Mantle Source and Magmatic Evolution of the Dying Spreading Ridge in the South China Sea

The genesis of seamount chains along fossil spreading ridges reflects the mantle melting dynamics of dying spreading center. The South China Sea developed a seamount chain along its fossil ridge shortly after cessation of spreading. International Ocean Discovery Program Expedition 349 has recovered three volcanic layers, a shallow volcanic breccia layer and two deeper (upper and lower sections) basalt layers, at Site U1431 near the fossil ridge of the South China Sea. Despite the mid-ocean ridge basalt (MORB)-like geochemistry of the lower and upper section basalts, they show distinct geochemical differences that indicate a change of mantle source and a transition from spreading to intraplate volcanism. While the lower section basalts could reflect the last stage volcanism before the cessation of ridge spreading, the upper section basalts with whole-rock MgO up to 20wt.% were formed by intraplate volcanism with significant olivine accumulation once the spreading center had become inactive. The MORB type basalts at Site U1431 have Hawaii-like high olivine Ni and Fe/Mn and low olivine Ca and Mn and bulk-rock low CaO that are distinctly different from normal global MORBs, suggesting an anomalous mantle source in lithology. We suggest that an eclogite-/pyroxenite-rich component, possibly sourced from the Hainan hot spot, played a fundamental role in the transition from spreading to intraplate volcanism of the dying spreading ridge of the South China Sea.