南海玄武岩:扩张洋脊与海山

Basalts in the South China Sea:Mid-ocean ridges and seamounts

南海存在两种火山岩:洋中脊玄武岩(MORB)和洋岛玄武岩(OIB)。国际大洋发现计划(OIB)第349、367、368、368X航次在南海海盆的成功钻取,获得了南海初始扩张(~34Ma)和停止扩张(~15-16 Ma)前的洋壳样品。南海东部、西南次海盆及北缘洋-陆过渡带代表海盆发展的不同阶段,具有不同的地幔潜能温度、物质组成和洋脊扩张速度,因此产生的洋中脊玄武岩成分差异显著:南海地区在扩张晚期及停止扩张之后存在大规模地幔上涌,与其周缘地区的持续俯冲有关,产出的海山OIB不同于地幔柱活动产生的火山链。南海虽小,但蕴含的信息异常丰富,是窥探地球深部难得的天然窗口。

There are two types of volcanic rocks in the South China Sea(SCS): the mid-ocean ridge basalts(MORB) and the ocean island basalts(OIB). The International Ocean Discovery Program(IODP) Expeditions 349, 367, 368 and 368 x have successfully drilled out the basement of the SCS basin, for the first time, with samples of the oceanic crust during the initial opening(~34 Ma) and the final spreading(~15-16 Ma) of the SCS. The East Subbasin, the Southwest Subbasin and the continent-ocean transition(COT) zone in the northern margin of the SCS represent different evolution stages of the basin. Due to the differences of the mantle evolution, the mantle potential temperature and the recycled materials in the mantle sources, the MORBs generated at the mid-ocean ridges with different spreading rates show distinct compositions. The large-scale mantle upwelling beneath the SCS during the post-spreading, probably induced by the continuous subduction in the surrounding area,has produced the seamounts at the fossil ridges in the SCS, unlike the volcano chain generated by the mantle plumes. Although the SCS is a small marginal sea, it has recorded incrediblly abundant information and thus provides a rare window for probing the deep earth.