Adakite Volcanism at Continental Margin and Associated Problems. Part II. Adakites from the Sea of Okhotsk, Kamchatka, and Bering Sea Regions: Typification and Genesis

摘要

Based on a compilation of the vast literature, we typify the peculiar mantle-derived adakitic volcanic rocks in the Sea of Okhotsk, Kamchatka, and Bering Sea regions. The low-silica (LSA) and high-silica (HSA) types are distinguished in each region. The prevailing adakite type corresponds to the bimodal basalt-andesite series in the Kamchatka region and the basanite-dacite series in the Bering Sea region. In the Kamchatka region, the formation of LSA was related to the fractionation and melting of pyroxene, while the origin of HSA was related to the fractionation, melting, and breakdown of amphibole. Alkaline metasomatism plays a subordinate role in these processes. In the Bering Sea region, the formation of both types of adakites was mainly controlled by amphibole, with the assistance of alkaline metasomatism. The isotope-geochemical compositions of the adakites and within-plate volcanic rocks shows that both these rock types can be equally regarded as indicators of plume petrogenesis and geodynamics. Based on a comparison of the compositions of the adakites and glasses derived by xenolith melting, a two-stage genetic model is proposed: formation of black pyroxenites through melting of mantle lherzolites and generation of the entire rock complex owing to the repeated melting of the same lherzolites and newly formed pyroxenites. The formation of adakitic and within-plate signatures in the volcanic products beginning from the Eocene indicates the transition of the spacious territory of the NE continental margin from subduction-related compression to a setting of rift extension.