Home – Home – Jornals – Russian Geology and Geophysics 2003 number 12

This paper prepared to the 100th birthday of Academician Yu.A. Kuznetsov follows the main steps of his life and professional career. The most important scientific works by Yu.A. Kuznetsov are reviewed, which are concerned largely with problems of petrography and petrology, with relationships between magmatism and tectonics, magmatism and ore formation. A considerable place is occupied by analysis of the history of the Kuznetsov school of thought, involved with igneous formations, state of the art of this line of research, and the problems lying ahead.

Periodicity of plume magmatism has been reviewed in this paper. It also demonstrates a correlation between trap magmatism and Permian-Triassic granitoids as well as general regularities of collisional and postcollisional granitoid magmatism and a role of plumes in their manifestation. It has been established that the Permian-Triassic superplume is expressed over vast spaces of Asia in two forms: (a) at places with cold lithosphere in the form of Siberian and Emeishan traps; (b) at places with thickened lithosphere and crust, as a result of preceding collision events, in the form of syenites and A-type granites. Effusions of the Siberian traps and intrusion of associated granitoids are largest-scale events in the Earth's history. At their peaks, the effusions of both Siberian and Ontong Java plumes were about 8-10 x 10⁶ km³ for one myr. Given that gaps between effusions take 90 % of the time, the effusions could reach 100 km³ a year or, with intrusions taken into account, twice as much.
The superplumes appearing at intervals of 120, 250 myr and, possibly, about 360 and 485 myr are good labels for event synchronization on a global scale. An interval of 120 myr can also be supposed for the Late Precambrian superplumes: at about 610, 730, and 850-860 Ma. Smaller local plumes are expressed between superplumes, with a periodicity of about 30 myr.

The paper considers magma-forming fluid systems of continental lithosphere in which T is equal or slightly above the solidus of the newly forming melt. The fluids should have an enthalpy (H, kcal/mole) high enough to maintain melting, and the magma systems should be open to ensure supply of heat carriers. The origin depth and enthalpy of the latter are key elements in the energy balance of the magma source. The most voluminous magma-forming fluids which induce melting in crust and mantle originate in the asthenosphere and in the outer core, two Earth's major fluid-bearing systems. T, P, and H of fluids separated from the outer core and asthenosphere in the period from Archean through Cenozoic were estimated by thermodynamic modeling. The major- and trace-element compositions of fluids were investigated for three stages of plutonism (443 11, 412 6, and 360 20 Ma) that produced the Zerenda complex. Two first stages result from upper mantle depletion, and the latest granites, with prominent rare-metal enrichment, record effects of high-F asthenospheric fluid systems on the preexisting granites.

Igneous rocks in oceanic and continental rifts are derived from depleted mantle or from deeper lower mantle. Within-plate magmatism may be related to hot mantle fields. The Phanerozoic history of magmatism in Siberia was to a great extent controlled by the drift of the continent over the Central Asian (Atlantic-African) hot field.

The structure of the Caledonian area of the Central Asian fold belt is determined by Caledonian folded formations made up of Vendian-Cambrian ophiolite and island-arc complexes and by terranes or microcontinents of older crust, located between them. The corresponding structural zones differ in the Nd isotope composition of their crust, which is determined from analysis of granitoids. The crust of the Precambrian terranes is characterized by negative _Nd(T) values. Most of the terranes (Hangayn, Barguzin-Vitim, Sangilen) have _Nd(T) = +0.5...-10, thus corresponding to crust with a model Nd isotopic age T_Nd(DM-2st) = 1000-1700 Ma. This allows uniting the above terranes into a

Plutonic and volcanic assemblages in the Central Asian fold belt provide a clue to geodynamic reconstructions of the Paleoasian ocean which existed in its place. Some assemblages bear clearly pronounced signature of their geodynamic environments and serve as indicators. Assemblages that originated in oceanic, island-arc, collisional, and postcollisional settings differ in specific structural-magmatic zonation.

Three large batholiths were formed in the Central Asian fold belt in the period 310-190 Ma: Angara-Vitim, Hangayn, and Henteyn-Daurian. They are made up of granitoids of variable composition - from tonalites and plagiogranites to granosyenites and rare-metal granites, with a predominance of normal granites. The formation of each batholith lasted about 20-30 myr. For example, the Angara-Vitim batholith was formed in the period 320-290 Ma, the Hangayn one, 275-250 Ma, and the Henteyn batholith, 225-195 Ma. The batholiths are localized in zonal petrographic provinces and are framed by rifting zones with basaltic, bimodal, and peralkaline-granite magmatism.
According to isotope-geochemical data, the batholith granitoids have the same isotopic Nd composition as the host crust. The rocks of the Angara-Vitim and Hangayn batholiths, formed within terranes with

The Pamir-Himalayan and Central Asian intracontinental fold belts developed to follow different geodynamic scenarios. In the first case, it was an ultimate version of hard collision, when cratons directly interacted with the Early Precambrian crust and thick lithosphere mantle. The second case was a soft collision, which came to an end in Late Paleozoic-Early Mesozoic time, without reaching the stage when the Siberian, Sino-Korean, and Tarim cratons would be struck. Direct geophysical observations cannot be used for ancient epochs, but the thickness of lithosphere of colliding plates and microplates can be inferred from Sr-Nd isotope characteristics of granitoid batholiths, which depend on average composition and age of the crust, thus indirectly indicating the thickness of a genetically related underlying lithosphere mantle. The upper mantle dynamics has been analyzed for different stages of collisional orogeny. It has been concluded that at the moment of inversion (the beginning of the early collision stage), a slab is detached and an asthenosphere swell appears in the vicinity of the future collisional building immediately beneath the Moho discontinuity. As a result, short-term anomalous temperature gradients appear in the lower crust, large-scale melting occurs, and bimodal volcanic series form, which, on the one hand, still retain suprasubduction geochemical labels but, on the other hand, reflect the composition of the lower crust subjected to advanced melting. Then the collisional orogeny follows the classical scenario of the thickening of the crust and its lithosphere root, covering the period from the end of the early orogeny stage through the late orogeny stage. The time of formation and extent of an orogen depend on the thickness of colliding plates, and the composition of granitoid batholiths is directly correlated with the composition of the geologic environment. The relationship with the mantle, if any, is expressed in specific forms, e.g., in the form of rifts in the orogen foreland during a frontal collision or in the form of feathering convergent and divergent strike-slip faults when the collision is oblique. The dynamics of development of collisional orogens radically changes at the postcollision (taphrogenic) stage. With a greater thickness of the lithosphere root (Pamirs-Himalayas), the density instability causes the lithosphere delamination, and asthenosphere flows move beneath the Moho, thus causing a drastic rise in relief, followed by the orogen's collapse. With a smaller thickness of the lithosphere (arc-arc, arc-seamount, arc-microcontinent, and other collisions), there is no delamination, and the orogen's breakup is due only to gravitation landslides and detachments in the crust. Central Asia is unique for the presence of a large lower-mantle plume. Therefore, the processes of the seemingly classical soft collision actually led there to the initiation of local plumes beneath folded orogens. A model is proposed for the induced plumes that permits the formation of giant granitoid batholiths or their source areas at the postcollision stage, as well as their specific composition combining plume and collision characteristics.

Previous research has shown that the sections of the vast volcanogenic trap field of the Siberian Platform bear analogs of oceanic A- and C-G-type plateau basalts. Comparative analysis of volcanic rocks shows that basalts of the Siberian Platform and the Ontong Java Plateau are similar in composition and differ considerably from MORB. This suggests that the geodynamic processes that formed great volumes of magmas in the former two regions were similar and differed from the deep-level processes that occurred beneath MOR. Basalts of both the Siberian Platform and the Java Ontong Plateau might have resulted from the ascent of plumes that acted, most likely, by the mechanism of huge

In the Silurian-Carboniferous (440-330 Ma), the Urals experienced subduction-related magmatism, and the following intrusive magmatites are recognized here:
1. Volcanic, mainly high-level complexes including:
a) Silurian gabbro-granitoid and gabbro-syenite series of the Tagil volcanogenic zone;
b) Middle-Late Devonian gabbro-granitoid series of the Magnitogorsk volcanic zone (360 Ma).
2. Early Silurian anorthosite-plagiogranite series of the Pt-bearing belt.
3. Silurian layered wehrlite-gabbro-plagiogranite complexes and accompanying parallel spreading dikes associated with ophiolites (Akkermanovka and Kirpichnaya Massifs on the eastern and western periphery of the Khabarny ophiolite and Reft Massif in the Central Urals).
4. Deep-level and medium-level tonalite-granodiorite complexes dated to 355-360 Ma (Chelyabinsk complex) and 320-340 Ma (Verkhisetsk complex).
The subduction is responsible for many peculiarities of the Urals magmatites of this age: high content of water in magmatic melts, which increased during the evolution of the mobile belt; calc-alkalic composition of the rocks; predominantly hornblende type of gabbroids; wide occurrence of primary epidote and sphene; positive anomalies of large-ion lithophile elements (Ba, Sr) and negative anomalies of HFSE (Nb, Zr, Hf, Ti) on spidergrams, typical of most S-C₂ Urals igneous rocks of basic and normal compositions. These regularities are disturbed only in those series that are related to taphrogenic structures and disjoining, though the latter, most likely, resulted from subduction.

Phase layering in mafic and ultramafic intrusions cannot be explained in terms of convection and accumulation in magma chamber as convection of melt rich in cumulus crystals is impossible. Intrusion of heterogeneous magma with 40-50 vol.% crystal phase is rather associated with compaction whereby intercumulus liquid is squeezed out toward the roof of the chamber.

The Late Paleozoic-Early Mesozoic Erdenetiyn-Ovoo magmatic center in Northern Mongolia includes the plutonic Selenge and subvolcanic ore-bearing porphyry complexes different in age and emplacement conditions but similar in major-element chemistry (calc-alkaline patterns, higher alkalinity, Na enrichment over K, etc.) and initial ⁸⁷Sr/⁸⁶86Sr ratios (about 0.7039-0.7044). Over 300 chemical analyses processed by correlation, cluster, and factor analysis techniques demonstrate general similarity and features of difference in the Selenge and porphyry complexes. Some element abundances in rocks of the same type in the compared complexes show different distribution functions. Correlation and cluster analysis reveal weaker linkage among elements in porphyry than in the Selenge granitoids. The composition of the Selenge complex in factor diagrams bears a stronger effect of differentiation and alkalinity, and porphyry magmatism, especially the ore-producing late phases, was also controlled by oxidation state. The weaker role of alkalinity in porphyry may be accounted for by loss of alkalis during separation of fluids from the magma. The general and specific features of major-element chemistry indicate that the Selenge and porphyry complexes originated at different stages of a single long-existing magma system of deep origin in which the conditions were favorable for concentration of ore components in late melts.

Analysis of the dynamics of development and the state of the art of regional geological research in Russia, in particular geological mapping, shows that the situation comes to a crisis. A multiscale geologo-geographic system is proposed as a stabilizing factor. Modified serial (regional) legends for medium- and small-scale national geological maps can serve as the informational core of this system. They systematize the recent regional geological data for the territory of Russia. To fulfill this function, the existing serial legends require a unified approach to the structurization of the information on geological structure and minerogenic potential of regions. The modified serial legends, jointly with geological maps, can be used as multilevel regional geoinformation systems, GIS, integrated into the single national system of Russia. A set of concepts, terms, and standard procedures is proposed for ranking of mappable units, geological zoning of areas (regionalization), and generalization of geologically mappable units as the map scales change from large through small to general. To improve the All-Russian system of mappable units, it is reasonable to make a more active use of formational analysis in the practice of present-day geological mapping.