三江北段贡觉三叠纪火山岩年代学、地球化学特征及其地质意义  

Chronology and Geochemistry of Gonjo Triassic Volcanic Rocks in Northern Part of Nujiang-Lancangjiang-Jinshajiang and Its Geological Significance

作　　者：段瑶瑶 唐渊[1] 秦雅东[1] 巩小栋[1] 刘函[1] Duan Yaoyao;Tang Yuan;Qin Yadong;Gong Xiaodong;Liu Han(Chengdu Center of China Geological Survey,Chengdu 610081,China)

机构地区：[1]中国地质调查局成都地质调查中心,四川成都610081

出　　处：《地球科学》2020年第8期2920-2931,共12页Earth Science

基　　金：中国地质调查局项目(No.DD20190053)。

摘　　要：三江北段贡觉三叠纪火山岩是金沙江古特提斯洋向西俯冲消减及随后的弧陆碰撞造山作用的产物,主要由玄武岩、安山质熔结凝灰岩和安山岩等组成,对探讨金沙江古特提斯构造-岩浆演化过程具有重要意义.对贡觉火山岩进行了LAICP-MS锆石U-Pb定年和全岩主、微量元素研究.其中代表性样品安山质熔结凝灰岩的LA-ICP-MS锆石U-Pb定年结果显示,锆石23个分析点的206Pb/238U年龄为243~233 Ma,206Pb/238U加权平均年龄为238.3±1.3 Ma(MSWD=1.7,n=23),代表了岩浆的结晶年龄为中-晚三叠世.地球化学分析表明,火山岩以低钛(0.52%~0.91%)、中镁(3.38%~7.22%)、富钠(3.63%~4.66%)、贫钾(0.09%~4.05%)、富集轻稀土元素(LREE/HREE=4.09~13.27)和大离子亲石元素(Th、U、La、Sm)、亏损高场强元素Nb和Ta(Nb=2.51×10^-6~24.5×10^-6、Ta=0.22×10^-6~1.89×10^-6)为特征,La/Nb值(2.13~2.89)较高、Nb/Th值(1.12~1.62)相对较低,具有与岛弧型或晚碰撞造山型火山岩相似的主-微量元素组成特征.贡觉中-晚三叠世火山岩代表了金沙江古特提斯演化过程中晚碰撞造山阶段的岩浆响应,推测是晚碰撞过程中残留的金沙江洋壳物质与岩石圈地幔物质相互作用的结果.The Gonjo Triassic volcanic rocks in the northern area of the Nujiang-Lancangjiang-Jinshajiang are the products of subduction of the Jinshajiang Paleo-Tethyan Ocean to the west and subsequent arc-continent collision-orogeny effect.It is mainly composed of basalt,andesite fused tuff and andesite,etc.The study of Gonjo volcanic rocks is of great significance for the exploration of the Jinshajiang Paleo-Tethyan tectonic-magmatic evolution process.In this paper,it presents experiments on Gonjo volcanic rocks of LA-ICP-MS zircon U-Pb dating and master-trace element research.The LA-ICP-MS zircon U-Pb dating of representative samples of andesite fused tuff shows that the age of206 Pb/238 U of 23 analysis points of zircon is 243-233 Ma.The LA-ICP-MS zircon U-Pb age of the representative sample of andesite fused tuff is 238.3±1.3 Ma,which indicates that the magmatic crystallization age is Middle-Late Triassic.Geochemical analysis shows that volcanic rocks contains low titanium(0.52%-0.91%),medium magnesium(3.38%-7.22%),rich sodium(3.63%-4.66%),depleted potassium(0.09%-4.05%),enriched light rare earth elements(LREE/HREE=4.09-13.27)and large ionic lithophilic elements(Th,U,La,Sm),deficient high field strength elements Nb and Ta(Nb=2.51×10^-6-24.5×10^-6,Ta=0.22×10^-6-1.89×10^-6),relatively high La/Nb(2.13-2.89)ratio and relatively low Nb/Th(1.12-1.62),which possesses the composition characteristics of main-trace elements similar to island arc or late collision orogenic volcanic rocks.It is concluded that the Middle-Late Triassic Gonjo volcanic rocks represent the magmatic response of the Jinshajiang during the evolution of Paleo-Tethys,which is probably the result of the interaction between the Jinshajiang oceanic crust material and the lithospheric mantle material remaining during the late collision.

关 键 词：锆石U-PB定年 地球化学 岛弧型火山岩 金沙江结合带 地质年代学 

分 类 号：P597[天文地球—地球化学]