Petrologic and REE Geochemical Characters of Burnt Rocks
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摘要: 烧变岩的研究有益于对煤层形成以来所经历的构造运动、古气候和古地理的探讨.为了获得有关烧变岩基本特征的数据资料, 本文利用扫描电镜、能谱测试、古地磁及ICP-MS等测试手段对陕北神木地区烧变岩进行了系统的岩石学及稀土元素地球化学的分析研究.剖面上将烧变岩分成烧熔岩和烧烤岩两个序列.扫描电镜观察及能谱测试显示, 各矿物均显示烧熔迹象, 除伊利石外未发现其他类型粘土矿物; 磁化率测试显示烧变岩具异常高的磁化率; 地球化学分析显示烧变岩稀土元素配分特征近于沉积岩(原岩) 特征; 纵向剖面上, 随烧变程度增大(由烧烤岩至烧熔岩) 其稀土元素总量逐渐减小, 烧熔岩稀土元素总含量较烧烤岩的要明显低; 另外, 烧熔岩表现为较明显的Ce负异常, 而烧烤岩则表现为无Ce异常, 有些甚至表现为偏正异常.Abstract: The study of burnt rocks faciliates the discussion on the tectonic movement, paleoclimate and paleogeography that coal seams were subject to after they were formed.In order to obtain the basic data on the features of the burnt rocks, a systematic study of petrology and REE geochemistry on burnt rocks in Shenmu, North Shaanxi Province, has been done, using the methods of SEM, EDS, susceptibility measurements and ICP-MS.The burnt rocks are divided into two series in the section: the melted rocks and the baked rocks.SEM and EDS analyses reveal that all the minerals show burnt and melted traces, and there are no clay minerals except illite found in the burnt rocks.Susceptibility measurements reveal that the burnt rocks have abnormally high susceptibility values, whereas a geochemical analysis shows that the REE distribution pattern of burnt rocks is similar to that of sedimentary rocks (initial rocks).In the longitudinal section, the REE gradually decreases as the burning degree increases (from baked rocks to melted rocks), and the total REE of melted rocks is obviously lower than that of baked rocks.Besides, the melted rocks show apparent negative Ce anomalies, while the baked rocks show no anomaly of Ce, and sometimes even show positive anomalies.
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Key words:
- burnt rock /
- mineral petrology /
- susceptibility /
- REE /
- negative Ce anomaly
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图 1 研究区区域地质简图
1.上三叠统瓦窑堡组和永平组; 2.早侏罗世富县组; 3.早中侏罗世延安组; 4.中侏罗世直罗组; 5.剖面所在位置
Fig. 1. Simplified geological map of the study area showing the section localities
图 2 烧变岩纵向剖面及煤灰与烧熔岩照片
a和b.烧变岩纵向剖面, 其中1.烧变残留煤层; 2.煤灰层; 3.烧烤岩层; 4.烧熔岩层; c.煤灰; d.熔融状烧熔岩; e.炉渣状烧熔岩
Fig. 2. Photos of burnt rock in profile, and coal ash, melted rocks
图 3 环境扫描电镜下烧变岩中矿物特征
a、b.烧熔岩中不同放大倍数下具烧熔边痕迹的长石; c.烧烤岩中具烧熔边的长石
Fig. 3. Feature of minerals in burnt rocks observed through SEM
图 4 环境扫描电镜下呈不规则鳞片状(a) 和丝缕状(b) 的伊利石
Fig. 4. Illite with irregular lepidosome features (a) and filamented features (b) shown under the SEM
表 1 研究中所选样品统计
Table 1. Samples used in this study
表 2 烧变岩样品中稀土组成(10-6)
Table 2. The REE compositions of burnt rocks
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