宮守超苦鉄質岩体の物理的性質

Abstract

Physical properties of 61 rock sample collected from 22 outcrops of the Miyamori ultramafic complex in the Kitakami Mountains are reported in this paper. Their density, P-wave velocity, magnetic susceptibility, Curie temperature, and natural remanent magnetization were newly measured and discussed based on the petrographic data. The western part of the Miyamori ultramafic complex was intruded by an early Cretaceous granite, which thermally affected one-third of this complex. The characteristics of data are summarized as follows. Rock density varies from 2.56 to 3.15 g/cm3 with an average of 2.76 g/cm3 (gabroic rocks are excluded). P-wave velocity varies from 4.26 to 6.76 km/s with an average of 5.83 km/s. Magnetic susceptibility varies from 1.2 to 14.4×10−3 (cgsemu) and the Köigsberger ratio varies widely from 0.3 to 4.9, with ratio less than 1.1 for most of the strongly metamorphosd rocks. Curie temperature is approximately 578°C in 65 percent of the rock samples, indicating the presence of magnetite, whereas many strongly metamorphosed rocks have two Curie temperatures for hematite, magnetite and titanomagnetite. One sample have three Curie temperatures for hematite, magnetite and titanomagnetite. The declination of natural remanent magnetization of the rocks are mostly plotted in the northwest and all the inclinations plot downward. The average of inclination and declination are 50°N and 35°W, respectively. In preliminary palaeomagnetic observations by AC demagnetizations, the mean direction of 18 outcrops show the results similar to that of natural remanent magnetization (51°N, 39°W). Significant difference was not recognized between the strongly metamorphosed and the less metamorphosed rocks. It is evident that the decreasing tendency of rock density and P-wave velocity with increasing magnetic susceptibility was caused by serpentinization. But it is not clear when serpentinization occurred before and/or after the tectonic emplacement to the shallower level or late stage tectonic disturbance (about 500°C; Shibata et al., 1992) of the Miyamori ultramafic complex.