MELANOCRATIC BASEMENT OF THE PEKUL'NEY RANGE, CHUKOTKA, AND TRENDS OF LITHOSPHERIC EVOLUTION IN PALEO-OCEANIC ZONES OF THE NORTHWEST PACIFIC FRINGE

Abstract

The melanocratic rock association considered represents the basement of the outer part of the Northwest Pacific paleo-oceanic fringe. It is confined to the zone where oceanic crustal complexes taper among crystalline rock formations of the pre-Riphean continental crust. This rock association includes two complexes. The first complex, the metamaficgabbroic, is present in the imbricate-nappe structure of the Pekul'ney Range in the basement of the autochthon. The second complex, the eclogite-spjnel peridotite-metagabbro, is incorporated into nappes of the allochthon. The studied rock complexes together with ophiolite series of the Koryak Mountai that replace them laterally reflect the directional stage-by-stage process which converted the laterally inhomogeneous Early Precambrian crust of the lithosphere of mafic zones to an oceanic lithosphere. The first stage of that process, the pre-oceanic (> 2700 m. y.), is now represented by eclogites and amphibole-garnet-two pyroxene schists, incorporated into nappes of the Pekul'ney Range and forming inclusions in ultramafic rocks of the Koryak Mountains.