Middle Paleoproterozoic tectonic evolution of the North China Craton: Constraints from the A-type granites in the Wutai area

The Paleoproterozoic tectonic evolution of the Trans-North China Orogen of the North China Craton (NCC) has long been controversial. One of the key factors is the difference in the understanding of the A-type granites in the middle Paleoproterozoic. In this study, whole-rock geochemistry and Nd isotope, zircon U-Pb and Hf-O isotope, and zircon H2O content are reported for middle Paleoproterozoic syenogranite, monzogranite, and alkali-feldspar granite in the Wutai area. LA-ICP-MS zircon dating results show that the intrusions have emplacement ages from 2117 to 2133 Ma. The 2.1 Ga granites in the Wutai area exhibit elevated SiO2 (72.20-78.01 wt%), K2O + Na2O (7.32-8.78 wt%) and Zr + Nb + Y + Ce (> 528 ppm) concentrations, high FeOt/(FeOt + MgO) (0.78-0.89), 10000*Ga/Al (> 3) and K2O/Na2O (1.0-1.9) ratios, low TiO2/MgO ratios (0.4-1.4), and significant depletion of Ba, Sr, Eu, as well as high zirconium saturation temperatures (785-892 degrees C), showing typical characteristics of aluminous A-type granites. The samples display characteristics of relatively enriched zircon Hf isotopes (epsilon Hf(t) = -5.6 to + 2.7), whole-rock Nd isotopes (epsilon Nd(t) = -2.9 to + 0.3), and old two-stage Hf model ages (2.56-3.05 Ga) similar to those of Neoarchean TTG gneisses in the same area. Furthermore, the oxygen isotope composition of zircon (delta O-18 = 4.7 to 7.4 parts per thousand) are lower than those of zircons from the A-type granites derived from meta-sedimentary melts, indicating that the A-type granites are predominantly derived from the partial melts of Archaean TTGs. These granites contain both ilmenite and magnetite, indicating a relatively oxidizing environment. Zircon (log(fO(2)) = -14.9 to - 7.8) and biotite (log(fO(2)) = -13.5 to - 11) also show high oxygen fugacity. The water content of zircon is 104-842 ppm, with an average of 414 ppm, showing a relatively wet environment. These oxidised and water-rich A-type granites are likely to form in a back-arc extensional setting. Combined with the published NCC data, the Trans-North China Orogen is inferred to be an Andean-type continental arc setting in the middle Paleoproterozoic, suggesting the possibility of continuous subduction in the Paleoproterozoic.