维西地区中三叠统上兰组物源分析及构造背景判断

Provenance Analysis and Tectonic Setting Judgment in Shanglan Formation of Middle Triassic in Weixi Area

前人对于金沙江构造带内三叠纪碎屑岩形成和演化的研究甚少,影响了对金沙江洋盆演化历史的整体认识.维西地区上兰组为砂板岩夹灰岩沉积,与原岩图解结果一致.变质砂岩碎屑锆石U-Pb年龄分布于426~603 Ma、713~980 Ma、1 000~1 600 Ma和1 600~2 500 Ma四个年龄段,年龄信息主要记录了来自扬子陆块及其周缘老基底、同泛非期及加里东期的物质来源;∑REE含量略高于上地壳值且具明显的轻重稀土分异特征,(La/Yb)N=5.23~12.33,具有轻微的δEu异常.碎屑组分特征及判别图解显示其物源主要来自于碰撞造山带及少量火山弧造山带,源岩性质属于再循环的古老沉积物及长英质火山岩类,构造环境为大陆岛弧及活动大陆边缘.以上研究表明上兰组形成于碰撞挤压坳陷构造背景,结合区域构造演化综合判断其为一种弧后前陆盆地构造环境.

There was little research on the formation and evolution of the Triassic clastic rocks in the Jinshajiang tectonic belt,undermining the overall understanding of the evolution history of the Jinshajiang ocean basin.It is found in this study the protolithes of Shanglan Formation are mainly sandstone and shale,which is in agreement with the classification discriminate diagram of clastic rocks.The U-Pb dating of detrital zircon show 4 age groups,which are 426-603 Ma,713-980 Ma,1 000-1 600 Ma and 1 600-2 500 Ma respectively,indicative of the provenance from basement of the Yangtze block and its vincinity,magmatism of homochronous Pan-African and Caledonian.The content of∑REE are slightly higher compared with that of upper crust.The LREE and HREE show obvious fractionation,（La/Yb）N=5.23-12.33.Meanwhile,δEu is slightly abnormal.The characteristics of detrital component and discrimination diagram show the provenance are mainly from collisional orogenic beltand a little from volcanic arcs.The quality of the matters belong to old sediment and quartz-feldspathic volcanic.The tectonic setting of provenance in Shanglan Formation is mainly continental arc and active continental margin.The information above common indicates that the tectonic setting of Shanglan Formation is depression which was formed by extruding of subduction collision.Combined with regional tectonic evolution,we consider its tectonic environment was retroarc foreland basin.