张世民
中国地震局地壳应力研究所, 地壳动力学重点实验室, 北京 100085
基金项目: 中国地震局地壳应力研究所中央级公益性科研院所基本科研业务专项项目（批准号：ZDJ2018-03）和国家自然科学基金项目（批准号：41802226）共同资助
详细信息
作者简介: 姜大伟, 男, 30岁, 助理研究员, 构造地质学研究, E-mail:jiangdawei12@163.com
中图分类号: P931.2;P546;P595 收稿日期:2019-07-28
修回日期:2019-10-23
刊出日期:2020-01-25
Major element characteristics and its significance of ancient alluvial fans in Ya'an, Sichuan Province
Jiang Dawei,Zhang Shimin
Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, China Earthquake Administration, Beijing 100085
MSC: P931.2;P546;P595
--> Received Date: 28 July 2019
Revised Date: 23 October 2019
Publish Date: 25 January 2020
摘要
摘要:第四系是与人类生活关系密切的地层，山区地带的第四系主要由河流沉积物组成，这些沉积物的表面风化程度具有重要的环境意义。四川省雅安市地区发育了中更新统砾石层构成的古冲积扇名邛冲积扇和丹思冲积扇，现今废弃于岷江与青衣江之间，遭受了不同程度的侵蚀风化。对两个冲积扇不同位置采集样品，尝试利用主量元素分析冲积扇的风化程度。经过实验发现，主量元素分析可以很好地应用于第四系的测试，砾石间填充的基质砂能够合理反映沉积物经历的风化过程。实验结果表明，沉积物主量元素中长石矿物元素Ca、Na大量流失，而稳定矿物元素Si、Ti等偏高，说明冲积扇经历了长期的风化淋溶作用。A-CN-K图解体现出冲积扇经历了早期的斜长石风化，已经进入以钾长石和伊利石风化为标志的中期阶段。Pettijohn图解表明名邛冲积扇样品含有更多的石英砂岩，沉积物的成熟度较高。这些指标说明名邛冲积扇相比丹思冲积扇经历了更强的风化作用。结合冲积扇的年龄发现，风化速度随年龄的增长呈现减速的特征，体现了风化作用的阶段性
关键词: 雅安/
古冲积扇/
主量元素/
样品采集/
环境意义
Abstract:The Quaternary sediments is necessary for human living and traditional agricultural planting. In the mountainous areas, the Quaternary sediments is mainly composed of river sediments, and the degree of surface weathering of these sediments is directly related to whether the soil is fertile. An ancient alluvial fan composed of a Middle Pleistocene gravel layer has been developed in the Ya'an area of Sichuan Province. It is now abandoned between the Minjiang River and the Qingyijiang River and is an important agricultural production area in the region. The ancient alluvial fan in the study area can be divided into two parts in space:the north part is called the Mingqiong alluvial fan, and the south part is called the Dansi alluvial fan. In this paper, major element samples are collected from different areas of the alluvial fan. Totally, ten samples were collected from the Mingqiong alluvial fan and fifteen samples were collected from the Dans alluvial fan. The ten samples of Mingqiong alluvial fan belong to three outcrops M1 (located in Jiaguan Town, latitude:30°14'41.46", longitude:103°13'48.01", two samples), M2 (located in Heizhu Town, latitude:30°14'50.74", longitude:103°20'59.16", two samples), and M3 (located in Hongxing Town, latitude:30°8'24.09", longitude:103°17'14.52", six samples). The fifteen samples also belong to three outcrops D1 (located in Danleng County, latitude:30°4'17.49", longitude:103°29'39.9", two samples), D2 (located in Zhoujiachong Village, latitude:29°51'8.7", longitude:103°39'56.01", two samples), and D3 (located in Qingshen County, latitude:29°56'5.48", longitude:103°48'4.45", eleven samples). After the selected outcrops were cleared, the matrix sand filled between the gravel in the original gravel layer is collected to ensure that the sediment is a native formation rather than a later artificially accumulated soil. At the same time, we tried to avoid the direct influence of plant roots, the flow of small rivers, and the mixing of substances from human activities. The samples were packaged using a sealed bag and later sent to the laboratory for testing. In the laboratory, the major elements, Si, Al, Ca, Fe, K, Mg, Mn, Na, P, and Ti are used to analyze the weathering degree of the alluvial fan. The samples were tested in the Key Laboratory of Crustal Dynamics, China Earthquake Administration. The testing method is called Alkali Melting. Reagents including anhydrous lithium tetraborate, anhydrous lithium metaborate, and lithium fluoride are used to melt the samples. The testing instrument is Panalytical Axios XRF. The composing of major elements of sediments shows the feldspar mineral elements, Ca and Na were largely lost, while the composing of stable mineral elements such as Si and Ti was high, indicating that the alluvial fan experienced long-term weathering and leaching. Through the A-CN-K diagram, it is found that the weathering of alluvial fans has experienced the early process of removing Na and Ca. The plagioclase in the near-surface sediments has disappeared almost completely, and the weathering has entered into the middle stage. In the middle stage, the K-feldspar and illite are further becoming weathered. Comparing the samples of the Mingqiong alluvial fan and the Dansi alluvial fan, it is found that the overall weathering of Mingqiong fan is deeper, indicating that its formation may be earlier. According to the Pettijohn diagram, the samples of the Mingqiong alluvial fan contain more quartz sandstone, indicating that the maturity of the sediment is high. The lithology maturity also indicates that the formation of the Mingqiong alluvial fan may be earlier. Taking the age of the alluvial fan into consideration, it is found that in the warm and humid climate, the weathering effect increases with time and the weathering speed becomes slower, which may be related to the stage of weathering. In the early stage of weathering, the loss of Na and Ca elements is fast, while in the middle of weathering, the loss of K element is slower. Totally, results in the paper indicate that the major element analysis can be applied to the Quaternary sediments. The matrix sand filling the gravels can reasonably reflect the weathering process of the sediments.
Key words:Ya'an/
ancient alluvial fan/
major element/
sample collection/
environmental significance
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