Petrology and geochemistry of Devonian cutinitic liptobioliths from northwestern China

Devonian cutinitic liptobioliths from northwestern China are notable for their distinct papery, sheet-like texture and unusually high cutinite content (>60%). They are bituminous coals with an exceptionally high yield of volatile matter and hydrogen content. Their measured random values of vitrinite reflectance range from 0.60% to 0.68%. Cutinitic liptobioliths are mainly composed of ribbon-like cutinites and collotelinites, usually showing alternations under the microscope. The cutinites are thick-walled types, with a width of 50-300 mu m. The ribbonlike collotelinites are sandwiched by cutinites, representing a section through a ribbon-like stem. Fissures or cleats, caused by shrinkage, can be observed in collotelinites. Other macerals, particularly inertinite, are only present in small amounts. Lycopsids are suggested to be the dominant coal-forming plants based on spore and plant fossil research. The coal petrological characteristics and redox-sensitive trace element ratios indicate an anoxic peat depositional environment with still water or a weak current, different from the favorable conditions for massive cutinite accumulation. The formation of cutinitic liptobioliths was attributed to the unique structure and composition of Devonian land plants rather than the peat depositional conditions. During the Middle Devonian, well-developed cuticles and undeveloped woody tissues in early land plants may have resulted in the deposition of cutinitic liptobioliths. This is consistent with the fact that nearly all such coal deposits occur in the Middle Devonian. Low boron and sulfur contents in the coals support a freshwater depositional condition, which is different from that of the Luquan Devonian cutinitic liptobiolith from southwestern China. The dominant sediment-source regions for the studied cutinitic liptobiolith and its host rocks are the Xiemisitai-Shaerbuerti Mountains, as indicated by their elemental compositions. However, widespread hydrothermal solutions resulted in the enrichment of Nb, Ta , Zr, and Hf elements and heavy rare earth elements (HREEs) in the liptobioliths and their host rocks.

Automated summary

Liptobioliths in northwestern China are unique coal deposits with high cutinite content, likely formed by lycopsids during the Middle Devonian. The depositional conditions indicate freshwater environment and the enrichment of certain elements due to hydrothermal solutions.