Meso- to Neoarchean geodynamic transition of the North China Craton indicated by H2O-in-zircon for TTG suite

Tonalite-trondhjemite-granodiorite (TTG) suites constitute nuclei of many ancient cratons, holding a key to reveal how continental crust formed and evolved in the early Earth. Archean to Paleo-Proterozoic TTG rocks are widely reported in the North China Craton (NCC), and have been widely used to infer continental crust growth and reworking. Previous whole-rock geochemical studies on these rocks had led to the proposal of Neoarchean subduction onset and the establishment of the Meso- to Neoarchean thermal state for this craton. However, whole-rock geochemical compositions can be modified by post-magmatic alternation or metamorphism. The content of water in zircon (a resistant mineral) have the potential to characterize the hydrous state of magma. Here, in order to track the NCC geodynamic evolvement from the Mesoarchean to the Paleoproterozoic, we measured the H2O-in-zircon content with a newly developed low background secondary ion mass spectrometer (SIMS) method for a Neoarchean-Paleoproterozoic (2.8–2.1 Ga) TTG suite in the central NCC. Our results show an abrupt zircon water content rise at the Paleoproterozoic-Neoarchean boundary (∼2.5 Ga), from ∼100 ppm for ∼2.8 Ga TTG rocks to > 1000 ppm for ∼2.5 and ∼2.1 Ga TTG rocks. In addition, zircon elemental ratios (e.g., U/Yb and Nb/Yb) also show a markable magmatic provenance change at the same time from MOR (mid-ocean-ridge)-type to arc-type. We link these changes to the initiation of plate subduction in the North China Craton.