台湾利吉混杂岩基性-超基性岩块来源与弧陆碰撞过程

The Lichi mélange in eastern Taiwan is the product of the collision of the Luzon Arc with the Eurasian Continent (arc-continent collision). The sources of all kinds of exotic blocks are the key factors for explaining how the Lichi mélange formed, especially for mafic and ultramafic blocks, which can represent oceanic crust and upper mantle. The source of these mafic and ultramafic blocks is the key point of studying the process of arc-continent collision and also has a very important significance on study of the tectonic characteristics of the seas nearby Taiwan and the tectonic evolution of the margins of the Western Pacific.. We are going to analysis petrology, mineralogy, geochemistry and chronology of the mafic and ultramafic rocks in Lichi mélange in the project, and compare with the data on the South China Sea and the Philippines Sea, and then we can discuss the original tectonic environment and tectonic activity of these mafic and ultramafic blocks, with the aim of the source of the mafic and ultramafic rocks. We will discuss the formation of the Lichi mélange on the basis of lithologic association and mixing form of the mafic and ultramafic rocks combined with other blocks sources, and establish the formation model of the Lichi mélange for providing more details of the arc-continent collision in Taiwan.

现有研究表明台湾东部利吉混杂岩是由于吕宋岛弧与欧亚大陆发生碰撞而形成的，确定利吉混杂岩中的各种外来岩块的来源是解析混杂岩形成的关键因素，对代表洋壳和上地幔的基性-超基性岩来源研究是了解混杂岩形成的必要条件之一，也是解析弧陆碰撞过程的重点，对研究台湾附近海域构造特征、西太平洋板块边缘构造演化过程都具有十分重要的意义。本项目通过对利吉混杂岩基性-超基性岩的岩石学特征、矿物组合、地球化学特征以及年代学等分析，结合邻近的南海、菲律宾海等相关资料，探讨这些基性和超基性岩块的原始构造环境及其在混杂岩形成过程中的构造活动，进一步判断这些基性-超基性岩来源，从混杂岩基性-超基性岩组合形式和混入方式的角度，结合其它外来岩块的来源，探讨利吉混杂岩的形成过程，建立利吉混杂岩的演化模式，细化弧陆碰撞的演化过程。

利吉混杂岩位于吕宋岛弧与欧亚大陆发生碰撞（弧陆碰撞）关键地带，是菲律宾海板块和欧亚板块结合带重点研究的区域，其中出露的蛇纹岩可以为研究弧陆碰撞造山过程提供重要细节信息。本项目岩石学和矿物学分析得到利吉混杂岩中蛇纹岩岩块原岩为橄榄岩。利吉混杂岩蛇纹岩的全岩主量元素分析表明，Al2O3含量较低，也即说明原岩为橄榄岩，稀土元素配分曲线与混杂岩中未蚀变的辉长岩稀土元素配分曲线形态一致，初步证明蛇纹岩与辉长岩一致来源。通过锆石U-Pb年代学分析，得到代表洋壳年龄的橄榄岩形成年龄为17.7±0.5Ma，结合南海洋壳、菲律宾海洋壳年龄和构造背景，判断利吉混杂岩中的蛇纹岩（蛇纹石化橄榄岩）来自马尼拉俯冲系统的弧前盆地，由于第一次向东逆冲过程中形成的花东海脊在变形程度和规模上都比较小，逆冲断层不足以达到弧前盆地基底的深度，而第二次向西的逆冲作用使得弧前盆地整体逆冲变形、隆升成山脉，形成现今的利吉混杂岩，因此来自弧前盆地基底的超基性岩是在利吉混杂岩形成过程中经第二次逆冲混入的。明确蛇纹岩岩块来源可以为利吉混杂岩形成提供细节信息，为研究弧陆碰撞造山活动和板块运动提供可靠依据和细节过程。