Vol 12, No 3 (2018)

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.

Complex mineralogical, geochemical, and geochronological studies of the gabbroids from the Dzhigdinskii Massif located in the western part of the Dzhugdzhur–Stanovoy Superterrane are performed. It is established that the age of the rocks from the Dzhigdinskii Massif is Middle Triassic (244 ± 5 Ma), rather than Early Archean, as was previously assumed. The age of the Dzhigdinskii Massif is close to the age of the formation of the other Triassic gabbroid massifs, such as the Amnunaktinskii (~240 Ma), Lukindinskii (~250 Ma), and Luchinskii (~248 Ma) in the southeastern environ of the North Asian Craton. One of the stages in the formation of the Selenga–Vitim volcanoplutonic belt falls in this period as well. This indicates that the Selenga–Vitim volcanoplutonic belt, along with the granitoids and volcanic rocks, is composed of ultrabasic–basic and basic massifs and that this belt is superposed on the structures of the Selenga–Stanovoy Superterrane, as well as on the western part of the Dzhugdzhur–Stanovoy Superterrane. The gabbro, gabbro–diorite, and series of gabbro and gabbro–diorite with high sodic alkalinity from the Dzhigdinskii Massif show obvious geochemical features of duality, including combination of intraplate and super-subduction origin. In this relation, we can assume that the origin of the gabbroids of the Dzhigdinskii Massif is related to the detachment of the oceanic lithosphere and its subduction into the mantle with the formation of an “asthenospheric window.”