This study reports the mineral assemblages, microstructures, P?T estimates and monazite U?Th?Pb ages from pelitic schists of the Nam Co Formation, Son La area, Song Ma suture zone, northwestern Vietnam, to delineate its pressure?temperature?time?deformation evolution. The Nam Co Formation in the study area is structurally characterized by an antiformal metamorphic core complex. Pelitic schists in the core of this complex belong to the garnet zone, consisting of quartz + chlorite + muscovite + garnet + albite with accessory apatite, zircon, monazite and ilmenite. On the other hand, the rims of this complex are consistent in metamorphic grade with the biotite zone of quartz, chlorite, muscovite, plagioclase, and with or without biotite. In the garnet zone, both albite and garnet occur as a porphyroblast. The curved to sigmoidal inclusion trails were defined by graphitic materials within porphyroblastic albite. The earlier foliations were truncated by the major foliation. The pre-Dn growth of porphyroblastic albite is also corroborated by the stylolitic halos and microfaults around the porphyroblasts. Garnet uncommonly occurs as a small inclusion within porphyrobalstic albite. Compositions of the inclusion are represented by the high almandine and relatively low spessartine contents (Xalm=80?83 & Xsps=7?8), in marked contrast to those of porphyroblastic garnet with high spessartine contents (Xalm=62?75 & Xsps=18?31). The above microstructural and compositional features suggest two temporally distinct crustal-thickening events in the region. The peak metamorphic conditions estimated from the garnet-chlorite geothermometer and the garnet-plagioclase-muscovite-quartz geobarometer are in the range of 450?490oC and 7?13 kbar. The SHRIMP U?Th?Pb isotopic analyses of monazite from the garnet zone yielded the lower intercept age of 234 ± 10 Ma. This age is interpreted as the timing of collision between the Indochina and South China cratons.