Differentiation of lamproitic magma; case study of Mesozoic high-K dikes of the Ryabinovyi Massif (central Aldan)

--Studies of the mineral-petrographic and geochemical compositions of high-potassic lamprophyric dikes of the Tobuk complex, manifested at the Ryabinovyi plutonic massif (Central Aldan Mesozoic magmatic province, Russia), have shown that these dikes compose a single fractionation series formed from a high-Mg lamproitic parental melt in an intermediate chamber. The composition of the rocks ranges from olivine-diopside-phlogopite and diopside-phlogopite lamproites through minettes to microsyenites and syenite-porphyry. Early crystallization of high-Mg olivine and chromite in an intermediate chamber could produce cumulative dunites similar to those of the Inagli intrusion. Crystallization of olivine and chromite was followed by cotectic crystallization of olivine and clinopyroxene, then that of clinopyroxene and phlogopite, and, finally, eutectic crystallization of Na-rich clinopyroxene, phlogopite, and K-feldspar. Crystallization and gravitational differentiation of lamproitic melt was complicated by silicate-carbonate immiscibility, which is texturally manifested in minettes as carbonate-silicate globules and interstitial calcium and magnesium carbonates. Furthermore, compositional zoning of Sr in apatite and Ba in phlogopite and K-feldspar is considered to have resulted from the immiscibility. Separation of the carbonate-fluorite melt fraction might have led to formation of the carbonatite and fluorite-carbonatite schlieren and gangues which have been described in drill cores from the Ryabinovyi massif. In most of the geochemical and mineralogic features, the Ryabinovyi massif lamproites are similar to the low-Ti lamproites of the Mediterranean postcollisional belt and northern Vietnam and differ from typical high-Ti within-plate lamproites.