Petrogenesis and tectonic implications of the late Paleoproterozoic A-type rhyolites at the southwestern North China Craton

Abstract

The late Paleoproterozoic A-type granitoids are important for constraining of the lithotectonic and geodynamic processed involved in the creation of continental crust. The newly identified late Paleoproterozoic felsic extrusive rocks from the southwestern margin of the North China Craton (NCC) consist of rhyolites which show melting corrosion structure, autoclastic texture and fiber structure of mineral grains and have weighted mean SIMS zircon UPb ages of 1794.7 ± 3.6 Ma to 1780.3 ± 5.6 Ma. The rhyolite samples show high SiO2 (67.59–74.38 wt%) and K2O + Na2O (3.68–11.10 wt%), and low MgO (0.05–1.17 wt%), CaO (0.19–2.58 wt%) and P2O5 (0.08–0.16 wt%). They display aluminous A-type granite/rhyolite geochemical compositions, such as high A/CNK (0.78–1.23), FeOT/(FeOT + MgO) (0.81–0.99) and 10000Ga/Al (2.32–4.03) ratios, and high Zr + Nb + Ce + Y (824–1272 ppm) concentrations, as well as high zirconium saturation temperatures (TZr = 873–964 °C). These signatures, along with their low εHf(t) (−3.6 to −1.5) and εNd(t) values (−4.31 to −3.31) and old two-stage Hf model ages (TDMC = 2.70–2.57 Ga), indicate that the rhyolites were derived from partial melting of the Neoarchean felsic basement rocks. These volcanic rocks also have low zircon δ18O values ranging from 1.5‰ to 6.8‰, which were inherited from the Neoarchean 18O-depleted basement rocks that underwent high-T hydrothermal alteration, rather than resulted from contamination or later high-T water/rock interaction. The rhyolite rocks in this study, the earliest igneous rocks in the southwestern NCC after the Paleoproterozoic consolidation of the crystalline basement, were formed in a rift setting, marking the initial breakup of the Guyuan Rift in the southwestern NCC. The rhyolite rocks in combination with the Xiong'er volcanic rocks in the southern NCC mark the initial rifting of the NCC (Xiong'er Rift and Guyuan Rift) after cratonization (~1.85–1.8 Ga). The youngest retrograde metamorphism (1.80–1.79 Ga) in the NCC indicates that the transition from post-orogenic to intracontinental rift setting occurred at ca. 1.79 Ga.