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Detrital zircon U-Pb geochronology of a metasomatic calc-silicate in the Tsäkkok Lens, Scandinavian Caledonides

Barnes Cristopher J., Majka Jarosław, Bukała Michał, Nääs Erika, Rousku Sabine

Geology, Geophysics and Environment

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2021

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Vol. 47, no. 1

21--31

EN

The Tsäkkok Lens of the Seve Nappe Complex in the Scandinavian Caledonides comprises eclogite bodies hosted within metasedimentary rocks. These rocks are thought to be derived from the outermost margin of Baltica along the periphery of the Iapetus Ocean, but detrital records from the sedimentary rocks are lacking. Many metasedimentary outcrops within the lens expose both well-foliated metapelitic rocks and massive calc-silicates. The contacts between these two lithologies are irregular and are observed to trend at all angles to the high-pressure foliation in the metapelites. Where folding is present in the metapelites, the calc-silicate rocks are also locally folded. These relationships suggest metasomatism of the metapelites during the Caledonian orogenesis. Zircon U-Pb geochronology was conducted on sixty-one zircon grains from a calc-silicate sample to investigate if they recorded the metasomatic event and to assess the detrital zircon populations. Zircon grains predominantly show oscillatory zoning, sometimes with thin, homogeneous rims that have embayed contacts with the oscillatory-zoned cores. The zircon cores yielded prominent early Stenian, Calymmian, and Statherian populations with a subordinate number of Tonian grains. The zircon rims exhibit dissolution-reprecipitation of the cores or new growth and provide ages that span similar time frames, indicating overprinting of successive tectonic events. Altogether, the zircon record of the calc-silicate suggests that the Tsäkkok Lens may be correlated to Neoproterozoic basins that are preserved in allochthonous positions within the northern extents of the Caledonian Orogen.

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Magmatic episodes in the Holy Cross Mountains, Poland – a new contribution from multi-age zircon populations

Krzemińska Ewa, Krzemiński Leszek

Biuletyn Państwowego Instytutu Geologicznego

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2019

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nr 474

43--57

EN

This contribution reports on new U-Pb zircon age data from magmatic rocks from the Holy Cross Mountains (HCM) of Poland. The analyzed samples were taken from lamprophyre and diabase veins of Podkranów and Janowice-2 as well as from tuff horizon of Kielce Beds (Ludlow). Internal morphologies have been investigated by SEM-BSE and cathodoluminescence images and they have been used as a guide for the selection of genetically various type of grains, e.g. potential auto-, ante- and xenocrysts, that were analyzed by ion microprobe. The U-Pb age of the magmatic events at 414.2 ±6.6 Ma (Kielce tuff), 322 ±12 Ma (Podkranów, lamprophyre), and 300±10 Ma (Janowice-2, diabase) confirmed the time frame of known magmatic activity reported within the HCM, as determined by 40Ar/39Ar geochronology in previous studies. The zircon investigation revealed also multiple populations with record of an earlier pulse of magma system (antecrysts), as well as abundant xenocrysts.

PL

W artykule przedstawiono nowe oznaczenia wieku U-Pb cyrkonu w skałach magmowych z obszaru Gór Świętokrzyskich (HCM). Próbki pobrano z żył lamprofiru i diabazu z Podkranowa i Janowic-2 oraz z poziomu tufu w warstwach kieleckich (ludlow). Morfologię i wewnętrzne cechy budowy ziaren zbadano za pomocą obrazów SEM-BSE i katodoluminescencji SEM, stosując je jako przewodnik do selekcji genetycznie różnych typów ziaren, potencjalnych auto-, ante- i ksenokryształów, które były analizowane na mikrosondzie jonowej. Wiek U-Pb epizodów magmowych, 414,2 ±6,6 Ma (Kielce, tuf), 322 ±12 Ma (Podkranów, lamprofir) i 300 ±10 Ma (Janowice-2, diabaz), potwierdza znane z wcześniejszych publikacji ramy czasowe aktywności magmowej na obszarze HCM oznaczone metodą 40Ar/39Ar. Badania wieku U-Pb cyrkonu ujawniły także szereg populacji z zapisem wcześniejszych impulsów magmowych (antekryształy) i licznych ksenokryształów.

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U-Pb zircon geochronology of high-grade charnockites - exploration of pre-Mesoproterozoic crust in the Mazury Complex area

Krzemińska Ewa, Łukawska Aleksandra, Bagiński Bogusław

Acta Geologica Polonica

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2019

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Vol. 69, no. 4

489--511

EN

Charnockites – i.e., orthopyroxene-bearing felsic rocks – were formed in a deep-seated dry environment, either under plutonic or high-grade metamorphic conditions. Most charnockites known from the crystalline basement of Poland appear to be of Mesoproterozoic age (1.50–1.54 Ga), cogenetic with the Suwałki Anorthosite Massif, and associated with mangerite and granite members forming the AMCG suite of the Mazury Complex. Genetically distinct rocks, characterised by the presence of anhydrous minerals, e.g., orthopyroxene and garnet, were also recognised along 592 m of the Łanowicze PIG-1 borehole section, within the AMCG suite. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date the complexly zoned zircons. The ages of crystallisation of the charnockite protoliths from various depths at 1837 ± 7, 1850 ± 9, 1842 ± 6, and 1881 ± 16 Ma makes these rocks the oldest dated crust within this part of the Polish basement. The Łanowicze PIG-1 borehole section bears components from neighbouring tectonic domains known from Lithuania: the West and Middle Lithuanian (WL/MLD) domains considered as a continental margin at 1.84–1.86 Ga and the fragmented Latvia-East Lithuania (LEL) domain, where the oldest continental crust was generated at c. 1.89–1.87 Ga. The metamorphic zircon overgrowths document a high-grade event at 1.79 Ga and then constrained at 1.5 Ga. Dating of pre-Mesoproterozoic crust cryptic within the AMCG Mazury Complex provides valuable information on the nature of the pre-existing blocks formed during the long lasting Svecofennian orogeny.