A magnetic study of deformed rocks in the Fennoscandian Shield
1999 (English)Conference paper, Meeting abstract (Other academic)
In the central parts of the Fennoscandian shield a regional shearzone (The Storsjön Edsbyn Deformation Zone, SEDZ) cuts through three 1.70-1.85 Ga old granite-granodiorite bodies. During the emplacement of a c. 1.85 Ga old granodiorite it suffered from a regional high-grade metamorphosis that gave rise to a subhorizontal E-W striking foliation (SGU, 1992). This pattern rotates to N-S strike with a vertical dip approaching the SEDZ. Different generations of dolerite sills and dikes cut, and are cut, by the zone. AMS (Anisotropy of Magnetic Susceptibility) and susceptibility versus temperature measurements, together with AF and thermal demagnetisation, were performed on samples along three trajectories across the zone. Thin sections of selected samples were analysed. The AMS directions correlate well with tectonic fabrics and major structures interpreted from areomagnetic data, with the minimum AMS axis being parallel to the pole of tectonic foliation. In mylonites the AMS axes are tightly grouped with subvertical maximum axes and in less deformed rocks the maximum and intermediate axes form a girdle pattern. In deformed rocks most often only a soft remanence component can be established, which generally lies parallel to the maximum AMS axis. Samples from a metamorphic rock outside of the shearzone-area carry a stable remanence in AF-fields up to 160 mT, and after a correction by use of the AMS tensor, the mean remanence direction better fits the expected paleo-latitude and longitude defined by the APWP for the Fennoscandian shield. A dolerite dike that is cut by a brittle shearzone has a paleomagnetic age of c. 1.6 Ga, indicating that the SEDZ was active after the dikes emplacement, and a dolerite sill that cuts the shearzone-fabric has a paleomagnetic age of c. 1.25 Ga, an age which post dates its movements. Magnetite is generally the main magnetic carrier, but biotite dominates the magnetic properties in one of the granites. Hematite and amphiboles are also present. Examples of good correlation between mean susceptibility and the degree of AMS shows that the degree of AMS cannot be used as a measure of the magnitude of tectonic strain in these rocks.
Place, publisher, year, edition, pages
1999. Vol. 4, 626- p.
Research subject Applied Geophysics
IdentifiersURN: urn:nbn:se:ltu:diva-37629Local ID: bb3d15a0-8ecf-11dd-b0bd-000ea68e967bOAI: oai:DiVA.org:ltu-37629DiVA: diva2:1011127
European Union of Geosciences : 28/03/1999 - 01/04/1999
Godkänd; 1999; Bibliografisk uppgift: Terra Abstracts, Vol. 11; 20080930 (andbra)2016-10-032016-10-03Bibliographically approved