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Detrital rutile U-Pb geochronology of the Alpine convergence in the External Western Carpathians

Doliwa Zieliński Ludwik de, Potočný Tomáš, Bazarnik Jakub, Kooijman Ellen, Kośmińska Karolina, Mazur Stanisław, Majka Jarosław

Geotourism / Geoturystyka

2023

nr 1-2 (72-73)

84--84

The Carpathian Flysch Belt represents a Paleogene accretionary wedge (External Western Carpathians – EWC) located in front of the narrow Pieniny Klippen Belt zone and the Cretaceous Central Western Carpathian nappe stack. The Flysch Belt is formed of several nappes thrust over the slope of the European Platform in the Miocene. This study is focused on the uppermost Magura Nappe, which consists of the Rača, Bystrica and Krynica subunits. As there are no relics of pre-Miocene oceanic crust in the EWC, the sedimentary rocks of the Flysch Belt are the only source of information available about the Alpine collisional events. U-Pb geochronology was applied to detrital rutile from sandstones of the Magura Nappe in order to better understand the closure of the Alpine Tethys in the Western Carpathians. Ten medium-sized sandstone samples were collected across the Bystrica and Krynica subunits in the Nowy Targ region in southern Poland. The samples represent synorogenic clastic sediments with inferred deposition ages between the Late Cretaceous and Oligocene. Approximately 200 rutile grains were separated from each sandstone sample and around half of them were selected for further analyses. The age and appearance (shape, inclusions, zoning etc.) of the dated rutile show significant variations, suggesting derivation from various sources. The most prominent age peaks represent the Variscan (c. 400–280 Ma) and Alpine (c. 160–90 Ma) tectonic events which are well-pronounced in all but the oldest dated sample. It is also noteworthy that four distinct Alpine signals were detected in our rutile data set. The two most prominent peaks with ages of 137–126 Ma and 115–105 Ma are found in majority of the samples. In two sandstone samples, deposited between the Eocene –Oligocene and the Late Cretaceous–Paleocene, the youngest peak of 94–90 Ma appears. Another peak of 193–184 Ma is also present in these two samples, as well as in another sandstone deposited between the Paleocene and the Eocene. In addition, most samples show few Proterozoic ages (approx. 1770 Ma, 1200 Ma, 680 Ma and 600 Ma). Since metamorphic rutile requires relatively high pressure to crystallize, its formation in the course of an orogeny is possible in a subduction setting. Hence, our new age data may reflect tectonic events related to subduction of oceanic crust and overlying sediments. Tentatively, we propose that recognizable events include the Jurassic subduction of the Meliata Ocean (~180–155 Ma), the Early Cretaceous thrust stacking of the Veporic and Gemeric domains (140–105 Ma) and possibly the Late Cretaceous subduction of the Váh Ocean (c. 90 Ma). In addition to dating, the Zr content of the rutile formed during the Alpine orogeny was measured by electron microprobe at the AGH University in Krakow. The amount of Zr varies between 37–420 ppm in almost all grains, with the exception of 4 rutile grains where ~1100 ppm was reached. The Zr in rutile thermometer, based on the approach of Kohn (2020) was used to calculate the possible metamorphic conditions at 450–650°C and >7.5 kbar. This data set corroborates formation of the Alpine rutile under relatively high pressure and rather low to moderate pressure/temperature gradient, i.e. typical of subduction-related tectonic environments.

Early Devonian sinistral shearing recorded by retrograde monazite-(Ce) in Oscar II Land, Svalbard

Ziemniak Grzegorz, Manecki Maciej, Jeanneret Pauline, Walczak Katarzyna, Kośmińska Karolina

Mineralogia

2022

Vol. 53, iss. 1

82--108

The Southwestern Basement Province of Svalbard extends northward from Sørkapp Land in the south to Oscar II Land. In the north, the Müllerneset Formation characterized by polymetamorphosed Proterozoic sedimentary rocks crops out. In this study we used an integrated tectonic and petrochronological approach to gain an insight into the structural and metamorphic evolution of the unit and surrounding basement. The Müllerneset Formation consists of two separate tectonic blocks. NNW-SSE trending retrograde foliation is associated with mineral and stretching lineation and kinematic indicators consistent with left-lateral to oblique sinistral shearing in the western block. The eastern block is characterized by the opposite sense of shear that was overturned during the Eurekan event as evidenced by unconformably overlaying Carboniferous sedimentary rocks. Conventional geothermobarometry yields the prograde peak pressure metamorphic conditions of 6.6 - 7.1 kbar at 480 - 520°C followed by peak temperature at 5.1 - 5.9 kbar and 530 - 560°C. Subsequent retrograde greenschist facies overprint is related to left-lateral NNW-SSE trending shearing. Tiny monazite occurs within foliation or overgrows allanite-(Ce), thus is interpreted as growth along a retrograde path. Th-U-total Pb dating of monazite-(Ce) provided an early Caledonian age (ca. 450 Ma) and younger population of ca. 410 ± 8 Ma. This age is consistent with previously reported 40Ar/39Ar cooling ages (410 ± 2 Ma) of muscovite supporting a retrograde growth of monazite. Petrochronological evidence combined with structural observations suggests that the Müllerneset Formation has been tectonically exhumed in the Early Devonian due to the NNW-SSE trending left-lateral shearing. Coeval folding and thrusting in the remaining basement of Oscar II Land to the east indicate a transpressional regime of the deformation in the Early Devonian. Similarly oriented contemporaneous tectonic zones within the Southwestern Basement Province of Svalbard may account for the same set of shear zones dispersing the Ordovician subduction complexes along western Spitsbergen.

Wstępna charakterystyka geochemiczna amfibolitów i skał ultramaficznych terranu West Ny-Friesland, północny Spitsbergen

Bazarnik Jakub, Barker Abigail, Majka Jarosław, Kośmińska Karolina, Elvevold Synnøve, Bazarnik Mirosława

Przegląd Geologiczny

2021

Vol. 69, nr 7

406--410

The metamorphic Atomfjella Complex of the West Ny-Friesland Terrane, which belongs to the Eastern Basement Svalbard Province, is composed of four nappes, namely Dirksodden, Nordbreen, Rekvika and Finlandveggen. All these nappescomprise a granitic gneiss basement associated with a metasedimentary cover, both cut by numerous mafic dykes. At the top of the Atomfjella Complex, close to the boundary with the Mosselhalvaya Group (Nordaustlandet terrane), the lenses of ultramafic rocks also occur. Some authors suggested that they provide evidence for the presence ofa deeply rooted, large-scale tectonic boundary between the West Ny-Friesland and Nordaustlandet terranes. The performed geochemical characterization of amphibolites and ultramafic rocks showed that nearly all major elements (except Si and Fe) as well as LILE, have wide compositional ranges and no obvious trends (Bazarnik, Majka, 2021). It is conceivable that the Caledonian metamorphism may have affected K, Na and P, as well as LILE, and caused scatter of Al, Ti, Ca and Fe, and likely Si. The trace and REE elements plots are characterized mainly by trends that probably express the original magmatic processes. However, the elements that clearly deviate from these trends are disturbed due to either metamorphism or crustal assimilation. According to the Th/Yb vs Nb/Yb relationship, the studied rocks indicate generally low influence of crustal contamination, with only 3 samples in the field of MORB-OIB array (Fig. 4B). Besides the higher content of Mgandsome other minor differences in chemical composition, the ultramafic rocks exhibit trends similar to that of amphibolites. Based on this aforementioned similarity and the confirmed influence of the Caledonian metamorphism on both groups of rocks, we speculate about the common history of both groups of rocks. Moreover, thank sto the identification of metamorphic alterations in ultramafic rocks, it was proved that these rocks must be pre-Caledonian and, in turn, older than the alleged terrane boundary. Thus, the ultramafic bodies located close to the top of the Atomfjella Complex cannot mark the large terrane boundary and do not provide any evidence of a deeply rooted tectonic zone, but merely the result of ascension from deeper levels of the mantle.