Neoarchean subduction unveiled: Discovery of 2.5 Ga Super-Wet magmas in the Zanhuang Complex, Trans-North China Orogen

Abstract

<p>Neoarchean lithologies of the North China Craton (NCC) provide essential insights into early continental evolution. This study investigates newly identified dioritic gneisses from the Zanhuang Complex in the central Trans-North China Orogen (TNCO) using zircon U-Pb geochronology, whole-rock geochemistry, and Nd-Hf-O isotopes. The rock types include Luzhuang tholeiitic gabbro-dioritic (2493 ± 6 Ma) and quartz dioritic (2498 ± 7 Ma) gneisses, Longmen tholeiitic garnet-bearing dioritic (2493 ± 6 Ma) and quartz dioritic (2498 ± 8 Ma) gneisses, and Shicao calc-alkaline dioritic gneisses, which are intruded by 2500 ± 18 Ma garnet-bearing anatectic granite vein. Geochemical analysis reveals relatively low MgO concentrations and Mg# values, indicating partial melting of mafic rocks. The dioritic gneisses show positive whole-rock ε_Nd(t) values (+1.9 to + 4.9) and zircon ε_Hf(t) values (+2.0 to + 10.0), suggesting a mantle-derived origin. Oxygen isotope analysis indicates zircon δ¹⁸O values ranging from 5.9 to 7.7 ‰ (average 7.0 ‰), which are higher than typical mantle zircons, indicating a metasomatized mantle source. Hygrometric calculations show 6.2–6.8 wt% H₂O in Luzhuang dioritic gneisses and 12.4–14.5 wt% H₂O in Longmen garnet-bearing varieties. The exceptionally high water content in Longmen dioritic gneisses (super-wet magmas) further supports the hypothesis of a subduction-related metasomatic process in the mantle source. These findings provide definitive evidence for Neoarchean subduction processes.<br></p>