V. V. Vrublevskii

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.

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

Abstract40Ar/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.

Zircon age range and sources of alkaline rocks from the Kurgusul intrusion, Kuznetsk Alatau: The first U-Pb (SHRIMP II) and Sm-Nd isotope data

U-Pb isotope analysis of basic feldspathoid rocks (juvites) of the Kurgusul intrusion, NE Kuznetsk Alatau, revealed three generations of zircons of various ages (∼1.3–1.5 Ga; 484.3 ± 5.5 and 393.6 ± 9.2Ma). This suggests several stages of regional alkaline basic magmatism in the Middle Cambrian-Early Ordovician and Early-Middle Devonian and melting of the Mesoproterozoic continental crust, which form part of the basement of the Kuznetsk Alatau terrane. The trace element geochemical data indicate a continental margin setting characterized by the complex interaction of a plume-related, supra-subduction zone and crustal materials. A possible model of the magmatic evolution assumes that the ascent of a plume at the Cambrian-Ordovician boundary induced generation of the initial alkaline basaltic magmas and metasomatism and erosion of the lower lithosphere. Repeated partial melting of the lower crust after a 100 Myr period produced new magma batches of alkaline composition, which inherited zircons from the preceding episode of magma generation. The inherited juvenile magmatic source (PREMA + E-type MORB + EM) is confirmed by similarities in the Sm-Nd isotopic signatures (ɛNd(T) ≈ +4.5 to +5.7, T(Nd)DM ≈ 0.8–0.9 Ga) of derivatives of regional alkaline basic complexes of different ages.

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.

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.

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.

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 (Pbс/Pbt = 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

The isotope geochemistry (εNd(t) 4.8–5.4, 206Pb/204Pbin 18.05–18.36, 207Pb/204Pbin 15.53–15.57, 208Pb/204Pbin 37.59–37.83, 87Sr/86Sr(t) 0.7048–0.7057, δ18OSMOW 8–10.5‰) and trace element composition of the Kiya–Shaltyr gabbro–urtite pluton allow us to suggest a heterogeneous source and complex geodynamic settings of the Devonian alkali magmatism in the Kuznetsk Alatau. It is assumed that its evolution took place under conditions of partial mingling of matter of the depleted (PREMA) and enriched (EM) mantle with crustal contamination of the evolving melt. Such an interaction could have been a result of superposition of a mantle plume and an active margin (OIB and IAB components). In fold belts this led to the formation of hybrid high-alumina foidoite magmas.