The Precambrian of Gotland, a key for understanding the Proterozoic evolution in southern Fennoscandia

Abstract

Gotland is an island in the central part of the Baltic Sea, with up to 800 m Palaeozoic sedimentary rocks on top of a Precambrian basement belonging to the East European Craton. In this contribution, two major ductile deformation zones (Lickershamn - Östergarn and Lilla Karlsö-Ronehamn) are recognized and connected with the Vingåker-Nyköping and Linköping-Loftahammar Deformation Zones in the adjacent Fennoscandian Shield. These deformation zones constitute the borders between three main Precambrian segments that are correlated with crustal units within the Fennoscandian Shield and concealed parts of the East European Craton east of the Baltic Sea.The Fårö-Northern Gotland segment is dominated by continental Jotnian sandstones and Svecofennian metasedimentary rocks, separated from each other by a fault and an associated dolerite dyke. The metasediments show a specific provenance pattern with 3.29 Ga, 2.95–2.63 Ga and 2.11–1.96 Ga sources, devoid of < 1.90 Ga detrital zircons. The Alby granite truncates the metasedimentary rocks of the Fårö-Northern Gotland segment and is part of the 1.58 Ga Riga rapakivi batholith. The Central Gotland segment is dominated by metabasalts and 1.90–1.88 Ga granitoids and is correlated with the Tiveden and Valdemarsvik areas in the Fennoscandian Shield. The amphibolites in the Southern Gotland segment are correlated with 1.87–1.86 Ga metabasalts, intercalated with the Västervik quartzites in the Fennoscandian Shield. They were intruded by TIB 0 granitoids, recognized at Frigsarve and correlated with the Askersund-Loftahammar granitoids in the Fennoscandian Shield. The TIB 0 granitoids on Gotland can also be followed to the east, first to the E6-1 offshore drill hole, 30 km west of the Latvian coast and further east into southwestern Latvia and western Lithuania as a major component in the Mid-Lithuanian Domain. The Kvarne granitoid on southernmost Gotland is correlated with the TIB 1a generation in the Fennoscandian Shield and with granitoids within the Mid-Lithuanian Domain.1.48 Ga small stitching plutons on southern Gotland penetrate the TIB 0 and 1a granitoids and are correlated with the Götemar and Karlshamn plutons in the Fennoscandian Shield and several plutons in western and southern Lithuania.