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柳江盆地髫髻山组凝灰岩地球化学与熔体包裹体水含量特征

Geochemical Characteristics and Water Content of Melt Inclusions in the Tuff of the Tiaojishan Formation, Liujiang Basin

  • 摘要: 水作为岩浆体系中最主要的挥发组分,对岩浆的形成和演化有重要的影响,柳江盆地髫髻山组岩浆岩是燕山期火山活动的重要产物,尽管前人对其地球化学特征进行了大量研究,但关于柳江盆地燕山期岩浆中的水含量仍不清楚。熔体包裹体记录了原始岩浆信息,是获取岩浆水含量特征的最直接样品。本文基于全岩地球化学分析,利用标准样品建立了显微激光拉曼光谱定量熔体包裹体水含量的标定曲线,并对柳江盆地髫髻山组下部流纹质岩屑-晶屑凝灰岩中石英斑晶内的原生熔体包裹体进行了水含量定量分析。结果表明:髫髻山组下部凝灰岩样品具有富Si、Al、大离子亲石元素富集、高场强元素亏损、轻稀土富集、重稀土亏损、负Eu异常、Sr含量低等特点;熔体包裹体中水含量的定量分析结果为0.99%~4.98%,平均水含量为2.62%,与前人统计的酸性岩浆水含量基本一致。地球化学特征和熔体包裹体水含量分析结果共同揭示了研究区髫髻山期早期为富水酸性岩浆。结合髫髻山期样品的熔体包裹体水含量测定结果及其早期的大规模火山喷发背景,本文认为岩浆中高含水量增强了岩浆系统的喷发动力,是诱发研究区髫髻山期早期大规模火山爆发的有利因素之一。

     

    Abstract: Water, as the primary volatile component in magmatic systems, has a significant impact on the formation and evolution of magma. The Tiaojishan Formation igneous rocks in the Liujiang Basin are significant products of Yanshanian volcanic activity. Although previous studies have extensively explored their geochemical characteristics, the water content of the magma in the Liujiang Basin during Yanshanian volcanic activity remains unclear. Melt inclusions, which capture the original magmatic information, serve as the most direct samples for determining the water content of magma. Based on geochemical analysis, this study quantitatively determines the water content in melt inclusions using laser Raman spectroscopy with standard samples. The results show that the lower tuff samples of the Tiaojishan Formation are characterized by high Si and Al contents, enrichment in LILEs, depletion in HFSEs, enrichment in LREEs, and depletion in HREEs. The water content in melt inclusions reveals a range of 0.99% to 4.98%, with an average of 2.62%. These characteristics jointly indicate the water-enriched acidic magmatic activity during the early Tiaojishan period in this area. Combining the water content of melt inclusions with the large-scale volcanic eruptions in the stage, this study suggests that high water content in the magma enhanced the eruptive dynamics of the magmatic system, making it a contributing factor to the large-scale volcanic eruption. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202404030074.

     

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