Perovskite U-Pb ages and the Pb isotopic composition of alkaline volcanism initiating the Penno-Carboniferous Oslo Rift

The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period of some 50 million years through a process characterized by moderate extension and widespread magmatism. The overall tectonic situation places the Oslo Rift in a post-collisional, dextral transtensional setting related to the convergence between Baltica, Laurentia, Gondwana and Siberia during assembly of Pangea, the location probably reflecting the control by pre-existing lithospheric structures. Although a detailed understanding of these factors and processes relies strongly on having a good age control, the dating of mafic to ultramafic alkalic volcanic units formed during initial rifting has been a very challenging task. In this study we have successftilly employed perovskite from melilitic and nephelinitic volcanic rocks, together with magmatic titanite in a more evolved ignimbrite, to obtain ID-TIMS high-precision U-Pb ages. Three samples from various levels of the Brunlanes succession, in the southernmost exposures of the Oslo Graben, yield ages of 300.2 +/- 0.9, 300.4 +/- 0.7 and 299.9 +/- 0.9 Ma. A melililitic tuff at the base of the Skien succession further to the northwest yields a slightly younger age of 298.9 +/- 0.7 Ma. The initial Pb compositions derived mainly from coexisting pyroxene, apatite and homblende are characterized by extremely radiogenic initial Pb-206/Pb-204 ratios (up to 21.3) that confirm a provenance of these early alkaline basalts from HIMU-type sources. The U-Pb ages coincide with the Gzhelian age inferred from fossils in the upper part of the basal rift sedimentary fill of the Asker Group, and postdate the underlying basal sedimentary sequences by some 10 million years, pointing to a relatively rapid initiation of the rifting process. (c) 2007 Elsevier B.V. All rights reserved.