N. I. Volkova

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.

Hollow silicate microspheres from fly ashes of the Chelyabinsk brown coals (South Urals, Russia)

Abstract Fly ashes from the South Uralian thermal power stations (TPS), using brown coals of the Chelyabinsk basin as combustible, were under study. The phase formation processes, proceeding in an ash fraction of these coals during their combustion, were investigated. The data on chemical composition of fly ashes are presented. Special attention was given to hollow silicate microspheres extracted from the wastes. With the use of a complex of physicochemical methods, morphology and chemical composition of microspheres are described in detail. Globules of all known morphological types made up of silicate glasses of the two discrete compositions, K–Al–Si and Fe–Al–Si, with a predominance of the first type, were found. The phase composition of mineral impurities in the Chelyabinsk coals is recognised optimal for originating hollow silicate microspheres during industrial coal combustion. The microspheres from the South Uralian fly ashes are comparable to microspheres having practical application in size, wall thickness, shape, morphology of a surface, chemical composition and homogeneity.

Age and thermal history of the Maksyutov metamorphic complex: 40Ar/39Ar evidence

The 40Ar/39Ar method with stepwise heating was used to date phengite and glaucophane in the contact zone of garnet glaucophanite an omphacite-garnet rock (eclogite) from the lower unit of the Maksyutov metamorphic complex. The correlation of the measured age and the sizes of the phengite flakes indicates that the behavior of radiogenic Ar in them was controlled by the mechanisms of volumetric diffusion. Taking into account the fact that all of the rocks have the same thermal history, the dates most close to the age of metamorphism are those of the largest phengite flakes from garnet glaucophanite: 392 Ma. The age values obtained on phengite from an omphacite-garnet rock sampled at the maximum distance from the contact are equal to 378 Ma and correspond to the time when the rocks cooled to temperatures below 350°C. The results of numerical simulations indicate that the metamorphic age is no younger than 400 Ma, and the linear cooling rate can be estimated at 3.40 −0.75/+1.24°C/m.y. The maximum values of the phengite ages are consistent with the dates of glaucophane from three rock samples: 389–411 Ma.

Geochemical signatures for eclogite protolith from the Maksyutov Complex, South Urals

Abstract We conducted a geochemical study of eclogites (40 samples) from a boudin of the Lower Unit of the Maksyutov Complex in the South Urals in order to determine their protolith nature. The eclogites have major element compositions corresponding to quartz-bearing hypersthene basalts. Trace-element characteristics of the eclogites further suggest that they resemble enriched-type of tholeiites such as E-MORB. The compositional variation of eclogites was likely caused by fractional crystallization of parental melt under hypabyssal conditions, during its intrusion in thinned continental crust shortly before subduction. The high-pressure metamorphism has not affected significantly the major- and trace-element signatures of the protoliths. The compositions of co-existing minerals from the distinguished rock groups do not show significant distinctions. The considerable scatter of P – T estimates of metamorphic conditions does not depend on whole-rock composition. Therefore, the eclogitization was preceded by a chemical differentiation of an initial magmatic source, which is responsible for co-existence of rocks of variable composition in the same boudin. Dikes or sills of tholeiite basalts having geochemical characteristics of E-MORB could be the protoliths for the Maksyutov eclogites.

Fayalite and kirschsteinite solid solutions in melts from burned spoil-heaps, South Urals, Russia

Individual grains of calcian fayalite and ferroan kirschsteinite, as well as fayalite-kirschsteinite intergrowths are observed in the groundmass of basic crystallised melts, or parabasalts, from burned spoil-heaps of the Chelyabinsk brown-coal basin. Exsolved fayalite and kirschsteinite rims surround the grains of fayalite and early Mg-Fe olivine. The chemical study of the olivines has shown that during their crystallisation they were becoming enriched in fayalite and larnite and depleted in forsterite. The intergrowths of ferroan kirschsteinite (> 20 wt.% of CaO) and calcian fayalite ( 8.5 wt.%. The exsolution temperatures were estimated to 980-800 °C. The main reasons for the appearance of the Ca-Fe olivine in the parabasalts are the composition of the initial melt enriched in FeO and CaO, fractional crystallisation resul ting in further enrichment in iron of the residual low-silica melt, and reducing conditions during olivine crystallisation and exsolution.

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

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.

Geochemistry of metamorphosed pillow basalts of the Chara Zone, NE Kazakhstan

Geochemical study of the metamorphosed pillow lavas of the Chara Zone revealed that their protoliths have N-MORB compositions. Elevated concentrations of K, Rb, Cs, and lower Ca may be related not to blueschist metamorphism, but rather they may be inherited from underwater alteration of parental basalts. Comparative analysis of the compositions of massive glaucophanites and vein rocks demonstrated the relative mobility of Sr, U, P, Ba, Rb, K, Cs, Ca, and LREEs, at least on the local scale. Their mobility has been provided by fluids circulating in the subduction zone, where the fluids have been generated through slab dehydratation processes. These studies of metamorphosed pillow lavas of the Chara Zone displayed only local alterations in the bulk composition of their protoliths, which evidences a low intensity of fluid flows: this allowed preservation of the geochemistry of oceanic basalts.

Trace Element Distribution in Mineral Inclusions in Zoned Garnets from Eclogites of the Atbashi Range (South Tianshan)

Ion microprobe data for minerals from the eclogites of the Atbashi Range (South Tianshan) constrain the distribution of trace (Rb, Sr, Ba, Cr, V, Zr, Hf, Nb, Ta, U, Th, and Y) and rare-earth elements (REE) in zoned garnets and mineral inclusions in them. This study showed that garnets from the Atbashi eclogites are the main hosts for heavy REE; epidotes are important hosts for REE, Y, Sr, Th, and U; and omphacites are depleted in almost all trace elements compared with the bulk-rock compositions. Garnet, as well as epidote and omphacite inclusions exhibit systematic rimward depletion in a number of trace elements, which is related to the depletion of the rock matrix in these elements during crystallization. Deviations from this trend, including the enrichment of garnet rims in HREE and strong variations in the REE contents of garnets and mineral inclusions, can be explained by metamorphic reactions involving the destabilization of REE-bearing minerals. Our data suggest that the mobility of trace elements under eclogites-facies conditions is mainly controlled by the stability of certain minerals.

Eclogites in the Chara zone, NE Kazakhstan: New geochemical and geochronological data

N. I. Volkovaa, b, V. A. Simonova, c, A. V. Travina, b, c, S. I. Stupakova, and D. S. Yudina, c a Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090 Russia b National Research Center Tomsk State University, pr. Lenina 36, Tomsk, 634050 Russia c Department of Geology and Geophysics, Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090 Russia e-mail: nvolkova@igm.nsc.ru Received July 28, 2014; in final form, November 24, 2014