Petrology of the Late Jurassic ultramafic-mafic Veselki massif,southeastern framing of the Siberian craton

The Veselki peridotite-websterite-gabbronorite massif was dated by U-Pb zircon method at 154 ± 1 Ma. This is the first Late Jurassic date obtained for ultramafic-mafic massifs in the eastern part of the Selenga-Stanovoy superterrane bounding the southeastern margin of the Siberian craton. The mineralogical specifics of the massif is expressed in the presence of three-pyroxene assemblage [bronzite-pigeonite-augite (diopside)] and exsolution lamellae of Cr-spinel and Cr-magnetite in Fe-Cr picotite and suggests unstable crystallization at shallow depths. Geochemical similarity between the Upper and Lower series attests to their genetic relation through intrachamber differentiation. The massif was generated from a highly evolved melt, as is seen from the LREE enrichment (La/Yb)_N = 3.89–30. Plagioclase varieties display a weak positive Eu anomaly (Eu/Eu* = 1.1–1.25), whereas other rocks have an insignificant negative Eu anomaly (Eu/Eu* = 0.85–0.97). Model calculations show that parental melt was close to subalkaline picrite, which evolved along two fractionation trends into dunites and subalkaline gabbroids and monzodiorites.     

Polyphase Inclusions in Chromium Spinels from Upper Triassic Gravelites from the Northeastern Part of the Siberian Platform

Doklady Earth Sciences - A comparative study of the chemical compositions of chromium spinel and polyphase inclusions has been carried out. Chromium spinels were separated from the concentrate of...     

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.     

Cr-spinel assemblage from the Upper Triassic gritstones of the northeastern Siberian Platform

An assemblage of Cr-spinels widespread in Carnian (Upper Triassic) diamondiferous deposits in the northeastern Siberian Platform is studied. Analysis of their morphology and chemical composition has revealed two dominant varieties of Cr-spinels and has demonstrated certain regularities in their distribution in the study area. Correlations have been established between the areal distribution of the recognized types of Cr-spinels and diamond varieties typical of kimberlite sources and between the distribution of Cr-spinels and rounded diamond dodecahedrons. The phase and chemical compositions of polyphase inclusions in the Cr-spinels are studied. The spatial arrangement of inclusions along the crystal growth zones indicates their primary genesis and trapping from the melt during crystallization. Compositional features of some minerals in the inclusions—SiO₂ impurity in apatite and high CaO contents (0.2-0.8 wt.%) in olivines—point to a nonkimberlite source of these Cr-spinels. The presence of K- and Na-containing phases and calcite in the inclusions indicates saturation of the initial melt with alkalies, Ca, and CO₂. The data obtained suggest that the numerous Late Vendian diatremes in K-rich alkaline basites of the Olenek Uplift area are the source of the dominant Cr-spinel variety.     

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

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.     

The Late Jurassic age and geochemistry of ultramafic-mafic massifs of the Selenga-Stanovoy superterrane (southern framing of the North Asian craton)

We present new data on the age and geochemistry of the Veselyi and Petropavlovsk ultramafic-mafic massifs of the Selenga-Stanovoy (West Stanovoy) superterrane on the southeastern framing of the North Asian craton. The massifs are composed of rocks of peridotitewebsterite-gabbro and peridotite-gabbro-monzodiorite associations, respectively. The latter combine normal, subalkalic, and alkaline rocks and thus are of diverse composition: from ultrabasites and pyroxenites through gabbroids to monzodiorites. The U-Pb zircon age of these massifs is 154 ± 1 and 159 ± 1 Ma, respectively, which permits them to be referred to as the youngest rocks of ultramafic-mafic complexes on the southern framing of the North Asian craton. The rocks of the studied massifs are enriched in LILE (K, Rb, Sr, Ba, LREE) and are depleted in HFSE (Zr, Nb, Hf, Ta). These rocks formed, most likely, in the rear of subduction zone or in the setting of the subducting-slab detachment.     

New data on the Upper Devonian biostratigraphy and sedimentology of Stolb Island (Lena River delta)

The obtained new data on conodonts, brachiopods, ammonoids, and ostracods from the Stolb Island section indicate the predominantly Early Famennian (triangularis–Lower rhomboidea Zones) age of sedimentation. The Upper Kellwasser global biotic event, which marks the Frasnian/Famennian boundary, has been distinguished for Arctic Siberia for the first time. It was found that carbonate-terrigenous sequences accumulated in basinal sedimentary environments. Fine-clastic material was supplied to the sediments from different sources. A flow of dolomite debris is associated with the most distant source—areas of evaporite sedimentation. The presence of calcareous fragments is due to destruction of skeletal material (close provenance areas). Siliciclastics, which make up a considerable part of the sediments, were produced by partial rewashing and eolian differentiation of felsic pyroclastics. The existence of organic-rich horizons and beds of well-washed and well-sorted clastic sequences suggests that the Upper Devonian sediments have a high general petroleum potential.     

Thermomechanical Modeling of the Formation of Multi-Сhamber Intrusions for Identifying the Relationship of Plutonic Metamorphism with Gabbro-Diorite Massifs of Western Sangilen,Tuva, Russia

Geotectonics - For the first time, a thermomechanical model of a system of multilevel magma chambers above a mantle heat source has been developed to explain the formation mechanisms of intrusive...