Rift-related igneous activity and metallogenesis in SW Bergslagen, Sweden

Abstract

The Bergslagen ore belt in central Sweden comprises a mid-Proterozoic complex of broadly coeval volcano-sedimentary and plutonic rock units, showing overlapping ages between 1.9 and 1.84 Ga, and formed in a rift tectonic setting. The volcano-sedimentary rock unit of this complex is referred to as the Bergslagen supracrustal sequence and divided into the lower leptite, the middle leptite, and the upper leptite and slate groups. In SW Bergslagen, the development of the plutono-volcano-sedimentary complex began about 1.9 Ga ago with the early volcanic phase of extrusion of lower leptite group felsic volcanics. In the subsequent initial rift phase, branching basins were formed in which felsic volcanics and iron-formations of the middle leptite group were deposited. The succeeding rift phase was marked by the deposition of felsic volcanics, volcaniclastic and other sediments of the upper leptite and slate group in rift phase basins. The rift phase is characterized by bimodal mafic-felsic volcanism and plutonism. Metabasic flows are found between the felsic metavolcanics and metasediments in the rift basins, whereas metabasic sills and dykes are associated with granite intrusions in the horsts between the rift basins. The metabasic rocks are referred to as the Bergslagen metabasic rock suite and the granites as the Bergslagen older granite suite. The alkaline to subalkaline chemistry and anorogenic suite trace element enrichments of the older granites, their close affinities to associated silicic volcanics, the continental tholeiitic character of the metabasite rock suite, and the outspoken bimodality of the rift phase magmatism, all support the contention of a rift tectonic setting. Manganese and sulfide ores are found as syngenetic exhalative-sedimentary deposits in the rift basins and epigenetic replacement deposits in older formations below the floor of and in the horsts between the rift basins. These ore deposits and associated metasomatic alterations are related to widespread sub-seafloor hydrothermal metamorphism in connection with hydrothermal convection systems driven by the heat of the various rift phase mafic and felsic intrusions and extrusions. The rift phase terminated with cessation of volcanism and was followed by the post-rift phase of tectonic compression and the intrusion of about 1.84 Ga old hybride rocks of the Hyttsjö gabbro-tonalite suite.