日本の新旧オフィオライトの岩石磁気学的特徴

抄録

Magnetic properties of the pillow basalts, dike basalts and ultramafic rocks of the ophiolite suite of the Boso-Miura Peninsula (Cenozoic) are compared with those of the ophiolite suite in the Maizuru district (Paleozoic). From the dike basalt of Boso Peninsula to the least altered dike basalts of Miura Peninsula and Maizuru district, the average intensity of the natural remanent magnetization (NRM) decreases from a maximum of 1.5×10^-3 to 8×10^-4 (emu/cc).
The Köenigsberger ratios (Qn) of these dike basalts range from 3.2 to 0.4. Thermomagnetic analyses for the dike basalts from the Cenozoic ophiolite suite exhibit the characteristic behavior of low temperature-oxidized cation deficient titanomagnetite or titanomaghemite.
In contrast the dike basalt of the Paleozoic ophiolite suite does not have a clear irreversible thermomagnetic curve. The pillow basalts of the Cenozoic ophiolite have NRM intensities from 3×10^-3 to 6×10-4 (emu/cc) and Qn-values from 8 to 5. Thermomagnetic analyses of the Cenozoic pillow basalts are quite similar to those of the Cenozoic dike basalts. In the Paleozoic ophiolite suite of the Maizuru district, pillow stractures are not developed in the basalt, but auto-brecciated pillow basalt are present. These auto-brecciated pillow basalts have NRM intensities of abont 1×10^-4 (emu/cc) and Qn-values of about 0.3.
The ultramafic rocks of the two ophiolite suites show wide range of NRM intensities from 1×10^-3 to 1×10^-4 (emu/cc). Thermomagnetic analyses of the ultramafic rocks have reversible thermomagnetic curves for pure magnetite.