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四川盆地南部五峰组—龙马溪组钾质斑脱岩地球化学特征

关键词：斑脱岩；五峰组；龙马溪组；四川盆地

四川盆地南部五峰组—龙马溪组钾质斑脱岩地球化学特征
王高翔，陈雷，陈鑫
西南石油大学地球科学与技术学院，四川成都610500
Email:yuanyang0609@126.com Tel:15928097980
摘要：斑脱岩层，作为特殊地质事件的产物，记录了火山活动沉积，具有良好的地层对比作用。四川盆地上奥陶统五峰组-下志留统龙马溪组黑色页岩伴生多层由火山灰搬运沉积形成的斑脱岩层，这指示当时频繁的火山运动。笔者以岩心观察为基础，通过全岩/黏土X射线衍射分析、高精度ICP-MS质谱仪等测试技术，对四川盆地南部8个斑脱岩样品进行矿物成分分析、主微量元素分析，得到以下几点结论：1、四川盆地南部五峰组-龙马溪组斑脱岩为钾质斑脱岩，主要是由粘土矿物和非晶体组成，其次含有石英、长石、黄铁矿、碳酸盐矿物等，偶可见锐钛矿；粘土矿物主要包括伊利石-蒙脱石混层矿物和伊利石。2、斑脱岩中SiO2平均含量为46.69%，Al2O3平均含量为24.35%，K2O平均含量为6.37%；根据K2O-SiO2图解及Zr/TiO2-Nb/Y图解，研究区斑脱岩样品值落入流纹英安岩-安山岩之间，说明原始岩浆为基性-中性。3、与北美页岩相比，随微量元素不相容程度降低，五峰组-龙马溪组钾质斑脱岩微量元素标准化值呈逐渐增高趋势，亏损Cr、Ta、Zr、Hf等高场强元素，富集Rb、Co、Cs、Th。4、通过球粒陨石和北美页岩标准化，五峰组-龙马溪组稀土元素配分图均以“右倾”为特征，轻稀土富集、重稀土亏损，具不明显Eu负异常。5、由Th-Hf-Ta构造环境判别图解得到其原始岩浆喷发环境主要与火山弧环境相关，推测与晚奥陶世-早志留世华夏古陆与扬子地块碰撞增生后大陆边缘地区的火山喷发有关。6、斑脱岩对页岩气成藏具有积极意义，一方面表现在火山灰入海为海洋生物提供营养物质从而提高海洋生产力，另一方面表现为火山喷发后的缺氧环境是有机质埋藏和保存的有利条件。
关键词：斑脱岩；五峰组；龙马溪组；四川盆地
 

Geochemical characteristics of K-bentonite in Wufeng-Longmaxi Formation in south Sichuan Basin
Gaoxiang Wang, Lei Chen, Xin Chen
School of Geoscience and technology ,Southwest Petroleum University,Chengdu 610500,China
Abstract：The Bentonite layers, as the product of special geological events, recorded the volcanic sedimentation and play a significant role for stratigraphic correlation. The black shale of the Upper Ordovician Wufeng Formation to the lower Silurian Longmaxi Formation in the Sichuan Basin is associated with multiple bentonite layers formed by the volcanic ash, which indicate frequent volcanic movements at that time. Based on core observation，XRD analysis，XRF analysis and ICP-MS analysis, the mineral composition and main trace elements of eight bentonite samples in south Sichuan Basin were analyzed，and the conclusions are as follows: 1.The bentonite of the Wufeng-Longmaxi Formation in the south Sichuan Basin is K-rich, mainly contains of clay mineral and amorphous mineral, and it’s secondary components are quartz, feldspar, pyrite, and carbonate minerals with few anatase. The clay mineral is dominated by illite/smectite mixed layers and illite. 2. The average content of SiO2，Al2O3 and K2O is 46.69%, 24.35% and 6.37%, respectively. According to K2O-SiO2 diagram and Zr/TiO2-Nb/Y diagram, the data are mainly located in the rhyodacite to andesite zone，which shows that the original magma was basic- neutral. 3.Comparing with the NASC, with the decrease of the incompatibility of trace elements, the standardized value of trace elements in K-bentonite of Wufeng-Longmaxi Formation shows a gradually increasing trend. The content of Cr, Ta, Zr and Hf is low, while the content of Rb, Co, Cs and Th is high. 4. According to chondrite and NASC standardization，the distribution model of rare earth elements is the “ right-leaning”. LREE are rich and HREE are poor, and there is mildly negative anomaly of Eu. 5. The original magma eruption was related to volcanic arc tectonic setting according to the tectonic environment discriminant diagram of the Th-Hf-Ta, which is related to the volcanic eruption in the continental margin after the collision and accretion between the Cathaysia Block and the Yangtze Block during the late Ordovician and early Silurian. 6.The bentonite has positive significance for the shale gas accumulation. On the one hand, after entering the sea, the volcanic ash provides nutrients for marine life and thus improves marine productivity. On the other hand, the hypoxic environment after volcanic eruption is a favorable condition for the burial and preservation of organic matter.
Key words:Bentonite; Wufeng Formation; Longmaxi Formation; Sichuan Basin