北秦岭-祁连结合区大草滩群碎屑锆石U-Pb年代学研究  

Detrital Zircon U-Pb Geochronology of Dacaotan Group in the Conjunction Area of North Qinling and Qilian

作　　者：陈义兵[1,2] 张国伟[1] 鲁如魁[1] 梁文天[1] 第五春荣[1] 郭秀峰[1] 

机构地区：[1]大陆动力学国家重点实验室西北大学地质学系,西安710069 [2]中国科学院广州地球化学研究所,广州510640

出　　处：《地质学报》2010年第7期947-962,共16页Acta Geologica Sinica

基　　金：国家自然科学基金重点项目(编号40234041);中国博士后科学基金(编号20070411138);中石化海相前瞻性项目(编号YPH08012);西北大学大陆动力学国家重点实验室开放课题基金(编号06LCD17);自主研究课题(编号BJ081331)的资助

摘　　要：应用LA-ICP-MS法对北秦岭-祁连结合区的晚泥盆世大草滩群进行了系统的碎屑锆石U-Pb年代学研究。地层剖面从下至上,大草滩群三个岩组的碎屑锆石U-Pb年龄谱具有以下特征:①古生代年龄组分(<550Ma)所占的比例依次变小(分别为36%,32%,25%),且最年轻的锆石年龄值也是逐渐变小(分别为403±5Ma,385±5Ma,375±6Ma),这一特征反映的是大陆边缘岩浆弧前缘同岩浆活动的隆升-剥蚀和沉积。②750～2600Ma年龄组分所占的比例逐渐增大(分别为54%,65%,72%),且主要的峰值年龄向老的方向变化,这一特征反映的是往造山带深部基底和大陆内部逐渐延伸的隆升和剥蚀。本研究工作阐明大草滩群的碎屑沉积物来自多种构造环境中的岩石地层单元,主要来自北秦岭-祁连微陆块基底,其次是来自古生代洋壳持续俯冲形成的大陆边缘岩浆弧和加里东期碰撞造山带再旋回物质,还有少量来源于华北克拉通西部地块的物质成分。晚泥盆世大草滩群陆相粗碎屑沉积组合在构造位置上是处于弧(微陆)-大陆碰撞造山带的南缘与安第斯型大陆边缘岩浆弧的弧前盆地这一构造叠加复合地区,是弧(微陆)-大陆碰撞造山作用以后,洋壳持续俯冲造山作用阶段同火山-岩浆活动的沉积响应。Detrital zircons LA-ICP-MS U-Pb dating was performed systematically on the Late Devonian Dacaotan Group in the conjunction area of North Qinling and Qilian.Three formations of Dacaotan Group are characterized from lower to upper by the followings.①With decreasing stratigraphic age,the proportion of pre-550 Ma grains in the three units decreases from 36% to 32%,and then to 25%.The minimum age of the youngest detrital-zircon population decreases upward from 403 5Ma in the bottom,through 385 5Ma in the middle,to 375 6Ma in the top of the group.These trends are consistent with the hypothesis that the Dacaotan Group represents a syn-magmatic sedimentary basin,deposited in front of the active continental-margin magmatic arc.②The systematic increase in the proportion of the 750~2600Ma zicons from 54% to 65%,and then to 72% upward with stratigraphy height,is accompanied by an increase in the peak age of Proterozoic zircons on the age histograms.These trends may reflect increased uplift and erosion of the orogen basement,and/or inward denudation of the continent crust with the transverse drainage system.The detrital zircon geochronological results from the Dacaotan Group provide detailed insights into both the provenance of sedimentary formations,and their tectonic history.Our data confirm that the basement of North Qinling-Qilian microcontinent was the predominant source area for the sediments,the active magmatic arc and recycled collision orogen should be another important source area,and some detritus was derived further from the western block of the North China Craton.In the middle Devonian,an early arc（microcontinent）-continent collisional orogen was overprinted by the Andean-type magmatic arc along the southern convergent margin of North China continent.Large volume of molasse sediments has been shed into forearc basins at this time,presumably resulted from the combined effects of the collision and the southern oceanic crust subduction northwardly.Siliciclastic rocks of the Upper Devonian Dacaotan Group reco

关 键 词：大草滩群 LA-ICP-MS 碎屑锆石U-PB年龄 沉积响应 北秦岭-祁连结合区 

分 类 号：P597.3[天文地球—地球化学]