Rock magnetic properties and their anisotropy from the Outokumpu assemblage in the Outokumpu deep drilling, Finland
Final Report Abstract
The terrane boundary between the Archaic and Proterozoic shield in Eastern Finland is characterized by high magnetic anomalies. These anomalies occur in the Outokumpu (OKU) assemblage of the the 2516 m deep OKU Deep Drill Hole comprising of serpentinized mantle material, black schist, skarn and quartz rocks in the depth interval 1314 and 1515 m. We observed that the strongest magnetic anomalies are related to three metaperidotite bodies (1328 - 1364 m, 1408 - 1432 m, 1466 - 1473 m) with a significant scattering of magnetic susceptibility from 0.04 to 192.7*10-3 SI and natural remanent magnetization from 0.06 mA/m to 45.1 A/m indicating a heterogeneous distribution of ferrimagnetic minerals in the ultramafic rocks. The main ferrimagnetic minerals are magnetite and pyrrhotite. A variation of the Königsberger ratio (Q) from below 1 up to 16 indicates that the ferrimagnetic metaperidotites in the assemblage are able to carry a remanent magnetization (Q ratio > 1). The strong scattering of rock magnetic properties is interpreted to reflect a complex geological history of the studied rocks. Magnetite is known to form during the serpentinization of ultramafic rocks on the sea floor or during subduction-related processes. In the OKU assemblage it was partially decomposed during early deformation stages by listwaenite-birbirite-type carbonate-quartz alteration. During subsequent events of regional deformation, the magnetic minerals were deformed or some new formation of magnetic minerals occurred. Microfabric and magnetic fabric investigations were done in order to examine deformation within the OKU assemblage during the collisional deformation event 1.9 Ga ago. Welldefined magnetic axes (kmax, kmin) and nearly horizontal magnetic foliation subparallel to the macroscopic foliation (mostly defined by talc minerals) occur at the top and the bottom of the OKU formation. Low- and high-field AMS measurements showed that ferri- and paramagnetic subfabrics are coaxial. The anisotropy values (P’) are high and range between 1.3 and 2.4 in the top and between 1.4 - 3.6 in the bottom unit, and the shape factor (T) trends to more oblate shapes (T > 0). The central anomaly shows the highest magnetic susceptibility, which is related to a high content of magnetite and pyrrhotite, mostly concentrated in huge aggregates and veins. Some of the samples show extremely high P’ values up to 5.6. The intercalated paramagnetic units (talc-carbonate and skarn rocks) in between the three serpentinite bodies show mostly triaxial magnetic fabrics, and low P’ values (<1.3) with a high variation of T from prolate to oblate shapes. The alignment of magnetic minerals parallel to the structural foliation and the high P’ values in the ferrimagnetic metaperidotite indicates that the magnetic fabric was acquired in a high strain zone. It is suggested that the magnetic fabrics represent shear zone fabrics, which have been formed during early obduction-related deformation stages of the Svecofennian orogeny. Later regional deformation caused folding and thrusting of the C-planes and is responsible for a stacked sequence of presumably one, formerly coherent, metaperidotite body.
Publications
- (2007) Magnetic logging, electrical tomography and rock magnetism in the Outokumpu drill hole, in: Kukkonen I.T., (Editor), 2007. Outokumpu Deep Drilling Project, Second International Workshop, May 21-22, 2007, Espoo, Finland. Programme and Extended Abstracts. Geological Survey of Finland, Report Q10.2/2007/29, 61-6
Hördt, A., Danckwardt, E., Kontny, A., Kück, J., Laaß, D., Leven, M., Steveling, E., Stoll, J.
- (2007) Magnetic logging, electrical tomography and rock magnetism in the Outokumpu drill hole, IODP- ICDP Kolloquium, Potsdam
Hördt, A., Danckwardt, E., Kontny, A., Laaß, D., Leven, M., Steveling, E., Stoll, J.
- (2009) Correlation of petrophysical logging data with core data for the depth interval 1300-1530 m of the OKU drilling. Outokumpu Deep Drilling Project, 3rd International Workshop, Espoo, Nov. 12-13, 2009, 86-90
Dietze, F., Kontny, A., Virgil, C., Kück, J., Hördt, A., Airo, M.-L.
- (2009) Rock magnetic properties and their anisotropy from the Outokumpu assemblage in the Outokumpu deep drilling, Finland. IODP/ICDP Kolloquium Potsdam, 16.-18.-03.2009, 44
Dietze, F., Virgil, C., Kontny, A., Airo, M.-L.
- (2009) Rock magnetic properties and their correlation with mineralogy / geochemistry for the serpentinite sequence of the OKU drillhole (Eastern Finland). Geophysical Research Abstracts, 11, EGU2009-8261
Dietze, F., Kontny, A., Airo, M.-L., Kramar, U.
- (2009) Rock magnetic properties of the Outokumpu rock type assemblage as function of mineralization process. 3rd International Workshop, Espoo, Nov. 12-13, 2009, 77-80
Kontny, A., Dietze, F.
- (2010) Magnetic fabric analysis of the Outokumpu serpentinite body in the upper crust of Eastern Finland. American Geophysical Union, Fall Meeting 2010, abstract GP41A-1028
Dietze, F., Kontny, A.
- (2011) A study of magnetic properties of serpentinites from the Outokumpu deep drill hole, Finland. In: Outokumpu Deep Drilling Project 2003-2010, edited by I.T. Kukkonen, Geological Survey of Finland, Special Paper 51, 133-150
Dietze, F., Kontny A.
- (2011) Three-component magnetic logging in the Outokumpu deep drill hole. In: Outokumpu Deep Drilling Project 2003-2010, edited by I.T. Kukkonen, Geological Survey of Finland, Special Paper 51, 119-132
Virgil, C., Hördt, A., Leven, M., Steveling, E., Kück, J., Dietze, F.
- Magnetic fabrics in deformed metaperidotites from the Outokumpu Deep Drill Core, Finland: implications for major crustal shear zones. Tectonophysics, Vol. 629. 2014, pp. 224–237.
Kontny, A., Dietze, F.
(See online at https://doi.org/10.1016/j.tecto.2014.04.011)
