Thermotectonic evolution of the Eastern Segment of southwestern Sweden: Tectonic constraints from U-Pb geochronology

The Eastern Segment forms a complex crustal terrane that occupies the southeastern part of the Southwest Scandinavian Domain adjacent to the less deformed 1.65-1.85 Ga Transscandinavian Igneous Belt (TIB). New U-Pb data from the Eastern Segment indicate that orthogneisses and granitoid rocks have protolith ages equivalent to the later magmatic phases of the TIB and that the earliest thermotectonic episodes in the Eastern Segment occurred between 1.70 and 1.61 Ga during the Gothian Orogeny. The comparable ages for Eastern Segment gneisses and TIB rocks represent permissive evidence for the hypothesis that the orthogneiss protoliths intruded into the western margin of Fennoscandia and are not exotic with respect to the pre-Gothian craton. The eastern limit of Gothian deformation is thus interpreted to be an intracratonic deformation front that coincides with the later Sveconorwegian deformation front along the Protogine Zone south of Lake Vänern. The post-Gothian anorogenic period was interrupted, in at least the eastern part of the Eastern Segment, by a thermal and magmatic event at 1.47 Ga that generated granitic dykes and (re)crystallized titanite. Sveconorwegian U-Pb ages and field observations are compatible with a thrusting and exhumation of western supracrustal terranes over the Eastern Segment during the Sveconorwegian Orogeny, followed by isostatic unroofing and cooling of the lower deck at 950-930 Ma. Available, albeit limited, Sveconorwegian titanite ages in the Eastern Segment young from east to west, ranging from 950 to 930 Ma, and are substantially younger than those dated in the western terranes of SSD.