海南岛晨星早石炭世高度亏损N-MORB型玄武岩及其地质意义

海南岛地处印支陆块和华南陆块的交界地带，具有复杂的构造演化史，其拼合机制及精细时代一直备受争议.在海南岛昌江-琼海断裂以北的晨星地区识别出一套具高ε_Nd(t)值的火山岩，对其进行了全岩40Ar-39Ar年代学及主、微量元素和同位素地球化学研究.结果表明，该火山岩的全岩40Ar-39Ar坪年龄为328.1±2.6 Ma，代表了其喷发年龄；该变基性岩SiO₂含量为46.92%~52.58%，具有低TiO₂、低K₂O、高Al₂O₃的特征；稀土元素丰度为15.1×10-6~28.7×10-6, 呈现N-MORB型微量元素配分曲线, ε_Nd(t)高至9.02~9.85，显示出高度亏损的地球化学特征；其成因被解释为源于受流体交代作用改造的MORB型地幔源区.综合分析表明晨星地区变基性岩的形成可能与古特提斯洋陆俯冲格局下华南与印支陆块间的弧后盆地或有限洋演变有关. 

Extremely Depleted Carbonferous N-MORB Metabasite at the Chenxing Area (Hainan) and Its Geological Significance

Hainan Island, located between the Indochina and South China Blocks, has undergone compound structural overprinting and complex tectonic evolution. However, the Late Paleozoic tectonic regime and amalgamation history of the South China with Indochina Blocks are still on debate. Our investigation identified the highly-depleted greenschist-facies basalt at the Chenxing area in the north of the Changjiang-Qionghai fault, Central Hainan Island. This paper presents a set of new geochronological and geochemical data for these metabasites. The representative sample for Chenxing metabasites gave the plateau age of 328.1±2.6 Ma, representing the eruption age. They have SiO₂ ranging from 46.92 % to 52.58 %, TiO₂ from 0.48 % to 1.12% and Al₂O₃ from 12.34 % to 17.45 %, and could be classified as basalt. These samples show lower K₂O contents, and HFSEs and REEs contents relative to N-MORB. They are characterized by the left-sloping REE pattern and multi-elemental pattern with a geochemical affinity to N-MORB. These samples show extremely high ε_Nd(t) values ranging from 9.02 to 9.85. Such geochemical characteristics for the Chenxing metabasites show the derivation of the MORB source modified by slab-related fluid. In combination with other available data, the Chenxing metabasites in Central Hainan are formed in the back-arc basin or a restricted ocean basin between the South China and Indochina blocks during the Carboniferous in response to the subduction of the Paleotethyan main ocean.