Late Triassic‐Early Jurassic abnormal thermal event constrained by zircon fission track dating and vitrinite reflectance in Xishan coalfield, Qinshui Basin, central North China

Abstract

Xishan coalfield, Shanxi, is located in the northwest of the Qinshui Basin, central North China. It is notable for its varieties of coal rank ranging from high volatile bituminous coals to anthracite as well as having abundant coalbed methane resources. Zircon fission track (ZFT) analyses were carried out on the zircons in 2 Upper Carboniferous and 5 Lower‐Middle Permian sandstones, and vitrinite reflectance of Late Carboniferous and Early Permian coals were measured to determine the timing of thermal events and maximum paleo‐temperatures, which were responsible for coal maturation and coalbed methane generation. Maximum paleo‐temperatures calculated from vitrinite reflectance values reached to about 232 and 223 °C in Late Carboniferous and Early Permian coals, respectively, and the estimated paleo‐temperature gradient was 11.84 °C/100 m, representing an intensive abnormal thermal event. Results of the ZFT dating indicated that 5 samples failed the χ2‐test and 2 samples passed the test. The decomposition results of the 5 samples divided their age populations into 3 periods: (a) older ages (537, 584, and 802 Ma) than sandstones ages, (b) close to or slightly older than their depositional ages (289, 301, and 331 Ma), and (c) younger than the depositional ages (181–215). The 2 samples that passed χ2‐test yield the central ages of 168 ± 7 Ma and 190 ± 8 Ma, respectively, younger than the deposition age. The close to or older ages than the sandstones depositional ages represent the tectonothermal events occurring in their source areas; the younger ages indicate the existence of the postdepositional tectonothermal event. The agreement of the partly annealing temperature zone (210–300 °C) of zircon fission tracks with the calculated maximum paleo‐temperatures from vitrinite reflectance suggests a Late Triassic‐Early Jurassic abnormal thermal event with the formation time of the present coal rank being 181–215 Ma, rather than a unique intrusion at 95–135 Ma on the western margin of coalfield as previously believed. Combined with other ZFT ages regionally, this abnormal event also occurred in the southern as well as the northern parts of the Qinshui Basin. The Late Triassic‐Early Jurassic intensive extension in the North China Craton is the geodynamic setting of this tectonothermal event.