Trace element geochemistry of altered volcanic ash layers (tonsteins) in Late Permian coal-bearing formations of eastern Yunnan and western Guizhou Provinces, China
Trace element geochemistry of altered volcanic ash layers (tonsteins) in Late Permian coal-bearing formations of eastern Yunnan and western Guizhou Provinces, China
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DOI：
10.1016/s0166-5162(00)00017-3
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发表时间：
2000-09
影响因子：
5.6
通讯作者：
Yiping Zhou;B. Bohor;Youliang Ren
中科院分区：
文献类型：
--
作者：
Yiping Zhou;B. Bohor;Youliang Ren
采用仪器中子活化分析（INAA）对云南东部和贵州西部晚二叠世煤系煤层中30个同沉积火山灰成因的砂岩和碎屑粘土样品进行了微量元素组成测定。其微量元素地球化学特征与碎屑粘土具有明显的区别，因为碎屑粘土具有独特的火山灰成因。碎屑粘土具有较高的V、Ti、Sc、Cr、Co、Ni含量，较低的Th、U含量和Th/U比值，较小的Eu负异常（Eu/Eu* 0.63 ~ 0.93）。总体而言，这些微量元素特征与研究区西缘侵蚀区玄武岩组成相似的基性烃源岩相一致。岩体中V、Ti、Sc、Cr、Co、Ni含量低，Th/U比值高，Eu负异常明显（Eu/Eu*一般在0.2 ~ 0.4之间），具有硅质岩浆源特征。组内煤组下部（P2.1）矿体的Ti、Zr、Hf、Nb、Ta和稀土元素含量高于中上部（P2.2+3）矿体。在Hf-Ta、Ti-Ta、Ti-V、Hf-Sc、Lu-Hf和Lu-Th微量元素判别图（散点图）中，p2.1层位的重晶石始终落在孤立分布区域，与P2.2+3层位的重晶石相分离。这些结果表明，来自两个不同层位的火山火山的物质来源可能来自两种性质截然不同的火山灰落体。通过对不同岩浆岩间微量元素含量和组合的比较，发现p2.1层位的矿源物质主要由钙碱性、贫硅火山灰（类似流纹岩岩浆）组成，而P2.2+3层位的矿源物质则明显以富硅富钾火山岩（流纹岩岩浆）为主。因此，来自两个不同层位的矿石具有独特的地球化学性质，这些地球化学性质在很大的横向范围内保持不变。将微量元素组成与矿物学和结构观测相结合，可以建立顿斯坦地层学，从而促进更精确和可靠的煤层对比。
Trace element compositions were determined (by instrumental neutron activation analysis; INAA) in 30 samples of synsedimentary volcanic ash-derived tonsteins and detrital claystones from coal seams within the late Permian coal-bearing formation of eastern Yunnan and western Guizhou Provinces, China. The characteristics of trace-element geochemistry in the tonsteins can be distinguished from those of detrital claystones because of the former's unique volcanic-ash origin. The detrital claystones are characterized by their relatively high content of V, Ti, Sc, Cr, Co and Ni, relatively low content of Th and U, Th/U ratio, and small negative Eu anomaly (Eu/Eu* 0.63–0.93). Overall, these trace element characteristics are consistent with a mafic source similar to the composition of basalt rocks in the erosional region on the western edge of the study area. In contrast, the tonsteins are low in V, Ti, Sc, Cr, Co and Ni contents and have a high Th/U ratio with a distinct negative Eu anomaly (Eu/Eu* normally in the range of 0.2–0.4), consistent with a silicic magmatic source. Within the group of tonsteins, those from the lower section (P2.1) of the coal-bearing formation are relatively high in Ti, Zr, Hf, Nb, Ta and rare earth elements (REE), as compared to those from the middle and upper sections (P2.2+3). In trace-element discrimination diagrams (scatter plots) of Hf–Ta, Ti–Ta, Ti–V, Hf–Sc, Lu–Hf and Lu–Th, tonsteins from the P2.1horizon always fall in isolated distribution areas, separate from the tonsteins of the P2.2+3horizon. These results suggest that the source materials of tonsteins from the two separate horizons were probably derived from volcanic ash falls of two distinctly different natures. Based on a comparison of the concentrations and assemblages of trace elements between various magmatic rocks, the source materials of tonsteins from P2.1horizon were mostly composed of calc-alkalic, silica-poor volcanic ash (similar to rhyodacitic magma), whereas those from P2.2+3were apparently more siliceous and K-rich (rhyolitic magma). Thus, tonsteins from the two different horizons are characterized by unique geochemical properties, which remain constant over a wide lateral extent. Integration of trace-elemental compositions with mineralogical and textural observations makes possible the establishment of tonstein stratigraphy, thus, facilitating more precise and reliable coal-seam correlations.