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2 Przegląd Geologiczny

2 2024

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Wykorzystanie mikrosondy jonowej SHRIMP IIe/MC do weryfikacji wieku skał krystalicznych w górach Chasagt (zachodnia Mongolia)

Wójcik Antoni, Krzemińska Ewa, Sikora Rafał, Mikulski Stanisław Z., Czupyt Zbigniew J.

Przegląd Geologiczny

2024

Vol. 72, nr 12-1

744--749

Over 60 years ago, the first age determination of granitoids was performed by the potassium-argon method, initiating a long-standing tradition of geochronological investigations at the PGI, as well as field exploration by the Polish Geological Expeditions in Mongolia. However, the early results require verification due to advances in equipment and methodology. One of the first projects carried out after the installation and commissioning of the SHRIMP ion microprobe at the PGI was the study of age of the several igneous rocks sampled in western Mongolia by a new Polish expedition. The fieldwork was carried out in the area of the Khasagt Mountains. The U-Pb SHRIMP analysis of the zircons revealed an emplacement age of the reddish granite and mafic granodioritic bodies in the range of816-775 Ma, accompanied by second generation of igneous rocks intruded at 783-768 Ma. These new data document a widespread late Tonian magmatic activity in this area. Moreover, a much younger Middle Permian magmatic episode between 276 and 268 Ma has been recognized. All these results represent progress in the correlation of the Mongolian terranes and allow for a partial correction of the chronostratigraphic position of the units, updating the geological map of Mongolia.

Skład izotopowy (U-Pb, O) cyrkonów z waryscyjskich skał magmowych w strefie uskokowej Kraków–Lubliniec i obszarach przyległych w badaniach mikrosondą jonową SHRIMP IIe/MC

Mikulski Stanisław Z., Krzemińska Ewa, Czupyt Zbigniew J.

Przegląd Geologiczny

2024

Vol. 72, nr 12-1

735--739

In situ analyses of zircon oxygen isotope compositions integrated with U-Pb dating have been used as a tracer of igneous processes on the Małopolska and Upper Silesia Blocks in the Kraków-Lubliniec Fault Zone (KLFZ). This integration provides one of the most robust records of primary magmatic oxygen isotope ratios, making them an important archive for crustal evolution considerations. The sensitive SHRIMP IIe/MC high-resolution microprobe was used to distinguish differences in melt components related to the Carboniferous-Permian magmatism and Mo-Cu (W) mineralization. The compilation of zircon oxygen isotope ratios from several samples from the KLFZ reveals variable magmatic δ18O values, interpreted as mixing of the mantle (δ18O ~ 5.3 ±0.6%) and crustal melts (δ18O >6%), with no contamination by sediments (i.e. δ18O >10%o). There is also a systematic record of the influence of hydrothermal processes with δ18O values <4%%. These results can be potentially used as a database, presented as a map of characteristic δ18O values collected as part of the tasks of the Polish Geological Survey. The starting point for this database could be a collection of about 600 oxygen isotope analyses of zircons from 20 samples of previously dated zircons from the MB in the KLFZ, accompanied by about 260 oxygen isotope analyses from 14 samples from Variscan rocks of the Sudetic area (southern Poland).