Petrological implications of the Early Mesozoic lamprophyre dikes and related Tarkhata syenites (SE Altai and NW Mongolia)

This paper presents new data derived from field sampling and from a thorough description of lamprophyres located in southeastern Altai and northwestern Mongolia in terms of their mineralogy, textures, and chemical composition. The swarms of alkaline mafic dikes in the area coexist with granosyenite-monzodiorite and gabbro-dolerite intrusions and spatially coincide with an ore district of Sb-Hg, Ag-Sb, Ni-Co-As, Cu-Mo-W, and CaF2 hydrothermal mineralization. All lamprophyres belong to the Early Mesozoic Chuya complex formed in an intracontinental enviroment. Their distribution and orientation is controlled by two large fault zones. The Chuya dikes were investigated at two localities, namely, Yustyd and South-Chuya. The Yustyd lamprophyres intrude Middle-Upper Devonian black shale of the Yustyd depression. At South Chuya, lamprophyres, together with the Tarkhata granosyenite-monzodiorite complex, are hosted by Cambrian and Ordovician metamorphic rocks of the South-Chuya Range. Ar-Ar (phlogopite) and U-Pb (SHRIMP, zircon) ages of the lamprophyre dikes indicate long and continuous period of the formation of the Chuya complex (250-235 Ma). Major- and trace-element compositions of the lamprophyres from both localities and of the syenite indicate their origin from the same magma source. The textures and structures of the lamprophyre and plutonic rocks, their mineral assemblages and the chemistry of the rock-forming minerals provide clues to the evolution of the parental alkaline mafic magma and fluid regime.