High-pressure ultramafics in the lower crustal rocks of the Pekul’ney complex, central Chukchi Peninsula. 2. Internal structure of blocks and ultramafic bodies, geologic and geodynamic setting of rock formation

Abstract

The detailed mapping of the blocks of the Pekul’ney complex revealed that cumulate ultramafics occur as separate tabular bodies among metamorphic rocks and are only fragmentarily observed in some of the blocks. Within these bodies, different types of ultramafics are regularly and multiply intercalated, forming banded structures, which supports their assignment to a single cumulate series. The tabular ultramafic bodies investigated in different blocks of the Pekul’ney complex are from 350 to 1100 m thick, and their internal structure is made up of intercalated regular rhythms of dunites-peridotites and olivine pyroxenites-olivine-free ultramafics (garnet, ceylonite, and clinozoisite clinopyroxenites, websterites, and hornblendites) and units of irregularly interlayered dunites, peridotites, and olivine pyroxenites. The thickness of individual regular rhythms ranges from 50 to 410 m. The cumulate ultramafics of the Pekul’ney complex were derived from a water-rich highly magnesian primary melt, which was equilibrated with mantle harzburgites, within a wide temperature range at pressures of 11–13 kbar in the geodynamic setting of the base of an ensialic arc. The Pekul’ney complex can be considered as a reference object for the petrological and geochemical investigation of the evolution of suprasubduction mantle melts during their high-pressure fractionation.