Home – Home – Jornals – Russian Geology and Geophysics 2006 number 1

The Earth has lived through three major climate periods: the Archean period when no glaciation appeared on the hot Earth, the Late Archean-Middle Riphean period of occasional glacial spells, and the period of rhythmic glacials from the Late Riphean to Present. The general climate trend was controlled by gradual cooling of the Earth's surface, and alternation of cold and warm cycles was influenced by volcanism. Eruptions along convergence plate boundaries provoked glacial events while pulses of within-plate magmatism corresponded to warm times. More control, especially during cold cycles, came from the position of continents and the presence of large continental mountain systems which governed the circulation of air and oceanic currents and the scale of chemical weathering. Finally, the most regular climate periodicity has been due to orbital forcing (Milankovitch cycles).
The Late Cenozoic climate events in Asia were mostly related to mountain growth in the zone of the India/Eurasia collision. The earliest strong cooling in the Northern Hemisphere at 2.8-2.5 Ma fits the time when the Tibetan Plateau shaped up in its present form and numerous ridges rose in Central Asia. The Late Cenozoic mountain building in South and Central Asia covered a total area exceeding 9 · 10⁶ km².
The Central Asian climate for the past 3 Myr was controlled by orbital forcing and changed in Milankovitch cycles (glacials and interglacials). The climate events in the Baikal record match the glacials and interglacials corresponding to pulses of flood-basalt magmatism in the mountains around Lake Baikal. Volcanics coeval to cold periods bear signature of eruption upon glaciers, i.e., all cold events recorded in the Baikal climate archive for the past 1.8 Myr were accompanied by mountain glaciation. The Brunhes chron included at least eight such glacials.

Turkic-type orogeny, a kind of collisional orogeny involving the growth and eventual apposition of very large subduction-accretion prisms, commonly but not necessarily with significant net crustal growth, leads to rising sea level, low ⁸⁷Sr/⁸⁶Sr ratio in seawater and an equable, generally ice-cap free

We discuss the structure of the Yenisei Ridge fold and thrust belt, its Paleoproterozoic and Neoproterozoic geodynamic evolution, and the related granitoid magmatism. Many previous studies interpreted the Yenisei Ridge as a fold belt upon Archean and Paleo-Proterozoic basement composed of high-grade metamorphic and igneous rocks discordantly overlain by Mesoproterozoic-Neoproterozoic sediments metamorphosed under greenschist-facies conditions. We used the available and new geological, petrological, geochemical, and U-Pb zircon data to reveal several terranes of different ages and compositions which were assembled in the course of Paleoproterozoic and Neoproterozoic collision-accretionary movements on the western periphery of the Siberian craton. We suggest that the fold and thrust belt evolved in four major tectonic stages at 1900-1840, 880-860, 760-720, and 700-630 Ma. The earliest event was associated with high-grade metamorphism (granulite to amphibolite facies) and emplacement of the Taraka granites within the Angara-Kan terrane. The following event occurred at 880-860 Ma, but the Eruda, Kalamy, and Teya granites had rather emplaced beyond the Yenisei Ridge within the Central Angara terrane before it collided with Siberia. The latter collision (the third event) was apparently responsible for the Chirimba, Ayakhta, and Glushikha granites (760-720 Ma). The fourth event in the Neoproterozoic-Vendian (700-630 Ma) is constrained by the age of island-arc and ophiolite complexes and their obduction onto the Siberia cratonic margin. The same activity (650-630 Ma) in the central part of the fold and thrust belt produced the Tatarka complex composed of A -type granites, nepheline syenites, and carbonatites of mantle and crust-mantle origin.

New data on tectonics, magmatism, deposition history, paleomagnetism, and geochronology were used to reconstruct the geodynamic evolution of Caledonides and Hercynides, and the related late Baikalides, in a large orogenic area on the southern periphery of the Siberian craton. The study region includes the Baikal-Patom fold-thrust belt and a collage of terranes in the south which accreted to Siberia in the latest Riphean and in the Early and Late Paleozoic. The terranes are fragments of Riphean and Paleozoic island arcs, active continental margins, oceanic crust (ophiolites, seamounts, etc.), turbidite basins, continental slopes and shelves or belong to cratonic terranes (microcontinents) composed of Precambrian basement rocks. The accretion provided southward (in present coordinates) growth of the continental lithosphere of the craton. The accreted terranes were subjected to large-scale strike-slip faulting, repeated deformation, collisional and within-plate plutonism, and metamorphism of different temperature facies.

Metamorphic complexes become exposed at the surface by three basic exhumation mechanisms (i) tectonic transport by return flow in accretionary wedges, overthrusting, and extrusion tectonics in convergent orogens, (ii) tectonic erosion (unroofing) under large-scale crustal extension, and (iii) diapirism (buoyant rise of low-density lower crust or subducted upper crust).

A model for free-convective flows in the asthenosphere beneath the ocean has been derived. The thermophysical model for the asthenosphere beneath a mid-oceanic ridge is a horizontal layer being heated from the side (in the vicinity of the ridge axis) and cooled from above, with the sole adiabatic. Laboratory modeling yielded velocity and temperature fields in the horizontal layer in the boundary-layer regime. Requirements for the correct determination of the velocity and temperature fields in the asthenosphere have been defined from the results of thermophysical modeling. The asthenosphere viscosity and maximum temperature difference in the asthenosphere in the vicinity of the MOR axis have been estimated. Velocity and temperature fields in the asthenosphere layer have been obtained under slow-spreading conditions. Stability fields of the main deep-seated parageneses and a zone of partial melting in the asthenosphere have been established on the basis of the experimental field of temperature and streamlines, using the velocity field obtained by laboratory modeling and experimental state diagram of peridotite. The width of the partial-melting zone could average 5-7 km (on the one side of the ridge), and its depth, about 80 km. Depthward, the gabbroid associations grades into spinel peridotites, which in turn give way to garnet peridotites. At depths of more than 400 km olivine grades into ringwoodite.

Data on compositions and parageneses of coexisting garnets and pyroxenes included in coesite-bearing diamonds from different deposits of the world and in xenoliths of coesite eclogites have been summarized. Remarkably, the diamonds with coesite inclusions (over 250 samples) have been detected in all deposits in operation. Diamonds with coesite as well as xenoliths of coesite eclogites contain all the parageneses represented in eclogite (E-type) diamonds in kimberlites and mantle-derived eclogites. These parageneses include a wide continuous series of compositions from websterites (pyroxenites) to kyanite eclogites, grospydites, and calc-silicate compositions. Data on exclusively wide variations of composition of mantle-derived coesite-bearing rocks and the available data on oxygen isotopy of coesite as well as carbon and nitrogen isotopes of diamonds with coesite inclusions suggest that coesite-bearing eclogite parageneses of kimberlites and lamproites are the result of subduction of ancient oceanic crust. Similar features of diamonds have been recorded in coesite-bearing in diamondiferous UHP metamorphic rocks.

Banded samples of calc-silicate rocks with contrasting diamond contents from the Kumdy-Kol' metamorphic diamond deposit, North Kazakhstan, were examined. The specimen Kar-4 abounds in diamonds. At the same time, the specimen Kar-98-6 contains few diamond crystals, and no diamonds have been found in Kar-200. The data obtained show that the composition of minerals and zoning character may considerably vary even within one sample. In particular, in the specimen Kar-200 the content of grossular component in garnets varies from 81 to 57%. Pyroxenes from different layers are distinguished by contents of potassium impurity. For example, the pyroxene from layer A contains up to 0.6% K₂O and phengite lamellae, whereas the pyroxenes from other layers contains no more than 0.2% potassium. Study of calc-silicate rocks under the scanning electron microscope has shown that dolomite is substituted by an aggregate of tremolite, talc, and calcite. Garnets from the sample Kar-4 contain rounded inclusions of dolomite, with globules in their central part composed of magnesite, amorphous hydrous silica, dolomite, and calcite. These globules are interpreted as a carbonatite melt. As a result of our studies, the following stages were distinguished in the metamorphic evolution of calc-silicate rocks. The protoliths of calc-silicate rocks are clayey calcareous dolomites. At the prograde stage of metamorphism, these rocks underwent partial melting initiated by a hydrous fluid that appeared on zoisite decomposition. At higher pressures and temperatures, the decomposition of phengite led to the formation of a high-K fluid. Diamonds are crystallized just at this stage. At the retrograde stage, in the stability field of zoisite, garnet was substituted by pyroxene-zoisite-calcite symplectite. At the final stage of exhumation, the carbonate rocks interacted with a Si-enriched hydrous fluid.

Analysis of new geochemical information of distribution of petrogenetic and rare, including rare-earth, elements in Fe- and Al-rich metapelites of the Korda (Yenisei Ridge) and Amar (Kuznetsk Alatau) Formations gave a clue to the reconstruction of the composition and origin of their protoliths. It has been established that these rocks are the redeposited and metamorphosed products of the Precambrian kaolinite-type crusts of weathering, while petro- and geochemical differences between them are due, chiefly, to different conditions of formation. The protolith of the Korda Formation metapelites was produced by erosion of the post-Archean complexes of rocks chiefly of granitoid composition and accumulation in continental-margin shallow basins in the conditions of humid climate. Geochemical characteristics of deeper primary deposits of the Amar Formation suggest a dominating role of volcanogenic material of basic composition in the erosion zone. These results agree with data of lithological-facies analysis and geodynamic reconstructions of evolution of geological complexes of the Yenisei Ridge in the Middle Riphean and of the Kuznetsk Alatau in the Vendian. A conclusion is made that rare-earth elements had limited migration mobility during contact and collisional metamorphism.

Cr-diopside group mantle xenoliths from Late Cenozoic basaltoids of the Udokan volcanic field located at the boundary of the Aldan Shield and Baikal-Vitim terrane have been studied. Slightly depleted lherzolites are predominant xenoliths in the central part of the field (Pliocene basanites of Lake Kuas), whereas depleted harzburgites prevail in its northern part. The composition of the Udokan peridotites suggests that they are components of the Phanerozoic oceanic mantle subducted beneath the Siberian craton rather than the Archean mantle of the Aldan Shield.
Xenoliths of Lake Kuas are divided into two series: harzburgite-lherzolite and lherzolite-websterite. The latter series probably represents ancient mantle, whereas the former might have been formed through the later interaction of peridotites with a hypothetic silicate melt, which was probably accompanied by Na-amphibole metasomatism. The Kuas spinel harzburgites and dunites are characterized by higher equilibrium temperatures (1000-1050

A specific complex of metaultrabasites and metabasites has been first mapped and comprehensively studied in the central part of the Aldan-Stanovoi Shield. The complex rocks break through granulites (Kurumkan, Nimnyr, and Fedorov Formations) of the Nimnyr terrane of the Paleoproterozoic collision belt. The age of granulite metamorphism is estimated at 2.0-1.9 Ga, and the age of syncollisional granites, at 1907-1920 Ma. The structure of the southern part of the terrane was determined from successive changes of two types of deformational parageneses of collisional genesis, early domal and late slip. It has been established that metabasite and metaultrabasite bodies intruding into syncollisional granites were deformed by asymmetric folds with steep bends formed during slip motions. The metamorphism grade of metabasites and metaultrabasites corresponds to granulite facies. ⁴⁰Ar/³⁹Ar dating of high-temperature amphiboles from two metabasite samples yielded 1903 ± 16 and 1908 ± 15 Ma. In chemical composition the complex rocks correspond to normal rocks of tholeiitic series. The REE pattern (strong enrichment with LREE, La/Yb = 2-9.5), REE ratios, and LILE and HFSE enrichment of these rocks due to the primitive mantle show their similarity to rocks of plume magmatism. Great variations in contents of HREE (3 to 20 times higher relative to chondrite) seem to be due to the melting of compositionally different mantle rock materials with the participation of asthenospheric or lower-crustal matter. Models for the lithosphere delamination and slab detachment are proposed for the explanation of plume magmatism during the collision of Precambrian terranes.

A specific complex of different types of mineralization (early Cu-Ni-Pt and Ni-Co-As and late Hg, Au-Hg, and porphyry Cu-Mo) has been revealed in the areas of influence of the Siberian and Tarim mantle plumes. In some regions, Mo-W, Sn-W, Ag-Sb, hydrothermal Fe-skarn, Fe-Ti (apatite), REE-Ta-Nb-carbonatite, and other types of mineralization have been found. The central parts of the areas host large Cu-Ni-Pt deposits, whereas the peripheries are made up of Ni-Co-As, Hg, Au-Hg, and Cu-Mo ores. In some ore districts, the largest commercial deposits are confined to rift structures or deep-fault zones. Formation of large ore deposits was determined by the spatial co-occurrence of plume magmatism and within-plate rifting and the active mantle-crust interaction.

Analysis of geological and isotope-geochronological data on the Sayan-Baikal-Muya belt has shown that subduction and the subsequent collision of microcontinents with island arc were essential in the Neoproterozoic-Early Paleozoic. They influenced the formation, localization, and preservation of gold mineralization in ophiolite belts and the formation of ore clusters with diverse types of precious-metal deposits and mineralization: gold-PGE, in blueschist rocks; high-temperature (>400