勉略构造带硅质岩元素地球化学特征及其形成环境  

Elemental geochemistry characteristics and forming environment of cherts in the Mianlue tectonic zone

作　　者：戢兴忠[1] 李楠[1] 张闯[2] 邱昆峰[1] 华北[1] 余金元 吴春俊 韩日[1] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,北京100083 [2]核工业北京地质研究院,北京100029 [3]武警黄金部队第十二支队,成都610036

出　　处：《岩石学报》2014年第9期2619-2630,共12页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划(2009CB421008);国家自然科学基金项目(41030423);公益性行业科研专项经费项目(201411048);中国地质调查局地质调查项目(1212011121090);中国地质大学(北京)基本科研业务费专项资金资助项目(2-9-2014-055)联合资助

摘　　要：勉略构造带古生代硅质岩发育,其对西秦岭造山带古生代沉积环境与构造背景具有重要指示意义。本文研究了勉略构造带西段文县地区泥盆纪硅质岩的野外露头产状、岩石学与矿物学及全岩主微量元素地球化学特征,对比分析了该带东段硅质岩性质,综合剖析了勉略构造带古生代硅质岩的沉积环境,探讨了西秦岭造山带古生代岩相古地理环境及大地构造背景。文县硅质岩有灰白色块状硅质岩和灰黑色角砾状硅质岩两种类型,两者均具层状、条带状等沉积构造,并与上覆、下伏地层整合接触,为典型热水沉积硅质岩。岩石主要组成矿物为细晶-微晶石英,含有少量的方解石、绢云母及泥质等。岩石地球化学分析显示,硅质岩SiO2含量较高,为84.85%~99.07%,平均为91.99%,且灰白色硅质岩质地更纯,SiO2含量明显高于灰黑色硅质岩。硅质岩主微量、稀土元素Fe2O3/TiO2-Al2O3/（Al2O3＋Fe2O3）、V/Y-Ti/V及LaN/CeN-Al2O3/（Al2O3＋Fe2O3）图解表明该区硅质岩主要形成于大陆边缘及近大陆边缘环境。稀土元素显示灰黑色角砾状硅质岩稀土总量较高,为171.3×10-6~236.1×10-6,而灰白色块状硅质岩仅为11.96×10-6~51.06×10-6,表明前者所含陆源物质组分更多;稀土元素北美页岩标准化后两者整体显示了相似的轻重稀土分异不明显的平坦型稀土配分模式,其中灰黑色硅质岩δCe为0.95~1.05、δEu为0.66~0.87、（La/Yb）SN值为0.95~1.10,反映基本无Ce异常、弱Eu负异常及轻重稀土分异不明显;灰白色硅质岩的δCe为1.07~1.12、δEu为0.70~0.90、（La/Yb）SN值为1.64~4.07显示了弱Ce正异常、弱Eu负异常及弱轻重稀土分异,两者稀土元素特征与世界典型大陆边缘（裂陷）盆地硅质岩相近。文县硅质岩地质、元素地球化学特征表明其整体形成于大陆边缘及近大陆边缘沉积环境,但相比于灰白色块状硅质岩,灰黑色角砾状硅质岩有更多陆源物Lots of Paleozoic chert outcrops were found in Mianlue tectonic zone which can reveal the sedimentary environment and tectonic background of western Qinling orogenic belt. In this paper, researches on the Devonian cherts from Wenxian which is located in the west of Mianlue tectonic zone have been done, including field work of the cherts outcrops, petrology and mineralogy of the cherts, major and trace elements of the cherts, and the sedimentary environment of cherts have also been analyzed. Moreover, combined with the studies related to cherts in the east of Mianluc tectonic zone, we conclude the sedimentary environment of Mianlue tectonic belt and the tectonic background of western Qinling orogenic belt in Paleozoic. Cherts from Wenxian can be divided into grey- white blocky and grey-dark brecciated cherts, and both of them are hot water sedimentary cherts with bedding or banded structure and conformity to upper and underlying strata in the field. And they are mainly made up of fine crystalline to microcrystalline quartz grain （84. 85% -99. 07%, average value is 91.99% ） with few calcite, sericite and mud material, while the grey-white cherts are purer than the other one with more SiOv According to the Fe2O3/TiO2-A12O3（ A12O3 ＋ Fe2O3 ）, V/Y-Ti/V and LaN/CeN-A12O3/（ A12O3 ＋ Fe2O3 ） discrimination diagrams, cherts are mainly formed in or near the continental margin. It is noticeable that the grey-white cherts are affected by materials from continental margin less than the grey-dark one based on the fewer ~ REE contents （ 11.96 ~ 10 -6 ~51.06 ~ 10-6 compare to 171.3 ~ 10 -6 - 236. 1 ~ 10-6 ）. The rare earth patterns of the two kinds of cherts are similar with flat model after North American Shale Standardization （NASS）. And the feature of cherts which shows no Ce anomaly （grey-dark is 0. 95 ~ 1.05 ） or weak Ce positive anomaly （ grey-white is 1.07 - 1.12）, Eu weak negative anomaly （ grey-dark cherts is O. 66 - 0. 87, the other is 0. 70 ~ 0. 90）, and low （La/Yb） sN value （ g

关 键 词：硅质岩 元素地球化学 沉积环境 构造背景 勉略构造带 西秦岭 

分 类 号：P588.2[天文地球—岩石学] P595[天文地球—地质学]