ISOTOPIC COMPOSITION OF LATE NEOGENE K-NA ALKALINE BASALTS OF EASTERN KAMCHATKA: INDICATORS OF THE HETEROGENEITY OF THE MANTLE MAGMA SOURCES

Abstract

Isotopic composition of Sr, O, Nd, and Pb was determined in K-Na alkaline gabbroids and basaltoids that formed in eastern Kamchatka during Middle Miocene (gabbroids of the subvolcanic complex) and Late Miocene (basaltoids of the volcanic complex) time, before the origin of the Eastern Kamchatka Volcanic Belt. The variations in the isotopic parameters of the gabbroids and basaltoids are, respectively, as follows: 87Sr/86Sr = 0.70381-0.70465 and 0.70362-0.70512, no data on δ18O and δ18O ranging from +6.0 to +10.4‰, εNd ranges from +3.8 to +10.4 and from +2.0 to +7.1, 206Pb/204Pb = 17.818-18.164 and 17.880-18.105, 207Pb/204Pb = 15.427-15.475 and 15.479-15.537, 208Pb/204Pb = 37.542-37.964 and 37.950-38.239. Systematic covariations were detected for isotopes of Sr, O, Nd, and Pb with one another and with some geochemical parameters of the rocks. This led us to propose a two-component mixing model for the initial magmas, which were produced by the melting of the material of an enriched mantle plume (EM 1) and the depleted mantle of the MORE type. Isotopic data provide further evidence that the sources of the Late Cenozoic volcanics of the within-plate and island-arc geochemical types were different.