Distribution of rare earth elements among rock-forming and accessory minerals: A case study of Variscan granites from the Krušné Hory Mts., Czech Republic
Abstract: Eight granite samples representing the main petrographic and geochemical types of Variscan granites from the Krušné Hory/Erzgebirge area were studied to decipher the distribution of rare-earth-elements (REE) among all rock-forming, minor, and accessory minerals. The chemical composition (REE contents) of minerals was determined using electron microprobe and laser ablation ICP MS, while the quantitative abundance of individual minerals was determined by automated mineralogy (TIMA technology). Monazite and xenotime + zircon are dominant hosts of both LREE and HREE respectively, especially in strongly peraluminous S-type granites. In less-fractionated Ca-richer facies, allanite is also an important carrier of LREE. In slightly peraluminous A-type granites, monazite + fluorite (including its alteration products) dominates as LREE hosts, while fluorite (including its alteration products), with small contributions of zircon and xenotime, hosts the HREE. The importance of rock-forming silicates for REE distribution is limited; the relatively highest contribution was found in plagioclase (up to approx. 15 % of LREE in S-type granites).
Early-diagenetic dolomitization of Middle Triassic platform/ramp carbonates driven by geothermal convection in the Bükk Mts. (North Hungary)
Abstract: Shallow marine carbonates of the Anisian Hámor Dolomite Formation in the Bükk Mountains, NE Hungary were studied to determine the mechanism and controlling factors of the dolomitization. Petrographic features, along with C and O stable isotope properties of the investigated rocks, indicate near-surface/shallow burial dolomitization of the shallow, subtidal–peritidal carbonate succession. This occurred via long-term circulation of relatively low-temperature fluid of sea-water origin. Geothermal convection may have been the driving force of this circulation. For application of this model, we need to assume that segmentation of a previously-established shallow ramp had already initiated in the Western Neotethys earlier in the middle Anisian. Unfortunately, we have only indirect evidence of this in the studied area. Still, the structural evolution and the related paleogeographic setting may have been the basic controlling factors of the pervasive early diagenetic near-surface/shallow burial dolomitization of the Hámor Formation. The coarse crystalline dolomite cement in the fractures and pores was precipitated from relatively high temperature (cc. 170 °C) water. Comparing the stable isotope values of the bulk rock and the fracture-occluding dolomite cement phase suggests a host-rock buffered fluid flow probably in the Late Cretaceous deformation phase.
Mineralogy and genesis of sapphire in corundum-bearing xenoliths from the Miocene andesites in the Záhradné, Hubošovce and Vechec quarries in the Slanské vrchy Mountains (Slovakia)
Abstract: Sapphires crystals were identified in the corundum-bearing xenoliths in Miocene (Upper to Middle Sarmathian) andesites of the Slanské vrchy Mountains, eastern Slovakia at the Záhradné, Hubošovce and Vechec localities. The sapphire crystals occur in (1) micaceous xenoliths, which are built mostly of dark mica from annite–phlogopite series (biotite), K-feldspars, plagioclase with abundant inclusions of hercynite, ilmenite and Ti-rich magnetite, locally with pyroxenes of enstatite-ferosilite series at the localities Záhradné and Hubošovce. Less abundant are sapphires occurring in cordieritic xenoliths (2), dominantly consisting of cordierite, plagioclase and sillimanite with minor hercynite and ilmenite inclusions discovered in the Vechec quarry. Sapphires are dark blue to light blue with vitreous to diamond lustre and no visible pleochroic colour change forming mostly pseudohexagonal tabular, locally more complex euhedral to subhedral crystals up to 2.0 mm in size with triangular-shaped patterns on crystal faces. Raman spectroscopy showed characteristic corundum peaks at 419 cm−1 A1g mode and 384 cm−1 Eg mode with other peaks assigned to Al2O3 crystal vibrations. Chemical composition (EPMA, LA-ICP-MS) shows typical content of 98.12–99.60 wt. % Al2O3 with increased concentrations of Fe, Ti, Cr, V, Mg and Ga, locally also Na, Ca, K, B and Li. Genesis of corundum-bearing xenoliths interpreted from paragenetic observations and geochemical data shows clear metamorphic trend. Formation of sapphires was caused by incorporation of Al-rich precursor metasediments depleted in silica into the magmatic reservoir, which caused thermal overprint of the precursor mineral assemblage and led to the formation of high temperature mineral association suitable for corundum crystallization. Furthermore, sapphire crystals from the Hubošovce quarry exhibit spinel coronas, which are typically developed from destabilization of corundum during their retrograde development.
Upper Cenozoic conglomeratic formations as a rock record of the turning points in the evolution of the Carpathian orogen and its foreland
Abstract: This paper presents the upper Cenozoic post-collisional terrestrial conglomeratic formations of the Eastern and South-Eastern Carpathian foreland subjected to sedimentological and geomorphological analyses including an in-depth review of previous investigations. These formations embrace gravels as the prominent component, together with sands and muds. The unit is up to 190 m thick. The conglomeratic formations are represented by erosional remnants in the Eastern Carpathians, whilst they occur as a continuous body (the Cândeşti Formation) in the South-Eastern Carpathians. The Eastern Carpathian formation ranges in age from the late Serravallian to early Tortonian stages, whilst the Cândeşti Formation extends from the late Early to the late Middle Pleistocene. These formations are separated by 10 Myr, supporting the view the Carpathians and their foreland from North to South evolved diachronously. Their affinity is determined by the sedimentary environment of alluvial fans within the wedge-top depozone of the foreland. This environment arose from simultaneous occurrence of rare intervals of intense floods and presence of highly erodible rocks in fan-supply catchments. Most of the fans are of hyperconcentrated flow-dominated type, whilst part of those within the Eastern Carpathians are of debris-flow-dominated type. The fans’ origin was provided by uplifted orogen and stable or subsiding foreland, giving both high gradient and orographic precipitation. The fans accumulated following a time lag after the post-collisional orogen uplift. The facies and architectural differences between the formations are associated with specific sedimentation and regional tectonics. In the Eastern Carpathians thin conglomeratic formation was deposited above the regional angular unconformity by predominant progradation in conditions of the tectonic quiescence and low accommodation space. In the South-Eastern Carpathians the fluvial fine-grained sedimentation gradually passed into the thick conglomeratic Cândeşti Formation during the attenuated subsidence of the Focşani Depression. As the accommodation space decreased, the aggradation of the formation gave way to its progradation.
