“Moist MORB” axial magmatism in the Oman ophiolite: The evidence against a mid-ocean ridge origin
“Moist MORB” axial magmatism in the Oman ophiolite: The evidence against a mid-ocean ridge origin
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DOI：
10.1130/g33904.1
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发表时间：
2013-04
期刊：
影响因子：
5.8
通讯作者：
C. MacLeod;C. Lissenberg;L. Bibby
中科院分区：
文献类型：
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作者：
C. MacLeod;C. Lissenberg;L. Bibby
阿曼蛇绿岩具有轴向火山岩套和伴生的席状岩墙杂岩，其成分与现代大洋中脊玄武岩(MORB)相似。它的内部结构被许多人认为可以直接与东太平洋隆起的海洋岩石圈相媲美。然而，关于蛇绿岩形成的地球动力学背景以及可以在多大程度上进行类比，长期以来一直存在争议，因为类似MORB的轴向火山被包括贫化弧拉斑玄武岩和博安岩在内的熔岩覆盖。对一些人来说，这意味着整个蛇绿岩形成于俯冲带上方；另一些人则坚持认为，蛇绿岩形成于真正的开阔大洋莫尔，产生弧形岩浆所需的水来自下降的近轴热液或来自古代俯冲。一种流行的折衷假设是，轴向岩套形成于真正的MOR，而后来的岩浆作用证明了逆冲作用的开始。我们通过重新检查早期轴向熔岩和岩墙的“类MORB”特征来检验这些模型。我们表明，分馏趋势要求水的存在浓度明显高于任何开阔大洋的MORB；相反，趋势与弧后盆地和大洋内弧前火山的趋势相同。通过证明整个蛇绿岩是在一个含水系统中形成的，我们排除了所有阿曼蛇绿岩形成于远洋MOR的模式；相反，它形成于(可能是新开始的)俯冲带上方的海底扩张中心。
The Oman ophiolite has an axial volcanic suite and associated sheeted dike complex similar in composition to modern mid-oceanic ridge basalt (MORB). Its internal structure is regarded by many as being directly comparable to ocean lithosphere from the East Pacific Rise. However, there has long been controversy over the geodynamic setting in which the ophiolite formed, and the extent to which the analogy can be drawn, because the MORB-like axial volcanics are overlain by lavas that include depleted arc tholeiites and boninites. To some, this implies that the entire ophiolite formed above a subduction zone; others maintain that it formed at a true open-ocean MOR, and that the water required to generate the arc-like magmas derived from descending near-axis hydrothermal fluids or from ancient subduction. A popular compromise posits that the axial suite formed at a true MOR and the later magmatism documents the initiation of obduction. We test these models by reexamining the “MORB-like” character of the early axial lavas and dikes. We show that fractionation trends require the presence of water at concentrations significantly higher than any open-ocean MORB; instead, trends are identical to those of backarc basin and intraoceanic forearc volcanics. By showing that the entire ophiolite formed in a hydrous system, we rule out all models in which the Oman ophiolite was generated at an open-ocean MOR; instead, it formed at a submarine spreading center above a (probably newly initiated) subduction zone.