Browsing by Author "Bohm, Katja"

Mid-Proterozoic mafic dyke swarms in southern Finland are associated with rapakivi magmatism. The dyke swarms are commonly referred to as “Subjotnian” (1.64–1.54 Ga), being older than the rift-filling Jotnian sandstones. Mafic rocks from five dyke swarms located in Åland, Satakunta, Häme, Suomenniemi and Sipoo were studied in this thesis. An X-ray fluorescence (XRF) analysis was made of 110 rock samples from 101 mafic dykes and one mafic intrusion. The analyses were made of the same rock samples as previous paleomagnetic studies. Overall, the Subjotnian mafic dykes in southern Finland are hyperstene-normative tholeiitic basalts or basaltic andesites with varying MgO contents (3–15 wt%). Some dykes show alkaline features with higher total alkali and/or Nb/Y values. They vary from quartz- to olivine-normative types. The dykes of the Åland swarm form two geohemical groups. The division is accompanied with a switch in magnetic polarity and distinct virtual geomagnetic pole positions. These observations imply that two separate magmatic events/pulses that have an age difference have taken place in Åland. The Satakunta dykes form two geochemical groups of which the other includes presumably Svecofennian dykes that show high Nb/Y values at given Zr/Y ratios. The dykes of the Häme swarm form three geochemical groups. Although some Suomenniemi dykes show geochemical and paleomagnetic affinities to Häme dykes, they probably represent a distinct igneous event of the event that formed the nearby Häme swarm. The Sipoo dykes are very homogeneous in their geochemistry and can be distinguished from the emplacement events that formed the other Subjotnian swarms. The Subjotnian dyke swarms in southern Finland that are believed to have emplacement ages of >1.63 Ga (Häme, Suomenniemi and Sipoo swarms in S-SE Finland) generally have higher Nb/Y (and Zr/Y) values than the dyke swarms that are believed to record younger magmatic events at <1.58 Ga (Åland and Satakunta swarms in SW Finland). Some Satakunta dykes, however, have geochemical and/or paleomagnetic implications that suggest they have an older Subjotnian age than the dated 1.57 Ga dyke in Satakunta. Further chronological work on the Satakunta dyke swarm is needed to verify the age of the dykes. Many of the Subjotnian dykes show a secondary magnetization component, called the “B-component”, whose direction is always close to, but distinct of, the Present Earth Field (PEF) at the sampling location. There was no correlation between the B-component and the magma types of the dykes. The B-component occurs mostly in dykes that are very altered. Thus, the results support previous suggestions that the B-component formed due to hydrothermal alteration of the rocks and the subsequent formation of new magnetic minerals.