川北九寨沟地区隆康熔结凝灰岩锆石LA-ICP-MSU-Pb年龄--勉略古缝合带西延的证据  

Zircon LA-ICP-MS U-Pb dating of the Longkang ignimbrite in the Jiuzhaigou area, Sichuan, China: Evidence of the westward extension of the Mianlüe suture

作　　者：秦江锋[1] 赖绍聪[1] 张国伟[1] 第五春荣[1] 李永飞[2] 

机构地区：[1]西北大学大陆动力学国家重点实验室,西北大学地质学系,陕西西安710069 [2]中国地质调查局沈阳地质矿产研究所,辽宁沈阳110032

出　　处：《地质通报》2008年第3期345-350,共6页Geological Bulletin of China

基　　金：国家自然科学基金重点项目(批准号:40234041);高等学校优秀青年教师教学科研奖励计划项目(批准号:教人司2002383号);西北大学研究生自主创新项目(批准号:07YZZ26)联合资助

摘　　要：康县-琵琶寺-南坪构造带内出露的一套变质沉积-火山岩系与勉略洋的构造演化具有密切的关系。在该构造带西段九寨沟塔藏-隆康地质剖面东端出露近百米厚的安山质熔结凝灰岩、层凝灰岩,在隆康、牙扎沟附近采集了熔结凝灰岩样品(JZG-L),选出百余颗锆石。锆石LA-ICP-MSU-Pb分析结果表明,锆石发育典型的岩浆锆石韵律环带,U和Th含量分别为35×10-6～750×10-6和31×10-6～717×10-6,Th/U比值介于0.44～1.30之间,为典型的岩浆锆石,该样品获得的206Pb/238U加权平均年龄为246.2Ma±2.8Ma(MSWD=2.6,n=12,2σ)。此外,样品中还包括其他多种类型的碎屑锆石,其中最老的碎屑锆石的谐和年龄为(2481±33)Ma,表明九寨沟地区存在前寒武纪基底。结合区域地质及该区火山岩的发育特征,246Ma的锆石U-Pb年龄的获得为勉略缝合带西延至南坪一带提供了重要的同位素年代学证据。A suite of metaseclimentary-volcanic rocks exposed in the Kangxian-Pipasi-Nanping tectonic zone is closely related to the tectonic evolution of the Mianliie ocean. Andesitic ignimbrite and tuffite nearly a hundred meters thick are exposed at the eastern end of the Tazang-Longkang section in the western segment of the Kangxian-Pipasi-Nanping tectonic belt. An ignimbrites sample （JZG- L） was collected near Yazagou at the eastern end of the Tazang-Longkang section, fi：om which more than one hundred zircon grains were selected. The zircon LA-ICP-MS U-Pb analysis shows that the zircons are of typical magmatic origin with oscillatory zoning, higher U （35×10^6-750×10^-6）Th（31×10^-6-717×10^-6） and Th/U（0.44-1.30）. The ^206Pb/^238U ratios are in good agreement with the analytical precision, yielding a weighted mean age of 246.2±2.8 Ma （MSWD=2.6; n=12, 2 σ）. Some detrital zircons have also been observed, which have the oldest concordia age of 2481±33 Ma, indicating the existence of Precambrian basement in the Jiuzhaigou area. According the regional geology and characteristics volcanic rocks in the study area, the 246.2±2.8 Ma zircon U-Pb age suggests the occurrence of Late Permian orogenic volcanism and provides important isotope geochronological evidence for the westward extension of the Mianliie suture to the Nanping area.

关 键 词：隆康熔结凝灰岩 锆石LA-ICP-MSU-PB年龄 勉略古缝合带 

分 类 号：P597.3[天文地球—地球化学] P542.4[天文地球—地质学]