Evidence of magmatic versus hydrothermal zircon ages and mantle-crust interactions obtained from the Carboniferous alkaline intrusions at the margin of East European Craton (NE Poland)

The Mazury Alkaline Intrusions (MAI), consisting of both silica-oversaturated and undersaturated rocks, are situated at the tectonically weakened western margin of the East European Craton (EEC). SHRIMP zircon U-Pb dating of nine syenitic samples, combined with zircon δ¹⁸O oxygen isotope data and mineral chemistry, provides insight into the Carboniferous magmatic-to-hydrothermal evolution of the MAI. Three magmatic pulses, at 352 ± 3 Ma, 344 ± 3 Ma and 337 ± 2 Ma, have been recognized. Each was followed closely by hydrothermal activity, dated at 351 ± 3 Ma, 347 ± 5 Ma and 333 ± 12 Ma, respectively. The hydrothermal stages were identified from hydrothermal zircon in the silica undersaturated syenites; its porous texture, its high abundance of non-formula elements (LREE, Y, Th, U) and its depletion in ¹⁸O, in some cases resulting in negative δ¹⁸O values. The zircon with higher δ¹⁸O values in the silica oversaturated syenites reflects crystallization from crustal melt, consistent with negative whole rock εNd(t) values ranging from −3.19 to −5.02, indicative of melts generated in the lower crust. Positive whole-rock εNd(t) values, from + 2.34 to + 0.86, obtained from undersaturated nepheline syenites indicate a depleted mantle source. T_DM model ages calculated for rocks from the MAI, and from the Devonian Kola province and Permian Oslo rift, reveal a similar interval of 0.74–0.62 Ga, pointing to common geodynamic processes having affected the EEC margins during the Neoproterozoic break-up of Rodinia. The intraplate MAI differs from the typical late Paleozoic rift-related alkaline provinces of the EEC. The newly identified phases of the MAI magmatism coincide in time with the Variscan continent/continent collision. This might have led to transmission of lithospheric-scale tectonic stress into the far foreland of the orogen, including the western EEC, and possibly also to the strike-slip reactivation of pre-existing tectonic zones, resulting in local mantle decompression.