东台湾蛇绿岩的源区交代作用及南海俯冲板片的脱水和熔融过程

The Manila trench has been a hot intra-oceanic subduction zone since the middle Miocene when the eastward subduction of the South China Sea (SCS) beneath the Huatung basin took place. The middle Miocene East Taiwan Ophiolite (ETO) is outcropping within the fossil forearc basin of the Taiwan active arc-continent collision zone. A new tectonic fingerprinting argues that the ETO is a piece of forearc basement, which was generated by mantle decompression along a near‐trench spreading center due to the sinking of the SCS lithosphere. Thusly, the ETO is getting to be an ideal lab to reveal a hot intra-oceanic subduction process. This program will carry out two works on the ETO to reveal its source metasomatism which was contributed by the dehydration and fusion of the SCS subducting slab. The first one is the water contents of the ETO basaltic crusts. The high contents of water of the ETO magma should derive from the dehydration of the SCS subducting slab. Secondly, the adakites of the ETO generated by fusion of the hot SCS subducting slab. Metasomatism of the forearc mantle by slab melts would give birth of elementally enriched and isotopically enriched or depleted forearc basalts of the ETO. This program connects the high contents of water and the adakites of the ETO with the dehydration and fusion of the hot SCS subducting slab. The ETO is an opposite case of the Izu-Bonin-Mariana forearc, where the cold Pacific slab can dehydration merely. If indeed, this will broaden our knowledge of subduction process.

马尼拉海沟是南海自中中新世开始向东俯冲消亡的地方，其北段构成典型洋内热俯冲带。最新研究揭示，出露于台湾活跃弧-陆碰撞造山带残留弧前盆地内的中中新世东台湾蛇绿岩，实际上形成于南海初始俯冲的弧前扩张。因此，东台湾蛇绿岩成为了解南海热初始俯冲岩浆响应的天然窗口。本项目拟主要开展两方面研究内容，其一测量东台湾蛇绿岩岩浆系统的水含量，认为高的含水量归因于俯冲板片脱水对弧前地幔的流体交代作用；其二揭示埃达克质岩对基性洋壳地球化学组成变化的支配作用，认为埃达克质熔体形成于南海俯冲板片的高压部分熔融，熔体的交代作用使弧前地幔元素组成更富集、同位素组成更富集或更亏损。本项目借助东台湾蛇绿岩的岩浆演化历史，将能充分了解南海初始俯冲过程中年轻热板片的脱水和熔融过程，是对太平洋板块沿伊豆-小笠原-马里亚纳初始俯冲过程中年老冷板片脱水过程的补充，这将极大丰富和拓展对俯冲带内部过程的认识。

南海沿马尼拉海沟向东俯冲消亡，其俯冲带上覆发育绵延的火山弧和宽阔的弧前，非常类似于太平洋板块西向俯冲形成的伊豆-小笠源-马里亚纳（IBM）岛弧-弧前体系。台湾弧-陆碰撞造山带是马尼拉海沟增生楔-弧前-岛弧体系的陆上延续，本项目对位于台湾造山带增生楔和岛弧之间的东台湾蛇绿岩套开展详细的地质调查、年代学和岩石地球化学研究，确认其洋壳层序以N-MORB型岩石为主，并首次在蛇绿岩套的壳-幔过渡带至地幔橄榄岩层序中发现埃达克岩脉体和E-MORB型辉绿岩岩墙。年代学和岩石地球化学分析揭示埃达克岩脉体与蛇绿岩套所代表的海底扩张同期，是俯冲板片的高压部分熔融成因，唯有初始俯冲引起的海底扩张大地构造背景才能合理解释东台湾蛇绿岩套埃达克岩脉体的形成，指示东台湾蛇绿岩套是台湾弧-陆碰撞引起构造侵位的吕宋弧前。东台湾蛇绿岩的N-MORB型样品呈现不均一的全岩同位素组成，其单斜辉石的红外光谱和矿物化学分析揭示其原始岩浆含有相对开阔大洋洋壳更高的水含量，支持弧前扩张地质构造背景，同时记录了弧前扩张的亏损地幔受含水流体和埃达克质熔体的差异化交代作用。对东台湾蛇绿岩套E-MORB型岩石进一步研究，揭示其形成于富Nb熔体对弧前地幔楔的交代再富集作用，原始富Nb熔体是俯冲板片进一步榴辉岩化之后的部分熔融，对应于年轻南海的初始俯冲。本项目通过台湾岛弧-陆碰撞造山带弧前位置的东台湾蛇绿岩重建了南海的初始俯冲，揭示了初始俯冲过程中年轻南海俯冲板片的脱水熔融和地幔楔的交代富集等过程。