Age and source material of the Kingash ultramafic-mafic massif,East Sayan

The REE distribution patterns and Nd whole-rock and mineral isotope ratios of the Kingash ultramafic-mafic massif enabled us to propose a multistage history for its evolution at 1410 and 875 Ma. These stages reflect the magmatic evolution of the Siberian paleocontinent margin during the Late Precambrian. The age of metamorphism of the massif during collision and accretion in the Early Paleozoic (∼500 Ma) was obtained based on a Sm-Nd mineral isochron from rheomorphic veined albitite. The Nd and Sr isotopic compositions of rocks from the Kingash massif suggest mantle sources for picritic and basic magmas, which are thought to have originated by mixing of different proportions of depleted (PREMA or DM) and enriched (EM) melts. The initial isotope ratios of the parental melts transformed during interaction with Sr-rich material from the host metasedimentary complexes.     

OIB-type basalts of the Irbistu River basin (Southeast Mountain Altai): Evidence for the HIMU component in the magmatic source

It has been found that basalts of the Irbistu River basin (southeast Altai Mountains) belong to the intraplate OIB type. The peculiarities of their material composition do not indicate interaction effects between the basic melt and continental crust, and signifies that there was a magmatic source in the moderately depleted mantle comparable to the HIMU domain. It is suggested that basalts of the Irbistu River basin, along with other manifestations of similar volcanites in the Altai Mountains, reflect the Neoproterozoic-Early Paleozoic stage of the North Asian plume’s activity during closure of the Paleoasian Ocean.     

The alkaline and carbonatitic rocks of Gorny Altai (Edel'veis complex) as indicators of Early Paleozoic plume magmatism in the Central Asian Fold Belt

The minor intrusions of the Edel'veis alkaline–carbonatite complex are bounded by the spurs of the North Chuya Ridge in southeastern Gorny Altai. According to Ar–Ar isotope data, the complex formed in the Middle Cambrian (～507 Ma). All of its components (alkali clinopyroxenite–melanogabbro–alkali syenite + Ca-carbonatite) occur in only one pluton. Silicate igneous rocks are equivalent in silica content and alkalinity to potassic alkaline and subalkalic mafic rocks. Apatite-phlogopitic Ca-carbonatites are enriched in P₂O₅ (up to 3.6 wt.%), Sr (～2500–5500 ppm), and REE (up to ～2000 ppm) and are, presumably, of liquation genesis. A PREMA-type plume component was a predominant magma source for the complex (?_Nd(T) = +6.56 to +6.85). According to isotope data (⁸⁷Sr/⁸⁶Sr(T) ～ 0.7032–0.7039; δ¹⁸O ～ 7.5–14.9‰; δ¹³C ～ –2.7 to –8.4‰), the fractionation of the melts was accompanied by their crustal contamination. The trace-element composition of the rocks suggests that the complex developed on a continental margin and its development was accompanied by late-collisional rifting and the mixing of moderately depleted (PREMA) and enriched suprasubductional lithospheric mantle (EM I or EM II) with continental crust. It is presumed that the alkaline and carbonatite complexes in the western Central Asian Fold Belt are of primary plume origin and form a LIP within this belt together with other associations produced by Early Paleozoic (510–470 Ma) magmatism.     

The age and petrologic and geochemical conditions of formation of the Kogtakh gabbro-monzonite complex in the Kuznetsk Alatau

We present new data on the U-Pb age of zircons, geochemistry of trace elements, and isotope (Nd, Sr, O) composition of rocks of the Kogtakh gabbro-monzonite complex on the eastern slope of the Kuznetsk Alatau Ridge. The established age of zircon in the rocks of the main intrusive phases (500-480 Ma) is taken as the time of formation of the Kogtakh complex in the Late Cambrian-Early Ordovician, during the accretion-collision stage of evolution of the Central Asian Orogenic Belt. The distribution and ratios of LILE and HFSE in the rocks suggest that the intrusions proceeded under interaction of the PREMA + EM/OIB mantle plume and the suprasubductional lithospheric IAB mantle. The mantle-crust interaction led to the heterogeneous isotopic composition of neodymium in the magma source (?_Nd(T) ~ 3.5-5.4). The mixing of different mantle and continental-crust materials resulted in an increase in isotope parameters: (⁸⁷Sr/⁸⁶Sr)_T ~ 0.7039-0.7052, ?_Sr(T) ~ 0-19, and δ¹⁸O ~ 6.5-8.8‰ (SMOW). The REE ratio in the least differentiated gabbroids indicates different depths of probable magma chambers and the formation of their primary (Tb/Yb_PM > 1.8) mafic melts at different degrees of melting of the model garnet peridotite.     

Neoproterozoic carbonatite magmatism of the Yenisei Ridge, Central Siberia: 40Ar/39Ar geochronology of the Penchenga rock complex

⁴⁰Ar/³⁹Ar isotopic ages of magnesioarfvedsonite (725.9 ± 6.3 myr) and phlogopite (637.6 ± 5.7 myr) from carbonatites of the Penchenga rock complex located in the South Trans Angara segment of the Yenisei Ridge have been determined. Based on this, the formation of the rock complex is associated with the start of the period of Neoproterozoic (725–700 Ma) rifting and alkaline magmatism in the region, the latter being associated with the former. The appearance of zones penetrable for magma of the “disseminated” rifting type is associated with the development of late collision shear dislocations in a consolidated marginal continental crust. The primary melts had a plume nature, but matter from different mantle sources intermixed in the environment of the changing geodynamic regime. The younger age of the mica was apparently caused by partial remelting of carbonatites under the influence of heat and fluids generated during the formation of the adjacent large granite massif of a younger age.     

Late Precambrian OIB magmatism in the Kuznetsk Alatau,Siberia: Geochemical features of the Kulbyurstyug Formation volcanics

Basaltoids of the Vendian–Cambrian Kulbyurstyug volcanic complex in the eastern part of Kuznetsk Alatau have a high content of titanium (TiO₂ 3–4 wt %). They are relatively enriched with LILE and HFSE (Ba 360–900, Zr 160–726, Nb 66–101, ΣREE up to 225–329 ppm), and demonstrate a fractionated spectrum of REEs (La/Yb ～ 13–17), high Nb/U (44–66), and low Th/Ta and Th/Nb. These features are comparable with derivatives of OIB (Ocean Island Basalts) magmatism. The rock geochemistry suggests the possible formation of an initial mafic melt in the garnet-bearing peridotite mantle with 2–4% degree of melting and the presence of a small amount of spinel. By the age and composition, the studied volcanics are correlated with the OIB magmatism products, occurring in the adjacent Gorny Altai and also considered to be derivatives of intraplate magmatism of Riphean–Early Cambrian age. This magmatism was caused by plume activity during formation of the Paleo-Asian Ocean.     

Andradite–Morimotoite Garnets as Promising U–Pb Geochronometers for Dating Ultrabasic Alkaline Rocks

U–Pb geochronological studies of garnet of the andradite–morimotoite series and Sm–Nd geochronological studies of this garnet and apatite from the Chikskii Massif (Tuva-Mongolia microcontinent) were carried out. The garnet studied is characterized by relatively high concentrations of U (14–16 ppm) and by a low level of common Pb (Pb_с/Pb_t = 0.07–0.1). The concordia age of garnet is 492 ± 2 Ma (MSWD = 0.01, probability 92%) and matches within the error with the Sm–Nd age determined by the isochrone for apatite, garnet, and bulk rock (489 ± 9 Ma, MSWD = 0.86). This allows us to consider calcic garnets of the andradite–morimotoite series as promising mineral geochronometers for U–Pb dating of ultrabasic alkaline rocks.     

Parental Sources of High–Alumina Alkaline Melts: Nd,Sr, Pb,and O Isotopic Evidence from the Devonian Kiya–Shaltyr Gabbro–Urtite Intrusion,South Siberia

Doklady Earth Sciences - The isotope geochemistry (εNd(t) 4.8–5.4, 206Pb/204Pb in 18.05–18.36, 207Pb/204Pbin 15.53–15.57, 208Pb/204Pb in 37.59–37.83, 87Sr/86Sr(t)...     

Isotope composition and magma sources of the Kresty volcanopluton in Polar Siberia

The first data are presented on the distribution of rare elements and isotope composition of neodymium and strontium in the rocks of the alkaline-ultramafitic association of the Kresty volcanoplutonic structure (one of the probable satellites of the Gulinian massif being the largest in the Maimecha-Kotui alkaline province in Polar Siberia). On the basis of the results of geochemical studies performed and formerly obtained geophysical and isotope data, a new model is proposed for the plume nature of magmatism in the region, based on the action of a single mantle source of the PREMA type and its active interaction with the matter of the Earth’s crust.     

Early Paleozoic alkaline magmatism of the Altai Mountains: 40Ar-39Ar geochronology data for the Edel’veis complex

The isotopic age by⁴⁰Ar-³⁹Ar (506.8 ± 3.5 Ma) was determined for the rock-forming phlogopite from the alkaline carbonatite-containing Edel’veis complex in the southeast of the Altai Mountains. Based on this, the complex development is attributed to the formation of a large magmatic province at the Early Paleozoic stage (Caledonian, ～510–470 My ago) of the development of the Central Asian folded belt. By the features of substantial composition and the time of appearance, the Edel’veis complex is considered as a product of the primary phase of the activity of the mantle plume promoting the multiform magmatism of Central Asia in the Early and Middle Paleozoic. In view of the new geochronology data obtained, for the geodynamic evolution of the Gorno-Altai terrain, the authors propose to distinguish the three stages of the appearance of magmatism of increased alkalinity: the Early Paleozoic (Cambrian), Middle Paleozoic (Devonian), and Early Mesozoic (Triassic) stages.