Palaeomagnetism of the ophiolite complexes of Meldal, Sør-Trøndelag

Abstract

A palaeomagnetic investigation of the Early-Ordovician (middle to late Arenig) sheeted dyke complexes of Meldal has revealed a complex multicomponent magneti- zation history. Three components of remanent magnetization were identified on their differing dysbilities, 'L' (low stability), 'I' (intermediate) and 'H' (high stability). The 'L' component is condidered to be a recent overprint. The 'I' component corresponds to an in-situ palaeomagnetic pole at 91.9\u00B0E, 58.6\u00B0N, Dy=12.1,DX=14.3. By comparison with a reference apparent polar wander (APW) path for Baltica, 'I' is considered to be an overprint of Jurassic age. The 'H' component (with the highest magnetic stability) was identified in two block samples from Resfjell. In in-situ form it corresponds to a palaeo-pole at 99\u00B0E, 16.4\u00B0N and in tilt-corrected form (palaeo-horizontal simply rotated into present horizontal) to a palaeo-pole at 281\u00B0E, 13.4\u00B0N. Both in-situ and tilt-corrected versions of the data fall far away from reference APW paths for Baltica and Laurentia. We suggest that a series of (Silurian) tectonic rotations, more complicated than the rotation accounted for in our simple tilt- correction procedure, are responsible for this deviation. If the remanence component does indeed pre-date complicated Silurian tectonic rotations, as its deviation from the APW paths suggests, one must consider the possibility of the remanence dating-back to when the sykes were still part of the Iapetus Ocean floor. The palaeolatitude for the dykes implied by the remanence is 12.6\u00B0S (tilt-corrected) assuming that 'H' is of reversed polarity. This choice of polarity minimises the net vertical-axis rotation (in this case anticlockwise through 100 degrees or so) necessary to restore the pole to reference data sets. The palaeolatitude 12.6\u00B0S would place them near the Laurentian margin of the Iapetus Ocean (if the remanence is presumed t