南海周缘与板块俯冲相关的岩浆岩研究

The South China Sea is the joint that connects the Indian, Australian, Eurasian and Pacific plates. Its formations are therefore attributed to the interactions between these large plates. Plate subduction is the most important interaction among them, such that is the key to deciphering the formation and evolution of the South China Sea. Ophiolite in Taiwan and the Philippines are relics of oceanic plate formed during the subductions of the Pacific, the New Tethys, the South China Sea and the Philippine Sea. Arc magmatisms in the Indonesia and several other places around the South China Sea were formed during the subduction of the Tethys Ocean. Detailed studies on these ophiolites and arc magmatic rocks provide unique perspectives on the interactions among major plates and the formation of the South China Sea. This proposal plans to look at the South China Sea through ophiolite and arc magmatic rocks from surrounding regions. Based on preliminary studies, we plan to carry out integrated investigations, on ophiolites from Taiwan, the Philippines, etc, and arc magmatic rocks from the Indonesia, including in-situ analyses, mixed double spike high U-Pb dating on zircons from plagiogranite and gabbro; Argon-Ar dating, chemical and isotopic compositions of volcanic glassed from ophiolite suits using laser ablation ICP-MS, high resolution ion probe. The distribution of adakites that formed in the Late Cretaceous to the Miocene will be the most important key of this study, which will provide clues to decipher the subduction of the Tethys spreading ridge. These results will be incorporated with global tectonics, as well as previous results, aiming to place firmer constraints on the formation ages and tectonic regimes of these ophiolite, arc magmatic rocks and the interaction among the Pacific, Philippine, Australian and Eurasian plates, understand the formation and evolution of the South China Sea.

南海位于印度、澳大利亚、欧亚和太平洋四大板块交汇处，是上述板块相互作用的产物。板块俯冲是上述4大板块相互作用的主要方式之一，是研究南海形成演化的重要切入点。在我国台湾省及菲律宾有大量的蛇绿岩，是多次俯冲留下的洋壳残片；印尼等地的岛弧岩浆岩是特提斯板块俯冲的产物。蛇绿岩和岛弧岩浆岩从不同侧面记录了板块相互作用的历史及南海形成演化。本研究拟跳出南海看南海，根据预研究，选取我国台湾省和菲律宾等地典型新生代蛇绿岩、印尼等地晚白垩世到中新世的岛弧岩浆岩，开展系统的地球化学研究，测定蛇绿岩、岛弧岩浆岩的形成时代和地球化学特征，分析其构造属性。重点解析晚白垩世-中新世之间南海南部诸岛的埃达克岩的时空分布，追踪特提斯扩张洋脊消失的历史，结合国内外最新研究成果，解析四大板块相互作用的历史，重点剖析特提斯洋壳俯冲的历史及其与南海、古南海的内在联系，为揭示南海的形成演化提供更坚实的约束条件。

项目执行期间，我们从南海周缘（从印尼-菲律宾-伊朗，到我国的台湾、广东、云南、西藏等广大区域）的各种岩浆岩入手，系统研究早白垩世以来各种典型岩石的形成时代，对其构造背景进行了厘定。在此基础上，厘定了新特提斯洋消亡的过程。获得的主要认识有：1. 我们利用埃达克岩和A-型花岗岩，对新特提斯洋脊俯冲进行了限定。论证新特提斯洋脊俯冲的时间发生在~100-110 Ma左右，形成了石菉埃达克岩及相应的斑岩铜金矿床和青藏高原107-80 Ma的一系列埃达克岩和埃达克质紫苏花岗岩，并且在冈第斯北部形成了大规模的花岗岩；洋脊俯冲引起的岩浆岩影响到了广东、广西、西藏等广大的地区，在形成埃达克岩后，形成了大量与钨锡成矿有关的A-型花岗岩岩体，从广东经东南亚到云南，构成了一条全球最大的锡矿带，占全球锡储量的70%左右。这些岩体和锡（钨）矿与俯冲板块后撤密切相关。2. 结合构造分析，提出南海的形成应该与弧后拉张有关。俯冲板块后撤引发拉张，当时的构造格局与美国西部有一定的可比性。南海下面有大量的俯冲再循环的物质。“海南地幔柱”与俯冲再循环物质有关，与南海的形成无关。3. 研究了菲律宾埃达克岩、岛弧火山岩等，与海南岛和广东的同类岩石进行了对比。初步判断菲律宾到海南岛一线为太平洋构造域和特提斯构造域转折的区域。在此基础上，建立了南海形成演化的模型：南海与新特提斯洋的闭合有关，新特提斯洋自~125Ma起开始向北俯冲，最初的俯冲属于正常俯冲，即俯冲板块的前端老。这种俯冲很可能是造成古南海拉张的主要动力过程。随后洋脊俯冲开始，构造体制有弧后拉张转变为挤压，造成古南海的闭合。俯冲的前缘板块掉入现今广东南部与海南岛之间；俯冲的后缘板片深入内陆，影响到了华南、西藏等地，形成了埃达克岩。随后的俯冲板片后撤，引起了弧后拉张、形成了大规模的A2型花岗岩岩浆活动和锡（钨）成矿，南海的形成是弧后拉张的结果。