Vol 54, No 9 (2016)

The study of melt microinclusions in olivine megacrysts from meimechites and alkali picrites of the Maimecha–Kotui alkali ultramafic and carbonatite province (Polar Siberia) revealed that the melt compositions corrected for loss of olivine due to post-entrapment crystallization of olivine on inclusion walls (differentiates of primary meimechite magma) match well to the composition of nephelinites and olivine melilitites belonging to carbonatite magmatic series. Modeling of fractional crystallization of meimechite magmas results in the high-alkali melt compositions corresponding to the silicate–carbonate liquid immiscibility field. The appearance of volatile-rich melts at the base of magma-generating plume systems at early stages of partial melting can be explained by extraction of incompatible elements including volatiles, by near-solidus melts at low degrees of partial melting, and meimechites are an example of such magmas. Subsequent accumulation of CO₂ in the residual melt results in generation of carbonate magma.

It is shown that the replacement and long evolution of miaskitic zircons led to the formation of two main age groups: 420–380 Ma (I) and 260–240 Ma (II). The age of miaskites is estimated at 440–445 Ma. Zircons I bear traces of fragmentation, dissolution, and replacement; they have “flat” REE patterns typical of metasomatic (hydrothermal) types, which is caused by allochthonous nature of the studied miaskites. Zircons II with differentiated REE patterns are similar to magmatic varieties, but have metamorphic origin. Mineralogical–geochemical and age characteristics of zircons in combination with structural–compositional features of miaskites define their metasomatic nature. The origin of the early zircon generations was related to the Ordovician rifting, while late generations were formed during shear deformations at the final stage of the evolution of the Uralian orogen.

Mineralogical–geochemical studies of zircon from the Ichet’yu occurrence revealed unusually high Y and HREE contents (correlative with the P content) in the inner parts and zones of approximately 10% of the grains. They represent the intermediate members of the zircon–xenotime join with a heterovalent scheme of isomorphism Zr⁴⁺ + Si⁴⁺ → (Y + HREE)³⁺ + P⁵⁺. Geochronological and mineralogical–geochemical data suggest that the Middle Timan basement (the most probable source of zircon of the Ichet’yu occurrence) is made up of the Paleoproterozoic rocks and possibly represents a continuation beneath the Mezen syneclise and Middle Timan of the Paleoproterozoic collisional structure, to which the Arkhangelsk diamond province is confined.

Major and trace element distribution in the bottom sediments from Hole 13 drilled in Lake Grand, Magadan district, was studied using the method of principal components. It was established that geochemical characteristics are correlated with environmental changes. The sediments of cold MIS2 and MIS4 are characterized by the enriched TiO₂, MgO, Al₂O₃, Fe₂O₃, and Cr and low Na₂O, K₂O contents, which is related to the grain-size composition of sediments. Sediments of warm stages show an opposite tendency. High concentration peaks of iron, phosphorus, and manganese correspond to the accumulation levels of vivianite and ferromanganese rocks. Silica is represented by biogenic and abiogenic varieties. Maximum SiO₂ contents were found in the Late Holocene sediments and mark the high biological productivity of the basin. Revealed variations of some elements are correlated with the Heinrich events.