Genesis of a synmagmatic charnockite associated with the Weinsberg granite, southern Bohemian Batholith, northern Austria (original) (raw)
Related papers
Journal of Petrology, 2013
The Varberg^Torpa charnockite^granite association (Varberg, SW Sweden) consists of the magmatic Varberg charnockite (1399 AE 6 Ma) and the Torpa granite (1380 AE12 Ma). The Torpa granite is both continuous and, based on its whole-rock geochemistry, synmagmatic with the Varberg charnockite. The granite body also contains a number of charnockite inliers. P^T estimation using garnet^clinopyroxene and orthopyroxene^clinopyroxene Fe^Mg exchange thermometry and garnet^orthopyroxene^plagioclase^quartz barometry gives temperatures and pressures (750^8508C; 800^850 MPa) that most probably approximate the P^Tconditions during emplacement of the charnockite compared with a lower crystallization temperature (650^7008C) for the granite. The earliest recognized fluid inclusions in both the granite and charnockite consist of H 2 O^CO 2 mixtures (H 2 O volume fraction 0·2^0·7). Fluid inclusions in the charnockite are characterized by high CO 2 densities (up to 1·0 g cm À3 ; 40^90% bulk CO 2 ), of probable magmatic origin, and are best preserved in garnet, plagioclase, and fluorapatite (in order of decreasing CO 2 densities), and sometimes also in clinopyroxene. Fluid inclusions with the highest CO 2 densities (1·08^1·10 g cm À3 ) are found in quartz (T h^3 1 to^368C) and may have originated under high P^T conditions during emplacement and cooling of the charnockite. Magmatic fluids in the granite correspond to aqueousĉ arbonic inclusions with an estimated bulk composition (mol %) of H 2 O 73%, CO 2 25%, NaCl 2%. The salinity of the solutes in the granite (typically 14^20 wt % NaCl-eq. ) is generally higher than for the charnockite (0^8 wt % NaCl-eq. ). Field, petrographic, mineralogical, geochemical, and fluid inclusion evidence indicates that, compared with the H 2 O-rich granite, the magma responsible for the charnockite had a preponderance of CO 2 over H 2 O, which lowered the H 2 O activity in the melt, stabilizing ortho-and clinopyroxene.This evidence also supports the idea that the granite and charnockite were derived from a common source magma (most probably a fluid-rich basalt at the base of the crust) as a result of fractional crystallization.
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.
Lithos, 2015
An uncommon assemblage of primary and secondary accessory REE minerals was identified in a Permian A-type granite clast in polymict conglomerates intercalated in the Cretaceous flysch sequence of the Pieniny Klippen Belt, Western Carpathians, northwest Slovakia. A detailed electron-microprobe study of the granite reveals extensive subsolidus alteration of primary magmatic allanite-(Ce) to ferriallanite-(Ce) and fluorapatite. The Y, Ce-rich fluorapatite was replaced by the dissolution-reprecipitation process to the britholite group mineral members: fluorbritholite-(Y), britholite-(Y), fluorcalciobritholite, and its hydroxyl-dominant analogue ("calciobritholite"). Britholite-(Y) contains up to 5.2 wt.% ThO 2 (0.15 apfu Th); the highest Th content yet reported in naturally occurring Y-dominant britholites. Moreover, the alteration of (ferri)allanite-(Ce) resulted to complex pseudomorphs and overgrowths, including mainly REE carbonate phases: [synchysite-(Ce) to its hydroxyl-dominant analogue "hydroxylsynchysite-(Ce)", bastnäsite-(Ce)] and calcite, rarely monazite-(Ce), epidote, clinochlore, titanite, TiO 2 phase, and pseudorutile. In some cases, secondary carbonate minerals (mainly synchysite and calcite) replaced a substantial part of former allanite crystals. Moreover, primary magmatic biotite (annite) was partly transformed to acicular stilpnomelane. Textural and compositional data indicate extensive replacement and breakdown of the primary magmatic allanite and apatite by aqueous fluids rich in fluorine and carbon, liberated during a younger post-magmatic, low-temperature hydrothermal-metamorphic overprint of the granite.
2004
This paper is devoted to investigation of quartz-feldspar granite pegmatites and quartz-calcite veins from Mórágy and Erdősmecske Granitoid, South Hungary, by microprobe and fluid inclusion studies. The result is the first verification of the presence of native gold, arsenopyrite, stibnite, cassiterite and scheelite. Three types of constituents can be divided into the following groups: "granitophile", early-formed accessory minerals: ilmenite, rutile, thorite, garnet, allanite, titanite. zircon, apatite, monazite, Y-, Ce-La-Ca phosphates, etc. "granitophile", high temperature-hydrothermal-minerals: arsenopyrite. molybdenite, cassiterite and scheelite, which sometimes associated with other sulphides (pyrite, chalcopyrite, fahlore, sphalerite, and galena). "epithermal" type, low temperature hydrothermal assemblage:-gold, stibnite, barite, carbonate phases including bastnasite. Ce-LaNd carbonates and CaFe -Mn-Mg double carbonates. The gold-stibnite pair can be interpreted as a decomposition of the Sb-bearing ligand during the transportation of gold. In granite environments, the Au-Sb association is not infrequent. The microthermometric study with limited number of fluid inclusion samples does not allow detailed interpretations. However a few remarks can be made. The Th distribution shows three maxima-140-150 °C, 200-210 °C and 260-270 °C, suggesting a wide range of fluid temperature history both in the pegmatite and the grown-up (vein) quartz-calcite materials. The uncertainty of the genetic type of inclusions does not allow speculations regarding to the fluid evolution from high PT conditions. The registered Th data set reflects some "late" history of fluids, without any time-related considerations. The fluid character is mainly Na-Ca-Cl type. The influence of Ca-bearing fluids can be traced also by the following facts: a) Garnet overgrowth by Ca-rich rim, b) Allanite overgrowth by Ca-rich rim, c) Decomposition-redistribution process of REE-bearing minerals like monazite, allanite, etc. to REE-bearing carbonate species like bastnasite, or other REE-bearing carbonate during a "hydrothermal" process, d) Formation of "sterile", late carbonate-bearing veinlets. The calculated salinity (expressed in NaCI eq.wt. %) is low: 6-0 NaCl eq.wt.% The Th/c plot shows the trapping of fluids in a wide density range. Comparing to the similar data of Velence Mts., it exhibits the presence of significantly lower salinity fluids in Erdősmecske-Mórágy Granitoid. These data suggest an overlapping as a result of subsequent tectonic and fluid rejuvenation events.
Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 2004
The prominent felsic granulites in the southern part of the Bohemian Massif (Gföhl Unit, Moldanubian Zone), with the Variscan (w340 Ma) high-pressure and high-temperature assemblage garnet+quartz+hypersolvus feldspar kyanite, correspond geochemically to slightly peraluminous, fractionated granitic rocks. Compared to the average upper crust and most granites, the U, Th and Cs concentrations are strongly depleted, probably because of the fluid and/or slight melt loss during the high-grade metamorphism (900-1050(C, 1·5-2·0 GPa). However, the rest of the trace-element contents and variation trends, such as decreasing Sr, Ba, Eu, LREE and Zr with increasing SiO 2 and Rb, can be explained by fractional crystallisation of a granitic magma. Low Zr and LREE contents yield w750(C zircon and monazite saturation temperatures and suggest relatively low-temperature crystallisation. The granulites contain radiogenic Sr ( 87 Sr/ 86 Sr 340 = 0·7106-0·7706) and unradiogenic Nd ( 340 Nd = 4·2 to 7·5), indicating derivation from an old crustal source. The whole-rock Rb-Sr isotopic system preserves the memory of an earlier, probably Ordovician, isotopic equilibrium.
Journal of the Czech Geological Society, 2002
1 Petrology, geochemical character and petrogenesis of a Variscan post-orogenic granite: case study from the evìtín Massif, Moldanubian Batholith, Southern Bohemia Petrologie, geochemický charakter a petrogeneze pozdnì variské granitové intruze: pøíklady ze evìtínského masívu, moldanubický pluton, jiní Èechy (16 figs, 6 tabs) VOJTÌCH JANOUEK STANISLAV VRÁNA VOJTÌCH ERBAN Czech Geological Survey, Klárov 3/131, 118 21 Prague 1, Czech Republic
The Canadian Mineralogist, 2010
The mechanisms of differentiation and the source rocks of hornblende-biotite granitoids from the 1.5 Ga Mazury Complex, in the East European Craton in northeastern Poland, were investigated with major and trace elements and Sr-Nd isotopes on drill-core samples from six localities. The rock suites show metaluminous, ferroan, potassic and mostly alkali-calcic characters, together with high contents of incompatible elements typical of A-type granitoids. The presence of magnetite and a low Fe/(Fe + Mg) value of the hornblende indicate rather oxidized conditions of crystallization. In Harker diagrams, the major elements plot on a nearly continuous trend from 43 to 67 wt% SiO 2 . From 56 wt% SiO 2 onward, the overall trend overlaps with the Tranevåg liquid line of descent, defined for hornblende-biotite granite in southern Norway. Most trace-element concentrations show decreasing trends with increasing SiO 2 . The rare-earth-element concentrations are controlled by the apatite contents of the samples. The overall geochemical trend results from fractional crystallization and can be modeled by subtraction of mafic-mineral-rich cumulates. The suite is formed from melts at different degrees of fractionation, laden with various amounts of cumulus minerals. The initial Nd ranges from -3.3 to -6.8, with relatively low values of the initial Sr isotope ratio (0.702-0.707). Because of the absence of Archean rocks in this part of the East European Craton, most Nd negative values are consistent with melting of a juvenile crust extracted from the mantle at ca. 2.0-2.2 Ga. In the Mazury Complex, the parent magma for the 1.5 Ga Suwalki anorthosite was also formed by the melting of juvenile crust within the same time range. The Mazury batholith was emplaced along a linear zone of weakness, which facilitated melting of the lower crust. The melting products were a hornblende-biotite granite suite, oxidized and H 2 O-rich, associated with an anorthosite-ferrodiorite suite, formed under dry and more reduced conditions. This is another line of evidence that, in anorthosite -mangerite -charnockite -granite (AMCG) complexes, two different crustal source-rocks can produce two different suites of rocks during the same melting episode.
Petrology
The paper presents data on the transformations in the mineralogy and mineral assemblages of quartz and feldspar free magnesian orthopyroxene-clinopyroxene-biotite metaultramafic granulites con verted into orthopyroxene-clinopyroxene-potassic feldspar-plagioclase-quartz charnockite. Within a 4 to 6 m aureole around the fluid conduit, the granulite has been affected by Na-K-Si-H 2 O-CO 2-Cl-F brine. Thereby the magnesian metaultramafite has undergone progressive debasification and leucocratization: potassic feldspar, oligoclase, and quartz have been metasomatically formed in the interstitial space between the mafic minerals and replaced ortho and clinopyroxene and biotite. The Fe mole fractions X Fe of all mafic minerals increase toward the fluid conduit as follows: from 0.25 to 0.54 for Opx, from 0.15 to 0.32 for Cpx, and from 0.16 to 0.56 for Bt. The whole rock compositions suggest that Na, K, and Si have been introduced and Mg, Fe, and Ca removed from the protolith, so that the metaultramafic rock has gradually been transformed first into a mesocratic rock and then, when partial melting started in its most significantly debasified domains, into leucocratic nebulitic migmatite with skialiths of the modified granulite, then into charnockite migma, and eventually into magma. The composition of the charnockite forming fluid was estimated as = 0.6, X (Na,K)F = 0.3, and = 0.1. The unusual F rich composition of the fluid is reflected in that both Bt and Hbl are enriched in F and contain almost no Cl. The P-T parameters of the process, which took place at the metamorphic peak, were T ~ 780°C, P = 8.5 kbar. Material balance plots of the rocks revealed three petro logical trends of the charnockite forming process controlled solely by the composition of the brine: (a) a trend that did not produce either Bt or Hbl during the metasomatic and anatectic stages, (b) that associated with intense amphibolization and biotitization during the metasomatic stage, and (c) a trend associated with the origin of melanocratic metasomatic Opx-Cpx-Bt-Hbl ± Pl selvages around newly formed charnocki toids, with the composition of the selvages close to the melanocratic veins produced in the peripheries of the charnockitization zone by the rapid redeposition of Mg, Fe, and Ca mobilized in the course of debasification. It follows that charnockitization proceeded according to the model of nonisochemical migmatization in an open system, a process driven first of all by the inflow of deep brines. This process differed strongly from sim ple closed system partial melting induced by an increase in temperature.
Charnockite microstructures: From magmatic to metamorphic
Geoscience Frontiers, 2012
Charnockites sensu lato (charnockite-enderbite series) are lower crustal felsic rocks typically characterised by the presence of anhydrous minerals including orthopyroxene and garnet. They either represent dry (H 2 O-poor) felsic magmas that are emplaced in the lower crust or granitic intrusions that have been dehydrated during a subsequent granulite facies metamorphic event. In the first case, postmagmatic high-temperature recrystallisation may result in widespread metamorphic granulite microstructures, superimposed or replacing the magmatic microstructures. Despite recrystallisation, magmatic remnants may still be found, notably in the form of melt-related microstructures such as melt inclusions. For both magmatic charnockites and dehydrated granites, subsequent fluid-mineral interaction at intergrain boundaries during retrogradation are documented by microstructures including K-feldspar microveins and myrmekites. They indicate that a large quantity of low-H 2 O activity salt-rich brines, were present (together with CO 2 under immiscible conditions) in the lower crust. ª
Lithos, 2019
The complementary roles of granites and rocks of the granulite facies have long been a key issue in models of the evolution of the continental crust. "Dehydration melting", or fluid-absent melting of a lower crust containing H 2 O only in the small amounts present in biotite and amphibole, has raised problems of excessively high temperatures and restricted amounts of granite production, factors seemingly incapable of explaining voluminous bodies of granite like the Archean Closepet Granite of South India. The existence of incipient granulite-facies metamorphism (charnockite formation) and closely associated migmatization (melting) in 2.5 Ga-old gneisses in a quarry exposure in southern India and elsewhere, with structural, chemical and mineral-inclusion evidence of fluid action, has encouraged a wetter approach, in consideration of aqueous fluids for rock melting which maintain sufficiently low H 2 O activity for granulite-facies metamorphism. Existing experimental data at elevated T and P are sufficient to demonstrate that, at mid-crust pressures of 0.5-0.6 GPa and metamorphic temperatures above 700°C, ascending immiscible CO 2-rich and concentrated alkali chloride aqueous fluids in equilibrium with charnockitic (orthopyroxene-bearing) gneiss will inevitably begin to melt granitic rocks. The experimental data show that H 2 O activity is much higher (0.5-0.6) than previously portrayed for beginning granulite facies metamorphism (0.15-0.3). Possibilities for metasomatism of the deep crust are greatly enhanced over the ultra-dry models traditionally espoused. Streaming of ultrasaline fluids through continental crust could be a mechanism for the generation of the discrete mid-crust layer of migmatites suggested to characterize younger tectonometamorphic regions. The action of CO 2-rich and hypersaline fluids in Late Archean metamorphism and magmatism could record the beginning of large-scale subduction of volatilerich surficial materials.