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Although seismic and hydrothermal data show evidence of magma–seawater interaction in slow spreading-rate ridges, such a process has been rarely observed in situ in lower oceanic crust. In this study, we reported petrological and mineral data on the plutonic rocks from the lower oceanic crust of the Lagkorco ophiolite from central Tibet, which shows geological and petrological features indicative of a slow-spreading ridge origin. Both troctolites and gabbros were derived from depleted magmas containing 0.5–1.0 wt.% H2O. Petrology and geochemistry indicate that troctolites have experienced two stages of melt impregnation: (1) early, water-poor melt impregnation in a dunitic matrix and (2) late, hydrous melt impregnation in a troctolitic matrix. Stage (1) generated oikocrysts of clinopyroxene and plagioclase within an olivine-rich matrix, while stage (2) formed brown amphibole. The high Cl (up to 0.25 wt%) contents of the brown amphibole and the fast cooling of the troctolites (~ 0.1 °C/year) suggest that stage (2) melt impregnation was associated with the hydration of the intercumulus melts by seawater infiltration. Together with the strong arc affinities of basalts, we suggest that Lagkorco ophiolite was formed at a nascent, slow-spreading back-arc basin with a fault-controlled crustal accretion. The deep-rooted faults may provide the channels for the downward seawater infiltration into the hot, impregnation zone near the crust–mantle transition zone. This study provides an example of seawater–magma interaction in the nascent back-arc lower oceanic crust with implications for the interplay among tectonic, hydrothermal, and magmatic processes at the slow-spreading ridges. [ABSTRACT FROM AUTHOR]