东劳扩张中心玄武质岩石的地球化学特征及其对地幔源区与板片俯冲效应的约束

Back-arc basin, as the mainly component in the "trench-arc-back arc" system at convergent margins, is a natural laboratory for studying the geochemical reservoir and the effects of slab-subduction. Lau basin, located in the southwest of the Pacific Ocean, is one of the classic modern back-arc basins. The Eastern Lau Spreading Center (ELSC) showed the significant changes in petrographical, geochemical and geophysical characteristics along strike in consistent with the proximity to the volcanic arc to the west, probably indicating the control of slab-subduction on axial magmatism. Meanwhile, the geochemical and geophysical discontinuities (e.g., Nb, Ta) were also found in the Central Eastern Lau Spreading Center that probably suggested variances in mantle source, subducted materials and effects of slab-subduction. However, the mechanism for these variances is still in debate. The project aims at the basaltic rocks in cruise "DY115-19-4" in ELSC , based on petrology, whole-rock major and trace element and Sr-Nd-Pb-Hf-Os isotopic compositions, to reveal the compositions of primary magma, potential mantle domains, the sources of subducted materials and to discuss the effects of slab-subduction on axial magmatism in different stages of basin evolution. Furthermore, the mantle heterogeneity beneath the ELSC and the effects of Pacific oceanic lithosphere subduction on the geochemistry of lavas will also be discussed, providing a better insight into the models of evolution of back-arc basins and the global chemical recycling.

弧后盆地是汇聚板块边缘"沟-弧-盆"体系的重要组成部分，是研究地球化学储库和板片俯冲效应的理想场所。位于西南太平洋的劳盆地是一个典型的现代弧后盆地，其中的东劳扩张中心沿走向存在明显的岩石学、地球化学和地球物理特征的梯度变化，并与岛弧的趋近程度密切相关，反映俯冲对其岩浆作用的影响。同时，东劳扩张中心的中部存在地球化学特征的明显跳跃（Nb、Ta等），可能与地幔源区、俯冲物质及俯冲影响程度的差异有关。而这些差异的形成机制尚未明确。申请人拟对DY115-19-4航段采集的玄武质岩石/玻璃进行系统的岩石学、全岩主量、微量元素和Sr-Nd-Pb-Hf-Os同位素特征的研究，反映原始岩浆成分、甄别地幔端元和俯冲物质来源，了解处于弧后盆地不同发育阶段的区域受俯冲影响的程度，进一步探讨俯冲作用对东劳扩张中心熔岩成分的控制及地幔源区的不均一性，以对现有弧后盆地地质模型的检验和全球化学循环的研究提供可能的启示。

西南太平洋的东劳盆地是一个典型的现代弧后盆地，是研究“沟-弧-盆”体系中地球化学储库和板片俯冲效应的理想场所。申请人针对 DY115-19-4 航段在东劳扩张中心采集的玄武质岩石进行系统的岩石学、全岩主量和微量元素的研究表明：东劳扩张中心南部瓦路法脊（VFR）和中部（CELSC）的熔岩均为典型的弧后拉斑质玄武-安山熔岩。这些样品Mg# 很低，暗示他们前期经历了橄榄石或辉石的分离结晶作用。现主要受斜长石和磁铁矿结晶分异作用的控制。新鲜样品类似于MORB的Fe/Mn比值（CELSC的比值为52-60，VFR的为50-61）以及大于1的拉斑指数（THI）指示这些玄武质岩石很可能来自干（H2O<2wt%）的地幔橄榄岩源区。VFR和CELSC的玄武质岩石大部分样品富集大离子亲石元素 (LILE)，亏损高场强元素 (HFSE)，并显示强烈的Nb-Ta负异常、Pb正异常，而TVG-12系列样品微量元素显示Sr、Ti负异常，和其他CELSC玄武质岩石明显不同，同时Nb-Ta负异常明显较弱。结合矿物化学的特征（如橄榄石、辉石和斜长石），共同指示了VFR和CELSC玄武质岩石其地幔源区很可能是有差异的，同时也表明CELSC在小尺度上岩石地化特征也具有不连续性。不同的Th/Nb和U/Th比值指示CELSC岩石主要是受到了俯冲流体的交代作用，而VFR玄武质岩石主要是受到了俯冲沉积物的交代作用。