南海北部渐新世以来的稀土元素地球化学特征及其意义
REE GEOCHEMICAL CHARACTERISTICS IN THE NORTHERN SOUTH CHINA SEA SINCE THE OLIGOCENE
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摘要: 对南海北部珠江口盆地沉积物样品进行的稀土元素研究表明,该地区∑REE平均值为150.62×10-6,接近中国黄土的稀土总量值,具有明显的亲陆性,Eu含量明显呈现负异常,δEu值随着离海岸距离的增大而逐渐减小,显示北部陆源物质向南逐步扩散的过程。通过对比分析珠江口盆地与ODP1148站深海沉积物稀土元素地球化学特征,发现它们的变化趋势具有极高的一致性。同时还显示,自渐新世以来南海北部地区发生多期构造运动,主要地球化学不连续面发生在13.8、16、23.8及32Ma等时期。尤其是发生在渐新世末23.8Ma的构造运动改变了南海北部地区的物源供应,加大了各井沉积物间的分异程度,从根本上影响了南海北部地区的沉积环境及充填特征,具有重要的地质意义。Abstract: The average abundance of REE in samples obtained from the Pearl River Mouth basin in the northern South China Sea (SCS) is 150.62×10-6, close to the Loess record, reflecting a continental origin. Negative anomaly of Eu is obvious in these samples, with δEu values gradually decreasing from nearcoast to off-shore boreholes, which displays a stepwise diffusion of terrigenous matter from north to south. The REE geochemistry characteristics from the Pearl River Mouth basin show high consistency in their variation trends with those from ODP Site 1148. Stepwise tectonic movements occurring since the end of Oligocene caused major geochemical discontinuities mainly at 13.8 Ma, 16 Ma, 23.8 Ma, and 32 Ma, the most impressive of which at 23.8 Ma has changed the sediment supply in the northern SCS and enlarged differentiations of the sediment composition between various boreholes. Therefore, this 23.8 Ma event exerted a profound influence on the sedimentary environment and sedimentary fill characteristics in the northern SCS.