Nd, Sr isotopes and elemental geochemistry of surface sediments from the South China Sea: Implications for Provenance Tracing
Nd, Sr isotopes and elemental geochemistry of surface sediments from the South China Sea: Implications for Provenance Tracing
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DOI：
10.1016/j.margeo.2012.05.007
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发表时间：
2012-08
期刊：
影响因子：
2.9
通讯作者：
G. Wei;Yuanfang Liu;Jinlong Ma;Luhua Xie;Jianfang Chen;W. Deng;Song Tang
中科院分区：
文献类型：
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作者：
G. Wei;Yuanfang Liu;Jinlong Ma;Luhua Xie;Jianfang Chen;W. Deng;Song Tang
对南海区中国海区表层沉积物的矿物学、主量元素和微量元素以及钕和锶同位素进行了记录，以探讨其在物源追踪中的适用性。结果表明，仅凭矿物成分并不能清楚地确定南海大块沉积物的来源。南海沉积物的Nd同位素组成具有明显的带状分布特征。中国南部近海沉积物的εNd值最负(−13.0~−10.7)，而印度支那近海沉积物的Nd值略为正(−10.7~−9.4)。婆罗洲近海沉积物的ND同位素组成更高，εNd值在−8.8~−7.0之间，菲律宾弧南近海沉积物的εNd值最大，为−3.7~+5.3。ε沉积物的这种带状分布与排泄到相应区域的水系提供的沉积物的Nd同位素组成很好地吻合，表明Nd同位素组成可以作为南海沉积物物源追踪的一个合适的指标。相比之下，由于其他地区沉积物的87Sr/86Sr比值重叠，锶同位素组成只能用于识别中国南部近海和菲律宾弧南近海的沉积物。在La-Th-Sc区分图中，类似的带状分布也很明显。南海西缘的沉积物，如北部湾、海南岛近海、印度支那近海和同一油田中的舜达陆架地块，而南海东北部近海、婆罗洲近海和菲律宾南部弧地近海的沉积物分布在不同的油田中。因此，La-Th-Sc判别图解结合Nd同位素可以追踪南海沉积物的物源。利用这一方法，我们重新评价了大洋钻探计划1148站位晚渐新世以来沉积物的物源变化。结果表明，23.8 Ma以后沉积的沉积物(大于455mcd：m的复合深度)主要来自南中国地块东部，来自南中国地块内部的贡献可以忽略不计。26 Ma以前(477Mcd以下)沉积的沉积物主要来自北巴拉望大陆地体，可能保留了中国南部沿海晚中生代花岗岩类物质的地球化学特征。为此，北巴拉望大陆地体目前位于菲律宾南部弧内，但在渐新世晚期靠近ODP1148点。与南海洋壳伸展有关的火山物质的风化产物也对这些沉积物起到了作用。26-23.8 Ma期间沉积物来源的快速变化可能是北巴拉望大陆地体突然停止沉积物供应的结果。我们认为，在此期间，北巴拉望大陆地体随着南海洋壳的伸展而向南漂移，当盆地足够大时，从南部到北坡ODP1148站的沉积物供应可能已经停止。
The mineralogy, major and trace elements, and neodymium and strontium isotopes of surface sediments in the South China Sea (SCS) are documented with the aim of investigating their applicability in provenance tracing. The results indicate that mineralogical compositions alone do not clearly identify the sources for the bulk sediments in the SCS. The Nd isotopic compositions of the SCS sediments show a clear zonal distribution. The most negative εNdvalues were obtained for sediments from offshore South China (−13.0 to −10.7), while those from offshore Indochina are slightly more positive (−10.7 to −9.4). The Nd isotopic compositions of the sediments from offshore Borneo are even higher, with εNdranging from −8.8 to −7.0, and the sediments offshore from the southern Philippine Arc have the most positive εNdvalues, from −3.7 to +5.3. This zonal distribution in εNdis in good agreement with the Nd isotopic compositions of the sediments supplied by river systems that drain into the corresponding regions, indicating that Nd isotopic compositions are an adequate proxy for provenance tracing of SCS sediments. Sr isotopic compositions, in contrast, can only be used to identify the sediments from offshore South China and offshore from the southern Philippine Arc, as the87Sr/86Sr ratios of sediments from other regions overlapped. Similar zonal distributions are also apparent in a La–Th–Sc discrimination diagram. Sediments from the west margin of the SCS, such as those from Beibuwan Bay, offshore from Hainan Island, offshore from Indochina, and from the Sunda Shelf plot in the same field, while those offshore from the northeastern SCS, offshore from Borneo, and offshore from the southern Philippine Arc plot in distinct fields. Thus, the La–Th–Sc discrimination diagram, coupled with Nd isotopes, can be used to trace the provenance of SCS sediments. Using this method, we re-assessed the provenance changes of sediments at Ocean Drilling Program (ODP) Site 1148 since the late Oligocene. The results indicate that sediments deposited after 23.8Ma (above 455 mcd: meters composite depth) were supplied mainly from the eastern South China Block, with a negligible contribution from the interior of the South China Block. Sediments deposited before 26Ma (beneath 477 mcd) were supplied mainly from the North Palawan Continental Terrane, which may retain the geochemical characteristics of the materials covered on the late Mesozoic granitoids along the coastal South China. For that the North Palawan Continental Terrane is presently located within the southern Philippine Arc but was located close to ODP Site 1148 in the late Oligocene. The weathering products of volcanic material associated with the extension of the SCS ocean crust also contributed to these sediments. The rapid change in sediment source at 26–23.8Ma probably resulted from a sudden cessation of sediment supply from the North Palawan Continental Terrane. We suggest that the North Palawan Continental Terrane drifted southwards along with the extension of the SCS ocean crust during that time, and when the basin was large enough, the supply of sediment from the south to ODP Site 1148 at the north slope may have ceased.