Element Geochemistry and Depositional Setting of the Chert in Devonian，Nanning Area，Guangxi

Key words: 
• Guangxi Devonian /
• bedded chert /
• hydrothermal activity /
• basin evolution

Abstract: Thick chert series occur from the Upper Emsian through Frasnian strata in the Nanning area, South China. These siliceous deposits are characterized by black to dark grey, thinbedded chert intercalated with light grey tuffaceous beds, which tend to be brownish yellow in the upper portion of the Frasnian. The bedded chert has very high SiO2contents (usually ＞90％) and relatively high contents of TFe2O3（total iron）(avg. 3.32%). By contrast, the contents of other major elements, i.e. Al2O3, TiO2, MgO and MnO are extremely low (avg. 0.71％, 0.04％, 0.10％ and 0.02％, respectively). Some major element ratios of Fe/Ti, (Fe+Mn)/Ti and Al/(Al+Fe+Mn) are 92.12 and 0.25 in average, respectively. Trace element ratios of Ti/V and U/Th are 4.57 and 4.09 in average, of which the U/Th ratio reached a maximum in the early Frasnian. These geochemical features suggest a hydrothermal influence upon the precipitation of the bedded chert, and a strongest hydrothermal activity in the Early Frasnian. The hydrothermal activity was likely a reflection of deepseated basement faulting. In this case, the strongest hydrothermal venting in the early Frasnian point towards a strongest extension process of the rift basin, reconciling the scenario of temporal distributions of chert and lithofacies in Devonian in South China.