Why Middle Jurassic coal seams generally lack volcanic-ash-altered kaolinitic layers and critical metals enrichment compared to the Carboniferous-Permian in the Ordos Basin, North China?

• Volcanic ash input during the Middle Jurassic coal accumulation in the Ordos Basin was limited. • Source regions during the Middle Jurassic coal accumulation in the Ordos Basin lack critical metals-enriched rocks. • Minimal hydrothermal activity has influenced the Middle Jurassic coal in the Ordos Basin. The North China Craton (NCC) experiences two major coal-forming episodes: the Late Carboniferous–Early Permian and the Early–Middle Jurassic. Despite the tectonically active setting of the NCC during the Middle Jurassic, volcanic-ash-altered kaolinitic layers and an enrichment of critical metals are notably lacking in Middle Jurassic coal seams relative to those of the Carboniferous–Permian, and the causes of this disparity remain unclear. This study investigates the contributions of inorganic inputs during peat accumulation and the influence of hydrothermal activity on Middle Jurassic coal seams in the Ordos Basin, aiming to clarify the geological controls underlying this disparity. The clastic materials of the coal and partings were derived from the weathering of uplifts around Ordos Basin, without a critical metals-rich source rocks therein. The peat accumulation during the Middle Jurassic did not coincide with intensive magmatic events in adjacent tectonic domains (Mongol–Okhotsk, Paleo-Pacific, and Bangong-Nujiang Tethys), resulting in limited volcanic ash input. Although the No. 6 coal seam contains XRD-detectable apatite, it is authigenic, forming during peat deposition or early diagenesis, and is not significantly enriched in rare earth elements. Moreover, the intense Late Jurassic–Early Cretaceous magmatic–hydrothermal activity linked to rollback of the subducted Paleo-Pacific Plate in the eastern NCC did not extend to the Ordos Basin. Therefore, the lack of volcanic-ash-altered kaolinitic layers and an enrichment of critical metals in the Middle Jurassic coal seams is primarily attributed to the limited supply of critical metal-enriched detrital material and volcanic ash, and negligible hydrothermal influence during and after peat accumulation.