Home – Home – Jornals – Russian Geology and Geophysics 2015 number 8

The composition and mechanisms of formation of continental crust in Gorny Altai and the role of granitoid magmatism in its evolution are considered. Geochemical and isotope data for major types of rocks of primary crust and for Early-Middle Paleozoic granitoids of the region are presented. The role of granitoids as indicators of different stages of the continental-crust evolution is discussed. A review of the main models of continental-formation crust is made, and their applicability to the Gorny Altai segment of the Central Asian Fold Belt is shown. Based on the complex of geological, geochemical, isotope, and geochronological data, it has been established that the formation of continental crust in the Early and Late Caledonian terranes of Gorny Altai proceeded nearly synchronously (in the Middle-Late Devonian). In the Early Caledonian terranes, this process was the consequence of the multistage fractionation of primary juvenile crust of basic composition, and in the Late Caledonian ones it was the result of one-cycle intracrustal melting of hybrid andesitic crust rich in recycled material.

The Kalaxiange’er porphyry copper ore belt is situated in the eastern part of the southern Altai of the Central Asian Orogenic Belt and forms part of a broad zone of Cu porphyry mineralization in southern Mongolia, which includes the Oyu Tolgoi ore district and other copper-gold deposits. The copper ore bodies are spatially associated with porphyry intrusions of granodiorite, quartz diorite, quartz syenite, and quartz monzonite and have a polygenetic (polychromous) origin (magmatic porphyry, hydrothermal, and supergene). The mineralized porphyries are characterized by almost identical REE and trace element patterns. The Zr/Hf and Nb/Ta ratios are similar to those of normal granite produced through the evolution of mantle magma. The low initial Sr isotope ratio I Sr, varying within a narrow range of values (0.703790-0.704218), corresponds to that of primitive mantle, whereas the εNd( T ) value of porphyry varies from 5.8 to 8.4 and is similar to that of MORB. These data testify to the upper-mantle genesis of the parental magmas of ore-bearing porphyry, which were then contaminated with crustal material in an island-arc environment. The isotopic composition of sulfur (unimodal distribution of δ34S with peak values of -2 to -4‰) evidences its deep magmatic origin; the few lower negative δ34S values suggest that part of S was extracted from volcanic deposits later. The isotopic characteristics of Pb testify to its mixed crust-upper-mantle origin. According to SHRIMP U-Pb geochronological data for zircon from granite porphyry and granodiorite porphyry, mineralization at the Xiletekehalasu porphyry Cu deposit formed in two stages: (1) Hercynian “porphyry” stage (375.2 ± 8.7 Ma), expressed as the formation of porphyry with disseminated and vein-disseminated mineralization, and (2) Indosinian stage (217.9 ± 4.2 Ma), expressed as superposed hydrothermal mineralization. The Re-Os isotope data on molybdenite (376.9 ± 2.2 Ma) are the most consistent with the age of primary mineralization at the Xiletekehalasu porphyry Cu deposit, whereas the Ar-Ar isotopic age (230 ± 5 Ma) of K-feldspar-quartz vein corresponds to the stage of hydrothermal mineralization. The results show that mineralization at the Xiletekehalasu porphyry Cu deposit was a multistage process which resulted in the superposition of the Indosinian hydrothermal mineralization on the Hercynian porphyry Cu mineralization.

The cocrystallization coefficient of Mn and Fe (DMn/Fe) in magnetite crystals is determined in hydrothermal-growth experiments with internal sampling at 450 and 500 ºC and 100 MPa (1 kbar). It is weakly dependent on temperature in the studied PT -region and is constant over a wide range of Mn/Fe values. This permits using the magnetite composition as an indicator of Mn/Fe in the fluid under equilibrium: (Mn/Fe) aq ≈ 100 (Mn/Fe) mt . Since Mn is often a macrocomponent of the fluid and a microcomponent of magnetite, local analysis of fluid inclusions for Mn might help to determine Fe even in iron minerals. This will permit evaluation of the contents of other ore metals if the DMe/Fe values are known. For fine crystals (<0.1-0.2 mm) with low contents of Mn (<0.01-0.02%), it is necessary to take into account the fractionation of Mn into the surficial nonautonomous phase, in which its content can reach several percent. Comparison of these data with earlier data on the distribution of Mn in the system magnetite-pyrite-pyrrhotite-greenockite-hydrothermal solution shows that DMn/Fe remains constant in the presence of sulfur and sulfides. Precipitation of magnetite, in which Mn is a compatible admixture, cannot affect radically Mn/Fe in the solution because of the low DMn/Fe value. This effect is still more unlikely for pyrrhotite and pyrite, in which Mn is an incompatible admixture. The most probable mechanism of Mn fractionation into the solid phase is crystallization of FeOOH at lower temperatures. This is indirectly supported by the strong fractionation of Mn into the nonautonomous oxyhydroxide phase on the surface of magnetite crystals. The necessity of a more rigorous validation of “the new Fe/Mn geothermometer for hydrothermal systems” is substantiated.

The problem of modeling of real parageneses has been solved by minimization of Gibbs thermodynamic potential for metapelites of the Okhotsk granulite complex. Model mineral assemblages completely reproduce the composition of minerals and their modal contents in the studied rocks. This fundamental fact directly verifies the solution of the problem, proving the validity of the principle of local equilibrium in the studied assemblages and the agreement of all thermodynamic data accepted on the modeling. The pressure and temperature during the metamorphism of granulites of the Okhotsk complex, estimated by modeling, are 5.2-7.0 kbar and 620-770 ºC, which corresponds to the near-boundary conditions of the amphibolitic and granulitic facies. Model mineral assemblages similar to real parageneses in the composition of minerals and their modes can be successfully obtained with the Selektor software under conditions of both inert and moving water. The composition of the external metamorphic fluid and the approximate weight ratio of fluid to rock have been determined. The oxidation potential of this fluid is similar to the potential of oxygen at the buffer C-CO-CO ₂ if the fluid/rock ratio is 0.03-0.30 and the low partial pressure of water varies from 1.80 to 0.35 kbar. The Okhotsk metamorphic complex is not an analog of the granulites of the southern Aldan Shield, because considerably higher pressure and temperature are typical of the latter.

Possible sources of gallium in hydrothermal-sedimentary ferromanganese crusts of the Belyaevsky Seamount edifice (Central Basin, Sea of Japan) are considered. Studies with successive selective leaching have shown that ~80% of Ga are present in the manganese fraction. The Changbaishan Volcano ash with up to 35.3 ppm Ga has been found in the marine sediment column located in the immediate vicinity of the Belyaevsky Seamount. This suggests that Ga of the Fe-Mn crusts of the seamount was supplied with the ash of volcanic rocks containing up to 300 ppm Ga.

The results of electron probe microanalysis of several rock-forming minerals by wavelength-dispersive spectrometry (WDS) and energy-dispersive spectrometry (EDS) are compared, and the metrological characteristics of both methods are studied. The measurements were made with the use of a JXA-8100 (JEOL) microanalyzer with five wavelength-dispersive spectrometers and a MIRA 3 LMU (Tescan) scanning electron microscope equipped with an INCA Energy 450 Xmax 80 (Oxford Instruments) microanalysis system. Specimens with olivine, garnet, pyroxene, ilmenite, and Cr-spinel grains were analyzed. The variation coefficients that characterize the repeatability of a single determination are found to be ~0.5% for WDS and ~0.9% for EDS in the compositional range of the main components ( C > 10%). For minor components (1% < C < 10%), the variation coefficients are 1.4% and 3.0%, respectively, and for impurities (0.3% < C < 1%), 2.7% and 13%, respectively. For lower contents EDS is almost inapplicable. The ratio of the results obtained by the two methods is reproduced with high precision: For major components, the variation coefficient is 0.56%; for minor components, 1.7%; and even for impurities, it is ~8%. The magnitude of the bias is between 0.2 and 3.2 rel.%, which is acceptable. The results show that the accuracies of WDS and EDS are similar for measuring major and minor components of rock-forming minerals. Energy-dispersive spectrometry is inferior to wavelength-dispersive spectrometry for impurities and is completely inapplicable for still lower contents. This method is easier to implement, and the results are available soon after switching on the instrument. Wavelength-dispersive spectrometry needs more time for preparation, but it ensures a precise high-efficiency large-scale analysis of samples of similar compositions, even when the element contents are lower than 1%.

Sedimentary indicators of catastrophic glacial megafloods-plane-bedded angular gravel, cobbles, and boulders-are described in several sections of the high terraces of the Chuya River valley. The principal difference of these sediments from typical alluvium of this area is demonstrated. The clast roundness, grain size of clasts, and sedimentary structures of the high-terrace deposits of the Chuya and Katun’ valleys indicate the same facies originated through megaflood deposition. These results are at odds with ideas of alluvial, glacial, or glaciofluvial genesis of the high-terrace deposits of the Chuya River.

The Late Jurassic evolution of Boreal and Arctic basins is reflected in the widespread deposition of organic-rich black shales (source rocks). In this connection, the priority should be placed on the development and refinement of zonal schemes for the Upper Jurassic of the Laptev Sea coast based on ammonites, foraminifers, ostracods, dinocysts, and spores and pollen from reference sections as the basis for stratigraphic, paleogeographic, and facies studies. The Upper Jurassic and Lower Cretaceous reference section of interest is located on the left side of the Anabar Bay of the Laptev Sea (Nordvik Peninsula, Urdyuk-Khaya Cape). An uninterrupted and continuous section from Upper Oxfordian to Lower Valanginian is exposed in coastal cliffs and consists mainly of silty clay deposits with abundant macro- and microfossils. A reliable biostratigraphic subdivision of the Upper Jurassic interval of this section was taken as the basis for the assessment of the correlation potential of different fossil groups and subsequent interregional correlations, facies analysis, and detailed paleogeographic reconstructions of the study area. The analysis of variations in the composition of macrobenthic communities and microphytoplankton and terrestrial palynomorph assemblages and the biofacies analysis allowed the reconstruction of the evolution of marine paleoenvironmental settings in the western part of the Anabar-Lena sea and in the terrestrial settings in the adjacent land area of Siberia.

The processing algorithms for high-frequency induction resistivity data are applied to logs acquired at different stages of well construction. Open-hole induction logging while reaming of vertical wells provides a priori information on geology and resistivity distribution. The resulting resistivity model can make reference in geosteering for deviated and horizontal drilling. Algorithms for inversion of high-frequency induction responses from layered media are used in a software package for processing LWD data. The software provides real-time inversion to recover resistivities and depths to layers in oil and gas reservoirs penetrated by wells of a complex trajectory. It also allows checking the inversion quality by analyzing the sensitivity of tool responses to model parameters with reference to the tool specifications.

The city of Izmir, located at the western end of Turkey, has experienced many strong earthquakes throughout its history. The southern coast of Izmir Bay, one of the most densely populated areas of Izmir, is located on deep alluvial sediments. It is important to determine the effect of local soil conditions on dynamic ground response in the study area, where thick loose water-saturated alluvial sediments exist. A database including geotechnical and geological information on the study area is constructed. Majority of the site is classified as D and E according to NEHRP provisions. Dynamic site response analyses are performed with EERA by utilizing the field and laboratory test results and earthquake time histories of moderate-scale earthquakes such as 1977 Izmir ( ML = 5.3), 2003 Urla ( Md = 5.6), and 2005 Uzunkuyu-Urla ( ML = 5.9), which occurred in and nearby Izmir. In addition, a scenario ground motion generated by the Izmir Fault with a magnitude of 6.5, having an average distance of 10 km to the study area, is also considered. The output data obtained from the dynamic site response analyses are evaluated, and maps displaying variation in dynamic parameters on ground surface are prepared for the southern coast of Izmir Bay, Turkey. Consequently, the dynamic analyses performed with the soil models constituted for the study area verified the damage occurred in a close distance event of 1977 Izmir earthquake. The scenario earthquake resulted in peak ground accelerations more than 0.6 g at the eastern and western ends of the study area. However, long distance events resulted in spectral amplifications by up to 5 times. With this study, it is emphasized that local soil conditions should be evaluated individually in the area of interest. Generation of a site-specific design spectrum is recommended for the areas located on deep alluvial sediments.