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

Zi Jianwei; Li Lixing; Li Yike; Rasmussen Birger; Ke Changhui

doi:10.3799/dqkx.2023.027

Abstract：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 emplace-ment age of the carbonatite dykes is critically important for understanding the genesis and geodynamic drivers of the giant REE de-posits.Dating carbonatites has proven challenging because they are commonly overprinted by later tectono-thermal events.Previ-ous 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 con-firms 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 su-percontinent(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 anal-ysed 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 geo-dynamic drivers and genesis of the giant REE deposits.