Palaeomagnetism of a Caledonian ophiolite complex, Karmøy, southwest Norway

Abstract

Abstract The palaeomagnetism of the Karmoy ophiolite suite emplaced at the southwest extremity of the Scandinavian Caledonides in Middle-Upper Ordovician times is described. Low intensities of magnetisation and low Q n values in the bulk of the constituent facies are linked to the effects of low grade metamorphism, but ultramafic pods that have experienced serpentinisation yield well-defined single component magnetisations. A predominant (A) component of magnetisation isolated in 35 samples yields a mean of D I = 51° 58° (α 95 = 5°) . This population is discrete from a subsidiary (B) component recognised in 17 samples with a mean direction of 145° 63° (α 95 = 8) ; a minor (C) component ( D I = 330° 60° , α 95 = 8° ) is also identified and small numbers of anti-parallel components are identified in each case. An Upper Ordovician ignimbrite from Kattnakken, northeast of Karmoy, yields an in situ component of D I = 270° 38° ; together with similar components recognised elsewhere, this mean ( ‘D’ ) direction is most closely compatible with directions recognised in Upper Silurian-Lower Devonian rocks from the Scottish sector of the Caledonides; it is interpreted as an overprint of this age, but may include a component of local rotation. Two interpretations are considered for the remanence record in the Karmoy complex. The ‘A’ component could record the influence of a thermal event linked to uplift of the continental margin and development of the adjoining North Sea Basin in Mesozoic times if this margin has been tilted seawards by ∼ 20° since magnetisation. No plausible tectonic model can bring the ‘B’ component into conformity with the post-300 Ma apparent polar wander (APW) path and it is alternatively proposed that both the ‘A’ and ‘B’ components record Upper Ordovician post-emplacement remanences. With this interpretation the data complement a range of palaeomagnetic results from Middle-Upper Ordovician igneous and metamorphic rocks from North Atlantic bordering terranes yielding intermediate to high palaeolatitudes and contrasting with older and younger data. Collectively they imply that the Iapetus Ocean had effectively closed by these times, and that large APW movements took place between mid-Ordovician and mid-Silurian times.