Eocene accretion at Kamchatka and a pulse of mantle plume magmatism

The paper presents newly obtained chemical analyses of metamorphosed mafic volcanic rocks from the upper reaches of the Krutogorova River, southwestern Kamchatka. According to their geochemistry, these mafic rocks and the orebearing mafic rocks of th e cortlandite-gabbro-norite association at the Kuval� orog occurrence of Ni mineralization in Kamchatka were produced by melting undepleted mantle at its adi� abatic decompression (without involvement of subductionrelated fluids and melts). The significant crystal� lization depths of the small cortlandite-gabbro-norite intrusions are confirmed by our pressure evaluations ( P = 8 kbar) by a newly developed amphibole barometer; the crystallization depths of the small subvolcanic intrusions was much shallower. The physical mechanism of derivation of the primitive partial mantle melts is discussed using simple numerical simulations of processes related to the onset of subduction and oceanic slab breakoff. Geochemical data and results of our numeri cal simulations testify that the picrobasaltic magmas were generated in ascending flows induced in the upper mantle by complicated geodynamic processes during the accretion of the Achaivaam-Valaginskii island arc to the Eurasian continent in the Eocene.