The Palaeoproterozoic Hamrånge and Loos formations in central Sweden contain similar metavolcanic and sedimentary rocks that have been considered to make up the upper part of the Svecofennian stratigraphy. Both formations occur in kilometer scale refolded synforms with similar deformation history, rock types, stratigraphy and metamorphic grade.
The surrounding granitoids (of Ljusdal type) are interpreted and partly demonstrated to be somewhat younger than the supracrustal rocks dated at 1.86 Ga in Loos, and 1.89 Ga in Hamrånge (Bergman et al., this volume), and consequently inferred to intrude the latter. The ca. 1.85 Ga Ljusdal granitoids, referred to as calc-alkaline, early orogenic tonalite to granodiorite, is somewhat younger than similar rocks in the Bothnian basin to the north and in the Bergslagen arc to the south that have been dated at 1.89-1.87 Ga.
The supracrustal sequences of both Hamrånge and Loos show primary features like pillows and amygdules in the basalts and cross-bedding and ripple marks in the sediments, revealing that they were deposited in water. Geochemical analyses of the basalts show that they are altered and spilitic to some extent.
The volcanism at Hamrånge was probably a continuous felsic to mafic extrusive event, rather than bimodal like in Loos and western Bergslagen. The rocks from both areas show all signatures of being tholeiitic, but the geochemical results show no signs of a continental extensional setting like that interpreted for the western part of Bergslagen.
The geochemical signatures of Hamrånge basalts suggest a volcanic arc (VAB) setting, probably of oceanic origin. Similar rocks from Loos show several features indicating a mid ocean ridge (MORB) environment 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) or intra arc basins.
2006. 184- p.