Petrogenesis of post-spreading seamount volcanism in the South China Sea

The ocean crust of the South China Sea basin was created by a short-lived rifting event from ~32-16 Ma. Since then, there has been continued magmatism – “post-spreading volcanism” – that forms numerous seamounts throughout the basin. Only a few of these submarine seamounts have been sampled, so knowledge of what causes this magmatism is still uncertain. I analyzed seamount samples from four separate dredging cruises (Vema36 1979, Sonne23 1982, Estase 1984, Nanhai 1985) for abundances of a wide range of chemical elements. I used these data, together with a compilation of published chemical data on the seamounts and related onshore volcanism in China, Vietnam, and Thailand, to investigate details of how melt was formed from the mantle, with a particular emphasis on the rare earth element group.

The ratio of the elements Lanthanum (La) to Samarium (Sm) is sensitive to the amount of melting that took place: high values indicate less melting for the ‘post-spreading’ volcanism compared to the ocean crust formation. There is a broad tendency for La/Sm to increase with time, indicating a decrease in the extent of melting, but the data are too sparse for this to be a robust conclusion. The ratio of Dysprosium (Yb) to Ytterbium (Yb) is sensitive to the depth at which melting occurred. There is a broad trend of increasing Dy/Yb with time for the ‘post-spreading’ volcanism, consistent with melting under a progressively thicker lithosphere as it cools. Higher Dy/Yb values on the basin margins indicate deeper melting beneath thicker continental lithosphere.