Magnetic properties of Cenozoic ophiolites in the Hayama-Mineoka and Setogawa belts, South-Central Honshu, Japan.

Abstract

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.