恰什坎萨伊沟玄武岩年代学、地球化学特征及其对北阿尔金洋盆闭合过程的制约

Geochronology and Geochemistry of the Qiashikansayi Basalt and Its Constraint on the Closure Progress of the North Altyn Ocean

北阿尔金恰什坎萨伊沟红柳泉以南早古生代蛇绿岩剖面发育两段玄武岩岩片。北段玄武岩锆石LA-ICP-MSU-Pb定年与前人对南段玄武岩定年结果(晚奥陶世)相近,表明该玄武岩形成于中—晚奥陶世;样品TR050-10-1锆石热干扰年龄为421～412Ma、395～388Ma和283～228Ma,TR050-13-1锆石热干扰年龄为442～434Ma、415Ma和241～221Ma。地球化学特征显示北段玄武岩和南段玄武岩的岩浆演化过程中均以单斜辉石结晶分异为主,仅表现极轻微地壳物质混染;二者均来自尖晶石二辉橄榄岩地幔源区,前者源区为原始尖晶石二辉橄榄岩相,后者源区为原始—亏损尖晶石二辉橄榄岩过渡相,前者的部分熔融程度更高。微量元素、稀土元素及构造判别图解指示其为形成于聚合背景下的E-MORB型拉斑玄武岩,具弧后盆地性质。结合区域资料,提出北阿尔金早古生代洋盆4阶段闭合模式:①早期俯冲阶段(∈2～O1),②晚期俯冲阶段(O1～O3),③俯冲-碰撞转换阶段(O3～S2),④后碰撞造山阶段(S3～D2)。

Two segments of the basalt develop within the early Paleozoic ophiolite section located south of Hongliuquan of Qiashikansayigou in the north Altyn Tagh Mountains.LA-ICP-MS U-Pb dating on the zircon collected from basalt in the north segment is basically in agreement with that（Late Ordovician） by previous studies,indicating that the basalt formed in mid-late Ordovician.Three thermal disturbance ages are 421~412 Ma,395~388 Ma and 283~228 Ma for zircons in sample TR050-10-1 and 442~434 Ma,415Ma and 241~221 Ma for zircons in sample TR050-13-1.Geochemical characteristics show that magma evolution of the basalts in both north and south segments was dominated by clinopyroxene fractionation,in company with slight hybridization of crust materials.Both basalts were all derived from spinal-lherzolite-bearing mantle,with the north basalt being spinal-lherzolite mantle and highly partial melting,and the south basalt being plausible-depleted lherzolite mantle.Analysis of trace elements and REE,and tectonic discrimination diagram show that the basalts are the E-MORB-type tholeitte formed under the background of convergence and characterized by back-arc basin.Combined with the regional geological data,this study suggests a model of four-stage closure progress for the North Altyn Ocean：：early stage of subduction（∈2-O1）,late stage of subduction（O1-O3）,tectonic environment transfer from subduction to collision（O3-S2）,and post-collisional orogenic period（S3-D2）.