Preservation of a magnetization carried by magnetite-inclusions during amphibolite grade metamorphic conditions? An example from the Sarek Dykeswarm, Northern Swedish Caledonides

Abstract

An outcrop consisting of a 608 Myr old sheeted dyke complex and dyke-intruded metasediments has been investigated palaeomagnetically. It is located in the interior of a nappe the basal and upper parts of which suffered penetrative deformation and amphibolite grade metamorphism during the Caledonian orogeny. The only trace from the orogeny at the outcrop is a widely spaced network of brittle and ductile shear zones crosscutting dyke contacts. Foliated amphibolite consisting of hornblende, quartz, plagioclase, garnet and sphene occur locally within them. The opaque mineralogy of the dykes differs from that commonly found in continental dykes, with ilmenite and pyrrhotite being the major visible opaque phases, and there are indications that magnetite inclusions exsolved from silicates carry a primary remanence even though 39Ar-40 Ar mica ages were reset at ca. 430 Myr during the orogeny. A secondary component carried by pyrrhotite is less prominent but falls on the Baltican APW path at 430 Myr. The inferred primary component has a large directional uncertainty, but indicates low palaeolatitudes for the Baltscandian margin during rifting.