A Palaeoproterozoic metamorphic amphibolite to granulite facies transition in Vesteralen, northern Norway, or a tectonic boundary? Insights from magnetic data.

The nature of the Palaeoproterozoic metamorphic amphibolite to granulite facies transition in Vesteralen, northern Norway, recently has been subject to debate. The transition has previously been considered to reflect a metamorphic gradient of a Palaeoproterozoic orogenic event. An alternative interpretation is that this transition represents a tectonic boundary. We investigated this controversy using ground-and-aeromagnetic data coupled with petrophysical and rock magnetic properties. Previously aeromagnetic data has been used to delineate this metamorphic transition. Generally the granulate-facies rocks correlate with magnetic highs (positive anomalies) while the amphibolite-facies rocks are more closely related to magnetic lows (below background). A ground magnetic survey of > 25 km was run along Sortlansundet on the western side of Hinnoy Island from the granulite rocks in the southwest to the amphibolite rocks in the northeast. In addition 23 paleomagnetic sites were collected along the traverse. Density, susceptibility, and natural remanent magnetization (NRM) were measured on 150 samples. Susceptibility values range over 4 orders of magnitude, from a high of 5 x 10^-2 to a low of 7 x 10^-5 with a general decrease in susceptibilities values from the southwest to the northeast. However, these values are not evenly distributed over the profile. Q values (NRM/Induced magnetization) in both the granulite-facies and the amphibolite-facies rocks are generally <1, indicating that the induced magnetization dominates over the NRM. In addition the site NRM directions are variable resulting in a vector NRM site averages significantly lower than the scalar averages. This dispersion in NRM directions further lowers the contribution from the remanent component. The amphibolite samples in the most southwestern part of the traverse show large variability in NRM and susceptibility values. The ground magnetic traverses found large variability in the magnetic signature in the amphibolite-facies rocks with no strong indication of a sharp boundary. Overall our results support the interpretation that this boundary, as delineated by the magnetic data, more likely reflects a tectonic boundary than a transition in metamorphic grade. Paleomagnetic data will help to illuminate further on our interpretation.