The origin of the earliest Jurassic basaltic rocks in southern Jiangxi Province, southeastern China: Implications for interaction between the asthenosphere and metasomatised lithosphere

Abstract Compositions of the earliest Jurassic basalts (190–188 Ma) from the the Baimianshi, Changpu, and Dongkeng basins in southern Jiangxi Province provide insight into the nature of their mantle sources as well as evidence for asthenosphere–lithospheric mantle interaction beneath Southeastern China. We report for the first time elemental and Sr Nd isotopic data for the basalts collected from new drill cores. The basalts are mainly tholeiitic and subordinate alkaline basalts. The alkaline basalts have high TiO2 contents (3.13–4.19 wt%), and high Ti/Y (596–699) and Nb/Y ratios (0.85–0.92) and Nb/La ratios (0.80–0.95). These basalts are characterised by ocean island basalt (OIB)-like trace element characteristics, and have primitive mantle-like eNd(t) values (−0.85 to +0.33) and moderately elevated (87Sr/86Sr)i ratios (0.7074–0.7082). The tholeiitic basalts have a large range of TiO2 contents (1.26–3.26 wt%) and Nb/La ratios (0.40–0.88), plus relatively low Ti/Y (135–534) and Nb/Y ratios (0.32–0.69). Based on Nb/La ratios, the tholeiitic basaltic rocks can further divided into two sub-types: high Nb/La (Nb/La > 0.70) and low Nb/La (Nb/La  Ta depletions (Nb/La = 0.73–0.88), mostly positive eNd(t) values (−0.04 to +2.5) and slightly elevated (87Sr/86Sr)i ratios (0.7061–0.7088), whereas the low Nb/La tholeiitic basalts exhibit moderate to large Nb Ta depletions (Nb/La = 0.40–0.65), much lower eNd(t) values (−7.4 to −0.60) and higher (87Sr/86Sr)i ratios (0.7082–0.7115). In particular, the two low Nb/La tholeiitic basalts (termed as “high-Ba tholeiitic basalts” in this study) are characterised by island arc-like trace element features, such as significant enrichments in Ba and Th, and depletions in Nb, Ta, P, and Ti (Nb/La = 0.40). The two high-Ba basalts also exhibit significantly lower eNd(t) values (−7.3 to −7.4) and higher (87Sr/86Sr)i ratios (0.7112–0.7115). Based on the results, we propose that the parental magmas of the alkaline basalts were produced by low degrees (& 5%–7%) of partial melting of an asthenospheric mantle source at the depth within the garnet–spinel-bearing mantle with more garnet than spinel, the parental magmas of the high Nb/La tholeiitic basalts were generated by moderate degrees (& 15%–20%) of partial melting of a depleted asthenospheric mantle at a relatively shallow depth within the garnet–spinel stability field, the parental magmas of the high-Ba tholeiitic basalts were derived from lithospheric mantle that had been metasomatised by subducted sediment-derived melts and/or fluids. We envision the formation of tholeiitic to alkaline basaltic magmas in the earliest Jurassic in South China records was the result of interactions between the asthenosphere and metasomatised lithosphere during upwelling of the asthenospheric mantle that was triggered by the foundering and/or delamination of a subducted flat slab beneath this region and steep NW-ward palaeo-Pacific subduction.