Polyphase Inclusions in Chromium Spinels from Upper Triassic Gravelites from the Northeastern Part of the Siberian Platform

A comparative study of the chemical compositions of chromium spinel and polyphase inclusions has been carried out. Chromium spinels were separated from the concentrate of the Carnian Stage (T₃) rock sample collected at the northeastern fringe of the Siberian platform. The chromium spinels show a wide variety of compositions, most of which are of the “Kurung” type, which is considered to be a deceptive indicator of a kimberlite. They contain inclusions in the size range of 10–100 μm and contain several mineral phases. The most frequent inclusions are olivine and clinopyroxene; additional phases are nepheline, K–Fsp, Ti–Mg–Ca–amphibole, Ti–biotite (phlogopite), apatite and perovskite. Ilmenite and iron sulfides are less common. The data obtained on the composition of these inclusions indicate differences in the conditions of their formation and possibly different types of parental sources. A possible type of provenance source could be potassium alkali basites.