Neoarchean to Paleoproterozoic tectonic evolution of the Khondalite Belt in the North China Craton: Constraints from ca. 2.5 Ga and ca. 2.1 Ga charnockites in Zhuozi

Abstract

The Khondalite Belt (KB), a Paleoproterozoic collisional orogen in the North China Craton (NCC), lacks a well-established pre-collisional tectonic evolution due to the paucity of late Neoarchean to middle Paleoproterozoic magmatic rocks. This study focuses on the poorly dated charnockites in the Zhuozi area, the eastern segment of the KB. Through an integrated field investigation, petrography, SHRIMP and LA-ICP-MS zircon U-Pb dating, zircon Lu-Hf isotopic analysis, and whole-rock geochemistry, this study determines the formation ages, petrogenesis, and tectonic settings of the targeted charnockites, and further constrains the Neoarchean to Paleoproterozoic tectonic evolution of the KB. The charnockites crystallized at ca. 2.5 Ga and ca. 2.1 Ga and were metamorphosed at ca. 1.9 Ga. The ca. 2.5 Ga charnockites are magnesian and exhibit high Ba-Sr affinity, with SiO2, MgO contents and Mg# values of 66.20–71.43 wt%, 1.10–2.34 wt% and 34–49, respectively. Most samples are enriched in light rare earth elements (LREEs), Ba, Sr (Sr/Y = 11.79–34.94), and Eu (Eu/Eu* = 0.56–1.67), but depleted in heavy rare earth elements (HREEs, LaN/YbN = 10.14–42.81), Th, Nb, Ta, P, and Ti. The magmatic zircons have εHf(t) ranging from + 2.8 to + 5.8 and Hf model ages (TDM2) of 2711–2822 Ma. In comparison, the ca. 2.1 Ga charnockites are ferroan and exhibit A-type granite affinity, with higher SiO2 (73.23–77.73 wt%) but lower MgO (0.03–0.27 wt%) contents and Mg# values (2–13). Most samples are enriched in LREEs, Rb, Zr, and Hf, but depleted in HREEs (LaN/YbN = 12.72–23.41), Nb, Ta, Sr (Sr/Y = 1.51–5.58), P, Ti, and Eu (Eu/Eu* = 0.24–1.11). The magmatic zircons have εHf(t) ranging from −4.0 to +0.8 and TDM2 of 2749–2916 Ma. The available data suggest that the two episodes of charnockites are homologous (Mesoarchean-Neoarchean TTGs and related intermediate-mafic rocks), with ca. 2.5 Ga and ca. 2.1 Ga charnockites formed at relatively high-pressure (subduction-collision) and low-pressure (intracontinental rifting) conditions, respectively. Combined with previous studies, the precursor of the KB may experience a subduction-collision process at ca. 2.6–2.45 Ga, followed by long-term intracontinental rifting events at ca. 2.45–2.0 Ga prior to the final subduction-collision at ca. 2.0–1.9 Ga.