Archaean-Proterozoic geochronology of the Vallen-Alhamn area, northern Sweden

A minor massif of approximately 2700 Ma Archaean granodiorite and tonalite is exposed in a Svecofennian environment at Vallen-Alhamn, near to the Arch aean-Proterozoic Palaeoboundary in northern Sweden. Close to this massif oc cur conglomerate-like rocks (in part interpreted as a magmatic, hydraulic b reccia) with fragments of Palaeoproterozoic metavolcanites and granitoids a s well as of the Archaean granitoids. The present work comprises dating of the granitoids of the massif, c. 2700 and 1900 Ma granitoid fragments in tw o occurrences of conglomerate-like rocks, and the volcanogenic matrix of on e of the conglomerate-like rocks. The age of igneous-looking zircons in the matrix (1871 +/- 2 Ma) is interpreted to be close to the time of formation of the conglomerate-like rock, and points to an Upper Svecofennian stratig raphic position. The bedrock of the Vallen-Alhamn area is thus a unique exa mple of Archaean rocks in close geographic association with Upper Svecofenn ian supracrustal or shallow-level intrusive rocks.