Geochemical Characteristics and Sedimentary Environments of Cherts from the Paleoproterozoic Xiong′er Group in the Southern Part of the North China Block

Key words: 
• North China block /
• Xiong′er Group /
• chert /
• geochemical characteristics /
• sedimentary environment

Abstract: Palaeoproterozoic cherts from Xiong′er Group in the southern part of the North China block formed in sedimentary rock, but a few in volcanic rocks. The distributions of major and trace elements for cherts demonstrate that cherts were affected by both the hydrothermal process and hydrogenous process. The hydrothermal process might be related with Xiong′er Group volcanism, its geochemical characteristics show the lower concentrations of TiO2, MgO, Al2O3, ∑REE , the lower ratio of A1/(A1+Fe+Mn) and U/Th>1, the NASCnormalized flat REE patterns with weak negative Ce anomalies and positive Eu anomalies. The hydrothermal process was less influenced by the input of terrigenous component due to quick subsidence. However, the hydrogenous genesis cherts were which showed LREE enrichment (LaN/YbN=1.41~9.04) and the high ∑REE, influenced by the input of terrigenous component. The lower ratio of Cr/Th, the chondritenormalized REE patterns and primitive mantlenormalized traceelement patterns for cherts are consistent with the volcanic rocks of the Xiong’er suggest that the single provenance. The cherts were mainly derived from the mixture of basalticandesite and daciterhyolite from Xiong′er Group. The characteristics of Al2O3/(Al2O3+Fe2O3), Ce/Ce*(0.77~0.94) and LaN/CeN(1.18~1.47) implied a setting of passive continental margin. The hydrothermal activity is interpreted as having been related to the intensity extensional background in the Xiong′erZhongtiao Aulacogen, as early as the Palaeoproterozoic. According to the geological characteristics and the ratio of elements, such as Sr/Ba, Th/U, CaO/(CaO+MgO), the cherts deposited in both marine and marineterrigenous facies. As a result of transgression toward north, southern part of Henan was in the marine environment, but northern part of Henan was marineterrigenous, some areas still in continent environment.