Alkaline magmatic rocks are important hosts of rare earth elements (REE) and high field-strength elements (HFSE, such as Nb and Zr) deposits. Previous studies have suggested the Nb-REE enriched alkaline rocks were derived from mantle sources metasomatized during subduction-related crustal recycling processes. However, details on how these rare metal enriched mantle reservoirs form are poorly known. In this study, we present detailed petrographic and geochemical data for the Nb-REE enriched alkali basalt and trachyte from Tianbao volcanics located in South Qinling, Central China. U-Pb dating of titanite from alkali basalt and trachyte yielded identical ages of -430 Ma. The alkali basalt in Tianbao contains high TiO2, total alkali and Na2O/K2O, and relative enrichment of LREE, Th, U, Nb, Ta, Zr and Hf. Compared to the alkali basalt, the trachyte has lower Ti and P, but higher Al, Na, K, REE, Nb and Zr contents. Moreover, they have similar Sr-Nd-Pb isotope compositions with low initial 87Sr/86Sr (0.70346 to 0.70566), positive epsilon Nd(t) (+3.61 to +4.61), and moderate 206Pb/204Pb (17.59 to 18.61), 207Pb/204Pb (15.54 to 15.63) and 208Pb/204Pb (37.64 to 38.65). SiO2 content shows a slightly positive correlation with (87Sr/86Sr)i and negative correlation with epsilon Nd(t), indicating that crustal contamination was involved in the evolution of at least the more siliceous magmas. MELTS modeling reveals that trachyte can evolve from the alkali basalt by fractional crystallization of clinopyroxene, Fe-Ti oxides, feldspar, biotite and apatite with <5% contamination of local metasedimentary or felsic rocks. The batch melting modeling indicates that the alkali basalt could be formed via mixing of a low-degree (F = 5%) melt of mantle peridotite (-82%) and a high-degree (F = 30%) melt of metasomatic hornblendite veins (-18%). The initial Sr-Nd-Pb isotopes of these volcanics are consistent with those of global ocean island basalts, with mixed components of depleted MORB, HIMU and EMI type mantle sources. These data, together with the regional tectono-magmatic framework, are consistent with a model for magma generation from a mantle metasomatized by the Neoproterozoic subducted oceanic crust. Our genetic model highlights the importance of subduction-related slab metasomatized mantle for the generation of Nb-REE enriched alkaline magmatism and can be also applicable to other alkaline intrusions associated with or without carbonatites around the world.
