Initiation of Mesoarchean nascent subduction zone in the North China Craton

Exploring the nature of Archean plate tectonics is crucial for understanding the geodynamic evolution of early Earth. While previous studies have focused on (ultra-)mafic metavolcanic and tonalite−trondhjemite−granodiorite (TTG) rocks, the crust-mantle interactions of Archean intermediate magmatism have received much less attention. Mesoarchean (ca. 2936−2855 Ma) dioritic and tonalitic-trondhjemitic magmatism occurred in the Jiaobei terrane, North China Craton. While low-silica (65.67−68.82 wt%) TTG magmatism persisted throughout the entire magmatic period, both the high-silica (69.79−74.54 wt%) TTG and biotite dioritic (MgO of 1.52−2.45 wt%) magmatism were transient (ca. 2922−2901 Ma). They were mainly sourced from metabasaltic crust, though the biotite dioritic and low-silica TTG rocks show variable mantle input. In comparison, the hornblende dioritic magmatism (MgO of 2.07−5.00 wt%) occurred later (ca. 2912−2865 Ma) and was derived from melt-metasomatized mantle sources. Spatially, the magmatism became younger from northeast to southwest. Combined with increasing crustal melting pressures after ca. 2920 Ma and mildly elevated zircon δ18O (up to +6.8‰) of TTG rocks, a Mesoarchean nascent subduction zone is proposed for the Jiaobei terrane that is featured by the recycling of supracrustal basaltic rocks to thickened lower crust (forming high-pressure and high-δ18O TTG) and subsequent melt-related mantle metasomatism (forming relatively high-Mg hornblende dioritic rocks). The recycled supracrustal materials may have lubricated the basal part of the thickened crust and then triggered the opening of a nascent mantle wedge. Further compilation of data from global Archean dioritic and TTG rocks reveals chemical discontinuities at ca. 3.6−3.5 Ga and after ca. 3.0 Ga; these periods are marked by the occurrence of both high-Mg (median MgO >3 wt%) dioritic magmatism and high-(La/Yb)N (median >30) and high-δ18O (median >5.9‰) TTG magmatism. These data, along with independent geological evidence, suggest that the nascent subduction processes may have initiated locally at ca. 3.6−3.5 Ga but globally after ca. 3.0 Ga.