The identification of ancient activity of large igneous provinces, which may be surface manifestations of giant mantle plumes, is of primary importance in understanding the evolution of the Earth system, because such major magmatic redistribution could have profound effects on the global environment. The immobile trace element compositions of metamorphosed basaltic rocks called greenstones found in accretionary complexes of southwest Japan suggest origins in accreted oceanic lavas at Pacific hotspots, rather than at mid-oceanic ridges. Furthermore, some of these greenstones have geochemical characteristics typical of hotspot lavas in the present superswell region of the South Pacific Ocean. Available age data show that the hotspot lavas formed at 350–300 Ma. Greenstones with eruption ages identical to those in southwest Japan are widely distributed in the circum-Pacific region. These observations provide evidence for activity of a giant Pacific mantle plume during the Carboniferous. The onset of such giant plume magmatism may have preceded that of the geomagnetic quiet zone.