PETROLOGY OF THE LAMPROITE AND GRANITE–SYENITE SERIES OF THE BILIBIN (<i>Lomam</i>) MASSIF (<i>Aldan–Stanovoy Shield</i>)

Abstract The Bilibin massif is a Mesozoic intrusion located in the southeastern part of the Aldan high-potassium igneous province of the Aldan–Stanovoy Shield of the Siberian Platform. This massif consists of the alkaline–mafic–ultramafic and granite–syenite phases, which form a concentric structure with the rocks becoming more silica-rich from periphery to center. Earlier studies proposed that these phases formed either from different parental magmas or from a common lamproitic magma via magmatic differentiation. In this study, we examined a representative set of rock samples from the Bilibin massif: phlogopite clinopyroxenites, melashonkinites, shonkinites, alkali syenites, quartz syenites, and granites. Mineralogical–petrographic, geochemical, and isotope–geochemical data imply that the series of differentiation of lamproitic magma comprises rocks from clinopyroxenites to shonkinites and possibly alkali syenites, which form the first phase. The quartz syenites and granites, which form the second phase, belong to a separate magmatic series. According to geochemical data and the Nd, Sr, and O isotope composition of the quartz syenites and granites, the magmas which formed the syenites and granites were derived from the lower crust rather than from the lithospheric mantle and originated owing to either crustal melting during mantle-derived magmatism or collisional tectonics at the southern margin of the Siberian Platform in the Mesozoic Era. Rocks of the lamproite series allow us to consider it as an example of complete differentiation of a mantle lamproitic melt, with the following stages of cotectic crystallization: olivine + chromite, olivine + clinopyroxene + chromite, olivine + clinopyroxene + phlogopite, clinopyroxene + phlogopite + leucite, and clinopyroxene + phlogopite + K-feldspar. Spot analyses of trace elements in clinopyroxene, phlogopite, leucite, and apatite allow estimation of the melt–mineral partition coefficients in such a system.