Evolution of Jurassic–Cretaceous magmatism in the Khambin volcanotectonic complex (western Transbaikalia)

The Khambin volcanotectonic complex is a horst framing the Late Cretaceous Lake Gusinoe basin in the northwest. This complex is due to the intracontinental rift conditions which existed in western Transbaikalia in the Late Mesozoic. They gave rise to a system of subparallel grabens and horsts in present-day topography. The magmatic evolution of this complex spans from 159 to 117 Ma and is divided into three stages. The first stage (159–156 Ma) witnessed the formation of thick (up to 1500 m) volcanic masses of trachybasalts, basaltic trachyandesites, trachytes, trachydacites, trachyrhyolites, and pantellerites. The next two stages were the formation of isolated ancient volcanoes (127–124 Ma) composed of trachybasalts, basaltic trachyandesites, phonotephrites, tephriphonolites, and alkali trachytes and the formation of the Murtoi (Lake Gusinoe) essexite dike (122–117 Ma). The main trends for igneous associations from early to late stages are reduced magmatism and reduced rock diversity because of the decreasing portion of felsic volcanic rocks. Mafic rocks show an increase in total alkalinity, the content of incompatible elements (Th, U, K, Rb, Pb, Nb, Ta, Zr, Hf), total REE content, and the LREE/HREE ratio. The Sr–Nd isotopic composition of these rocks remained nearly constant and corresponds to that of OIB-EMII mantle sources. Compositional variations are attributed to a time-dependent decrease in the degree of partial melting of a similar magma source.