西藏班公湖蛇绿混杂岩中硅质岩的地质地球化学特征及其形成构造环境

Geological and Geochemical Characteristics and Forming Environments of Siliceous Rocks in the Bangong Lake Ophiolite Mélange,Tibet

蛇绿混杂岩是研究洋陆转换、板块构造向板内构造转换过程的关键证据。本文以西藏班公湖蛇绿混杂岩中的硅质岩为研究对象,通过岩石学以及主量、稀土元素地球化学特征研究,分析了硅质岩成因和沉积环境。研究区硅质岩呈夹层产出在沙木罗组或断片产出在玄武岩中,具有隐晶质结构、生物碎屑结构和块状构造,含有晚侏罗世—早白垩世放射虫。硅质岩w(SiO_(2))为73.80%~90.44%,w(Al_(2)O_(3))为3.67%~12.33%;MnO/TiO_(2),Fe_(2)O_(3)/SiO_(2),Fe_(2)O_(3)/TiO_(2)和Al_(2)O_(3)/(Al_(2)O_(3)+Fe_(2)O_(3))分别为0.14~0.91,31.44~141.88,1.83~11.80和0.66~0.96;稀土元素北美页岩标准化后显示平坦型稀土配分模式,δCe值为0.77~2.03,Ce整体呈负异常特征,δEu值为0.99~2.11,Eu整体呈弱的正异常特征;(La/Yb)_(N),(La/Ce)_(N),(La/Lu)_(N),(La/Sm)_(N),(Ce/Yb)_(N)和(Gd/Lu)N值分别为0.43~1.12,0.51~1.27,0.44~1.26,0.59~1.12,0.41~2.01和0.82~1.16。硅质岩的地球化学特征指示,班公湖蛇绿混杂岩中伴生的硅质岩为生物成因,形成明显受陆源物质的影响,其形成环境并非典型的大陆边缘环境,而是一个与大陆边缘有一定距离的大洋盆地到大陆边缘构造环境。

The ophiolite mélange is an ideal laboratory for studying the process of ocean-continent transition and plate tectonics to intracontinental tectonic transition.In this paper,the petrology and geochemical characteristics of major and rare earth elements of siliceous rocks were studied to judge the origin and forming environment of Bangong Lake ophiolite mélange in Tibet.The siliceous rocks occur as intercalated beds in Shamuluo Formation or fault-fragments in basalt,exhibit cryptocrystalline texture,bioclastic texture and massive structure,and contain abundant Late Jurassic-Early Cretaceous radiolarite.They have a SiO_(2)content of 73.80%to 90.44%,and a high Al_(2)O_(3)content of 3.67%to 12.33%.The ratios of MnO/TiO_(2),Fe_(2)O_(3)/SiO_(2),Fe_(2)O_(3)/TiO_(2),and Al_(2)O_(3)/(Al_(2)O_(3)+Fe_(2)O_(3))range from 0.14 to 0.91,31.44 to 141.88,1.83 to 11.80,and 0.66 to 0.96,respectively.After standardization of rare earth elements in North American shale,it shows a flat rare earth distribution pattern,with Ce showing a negative anomaly overall and Eu showing a weak positive anomaly overall;δCe values is 0.77 to 2.03,δEu value is 0.99 to 2.11.The values of(La/Yb)_(N),(La/Ce)_(N),(La/Lu)_(N),(La/Sm)_(N),(Ce/Yb)_(N),and(Gd/Lu)_(N)are 0.43 to 1.12,0.51 to 1.27,0.44 to 1.26,0.59 to 1.12,0.41 to 2.01,and 0.82 to 1.16,respectively.After standardization,the rare earth element chondrite shows a right-leaning rare earth distribution pattern,with overall Ce showing weak positive anomalies and overall Eu showing negative anomalies.The authors propose that the formation environment of the siliceous rocks is not a typical continental margin,but rather a certain distance from the continental margin between ocean basin and continental margin.In combination with the results of area studies,the formation environment of the Bangong Lake ophiolite mélange is preliminarily identified as a back arc ocean basin environment,which is the product of the northward subduction of the Bangong Lake-Nujiang Ocean basin in the Late Jurassic.