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East Qinling Orogen,Fangcheng alkaline complex, Neoproterozoic,
Assimilation and fractional crystallization, Nb mineralization, Post-collisional extension
Abstract:
An integrated study was carried out on samples from the Fangcheng alkaline complex so as to establish the origin and evolution of the Nb-rich alkaline felsic magmas. The Fangcheng complex is composed of nepheline syenites, biotite syenites, alkali-feldspar syenites, quartz syenites and syenogranites. Titanite and zircon U-Pb dating reveals that these rocks were simultaneously emplaced between 867 and 833 Ma. They are strongly enriched in HFSEs (e.g., Nb and Zr) and LREEs, with obvious depletions of Ba, Sr and Eu. Notably, all Fangcheng alkaline rocks are enriched in Nb, and this is particularly true for the nepheline syenites (Nb = 161–461 ppm). The nepheline syenites and biotite syenites yield relatively high εNd(t) values (−0.7 to −1.5) and initial 208Pb/204Pb ratios (36.12–37.61) as compared to the quartz syenites and syenogranites (−4.5 to −4.9 and 23.40–36.72, respectively). Likewise, lower zircon εHf(t) values (−4.2 to −6.9) are recorded by the quartz syenites and syenogranites. Based on our results, we suggest that the Fangcheng alkaline rocks were generated via low degrees of partial melting of an enriched mantle during which the fractionation of an alkali basaltic melt generated a nepheline syenite magma. An ongoing segregation of sodic minerals caused the parent melt to evolve towards a potassium-rich composition, leading to the generation of the biotite syenites and alkali-feldspar syenites. Ultimately, the quartz syenites and syenogranites represent the end products of a highly evolved alkaline magma containing a significant degree of contamination by siliceous-rich crustal materials. The enrichment of Nb in all lithologies was controlled by both magmatic and hydrothermal processes. In an initial F- and HFSE-rich magmatic system, prolonged fractionation of Nb-poor minerals led to the concentration of both F and Nb in the residual melts, as is recorded by the Nb-rich titanite. Hydrothermal metasomatism by Na- and F-rich fluids induced further remobilization and subsequent precipitation of Nb to form secondary titanite, pyrochlore, euxenite and rutile in a post-magmatic stage. We infer that the Neoproterozoic Fangcheng alkaline rocks were emplaced in a post-collisional extensional regime, implying that the assembly of the Rodinia supercontinent might have been completed prior to ca. 833 Ma.
