中祁连地块西段青白口系其它大坂组英安质凝灰岩LA-ICP-MS锆石U-Pb年龄及其地质意义  

Zircon LA-ICP-MS U-Pb age of dacitic tuff from the Qitadaban Formation,Qingbaikou System,in western part of Middle Qilian Block and its geological significance

作　　者：张金明[1,2,3] 陈光庭 才航加[1,2,3] ZHANG Jinming;CHEN Guangting;CAI Hangjia(Qinghai Institute of Geological Survey,Xining,810012;Qinghai Province Key Laboratory of Geological Process and Mineral Resources of Northern Qinghai—Tibet Plateau,Xining,810012;Qinghai Engineering Research Center of Remote Sensing Big Data,Xining,810012)

机构地区：[1]青海省地质调查院,西宁810012 [2]青藏高原北部地质过程与矿产资源重点实验室,西宁810012 [3]青海省遥感大数据工程技术研究中心,西宁810012

出　　处：《地质论评》2023年第2期747-760,共14页Geological Review

基　　金：青海学者专项(编号:QHS201802);国家重点研发计划项目(编号:2021YFC2901902);青海省天峻县苏里地区1∶5万五幅区域地质调查项目(编号:1212011140088)的成果。

摘　　要：新元古界青白口系其它大坂组是中祁连地块西段基底岩系之一,对研究中祁连地块元古宙地质构造演化具有重要意义。笔者等通过1∶5万区调工作在其它大坂组中发现了英安质晶屑岩屑凝灰岩夹层,并对凝灰岩进行了锆石U-Pb年代学、岩石地球化学研究。测年结果表明样品中锆石年龄主要分3组。结合区域地质资料,笔者等认为第一组锆石(1456~1524 Ma)和第二组锆石(1018~1021 M)为外来锆石或/碎屑锆石,分别为物源区中元古代早期Columbia超大陆裂解事件和Rodinia大陆聚合事件(格林威尔造山作用)在中祁连地块中的响应;第三组锆石(946~964 Ma)的n(^(206)Pb)/n(^(238)U)加权年龄为958±3 Ma(MSWD=1.4),代表英安质凝灰岩的形成时代。岩石地球化学分析表明,其它大坂组英安质晶屑岩屑凝灰岩样品属于过铝质(A/CNK=1.37~1.75)、高钾(K_(2)O>Na_(2)O,K_(2)O=4.48%~4.86%)、钙碱性(σ=0.89~1.26)火山岩,稀土总量为(149.7~156.4)×10^(-6),(La/Yb)_(N)为10.15~10.61,具负Eu异常(δEu=0.62~0.63),富集K、Rb、Ba、Th等大离子亲石元素,亏损Nb、Ti等高场强元素,显示出与岛弧钙碱性火山岩相似的特点,形成于活动大陆边缘环境,是中祁连地块中—新元古代早期Rodinia超大陆形成地质事件的响应。Objectives:The Qitadaban Formation of the Qingbaikou System,Neoproterozoic,is an important component of metamorphic basement in the western part of the Central Qilian Block,being of great significance to understanding the Proterozoic geological and tectonic evolution of the Qilian Block.The research work on dacite tuff of the Qitadaban Formation has been carried out in this paper.Methods:Based on the field work,we finished microscope observation,whole-rock geochemical analysis and LA-ICP-MS zircon U-Pb dating for the dacitic tuff in this paper.Results:The zircons from the rock can be divided into three groups in age.Combined with regional geological data,it is considered that the first and the second age groups of zircons(1408~1410 Ma,1018~1021 Ma)from the dacitic tuff are responding to the Columbia supercontinent breakup and Rodinia continental convergence(Greenwell Orogeny)in the Qilian Block.The third group of zircons(946~964 Ma)has a mean-weighted n(206Pb)/n(238U)age of 958±3 Ma(MSWD=1.4),interpreted as the formation age of the tuff.Petrogeochemical analysis shows that the dacitic tuffs are peraluminous(A/CNK=1.37~1.75),high potassium(K_(2)ONa_(2)O,K_(2)O=4.48%~4.86%)and calc-alkaline(σ=0.89~1.26).TheΣREE=149.7×10^(-6)~156.4×10^(-6),(La/Yb)N=10.15~10.61,andδEu=0.62~0.63;with enrichment in large ion lithophile elements(K,Rb,Ba,Th)and deplation in high field strength elements(Nb,Ti).Conclusions:The dacite tuff show similar characteristics with island arc calc-alkaline volcanic rocks in geochemical composition.Thus,they formed in an active continental margin environment and was a response to the formation of the Rodinia supercontinent in the early Neoproterozoic in the Qilian Block.

关 键 词：英安质凝灰岩 其它大坂组 LA-ICP-MS U-Pb测年 青白口系 中祁连地块西段 

分 类 号：P588.2[天文地球—岩石学] P597.3[天文地球—地质学]