Table of Contents for the Journal of GEOsciences. List of articles from the latest print issue.http://www.jgeosci.orgen-US http://www.jgeosci.org/jgeosci_cover.jpghttp://www.jgeosci.org Mlčoch B, Konopásek J; Vol. 55, issue 2, pages 81 - 94
The boundary between the Saxothuringian and the Teplá-Barrandian zones at the western margin of the Bohemian Massif represents an important tectonic suture of the Central European Variscides. However, most of this boundary is covered by Late Carboniferous and younger sedimentary and volcanic rocks, which prevent direct observation of particular geological units. We present a compilation of geological and depth measurement data from 12,134 exploration boreholes that reached the basement of the volcanic and sedimentary infill in the area of the Eger Graben in the north-western Bohemia, and correlate covered geological units with those exposed on the present day surface. The resulting compilation reveals the relief of the sedimentary basins basement and interprets the real extent of the basement geological units in the western part of the Bohemian Massif. It also shows the position of the contact between units with the Saxothuringian and the Teplá-Barrandian affinities and suggests the boundary between rocks with Devonian metamorphic record and those metamorphosed during the Early Carboniferous period of the Variscan tectonometamorphic cycle. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.068_2010_2_mlcoch.pdf Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.068_2010_2_mlcoch.pdf Rapprich V, Erban V, Fárová K, Kopačková V, Bellon H, Hernández W; Vol. 55, issue 2, pages 95 - 112
New results of detailed geological mapping, K-Ar dating and geochemical study of the Conchagua Peninsula in eastern El Salvador are presented. Volcanism in the area was controlled by intersection of three tectonic structures, the trench-parallel Central Graben, perpendicular Comayagua Graben, and the Guayape Fault Zone. The age of the volcanic activity spans from Miocene to Quaternary, however, the volcano itself is extinct. The basement is built of the welded rhyolitic Playitas ignimbrite, which extends as far as to the Island of Zacatillo. The pyroclastic rocks of La Unión unit (mean K-Ar age: 13.3 ± 3.7 Ma) display signs of mingling between basaltic and dacitic magmas (banded pumice, deposits containing both mafic scoria and felsic pumice fragments), and this is interpreted as a result of eruptions triggered by injection of a basaltic magma into a dacitic magma chamber. Lavas and pyroclastic flow deposits of the subsequent Pozo unit are poorly exposed and strongly altered. Following effusive activity hereby defined as Pilón Lavas was dominated by andesite and basaltic andesite lavas. Pleistocene volcanic activity is represented by the Pre-Conchagua edifice (1.6 ± 0.6 Ma), Cerro Montoso, El Bable and Juana-Pancha. Regarding the trace element composition, some lavas of the Pre-Conchagua - Juana-Pancha are distinct from common volcanic front products (lower Zr/Nb, Th/Nb, Ba/Nb), resembling the lavas of Tegucigalpa volcanic field to the north, which is located behind the volcanic arc in the Comayagua Graben. Behind-arc extensional tectonics could have facilitated the magma genesis via decompression melting of the mantle wedge. The current Conchagua Volcano consists of two cones, Ocotal and Banderas, built by repeated Strombolian eruptions associated with effusions of basaltic lavas. The uppermost unit consists of a white tuff preserved in the sedimentary fill of several tectonic valleys west of Conchagua. The white tuff was interpreted as distal fall-out of the Tierra Blanca Joven eruption of the Ilopango Caldera. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.069_2010_2_rapprich.pdf Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.069_2010_2_rapprich.pdf Hönig S, Leichmann J, Novák M; Vol. 55, issue 2, pages 113 - 129
Layered, garnet-bearing aplite-pegmatite dykes of the Hlína granitic suite, ˜2 to 50 m thick and up to ˜200 m long with general NW-SE orientation and dip 40-80° to NE or SW, cut granodiorites to granites at the SW part of the Brno Batholith, Brunovistulicum, Czech Republic. Aplite-pegmatite bodies are characterized by alternation of two main textural units: (i) fine-grained aplite unit with garnet stripes and (ii) coarse-grained pegmatite unit with oriented Qtz, Plg and Kfs comb-like crystals known as Unidirectional Solidification Textures (UST). Both units developed in zones with the thickness varying from several cm to ˜1-2 m for aplite unit, and ˜10 cm for the UST unit. All rock types are characterized by high contents of SiO₂ (74.6-75.7 wt. %), K₂O (4.61-4.94 wt. %), Na₂O (3.82-4.21 wt. %), moderate concentration of CaO (0.94-1.11 wt. %), and low to very low concentrations of Fe₂O₃^T (0.62-0.93 wt. %), MgO (0.02-0.03 wt. %), and TiO₂ (≤ 0.03 wt. %). Low K/Rb (212-241) and high K/Ba ratios (1034-2303) with deep Eu anomaly indicate high degree of fractionation. Both textural units consist of perthitic microcline, plagioclase An_15-8 and quartz. Total amount of accessory minerals is typically very low, commonly < ˜1 vol. % in aplite unit, and in the UST unit they are almost absent. Accessory minerals include relatively common Y-rich garnet Sps_42-38 Alm_32-28 And_15-7 Grs_21-15 Prp_2-1 (1.10 wt. % Y₂O₃, 0.53 wt. % Yb₂O₃ and 0.20 wt. % Er₂O₃) with oscillatory and sector zoning. Other minerals closely associated with garnet are extremely rare: magnetite, chloritized biotite, muscovite, Ta-rich titanite I, Al, F-rich titanite II, and ilmenite. Primary zircon, xenotime-(Y), monazite-(Nd), fersmite, ferrocolumbite, REE,Y-rich pyrochlore are strongly altered. Geochemical and mineralogical features of the Hlína aplite-pegmatites are characterized by (i) subaluminous chemistry with Nb >> Ta and HREE >> LREE, A-type (NYF) affinity, indicated by elevated concentrations of Y, REE (especially HREE), Zr, U, Th, Nb, and Ta, (ii) occurrence of specific accessory minerals including Y-rich garnet, and (iii) a remarkable, almost complete absence of micas and other minerals with volatiles (B, F, P and also H₂O). The studied suite differs from all other granitic rocks with UST described to date, which are typically characterized by volatile-bearing minerals and peraluminous signatures. Magmatic layering involving garnet is explained by formation of a boundary chemical layer bordering the surface of the coarse-grained UST unit. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.065_2010_2_honig.pdf Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.065_2010_2_honig.pdf Kubiš M, Broska I; Vol. 55, issue 2, pages 131 - 148
Boron- and fluorine-rich highly evolved granites in the Betliar area (Gemeric Unit, Western Carpathians) represent composite intrusion that formed probably during two distinct magmatic episodes. During the first stage, evolved granitic magma originating from an underlying volatile-rich reservoir intruded into an open fault system in the form of sill-like bodies and crystallized as equigranular or medium- to fine-grained rocks. The subsequent volatile flux enhanced post-magmatic alterations of the solidified granites and led to the formation of greisens with elevated amounts of tourmaline and Nb-Ta-W-Th phases. During the second stage, magma from a deeper magmatic reservoir intruded as a mush containing K-feldspar, albite, mica and quartz phenocrysts and gave rise to porphyritic granites. Partial dissolution and corrosion of the phenocrysts was enhanced by a pressure drop during emplacement of the porphyritic granites into middle-crustal level where the volatile-rich residual melt rapidly crystallized and is now preserved as quartz-albite-K-feldspar matrix with tourmaline and other accessories. Monazite and zircon geochronology indicates that the process occurred during Middle and Late Permian, and possibly extended to Early Triassic. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.066_2010_2_kubis.pdf Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.066_2010_2_kubis.pdf Sejkora J, Ozdín D, Ďuďa R; Vol. 55, issue 2, pages 149 - 160
Brandholzite, a hydrated hydroxide of Mg²⁺ and Sb⁵⁺, was found at the Krížnica deposit near Pernek (Malé Karpaty Mountains, Slovak Republic); it is the third occurrence of this mineral phase worldwide. Crystals and crusts of brandholzite occur as surface coatings on altered shales mineralized with stibnite and pyrite. Associated minerals include roméite-like phases, sulphur, aragonite, gypsum, sénarmontite and Sb-rich Fe-hydroxides. Brandholzite forms euhedral tabular crystals of hexagonal habit, typically 0.5 to 0.8 mm across, exceptionally up to 2 mm long. The crystals occur as isolated, or in randomly grouped individuals, creating inconspicuous crusts up to several cm². Brandholzite crystals are colourless, transparent or showing white clouding. No fluorescence was observed under either short- or long-wave UV radiation. The average electron microprobe analysis gave 6.11 MgO, 0.52 CaO, 0.16 FeO, 55.89 Sb₂O₅, (37.32) H₂O if recast to 100 wt. %. The calculated formula is (Mg_0.88Ca_0.05Fe_0.01)_Σ0.94(H₂O)_6.00[Sb(OH)₆]_2.00. The XRD pattern was indexed in a trigonal setting (space group P3), with refined unit-cell parameters a = 16.1076(9), c = 9.8628(9) Å and V = 2216.1(2) ų. The thermogravimetric curve shows total weight loss of 34.6 wt. % (over the interval of 20-800 °C), caused by H₂O release. The initial rapid weight decrease (20-140 °C) was followed by a minor continuous weight loss up to 800 °C. Brandholzite from Pernek and its mineral association were probably formed by sub-recent weathering processes in an abandoned mines environment. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.064_2010_2_sejkora.pdf Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.064_2010_2_sejkora.pdf Laufek F, Sejkora J, Dušek M; Vol. 55, issue 2, pages 161 - 167
The crystal structure of pyrargyrite, Ag₃SbS₃, from Příbram-Háje (Czech Republic) was refined from single-crystal X-ray diffraction data in the space group R3c to an R factor of 0.0112. Unit-cell parameters are a = 11.0464(3) Å, c = 8.7211(2) Å, V = 921.60(4) ų, Z = 6. In order to mimic the spread of electron density of silver, the non-harmonic Gram-Charlier development of the silver atomic displacement parameters was applied. A reasonable triangular shape of the electron density maximum centred in Ag position was obtained. The analysis of potential barriers between Ag sites reveals that silver transfer is equally probable via additional P site not only within the Ag-S-Ag spirals and Ag-Ag chains, but also between these spirals and chains. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.067_2010_2_laufek.pdf Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.067_2010_2_laufek.pdf