中祁连北缘奥陶纪岩浆弧地层

奥陶纪是祁连山加里东期构造带俯冲-碰撞格局奠定时期。通过区域填图和沉积环境分析及岩石地球化学研究，系统分析了沉积及岩浆作用过程， 认为在肃北县野牛台地区的晚奥陶世地层是中祁连地块西段北缘岩浆弧的组成部分。该地层为一套浅变质的基性-中性火山岩、碎屑岩及少量含化石灰岩组合，由下部海相安静深水还原环境向上部近岸水下河道-滨岸过渡，上部夹有冲积扇-河流相沉积，物源区为近源的岩浆弧和古老结晶基底，反映出自下向上水体变浅、盆地收缩的进积序列。其中的火山岩自下向上由基性向中性转变、由岛弧拉斑-钠质碱性玄武岩向钾质碱性玄武岩过渡，反映出与挤压构造环境相关的成熟岩浆弧火山岩组合特点。晚奥陶世岩浆弧地层、侵入岩与其北的同期构造蛇绿混杂岩带的相伴产出，暗示北祁连西段该时期洋壳在北部向北俯冲的同时，南部还向南俯冲并形成汇聚大陆边缘。

Ordovician Magmatic Arc Stratum of the Middle Qilian Block

Seduction-collision of the Caledonian tectonic belt in the Qilian unit formed in the Ordovician period.By geologic mapping,analysis of environment of sedimentation and study of rock geochemistry,it is considered that the rock formation of Late Ordovician epoch in Yeniutai,Subei County is the important part of magmatic arc of the northern margin in the western part of the middle Qilian block.Companied with the intermediate-acid intrusive rocks of Late Ordovician volcanic arcs,this stratum is mainly composed of slightly metamorphosed basic and intermediate volcanic rocks and conglomerate,sandstone,slate and a few fossil-bearing limestones from bottom of peaceful and deep water reducing environment of marine facies to the upper of alongshore underwater river-estuarine environment with alluvial-fluvial sedimentary sandwiched in the upper part.Material source is the magmatic arc intrusive rock,volcano-sedimentary rock and antiquity crystalline basement in the neighbor area,which reflects a progradational sequence in ascending order with lower water and shrinkage of the basin.The transformations of volcanic rock in ascending order are from basic to middle volcanic rock,from tholeiite of island arc sodium alkaline basalts to potassium alkaline basalts,from extravasations facies to burst facies,generally reflecting the characteristic of mature magmatic arc volcanic rock assemblage which is related to extrusion of the tectonic environment.The assemblage of magmatic arc stratum,intrusive rock and tectonic-ophiolitic mélange belt suggests characteristic of the continental margin proximal of Late Ordovician oceanic crust in west section of northern Qilian Mountains south-dipping.