Petrogenesis of two episodes of Neoarchean TTG gneisses (-2.72 Ga and 2.56-2.54 Ga) from the Zhongtiao Mountains, North China Craton: Implications for crustal evolution

The petrogenesis of tonalite-trondhjemite-granodiorite (TTG) suites can provide key clues for the formation and evolution of continental crust. Neoarchean TTG gneisses are widely distributed in the Zhongtiao Mountains (ZTM), North China Craton (NCC). In this contribution, to better understand Neoarchean crustal evolution of the NCC, two discrete episodes of TTG gneisses (similar to 2.72 Ga and similar to 2.56-2.54 Ga) from the ZTM are investigated. New results approve that the two episodes of TTG gneisses are all characterized by high SiO2, Na2O contents, (La/Yb)(N) and Sr/Y values, but low K2O/Na2O ratios. These geochemical features are similar to the Archean TTG suites and Phanerozoic high-SiO2 adakites, suggesting that they were both derived from partial melting of mafic rocks. The characteristics of trace elements (e.g., La/Yb, Sr/Y, Y/Yb, (Ho/Yb)(N) and Nb/Ta ratios) imply that both of them correspond to MP-TTGs, and garnet-amphibole exist in the residue phase. The similar to 2.72 Ga TTGs show high Mg-# values (ave. = 51), Cr (4.87-104 ppm) and Ni (6.37-55.7 ppm) contents, and negative Nb, Ta anomalies. Some high Nb/Ta ratios (up to 17.6) imply that rutile might be involved in the residual phase, which should require a higher pressure. Zircons from these rocks yield positive epsilon(Hf)(t) values of +4.0 similar to +6.8 (ave. = +5.6), with two-stage Hf model ages (T-DM2) of similar to 2.85-2.70 Ga, indicating a juvenile basaltic crustal source. Hence, we interpret that similar to 2.72 Ga TTGs in the ZTM were derived from partial melting of mafic rocks, and more likely related to subduction process. In contrast, the similar to 2.56-2.54 Ga TTGs have lower Mg-# values (39-54, mostly <45), Cr (6.02-22.8 ppm) and Ni (4.93-15.7 ppm) contents. They exhibit more evolved zircon Hf (average epsilon(Hf)(t) = +3.21) and whole-rock Nd (epsilon(Nd)(t) = +1.87) isotopic compositions, and their T-DM2 ages are concentrated at similar to 2.85-2.68 Ga. These similar to 2.5 Ga TTGs in the ZTM show similar geochemical features with contemporary TTGs in the XQL, DF and EZT of the southern NCC, identical to melts from partial melting of mafic rocks under a thickened lower crust. The similar to 2.5 Ga TTGs, A-type granites and mafic rocks in the ZTM jointly indicate a complicated crust-mantle process at different depths related to mantle plume activity during the late Neoarchean. Based on comparison of the spatial-temporal distribution and geochemical features of similar to 2.7 Ga and similar to 2.5 Ga TTGs in the NCC, we propose that Neoarchean crustal evolution of the ZTM is similar to the whole NCC, which recorded a significant crustal growth event at similar to 2.7 Ga, and an important crustal reworking event associated with some juvenile crustal growth at similar to 2.5 Ga.