta, Ti and Y. The synthetic studies of Sr-Nd isotopes indicate two end-members for the formations of the three types of mafic dykes: depleted asthenospheric mantle and metasomatized lithospheric mantle. The upwelling of asthenosphere at ca. 760 Ma led to partial melting of the pre-existed metasomatized lithospheric mantle to generate the Group-1 rocks with arc-like geochemical features. Continued extension resulted in low-degree partial melting of asthenospheric mantle and subsequent interaction with Group-1 magmas form the Group-2 rocks. The Group-3 rocks were resulted from AFC processes of the Group-2 magmas. Based on our new data and the available U-Pb ages of Neoproterozoic magmatic rocks, the rifting of South China from Rodinia should have not happened before ca. 820 Ma.