Origin of Baotoudong syenites in North China Craton: Petrological, mineralogical and geochemical evidence

Abstract

Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Triassic alkaline magmatic belt along the northern margin of the North China Craton (NCC). Zircon U-Pb age, petrological, mineralogical and geochemical data of the pluton were obtained in this paper, to constrain its origin and mantle source characteristics. The pluton is composed of nepheline-clinopyroxene syenite and alkali-feldspar syenite, with zircon U-Pb age of 214.7±1.1 Ma. Diopside (cores)-aegirine-augite (rims), biotite, orthoclase and nepheline are the major minerals. The Baotoudong syenites have high contents of rare earth elements (REE), and are characterized by enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE; e.g., Rb, Ba, Sr), depletion in heavy rare earth elements (HREE) and high field strength elements (HFSE). They show enriched Sr-Nd isotopic compositions with initial 87Sr/86Sr ranging from 0.7061 to 0.7067 and e Nd( t ) values from –9.0 to –11.2. Mineralogy, petrology and geochemical studies show that the parental magma of the syenites is SiO2-undersaturated potassic-ultrapotassic, and is characterized by high contents of CaO, Fe2O3, K2O, Na2O and fluid compositions (H2O), and by high temperature and high oxygen fugacity. The syenites were originated from a phlogopite-rich, enriched lithospheric mantle source in garnet-stable area (>80 km). The occurrence of the Baotoudong syenites, together with many other ultrapotassic, alkaline complexes of similar ages on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids from the subducted, altered paleo-Mongolian oceanic crust, and the northern margin of the craton has entered into an extensively extensional regime as a destructive continental margin in Late Triassic.