Formation Mechanism Of Synchronous Late Neoarchean Na-and K-rich Granitoid Magmatism In The Eastern Liaoning Province

The late Archean Earth is marked by gradual appearance of voluminous K-rich granitoid rocks within different cratons.Although they were commonly considered to be formed following long-term TTG magmatism,coeval Na-and K-rich granitoid rocks were recently identified in some terranes,which have the potential to provide valuable insights into the late Archean evolution of continental crust as well as related geodynamic regimes.In this study,a series of?2.57-2.52 Ga coeval but 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,negative Eu anomalies and depleted zircon?Hf(t₂)values(+2.1-+7.4),suggest that they were differentiated from a depleted mantle source that was metasomatized by slab-derived fluids.The 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.Petrogenetic studies indicate that the trondhjemitic gneisses were not subjected to evident magma differentiation,and are inferred to have been formed by the partial melting of amphibolites/greywackes at diverse crustal levels,with some inputs of mantle materials in the earlier subgroup.In comparison,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?Hf(t₂)(+2.0-+9.3)values.The monzo-/syenogranitic gneisses show variable A/CNK(0.78-1.32)and zircon?Hf(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?Hf(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 through 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 continental crust maturation,and this tectonic scenario acted as a trigger for granitoid diversification during both subduction and subsequent collisional stages.