Home – Home – Jornals – Russian Geology and Geophysics 2006 number 11

In southern Israel and on the Sinai Peninsula, ultrapotassic quartz porphyries (UPQP) with 6.5-10 wt.% K₂O and 0.1-2.5 wt.% Na₂O were found in a bimodal dike suite that formed at the final stage of the Pan-African orogeny. The suite is made up mainly of quartz porphyry (4-5.5 wt.% K₂O); mafic rocks amount to ~5%. The UPQP form rare dikes or patches in quartz porphyry dikes. These are typical igneous rocks with microgranophyric and spherulitic matrices. There is no mineralogical evidence for the gain of K at the postmagmatic stage. Evidence for low-temperature adularization found in some ultrapotassic rhyolites from other world areas has not been revealed either. Alkali-feldspar phenocrysts in the UPQP and quartz porphyries have high contents of orthoclase (≥ 85-90%). Study of melt inclusions in quartz phenocrysts in both the UPQP and ordinary quartz porphyries showed that the phenocrysts crystallized from magmas of quartz porphyry composition that contained 2-3 wt.% H₂O, up to ~1% F, and 0.1-0.15% Cl. Scanning electron microscope studies showed that many alkali quartz porphyry dikes have a heterogeneous matrix. Rounded and oval segregations (0.3-15 mm across) with microgranophyric and spherulitic textures amount to ~50%. They are enriched in K₂O and are compositionally similar to typical UPQP. In the microcrystalline aggregates hosting these segregations Na dominates over K.
To account for the UPQP generation, a model is proposed for the disequilibrium crystallization of silicic magma during its flow along fractures in cooled country rocks. The magma was of alkali rhyolite composition and contained 5-7% phenocrysts. At the early stages of the matrix crystallization, spherulitic and microgranophyric segregations formed. They were enriched in K, which is typical of a haplogranite system at the beginning of crystallization. Since the system remained closed for major components, the portion of Na in the residual melt increased. The disequilibrium crystallization conditions inhibited a chemical interaction between early and late phases. During the

High-potassium rocks have been found among ultrabasites of the Il'chir ophiolite nappe. Stock- and sill-like bodies are composed of greenish-gray to dark-gray holocrystalline (sometimes, porphyritic) rocks. Three rock varieties are recognized: albitized monzonitoids (wt.%): SiO₂ - 53.2-53.8; Al₂O₃ - 14.6-14.9; MgO - 3.3-8.3; Na₂O - 5.0-5.9; and K₂O - 0.05-0.1; syenites (orthoclasites) (wt.%): SiO₂ - 63.0-66.1; Al₂O₃ - 17.2-18.6; Na₂O - 0.2-3.1; and K₂O - 8.2-16.7; granosyenites (wt.%): SiO₂ - 71.5-77.3; Al₂O₃ - 11.5-13.5; Na₂O - 0.1-1.72, and K₂O - 9.2-10.7. The K-richest varieties are composed of allotriomorphic grains of perthites, K-antiperthites, and, sometimes, plagioclase. Biotite, amphibole, and chlorite occur in small amounts (1-5 vol.%). The rocks are strongly altered to blastocataclasites with a feldspathic groundmass. Along with intense shearing and pseudoporphyritic texture (porphyroclasts), a distinctive feature of K-feldspar and plagioclase preserved as porphyroclasts in relics is their carbonization. Neogenic K-feldspar, albite, and dark-colored minerals lack cloud-like carbon pigmentation. Potassium-rich rocks have high contents of Ba and Rb and show a distinct Eu minimum on their REE patterns. According to Rb-Sr dating, these rocks have an age of 419 ± 30 Ma (⁸⁷Sr/⁸⁶Sr = 0.7061 ± 0.0021), which is, most likely, close to the age of the last metamorphism. Indirect data evidence that primary island-arc rocks might have formed in island-arc setting, possibly, in the foreland of island arcs.

Based on Ovchinnikov's genetic-type formula, geologo-genetic models have been constructed for Mesozoic gold deposits localized in chamber-dome and chamber-depression structures in the Transbaikalian area of the Mongolo-Okhotsk fold belt. This formula includes 12 factors that characterize ore formation processes, their depth of occurrence and geodynamic settings, the sources of ore matter and ore-forming solution, energy of ore formation processes, composition of ore-forming solution, environment and mechanism of ore deposition, zoning, interaction of ores with host rocks, and thermodynamic setting of ore deposition.

Mathematical properties of the inverse dynamic seismic problem for determining the elasticity parameters of a homogeneous layered anisotropic medium are investigated for the case of orthorhombic symmetry. Analysis of the mathematical properties of the inverse problem allows predicting which parameters can be estimated to a satisfactory accuracy. The suggested inversion algorithm is tested in numerical experiments for determining elastic parameters of a thinly stratified medium including several anisotropic layers.

We investigate the stability of oil-water layered systems at arbitrary values of physical parameters. It is essential to distinguish the known electric-hydrodynamic instability from parametric resonance. Parametric resonance in layered systems under alternating current shows the presence of two frequency bands which differ in behavior of foreign charge storage. We report numerical experiments on the nonlinear behavior of parametric resonance in layered systems subject to acoustic excitation. Periodic pulse impact on the boundary of a layered system causes an increase of several orders of magnitude in the amplitude of its elastic stress relative to the initial distribution.

The preseismic stress field on the western flank of the Sunda seismic arc before the great Sumatra-Andaman earthquake modeled from earthquake mechanism data shows a heterogeneous pattern in the area of the pending source. The first motion in the source of the coming great event occurred within the region of high deviatoric and isotropic stresses near their steepest gradients. Seismic radiation in the earthquake culminated in the zone of lowest stress extending for about 350 km along the northern edge of Sumatra Island as far as the Nicobar Islands. The revealed stress distribution in the earthquake nucleation area agrees well with laboratory experiments on rock failure in which low and medium stresses correspond to brittle failure. The suggested model of the metastable state of rocks in seismogenic regions includes the presence of different stress zones and the formation of a zone of high stress gradient in the place of the pending bigger earthquake.

Four climatholiths of the Upper Pleistocene are used in Siberia as regional stratigraphic horizons: Kazantsevo, Zyrjanka, Karginsky, and Sartan. Their geochronological basis is discussed in this paper. A conclusion is made that they are erroneously correlated in the official stratigraphic scheme. New detailed data obtained by international teams on key sections of Arctic Siberia suggest an alternative division of the Upper Pleistocene. A new version of chronostratigraphic scheme of West Siberia is proposed for discussion. The new scheme for the lower Upper Pleistocene is based on well-known key sections of the Karginsky and Zyrjanka Horizons. The main thermomer of the Upper Pleistocene is represented by the Malaya Kheta and Karginsky strata with geochronometric marks corresponding to early marine isotope stage 5. The complex of the last glaciation (Zyrjanka Horizon) is dated within a broad interval from 100 to 50 kyr BP. Two new horizons with stratotypes on the Yamal Peninsula are proposed for the second half of the Upper Pleistocene. These are the cold Var'yakha interstadial about 50-30 kyr BP and the Syoyakha cryomer 30-11 kyr BP. The new horizons are in good paleoclimatic correspondence with the Middle and Late Pleniglacial of Europe.