Geochronology and palaeomagnetism of the Hunnedalen dykes, SW Norway: implications for the Sveconorwegian apparent polar wander loop

The post-Sveconorwegian Hunnedalen dyke swarm intrudes the high-grade Prote rozoic gneiss complex of SW Norway. The dykes yield a Sm-Nd whole-rock-mine ral date of 855 +/- 59 Ma and a Ar-40/Ar-39 biotite plateau date of 848 +/- 27 Ma (2 sigma). The consistency of these two age determinations suggests that 850 Ma approximates the intrusion age of the swarm. Palaeomagnetic dat a from the dykes match palaeomagnetic directions obtained from Late Svecono rwegian massive-type anorthosites (932-929 Ma) in SW Norway. The collective palaeomagnetic data (mean pole: latitude = 43.7 degrees S, longitude = 213 .3 degrees E, A(95) = 4.6 degrees) have uniform magnetic polarity, and, whe n considered in the context of the new geochronologic data, require that re manence acquisition in Sveconorwegian rocks (massif-type anorthosites and g neissic basement) of SW Norway is, in fact, not Sveconorwegian in age, but rather younger and related to a regional metamorphic/hydrothermal feature a t ca. 850 Ma. The new age data question the validity of a counter-clockwise Sveconorwegian apparent polar wander loop in the Baltic data-sets and ther efore require a critical re-evaluation of Rodinia Supercontinent fits based on palaeomagnetic data. (C) 1999 Elsevier Science B.V. All rights reserved .