ICP AES, microprobe, and X-ray powder diffraction data for garnets from metamorphic rocks in the Sakar region, SE Bulgaria (original) (raw)

2006, Geochemistry, Mineralogy and Petrology

Abstract. In order to obtain informative data on the crystal chemistry of garnets from metamorphic rocks of the Zhulti Chal and Ustrem Formations in the rim of the Sakar pluton (SE Bulgaria), their chemical composition, trace elements, unit cell parameters and compositional zoning were investigated. All garnets are almandine rich. For samples from the Zhulti Chal Formation the molar percentage of almandine range from 70.2 to 79.0, of grossular – from 4.2 to 16.5, of pyrope – from 5.0 to 14.0 and of spessartine – from 3.6 to 11.4. For samples from the Ustrem Formation the molar percentage of almandine range from 72.7 to 74.8, of grossular – from 9.8 to 14.5, of pyrope – from 8.0 to 10.4 and of spessartine – from 3.2 to 6.4. The values of the unit cell parameter of garnets from the Zhulti Chal Formation range between 11.544(4) and 11.597(3) Å while those from the Ustrem Formation – between 11.552(3) and 11.583(3) Å. The (FeO+MgO) / (CaO+MnO) oxide ratio and the unit cell parameters allow suggesting that samples from the Zhulti Chal Formation were formed in more variable temperature conditions of metamorphism in comparison with samples from the Ustrem Formation. The following trace elements were determined in the studied garnets: P2O5 (0.03-0.20 wt.%), SO3 (<0.03-0.37 wt.%), Ba (10-92 ppm), Co (<10-18 ppm), Cr (63-128 ppm), Ni (<10-40 ppm), Sr (6-36 ppm), V (56-115 ppm), Zn (104-218 ppm), Zr (15-31 ppm). The zoning paths of the studied garnets, except sample No 2, show normal type of compositional zoning. It is of prograde genesis. Only in the garnet rims the retrograde features were observed. The examination of the growth zoning features of sample No 2 from the region of the Orlov Dol village shows complex compositional zoning, which is a possible indication for growth under polymetamorphic conditions. Key words: garnet, chemical composition, trace elements, unit-cell parameter, metamorphism.