Magnetic Characteristics of Sedimentary Rocks from the Tsengwen-chi and Erhjen-chi Sections in Southwestern Taiwan

Four basic magnetic parameters (κ, SIRM, Hcr, MDF(subscript SIRM))and two thermal magnetic analyses were investigated in this study to characterize the magnetic properties of magnetite-, pyrrhotite- and greigite-bearing rocks of the Tsengwen-chi and Erhjen-chi sections. These rocks have very similar behaviors in the acquisition of isothermal remanent magnetization (IRM) in spite of bearing with different magnetic minerals. Because of the difference in magnetic mineral concentration and grain size, greigite-, pyrrhotite-, and magnetite-bearing rocks have relatively high, intermediate and low parameter values (κ, SIRM, Hcr, MDF(subscript SIRM)), respectively. Thermomagnetic analysis in a nitrogen atmosphere has further revealed that each magnetic mineral has its own diagnostic magnetization curves and that greigite could have transformed to pyrrhotite and magnetite, pyrrhotite to magnetite, and magnetite to hematite. Bulk magnetic susceptibility has also shown remarkable drop in greigite-bearing rocks below 320℃, while there are minor and almost noneffective susceptibility changes in pyrrhotite- and magnetite-bearing rocks below 400℃, respectively.