Fragments of Metasomatized Forearc: Origin and Implications of Mafic and Ultramafic Xenoliths From Kharchinsky Volcano, Kamchatka

Abstract

AbstractDunite, harzburgite, and clinopyroxenite xenoliths from Kharchinsky volcano, Kamchatka, have abundances and ratios of incompatible trace elements similar to those in arc volcanic rocks (elevated Ba/Th, La/Yb, Nd/Hf, and Sr/Y). All orthopyroxenes and some clinopyroxenes in the peridotites have U‐shaped rare‐earth element patterns. Negative Ce anomalies are present in orthopyroxenes with Ce/Ce* as low as 0.01 and down to 0.22 in whole‐rock peridotite data. Ce anomaly growth is linked to increasing La/Sm and enrichments in Rb, U, Pb, and Ba over La and Ce. Isotopes (Pb, Sr, Nd, and Hf) indicate pelagic sediment, and hydrothermal crusts play no role in Ce anomaly development. Instead, Ce anomalies appear to be products of fluid transport and elemental scavenging under oxidizing conditions beneath the deep forearc. Textures and compositions of aluminous green spinels indicate most of the peridotites were partially melted and recrystallized at depth. Veins and pockets of amphibole reflect impregnation late in the petrogenesis of the rocks by melts similar to Kamchatka basalts. Orthopyroxenite xenoliths are fine‐grained with fibrous orthopyroxene that has high‐Mg/Mg + Fe (up to 0.96) and generally lower CaO and Al2O3 compared to peridotite orthopyroxenes and perhaps formed by reaction of siliceous fluids with olivine. Kharchinsky xenoliths have Pb, Sr, and Nd isotopes similar to Kamchatka volcanic rocks, but Hf isotopes in clinopyroxenites and gabbros are more radiogenic by 1–3 epsilon units. Patterns in isotopic data indicate a compositional change in the source of Kamchatka volcanism within the past 20 million years.