Crystal chemical features of garnets from metamorphic rocks of Zhalti Chal and Ustrem Formations from the frame of Sakar pluton, SE Bulgaria.

Tzankova, N., O. Petrov, M. Kadiyski. 2006. Crystal chemical features of garnets from metamorphic rocks of Zhalti Chal and Ustrem Formations from the frame of Sakar pluton, SE Bulgaria. 2006. – Comptes rendus de l’Academie bulgare des Sciences, 59, 5, 531-538.

Abstract
To elucidate the garnet forming processes and their relation to the metamorphism in the rim of the Sakar pluton (SE Bulgaria) the crystal chemical features of garnets (chemical composition, admixtures, unit cell parameter, and compositional zoning) in metamorphic rocks of Zhalti Chal and Ustrem Formations are studied. The garnets are almandine rich. For the samples from Zhalti Chal Formation the molar percentages of almandine are in the range 70.23-78.96, of grossular – 4.24-16.53, of pyrope – 5.01-13.97 and of spessartine – 3.56-11.37. For the samples from Ustrem Formation the molar percentages of almandine are in the range 72.67-74.76, of grossular – 9.83-14.46, of pyrope – 8.02-10.37 and of spessartine – 3.20-6.44. The values of the unit cell parameter of the garnets from Zhalti Chal Formation range from 11.544 to 11.597 Å while those from the Ustrem Formation – from 11.552 to 11.583 Å. The oxide ratio (FeO+MgO)/(CaO+MnO) and the unit cell parameter of the garnets show that the samples from Zhalti Chal Formation formed in more variable P-T conditions of metamorphism compared to the samples from Ustrem Formation. The zoning paths of all studied garnets, except sample 2, are of the normal type characterizing a prograde genesis. Retrograde features were observed only in the garnet rims. Examination of the growth zoning of sample 2 reveals a complex compositional zoning – indication for growth under polymetamorphic conditions.

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