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 ore-bearing mafic rocks of the cortlandite-gabbro-norite association at the Kuvalorog occurrence of Ni mineralization in Kamchatka were produced by melting undepleted mantle at its adiabatic decompression (without involvement of subduction-related fluids and melts). The significant crystallization 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 break-off. Geochemical data and results of our numerical 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.