Journal of GEOsciences Table of Contents for the Journal of GEOsciences. List of articles from the latest print issue.http://www.jgeosci.orgen-US Journal of GEOsciences <![CDATA[ MORB melt metasomatism and deserpentinization in the peridotitic member of Variscan ophiolite: an example of the Braszowice–Brzeźnica serpentinites (SW Poland) ]]> Wojtulek PM, Puziewicz J, Ntaflos T; Vol. 62, issue 3, pages 147 - 164
The Variscan Braszowice-Brzeźnica Massif (SW Poland) consists of gabbros and serpentinized peridotites with gabbro veins. Antigorite serpentinites form the western part of the Massif, whereas tremolite peridotites, tremolite serpentinites and lizardite-chrysotile serpentinites are found at the contact with granite intrusion in the east. Sparse relics of clinopyroxene, olivine and chromite were studied within the antigorite serpentinites. Clinopyroxene I (Mg# 90.9-93.47, 1.92-3.80 wt. % Al2O3) occurs in the neighbourhood of gabbro veins. Its REE patterns are similar to those of clinopyroxene from the mid-ocean ridge gabbros. Clinopyroxene II (Mg# 96.0-97.0) is Al-poor (≥ 0.10 wt. % Al2O3). Olivine I (Fo = 90.1-92.3) contains 0.32-0.50 wt. % NiO, whereas olivine II (Fo = 86.0-91.2) is Ni-poor (0.01-0.25 wt. % NiO) and contains micrometric magnetite intergrowths. Chromite I (Cr# 44.9-54.0, Mg# 45.0-52.1, < 0.17 wt. % TiO2) is associated with olivine I and clinopyroxene I, whereas chromite II (Cr# 43.2-51.4, Mg# = 34.6-47.7, 0.49-0.74 wt. % TiO2) occurs in serpentinites penetrated by gabbro veins.
The serpentinites of the Braszowice-Brzeźnica Massif were formed supposedly immediately below the paleo-Moho in the ocean-spreading setting. Chemistry of clinopyroxene I from antigorite serpentinites resembles, in terms of major elements and REE, clinopyroxenes that originate due to MORB-like melt percolation through abyssal peridotites. The coexisting olivine I with both chromite I and II supposedly shared a similar origin. Composition of chromite suggests the back-arc setting of the Braszowice-Brzeźnica Massif. Clinopyroxene II and olivine II have major-element compositions indicative of metamorphic origin at expense of serpentine ± magnetite (deserpentinization). The deserpentinization assemblage occurring in serpentinites (antigorite-olivine-clinopyroxene) was formed probably under low-grade metamorphic conditions. The tremolite-bearing rocks record the thermal metamorphism by granite intrusion. ]]> Original Paper
<![CDATA[ Textural records and geochemistry of the Kermanshah mantle peridotites (Iran): implications for the tectonic evolution of southern Neo-Tethys ]]> Moradpour A, Zarei Sahamieh R, Ahmadi Khalaji A, Sarikhani R; Vol. 62, issue 3, pages 165 - 186
Kermanshah Ophiolite Complex (NW Iran), located along the Main Zagros Thrust Zone, marks the ophiolitic suture between the Arabian and Sanandaj-Sirjan continental blocks. N-MORB-normalized multielement patterns and chondrite-normalized REE patterns in the Kermanshah mantle restites show depletion in incompatible elements concentrations with respect to the depleted MORB mantle (DMM). In the V vs. Yb diagram, Harsin-Sahneh-Norabad and Miyanrahan peridotites (Kermanshah Ophiolite Complex), due to their relatively low V contents, fall close to the QFM buffer. Calculated TiO2 and Al2O3 compositions in the parental melt that were in equilibrium with chromian spinel are consistent with supra-subduction zone -type compositions. Besides, calculated FeO/MgO ratios in the parental melts are comparable with those in the boninites from Oman ophiolites. Olivine from peridotite restites is highly magnesian (Fo89-93) with NiO contents of 0.2-0.4 wt. %, consistent with formation in a forearc environment. This is also proven by the Cr# and Mg# values as well as TiO2, Cr2O3, and Al2O3 concentrations in chromian spinel of ultramafic rocks. Accordingly, petrogenetic modeling indicates that the Kermanshah ultramafic rocks may represent the residual mantle after extraction of 13-23% of boninitic-type melts. Decompression, melting and melt-rock reaction related textures are widespread in the Kermanshah mantle restites. Field relationships and geochemical evidence reveal that the studied ophiolites were a part of a rifted basin at the ocean-continent transition zone formed in the south Neo-Tethyan Ocean. ]]> Original Paper
<![CDATA[ Primary and secondary textures of dolomite in Eppawala carbonatites, Sri Lanka: implications for their petrogenetic history ]]> Madugalla NS, Pitawala A, Manthilake G; Vol. 62, issue 3, pages 187 - 200
Textural studies of carbonate minerals over the past three decades revealed that their textures are useful tool for understanding of petrogenesis of carbonatites. Petrographic, cathodoluminescence (CL) and electron-microprobe studies on textures of calcite and dolomite were performed for interpretation of evolution of Eppawala carbonatites in Sri Lanka. The studied carbonatites are dominated by calcite with subordinate dolomite. Calcites occur in two different morphological forms, reflecting two generations: as grains with dolomite inclusions (type-1) and dolomite-free (type-2) ones. Dolomites were subdivided into five distinct morphological types: randomly distributed, coarse-grained dolomite (type-1), rod-shaped or vermicular dolomite microcrysts within the type-1 calcite (type-2), inclusions of dolomite within the type 1 calcite forming plug- or wedge-shaped arrangements (type-3), dolomite microcrysts along the grain boundaries of type 1 calcite (type-4) and clusters of dolomite crosscutting the type 1 calcite (type-5).
The geochemical results indicate that these five morphological types accounts for three different generations of dolomites. Type-1 dolomite and type-1 calcite are interpreted as primary magmatic. Type-2 and type-3 represent exsolved dolomite formed by exsolution from type-1 calcite. Type-4 and type-5 dolomites are recrystallized and reorganized dolomites of exsolved type-2 and type-3 dolomites. Type-2 calcite reflects later recrystallization event. The composition of type-1 calcite indicates minimum temperatures of exsolution of c. 650 °C. The exsolution and recrystallization kinetics reflected the equilibration of carbonatite magma at two crustal depths during the petrogenesis of Eppawala carbonatite. The re-localization may have been related to the deformations experienced by the country rocks. ]]> Original Paper
<![CDATA[ Línekite, K2Ca3[(UO2)(CO3)3]2.8H2O, a new uranyl carbonate mineral from Jáchymov, Czech Republic ]]> Plášil J, Čejka J, Sejkora J, Hloušek J, Škoda R, Novák M, Dušek M, Císařová I, Němec I, Ederová J; Vol. 62, issue 3, pages 201 - 213
Línekite, K2Ca3[(UO2)(CO3)3]2.8H2O, is a new uranyl tricarbonate mineral from Jáchymov, Western Bohemia, Czech Republic. It occurs in association with grimselite, andersonite, liebigite, čejkaite, schröckingerite, agricolaite, ježekite and braunerite. Línekite forms from uranium-rich aqueous solutions and its origin is associated with post-mining processes. Línekite is orthorhombic, space group Pnnm, with a = 17.0069(5) Å, b = 18.0273(5) Å, c = 18.3374(5) Å and V = 5622.1(2) Å3, and Z = 8. It forms tabular, mostly isometric crystals, up to c. 0.5 mm across, typically in multiple intergrowths. The color is pale olive to khaki green and it has a greenish white to yellowish white streak. Crystals are transparent and have vitreous luster. The Mohs hardness is estimated to be between 2 and 3. Línekite is brittle with an uneven fracture and perfect cleavage on {100} and very good cleavage on {010}. It exhibits intense greenish yellow luminescence under both short- (254 nm) and long-wave (366 nm) UV radiation. The calculated density is 2.922 g/cm3. The mineral is biaxial (+) with indices of refraction, α = 1.546(2), β = 1.550, γ = 1.562(2). The 2Vobs is moderate; the calculated 2V is +60°. Optical orientation: Y = a, X = b, Z = c. The electron microprobe analyses (average of 28) provided: Na2O 0.06, K2O 6.89, CaO 14.11, CuO 0.12, UO3 48.76, CO2* 22.51, H2O* 12.20 (˜12.9 from TG) (*calculated), total 104.65 wt%. The empirical formula (based on 30.22 O apfu) is: (K1.73Na0.02)Σ1.75(Ca2.97Cu0.02)Σ2.99[(UO2)(CO3)3]2.02(H2O)8.00. The Raman and infrared spectra exhibit prominent features consistent with the mineral being a hydrated uranyl tricarbonate, with fundamental vibrations of H2O molecules, CO32- anions and UO22+ ions. The seven strongest powder X ray diffraction lines are [dobs in Å (hkl) Irel]: 8.627 (200) 100, 6.436 (022) 60, 5.935 (212) 11, 5.153 (222) 43, 4.592 (004) 19, 4.505 (040) 12 and 4.053 (204) 15. The structure of línekite was refined from single-crystal X-ray data to R = 0.034 for 4468 unique observed reflections (Iobs > 3σI). The structure consists of prominent (Ca(H2O)2[(UO2)(CO3)3])2- layers parallel to (100), which define a square grid, leading to a strong tetragonal pseudosymmetry of línekite. Between the layers, disordered K+ cations and H2O molecules are localized. The structure is closely related to other uranyl tricarbonate minerals, e.g., albrechtschraufite and andersonite, due to the presence of a very characteristic paddle-wheel motif, Ca[(UO2)(CO3)3]4Ca. ]]> Original Paper