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）352航次获取了博宁弧前区域与俯冲起始（subduction initiation）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站位（U1440与U1441）采获了玄武岩（basalt）及其相关岩石；另外两个位于远离海沟的较浅水区的站位（U1439与U1442）则采获了玻安岩（boninite）及其相关岩石。两个区域的钻探最终均钻遇粗玄岩（dolerite），其被推断为席状侵入岩。这些玄武岩显然是在俯冲起始后即刻喷发的，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相似，几乎或完全没有俯冲板片（slab）的物质补给。随着俯冲作用持续演化且火山作用向远离海沟的方向迁移，后续形成玻安岩的熔融作用涉及更为亏损的地幔物质，以及温度更高、来源更深的俯冲组分。该火山序列与众多蛇绿岩所记录的火山序列高度相似，这佐证了俯冲起始、弧前扩张与蛇绿岩成因之间存在直接关联。