Structural and petrological data suggest that the Xigaze ophiolite from the Yarlung Zangbo Suture Zone (YZSZ) in south Tibet was a typical slow-spreading ridge. A new field, geochemical, mineral, and U-Pb zircon dataset of plagiogranite intrusions were used to constrain the dynamic processes of oceanic accretion in this slow-spreading ridge. Plagiogranites mainly occur as dykes or intrusions intruded into the whole sequence of the ophiolite and have a similar orientation to the dolerite dykes developed in the late stage of detachment faulting. U-Pb zircon ages of 122–123 Ma were obtained for two types of plagiogranites and associated dolerite dykes. Detailed geochemical and mineralogical examinations suggest that the plagiogranites are the product of low-pressure (2–3 kbar) fractional crystallization of mid-ocean ridge basalt-like magma and unlikely to have been derived from the partial melting of hydrous gabbroic rocks. The complex cross-cut relationship between the plagiogranites and ophiolite sequence reflects that they are controlled by small discontinued melt lenses rather than a big magma chamber under the ridge axis and reveals multiple injections during the oceanic crust accretion. The formation of plagiogranites possibly reflects the complex characteristic of oceanic accretion at slow-spreading ridges, time-dependent on structural (external) and magmatic (internal) processes.