New geochemical and isotope (Rb-Sr and Sm-Nd) data for the rocks of the alkali-ultramafic Konder massif of the Aldan Shield (Khabarovsk Territory, Russia)

Abstract

The chemical composition and Rb-Sr and Sm-Nd isotope characteristics of various rocks that make up the alkali-ultramafic Kondyor massif of the Aldan Shield were studied: dunites, clinopyroxenites, gabbro, kosvites, alkaline syenites. Ultrabasic rocks of the early stage (dunites, clinopyroxenites) have a significant metasomatic alteration associated with the intrusion of a dike complex of alkaline rocks. The petrochemical features of dunites and clinopyroxenites indicate that these rocks are cumulative phases that successively crystallized from picritic melt at an early stage of the formation of the massif. Gabbro and kosvites of the marginal series were formed from the residual melt after the crystallization of dunites and clinopyroxenites. The Rb-Sr and Sm-Nd isotopic characteristics in the rocks of the early intrusion stage (dunites, clinopyroxenites, gabbro) indicate that these rocks were formed with the participation of the host rock contamination processes. The isotope-geochemical characteristics of the rocks of the dike complex (kosvites and alkaline rocks of the dunite core) indicate that these rocks crystallized from a deep melt that was not significantly contaminated by the continental crust. Differences in isotopic characteristics between ultramafic rocks and rocks of the dike complex of the dunite "core" - kosvites and alkaline rocks, suggest at least two sources of melts that participated in the formation of the Konder massif: (1) an early, deep-seated melt that formed ultrabasic rocks at the upper level, and also kosvites, plagioclase pyroxenites and gabbro of the outer “ring” with a significant degree of contamination; (2) late, differentiated (monzonite and alkaline-syenite) deep-seated melt (far from the contaminant) that became the source of kosvite and alkaline rock dikes in the central part of the Konder massif.