A paleomagnetic and AMS study of the Rätan granite of the Transscandinavian Igneous Belt, central Sweden

Abstract

The 1.85-1.67 Ga Transscandinavian Igneous Belt (TIB) forms a major Paleoproterozoic igneous complex in the western part of the Fennoscandian Shield. Different tectonic models propose that the TIB was formed in a tectonic regime governed either by compression or by extension. This paper presents an analysis of anisotropy of magnetic susceptibility (AMS) and paleomagnetic data from the Rätan granite (1.70 Ga), which forms a large pluton in the central part of the TIB. The major aim of the study is to test the existing tectonic models and to define a paleomagnetic pole position for the Rätan granite. Three contrasting magnetic fabric domains are identified. In the eastern and western domains the AMS fabric is characterized by NW-SE to N-S trending subvertical foliation planes and a girdle distribution of the maximum and intermediate anisotropy axes. The central domain is characterized by subhorizontal magnetic foliation planes and lineations that cluster in a NW-SE trending direction. We suggest that the Rätan granite was emplaced in a tectonic regime governed by NW-SE directed extension and that the pluton was fed obliquely with a magma source situated to the present northwest. A positive baked contact test is demonstrated for a basic dyke suggesting that the Rätan granite has not been re-magnetized since c. 1.6 Ga. A paleomagnetic formation mean direction is defined for the granite (decl. = 2°, incl. = 59°) that yields a paleopole position of Plat = 67° and Plon = 190°.