Na Wang , Joan S. Esterle , Sandra Rodrigues , James C. Hower , Shifeng Dai
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Based on 11 samples at 50-cm horizontal intervals, random vitrinite reflectance value (R<sub>r</sub>) declines from 5.41% at the contact to the background value of 2.05% at 5-m distance from the contact. Towards the dike, vitrinite anisotropy increases and the microtexture shows development of circular, coarse mosaic, and ribbon anisotropic coke textures and devolatilization vacuoles. Natural coke textures respond to an estimated temperature > 570 °C at the intrusion/coal contact. However, the occurrence of coke structures within a semianthracite reveals a local and regional thermal evolution. Coke textures suggest that coal rank was high volatile A bituminous coal at the time of intrusion. This has implications for burial history, heating, and subsidence rates. Assuming a paleogeothermal gradient of 40 °C/km, high volatile A bituminous coal (1% R<sub>r</sub>) would have been buried to 3 km at the time of the Jurassic intrusion. 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引用次数: 0
摘要
火成岩侵入会引起煤炭和矿物的改变。蚀变程度取决于原位煤的等级和宏观矿物组成、侵入温度、热源远近和侵入体大小。在二叠纪峰峰煤田,煤炭等级从高挥发烟煤到无烟煤,由南向北递增约 50 公里。煤层普遍被侵入,侏罗纪-白垩纪火成岩侵入体加剧了区域等级的提升,而不仅仅是埋藏煤化。对峰峰煤田北部大树村煤矿的半无烟煤进行的岩相检查显示,焦炭微观结构有所发展,2 号煤层中接近堤坝的反射率增加。根据 11 个水平间隔为 50 厘米的样本,随机玻璃光泽反射率值(Rr)从接触面处的 5.41% 下降到距离接触面 5 米处的背景值 2.05%。向堤坝方向,矾土各向异性增加,微纹理显示出圆形、粗镶嵌、带状各向异性焦炭纹理和脱溶空泡的发展。天然焦炭纹理与侵入体/煤接触处的估计温度 > 570 °C 相符。然而,半无烟煤中出现的焦炭结构揭示了局部和区域的热演化。焦炭纹理表明,侵入时煤炭等级为高挥发性 A 级烟煤。这对埋藏历史、加热和沉降速度都有影响。假设古地热梯度为 40 °C/km,那么在侏罗纪侵入时,高挥发性 A 烟煤(1% Rr)将被埋藏 3 km。入侵将高挥发性烟煤变质为天然焦炭,随后更大规模的区域变质作用又将煤变质为半无烟煤。
Insights on the regional thermal evolution from semianthracite petrology of the Fengfeng coalfield, China
Igneous intrusions induce alterations in coal and minerals. Alteration degree depends upon the in-situ coal rank and maceral composition, the intrusion temperature, heat source proximity, and intrusion size. In the Permo-Carboniferous Fengfeng coalfield, coal rank increases over some 50 km distance south to north from high volatile bituminous coal to anthracite. The coal seams were commonly intruded, and the regional elevation of rank was augmented by Jurassic-Cretaceous igneous intrusions, not just burial coalification. The petrographic examination of a semianthracite from the Dashucun mine in the north of the Fengfeng coalfield shows development of coke microstructure and increased reflectance approaching the dike in the No. 2 Seam. Based on 11 samples at 50-cm horizontal intervals, random vitrinite reflectance value (Rr) declines from 5.41% at the contact to the background value of 2.05% at 5-m distance from the contact. Towards the dike, vitrinite anisotropy increases and the microtexture shows development of circular, coarse mosaic, and ribbon anisotropic coke textures and devolatilization vacuoles. Natural coke textures respond to an estimated temperature > 570 °C at the intrusion/coal contact. However, the occurrence of coke structures within a semianthracite reveals a local and regional thermal evolution. Coke textures suggest that coal rank was high volatile A bituminous coal at the time of intrusion. This has implications for burial history, heating, and subsidence rates. Assuming a paleogeothermal gradient of 40 °C/km, high volatile A bituminous coal (1% Rr) would have been buried to 3 km at the time of the Jurassic intrusion. The intrusion metamorphosed high volatile bituminous coal to natural coke and subsequent larger-scale regional metamorphism then metamorphosed the coal to semianthracite.
期刊介绍:
The International Journal of Coal Geology deals with fundamental and applied aspects of the geology and petrology of coal, oil/gas source rocks and shale gas resources. The journal aims to advance the exploration, exploitation and utilization of these resources, and to stimulate environmental awareness as well as advancement of engineering for effective resource management.