Integrated garnet and zircon petrochronology of corona formation in highgrade rocks from granulitefacies 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 LuHf and Sm-Nd alongside zircon UPb 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 texturepredominantly garnet, with minor clinopyroxene and quartzformed 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. LuHf dating of garnet gave an age of 1864 3 Ma, significantly younger than the metamorphic zircons' UPb 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 LuHf date reflects the period of garnet growth, approximating the time of anhydrous corona assembly formation. The UPb 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 SmNd dating, yielding an age of 1817 6 Ma, aligns with the amphibole-biotite LuHf date of 1819 16 Ma, indicating a gap of approximately 47 Myr from the garnet LuHf date. This suggests a phase of rapid growth followed by fast cooling. The coinciding ages of garnet SmNd and amphibole-biotite SmNd and LuHf, 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 LuHf and SmNd dating, is particularly effective for rocks with explicit textures, enabling precise extraction of temporal intervals in metamorphic processes.