内蒙古克什克腾旗五道石门基性火山岩锆石U-Pb年龄及其地质意义  

Zircon U-Pb dating of the mafic lava from Wudaoshimen,Hexigten,Inner Mongolia and its geological significance

作　　者：王炎阳 徐备[1] 程胜东 廖闻[1] 邵军[2] 汪岩[2] 

机构地区：[1]北京大学造山带与地壳演化教育部重点实验室、京大学地球与空间科学学院,北京100871 [2]沈阳地质矿产研究所,沈阳110034

出　　处：《岩石学报》2014年第7期2055-2062,共8页Acta Petrologica Sinica

基　　金：国家重点基础研究发展计划(2013CB429806);国家自然科学基金项目(40872145、41121062);中国地质调查局项目(1212011220899)联合资助

摘　　要：内蒙古克什克腾旗五道石门水库出露一套由灰绿色玄武岩、枕状玄武岩和硅质粉砂岩组成的火山-沉积岩系。枕状玄武岩主要出露于水库大坝南岸,枕体呈不规则椭球状,长轴约0.4~1.0m,边部具有冷凝边,内部气孔和杏仁状构造较发育。镜下观察其矿物组成主要为细长条状斜长石和少量辉石,间粒结构。前人研究认为它们属于早古生代蛇绿岩,本次研究对枕状玄武岩样品进行了LA-ICP-MS锆石U-Pb年龄测试。锆石测年结果大致可分为四组,依次为262~290Ma（A组,15颗）、301~345Ma（B组,5颗）、446~572Ma（C组,6颗）和763~948Ma（D组,5颗）。其中A组锆石呈长柱状,自形,长宽比约2∶1~3∶1,Th/U值介于0.22~0.98;振荡环带较宽,体现了基性岩浆锆石的特征。15个点的谐和年龄为277±3Ma,应代表岩石的形成年龄,表明五道石门枕状玄武岩形成于早二叠世晚期,应属大石寨组。其余三组锆石多呈自形-半自形,粒状或短柱状,振荡环带较窄,且没有270~280Ma的生长边,表明可能是基性岩浆上升过程中从围岩捕获的锆石,表明五道石门玄武岩可能发育在具有晚元古-古生代基底的陆壳内,暗示早二叠世晚期本区处于板内拉张环境。五道石门枕状玄武岩曾被认为是早古生带大洋蛇绿岩,但综合我们的年代学结果和对锆石类型的分析,它应属早二叠世晚期伸展环境中基性岩浆活动的产物。A series of volcanic-sedimentary rocks, composed of gray-green basalts, pillow basalts in lower part and siliceous sihstone in upper part, was recognized in Wudaoshimen area, Hexigten Banner, Inner Mongolia. Previous studies considered them as part of the Early Paleozoic ophiolite suite. In this paper, a pillow basalt sample was collected to date U-Pb ages of zircons by using LA- ICP-MS analytical techniques. Existed south of the Wudaoshimen dam, the pillows are irregular ellipsoidal with long axis ranging from 0. 4 - 1.0m, and have obvious condensation rims and little pores inside. The pillow basalt sample display typical intergranular texture with plenty of narrow-strip feldspar and some tiny pyroxene under microscope. Most zircons are colorless, transparent, and euhedral to subhedral in shape. Cathodoluminescene （CL） imaging shows most of them have oscillatory zones. Combined with their high Th/U ratios （ 〉 0. 1, except for one spot of 0. 08 ）, it is suggested that these zircons are of igneous origin. The 31 zircon analyses are concordant, and show four groups： 261 - 290Ma （group A, n = 15）, 301 - 345Ma （group B, n =5）, 446 -572Ma （group C, n =6） and 763 -948Ma （group D, n =5）, respectively. Group A zircons are euhedral and columnar with length：width ratios of 2 ： 1 -3 ： 1, and display typical widely-spaced oscillatory zoning, which indicates they crystallized at relatively high temperature. Fifteen zircon analyses yield a concordant age of 277 ± 3Ma （MSWD = 0. 02）, representing that the Wudaoshimen pillow basalts formed in the late Early-Permian and belong to the Dashizhai Formation. Group B zircons are euhedral and columnar, while group C and D zircons are subhedral-euhedral and granular, respectively. Furthermore, most of these three group zircons display closely-spaced concentric oscillatory zoning. The remarkable differences between group A and the other three groups suggest different source. Combined with the lack of magmatic rims with age of 270 ~ 280Ma in older zi

关 键 词：大石寨组 枕状玄武岩 锆石U-PB年龄 兴蒙造山带 

分 类 号：P588.145[天文地球—岩石学] P597.3[天文地球—地质学]