SOME SUBORDINATE MINERALS OF ALKALINE-ULTRABASIC ROCKS FROM THE NORTH-WESTERN PART OF THE UKRAINIAN SHIELD AND THEIR PETROGENETIC MEANING

Abstract

Microprobe analyses of perovskite, Ti-andradite and joanite from alkaline-ultrabasic rocks of the Ukrainian Shield north-western part are given for the first time. Certain peculiarities of the chemistry of minerals were revealed: the content of TiO2 reaches 10.5 % in Ti-andradite, and an increased content of Сr2О3 (1.3–1.5 %) was recorded in juanite, which is characteristic of other minerals of these rocks (pyroxenes, including Сr-diopside, am-phiboles). In addition to the previously described high-chromium (deep) Cr-spinelides, chromi-um-containing pyrope (2.14–6.33 % Сr2О3) was found, which indicates the deep (mantle) nature of the investigated rocks. Perovskite, Ti-andradite and juanite (pseudomorphoses of melilite) are characteristic (indica-tor) minerals of alkaline-ultrabasic hypabyssal intrusions of the Ukrainian Shield north-western part. On the other hand, they were not found in the deeply eroded intrusions of the same rocks in the other regions of the Shield (Chernihivka massif in the Azov region, Proskurivka and Anto-nivka ones – in the Pivdennyi Buh basin).