Search

Eight granite samples representing the main petrographic and geochemical types of Variscan granites from the Krusné 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). [ABSTRACT FROM AUTHOR]

Zircon from four plutons of peralkaline granites and quartz-bearing syenites, differing in geotectonic positions, petrological and mineralogical compositions, and contents of volatile and trace elements, was studied using SEM, CL, and EPMA with the intention to define typical textural and chemical features of zircon from peralkaline rocks. In strongly peralkaline Na-pyroxene-bearing rocks represented by the Khan Bogd and Khalzan Buregte plutons (Mongolia), the primary zircon is scarce or missing. Most zircon grains are secondary, originating in hydrothermal stage from primary Zr silicates. They often form globular or radial aggregates. Chemical compositions of zircon in these rocks typically show high contents of Y, moderate contents of REE (thus high Y/Yb values) together with low contents of U and Th and low analytical totals. In mildly peralkaline mica-bearing rocks represented by Ivigtut stock (Groenland) and Madeira pluton (Brazil), the exclusive primary Zr mineral is zircon, mostly of orthomagmatic origin. Its analytical totals approach 100 wt%, enrichment in HREE, resulting in low Y/Yb values, is typical. Zircon populations from two types of peralkaline granitoids can be distinguished from each other and from zircon from S-type granites based on combination of the Zr/Hf, Y/Yb, and U/Th values, or on the Y-Hf-P ternary diagram. [ABSTRACT FROM AUTHOR]

A unique case of low-temperature metamorphic (hydrothermal) overprint of peraluminous, highly evolved rare-metal S-type granite is described. The hidden Dlhá dolina granite pluton of Permian age (Western Carpathians, eastern Slovakia) is composed of barren biotite granite, mineralized Li-mica granite and albitite. Based on whole-rock chemical data and evaluation of compositional variations of rock-forming and accessory minerals (Rb-P-enriched K-feldspar and albite; biotite, zinnwaldite and di-octahedral micas; Hf-(Sc)-rich zircon, fluorapatite, topaz, schorlitic tourmaline), the following evolutionary scenario is proposed: (1) Intrusion of evolved peraluminous melt enriched in Li, B, P, F, Sn, Nb, Ta, and W took place followed by intrusion of a large body of biotite granites into Paleozoic metapelites and metarhyolite tuffs; (2) The highly evolved melt differentiated in situ forming tourmaline-bearing Li-biotite granite at the bottom, topaz-zinnwaldite granite in the middle, and quartz albitite to albitite at the top of the cupola. The main part of the Sn, Nb, and Ta crystallized from the melt as disseminated cassiterite and Nb-Ta oxide minerals within the albitite, while disseminated wolframite appears mainly within the topaz-zinnwaldite granite. The fluid separated from the last portion of crystallized magma caused small scale greisenization of the albitite; (3) Alpine (Cretaceous) thrusting strongly tectonized and mylonitized the upper part of the pluton. Hydrothermal low-temperature fluids enriched in Ca, Mg, and CO2 unfiltered mechanically damaged granite. This fluid-driven overprint caused formation of carbonate veinlets, alteration and release of phosphorus from crystal lattice of feldspars and Li from micas, precipitating secondary Sr-enriched apatite and Mg-rich micas. Consequently, all bulk-rock and mineral markers were reset and now represent the P-T conditions of the Alpine overprint.

The complex of leucocratic granitoid rocks at Přibyslavice (Fig. 1), composed of muscovite-tourmaline orthogneiss, muscovite granite and several types of pegmatites, is well known for numerous finds of interesting minerals like large almandine crystals (Breiter et al. 2005b), Li-Fe-Mn phosphates (Povondra et al. 1987), nigerite (Čech et al. 1978), columbite and cassiterite (Šrein et al. 2004, Breiter et al. 2006), lepidolite (Šrein et al. 2004) and oxy-schorl (Bačík et al. 2013). Less attention has been paid so far to the petrology of orthogneisses, granite and pegmatites and their genetic relationships. This study aims, based on the chemical composition of quartz and muscovite, to assess possible genetic links among all these rocks of granitoid composition including an associated cassiterite-bearing quartz vein with a B, Ta-rich metasomatic halo. Major elements in micas were analyzed using electron microprobe, and trace elements in both quartz and mica were determined using laser ablation-ICP-MS according to methods described in Breiter et al. (2017, 2020). About 550 spot analyses of quartz and 220 spot analyses of mica allow reliable definition of the typical composition of quartz and mica from all types of studied rocks (Tables 2, 3). Some genetic relationships are visualized in Figs. 2 and 3. The Přibyslavice orthogneiss is geochemically more evolved than petrographically similar orthogneisses through entire Moldanubicum as expressed not only in bulk rock chemical composition but also in trace element composition of quartz (higher Al, Ge and Li contents, Fig. 2) and muscovite (higher Li, Nb, Ta, Sn and W contents, Fig. 3). Pegmatoids at Přibyslavice and nearby Březí, forming small nests in orthogneisses with a gradual mutual transition, are interpreted as in situ anatexites. The direct genetic link between the granite intrusion and the quartz vein with cassiterite and B, Ta-rich metasomatites (tourmaline + Ta-rutile) is supported by the Sn, Nb and Ta enrichment of granite. It is highlighted by the relative distribution of Nb and Ta, with Nb preferentially bonded to muscovite in granite and Ta segregated into a fluid (see e.g. Stepanov et al. 2014). Chemical and mineral similarity between the granites and the orthogneiss suggests a common source lithology located deep below the present surface. During the first melting of the protolith in the late Cambrian (Vrána - Kröner 1995), a boron-rich melt was formed, which extracted most boron from the source and facilitated the origin of tourmaline orthogneiss. The second, Variscan melting of the same or a very similar protolith produced a melt slightly enriched in F and Li. We assume that the Přibyslavice granite represents only a small proportion of this melt, strongly enriched in Sn, W, Nb and Ta due to fractionation. Chemistry and mineralogy of several types of associated pegmatites suggest their relation rather to the granite melt than to the anatexis of the orthogneiss. The difference in their chemical composition including the content of water, F, and Li reflects the timing of separation from the parental magma and the distance of the melt transport upwards. During the transport, internal fractionation culminated in the "Li-pegmatite" by crystallization of its quartz-lepidolite core. [ABSTRACT FROM AUTHOR]

The aim of this paper is to provide an overview of Li contents and their trends in pre-Variscan and Variscan granitoids in the Czech part of the Bohemian Massif. All igneous systems with significant Li enrichment are described (Fig. 1). Among other plutons, only some typical examples are selected. A special attention is given to rocks with sufficiently robust data sets on Li contents. Bulk-rock contents of Li in granitoids were assessed within several projects of the Czech Geological Survey (CGS) and the Institute of Geology of the Czech Academy of Sciences (GLÚ) in 1985-2019. Lithium was determined mostly by the AAS method in laboratories of the CGS. Analyses of micas can be divided into three groups according to the methodology: (i) classic wet complete chemical analyses of mica concentrates (from years 1985-90); (ii) a combination of AAS-Li analyses of mica concentrate and determination of other elements using a microprobe in thin sections (mostly from years 1990-2010); and (iii) a combination of Li analyses and trace element analyses using laser-ablation ICP-MS and major elements using a microprobe. The method of laser ablation was performed at Faculty of Sciences, Masaryk University in Brno, microprobe analyses in GLÚ and wet chemical analyses in CGS. Pre-Variscan granitoids in the Bohemian Massif are generally Li-poor (Fig. 2). Relatively highest Li values (0.08 wt.% Li2O + elevated P, B, and Sn) were found in the Blaník-type orthogneiss in the NE part of the Moldanubicum. Lithium is hosted in micas: ca. 0.1-0.6 wt.% Li2O in biotite and 0.2-0.4 wt.% Li2O in muscovite. Among Variscan igneous rocks (Fig. 3), Li is slightly enriched in some two-mica and muscovite granites in the Moldanubicum, and particularly in Li-biotite and zinnwaldite granites in the Krušné Hory - Erzgebirge (Saxothuringicum). Two-mica granites of the Eisgarn type in the South Bohemian pluton in the Moldanubicum contain up to 0.05 wt.% Li2O; small bodies of late muscovite granites near the Czech-Austrian border up to 0.16 wt.% Li2O. Biotite and muscovite from the two-mica granites contain about 0.1-0.5 wt.% Li2O and 0.03-0.14 wt.% Li2O, respectively. Muscovite of the Homolka muscovite granite contains up to 1.0 wt.% Li2O. In the Kreuzstein granite (0.12 wt.% Li2O), situated at Czech-Bavarian border in the NW edge of the Moldanubicum, zinnwaldite is the Li carrier; this makes this granite more similar to evolved Saxothuringian granites. Variscan granites in the Erzgebirge should be divided into two groups. The first group includes strongly peraluminous granites which dominate the western part of the area forming the Smrčiny-Fichtelgebirge, Nejdek-Eibenstock and Slavkovský les plutons (Fig. 4). These granites evolved from biotite to zinnwaldite facies containing 0.04-0.20 wt.% Li2O. Hydrothermal greisens, forming cupolas in the uppermost part of granite intrusions at Krásno, are also enriched in Li (0.6 wt.% Li2O), while vein-shaped and pericontact greisens in northern and northeastern parts of the Nejdek pluton are Li-poor (usually below 0.1 wt.% Li2O). The second group, only slightly peraluminous A-type granites, form a large volcano-plutonic complex of the Teplice caldera in the eastern Erzgebirge (Fig. 5). While early rhyolite tuffs are Li-poor (< 0.05 wt.% Li2O), later granites contain up to 0.2 wt.% Li2O in zinnwaldite facies in the upper parts of the cupolas at Cínovec and Krupka. Hydrothermal greisenization in the uppermost parts of cupolas increased the Li contents up to 0.4-1 % wt.% Li2O (0.6 wt.% Li2O on average). The only Li carrier is represented by trioctahedral Li-Fe micas with the contents from 0.25 wt.% Li2O in biotite to 4.8 wt.% Li2O in zinnwaldite (Fig. 6). The Cínovec-Zinnwald represents the only potential Li deposit in the Czech Republic. [ABSTRACT FROM AUTHOR]

Occurrence of so-called "vein greisens" is one of characteristic features of the Variscan peraluminous granites in the western part of the Krušné hory/Erzgebirge area (Nejdek-Eibenstock Pluton, Horní Blatná body). The "veins" actually represent steeply dipping zones consisting of tens to hundreds of individual roughly parallel cm- to dm-thick stringers of metasomatic greisen reaching total thickness to several meters, and lengths of hundreds of meters. They mostly consist of quartz and Li-bearing mica with some topaz and cassiterite. Greisens of this type were mined at Přebuz, Rolava and Horní Blatná since the 15th century until 1945. The aim of this study is to distinguish magmatic and hydrothermal quartz in greisen, i.e. to differentiate relics of the original magmatic quartz from quartz originated hydrothermally during greisenization. We studied a series of samples from the historic mine Streitpingen situated in the Horní Blatná granite body near the village of Potůčky. The parent rock of greisens is medium-grained alkali-feldspar granite composed of 40 vol.% quartz, 29 vol.% albite, 20 vol.% perthitic K-feldspar and 9 vol.% Li-enriched biotite with small amount of topaz, apatite, rutile, monazite and zircon (Tables 1, 2). The zone of greisenization is up to 5 m thick and enriched with quartz (87 vol.%) and topaz (9 vol.%). The content of mica decreased to 3 vol.% and both feldspars disappear. Greisen is penetrated by monomineralic up to 10 cm thick quartz veins (> 97 vol.% quartz) with abundant tiny cavities. Veins are composed of clear long columnar crystals (5 mm diam., up to 5 cm long) which are coated with a thin layer (~ 1 mm) of milky white quartz. For comparison, we analyzed also quartz from quartz-tourmaline fillings of miarolitic cavities (5-10 cm diam.) in granite, traditionally called as "tourmaline suns", which are widespread in the central and northern parts of the Nejdek-Eibenstock Pluton (Schust et al. 1970). Internal structure of analyzed quartz grains visualized by cathodoluminescence (CL) is shown in Figs 1 and 2. Trace elements in quartz were analyzed using laser-ablation ICP-MS (for detail of the method used see Breiter et al. 2013). Average contents of the analyzed element in selected quartz crystals are shown in Table 3, all the individual analyses of selected elements are shown in Fig. 4. Contents of Al, Ti, and Li across selected crystals are given in Fig. 5; position of these profiles is shown in Figs 2b, c and 3. Aluminum, Li, and Ti are the most abundant trace elements in quartz. The content of Al in phenocysts of magmatic quartz and quartz from tourmaline suns corresponds roughly to 400 ppm (range 300-500 ppm), whereas only 115-270 ppm were found in the greisen quartz. The Al-contents in hydrothermal quartz strongly fluctuate: the clear domains contain 50-340 ppm Al, while 500-2400 ppm were detected in cloudy crystal cores, and 1100-4800 ppm Al in the milky white crystal rims. Content of Ti in magmatic quartz fluctuates between 40 to 100 ppm with extreme values up to 200 ppm near the margins of some crystals. Quartz from tourmaline suns contains 10-40 ppm Ti, while quartz from the greisen approximately 20 ppm Ti, and the hydrothermal quartz from veins generally contains < ppm Ti. The contents of Li in magmatic quartz and quartz from tourmaline suns range between 30-50 ppm. Contents of Li in quartz from greisen and clear domains of hydrothermal quartz are rather smaller, approximately 10-30 ppm. The highest Li-values in the range of 70-170 ppm were found in the milky hydrothermal quartz. The contents of Li and Al reveal positive correlation. Our preliminary research has shown that quartz originated during metasomatic greisenization differs from the primary magmatic quartz by lower intensity of cathodoluminescence and significantly lower concentrations of trace elements Al, Ti, and Li. [ABSTRACT FROM AUTHOR]

Two studied granodiorites from the Derflice village (east of the town of Znojmo, southern Moravia) in the NE-part of the Dyje- (Thaya-) Massif (Fig. 1) are homogeneous fine-grained rocks composed of quartz (35 wt%), altered plagioclase (35 wt%), potassium feldspar (25 wt%) and chlorite (5 wt%) with accessory amounts of muscovite, apatite, hematitized ilmenite, and zircon. Appropriate petrological name of these rocks are chloritized biotite granodiorites. Chemically and mineralogically (Table 1, Fig. 2), the Derflice granitoids resemble granodiorites of the Tetčice suite from the western branch of the Brno Massif. Representative oscillatory zoning (Fig. 3) revealed in cathodoluminescence (CL) images of zircon grains in all studied samples is shown in Fig. 1, and U-Pb concordia diagrams for zircons from the Thaya (Dyje) Massif are in Fig. 4. Dating of samples DF-B and DF-C yielded concordia ages of 603±3 Ma and 603±5 Ma (2σ), respectively (for geochronological data of individual zircon grains see Table 2), and are interpreted as the Upper Proterozoic intrusive age of the granodiorite. These ages are close to the age of zircons from granodiorite of the nearby Western Granitoid Complex of the Brno Massif (603±3 Ma, 2σ; Soejono et al. 2016, in review). Comparable ages supporting the idea that the granitoids of the Thaya Massif are assumed to be a south-western continuation of the Western Granitoid Complex of the Brno Massif. The presence of granitic rocks in the Thaya (Dyje) Massif with the Upper Proterozoic (Ediacaran) crystallization ages provides further evidence for a widespread Cadomian magmatic activity along the northern margin of Gondwana. [ABSTRACT FROM AUTHOR]