Petrological, mineralogical, and geochemical study of pyrometamorphic rocks in the northeast margin of Ordos Basin, China: Insight into high-temperature coal combustion events

Abstract

Coal combustion and coal fires occur globally, and pyrometamorphic rocks produced by coal combustion and coal fires are widely distributed in the northeastern Ordos Basin, China. Studies on the characteristics and genetic environment of pyrometamorphic rocks within the study area remain limited. In this light, the paper explored the petrological, mineralogical, and geochemical properties of pyrometamorphic rocks in the northeast margin of Ordos Basin, China. Macroscopic features and petrology were examined through fieldwork and optical microscopy. SEM-EDS (Scanning Electron Microscope-Energy Dispersive Spectrometer) and XRD (X-ray diffraction) analysis provided detailed mineralogical information. Major and trace elements, including rare earth elements (REE), were quantified using ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) and ICP-MS (Inductively Coupled Plasma Mass Spectrometry), respectively. The results indicate two types of pyrometamorphic rocks consisting of clinkers and paralavas. Typical high-temperature minerals such as tridymite, cristobalite, mullite, cordierite, sekaninaite, hematite, and magnetite were detected in clinkers and paralavas. Large ion lithophile elements such as Rb, Sr, Cs and Ba vary greatly in pyrometamorphic rocks, which may be caused by heating. Both the clinkers and paralavas show Se, As, Mo, Sn, and Tl anomalies, indicative of hydrothermal activities following the pyrometamorphic event. The clinkers appear enriched in LREE, while the paralavas exhibit enrichment in HREE. REE distribution patterns normalized to UCC (upper continental crust) display no apparent δCe and δEu anomalies in clinkers and paralavas. This study contributes to the understanding of pyrometamorphic rocks by providing new insights into their genesis and high-temperature formation conditions.