Home – Home – Jornals – Russian Geology and Geophysics 2011 number 1

The paper presents ³⁹Ar-⁴⁰Ar and U-Pb (SHRIMP zircon method) geochronological data on minor picrodolerite intrusions fr om western Mongolia. Picrodolerite magmatism in western Mongolia took place within different age intervals and in different geodynamic settings: accretionary-collisional Є_1-2, ~510 Ma (Ььreg Nuur association, Hayrhan massif), intraplate D₁, 410-390 Ma (Tsagaan Shuvuut Range, Mor't Uula massif), intraplate D₃-C₁, 345-360 Ma (Altan Gadas, Tavtyn Hundiyn, and Hu Tsan Bulak massifs), island-arc C₂, 315-335 Ma (Dzahuy, Yaryn Had, and Javhlant massifs), and intraplate P₁, ~270 Ma (Dzaraa Uula massif).
Petrographic, mineralogical, and geochemical data permit distinguishing derivates of picrobasaltic (12-14 wt.% MgO) (Ььreg Nuur association, Tsagaan Shuvuut Ridge, Javhlant and Yaryn Had massifs) and melabasaltic melts (7-10 wt.% MgO) (Mor't Uula, Altan Gadas, Dzahuy, and Dzaraa Uula) among picrodolerite magmatism in western Mongolia. Picritoids in these associations resulted from early olivine fractionation.
The Early Devonian picrites and picrodolerites of the Tsagaan Shuvuut Range and the olivine dolerites of the Mor't Uula massif can be assigned mafic magmatism in the Devonian large igneous province (LIP) (North Mongolian megablock). Late Devonian-Early Carboniferous picrodolerite intrusions in the Baruun Huuray zone (Altan Gadas) and Mongolian Altay (Tavtyn Hundiyn) are related to the Tien Shan LIP. Bimodal volcanism on the southern margin of the Hangayn Mts. (Dzaraa Uula), in the eastern segment of the Hanhuhiy Range (Dzagday Nuur, Hara Teg), and in the Argalantu trough (Tegshiyn Gol, Muhur Shurgah, and Deed Shurgah massifs) might be related to the Tarim LIP, wh ere they are part of an Early Permian volcanoplutonic association. Carboniferous picrodolerite massifs in the South Mongolian megablock and the Trans-Altai Gobi formed in subduction-related settings (Dzahuy, Yaryn Had, Javhlant).

Using geological, geophysical, and thermophysical parameters we performed a numeric modeling of the thermal regime of a collisional process by the example of the Yenisei Ridge Neoproterozoic orogen in the southwestern framing of the Siberian Platform. The results yielded by the described 3D numerical models and by the one-dimensional parametric modeling of the thermal effect of crustal matter differentiation allowed reconstructing the main tectonothermal processes of the collisional stage of this structure formation. The performed modeling allows taking into account the local particularities of the crustal thermal state of the structure under discussion and at the same time it allows determining the general regularities typical of orogens on postcollisional stages. We have established that it is the action of three factors that significantly influences the thermal regime: the radiogenic heat of the intrusions, high heat flow anomalies, and clustering of the Glushikha complex leucogranitic bodies in the central region of the Central Angara terrane. Our studies have shown that tectonic processes combined with diachronous magmatic activity can significantly affect the course of collisional orogens' thermal history.

Chromium- and vanadium-bearing metamorphic derivates of siliceous-carbonate sediments in the Sludyanka granulite-facies complex, southern Baikal, contain high-Cr clinopyroxenes that belong to the diopside-kosmochlor-natalyite join (CaMgSi₂O₆-NaCrSi₂O₆-NaVSi₂O₆). The ternary join includes isomorphic binary joins of which the diopside-kosmochlor join with 94% kosmochlor (Kos) end-member has been studied in detail and found to be virtually complete. As indicated by signature of diopside-eskolaite reactions in the minerals, kosmochlor and high-Cr members of the join formed at the expense of metamorphic eskolaite as a result of Na input after progressive metamorphism. There is no miscibility gap along the diopside-kosmochlor join; the incompleteness of the kosmochlor formation reaction and the coexistence of its compositionally different members are rather due to kinetic factors associated with low mobility of Cr. Thus, the mechanism of Cr incorporation in clinopyroxenes does not depend on PT -conditions of metamorphism.

The following structural elements have been recognized to constitute the tectonic demarcation of Central Asian Foldbelt: (1) The Kazakhstan-Baikal composite continent, its basement formed in Vendian-Cambrian as a result of Paleoasian oceanic crust, along with Precambrian microcontinents and Gondwana-type terranes, subduction beneath the southeastern margin of the Siberian continent (western margin in present-day coordinates). The subduction and subsequent collision of microcontinents and terranes with the Kazakhstan-Tuva-Mongolia island arc led to crustal consolidation and formation of the composite-continent basement. In Late Cambrian and Early Ordovician, this continent was separated from Siberia by the Ob'-Zaisan ocean basin. (2) The Vendian and Paleozoic Siberian continental margin complexes comprising the Vendian-Cambrian Kuznetsk-Altai island arc and the rock complexes of Ordovician-Early Devonian passive margin and Devonian to Early Carboniferous active margin. Fragments of Vendian-Early Cambrian oceanic crust represented by ophiolite and paleo-oceanic mounds dominate in the accretionary wedges of island arc. The Gondwana-type continental blocks are absent in western Siberian continental margin complexes and supposedly formed at the convergent boundary of a different ocean, probably, Paleopacific. (3) The Middle-Late Paleozoic Charysh-Terekta-Ulagan-Sayan suture-shear zone separating the continental margin complexes of Siberia and Kazakhstan-Baikal. It is composed of fragments of Cambrian and Early Ordovician oceanic crust of the Ob'-Zaisan basin, Ordovician blueschists and Cambrian-Ordovician turbidites, and Middle Paleozoic metamorphic rocks of shear zones. In the suture zone, the Kazakhstan-Baikal continental masses moved westward along the southeastern margin of Siberia. In Late Devonian and Early Carboniferous, the continents amalgamated to form the North Asian continent. (4) The Late Paleozoic strike-slip faults forming an orogenic collage of terranes, which resulted from Late Devonian to Early Carboniferous collision between Kazakhstan-Baikal and Siberian continents and Late Carboniferous to Permian and Late Permian to Early Triassic collisions between East European Craton and North Asian continent. As a result, the Vendian to Middle Paleozoic accretion-collisional continental margins of Siberia and the entire Kazakhstan-Baikal composite continent became fragmented by large-amplitude (up to a few thousand kilometers) strike-slip faults and conjugate thrusts into several strike-slip terranes, which mixed with each other and thus disrupted the original geodynamic, tectonic, and paleogeographic demarcation.

A brief review of Ordovician blueschist complexes on the southwestern framing of the Siberian craton is presented in order to place further age constraints on the tectonic evolution of the Central Asian orogenic belt (CAOB). Three different blueschist localities (imbricated slices of blueschists in the Uimon Zone, Gorny Altai; a blueschist unit in the Kurtushiba ophiolite belt; blocks of blueschists and eclogites in a serpentinite melange of the Chara zone, northeastern Kazakhstan) are considered. We obtained ⁴⁰Ar/³⁹Ar dates for white micas and sodic amphiboles from blueschists of the Uimon Zone (490-485 Ma), Kurtushiba belt (470-465 Ma), and Chara zone (450 Ma), suggesting the Ordovician ages of the subduction/exhumation of these complexes. The dates obtained also coincide with the metamorphic ages of many blueschist belts in North China, which allows us to distinguish the Ordovician stage of accretion-collision events in the tectonic evolution of the CAOB.

Diamond microinclusions provide the unique opportunity to study the composition of the mineral-forming medium. The paper presents the first data on the composition of cloudy microinclusions in the cores of octahedral diamonds from the Internatsional'naya pipe. These zones are of cuboid shape and have a fibrous internal structure. Here, the microinclusions form a continuous trend from chloride-carbonate to carbonate composition. Their composition slightly overlaps that of the microinclusions in cuboids from the same pipe, but in a carbonate-enriched zone. Also, data on N aggregation suggest that the cubic zones which formed before octahedra crystallized at higher temperature or spent considerably more time in the mantle than cubic crystals and fibrous coats of type IV diamonds.

U-Pb dating (SHRIMP-II) and study of the internal structure and composition were carried out for zircon from hypersthene gneiss from the Irkut granulite-gneiss block (Sharyzhalgai uplift in the southwestern Siberian craton). Three generations of zircon have been revealed in the hypersthene gneiss, which differ in zoning pattern, U and Th concentrations, and REE distribution. Zircon cores with growth zoning relics show a REE pattern typical of magmatic zircon: with a high (Lu/Gd)_n value (11-36) and a distinct Ce anomaly (Ce/Ce* = 15-81). They belong to early magmatic generation with an age of ≥3.16 Ga. Multifaced soccerball crystals, rims, and unzoned cores of zircon belong to metamorphogene generation; they are depleted in REE and show a lower (Lu/Gd)_n value (1.1-9.2) than the magmatic cores. This zircon generation formed as a result of the Mesoarchean high-temperature metamorphism at ~3.04 Ga. The latest zircon generation includes thin outer rims with low (Lu/Gd)_n (11-12.4) and Th/U (0.02-0.05) values and long-prismatic crystals with an oscillatory zoning, which resulted from the Paleoproterozoic (~1.85 Ga) granulite metamorphism and partial melting. The different ages of high-temperature metamorphism in the granulite-gneiss (~3.04 and 2.55-2.6 Ga) and granite-greenstone (~3.2 Ga) blocks of the Sharyzhalgai uplift reflect the independent tectonothermal and geodynamic evolution of crust in these structures, up to the final amalgamation in the Paleoproterozoic (1.88-1.85 Ga).

The North Kokchetav tectonic zone is located between the Kokchetav HP-UHP metamorphic belt and the Stepnyak zone of Ordovician island arc and oceanic complexes. The Kokchetav zone is a collage of nappes (thrust sheets) that consist of basement gneiss and sedimentary rocks of the Kokchetav microcontinent, granite gneiss, mica schists with eclogite blocks, the Shchuch'e ophiolite, Middle Proterozoic felsic volcanics, and Arenigian siliceous-terrigenous sediments with olistostromes. The latter are of gravity-sliding origin, and their clastic material includes quartz-muscovite and quartz-garnet-muscovite schists, gneiss, dolomite, and amphibolite. The sheet boundaries are marked by mylonite and Early Ordovician mica schists (⁴⁰Ar/³⁹Ar ages of syntectonic muscovite are 489-469 Ma). The North Kokchetav collage of compositionally diverse thrust sheets can be interpreted as a collisional zone. According to geological evidence, tectonic activity in the zone lasted as late as the Middle Ordovician. Syncollisional thrusting in the North Kokchetav zone was coeval with the latest dynamic metamorphic event in the Kokchetav belt. All events of retrograde metamorphism and exhumation of HP and UHP rocks in the belt are of Cambrian ages, i.e., the rocks had been exhumed prior to the Early-Middle Ordovician collisions and the related orogeny.

Geological, isotopic, and geochemical data permitted distinguishing the Mesoproterozoic (1.6-1.05 Ga), Early Neoproterozoic (1.05-0.8 Ga), and Late Neoproterozoic (0.8-0.6 Ga) stages of magmatism and crustal evolution in the Yenisei Ridge. Each of them contributed to the regional Au metallogeny. In the Early Mesoproterozoic, crustal destruction and stretching in the southwestern Siberian craton (Yenisei Ridge) led to the initiation of a pericratonic trough, the formation of rift mafic associations (Rybnaya-Panimba volcanic belt), and the accumulation of fine-grained terrigenous sediments (Sukhoi Pit Group). Black carbonaceous shales and the picrite basalt-basalt association were enriched in Au. In the early Neoproterozoic, the terrigenous strata of the Sukhoi Pit Group were deformed, metamorphosed, and granitized as a result of the Grenville orogeny. Granite-gneiss domes formed in the earlier, syncollisional, period (1.05-0.95 Ga) of this stage, and K-Na granitoid plutons formed in the late collisional one (0.88-0.86 Ga). Premineral metasomatites formed in the zone where these plutons influenced enclosing black shale-terrigenous strata. They host Au deposits, which formed later. The formation of quartz-reef zones correlates with that of thrust nappes (0.85-0.82 Ga). The latter is genetically related to the final stage of the evolution of a collisional orogen. In the Late Neoproterozoic, rift and intraplate magmatism was most intense and frequent (780, 750, 700, 670-650 Ma) in the Tatarka-Ishimba fault system. Manifestations of Mesoproterozoic volcanism and all the Au deposits of the Central metallogenic belt in the Yenisei Ridge are concentrated here. The three periods of gold-arsenopyrite-quartz, gold-sulfide, and Au-Sb mineralization correlate well with the initiation and evolution of rift structures and the manifestations of intraplate magmatism at 800-770, 720-700, and 670-650 Ma. The tectonomagmatic processes which took place in these periods might have been crucial in Au ore accumulation.

Based on study of fluid and melt inclusions in minerals from igneous rocks and associated ore-metasomatic objects, we consider the formation conditions of oxidized fluids produced at the final stages of differentiation of alkali-basic, alkaline, lamproitic, and some granitoid melts. These fluids are characterized by wide variations in composition, concentrations, and physicochemical parameters (PT, Eh, pH, etc.) and are of sulfate-chloride, sulfate-carbonate, sulfate-fluoride, fluoride-sulfate, essentially sulfate, and other types. The specific composition of these magmatogene fluids with a high extractive power ensures effective removal of ore-forming elements (Fe, Mn, Co, N, Ag, Cu, Pb, Zn, Mo, W, Bi, U, REE, etc.) from melt and their trapping from the host rocks. The set of these elements is determined by the PTX -parameters, conditions of fluid separation from melts, composition of fluid-generating magmas, and geochemical composition and metal-bearing capacity of rocks through which the fluids migrate. These factors significantly determine the metallogeny of alkaline, alkali-basic, and some granitoid complexes and associated mineralization.