博格达晚古生代岛弧的沉积岩石学证据

Sedimentological Evidence of Late Palaeozoic Bogda Island Arc

博格达地区上古生界以极其发育的火山碎屑岩、火山岩、火山碎屑沉积为显著特征,并且具有如下沉积建造组成及特点:(1)石炭系火山岩为CA系列,下二叠统火山碎屑岩为CA+TH系列,且都形成在岛弧—造山环境下;(2)早期发育巨厚的岛弧型安山质火山—沉积建造;(3)下二叠统砂岩化学成分以低Si0_2Al_2O_3(<6)和低K_2O/(Na_2O+CaO)(<0.5)为特征.SiO_2/Al_2O_3—K_2O/(Na_2O+CaO)双变量图解分析表明其形成在大洋岛弧—活动大陆边缘环境下;(4)下二叠统浊积砂岩骨架矿物组分的QmFLT,QpLvLs图解分析揭示物源区为弧造山带。以上特点综合表明博格达在晚古生代为岛弧。

There are a lot of pyroclastic rocks, volcanic rocks and volcaniclastic sediments in Bogda Mountainm. There are two different views on their sedimentary enviroment: rift and island arc. In this paper it is considered that Late Palaeozoic Bogda Mountain was an island arc based on the research of sedimentology in combination with plate Tectonic theory. 1. Diagram of Na2O + K2O-Fe2O3 + FeO-MgO (AFM), diagram of TiO2-Al2O3-K2O and diagram of log2 versus log2 of the chemical pyroclastic rocks indicate that Carboniferous volcanic rocks are attrituted to CA (calc -alkalian) series and Lower Permian pyroclastics rocks to CA+TH (tholeiite) series, all forming under he environment of island arc orogen. 2. The oldest formaton are sea -facies andesitic volcanic - sedimentary formation with typical sea -fa-cies animal group, in which volcanic rocks and pyroclastic rocks are more than 90% and andesitic volcanic rocks are major component (±85%). Clastic components are made up of pyroclasts and without clasts of metamcrphic rocks. 3. The chemical compositions of Lower Permian sandstones are characterized by low SiO2/Al2O3 (6) and low K2O/ (Na2O+CaO)(<0. 5). Divariant diagram of SiO2/Al2O3-K2O/ (Na2O+CaO) shows that they were formed under the environments of volcanic island arc to residual oceanic basin. 4. QmFLt and QpLvLs of sandstones illustrate that they were derived from island arc orogen. All above mentioned illustrate that Late Palaeozoic Bogda area were belong to island arc and abandance volcanic component derived from volcanism of island arc.