Geological implications of elements of the Pleistocene mudstone with different organism compositions and enrichment environments in the Qaidam Basin, China

Trace elements and rare earth elements (REEs) of two kinds of organic facies samples representing marginal and more basin-center deposits from Pleistocene lacustrine mudstones in the central Qaidam Basin were studied to understand the provenance, palaeotectonic setting, hydrothermal activity, palaeoredox conditions and sedimentary rate. The results show that the lacustrine mudstones were mainly derived from felsic sources with little contribution from ancient crustal sediments and no ultramafic (ophiolitic) source. The mudstones were deposited in a continental island arc tectonic setting, which is consistent with the tectonic evolution of the Cenozoic basin. Both two organic facies samples were hydrothermal in origin based on the ternary diagram of Ni-Zn-Co and normalized REE patterns. However, this does not mean that the water column in paleolake was affected by hydrothermal fluids in situ . This signal might indicate hydrothermal origins from hot springs related to active faults around the basin rather than the deep hydrothermal fluids entering the sediments via deep faculties based on the comprehensive analyses of normalized REE patterns, negative Eu anom (Eu anomaly), Y/Ho, Sm/Yb, and Eu/Sm. Redox proxies including U/Th, Ni/Co, and Mn anom values, are more sensitive for the studied samples indicating that most of the organic facies A samples were deposited under an oxygen-depleted condition, while the organic facies B samples were deposited under oxygen-rich conditions. Redox proxies of Ce anom values are unavailable for the organic facies B samples due to hypersaline environments, and V/Cr and V/(V + Ni) are invalid for the organic facies A samples, possibly because of their organism composition. The low La n /Yb n values indicate high sedimentation rates, which is consistent with the average sedimentation rates of approximately 0.43 to 1.1 km/Ma. However, the La n /Yb n is more likely affected by the provenance of the studied samples, so it should be used with caution.