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    宁夏灵武侏罗系直罗组碎屑岩地球化学特征及源区构造背景分析

    Geochemical characteristics of clastic rocks from the Jurassic Zhiluo Formation in Lingwu, Ningxia and analysis of tectonic background of the source area

      摘要
    • 摘要:
      研究目的 直罗组在整个侏罗纪演化过程中处于湖进与湖退序列的转换时期，其构造背景对研究鄂尔多斯盆地西缘断褶带发育的启动时限意义重大。
      研究方法 本文采用XRF和ICP-MS对宁夏灵武侏罗系直罗组16件碎屑岩样品进行了主微量元素及稀土元素测试分析，旨在揭示灵武直罗组碎屑岩地球化学特征及源区构造背景。
      研究结果 主量元素间相关系数较低，微量元素及稀土元素自身相关系数较高，Al2O3与微量元素Co、Ni、Cr、V、Sc、Li、Cs、Be、Ga、Tl、Cu、Pb、Zn、Sn等元素之间相关系数多大于0.9，TiO2与Nb之间的相关系数为0.98，整体显示了沉积物源以陆源碎屑为主；富集集中型（K>1、CV>1）元素有Zr、U、CaO元素；富集分散型（K>1、CV＜1）元素有Al2O3、Fe2O3、MgO、Sr、Ba、U、Co、V、Sc、Li、Pb；贫乏分散型（K＜1、CV＜1）元素有SiO2、TiO2、MnO、Na2O、K2O、P2O5、Rb、Th、Nb、Ta、Ni、Cr、Cs、Be、Ga、Tl、Cu、Zn、As、Sn及稀土元素；缺乏贫乏集中型（K＜1、CV>1）元素。
      结论 主量元素特征氧化物组合的平均值TiO2（0.54）、TFe2O3+MgO（4.58）、Al2O3/SiO2（0.18）与活动大陆边缘相似，微量元素蛛网图显示直罗组岩石富集Rb、K、U等大离子亲石元素，强烈亏损Nb、Sr、P、Ti，稀土元素球粒陨石标准化配分曲线总体表现为轻稀土富集，重稀土相对亏损的平缓右倾模式，泥岩CIA（70.76~81.88），砂岩ICV（1.02~1.6）>泥岩ICV（0.7~1.14），泥岩ICV基本小于或等于1，指示灵武直罗组没有或经历了很弱的再旋回作用，属于构造活动背景下的初次沉积，分化程度中等。地球化学特征表明灵武直罗组源区构造背景主要与活动大陆边缘相关，与大陆岛弧也有较多联系，总体显示为活动大陆边缘或活动陆缘消减带，而且可能为安第斯型活动大陆边缘。

       

      Abstract:
      This paper is the result of geological survey engineering.
      Objective The Zhiluo Formation represents a transitional phase in the lake's advancement and retreat within the broader context of Jurassic geological evolution. Its tectonic background holds paramount importance in exploring the inception timeline of fault-fold belt development along the western margin of the Ordos Basin. This study focuses on the Zhiluo Formation, specifically clastic rocks, from the Jurassic period in the Lingwu area, Ningxia.
      Methods We conducted analyses using X-ray and ICP-MS techniques to uncover the geochemical characteristics of clastic rocks and the structural context of the source region.
      Results Our results reveal that major element correlation coefficients are generally low, while trace elements and REE exhibit high values. Notably, correlation coefficients between Al2O3 and trace elements such as Co, Ni, Cr, V, Sc, Li, Cs, Be, Ga, Tl, Cu, Pb, Zn, and Sn exceed 0.9, and the correlation coefficients between TiO2 and Nb are at 0.98. This suggests that the primary source of sediment is terrigenous clastic material. Enrichment concentration-type elements (K>1, CV>1) include Zr, U, and CaO, while enrichment dispersion-type elements (K>1, CV < 1) consist of Al2O3, Fe2O3, MgO, Sr, Ba, U, Co, V, Sc, Li, and Pb. Poorly dispersive elements (K < 1, CV < 1) encompass SiO2, TiO2, MnO, Na2O, K2O, P2O5, Rb, Th, Nb, Ta, Ni, Cr, Cs, Be, Ga, Tl, Cu, Zn, As, Sn, and REE. No elements fall under the poor concentration type (K < 1, CV>1).
      Conclusions The average values for key oxide combinations of main elements are TiO2 (0.54), TFe2O3+MgO (4.58), Al2O3/SiO2 (0.58), and Al2O3/SiO2 (0.18), indicating similarity to an active continental margin. A trace element spider diagram demonstrates the rocks of the Zhiluo Formation's richness in Rb, K, and U, which are large ion lithophile elements, and a significant deficit in Nb, Sr, P, and Ti. The chondrite-standardized REE distribution curve depicts a gentle right-leaning pattern, signifying LREE enrichment and HREE relative depletion. In terms of geochemical indices, the CIA (70.76-81.88) for mudstone exceeds the ICV (1.02-1.6) for sandstone, while mudstone ICV (0.7-1.14) remains mostly less than or equal to 1. This suggests that the Zhiluo Formation in the Lingwu area exhibits little to no recycling and is characteristic of primary sedimentation in the context of tectonic activity with a moderate degree of differentiation. Correlation diagrams indicates that tectonic background of the source region is primarily associated with an active continental margin. Additionally, it exhibits some links to continental island arc systems, suggesting an active continental margin or an active continental margin subduction zone, potentially resembling an Andean-type active continental margin.

       

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