Gravimetrical and geochemical studies of 1.8 Ga old granites in the Strängnäs-Enköping area, south central Sweden

Abstract

Abstract The Central Swedish Gravity Low is a distinct belt of gravity lows striking west-east from the Transscandinavian Igneous Belt to the Baltic Sea. Large homogeneous intrusions of c. 1.8 Ga old potassium rich granites with high contents of elements such as Y, HREE, and Nb, typical of granites generated in tensional environments, coincide with the gravity lows in the Fellingsbro-Lisjö area in the west and the Vallentuna area in the east. Between these areas only smaller and more heterogeneous granites of this age are exposed. The gravity measurements in the Strängnäs-Enköping area, however, clearly show the existence of large granite bodies beneath the present surface. The present and previous gravimetrical and lithogeochemical investigations suggest that c. 1.8 Ga ago, a tensional zone, now striking west-east, existed in the older crust. As a result of fusion of the lower crust in the tensional zone, a huge granitic ridge was formed along what is now the Central Swedish Gravity Low. The ridge was a heterogeneous mixture of granitic melts and solid residues. The Fellingsbro-type granites rose from this heterogeneous ridge through tensional fractures, and homogeneous intrusions were formed. The structural pattern of the tensional zone has a geometry remarkably similar to a modern oceanic ridge.