| 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 |
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| 引用本文： | 张华锋,王浩铮,豆敬兆,张少颖.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[J].岩石学报,2015,31(6):1518-1534. |
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| 作者姓名： | 张华锋 王浩铮 豆敬兆 张少颖 |
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| 作者单位： | 中国地质大学地球科学与资源学院, 北京 100083;中国科学院地质与地球物理研究所, 北京 100029;中国地质大学地球科学与资源学院, 北京 100083;中国地质大学地球科学与资源学院, 北京 100083 |
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| 基金项目： | 本文受973项目(2012CB416603)、国家自然科学基金项目(40772122、41210003)和高校教育基金(53200959742)联合资助. |
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| 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,岩浆可能存在过独居石的分异作用。高铝TTG岩系主要由新生基性地壳在相对高压下部分熔融而成,源区残留相以石榴石+辉石+角闪石以及无或少量斜长石为特征。岩浆经历过角闪石和辉石分离结晶作用,铕正异常增大的现象可能与斜长石堆晶有关。本区同时形成高铝和低铝TTG岩系的机制还需深入研究。俯冲机制、地幔柱机制以及二者共同作用等机制均能解释TTG的成因。依据本区同期还形成大量辉长质-闪长质岩浆和稍晚(2.5~2.45Ga)形成的钾质花岗岩类岩浆的侵入活动,我们认为本区高铝和低铝TTG岩系分别来自底侵作用导致的下地壳不同深度不同程度的部分熔融有关。引起底侵作用的机制可能与地幔柱或地幔柱与板块俯冲共同作用有关。
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| 关 键 词： | TTG 岩石成因 地球化学 怀安陆块 华北克拉通 |
| 收稿时间： | 2014/6/30 0:00:00 |
| 修稿时间： | 3/1/2015 12:00:00 AM |
Geochemistry and genesis of the Late Archean Low-Al and High-Al TTGs from the Huai'an terrane, North China Craton |
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| ZHANG HuaFeng,WANG HaoZheng,DOU JingZhao and ZHANG ShaoYing.Geochemistry and genesis of the Late Archean Low-Al and High-Al TTGs from the Huai'an terrane, North China Craton[J].Acta Petrologica Sinica,2015,31(6):1518-1534. |
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| Authors: | ZHANG HuaFeng WANG HaoZheng DOU JingZhao and ZHANG ShaoYing |
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| Institution: | School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China |
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| Abstract: | 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 SiO2 contents. Zircon SHRIMP U-Pb dating yielded a formation age of 2.53Ga, 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 SiO2 (76%~79%), with low Al2O3 (11.01%~12.61%), CaO (1.27%~1.59%), MgO (0.74%~0.24%) contents and highly variable Mg# (18~53). In contrast, major elements of the High-Al TTGs vary largely, for example, SiO2 is between 63%~77%, Al2O3 is 13.2%~17.77%, CaO is 1.8%~5.78%, MgO 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 Low-Al 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), indicating that they have different sources and petrogenesis. Based on a combination of zircon U-Pb and Lu-Hf isotopes and whole-rock geochemistry, we consider that the Low-Al trondhjemitic magma was derived by partial melting of juvenile crust leaving a residue composed of pyroxene and plagioclase, without or with minor amphibole, and experienced the crystallization of monazite en route. The High-Al TTGs were generated by partial melting of juvenile crust at relatively high pressure with a residual mineral assemblage of garnet, amphibole, pyroxene and minor plagioclase, and the magma experienced fractional crystallization of hornblende and clinopyroxene with some extent of feldspar accumulation. It is proposed that the Huai'an TTGs were generated by anatexis of lower crust at various depths as a consequence of the Late Archean mantle plume activity or the combined effect of mantle plume and slab subduction. |
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| Keywords: | TTG Petrogenesis Geochemistry Huai'an terrane North China Craton |
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