LAUSR

Abstract Adakitic rocks of the Sikhote-Alin area were emplaced during two main periods: the Cretaceous (132–98 Ma) and Eocene (46–39 Ma). These rocks primarily occur in the Khanka Block and, less commonly, in the Sikhote-Alin Orogenic Belt. The adakitic rocks record the following chemical compositions: SiO2 = 57–74%, Al2O3 = 15–18%, Na2O = 3.5–6.1%, K2O = 0.7–3.2%, Na2O/K2O = 1.1–3.9, Sr/Y = 33–145, and (La/Yb)N = 11–53. The HREE and HFSE in these rocks are remarkably depleted. The Early Cretaceous adakites record eNd(T) = −1.0 to +3.2 and ISr = 0.7040–0.7090, and the Eocene adakitic rocks record Nd(T) = −2.0 to +2.2 and ISr = 0.7042–0.7058. Adakitic features suggest different modes of magma generation; a comparison of the Sr/Y and La/Yb ratios and geochemical data on Harker diagrams between the two periods of adakitic rocks reveals differences in their petrogenesis. The Cretaceous adakites may have been generated by the partial melting of meta-basic rocks in a subduction zone, accompanied by the emplacement of volcanic arc granitoids. Therefore, the subduction of the Paleo-Pacific Plate beneath the Sikhote-Alin was probably initiated during this time. The Eocene rocks, which record increasing adakitic features with increasing silica content, are most likely the product of andesite that underwent fractionation of mineral assemblage including clinopyoxene, orthopyroxene, garnet and amphibole. These rocks and associated basalts and rhyolite were formed after Cretaceous arc magmatism in the Sikhote-Alin area and were most likely generated by rollback of the subducting Pacific Plate after the Eocene. Abundant adakitic granitoids of Early Cretaceous and Eocene age occur in the Kitakami and Abukuma Mountains of NE Japan. Consequently, it is highly probable that a geological correlation existed between Sikhote-Alin and North Japan, particularly before the opening of the Japan Sea.