Garnet and zircon petrochronological study of the garnet clinopyroxenite from Eastern Hebei terrane，North China Craton——no evidence for Archean eclogite-facies metamorphism？

Plate tectonics represents a forefront of research in earth sciences, with the establishment of modern-style plate tectonics remaining one of the key unresolved issues. Debates continue about its initiation, with estimates ranging from the Hadean to the Neoproterozoic or Cambrian periods, spanning nearly Earth's entire history. The North China Craton (NCC), as one of the world's oldest cratons, encapsulates a geological history from 3.8 to 1.8 billion years ago, offering valuable insights into early Earth's evolution. Recent work by Ning et al. (2022) highlighted eclogite-facies garnet clinopyroxenite in the Eastern Hebei terrane of the NCC, dated at over 2.47 billion years, suggesting the presence of modern-style plate tectonics by the end of the Archean era.

To further investigate the timing and metamorphic processes of the garnet clinopyroxenite, we conducted a comprehensive garnet and zircon petrochronological study. The garnet clinopyroxenite is primarily composed of clinopyroxene and garnet, with accessory minerals including plagioclase, quartz, titanite, magnetite, and ilmenite. Garnet compositional profiles show minimal chemical zonation, with a gradual decrease in X_Prp (0.27–0.23) from core to rim, indicative of diffusional resorption driven by retrogression. Conventional thermobarometric calculations suggest peak metamorphic conditions between 761–833°C and 1.0–1.1 GPa, not reaching eclogite-facies. Zircon analysis revealed depleted LREE, positive Ce, and negative Eu anomalies, with most zircons indicating a metamorphic age of 2478 ± 12 Ma. A subset of zircons provided a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1770 ± 11 Ma, displaying flat HREE patterns potentially coexisting with garnet. U–Pb data suggest the Eastern Hebei terrane's garnet clinopyroxenite records Neoarchean (ca. 2.50 Ga) and Paleoproterozoic (ca. 1.8 Ga) metamorphic ages. However, the diverse range of zircon U–Pb ages and their ambiguous textural associations complicate the correlation of P–T conditions with specific timeframes. Thus, the peak metamorphic conditions of garnet clinopyroxenite, as estimated by conventional thermobarometry, cannot be conclusively dated to either ca. 2.50 Ga or ca. 1.8 Ga. This ambiguity limits the evidence for Archean eclogite-facies metamorphism. Future garnet-whole rock Sm–Nd/Lu–Hf isochron analysis may offer more definitive dating of metamorphic events and enhance understanding of the garnet clinopyroxenite's thermal history.