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High Temperature Reduced Granulite-Facies Nature of Garnetites in the Khabarny Mafic–Ultramafic Massif, Southern Urals: Evidence from Fluid and Mineral Analysesстатья
Статья опубликована в высокорейтинговом журнале
Статья опубликована в журнале из списка Web of Science и/или Scopus- Авторы: Bakker Ronald J., Pushkarev Evgenii, Biryuzova Anna P.
- Журнал: Journal of Petrology
- Том: 61
- Номер: 6
- Год издания: 2020
- Издательство: Oxford University Press
- Местоположение издательства: United Kingdom
- DOI: 10.1093/petrology/egaa066
- Аннотация: High-grade metamorphic rocks underlying the intrusive layered dunite–pyroxenite–gabbronoriteEast-Khabarny Complex (EKC) are integrated in the complex Khabarny mafic–ultramafic Massif inthe Sakmara Allochthon zone in the Southern Urals. These rocks are associated with hightemperatureshear zones. Garnetites from the upper part of the metamorphic unit close to the contact with EKC gabbronorite are chemically and texturally analysed to estimate their formation conditionsand fluid regime. Fluids provide crucial information of formation conditions and evolutionof these garnetites during high-grade metamorphism, and are preserved in channel positions withinSi6O12 18 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 to80 vol. %), cordierite and quartz. Accessory minerals are rutile, ilmenite, graphite, magnetite and cristobalite. Granulite-facies metamorphic conditions of the garnetites are estimated with the garnet–cordierite–sillimanite–quartz geothermobarometer: temperatures of 740 to 830 °C and pressures of 770–845 MPa. The average garnet composition in end-member concentrations is 485 mole % almandine (639), 347 mole % pyrope (633), 103 mole % spessartine (611), 18 mole % grossular (615), and 15 mole % andradite (615). The cordierite electron microprobe analyses reveal an average Mg2þ fraction of 0796001 in the octahedral site. Relicts of a strong positive temperature anomaly (up to 1000 °C) are evidenced by the preservation of cristobalite crystals in garnet and the high titanium content of quartz (003160008 mass % TiO2) and garnet (0316016 mole % endmember Schorlomite-Al). The fluid components H2O, CO2, N2 and H2S are detected in cordierite, which correspond to a relatively 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 0349 to 0367 gcm-3. The CH4-rich fluid may represent peak-temperature metamorphic conditions, and is consistent with temperature estimation (1000 °C) from Ti-in-garnet and Ti-in-quartz geothermometry. Tiny CH4-rich fluid inclusions (diameter 05 to 2mm) are also detected by careful optical analyses in garnet and at the surface of quartz crystals that are included in garnet grains. Graphite in fluid inclusions precipitated at retrograde metamorphic conditions around 300–310627 °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 (wu¨ stite–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 result of magmatic underplating that provided an appearance of a positive thermal anomaly, and further joint emplacement (magmatic and metamorphic rocks together) into upper crustal level as a high temperature plastic body (diapir).
- Добавил в систему: Пушкарев Евгений Владимирович