试谈南海海盆地壳属性问题——由南海海盆及其邻区玄武岩的比较研究进行讨论

ON THE PROBLEM ABOUT THE CRUST'S ATTRIBUTION OF SOUTH CHINA SEA BASIN——DISCUSSION FROM COMPARATIVE STUDY ON BASALTS OF SEAMOUNTS IN SOUTH CHINA SEA BASIN AND THE NEIGHBORING AREAS

根据对南海海盆中的海山玄武岩和雷琼、礼乐滩玄武岩的比较,对“南海海盆为洋壳”之说表示怀疑。南海海底火山并非象以往认为的那样均是在“海底扩张”结束之后才形成的。南海海山、雷琼及礼乐滩新生代玄武岩的同类型岩石相比较,它们的主元素、K_2O/Na_2O值、大洋系数KO_1以及(La/Yb)_N、Zr/Nb、Rb/Sr、Nb/Yb、La/Nb等微量元素比值变化范围与微量元素、稀土配分型式基本相似.属LREE、LILE和HFSE富集类型.并具相似的Sr-Nd-Pb同位素比值和变化范围。研究表明,南海海山玄武岩更类似于大陆玄武岩,而与大洋环境中生成的玄武岩差异更大。因此,玄武岩岩石地球化学资料不支持“南海海盆为洋壳”的观点。起码,这里不属于人们通常所认为的那种新生洋壳。

According to a comparative study on the basalts from seamounts in the South China Sea Basin (SCSB) and those from Leizhou Peninsula-North Hainan and Reed Bank Seamount, this paper takes a sceptical attitude to the view that the crust oF SCSB is an oceanic crust.The seamounts in SCSB arc not all formed by the post-spreading volcanism. Some were formed calier than the period or the same time when the so-called sea-floor spreading took place. In the same type of basalts from the seamounts in SCSB and from Leizhou Peninsula-North Hainan and Reed Bank Seamount, the content of major elements, the value of K_2O/Na_2O and KO_1(KO_1=MgO+2TiO_2-3K_2O), the variance of (La/Yb)_N, Zr/Nb, Rb/Sr, Nb/Yb, La/Nb, and the patterns of trace elements and REE are quite similar. Also, the rocks have similar Sr-Nd-Pb isotopic ratios and variants. It is shown that the basalts of seamounts in SCSB have a closer affinity to continental basalt than to oceanic basalt. And therefore, the author believes that the data of lithology and geochemistry, also chronology of the basalts do not support the view that the crust of SCSB is an oceanic crust. At least, the crust of SCSB does not belong to the typical new-born oceanic crust generally recognized.