Chemical compositions of white micas in eclogites and country-rock gneisses from the Aktyuz area, northern Kyrgyz Tien-Shan

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The modes of occurrence and chemical compositions of white micas in the eclogites and surrounding country-rock gneisses in the Aktyuz area in Northern Kyrgyz Tien-Shan have been described. In the eclogites, white micas occur in the matrix and as inclusions within garnet and amphibole. They are classified as phengite, paragonite and muscovite. The highest Si contents (6.89-7.14 p.f.u.) are observed in phengite inclusions in garnets and in the cores of matrix phengites. Muscovite (Si = 5.96-6.09 p.f.u) occurs at the outermost rims of the matrix phengites. In the pelitic gneisses, phengites occur in the matrix and as inclusions within garnet, plagioclase and zircon. The cores of the matrix phengites and phengite inclusions within zircons have the highest Si contents of up to 6.83 p.f.u. and 6.88 p.f.u., respectively. The Si contents in the phengites suggest that the eclogites and the country rock gneisses experienced relatively high-pressure metamorphism. The muscovite rims of the matrix phengites indicate relatively low-pressure and high-temperature metamorphic conditions in the eclogites.

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