藏南彭错林硅质岩地球化学特征及沉积环境分析

Study on Geochemical Characteristics and Depositional Environment of Pengcuolin Chert,Southern Tibet

通过野外地质调查和系统的岩石化学、常量元素、微量元素以及稀土元素等研究,探讨了藏南彭错林硅质岩的地球化学特征及其沉积环境。研究表明,呈厚层状产出的绿色、棕褐色硅质岩具有较高的SiO2含量,局部受Fe、Mn矿化,Al/(Al+Fe+Mn)比值小,在Fe-Mn-Al三角成因判别图解中,大部分硅质岩样品落入热水沉积区域,微量元素Sr、Zr、Cu、Zn和Ba含量较高,稀土元素∑REE较低,Ce负异常,Eu正异常,北美页岩标准化后配分模式稍向左倾,体现出热水沉积特征;在沉积环境判别图100×Fe2O3/SiO2-100×Al2O3/SiO2、Fe2O3/(100-SiO2)-Al2O3/(100-SiO2)和Fe2O3/TiO2-Al2O3/(Al2O3+Fe2O3)中,硅质岩样品投影到洋中脊和深海区域,说明形成于大洋中脊和深海盆地环境;硅质岩常量和稀土参数变化特征图显示,在剖面中PCL-1、PCL-5和PCL-7采样位置更接近热液中心;硅质岩的形成与板块的汇聚消减产生的以洋中脊和海底热点为中心的洋壳热水系统有关。

The geochemical characteristics and depositional environment of Pengcuolin chert are studied based on the field investigations and analysis on major elements, trace elements and rare earth elements. The massive chert is green and brown, being high in SiOz content, locally enriched in Fe and Mn, Al/（Al＋Fe＋Mn） ratio being low. In Fe- Mn- Al triangle diagrams, most chert samples fall into the hydrothermal region. Trace elements such as Sr, Zr,Cu, Zn and Ba are higher and 5＇.REE is lower, with Ce negative anomaly and Eu anomaly. Their NASC- normalized REE distribution patterns are slightly left- leaning, indicating their hydrothermal origin. In sedimentary environment discrimination diagrams such as 100×Fe2O3/SiO2 - 100 × At2O3/SiO2, Fe2O3/（100- SiO2 ）- Al2O3/（100 -- SiO2 ） and Fe2O3/TiO2 -Al2O3/（Al2O2 ＋Fee O3 ）, chert samples fall into the areas of the mid-ocean ridges or deepsea. In parameter variation diagrams, samples of PCL - 1 ,PCL - 5 and PCL - 7 are closer to the hydrothermal center. It is shown that the Pengcuolin chert, southern Tibet was formed by the hydrothermal systems at convergence and subduction sites of plates.