Zircon U-Pb-Hf isotopes and geochemistry of Neoarchean TTG gneisses in the Guyang area of the Yinshan Block: Constraints on petrogenesis and tettonic implications

As a well-preserved Archean basement, the Yinshan Block in North China Craton preserved large numbers of Neoarchean rock assemblages, including the supracrustal rocks, tonalitic-trondhjemitic-granodioritic gneisses (TTG gneisses), K-rich granitoids and diorites (sanukitoids). Previous studies have been carried out on the Neoarchean TTG gneisses in this block, but the petrogenesis and tectonic implications of the TTG gneisses are still controversial, which greatly hinders the understanding of the formation and evolution of the Neoarchean continental crust in this region. In this study, a new TTG gneiss assemblage has been identified in the granite-greenstone belt of western Guyang, Inner Mongolia, and its petrology, geochronology and petrogeochemistry have been studied. The LA-ICP-MS zircon U-Pb dating results indicate that the formation age of the TTG gneiss assemblage is 2550 similar to 2524Ma; while the Lu-Hf isotope analysis results show an epsilon(Hf)(t) value ranging from +0.54 to +4.96, and a t(DM)(c) model age ranging from 2985Ma to 2715Ma. These ages are similar to the magmatic age (2.7Ga) of the oldest trondhjemite gneiss in the Yinshan Block, suggesting that they may have originated from partial melting of a newly formed mafic crust at 2.7Ga. The whole-rock geochemical analysis results show that the TTG gneiss assemblage has high SiO2 (63.32%similar to 70.80%) and Al2O3 (15.19%similar to 16.55%) contents, as well as high Sr (410x10(-6)similar to 1532x10(-6)) and low Y (2.61x10(-6)similar to 14.04x10(-6)) contents with relatively high (La/Yb)(N) (10.31 similar to 56.85) and Sr/Y (44.32 similar to 349.1) ratios. The light rare earth elements (LREEs) and Large-ion Lithophile Elements (LILEs) of the gneiss are relatively enriched, which are similar to the geochemical characteristics of the typical adakitic rocks. In addition, the TTG gneiss assemblage has relatively low MgO (0.60%similar to 2.60%), Ni (3.12x10(-6)similar to 11.12x10(-6)), Cr (3.77x10(-6)similar to 18.11x10(-6)) contents and Mg-# (34.88 similar to 53.45), indicating that it is originated from the adakitic magma of a thickened continental crust. Therefore, the Neoarchean TTG gneisses from Guyang area may have been formed by partial melting of a thickened mafic lower crust during 2.55 similar to 2.52Ga. Based on the previous studies, it is further hypothesized that the Yinshan Block underwent two periods of magmatic events during the Neoarchean era: the first magmatic event occurred at 2.7Ga, forming a thickened mafic crust as well as a small amount of similar to 2.7Ga TTG rocks; while the second magmatic event occurred in 2.55 similar to 2.50Ga, forming an extensive area of Neoarchean TTG rocks and other granitic rock associations in the Yinshan Block. The geodynamic mechanism dominated by mantle plume can better explain the partial melting of the thickened mafic lower crust and the formation of TTG magmatic activities.