Belsky and 3 co-authors reported in 2013 that allanite (orthite) — a rare earth silicate from epidote group commonly known as accessory mineral of granitoids has been analyzed in this work. Ore-bearing veins explored at the Anadol (Eastern part of Azov Megablock, the Ukrainian Shield) and known as "dike" are unique because allanite is the main mineral of rock formation.
Opening of almost monomineralic allanite veins among granites naturally led to the question of the conditions of its formation. Chemical composition of the Anadol allanite and composite is shown that the mineral from the Anadol "dike", as well as some other findings in the Azov Megablock, are characterized by volatile content of major elements, including REE, Fe, Al and Ca within one ore occurrence, which distinguishes it from the Japan island arc allanite
and others.
The ratio of Fe2+/Fe3+ is especially volatile, as a consequence of changing oxygen fugacities (fO2), which does not allow using the discriminatory chart of M. Hoshino et al. (2005) to identify the tectonic conditions of formation of "dikes". Composition of volatile impurities of the Anadol allanite obtained as a result of gas analysis of products of high-temperature pyrolysis, also has wide variations in the content of H2O, CO2, H2, CO, N2.
There are two generations of allanite — early and late ones, identified in the Anadol ore occurrence. They differ in content of major and minor components. The amount of water in the products of pyrolysis increases from early to late generation, which is associated with a decrease in the formation temperature of the generations. The amount of CO2 decreases that is seen as a consequence of reducing of fluid pressure and the values of H2O/H2 and CO2/CO
increases as a reflection of the growth potential of oxygen to end of crystallization. Mean values of H2O/H2 and CO2/CO pyrolysis products in both generations are by an order of magnitude greater than those for massive entourage of the "dike".
On these indicators, the "dike" veined rock and allanite belong to the most oxidized ones and have not analogues in the Ukrainian Shield. High oxygen
potential during crystallization of allanite late generation was first recorded in formation of NO in the high-temperature pyrolysis products of minerals. The data of pyrolysis gas chromatography and previously obtained results of the study of fluid inclusions in quartz allow the authors to conclude that the formation of Anadol ore-bearing veins was accompanied by a significant reduction in the temperature from >850 (early allanite generation) to <450
(later allanite generation) and 170—140°C (quartz), and a moderate decrease in fluid pressure at the time of the late allanite crystallization was <<30 MPa.
High-oxygen potential in the mineral formation environment was caused by the shallow depth of formation of "dike" and alkaline
characteristics of the parent fluid.
See ISSN 0204-3548. Mineral. Journ. (Ukraine). 2013. 35, No. 1 - for similar studies in Ukraine.
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