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 Vondrovic L, Verner K, Buriánek D, Halodová P, Kachlík V, Míková J; Vol. 56, issue 4, pages 343 - 357
The calc-alkaline Miřetín Pluton (newly dated at 346 Ma ± 5 Ma; an U-Pb age obtained by laser-ablation ICP MS method on zircons) is a NNE-SSW elongated intrusive body emplaced into the upper- to mid- crustal rocks of the Polička Unit (eastern Teplá-Barrandian Zone; Bohemian Massif). Its composition reveals similarities to other calc-alkaline granitoids, which are mostly interpreted as products of magma mixing between the basic magmas derived from mantle wedge above a subduction zone with crustally-derived acid melts. The conditions of magma crystallization estimated at 653-681 °C and 0.29-0.43 GPa roughly correspond to peak metamorphic evolution of the host volcano-sedimentary rocks of the northwestern part of the Polička and Hlinsko units. The Miřetín Pluton was emplaced into a NNE-SSW oriented transpressional domain, well recognized on a regional scale along the eastern margin of the Teplá-Barrandian Zone. During, or shortly after, the magma emplacement, the Miřetín Pluton was affected by pervasive submagmatic to high-T solid-state deformation, reflecting an additional strain increment of regional transpression in a narrow zone of thermal softening. Sharply superimposed low-T solid-state fabric preserved along the western part of the Pluton was connected with normal shearing between the Polička Unit at the bottom and the overlying Hlinsko Unit after 335 Ma. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.109 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.109 Vrána S; Vol. 56, issue 4, pages 359 - 374
Manganese-rich garnet-quartz rocks and gneisses occur in the Varied Group of the Moldanubian Zone, Bohemian Massif, in close association with amphibolites, marbles and accompanying graphite gneisses. Fine-grained garnets contain (mol. %) 26-37 spessartine, 36.8-45.9 almandine, 11.1-14.3 pyrope, 2.9-21.0 grossular. Minor amphibole present in some samples is ferrimagnesiohornblende with 0.17-0.22 Mn pfu. Accessory ilmenite contains 24-34 mol. % pyrophanite and 1.7-5.8 hematite. Some closely associated impure calcite marbles (or amphibolites) carry Ti-bearing andradite, epidote, diopside-hedenbergite, and accessory magnetite.
Data from the Varied Group indicate that manganese enrichment took place both under oxidizing and reducing conditions, but the Mn-garnet-quartz rocks are oxidic. Normalization of major-element contents in the Mn-rich rocks by average abundances in Varied Group paragneisses shows ten- to hundred-fold enrichment in MnO and a slight to moderate increase in CaO and P₂O₅. Values for Na₂O and K₂O indicate severe depletion in some samples, but contents of other oxides are close to unity. Comparison of chondrite-normalized REE patterns in Mn-rich rocks with data for ordinary paragneisses (Varied Group) also indicates that detrital component in Mn-rich rocks was closely comparable to material supplied for protolith of paragneisses. This permits to ascribe the spike of Mn to likely short-lived exhalative processes.
Manganese-rich garnet-quartz rocks and gneisses remained unnoticed till now mainly owing to a small thickness of their layers, typically less than 1 m. Structural relations in regions carrying Mn-rich rocks indicate their occurrence in relict domains of preserved D₁ structures with NW-SE trending foliation. It is suggested that in regionally prevalent areas dominated by superimposed refoliation (D₂, D₃) the thin layers of Mn-rich rocks were likely reduced to boudins dispersed in paragneiss matrix. Comparison with published data for similar Mn-rich rocks abroad (including the so-called coticules) strongly indicates a lithostratigraphic correlation potential of Mn-rich garnet-quartz rocks. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.106 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.106 Nkouandou OF, Temdjim R; Vol. 56, issue 4, pages 375 - 387
Spinel-bearing lherzolite xenoliths have been recovered from Mio-Pliocene alkaline basalt flows from the Ngao Voglar volcano, 35 km northwest of Ngaoundere in the Adamawa volcanic Massif (Cameroon). They have been examined to characterize the petrography, mineralogical composition, and equilibrium conditions of the upper mantle beneath the Ngaoundere region. The xenoliths exhibit protogranular textures and consist of four main minerals: olivine (Fo_89-90); Mg-enstatite (En_89-91Wo₁Fs_8-10), Cr-diopside (En_49-52Wo_44-49Fs_1.5-5) and spinel (Mg# ˜79.2, Cr# ˜10.7).
Thermobarometric calculations show equilibrium temperatures ranging between 850 and 950 °C and pressures of 8 to 17 kbar consistent with the spinel lherzolite stability field. These data suggest that the xenoliths come from a depth of 28-31km in the uppermost mantle situated just below a thinned crust; they are in agreement with the geophysical data previously determined in the Adamawa Massif. On the basis of these features, and considering the evidence for textural, mineralogical, and chemical equilibrium in the studied xenoliths prior to their entrainment in the host magma, we conclude that the source of these xenoliths was a chemically and petrographically homogeneous, spinel lherzolite lithospheric mantle. But the occurrence of mantle-derived xenoliths of various types (dunite, lherzolite, wehrlite, harzburgite, websterite, clinopyroxenite, and orthopyroxenite) in alkali basalts from many other localities of Cameroon (Oku Massif, Lake Nyos area and Mount Cameroon) is consistent with upper-mantle heterogeneities on a regional scale and implies that the nature of the upper mantle beneath the continental sector of the Cameroon Volcanic Line varies under its different volcanic centres. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.108 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.108 Pieczka A, Buniak A, Majka J, Harryson H ; Vol. 56, issue 4, pages 389 - 398
A tourmaline depleted in Si has been found in the cement of Upper Rotliegend aeolian sandstone overlaying Lower Rotliegend volcanic rock in the ‘Ługi-1’ prospecting borehole near Jarocin in the Fore-Sudetic Monocline, south-western Poland. Tourmaline, representing Mg-bearing foitite, crystallized around quartz grains in the form of radial aggregates of tiny crystals, reaching only 1-2 μm in diameter. Due to the very small size of the tourmaline crystals and the presence of significant contents of transitional metals in the crystal lattice, only direct determination of B₂O₃ in nm-sized spots enables evaluation of ^[4]Al and ^[4]B. Compositions of the tourmaline in successive analytical spots show that Si deficiency is supplemented both by ^[4]Al (0.00 to 0.48 Al apfu) as well as by ^[4]B (0.00 to 0.83 B apfu) at varying proportions. The origin of the mineral was related to diagenesis of evaporate sediments inducing reactions of quartz and clay minerals as primary components of the grain facies in the tourmaline-bearing sandstone with Ca saturated pore brines rich in Cl^- and (BO₃)^3-. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.105 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.105 Sejkora J, Ozdín D, Laufek F, Plášil J, Litochleb J, ; Vol. 56, issue 4, pages 399 - 408
The rare Hg-sulfosalt, marrucciite, with ideal formula Hg₃Pb₁₆Sb₁₈S₄₆, was found in the Gelnica ore deposit (Spišsko-Gemerské Rudohorie Mts., Slovak Republic). It occurs as acicular crystals up to 0.5 mm in length and fine-grained aggregates up to 1 mm in quartz (+siderite/dolomite) gangue in association with other sulfosalts (zinkenite, scainiite, boulangerite, chalcostibite, bournonite) and cinnabar. It is opaque, dark steel grey to grayish black, sometimes tarnished to a bronze-brown tint, has a dark grey streak with a very weak reddish tint and a metallic luster. Marrucciite is very brittle but thin acicular crystals are flexible. One good cleavage parallel to the elongation of the crystals was observed. The VHN_20g microhardness is 85 (69-101) kp/mm² and corresponds to a Moh’s hardness of c. 2; the calculated density is 6.00 g.cm^-3. In reflected light, marrucciite is white to grayish white with a weak yellowish tint, distinct bireflectance and white-white grey pleochroism. Under crossed polars, it has distinct anisotropy, with rotation tints in shades of grayish brown and dark brown. Dark red internal reflections were observed only rarely. Marrucciite is monoclinic, space group C2/m with a 48.12(1), b 4.1083(2), c 23.990(5) Å, β 118.76(2)^o, V 4158.(3) ų. Its average composition (electron microprobe data in wt. %) is Cu 0.03, Hg 7.83, Cd 0.02, Fe 0.06, Zn 0.02, Pb 43.13, Sn 0.03, Sb 29.15, Bi 0.12, S 19.71, Cl 0.05, total 100.15. The resulting empirical formula on the basis 83 of apfu is Hg_2.93(Pb_15.63Fe_0.08Cu_0.04Cd_0.02Zn_0.02Sn_0.02)_Σ15.81 (Sb_17.97Bi_0.04)_Σ18.01(S_46.14Cl_0.11)_Σ46.25.
The crystal structure of marrucciite from Gelnica contains two Hg positions (6 atoms), 7 Pb positions (28 atoms), 8 Sb positions (32 atoms), two mixed Pb-Sb positions (refined as Pb_0.39Sb_0.61 and Pb_0.61Sb_0.39) and 23 S positions (46 atoms). ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.107 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.107