| 摘要: |
| 壳幔黏性结构是影响南海中央海盆扩张期后海山链岩浆活动的主要因素, 研究其岩浆活动发育机制具有重要的科学意义。利用重、磁、测深及岩样分析数据提供的地壳结构和热力学参数, 通过FEM数值模拟, 研究了南海中央海盆珍贝-黄岩海山链之下黏性结构与岩浆熔融、运移活动的关系。根据黏性主要受压力和温度控制, 但熔融期间的散热及脱水会增加黏性, 设计了三种不同的黏性结构模型。计算结果表明: 三种垂向黏性结构模型在不同的温度条件下, 都可以使地幔熔融区最大熔融程度达到20%～25%。岩浆熔融程度与地幔热结构、熔融潜热和含水量有关, 并受扩张速率影响, 慢速扩张脊下的岩浆熔融程度相对较低。岩浆运移在扩张期后主要依靠减压熔融浮力, 垂直上升至熔融区顶面后沿上倾面向脊轴运移。接近脊轴熔融带, 部分熔融程度高, 远离轴部熔融带, 部分熔融程度较低。主要结论: 南海中央海盆海底扩张期, 海山链之下10 km 深度具备形成拉斑玄武岩浆的条件; 扩张期末, 该地区25 km 深度具备形成碱性玄武岩浆的条件。 |
| 关键词: 南海珍贝-黄岩海山链 扩张期后岩浆活动 黏性结构 减压熔融与上浮 |
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| 基金项目:国家自然科学基金(41174085); 国家重点基础研究发展计划“973”课题(2007CB411704) |
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| The thermal simulation of magma activities of the seamount chain after |
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| Abstract: |
| On the basis of the gravity data, magnetic data, bathymetric data, rock sample analysis data (crustal structure and the thermal parameters) and the FEM numerical simulation, we researched the relation among the viscosity, melting and migration issues beneath the Zhenbei-Huangyan seamount chain. The result of our analysis was the viscosity was controlled by pressure and temperature but it increase with thermal dissipation and melting dehydration or decrease caused by the residual magma. According to the result, we designed three different viscosity structure models. The calculation results showed that the maximum melting rates of the three vertical viscosity structure models in the mantle melting zone were all bigger than 20%～25%. The melting degree of the magma activities was related to the mantle thermal structure, melting latent heat and water content, meanwhile it was affected by the expansion rate. For example, the melting rate of the slow spreading ridges slower lower than others. After the spreading period, the magma vertically rose to the top interface of the melting zone mainly by the decompression melting buoyancy, and migrated to the ridge axis along the dumping area. We discussed the factors which affected the melting and migration. With the increase of the distance to the ridge axis, the potential melting degree became lower. Thus we can come to a conclusion that during the peak spreading period of the South China Sea’s central basin, the area which was 10 km below the seamount chain had the condition to form the tholeiitic magma; at the end of the spreading period, the area in the depth of 25 km had the condition to form alkaline basalt magma. |
| Key words: Zhenbei-Huangyan seamount chain in the South China Sea magma activities after the spreading period viscosity structure decompression melting and buoyancy |
