中国北方烧变岩的分布、特征及研究意义*

中国北方烧变岩分布广泛,见于以昆仑山—秦岭—大别山为界的中国北方地区多个沉积盆地边缘,但地质学家对其研究较为薄弱。野外调查显示中国北方烧变岩主要以4种方式分布: (1)大面积连片分布,(2)沿山脉走向的线状分布,(3)沿河流下切河谷分布,(4)为第四系黄土覆盖。烧变岩主要为侏罗系、石炭—二叠系煤层自燃所致。各类型的烧变岩在中国烧变岩区不同程度发育,具瓷化结构、白化结构、烧熔结构、残余构造、气孔构造、微柱状节理、角砾状构造、垮塌构造、裂缝充填等结构和构造。其颜色以砖红色、赭红色、棕色、钢灰色最为典型,岩石中发育鳞石英、方石英、堇青石、铁堇青石、莫来石等典型高温矿物。中新世以来,中国西部、北部地区构造活动强烈,干旱气候加剧,烧变岩也主要形成于这一时期,但煤层自燃与构造运动、环境变化、古野火事件的耦合关系还鲜有提及,其所蕴含的各种地质信息值得在今后的研究中予以关注。

Distribution,characteristics and significances of burnt rocks in northern China

Burnt rocks are widely distributed in many sedimentary basins located in the northern China that are bounded by Kunlun mountain-Qinling-Dabie mountain. However,few studies have focused on the origin,age and geological significance of these kinds of metamorphic rocks. Based on our field investigation,four distribution modes of the burnt rocks are found: (1)large area distribution,(2)linear distribution along the mountain range alignment,(3)distribution along the river incised-valley and(4)covered by the Quaternary loess. These burnt rocks were mainly caused by the self-ignition of Jurassic and Carboniferous-Permian coals. Various burnt rocks developed in the burnt rock area on different levels. The vitrified structure,whitened structure,lava structure,residual structure,pore structure,columnar jointing structure,breccia structure,fissure structure and filling minerals are often found in these burnt rocks. The burnt rocks are mainly red,mixed with brown,black and steel-gray colors. The typical high temperature minerals as cristobalite,tridymite,cordierite,sekaninaite and mullite are common in these burnt rocks. The active tectonic movements and arid climate in northwestern China triggered the formation of burnt rocks since the Miocene. The relationship between the palaeotectonic,palaeoclimate,palaeo-wildfire and the coal self-ignition,however,has been seldomly studied. Such information implied from the formation of burnt rocks should be paid more attention in the future work.