No abstract

Tholeiitic pillow basalt dredged from the Hakuho seamount located in the Kinan seamount chain, the Shikoku Basin, has peculiar magnetic properties; wide range intensity and high stability of natural remanent magnetization (NRM), low susceptibility, high initial Curie temperature and relatively small lattice constant. Four specimens of the pillow basalt have been cored from rim to center of one boulder sample. The NRM intensity decreases from center to rim, but the stability increases from center to rim. Reduced intensity of NRM in the rim is possibly caused by a high degree of low temperature oxidation. High magnetic stability is due to the very fine size of ferromagnetic minerals.

Directions of stable natural remanent magnetization as well as thermomagnetic and opaque-mineralogical characteristics of alkali basalts occurring in Takakusayama and Ryuso districts immediately east (in the oceanside) of the Setogawa ophiolite belt indicate that the alkali basalts were formed as seamounts or post-spreading volcanics, partly emerged at one time, at a site not far from the present location. The masses of alkali rocks were dissected by the normal faults with a strike of N40 E and then accreted along the outer margin of the Setogawa ophiolite belt.

Intensity and directions of natural remanent magnetization and their stability against alternating field demagnetization as well as thermomagnetic behavior, opaque mineralogy and chemical composition of constituent ferromagnetic minerals have been extensively investigated with pillow and dyke basalts, dolerites, gabbros and ultramafic rocks composing the Hayama-Mineoka and Setogawa ophiolite complexes.It has been demonstrated by the present work that degree of alteration of most basalts in these ophiolite complexes is low and comparable to that of the oceanic basalts. Titanomagnetite is subjected to low-temperature oxidation mostly owing to bleaching of ferrous ions but directions of the original TRM appear to be unaffected by the oxidation, although its intensity may be reduced and additional CRM may be acquired. Hydrothermal alteration recognized in a restricted area substantially reduces intensity of NRM and causes scatter in directions. Ultramafic rocks have stable and relatively large NRM, which is carried by fine magnetite particles precipitated along margins of olivine crystals.