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.