Petrologiâ

The role and conditions of liquid immiscibility and crystallization of sulfide phase during evolution of subduction related magmas remains to be a debated topic, which bears relevance to mechanisms of porphyry copper deposit formation and evolution of the continental crust. We studied rare volcanic rocks with inclusions of magmatic sulfides in olivine – the basalts of Medvezhya Mount in the Avachinsky group of volcanoes. The rocks belong to primitive (Mg# = 66 mol. %) middle-K island arc olivine basalts. Olivine with normal zoning predominate (~98%) among phenocrysts. The olivine compositions are typical for Kamchatka basalts, except for an unusual trend of increase of MnO content from 0.20 to 0.55 wt. % and decrease of Fe/Mn from 60 to 35 with change of olivine composition from Fo_87.8 to Fo_78.2. Olivines of this group contain numerous inclusions of minerals of the spinel group varying in composition from chromium spinel to magnesian magnetite. Olivine phenocrysts with sulfide inclusions are characterized by the absence or weak reverse zoning and reduced contents of Ca, Ni, Mn, Cr, and Al. The estimated crystallization temperatures for olivines of the prevailing type are 1036–1241°C, for sulfide-bearing olivines – 1010–1062°C. The data suggest that crystallization of the main olivine population occurred under relatively shallow conditions and was accompanied by strong oxidation of the magmas. On the contrary, the zoning and composition features of sulfide-bearing olivine suggest its xenogenic origin and probable crystallization at conditions of deep crust from low temperature water-rich and/or low-Ca magmas. The results obtained confirm the possibility of saturation of oxidized island-arc magmas with sulfide phase at lower crustal conditions, but show that this process is rare and not typical for low-pressure crystallization stage.

In 2021 a unique event occurred on Klyuchevskoy volcano (Kamchatka). After more than 30 years dominance of terminal eruptions a flank vent was formed. It was named after corresponding member of the Academy of Sciences USSR G.S. Gorshkov. The eruption began immediately after the end of the summit crater activation in 2020–2021 at an altitude of 2850 m in the northwestern part of the volcano, where no manifestations of flank volcanism were observed earlier. This paper presents geochemical and isotopic Sr-Nd-Pb-O data for lavas of the summit and flank eruptions of the Klyuchevskoy volcano in 2020–2021. A comparative petrographic analysis was carried out and the chemical composition of the Ol, Cpx, Pl phenocrysts of these lavas was analyzed. The lavas of both eruptions are alumina andesitic basalts of normal alkalinity. Variations of basic oxides in the lavas of the summit eruption and in the lavas of the G.S. Gorshkov vent are SiO₂ 53.1–53.2 wt. % and 51.6–53.2 wt. %, MgO 5.6 wt. % and 5.5–6.0 wt. %; respectively. Estimates of temperatures and pressures of plagioclase crystallization showed that for lavas of the summit eruption they were 1210–1118°С and 12.3–3.6 kbar, for lavas of the flank vent 1203–1119°С and 9.0–3.3 kbar. The content of the main elements, the proximity of the conditions of plagioclase generations, and the features of variations in the chemical composition of Ol, Cpx, and Pl phenocrysts in the lavas of both eruptions indicate the genetic relationship of the magmas that fed the summit and flank eruptions. The lavas of the summit eruptions of 2016, 2020–2021, as well as the lavas of the previous summit eruptions of the Klyuchevsky volcano, are characterized by highly sustained Sr-Nd-Pb isotopic characteristics: ⁸⁷Sr/⁸⁶Sr = 0.703625–0.703626, ¹⁴³Nd/¹⁴⁴Nd = 0.513085–0.513102, ²⁰⁶Pb/²⁰⁴Pb = 18.3148–18.3179). Isotopic ratios ²⁰⁷Pb/²⁰⁴Pb (15.5022–15.5107) and ²⁰⁸Pb/²⁰⁴Pb (37.9597–38.0143) are significantly higher for the lavas of the last summit and flank eruptions than for all Klyuchevskoy’s lavas of the past, which indicates more complex processes of magma evolution at crustal levels. The values of δ¹⁸O = 6.49–7.39 rel. units (SMOW) in the lavas of the considered eruptions are consistent with previously published data on the Klyuchevsky volcano. The lavas of the Gorshkov vent are enriched with Ba, Zr, Sr and other incompatible elements at constant values of MgO in comparison with the lavas of the last summit eruptions, which indicates differences in the ways of evolution of their magmas. Sharply increased values of the ⁸⁷Sr/⁸⁶Sr ratio (0.703673–0.703743) in the lavas of the G.S. Gorshkov vent, not previously observed in the lavas of the Klyuchevsky volcano, testifies to intensive assimilation of crustal matter by the initial melts in the northwestern part of the volcano.

The paper presents petrographic, mineralogical, and geochemical data on dunites, pyroxenites, peridotites, and gabbroids of the Kamchatsky Mys ophiolite. These data were acquired to distinguish cogenetic assemblages of igneous rocks, gain an insight into their geodynamic settings, and test various criteria of genetic links between the different magmatic rocks of ophiolites. The ultramafic and mafic rocks are shown to belong to two series, which differ in the compositions of the primary minerals, bulk rocks, and estimated trapped melts. The rocks of these series are found out to have been produced by geochemically different melts in different geodynamic settings, and during different episodes of mantle magmatism. The rocks of the high-Ti series (gabbro of the Olenegorsk massif, dunite and melanogabbro xenoliths in them, and vein gabbro in these xenoliths) crystallized from N-MORB melts in an oceanic spreading center. The rocks of the low-Ti series (dunite, pyroxenite, and gabbro veins in the residual spinel peridotites of the Mount Soldatskaya massif, as well as pyroxenite, peridotite, and gabbro alluvium and diluvium in the central and western parts of the peninsula) crystallized from water-rich boninite melts in relation to initial subduction magmatism. Taken into account the absence of boninite lavas from the Kamchatsky Mys ophiolite, the plutonic ultramafic rocks (including the rocks of the veins) might be the only evidence of subduction boninitic magmatism in the ophiolites. It was demonstrated that conclusions about the geodynamic settings of plutonic ultramafic and mafic rocks and recognition of cogenetic relations of these rocks with spatially associated basalts are more reliable when derived from the compositions of the trapped melts, which are estimated from their bulk geochemistry and primary mineral compositions, than when they are based on the mineral compositions only.