Abstract: In this paper, the precise dating of LA-ICP-MS zircon U-Pb and Hf isotope analysis and geochemical analysis were performed on Baiheshan quartz diorite from Baishan magmatic arc, Inner Mongolia.The results show that the formation age of the rock is 350.8±1.5 Ma and the ε_Hf(t)values of magmatic zircon are between 5.28 and 9.88 with crustal model ages ranging from 724 Ma to 1018 Ma.Geochemical data show that Baiheshan quartz diorite is a set of I type calc-alkaline rocks.All the samples have rich alkali(Na₂O+K₂O=6.03%~6.96%)and sodium(Na₂O/K₂O>2), low values of Mg^#(35.48~39.78)and low content of Ni(2.06×10^-6~5.54×10^-6)and Cr(2.11×10^-6~7.93×10^-6).Light rare earth elements(LREEs)are enriched in the chondrite-normalized REE patterns while heavy rare earth elements(HREEs)fractionation and negative Eu anomalies are not obvious.They are enriched in large ion lithophile elements such as Rb, K and U and depleted in high field strength elements(HFSEs)such as Nb, Ta, P and Ti.The geochemical composition indicates that the rock has the genetic characteristics of the subduction zone.The zircon U-Pb age and petrogenesis of the quartz diorite prove that it is the earliest arc magmatic record that was first discovered in the Late Paleozoic Baishan magmatic arc, which is clearly related to the subduction of the Hongshishan-Baiheshan Ocean.It is shown that the subduction of Hongshishan and Baiheshan Ocean occurred before ~350 Ma, which is 22 Ma earlier than the previous conclusion about the start of the ocean subduction.