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Metalliferous umbers and red shales occur as unique products of the Kula‐Pacific ridge‐forearc collision in the Late Cretaceous Shimanto Supergroup, an accretionary complex in Japan. These umbers are closely associated with greenstones of mid‐ocean ridge basalt (MORB) origin and are regarded as hydrothermal metalliferous precipitates related to MOR‐type volcanism. The umbers and red shales were deposited in the trench area where both terrigenous detritus from land and hydrothermal metalliferous particulates from a MOR were supplied simultaneously. Besides a predominance of Fe and Mn, the umbers exhibit remarkable enrichments in P, V, Co, Ni, Zn, Y, Mo, rare earth elements (REEs), and Os relative to continental crustal abundances. The X/Fe (X = Mn, P, V, Co, Ni, Zn, Y, and REEs) ratios and PAAS‐normalized REE patterns of the umbers are very similar to those of modern hydrothermal plume …