Multi-stage magmatic evolution and genetic model of carbonatite and syenite in the Yangyugou Complex, Central China: Insights from U–Pb geochronology and Nd isotopes

Abstract

Carbonatites and syenites are commonly associated in geological complexes, but their genetic relationship has not been fully understood. This study, by integrating chronology and isotopic geochemical analyses, provides new constraints on the traditionally held “cogenetic and contemporaneous carbonatite–syenite” model, suggesting that the two units in the Yangyugou Complex did not derive from the same source at the same time, and further reveals the magmatic evolutionary processes controlling their temporal and spatial association. We studied the Yangyugou Complex, located within the Qinling Orogenic Belt, through monazite and rutile U–Pb geochronology in carbonatites, as well as in-situ analyses of trace elements and Nd isotopic compositions in apatite and monazite in syenites and carbonatites, respectively. U–Pb geochronology of rutile and monazite from carbonatites indicates consistent crystallization ages of ∼ 240 Ma, corresponding to Late Triassic magmatic activity. There are significant differences in Nd isotopic compositions between apatite from syenites and monazite from carbonatites in the Yangyugou complex: the ε Nd (t) isotopic signatures of apatite extend from − 16.1 to − 10.8, while those of monazite range from –20.3 to –18.6, indicating an essential difference in their melt sources. Compared with other carbonatites in the Qinling Orogenic Belt, the Yangyugou carbonatite exhibits more negative ε Nd (t) values, indicating stronger involvement of crustal materials and/or subducted sediment-derived components. In this study, a new model is proposed in which the Yangyugou Complex in the Qinling Orogenic Belt experienced two distinct magmatic events. The first, Silurian syenite (∼438.9 ± 3.3 Ma), generated by lower crust partial melting and metasomatized mantle source, while the second, Late Triassic carbonatite magmatism mixed with REE- and CO 2 -enriched slab fluids and melt of deep-sea sediments (∼240 Ma), intruded along structural pathways into the pre-existing syenite. Their contrasting ages and sources indicate that the two rock types are genetically independent, reflecting the complex multi-stage magmatic evolution of the belt.