地质力学学报  2021, Vol. 27 Issue (3): 475-490
引用本文
綦琳, 乔彦松, 刘宗秀, 王燕, 彭莎莎. 陇东新近纪红粘土与第四纪黄土地球化学特征及其物源和风化指示意义[J]. 地质力学学报, 2021, 27(3): 475-490.
QI Lin, QIAO Yansong, LIU Zongxiu, WANG Yan, PENG Shasha. Geochemical characteristics of the Tertiary and Quaternary eolian deposits in eastern Gansu province: Implications for provenance and weathering intensity[J]. Journal of Geomechanics, 2021, 27(3): 475-490.
陇东新近纪红粘土与第四纪黄土地球化学特征及其物源和风化指示意义
綦琳1,2, 乔彦松1,2,4, 刘宗秀1,2, 王燕1,2, 彭莎莎3    
1. 中国地质科学院地质力学研究所, 北京 100081;
2. 新构造运动与地质灾害重点实验室, 北京 100081;
3. 中国科学院广州地球化学研究所, 广东 广州 510640;
4. 中国地质调查局新构造与地壳稳定性研究中心, 北京 100081
摘要:甘肃省平凉市灵台县邵寨镇剖面风尘堆积底界年龄大约为5.23 Ma B.P.,通过对该剖面新近纪红粘土与第四纪黄土-古土壤序列的常量元素、微量元素、Nd同位素的测试,分析其在物源和风化方面的指示意义,发现新近纪红粘土与第四纪黄土具有相似的常量、微量元素UCC标准化曲线和稀土元素球粒陨石标准化曲线,指示二者皆来自广阔的物源区,经过了相似的搬运过程,并在搬运中得到充分混合。新近纪红粘土的MgO、Li、Cs、Bi含量较高,Na2O、稀土元素La-Lu、Y含量较低。风化参数Na2O/Al2O3、化学风化参数CIA以及Al2O3-CaO+Na2O-K2O (A-CN-K)图,均显示新近纪红粘土比第四纪黄土经历了更为强烈的风化过程。新近纪红粘土的稳定元素比值(TiO2/Al2O3,SiO2/Al2O3,SiO2/TiO2,Zr/Hf,Nb/Ta,Lu/Hf,Y/Ho,Th/Nb和Hf/Nb)、稀土元素总量、轻稀土与重稀土的分异程度、轻稀土内部分异程度、重稀土内部分异程度、Ce和Eu的异常程度、同位素εNd(0) 值等,皆与第四纪黄土无太大差异,指示二者物质来源一致。粒度以及风化强度的差异,可能是导致新近纪红粘土与第四纪黄土常量和微量元素含量差异的主要原因。
关键词风尘堆积    红粘土    地球化学    物源    黄土高原    
DOI10.12090/j.issn.1006-6616.2021.27.03.043     文章编号:1006-6616(2021)03-0475-16
Geochemical characteristics of the Tertiary and Quaternary eolian deposits in eastern Gansu province: Implications for provenance and weathering intensity
QI Lin1,2, QIAO Yansong1,2,4, LIU Zongxiu1,2, WANG Yan1,2, PENG Shasha3    
1. Chinese Academy of Geological Sciences, Beijing 100081, China;
2. Key Laboratory of Neotectonic Movement and Geohazard, Institute of Geomechanics, Beijing 100081, China;
3. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China;
4. Research Center of Neotectonism and Crustal Stability, China Geological Survey, Beijing 100081, China
Abstract: An eolian deposit section named Shaozhai, which has been developed since the Tertiary in eastern Gansu province, was analyzed for major-trace elements and Sm-Nd isotopic compositions to explore the relationship for the provenance and weathering intensity between the Tertiary red clay and Quaternary loess. The magnetostratigraphy data indicates that the basal age of the typical eolian deposits is about 5.23 Ma B.P.. We observed both similarities and obvious differences in the Tertiary red clay and Quaternary loess. Specifically, the Tertiary red clay has similar UCC-normalized major-trace element abundances and chondrite-normalized rare earth element abundances as the Quaternary loess, demonstrating that dust materials making up these sediments likely have experienced similar and numerous recycling processes prior to deposition. The Tertiary red clay has relatively high abundances of MgO, Li, Cs, Bi and low abundances of Na2O, La-Lu, Y. In comparison with the Quaternary loess, chemical weathering parameters such as Na2O/Al2O3, CIA, combined with the A-CN-K diagram suggest that the Tertiary Red clay has experienced a stronger weathering intensity. There is not much difference in the TiO2/Al2O3, SiO2/Al2O3, SiO2/TiO2, Zr/Hf, Nb/Ta, Lu/Hf, Y/Ho, Th/Nb, Hf/Nb, total rare earth elements, differentiation degree between the light and heavy rare earth element, internal differentiation degree of the light rare earth element, abnormal degree of Ce and Eu, εNd(0) of the Tertiary red clay and Quaternary loess, demonstrating a similar provenance for them. The finer grain size and stronger weathering of the Tertiary red clay may be responsible for the difference in major and trace element content for the Tertiary red clay and Quaternary loess.
Key words: eolian deposits    red clay    geochemistry    provenance    Chinese Loess Plateau    
0 引言

在中国北方黄土高原地区第四纪黄土-古土壤沉积序列之下,普遍发育了一套第三纪红色土状堆积,二者共同构成连续的陆相沉积记录,较好地记录了东亚季风第三纪以来的演化信息,成为研究古环境、古气候变化的理想材料(Ding et al., 1998Sun et al., 1998An et al., 2001Guo et al., 2002)。与第四纪黄土-古土壤序列相比,对第三纪红粘土的研究起步较晚。从90年代中期开始,第四纪黄土-古土壤序列的研究方法与手段引入红粘土研究中,使红粘土的研究得到突飞猛进的发展(朱日祥等,1996孙东怀等,1997顾兆炎等, 1999, 2000, 2006安芷生等,2000郝青振等,2000杨石岭和丁仲礼,2000Ding et al., 2001a彭淑贞和郭正堂,2007),并逐渐成为晚新生代环境演化研究的热点。

目前,大多学者认为第三纪红粘土与第四纪黄土-古土壤序列一样,皆为风成成因(丁仲礼等, 1997, 1998鹿化煜和安芷生,1999彭淑贞和郭正堂,2000Ding et al., 2001b),但其物质来源是否一致,仍有争议。杨杰东等(2005)Wang et al.(2007)基于Nd同位素研究,认为晚第三纪红粘土和第四纪黄土-古土壤序列具有基本相同的物源系统。梁美艳等对中新世黄土层、上新世三趾马红土以及第四纪黄土常量元素、微量元素、稀土元素地球化学特征对比研究显示,三者具有相似的地球化学组成,表明均来源于广阔的物源区且具有沙漠黄土的特征(梁美艳等,2006Liang et al., 2009);中新世黄土层、上新世三趾马红土部分元素与第四纪黄土有差异是因为受到粒度和温湿气候的影响。Sun(2005)对黄土高原中部陕西泾川剖面Nd同位素组成研究表明,第四纪黄土的143Nd/144Nd比值低于红粘土,表明风尘的来源在2.58 Ma B.P.前后发生了重大变化。Sun and Zhu(2010)后来进行的Pb同位素和微量元素研究,显示二者浓度在2.6 Ma B.P.前后表现出明显的变化,进一步肯定第三纪红粘土的物质来源在一定程度上不同于第四纪黄土,且因晚新生代的构造运动造成高海拔山地的冰蚀作用加强,导致较多的年轻的地壳物质被搬运到黄土高原地区沉积下来。李云等(2014)通过碎屑锆石U-Pb定年研究后也认为第四纪黄土和第三纪红粘土碎屑锆石物源在2.6 Ma B.P.和3.6 Ma B.P.前后发生了明显变化;第四纪黄土中的碎屑锆石来源于北部的戈壁沙漠,而红粘土的锆石物源受近源基岩的影响较大。

风尘堆积的地球化学特征与其物质来源区域的母岩密切相关, 因而经常用来作为追踪物源区的指标(Taylor et al., 1983Liu et al., 1993)。文章首先对黄土高原灵台邵寨地区风尘堆积剖面进行系统的磁性地层研究,继而对该剖面新近纪红粘土和第四纪黄土的常量、微量、Sm-Nd同位素组成开展对比研究,以揭示这两个时期风尘堆积地球化学特征的异同,并对其蕴含的物源、风化指示意义进行探讨。

1 地层划分、样品选取及实验方法

邵寨剖面位于黄土高原中部、甘肃省平凉市灵台县东南约20 km的邵寨镇(34°59′26.1″N;107°45′14.1″E, 1061 m;图 1)。剖面总厚度275.7 m,上部为第四纪黄土-古土壤序列;中部为红粘土层,发育大量钙质结核层,结核直径2~10 cm,结核层厚1~100 cm;底部为流水改造后的沉积物,具水平层理,厚约23.7 m。

图 1 邵寨剖面位置图 Fig. 1 Map showing the location of the Shaozhai section in the Chinese Loess Plateau

分别按照50 cm和25 cm间距在第四纪黄土和红粘土中采集定向样品用于古地磁测试,共获得780块样品。在中国地质科学院地质力学研究所古地磁实验室完成古地磁测试。采用美国ASC公司生产的TD-48型全自动热退磁仪进行热退磁,采用美国产2G-755型超导磁力仪测试剩磁。首先测量样品的天然剩磁,然后对所有样品进行系统热退磁,退磁温度为100 ℃、150 ℃、200 ℃、250 ℃、300 ℃、350 ℃、400 ℃、450 ℃、500 ℃、520 ℃、550 ℃、585 ℃、600 ℃、620 ℃、675 ℃。古地磁研究显示(图 2),该剖面的典型风尘堆积底界与Gilbert反向极性期底界对应,年龄约为5.23 Ma B.P.,第四纪黄土与红粘土界线与M/G界线对应,位于161.1 m处。

图 2 邵寨剖面岩性地层及磁性地层 Fig. 2 Lithostratigraphy and magnetostratigraphy of the Shaozhai section

按照10 cm间距采集磁化率样品。在中国地质科学院地质力学研究所第四纪地质与环境实验室采用英国产的Bartington MS2磁化率仪完成磁化率测试。野外地层观察及磁化率研究显示(图 2),在第四纪黄土中磁化率测试结果与野外地层划分具有较好的对应关系,即磁化率的峰值、谷值分别与野外划分的古土壤层(S层)和黄土层(L层)相对应,邵寨剖面第四纪黄土-古土壤序列包含了从L2到L33的地层。

以磁性地层研究结果为基础,自上而下在邵寨剖面第四纪黄土-古土壤序列中选取了3个黄土样品(L2、L8、L27)和8个古土壤样品(S4、S5、S10、S11、S18、S24、S29、S32)、在新近纪红粘土中选取了19个样品进行常量元素和微量元素测试,并对其中的7个第四纪样品和14个红粘土样品进行Sm-Nd同位素测试。采样深度及层位示于图 2表 1

表 1 邵寨剖面样品常量元素数据表(单位%) Table 1 Major element concentrations (%) of the samples from the Shaozhai section

在中国地质科学院国家地质实验测试中心完成常量元素和微量元素测试。已报道的淋溶实验证明(陈骏等,1996刘连文等,2002),醋酸溶液可以完全溶解成壤阶段产生的碳酸盐物质,对硅酸盐等的影响较小。去除成壤碳酸盐后的酸不溶组分代表了原始粉尘中的稳定组分,具有示踪源区的潜力。因此在室温环境下将用于常量、微量元素测试的样品浸泡在1 mol/L醋酸溶液中24小时。将酸不溶物用去离子水洗涤后烘干,研磨至200目以下。采用3038E型光谱仪测定常量元素。MnO和P2O5测试误差较大(±10%),其他元素测试误差均<3%。由于MnO和P2O5测试误差较大,此次不对其进行分析。使用ICP-MS等离子质谱仪测试微量元素,测试误差<10%。称取1 g风干样,置于950 ℃的马弗炉中煅烧1小时后再称量,计算煅烧前、后质量差,获得烧失量(LOI)。

为降低近源粗颗粒物质对测试结果的影响,提取粒径小于20 μm组分进行Nd同位素测试分析。具体做法是:首先对样品进行去除有机质和钙质处理,然后洗酸;加入六偏磷酸钠分散剂,放入超声振荡器使其分散;过湿筛将大于63 μm的组分去除;将剩余的混合样品装入沉降桶;依据斯托克斯法则利用重力沉降法获取样品。在中国地质科学院地质研究所完成Nd同位素测试,使用仪器为Nu Plasma HR型MC-ICP-MS质谱仪。使用146Nd/144Nd=0.7219校正同位素的质量分馏;用国际标准样品JMC检验实验流程的分析误差和测试结果的准确度,JMC标样测定值为143Nd/144Nd=0.511123±10(2σ),测试误差<10%。

2 地球化学测试结果及数据分析 2.1 常量元素

首先对第四纪黄土和新近纪红粘土样品进行烧失量校正,然后将其与上地壳平均化学组成(UCC)(Taylor and Mclennan, 1985McLennan,2001)进行标准化处理(表 1图 3)。第四纪黄土和新近纪红粘土的常量元素组成均以SiO2、Al2O3和Fe2O3为主,除CaO和Na2O含量明显低于UCC外,其余常量元素含量与UCC接近。与第四纪黄土相比,新近纪红粘土中的MgO含量较高,而Na2O含量略低。新近纪红粘土不同样品间MgO变异系数较大,第四纪黄土中的Fe2O3和CaO变异系数较大。无论在新近纪红粘土还是在第四纪黄土中,SiO2、Al2O3和TiO2变异系数较其他元素小。

a—邵寨剖面所有样品的UCC标准化值;b—新近纪红粘土与第四纪黄土UCC标准化均值及标准差值 图 3 邵寨剖面样品常量元素的UCC标准化值 Fig. 3 UCC-normalized abundances of major elements for the samples from the Shaozhai section. (a) Data for all studied samples. (b) Average and standard deviation for the Tertiary red clay and Quaternary loess.

一般而言,SiO2、Al2O3、TiO2在初级到中等风化强度下是抗风化的,能够较好的反映母岩信息(Broecker and Peng, 1982Li,1982文启忠,1989)。因此,学者们常用TiO2/Al2O3、SiO2/TiO2和SiO2/Al2O3进行物源判断(Sugitani et al., 1996Young and Nesbitt, 1998Sheldon and Tabor, 2009Qi and Qiao, 2014杨帅斌等,2017)。新近纪红粘土的TiO2/Al2O3、SiO2/Al2O3和SiO2/TiO2与第四纪黄土差异并不明显(图 4),指示物质来源较为一致。

a—SiO2/Al2O3-SiO2/TiO2; b—TiO2/Al2O3-SiO2/TiO2; c—SiO2/Al2O3-TiO2/Al2O3 图 4 邵寨剖面样品不易迁移常量元素比值图 Fig. 4 Plots of immobile major element ratios for the samples from the Shaozhai section. (a) SiO2/Al2O3 versus SiO2/TiO2. (b) TiO2/Al2O3 versus SiO2/TiO2. (c) SiO2/Al2O3 versus TiO2/Al2O3

Na2O、K2O和CaO含量容易受风化影响而发生迁移。一般用Na2O/Al2O3与K2O/Al2O3比值反映风化过程中Na和K元素的迁移情况(Garrels and Mackenzie, 1971)。与第四纪黄土相比,新近纪红粘土的Na2O/Al2O3比值明显偏低,K2O/Al2O3比值没有太大差异,指示第四纪黄土和新近纪红粘土均处于斜长石风化阶段,尚未达到钾长石风化阶段(图 5)。A-CN-K能较好地反映风化作用的阶段以及未来发展趋势(Nesbitt et al., 1980)。A-CN-K图(图 6)显示,新近纪红粘土与第四纪黄土样品风化趋势与A-CN线平行,均处于去除Na和Ca的早期阶段,以斜长石分解为特征,产物为伊利石和蒙脱石。新近纪红粘土样品更为靠近A-K线,指示经历了较强的风化作用。化学蚀变指数(CIA)常用来衡量岩石或土壤的风化程度(路硕等,2019),其计算公式是:CIA=[Al2O3/(Al2O3+CaO*+K2O+Na2O)]×100,其中CaO*是硅酸盐中的CaO含量(Nesbitt and Young, 1982)。早期的研究(Fedo et al., 1995)表明,岩石化学风化会经历从初期(CIA=50~60)到中期(CIA=60~80)再到极端(CIA>80)风化的过程。在文中,新近纪红粘土CIA的范围是66.6~71.2(平均值为69.3),第四纪黄土CIA的范围是63.2~70.2(平均值为66.9),均处于中期风化阶段,但新近纪红粘土所经历的化学风化作用更强。

图 5 邵寨剖面样品的Na2O/Al2O3-K2O/Al2O3 Fig. 5 Na2O/Al2O3 versus K2O/Al2O3 diagram for the samples from the Shaozhai section

图 6 邵寨剖面样品A-CN-K(Al2O3-CaO+Na2O-K2O)图 Fig. 6 A-CN-K (Al2O3-CaO+Na2O-K2O) triangular diagram for the samples from the Shaozhai section
2.2 微量元素

新近纪红粘土与第四纪黄土的微量元素含量如表 2表 3所示,经UCC标准化后获得图 7。与UCC相比,新近纪红粘土与第四纪黄土表现出了相似的分布模式,即相对富集Li、Cs和Bi,明显亏损Be和Sr。新近纪红粘土与第四纪黄土的微量元素含量有轻微差异。新近纪红粘土的Li、Cs和Bi含量略高于第四纪黄土,稀土元素La-Lu、Y含量略低于第四纪黄土。无论是在新近纪红粘土还是在第四纪黄土中,Be、Ga、Sr、Nb、Ba、Tl和U元素含量变异系数较小。

表 2 邵寨剖面样品微量元素数据表(单位μg/g) Table 2 Trace element concentrations (μg/g) of the samples from the Shaozhai section

表 3 邵寨剖面样品稀土元素数据表(单位μg/g) Table 3 Rare earth element concentrations (μg/g) of the samples from the Shaozhai section

a—邵寨剖面所有样品的UCC标准化值;b—新近纪红粘土与第四纪黄土UCC标准化均值及标准差值 图 7 邵寨剖面样品微量元素的UCC标准化值 Fig. 7 UCC-normalized abundances of trace elements for the samples from the Shaozhai section. (a) Standard data for all studied samples. (b) Standard average and standard deviation for the Tertiary red clay and Quaternary loess.

常采用化学性质不活泼的元素及其比值进行物源示踪(Qiao et al., 2011)。由于Zr、Nb、Ta、Hf、Lu、Ho、Y和Th都是高场元素,其中,Zr和Hf、Nb和Ta、Y和Ho、Lu和Hf分别具有相近的离子半径和相似的地球化学行为,是紧密共生的元素对(Bau,1996Bouvier et al., 2008),它们在风化过程中属于不活泼元素,在风化极为强烈的情况下仍然不发生淋失(Condie,2005),此外,Th元素几乎不受风力分选的影响(Rollinson,1993Yang et al., 2003Ozkan and Ayaz-Bozdag, 2011)。这些元素多在抗风化的副矿物中富集,其含量和比值在不同岩石类型中差别较大,沉积后受风化影响较小,是理想的物源示踪指标(Marques et al., 2004)。新近纪红粘土与第四纪黄土的Zr/Hf、Nb/Ta、Lu/Hf、Y/Ho、Th/Nb和Hf/Nb比没有明显区别(图 8),指示了物质来源的一致性。

图 8 邵寨剖面样品不易迁移微量元素比值图 Fig. 8 Plots of immobile trace element ratios for the samples from the Shaozhai section
2.3 稀土元素

稀土元素(REE)也是常用的物源示踪物(McLennan,1989Yang et al., 2007),它是一个地球化学性质很相似的元素组,随岩石类型不同元素含量发生变化,在风化、搬运及沉积过程中其组成变化较小,所携带物源区信息一般不会丢失。将样品与球粒陨石中相应的各稀土元素的含量(Taylor and Mclennan, 1985McLennan,2001)进行对比求值,通过球粒陨石标准化后可以直观地观察到样品对于球粒陨石的分异程度。新近纪红粘土与第四纪黄土的球粒陨石标准化曲线十分相似,均呈现出轻稀土元素富集、重稀土元素平坦、Eu元素亏损的特点(图 9),但新近纪红粘土稀土元素含量略低于第四纪黄土。

a—邵寨剖面所有样品的球粒陨石标准化值;b—新近纪红粘土与第四纪黄土球粒陨石标准化均值及标准差 图 9 邵寨剖面样品稀土元素球粒陨石标准化分布模式图 Fig. 9 Chondrite-normalized abundances of trace elements for the samples from the Shaozhai section. (a) Data for all studied samples. (b) Average data and standard deviation for the Tertiary red clay and Quaternary loess.

尽管新近纪红粘土中的稀土元素La-Lu和Y含量略低于第四纪黄土,但其稀土元素总量(ΣREE)、轻稀土与重稀土的分异程度(LREE/HREE)、轻稀土内部分异程度(LaN/SmN)、重稀土内部分异程度(GdN/YbN)、Ce和Eu的异常程度(Ce/Ce*、Eu/Eu*)与第四纪黄土并无太大差异(图 10),反映出物质来源相对一致。

图 10 邵寨剖面样品的LREE/HREE-∑REE、LaN/SmN-GdN/LuN、Eu/Eu*-Ce/Ce*图解 Fig. 10 LREE/HREE versus ∑REE, LaN/SmN versus GdN/LuN and Eu/Eu* versus Ce/Ce* diagrams for the samples from the Shaozhai section
2.4 Sm-Nd同位素

Sm和Nd具有较为相似的化学性质,在风化、搬运、沉积过程中基本上保持了源岩的信息(Nakai et al., 1993)。Sm和Nd在自然界中分别有7个同位素,除147Sm以106Ga的半衰期α衰变成143Nd外,其他放射性同位素因为半衰期过长,可以作为稳定同位素看待。不同成因和时代的岩石,其143Nd/144Nd不相同,因而可以用来进行物源区的区分(Goldstein et al., 1984Revel et al., 1996Grousset and Biscaye, 2005)。由于自然界中143Nd/144Nd比值变化范围较小,因而经常采用球粒陨石标准化值εNd(0)来代替,εNd(0)=(143Nd/144Nd样品/143Nd/144Nd球粒陨石-1)×104。新近纪红粘土与第四纪黄土的143Nd/144Nd和εNd(0)没有明显差异性(图 11),表明两者的物质来源基本一致。

表 4 邵寨剖面样品Sm-Nd同位素数据表 Table 4 Sm-Nd data of the samples from the Shaozhai section

图 11 邵寨剖面样品εNd(0)变化图 Fig. 11 Variations in εNd(0) of the samples from the Shaozhai section
3 地球化学特征对物源及风化的指示意义

新近纪红粘土与第四纪黄土-古土壤序列的常量元素、微量元素的组成与上地壳平均化学组成基本相同,在常量元素和微量元素UCC标准化图中具有相似的分布模式,具有相似的稀土元素球粒陨石标准化曲线,证明它们与上地壳存在着十分密切的成因联系,在沉积之前都经过了相似的、多次的搬运与沉积,是高度混合的产物,也证明了新近纪红粘土与第四纪黄土同样为风尘堆积物。

由于利用地球化学方法示踪风尘堆积物物源的本质是看矿物成分的差异,矿物成分不仅受控于岩浆结晶分异,在矿物形成后还会受到风化以及沉积分异的影响。因此,在对风尘堆积物进行物源判断时,需要选择化学性质相对稳定的地球化学指标,避免沉积分选和风化改造对物源判断产生的影响。

新近纪红粘土与第四纪黄土地球化学特征主要差异,体现在新近纪红粘土具有较高的MgO、Li、Cs、Bi含量,较低的Na2O,以及略低的稀土元素La-Lu和Y含量。由于MgO、Na2O属于易迁移元素,而Li、Cs、Bi元素易受到分选作用影响,在细粒级沉积物中富集(Nesbitt et al., 1980, 1996),故一般不作为物源判别指标。

稀土元素由于化学性质稳定,在风化搬运沉积时组成变化小,因而是常用的物源示踪指标(McLennan,1989Yang et al., 2007)。尽管新近纪红粘土稀土元素La-Lu、Y含量略低于第四纪黄土,但其他稀土表征参数与第四纪黄土没有明显差异,指示新近纪红粘土与第四纪黄土物质来源相对一致。除了稀土元素表征参数相似,新近纪红粘土常量元素比值(TiO2/Al2O3,SiO2/Al2O3,SiO2/TiO2)、微量元素比值(Zr/Hf、Nb/Ta,Lu/Hf,Y/Ho,Th/Nb和Hf/Nb)、同位素εNd(0) 值,与第四纪黄土所见并无太大差异,进一步说明两者源岩化学成分一致,即粉尘的物源来源一致。

尽管稀土元素经常被用作物源示踪指标,但有研究显示稀土元素容易受到分选作用和风化强度的影响。一般研究(吴明清等,1991李福春等,2002)认为,随着沉积物的粒度由粗变细,稀土元素含量会随之增加;但李双林(2001)则认为粒度对稀土元素总量的影响很复杂,在不同时期存在着明显差别。根据已报道的对渭南黄土剖面以及下蜀土剖面稀土元素研究(刁桂仪和文启忠,2000李徐生等,2006)表明,风化淋滤作用会导致稀土元素迁移,从而导致古土壤层中的稀土含量略低于黄土层。在文中,新近纪红粘土的颗粒总体较第四纪黄土细,Na2O/Al2O3和CIA指示新近纪红粘土所经历的风化强度强于第四纪黄土,因而容易吸附铁镁质矿物以及Li、Cs和Bi元素,淋失Na2O和稀土元素,从而导致MgO、Li、Cs和Bi的相对富集,以及Na2O、稀土元素La-Lu和Y的相对亏损。

4 结论

通过对陇东邵寨剖面新近纪红粘土与第四纪黄土-古土壤序列进行地球化学特征分析,文章得出以下认识。

(1) 新近纪红粘土与第四纪黄土具有相似的常量、微量元素UCC标准化曲线,以及稀土元素球粒陨石标准化曲线,指示二者均来自广阔的物源区,经过了相似的搬运过程,并在搬运中得到了充分混合。

(2) 新近纪红粘土与第四纪黄土在地球化学特征方面有较多的相似性,常量、微量稳定元素比值、稀土元素总量、轻稀土与重稀土的分异程度、轻稀土内部分异程度、重稀土内部分异程度、Ce和Eu的异常程度、同位素εNd(0) 值与第四纪黄土并无太大差异,指示两者物质来源一致。

(3) 新近纪红粘土比第四纪黄土经历了更为强烈的风化过程,二者粒度组分及风化程度的差异,可能是导致新近纪红粘土与第四纪黄土常量、微量元素含量差异的主要原因。

参考文献/References
AN Z S, SUN D H, CHEN M Y, et al., 2000. Red clay sequences in Chinese Loess Plateau and recorded paleoclimate events of the late Tertiary[J]. Quaternary Sciences, 20(5): 435-446. (in Chinese with English abstract)
AN Z S, KUTZBACH J E, PRELL W L, et al., 2001. Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan Plateau since Late Miocene times[J]. Nature, 411(6833): 62-66. DOI:10.1038/35075035
BAU M, 1996. Controls on the fractionation of isovalent trace elements in magmatic and aqueous systems: evidence from Y/Ho, Zr/Hf, and lanthanide tetrad effect[J]. Contributions to Mineralogy and Petrology, 123(3): 323-333. DOI:10.1007/s004100050159
BOUVIER A, VERVOORT J D, PATCHETT P J, 2008. The Lu-Hf and Sm-Nd isotopic composition of CHUR: constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets[J]. Earth and Planetary Science Letters, 273(1-2): 48-57. DOI:10.1016/j.epsl.2008.06.010
BROECKER W S, PENG T H, 1982. Tracers in the sea[M]. New York: Eldigio Press, 26-31.
CHEN J, WANG H T, LU H Y, 1996. Behaviours of REE and other trace elements during pedological weathering-evidence from chemical leaching of loess and paleosol from the Luochuan section in Centra China[J]. Acta Geologica Sinica, 70(1): 61-72. (in Chinese with English abstract)
CONDIE K C, 2005. High field strength element ratios in Archean basalts: a window to evolving sources of mantle plumes?[J]. Lithos, 79(3-4): 491-504. DOI:10.1016/j.lithos.2004.09.014
DIAO G Y, WEN Q Z, 2000. Rare earth elements in the Weinan Loess section[J]. Marine Geology & Quaternary Geology, 20(4): 57-61. (in Chinese with English abstract)
DING Z L, SUN J M, YANG S L, et al., 1998. Preliminary magnetostratigraphy of a thick eolian red clay-loess sequence at Lingtai, the Chinese Loess Plateau[J]. Geophysical Research Letters, 25(8): 1225-1228. DOI:10.1029/98GL00836
DING Z L, SUN J M, ZHU R X, et al., 1997. Eolian origin of the red clay deposits in the loess plateau and implications for Pliocene climatic changes[J]. Quaternary Sciences, 17(2): 147-157. (in Chinese with English abstract)
DING Z L, SUN J M, YANG S L, et al., 1998. Magnetostratigraphy and grain size record of a thick red clay-loess sequence at Lingtai, the Chinese Loess Plateau[J]. Quaternary Sciences, 18(1): 86-94. (in Chinese with English abstract)
DING Z L, YANG S L, HOU S S, et al., 2001a. Magnetostratigraphy and sedimentology of the Jingchuan red clay section and correlation of the Tertiary eolian red clay sediments of the Chinese Loess Plateau[J]. Journal of Geophysical Research, 106(B4): 6399-6407. DOI:10.1029/2000JB900445
DING Z L, SUN J M, YANG S L, et al., 2001b. Geochemistry of the Pliocene red clay formation in the Chinese Loess Plateau and implications for its origin, source provenance and paleoclimate change[J]. Geochimica et Cosmochimica Acta, 65(6): 901-913. DOI:10.1016/S0016-7037(00)00571-8
FEDO C M, NESBITT H W, YOUNG G M, 1995. Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance[J]. Geology, 23(10): 921-924. DOI:10.1130/0091-7613(1995)023<0921:UTEOPM>2.3.CO;2
FERRAT M, WEISS D J, STREKOPYTOV S, et al., 2011. Improved provenance tracing of Asian dust sources using rare earth elements and selected trace elements for palaeomonsoon studies on the easternTibetan Plateau[J]. Geochimica et Cosmochimica Acta, 75(21): 6374-6399. DOI:10.1016/j.gca.2011.08.025
GALLET S, JAHN B M, VAN VLIET LANOE B, et al., 1998. Loess geochemistry and its implications for particle origin and composition of the upper continental crust[J]. Earth and Planetary Science Letters, 156(3-4): 157-172. DOI:10.1016/S0012-821X(97)00218-5
GARRELS R M, MACKENZIE F T, 1971. Evolution of sedimentary rocks[M]. New York: Norton.
GOLDSTEIN S L, O'NIONS R K, HAMILTON P J, 1984. A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems[J]. Earth and Planetary Science Letters, 70(2): 221-236. DOI:10.1016/0012-821X(84)90007-4
GROUSSET F E, BISCAYE P E, 2005. Tracing dust sources and transport patterns using Sr, Nd and Pb isotopes[J]. Chemical Geology, 222(3-4): 149-167. DOI:10.1016/j.chemgeo.2005.05.006
GU Z Y, DING Z L, XIONG S F, et al., 1999. A seven million geochemical record from Chinese red-clay and loess-paleosol sequence: weathering and erosion in northwestern China[J]. Quaternary Sciences, 19(4): 357-365. (in Chinese with English abstract)
GU Z Y, LAL D, GUO Z T, et al., 2000. Geochemistry of cosmogenic 10Be in loess-paleosol sequences and red clay in the Loess Plateau[J]. Quaternary Sciences, 20(5): 409-422. (in Chinese with English abstract)
GU Z Y, GUO Z T, LAL D, et al., 2006. 10Be concentration relation to chemical compositions of Chinese loess and red clay as a potential dating method[J]. Quaternary Sciences, 26(2): 244-249. (in Chinese with English abstract)
GUO Z T, RUDDIMAN W F, HAO Q Z, et al., 2002. Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China[J]. Nature, 416(6877): 159-163. DOI:10.1038/416159a
HAO Q Z, GUO Z T, PENG S Z, 2000. A preliminary study on the magnetic properties of the Tertiary red earth in the Longxi area[J]. Quaternary Sciences, 20(5): 447-456. (in Chinese with English abstract)
LI F C, XIE C R, PAN G X, 2002. Paleoclimatic implication of distribution of Rb, Rb/Sr and magnetic susceptibility in loess and paleosols from Laohushan profile, Nanjing[J]. Marine Geology & Quaternary Geology, 22(4): 47-52. (in Chinese with English abstract)
LI S L, 2001. Geochemistry of rare earth element in sediments at HY126EA1 hole in the continental shelf of the East China Sea[J]. Acta Oceanologica Sinica, 23(3): 127-132. (in Chinese with English abstract)
LI X S, HAN Z Y, YANG D Y, et al., 2006. REE geochemistry of Xiashu loess in Zhenjiang, Jiangsu province[J]. Acta Pedologica Sinica, 43(1): 1-7. (in Chinese with English abstract)
LI Y H, 1982. A brief discussion on the mean oceanic residence time of elements[J]. Geochimica et Cosmochimica Acta, 46(12): 2671-2675. DOI:10.1016/0016-7037(82)90386-6
LI Y, SONG Y G, NIE J S, et al., 2014. Tracing the provenance of loess and red clay on the Chinese Loess Plateau using the U-Pb dating and single-size zircon size[J]. Geological Review, 60(2): 380-388. (in Chinese with English abstract)
LIANG M Y, GUO Z T, GU Z Y, 2006. Geochemical characteristics of the Miocene eolian deposits and comparison with the Pliocene and Quaternary eolian deposits[J]. Quaternary Sciences, 36(4): 657-664. (in Chinese with English abstract)
LIANG M Y, GUO Z T, KAHMANN A J, et al., 2009. Geochemical characteristics of the Miocene eolian deposits in China: their provenance and climate implications[J]. Geochemistry, Geophysics, Geosystems, 10(4): Q04004. DOI:10.1029/2008GC002331
LIU C Q, MASUDA A, OKADA A, et al., 1993. A geochemical study of loess and desert sand in northern China: implications for continental crust weathering and composition[J]. Chemical Geology, 106(3-4): 359-374. DOI:10.1016/0009-2541(93)90037-J
LIU L W, WANG H T, CHEN Y, et al., 2002. Chemical leaching of loess deposits in China and its implications for carbonate composition[J]. Acta Petrologica et Mineralogica, 21(1): 69-75. (in Chinese with English abstract)
LU H Y, AN Z S, 1999. Comparison of grain-size distribution of red clay and loess-paleosol deposits in Chinese loess Plateau[J]. Acta Sedimentologica Sinica, 17(2): 226-232. (in Chinese with English abstract)
LU S, YIN G M, SONG W J, et al., 2019. Geochemical characteristics and paleoclimate implications of Hefei Xiashu loess[J]. Journal of Geomechanics, 25(3): 428-439. (in Chinese with English abstract)
MARQUES J J, SCHULZE D G, CURI N, et al., 2004. Trace element geochemistry in Brazilian Cerrado soils[J]. Geoderma, 121(1-2): 31-43. DOI:10.1016/j.geoderma.2003.10.003
MCLENNAN S M, 1989. Rare earth elements in sedimentary rocks: influence of provenance and sedimentary processes[J]. Reviews in Mineralogy and Geochemistry, 21(1): 169-200.
MCLENNAN S M, 2001. Relationships between the trace element composition of sedimentary rocks and upper continental crust[J]. Geochemistry, Geophysics, Geosystems, 2(4): 2000G. DOI:10.1029/2000GC000109
MUHS D R, BUDAHN J R, JOHNSON D L, et al., 2008. Geochemical evidence for airborne dust additions to soils in Channel Islands National Park, California[J]. GSA Bulletin, 120(1-2): 106-126. DOI:10.1130/B26218.1
NAKAI S I, HALLIDAY A N, REA D K, 1993. Provenance of dust in the Pacific Ocean[J]. Earth and Planetary Science Letters, 119(1-2): 143-157. DOI:10.1016/0012-821X(93)90012-X
NESBITT H W, MARKOVICS G, PRICE R C, 1980. Chemical processes affecting alkalis and alkaline earths during continental weathering[J]. Geochimica et Cosmochimica Acta, 44(11): 1659-1666. DOI:10.1016/0016-7037(80)90218-5
NESBITT H W, YOUNG G M, 1982. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites[J]. Nature, 299(5885): 715-717. DOI:10.1038/299715a0
NESBITT H W, YOUNG G M, MCLENNAN S M, et al., 1996. Effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments, with implications for provenance studies[J]. The Journal of Geology, 104(5): 525-542. DOI:10.1086/629850
OZKAN A M, AYAZ-BOZDAG A, 2011. Geochemistry and provenance of Maastrichtian clastic rocks in the Dikmendede Formation of Orhaniye in Kazan-Ankara-Turkey region[J]. Acta Geologica Sinica (English Edition), 85(5): 1067-1083. DOI:10.1111/j.1755-6724.2011.00541.x
PENG S Z, GUO Z T, 2000. A preliminary study on ree of the late tertiary laterite in Xifeng area[J]. Marine Geology & Quaternary Geology, 20(2): 39-43. (in Chinese with English abstract)
PENG S Z, GUO Z T, 2007. Clay mineral composition of the Tertiary Red Clay and the Quaternary loess-paleosols as well as its environmental implication[J]. Quaternary Sciences, 27(2): 277-285. (in Chinese with English abstract)
QI L, QIAO Y S, 2014. Geochemical characteristics of eolian deposits on the eastern margin of the Tibetan Plateau and implications for provenance[J]. Acta Geologica Sinica (English Edition), 88(3): 963-973. DOI:10.1111/1755-6724.12249
QIAO Y S, HAO Q Z, PENG S S, et al., 2011. Geochemical characteristics of the eolian deposits in southern China, and their implications for provenance and weathering intensity[J]. Palaeogeography, Palaeoclimatology, Palaeoecology, 308(3-4): 513-523. DOI:10.1016/j.palaeo.2011.06.003
REVEL M, SINKO J A, GROUSSET F E, et al., 1996. Sr and Nd isotopes as tracers of North Atlantic lithic particles: paleoclimatic implications[J]. Paleoceanography, 11(1): 95-113. DOI:10.1029/95PA03199
ROLLINSON H R, 1993. Using geochemical data: evaluation, presentation, interpretation[M]. New York: Longman Scientific and Technical.
SHELDON N D, TABOR N J, 2009. Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols[J]. Earth-Science Reviews, 95(1-2): 1-52.
SUGITANI K, HORIUCHI Y, ADACHI M, et al., 1996. Anomalously low Al2O3/TiO2 values for Archean cherts from the Pilbara Block, Western Australia-possible evidence for extensive chemical weathering on the early earth[J]. Precambrian Research, 80(1-2): 49-76. DOI:10.1016/S0301-9268(96)00005-8
SUN D H, LIU D S, CHEN M Y, et al., 1997. Magnetostratigraphy and palaeoclimate of Red Clay sequences from Chinese Loess Plateau[J]. Science in China Series D: Earth Sciences, 40(4): 337-343.
SUN D H, SHAW J, AN Z S, et al., 1998. Magnetostratigraphy and paleoclimatic interpretation of a continuous 7.2 Ma Late Cenozoic eolian sediments from the Chinese Loess Plateau[J]. Geophysical Research Letters, 25(1): 85-88. DOI:10.1029/97GL03353
SUN J M, 2005. Nd and Sr isotopic variations in Chinese eolian deposits during the past 8 Ma: implications for provenance change[J]. Earth and Planetary Science Letters, 240(2): 454-466. DOI:10.1016/j.epsl.2005.09.019
SUN J M, ZHU X K, 2010. Temporal variations in Pb isotopes and trace element concentrations within Chinese eolian deposits during the past 8 Ma: Implications for provenance change[J]. Earth and Planetary Science Letters, 290(3-4): 438-447. DOI:10.1016/j.epsl.2010.01.001
TAYLOR S R, MCLENNAN S M, MCCULLOCH M T, 1983. Geochemistry of loess, continental crustal composition and crustal model ages[J]. Geochimica et Cosmochimica Acta, 47(11): 1897-1905. DOI:10.1016/0016-7037(83)90206-5
TAYLOR S R, MCLENNAN S M, 1985. The continental crust: its composition and evolution[M]. London: Blackwell Scientific Publications, 57-72.
WANG Y X, YANG J D, CHEN J, et al., 2007. The Sr and Nd isotopic variations of the Chinese Loess Plateau during the past 7 Ma: implications for the East Asian winter monsoon and source areas of loess[J]. Palaeogeography, Palaeoclimatology, Palaeoecology, 249(3-4): 351-361. DOI:10.1016/j.palaeo.2007.02.010
WEN Q Z, 1989. Loess geochemistry in China[M]. Beijing: Science Press, 71-133. (in Chinese)
WU M Q, WEN Q Z, PAN J Y, et al., 1991. Rare earth elements in the Malan Loess from the middle reaches of the Huanghe River[J]. Chinese Science Bulletin, 36(16): 1380-1385.
YANG J D, CHEN J, ZHANG Z F, et al., 2005. Variations in 143Nd/144Nd and 87Sr/86Sr of Lingtai profile over the past 7 Ma[J]. Geochimica, 34(1): 1-6. (in Chinese with English abstract)
YANG S B, QIAO Y S, PENG S S, et al., 2017. Geochemical characteristics of eolian deposits in the northeastern margin of the Tibetan Plateau and implications for provenance and weathering intensity[J]. Quaternary Sciences, 37(1): 1-13. (in Chinese with English abstract)
YANG S L, DING Z L, 2000. Seven million-year iron geochemistry record from a thick eolian red clay-loess sequence in Chinese Loess Plateau and the implications for paleomonsoon evolution[J]. Chinese Science Bulletin, 46(4): 337-341.
YANG S Y, JUNG H S, LIM D I, et al., 2003. A review on the provenance discrimination of sediments in the Yellow Sea[J]. Earth-Science Reviews, 63(1-2): 93-120. DOI:10.1016/S0012-8252(03)00033-3
YANG X P, ZHU B Q, WHITE P D, 2007. Provenance of aeolian sediment in the Taklamakan Desert of western China, inferred from REE and major-elemental data[J]. Quaternary International, 175(1): 71-85. DOI:10.1016/j.quaint.2007.03.005
YOUNG G M, NESBITT H W, 1998. Processes controlling the distribution of Ti and Al in weathering profiles, siliciclastic sediments and sedimentary rocks[J]. Journal of Sedimentary Research, 68(3): 448-455. DOI:10.2110/jsr.68.448
ZHU R X, PAN Y X, DING Z L, et al., 1996. Magnetic property of red clay[J]. Quaternary Sciences, 16(3): 232-238. (in Chinese with English abstract)
安芷生, 孙东怀, 陈明扬, 等, 2000. 黄土高原红粘土序列与晚第三纪的气候事件[J]. 第四纪研究, 20(5): 435-446. DOI:10.3321/j.issn:1001-7410.2000.05.004
陈骏, 王洪涛, 鹿化煜, 1996. 陕西洛川黄土沉积物中稀土元素及其它微量元素的化学淋滤研究[J]. 地质学报, 70(1): 61-72.
刁桂仪, 文启忠, 2000. 渭南黄土剖面中的稀土元素[J]. 海洋地质与第四纪地质, 20(4): 57-61.
丁仲礼, 孙继敏, 朱日祥, 等, 1997. 黄土高原红粘土成因及上新世北方干旱化问题[J]. 第四纪研究, 17(2): 147-157. DOI:10.3321/j.issn:1001-7410.1997.02.007
丁仲礼, 孙继敏, 杨石岭, 等, 1998. 灵台黄土-红粘土序列的磁性地层及粒度记录[J]. 第四纪研究, 18(1): 86-94. DOI:10.3321/j.issn:1001-7410.1998.01.011
顾兆炎, 丁仲礼, 熊尚发, 等, 1999. 灵台红粘土和黄土-古土壤序列的地球化学演化[J]. 第四纪研究, 19(4): 357-365. DOI:10.3321/j.issn:1001-7410.1999.04.008
顾兆炎, LAL D, 郭正堂, 等, 2000. 黄土高原黄土和红粘土10Be地球化学特征[J]. 第四纪研究, 20(5): 409-422. DOI:10.3321/j.issn:1001-7410.2000.05.002
顾兆炎, 郭正堂, LAL D, 等, 2006. 黄土和红粘土中宇宙成因核素定年的潜力: 10Be浓度与化学成分的关系[J]. 第四纪研究, 26(2): 244-249. DOI:10.3321/j.issn:1001-7410.2006.02.012
郝青振, 郭正堂, 彭淑贞, 2000. 陇西第三纪红土磁学性质初步研究[J]. 第四纪研究, 20(5): 447-456. DOI:10.3321/j.issn:1001-7410.2000.05.005
李福春, 谢昌仁, 潘根兴, 2002. 南京老虎山黄土剖面的磁化率及Rb和Rb/Sr对古气候的指示意义[J]. 海洋地质与第四纪地质, 22(4): 47-52.
李双林, 2001. 东海陆架HY126EA1孔沉积物稀土元素地球化学[J]. 海洋学报, 23(3): 127-132. DOI:10.3321/j.issn:0253-4193.2001.03.016
李徐生, 韩志勇, 杨达源, 等, 2006. 镇江下蜀黄土的稀土元素地球化学特征研究[J]. 土壤学报, 43(1): 1-7. DOI:10.3321/j.issn:0564-3929.2006.01.001
李云, 宋友桂, 聂军胜, 等, 2014. 基于U-Pb定年和单颗粒锆石粒径分析示踪中国黄土高原黄土和红粘土物源[J]. 地质论评, 60(2): 380-388.
梁美艳, 郭正堂, 顾兆炎, 2006. 中新世风尘堆积的地球化学特征及其与上新世和第四纪风尘堆积的比较[J]. 第四纪研究, 36(4): 657-664. DOI:10.3321/j.issn:1001-7410.2006.04.023
刘连文, 王洪涛, 陈旸, 等, 2002. 黄土醋酸淋溶实验及其碳酸盐组分的地球化学特征[J]. 岩石矿物学杂志, 21(1): 69-75. DOI:10.3969/j.issn.1000-6524.2002.01.009
鹿化煜, 安芷生, 1999. 黄土高原红粘土与黄土古土壤粒度特征对比: 红粘土风成成因的新证据[J]. 沉积学报, 17(2): 226-232. DOI:10.3969/j.issn.1000-0550.1999.02.011
路硕, 尹功明, 宋为娟, 等, 2019. 合肥下蜀土地球化学特征及其古气候意义[J]. 地质力学学报, 25(3): 428-439.
彭淑贞, 郭正堂, 2000. 西峰地区晚第三纪红土稀土元素的初步研究[J]. 海洋地质与第四纪地质, 20(2): 39-43.
彭淑贞, 郭正堂, 2007. 风成三趾马红土与第四纪黄土的粘土矿物组成异同及其环境意义[J]. 第四纪研究, 27(2): 277-285. DOI:10.3321/j.issn:1001-7410.2007.02.013
孙东怀, 刘东生, 陈明扬, 等, 1997. 中国黄土高原红粘土序列的磁性地层与气候变化[J]. 中国科学(D辑), 27(3): 265-270. DOI:10.3321/j.issn:1006-9267.1997.03.007
文启忠, 1989. 中国黄土地球化学[M]. 北京: 科学出版社, 71-133.
吴明清, 文启忠, 潘景瑜, 等, 1991. 黄河中游地区马兰黄土的稀土元素[J]. 科学通报, 36(5): 366-369.
杨杰东, 陈骏, 张兆峰, 等, 2005. 距今7Ma以来甘肃灵台剖面Nd和Sr同位素特征[J]. 地球化学, 34(1): 1-6.
杨帅斌, 乔彦松, 彭莎莎, 等, 2017. 青藏高原东北缘黄土的地球化学特征及其对物源和风化强度的指示意义[J]. 第四纪研究, 37(1): 1-13.
杨石岭, 丁仲礼, 2000. 7.0 Ma以来中国北方风尘沉积的游离铁/全铁值变化及其古季风指示意义[J]. 科学通报, 45(22): 2453-2456. DOI:10.3321/j.issn:0023-074X.2000.22.018
朱日祥, 潘永信, 丁仲礼, 1996. 红粘土的磁学性质研究[J]. 第四纪研究, 16(3): 232-238. DOI:10.3321/j.issn:1001-7410.1996.03.005