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 (R) 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) 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.

中文翻译：

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峰峰煤田半无烟煤岩石学对区域热演化的认识


火成岩侵入引起煤炭和矿物的变化。蚀变程度取决于原位煤阶和煤质组成、侵入温度、热源接近度和侵入尺寸。在二叠纪-石炭系峰峰煤田，从南向北约 50 公里的距离内，煤级从高挥发性烟煤逐渐增加到无烟煤。煤层普遍被侵入，侏罗纪-白垩纪火成岩侵入（而不仅仅是埋藏煤化作用）增强了区域等级的升高。对峰峰煤田北部大树村矿的半无烟煤进行岩相检查，结果显示，2 号煤层附近焦炭微观结构发达，且反射率增加。基于水平间隔为 50 厘米的 11 个样品，随机镜质体反射率值 (R) 从接触处的 5.41% 下降到距接触处 5 米处的背景值 2.05%。朝向岩脉，镜质体各向异性增加，微观结构显示出圆形、粗镶嵌和带状各向异性焦炭结构和脱挥发分空泡的发展。天然焦炭质地对侵入/煤接触处的估计温度 > 570 °C 做出反应。然而，半无烟煤中焦炭结构的出现揭示了局部和区域的热演化。焦炭结构表明，入侵时煤炭等级为高挥发性 A 烟煤。这对埋藏历史、加热和沉降率都有影响。假设古地温梯度为 40 °C/km，在侏罗纪侵入时，高挥发性 A 烟煤 (1% R) 会被埋藏至 3 公里。 侵入将高挥发分烟煤变质为天然焦炭，随后更大规模的区域变质作用将煤变质为半无烟煤。