Late Archean K-rich intermediate magmatism driven by deep supracrustal recycling

Archean K-rich intermediate rocks hold important information of crust-mantle geodynamic evolution of the early Earth. However, the nature of their magma sources and thermodynamic origin remain enigmatic. Two distinct groups of late Neoarchean (~2551–2498 Ma) K-rich intermediate rocks were identified in the North China Craton. The high-Mg group is compositionally similar to low-Ti sanukitoids, and includes mostly quartz diorites and monzonites, which show high MgO (2.5–6.7 wt%) and Mg# (38.6–65.1), but low TiO2 (0.4–0.9 wt%), with zircon ƐHf(t) of +2.3 to +7.4 and δ18O of +5.2 ‰ to +6.5 ‰. In comparison, the high-Fe-Ti group rocks are titanite-bearing monzonites and compositionally similar to the very rare high-Ti sanukitoids that were previously only found in the Kaapvaal and Superior cratons. They show higher TiO2 (1.1–1.6 wt%) and Fe* (FeOT/(FeOT + MgO), 0.75–0.86), lower Mg# (22.9–37.5), with lower zircon ƐHf(t) (+1.3 to +5.5) but heavier δ18O (+5.6 ‰ to +7.1 ‰). Zircon HfO isotopic and thermodynamic modeling suggest that the two groups of intermediate rocks were derived from variably metasomatized mantle sources. In detail, the high-Mg rocks originated from a shallow mantle source (~2 GPa) metasomatized by ~15% sediment melts and ~ 25% oceanic crust melts. In contrast, the high-Fe-Ti rocks were differentiated from a deeper (~3–4 GPa) mantle source metasomatized by ~30% sediment melts. Importantly, our modeling data reveal a critical control of melting pressures on the TiO2 contents of intermediate melts from metasomatized mantle sources. Therefore, the scarcity of high-Ti sanukitoids may thus indicate very rare deep subduction in the Archean. Combined with the coeval increase of K2O/Na2O, Sr + Ba, TiO2, and Fe* of global late Archean intermediate rocks, we propose that it is the enhanced mantle recycling of supracrustal materials that may have triggered late Archean voluminous and diverse intermediate magmatism, likely evolved under a gradually stabilized plate tectonic system.