Petrogenesis of Two Types of Archean TTGs in the North China Craton: A Case Study of Intercalated TTGs in Lushan and Non-intercalated TTGs in Hengshan |
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| Authors: | SHAN Houxiang ZHAI Mingguo and DEY Sukanta |
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| Institution: | 1 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2 University of Chinese Academy of Sciences, Beijing 100049, China,1 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China and 3 Department of Applied Geology, Indian School of Mines, Dhanbad 826004, India |
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| Abstract: | TTG (Tonalite-Trondhjemite-Granodiorite) gneisses, a major component of Precambrian continental crust, play a significant role in understanding the process and mechanism of the crustal evolution in the early periods of the Earth. In terms of field occurrence, there are two kinds of Archean TTGs in the NCC (North China Craton): intercalated and non-intercalated TTGs. In this contribution, we make a comprehensive comparison of these two types of TTGs from the typical areas (Lushan and Hengshan) in the NCC with an aim to constrain their petrogenesis. The results suggest that they have similar mineral assemblages of Pl + Qtz + Bt ± Amp ± Kfs but different field appearances and geochemical compositions, thus probably reflecting different source materials and tectonic settings. Differences in the contents of characteristic elements, such as Sr, REE and HFSE, suggest that the non-intercalated TTGs in Hengshan were generated at deeper levels than those of intercalated TTGs in Lushan. Constraints from element contents and geochemical modeling results are consistent with derivation from dual sources involving both garnet amphibolite and rutile-bearing eclogite residues for the non-intercalated TTGs in Hengshan, whereas the compositions of intercalated TTGs in Lushan indicate that they were formed by partial melting with amphibolite to garnet-amphibolite residues. Moreover, accumulation of plagioclase is also required in the petrogenesis of intercalated TTGs in Lushan, at least for part of them. In addition, the non-intercalated TTGs in Hengshan display distinctly higher MgO, Mg#, Cr and Ni values and lower SiO2 average contents compared to the intercalated TTGs in Lushan. These features suggest that the former magma, at least a part, might have interacted with the mantle wedge during ascent. Considering all the above factors and in combination with the whole-rock Nd and zircon Hf isotopic data, it is suggested that the non-intercalated TTGs in Hengshan were produced by partial melting of subducted slab contaminated by the overlying mantle wedge at deeper levels and high pressures, whereas the intercalated TTGs in Lushan were generated by melting of the thickened lower crust at lower pressures and shallower depths. The tectonic settings of the two types of TTGs shed new light on the growth of the NCC. |
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| Keywords: | TTGs petrogenesis crustal growth Archean North China Craton |
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