大同新生代火山岩浆的快速喷发——来自橄榄石地幔捕掳晶的证据
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P588.14+5;P588.11

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中央高校基本科研业务费(2652018120);中国地质大学(北京)"求真学人"计划(265QZ201901);国家自然科学基金优秀青年科学基金(41922012);中国地质大学(北京)创新创业训练计划(S202111415017)


Rapid ascent of basaltic magma beneath the Datong Cenozoic volcanic field: Evidence from mantle olivine xenocrysts
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    摘要:

    量化研究幔源岩浆从源区运移至喷发或者侵位的时间尺度,对理解基性岩浆作用具有重要意义。然而,对于岩浆的喷发和就位的时间尺度研究仍缺乏有效的约束,位于我国华北克拉通北部山西大同新生代火山岩群是理想的研究地区。本文以该火山群~0.2 Ma喷发的神泉寺碱性玄武岩为研究对象,重点研究其中携带的地幔橄榄石捕掳晶来约束喷发前的时间尺度。通过对其开展详细的矿物化学研究,发现地幔捕掳晶核部的Fo值高达97.7,为极富镁橄榄石,结合其极低的Ca、Mn和Ni含量特征,认为它们捕获自被交代的地幔橄榄岩。另外,地幔橄榄石捕掳晶发育明显的CaO成分环带,表明其在地壳岩浆系统内经历了复杂的岩浆演化过程。地幔橄榄石捕掳晶反应边宽度变化很大,说明它们在源区被捕获时及在运移过程中经过了多次破碎过程。橄榄石捕掳晶最边缘的Fo值为70左右,平衡计算表明它们在边部已与主岩浆(碱性玄武岩)达到平衡。Fe-Mg元素扩散计时结果显示,橄榄石地幔捕掳晶仅在岩浆中滞留了几个月的时间。对于40~70km的岩石圈地幔厚度来说,岩浆平均上升速率最快可能超过500m/d。

    Abstract:

    Quantitatively determining the timescale during mantle-derived magma ascent from the source to eruption or emplacement is essential for the understanding of magmatism, however, the timescales of eruption/emplacement are still poorly constrained. Shanxi Datong Cenozoic volcanic field, north of the North China Craton, is an ideal area for the investigation. In this study, we focused on the mantle olivine xenocrysts entrained in ca. 0.2 Ma Shenquansi alkali basalt, and its timescale of residence in the host magma prior to eruption. According to mineral chemistry, cores of these mantle olivine xenocrysts have Fo values up to 97.7, which can be defined as extremely magnesian olivine. They are also characterized by the extremely low contents in Ca, Mn and Ni, suggesting they were captured from metasomatized mantle peridotite. Moreover, both of the mantle olivine xenocrysts display complex CaO profiles, attributed to complex magmatic processes in the magma plumbing system. The reaction rim widths of one olivine xenocryst vary significantly, implying it has experienced multiple crack processes when captured or dur- ing transport. The Fo values of the mantle olivine xenocrysts rims are about 70, indicating they are in diffusion equilibrium with the host magma (alkali basalt) at rims. Timescales obtained by Fe-Mg diffusion chronometry of olivine mantle xenocrysts show that they have only resided in the magma for months. For a lithospheric mantle thickness of 40~70 km, the fastest average ascent rate may exceed 500 m/d.

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潘荣昊,朱磊,王思佳,等, 2022. 大同新生代火山岩浆的快速喷发——来自橄榄石地幔捕掳晶的证据[J]. 岩石矿物学杂志, 41(3):519~536.
PAN Rong-hao, ZHU Lei, WANG Si-jia, et al, 2022. Rapid ascent of basaltic magma beneath the Datong Cenozoic volcanic field: Evidence from mantle olivine xenocrysts[J]. Acta Petrologica et Mineralogica, 41(3): 519~536.

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  • 收稿日期:2021-04-20
  • 最后修改日期:2022-02-08
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  • 在线发布日期: 2022-05-17
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