| The Permian was a significant period for chert depositions, and they were widely developed in the South China. During the deposition process, a series of contemporaneous faults had been produced, which may cause volcanic activities. It could change the local geochemical characteristics to form chert depositions. The Dalong formation was a typical set of chert depositions in the Northwest of Sichuan Basin. The geochemical characteristics and formative factor could provide key information about local construction as well as sedimentation. Based on sedimentology research, combined with field profile, slice observation, features analysis of major elements, trace elements and rare earth elements(REE), this paper mainly studied the sedimentary and geochemical characteristics of chert depositions in Shangjianggou profile.According to the field data and sedimentary characteristics, the Shangjianggou profile could be further divided into 17 layers, which belong to carbonate slope and trough depositions. The underpart of lower Dalong formation(1 to 6 layers) is in carbonate slope facies, where the sediments consist of lime carbonate, siliceous and argillaceous deposits. Biogroup is poor amd monotonous and only a few plankton and neuston can be seen. The upper part of lower Dalong formation(7 to 9 layers) and the whole upper Dalong formation(10 to 17 layers) were trough depositions. The sediments in the upper part of lower Dalong formation were gray-black laminated muddy cherts, siliceous limestones with siliceous shales and black shales. The basin partly rose during the bottom deposition in the upper Dalong formation, carbonate keep growing with silicon and argillaceous components decreasing. The top part of the upper Dalong formation deposited in relatively high-energy water, mainly consisted of dark gray banded cherts and siliceous micrite.Through the analysis of slice observation, the chert depositons were mainly consisted of radiolarite. Ostracoda only appeared in the underpart of Dalong formation while radiolarian and ammonite showed an enrichment tendency, which suggested that the main depositional environment of Dalong formation was in transgressive period. The sea level kept growing and the flow at the bottom nearly stagnated, making it difficult for benthos to live, such as ostracoda. Meanwhile, around sea surface, the water was in good circulation, where is the perfect area for nekton and plankton as well as neuston, including radiolarian and ammonite.The major elements showed that the chert depositions in Dalong formation had high Al2O3 content with low MgO and Na2 O. Except the Mn, the other major elements including Ti, Al, Fe, P, Mg, Ca and K were higher than those in classic biogenic cherts or in classic hydrothernal cherts 1 to 3 times, which may be caused by the impact of terrigenous debris injection near the continental margin. The content of Ti, Al, Mn, Ca and K were closer to classic biogenic cherts. Through the analysis of Al/(Al+Fe+Mn), MgO content, K2O/Na2 O, making Al-Fe-Mn triangular graph and scatter diagrams of SiO2-Al2O3, SiO2-(K2O+Na2O), SiO2-MgO and Fe2O3-FeO, it clearly suggested that the chert depositions in Dalong formation were affected by several factors, including biological action, volcanism and chemical action.The trace element characteristics appeared relative enrichment of Ni, and Co is relatively poor. The content of As from samples exceeded the average content in clay rock and normal seawater. The Hg content was far greater than its average abundance in clay rock. The Ba content was relatively low and the Sr content was relatively high. The U showed relative enrichment while the Th was relative poverty. In addition, the chert depositions in Dalong formation also contain more Sb. With the similar feature like major elements, some of the trace elements were higher than those in classic biogenic cherts or in classic hydrothernal cherts, and some of them are between these two types of cherts, especially the Co, Ni, Zn and Th were close to classic hydrothernal cherts.The total of rare earth elements ΣREE was not high, and the heavy rare earth elements appeared medium enrichment. According to the allocation model curve of REE, δCe and δEu, it showed obvious negative anomaly of Ce and slightly negative anomaly of Eu, which indicates the characteristics of hot-water deposition.According to the sedimentary characteristics and geochemical characteristics, the chert depositons in the Dalong formation were mainly consisted of radiolarite, belonging to biological cherts. There were 2 kinds of silicon source, including biological source and volcanism source. The biological silicon source was predominant in the early sedimentation of Dalong formation. After the Mount Emei mantle plume event, except the biological source, the middle-late sedimentation of Dalong formation also receive the SiO2 from volcanism.The sedimentary characteristics and formation mechanism of the chert depositions in Dalong formation faithfully recorded the evolution of local construction and palaeogeography. On the one hand, it indicated the contemporaneous volcanogene hydrothermal activities caused by Mount Emei mantle plume event. On the other hand, it also reflected the impact from volcanogene hydrothermal activities in local depositions. The research results not only have important theoretical significance, but also provide key information for late Permian tectonic evolution analysis and geological event study in the Northwest of Sichuan Basin. |
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