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首页> 外文期刊>Contributions to Mineralogy and Petrology >Geochemistry of Eocene-Early Oligocene low-temperature crustal melts from Greater Himalayan Sequence (Nepal): a nanogranitoid perspective
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Geochemistry of Eocene-Early Oligocene low-temperature crustal melts from Greater Himalayan Sequence (Nepal): a nanogranitoid perspective

机译：来自较大喜马拉雅序列（尼泊尔）的何种寡核苷酸低温外壳熔体地球化学：纳米毒素观点

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摘要

Despite melt inclusions in migmatites and granulites provide a wealth of information on crustal anatexis in different geological scenarios, a complete compositional study (including trace elements and H2O) is yet to be made for the Himalayan rocks. In this contribution, we focus on nanogranitoids occurring in peritectic garnet of migmatites from Kali Gandaki valley in central Nepal (Greater Himalayan Sequence, GHS). The microstructural position of the nanogranitoids proves that these melts were produced at 650-720 degrees C and 1.0-1.1 GPa, during the Eohimalayan prograde metamorphism (41-36 Ma) associated with crustal thickening. Nanogranitoid compositions (mostly granodiorites, tonalities and trondhjemites) resemble those of experimental melts produced during H2O-present melting of meta-sedimentary rocks. They have variable H2O concentrations (6.5-14.4 wt%), which are similar to the expected minimum and maximum values for crustal melts produced at the inferred P-T conditions. These compositional signatures suggest that melt formation occurred in the proximity of the H2O-saturated solidus, in a rock-buffered system. The low-to-very low contents of Zr (3-8 ppm), Th (0.1-1.2 ppm) and LREE (4-11 ppm) along with the weak-to-moderate positive Eu anomalies (Eu/Eu* = 1.2-3.3), the high B concentrations (200-3400 ppm) and the high U/Th ratio (up to 21) point to the lack of equilibration between melt and accessory minerals and are consistent with melting of plagioclase at low temperature. Kali Gandaki nanogranitoids record the beginning of melting in a H2O-present system that, in other GHS localities, may have produced voluminous crustal melts. We demonstrate that compositional comparison with nanogranitoids may be useful to reconstruct the petrogenesis of Eohimalayan granitoids.
机译：尽管Migmatites和Magulites中的熔体夹杂物，但在不同地质情景中提供了关于地壳anatexis的丰富信息，但尚未为喜马拉雅岩石制造完整的组成研究（包括痕量元素和H2O）。在这一贡献中，我们专注于来自Kali Gandaki Valley的Migmatites的覆盖物中的纳米蛋白在尼泊尔中部（大喜马拉雅序列，GHS）。纳米蛋白的微观结构位置证明，在与地壳增厚相关的Eohimalayan Proprade变质（41-36mA）期间，在650-720℃和1.0-1.1GPa下产生这些熔体。纳米蛋白组合物（大多是Granodiorites，色调和Trondhjemites）类似于在H2O-目前沉积岩中产生的实验熔融的实验熔体。它们具有可变的H2O浓度（6.5-14.4wt％），其类似于在推断的P-T条件下产生的地壳熔体的预期最小值和最大值。这些成分签名表明，在岩石缓冲系统中，在H2O饱和固体的附近发生熔体形成。 Zr（3-8ppm），TH（0.1-1.2ppm）和lee（4-11ppm）的低至极低含量以及弱于中度态欧欧欧马目（EU / EU * = 1.2 -3.3），高B浓度（200-3400ppm）和高U / Th比（最多21）点缺乏熔体和辅助矿物质缺乏平衡，并且与低温下Plagioclase的熔化一致。 Kali Gandaki Nanogranitoids记录在H2O-invaly系统中融化的开始，在其他GHS的地方，可能产生了大量的地壳熔体。我们证明与纳米蛋白的组成比较可用于重建Eohimalayan花岗岩的纤维化。

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