大洋下地壳堆晶岩组成及其对蛇绿岩研究的启示

大洋下地壳是大洋岩石圈和蛇绿岩的重要组成部分，在洋中脊及俯冲带演化以及蛇绿岩成因研究中具有重要的意义。本文总结了不同构造环境形成的大洋下地壳堆晶岩的岩石组合、地球化学组成，以建立起适用于蛇绿岩中堆晶岩的构造环境判别标志。洋中脊和俯冲相关环境堆晶岩在Pearce图解上均区别于对应的熔岩成分，表明蛇绿岩中的堆晶岩无法应用Pearce图解进行构造环境判别。不同构造环境产出的堆晶岩在岩石组合、结晶顺序和地球化学上存在明显差异：(1)绝大多数洋中脊堆晶岩和弧后盆地堆晶岩较为类似，反映其来源于洋中脊玄武岩型母岩浆低压、贫水体系的分离结晶；(2)中大西洋脊DSDP 334的洋中脊堆晶岩较为类似弧前堆晶岩，是海水蚀变难熔橄榄岩重熔或混染的产物；(3)弧前堆晶岩的岩石学、地球化学特征与亏损的富水、富硅的玻安质熔体的低压分离结晶过程相吻合；(4)洋岛堆晶岩的特征与相对贫水、成分富集的洋岛玄武岩高压分离结晶的特征相吻合。最后，本文总结了应用堆晶岩进行蛇绿岩构造环境判别的一系列岩石学、地球化学指标，并结合日喀则蛇绿岩中的堆晶岩体和辉长岩脉的实例论述堆晶岩在蛇绿岩研究中的应用。

Compositions of cumulates in lower oceanic crust and implications for ophiolite studies

Lower oceanic crust is the essential component of oceanic lithosphere and ophiolite and is, thus, critical to the studies on the evolution of mid-ocean ridge and supra-subduction zone and the origin of ophiolite. Here, we summarize the lithological and geochemical compositions of the lower oceanic crustal cumulates to build up the proxies for tectonic setting discriminations. In Pearce diagrams, both mid-ocean ridge and subduction-related lower oceanic crustal cumulates contrast with the corresponding lavas indicating that Pearce diagrams cannot be applied in the cumulates. The cumulates formed at different environments exhibit contrasting features of lithological associations, crystallization sequence and geochemistry. Most mid-ocean ridge and back-arc basin cumulates are nearly identical, implying they were formed by the low-pressure fractional crystallization of water-poor MORB-like melts. Cumulates from DSDP 334 drilling hole at Mid-Atlantic Ridge are similar to forearc cumulates, representing the product of re-melting or assimilation of altered mantle. The petrological and geochemical characteristics of forearc cumulates are compatible with the low-pressure fractional crystallization of a hydrous, Si-rich and depleted melt. Oceanic island cumulates show features indicative of high-pressure crystallization of a relatively water-poor and enriched melt. Finally, we summarize the essential petrological and geochemical proxies for tectonic setting discriminations for the cumulates in ophiolite and then re-interpret the origin of cumulates and gabbroic veins in Xigaze ophiolite to explain the applications of oceanic cumulates in ophiolite tectonic setting discriminations.