Omphacite Melting and the Destruction of Early High‐Pressure Rock Records
Omphacite Melting and the Destruction of Early High‐Pressure Rock Records
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DOI：
10.1029/2023jb027395
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发表时间：
2023-11
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影响因子：
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通讯作者：
Shuaiqi Liu;Guibin Zhang;Lifei Zhang;A. A. G. Webb-A.
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作者：
Shuaiqi Liu;Guibin Zhang;Lifei Zhang;A. A. G. Webb-A.
榴辉岩的存在标志着高压变质作用和地壳深冷埋藏，因此被广泛认为是现代板块构造开始的指标。然而，榴辉岩很少保存在许多大量麻粒岩叠印的造山带中，这在古代变质地体（即太古代和古元古代地体）中尤其常见。在这里，我们表明榴辉岩熔融过程可能是高压记录保存不良的主要原因。我们通过对榴辉岩和喜马拉雅中部分离的厘米级无色体进行详细的岩石学、年代学和地球化学分析，证明了榴辉岩的熔化。喜马拉雅中部的榴辉岩受到强烈的麻粒岩相变质作用的影响，使得绿辉石仅保存稀疏。热力学模拟结果表明，榴辉岩经历了两种类型的深熔反应：绿辉石存在下的高压多硅白云石脱水熔融，以及随后折返过程中绿辉石主导的熔融。这些熔化反应的诊断特征包括 (a) Kfs-Pl-Qz 多相固体包裹体和 Cpx-Pl-Qz 多矿物包裹体的出现； (b) 无色体中高 Na2O/K2O 熔化和富钠斜长石； (c) 绿辉石和隐色体之间一致的微量元素模式； (d) 单斜辉石中存在明显的 Na2O 环带。以绿辉石为主的熔化的特点是硬玉分解，将钠和铝释放到熔体中。这种熔化机制随后形成钠含量较低的单斜辉石和高 Na2O/K2O 熔体。由于高热梯度和强烈的麻粒岩相叠印，这些喜马拉雅发现似乎与早期地球探索相关，这意味着以绿辉石分解为主的榴辉岩的部分熔融可能抹去了现代板块构造的早期高压记录。
The presence of eclogite marks high‐pressure metamorphism and deep, cold burial of crust, and is therefore a widely accepted indicator for the onset of modern‐style plate tectonics. However, eclogite is rarely preserved in many heavily granulite‐overprinted orogens, which are particularly common across ancient metamorphic terranes (i.e., Archean and Paleoproterozoic terranes). Here, we show that eclogite melting processes may be principally responsible for the poor preservation of high‐pressure records. We demonstrate eclogite melting via detailed petrological, geochronological, and geochemical analyses on eclogites and separated centimetric leucosomes from the central Himalaya. The central Himalayan eclogites were overprinted by strong granulite‐facies metamorphism, such that omphacite is only sparsely preserved. Thermodynamic modeling results indicate the eclogite experienced two types of anatectic reactions: phengite dehydration melting at high‐pressure in the presence of omphacite, and subsequent omphacite dominated melting during exhumation. Diagnostic features for these melting reactions include (a) the occurrence of Kfs‐Pl‐Qz multiphase solid inclusions and Cpx‐Pl‐Qz polymineralic inclusions; (b) the high Na2O/K2O melts and Na‐rich plagioclase in leucosomes; (c) the consistent trace elements patterns between omphacite and leucosomes; and (d) obvious Na2O zonation in clinopyroxene. Omphacite dominated melting is characterized by the jadeite breakdown, releasing Na and Al into the melts. This melting mechanism subsequently forms a less sodic clinopyroxene and high Na2O/K2O melts. These Himalayan findings appear relevant to early Earth explorations because of the high thermal gradients and intense granulite‐facies overprints, implying that partial melting of eclogite dominated by omphacite breakdown could have erased early high‐pressure records of modern‐style plate tectonics.