Late stages in the evolution of the late Mesozoic East Mongolian volcanic areal: rock age and composition

The Mandakh-Madal-Gobi (MMG) zone of alkali basalt magmatism has been delineated in the Late Mesozoic East Mongolian volcanic areal. It comprises clusters of igneous rock bodies and isolated stocks, domes, sills, laccoliths, dikes, and limited fragments of lava flows composed of tephrite, phono-tephrite, and trachybasalt. Two pulses of magmatism in the MMG zone have occurred in the Late Cretaceous (about 85 Ma) and Early Cenozoic (about 50 Ma). Recognition of this zone and deciphering of its formation history demonstrated that the development of the entire East Mongolian volcanic areal had the same regularities as those identified in other major regions of the Late Mesozoic Central Asia magmatic province. This indicates that the areal undoubtedly belongs to the latter. These facts support the conclusion that the correlation observed through the zone’s evolution between reduction in the volume of igneous products and the change in their composition towards the OIB was apparently determined by a decrease in the effect of thermal sublithospheric mantle melts upon a metasomatically enriched lithospheric mantle, leading to its gradual elimination from the magma sources.

Sources of eocene magmatism of Western Kamchatka (according to geochemical and isotope Sr-Nd-Pb characteristics of basites)

The isotope-geochemical characteristics of the Eocene-Oligocene magmatic rocks of Western Kamchatka were studied. It is shown that the igneous rocks of the Eocene (45-53 Ma) Kinkil complex of Western Kamchatka are characterized by geochemical signs of super-subduction volcanism. Their isotopic composition of Sr, Nd and Pb, low concentrations of HFSE and HREE relative to the composition of MORB, suggest the formation of primary melts from depleted or poorly enriched in isotopic composition of the mantle wedge sources in different contaminated quartz-feldspath sialic sediments. From the end of the Middle Eocene on the territory of Western Kamchatka, K-Na alkaline-basalt magmatism (46-31 million years), whose geochemical characteristics are similar to E-MORB, as well as ultrapotassic alkaline-basalt magmatism, which continued to develop to the early Miocene (35-17 million years). The geodynamic nature of Late Paleogene alkaline magmatism involves the implementation of processes of diffuse rifting.

On time of the triassic rifts system origin in West Siberia

It is commonly supposed that a very substantial volume of early basalt magmatism effused synchronously on Siberia platform and West Siberia in a very short time interval at 249.4 ± 0.5 Ma (Reichow et al., 2002, etc.). This magmatism and induced climate change are considered as a main reason of the most catastrophic in the Earth history extinction at the border of Permian and Triassic time. But these conclusions were based on incomplete and unrepresentative data on West Siberia. We have obtained by analysis of pyroxenes monofraction from kainotype basalts of Guslinskaya P-430 well Ar-Ar age 268.4 ± 7.5 Ma. In Taurovskaya 503 well this age is 268.1 ± 7.5 Ma. Hence, volcanism in axial rift zones of the basement of West Siberia plate began earlier than that considered before and significantly earlier than on Siberia platform.