SUMMARY

Geodynamic context and mantle source of the East Vietnam Sea
spreading- related volcanic rocks in Vietnam and neighboring areas

Nguyễn Hoàng

The opening of the East Vietnam Sea (EVS), leading to the activity of syn-spreading (33-16 Ma) and post-spreading (<16 Ma to today) volcanic activity. Syn-opening magma making up thick layers of tholeiitic basalt with a geochemical composition close to the refractory mid-ocean ridge basalt (MORB) is mainly distributed inside the EVS. The post-opening magma is widely spread not only inside the EVS, but also extends to South and SE China, Hainan Island, Southern Laos (Bolaven), Khorat Plateau (Thailand), and Vietnam showing typical intraplate geochemistry. Samples were collected at a number of places in 3 Indochina countries, coastal areas, and continental shelf of Vietnam to analyze for eruption age, petrographical, geochemical, and isotopic composition. The results together with compiled data from the literature sources are used for comparison to understand the similarities and differences between regions, which show that basalts in different areas have the different rare earth element characteristics. However, they all show signs of melting from spinel peridotite source to garnet peridotite over time. Radiogenic isotope data show that different basalts are distributed into different fields, regardless of the age of the eruption, indicating that the mantle source is largely dependent on space. That is, basalt eruptions of different areas have different source characteristics, not originating from a Hainan mantle plume, if it ever exists. From the results obtained in this report, we propose a suitable geodynamic model explaining the relationship between the opening of the East Vietnam Sea and volcanic activity following the collision tectonics of the Indian plate and Eurasia since the Eocene.

Keywords: East Vietnam Sea spreading, basalt, geochemistry, isotope, mantle flow.