Origin of Northeast Fujian Basalts and Limitations on the Heterogeneity of Mantle Sources for Cenozoic Alkaline Magmatism across SE China: Evidence from Zircon U-Pb Dating, Petrological, Whole-Rock Geochemical, and Isotopic Studies

• Main Authors: Guishan Zhang, Ren Peng, Hongxin Qiu, Hanjie Wen, Yonggang Feng, Baoyun Chen, Lei Zhang, Shen Liu, Taotao Liu
• Format: Article
• Language: English
• Published: MDPI AG 2020-08-01
• Series: Minerals
• Subjects: alkali basalts; pyroxenes; carbonated peridotite; continental rift; NE Fujian
• Online Access: https://www.mdpi.com/2075-163X/10/9/770

Cenozoic alkali basalts in Southeast (SE) China generally are genetically related to intracontinental rifting. Hence, they can be used to probe the nature of their underlying mantle sources and aid studies of the tectonic background in this region. This paper focuses on the Shanhoujian alkali basalts located in Bailing County, northeastern Fujian, SE China. We herein report their petrology, whole-rock major, and trace element geochemistry, and Sr-Nd isotopic composition and provide a new zircon U-Pb age for the basalts (~40 Ma, Eocene). These data help to constrain the petrogenesis of alkali basalts, their mantle source, and tectonic settings. The basalts are characterized by high Mg^# (58.21–63.52) with Na₂O/K₂O > 1. MgO content is weakly correlated with CaO and Cr content but shows no correlation with Ni and Fe₂O₃ (total). Such features suggest that fractionation of clinopyroxene rather than olivine was important. In terms of trace elements, the alkali basalts display: (1) enrichment in La, Ce, Rb, Ba, Nb, and Ta and depletion in K, Pb, Zr, Hf, and Ti and (2) notable fractionation of light rare earth elements from heavy rare earth elements. Determined (⁸⁷Sr/⁸⁶Sr)<i>_i</i> is in the range of 0.7041–0.7040 and ε_Nd (t) is between +3.2 and +3.3. The Shanhoujian alkali basalts show a notable affinity to oceanic island basalts (OIBs) with little assimilation of crustal materials. They were derived from a pyroxenite and carbonated peridotite mantle source metasomatized by sediments carried by the subduction plate at different depths. The primary magmas of these basalts were derived from partial melting of this metasomatized mantle source during upwelling of the asthenospheric mantle as an intracontinental rift formed through extension in this part of SE China.