The stability of xenotime in high Ca and Ca-Na systems, under experimental conditions of 250-350°C and 200-400 MPa : the implications for fluid-mediated low-temperature processes in granitic rocks

The stability of xenotime was tested by experiments in the presence of a silicate mineral assemblage and two different fluids, 2M Ca(OH)_2 or Na_2Si_2O_5 + H_2O, under P-T conditions of 200-400 MPa and 250-350°C. The xenotime was stable in runs with 2M Ca(OH)_2, replicating the low-temperature metasomatic alterations of granitic rocks, except in experiment at 350°C and 400 MPa, where some (Y,REE)-rich fluorapatite formed. Experiments with Na_2Si_2O_5 + H_2O resulted in significant xenotime alteration and partial replacement by an unknown (Y,HREE)-rich silicate, and in the formation of minor amounts of (Y,REE)-rich fluorapatite. The latter indicate preferential partitioning of Y and REE into silicates over phosphates during low-temperature, metasomatic processes in a high Na-Ca system, similar to peralkaline granitic rocks

Stability of monazite and disturbance of the Th-U-Pb system under experimental conditions of 250-350^{\circ}C and 200-400 MPa

This experimental study provides important data filling the gap in our knowledge on monazite stability under conditions of fluid-mediated low-temperature metamorphic alteration and post-magmatic hydrothermal alterations. The stability of monazite and maintenance of original Th-U-total Pb ages were tested experimentally under P-T conditions of 250-350 ^{\circ}C and 200-400 MPa over 20-40 days. The starting materials included the Burnet monazite + K-feldspar ± albite ± labradorite + muscovite + biotite + SiO_2 + CaF_2 and 2M Ca(OH)_2 or Na_2Si_2O_5 + H_2O fluid. In the runs with 2M Ca(OH)_2, monazite was unaltered. REE-enriched apatite formed at 350 ^{\circ}C and 400 MPa. The presence of the Na_2Si_2O_5 + H_2O fluid promoted the strong alteration of monazite, the formation of secondary REE-enriched apatite to fluorcalciobritholite, and the formation of REE-rich steacyite. Monazite alteration included the newly developed porosity, patchy zoning, and partial replacement by REE-rich steacyite. The unaltered domains of monazite maintained the composition of the Burnet monazite and its age of (or close to) ca. 1072 Ma, while the altered domains showed random dates in the intervals of 375-771 Ma (250 ^{\circ}C, 200 MPa run), 82-253 Ma (350 ^{\circ}C, 200 MPa), and 95-635 Ma (350 ^{\circ}C, 400 MPa). The compositional alteration and disturbance of the Th-U-Pb system resulted from fluid-mediated coupled dissolution-reprecipitation. In nature, such age disturbance in monazite can be attributed to post-magmatic alteration in granitic rocks or to metasomatic alteration during metamorphism. Recognition of potentially altered domains (dark patches in high-contrast BSE-imaging, developed porosity or inclusions of secondary minerals) is crucial to the application of Th-U-Pb geochronology

Th-U-total Pb timing constraints on the emplacement of the granitoid pluton of Stolpen, Germany

Monazite from the Stolpen monzogranite (SE Germany) was studied to constrain the Th-U-total Pb age of pluton formation. Monazite grains demonstrate subtle to distinct patchy zoning related to slight compositional variations. Textural and compositional characteristics indicate that the monazite formed in a single magmatic event in a slightly heterogeneous system, and was only weakly affected by secondary alteration, which did not disturb the Th-U-Pb system. Chemical dating of the monazite gave a consistent age of 299 ± 1.7 Ma. The current study presents the first geochronological data for the Stolpen granite. It provides evidence that Stolpen is the youngest Variscan granitic intrusion in the Lusatian Granodiorite Complex and indicates that magmatic activity related to post-collisional extension in this region lasted at least 5my longer than previously assumed

Detrital zircon U-Pb and Hf constraints on provenance and timing of deposition of the Mesoproterozoic to Cambrian sedimentary cover of the East European Craton, Belarus

International audienceThe sedimentary cover of the East European Craton (EEC) is unique because of its low degree of diagenetic alteration that allows preservation of the original “source to sink” relationships. The present study provides U-Pb and Hf zircon data for the entire Proterozoic sedimentary section of the EEC based on samples from five boreholes in Belarus within the Volyn-Orsha Basin, one of the most important sedimentary basins of the craton. Twenty-one samples of mudstones and sandstones were selected for detrital zircon U-Pb geochronology, supplemented by the Hf isotope analyses of zircons from 6 samples representing different U-Pb age spectra and bulk rock XRD mineralogy of all mudstone samples collected from the studied boreholes. Five clastic successions in the Volyn-Orsha Basin are characterized by different sources of detrital material: (1) The Mesoproterozoic Pinsk Suite with a narrow population of c. 2.0 Ga zircons, (2) The Orsha Suite with a broad 1.3–3.2 Ga zircon age distribution, (3) Glacial sediments of the Vilchitsy Series with an age spectra similar to the Orsha Suite, except for a c. 1.0 and 1.2 Ga cluster, (4) The Volyn and Valdai Series, including lowermost Cambrian, with a narrow trimodal population of 0.5, 1.5, and 1.8 Ga zircons, and (5) lower Cambrian (?) sediments with a diffused zircon age spectrum, including a 500–700 Ma cluster. Maximum depositional ages were constrained for the Vilchitsy Series at 977 ± 6 Ma and for the Volyn Series at 579–545 ± 4 Ma. Combined Hf zircon data indicate four episodes of new continental crust generation at 3.3, 2.8, 2.1–2.3 and 1.8 Ga, suggestive of source terrains within the crust of the present-day EEC. These sources experienced subsequent reworking of crust at c. 1.8 Ga and 550–600 Ma. Only a lower Cambrian sample lacks any trend or clustering within the Hf data probably due to mixing of zircons from exotic and local sources. Paleogeographic models explaining these provenance signals in terms of intracratonic erosion and sediment transport are presented

Warunki ciśnień i temperatur metamorfizmu wysokiego stopnia w Górach Sowich, Sudety Zachodnie

P-T conditions of metamorphism that affected the Góry Sowie Mts gneisses (West Sudetes, SW Poland) were determined with use of garnet-biotite (GB) and muscovite-biotite (MB) geothermometry, and muscovite geobarometry on selected rocks. Granulite and sillimanite-bearing layered gneiss from the Bystrzyckie Lake region (northern part of the Góry Sowie Mts) revealed GB temperatures 652 ± $35^{\circ}C$. To the south, layered gneiss and metapegmatite from the Przygórze area yielded similar GB temperatures (660 ± $28^{\circ}C$), lower MB temperatures (613 ± $25^{\circ}C$) and pressures 6.4 ± 1.4 kbar. In the central part of the massif, finely laminated gneiss and diatexite from the Potoczek region revealed MB temperatures 596 ± $24^{\circ}C$ and pressures 5.2 ± 0.7 kbar. A similar temperature (600 ± $25^{\circ}C$) was obtained out of a flaser gneiss from the Kietlice region in the Fore-Sudetic part of the Góry Sowie Block. Differences between GB and MB results might be related to different speed of ion diffusion between coexisting minerals. We suggest that reported data are related to initial midcrustal exhumation and coeval amphibolite facies metamorphism

Migmatization and large-scale folding in the Orlica-Śnieżnik Dome, NE Bohemian Massif : pressure, temperature, time, deformation constraints on Variscan terrane assembly

New research indicates that there is stratigraphic and structural continuity between the Młynowiec and Stronie Formations of the Orlica-Śnieżnik Dome (OSD), Bohemian Massif, and reveals the primary importance of the collision of Armorican Terrane Assemblage and Brunovistulian terrane in Variscan development of the OSD. The Młynowiec Formation is a locally migmatized metasedimentary unit and understanding its tectonometamorphic history is crucial for making regional-scale Variscan reconstructions. Structural studies, pseudosection modelling in MnNCKFMASH system and microprobe monazite dating revealed that this Variscan collision led to crustal thickening of the OSD, tectonic burial of the Młynowiec Formation to depths corresponding to 10-11 kbar and progression of regional metamorphism at 361 ± 5 Ma. Continuous underthrusting of the Brunovistulian terrane led to exhumation of the OSD metasediments and the formation of subhorizontal, N-S-trending tight folds and a peak-metamorphic axial planar foliation. This episode took place at 650 °C and 6-7 kbar in the non-migmatized rocks of structurally upper parts of the Młynowiec Formation, and at 700 °C and 6-7 kbar in its structurally lower parts, where metamorphism was accompanied by localised in-situ migmatisation. During final consolidation of a mosaic of terranes, top-to-the-NE tectonic movements reactivated former fabrics, produced further uplift and cooled the rocks to greenschist facies. This pervasive shearing could be also responsible for the metasomatic event at ~ 330 Ma. The continuation of the NE-transpression of the OSD involved its interactions with adjacent crustal units in the east and in the north, which led to large-scale folding represented now by synforms and associated antiforms. The metamorphic structure of the rocks subjected to the Barrovian-type metamorphism was rebuilt so that the migmatitic rocks outcrop in the cores of large-scale antiforms, and the lithological contacts, penetrative foliation and metamorphic isograds dip locally at different angles (shallow, moderate to high) and at different azimuths (E-, NE-, N, NW and W)

Pierwotny i wtórny monacyt w skalcytyzowanym klaście gnejsu z Bukowca (jednostka śląska, Zachodnie Karpaty Zewnętrzne)

Primary and secondary monazite-(Ce) in a calcitized gneiss clast from Bukowiec near Rożnowskie Lake (the Silesian Unit, Western Outer Carpathians) has been analysed using SEM-EDS methods. Secondary monazite-(Ce) formation probably took place prior to calcitization of the gneiss