Olivine megacrysts in alkali-basaltoid dikes of the Western Sangilen, 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, South-Eastern Tuva. The composition of olivine (Mg# 81–83, NiO 0.2–0.3 wt. %) differs sharply from the composition of olivines from mantle and gabbroid parageneses. It is shown that olivine is a liquidus phase that crystallized from an alkali basaltic melt at pressures of 10–13 kbar in intermediate magma chambers located at depths of 34–43 km, which corresponds to the level of the crust-lithospheric mantle boundary for the Western Sangilen.