Geochronology, petrology, and geochemistry of the Yaojiazhuang ultramafic-syenitic complex from the North China Craton

The Yaojiazhuang ultramafic-syenitic complex is one of the representative Triassic alkaline plutons on the northern margin of the North China Craton (NCC). Based on detailed study of the zircon U-Pb age, petrological, mineralogical, and geochemical data of the complex, the characteristics of the magmas system, the petrogenesis of different rock types, and the nature of the mantle source were discussed to provide new constraints on the origin and tectonic setting of the Triassic alkaline belt. Cumulus ultramafic rocks, clinopyroxene-syenites and syenites are the main rock types of the complex. The zircons from the syenites yielded a U-Pb age of 209 Ma. Diopside-augite, biotite, and sanidine-orthoclase are the major minerals, with subordinate apatite and magnetite. Rocks from the complex are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), depleted in high field strength elements (HFSE) and heavy rare earth elements (HREE), and the initial 87Sr/86Sr ranges from 0.7057 to 0.7061 and ɛ Nd(t) from −9.4 to −11.4. Mineralogy and geochemical data demonstrate that the parent magma of the complex is SiO2-undersaturated ultrapotassic alkaline-peralkaline, and is characterized by high CaO content and fluid compositions (P2O5, CO2, H2O), and by high oxygen fugacity and high temperature. The complex was originated from a phlogopite-clinopyroxenite-rich lithospheric mantle source in the garnet-stable area (> 80 km) that had previously been metasomatized by melts/fluids from altered oceanic crust. The parent magma has been contaminated by little ancient TTG gneisses during magma emplacement. The development of the Yaojiazhuang complex indicates that the northern margin of the NCC has entered into an extensively extensional regime in the Late Triassic.