Geochronology of Tectonothermal Events in the Parautochthonous Eastern Segment of the Sveconorwegian (Grenvillian) Orogen, Southwestern Sweden

Timing of deformation and metamorphism yield important information for understanding the development of orogenic processes and correlation of mobile belts and continents. This thesis adds new geochronological data constraining the age of protoliths and events of reworking in the frontal region of the c. 1.2-0.9 Ga Sveconorwegian (Grenvillian) Orogen in SW Sweden. The Eastern Segment mainly consists of c. 1.80-1.67 Ga orthogneisses and is the parautochthonous belt of the Sveconorwegian Orogen, SW Sweden. It is bounded my major zones of ductile deformation, the Mylonite Zone (MZ) in the west and the Protogine Zone (PZ)/Sveconorwegian Frontal Deformation Zone (SFDZ) in the east. Metamorphic grade increases from north to south reaching high-pressure granulite conditions. The oldest event of reworking, associated with extensive veining, anatexis and intrusion of igneous rocks, took place at c. 1.46-1.42 Ga and mainly affected the high-grade, southern part of the Eastern Segment. This event is recorded by formation of new zircon and recrystallization in primary zircon in variably veined and ductily deformed, c. 1.7 Ga old orthogneisses. Coeval deformation has not been observed but local deformation is possible, or even probable. Timing of deformation north of Lake Vänern is directly dated to 975-955 Ma, using U-Pb on zircons and titanites. This event of reworking was the first affecting the orthogneisses north of lake Vänern. South of lake Vänern, findings of eclogites, approximately 20 km east of the Mylonite Zone, demonstrate that at least some parts of the Eastern Segment was depressed to >50 km during Sveconorwegian convergence prior to c. 970 Ma. Peak temperature around 750 °C was reached before c. 955 Ma during decompression and deformation under high-P granulite and amphibolite conditions. Radiometric data, preservation of eclogites and prograde-zoned garnet, demonstrate that the early stage of decompression was rapid. The pre-kinematic character of c. 1.46-1.42 Ga dykes/veins and c. 1.40-1.37 Ga plutonic rocks, together with c. 0.96 Ga synkinematic segregates evidence a Sveconorwegian age of deformation and high-grade metamorphism. The regional significance of this reworking is demonstrated by zircon ages (1.0-0.95 Ga) in high-grade metamorphic rocks from 15 widely distributed localities. Following is concluded: (1) The oldest rocks in the Eastern Segment represent reworked equivalents to younger rocks of the c. 1.85-1.67 Ga Transscandinavian Igneous Belt and c. 1.45-1.40 Ga granites, forming large portions of the crust east of the SFDZ. (2) The first event of reworking took place at 1.46-1.42 Ga (southern part of the Eastern Segment). (3) Sveconorwegian ductile deformation was coeval (975-955 Ma) in the Eastern Segment. (4) The Mylonite Zone is a lithological boundary separating the parautochthonous Eastern Segment from overlying allochthonous terranes. (5) Reliable zircon ages in poly-metamorphic high-grade rocks can only be accomplished using microscale U-Pb techniques. Recent U-Pb ages of baddeleyite from the Blekinge-Dalarna Dolerite dykes east of, and semi-parallel to, the PZ/SFDZ indicate that exhumation of the southern Eastern Segment was coeval with extension-related magmatism. Exhumation of high-grade rocks in the Eastern Segment at 960-920 Ma is attributed to extensional movements along the MZ (west-dipping) and the PZ (east-dipping)