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

Isotopic composition of Sr, O, Nd, and Pb was determined in K-Na alkal ine gabbroids and basaltoids that formed in eastern Kamchatka during M iddle Miocene (gabbroids of the subvolcanic complex) and Late Miocene (basaltoids of the volcanic complex) time, before the origin of the Ea stern Kamchatka Volcanic Belt. The variations in the isotopic paramete rs of the gabbroids and basaltoids are, respectively, as follows: Sr-8 7/Sr-86 = = 0.70381-0.70465 and 0.70362-0.70362, no data on delta(18)O ranging from +6.0 to +10.4 parts per thousand, epsilon(Nd) ranges fro m +10.4 to +2.0 and from +2.0 to +7.1, Pb-206/Pb-204 = 17.818-18.164 a nd 17.880-18.105, Pb-207/Pb-204 = 15.427-15.475 and 15.479-15.537, Pb- 208/Pb-204 = 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 w ith some geochemical parameters of the rocks. This led us to propose a two-component mixing model for the initial magmas, which were produce d by the melting of the material of an enriched mantle plume (EM 1) an d the depleted mantle of the MORB 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.