Plate tectonic settings of the Svecofennian Palaeoproterozoic volcanic rocks at Hamrånge and Loos, south central Sweden, based on geochemical data

Abstract

The Palaeoproterozoic Hamrånge and Loos groups in central Sweden contain similar metavolcanic and metasedimentary rocks that have been considered to make up the upper part of the Svecofennian stratigraphy. Both occur in kilometre scale refolded synforms with partly similar deformation history, rock types, stratigraphy and metamorphic grade. Water deposition of the supracrustal sequences of both the Hamrånge and Loos areas is shown by primary features like pillows in the metabasalts and cross-bedding and ripple marks in the metasedimentary rocks. Geochemical analyses of the metabasalts demonstrate that they are altered and spilitic to some extent. The volcanism at Hamrånge was probably a continuous felsic to mafic suite, rather than bimodal like in Loos and western Bergslagen. The mafic rocks from both areas show all signatures of being tholeiitic, but with obvious differences. The metabasalts from Hamrånge have signatures suggesting an oceanic volcanic arc (VAB) setting. Similar rocks from Loos show several features indicating a mid ocean ridge (MORB) setting but also some signatures of a volcanic arc setting. The results from Loos can be explained by a transitional setting with a VAB influenced MORB found in back arc basins (BAB). The geochemical results from the two areas show no signs of a continental extensional or within plate (WP) setting as suggested for the Bergslagen area to the south.