Early Cambrian Syenite and Monzonite Magmatism in the Southeast of the East European Platform: Petrogenesis and Tectonic Setting

Abstract—The paper reports new geochronological, petrological, and isotope-geochemical data on the syenites and alkali syenites of the Artyushki massif, and the monzonites of the Gusikha massif. These massifs are located along the southwestern and northeastern margins of the Pachelma aulacogen, in the southeastern part of the East European Platform (EEP). They have Early Cambrian ages of 524 ± 3 (Artyushki) and 514 ± 2 Ma (Gusikha) obtained by the U-Pb zircon method and similar ages of amphibole and K-feldspar by the ⁴⁰Ar/³⁹Ar method. This time period has previously been regarded as amagmatic in the EEP evolution. The Artyushki massif is made up of Amp–Cpx syenite porphyries and Grt–Cpx alkali syenite porphyries and their fenitized varieties. As compared to the Amp–Cpx varieties the Grt–Cpx rocks are more peralkaline (A/NK > 0.9) and have higher LREE and HFSE, and fractionated HREE patterns. The metasomatized (fenitized) varieties are more potassic and bear geochemical evidence of fluid reworking (high Y/Ho ratios, significant Zn variations, and etc.). Bulk samples have weakly radiogenic Sr isotopic compositions: (⁸⁷Sr/⁸⁶Sr)₅₂₀ are within 0.703066–0.703615. The values of ε_Nd(520) vary from –0.69 to +1.64. The Grt–Cpx syenite porphyries have the positive ε_Nd(520), while the Amp–Cpx and fenitized syenite porphyries feature negative ε_Nd. The Gusikha massif consists of biotite–amphibole and biotite monzonites. Similar to the Artyushki syenites in SiO₂ contents, the Gusikha monzonites have higher Mg# (0.22–0.54 and 0.34–0.71 for the Artyushki and Gusikha massifs, respectively). They are also characterized by a negative Nb–Ta anomaly (Nb/Nb* = 0.5), high Ва/Sr ratio, and highly radiogenic (⁸⁷Sr/⁸⁶Sr)₅₂₀ = 0.705204 and 0.705320. Their Nd-isotopic compositions correspond to ε_Nd(520) = –6.7 and ‒7.0. Two melts contributed to the formation of the Artyushki massif. One was a strongly contaminated melt (Amp–Cpx syenite porphyries, the other was weakly contaminated (Grt–Cpx syenite porphyries). The main contribution was phonolitic melt derived from the melting of a moderately metasomatized (carbonate- and amphibole-bearing) shallow lithospheric mantle. The earliest and deepest melt portions were carbonate–silicate in composition. The geochemical, as well as the Sr and Nd isotopic compositions of the Gusikha monzonites indicate a predominant crustal contribution and pervasive reworking of the lithospheric mantle beneath southeastern Volgo–Uralia of the EEP in the Mesoproterozoic. Both massifs feature the geochemistry of within-plate and supra-subduction derivatives, which suggests a postorogenic tectonic setting of the magmatism. The presence of the Early Cambrian postorogenic magmatism within the East European Platform/Baltica is direct evidence for the involvement of Baltica in the collisional and/or accretionary events during the terminal Neoproterozoic–the beginning of the Paleozoic. This suggests reworking of the lithospheric mantle of Baltica during its collision with Timanian and East Avalonian/Cadomian terranes, including Scythia.