Characteristics,ages And Geological Significance Of The Jurassic Combustion Metamorphic Rocks In Northwestern China

Spontaneous combustion of coals is a common geological phenomenon in the nature.As the most important product of spontaneous combustion of coal seams,combustion metamorphic(CM)rocks are widely distributed around the world(e.g.,in northwestern China).Only limited attention,however,has been paid to the research to characteristics and ages of CM rocks in China.Combined with field geological surveys,sampling and previous studies,comprehensive methods such as thin section analyses,scanning electron microscopy(SEM),X-ray diffraction(XRD),X-ray fluorescence(XRF),inductively coupled plasma mass spectrometry(ICP-MS),(U-Th)/He dating and apatite fission-tracks(AFT)dating were used to investigate the CM rocks yielding from the Jurassic in northwestern China in detail.The petrologic,mineralogical,geochemical and geochronological characteristics of the CM rocks in the Jurassic successions were investigated in this study.Through the present research,the formation age of combustion metamorphic rocks(i.e.,the spontaneous combustion age of Jurassic coal seams)was dated.Importantly the relationship between combustion metamorphic rocks and Cenozoic tectonic movements or river erosion was confirmed.Furthermore,the origin of white sandstone associated with the red CM rocks was extensively discussed.The above research has improved significantly the understanding of the geological information of the CM rocks.Affected by factors such as characteristics of coal seam distribution,climatic conditions,and human mining activities,the present spontaneous combustion of coal seams occurs mainly in northwestern China.The CM rocks are mainly distributed in the margins of sedimentary basins in northwestern China(such as the Ordos,Yili,Junggar Basin,Tuha and Tarim Basins)or the margins of orogenic belts(such as Tianshan and Helan Mountains).Field observation allowed us to divide the products of spontaneous combustion of coal seams into three categories,namely metamorphic coal series,coal-fire gas and gas-vent minerals series,as well as CM rock series.Results showed that,the CM rocks yielding from the Jurassic in northwestern China are mainly red,mixed with white,yellow,black,and steel gray colors.The main structural features include residual structure,pore structure,columnar joint structure,breccia structure,and lava structure.The CM rocks contain a variety of high temperature minerals,including cristobalite,tridymite,cordierite,sekaninaite,mullite,magnetite,and magnetite,which can be identified as high-temperature phases associated with coal combustion.The main components of CM rocks are Si O₂ and Al₂O₃,with basically stable concentration.Fe₂O₃ shows a relatively high content,and is most abundant in the melt rock.Meanwhile,the content of Si O₂ in CM rock is negatively correlated with that of Mg O and Al₂O₃.In addition,the?Eu in CM rocks show negative anomaly,and no anomaly is indicated for the?Ce.The total amount of rare earth elements(?REE)of sintered rock is higher than that of melt rock.The(U-Th)/He ages of zircons in CM rocks in northeastern Ordos Basin are mainly concentrated in two periods,1.74±0.19 Ma and 4.52±0.77 Ma?4.75±0.37 Ma,respectively.Here,the ages of CM rocks are consistent with the river erosion time.Comprehensive analyses suggest that the spontaneous combustion of coal seams(i.e.,the formation of CM rocks)in this area are mainly due to the river erosion.In the northern margin of the Yili Basin,the(U-Th)/He age of zircon is 0.84±0.22 Ma,which is in good agreement with the structural uplift of the Tianshan Orogenic Belt,indicating that the uplift of the Tianshan Orogenic Belt controls the spontaneous combustion of coal seams.In the Liuhuanggou Area in the northern margin of the Tianshan Mountains,investigations have been carried out for the ages of the Tianshan uplift and river terraces.The apatite fission-tracks ages and cross-cutting relationship between the combustion metamorphic rocks and terrace deposits are studied as well.All of the results make it possible to determine that at least four phases of coal fire activity occurred,spanning from Late Miocene to Holocene.Considering the fact that the formation of CM rocks could be linked to tectonic uplift and river erosion,and the consistency in term of time,it is thought that the temporal and spatial distribution of CM rocks can be used to provide a new idea for studying the age and rate of tectonic uplift and river erosion.Investigations in the northeastern Ordos Basin also highlighted the occurrence of the peculiar,porous,and permeable white sandstones which appears commonly associated with clinkers or coal seams.It is composed of quartz,feldspar grains and kaolinite cements.It is suggested that the white color of this sandstone could be due to coal fire-related kaolinization of a sandstone protolith produced by the acidic low-temperature hydrothermal circulation of rain waters during the period of coal fire activity.