太古宙TTG的Nb/Ta变化特征:对“Nb-Ta悖论”的启示

Nb/Ta variations in Archean TTG:Implications for“Na-Ta paradox”

英云闪长岩-奥长花岗岩-花岗闪长岩(TTG)是地球早期大陆地壳最重要的组成部分。TTG的Nb/Ta比值变化不仅与它的成因相关,而且与早期构造环境和地壳分异过程关系紧密。本文选择阴山地块出露的TTG片麻岩及下地壳斜长角闪岩/麻粒岩包体作为研究对象,开展了寄主花岗闪长岩和同源镁铁质包体中的角闪石和黑云母的原位微区矿物的微量元素分析工作,以及TTG与非同源斜长角闪岩包体的全岩主微量元素分析工作。矿物化学研究结果表明,花岗闪长岩和同源镁铁质包体的角闪石具有相似的Mg^(#)值,但是两者具有明显不同的Nb/Ta比值。镁铁质包体中的角闪石更富Cr、Ta,Nb/Ta比值为30~50;TTG岩石中的角闪石Cr和Ta含量偏低,但具有更高的Nb/Ta比值(38~70)。TTG和镁铁质包体中的角闪石Cr含量与Nb/Ta具有较好的负相关关系。全岩地球化学分析结果揭示,TTG片麻岩的具有高Nb/Ta比值(13~65,平均值31),斜长角闪岩和麻粒岩包体具有变化的Nb/Ta比值(10~56)。太古宙绿岩带中玄武质岩石的Nb/Ta平均值为~15,阴山地块斜长角闪岩和麻粒岩包体具有高的Nb/Ta比值,反映了这些代表基性下地壳的岩石经历了部分熔融作用或后期的交代作用,使其Nb/Ta比值发生改变。研究区具有高Nb/Ta比值的TTG可能来源于高Nb/Ta比值基性下地壳部分熔融,并继承了源区高Nb/Ta比值的特征。通过本文研究揭示,高Nb/Ta比值的TTG并非一定形成于俯冲带洋壳榴辉岩相部分熔融,下地壳富角闪石和黑云母的岩石部分熔融是形成高Nb/Ta比值TTG的一种重要途径。

The Archean continental crust predominantly consists of Tonalite-Trondhjemite-Granodiorite(TTG)associations.Nb/Ta variations in TTG are not only related to their petrogenesis,but also associated with Archean crustal differentiation and tectonics on early Earth.In this study,we provide detailed the whole-rock(WR)major and trace elements,and trace element analysis of biotite and amphibole in both TTG and mafic enclaves from the Yinshan Block.Amphiboles from the granodiorite and co-genetic mafic enclave have similar Mg^(#)values but clearly different Nb/Ta ratios.Higher Nb/Ta ratios are in the amphiboles from the granodiorites than those from the mafic enclaves(38~70 vs.30~50).Amphiboles in the mafic enclaves have higher Ta and Cr contents but lower Nb/Ta ratios,in comparison with those from the host granodiorite.Nb/Ta ratios in amphibole decrease with the increasing of Cr contents.The WR geochemical data show that TTGs from the Yinshan Block have high Nb/Ta ratios of 13~65(av.31),and the mafic enclaves have variable Nb/Ta ratios of 10~56.As the average Nb/Ta ratio of the basalts from the Archean greenstone belt is about 15,variable Nb/Ta ratios of amphibolite and mafic granulite from the Yinshan Block reflect that they must have experienced partial melting or later metasomatism in the lower crust.Partial melting of high Nb/Ta amphibolites and granulites may produce high Nb/Ta TTG melts,as the melts would inherit high Nb/Ta ratios from their source.Therefore,we suggested that high Nb/Ta TTG is not necessarily formed through high-pressure eclogite-facies partial melting of the hydrous oceanic crust in the subduction zone.Partial melting of the biotite/amphibole-rich residues is one of the crucial mechanisms for producing high Nb/Ta TTG.