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7473813 
Journal Article 
Nb/Ta variations in Archean TTG: Implications for "Na-Ta paradox" 
Wang, D; Guo, JH; Ma, XD 
2021 
1 
Yanshi Xuebao / Acta Petrologica Sinica
ISSN: 1000-0569 
37 
2 
341-355 
Chinese 
CCC:000629649100002 
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 similar to 70 vs. 30 similar to 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 similar to 65 (ay. 31), and the mafic enclaves have variable Nb/Ta ratios of 10 similar to 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. 
Physical Sciences - Other Topics; Archean, TTG, Lower crust, Nb/Ta ratio; north china craton, trondhjemite-granodiorite ttg, early; continental-crust, xi ulanbulang area, yinshan block, metamorphic rocks,; trace-elements, fractionation, constraints, evolution