Mineralogical and geochemical constraints on the origin and evolution of albitites from Dmytrivka at the Oktiabrski complex, Southeast Ukraine

Abstract

Albitites from Dmytrivka quarry form veins like bodies within fenitized alkali granites of the Oktiabrski complex. They are mainly composed of albite, and subordinately K-feldspar, zircon, aegirine, astrophyllite, pyrochlore and ilmenite. Albitites are characterized by high alkalies (Na2O + K2O) content coupled with low MgO, CaO, and SiO2 concentrations relative to host alkali granites. They show inversion of REE patterns (negative slopes for LREE and positive slopes for HREE), enrichment in HFSE (i.e. Zr, Nb, Ta), and strong Eu anomalies. The existence of rare composite of M-type and W-type REE tetrad effect in albitites indicates their complex genesis, i.e. formation during the coexistence of the melt with aqueous phase, later being superimposed by further alterations with hydrothermal solutions. The magmatic origin of albitites (evolved from highly differentiated A-type magma and emplaced at shallow depths) is documented by the presence of flow and snowball textures, zircon-aegirine glomerocrysts, no traces of replacement interfaces and reaction zones between K-feldspar and albite as well as no smooth, gradual changes in bulk-rock compositions between alkali granite, fenite vs albitite. On the contrary, the strong influence of pervasive, post-magmatic high-temperature (381–392 °C evidenced by CM geothermometer), and highly-oxidizing alkaline-bearing metasomatizing fluids on albitites is revealed by the occurrence of polygonal textures of albite laths, red CL luminescence of feldspars typical of fenitized rocks as well as crystallization of astrophyllite at the expense of arfvedsonite. Additionally, the presence of fluid-mineral interactions is evidenced by the development of secondary porosity and patchy zoning within zircon crystals coupled with the presence of blue (Dy3+-Tb3+-activated) and yellow (SiO4-Tb3+-activated) domains within them. All these textural, mineralogical and geochemical features observed in albitites from Dmytrivka suggest complex, magmatic-hydrothermal nature of the rocks.