Spinel–Sapphirine Reaction Structures in the Garnet Metaultramafic Rocks of the Omolon Massif: Petrogenesis and Geological Interpretation (Northeast Asia)

Spinel–sapphirine reaction structures are studied in detail in the sapphirine gedritites which form a small segregation in one of the garnet metaultramafic bodies framing the Aulandzha charnockitic dome (the pre-Riphean basement of the Omolon Massif). It is established that the sapphirine gedritites resulted from the retrograde evolution of the garnet–spinel metaultramafic rocks, the formation temperature of which may have exceeded 900°C at a probable pressure no more than 7 kbar. It is shown that the spinel–sapphirine reaction structures were formed under conditions of elevated O₂ potential close to the magnetite–hematite buffer. Subsequent diaphthoresis of the metaultramafic rocks was accompanied by an increase in the H₂O potential and decrease in temperature to at least 760°C. This explains the formation of another group of diaphthorites, garnet gedritites, the index minerals of which are sodium gedrite and calcic plagioclase with strong inverse zoning. A comprehensive analysis of the 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 1.9 Ga (middle of the second half of the Early Proterozoic, according to the General Stratigraphic Scale of Russia). The unusually high value of the O₂ potential calculated for the spinel–sapphirine reaction structures; the unique magnesian–alumina composition of the gedritites; and the extremely high contents of Zr, Ba, Rb, and Hf allow O.V. Avchenko to hypothesize that the protoliths of these scarce rocks were products of weathering crust after orthomagmatic ultramafic rocks. In this case, the calculated parameters for the formation of the spinel–sapphirine reaction structures may indicate that the value of the O₂ potential on the Earth’s surface in the Paleoproterozoic corresponded to the magnetite–hematite buffer.