Palaeomagnetic and rock magnetic investigations of the Sipoo Subjotnian quartz porphyry and diabase dykes, southern Fennoscandia

Abstract

The Sipoo quartz porphyry dyke swarm (UPb zircon age 1.63 Ga) in southern Finland carries an intermediate to high coercivity remanent magnetization A N ( D = 22.2°, I = −1.8°, k = 34.2, α95 = 13.3°, n = 5 dykes) which is regarded as primary. The Sipoo diabase dykes, emplaced slightly before the granite porphyries, yield a highly scattered antiparallel direction A R ( D = 198.0°, I = −6.5°, k = 6.2, α95 = 40.1°, n = 4 dykes) isolated in intermediate to high coercivities. Based on positive contact test results, remanence A R is primary in the diabases. The best preserved diabase dyke, as determined by petrographic and bulk magnetization properties, carries both the A R magnetization and the A N magnetization as a secondary component in some samples. The most altered diabase dyke is almost completely overprinted by A N . The overprinting has taken place as a result of hydrothermal alteration and formation of new magnetite during the emplacement of quartz porphyry dykes and rapakivi granite. The quartz porphyries and diabases carry mixed SD and MD magnetic grain sizes, the large MD grains being responsible for a relatively large Present Earth's Field (PEF) magnetization. In addition, a high coercivity magnetization B ( D = 23.9°, I = 52.2°, k = 215.8, α95 = 6.3°, n = 4 dykes), presumably a chemical overprint, has been isolated in the quartz porphyries. The B magnetization is interpreted to be either an intermediate direction between A N and PEF with no geological significance, or to represent a separate reactivation of the crust at about 1.32 Ga ago.