Genesis of the Fellingsbro-type granites: Evidence from gravity measurements and geochemistry

Abstract

The Fellingsbro granite sensu stricto is a porphyritic coarse-grained rock characterized by red angular microcline megacrysts. A total of 52 samples of the Fellingsbro granite and the Lisjö granite (of the same type and situated 20 km to the northeast) have been analysed for major elements and Ba, Rb, Sr, Cu, Zn, Pb, Ga, Y, Zr, Nb, Mo, Th and U. Ten samples were analysed for REE, Ta, Hf, Cs and Sc. Both intrusions belong to a belt of ca. 1.78 Ga old granites which coincides with a major negative gravity anomaly, the so-called Central Swedish Gravity Low, We have carried out gravity measurements and obtained a fairly dense regional gravity network from the Fellingsbro area to Västerås. In addition, an approx. 60 km long profile with gravity stations ca. 200 m apart was measured across the Lisjö granite. The porphyritic Fellingsbro and Lisjö granites were generated in a tensional environment. They rose from a huge granite ridge through structurally controlled root zones, flowed out and formed mushroom-shaped intrusions. Even-grained granites occurring in the Fellingsbro and Lisjö areas were generated through a mixing between the original granitic magma and locally remobilized country rocks. The porphyritic granites are of the same age and have similar geochemical and geophysical properties as the Småland-Värmland granites. We speculate that both groups were generated in a tensional environment on the continental side of a major subduction-related calc-alkaline belt; remnants of which may be preserved in the Southwest Swedish Gneiss Region. The Lisjö and Fellingsbro granites were generated more towards the interior of the pre-existing ca 1.9 Ga old continent.

Key Words:

• Fellingsbro granite
• Lisjö granite
• south central Sweden
• density
• susceptibility
• gravity measurements
• subsurface shape of granites
• geochemistry
• genetic model
• tensional environment