板块俯冲起始机制-来自伊豆小笠原俯冲体系的地球化学制约

Plate tectonics is the cornerstone of solid earth science. How plate subduction initiated and how to create a new subduction zone are main issues and hot topics of solid earth science. The controversy of the subduction initiation mechanism focused on spontaneous or induced subduction. The spontaneous subduction may happen at passive margin collapse, transform collapse and plume head margin collapse, while induced subduction formed as a result of changing of plate motions. Izu-Bonin subduction zone in the west Pacific is recognized as one of the best place to study subduction initiation. The applicant attended International Ocean Drilling expedition 351 drilled at Izu-Bonin back arc basin and got relative basalt samples formed as magma response of the subduction initiation. This application chooses these basalt to determine helium isotope of pyroxene, osmium isotope of spinel, and major and trace elements of minerals in the basalt, to trace source characteristics of the basalt and the magma forming conditions. The role of mantle plume, ridge related transform fault, ancient continental origin, that played in subduction initiation mechanism will be studied to discriminate spontaneous or induced subduction at Izu-Bonin, and reveal mechanism of subduction initiation based on geochemical information.

板块构造理论是固体地球科学的基石。板块俯冲如何开始、如何形成新的俯冲带是固体地球科学研究的重点和热点问题。关于板块俯冲起始机制的争论主要集中在自发式俯冲还是诱导式俯冲。其中，自发式俯冲可能发生在被动陆源的洋陆过渡带、大洋中的断裂带（转换断层）、以及大火成岩省周边；而诱导式俯冲通常是大洋板块受力而改变向大陆俯冲方向。西太平洋伊豆小笠原俯冲体系是公认的研究俯冲起始机制的最佳天然场所之一。申请人参加了国际大洋钻探在该地区进行的钻探，获得了俯冲起始阶段形成的弧后盆地玄武岩岩心样品。本项目以该航次钻到的弧后盆地玄武岩为研究对象，通过对玄武岩中辉石氦同位素，尖晶石锇同位素以及单矿物主微量元素的测定，对起始俯冲形成的玄武岩源区特征、岩浆形成条件等进行研究，探索地幔柱组份，扩张脊相关的断层，以及古老大陆起源在俯冲起始机制中的作用，从而为判别区内自发式俯冲还是诱导式俯冲，揭示俯冲起始机制提供地球化学制约。

板块构造理论是固体地球科学的基石。板块俯冲起始是研究板块构造理论的难点，也是固体地球科学研究重点和热点问题。板块俯冲起始机制主要的争议是自发式俯冲起始和诱导式俯冲起始。西太平洋伊豆小笠原俯冲体系是研究板块俯冲起始的最佳天然场所之一。本项目对国际大洋钻探在该体系奄美盆地钻取的玄武岩样品，开展了全岩及单矿物主微量元素的研究，氦同位素研究和锇同位素研究。结果显示奄美盆地玄武岩形成于俯冲起始阶段末期向岛弧起源转折的这个关键点。玄武岩具有初始岩浆的特征，记录了的俯冲起始阶段地幔源区特征是难熔的、亏损的、还原的地幔源区。全岩及单斜辉石都具有比大洋中脊玄武岩更亏损的轻稀土特征，指示了难熔、亏损的地幔源区未收到俯冲组分的加入。初始的尖晶石具有低铁比值指示岩浆来自还原环境，未受到俯冲组分的改变。全岩稀土元素特征，单斜辉石和尖晶石组分都指示奄美盆地玄武岩与大洋中脊玄武岩，弧后盆地玄武岩，岛弧及洋岛玄武岩的组分都有所不同。低氦同位素比值和锇同位素比值都未显示地幔柱和大陆组分。奄美盆地玄武岩温压结果显示岩浆具有较高的温度，来自60Km深处的尖晶石橄榄岩相，并且岩浆从该深度迅速上升至地表，指示俯冲起始阶段存在挤压环境，该成果为新生代西太平洋诱导式俯冲提供了关键证据。