THE SPINEL-SAPPHIRINE REACTION STRUCTURES IN THE GARNET METAULTRAMAFIC ROCKS OF THE OMOLON MASSIF: PETROGENESIS AND GEOLOGICAL INTERPRETATION (NORTHEAST ASIA)
Spinel-sapphirine reaction structures in sapphirine gedritites which form a small segregation within one of the bodies of garnet metaultramafic rocks framing the Aulandzha charnockitoid dome (the pre-Riphean basement of the Omolon Massif, Northeast Russia) have been studied in detail. It is established that sapphirine gedritites resulted from the regressive evolution of garnet-spinel metaultramafic rocks, the formation temperature of which could exceed 900° C at a probable pressure not exceeding 7 kbar. It is shown that spinel-sapphirine reaction structures were formed under conditions of an increased O<sub>2</sub> potential close to the buffer equilibrium of magnetite-hematite. Retrograde metamorphism was accompanied by an increase in the potential of H<sub>2</sub>O and a decrease in T to at least 760° C. This process reasons the development of another group of diaphthorites - the garnet gedritites, whose index-minerals are sodium gedrite and basic plagioclase with strong inverse zoning. A comprehensive analysis of new petrological data and published materials on the isotope-geochronological study of the pre-Riphean basement of the Omolon Massif makes it possible to attribute the formation of the sapphirine gedritites to the turn of 1.9 Ga (the middle of the second half of the Early Proterozoic, according to the General Stratigraphic Scale of Russia). The unusually high value of the O<sub>2</sub> potential calculated for spinel-sapphirine reaction structures, the unique magnesian-alumina composition of gedritites, extremely high concentration of Zb, Ba, Rb, and Hf offer a conclusion that the gedritites are the products of crust weathering of orthomagmatic ultramafic rocks. In this case, the calculated parameters for formation of the spinel-sapphirine reaction structures may indicate that in the early Proterozoic the value of the O<sub>2</sub> potential on the Earth's surface most probably corresponded to the magnetite-hematite buffer.