Causes of the Occurrence of A-Type Volcanic Rocks in Active Continental Margins (Southern Sikhote-Alin, Russian Far East)

Abstract

Abstract ––New isotope-geochemical data on the volcanic complexes of the South Yakut and Martel volcanic depressions in southern Primorye are presented. Their formation in the early Eocene (54.3 Ma) and Late Cretaceous (83.5 Ma), respectively, is evidenced by U–Pb zircon dating (LA-ICP-MS). Based on the geochemical characteristics, it is concluded that the volcanics are typical A-type igneous rocks. Their formation coincides with the sudden change in the vector of motion of the Pacific slab with respect to the continent in the Campanian and Paleocene–Eocene, which caused destruction of the slab with its probable discontinuity and the injection of the subslab asthenosphere. The effect of mantle fluids on the continental lithospheric-rock melting determined the generation of magmas with the specific geochemical features of A-type igneous rocks. The regularities of their composition are due to the deep-seated reduced F-rich fluids that caused the intense differentiation of magmas accumulating fluidized melts enriched in mobile components in the apical part.