安徽省巢湖地区二叠纪栖霞组、孤峰组硅质岩地球化学特征对比及大地构造背景分析

Geochemical Characteristics Comparison and Tectonic Background Analysis of Siliceous Rocks from Qixia Formation and Gufeng Formation of Permian in Chaohu Area,Anhui Province

为探讨下扬子地区二叠系硅质岩的硅质来源和形成背景,以巢湖地区二叠纪栖霞组和孤峰组硅质岩为研究对象,收集了前人大量有关栖霞组和孤峰组的研究成果和数据,并对采自栖霞组和孤峰组硅质岩样品进行了主量元素、微量元素、稀土元素测试。栖霞组、孤峰组硅质岩样品测试结果显示,硅质岩的稀土元素经北美页岩标准化后的配分曲线近似平坦型向左倾斜,有弱Ce负异常,且HREE弱富集,反映了热水沉积硅质岩的特征。研究区硅质岩样品的Al-FeMn三角图中,样品投点分布较广,即Al比值变化范围较大,认为是岩浆在喷发过程中受硅-铝质陆壳混染作用的结果。结合沉积环境判别指标分析认为:栖霞组硅质岩沉积时期其沉积环境主要为大陆边缘环境;孤峰组硅质岩沉积时期其沉积环境为大陆边缘-海盆过渡环境。从栖霞组到孤峰组沉积时期,海侵规模不断变大,水体变深,海盆面积扩大,研究区在不同时期沉积环境存在差异。研究区栖霞组、孤峰组硅质岩的沉积与大陆边缘裂陷活动紧密相关。

Aiming to discuss the origin and sedimentary background of Permian chert in Yangtze region, we took the siliceous rocks as samples from Qixia Formation and Gufeng Formation in Chaohu area, which located in the northern margin of the Yangtze block. In this study, we collected previous research data of these two formations and systematically analyzed major, trace and rare elements data of siliceous rocks collected from the two forma-tions. Test results of samples which we got in this study show that rare earth elements （REEs） pattern curves lean slightly to the left, with slight Ce negative anomaly and H R E E relative enrichment, reflects the characteris-tics of the hydrothermal chert. The points pitched on the Al-Fe-Mn diagram of Qixia Formation and Gufeng For-mation siliceous rocks indicate that most of the samples are high in Al. In this paper, we proposed that magma suffered the contamination of the Si-Al rich continental crust during the eruption, with high Al ratio. Combined with diagrams of sedimentary environments we came to a conclusion that cherts in Qixia Formation and Gufeng Formation were deposited at continental margin, continental margin-ocean basin, respectively. From Qixia Peri-od to Gufeng Period, transgression scale became large, the depth of seawater became large, the area of ocean basin expanded, which made sedimentary environment different. The siliceous rocks sedimentation in both Qix- ia Formation and Gufeng Formation was closely related to continental margin chasmic activities.