A revised geochronology for the Blekinge Province, southern Sweden

Abstract

Zircon grains from nine samples of metavolcanic rocks, gneisses, granitoids and migmatites from the Blekinge Province in southeasternmost Sweden have been dated by U–Pb using ion microprobe. The results suggest that most of the Blekinge bedrock was formed within a narrow time interval of 1.77–1.75 Ga, including the Västanå supracrustal formation and ‘coastal gneisses’ previously dated to c. 1.70 Ga. One sample, the 1.81 Ga Nättraby gneissic granite, appears to represent a sliver of slightly older proto-crust. An age of c. 1.75 Ga for an aplitic granite crosscutting the deformed Tving granitoids in eastern Blekinge brackets their deformation to between 1.77 and 1.75 Ga, whereas zircon grains of similar age from a migmatite neosome at Lindö may be inherited, making the result inconclusive. Thin metamorphic zircon overgrowths and resetting of the U–Pb system in titanite indicate a regional tectonothermal event at 1.45 to 1.40 Ga, accompanying the intrusion of the Karlshamn-type granites. The crust of the Blekinge Province thus was formed at 1.77–1.75 Ga from relatively juvenile sources in a subduction-related environment along the southern edge of Fennoscandia. It was deformed and partly uplifted relative to the undeformed TIB-1 granitoids of the Småland block further north, prior to the intrusion of the Karlshamn-type granites at 1.45 Ga.