ELSEVIER Precambrian Research 77 (1996) 91-103 Geological, geochemical and geochronological evidence for a new Palaeoproterozoic terrane in southeastern Sweden Joakim Mansfeld 1 Department of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden Received 27 October 1994; revised version accepted 19 July 1995 Abstract The geological and geochronological relationships between a rhyolite, belonging to the Transscandinavian Igneous Belt (TIB), and an intrusive suite ( the B~ickabyintrusion), southeast Sweden, have been investigated by means of bedrock mapping, geochemistry and radiometric age determinations ( U-Pb on zircons). Field relations suggest that the rhyolite rests unconformably on the B~ickabyintrusion, which previously has been correlated with ~ 1.9 Ga granitoids within the Svecofennian Domain. The rhyolite yielded an age of 1800 + 8 Ma and a tonalite belonging to the B~ickabyintrusion basement yielded an age of 1834 + 3 Ma. The age of the rhyolite is slightly older ( 10-15 Ma) than adjacent granitoids belonging to the Transscandinavian Igneous Belt, and is considered as the earliest magmatic expression of TIB in the area. The age and the geochemical trends for the B~ickaby intrusion is significantly different from that of the surrounding TIB granitoids and indicate a rock forming event at about 1830-1840 Ma in southeastern Sweden represented by the Oskarshamn-J6nk6ping Belt. This event is significantly younger than the older Svecofennian rocks within the Svecofennian Domain proper, indicating a hitherto unidentified geological terrane in southeastern Sweden. 1. Introduction The Proterozoic bedrock in southeastern Sweden is dominated by the Smhland-Varrnland igneous belt (also known as the Sm~dand-Varmland granite-por- phyry belt), which forms a part of the Transscandina- vian Igneous Belt (TIB). The Sm~land-V~mland igneous belt consists of various granitoids and acid volcanic rocks (Sm~land porphyries) which are con- sidered to be comagmatic (Geijer, 1963; Persson, 1973, 1974). In addition, a belt of older igneous and sedi- mentary rocks, which have been correlated with rocks in the Svecofennian Domain further to the north, is found southeast of Lake V~ittern (Welin et al., 1966; Aberg, 1978) (Fig. 1). Understanding of the Palaeo- Also at: Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, S-104 05 Stockholm, Sweden. 0301-9268/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI030 1-9268 ( 1995)00046- 1 proterozoic crustal evolution in southeastern Sweden is of importance for the interpretation of the Svecofen- nian crust-forming event and the evolution of the south- ern part of the Fennoscandian Shield. Although the age of the Sm~land-Varrnland granitoids in southeastern Sweden is well established (Mansfeld, 1991 ), very few and rather inaccurate ages and field relationships are available for other rock units in southeastern Sweden. This paper describes and discusses field relationships, U-Pb zircon ages and geochemical characters of two igneous suites situated in the Vetlanda area, southeast- ern Sweden. 2. Geological background The Svecofennian Domain (Ga~l and Gorbatschev, 1987) in Sweden and Finland consists of two different