The Neoarchean Era, a crucial period for the growth of global continental crust, witnessed two significant magmatic events at ca. 2.7 Ga and 2.6−2.5 Ga. These events are of vital importance for comprehending the tectonic regime of early Earth. However, there exists considerable controversy regarding this geodynamic regime. Hence, to address this scientific puzzle, we conducted comprehensive investigations into the geochronological, geochemical, and Nd-Hf isotopic compositions of newly identified Neoarchean granitoids in the Yinshan block of the North China craton. Zircon U-Pb dating reveals that these granitoids were formed at 2.55−2.52 Ga. Monzogranitic gneisses exhibit high SiO2 and Al2O3 contents, along with high Sr/Y and LaN/YbN ratios. Meanwhile, they have low MgO, Cr, and Ni contents. Furthermore, they present variable εHf(t) (+3.5 to +7.8) and εNd(t) (+1.27 to +5.08) values, suggesting that they were derived from partial melting of tonalite-trondhjemite-granodiorite rocks, which themselves were formed by partial melting of earlier thickened (mafic) lower crust under high-pressure conditions. The dioritic gneiss shows moderate SiO2 content with comparatively high MgO, Al2O3, and Cr contents and relatively low (La/Yb)N ratios, showing geochemical affinity to Archean sanukitoids. Concurrently, they exhibit variable εHf(t) (+3.2 to +9.0) and εNd(t) (+3.18 to +4.14) values. Therefore, these dioritic gneisses were derived from partial melting of thickened (mafic) lower crust induced by upwelling mantle−derived mafic magma, while incorporating minor mantle-derived materials. Combined with previous studies, we propose that during the Neoarchean Era, significant crustal accretion likely occurred in the Yinshan block of the North China craton due to a synergetic interaction between diminished activity of the mantle plume and the concurrent initiation of plate tectonics.
Wang et al. (Fri,) studied this question.