A. G. Vladimirov

Thermochronology of the Chernorud granulite zone, Ol’khon Region, Western Baikal area

Structural-petrologic and isotopic-geochronologic data on magmatic, metamorphic, and metasomatic rocks from the Chernorud zone were used to reproduce the multistage history of their exhumation to upper crustal levels. The process is subdivided into four discrete stages, which corresponded to metamorphism to the granulite facies (500–490 Ma), metamorphism to the amphibolite facies (470–460 Ma), metamorphism to at least the epidote-amphibolite facies (440–430 Ma), and postmetamorphic events (410–400 Ma). The earliest two stages likely corresponded to the tectonic stacking of the backarc basin in response to the collision of the Siberian continent with the Eravninskaya island arc or the Barguzin microcontinent, a process that ended with the extensive generation of synmetamorphic granites. During the third and fourth stages, the granulites of the Chernorud nappe were successively exposed during intense tectonic motions along large deformation zones (Primorskii fault, collision lineament, and Orso Complex).The comparison of the histories of active thermal events for Early Caledonian folded structures in the Central Asian Foldbelt indicates that active thermal events of equal duration are reconstructed for the following five widely spiced accretion-collision structures: the Chernorud granulite zone in the Ol’khon territory, the Slyudyanka crystalline complex in the southwestern Baikal area, the western Sangilen territory in southeastern Tuva, Derbinskii terrane in the Eastern Sayan, and the Bayankhongor ophiolite zone in central Mongolia. The dates obtained by various isotopic techniques are generally consistent with the four discrete stages identified in the Chernorud nappe, whereas the dates corresponding to the island-arc evolutionary stage were obtained only for the western Sangilen and Bayankhongor ophiolite zone.

U-Pb Isotopic Age, Composition, and Sources of the Plagiogranites of the Kalba Range, Eastern Kazakhstan

Of special importance in the evolution of the Paleoasian ocean is the Carboniferous‐Permian stage, which reflects the closure of the oceanic basin and formation of the structure of the Central Asian fold belt (CAFB) in its present-day form [1]. This is also the time (320‐280 Ma) when the largest Angara‐Vitim granitoid batholith and conjugate synplutonic basite dikes in Transbaikalia, granitoid intrusions and bimodal basalt‐rhyolite series of the Gobi‐Tien Shan zone of South Mongolia, and the Kalba‐Narym and ZharmaSaur batholithic bodies and preceding subalkaline gabbroids, picritoids, high-Al plagiogranitoids, and paleovolcanic structures of the central type in Eastern Kazakhstan were formed. Unlike the Permian‐Triassic stage, which was proved to be related to the Siberian superplume [2], the geodynamic nature of the Carboniferous‐Permian magmatism of the CAFB remains controversial. The plume model was proposed for the Angara‐Vitim batholith [3], and break-off of lithospheric plates in the collision zone between the Kazakhstan and Siberian paleocontinents was suggested for the batholithic belts of Eastern Kazakhstan [4]. These models believe that the collision-related mantle was supplied either with autonomous plumes or with asthenospheric fingers produced by mantle extinction at the divergent margins of lithospheric plates. The solution of this problem consists in studying key magmatic complexes (composition, structure, age), with a special emphasis on the sources of magmatic melts. One such key complex is the plagiogranite of the Kalba Range traditionally distinguished in the Kunush Complex. One group of researchers suggested an Early Carboniferous age and island arc affinity [5], while others consider them as collision-related Late Carboniferous rocks [6, 7], or as products of the Tarim plume [8]. The analysis of the tectonic position of the magmatic complexes of Great Altai (according to N.A. Eliseev) and their correlation with sources [8, 9] sheds no light on this controversary. This article is aimed at accurate dating of the plagiogranites of the Kalba‐Narym zone for refining their position in the magmatic scheme, as well as for estimating the formation conditions and sources of plagiogranite magmas based on their composition and geochemical modeling of the protolith‐melt system. We studied the plagiogranites from the Zhilandy and Tochka massifs (Fig. 1). The Zhilandy Massif forms an isometric body, with medium-grained biotite granites in the central part and fine-grained porphyritic varieties and plagiogranite porphyry dikes in the marginal parts. The plagiogranites consist of large biotite, short-prismatic zoned andesine‐oligoclase, and single grains of orthoclase‐microperthite and green hornblende. The Tochka Massif is made up of NW-extended minor bodies and dikes of fine-grained porphyritic biotite plagiogranites and plagiogranite porphyries. The plagiogranites of the Tochka Massif differ macroscopically from the Zhilandy rocks owing to superimposed processes of mylonitization; however, microscopically, they have similar petrographic and mineral composition. The dikes of plagiogranite porphyries of both the massifs contain phenocrysts of short-prismatic zoned plagioclase, quartz, and biotite embedded in the microcrystalline groundmass. In both cases, the plagiogranites intrude black shales of the Takyr Formation ( D 3 –C 1 ) and are cut by the Early Permian granitoids of the Kalba batholithic belt [5‐7, 10].

Convex-along-rays functions and star-shaped sets

We study abstract convex functions with respect to the set of all abstract affine functions of the form x→ mini=l,…,n+l[li,x] - c. We describe large classes of abstract convex functions and give conditions guaranteeing subdifferentiability of these functions. Some applications to the study of problems of global optimization are discussed.

Spontaneous Resonances and the Coherent States of the Queuing Networks

We present an example of a highly connected closed network of servers, where the time correlations do not vanish in the infinite volume limit. The limiting interacting particle system behaves in a periodic manner. This phenomenon is similar to the continuous symmetry breaking at low temperatures in statistical mechanics, with the average load playing the role of the inverse temperature.

Uranium in saline lakes of Northwestern Mongolia

Analysis of major- and trace-element compositions of water in hypersaline soda closed basin lakes of Northwestern Mongolia and Chuya basin (Gorny Altai) shows high enrichment in 238U (up to 1 mg/l). Proceeding from new data, uranium accumulation in water has been attributed to (i) location of the lakes and their watersheds in potential provinces of U-bearing rocks and (ii) uranium complexing with carbonate in presence of carbonate (bicarbonate) anions. Among the explored hypersaline soda lakes of the area, the greatest uranium resources are stored in Lake Hyargas Nuur (about 6000 ton).

U-Pb Dating and Sm-Nd Systematics of Igneous Rocks in the Ol'khon Region (Western Baikal Coast)

The Ol’khon region represents a fragment of the West Baikal collision belt that resulted from Early Caledonian accretionary‐collisional processes related to the closure of the Paleo-Asian ocean in the southern margin (in present-day coordinates) of the Siberian Craton [1‐3]. This communication is dedicated to analysis of the internal structure and age of protoliths from this region based on U‐Pb dating and the Sm‐Md isotopic composition of igneous complexes. The U‐Pb (SHRIMP-II) age of rocks was measured at the Center of Isotopic Studies (VSEGEI, St. Petersburg) using the technique described in [4]. The Sm‐Nd isotope composition of bulk rock samples was determined by the standard method [5] at the Laboratory of Isotope Geochronology and Geochemistry (Geological Institute, Kola Scientific Center, Russian Academy of Sciences, Apatity). Figures 1 and 2 demonstrate experimental results obtained for two periods (500‐485 and 475‐465 Ma ago) (Figs. 1, 2), which reflect the scale and spatial distribution of synmetamorphic and intrusive igneous rocks in the southwestern part of the Ol’khon region. 1

Gabbro-granite intrusive series and their indicator importance for geodynamic reconstructions

One of the problems faced by researchers when paleogeodynamic reconstructions are carried out for deeply eroded orogenic terranes is the limited usage of geological and isotopic geochemical data on volcanic associations. The utilization of information on gabbroids and granitoids considered separately also fails to resolve this problem. The convergence of features of arc, collisional, and within-plate magmatic processes leads the researcher to search for indicator plutonic associations, such as paired gabbrogranite intrusive series. The latter were distinguished using geoinformation databases (including those compiled by the authors of this paper), which were composed for the Early Caledonides in the Altai-Sayan folded area and adjacent territories. This makes it possible to characterize mantle-crustal magmatism in suprasubductional and collisional environments and the conditions under which these geodynamic regimes (plate- and plume-tectonic factors) interact. This paper presents estimates of the composition of the parental basic magmas, distinctive features of their differentiation, and the compositional specifics of the accompanying extensive granite-forming processes. The example of the Altai-Sayan folded area and adjacent territories is employed to correlate the composition of the basic-ultrabasic and granitoid magmas and, on this basis, distinguish (a) differentiated gabbro-tonalite-plagiogranite intrusive series corresponding to accretionary-collisional geodynamic environments, and (b) gabbro-monzonite-granosyenite-potassic granite intrusive series, which were produced when the accretionary-collisional system was affected by a plume.

U-Pb Isotopic Dating of Zircons (SHRIMPII) from Granulites of the Ol'khon Region of Western Baikal Area

Based on the first isotopic geochronological data obtained by the classic U–Pb method by zircons [1–3], it was established that magmatic and metamorphic rocks of the Ol’khon region are of Early Paleozoic age. The age of granulite metamorphism of the Chernor� udskaya zone rocks spreading as a narrow band (with a width of <1 km) along the boundary of the Siberian Craton was 485 ± 5 Ma. Data of the SHRIMP ion microprobe for U–Pb isotopic dating of zircons [4, 5] confirmed previously obtained results: age estimations were 507 ± 8 Ma, for granulites in the northeastern part of the Chernorudskaya zone (Khadarta Penin� sula) and 498 ± 7 Ma, for the Khoboi Cape (Ol’khon Island). However, problems concerning age estima� tions for the protolith of these rocks remained unsolved. Our study is aimed at solution of these prob� lems.

The Khao Que-Tam Tao gabbro-granite massif, Northern Vietnam: A petrological indicator of the Emeishan plume

New data obtained on the Khao Que-Tam Tao gabbro-granite pluton (Northern Vietnam) are discussed. It was established that this pluton was formed at the Permian-Triassic boundary (250.5 ± 3.2 Ma, 40Ar/39Ar isotopic age). Morphologically, it represents a hypabyssal fracture intrusion. The first stage was marked by the intrusion of the picrobasaltic melt, the differentiation of which resulted in the formation of the layered peridotite-gabbro series and the quartz-bearing monzodiorites and granophyres in its endocontact at the final stage. At the second stage, the Khao Que peridotite-gabbro massif was broken in its central part by a fault, along which the Tam Tao granodiorite-granite massif was localized. Numerical simulation using the COMAGMAT program for the basic system and geochemical estimates for the granite system allow the statement that the mafic and granitic melts evolved independently, and their final products were quartz-bearing monzodiorite and granophyre, on the one hand, and aplites and pegmatites, on the other hand. The compositional correlation of the Permian-Triassic magmatic associations in Northern Vietnam (the Nui Chua gabbro pluton and the Khao Que-Tam Tao gabbro-granite and Pia Bioc granite plutons) and in Southeast China (flood basalts) allows these complexes to be regarded as a part of a single large igneous province produced by the Emeishan plume activity.

Thermodynamic modeling of the behavior of Uranium and Arsenic in mineralized Shaazgai-Nuur Lake (Northwest Mongolia)

Highly mineralized closed lakes on the territory of ore regions of Mongolia are of special interest in relation to the search for nonconventional sources of metals. Water of soda Shaazgai-Nuur Lake contains ~1 mg/L U, and the content of the undesirable admixture of As is up to 300 μg/L. Uranium and Arsenic speciation in solution and in the bottom sediments of the lake was estimated using thermodynamic modeling, and a method of their separation was suggested. Calculation of the models of sorption of these elements by goethite and calcite showed that at pH 9.4 typical of natural water it could be effective only at a high concentration of FeOOH sorbent. In this case, at pH <5 and >8 (the area of U sorption), As may be removed by simple filtering of solutions from the suspension upon additional coagulation.