Metamorphic mafic metavolcanic rocks from the upper stream of Krutogorova river (SW Kamchatka) are characterized geochemically. In accordance with presented data these mafic rocks along with Ni ore-bearing rocks of the kortlandite-gabbronorite formation from Kuvalorog (Kamchatka) had been formed at the melting of the undepleted mantle during adiabatic decompression (without involvement of the subductive fluids and melts). Relatively large formation depth (P=8 kbar) of the small kortlandite-gabbro-norite inrusions is confirmed by means of our new amphibole barometer, formation depth of the subvolcanic sills of Krutogorova river is substantially smaller (P=2-4 kbar). Physical mechanism of the primitive mantle melts formation is discussed based on the results of the simplified numerical modelling of the subduction initiation and slab delamination. Geochemical and numerical data stay for the formation of the picrobasaltic magmas in the ascending upper mantle flows caused by the complica...
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