湄公河三角洲第四系沉积物地球化学特征及其地质意义

Geochemical Characteristics and Geological Significance of Quaternary Sediments in the Mekong Delta

湄公河三角洲是由发源于青藏高原的世界第7长河——湄公河于南海西南部入海口处沉积而成.由于缺少可靠的地球化学资料和系统研究,湄公河三角洲地区第四系沉积物源性质尚不明确,制约了对湄公河三角洲源汇系统的进一步认识.本文对湄公河三角洲第四系沉积物进行了重矿物、主量元素和微量元素的地球化学特征分析,结果表明:所有样品稀土元素球粒陨石标准化分布均表现为负Eu异常,为典型的陆源沉积.全新统样品ZTR指数为2~13,更新统样品ZTR指数为21~69;相关图解表明全新统样品主要受分选影响,更新统样品主要受旋回影响.更新统样品最远物源可能为青藏高原东部,经过多次沉积旋回,由古湄公河搬运并再次沉积于湄公河三角洲,而全新统样品成分成熟度较低,为近源沉积,可能为大叻区基岩产物.湄公河三角洲不同时期沉积物源区变化,可能与湄公河和湄公河三角洲在第四系的沉积过程和演化有关.

Originating from Qinghai-Tibet Plateau, the Mekong River is seventh longest river of the world, forming the Mekong River Delta in southwestern South China Sea. Due to lack of reliable geochemical data and systematic research, the properties of Quaternary sediments in the Mekong Delta are still unclear, which restricts further understanding of the source and sink system of the Mekong River. Based on the analysis of heavy minerals, geochemical characteristics of major elements and trace elements of samples from Mekong Delta, all the samples present negative chondrite-normalized Eu anomaly, indicating typical terrigenous deposition. With respect to the Pleistocene samples, the ZTR index ranges between 21 and 69. By contrast, the ZTR index of the Holocene samples ranges from 2 to 13. The correlation diagrams show that the Holocene samples are mainly affected by sorting and the Pleistocene samples are mainly affected by sedimentary recycling. The provenance of sediments from the Pleistocene with high component maturity was probably derived from eastern part of the Tibet, which has experienced several depositional recycling and have been transported by Paleo-Mekong River to the Delta. However, with low component maturity,the sediments of the Holocene are proximal source, and most likely from basement of Dalat area nearby. Variation of provenance of Mekong River Delta during different depositional periods may be related to the tectonic activities, depositional processes and evolution of the Mekong River and the Mekong River Delta in Quaternary.