Integrated garnet and zircon petrochronology of corona formation in high‐grade rocks from granulite‐facies rocks in the North China Craton

Corona textures in high-grade metamorphic rocks, characterized by specific mineral assemblages, have been extensively studied, yet their direct dating remains a challenge. In an effort to ascertain the formation time and duration of corona textures, we undertook a comprehensive petrochronological study, utilizing garnet Lu–Hf and Sm-Nd alongside zircon U–Pb dating, on a corona-bearing granulite from the Miyun area in the North China Craton. This region, situated at the intersection of the Archean Eastern Block and the Paleoproterozoic Trans-North China Orogen, provides valuable insights into polymetamorphic processes.

Through pseudosection modeling and conventional thermobarometry, we determined that the corona texture—predominantly garnet, with minor clinopyroxene and quartz—formed at approximately 0.9 GPa and 740 °C. This was followed by retrogression at ~0.5 GPa and 670 °C, leading to the growth of amphibole and biotite. Lu–Hf dating of garnet gave an age of 1864 ± 3 Ma, significantly younger than the metamorphic zircons' U–Pb age of ca. 2454 Ma. This discrepancy suggests a disequilibrium in rare earth elements (REE) between garnet and zircon. Comparative analysis of REE concentrations in zircon and garnet, along with experimental data and lattice strain modeling, indicated that zircons deviated from the equilibrium array, implying their (re)crystallization independent of garnet. The preservation of Lu zoning in garnet suggests that the Lu–Hf date reflects the period of garnet growth, approximating the time of anhydrous corona assembly formation.

The U–Pb zircon age of ca. 2454 Ma indicates an earlier, distinct Archean metamorphic event, which is a common feature in high-grade metamorphic rocks throughout the Eastern Block of the North China Craton. The garnet Sm–Nd dating, yielding an age of 1817 ± 6 Ma, aligns with the amphibole-biotite Lu–Hf date of 1819 ± 16 Ma, indicating a gap of approximately 47 Myr from the garnet Lu–Hf date. This suggests a phase of rapid growth followed by fast cooling. The coinciding ages of garnet Sm–Nd and amphibole-biotite Sm–Nd and Lu–Hf, given their different closure temperatures, point to more than mere cooling, indicating the formation of anhydrous coronae around 1.86 billion years ago, followed by hydrous retrogression ca. 1.82 Ga.

The presence of both Paleoproterozoic and Archean ages in the Miyun granulites correlates with significant tectonic episodes in the Trans-North China Orogen and the Eastern Block, respectively. This correlation suggests that the Miyun region experienced overlapping metamorphic events associated with both these tectonic units. Our approach, integrating Lu–Hf and Sm–Nd dating, is particularly effective for rocks with explicit textures, enabling precise extraction of temporal intervals in metamorphic processes.