Geochronology and geochemistry of ca. 2.48 Ga granitoid gneisses from the Yudongzi Complex in the north-western Yangtze Block, China

The Early Precambrian magmatism in the Archean-Palaeoproterozoic basement Yudongzi Complex is critical for understanding the early crustal formation and evolutionary history of the Yangtze Block in the South China Craton. In this paper, combined zircon UPb geochronological, zircon Hf isotopic, and whole-rock geochemical studies are carried out on the granitoid gneisses from the Yudongzi Complex, north-western Yangtze Block. The magmatic zircons from the granitoid gneiss yield a UPb upper intercept age of 2,477 +/- 18Ma, interpreted as the crystallization age of the granitoid gneiss. The granitoid gneiss samples are characterized by high Al2O3 contents, moderate to high Sr/Y, (La/Yb)(N) and Na2O/K2O ratios, no obvious Eu and Sr anomalies, and depleted Nb, Ta, and Ti values that are similar to those of Archean trondhjemite-tonalite-granodiorite (TTG) suites, indicating that the investigated rocks belonging to TTG-series rocks. They display pronounced enriched-LREE patterns with high La/Yb values and relatively high Zr/Sm and Eu/Eu* but low Nb/Ta and Dy/Yb ratios, implying that these rocks were formed by partial melting within amphibolite-facies conditions with mainly residual amphibole, garnet, and accessory rutile in the magma source. These rocks also display apparently low MgO, Mg-#, Ni, and Cr contents, suggesting derivation from partial melting of thickened ancient lower crust. Zircon epsilon(Hf)(t) values from the granitoid gneiss sample vary from -10.1 to -6.9 with T-DM2 of ca. 3.59-3.40Ga, what demonstrates their crustal source nature as well. Thus, taking all these factors together, we propose that the ca. 2.48Ga granitoid gneisses in the Yudongzi Complex might have been produced by partial melting of ancient crust material under amphibolite-facies conditions, with primarily residual assemblages of amphibole, garnet, and attached rutile. Taking together our new data and such from previous studies, the Yangtze Block has been probably involved into a significant reworking of ancient crust event during the Early Palaeoproterozoic time.