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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.