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

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Abstract

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 successfully 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 hornblende are characterized by extremely radiogenic initial 206Pb/204Pb 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 post-date the underlying basal sedimentary sequences by some 10 million years, pointing to a relatively rapid initiation of the rifting process.

Introduction

Intracontinental rift systems are zones of rupture and deformation of the crust and generally also sites of magmatism. Their development is related to stress imposed on the lithosphere, and to heating or decompression in the underlying mantle. Interpretations of the actual processes and their causes are often controversial and open to debate (e.g. White and McKenzie, 1989, Anderson, 1994). Alkaline basaltic rocks are a common occurrence in continental rifts, especially in the earliest stages of extension, and their genesis is commonly discussed either in terms of partial melting of sublithospheric, enriched mantle domains, or of mantle plumes, or combinations thereof (e.g. Wilson et al., 1995, Späth et al., 2001, Furman et al., 2004, Neumann et al., 2004). It is apparent that the locations of at least some of the continental rifts are controlled by pre-existing lithospheric anisotropies, which can also exert some control on asthenospheric flow patterns and melt generation, as shown for the Baikal Rift (Lebedev et al., 2006).

The Oslo Rift is one well studied example that combines many of the classical features of continental rifts. It developed over a period of some 50 million years, from the end of the Carboniferous throughout much of the Permian, in the foreland of the Variscan Orogen. The rifting was one of the events that accompanied the larger scale assembly of Pangea. It was characterized by a low degree of extension (Pallesen, 1994) and by multistage magmatic activity. The general tectonic setting suggests that rifting was mainly a consequence of regional stretching and thinning of the lithosphere (e.g. Neumann et al., 2004), but the alternative of a mantle plume has also been contemplated (Torsvik et al., 2007). The timing of rifting and magmatism is one of the important parameters for constraining rates of magma emplacement and it is also essential for establishing larger scale correlations. Although quite extensive, the existing geochronological data base for the Oslo Rift has been obtained largely from Rb–Sr whole-rock dating (e.g. Sundvoll and Larsen, 1990, Sundvoll and Larsen, 1993). This system is very susceptible to chemical alteration and Rb–Sr ages of the Oslo Rift can be some 10–30 million years too young because of the protracted hydrothermal activity that accompanied the development of the rift (Dahlgren et al., 1996). Although, U–Pb dating using zircon and baddeleyite provides the means to circumvent this problem, such minerals are absent from the ultramafic to intermediate volcanic rocks that extruded in the initial stages of development of the rift. In this study we have constrained the age of alkaline volcanism using perovskite (CaTiO3), a mineral that has been employed with success in previous geochronological studies of kimberlites and undersaturated basaltic rocks (Heaman, 1989, Kamo et al., 2003, Heaman et al., 2004). We report here U–Pb and Pb–Pb data for perovskite from mafic and ultramafic alkalic rocks located in the Brunlanes and Skien areas, in the southernmost exposures of the Oslo Graben, complemented by U–Pb and Pb–Pb titanite (CaTiSiO5) data for a related ignimbrite, and discuss the implications for the evolution of the Rift.

Section snippets

Geological setting

The Oslo Rift comprises two main parts, the Oslo Graben (shown in Fig. 1A) and its extension southward in the North Sea, the Skagerrak Graben. The Oslo Graben is itself subdivided into several en-echelon segments displaced dextrally relative to each other and interpreted to reflect the larger scale dextral Variscan wrench faulting (Olaussen et al., 1994). The rift is located in Proterozoic crust composed mainly of 1600–1500 Ma arc sequences deformed and overprinted by metamorphism and

The Brunlanes and Skien basalts

The Brunlanes and Skien basaltic series (Fig. 1B) are two of the earliest volcanic assemblages that characterize stage 2 in the formation of the Oslo Rift.

The Brunlanes series consists of olivine–melilititic, melilite–nephelinitic and melilititic lavas in its lower parts, with basanitic flows becoming abundant in the upper part as the sequence evolves progressively into trachybasalt and then ignimbrite. In addition to the flows, there are ca. 8% pyroclastic rocks. The base is not exposed, but

Analytical procedure

The rocks were crushed and separated using, sequentially, jaw crusher, hammer mill, Wilfley table, magnetic separation and heavy liquids. Perovskite and titanite occur mainly in fractions of intermediate magnetic susceptibility and the fractions to be analyzed were handpicked under a binocular microscope. After washing and weighing the grains were transferred to Savillex vials, spiked and dissolved on a hot-plate in HF (+ HNO3) for several days. Spiking was done initially with a mixed 205Pb/235U

Rift chronology

The Brunlanes alkalic basalts are among the earliest magmatic products of the Oslo Rift. The ages obtained for this sequence at 300.4 ± 0.7 to 299.9 ± 0.9 Ma are, thus, a good estimate for initiation of magmatic activity during rifting (Fig. 5). The melilitic tuff at the base of the Skien succession yields a slightly younger age of 298.9 ± 0.7 Ma suggesting that the northward decrease in alkalinity may also reflect a temporal trend. If technically feasible, dating of the B1 tholeiitic basalts further

Conclusions

U–Pb dating of perovskite (and titanite) has established precise and highly reproducible ages for the otherwise geochronologically nearly intractable melililitic and nephelinitic volcanic rocks that represented the initiation of magmatism in the Oslo Rift. Data for the Brunlanes succession yield ages of 300.2 ± 0.9, 300.4 ± 0.7 and 299.9 ± 0.9 Ma and a tuff at the base of Skien succession yields a slightly younger age of 298.9 ± 0.7 Ma. These rocks are some 10 m.y. younger than underlying clastic

Acknowledgments

Morten Schjoldager is thanked for preparing the mineral separates and Gunborg Bye Fjeld for assistance with the isotopic work. Martin Timmerman, editor Richard W. Carlson and an anonymous reviewer provided helpful comments and suggestions.

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