Subduction initiation and ophiolite crust: new insights from IODP drilling

International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

国际大洋发现计划（International Ocean Discovery Program, IODP）第352科考航次采获了博宁弧前区域内与俯冲起始作用相关的火山作用高保真记录。两个位于海沟附近深水区域的钻探站位（U1440与U1441）采获了玄武岩及其伴生岩石；另外两个站位（U1439与U1442）地处远离海沟的较浅水区，采获了玻安岩及其伴生岩石。两处区域的钻探最终均钻入被推断为席状侵入岩的辉绿岩地层中。该套玄武岩显然形成于俯冲起始后的即刻喷发阶段，其地球化学组成与洋中脊快速海底扩张形成的极度亏损玄武岩高度相似，几乎不含或完全不含板片组分。随着俯冲作用持续推进、火山活动向远离海沟的方向迁移，后续形成玻安岩的熔融过程涉及更为亏损的地幔源区，以及更热、更深的俯冲带组分。这套火山岩序列与诸多蛇绿岩所记录的火山序列高度相似，佐证了俯冲起始、弧前扩张与蛇绿岩成因之间的直接关联。