中国北方侏罗系烧变岩露头区白色砂岩特征与成因分析^*

Characteristic and formation cause of white sandstones in the Jurassic outcrop area of burnt rocks in northern China

摘要

中国北方烧变岩广泛发育,常见侏罗系煤层于中生代、新生代自燃形成的烧变岩。在烧变岩露头区,白色砂岩常与红色烘烤岩伴生,但国内外对其关注不够,成因不明。本次研究对伊犁、准噶尔、吐哈、鄂尔多斯等盆地周缘烧变岩区野外露头进行了观察,采集白色砂岩样品进行薄片鉴定、扫描电镜及能谱等测试与分析,对白色砂岩岩石学、矿物学特征进行观察和描述,认为中国北方侏罗系白色砂岩常分布在烧变岩的上覆、下伏或侧边位置,颜色多以灰白为主,结构疏松,孔隙发育,碎屑颗粒以石英、长石矿物为主,填隙物主要为高岭石,发育特殊的流体流动所致的孔洞构造; 据其与烧变岩的空间相关性,推测白色砂岩的形成与煤层自燃有密切联系,煤层自燃导致高岭石大量生成、有色金属矿物或金属离子含量变化,这是砂岩呈现白色的主要诱因,因此烧变岩区所见白色砂岩为特殊类型的低温烘烤岩,其形成模式有3种: 河谷下切型、裂隙下切型和山丘下渗型。

Abstract

Burnt rocks are widely distributed across northern China,often formed by the spontaneous combustion of Jurassic coal beds during the Mesozoic and Cenozoic. White sandstones are commonly accompanied with the red burnt rocks caused by the spontaneous combustion of Jurassic coals in these areas. These white sandstones have received little attention,and their formation remains poorly understood. In this study,the white sandstones accompanied with burnt rocks are observed at the Jurassic outcrops in the margins of Yili,Junggar,Tuha and Ordos Basins. White sandstone samples were collected and analyzed through thin section identification,scanning electron microscopy(SEM),and energy spectrum analysis to examine their petrological and mineralogical characteristics. The results show that the white sandstones in the Jurassic coal-bearing sequence are distributed at the top,bottom or alongside of burnt rocks,with light-grey white color,loose structure,and well-developed porosity. The clastic grains are mainly quartz and feldspar minerals,and the interstitial material is mainly kaolinite. Unique pore structures caused by fluid movement are also observed. According to the spatial correlation with burnt rocks,we propose that the cause of these white sandstones is closely related to the spontaneous combustion of Jurassic coals. The massive production of kaolinite,removement of nonferrous metals and vanishment of organic matter are likely caused by coal combustion process,which result in the formation of white sandstones. Thus,these white sandstones represent a special type of low-temperature baked rock. Three genetic models are proposed for the formation of these white sandstones: (A)valley incision type,(B)fissure incision type and(C)hill infiltration type.

关键词

烧变岩 / 白色砂岩 / 沉积盆地 / 侏罗系煤层 / 中国北方

Key words

burnt rock / white sandstone / sedimentary basin / Jurassic coal / northern China

基金资助

*国家自然科学基金(编号: 42201006)资助