New Data on the Rock and Mineral Composition of Kharchinsky and Zarechny Volcanoes (Central Kamchatka depression): Heterogeneity of the Mantle Source and Peculiarities of Magma Evolution in Crust

Abstract

The Kharchinsky and Zarechny volcanoes and the Kharchinsky Lake zone of monogenetic cones are unique eruptive centers of magnesian lavas located above the northern margin of the Pacific Plate subducting beneath Kamchatka. This work presents new geochemical data on the composition of rocks (55 samples) and minerals (over 900 analyses of olivine, pyroxenes, amphibole, and plagioclase) of these centers analyzed by XRF and LA-ICP-MS (rocks) and electron microprobe (minerals). Most of the studied rocks are represented by magnesian (Mg# = 60–75 mol. %), medium-K basalts and basaltic andesites. Moderate-magnesian (Mg# = = 52–59 mol. %) basaltic andesites are present among the monogenic cones of the Kharchinsky Lake. The rare rock varieties include high-K basalts-basaltic andesites of dikes in the center of the Kharchinsky volcano and magnesian andesites (Mg# = 58–61 mol. %) of the extrusions of Zarechnу volcano. The distribution of trace element contents in these samples demonstrates the enrichment of large-ion lithophile elements, light REEs and depletion of high field strength elements and heavy REEs typical of arc rocks. High-K basalts and basaltic andesites show anomalous enrichment in Ba (1000 ppm), Th (3.8 ppm), U (1.8 ppm), Sr (800 ppm, Sr/Y 50) and light REE (La 20 ppm); their compositions are close to those of low-Si adakites. Basalts and basaltic andesites contain high-Mg olivine phenocrysts (up to Fo92.6) and clinopyroxene (Mg# up to 91 mol. %). The rocks show petrographic and geochemical signs of fractional crystallization along with the processes of mineral accumulation and magma mixing. Some of the olivine phenocrysts show high NiO contents (up to 5000 ppm) and elevated Fe/Mn ratio (up to 80), interpreted as evidence of participation of the pyroxenite source in the magma generation processes. The use of Ca/Fe and Ni/Mg ratios allowed us to distinguish the composition fields and evolution trends of olivines associated with different sources – peridotite and pyroxenite, formed by the reaction of mantle wedge peridotites and high-Si melts of the subducted oceanic crust. The new data are consistent with other evidence of melting of the subducted Pacific plate edge beneath the northern part of the Central Kamchatka depression at the Kurile-Kamchatka and Aleutian subduction zone junction and testify to significant heterogeneity of the mantle in this area.