High temperature reduced granulite-facies nature of garnetites in the Khabarny Mafic-Ultramafic Massif, Southern Urals: evidence from fluid and mineral analyses

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  • The Uralian Branch of Russian Academy of Sciences

Abstract

High metamorphic rocks underlying the layered dunite-pyroxenite-gabbronorite East-Khabarny intrusion (EKI) are integrated in the complex Khabarny mafic-ultramafic Massif in the Sakmara Allochthon zone in the Southern Urals and reveal strong deformation and high-temperature shear zones. Garnetites from the upper part of the metamorphic unit close to the contact with EKI gabbronorite were chemically and texturally analysed to estimate their formation conditions and volatiles environment. Fluids play an important role in the formation and evolution of these garnetites during high-grade metamorphism, and the fluid may be preserved in channel positions within Si6O1812- rings of cordierite, and in fluid inclusions in quartz and garnet. Minerals and fluid inclusions of the garnetites are studied with X-ray fluorescence spectrometry, electron microprobe analyses, Raman spectroscopy, and microthermometry.
The garnetites mainly consist of garnet (up to 80 vol.%), cordierite and quartz. Granulite-facies metamorphic conditions of the garnetites are estimated with the garnet-cordierite geothermobarometer: temperatures of 740 to 830 ˚C and pressures of 770 to 845 MPa. The average garnet composition in end-member concentrations is 48.5% almandine (±3.9), 34,7% pyrope (±3.3), 10.3% spessartine (±1.1), 1.8% grossular (±1.5), and 1.5% andradite (±1.5). The cordierite electron microprobe analyses reveal an average Mg2+ fraction of 0.79 ± 0.01 in the octahedral site. Relicts of a strong positive temperature anomaly are evidenced by the preservation of cristobalite crystals in garnet and the high titanium content of quartz (0.031 ± 0.008 mass% TiO2). The fluid components H2O, CO2, N2, and H2S are detected in cordierite, which correspond to the relative oxidized fluid environment that is common in granulites. In contrast, a highly reduced fluid environment is preserved in fluid inclusions in quartz nodules, which are mono-fluid phase at room temperature and composed of CH4 (>96 mole%) with locally minor amounts of C2H6, N2 H2S and graphite. The fluid inclusions occur in homogeneous assemblages with a density of 0.349 to 0.367 g·cm-3. Tiny CH4-rich fluid inclusions (diameter 0.5 to 2 µm) are detected by careful optical analyses in garnet and at the surface of quartz crystals within clusters in the centre of garnet grains. Graphite in fluid inclusions precipitated at retrograde metamorphic conditions around 300 to 310 ± 27 ˚C. Aragonite was trapped simultaneously with CH4-rich fluids and is assumed to have crystallized at metastable conditions.
The initial granulite facies conditions that led to the formation of a cordierite and garnet mineral assemblage must have occurred in a relative oxidized environment (QFM-buffered) with H2O-CO2-rich fluids. Abundant intrusions or tectonic emplacement of mafic to ultramafic melts from the upper mantle that were internally buffered at a WI-buffered (wüstite-iron) level must have released abundant hot CH4-rich fluids that flooded and subsequently dominated the system. The origin of the granulite-facies conditions is similar to peak-metamorphic conditions in the Salda complex (Central Urals) and the Ivrea-Verbano zone (Italian Alps) as a results of magmatic underplating that provided an appearance of positive thermal anomaly, and further joint emplacement (magmatic and metamorphic rocks together) into upper crustal level as a high temperature plastic body (diapir).

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Original languageEnglish
JournalJournal of petrology
Volume2020
DOIs
Publication statusPublished - 2020