Synchronous late Neoarchean Na- and K-rich granitoid magmatism at an active continental margin in the Eastern Liaoning Province of North China Craton

The appearance of voluminous K-rich granitoid rocks during Late Archean marks gradual maturation and stabilization of the continental crust. While most K-rich granitoid magmatism followed TTG magmatism, they occur synchronously sometimes, and this relationship is crucial for our understanding of the evolution of late Archean continental crust and its geodynamic setting. In this study, a series of ~2.57–2.52 Ga coeval and diverse granitoid rocks, including quartz dioritic-trondhjemitic and granodioritic to monzo-/syenogranitic gneisses, were identified in the southern Fushun area of Eastern Liaoning Province, North China Craton. The ~2.57 Ga quartz dioritic gneisses show moderate MgO (≤ 3.96 wt.%) and moderate to high Mg^# (37.3–75.5). Geochemical modeling, together with mildly fractionated REE patterns and negative Eu anomalies and depleted zircon ƐHf(t₂) (+2.1 − +7.4), suggest that they were differentiated from a depleted mantle source that was metasomatized by slab-derived fluids. The younger trondhjemitic gneisses (~2.55–2.52 Ga) are divided into two subgroups, i.e., an older subgroup (~2547–2540 Ma) characterized by mildly fractionated REE patterns and negative Eu anomalies, and a younger subgroup (~2533–2517 Ma) with strongly fractionated REE patterns and positive Eu anomalies. The trondhjemitic gneisses lack evidence for magma differentiation, and they are inferred to have been formed by reworking of amphibolites/greywackes at diverse crustal levels, with some inputs of mantle materials in the earlier subgroup. The coeval ~2550–2529 Ma granodioritic and monzo-/syenogranitic gneisses are characterized by high K₂O/Na₂O (0.67–2.45) but low MgO and Mg^# (mostly <2 wt.% and < 50, respectively). Geochemical modeling data indicate that the granodioritic gneisses are less differentiated, which could have been derived from the partial melting of high-K mafic rocks (e.g., regional late Neoarchean calc-alkaline meta-basaltic rocks), as further supported by the constant A/CNK (1.00–1.14) and zircon ƐHf(t₂) (+2.0 − +9.3) values. In comparison, the monzo-/syenogranitic gneisses show variable A/CNK (0.78–1.32) and zircon ƐHf(t₂) (−2.4 − +7.8) values. They are explained to be formed by the partial melting of mixed sources of high-K mafic and sedimentary rocks, with the primary magmas showing plagioclase and apatite fractionation. Taken together, the above granitoid rocks record gradually decreasing zircon ƐHf(t₂) values and increasing crustal thickness. Considering the petrogenetic information, regional geological data and the presence of some ~3.45–2.70 Ga crustal materials, the late Neoarchean granitoid magmatism of southern Fushun were likely generated via complex crust-mantle interaction processes at an active continental margin. It is further emphasized that Archean active continental margins are key sites for the initial maturation of the crust, and this tectonic scenario acted as a trigger for granitoid diversification during both the subduction and subsequent collisional stages.