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

Whole-rock laser-ablation Ar-40/Ar-39 dating of dolerite sills, penetrated at depths between 4479 and 5404 m during the drilling of the Kolva super-de ep 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 Ea rly Carboniferous (Visean-Tournaisian boundary) emplacement age of 351 +/- 2 Ma. The sills are intruded into Early Devonian (Lochkovian) sediments, wh ich 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 wi th the widespread Frasnian-Famennian magmatic activity of the EEP. Two samp les 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 lik ely to be a consequence of thermal re-equilibration and alteration. Major- and trace-element geochemical data suggest that the sills represent the int rusion, into the deeper parts of the Pechora basin, of subalkaline (tholeii tic) 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-are bas in associated with the Uralian subduction system, implying westward subduct ion 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 T ournaisian age are missing. (C) 1999 Elsevier Science B.V. All rights reser ved.