Mineralogy and petrogenesis of a Ba–Ti–Zr-rich peralkaline dyke from Šebkovice (Czech Republic): Recognition of the most lamproitic Variscan intrusion (original) (raw)
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Journal of Geosciences, 2008
Mafic and silicic dyke rocks, which occur in the Ag-U ore district of Jáchymov (Joachimsthal) in the Krušné hory (Erzgebirge) belong to kersantites, diorite porphyries, and granite porphyries, subordinately to spessartites. Dykes of minettes are located outside the district and penetrate the Nejdek-Eibenstock granitic massif. Mafic and silicic dykes exhibit diverse cross-cutting relationships and form in places composite dykes. Lamprophyres correspond to shoshonitic magmas with high abundances of compatible (Cr, Ni) and incompatible elements (Rb, Ba, P). All mafic dykes were affected by a postmagmatic alteration manifested by replacement of primary olivine and pyroxenes by amphibole, biotite and chlorite. Granite porphyries crystallized from leucocratic, weakly peraluminous, low-P magmas with moderate enrichments in Rb and depletions in Ca, Sr and Zr. By contrast, diorite porphyries are intermediate rocks containing 62-65 wt. % SiO 2 whose geological occurrence and major-element composition suggest origin by magma mixing involving lamprophyric and silicic melts, with simultaneous fractionation of biotite. The Th/U ratios in all dyke rocks correspond to chondritic and crustal values and demonstrate that whole-rock uranium contents were neither significantly increased nor affected by contemporaneous mineralization events. The stable isotope composition has been determined for whole rocks (O, C, S) and carbonate minerals (C, O). Wholerock δ 18 O values range between 1.6 to 7.8 ‰ SMOW and do not show any systematic variations with the rock types. The isotopic composition of carbonates is represented by the following values: δ 18 O = 19.6 to 25.0 ‰ SMOW and δ 13 C = -5.7 to -9.0 ‰ PDB. The high positive values of δ 18 O of carbonates are in a remarkable contrast to compositions of primary magmatic carbonates and indicate the carbonate formation during low-temperature hydrothermal event(s). Sulphides in kersantites and diorite porphyries yielded δ 34 S = 0.5 to 6.6 ‰ CDT, which document crustal in addition to mantle source of sulphur consistent with average value in the Saxothuringian crust (~ 5 ‰ CDT). Bulk magnetic susceptibility of dyke rocks shows a general decrease from mafic to silicic magmas in a sequence as follows: kersantites -500, minettes -350, diorite porphyries -350 and granite porphyries -50•10 -6 [SI]. Local increases up to 5000•10 -6 [SI] are due to accessory pyrrhotite in kersantites and diorite porphyries. Mafic dykes in the Western Krušné hory (Erzgebirge) are preferentially found within the Jáchymov-Gera tectonic zone and demonstrate spatial focusing of ascending mantle-derived melts. Such pathways may have served for transfer of ore-forming elements on the crustal scale by fluids of diverse origin.
Lithos, 2017
We report mineral textures, geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the Western part of the Moldanubian Zone at contact to the Teplá-Barrandian Block. The dykes have orientation almost perpendicular to the NNE-SSW trending Central Bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites. Based on mineral compositions and geochemistry, the dykes show close relations to Mg-K syenite plutons in the Moldanubian Zone. The two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine-grained matrix that consists of K-feldspar, phlogopite with small amounts of clinopyroxene and accessory quartz, apatite, titanite and sulphides of Fe, Cu, Ni. The syenite porphyry dyke cross cuts the vaugnerite. It contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs.
Journal of Geosciences, 2016
The study is focused on the composition of various types of Moldanubian dyke granites in the Bohemian Forest (Czech Republic). The studied area of about 200 km 2 is mainly in the northern environs of the Lipno dam lake on the Vltava River. This territory consists of metamorphic units such as Blanský les and Křišťanov granulite massifs associated with metasedimentary migmatite complexes of Monotonous and Varied units, intruded by Knížecí Stolec durbachite pluton and post-tectonic Variscan granitoids. The range of granite samples includes leucocratic rocks with muscovite, or with muscovite and biotite, and types with biotite as the single mica. Tourmaline-and garnet-bearing granites are less common. The set of 25 samples characterizes the composition of 20 dykes and small intrusions. A simple provisional division of granite samples into low-Ca (0.35-0.65 wt. % CaO) and medium-Ca (0.67-1.16 wt. % CaO) groups is used. Tourmaline granites (± Ms, Grt) contain schorl with 20-40 mol. % dravite. Garnets contain almandine and spessartine as the major components (c. 30 mol. % Sps) but the sample from the Hrad hill exhibits an outer zone with up to 32 mol. % Grs. Apatite occurs in several generations, especially in low-Ca granites, which have a significant phosphorus substitution in feldspars: 1) primary fluorapatite, 2) minute anhedral apatite (containing P unmixed from albite) characterized by up to c. 10 mol. % of chlorapatite component in predominating fluorapatite, 3) very rare (hydrothermal) hydroxylapatite filling brittle fractures in tourmaline. Accessory cordierite, originally present in some leucogranites, is altered to pinite (muscovite + chlorite ± biotite aggregate). Several samples from the Smrčina area contained cordierite with low Be, which has been unmixed as a newly formed tiny beryl in pinite. The dataset exhibits geochemical heterogeneity. Low-Ca and medium-Ca granites are distinct in the Ba-Rb-Sr ternary, as well as in the of Zr/Hf vs. Y/Ho and SiO 2 vs. A/CNK plots. The low-Ca dyke granites show numerous chemical differences from the granites of the plutonic bodies (such as the Eisgarn or Deštná types of the Moldanubian Batholith). The medium-Ca granite dykes split into the Smrčina type and remaining types of muscovite-biotite granites. Several types of chondrite-normalized REE patterns can be distinguished in terms of the total REE contents, the degree of LREE over HREE enrichment and magnitude of the Eu anomaly; most of the patterns show clearly a tetrad effect.
Geologica Carpathica, 2016
On the southeastern slope of the Baranec Mount in the Western Tatra Mountains (Slovakia) an apatite-rich pegmatite-like segregation was found in the subvertical fault zone cutting metapelitic rocks. Two zones: felsic (F) and mafic (M) were found, differing in mineral assemblages and consequently in chemistry. Fluorapatite crystals yield a LA-ICP-MS U-Pb age of 328.6 ± 2.4 Ma. A temperature decrease from 634 °C to 454 °C at a pressure around 500 to 400 MPa with oxygen fugacity increasing during crystallization are the possible conditions for formation of the pegmatite-like segregation, while secondary alterations took place in the temperature range of 340 – 320 °C. The Sr-Nd isotope composition of both apatite and whole rock point toward a crustal origin of the dike in question, suggesting partial melting of (P, F, H
Journal of Geosciences, 2008
Metabasic rocks form an important constituent of the Chýnov and Český Krumlov units belonging to the Varied Group (Moldanubian Zone, south Bohemia). The amphibolites are dominated by amphibolite-facies mineral assemblages of mainly tschermakitic amphibole and plagioclase. Hornblendes show compositional variation with Si ~ 6.5 apfu, Mg/(Mg + Fe) ~ 0.5 and (Na + K) A ~ 0.5 apfu. Garnet with clinopyroxene are subordinate and occur in a few samples only. No relics of previous greenschist-or granulite-facies assemblages have been observed, most likely due to the relatively simple metamorphic history. The petrology indicates rather close correlation of the Chýnov and Český Krumlov units. The similarities include presence of dolomite in carbonate bodies, graphite schists, rocks with marialitic scapolite, locally also Ti-andradite (± magnetite, epidote) oxidic assemblages and thin layers of Mn-rich garnet-quartz rocks. However, there is a major difference in the oxidation state. Most Chýnov amphibolites have Fe 2 O 3 /FeO = 0.70-1.00 and their protolith probably experienced an early incipient oxidation. Great deal of the parental basalts thus could have been effusive. The Český Krumlov amphibolites have Fe 2 O 3 /FeO ≤ 0.4, perhaps because they show much closer association with graphite schists that could have been responsible for the reduction of the adjacent rock units. The dataset is dominated by EMORB-like tholeiite basalts interpreted as having been derived by Early Palaeozoic melting of a strongly depleted mantle source (ε 50 Nd 0 = +8.6 to +9.4; T D Nd M = 0.43-0.50 Ga). This argues stoutly against Precambrian age of the Varied Group in south Bohemia. The composition of the remaining samples reflects contamination by upper continental crust (ε 50
Journal of the Czech Geological Society
Electron microprobe analyses were made on micas, amphiboles, feldspars, chlorites and accessory minerals in lamprophyric dykes (kersantites, minettes, spessartites), and in associated mafic diorite to tonalite porphyries (porphyrites) of the Kruné hory (Erzgebirge) area and Mariánské Láznì region in Western Bohemia (Czech Republic). All studied rocks are altered to various degrees during deuteric and/or postmagmatic stages of evolution. The only primary mafic mineral preserved in all rock types is Mg-biotite to phlogopite, in spessartites and some diorite porphyries also Ti-rich hornblende corresponding to titanian magnesiohastingsite to kaersutite. Magmatic biotites are relatively rich in Ti with limited variations in their Mg/Fe ratios, evidently re-equilibrated during cooling or re-heating. Olivine is totally replaced by pilitic pseudomorphs and by biotite-actinolite clots. Phenocrystic clinopyroxene is completely uralitized, often in well-preserved original shapes. The secondary amphiboles correspond to Si-rich magnesiohornblende to actinolite. Chlorite and epidote are rather scarce and their important occurrences are restricted to limited number of samples. Instead of chloritization typical of lamprophyres in other parts of the Bohemian Massif, most samples are affected by secondary biotitization. Effects of greisenization sensu stricto were observed in some lamprophyres from Krupka where Li-bearing dark micas rich in Si and Mg originated.
Mineralogy and Petrology, 1992
The Alcsutdoboz-2 (AD-2) core contains 12 magmatie dykes which belong to the Late Cretaceous lamprophyric-carbonatitic association of NE Transdanubia, Hungary. Petrographically, 11 dykes can be considered alkaline lamprophyre (mainly monchiquite), and the remainder might be called carbonatite. The lamprophyre dykes are similar to both alkaline lamprophyres and ultramafic lamprophyres in major element composition, whereas the carbonatite dyke has some features that are similar to carbonatites but others that are dissimilar. Nevertheless, both of the two types of AD-2 dykes possess higher LILE content than the ultramafic lamprophyres and kimberlites, but strongly differ from average earbonatite. Based on the REE pattern, crystal fractionation (mainly of olivine) and separation of a carbonate phase from the parental lamprophyric magma are proposed for genesis of the carbonatite dyke. These characteristics and the compositional zoning of clinopyroxene and mica suggest a complex crystallization history for these dykes. The likeliest origin for the parental lamprophyric melt is through a very small degree of partial melting of metasomatized mantle.
Journal of Petrology, 2020
The orogenic development after the continental collision between Laurussia and Gondwana, led to two contrasting associations of mantle-derived magmatic rocks on the territory of the Bohemian Massif: (i) a 340–310 Ma lamprophyre-lamproite orogenic association; and (ii) a 300–275 Ma lamprophyre association of anorogenic affinity. Major types of potassic mantle-derived magmatic rocks recognized in the orogenic and anorogenic associations include: (i) calc-alkaline to alkaline lamprophyres; (ii) alkaline ‘orthopyroxene minettes’ and geochemically related rocks grouped here under the new term lampyrite; and (iii) peralkaline lamproites. These three types significantly differ with respect to mineral, whole-rock and Sr–Nd–Pb–Li isotope composition and spatial distribution. The calc-alkaline lamprophyres occur throughout the entire Saxo-Thuringian and Moldanubian zones, whereas the different types of malte-derived potassic rocks are spatially restricted to particular zones. Rocks of the Car...
International Journal of Earth Sciences
The Nové Město Unit and the northern part of the Zábřeh Unit comprise back-arc basin mafic rocks metamorphosed during Variscan times. In both units, nearly identical metabasites comprise variously enriched tholeiites (from N-MORB to transitional E-MORB-OIB), high-Ca, low-silica boninites and low-Ti tholeiites. The tholeiites (Ti/V: 22–58, εNd540: + 7.6 to − 4.7) represent 10–15% melting at ca. 30–60 km and temperatures of 1380–1230 °C of a depleted MORB mantle-type (DMM) wedge heterogeneously modified shortly before fusion by OIB-like melts (enriched mantle possibly of EM1–EM2 type) presumably derived from decompression melting of upwelling asthenosphere. Much less common meta-boninites (Ti/V: 6–23, εNd540: + 6.7 to − 2.9) formed by 15–25% re-melting of residual mantle (DMM after ~ 15% melt extraction) at depths of 40–65 km and temperatures of ~ 1420–1300 °C. Scarce low-Ti meta-tholeiites (Ti/V: 18–19, εNd540: + 7.1) resulted from
Journal of GEOsciences, 2014
Variscan lamprophyres occur in the greisen tin-, tungsten-and molybdenum-ore district of Krupka in the Eastern Krušné hory/Erzgebirge (KHE). They belong to a bimodal dyke suite of aplites, felsic porphyries, microgranites and mafic dykes associated with late Variscan tin-bearing granites and include minettes, kersantites and spessartites, while vogesite reported earlier has not been confirmed. One altered mafic dyke is interpreted as original microdiorite. All lamprophyres are basic to intermediate rocks (47.3-56.9 wt. % SiO 2 ) with shoshonitic to ultrapotassic composition (3.1-7.5 wt. % K 2 O). The high concentrations of MgO (4.7-11.4 wt. %), molar Mg/(Mg + Fe) ratios (0.56-0.74) and abundances of compatible elements (350-800 ppm Cr, 130-360 ppm Ni) indicate that lamprophyres represent primary mantle melts that underwent no or little fractionation or contamination, and high LREE/HREE ratios point to magma formation in the stability field of garnet peridotite. In addition, high contents of potassium and LILE (50-370 ppm Li, 150-920 ppm Rb, 750-3100 ppm Ba) indicate metasomatic enrichment of the upper mantle prior to partial melting. The LILE-HFSE-REE patterns indicate involvement of slab components (subducted siliciclastic and carbonate sediments). Strong enrichment in U (6-29 ppm) and Th (17-75 ppm) is another characteristic feature of lamprophyres from the Eastern KHE and elsewhere in central Europe, and it is consistent with the metasomatic transport via oxidized saline fluids from the slab to the mantle wedge. The lamprophyres in the Krupka district were variably greisenized in the vicinity of granite greisens and Sn-W hydrothermal veins and their original minerals were replaced by an assemblage of lithian phlogopite, topaz, fluorite, apatite and titanium-bearing phases. During alteration, they were strongly depleted in Na 2 O, CaO, Sr and Ba, moderately depleted in REE, and enriched in Li, Rb, Cs, Sn and F. By contrast, Al and Zr behaved as immobile elements and their abundances indicate overall mass loss of 10-18 % during greisenization consistent with the increase of porosity, which facilitated the hydrothermal dissolution-precipitation reactions. The spatial association of greisens and lamprophyres suggests that the greisenizing fluids migrated along similar geological structures, which were previously accessible to the mantle-derived media (melts and/or fluids). On a local scale, the lamprophyre dykes intersected by greisen veins provided geochemical or lithological barrier, which favoured the cassiterite deposition. The timing of lamprophyre dykes also indicates that the mantle metasomatism beneath the KHE area occurred before the late Variscan granitic magmas were generated.