No 2 (2019)

From Kenorland to modern continents: tectonics and metallogeny
Yakubchuk A.S.

There are three stages in tectonic evolution of the Earth: (1) nucleation — from origin of protocratons to their assembly into Supercontinent Kenorland (2.7–2.5 Ga); (2) cratonization — from breakup of Kenorland (2.45 Ga) to the assembly of Columbia (1.85 Ga) and its reorganization into Rodinia (1.0–0.72 Ga); (3) modern plate tectonics — from breakup of Rodinia at 720 Ma until present. Analysis of time-space reorganizations of Archean granulite-gneiss terranes, which correspond to continental lithospheric keels, reveals five groups of protocratons (Nena, Ur, Congo-Sahara, NAsia and Atlantica) that remained almost intact during long time intervals.

After the breakup of Kenorland, the continental crust rotated counter-clockwise. NAsia and Atlantica the least rotated and drifted relative to Nena, however the latter was rotated by 180°. Congo-Sahara, Ur and Kalahari were the most rotated. The assembly and breakup of the supercontinents clearly correlates with secular changes in dominant types of base, precious and ferrous metal deposits, as well as formation and emplacement of diamonds.

Геотектоника. 2019;(2):3-32
Distribution of Cenozoic metalliferous coal deposits in Zeya-Bureya Sedimentary Basin (Eastern Siberia): Tectonic reconstruction and paleogeographic analysis
Sorokin A.P., Konyushok A.A., Kuzminykh V.M., Artyomenko T.V., Popov A.A.

Distribution of noble metals, rare metals and rare earth elements in the Paleocene and Lower-Middle Miocene coals of Zeya-Bureya sedimentary basin was analyzed. Reconstruction of the basin formation in the Mesozoic and Cenozoic was performed with detailed paleogeographic analysis of the Cenozoic coal-bearing sequences from the position of geodynamic features of the adjacent regions development. Geological events at the turn of the Cretaceous and Paleogene are considered. Comprehensive analysis of the metal content in the basin frame carried out.

We described conditions of migration and localization of trace elements. Given conditions arose mainly in the process of geodynamics developed on the northwestern flank of the Zeya-Bureya basin including the static orogens in the Paleocene — Miocene where the sedimentation was actively expanding. Microelements migration with capturing denudation plains occurred in stable processes of peat accumulation and localization of economically important components in streams associated with plains.

Contrasting forms of conjugation in the flexure-uplift system and uneven localization of microelements are determined by high level of geodynamic activity on the south-eastern margin of Zeya-Bureya sedimentary basin. The presence of gold throughout the strata of the Paleocene and Lower-Middle Miocen coal-bearing sediments in the Sergeevskoe, Yerkovetskoe and Raychikhinskoe deposits is established. The Sergeevskoe deposit coals are enriched with Be, Sc, V, Ga, Rb, Nb, Ta, REE + Y.

Геотектоника. 2019;(2):33-45
A tectonic remnant of the mesoarchean oceanic lithosphere in the Belomorian Province, Fennoscandian Shield
Slabunov A.I., Shchipansky A.A., Stepanov V.S., Babarina I.I.

The results of the detailed geological mapping, coupled with the isotope-geochemical study of a metamorphosed mafic-ultramafic complex known as the Central Belomorian Belt located in the Belomorian province of the Fennoscandian Shield, are reported.

The protholith of the complex is ~ 2.9—3.1 Ga old. It has been subjected to two 2.87 and 1.87 Ga structural-metamorphic reworking. This complex is one of the oldest in the Belomorian Province.

We present several lines of evidence showing that these lithologies constitute a tectonic remnant of the Mesoarchean oceanic lithosphere, rather than any other mafic-ulramafic complex from the other geodynamic settings. The Central-Belomorian high grade mafic-ultramafics reveal a clear geochemical coherency, which implies their genetic relationships. Their mafic protholiths stem from the partial melting of a mantle peridotite protholith. The petrologic modelling has shown that primary melts were formed in the garnet lherzolite field at a pressure of 3.5–3.8 GPa at ambient mantle potential temperatures of 1520–1550 °С which led to an emergence of ~ 25–30 thick oceanic crust. The available geochemical data suggest that the complex was formed at the initial stage of subduction. It marks the start of early continental crust-forming processes in the Belomorian Province.

Геотектоника. 2019;(2):46-71
Formation of the rear slope of the island arc of Chingiz Kaledonid ridge in eastern Kazakhstan
Samygin S.G.

Process of formation of the island-arc rear slope is considered on the example of the Upper Cambrian–Middle Ordovician arc found in the Chingiz ridge in eastern Kazakhstan. Its occurrence is shown at the end of volcanic activity in the island-arc structure, beginning at the end of the early Arenig (from the end of the Flos century of the Early Ordovician) with tephroturbidites appearance. After the cessation of volcanism, two sedimentation cycles were distinguished in the sedimentary section of the slope in the middle Ordovician: (1) transgressive when the island arc submerged, (2) and regressive when the Chingiz arc began to build up at the beginning of the Llanwyrn (Darrivilian) century.

The sedimentation was repeatedly accompanied by landslide processes, which ended in the middle of llanvirna (darrivilia) with the disruption of tectonic-gravity plate composed of Upper Cambrian volcanic rocks with limestone in the sole, resulting in the formation of coarsely fragmented mixtite at the allochthonous mass frontier, the further sedimentation on the rear slope stopped.

Keywords: the island-arc; rear slope; sedimentation cycles; landslide processes

Геотектоника. 2019;(2):72-80

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