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

Abstract

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°C), showing typical characteristics of aluminous A-type granites. The samples display characteristics of relatively enriched zircon Hf isotopes (εHf(t) = −5.6 to + 2.7), whole-rock Nd isotopes (εNd(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 (δ¹⁸O = 4.7 to 7.4 ‰) 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(fO₂) = −14.9 to − 7.8) and biotite (log(fO₂) = −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.