The Xiaoshuijing Syenogranite is the host of Mo mineralisation recently discovered in the Tengchong Terrane of southwestern China. Here, we present new zircon U-Pb ages and molybdenite Re-Os ages, whole-rock geochemical data, and zircon Hf isotopic data for the Xiaoshuijing Syenogranite hosting disseminated Mo mineralisation. The syenogranite's mineralogical and geochemical characteristics indicate it is a high-K, alkaline, and peraluminous granite. LA-ICP-MS analysis of zircon yields an age of 65 ± 1 Ma (n = 14, MSWD = 2.1), and yields negative zircon ε Hf (t) values ranging from −18.23 to −12.92, interpreted to have a crustal source. The Re-Os Mo age of 64 ± 2 Ma (n = 4, MSWD = 2.2) for the mineralisation is the same as the syenogranite's age, within error. The geochemistry also indicates that the syenogranite is related to a thickened crust resulting from the eastward subduction of the Neo-Tethyan oceanic lithosphere shortly before the collision of the Indian and Eurasian continental crusts.