Geochemistry and Provenance of the Early Cretaceous Lacustrine Clastic Sedimentary Rocks of Yixian Formation from the Sihetun Section, West Liaoning
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摘要: 辽西北票上园镇四合屯地区义县组沉积层是热河生物群珍稀化石的重要产出层位之一, 其中蕴含丰富的地壳演化和古地理环境演化信息.分析了四合屯剖面上12m厚的含鸟化石沉积岩层的50件样品的微量元素组成和其中10件样品的主量元素组成.结合前人已发表数据, 得出以下认识: (1) 该套沉积岩具有与下伏火山岩类似的地球化学特征: Mg#值较高, 大离子亲石元素含量较高.稀土元素的分异程度较高, 在微量元素分布蛛网图中表现为明显的Nb负异常和Pb正异常等.因此我们认为义县组高镁火山岩构成了该沉积岩的主要源区之一; (2) 样品的Eu/Eu*=0.48~1.81, 其中大部分样品具负Eu异常.正的Eu异常(7件样品) 主要由于含有长石造成; (3) 通过微量元素Th-Co-Zr/10沉积构造环境判别图解对区域构造环境进行分析, 结果表明该沉积岩物源区具有大陆岛弧和活动大陆边缘物源特征.Abstract: A wide range of well-preserved Jehol biota fossils have been found in the sedimentary rocks of the Yixian Formation from the Sihetun area in West Liaoning Province.We have analyzed major and trace element compositions of fifty samples of clastic sedimentary rocks from this section.Combined with data previously publicized by others, the conclusions are as follows: firstly, this set of sedimentary rocks share similar chemical characteristics to the associated volcanic rocks, with LREE enrichment, flat HREE, high-Mg#, high contents of large ion lithophile element (LILE) and variable REE anomalies.Their spider diagrams show significant negative Nb and positive Pb anomalies.It is deducted that the sedimentary rocks were mainly derived from the high-Mg# volcanic rocks from the Yixian Formation.Secondly, these sedimentary rocks show variable Eu anomalies (Eu/Eu=0.48-1.81) with the average value being 0.75.The positive anomaly is due to presence of plagioclase.Moreover, tectonic discrimination diagram of Th-Co-Zr/10 suggests that these sedimentary rocks were formed in a tectonic setting similar to the continental island arc and active continental margin.
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Key words:
- Sihetun /
- Mesozoic /
- sedimentary rock /
- trace element composition
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图 1 辽西四合屯地区区域地质简图及义县组剖面图(据王晓蕊等, 2005修改)
NC.华北克拉通; QL-DB.秦岭-大别造山带; SC.华南克拉通; YC.扬子克拉通
Fig. 1. Geological sketch map and section of the Yixian Formation in Sihetun area, West Liaoning Province
图 2 四合屯义县组沉积岩样品稀土元素模式分配图和微量元素分布蛛网图
a、b为Eu正异常(Eu/Eu* > 1.05) 组样品; c、d为无Eu异常(Eu/Eu*=0.95~1.05) 组样品; e、f为Eu负异常(Eu/Eu* < 0.95) 组样品; 阴影部分为四合屯义县组安山岩数据(王晓蕊等, 2005)
Fig. 2. Chondrite-normalized REE patterns (a, c, e) and primitive mantle-normalized spider diagrams (b, d, f) of Sihetun sedimentary rocks
图 3 辽西四合屯义县组沉积岩Sr与Eu/Eu*相关图及其与前人资料的对比
上地壳均值和华北克拉通内部数据来源于Gao et al.(1998); 义县组火山岩数据来自王晓蕊等(2005); 燕山杂岩体数据来自苏尚国等(2006); 燕山晚侏罗世火山岩数据来自李晓勇等(2006)
Fig. 3. Sr versus Eu/Eu* plot for Sihetun sedimentary rocks
图 4 辽西四合屯义县组沉积岩La/Th-Hf源区判别图解(Gu et al., 2002)
Fig. 4. La/Th Hf discrimination diagram of source rocks for Sihetun sedimentary rocks
图 5 辽西四合屯义县组沉积岩Th-Co-Zr/10判别图解(Bhatia and Crook, 1986)
A.大洋岛弧(OIA); B.大陆岛弧(CIA); C.活动大陆边缘(ACM); D.被动大陆边缘(PM)
Fig. 5. Tectonic discrimination diagram of Th-Co-Zr/10 for Sihetun sedimentary rocks
表 1 样品描述
Table 1. Description of samples
表 2 辽西四合屯地区义县组沉积岩样品主量元素含量(%)
Table 2. Major element compositions of sedimentary rocks from Yixian Formation in Sihetun area (%)
表 3 辽西四合屯地区义县组沉积岩样品微量元素含量(10-6)
Table 3. Trace element compositions of sedimentary rocks from Yixian Formation in Sihetun area (10-6)
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