Geochronological Study of Proterozoic Granitoids in the Gneiss Complex of South-Western Sweden

摘要

The thesis comprises Rb-Sr and U-Pb isotopic investigations. The aim of the study was to establish a geochronological framework for the whole of the Gneiss Complex and to study the influence of the metamorphic processes on the isotope systems. The Gneiss Complex is split into an Eastern and a Western segment. The latter is to a large extent made up of calc-alkaline granitoids (the Aamaal granitoids) known to have Rb-Sr whole rock ages of 1650-1700 Ma. These are interpreted as intrusion ages. The U-Pb data on zircons from Aamaal granitoids in this thesis yield ages of 1620 and 1500 Ma. The Rb-Sr systems of those rocks, which yield the lowest zircon ages are severely affected. A three-stage model for the evolution of the zircons is presented. The subsequent development of the Western Gneiss Segment, including south-eastern Norway, was essentially ensialic. When the existing age data are compiled, four periods of ensialic magmatism can be outlined: 1535-1570, 1350-1400, 1200-1270 and c. 900 Ma ago. Results from the first and third of these age-groups are presented. The zircons are in most cases interpreted to yield intrusion ages although rejuvenation is likely in some instances. The third age-group generally yields Rb-Sr isochrons which record the intrusion and whose initial Sr isotopic ratios of around 0.710 indicate rock formation by remelting of pre-existing crustal material. For the case reported here, Rb-Sr and zircon ages agree and the zircons are devoid of inherited material. In Eastern Gneiss Segment reddish microline granitoids predominate. The age data are as yet too scarce to allow a grouping. Ages of 1612, 1486 and 1140 Ma are reported. Rb-Sr whole rock dating of thin slabs yields ages between 1004 and 1071 Ma, which proves the importance of the Sveconorwegian rehomogenization. The Rb-Sr mineral ages range from 962 to 970 Ma. With 36 refs. (ERA citation 13:004567)