GEOCHEMICAL CHARACTERISTICS AND SEDIMENTARY ENVIRONMENT OF TRIASSIC SILICEOUS ROCKS FROM MUYINHE FORMATION IN CHANGNING-MENGLIAN BELT, SOUTHWEST YUNNAN
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摘要: 滇西南昌宁—孟连带牡音河组硅质岩已进行了较好的生物地层学和沉积学研究, 表明为中三叠世深水盆地沉积, 但对其沉积构造背景存在不同的认识. 本文尝试用分子有机地球化学方法, 并辅以无机地球化学的手段, 研究牡音河组硅质岩的物源环境, 分析沉积区构造背景. 研究结果表明硅质岩中正构烷烃碳数分布范围较宽C14—C36, 且具双峰型分布模式, 说明其有机质为陆生高等植物和水生生物混合来源; 萜类和甾类化合物特征说明研究区有机质来源总体上以水生生物为主, 同时有不同程度陆生高等植物混入. 牡音河组硅质岩Σw (REE) 为83.56 × 10-6 124.1 × 10-6, w (Ce)/w (Ce) 在0.760.99之间变化, w (La)n/w (Yb)n在0.771.98之间, 说明该盆地明显受陆源区物质影响. 另外, 该硅质岩的w (La)n/w (Ce)n和w (Al2O3)/w (Al2O3+Fe2O3) 图解及w (MnO)/w (TiO2) 比值也说明牡音河组硅质岩沉积过程中受到大陆作用. 两方面的特征互相印证, 说明研究区中三叠世为受陆地强烈作用的深水局限盆地. 这项研究还说明, 分子有机地球化学方法能够为硅质岩沉积环境的探讨提供更多的物源和环境信息.Abstract: Detailed studies of biostratigraphy and sedimentology of the siliceous rocks from the Muyinhe Formation in the Changning-Menglian belt, Southwest Yunnan indicate that the deposition occurred in a deep-water basin in Middle Triassic. However, its depositional tectonic setting is little understood. The main purpose of this article is to analyze its provenance and depositional tectonic background by using molecular organic geochemistry and inorganic geochemistry. The carbon number distribution of the normal paraffin in the siliceous rock is relatively wide, C14-C36, with two peaks, which implies that this organic matter is derived from both higher terrigenous plants and marine autochthonous organisms. The features of the steroid and terpenoid hydrocarbons suggest that the organic is mainly originated from marine sources mixed with terrigenous sources. The REE characteristics of the siliceous rock are as followed: total w (REE) ranges from 83.56 × 10-6 to 124.1 × 10-6, w (Ce)/w (Ce*) between 0.76 and 0.99, w (La)n/w (Yb)n from 0.77 to 1.98. These characteristics show that the basin was apparently influenced by terrigenous sources. In addition, w (MnO)/w (TiO2) ratio and w (La)n/w (Ce)n vs. w (Al2O3)/w (Al2O3+Fe2O3) plot consistently show that the chert was influenced by land sources during its depositional process. While the incorporating characteristics of the organic and inorganic in the siliceous rocks suggest that the studied area was a limited deep-water basin and was strongly influenced by terrigenous matter and that molecular organic geochemistry can provide more information about the provenance and sedimentary environment of siliceous rocks.
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Key words:
- biological marker compound /
- REE /
- depositional tectonic setting /
- siliceous rock /
- Middle Triassic /
- Southwest Yunnan
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图 1 研究区交通位置和地质简图
1.下-中三叠统牡音河组; 2.石炭—二叠系拉巴群; 3.石炭—二叠系灰岩; 4.下石炭统平掌组; 5.断层; 6.地质界线; 7.剖面位置
图 3 w (La)n/w (Ce)n-w (Al)/w (Al+Fe) 图解
Fig. 3. w (La)n/w (Ce)n-w (Al)/w (Al+Fe) diagram
表 1 反映环境的生物标志化合物参数
Table 1. Biomarker parameters reflecting depositional environment
表 2 生物标志化合物参数计算
Table 2. Calculation and interpretation of biomarker parameters
表 3 牡音河组硅质岩中稀土元素质量分数及特征
Table 3. REE mass fractions and features of chert from Muyinhe Formation
表 4 牡音河组硅质岩中常量元素质量分数及特征
Table 4. Mass fractions and features of major elements in siliceous rocks from Muyinhe Formation
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