广西南宁地区泥盆系硅质岩地球化学特征及沉积环境

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

泥盆纪时,广西南宁附近发育了一套从早泥盆世埃姆斯期到晚泥盆世弗拉斯期末的硅质岩建造,硅质岩多为薄层,灰黑到黑色,至弗拉斯阶上部变为灰黄色,水平纹理发育,常夹火山凝灰岩或与之互层。这套硅质岩SiO2含量高(一般>90%),其他主量化学成分均很低(一般<5%)。TFe2O3和含量相对比较富集(平均含量3.32%),Al2O3、TiO2、MgO和MnO相对贫乏(平均含量分别为0.71%、0.04%、0.10%和0.02%)。常量元素Fe/Ti、(Fe+Mn)/Ti和Al/(Al+Fe+Mn)比值的平均值分别为91.65,92.12和0.25,微量元素Ti/V和U/Th比值的平均值分别为4.57和4.09,其中U/Th比值在弗拉斯期早期达到最大。这些数据说明,硅质岩在形成时受到了明显的海底热液活动影响,而这种热液活动在弗拉斯期早期达到最强,后逐渐减弱,而热液活动的强度可能主要受盆地裂解(或张裂作用)的影响;因此,盆地拉张作用在弗拉斯期早期达到最强,这与硅质岩的空间分布是吻合的。

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 SiO2contents （usually 〉90% ） and relatively high contents of TFe203 （total iron） （avg. 3.32% ）. By contrast, the contents of other major elements, i.e. A1203, TiO2, 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 AI/（ A1 ＋ 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.