40Ar/ 39Ar dating, geochemistry and tectonic setting of Early Carboniferous dolerite sills in the Pechora basin, foreland of the Polar Urals

Abstract

Whole-rock laser-ablation 40Ar/ 39Ar dating of dolerite sills, penetrated at depths between 4479 and 5404 m during the drilling of the Kolva super-deep well in the Pechora basin, located in the northeastern part of the East European Platform (EEP) in the foreland of the Polar Urals, indicates an Early Carboniferous (Visean–Tournaisian boundary) emplacement age of 351±2 Ma. The sills are intruded into Early Devonian (Lochkovian) sediments, which constrains their maximum age to less than 400 Ma. They were previously considered to be of late Frasnian age (ca. 370 Ma) based on a comparison with the widespread Frasnian–Famennian magmatic activity of the EEP. Two samples from one of the sills are moderately altered and give younger ages (ca. 290–320 Ma) than the other samples. This apparent rejuvenation is most likely to be a consequence of thermal re-equilibration and alteration. Major- and trace-element geochemical data suggest that the sills represent the intrusion, into the deeper parts of the Pechora basin, of subalkaline (tholeiitic) basalt magmas generated by moderately high degrees of partial melting of a subduction-modified mantle source which may include an enriched mantle plume component. The inferred geodynamic setting is that of a back-arc basin associated with the Uralian subduction system, implying westward subduction beneath the EEP. The timing of sill emplacement corresponds to the age of a basal Carboniferous unconformity within the Kolva well at which Visean carbonates overlie a Frasnian–Famennian carbonate sequence and strata of Tournaisian age are missing.