第四纪研究  2020, Vol.40 Issue (1): 105-113   PDF    
四川雅安地区古冲积扇主量元素特征及其意义
姜大伟, 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)
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    
中图分类号     P931.2;P546;P595                     文献标识码    A

0 引言

四川雅安地区现存面积超过1750 km2的古冲积扇,在川西丘陵地区形成了河流相砾石层构成的大片第四系沉积区[1~2],也是区域上重要的农业生产区。在野外考察中发现,古冲积扇不同区域的沉积物具有差异性,似乎经历了不同的地表风化过程。

主量元素测试是阐述河流沉积物矿物特征的基础[3],在技术手段上已经颇为成熟,对南美、北美、欧洲、非洲等地的河流研究取得了很好的成果[4~9]。以往的研究侧重于分析现代河床沉积物、悬浮物,来反映河流物源[10~12]、源区风化[13]、区域构造演化[14]、水系演化[15]及河流污染等问题[16~17]。而主量元素分析是否可以有效的应用于古河流沉积物的研究中,前人的研究并不多。

从以往对现代河床沉积物的研究来看,主量元素特征能够很好的体现沉积物经历淋溶等化学变化过程[10~12]。另外,在国内的第四系钻孔研究中[18~19],以及黄土、红土等特征第四系研究中[20~24],主量元素可以有效反映沉积物的化学风化过程。因此利用主量元素分析古冲积扇沉积物的风化程度具备可行性。

本文通过雅安地区古冲积扇沉积物的主量元素分布特征,定量的分析古冲积扇不同区域的元素富集程度,揭示了沉积物所经历的风化过程,进而通过风化程度区分古冲积扇不同的形成期次。结合前期的冲积扇年代学结果,发现冲积扇风化速度具有与物质流失相对应的阶段性特征。研究成果证实了利用主量元素分析河流冲积物风化过程的可行性。

1 研究对象

四川雅安地区的古冲积扇是典型的湿润区山前地貌,也有学者称之为冲洪积扇[25]。与河流末端或季节性流水形成的洪积扇不同,这类冲积扇是在山区经常性水流的出山口形成,因山地河流河床比较固定,冲积物出山后可继续被带到很远的地方,通过河床的摆动,可以形成延伸很广、坡度较缓的扇状地形[26]。研究区古冲积扇从空间上被分成两部分:北侧的称为名邛冲积扇[27],南侧的称为丹思冲积扇[27]。两者现今都已经废弃,表面不再接受河流沉积物的补给,因此在丘陵地区呈现出较为平坦的第四纪地貌面。因构成冲积扇的砾石层之间填充有砂,适合作物生长,现今已被改造为农业种植区(图 1)。

图 1 雅安地区古冲积扇形态与取样位置 Fig. 1 Morphological feature of the ancient alluvial fans in Ya'an area and the location of samples

名邛冲积扇以名山县和邛崃市命名,也称为名邛砾石层,分布于川西前陆盆地名山至邛崃一带,成为青衣江与岷江之间的分水岭[28](图 1)。名邛冲积扇由名山县开始,向北东东方向呈带状分布,一直到邛崃市西侧的扇缘位置,纵长56 km,前缘宽约24 km,分布面积1050 km2,沉积厚度50~150 m(图 2a)。名邛冲积扇沉积特征往往是以石英岩为主的砾石与粘土混杂,前人研究结果表明其成分与青衣江上游宝兴、天全、荥经一带广泛分布的元古界地层一致,而与岷江上游大面积中酸性侵入岩的陆源成分有明显区别,判断其为古青衣江的河流沉积物[28~30]。沉积物现今已强烈风化,石英、长石为主的砾石仍较为坚硬,其他大部分火成岩、沉积岩都已风化破碎(图 2c)。

丹思冲积扇以丹棱县、思濛县命名[27],也称为丹思砾石层(图 1)。丹思冲积扇平面分布自西向东呈撒开的扇状,西起洪雅县,东至眉山县,南至夹江县[27],纵长34 km,前缘宽约28 km,分布面积700 km2,沉积厚度30~100 m(图 2b)。野外调查中发现丹棱以西丹思砾石层零星分布,遭受较严重侵蚀,丹棱以东则成片状分布。前人对丹思砾石层的砾石组分调查显示[27, 31],其砾石成分与名邛砾石层、青衣江现代阶地沉积皆有相似性(图 2d),而与岷江流域河流沉积地层有较大差别,推断丹思砾石层也是青衣江流域的古冲积扇。

图 2 古冲积扇地表形态与沉积特征 Fig. 2 Surface feature and depositional characteristic of the alluvial fan

对名邛冲积扇与丹思冲积扇分别采集了主量元素样品,其中名邛冲积扇采集样品10个,丹思冲积扇采集样品15个(图 1)。对选取的天然剖面进行清理,采集原始砾石层中砾石之间填充的基质砂,保证了沉积物为原生地层,而不是后期人工堆积物。同时尽量避开植物根系的直接影响,避开小型河流的流水作用,避免人类活动产生物质混入。样品使用密封袋进行封装编号,后期送入实验室测试。

名邛冲积扇采集了M1、M2和M3共3个剖面计10个样品(图 1)。M1剖面位于夹关镇(30°14′41.46″N,103°13′48.01″E),采集2个样品,采集地层位于冲积扇顶部砾石层,取样深度约1 m,砾石砾径2~10 cm,砾石间填充细砂(图 3a);M2剖面位于黑竹镇(30°14′50.74″N,103°20′59.16″E),采集2个样品,采集地层位于冲积扇顶部砾石层,取样深度约1.5 m,砾石砾径2~5 cm,砾石间填充粘土至细砂(图 3b);M3剖面位于红星镇(30°8′24.09″N,103°17′14.52″E),采集6个样品,采集地层位于冲积扇顶部砾石层,取样深度1~3 m,砾径5~20 cm,砾石间填充细砂(图 3c)。丹思冲积扇采集了D1、D2和D3共3个剖面计15个样品(图 1),其中D1剖面位于丹棱县(30°4′17.49″N,103°29′39.9″E),采集2个样品,采集地层位于冲积扇顶部砾石层,取样深度约2 m,砾石砾径3~20 cm,砾石间填充细砂至中砂(图 3d);D2位于周家冲(29°51′8.7″N,103°39′56.01″E),采集2个样品,采集地层位于冲积扇中上部砾石层,取样深度约3 m,砾石砾径3~15 cm,砾石间填充细砂(图 3e);D3位于青神县(29°56′5.48″N,103°48′4.45″E),采集11个样品,采集地层位于冲积扇中上部砾石层,取样深度1.0~3.5 m,砾石砾径5~20 cm,砾石间填充细砂(图 3f)。

图 3 冲积扇样品采集照片 Fig. 3 Photos showing the collecting of major element samples
2 实验方法

样品主量元素测试测试在中国地震局地壳动力学重点实验室完成,测试仪器为Panalytical Axios XRF,前处理采用碱熔法。首先将0.7 g岩石样品粉末与7 g X荧光光谱仪专用熔剂(无水四硼酸锂+无水偏硼酸锂+氟化锂)混合,滴加3~5滴溴化锂溶液;在高频炉上预热6 min,加热到950 ℃维持7 min,样品熔化,混合均匀,经9 min自冷和风冷后形成均质玻璃体。然后在Panalytical Axios X荧光光谱仪上分析,分析采用国标GSR-1,工作条件为50 kV,60 mA。用坩埚称0.7 g样品,放入马弗炉中烘烤后,将样品直接放入干燥器中,最后称量计算获得烧失量。

3 结果

主量元素测试结果列于表 1中,原始数据单位为百分比含量。样品编号1~10为名邛冲积扇样品,取样剖面分别为M1、M2和M3;样品编号11~25为丹思冲积扇样品,取样剖面分别为D1、D2和D3(图 1图 3)。

表 1 主量元素测试结果(%) Table 1 Testing results of the major element(%)

将代表名邛冲积扇、丹思冲积扇的样品主量元素含量与上地壳平均物质含量(UCC)正交化[33],并投影图 4中。从图 4发现沉积物除了Ca、Na、Mg等元素以外,其他元素与UCC较为接近,说明沉积物来源较广泛,经历了一定程度的混合过程。在矿物风化过程中,长石的差异消解产生SiO2、Na与K离子,黑云母及铁镁矿物风化产生Mg和K,硅酸盐矿物风化时,Si被流水带走而留下石英,这些Ca、Mg、K、Na等离子优先被降解流失[34]。样品主量元素结果显示亏损最多的物质为Na、Ca,反映了沉积物普遍发生了风化,K也有少量亏损的趋势(图 4)。

图 4 主量元素均一化结果 Fig. 4 Normalized results with upper continental crust(UCC)

其中名邛冲积扇(红色线条)的样品中,组成长石矿物的易流失元素Ca、Na等的含量都明显低于丹思冲积扇(蓝色线条),而稳定矿物的组成元素Si、Ti等的含量都高于丹思冲积扇(图 4),说明名邛冲积扇样品的风化程度要普遍高于丹思冲积扇,经历了更长时间的风化淋溶作用,导致长石降解,而残留SiO2的相对含量明显变多。拟合后的曲线(较粗线条)也显示了相同的变化趋势。

4 讨论 4.1 A-CN-K三角模型图

由于上地壳主要的成分斜长石、钾长石都属于易风化的不稳定矿物[34],因此大陆风化在广泛的地表都会产生作用。Nesbitt和Young[35]根据质量平衡原理、长石淋溶实验和矿物稳定性的热力学计算,提出A-CN-K来反映陆壳物质风化趋势。其中A代表Al2O3,CN代表(CaO+Na2O),K代表K2O。A-CN-K模型原理为:大陆风化的早期阶段以斜长石的风化为标志,进行了脱Na、Ca的过程,风化产物以伊利石、蒙脱石和高岭石为特征,风化趋势线平行于A-CN连线并指向A。这一阶段典型的风化过程:上地壳UCC至陆源页岩就位于这个趋势线上。同时,由于河流溶质代表了大陆风化过程中的可溶组分,其组成点落在风化趋势线的反向延长线上。当风化趋势点抵达A-K连线,表明风化剖面中斜长石全部消失,风化作用进入以钾长石和伊利石风化为标志的中级阶段,风化趋势平行于A-K线,进行脱K化过程。在大陆风化的晚期阶段,风化产物的组成落在A点附近,风化产物以高岭石-三水铝石-石英-铁氧化物组合为特征,进行脱Si过程。我们进一步将两套冲积扇的样品投影到A-CN-K三角模型图进行对比(图 5)。

图 5 主量元素A-CN-K图解 Fig. 5 The A-CN-K triangle graph of the samples

同时投影到A-CN-K三角模型中的还有上地壳沉积物(Upper Continental Crust,简称UCC)与陆源页岩[33]、世界沉积物综合指标(World Sediments,简称WS)[36]、洛川黄土[37]、宣城红土[38],以及模型边框中的标准岩石值[35]。结果显示(图 5),名邛冲积扇比丹思冲积扇的整体风化程度更强,已经进入了上地壳风化的第二个阶段,开始向A点靠近。冲积扇的整体风化程度远高于上地壳标准物质,并分布在黄土与红土之间,其中名邛冲积扇的样品风化基本全部高于陆源页岩。整体上看,名邛冲积扇经历的风化过程强于丹思冲积扇。

4.2 Pettijohn岩性图解

Pettijohn岩性分类可以定量化的反映沉积物的成熟度[39],我们将样品投影到Pettijohn岩性分布图中,分析两套扇体沉积物岩性成熟度的差异(图 6)。结果显示名邛冲积扇砾石填充基质含有更多的石英砂岩,长石降解比例很大,表明沉积物的成熟度较高,并且岩性变化趋势很一致(图 6)。因此,主量元素整体反映出,名邛冲积扇在形成时间上应该早于丹思冲积扇,虽然前文阐述砾石成分反映了两者整体物源一致,但基质砂所反映的成分说明两者经历了不同的风化过程。另一方面,名邛冲积扇早于丹思冲积扇形成,由于古青衣江的不断溯源侵蚀作用,侵蚀的物源区也有一定变化,两者的物源特征可能存在细微差别。

图 6 主量元素Pettijohn图解 Fig. 6 Pettijohn graph of the samples' major element
4.3 风化程度与年龄的关系

主量元素的分析结果体现出名邛冲积扇相比丹思冲积扇经历了更长的风化过程,表明主量元素相关分析方法也适用于第四系的研究工作中。尤其根据A-CN-K与Pettijohn图解等方式可以对冲积扇进行分期,本研究表明名邛冲积扇与丹思冲积扇是两期形成的,这与我们之前对冲积扇的年代学研究结果相符[2]。结合洪积扇的年龄(表 2),可以进一步探讨冲积扇风化程度随时间的变化规律。

表 2 名邛与丹思古冲积扇年龄[2] Table 2 Ages of Mingqiong alluvial fan and Dansi alluvial fan[2]

我们以冲积扇年龄为横坐标,将名邛冲积扇与丹思冲积扇的风化指数(CIA)进行投影来对比两者风化程度与年龄的关系(图 7)。其中,年龄与CIA取数据的平均值,误差棒为标准偏差。同时,将上地壳平均物质含量(UCC)[33]也投影至图 7中作为参考,近似代表未风化的原始沉积物。从图 7可以看出,冲积扇表现出了非线性的风化速度,表明冲积扇早期风化速度较快,当风化发展至一定程度,风化速度开始变慢,这与风化的阶段性有很大关系。冲积扇所处川西地区气候湿润,年平均气温12 ℃,年降雨量1600 mm[40],温润的气候可能令冲积扇快速的完成了早期风化阶段,易风化的斜长石组分很快发生淋滤。从数值上看(图 7),名邛冲积扇与丹思冲积扇都基本经历了早期以脱Na、Ca为标志的风化阶段,进入了以脱K过程的风化中期阶段[35],在风化中期阶段,可能由于钾长石等矿物相对稳定而导致风化速度减慢。因此从图 7可以看出,距今300 ka以内甚至200 ka以内冲积扇就已经进入脱K阶段,风化开始减速。

图 7 冲积扇风化程度与年龄的关系 Fig. 7 Relationship between the weathering degree and the age of alluvial fans
5 结论

(1) 四川雅安地区现存两片古冲积扇,对其近地表沉积物的主量元素研究表明,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,表明冲积扇经历了长期的风化淋溶作用。

(2) 通过A-CN-K图解发现,冲积扇的风化经历了早期的脱Na、Ca的过程,近地表沉积物中斜长石基本全部消失,风化作用已经进入以钾长石和伊利石风化为标志的中期阶段,风化趋势平行于A-K线,进行脱K化过程。对比名邛冲积扇与丹思冲积扇样品发现,名邛冲积扇整体风化程度更深,表明名邛冲积扇形成时间可能更早。

(3) 通过Pettijohn图解发现,名邛冲积扇样品含有更多的石英砂岩,长石降解比例很大,表明沉积物的成熟度较高,并且岩性分布变化趋势很一致。岩性成熟度同样表明名邛冲积扇的形成时间可能更早。

(4) 结合冲积扇的年龄发现,在温暖湿润气候下,风化作用随时间增长而体现出风化速度变慢的特征,这可能与风化作用的阶段性有关。风化早期流失Na、Ca元素速度快,而风化中期流失K元素速度较慢。

(5) 本文研究结果表明,主量元素测试方法可以很好地应用于第四纪沉积物中,并作为环境风化程度与土壤化的一个参考指标。在松散沉积物样品选取方面,应选择样品中对化学过程更敏感的细颗粒物质,如河流相砾石层中填充的基质砂颗粒,可以得到最佳效果。

致谢: 感谢审稿专家提出宝贵意见,感谢编辑老师的认真审阅和校对。

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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)

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