Home – Home – Jornals – Russian Geology and Geophysics 2012 number 8

The minor intrusions of the Edel'veis alkaline-carbonatite complex are bounded by the spurs of the North Chuya Ridge in southeastern Gorny Altai. According to Ar-Ar isotope data, the complex formed in the Middle Cambrian (~507 Ma). All of its components (alkali clinopyroxenite-melanogabbro-alkali syenite + Ca-carbonatite) occur in only one pluton. Silicate igneous rocks are equivalent in silica content and alkalinity to potassic alkaline and subalkalic mafic rocks. Apatite-phlogopitic Ca-carbonatites are enriched in P₂O₅ (up to 3.6 wt.%), Sr (~2500-5500 ppm), and REE (up to ~2000 ppm) and are, presumably, of liquation genesis. A PREMA-type plume component was a predominant magma source for the complex (ε_Nb (Т) = +6.56... +6.85). According to isotope data (⁸⁷Sr/⁸⁶Sr_(Т) ~ 0.7032-0.7039; δ¹⁸ О ~ 7.5-14.9 ‰; δ¹³ С ~ -2.7...-8.4 ‰), the fractionation of the melts was accompanied by their crustal contamination. The trace-element composition of the rocks suggests that the complex developed on a continental margin and its development was accompanied by late-collisional rifting and the mixing of moderately depleted (PREMA) and enriched suprasubduction lithospheric mantle (EM I or EM II) with continental crust. It is presumed that the alkaline and carbonatite complexes in the western Central Asian Fold Belt are of primary plume origin and form a LIP within this belt together with other associations produced by Early Paleozoic (510-470 Ma) magmatism.

The Sillai Patti carbonatite complex represents the second largest carbonatite body of the Peshawar Plain Alkaline Igneous Province of northern Pakistan. It is situated about 20 km west of Malakand, near Sillai Patti Village. Here, the carbonatite occurs along a fault in the form of a sheet striking in the NNE-SSW direction and dipping southward. The carbonatite body is about 12 km long and 2-20 m thick, predominantly intruded along the faulted contact of metasediments and granite gneiss but locally, within the metasediments.
A fission-track age of 29.40 ± 1.47 Ma was obtained for the Sillai Patti carbonatite complex. Close resemblance of the fission-track apatite age of this study with the fission-track as well as other high-temperature radiometric ages from the same and the neighboring carbonatite complexes of the alkaline belt of northern Pakistan suggests emplacement of the Sillai Patti carbonatite complex at higher crustal level and subsequent extremely fast cooling to near ambient temperatures (<60 ?C) required for the complete retention of fission tracks in apatite. The age data also point out that the fission-track age of 29.40 ± 1.47 Ma of this study is the age of intrusion of the carbonatitic magma of the Sillai Patti carbonatite complex to shallow, near-surface level.
Comparison of the uplift-induced denudation rates of the region with the world data clearly reflects the presence of a postcollisonal extensional environment in the region south of the Main Mantle Thrust during Oligocene time. This strongly negates the idea of the earlier workers of emplacement of the carbonatite complexes of the Loe-Shilman and Sillai Patti areas along thrust faults during the Oligocene.

Nabi Musa located at the northern tip of the Dead Sea at 31?48?N, 35?25?E is one of fifteen complexes of the Hatrurim Formation or the so-called "Mottled Zone" (MZ) which are fossil mud volcanoes. Self-ignition of methane during their eruptions in the Middle-Late Pleistocene caused combustion metamorphism of sediments. Melting foci have been discovered in two craters of Nabi Musa volcano, with numerous veins of paralavas having particular calcic-silicic compositions (Ca₂SiO₄- and CaSiO₃-normative). Their major- and trace-element spectra bear signature of a mixed sedimentary protolith consisting of Cretaceous marine carbonates, marl, and quartz sand. The paralavas inherit high Sr, P, and U enrichments, positive La/La* and Y anomalies, and a negative Ce/Ce* anomaly from calcareous marine sediments, including bituminous and apatite-rich chalks. The presence of quartz arenite in the protolith is responsible for relatively high Ti, Nb, Zr, and Hf, while the marl pelitic component accounts for MREE and LREE depletion. The suggested mixing models predict that the Nabi Musa paralavas result from combustion metamorphism of a sediment mixture with 53-60 wt.% chalk, 5-14 wt.% marl, and 27-44 wt.% quartz arenite. The history of mud volcanism at Nabi Musa began with small eruptions that mobilized gas and water from shallow (within 300 m) Turonian carbonate aquifers, and later explosive activity triggered violent gas blowouts from the older terrigenous reservoir of Aptian-Albian Nubian-type sandstone lying as deep as 1300-1500 m.

A camptonite dike swarm (Agardag alkali-basalt complex) in the western part of the Sangilen Upland abounds in mantle xenoliths. Mineralogical, petrographic, and petrochemical studies showed that the dikes are composed of lamprophyres of two groups, basic and ultrabasic. Ar/Ar dating of amphibole and phlogopite megacrysts yields an intrusion age for the dikes of 443.0 ± 1.3 Ma. ²⁰⁶Pb/²³⁸U dating of zircon from a glomeroporphyritic intergrowth in camptonite from one of the dikes yielded a core age of 489.0 ± 5.4 Ma. This corresponds to the time of formation of the Chzhargalanta granite-leucogranite complex (489.4 ± 2.6 Ma). The ²⁰⁶Pb/²³⁸U age of the zircon rim is 444.0 ± 7.5 Ma. The ages obtained by Ar/Ar dating of amphibole and biotite megacrysts and by U/Pb dating of the magmatic rim of zircon crystal from the camptonite coincide within the dating error, which indicates that the camptonite dikes formed in the Late Ordovician. These dikes are the oldest-known example of mantle-derived xenoliths in mafic volcanic rocks from an off-craton setting. These are samples of the Upper Ordovician lithospheric mantle.

The Cambrian section of the western limb of the Chekurovka anticline was studied, with layer-by-layer collection of animal fossils. This permitted us to refine the biostratigraphic division of this section. Four biostratigraphic units (beds) with small shelly fossils were identified: Lapworthella tortuosa , Lapworthella bella , Mobergella radiolata , and Rhombocorniculum cancellatum . They characterize most of the Tommotian and Atdabanian Stages of the Lower Cambrian. The Lower Tyuser Subformation and the Sekten Formation were constrained by small shelly fossils for the first time. Numerous trilobite finds helped to reveal intervals correlating with the Delgadella anabara-Nevadella and Judomia-Uktaspis (Prouktaspis) Regional Zones in the upper Atdabanian.
The upper Sekten Formation (black shales) forms the Kuonamka Formation. Within this interval, the following trilobites were found: Triplagnostus gibbus , Tomagnostus sibiricus , Pseudanomocarina sp., Eodiscus oelandicus , Paradoxides sacheri , and Ptychagnostus aff. atavus . They are widespread in the vast territory of the eastern Siberian Platform, within the Kuonamka Formation.
Also, Korobovia khorbosuonica , Charaulaspis postprimus , and Hebediscus convexa trilobites were observed, which had been found earlier in the Khorbosuonka River section (Olenek uplift). This permits a detailed correlation between these localities.
Similarity was observed between Triangulaspis annio trilobites and young Charaulaspis . This casts doubt upon the fact that the first Triangulaspis annio finds in the northeastern Siberian Platform belonged to the upper Atdabanian.

Organic matter of Cretaceous and Cenozoic deposits in the Indigirka-Zyryanka basin was studied by bituminological analysis and chromato-mass spectrometry. The regularities of distribution of alkanes and polycyclic aromatic hydrocarbons depending on the catagenesis of fossil organic matter are shown. Sesquiterpanes of the drimane series have been recognized in the Cretaceous deposits, and tetracyclic biomarkers of phyllocladane (kaurane), in the Cenozoic ones. The biomarkers were produced from terpenes of conifers, and the identified set of pentacyclic oleanenes are the products of angiosperms flourishing in the Paleogene and Neogene. As evidenced from geochemical indices, the Cenozoic deposits did not reach the PT -conditions of "the main oil formation zone" and should not be used for the quantitative evaluation of the regional petroleum potential. Nevertheless, the Cenozoic clayey units can be perfect isolating horizons for fluids migrating from more ancient, including Cretaceous, deposits.

Crustal kinematic characteristics such as strain and displacement rates are important in the monitoring of present-day processes in zones of seismic and industrial hazard. Strain measurements on different temporal and spatial baselines reveal potentially hazardous zones. Data on coseismic displacements and strain can be used to refine earthquake models, and long-term characteristics are important in searching for earthquake precursors and studying the rheology of the crust and fault zones. Tilt measurements in the adit of the Talaya seismic station (51.68?N, 103.64?E, South Baikal region) began in 1985; strain measurements, in 1990; and GPS measurements, in 2000.
The data reflect time variation in the local strain parameters and permit a comparison with the GPS data on the region and western Central Asia. The data on strain variations obtained in the Ala-Archa underground observatory (42.63?N, 74.50?E, North Tien Shan) are analyzed together with those obtained in the region by GPS methods.
Strain rates on very long baselines were determined using data from permanent IGS stations for Central Asia - the territory expanding from Dzungaria in the south to the Siberian Platform in the north and from the Tien Shan in the west to Lake Baikal in the east. We consider the example of using strain gaging and GPS data to construct a dislocation model and refine the parameters of the Kultuk earthquake (South Baikal region, 27 August 2008, M = 6.3).

Biopolymer-based drilling fluids affect data of high-frequency isoparametric induction logging. Highly conductive fluid invaded into drilling-induced flaws produces zones of high resistivity contrasts. Thus, the logs acquire new features that never appear in drilling with argillic mud. These effects have been simulated with regard to high resistivity contrasts for the case of flawed borehole walls, and the modeling results were checked against experimental data.

A mudcake formed on the borehole wall between a fluid-filled porous formation and the borehole fluid can affect Stoneley wave propagation used to estimate the formation permeability. The mudcake effect on the permeability dependence of radial oscillations of borehole fluid is investigated in a system with a source generating radial acoustic waves in the borehole and hydrophones (pressure sensors) that record the respective attenuating oscillations of the borehole fluid. The permeability of the porous formation is estimated from theoretical permeability dependence of the attenuation exponent, knowing the time dependence of the exponential attenuation of borehole fluid oscillations.

Inversion of laterally heterogeneous MT responses collected by a dense synchronous 2D array allows a new approach to estimating electrical anisotropy. The procedure includes solving an overdetermined equation system, assuming distant-dependent anisotropy in a smooth approximation. Inversion is performed using an effective conductivity tensor, which approximates some limited domain of the survey polygon. The algorithm for simulating 2D array MT responses with the effective conductivity tensor approach implies correlation of surface components of the EM field measured simultaneously at all stations within the chosen domain of the survey polygon. The suggested technique of data acquisition and processing ensures reconstruction of the full conductivity tensor from laterally variable components of the EM field.