Geochemical Characteristics and Tectonic Significance of Late Triassic Lamprophyre in the Beishan Region, Gansu Province

The Beishan region is located at the southern margin of the Central Asian Orogenic Belt (CAOB) and is a key area for studying the tectonic evolution of the CAOB. Lamprophyres are the product of low-degree partial melting of the subcontinental lithospheric mantle in the extensional background. Their formation age and petrogenesis can play a significant role in ascertaining the regional tectonic evolution. In this paper, systematic petrological, geochemical and zircon U-Pb chronology and Hf isotope studies were carried out on the lamprophyre in the Liuyuan area in southern Beishan region. The zircon U-Pb age of the Liuyuannan lamprophyre is (228.2±1.1) Ma (Late Triassic). The lamprophyre is rich in volatile component minerals such as phlogopite and hornblende. The Liuyuannan lamprophyre belongs to ultrapotassic lamprophyre, and its parent magma is an alkaline magma series. The Liuyuannan lamprophyre are characterized by pronounced negative Nb-Ta and Zr-Hf anomalies, positive zircon εHf (t) values ranging from 0.5 to 4.9, with an average value of +2.8. These data indicate that the mantle source was metasomatized by the subducted melts/fluids. The Mn/Fe and Ca/Fe ratios of olivine crystals in the Liuyuannan lamprophyre indicate that the mantle source is phlogopite-rich harzburgite mantle; trace element simulations indicate that the mantle source of Liuyuannan lamprophyre is an enriched peridotite-type mantle. Therefor, the source of the Liuyuan lamprophyre should be the lithospheric mantle metasomatized by subducting slab fluids. Combined with previous studies on the tectonic evolution of the southern of the CAOB, we believe that the Beishan region has entered intracontinental extensional environment in the Late Triassic. Decompression promoted low-degree partial melting of the lithospheric mantle metasomatized by subduction fluids, resulting in the formation of the Liuyuan lamprophyre.