V. I. Lebedev

Geochronology of igneous rocks and formation of the Late Paleozoic south Mongolian active margin of the Siberian continent

The succession of magmatic events associated with development of the Early Carboniferous-Early Permian marginal continental magmatic belt of southern Mongolia is studied. In the belt structure there are defined the successive rock complexes: the older one represented by differentiated basalt-andesite-rhyodacite series and younger bimodal complex of basalt-comendite-trachyrhyolite composition. The granodiorite-plagiogranite and banatite (diorite-monzonite-granodiorite) plutonic massifs are associated with the former, while peralkaline granite massifs are characteristic of the latter. First systematic geochronological study of igneous rock associations is performed to establish time succession and structural position of both complexes. Geochronological results and geological relations between rocks of the bimodal and differentiated complexes showed first that rocks of the differentiated complex originated 350 to 330 Ma ago at the initial stage of development of the marginal continental belt. This is evident from geochronological dates obtained for the Adzh-Bogd and Edrengiyn-Nuruu massifs and for volcanic associations of the complex. The dates are consistent with paleontological data. The bimodal association was formed later, 320 to 290 Ma ago. The time span separating formation of two igneous complexes ranges from several to 20–30 m.y. in different areas of the marginal belt. The bimodal magmatism was interrelated with rifting responsible for development of the Gobi-Tien Shan rift zone in the belt axial part and the Main Mongolian lineament along the belt northern boundary. Loci of bimodal rift magmatism likely migrated with time: the respective magmatic activity first initiated on the west of the rift system and then advanced gradually eastward with development of rift structures. Normal granitoids untypical but occurring nevertheless among the products of rift magmatism in addition to peralkaline massifs are assumed to have been formed, when the basic magmatism associated with rifting stimulated crustal anatexis and generation of crustal granitoid magmas under specific conditions of rifting within the active continental margin.

Crystalline complexes of the Tarbagatai block of the Early Caledonian superterrane of Central Asia

The oldest crystalline complexes of the Early Caledonian superterrane of Central Asia were formed in the Early Precambrian. They are exposed in the basement of microcontinents, which represent old cratonic fragments. Among the latters are the crystalline complexes of the Tarbagatai block previously ascribed to the Dzabkhan microcontinent. It was shown that the crystalline complexes of the Tarbagatai block have a heterogeneous structure, consisting of the Early Precambrian and later Riphean lithotectonic complexes. Structurally, the Early Precambrian complexes are made up of tectonic sheets of gneisses, migmatites, and gneiss granites of the Ider Complex that are cut by gabbroanorthosite massif. The Riphean Jargalant Complex comprises alternating hornblende crystalline schists and biotite (sometimes sillimanite-bearing) gneisses with marble horizons. The upper age boundary of the Riphean Complex is determined by the subautochthonous granitoids with age about 810 Ma. The presence of the Riphean high-grade rocks indicates that structures with newly formed crust were formed in the paleooceanic framing of the Early Precambrian blocks of the Rodinia supercontinent by the Mid-Late Riphean. Divergence that began at that time within old Rodinian cratons and caused rifting and subsequent break-up of the supercontinent was presumably changed by convergence in the paleooceanic area.

The Early Baikalian Crystalline Complex in the Basement of the Dzabkhan Microcontinent of the Early Caledonian Orogenic Area, Central Asia

Fragments of continental blocks or microcontinents are represented in the Early Caledonian orogenic area of Central Asia (or Early Caledonian superterrane); the largest of these are the Dzabkhan and Tuva-Mongolian microcontinents, with Early and Late Precambrian crystalline basements, respectively. In the linkage zone of these microcontinents, crystalline rocks of the Tarbagatai and Songino blocks that are considered as units of the Early Precambrian ensialic basement of the superterrane are also known. They are composed of strongly metamorphosed rocks formed during the Early Baikalian orogeny about 790 to 820 Ma. U-Pb zircon dating and Nd isotope studies revealed, within the northwestern Dzabkhan microcontinent, the Dzabkhan-Mandal zone of crystalline rocks associated with the Riphean crust-forming process. The age of the gneiss substrate of this zone is estimated as 1.3 to 0.86 Ga. An early episode of metamorphism is dated at about 856 ± 2 Ma. The data available so far indicate a heterogeneous structure of the Dzabkhan microcontinent basement represented by Early Precambrian and Early and Late Baikalian crystalline formations.

Rare-metal granitoids of the Ulug-Tanzek deposit (Eastern Tyva): Age and tectonic setting

95 Raremetal magmatism covering the age range of more than 1 billion years, from the Late Riphean to the Triassic, is widely abundant within the southern margin of the Siberian Platform and its folded sur� rounding. Late Riphean massifs of alkali-ultrabasic rocks with carbonatite of the Beloziminsk Complex, Ordovician-Triassic massifs of alkali granite (Arys� kan, Zashikhin, Ermakov, and others), as well as numerous massifs of lithium-fluorine granite includ� ing the spodumene variety, are related to the most wellknown manifestations of raremetal magmatism, which control the formation of large deposits of rare metals, rare elements, and rare earth elements. Until recently, the ideas about their age were based mainly on geological data. Only in the last few years have geo� chronological investigations, which allowed us to esti� mate the age and tectonic conditions of related mag� matic processes in geological evolution of the southern surroundings of the Siberian Platform, been carried out (1). However, the regularities of raremetal mag� matism in the structures of this region are not under� stood completely for some epochs. In particular, this relates to the epoch when the unique complex rare� metal-rareearth deposit UlugTanzek (Eastern Tyva) was formed. The existing estimations of age for host alkaline granitoids vary within very broad limits (229- 217 Ma, K-Ar method by potassic feldspar) and cover the whole Triassic (2). However, this age range in the region is amagmatic as a whole. Because of this, the reliability of age estimations obtained is doubtful and, therefore, there is a question about the geological events that controlled the formation of the deposit. In this paper we present the results of dating of alkaline granitoids of the UlugTanzek Massif by the U-Pb method for zircon and the Ar-Ar method. With account for the geochronological data obtained, we analyzed its position in the structure of magmatic areals of the same age and, on this basis, reconstructed the geodynamic environment of raremetal magma� tism in the corresponding epoch. The UlugTanzek Massif of alkaline granitoids is located in southeastern Tyva within the Sangilen part of the Tyva-Mongol microcontinent, where it intrudes metamorphosed Vendian-Cambrian terrige� nous-carbonate deposits of the Kaakhem structural- facial zone. The massif is composed of quartz-albite- microcline granite with complex (Ta, Nb, Zr, Hf, REE, Li, Th, U, and cryolite) ore.

Convergent boundaries and related igneous and metamorphic complexes in caledonides of Central Asia

Fragments of the crystalline complexes where Vendian metamorphism of moderate and elevated pressure predated Early Paleozoic metamorphism have been established in the accretionary-collisional domain of the eastern segment of the Central Asian Foldbelt (Early Caledonian superterrane of Central Asia). The geodynamic setting of the Vendian (∼560–570 Ma) South Hangay metamorphic belt located in the junction zone of the Baydrag Block and the Late Riphean (∼665 Ma) ophiolite complex of the Bayanhongor Zone is considered. The origination of this belt was related to the formation of the convergent boundary in the framework of the Zabhan microcontinent about 570 Ma ago. At the same time, an island-arc complex was formed in the paleo-oceanic domain. Metamorphism of elevated pressure indicates that Vendian structures with sufficiently thick continental crust were formed in the framework of the continental blocks. Vendian metamorphism is also established in the Tuva-Mongolia Massif and the Kan Block of the Eastern Sayan. These data show that the Late Baikalian stage predated the evolution of the Early Caledonian superterrane of Central Asia. The development of its accretionary-collisional structure was accompanied by Late Cambrian-Early Ordovician low-pressure regional metamorphism. Granulite-facies conditions were reached only at the deep levels of the accretionary-collisional edifice. The outcrops of crystalline complexes in the southern framework of the Caledonian paleocontinent are regarded as fragments of the Early Paleozoic Central Mongolian metamorphic belt.

Late Paleozoic alkali-granitic magmatism of Tuva and its relation to intraplate activity within the Siberian paleocontinent

Geological, petrologeochemical, and geochronological studies of the rocks from the Shivei alkali-granitic pluton were conducted. A pluton about 500 km2 in area is a part of the larger (more than 30 000 km2) Kaakhem magmatic area. The data obtained allow us to characterize the magmatic complex of the Shivei pluton as a bimodal association with picrobasalts, subalkali basalts, and subalkali and alkali granitic rocks differentiated from syenites to leucogranites. The SHRIMP_II zircon dating from quartz syenites and alkali granites indicate the Permian age of the pluton (293.8 ± 3.8 Ma and 297.1 ± 3.8 Ma, respectively). Mafic-alkali-granitic associations similar in age and type, which are traced in the meridional direction along the Eastern Sayan toward the Siberian Platform, were distinguished as the Eastern Sayan zone of the Late Paleozoic alkaline magmatism. Its location corresponds to the western periphery of the Angaro-Vitim batholite and fits well into the zonal structure of the Barguzin magmatic province. We relate the geodynamic position of the Barguzin province with the mantle plume that was overlapped by the edge of the Siberian Pale-ocontinent in the course of its Paleozoic migration above the African hot spot.

Ophiolites of Western Tyva as fragments of a late Vendian Island arc of the Paleoasian ocean

866 In recent years a huge volume of principally new geochronological and isotope geochemical informa� tion, which characterize multiage complexes of the Central Asian folbelt (CAFB) and determined signifi� cant success in the study of its formation, has been obtained. However, many problems of its early (Cale� donian) history remain poorly investigated. This is definitely explained by the fact that, as was repeatedly mentioned by different researchers, the Caledonian CAFB megablock is characterized by a mosaic–block structure controlled by a combination of Precambrian terrains and surrounding folded zones of properly Caledonian rocks. The structural dissociation of these zones, as well as participation of rock complexes formed under different geological environments (oce�

Composition, sources, and geodynamic nature of giant batholiths in Central Asia: Evidence from the geochemistry and Nd isotopic characteristics of granitoids in the Khangai zonal magmatic area

Data on the composition, inner structure, and magma sources of giant batholith in the Central Asian Orogenic Belt are analyzed with reference to the Khangai batholith. The Khangai batholith was emplaced in the Late Permian–Early Triassic (270–240 Ma) and is the largest accumulations (>150000 km2) of granite plutons in central Mongolia. The plutons are dominated by granites of normal alkalinity and contain subalkaline granites and more rare alkaline granites. The batholith is hosted in the Khangai zonal magmatic area, which consists of the batholith itself and surrounding rift zones. The zones are made up of bimodal basalt–trachyte–comendite (pantellerite) or basalt-dominated (alkaline basalt) volcanic associations, whose intrusive rocks are dominated by syenite and granite, granosyenite, and leucogranite. Both the batholith and the rift zones were produced within the time span of 270–240 Ma. Although the rocks composing the batholith and its rift surroundings are different, they are related through a broad spectrum of transitional varieties, which suggests that that the mantle and crustal melts could interact at various scale when the magmatic area was produced. A model is suggested to explain how the geological structure of the magmatic area and the composition of the magmatic associations that make up its various zones were controlled by the interaction between a mantle plume and the lithospheric folded area. The mantle melts emplaced into the lower crust are thought to not only have been heat sources and thus induced melting but also have predetermined the variable geochemical and isotopic characteristics of the granitoids. In the marginal portions of the zonal area, the activity of the mantle plume triggered rifting associated with bimodal and alkaline granite magmatism. The formation of giant batholiths was typical of the evolution of the active continental margin of the Siberian paleocontinent in the Late Paleozoic and Early Mesozoic: the Khangai, Angara–Vitim, and Khentei batholiths were formed in this area within a relatively brief time span between 300 and 190Ma. The batholiths share certain features: they consist of granitoids of a broad compositional range, from tonalite and plagiogranite to granosyenite and rare-metal granites; and the batholiths were produced in relation to rifting processes that also formed rift magmatic zones in the surroundings of the batholiths. The large-scale and unusual batholith-forming processes are thought to have occurred when the active continental margin of the Late Paleozoic Siberian continent overlapped a number of hotspots in the Paleo- Asian Ocean. This resulted in the origin of a giant anorogenic magmatic province, which included batholiths, flood-basalt areas in Tarim and Junggar, and the Central Asian Rift System. The batholiths are structural elements of the latter and components of the zonal magmatic areas.

Forming conditions and age of native silver deposits in Anti-Atlas (Morocco)

663 Mercury bearing silver and Ag and Hg intermetal lides are described in primary and oxidized ores from different type ore deposits: Ni–Co–Ag–Bi–U, mer cury, polymetallic, Ag–Sb, Ag–Hg, and others. At the same time, the economic concentrations of these min erals are recorded only in ores of the Ni–Co–Ag–Bi– U and Ag–Hg deposits. The Ag–Hg deposits repre sent a peculiar separate type of silver mineralization, which distinctly differs from other types by the domi nant role of native Hg silver (main ore mineral) and the presence of different sulfides and Ag sulfasalts. There are several ore areas with such mineralization: Anti Atlas in Morocco, Verkhoyansk region in Yaku tia, the Altai–Sayan region, and others. Anti Atlas, which hosts three known deposits of the Ag–Hg type (Imiter, Igoudran, Zgounder) and several ore occur rences including those in the ore field of the Bou Azzer Co deposit [1–3], is most famous among them. The Imiter deposit, development of which started as early as in the 8th century and continues at present, is the largest of these deposits. The Imiter and Igourdan deposits are located among Middle Proterozoic terrigenous rocks repre sented by sandstones, siltstones, and black shales, which are intruded by Neoproterozoic gabbro, diorite, and granodiorite bodies. They are unconformably overlain by the Upper Neoproterozoic effusive sequence composed of andesites, rhyolites, and ign imbrites with dikes and stocks of andesites, dolerites, rhyolites, and granite porphyries. The Neoproterozoic igneous rocks in the ore field of the Imiter deposit host age and type variable mineralization: small occur rences of Mo greisen, Au–Ag and polymetallic Ag bearing, Co–As, and barite ores. The Neoproterozoic rocks are overlain by Cambrian platform strata with post Ordovician dolerite, monzonite, and K microsy enite dikes. The biotite and K feldspar 40Ar–/39Ar ages of microsyenites determined at the Analytical Center of the Institute of Geology and Mineralogy (Novosi birsk) are estimated to be 204.5 ± 2.5 and 199.5 ± 2.4 Ma, respectively (Fig. 1).

Age of the Khangai batholith and challenge of polychronic batholith formation in Central Asia

The Central Asian fold belt (CAFB) exhibits many manifestations of granitoid magmatism. The granite formation is largely determined by the convergence of the Siberian continent with structures of the Paleo� Asian ocean, which was practically continuous from the Late Riphean until the Early Mesozoic. This pro� cess was responsible for development of differentage folded domains (Early and Late Baikalian, Cale� donian, Hercynian, and IndoSinian) and conjugate subductionrelated magmatic (including granitoid) belts. An important role in granite formation also belonged to mantle plumes in addition to conver� gence. Their influence on the lithosphere resulted in the formation of giant batholiths represented by accu� mulations of closely spaced large granite plutons (1). Such batholiths were formed in the Late Cambrian- Early Ordovician (Altai-Sayany), Early Permian (Angara-Vitim), Late Permian-Early Triassic (Khan� gai), and Late Triassic-Early Jurassic (Khentei). The individual batholiths are >120 000-150 000 km 2 in size. The geological position of batholiths is independent from basement structures, and they usually comprise several terranes differing in their geological, including magmatic, history. Therefore, batholiths frequently include granitoids independent of them with respect to their formation time and formation conditions. Nevertheless, such a cooccurrence of differentage granites within a single region provides grounds for assumption of the polychronic nature of batholiths and their very long development history (2). This problem is of principal significance for interpreting the geodynamic evolution of the Central Asian fold belt and understanding the nature of granitoid magma� tism. One of the possible approaches to its solution consists in defining differences in the distribution of igneous rocks of different age groups and different geodynamic origin. For example, igneous rocks of the convergence zone are usually characterized by a distri� bution along belts, while products of plume magma� tism are localized above their sources. Precisely such an approach was used for consider� ing the problem of polychronic granite formation in the Khangai batholith. Recent geochronological investigations have revealed the relatively significant scatter of granitoid ages ranging from 300 to 220 Ma with dates of 269-242 Ma being prevalent (3-5) (Fig. 1). At the same time, these data are insufficient for solving the abovementioned problem, since no geochrono� logical dates are available for several key massifs in the southern, central, and southwestern parts of the batholith. Therefore, geochronological investigations of granitoids were conducted in the Tatsin Gol, Erdene Tsogt, Eginaba, Buyant Gol, and Yarugin Gol massifs located in these areas of the batholith. The Tatsin Gol massif is the largest one in the south� eastern part of the Khangai batholith. It is a typical representative of the granodiorite-granite formation, or the Khangai intrusive complex. The massif is com� posed of hornblende-biotite and biotite granodior� ites, monzogranites, and granites. They were formed during two intrusion phases. The first phase is largely represented by granodiorites accompanied by quartz diorites and tonalities. The rocks of the second phase include mediumto coarsegrained monzogranites and granites usually with a distinct porphyroid texture. For geochronological investigations, a sample of light gray porphyroid biotite-hornblende granites (Sample KHAN�09/46) was taken from the central part of the