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In situ gamma-ray measurements of 40K, 232Th and 238U in high-grade metamorphic rocks from the Sowie Mountains (Sudetes, SW Poland)

Malczewski Dariusz, Żaba Jerzy

Geological Quarterly

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2019

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Vol. 63, No. 3

493--504

EN

This paper reports on the abundance of primordial radionuclides (40K,232K and 238U) in characteristic lithologies from the Sowie Mountains (SW Poland). In situ gamma-ray measurements were conducted at 12 localities hosting exposed augen gneiss, flaser gneiss, granulite, homophanized gneiss, hornblendite, layered gneiss, layered migmatite, migmatic gneiss, mylonitic gneiss and mylonitic granulite. The activity concentration of 40K varied from 180 Bq kg-1 (mylonitic granulite) to 845 Bq kg-1 (layered gneiss). The activity concentrations associated with 228Ac (232Th) varied from 10 Bq kg-1 (mylonitic granulite) to 53 Bq kg-1 (homophanized gneiss), while activity associated with 226Ra (238 U) varied from 9 Bq kg-1 (mylonitic granulite) to 43 Bq kg-1 (layered gneiss). An augen gneiss adjacent to the Intra-Sudetic Fault showed the highest combined Th + U activity (89 Bq kg-1). The average Th/U ratio (3.6) calculated for gneiss outcrops of the Sowie Mountains falls within the range reported for biotite gneiss (3.5-4). Subsurface outcrops from the Walim-Rzeczka underground complex did not give noticeably higher 238U activities. The results obtained forthe Sowie lithologies are compared with those obtained by similar methods for similar rocks as reported in literature sources.

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Wstępne wyniki badań petrograficznych archaicznych gnejsów Uivak (Prowincja Nain, Labrador)

Sałacińska A., Kusiak M. A.

Przegląd Geologiczny

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2017

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Vol. 65, nr 6

383--391

EN

The Saglek Block of the Nain Complex in Northern Labrador forms the westernmost part of the North Atlantic Craton. It is one of the few regions on Earth where early Archaean crust is preserved. It consists of regionally metamorphosed gneisses to either granulite-facies from 2.74 to 2.71 Ga or amphibolite-facies at ca. 2.70 Ga. The Saglek Block is dominated by two suites of metaigneous gneisses: the Uivak I TTG suite (>3.6 Ga) and the Uivak II augen gneisses (ca. 3.4 Ga). Four samples offelsic Uivak I orthogneiss were selected to petrographi- cal and mineralogical studies: Johannes Point Cove (L1419), TigigakyukInlet (L1434; L1440) and Big Island (L1443). Sample composition (granodiorite-tonalite) and mineralogy (mostly plagioclase and quartz, with minor K-feldspar and biotite, and accessory zircon and apatite) for three of the four samples are consistent with published data of the Uivak I gneiss. Sample L1434 contains ferroean diopside and hornblende, which are absent in the other samples. Based on detailed mineralogical analyses, authors concluded that more mafic sample, can not belong to the suite of the Uivak I gneisses. Most probably it represents the younger Uivak II. Further, geochronological study is going to be conducted to characterise both, Uivak I and Uivak II gneisses isotopically.

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Paleomagnetism and rock-magnetism of ordovician gneisses from the Leszczyniec unit (Rudawy Janowickie, Sudetes) : study of remagnetization effects

Kądziołko-Hofmokl M., Jeleńska M., Szałamacha M., Szałamacha J.

Acta Geophysica Polonica

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1998

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Vol. 46, nr 2

187-215

EN

Research performed for Leszczyniec hornblende gneisses comprises paleomagnetic and rock magnetic study. Thermomagnetic and microscopic investigations indicate that magnetic properties of the gneisses are carried mainly by minerals formed or altered and/or magnetized due to deformational stages. The most important changes are related to the stage D1 that took place in the amphibolite facies conditions and ended at the Devonian-Carboniferous boundary. Low field magnetic susceptibility and its anisotropy are in major part related to paramagnetic hornblende. The directions of magnetic lineation before tectonic correction remain in agreement with directions of stretching lineation found in the Leszczyniec (LE) unit. Paleomagnetic study reveals presence of nine components of characteristic remanence. Five of them, labeled T, A, A1, A2 and CP may be interpreted in terms of the geomagetic field or geomagnetic field and tectonics. Component T (in situ) represents Mesozoic overprint, components A and A1 (both in situ) represent Early Permian and Middle Carboniferous overprints, respectively. Components A2 and CP represent geomagnetic field from the pre-tectonic times. In order to interpret them we assumed anticlockwise rotation of the LE unit around the vertical intraplate axis by 80 remaining in agreement with its sinistral shearing connected with the Dl deformational stage stated by Mazur (1995). This procedure shifted the tectonically corrected A2 pole position to the Devonian segment of the reference APW path for Baltica and the CP pole to its Ordovician segment. These results suggest that the LE unit was close to the Baltica perhaps even since the Ordovician and became rotated anticlockwise by about 80 at the final phase of the D1 stage (Upper Devonian-Lower Carboniferous). Despite the apparent logic of the presented results, there is always a possibility that the A2 and CP components of NRM are artefacts as are probably the remaining four isolated components CN, BW, BE1 and BE2.