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首页> 外文期刊>Lithos: An International Journal of Mineralogy, Petrology, and Geochemistry >Late Triassic high-Mg andesite/dacite suites from northern Hohxil, North Tibet: Geochronology, geochemical characteristics, petrogenetic processes and tectonic implications
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Late Triassic high-Mg andesite/dacite suites from northern Hohxil, North Tibet: Geochronology, geochemical characteristics, petrogenetic processes and tectonic implications

机译：藏北北部呼和浩特地区晚三叠世高镁安山岩/ d石套件：地质年代学，地球化学特征，成岩过程和构造意义

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摘要

This study reports age, petrologic and geochemical data for andesites and dacites from the Late Triassic sedimentary strata of northern Hohxil, in the Hohxil-Songpan-Ganzi Block (northern Tibet), which constitutes the most voluminous Triassic strata on Earth. LA-ICP-MS zircon U-Pb analysis of dacite (210.4 ± 1.9 Ma) and whole rock ~(40)Ar-~(38)Ar analyses for both the andesites and dacites (211 ± 2 Ma and 210.9 ± 1.6 Ma) show that the rocks were almost contemporaneous. Both rock types are sodium-rich and calc-alkaline. The andesites, characterized by high MgO (up to 10 wt.%) or Mg~# (- 70), TiO2, Al2O3, Cr, Ni, La/Yb and Th/La, but low Nb/Ta ratios, are geochemically similar to sanukitoids in southeastern Japan. The dacites are strongly peraluminous, and have high Al2O3 and La/Yb, low Y and Yb, coupled with negligible to positive Eu and Sr anomalies, comparable to slab-derived adakites in the circum Pacific arc system. Both rocks exhibit strongly fractionated platinum group element patterns, with Pt/Pt* (Pt anomaly), (Pt/Ir)_N and Re/Os ratios higher than those of the primitive mantle. They have uniformly low sNd(t) values ( — 7.57—9.59) and high (~(86)Sr/~(87)Sr)_i ratios (0.7086-0.7106) that imply a continental rather than oceanic type magma source. We suggest that the northern Hohxil dacites were produced by partial melting of subducted sediments on the northward-subducting Songpan-Ganzi oceanic slab, and the high-Mg andesites were formed by subsequent interaction between the sediment-derived melts and mantle wedge peridotites. Taking into account the Triassic magmatic record from nearby regions, we suggest that the Late Triassic high-Mg andesite/dacite suites of northern Hohxil were generated in a forearc setting, and propose that double-sided subduction eventually closed the Songpan-Ganzi ocean during the Late Triassic.
机译：这项研究报告了Hohxil-Songpan-Ganzi区块（西藏北部）北部Hohxil的晚三叠世沉积地层中安山岩和dacits的年龄，岩石学和地球化学数据，该地区构成了地球上最大的三叠纪地层。 LA-ICP-MS镁铁矿（210.4±1.9 Ma）和整个岩石〜（40）Ar-〜（38）Ar分析安山岩和镁铝石（211±2 Ma和210.9±1.6 Ma）的锆石U-Pb分析表明岩石几乎是同期的。两种岩石类型都富含钠和钙碱性。镁锰矿的地球化学特征是高MgO（至多10 wt。％）或Mg〜＃（-70），TiO2，Al2O3，Cr，Ni，La / Yb和Th / La，但Nb / Ta比低到日本东南部的sanukitoids。 dacites的钙铝酸盐含量很高，Al2O3和La / Yb较高，Y和Yb较低，与Eu和Sr的正异常相差无几，与环太平洋太平洋弧系统中的板状玄武岩相当。两种岩石均表现出强烈的铂族元素分布格局，其Pt / Pt *（Pt异常），（Pt / Ir）_N和Re / Os比值均高于原始地幔。它们的sNd（t）值一致较低（-7.57-9.59），并且具有较高的（〜（86）Sr /〜（87）Sr）_i比（0.7086-0.7106），这意味着是大陆型而非海洋型岩浆源。我们认为北部的Hohxil dacites是由向北俯冲的松潘—甘孜洋洋板块上俯冲沉积物的部分融化产生的，而高镁安山岩是由随后的沉积物衍生的熔体与地幔楔形橄榄岩之间的相互作用形成的。考虑到来自附近地区的三叠纪岩浆记录，我们建议北部霍赫西尔的三叠纪晚期高镁安山岩/针铁矿套件是在前极环境中产生的，并建议在两次地震作用下进行双面俯冲最终封闭了松潘-甘孜海。三叠纪晚期。

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