The Thack Khe iron deposit, located in the Truong Son metallogenic belt of the northeast of the Indochina Block, is the largest skarn-related magnetite deposit in Southeast Asia. In this study, we performed zircon U-Pb geochronology, geochemistry and zircon Lu-Hf isotope analyses on biotite monzogranite related with the metallogenesis of the Thack Khe iron deposit. The geochemical results show that the biotite monzogranite rock is enriched in Si (SiO2 =69.82% to 71.8%) and depleted in Ca and Al, with TFeO/MgO=4.07 to 4.76 and slightly peraluminous (A/CNK=1.01 to 1.05). Enrichment of Rb, Th, Pb, Zr and depletion of Ba, Sr-P, Eu-Ti indicate the rock belongs to the aluminium A-type granite. The zircon U-Pb dating indicates that the biotite monzogranite of Phia Bioc complex from the Thack Khe iron deposit emplaced at 257.8±2.2 Ma, i.e. the Late Permian. The initial Hf isotope ratios (εHf(t)=-2.6 to -6.4) responds to tDM2 of 1296 Ma to 1505 Ma. The geochemical characteristic and Hf isotope suggest that the biotite monzogranite from the Phia Bioc complex in the Thack Khe iron deposit formed by the partial melting of the lower crustal igneous rock in the tectonic setting of a back-arc extension related to subduction of the Tethyan Majiang Ocean towards the Truong Son metallogenic zone.