Regional metamorphism in the Bergslagen Province, South Central Sweden

Abstract

Regional metamorphism in the Bergslagen Province of South Central Sweden reached but did not significantly extend beyond the granulite facies boundary. It occurred during a less than 20 Ma, and probably less than 5 Ma, period between 1.89 and 1.84 Ga. It resulted from burial metamorphism in an extensional basin under a moderately high geothermal gradient of at least 50°C km-1. Within this relatively warm terrane, submarine areas of recharge are delineated by zones with lower metamorphic facies. The high heat flow produced deep hydrothermal convection through the volcano-sedimentary pile resulting in extensive alteration, particularly alkali metasomatism, and concomitant ore formation. Migmatite development, related to incipient melting occurred in the pelite-infilled, deeper part of the basin. At this high heat flow, melting occurred at less than 13 km depth. The granitoids resulting from this process represent the urgranit of central Sweden and display mixed S- and l-type characteristics. Intrusive activity in a region with intense hydrothermal activity resulted in substantial explosive volcanic activity and the formation of large volumes of volcanoclastics. Sediment and subsequent volcanoclastic loading provided a virtually autocatalytic subsidence driving mechanism for the basin, resulting in extensive melting. The basin developed through lithospheric stretching and attenuation of an earlier continental crust. Nd isotopic systematics may be reinterpreted to suggest a substantial 2.2-2.0 Ga component in the older granitoids. This is consistent with other isotopic and geochemical data which strongly suggest an ensialic development for the district. The earlier crust is now represented by the older granitoids, which were produced by basement melting at the crust-mantle interface during burial. The pegmatite-rich younger granitoids are representatives of the L.8-1.7 Ga intrusives which mantle the Province to the west. These granitoids penetrate the Bergslagen crust sporadically; they may underlie the western edge and have given rise to an extensive veining and pegmatite formation, unrelated to the major metamorphic event. The development of Bergslagen is similar to Phanerozoic extensional basins, except that a deeper part of the crust is uncovered. In these deeper zones with high heat flow, major deformation was probably ductile rather than by brittle failure. The development of the Bergslagen basin immediately preceded plate collision at the end of the 1.9-1.8 Arizona-Finland Wilson cycle, which is expressed in Bergslagen as intensive folding rather than extensive thrusting.