Geologicheskii Vestnik

This article presents data on "exotic" sulfide-rare earth assemblages-monazite-(Ce) + pyrite and allanite-(Ce) + xenotime-(Y) + pyrite-discovered in metamorphosed carbonaceous schists of the Ishly Graben. The formation mechanism of these assemblages is determined by stress-pressure metamorphism, the P–T parameters of which are reconstructed from the compositions of metamorphic minerals. The highest-temperature stage is the amphibolization of igneous rocks (T = 500–600 °C and P = ~3–5 kbar), while the formation of light-colored micas in terrigenous sediments occurred at P ~2–10 kbar and T = 390–490 °C. During this process, the chemical composition of the fluid phase changed from CaCl₂ + NaCl to FeCl₂. Stress pressure leads to the formation of tensile fractures (pressure relief zones), in which, in the presence of clastogenic rare earth minerals, their dissolution and local enrichment of the fluid with rare earth elements (REEs) occur. Since the release of pressure in a local zone places the system in a state of sharp disequilibrium, the co-crystallization of "incompatible" minerals (sulfide + REE phosphate and sulfide + REE phosphate + REE silicate) is possible within this volume, forming complex intergrowths.

Southern Urals, rare earth minerals, sulfides, metamorphism, paragenesis

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