华北克拉通怀安陆块新太古代低铝和高铝TTG片麻岩的地球化学特征与成因  

Geochemistry and genesis of the Late Archean Low-Al and High-Al TTGs from the Huai'an terrane,North China Craton

作　　者：张华锋[1] 王浩铮[2] 豆敬兆 张少颖[1] 

机构地区：[1]中国地质大学地球科学与资源学院,北京100083 [2]中国科学院地质与地球物理研究所,北京100029

出　　处：《岩石学报》2015年第6期1518-1534,共17页Acta Petrologica Sinica

基　　金：973项目(2012CB416603);国家自然科学基金项目(40772122;41210003);高校教育基金(53200959742)联合资助

摘　　要：英云闪长岩-奥长花岗岩和花岗闪长岩（简称TTG）是太古宙高级变质地体的主要物质组成,对深入理解早期大陆生长及其机制具有重要的科学意义。目前,人们对其成因过程与机制仍有不同认识。本文以怀安陆块中广泛分布的TTG片麻岩为例,探讨其成因演化和机制。研究区位于华北克拉通中北部,主要由新太古代英云闪长岩及少量奥长花岗岩、花岗闪长岩组成。我们从该区识别出富硅富重稀土和负铕异常的低铝奥长花岗质片麻岩,形成时代与广泛分布的高铝TTG质片麻岩一致（锆石SHRIMP U-Pb年龄2.53Ga）。岩石地球化学数据显示,低铝奥长花岗质片麻岩的主量元素具有富SiO 2（76%-79%）,低Al2O3（11.01%-12.61%）、CaO（1.27%-1.59%）、MgO（0.74%-0.24%）和Mg#（18-53）等特征,而广泛分布的高铝TTG岩系的主量元素含量变化大,例如,SiO 2=63%-77%、Al2O3=13.2%-17.77%、CaO=1.8%-5.78%、MgO=0.18%-3.84%和Mg#=35-64。微量元素方面,低铝奥长花岗质片麻岩具有Eu/Eu＊负异常（除1个样品为弱正异常1.38外,其余样品分布在0.59-0.44）,富集重稀土（（La/Yb）N=4-7,（Gd/Yb）N=0.36-1.27）,而高铝TTG岩系从弱负铕异常到正异常（Eu/Eu＊=0.8-5.35）,轻重稀土分馏明显（（La/Yb）N=10-103、（Gd/Yb）N=1.97-5.72）。在微量蛛网图中二者的区别除重稀土明显存在区别外,低铝奥长花岗质片麻岩显示出Ba、Sr的相对亏损,而高铝TTG岩系则相反。二者Lu/Hf比值差异明显,低铝奥长花岗质片麻岩变化在0.1-0.16,而高铝TTG岩系变化在0.01-0.07。在Lu/Hf与相关元素以及SiO 2与相关元素哈克图解中,二者差异更加明显,表明它们之间不存在直接的成因联系。综合锆石U-Pb、Lu-Hf同位素特征以及岩石地球化学特征,我们认为低铝奥长花岗质片麻岩是低压下由新太古代新生基性地壳物质低程度部分熔融而成,源区残留矿物相以辉石＋斜长石为主,岩浆可能存在Trondhjemite, Tonalite and Granodiorite（ TTG） as dominated lithological assemblages in Archean high-grade metamorphic terranes can provide important clues for the history and mechanism of crustal growth. However,their genesis is still debated. The Huai＇an terrane located in the north-central region of the North China Craton,is mainly composed of Archean gray gneisses,which are dominated by TTG rocks. The Huai＇an TTG are predominantly tonalite with minor trondhjemite and granodiorite.Previously,we recognized some Low-Al trondhjemites from the Huai ＇an gray gneisses with pronounced negative Eu anomalies and enriched heavy rare earth elements（ REE） as well as high Si O2 contents. Zircon SHRIMP U-Pb dating yielded a formation age of2. 53 Ga,indicating their coeval magmas with the widespread High-Al TTGs,regionally. This work provides new data from Low-Al and High-Al TTGs to give a detailed investigation on their petrogenesis. As a result,the Low-Al trondhjemites are enriched in Si O2（ 76%~ 79%）,with low Al2O3（ 11. 01% ~ 12. 61%）,Ca O（ 1. 27% ~ 1. 59%）,Mg O（ 0. 74% ~ 0. 24%） contents and highly variable Mg#（ 18 ~ 53）. In contrast,major elements of the High-Al TTGs vary largely,for example,Si O2 is between 63% ~ 77%,Al2O3is13. 2% ~ 17. 77%,Ca O is 1. 8% ~ 5. 78%,Mg O is 0. 18% ~ 3. 84%,and Mg#is 35 ~ 64. In addition,the Low-Al trondhjemites exhibit pronounced negative Eu anomalies（ Eu / Eu＊is 0. 59 ~ 0. 44） and enriched heavy REE（（ La / Yb）N= 4 ~ 7,（ Gd / Yb）N=0. 36 ~ 1. 27）,whereas the High-Al TTGs show slightly negative to pronounced positive Eu anomalies（ Eu / Eu＊= 0. 8 ~ 5. 35） and relatively low heavy REE（（ La / Yb）N= 10 ~ 103,（ Gd / Yb）N= 1. 97 ~ 5. 72）. In primitive mantle normalized spidergrams,the LowAl trondhjemites are depleted in Sr and Ba,while the High-Al tonalites are enriched in the elements. Lu / Hf ratios of the Low-Al trondhjemite（ 0. 10 ~ 0. 16） are relatively higher than those of the High-Al rocks（ 0. 01 ~ 0. 07）,indicati

关 键 词：TTG 岩石成因 地球化学 怀安陆块 华北克拉通 

分 类 号：P588.345[天文地球—岩石学]