湖北竹山杨家堡组硅质岩成因及沉积环境分析

竹山地区位于扬子陆块北缘,寒武纪早期发育一套深水硅质岩系。该岩系由纯硅质岩夹少量白云岩、页岩所组成。硅质岩中微晶石英含量均超过85%,还含有少量碳质、燧石及微量绢云母。地球化学分析表明,该硅质岩的SiO₂含量较高,平均为93.4%;Al₂O₃和TiO₂含量较低,分别为0.19%~1.45%和0.01%~0.09%,TFe₂O₃为0.053%~0.342%,Al/(Al+Fe+Mn)比值为0.58~0.83,平均为0.71,Eu/Eu*为0.74~3.61,平均为1.65,表明其硅质主要来源于生物本身或其生命活动产物,海底热液活动亦提供了一部硅质物质;同时,硅质岩具有较低的TFe₂O₃/TiO₂比值(2.16~12.56,平均值为5.86)和Al₂O₃/(Al₂O₃+Fe₂O₃)比值(0.65~0.87,平均值为0.78),Y的正异常(Y/Ho值为34.81~60.19,平均为47.0)及Ce的负异常(Ce/Ce*值为0.28~0.92),表明它们形成于远离陆源物质干扰的边缘海盆环境,且Ce负异常从底部到顶部有逐渐减小的趋势,记录了寒武纪时期扬子北缘被动陆缘裂谷盆地不断扩大,水体逐渐变深的过程。

Petrogenesis and Sedimentary Environment of Siliceous Rocks of Yangjiabao Formation in Zhushan Area,Northwestern Hubei

Deepwater siliceous rocks developed during the Early Cambrian in Zhushan area which located on the north margin of the Yangtze block. These rocks mainly comprise pure siliceous rocks, with little dolomite and shale. The siliceous rocks are composed of microcrystalline quartz(content of more than 85%) and some carbon, flint and sericite. They show high SiO₂ content(93.4%, on average) by geochemical analysis. Contents of Al₂O₃ and TiO₂, are as low as 0.19%~1.45% and 0.01%~0.09%, respectively. And TFe₂O₃ accounts for 0.053%~0.342%. The ratio of Al/(Al+Fe+Mn) is from 0.58 to 0.83(0.73, on average), with Eu/Eu* being in 0.74~3.61 range(1.65, on average), which indicates the silicon comes from biological activities and hydrothermal injection. Meanwhile, the ratio of TFe₂O₃/TiO₂ and Al₂O₃/(Al₂O₃+Fe₂O₃) are evidently low to 2.16~12.56 and 0.65~0.87 Y is positive anomaly(Y/Ho value is 34.81~60.19) and Ce is negative anomaly(Ce/Ce* value is 0.28~0.92), implying that these rocks were deposited in an ocean basin with no input of terrigenous materials. Negative anomaly of Ce are less evident to upside, which records the expansion of the passive continental margin rift basin on the northern Yangtze block.