Jurassic intraplate magmatism in southern Hunan-eastern Guangxi: 40Ar/39Ar dating, geochemistry, Sr-Nd isotopes and implications for the tectonic evolution of SE China

The Mesozoic geology of SE China is characterized by intensive and widespread magmatism. However, the tectonic regime that accounted for the Mesozoic magmatism has been an issue with little consensus. A comprehensive study of 40Ar -39Ar dating, geochemistry and Sr-Nd isotopes has been conducted on basalts from southern Hunan and syenite intrusions from eastern Guangxi. Three episodes of Jurassic magmatism, i.e. alkaline basalts of c. 175 Ma in age, syenitic intrusions of c. 160 Ma and high-Mg basalts of c. 150 Ma, are identified. The older, c. 175 Ma alkaline basalts are characterized by low Sr (I-Sr = 0.7035-0.7040) and high Nd (epsilon(Nd)(T) = 5 to 6) isotopic compositions and OIB-like trace-element patterns (e.g. Nb/La > 1). In contrast, the younger, c. 150 Ma high-Mg basalts have high Sr (I-Sr c. 0.7054) and low Nd (epsilon(Nd)(T) c. -2) isotopic compositions and incompatible trace-element patterns of arc affinity. The c. 160 Ma syenitic intrusions display a relatively large range of Sr and Nd isotopic compositions (I-Sr = 0.7032-0.7082, epsilon(Nd)(T) = 5.5 to -4.1), with the Qinghu syenites having the lowest I-Sr, highest epsilon(Nd)(T) and OIB-type incompatible trace-element patterns analogous to the c. 175 Ma alkaline basalts. Such a secular variation in rock types and geochemical and isotopic characteristics reveals changes in melt segregation depth and mantle sources, which are inferred to have resulted from the post-Indosinian orogenic lithosphere extension and thinning. The c. 175 Ma alkaline basalts are suggested to have formed by small degrees of decompression melting of the astheno-sphere or an enriched lithospheric mantle source accreted by asthenosphere-derived melts during the initial extension. The c. 160 Ma syenitic and c. 150 Ma high-Mg basaltic rocks mainly originated from the enriched lithospheric mantle that melted owing to a raised geotherm caused by lithosphere thinning. This interpretation is at odds with the active continental margin related to the subduction of palaeo-Pacific plate, but consistent with continental rifting and extension for the Mesozoic of SE China.