S.G. Kryvdik. SPECIFICITY OF MINERAL PARAGENESISES IN CARBONATITES OF CHERNIHIVKA MASSIF (WESTERN AZOV AREA)
SPECIFICITY OF MINERAL PARAGENESISES IN CARBONATITES OF CHERNIHIVKA MASSIF (WESTERN AZOV AREA)
S.G. Kryvdik, http://orcid.org/0000-0002-8356-1115
M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NAS of Ukraine
34, Acad. Palladin Ave., Kyiv, Ukraine, 03142
Mineralogical journal 2019, 41 (3): 48-53
The carbonatites of the Chernihivka complex sharply differ from the similar carbonatites of other complexes by a number of mineralogical, petrological and geochemical features. Among the most spectacular is an unusually high iron content in olivines (up to Fa65 in carbonatites and Fa70 in ijolite-melteigites). Carbonatites and alkaline rocks show wide range variations in chemical composition of olivine, from Fa10—15 to Fa70. At the same time, the dependence between the iron index in olivine (and other femic minerals) and the oxidation of iron (crystallization of magnetite) is observed. Mg# index of olivine rises at more magnetite present. High-Fe olivines are associated with iron-rich calcite (1.4 % FeO) and dolomite (>5.8 FeO). As one reason of this dependence is considered to be the thermal dissociation of siderite (FeCO3) of carbonatite melt that is accompanied by formation of magnetite, CO, CO2 and C (graphite). As a result of magnetite grains sinking, the residual melt becomes enriched in MgO and magnesian olivines (and other minerals) crystallized. This fact is also proved by the results of isotope-geochemical studies. Calcites of enriched in iron association show d18О and δ13С values typical of carbonatites (+6.1 and –5.1, respectively), whereas in magnesian varieties (with magnetite) indicate deviation from "mantle" range (d18О from +8 up to +17.5 ‰, d13С to –9.5 ‰). The temperatures determined on isotopic data range from 1050 °С for enriched in iron to 971 °С for magnesian paragenesises. Such features of carbonatites of the Chernihivka complex are supposed to indicate abyssal conditions of their crystallization, with this complex being one of the most deeply eroded (up to 20 km).
Keywords: carbonatites with high-iron olivine (up to Fa65), deep erosion level, dissociation of the FеCO3 molecule.
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