Determination of magnetic properties of rocks by analysis of demagnetization curves: Hematite-ilmenite bearing rocks from SW Sweden

摘要

Granulites from the high-grade Sveconorwegian (corresponding to Grenvillian) gneiss terrane of SW Sweden were investigated in order to relate magnetic properties to the granulite facies mineralogy and the amphibolite facies retrograde mineralogy. We used standard paleomagnetic techniques, susceptibility measurements, electron and optical microscopy, and a new way to analyze demagnetization curves based on vector difference sums. The new technique allowed quantitation of the relative contributions of partial natural remanent magnetization (NRM) components as well as numerical extraction of thermal and coercive force (alternating field, AF) unblocking data. Two major components were identified, one stable which is carried by hematite-ilmenite, and one less stable carried by multidomain (MD) magnetite. The age of the hematite-ilmenite component is circa 930 Ma, and it has steep negative inclination, coercivities in excess of 150 mT, and unblocking temperatures between 540°C and 640°C. Two corresponding poles are reported (1) paleolatitude/paleolongitude (plat/plong) (218°/?24.3°) with Dp/Dm (9.4°/9.9°) (paleopole) and (2) plat/plong (203°/?51°) with Dp/Dm (4.4°/5.6°) (VGP). The MD magnetite component is a recent or Cretaceous-Paleogene, steeply positive inclined NRM component associated with strong induced magnetization (plat/plong 2°/?74° and Dp/Dm 11°/14°) (VGP). The MD magnetite was unblocked at conditions of 6–40 mT and 60–400°C. Our new analysis method was successful in showing that retrograde amphibole formation strongly decreases the importance of the hematite-ilmenite component on behalf of the MD magnetite. It has a diminishing effect on a negative aeromagnetic anomaly in the area. The 930 Ma components constitute 80–0% of the vector difference sums depending on rock composition. However, as this fraction goes down to 60%, the induced magnetization seems to outweigh the hematite-ilmenite influence on aeromagnetic anomalies due to its opposite direction. These rocks still have a strong and stable hematite-ilmenite dominated NRM.