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Abstract
A minor massif of approximately 2700 Ma Archaean granodiorite and tonalite is exposed in a Svecofennian environment at Vallen-Alhamn, near to the Archaean–Proterozoic Palaeoboundary in northern Sweden. Close to this massif occur conglomerate-like rocks (in part interpreted as a magmatic, hydraulic breccia) with fragments of Palaeoproterozoic metavolcanites and granitoids as 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 two occurrences of conglomerate-like rocks, and the volcanogenic matrix of one 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 stratigraphic position. The bedrock of the Vallen-Alhamn area is thus a unique example of Archaean rocks in close geographic association with Upper Svecofennian supracrustal or shallow-level intrusive rocks.