Geochemical nature of sub-ridge mantle and opening dynamics of the South China Sea
Geochemical nature of sub-ridge mantle and opening dynamics of the South China Sea
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DOI：
10.1016/j.epsl.2018.02.040
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发表时间：
2018-05
影响因子：
5.3
通讯作者：
Guoliang Zhang;Qing Luo;Jian Zhao;M. Jackson;Lishuang Guo;Li-feng Zhong
中科院分区：
文献类型：
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作者：
Guoliang Zhang;Qing Luo;Jian Zhao;M. Jackson;Lishuang Guo;Li-feng Zhong
印度式地幔（即，在208 Pb/204 Pb对206 Pb/204 Pb的图上，位于北半球基准线之上的Pb（208 Pb/204 Pb对206 Pb/204 Pb）被认为是“南半球”的地球化学特征，其来源仍然是个谜。南海是北方半球欧亚大陆裂陷后形成的拉张盆地，但其岩浆地壳的地球化学性质尚未得到充分研究。IODP第349次考察首次在西南次盆地（地点U1433和U1434）和东部次盆地（地点U1431）发现了被厚沉积物覆盖的海底玄武岩。微量元素分析表明，西南次盆地由富集的（E）-MORB型玄武岩组成，东部次盆地由正常（N）-MORB型和E-MORB型玄武岩组成。根据Sr-Nd-Pb-Hf同位素组成，两个次盆地的玄武岩均为印度型MORB，西南次盆地的玄武岩显示同位素组成（即，206 Pb/204 Pb为17.59-17.89）明显不同于东次盆地（即，206 Pb/204 Pb为18.38-18.57），反映了亚盆地尺度地幔成分的不均一性和不同的地幔成分演化历史。两种不同的富集型地幔端元（EM 1和EM 2）是南海印度型地幔成因的主要原因。根据Sr-Nd-Pb-Hf同位素组成模拟了海南地幔柱和下陆壳的影响。结果表明，海南地幔柱的影响可以解释东部次盆地玄武岩206 Pb/204 Pb的升高，而下陆壳的再循环作用可以解释西南次盆地玄武岩206 Pb/204 Pb的降低。基于海南地幔柱在洋脊岩浆活动中的强烈地球化学印记，我们认为海南地幔柱可能促进了南海的开启，在此过程中，海南地幔柱向洋脊下地幔贡献了富集的组分，并导致了热侵蚀和下陆壳向对流地幔的返回。这些结果表明印度型地幔的原位起源，有助于理解北方伸展盆地“南半球”地球化学异常的成因。
The Indian-type mantle (i.e., above the north hemisphere reference line on the plot of208Pb/204Pb vs.206Pb/204Pb) has been considered as a “Southern Hemisphere” geochemical signature, whose origin remains enigmatic. The South China Sea is an extensional basin formed after rifting of the Euro-Asia continent in the Northern Hemisphere, however, the geochemical nature of the igneous crust remains unexplored. For the first time, IODP Expedition 349 has recovered seafloor basalts covered by the thick sediments in the Southwest sub-basin (Sites U1433 and U1434) and the East sub-basin (Site U1431). The Southwest sub-basin consists of enriched (E)-MORB type basalts, and the East sub-basin consists of both normal (N)-MORB-type and E-MORB-type basalts based on trace element compositions. The basalts of the two sub-basins are Indian-type MORBs based on Sr–Nd–Pb–Hf isotope compositions, and the Southwest sub-basin basalts show isotopic compositions (i.e.,206Pb/204Pb of 17.59–17.89) distinctly different from the East sub-basin (i.e.,206Pb/204Pb of 18.38–18.57), suggesting a sub-basin scale mantle compositional heterogeneity and different histories of mantle compositional evolution. Two different enriched mantle end-members (EM1 and EM2) are responsible for the genesis of the Indian-type mantle in the South China Sea. We have modeled the influences of Hainan mantle plume and lower continental crust based on Sr–Nd–Pb–Hf isotope compositions. The results indicate that the influence of Hainan plume can explain the elevated206Pb/204Pb of the East sub-basin basalts, and the recycling of lower continental crust can explain the low206Pb/204Pb of the Southwest sub-basin basalts. Based on the strong geochemical imprints of Hainan plume in the ridge magmatism, we propose that the Hainan plume might have promoted the opening of the South China Sea, during which the Hainan plume contributed enriched component to the sub-ridge mantle and caused thermal erosion and return of lower continental crust to the convective mantle. These results imply anin situorigin of the Indian-type mantle that can help understand the genesis of the “Southern Hemisphere” geochemical anomaly in the Northern Hemispheric extensional basin.