PYROXENES FROM FENITES OF THE EAST-AZOV REGION

Abstract

We researched the pyroxenes from alkaline metasomatites of East-Azov region (Ukrainian Shield) which have been discovered in Kalmius-Yelanchytskyi massif and near Oktyabrskyi and Pivdennokalchytskyi (South Kalchyk) massifs. These alkaline metasomatites are called fenites. Petrographic and geochemical studies of the rocks, many chemical and microprobe analyses of minerals and rocks, determination of the C, O, Sr isotopic composition of these rocks have been performed during recent years. All this confirms the conclusion that these rocks relate to the fenites. One of the main rock-forming mineral of these metasomatites is alkaline pyroxene (aegirine, aegirine-salite). The composition, forms of existence and paragenesis of alkaline min-erals in fenites and other alkaline rocks are characterized in this paper. As the observations in thin sections shows, at early stages of alkalinity of granitoides, new formed feldspar and aegirine contained or blue-green amphibole confined to the borders of quartz and biotite. Further, with increasing intensity of the process of alkaline metasomatism, these new formations grow. This quartz can completely substituted by albite and microcline, and partly ae-girine or move to the next zone. This obviously depends on the amount of quartz and silica neces-sary for the formation of alkali feldspar and aegirine. This is because the albite, microcline and aegirine are richer in silica than the original plagioclase and biotite, respectively. That’s why di-rect substitution biotite on aegirine is not observed, the formation of the latter is confined to the boundaries of biotite and quartz Fenites are formed on quartz-containing rocks. These are often granitoids, sometimes quartz-ites. The main minerals of fenites – alkali pyroxenes and amphiboles. Aegirine-containing pyrox-enes and aegirine-salite are formed at early stages in hypabyssal conditions. Mafic minerals of late stages of this process are presented by riebeckite-arfvedsonite (hypabyssal facies) or aegirine-salite, richterite, and Ca-Na-amphiboles (abyssal facies).