摘要
通过对辽西彰武县以东大四家子乡高城窝堡村义县组标准剖面中生代火山岩锆石U-Pb年代学研究表明,其火山岩年龄为122.4±0.4Ma,属早白垩世.对该区域23件典型火山岩样品的地球化学研究表明,除3件流纹岩样品外,其余样品具有高镁埃达克岩地球化学特征(SiO2=56.46%～65.14%、Al2O3=14.60%～17.19%、Mg#=50～59、Sr=501～700μg/g、Yb=1.04～1.54μg/g、Y=12.0～17.5μg/g、Eu/Eu*=0.85～0.97、Sr/Y=29～46、LaN/YbN=13～28),同位素上具有高的初始87Sr/86Sr(0.705464～0.705812)比值,低的εNd(122Ma)(-6.12～-12.80)值特征,同时样品中存在具有反环带的辉石斑晶,辉石中稀土元素含量分布存在着从核部到边部逐渐降低的趋势,且存在负铕异常(Eu/Eu*=0.64～0.76).结合前人对彰武义县组下部火山岩的研究,笔者倾向认为该套火山岩的成因是拆沉作用与岩浆混合作用共同作用的结果,即拆沉作用导致软流圈地幔物质上涌加热下地壳形成的长英质岩浆,与来自地幔由拆沉作用形成的埃达克质高镁安山岩浆混合形成.
Zircon U-Pb dating by LA-ICPMS reveals that the age of Mesozoic volcanic rocks from the Zhangwu area in west Liaoning Province is 122.4± 0.4 Ma, which belongs to Early Cretaceous. Geochemical compositions of 23 typical volcanic samples are studied. The results show that most samples except three rhyolites have similar geochemical characteristics of high-Mg adakite （SiO2 =56.46%-65.14%,Al2O3 =14. 60%--17.19%,Mg^# = 50--59,Sr=501- 700μg/g,Yb= 1.04- 1.54μg/g ,Y= 12.0- 17.5μg/g,Eu/Eu^＊ = 0.85 - 0.97 ,Sr/Y = 29 - 46, Lay/Yby = 13 - 28）. They also have high initial ^87Sr/^86Sr （0. 706 39-0. 706 47） and low Nd （122 Ma） values （- 6.43 to - 12.26） that are inconsistent with the original ones from slab melting. Pyroxene phenocrysts are characterized by reversed compositional zonning. Rare earth element concentrations decrease from pyroxene core to rim and also show negative Eu anomalies （Eu/Eu^＊= 0.64-0. 76）. Combined with the previous studies on volcanic rocks from the low part of Yixian Formation in Zhangwu area, we propose that our samples from Zhangwu area resulted from delamination and magma mixing. A crustally derived felsic magma resulted from partial melting of the low crust, heated by asthenosphere upwelling and subsequently mixed with a mantle-derived high-Mg adakitie melt during magma storage or ascent to surface to form the high-Mg adakites in the Zhangwu area.
出处
《地球科学（中国地质大学学报）》
EI
CAS
CSCD
北大核心
2008年第2期151-164,共14页
Earth Science-Journal of China University of Geosciences
基金
教育部创新团队研究计划项目(NoIRT0441)
国家自然科学基金委创新研究群体科学基金项目(No40521001)
高等学校学科创新引智计划项目(NoB07039)
关键词
高镁埃达克岩
岩浆混合
拆沉作用
中生代火山岩
辽西
high-Mg adakites
magma mixing
delamination
Mesozoic volcanic rocks
West Liaoning Province.
