Geochronology and geochemistry of Cenozoic basalts from eastern Guangdong, SE China: constraints on the lithosphere evolution beneath the northern margin of the South China Sea

The 40Ar-39Ar dating reveals three episodes of basaltic volcanism in eastern Guangdong of SE China since the late Eocene (i.e., 35.5, similar to 20 and 6.6 Ma). The Miocene alkali olivine basalts (similar to 20 and 6.6 Ma) have OIB-like trace element characteristics, which is coupled with low (Sr-87/Sr-86)(i), high epsilon(Nd)(t), and high epsilon(Hf)(t). In contrast, the late Eocene basalts (35.5 Ma) have overall characteristics of "Island Arc" basalts with strong negative Ta-Nb-Ti anomalies in the primitive mantle-normalized multi-element diagram with high (Sr-87/Sr-86)(i), negative epsilon(Nd)(t), and relatively low epsilon(Hf)(t). All basalts have unexpectedly high Pb-207/Pb-204 and Pb-208/Pb-204, delineating a DUPAL signature in the sources. The late Eocene Arc-like basalts may reflect contributions of relict ancient metasomatized mantle lithosphere that melted as the result of extension-induced asthenospheric upwelling and heating, whereas the Miocene OIB-like basalts may represent partial melting of the asthenospheric mantle beneath the thickened lithosphere. We propose that the Cenozoic basaltic volcanism in eastern Guangdong records an overall lithospheric thickening process beneath SE China, that is, a continental rift system from its maximum extension in the late Eocene to its waning in the Miocene. This interpretation is consistent with the evolution of the South China Sea, whose origin is most consistent with the development of a passive continental margin. The seafloor spreading of the South China Sea during similar to 32-16 Ma may not result from the effect of the "Hainan" mantle plume, but rather played a positive role in allowing the mantle plume to express on the surface.