安徽狸桥地区晚石炭世硅质岩的发现及其地质意义

Discovery and Geological Significance of the Late Carboniferous Siliceous Rock in the Liqiao Area,Anhui Province

安徽狸桥地区出露一套硅质建造,其赋存于黄龙组灰岩与五通组石英砂岩之间,呈层状产出,显微镜下主要由隐晶质石英组成,具角砾结构,未见硅质生物,局部有硅化、褐铁矿化现象。其地球化学特征显示,本区硅质岩Si O2含量为93.57%~98.06%(平均为97.22%),∑REE含量低,为4.89×10^(-6)~18.40×10^(-6)(平均为11.29×10^(-6)),δCe为0.50~0.68(平均为0.58),呈负异常,δEu为1.13~2.98(平均为1.49),呈明显的正异常。Al/(Al+Fe+Mn)值为0.11~0.64(平均为0.45),Al_2O_3,TiO_2与Si O2相关性较差,而Fe2O3和Si O2呈明显的正相关。结合双变量SiO_2-Al_2O_3、SiO_2-MgO,三变量Al-Fe-Mn,Fe-Mn-(Ni+Co+Cu)×10图解,指示其为热水沉积成因;其δEu、Mn O/TiO_2、Fe2O3/TiO_2和Al_2O_3/(Al_2O_3+Fe_2O_3)值分析,其沉积环境为远离陆源物质供给的大陆边缘,受到强烈的热水活动影响。综上表明:晚石炭世时期,狸桥地区处于受基底深断裂及同沉积断裂控制的二级断陷盆地内,可能由于地壳的持续拉张、减薄,导致火山热液或喷气活动沿基底深断裂在海底火山喷发间歇式或旋回期发生热水喷流作用,形成了具有热水喷流沉积成因的角砾状硅质岩。这一发现对于长江中下游地区晚古生代热水沉积硅质岩的研究提供一定的借鉴,也为长江中下游地区至少在石炭纪时存在一定的热水活动提供了有利的证据。

A suit of siliceous formation is found in the Liqiao area of Anhui Province,which occurs between the Huanglong Formation limestone and Wutong Formation quartz sandstone. It is mainly composed of aphanitic quartz seen under microscope,with gravel structure,no siliceous organism,local limonite,silicification and recrystallization phenomenon. An analysis of major elements shows that the SiO 2 content of siliceous rock is 93. 57% ~98. 96%（ 97. 22% on average）,ΣREE content is as low as 4. 89 × 10^（-6）~ 18. 40 × 10^（-6）（ 11. 29 × 10-6on average）,δCe is 0. 50 ~ 0. 68（ 0. 58 on average）,implying a negative anomaly; δEu is 1. 13 ~ 2. 98（ 1. 49 on average）,indicative of a significant positive anomaly; Al/（ Al ＋ Fe ＋ Mn） is 0. 11~ 0. 64（ 0. 45 on average）; Al2O3 and TiO 2are poorly correlated with SiO2,and Fe2O3 has a positive correlation with SiO2. In combination with the SiO2-Al2O3,SiO2-MgO binary,and Al-Fe-Mn,Fe-Mn-（ Ni ＋ Co ＋ Cu） × 10 ternary discriminant diagrams,the siliceous rock samples mainly fall within the hydrothermally deposited areas. The graphs of δEu,MnO/TiO2,Fe2O3/TiO2 and Al2O3/（ Al2O3＋ Fe2O3） show that they were formed under the influence of tandem interference and hot water activity,which was in a continental margin depositional environment. It is inferred that during the Late Carboniferous period,the Liqiao area was in a second-order fault-depression basin controlled by deep faults in the basement and syndepositional faults. Probably continuous extension and thinning of the crust led to the volcanic hydrothermal or hydrothermal activities along the basement faults during the intermittent or cyclical periods of seafloor volcanoes,resulting in the formation of a hydrothermal sedimentary siliceous rock with a brecciated structure.This finding provides some reference for research of the Late-Paleozoic hydrothermal sedimentary siliceous rock,also a piece of evidence for the existence of hydrothermal activity in the middle and lower reaches of the Yangtze River at least during the Carboniferous.