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Rezumat articol ediţie STUDIA UNIVERSITATIS BABEÅž-BOLYAI
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Rezumat: Metapelites are important rocks, sensitive to the condition of metamorphism. Thus they can be used to estimate the pressure and temperature of the metamorphism. Despite these characteristics, it is difficult to find low-grade metapelites (greenschist facies) possessing enough phases in equilibrium to have low-variance assemblages. Therefore, the estimation of the pressure and the temperature of the metamorphism in these rocks is difficult. Some investigators have considered this problem and have attempted to calibrate useful geothermometers for low-grade pelitic rocks. The most important geothermometers applicable to low-grade metapelites are chlorite-garnet (for garnet phyllites and garnet schists) and chlorite-chloritoid Fe-Mg exchange geothermometers. Chlorite-chloritoid-garnet and chlorite-chloritoid assemblages were found in metapelites of the Sanandaj-Sirjan metamorphic belt in southern Iran. Other phases in the rocks include muscovite, quartz, magnetite, titanite and zircon. Chloritoid, chlorite, garnet and muscovite were analysed using electron microprobe and SEM. The chemical study of the minerals indicates that chlorites are ripidolite to daphnite. Chloritoids are Fe-rich and garnets are essentially almandine with appreciable amounts of grossular and spessartin. The analysed white micas are muscovites with considerable amounts of paragonite end-member, a celadonite end-member being absent. One calibration of chlorite-chloritoid Fe-Mg exchange geothermometer and three different calibrations of chlorite-garnet Fe-Mg exchange geothermometer were applied to metapelites of the Sanandaj-Sirjan metamorphic rocks. Chlorite-chloritoid thermometry yields temperatures of cca 515-557ºC for the rocks. Chlorite-garnet thermometry gives temperatures between 442 and 460ºC. The application of these thermometers yields fairly close temperatures for regional metamorphism in the studied area within the chlorite zone, indicating a medium to high greenschist facies. The difference between the results from garnet-chlorite and chlorite-chloritoid thermometers is about 70ºC. Most probably, the temperatures obtained using the garnet-chlorite thermometer are more realistic. If this is true, then chloritoid-chlorite thermometer overestimates temperature about 70ºC. ... |
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