Petrogenesis of the Coeval High-Mg Diorites and Adakitic Granodiorites in Seolhwa Igneous complex: Insights into the parental melt diversity of early cretaceous magmatism in the Korean Peninsula

Diverse magmas were generated from various sources under an extensional tectonic setting in the Korean Peninsula during the Early Cretaceous. To investigate this process, we conducted petrological and geochemical analyses of the Seolhwa Igneous Complex (SIC), located along the mid-western margin of the Korean Peninsula. The SIC comprises a granodioritic northern suite (zircon U-Pb ages: 110–120 Ma) and a dioritic southern suite (110.70 ± 0.49 Ma). The two suites are spatially and temporally associated displaying coherent major and trace element trends, as well as enriched isotopic signatures, which suggests a comagmatic origin. The high concentrations of compatible elements (Mg# = 59.1–64.9; Ni = 66–210 ppm; Cr = 235–510 ppm) in the southern suite high-Mg diorites indicate derivation from a mantle source. However, their silica contents (SiO₂ = 56.4–61.1 wt%) are higher than those of primitive basalts at comparable Mg# values and are more akin to primitive andesites. The enriched isotopic signatures [(⁸⁷Sr/⁸⁶Sr) _i = 0.7136–0.7138; ε_Nd(t) = −15.1 to −14.1], together with arc-like geochemical features, suggest a metasomatized mantle source. The presence of inherited zircon cores in the adakitic northern granodioritic suite, along with correlations between SiO₂ contents and Sr-Nd isotopes [(⁸⁷Sr/⁸⁶Sr) _i = 0.7129–0.7168; ε_Nd(t) = −13.8 to −17.3], indicates significant crustal assimilation. Geochemical modeling suggests that the adakitic signature developed through assimilation–fractional crystallization processes from a parental melt similar to that of the southern suite. Primitive andesitic geochemical signatures comparable to the southern dioritic suite are observed in several other Early Cretaceous suites. We therefore propose that, although volumetrically minor, primitive andesitic melts were more widespread than previously recognized. Slab steepening and rollback-induced extension along the continental margin during the Early Cretaceous promoted partial melting of enriched subcontinental lithospheric mantle through decompression and thermal perturbation. Under such conditions, a spectrum of primitive mantle-derived melts, ranging from andesitic to basaltic, could be produced.