Detrital zircon geochronology of the Jurassic strata in the western Ordos Basin, North China: Constraints on the provenance and its tectonic implication

摘要

>U–Pb detrital zircon data from the Jurassic strata in the western Ordos Basin (WOB) show significant age differences between formations, from lower to upper including the Yan'an, Zhiluo, Anding, and Fenfanghe formations. The main peaks in the detrital zircon age spectra of the Yan'an Formation are at ca. 245, 444, 1813, and 2,387?Ma, which resemble those of the underlying Triassic strata in the WOB. Combined with the occurrence of reworked sporopollen and carbonate debris, the sediments of the Yan'an Formation are interpreted to be potentially recycled from the residual Triassic highlands. The overlying Zhiluo Formation contains detrital zircons mostly in the 260–290?Ma age range with only one peak at 269?Ma, reflecting primary derivations from the Permian granites in the Yinshan Belt to the north and/or the Alxa Block to the northwest. The detrital zircon age compositions of the Anding Formation, however, are most varied, with one prominent age peak at 274?Ma and several other minor peaks at 348?Ma, 447, 932, 1641, 2143, and 2514?Ma, reflecting complex origins. The uppermost syntectonic Fenfanghe Formation, contains detrital zircons with similar age compositions with the Yan'an Formation, once again interpreted as the result of recycled sedimentation. These two phases of multicycle sedimentation in the WOB reveals two intense tectonic events in the Late Triassic and Late Jurassic, whereas the provenance transition between the Yan'an and Zhiluo formations implies a less intense but extensive event in the Middle Jurassic. The complex age composition of the Anding Formation, however, are possibly as the result of climate change corresponding to the change of petrology and sediment colour. These Early‐Middle Mesozoic tectonic events evidenced by the U–Pb ages record the tectonic transition of the North China Block from the Tethys to the paleo‐Pacific tectonic domains.