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| 2. |
- Cámara, Fernando, et al.
(författare)
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Schorl-1A from Langesundsfjord (Norway)
- 2022
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Ingår i: Journal of Geosciences. - : Czech Geological Society. - 1802-6222 .- 1803-1943. ; 67:2, s. 129-139
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Tidskriftsartikel (refereegranskat)
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| 3. |
- Dietl, Carlo, et al.
(författare)
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Formation of tabular plutons : results and implications of centrifuge modelling
- 2008
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Ingår i: Journal of geosciences. - : Czech Geological Society. - 1802-6222 .- 1803-1943. ; 53:3-4, s. 253-261
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Tidskriftsartikel (refereegranskat)abstract
- Geophysical investigations reveal that many granitoid plutons possess a tabular shape: either laccolithic, lopolithic or phacolithic. In this study, the results of a centrifuge experiment are used to understand the formation mechanisms of these features. The model was build of a sequence of 14 differently coloured plasticine layers. Two buoyant layers - with a volume of c. 40 cm(3) each - were incorporated into the model stratigraphy at different depths to investigate, whether the rise and emplacement of buoyant material at different levels results in different intrusion structures. After centrifuging for 30 min at 700 G, both the buoyant layers had formed two lenticular sills (phacoliths) with aspect ratios (length/thickness) of 6 and 3.4 for the upper and lower phacoliths, respectively, directly above both pre-existing perturbations in the buoyant layers. During their movement, the buoyant phacoliths had pushed their roof plasticine upward. Simultaneously, their floor plasticine had subsided (bottom sinking). Subsidence of the floor material had choked the inflow of further buoyant material into the feeder channel of the developing sills and inhibited their further lateral growth. The observed forced downward movement of the plasticine floor of the forming PDMS (polydimethylsiloxane) phacoliths resembles the so-called "floor depression" of host rock material around an emplacing tabular pluton. Floor depression is supposed to be a very important vertical material transfer process, which provides space for the construction of lopo- and phacoliths. The subsidence of host material made space for the developing buoyant phacoliths, but also restricted their growth to a certain time slot before the influx of new buoyant material into the feeder dyke of the tabular intrusive body was shut off. Similarly, in nature, the growth of a tabular pluton might be limited not only by the rate of magma ascent and its physical properties, but also by the emplacement processes of the evolving pluton.
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| 4. |
- Majka, Jaroslaw, et al.
(författare)
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Multiple monazite growth in the Areskutan migmatite: evidence for a polymetamorphic Late Ordovician to Late Silurian evolution in the Seve Nappe Complex of west-central Jamtland, Sweden
- 2012
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Ingår i: Journal of Geosciences. - : Czech Geological Society. - 1802-6222 .- 1803-1943. ; 57:1, s. 3-23
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Tidskriftsartikel (refereegranskat)abstract
- Monazite from granulite-facies rocks of the angstrom reskutan Nappe in the Scandinavian Caledonides (Seve Nappe Complex, Sweden) was dated using in-situ U-Th-total Pb chemical geochronology (CHIME). Multi-spot analyses of a non-sheared migmatite neosome yielded an age of 439 +/- 3 Ma, whereas a sheared migmatite gave 433 +/- 3 Ma (2 sigma). Although the obtained dates are rather similar, a continuous array of single dates from c. 400 Ma to c. 500 Ma suggests possibly a more complex monazite age pattern in the studied rocks. The grouping and recalculation of the obtained results in respect to Y-Th-U systematics and microtextural context allowed distinguishing several different populations of monazite grains/growth zones. In the migmatite neosome, low-Th and low-Y domains dated at 455 +/- 11 Ma are considered to have grown under high-grade sub-solidus conditions, most likely during a progressive burial metamorphic event. The monazites with higher Th and lower Y yielded an age of 439 +/- 4 Ma marking the subsequent partial melting event caused by decompression. The youngest (423 +/- 13 Ma) Y-enriched monazite reveals features of fluid-assisted growth and is interpreted to date the emplacement of the Areskutan onto the Lower Seve Nappe. In the sheared migmatite, the high-Th and low-U (high Th/U) monazite with variable Y contents yielded an age of 438 +/- 4 Ma, which is interpreted to date the partial melting event. Relatively U-rich rims on some of the monazite grains again reveal features of fluid-assisted growth, and thus their age of 424 +/- 6 Ma is interpreted as timing of the nappes emplacement. These results call, however, for further more precise, isotopic (preferably ion microprobe) dating of monazite in the studied rocks.
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| 5. |
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| 6. |
- Pašava, Jan, et al.
(författare)
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Platinum-group elements (PGE) and their principal carriers in metal-rich black shales : An overview with a new data from Mo-Ni-PGE black shales (Zunyi region, Guizhou Province, south China)
- 2013
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Ingår i: Journal of Geosciences. - : Czech Geological Society. - 1802-6222 .- 1803-1943. ; 58:3, s. 213-220
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Tidskriftsartikel (refereegranskat)abstract
- Lower Cambrian Mo–Ni sulfidic black shales from the Huangjiawan mine (Guizhou Province, south China) have anomalousplatinum-group elements (PGE) concentrations (up to ~1 ppm in total). In order to identify principal PGE carriers,we used heavy mineral separates which were produced by innovative hydroseparation techniques. Subsequent detailedmineralogical study using electron microprobe did not result in the identification of discrete platinum-group minerals.Pyrite (grainy, not framboidal), millerite and gersdorffite that were found in our heavy concentrate were analyzed forPGE and Re. We found that they contain the following concentrations of PGE and Re: pyrite (up to 490 ppm Pt, 390ppm Pd and 220 ppm Rh), millerite (up to 530 ppm Pt, 430 ppm Pd and 190 ppm Rh) and gersdorffite (up to 410 ppmPt and 320 ppm Pd; no Rh detected). Rhenium was detected only in grainy pyrite (up to 1060 ppm). It was found thatdespite anomalous PGE concentrations, the Mo–Ni black shales do not contain any platinum-group minerals and thatthe PGE are bound to pyrite and Ni-sulfides (millerite and gersdorffite).
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| 7. |
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| 8. |
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| 9. |
- Pieczka, Adam, et al.
(författare)
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Si-deficient foitite with Al-[4] and B-[4] from the 'Lugi-1' borehole, southwestern Poland
- 2011
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Ingår i: Journal of Geosciences. - : Czech Geological Society. - 1802-6222 .- 1803-1943. ; 56:4, s. 389-398
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Tidskriftsartikel (refereegranskat)abstract
- A tourmaline depleted in Si has been found in the cement of Upper Rotliegend aeolian sandstone overlaying Lower Rotliegend volcanic rock in the 'Lugi-1' prospecting borehole near Jarocin in the Fore-Sudetic Monocline, south-western Poland. Tourmaline, representing Mg-bearing foitite, crystallized around quartz grains in the form of radial aggregates of tiny crystals, reaching only 1-2 mu m in diameter. Due to the very small size of the tourmaline crystals and the presence of significant contents of transitional metals in the crystal lattice, only direct determination of B2O3 in nm-sized spots enables evaluation of Al-[4] and B-[4]. Compositions of the tourmaline in successive analytical spots show that Si deficiency is supplemented both by Al-[4] (0.00 to 0.48 Al apfu) as well as by B-[4] (0.00 to 0.83 B apfu) at varying proportions. The origin of the mineral was related to diagenesis of evaporate sediments inducing reactions of quartz and clay minerals as primary components in the tourmaline-bearing sandstone with Ca saturated pore brines rich in Cl- and (BO3)(3-).
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