Neoarchean (2.7–2.8 Ga) accretion beneath the North China Craton: U–Pb age, trace elements and Hf isotopes of zircons in diamondiferous kimberlites

Zircon xenocrysts from the Early Paleozoic Mengyin and Fuxian diamondiferous kimberlites, on opposite sides of the translithospheric Tanlu fault in the North China Craton, have been used to probe the deep crust. Based on detailed studies of external forms and internal structures, the zircons can be roughly divided into magmatic and metamorphic zircons. There are no consistent differences in U, Th or Th/U between the two zircon populations. The trace-element affinities of the igneous zircons vary widely from mafic, through intermediate, to felsic rocks with minor populations similar to zircons from carbonatite and syenite. In situ isotopic analysis for U–Pb and Lu–Hf reveals that all zircons both from Mengyin and Fuxian kimberlites, regardless of their chemical affinities, have similar formation ages (2.5–2.7 Ga) and depleted mantle model ages (TDM, 2.7–2.8 Ga). These ages are also similar within error to those of zircons in mafic granulite xenoliths from the Fuxian kimberlites, implying that the lower crust was similar (Neoarchean cratonic basement) beneath the Mengyin and Fuxian areas, but the latter experienced stronger thermal modification in Proterozoic time. The magmatic zircons may be derived from late differentiates of underplated mafic magmas with some contamination by crustal components, and the metamorphic populations developed during heating events linked to the ongoing underplating. The addition of juvenile material (up to + 9.5 εHf) at this time (2.7–2.8 Ga) corresponds to a known major episode of global crustal formation.