Sources of Eocene Magmatism in Western Kamchatka by the Geochemical and Sr–Nd–Pb Isotope Characteristics of Basites

Abstract

The isotope and geochemical characteristics of Eocene–Oligocene igneous rocks of Western Kamchatka were studied. It was shown that igneous rocks of the Eocene Kinkil complex of Western Kamchatka (45–53 Ma) were characterized by the geochemical features of subduction volcanism. The isotope composition of the rocks by Sr, Nd, and Pb along with the low concentrations of HFSEs and HREEs related to the MORB composition point to the formation of primary melts from depleted or slightly isotope-enriched sources of the mantle wedge contaminated to various degrees by quartz–feldspar sialic sediments. From the end of the Middle Eocene, under the conditions of intraformational activation in Western Kamchatka, the K–Na-alkali–basalt magmatism appeared (46–31 Ma) with similar geochemical characteristics to those of E-MORB, along with the ultra-potassic alkali–basalt magmatism developing right up to the Early Miocene (35–17 Ma). The geodynamic nature of the Late Paleogene alkaline magmatism implies the occurrence of processes of diffuse rifting.