中美洲俯冲带南段上覆板块沉积物化学组分及其地质意义

在剥蚀型汇聚板块边缘,俯冲输入板块剥蚀上覆板块并将剥蚀物质带入俯冲隧道,随后,这些剥蚀物质与俯冲板块物质一同参与了俯冲带浅部与深部地幔的地球化学循环。构造地质学和地球物理研究显示中美洲俯冲带南段是典型的俯冲剥蚀型汇聚板块边缘,这为研究上覆板块俯冲剥蚀物质是否参与俯冲带物质循环过程提供了天然的实验室。由于目前仍然缺乏对该俯冲剥蚀机制的地球化学制约,综合大洋钻探计划(IODP) 344航次对中美洲俯冲带南部哥斯达黎加西部的俯冲板块和上覆板块开展了钻探工作,并获取了系统的岩芯样品。本文对IODP344航次取自上覆板块中陆坡和上陆坡的U1380和U1413站位中沉积物中的粗碎屑层位样品,开展了系统的主、微量元素与Sr-Nd-Pb-Hf同位素地球化学研究。这些沉积物粗碎屑层位组分与加勒比大火成岩省基底相似,指示它们可能来自加勒比大火成岩省基底在弧前出露的区域,可以代表上覆板块基底被俯冲剥蚀的物质组成。研究进一步探讨了晚中新世中美洲俯冲带南部大陆弧火山岩的成因,并指出上覆板块底部被剥蚀物质参与了中美洲俯冲带南部大陆弧火山岩岩浆过程,这为中美洲俯冲带南部存在俯冲剥蚀过程提供了直接的地质学证据。

Chemical composition of sediments from the upper plate at the southern Central American subduction zone and its geological significance

At the erosion convergent margins, the subduction input slab denuded the base of the upper plate and brought the eroded materials into subduction channel. Then, the eroded material of the base of upper plate, together with the subduction plate material, were involved in the crust-mantle interaction and deep mantle processes. Previous geological and geophysical studies have shown that the southern Central American (SCA) subduction zone is a typical convergence margin with subduction erosion, but there is still lack of a geochemical constraint on the subduction erosion mechanism. The Integrated Ocean Drilling Program (IODP) Expedition 344 have drilled the subducting plate and the upper plate offshore the Costa Rica at SCA subduction zone. Here, we report the comprehensive major, trace element and Sr-Nd-Pb-Hf isotopes analyses of coarse-grained layers from sites U1380 and U1413, which are located in the middle and upper slope of the upper plate. The results show that the chemical compositions of the coarse-grained sediments from these two sites are similar to the Caribbean Large Igneous Province (CLIP) basement component, indicating that these coarse-grained sediments may come from the exposed areas of CLIP basement at fore-arc region, which could represent the subducted eroded materials from the upper plate basement. Furthermore, we find that the eroded materials from the upper plate basement have been involved in the genesis of the volcanic lavas at SCA since Late Miocene, which in turn provide a direct geological insight for the existence of the subducted erosion process in the SCA.