PALEOMAGNETIC AND ROCK MAGNETIC INVESTIGATIONS OF THE SIPOO SUBJOTNIAN QUARTZ PORPHYRY AND DIABASE DYKES, SOUTHERN FENNOSCANDIA

The Sipoo quartz porphyry dyke swarm (U-Pb zircon age 1.63 Ga) in sout hern Finland carries an intermediate to high coercivity remanent magne tization A(N) (D = 22.2 degrees, I = -1.8 degrees, k = 34.2, alpha 95 = 13.3 degrees, n = 5 dykes) which is regarded as primary. The Sipoo d iabase dykes, emplaced slightly before the granite porphyries, yield a highly scattered antiparallel direction A(R) (D = 198.0 degrees, I = - 6.5 degrees, k = 6.2, alpha 95 = 40.1 degrees, 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 pr operties, carries both the A(R) magnetization and the A(N) magnetizati on as a secondary component in some samples. The most altered diabase dyke is almost completely overprinted by A(N). The overprinting has ta ken 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 mag netic grain sizes, the large MD grains being responsible for a relativ ely large Present Earth's Field (PEF) magnetization. In addition, a hi gh coercivity magnetization B (D = 23.9 degrees, I = 52.2 degrees, k = 215.8, alpha 95 = 6.3 degrees, n = 4 dykes), presumably a chemical ov erprint, has been isolated in the quartz porphyries. The B magnetizati on is interpreted to be either an intermediate direction between A(N) and PEF with no geological significance, or to represent a separate re activation of the crust at about 1.32 Ga ago.