Mineral inclusions in retrograde garnets from archean aluminous iron formations of the Voronezh crystalline massifs (features of the internal structure, crystallization conditions)

Abstract

In garnets from the Archean aluminous iron formations of the Voronezh crystalline massif, mineral inclusions of two types were found: solid-phase monomineral (pyroxenes, plagioclase, zircon, potassium feldspar, magnetite) and retrograde polymineral. Polymineral inclusions are crystalline mineral systems of siderite-pyrophyllite composition. The observed mineral paragenesis were formed as a result of the reaction: Opx + Pl + H2O + CO2 => Grt + Sd + Prf ± Cpx ± Mag. Thermometric estimates of garnet crystallization, obtained from mineral solid-phase inclusions (GOp geothermometry) are 636 ± 15 ° C (the stage of metamorphism M3). The stability temperature of the pyrophyllite-siderite association (350 to 550 ° C) can be considered as the lower boundary of the M3 metamorphism stage.