Home – Home – Jornals – Russian Geology and Geophysics 2010 number 7

Study of the chemical composition of clinopyroxene and garnet megacrysts from the Dak Nong sapphire deposit and model calculations have shown that megacrysts originated from the crystallization of alkali basaltoid magma in a deep-seated intermediate chamber at 14-15 kbar, which is close to the Moho depth (50 km) in this part of southeastern Asia. The chamber was a source of heat and CO₂ fluids for the generation of crustal syenitic melts producing sapphires and zircons. The formation conditions of sapphires and zircons are significantly different. The presence of jadeite inclusions in placer zircons points to high pressures during their crystallization, which is confirmed by the ubiquitous decrepitation of CO₂-rich melt inclusions. Sapphires crystallized from iron-rich syenitic melt in the shallower Earth's crust horizons with the participation of CO₂ and carbonate-H₂O-CO₂ fluids. The subsequent eruptions of alkali basalts favored the transportation of garnet and pyroxene megacrysts as well as sapphire and zircon xenocrysts to the surface. It is shown that sapphire deposits can be produced only during multistage basaltic volcanism with deep-seated intermediate chambers in the regions with thick continental crust. The widespread megacryst mineral assemblage (clinopyroxene, garnet, sanidine, ilmenite) and the presence of placer zircon megacrysts can be used as indicators for sapphire prospecting.

The Khambin volcanotectonic complex is a horst framing the Late Cretaceous Lake Gusinoe basin in the northwest. This complex is due to the intracontinental rift conditions which existed in western Transbaikalia in the Late Mesozoic. They gave rise to a system of subparallel grabens and horsts in present-day topography. The magmatic evolution of this complex spans from 159 to 117 Ma and is divided into three stages. The first stage (159-156 Ma) was the formation of thick (up to 1500 m) volcanic masses composed of trachybasalts, basaltic trachyandesites, trachytes, trachydacites, trachyrhyolites, and pantellerites. The next two stages were the formation of isolated ancient volcanoes (127-124 Ma) composed of trachybasalts, basaltic trachyandesites, phonotephrites, tephriphonolites, and alkali trachytes and the formation of the Murtoi (Lake Gusinoe) essexite dike (122-117 Ma). The main trends for igneous associations from early to late stages are reduced magmatism and reduced rock diversity because of the decreasing portion of felsic volcanic rocks. Mafic rocks show an increase in total alkalinity, contents of incompatible elements (Th, U, K, Rb, Pb, Nb, Ta, Zr, Hf), total REE contents, and the LREE/HREE ratio. The isotopic composition of Sr and Nd in these rocks remained nearly constant and corresponds to that of OIB-EMII mantle sources. Compositional variations are attributed to a time-dependent decrease in the degree of partial melting of a similar magma source.

Ophicalcites were earlier found in the Lower Devonian olistostromes overlapping cobalt-bearing massive sulfide deposits in the ultramafic rocks of the West Magnitogorsk paleoisland arc. They are composed of angular clastics of serpentinites and carbonates few millimeters to several centimeters in size, which are cemented with hematite-calcite and quartz-hematite-calcite matrix with aragonite, magnesite, and siderite admixtures. In chemical composition Cr-spinels from serpentinites of the ophicalcites are similar to those from the underlying serpentinites and are suprasubduction products of active continental margins. The ¹³C/¹²C and ¹⁸O/¹⁶O ratios of calcite from the breccia matrix are typical of hydrothermal deposits and are close to those of carbonate in sulfide ores and talc-carbonate metasomatites. Study of fluid inclusions from the calcite cement has shown that the ophicalcites formed from low- to moderate-temperature (100-280 °C) hydrothermal fluids as a result of post-ore hydrothermal emanations on ultramafic seafloor rocks similar to modern hydrothermal fields in MORs and island arcs. Hydrothermal and tectonosedimentation processes in the roof of ultramafic massifs at the vents of hydrothermal fluids led to erosion, redeposition, and cementation of ophicalcites of four types. The subsequent tectonic and gravitational processes resulted in their denudation and accumulation in olistostromes.

The dependence of trace-element concentration on the size of crystal in sample is experimentally studied by the example of gold distribution among single crystals of different sizes of hydrothermally grown pyrite, As-pyrite, and magnetite. The effect is modeled on the assumption that the Au uptake is due to a nonautonomous phase (NАP). The structurally bound gold admixture is estimated from the dependence of the average content of evenly distributed gold on the specific surface of average crystal (1.5, 0.5, and 0.7 ppm for pyrite, As-pyrite with 0.02-0.08 wt.% As, and magnetite, respectively). The gold concentrations in hypothetical "pure" NАPs have been estimated by the extrapolation of the concentration dependence to the characteristic size of an NАP. The coefficients of fractionation of Au into an NАP relative to the bulk phase are 1.1·10³, 3.5·10³, and 2.4·10³ for pyrite, As-pyrite, and magnetite, respectively. Thus, the above effect is comparable in magnitude with the known effect of trace-element trapping by defects of crystal structure. Arsenic admixture favors the fractionation of gold into an NAP. We also considered other manifestations of this effect and its significance for solving problems of experimental geochemistry and analytical chemistry of trace elements and mineral processing. The data obtained substantiate the new mechanism of uptake of incompatible elements (including noble metals) during endogenic ore formation as more common and more effective than classical adsorption, including reducing adsorption of mercury and noble metals on mineral phases.

The Late Mesozoic and Cenozoic location of volcanic zones in the Central Asian intraplate volcanic province has been reconstructed. The anomalous-mantle regions related to magmatism in the province changed in shape in the Cretaceous and Cenozoic. In the early Early Cretaceous, the anomalous-mantle regions spanned from 42? to 61?N (about 2000 km in latitude), and their location might have remained unchanged throughout the Cretaceous. Magmatism in the province took place in the lithospheric regions of the Eurasian Plate with a thickness close to or smaller than that of the oceanic lithosphere. Late Mesozoic magmas originated mainly from hydrated mantle sources with isotopic compositions typical of PREMA or EM-II. In the Early Cenozoic (50 Ma) the anomalous mantle was considerably less active than in the Early Cretaceous. Magmatic melts were generated only in two mantle regions: the local South Hangay hotspot and, apparently, the fairly extensive (at least 800 km wide) mantle region north and northeast of it. The entire anomalous mantle spanned from 46? to 59?N (about 1300 km in latitude). Magmas of OIB type originated from slightly hydrated sources with isotopic compositions typical of PREMA or EM-I. In the Miocene, the mantle might have again "ejected" heated decompressed anomalous matter. The ejection led to an outburst of magmatism and expansion of the volcanic province up to 2000 km in latitude. The lithosphere in all the volcanic zones was thin, including the entire Eurasian territory over the South Hangay hotspot.

Current deformation in Pribaikalia, Western and Central Mongolia, and Tuva has been studied fr om measured horizontal GPS velocities and respective computed strain and rotation rates using 1994-2007 data of the Baikal-Mongolian GPS triangulation network.
The GPS velocity field shows two main trends: an NE trend within Jonggaria, the Mongolian Altay, and the Great Lakes Valley and an SE trend in the Hangayn and eastern Gobi Altay mountains, and in the Transbaikalian block of the Amur plate. The velocity magnitudes and vectors are consistent with an SE motion of the Amur plate at a rate of ~2 mm/year.
The derived strain pattern includes domains of crustal contraction and extension recognized fr om the magnitudes of relative strains. Shortening predominates in the Gobi and Mongolian Altay and in the Khamar-Daban Range, where it is at µ₂ = (19.2 ± 6.0)·10^-9 yr^-1 being directed northeastward. Extension domains exist in the Baikal rift and in the Busiyngol-West Hangayn area, wh ere the crust is stretching along NW axes at µ₁= (22.2 ± 3.1)·10^-9 yr^-1. The eastern Hangayn dome and the Gobi peneplain on its eastern border show low and unstable strain rates. In central and northern Mongolia (Orhon-Selenge basin), shortening and extension are at similar rates: µ₂ = (15.4 ± 5.4)·10^-9 yr^-1 and µ₁ = (18.1 ± 3.1)·10^-9 yr^-1. The strain pattern changes notably in the area of the Mogod earthquake of 1967.
Most of rotation throughout Central Asia is clockwise at a low rate of about © = 6·10^-9 deg·yr^-1. High rates of clockwise rotation are observed in the Hangayn domain (18.1 ± 5.2)·10^-9 deg·yr^-1, in the Gobi Altay (10.4 ± 7.5)·10^-9 deg·yr^-1, and in the Orhon-Selenge domain (11.9 ± 5.2)·10^-9 deg·yr^-1. Counterclockwise rotation is restricted to several domains. One is in western Tuva and northwestern Great Lakes Valley of Mongolia (© = 3.7·10^-9 deg·yr^-1). Two more counterclockwise rotation regions occur on both flanks of the Baikal rift: along the craton edge and in basins of Transbaikalia on the rift eastern border, wh ere rotation rates are as high as (13.0 ± 3.9)·10^-9 deg·yr^-1, while rotation within the Baikal basin does not exceed the measurement error. Another such domain extends from the eastern Hцvsgцl area to the Hangayn northern foothills, with the counterclockwise rotation at a highest rate of (16.3 ± 2.8)·10^-9 deg·yr^-1.

Structures of dynamic metamorphism have been traditionally studied proceeding from their similarity with faults, according to stratigraphic criteria and with reconstructions of predeformation settings. Using the example of the Kedrovyi-Butachikha shear zone in Rudny Altai, we suggest to distinguish zones with abundant dynamic metamorphic rocks (tectonites) as a special class of structures. Their diagnostic features are (i) dense fault populations, with mostly strike slip geometry of motion and intense mechanic failure and rework of the substrate; (ii) generally coordinated orientations (anisotropy) of structural elements at all hierarchic levels; and (iii) ordered patterns of laminar and turbulent flow. Complexes of tectonites in the Kedrovyi-Butachikha shear zone have been classified into dynamic clastics, tectonic schists, tectonic mixtites, and mechanic metasomatites according to their lithological and structural features. The new classification is used to image the architecture of the dynamic metamorphic zone in a map model which shows the pattern of tectonite complexes with their substrate unevenly reworked by shear-induced metamorphism.

The discussed spline approximation in spatial data modeling for geosciences implies formulation of the variational problem in terms of functional minimization and allows simultaneous inversion for several surfaces. This modeling employs the following basic elements: stabilizers to define the common properties of unknown surfaces; differential operators to describe the unknown surfaces and their relation with the known fields; data specified locally at test points; partial differential equations similar to equations of mathematical physics for the properties of the surfaces of interest; elements of regression analysis, with the regression coefficients being calculated while solving the principal modeling problem; arbitrary amounts of direct or indirect information which is incorporated additionally into the functional on the basis of approximate conditions using weight coefficients as control parameters.
The suggested generalized formulation includes the concepts of global and local equations and strict and nonstrict relationships. This formulation, realized in the GST software, may apply to many surface modeling problems to be solved using second-order partial differential equations, with multiple criteria optimization of results and with the use of different auxiliary datasets.

Much information on the regional lithospheric structure may come fr om MTS data acquired by synchronous 2D arrays and processed with regard to the nonlocal response of a laterally inhomogeneous subsurface. We suggest to invert the nonlocal MT responses applying correlation of all surface horizontal and vertical components of the geomagnetic field recorded simultaneously at all stations. The inversion algorithm has been applied to 2004-2005 European observatory data of diurnal S _q variations for first five harmonics and yielded lateral conductivity patterns for different periods. The maps show spatial correlation between conductivity maxima and lithospheric thickness minima and, specifically, highlight the contours of the Pannonian basin, wh ere lithosphere is as thin as ~50 km, from seismic data.