Serpentinites of Different Tectonic Origin in an Exhumed Subduction Complex (New Caledonia, SW Pacific)
Serpentinites of Different Tectonic Origin in an Exhumed Subduction Complex (New Caledonia, SW Pacific)
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DOI：
10.1029/2022gc010395
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发表时间：
2022-07
期刊：
影响因子：
3.7
通讯作者：
Natalie H. Raia;D. Whitney;C. Teyssier;S. Lesimple
中科院分区：
文献类型：
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作者：
Natalie H. Raia;D. Whitney;C. Teyssier;S. Lesimple
由于蛇纹岩在地球地球化学和地球动力学过程中的重要性，人们已经做了大量的工作来辨别它们的母岩来源，包括区分来自俯冲板块的蛇纹岩和来自折返俯冲杂岩的上覆地幔。新喀里多尼亚岛(太平洋西南部)为了解新生代太平洋俯冲过程提供了一个难得的窗口。该岛在暴露保存完好的高压低温俯冲杂岩和世界上最大的超俯冲带蛇绿岩方面是独一无二的。关于俯冲杂岩中蛇纹岩的成因，以往的研究意见不一。本文分析了该杂岩中23个蛇纹岩的全岩主量元素、微量元素和稳定同位素(δD、δ~(18)O)组成。我们的数据揭示了两组不同的蛇纹岩：杂岩北部的I组样品普遍蛇纹岩，具有丰富的重稀土元素和δ18O值，介于+6.7‰和+10.2‰之间。南部蛇纹岩保存了残留的斜方辉石和橄榄石，微量元素亏损，δ18O值较低，介于+5.1~+8.0‰之间。我们解释第I组蛇纹岩来源于下沉的板块地幔，而第II组蛇纹岩来源于上覆地幔楔体，与构造上覆的新喀里多尼亚蛇绿岩的稀土元素地球化学特征极为相似。我们的结果表明，新喀里多尼亚俯冲杂岩是大洋俯冲带上覆地幔楔体和下沉板块中地幔夹带和折返的一种不同寻常的自然记录。
Owing to the importance of serpentinites for planetary geochemical and geodynamic processes, there has been much work discerning the origins of their parent rocks, including distinguishing between serpentinites derived from a subducting plate versus overlying mantle in exhumed subduction complexes. The island of New Caledonia (SW Pacific Ocean) provides a rare window into Cenozoic Pacific subduction processes. The island is unique in exposing both an exceptionally preserved high‐pressure, low‐temperature subduction complex and one of the largest supra‐subduction zone ophiolites in the world. Previous studies disagree on the origin of serpentinites in the subduction complex. In this study, we analyze 23 serpentinites from this complex for whole‐rock major and trace element geochemistry and stable isotope (δD, δ18O) compositions. Our data reveal two distinct groups of serpentinites: Group I samples in the northern portion of the complex are pervasively serpentinized, and exhibit enriched heavy rare earth element (REE) compositions and δ18O values between +6.7‰ and +10.2‰. In contrast, Group II serpentinites in the south preserve relict orthopyroxene and olivine, and show depleted trace element compositions and comparatively lower δ18O values between +5.1‰ and +8.0‰. We interpret Group I serpentinites to derive from downgoing plate mantle, whereas Group II serpentinites derive from overlying mantle wedge, exhibiting remarkable similarity to the REE geochemistry of the structurally overlying New Caledonia ophiolite. Our results establish the subduction complex in New Caledonia as an unusual natural record of the entrainment and exhumation of mantle from both the overlying mantle wedge and the downgoing plate in an oceanic subduction zone.