Magnetohydrodynamic oscillations due to precessional motion

Summary The motion of a non-viscous liquid medium with an ideal electrical conductivity, enclosed in the ellipsoidal cavity of an envelope subject to precessional motion, is investigated. Inside the cavity there is a toroidal magnetic field. It is proved that the precessional motion of the envelope generates forced oscillations of the medium with a period of 24 hours and two different types of eigen oscillations. Expressions for computing the amplitudes and frequencies of all types of oscillations are derived. The maximum values of disturbance fields are determined for geophysically acceptable parameters.     

Magmatic history and geophysical signature of a post-collisional intrusive center emplaced near a crustal-scale shear zone: the Plechý granite pluton (Moldanubian batholith,Bohemian Massif)

The Plechy pluton, southwestern Bohemian Massif, represents a late-Variscan, complexly zoned intrusive center emplaced near the crustal-scale Pfahl shear zone; the pluton thus provides an opportunity to examine the interplay among successive emplacement of large magma batches, magmatic fabric acquisition, and the late-Variscan stress field associated with strike-slip shearing. The magmatic history of the pluton started with the emplacement of the porphyritic Plechy and Haidmühler granites. Based on gravity and structural data, we interpret that the Plechy and Haidmühler granites were emplaced as a deeply rooted, ∼NE–SW elongated body; its gross shape and internal fabric (steep ∼NE–SW magmatic foliation) may have been controlled by the late-Variscan stress field. The steep magmatic foliation changes into flat-lying foliation (particularly recorded by AMS) presumably as a result of divergent flow. Magnetic lineations correspond to a sub-horizontal ∼NE–SW finite stretch associated with the divergent flow. Subsequently, the Třístoličník granite, characterized by steep margin-parallel magmatic foliation, was emplaced as a crescent-shaped body in the central part of the pluton. The otherwise inward-younging intrusive sequence was completed by the emplacement of the outermost and the most evolved garnet-bearing granite (the Marginal granite) along the southeastern margin of the pluton. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Silver-cobalt mineralization in the Upper Seymchan ore cluster,Northeastern Russia

This paper is focused on the Early Cretaceous Ag-Bi-Co-sulfoarsenide mineralization atypical of northeastern Asia, which contains diverse Co-Ni sulfoarsenides, Se-bearing Bi sulfotellurites, and Ag-Bi-Pb sulfosalts. The Upper Seymchan ore cluster is located at the boundary between the Paleozoic Omulevka Terrane of carbonate platform and the In’yali-Debin Synclinorium of the Kular-Nera Terrane. These ore-bearing sequences are represented by the Middle Jurassic terrigenous rocks that rest upon the Upper Triassic sandshale rocks of the upper structural stage. The sedimentary rocks are cut through by high-Al granitic plutons and younger granite-porphyry dikes. The orebodies that are superposed on igneous rocks were formed during (1) the quartz-chlorite-tourmaline stage of metasomatic alteration, (2) the main economic tourmaline-chlorite-quartz-sulfoarsenide vein stage, (3) the polysulfide-quartz stage with Ag, Se, Bi minerals, and (4) the postore quartz-calcite stage with fluorite. The epithermal veins of festoon chalcedony-like quartz with Sb-bearing arsenopyrite occupy a special position. In particular orebodies, the chlorite-quartz ore veins dominate at the upper levels, whereas the quartz-tourmaline veins occur at the lower levels. Wall-rock alteration is represented by metasomatic chloritization and tourmalinization up to the formation of monomineralic metasomatic zones. Sulfides and sulfoarsenides are distinguished by anomalous enrichment of sulfur in the light isotope (δ³⁴S = ?12.8 to ?16.7‰) in contrast to the sulfur isotopic composition of Sb-asenopyrite (?1.7‰) from the genetically different epithermal veins. The oxygen isotopic composition of calcite (the third stage) is uniform at all studied deposits and reveals a tendency to updip enrichment in δ¹⁸O within a vertical interval of 200 m. Quartz from ore-bearing and epithermal veins has an almost identical δ¹⁸O value (±2‰) but differs from quartz at the tin deposits related to granites of the Canyon Complex and enriched in the light isotope in its oxygen isotopic composition. The mineralization in the Upper Seymchan ore cluster, which is genetically linked to the Early Cretaceous calc-alkaline dike suite pertaining to the period of postcollision late orogenic extension, is formed from magmatic fluids diluted with meteoric water (salinity reaches 20 wt % NaCl equiv) at temperatures varying from 400-380°C to 220-150°C and under a pressure of 970 to 60 bar. The direct vertical mineral zoning is expressed in the change of mineral species with depth and in variable compositions and properties of particular minerals.