Carboniferous Magmatism in the Polar Urals

Abstract

Dikes and sills of dolerites, essexite–dolerites, diorites, monzodiorites, and lamprophyres belonging to the Musyur (Malyi Khanmey) hypabyssal complex (Polar Urals) cut all Ordovician–Middle Devonian oceanic and island arc formations. Based on the geological data and previous isotope dating, the age of this complex is estimated from the end of the Late Devonian to the Early Triassic. The 40Ar/39Ar dating of hypabyssal magmatic bodies (various intrusive phases) of the Musyur complex was performed for the first time for the northern part of the Urals. The following 40Ar/39Ar ages were obtained: 349 ± 3, 347 ± 9, 339 ± 4, 334 ± 3, and 313 ± 10 Ma. They recorded the collision of the Polar Ural island-arc system with the continent of Arct–Laurussia (Arctida + Baltica + Laurentia), which led to the formation of the Carboniferous Early Ural Orogen. The prevailing northwestern (transversely to the supposed orogen) strike of most dikes in the Toupugol–Khanmeyshor area, predominantly the subduction zone geochemical features of the rocks (low and moderate TiO2 contents (0.7–2.1 wt %), P2O5 (0.1–0.8 wt %), enrichment in large-ion lithophile elements (Cs, Rb, Ba, Pb, Sr) compared to high-field-strength trace elements, the Ta–Nb minimum, and Pb and Th–U maxima), and the Early Carboniferous age estimates (349–334 Ma) lead to the conclusion that they probably formed in a syncollisional setting. The orthogonal northeastern (along the orogen axis) strike of some dikes (essexite–dolerites in more southerly regions), significantly higher concentrations of TiO2 (3.1–3.6 wt %), P2O5 (0.6–1.3 wt %), high-field-strength (Ta, Nb) and most rare earth elements (Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu), which are more typical for rift-related and intraplate igneous rocks, and their Late Carboniferous age (313 Ma) may testify to their emplacement in a post-collision setting.