扬子地区二叠系硅质岩成因分析及沉积环境研究

PETROGENESIS AND SEDIMENTARY ENVIRONMENT OF THE PERMIAN CHERTS IN THE YANGTZE REGION

对扬子地区2个典型剖面即广元上寺剖面和罗甸纳水剖面的二叠系硅质岩稀土元素地球化学特征研究发现:扬子板块不同地区不同产状的硅质岩稀土元素特征存在差异,燧石条带和团块状硅质岩∑REE均小于层状硅质岩的∑REE;同一地区不同产状的硅质岩Ce/Ce*相差极小,而不同地区Ce/Ce*存在明显差异。硅质岩与围岩(灰岩和页岩)进行对比分析发现,页岩的∑REE最高,其次为层状硅质岩,再次为灰岩,最低为燧石条带。通过系统的岩石学和稀土元素地球化学特征研究表明,广元地区二叠系硅质岩以生物沉积为主,并有一定的热水沉积物质,其沉积环境为大陆边缘中下部;而黔南地区二叠系硅质岩是热水沉积为主,混有少量非热水成因的物质,其形成于远离陆源的洋盆环境。

The studies on Permian cherts from the two typical sections（Shangsi section of Guangyuan area and Nashui section of Luodian area） in the Yangtze region indicate that distict REE characteristics occur among banded,nubby and lamellated cherts in different areas.The ∑REE values in banded and nubby cherts are less than that in lamellated chert.There are semblable Ce/Ce＊ values in cherts of different occurrences in same area,but it is quite different in these two areas.In this paper,through comparison and analysis of cherts and surrounding rocks（including limestone and shale）,of them,the ∑REE values in shales are the highest,and the next is the lamellated cherts,then the limestone,but the lowest is banded chert.The studies on petrological characteristics and REE geochemical characteristics in cherts show that the Permian cherts in the Guangyuan area are mainly biological sedimentation,but are effected by hydrothermal sedimentation,and they are formed in the middle-lower part of continental shelf.While the Permian cherts in the Luodian area are mainly hydrothermal sedimentation,and the non-hydrothermal sedimentation is also interfused.They are formed in oceanic basin.