Petrogenesis of Coexisting Agpaitic and Potassic Syenites in Late Triassic Yangyuan Alkaline Complex, North China

Abstract Agpaitic nepheline syenite complexes are globally rare, yet their unique mineralogy and potential for critical metal mineralization make them an important focus of research. Here we investigate the Yangyuan Alkaline Complex from the circum-cratonic Triassic alkaline belt of the North China Craton. The complex mainly comprises sills divided into two series: an early phase of aegirine nepheline syenite series (ANS) and a late phase of alkali-feldspar syenite series (AFS). The ANS features agpaitic mineral assemblages, high alkalinity index (most >1.0), and high Na/K molar ratios (>2.0), whereas the AFS is orthoclase dominated and potassic. Both series are enriched in incompatible elements and have δ66Zn ranging from 0.37‰ to 0.39‰ for ANS and 0.29‰ to 0.43‰ for AFS, and an average εNd(t) of −3.23 for ANS and −4.43 for AFS. The Sr–Nd–Zn isotopes, together with in situ zircon Hf–O isotopic systematics, indicate that both the ANS and AFS are derived from an enriched lithospheric mantle source that was previously metasomatized by fluids/melts from recycled carbonate-bearing sediments. Phase-equilibrium relations and isotope mixing models indicate that the AFS cannot be evolved from the ANS via crystal fractionation and assimilation at upper crustal levels. In contrast, a combination of lower continental crustal assimilation and potential kaersutite fractionation may be important factors driving the transition from agpaitic-silica-undersaturated to potassic-silica-saturated magma systems. During ascent and emplacement at upper crustal levels, extensive fractional crystallization of sanidine, ±eudialyte, ±nepheline, and ±apatite resulted in an enrichment in high-field-strength elements (HFSE), Rb, Th U and rare earth elements (REE). Two different types of zircons—magmatic and hydrothermal—yield indistinguishable ages at c. 223–221 Ma, indicating near contemporaneous igneous crystallization and hydrothermal alteration. Hydrothermal metasomatism may have introduced post-magmatic HFSE−REE mineralization.