Home – Home – Jornals – Russian Geology and Geophysics 2008 number 7

This paper reviews available the mantle plume models and formulates seven questions, which can be solved using geological and geochemical data in terms of the thermochemical plume model.
The main features of thermochemical plumes, including calculated plume parameters, are presented. It has been estimated how the power of a plume depends on water concentration in melt at variable melt portions and diffusion coefficients (Lewis numbers). A correlation between the latter and temperature drop at the base of a plume has been shown.
The obtained plume parameters are compared with model plume localities, such as the Hawaiian and East African plumes, Siberian and Emeishan flood basalts, etc. Based on the oceanic plate movement pattern and He, Os, and Sr isotope data, the Hawaiian plume was proved to be generated within the D₂ lower mantle layer; the diameter of the plume channel is about 100 km, the plume channel rotated and migrated to 100-200 km every 2 Ma; the plume power is about 3 · 10⁸ kW, changing each 15 Ma. The reconstruction of complex core material-plume melts and Layer C-asthenosphere interactions is possible only by isotopic data and primary olivine composition.
The plume model was applied to large igneous provinces (Siberian, Emeishan, and Ontong Java LIPs) using detailed mechanisms of plume head interaction with the lithosphere. Taking into account the similar age and evolution of the Tunguska syncline (Siberian LIP) and West Siberia, the plume-lithosphere interactions are deduced from the relationships between sedimentary and magmatic units, different modes of rifting, and variations of Sr and Nd isotopes.

The paper reports data on the age, composition, and geodynamic environments of Permian ultramafic-mafic complexes in the Zaisan-Gobi zone in the eastern Gobi-Tien Shan rift belt. The synchronous formation of these complexes with Early Permian lava alkali and tholeiitic basalts discovered beneath the cover of the Tarim and Junggar platform blocks suggests that they are derivates of the Tarim plume related to the Permo-Triassic Siberian superplume. This is evidenced from the composition and metallogeny of the complexes similar to those of magnesian trap intrusions on the western periphery of the Siberian Platform.
In the area of influence of the Tarim plume, including the Zaisan-Gobi zone and other structures of the Gobi-Tien Shan belt, diverse Cu-Ni-PGE, Ni-Co-As, Au-Hg, Cu-Mo, and ore deposits related to plume magmatism formed. Their composition, formation conditions, and relationship with magmatism are discussed.

New structural and petrological data have been obtained for the zone of Siberia-Kazakhstan oblique collision for Permian time. In terms of classical tectonics, the area coincides with the Zaisan folded area produced by closure of the Char paleo-ocean in the Late Carboniferous. However, the extent, structure, and composition of magmatism at the Carboniferous-Permian (280 ± 10 Ma) and Permian-Triassic (250 ± 5 Ma) boundaries require an active control from Morgan-type lower mantle plumes (Tarim and Siberian plumes). Structure formation in the lithosphere and heat sources of magmatism have been simulated in a 3D model including lithospheric strain rates (with regard to viscosity layering) and subcontinental upper mantle convection. According to our model, heat supply from slab break-off and/or delamination of lithosphere is insufficient to maintain large-scale mantle-crustal magmatism in the case of oblique collision between 80-100 km thick plates (

We have shown that the occurrences of Permo-Triassic magmatism in northern Vietnam compose the southwestern and southeastern flanks of the Emeishan large igneous province, shifted along the large-amplitude Ailao Shan-Red River shear zone. Based on new geochronological (U-Pb and Ar/Ar) data, two stages of magmatism related to the Emeishan plume have been recognized. At the first, Permian, stage (260 Ma), abundant picritoids of the Song Da and Song Hien zones formed, and at the second, gabbro-syenite and gabbro-monzodiorite associations of the Lo Gam zone, layered peridotite-gabbro massifs of the Nui Chua complex, high-alumina granites of the Phia Bioc complex, and basalt-rhyolite association of the Song Hien zone were produced. The metallogenic specialization of the Permo-Triassic magmatism of northern Vietnam is considered.

We investigate the dynamics of multilevel subcontinental melting beneath a lithosphere of variable thickness as a model simulating generation of melts that produced flood basalt in the Siberian Trap province. The inferred sequence of volcanic events correlates with upper mantle decompression melting above hotspots and related conductive melting of metasomatized depleted lithosphere. Numerical experiments show that complex mafic magmatic systems of this kind are at least two-stage with a zone of decompression melting in upwelling mantle flow and a sub-Moho lithospheric melting region; the shallower lithospheric melting region can be disconnected from the mantle zone of decompression melting along the edges of the magmatic system; the lithospheric melting region has a particular layered structure with its top at a depth of 60-70 km and a maximum size exceeding the length of the erupted lava field; mafic melts produced at different levels of the system have different compositions.

Geological, petrological, geochemical, and isotope data from the Yenisei Ridge indicate three stages of rifting and attendant within-plate magmatism at 750, 700, and 670 Ma. The igneous rocks of the three stages are, respectively, metarhyolite-basalt, trachybasalt-trachyte, and alkali ultramafic (alkali picrite) associations. Magmatism was concurrent with terrigenous deposition of the Neoproterozoic Upper Vorogovka, Chingasan, and Chapa Groups. The volcanosedimentary complexes were deposited in narrow rift-like graben along faults. The earlier consolidated flanking uplifts of the graben experienced granitoid magmatism synchronously with rifting and within-plate volcanism. The respective plutonic events produced granitoid intrusions of the Ayakhta (760-750 Ma), Kutukas (690-700 Ma), and Middle Tatarka (~700 Ma) alkaline complexes, and the later (about 650-670 Ma) alkali ultramafic Chapa complex of carbonatites and metasomatites. Basalts and alkaline rocks are chemically similar to ocean-island and continental-rift basalts which have been reliably attributed to mantle plumes. Neoproterozoic rifting and within-plate magmatism were possibly related to the plume activity responsible for the breakup of Rodinia. These events in the Yenisei Ridge appear to be coeval with rifting and within-plate magmatic processes in other continental blocks which may have been parts of the Rodinia supercontinent.

We investigated local geology of Meso-Cenozoic basalt complexes in the Tien Shan and collected the first samples that represent nearly the entire area of within-plate basaltic magmatism in Central Asia (over 285,000 km²). According to their ⁴⁰Ar/³⁹Ar ages, the Tien Shan basalts erupted for a relatively short time span between 61 and 76 Ma, in the Late Cretaceous-Paleogene. The trace- and rare-earth-element compositions of rocks show that most of the studied basaltic series in the Tien Shan formed in within-plate magmatic systems related to mantle plume sources. Some melts bear evidence of crustal contamination. The compositions of melt inclusions and patterns of trace and rare-earth elements in them are consistent with the plume origin of the melts and with the evolution of magmatic systems toward an increase in K, Al, and Fe contents. The melt inclusion data show a direct dependence of the crystallization temperatures of parental melts of the Meso-Cenozoic Tien Shan basalts on their Mg#: 1220-1250

We report new isotope dates and metallogenic and chemical characteristics of granitoids from the Hangayn area (central Mongolia) with implications for the time and tectonic settings of their formation. Proximal U-Pb and Ar-Ar ages of 241.3 ± 1.5 Ma and 238.2 ± 2.5 Ma have been obtained for the Triassic Guchin Us and Hurmen Gol intrusions, respectively, and an Early Permian age (288.7 ± 2.3 Ma, Ar-Ar) for the Bayanulaan granite and for granite porphyry from the Saran Uul Cu-Mo-(Au) deposit. Granitoids of the Egiyn Davaa, Hangayn, and Shar Us Gol complexes in the southwestern Hangayn area are of two geochemical types. Some intrusions that were formerly attributed to these complexes have been reinterpreted as resulting from multiple plutonic pulses of different ages. According to our new data and published evidence, most of granitoids in the area may have been produced by Early-Middle Triassic (255-230 Ma) rather than Permian and Late Triassic-Early Jurassic events. The Hangayn granitoids are of low mineral potential. Gold mineralization in the area is confined within the Hangayn gold belt and appears to be independent of the intrusions.

In the 1980s, Ag-Sb deposits were discovered in a new ore cluster of the Delyun-Yustyd back-arc rift basin in southeastern Altai and northwestern Mongolia. The Delyun-Yustyd basin is filled with up to 9 km thick Devonian volcanosedimentary and terrigenous rocks lying over Vendian-Cambrian carbonate-terrigenous strata. Volcanosedimentary rocks are intruded by D₃-C₁ granites of the Yustyd complex, as well as by pre-granite mafic rocks, post-granite gabbro-diabase, diabase porphyry dikes, and younger lamprophyre dikes of the Chuya complex (245-236 Ma Ar-Ar biotite age). Ag-Sb ores in the Yustyd cluster formed at the Early Mesozoic stage at 240 ± 1.6 Ma (Early-Middle Triassic boundary) and were thus nearly coeval with the Chuya lamprophyre intrusion. The age of Cu-Ag-Sb-Hg mineralization is about 234.4 ± 1.0 Ma (Ar-Ar, sericite). Sb-Hg mineralization is the latest in the province (231.5 ± 1.0 Ma, Ar-Ar, sericite). Gradual change of Ag-Sb (siderite-tetrahedrite) ores to complex Cu-Ag-Sb-Hg (Hg-Ag-tetrahedrite) and Sb-Hg (cinnabar with Sb sulfosalts and stibnite) mineralization along the Kurai-Kobdo and Terekta-Tolbonur large faults is interpreted as development of stage metallogenic zoning along major ore-controlling faults.