Late Neoarchean high-grade regional metamorphism in the eastern North China Craton: New constraints from monazite dating in northern Liaoning

Abstract

Late Neoarchean tectono-thermal events in the eastern North China Craton have been comprehensively characterized in terms of mineralogy, petrology, geochemistry, metamorphic evolution, geophysics and geochronology. The Late Neoarchean metamorphic event is debated to correlate with vertical and/or horizontal tectonism, potentially an indicator as initiation of plate tectonics. With a growing number of zircon geochronological investigations, the precise timing of the Late Neoarchean metamorphic event should be questioned due to the complexity of post-Archean multiple tectono-thermal overprints. Here we demonstrate the capabilities of LA–ICP–MS U–Pb and LA–MC–ICP–MS Sm–Nd isotope measurements applied to monazite endowed with LREE-enriched geochemical peculiarities as well as zircon U–Pb–Hf isotope, to reevaluate the Late Neoarchean metamorphic event in northern Liaoning, eastern North China Craton. Monazites within the biotite two-feldspar gneiss from the Qingyuan greenstone belt are Ce-monazite in composition, with negligible amounts of the huttonite and brabantite components. The internally homogeneous and anhedral rounded granular monazites show textural equilibration in contact with plagioclase, quartz, and K-feldspar, indicating that monazite is a product of metamorphism. The apparent ages and Eu/Eu* values of monazite analyses show a positive correlation, which confirms the continuous formation of monazite and coeval feldspar during the metamorphic evolution. These metamorphic monazites yield a weighted mean 207Pb/206Pb age of 2493 ± 8 Ma and an upper intercept age of 2496 ± 8 Ma, consistent with the metamorphic zircon dates of 2492–2468 Ma. These results, along with monazite Sm–Nd isochron of 2459 Ma, thus provide a critical clue to understand the Late Neoarchean high-grade regional metamorphism in the eastern North China Craton. As a consequence, the Qingyuan greenstone belt was only partially influenced by the Paleoproterozoic tectono-thermal events, not largely as previously thought by some workers.