Delineation Of The Early Neoarchean (2.75 To 2.70 Ga) Crustal Growth And Reworking Processes In The Southeast Base Of Taihang Mountain, North China Craton

Numerous studies suggest that various secular geologic and geochemical transitions occurred between similar to 3.2 Ga and 2.5 Ga. During this age-window, the similar to 2.70 Ga tectono-thermal event of the Neoarchean is by far themost influential, and is unusual in terms of coeval mafic-felsic magmatic rocks which were interpreted to reflect widespread crustal accretion. Here we report the early Neoarchean TTG and potassic granite association preserved in the Yunmengshan area, Taihang Mountain, North China Craton (NCC). TTG and potassic granites emplaced at 2712 +/- 65 to 2644 +/- 25 Ma, followed by similar to 2.57 to 2.50 Ga metamorphism and partial anatexis. TTG has geochemical features corresponding to high-Al medium-pressure (MP) TTG. Their high CaO, low to moderate Al2O3/(FeOT + MgO), and positive epsilon(Hf) (t) values (+3.8 to +7.2) with T-DM2 of 2.96 to 2.75 Ga, indicate that they formed by partial melting of juvenile low-K mafic rocks, representative of crustal accretion. Besides, TTG can be subdivided into two groups based on Eu anomalies. Thosewith Eu/Eu*< 1.0 underwent advanced amphibole and plagioclase fractionation. The possible reason for theirweakly negative Eu anomalies is that plagioclase has negated the effects of amphibole fractionation which is theoretically accompanied by plagioclase removal. Those with positive Eu anomalies accord with "slab-melt" identification criterion (Sr > 300 ppm plus elevated Sr/Y> 40, (La/Yb)(N)> 12 and Eu/Eu*> 1.0). The high Eu/Eu*, Sr/Y and lowYb probably represent the contribution of amphibole fractionation and plagioclase accumulation during the magma evolution. The evidence favors an arc-related setting for the TTG. The early Neoarchean potassic granites are monzogranite to syenogranite, and formed shortly after TTG. They belong to high-K calc-alkaline to shoshonitic I-type granite, and display depleted and concave-upward REE patterns between middle and heavy REEs and higher Zr/Sm ratios (69.6 on average) than TTG (45.4). The calculated eHf (t) values are mainly positive (+1.4 to +7.0) with T-DM2 from 3.03 Ga to 2.72 Ga, together with low Al2O3 /(FeOT + MgO), low to moderate CaO and high K2O/Na2O ratios, indicating a high-K mafic crustal source with metapelite involvement. Meanwhile, we reviewed geochemical data of the early Neoarchean TTGs published in the NCC and other cratons abroad. The results show that samples from Trans-North China Orogen in NCC have lower Y contents, higher Sr/Y and (La/Yb)(N) ratios, and more depleted HREE than those from Eastern Block, which could be interpreted to reflect an increase in the depth of melting. Alternatively, according to previous studies and comparison globally, elevated Sr/Y values could also reflect increased consumption of plagioclase in melting reactions due to higher temperatures, or magma fractionation. Therefore, it mostly lacks a clear distinction between the "arc" and "non-arc" settings based on geochemistry. However, many researchers put forward evidence that subduction was well attested in the late Archean (3.0 to 2.5 Ga), but it might be unstable. (C) 2020 Elsevier B.V. All rights reserved.