—Unique magnesian–aluminous ultrabasic sapphirine–spinel and garnet gedritites of the Aulandzha block were studied. The deepest part of the Archean Stage of the Omolon massif is exposed in this district. The gedritites differ strongly in petrochemistry and geochemistry from other metaultramafites of this block. The difference is in their enrichment in alumina, zirconium, barium, rubidium, hafnium, and uranium and depletion in calcium and heavy rare-earth elements. The oxidizing potential in the sapphirine–spinel assemblage, estimated by physicochemical modeling, showed high oxygen fugacity in these rocks, close to the value of the magnetite–hematite buffer, which was never observed in ancient granulite complexes. The above petro- and geochemical features of the gedritites of the Aulandzha block, having an isotopic age of 1.9 Ga, are explained by the fact that these rocks are probably the crust of weathering of the enclosing metaultramafites. If this hypothesis is true, then the above rocks may indicate that the oxygen potential on the Earth’s surface corresponded to the magnetite–hematite buffer as early as the Paleoproterozoic.

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