A linkage between early Silurian Nb-REE enriched alkaline magmatism and Neoproterozoic subduction metasomatized mantle in South Qinling, Central China

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. UPb 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 εNd(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 εNd(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, FeTi oxides, feldspar, biotite and apatite with <5% contamination of local metasedimentary or felsic rocks.