Magmatic-Hydrothermal History of Bayan Obo REE-Rich Carbonatites Revealed by Monazite Th-Pb Geochronology of the Wu Dyke

Growing evidence suggests that mineralisation of the world-renowned Bayan Obo REE deposit is genetically linked to the intrusion of carbonatite dykes in Archean-Paleoproterozoic sedimentary rocks. Hence, accurate determination of the emplacement age of the carbonatite dykes is critically important for understanding the genesis and geodynamic drivers of the giant REE deposits. Dating carbonatites has proven challenging because they are commonly overprinted by later tectono-thermal events. Previous attempts to date the Bayan Obo carbonatites have yielded younger ages reflecting post-magmatic overprinting (typically 1 300 – 420 Ma). In this study, we collected samples from the No. 1 carbonatite dyke (Wu dyke) and its fenitised wall-rocks and carried out insitu (in polished thin section) U-Th-Pb dating of monazite by Sensitive High-Resolution Ion Microprobe (SHRIMP), which enables us to precisely reconstruct the magmatic and hydrothermal history of the dyke. Monazites in the calcite carbonatite, fenite and metasomatised quartz conglomerate are characterised by extreme depletion of U and high Th/U ratios. The oldest cluster of monazite analyses yielded a weighted mean ²⁰⁸Pb/²³²Th age of 1 401±39 Ma (MSWD=0.57, n=7), which is the oldest monazite age hitherto obtained from Bayan Obo, and is taken to approximate the onset of intrusion of the carbonatite dykes. This age confirms that the carbonatite magmatism at Bayan Obo predates the 1.33-1.30 Ga Yanliao Large Igneous Province, but coincides with the separation of North China Craton from the Western Australian Craton as part of the greater breakup of the Nuna/Columbia supercontinent (1 450-1 380 Ma). Our results also reveal repeated monazite growth during the late Mesoproterozoic through to the early Paleozoic (1 300-420 Ma) as widely documented in previous studies. Notably, our data define a prominent age peak at 529±17 Ma (MSWD=1.01, n=6) indicating Pan-African overprinting, supportive of recent reconstruction placing North China at the northern margin of Gondwana during the earliest Paleozoic. In addition, zircon crystals in the quartz conglomerate were also analysed in this study and yielded concordant ²⁰⁷Pb/²⁰⁶Pb dates between 2.1 Ga and 1.9 Ga, which are interpreted as detrital ages on the basis of crystal morphology and Th-U chemistry of the zircon. In summary, the new monazite geochronology data from the Wu dyke significantly refine the magmatic-hydrothermal timeframe of the Bayan Obo carbonatites, affording new insights into the geodynamic drivers and genesis of the giant REE deposits.