Confirmation of the Ordovician alkali syenite in Niandao implications for the Early Paleozoic tectonic evolution of North Qinling

Through detailed field geological surveys, combined with systematic studies of petrography, geochemical and zircon U-Pb chronology, the Ordovician alkali syenite is determined in Niandao area of Neixiang County, which was first discovered in North Qinling.The Niandao alkali syenite was involved in Zhuyangguan—Xiaguan fault belt and transformed into tectonic massif, however, the early intrusive contact with Qinling Complex was preserved, and intrusive rocks were mainly composed of hornblende syenite and aegirine-augite syenit.Samples are characterized by relatively low silicon(SiO₂ =52.09%~67.48%), total alkali(K₂O+Na₂O = 5.82%~11.75%), abundant aluminum(Al₂O₃=11.87%~17.81%), depleted magnesium(MgO=0.09%~1.99%), enrichment of LREE, Rb and K, depletion of Nb, Ta, P and Ti, indicate that the intrusion is aluminous A-type granite and further classified to A₂ type.The geochemical characteristics show that this pluton may be dominantly derived from partial melting of an enriched mantle modified by subduction-related components, and the magma produced was subjected to minor amounts of crustal contamination during its ascending process, and its formation might be related to an island arc environment.Zircon LA-ICP-MS U-Pb dating of hornblende syenite gave ²⁰⁶Pb/²³⁸U weighted mean ages of 457.4±1.8 Ma, showing that the Niandao alkali syenite emplaced in Late Ordovician.Combined with the regional geological data, the Niandao alkali syenite ought to be initial magmatism record of negative structure inversion of North Qinling that experienced in the Paleozoic stage of exhumation at ca.450 Ma, and its formation age should represent the initial time of negative structure inversion.