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Abstract

The paper presents data on the petrology and geochemistry of plutonic rocks dredged from the Stalemate Fracture Zone, Northwest Pacific Ocean, during Cruise SO201-1 of the R/V "Sonne". We proposed also the reconstruction of their formation conditions and interpretation of their tectonic evolution. The genesis of gabbroids found among plutonic rocks composing the Cretaceous-Paleogene basement of the northwestern part of the Pacific Ocean was related to magmatism at the ancient spreading center and provides record of the evolution of the parental magmatic melts of N-MORB. Along with related peridotites, basalts, and dolerites, these rocks can be attributed to the disintegrated the Cretaceous-Paleogene oceanic lithosphere of the Pacific Ocean. The shallow mantle beneath the ancient oceanic crust of this area is made up of depleted magmatic spinel lherzolite, harzburgite, and dunite. The fact that gabbro-diorite and diorite that are not genetically related to the rocks of the Cretaceous-Paleogene basement of the Northwest Pacific occur at the eastern termination of the Stalemate Fracture Zone possibly reflects the complicated structure of the tectonic collage of rocks of different age that were produced in different geodynamic environments and were later tectonically brought together near the frontal portion of the Aleutian island arc. Judging by the isotopic-geochemical characteristics of these rocks, they cannot be classed with the family of oceanic plagiogranites. Deformations of the oceanic basement can be discerned throughout the whole Stalemate Fracture Zone as brecciation and large-amplitude vertical displacements within the oceanic lithosphere.