Olivine Megacrysts in Alkaline Basalt Dikes of the Western Sangilen Region, Southeastern Tuva

Large (up to 4 cm) crystals of euhedral olivine and its intergrowths with clinopyroxene were discovered in the alkali–basaltoid dikes of the Western Sangilen region, Southeastern Tuva. The composition of olivine megacrysts (81–83 at % Mg#, 0.2–0.3 wt % NiO) is drastically different from that of olivines in mantle and gabbroid parageneses. Olivine is a liquidus phase crystallized from an alkaline basaltic melt at a pressure of 10–13 kbar in intermediate magma chambers located at a depth of 34–43 km corresponding to the level of the crust–lithospheric mantle boundary in the Western Sangilen region.